U.S. patent application number 14/268360 was filed with the patent office on 2015-02-12 for optopairs with temperature compensable electroluminescence for use in optical gas absorption analyzers.
This patent application is currently assigned to BAH HOLDINGS LLC. The applicant listed for this patent is BAH HOLDINGS LLC. Invention is credited to Sergey Suchalkin, MICHAEL TKACHUK.
Application Number | 20150041655 14/268360 |
Document ID | / |
Family ID | 52447805 |
Filed Date | 2015-02-12 |
United States Patent
Application |
20150041655 |
Kind Code |
A1 |
TKACHUK; MICHAEL ; et
al. |
February 12, 2015 |
OPTOPAIRS WITH TEMPERATURE COMPENSABLE ELECTROLUMINESCENCE FOR USE
IN OPTICAL GAS ABSORPTION ANALYZERS
Abstract
Optopair for use in sensors and analyzers of gases such as
methane, and a fabrication method therefor is disclosed. It
comprises: a) an LED, either cascaded or not, having at least one
radiation emitting area, whose spectral maximum is de-tuned from
the maximum absorption spectrum line of the gas absorption spectral
band; and b) a Photodetector, whose responsivity spectral maximum
can be either de-tuned from, or alternatively completely correspond
to the maximum absorption spectrum line of the absorption spectral
band of the gas. Modeling the LED emission and Photodetector
responsivity spectra and minimizing the temperature sensitivity of
the optopair based on the technical requirements of the optopair
signal registration circuitry, once the spectral characteristics of
the LED and Photodetector materials and the temperature
dependencies of said spectral characteristics are determined,
provides the LED de-tuned emission and Photodetector responsivity
target peaks respectively.
Inventors: |
TKACHUK; MICHAEL; (South
Setauket, NY) ; Suchalkin; Sergey; (Stony Brook,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BAH HOLDINGS LLC |
GLEN COVE |
NY |
US |
|
|
Assignee: |
BAH HOLDINGS LLC
GLEN COVE
NY
|
Family ID: |
52447805 |
Appl. No.: |
14/268360 |
Filed: |
May 2, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61862992 |
Aug 7, 2013 |
|
|
|
Current U.S.
Class: |
250/338.4 ;
250/339.06; 257/184; 438/24 |
Current CPC
Class: |
G01N 21/3504 20130101;
H01L 31/101 20130101; G01N 2201/0625 20130101; G01J 3/10 20130101;
G01N 21/61 20130101; H01L 25/167 20130101; H01L 31/03046 20130101;
H01L 33/30 20130101; G01J 3/0286 20130101; H01L 33/06 20130101;
Y02P 70/521 20151101; G01N 2201/062 20130101; G01N 21/255 20130101;
Y02P 70/50 20151101; H01L 31/105 20130101; Y02E 10/544 20130101;
G01J 3/108 20130101; H01L 31/167 20130101; G01J 3/42 20130101 |
Class at
Publication: |
250/338.4 ;
250/339.06; 438/24; 257/184 |
International
Class: |
G01N 21/35 20060101
G01N021/35; H01L 31/101 20060101 H01L031/101; H01L 31/167 20060101
H01L031/167 |
Claims
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27. A process for building an LED and Photodetector optopair for
use in optical sensors for analysis of a target sample gas analyte,
said optopair being able to compensate for the effect of
temperature variation on its signal comprising the following steps:
a. identifying the target sample gas analyte; b. establishing the
target sample gas analyte absorption band by determining its
shortest, longest, and maximum spectral peak wavelength; c. using
said shortest, longest and maximum spectral peak wavelengths to
determine the material systems for each of the LED and the
Photodetector respectively; d. determining the spectral
characteristics of the LED and Photodetector materials and the
temperature dependencies of said spectral characteristics; e.
identifying the target peak wavelength for each of the LED and
Photodetector emission and responsivity spectra respectively
through modeling of the LED emission and the Photodetector
responsivity spectra using the information generated by the
preceding steps and minimizing the temperature sensitivity of the
optopair as determined by the technical requirements of the signal
registration circuitry of the optopair, such that each of the
target peak wavelengths of the LED and Photodetector emission and
responsivity spectra respectively is de-tuned from the maximum
spectral absorption peak wavelength of the target sample gas
analyte, a portion said LED emission spectrum overlaps a portion of
said Photodetector responsivity spectrum, said overlapping spectral
portions remaining within said target sample gas analyte absorption
band and the area of said overlapping spectral portions within said
target sample gas analyte absorption band remaining constant
notwithstanding temperature variations; and f. finalizing the
optopair design and fabricating same.
28. The process of building the LED and Photodetector optopair of
claim 27 wherein the step of minimizing the temperature sensitivity
of the optopair as determined by the technical requirements of the
signal registration circuitry of the optopair further optionally
comprises the step of identifying the target peak wavelengths for a
multi-cascade LED emission spectrum.
29. (canceled)
30. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application Ser. No. 61/862,992 filed on Aug. 7, 2013, which is
incorporated by reference in its entirety as if more fully set
forth herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] This invention relates to an improved optical gas sensor
with temperature compensable performance for the detection and
determination of gas concentration by means of absorption
spectroscopy, using non-dispersive radiation. More particularly,
the present invention relates to an optopair for use in optical gas
sensors and/or optical absorption gas analyzers, the optopair
comprising an LED source and a corresponding Photodetector capable
of reliable and consistent qualitative and quantitative analysis of
gases at different temperatures, notwithstanding the optopair's
sensitivity to temperature changes.
[0004] 2. Prior Art
[0005] Absorption spectroscopy refers to spectroscopic techniques
that measure the absorption of radiation, as a function of its
frequency or wavelength, due to its interaction with a sample to be
analyzed (analyte). The analyte absorbs some of the radiation, as
it comes in contact with it. The amount of the radiation absorbed
by the analyte varies as a function of the frequency of the
radiation, and the concentration of the analyte (Beer-Lambert
Absorption Law).sup.1, and this variation determines the absorption
spectrum of the sample analyte. Thus, absorption spectroscopy is
employed as an analytical tool to determine the presence of a
particular analyte and, in many cases, to quantify the amount of
the analyte present. Sotnikova, G. Y., Low Voltage CO.sub.2-Gas
Sensor based on III-V Mir-IR Immersion Lens Diode Optopairs: Where
are we and How Far we Can Go? IEEE SENSORS JOURNAL, 2009
[0006] Absorption spectroscopy, as an analytical tool, works by
directing a generated beam of radiation at the analyte and
detecting the intensity of the radiation that passes through it.
The analysis of the transmitted radiation can be used to calculate
the absorption and from the absorption the concentration of the
analyte. The source, sample arrangement, and detection technique
vary significantly depending on the frequency range and the purpose
of the experiment.
[0007] Absorption spectroscopy methods utilize any type of
radiation. Provided however, that the type of radiation utilized
depends on the atomic structure and nature of chemical bonds of the
analyte. Many gases are highly absorbent in the infrared spectral
region. Absorption spectroscopy methods using infra-red radiation
have long been recognized as sensitive, stable, and reliable
methods for the detection and determination of the concentration of
gases, in among other things, atmospheric air. If the measured
parameter is the intensity of absorbed radiation at a fixed
frequency or within a fixed frequency range, the spectroscopic
method is called "non-dispersive." Such non-dispersive infra-red
absorption measurement methods are based on the gases' molecular
properties, which enable them to interact with and absorb infrared
radiation within a certain spectral range.
[0008] When the gases are placed in the path of the infra-red
radiation and the radiation's spectrum corresponds to the
absorption spectrum of the gases, the gases will absorb such
radiation..sup.2 Further, the gases will absorb the most when the
wavelengths of maximum radiation intensity correspond to, coincide
with, and match the wavelengths of maximum gas radiation
absorption. More specifically, in a typical infra-red atomic
spectroscopy method, the concentration of the gas of interest in a
sample is determined once the absorption of the infra-red radiation
by the gas has been detected and measured. As a result,
non-dispersive infra-red absorption spectroscopy methods are widely
used to detect many different gases, including carbon dioxide,
carbon monoxide, methane, ethane, hydrogen sulfide and so on.
.sup.2 Sotnikova, G. Y., Low Voltage CO.sub.3-Gas Sensor Based on
III-V Mid-IR Immersion Lens Diode Optopairs: Where are we and how
far can We go? IEEE Sensors Journal, 2009.
[0009] Infra-red absorption spectroscopy instrumentation comes in
more than one configuration. A typical "one channel" infra-red
instrument ("sensor") comprises a source of radiation (usually
infrared), such as an incandescent lamp or another electrically
heated element that serves as a blackbody emitter, e.g., a silicon
carbide rod or nichrome filament; a narrow bandpass filter arranged
to ensure that only radiation intensity absorbed by the gas of
interest is measured; a gas chamber for containing a sample
including the target gas of interest; and a photodetector for
detecting radiation transmitted by the sample and transforming the
energy of the detected radiation into an electrical signal whose
magnitude corresponds to the intensity of the detected
radiation.
[0010] "Two channel" infra-red sensors have a signal channel and a
reference channel. The signal channel operates in exactly the same
way as the "one" channel device described above, with the
transmission band of the band pass filter adjusted to the
absorption wavelength(s) of the gas of interest. The reference
channel usually works in another wavelength band, at which the
target gas species does not absorb. This provides a base line for
the signal channel. The differential signal between the signal and
reference channels, normalized on reference channel intensity,
gives an absorption signal which is stable with respect to any
intensity drift resulting from the radiation source (or
detector).
[0011] Another type of a "two channel" infra-red sensor comprises
two photodetectors and includes two separate gas cells into which
the emission from the radiation source is split along paths of
equal lengths. One cell is filled with nonabsorptive (inert) gas to
provide a reference channel, and the other with the sample gas
(including the gas of interest). Both photodetectors work on the
same wavelength (corresponding to an absorption wavelength of the
target gas analyte) resulting in a sensor that is relatively stable
and produces reliable results.
[0012] The instrument configurations described above present some
serious design drawbacks when attempting to modify them for
portable, in-situ field use, beyond the laboratory walls. For
example, requiring a separate, sealed gas reference cell containing
an inert gas results in an instrument configuration that is
expensive, bulky, heavy and unwieldy. Similarly, using an
incandescent bulb as the source of radiation in a portable
instrument does not make sense because incandescent bulbs, which
provide the necessary wide wavelength radiation band, result in a
radiation source that is slow to respond (typically, the response
time is more than 100 milliseconds) and requires significant power
(200 milliwatts or more). As such, the instrument configurations
described above are not suitable for portable, low power sensors
which ideally should operate at a power consumption of no more than
1-2 milliwatts.
[0013] To resolve these drawbacks, portable infra-red sensors have
been developed where semiconductors are used as both radiation
sources and radiation detectors. The radiation sources are
semiconductors that behave as light emitting diodes (LEDs).
Likewise, the radiation detectors are semiconductors that behave as
photodetectors (PDs). A number of such infra-red LED sensors are
disclosed in the Tkachuk U.S. Letters Pat. No. 7,796,265 titled
Optical Absorption Gas Analyzer, (the "265" patent), which is
hereby incorporated by reference in its entirety as if more fully
set forth herein.
[0014] The '265 patent discloses an analyzer comprising a chamber
for containing the sample in use; a radiation source assembly
arranged to emit radiation into the chamber; a first radiation
detector assembly arranged to detect radiation transmitted along a
first optical path through the chamber; a second radiation detector
assembly arranged to detect radiation transmitted along a second
optical path through the chamber, wherein the length of the second
optical path which the sample can intercept is shorter than that of
the first optical path; and a processor adapted to generate a
sensing signal S.sub.S based on the detected radiation transmitted
along the first optical path and a reference signal S.sub.R based
on the detected radiation transmitted along the second optical
path, and to determine the concentration of the target gas in the
sample based on a comparison of the sensing signal with the
reference signal.
[0015] By arranging for radiation to be detected along a second
optical path which is shorter than the first, the '265 patent
provides a reference channel which operates using the same
radiation as the signal channel, yet does not require the provision
of a separate (inert) cell, since both optical paths pass through
the same chamber. The relatively short length of the second optical
path with which the sample can interact (compared to that of the
first optical path) means that absorption in the reference channel
is suppressed and can be used to accurately compensate for drift
Preferably, the length of the second optical path with which the
sample can interact is made as short as possible, and in any case
significantly shorter than that of the first optical path. As a
result any losses caused by absorption in the reference path will
be small.
[0016] The '265 patent further discloses that LEDs or any other
fast-response radiation source (a response time of less than or
equal to 100 milliseconds, preferably less than 1 milliseconds,
still preferably less than 50 microseconds) can be used as the
radiation source assembly arranged to emit radiation into the
chamber, since both the signal and the reference channels can
operate at the same or overlapping wavebands. Further,
photodetectors and preferably photodiodes, can be used as the first
and second detector assemblies.
[0017] Based on the foregoing, it appears that use of LEDs in
portable infra-red instrument configurations may seem ideal. After
all, LEDs are very fast; their response time is of the order of
less than one micro-second. They provide greater selectivity of
radiation. They have corrosive medium stability and longer periods
of service and operation. And during all of that they consume very
little power..sup.3 Sotinikova, G. Y. et, al., Performance analysis
of diode optopair gas sensors, Proc. Of SPIE, 2009, Vol. 7356
73561T; Sotnikova, G. Y. et. al., Low Voltage CO.sub.2-Gas. Sensor
Based on III-V Mid-IR Immersion Lens Diode Optopairs: Where are we
and how far can We go? IEEE Sensors Journal, 2009; Gibson D. et.
al., A Novel Solid State Non-Dispersive Infrared CO.sub.2 Gas
Sensor Compatible with Wireless and Portable Deployment. Sensors,
2013, 13, 7079-7103.
[0018] By way of background, LEDs are semiconductor emitters.
Semiconductor emitters and detectors appear to be the most
prospective candidates for a optical absorbance gas analyzers and
sensors due to reliable processing technology of semiconductors,
high repeatability and precision of the semiconductor devices. The
technology of molecular beam epitaxy (MBE) allows atomic level
precision and makes possible production of multi-layer
semiconductor structures while preserving the crystalline order
throughout the entire structure.
[0019] One of the most important characteristics of the
semiconductor material is the magnitude of its so-called "band
gap". The band gap represents the energy range which is forbidden
for the charge carriers inside the material. The band gap separates
two allowed energy ranges: lower--"valence" band and
higher--"conduction" band. In intrinsic semiconductors at low
temperature the valence band is completely occupied by electrons
while the conduction band is empty. To be excited to the conduction
band, the electron has to obtain the minimum energy which is equal
to the band gap energy or, simply, to the band gap. Excited
electrons leave the vacancy in the valence band which behaves as a
positively charged particle "hole". When the excited electron
returns back to the valence band (or, in other words, recombines
with the hole), the minimum energy it can return is the bandgap
energy. If the energy is returned in the form of electromagnetic
emission, the wavelength of the emission is determined by the band
gap. So the band gap determines the emission spectrum of the
emitter and the responsivity spectrum of the detector.
[0020] In semiconductor alloys, the band gap magnitude can be
controlled through the alloy composition. Molecular Beam Epitaxy
(MBE) allows more precise way of the effective band gap control
through the dimensional quantization effect. In a very thin layer
of semiconductor materials (with the thickness of decades of the
atomic layers and less) the charge carrier energy is determined
both by the material bandgap and the thickness of the layer. If
this layer is able to retain the carriers it is called "quantum
well". The emitters based on quantum wells allow precise tuning of
the band gap by variation of the quantum well width.
[0021] LEDs are some of the simplest semiconductor light emitters.
Their composition and structure is such that when a current is
applied to them, they emit light. As is shown in FIG. 1, a typical
LED comprises two layers of semiconductor material. Each layer has
a different type of conductivity, i.e., either n-type conductivity
or p-type conductivity.
[0022] The conductivity type depends on the type of dopant
introduced into the semiconductor material during its formation.
Dopants are distinguished into "donors" and "acceptors."
Introduction into the semiconductor material of dopants, which are
"donors", produces an excess of mobile electrons, thereby resulting
in a conductivity type which is negative: n-type. Introduction into
the semiconductor material of dopants, which are "acceptors", leads
to a majority of mobile holes, thereby resulting in a conductivity
type which is positive: p-type.
[0023] As is shown in FIG. 2, in the region near the boundary
between n- and p-type materials the electrons recombine with the
holes and a depletion region is formed. The concentration of mobile
carriers in the depletion region is reduced The depletion region is
also characterized by built-in electric field. When the external
bias is zero, the diffusion currents of holes into the n-type layer
and electrons into the p-type layer are compensated by the drift
currents produced by the built-in electric field, so the net
current is zero. When positive bias (plus to p-type, minus to
n-type) is applied to the LED, as is shown in FIG. 3, the drift
component of the current is suppressed and the net diffusion
current appears in the structure. Electrons and holes meet in the
depletion region and recombine releasing energy in the form of
photons. This effect is called electroluminescence
[0024] The place where most of the electrons and holes recombine is
called the emitter region or active region of the LED. The color of
the light (corresponding to the energy of the photons released) and
the wavelength of the LED radiation, is determined by the energy
band gap (the energy difference in electron volts between the top
of the valence band (n-type) and the bottom of the conduction band
(p-type) of the LED.
[0025] Applications of semiconductor hetero-structures have led to
breakthroughs in the technology of light emitters. A semiconductor
hetero-structure is a layered semiconductor crystal. The layers
have different band gaps but same lattice constant, so crystalline
order is preserved throughout the entire stack of layers.
Introduction of narrow band gap layers (quantum wells), which can
accumulate injected electrons and holes provide for a high
recombination efficiency and precise control of the emission
wavelength. A typical band diagram of the quantum well (QW) LED is
presented in FIG. 5.
[0026] Molecular Beam Epitaxy ("MBE") growth technology allows
fabrication of quantum well LEDs, which can be used to create
efficient cascaded LEDs. A cascaded LED structure includes several
active regions connected in series. This injection scheme allows
electron recycling so, one electron can produce more than one
photon. Cascaded LEDs have increased power at a fixed current
Methods for the fabrication of cascaded LEDs are disclosed in the
following articles, Jung, et. al., Light-Emitting Diodes Operating
at 2 .mu.m With 10 mW Optical Power, IEEE PHOTONICS TECHNOLOGY
LETTERS, VOL. 25, NO. 23, Dec. 1, 2013; Prineas, et. al., Cascaded
active regions in 2.4 .mu.m GaInAsSb light-emitting diodes for
improved current efficiency. Applied Physics Letters 2006, 89,
211108; Crowder, J G., et. al., Mid-infrared gas detection using
optically immersed, room-temperature, semiconductor devices. Meas.
Sci. Technol. 2002, 13, 882-884; Li, W. et. al., InGaAsSbN: A
dilute nitride compound for midinfrared optoelectronic devices.
Journal of Applied Physics. 2003, Vol. 94, No. 7, 4248-4250;
Ashley, T. et. al., Uncooled InSb/In.sub.1-xAl.sub.xSb mid-infrared
emitter. Applied Physics Letters 1994, 64, 2433-2435; Shterenga, L.
et. al., Type-I quantum well cascade diode lasers emitting near 3
m. Applied Physics Letters 2013, 103, 121108; Krier, A. et. al.,
The development of room temperature LEDs and lasers for the
mid-infrared spectral range. Phys. Stat. Sol. (a), 2008, 205, No.
1, 129-143.
[0027] Likewise, by way of background, conventional photovoltaic
detectors, i.e., photo-diodes ("PD") are fabricated in a manner
that is similar to the fabrication of LEDs. In contrast to LEDs,
however, PDs absorb photons and produce an electrical signal whose
magnitude is determined by the intensity of the absorbed emission.
So PDs operate in a manner that is the reverse of the operation of
LEDs.
[0028] Conventional photo-diodes like LEDs comprise two layers of
semiconductor material. Each layer has a different type of
conductivity, i.e., either n-type conductivity or p-type
conductivity. Like in LEDs, the conductivity type of each of the
layers of the photo-diodes depends on the type of dopant introduced
into the semiconductor material during its formation. Introduction
into the semiconductor material of dopants, which are "donors",
produces an excess of mobile electrons, thereby resulting in a
conductivity type which is negative: n-type. Introduction into the
semiconductor material of dopants, which are "acceptors", leads to
a majority of mobile holes, thereby resulting in a conductivity
type which is positive: p-type.
[0029] As a result, like in LEDs, the photo-diodes have a depletion
region. Light in the form of photons absorbed in the depletion
region of a photo-diode generates electrons and holes which are
separated by the built in electric field. A schematic showing this
operation principle of a photovoltaic detector is shown in FIG. 4.
The motion of the generated electrons and holes in the depletion
region creates a current whose signal can be measured precisely;
but only when the signal to electron and hole generation noise
ratio is high.
[0030] In accordance with Beer's-Lambert Absorption law (see
discussion above), when the radiation produced by an LED passes
through an analyte gas sample, the analyte gas sample will absorb
energy as the radiation comes in contact with it. The amount of
energy the analyte gas sample absorbs will vary as a function of
frequency and wavelength of the radiation. The closer the frequency
and wavelength of the radiation is to the corresponding frequency
and wavelength of absorption line of the analyte gas sample, the
larger the absorption of energy by the analyte gas sample. The
larger the absorption the better and more reliable the signal to
the photodiode detector, by which the detection and measurement the
analyte gas sample is achieved.
[0031] Thus, in accordance with the foregoing, the following must
be present for optimal optical gas sensor performance. First, the
LED used as a radiation source and the photodetector used to read
the absorption signal must be optically and spectrally matched
(optopair). And, second, the energy and wavelengths of the
radiation generated by the LED and directed on the analyte gas
sample must coincide, correspond to and match the energy and
wavelength of the maximum absorption line within the analyte gas
sample's absorption spectrum.
[0032] However, due to the manner of LED production, it is very
difficult to manufacture LEDs that consistently and reproducibly
generate radiation whose energy and wavelengths coincide,
correspond to and match the maximum absorption line within the gas'
absorption spectrum. That is the reason why, many times optical gas
sensors of the type described herein above are equipped with
bandpass filters. The filters cut off all of the radiation
emissions outside the target sample gas band, and enhance the
relative magnitude of the absorbed emission. The filter is
characterized by the spectral transmission function
F(.omega.)=.THETA.(.omega.-.omega..sub.1)-.THETA.(.omega.-.omega..sub.2)
where .THETA. is the step function.
[0033] But the use of LEDs and the photodetectors in portable
infra-red instrument configurations, such as the optical gas
sensors discussed above, is not without problems even if there are
bandpass filters. First, the photodetectors discussed herein above
produce noise during the generation of electrons and holes that
could mask and interfere with the electric current generated
thereby.
[0034] Second it is very well known that both the LEDs and the
photodetectors are inherently sensitive to temperature changes. As
the temperature changes, the LED emission spectrum broadens, its
maximum spectral position shifts, and its intensity drops.
Likewise, the amplitude and spectral position of the photo-detector
responsivity drops and shifts respectively. Thus, these gas sensors
cannot be reliable and suitable for the detection of gases in
multiple environments having different temperatures. "The
temperature shift of spectral characteristics of a source and
detector of radiation inherent to all semiconductor elements and
photoresistors without exception leads to changes of the optical
sensor output signal and consequently, to errors in calculating the
gas concentration.".sup.4 .sup.4 Sotnikova, G. Y. et. al., Low.
Voltage CO.sub.2-Gas Sensor Based on III-V Mid-IR Immersion Lens
Diode Optopairs: Where are we and how far can We go? IEEE Sensors
Journal, 2009
[0035] More specifically, when an optical gas sensor equipped with
an LED as its radiation source is taken out of an environment
having one temperature and placed in an environment having a
different temperature, the spectral position and intensity of the
LED's radiation emission changes. As the temperature increases, the
LED's band gap energy changes, resulting in a decrease of its
luminescence intensity and a shift in its spectral position. And
when it does shift, it will no longer correspond to the absorption
line of the analyte gas sample, resulting in a much smaller
absorption of radiation by the gas sample, a poor signal to the
photodiode detector, and an unreliable detection and measurement of
the gas.
[0036] In other words, when the temperature increases the spectrum
maximum of the LED's emission radiation targeting the gas sample
analyte will no longer coincide or correspond to the maximum of the
absorption spectrum of the gas sample. As a result, the gas sample
will absorb less of the radiation that comes in contact with it.
The less the absorption of radiation by the gas sample, the less
the photo-detector signal will be. The less the photo-detector
signal, the more unreliable the detection and measurement of the
gas sample will be.
[0037] Likewise, as the temperature increases, the peak of the
photo-detector responsivity curve will shift to a lower wavelength
side and its amplitude will decrease. As a result, the LED and the
Photodetector will no longer be spectrally matched. This in turn
will produce a signal that either does not register or is so small
in value that it once again, it will be unreliable for the
detection and measurement of the target sample gas.
[0038] Thus, as can be seen from the foregoing, an LED's power
output and emitted radiation spectrum depend greatly on and will
vary with the temperature of the environment the LED is being used
in. Further, the LEDs' sensitivity to temperature changes is such
an inherent characteristic of the LEDs' composition and structure,
that many of the attempts to design around it up until now have
used a totally different approach. See for example U.S. Letters
Pat. No. 8,649,012 issued to Beckman et. al. and titled Optical Gas
Sensor which attempts to solve the foregoing problem but in a
manner that is totally different from the present invention. It
discloses a sensor having a light-emitting diode, a photo-sensor, a
measuring section between the light-emitting diode and the
photo-sensor, and a control and analyzing unit, which is set up to
determine the concentration of a gas in the measuring section from
the light intensity measurement by the photo-sensor. The control
and analyzing unit is set up to measure the forward diode voltage
over the light-emitting diode at a constant current, to determine
the temperature of the light-emitting diode from the detected
forward diode voltage over the light-emitting diode by means of a
preset temperature dependence of the forward diode voltage, and to
apply a correction function as a function of the light-emitting
diode temperature determined, with which the measurement is
converted to that of a preset temperature of the light-emitting
diode. See also U.S. Application Pub. No. US 20070035737 A1.
[0039] On the basis of the foregoing, it becomes evident that the
LED radiation source in a portable infra-red gas sensor together
with a photo-diode photodetector, chosen for optimal power output
and emitted radiation wavelength at a certain temperature, will not
have the same power output and emitted radiation at a different
temperature. Thus, a single sensor will not be reliable and
suitable for use in a wide environment temperature range. In other
words, one who wants to use an infrared LED gas sensor in a room
that is at 25.degree. Celsius, would not be able to reliably use
the same gas sensor in an environment that is at 60.degree.
Celsius. Use of a single infra-red LED gas sensor in multiple
environments with different temperatures is unreliable.
[0040] A way to resolve the LED's reliability issue due to its
sensitivity to temperature fluctuations, different from U.S.
Letters Pat. No. 8,649,012 issued to Beckman (see discussion herein
above) would be to use multiple sensors for measurements of gases
at different temperatures. But that would result in having to keep
multiple sensors at hand, one for each temperature at which a gas
would have to be identified and quantified; something that would be
extremely cumbersome, expensive and highly impractical.
[0041] Resolving the drawbacks in the prior art as discussed herein
above is of paramount importance because among other things,
reliable techniques for detecting methane and monitoring its
concentration are in high demand in many industrial areas such as
mining, construction, transportation of carbohydrate fuels and many
others. The techniques need to be accurate enough to detect methane
concentration below the methane lower explosive limit (LEL, 5% in
atmosphere). A minimum accuracy of 1% is a must.
[0042] Methane has a low chemical activity. Accordingly, the
chemical methods of methane detection are not effective and
absorption spectroscopy through optical detection as described
herein above, appears to be a promising path for methane sensing
and analysis. However, any methane sensor implementing absorption
spectroscopy must provide low power consumption and portability,
with a 5-10 cm limit for the optical path of the probe emission.
Further, it has to be able to feel changes of as low as 0.1% in the
probe emission intensity. Finally it has to be based on detection
of the change in the intensity of the probe emission in the
absorption spectral range which is typical for methane only. Since
no other atmospheric gases absorb in that range, any change of
light intensity within the methane spectral range will indicate
presence of methane and in accordance with Beers-Lambert Law of
Absorbance, the magnitude of the absorption will be proportional to
the concentration of methane gas sample analyte. Thus, the emission
spectrum of the optopair light source should match to the optopair
detector responsivity spectrum and the methane absorption band.
[0043] One of the strongest and most prominent vibration-rotation
absorption bands of methane is near wavelength .lamda.=3.3 .mu.m
(.about.3000 cm.sup.-1), which is within the mid-IR spectral range.
So the emitter and detector should have their spectral
characteristics peaked near 3.3 .mu.m.
[0044] The bandgap of the emitter and detector material for methane
sensing has to be around 0.367 eV which corresponds to .lamda.=3.3
.mu.m. This band gap belongs to relatively narrow band materials
and it means strong temperature dependence of the materials'
properties in the temperature range 240-330K. (-40 C to +60 C).
This is a serious obstacle for developing a semiconductor based
optopair which can reliably operate in this temperature range,
which is essential for outdoor stand-alone methane sensors. Upon
information and belief, no semiconductor methane sensors operating
in this temperature range exist.
SUMMARY OF THE INVENTION
[0045] Accordingly there still exists a need for an optical gas
sensor which is portable, light to carry, relatively inexpensive to
manufacture and produce, capable of consuming very little power and
equipped to deliver accurate and reliable qualitative and
quantitative analysis of gases at multiple temperatures and at low
concentrations. More particularly, there still exists a need for an
optical gas sensor capable of identifying and measuring the
absorbance of methane at below the lower explosive limit (LEL) of
methane, i.e., 5% by volume in atmosphere, based on methane's
absorption of infra-red radiation, across a broad range of
environment temperature. There still exists a need for an optopair
for use in a gas sensor that accurately and precisely detects and
measures the concentration of a gas, irrespective of temperature,
as a result of the optopair's structure and the compensable
temperature susceptibility of the optopair signal resulting
therefrom.
[0046] It is therefore an object of the present invention to
provide an apparatus, process for production thereof and absorption
spectroscopy technique which can produce a temperature compensable
optopair signal, and which as a result can reliably and effectively
detect and measure the absorbance of gas in environments across a
broad range of temperatures, at low concentrations.
[0047] It is a further object of the present invention to provide
an apparatus, process for production thereof and absorption
spectroscopy technique for reliably and effectively detecting and
measuring gases in environments across a broad range of
temperatures and at low concentrations, said apparatus having a low
power consumption and being highly portable.
[0048] It is still another object of the present invention to
provide an apparatus, process for the production thereof and
absorption spectroscopy technique for reliably and effectively
detecting and measuring methane gas in environments across a broad
range of temperatures, said apparatus being so sensitive that it is
able to detect changes in the emission energy transmitted through
the methane, of as little as 0.1%.
[0049] In accordance with the present invention there is provided
an optopair for use in optical sensors for the analysis of an
atmospheric gas sample ("target gas analyte"); namely the detection
and determination of the concentration of the target gas analyte in
atmosphere irrespective of temperature. The optopair comprises one
radiation source and one radiation detector. The radiation source
is an LED which has at least one active area capable of emitting
radiation whose spectral maximum is de-tuned from, i.e., does not
correspond or coincide with the maximum absorption spectrum line of
the absorption spectral band of the target gas analyte. The active
area of the LED can be a bulk, quantum-well or super-lattice active
area.
[0050] Alternatively, the LED can be cascaded, with two or more
cascades, each cascade having an active area, which can be a bulk,
quantum-well, or super-lattice active area. Or the cascaded LED can
have two or more cascades, each respectively having a bulk, quantum
well or super-lattice active area, one of which has a spectral
maximum that is de-tuned from the maximum absorption spectrum line
of the absorption spectral band of the target gas analyte.
[0051] The radiation detector in turn is a photodetector ("PD")
whose responsivity spectral maximum can be either dc-tuned from the
maximum absorption spectrum line of the absorption spectral band of
the target gas analyte, or completely correspond to and coincide
with the maximum absorption spectrum line of the absorption
spectral band of the target gas analyte. It can comprise a sequence
of a contact layer, a middle barrier layer and an n-type photon
absorbing layer arranged such that the top energy of the valence
band of the contact layer is not more than the bottom energy of the
conduction band of the n-type photon absorbing layer, and the
middle bather layer has an energy band gap significantly greater
than that of the photon absorbing layer. Optionally, its contact
layer can be a p-type contact layer.
[0052] The process of designing and fabricating an optopair
suitable for use in a gas sensor, for the detection and accurate
and precise measurement of a target gas analyte in a specific
temperature range, as for example in a temperature range from
-40.degree. C. to 60.degree. C., includes: the identification of
the wavelengths that define the shortest, longest, and maximum
spectral peak wavelengths of the absorption band of the target gas
analyte; the use of the wavelengths to determine the material
systems for the LED and the PD; the determination of the spectral
characteristics of the LED and PD materials and the temperature
dependencies of said spectral characteristics; the selection of the
LED emission spectrum target peak wavelength, de-tuned from the
maximum spectral absorption peak wavelength of the target sample
gas analyte and the selection of the PD responsivity spectrum
target peak wavelength, either de-tuned from, or in tune with, the
maximum spectral absorption peak wavelength of the target sample
gas analyte respectively, through modeling of the LED emission and
the Photodetector responsivity spectra using the information
generated by the preceding steps and minimizing the temperature
sensitivity of the optopair as determined by the technical
requirements of the signal registration circuitry of the optopair;
and if necessary, further minimizing the temperature sensitivity of
the optopair as dictated by the technical requirements of the
signal registration circuitry of the optopair by alternatively
identifying the target peak wavelengths for a multi-cascade LED
emission spectrum for use in the optopair.
[0053] These and other objects, advantages, features and
characteristics of the invention will be apparent from the
following description of a preferred embodiment, considered along
with the accompanying drawings.
BRIEF DESCRIPTION OF DM DRAWINGS
[0054] It is believed that the present invention will be better
understood from the following detailed description taken in
conjunction with the accompanying drawings, in which the numerals
represent identical elements and wherein:
[0055] FIG. 1 is a simple schematic of a typical semiconductor
diode (PRIOR ART);
[0056] FIG. 2 is a schematic band diagram of a homo-diode (PRIOR
ART);
[0057] FIG. 3 is a schematic showing the operation of an LED under
direct bias (PRIOR ART),
[0058] FIG. 4 is a schematic showing the operation principle of a
photovoltaic detector (PRIOR ART);
[0059] FIG. 5 is a band diagram of a GaSb-based QW LEDs (PRIOR
ART);
[0060] FIG. 6 is a schematic showing the operation principle of the
inventive nBp Photodetector used in the inventive optopair;
[0061] FIG. 7 is a schematic LED spectrum whose spectral maximum is
de-tuned from the maximum absorption spectrum line of the
absorption spectral band of the target gas analyte, at two
temperatures and the portion of such LED spectrum area remaining
within such absorption spectral band. The LED spectral portion
within the filter band remains relatively constant;
[0062] FIG. 8 is an experimental long wavelength edge of an LED
spectrum at a specific temperature;
[0063] FIG. 9 is a schematic Gaussian fit of the long wavelength
edge of the LED spectrum of FIG. 8 herein above;
[0064] FIG. 10 shows experimental data obtained on LED with
spectrum peak at 0.367 eV (hollow circles) corresponding to LED
spectrum generated via mathematical modeling;
[0065] FIG. 11 is S.sub.h.omega.0(T) at different positions of the
LED spectrum peak;
[0066] FIGS. 12 and 13 show experimental and modeled responsivity
spectrum and of the optopair photodetector respectively;
[0067] FIG. 14 is a schematic showing PD integral responsivity with
the filter pass band; experiment (hollow circles) and fit
(line);
[0068] FIG. 15 is a schematic showing simulated temperature
sensitivity of the opto pair corresponding to te LED emission peak
at 0.37 ev;
[0069] FIG. 16 is a schematic showing Temperature sensitivity of
the optopair with maximized signal at room temperature (heavy
black) and optopair with improved temperature stability (light
black);
[0070] FIG. 17 is a band diagram of the inventive optopair
photodetector;
[0071] FIG. 18 is a schematic of an nBp photo detector with reverse
temperature dependence of responsivity;
[0072] FIG. 19 is a schematic showing reciprocal compensation of
LED and PD temperature dependencies;
[0073] FIG. 20 is a schematic layout of a two cascade LED; and
[0074] FIG. 21 is a schematic showing a simulated spectrum of
two-cascade LED.
DETAILED DESCRIPTION OF THE INVENTION
[0075] The optopair of the present invention can be used with any
number of optical gas sensors, schematic views and detailed
descriptions of some of which are disclosed in the following
patents and patent applications, the disclosures of which are
incorporated and made a part of the present description by
reference, as if more fully set forth herein: Tkachuk, Optical
Absorption Gas Analyzer, U.S. Letters Pat. No. 7,796,265 B2 and
U.S. Pat. No. 8,665,424 B2; Tkachuk, Optical Gas and/or particulate
sensors, U.S. Letters Pat. No. 8,692,997 B2; Tkachuk, Gas Sensor,
U.S. patent application Ser. No. 13/426,494; Tkachuk, and Optical
Absorption spectrometer and method for measuring concentration of a
Substance, U.S. Letters Pat. No. 7,570,360 B1.
[0076] The optopair of the present invention comprises at least one
radiation source and one radiation detector, which as is described
in more detail herein below, are relatively optically and
spectrally matched.
[0077] The radiation source of the optopair of the present
invention is an LED, engineered to emit radiation having at least
one wavelength frequency, which is "de-tuned" from the peak
absorption wavelength/frequency of the target sample gas analyte.
In other words, the spectral maximum of the radiation generated by
the LED of the optopair is "de-tuned" from the maximum absorption
spectrum line of the absorption spectral band of the target gas
being analyzed. Alternatively, the LED of the optopair of the
present invention can be engineered to emit two or more radiations,
each optionally having a slightly different wavelength or frequency
from the other, and at least one of such two or more radiations
being "de-tuned" from the peak absorption wavelength or frequency
of the target sample gas.
[0078] As has been discussed above, optical methods of detection
and quantification of gases, as for example Non-Dispersive
Infra-Red (NM) methods, rely on the fact that many gases absorb at
a very specific wavelength of infra-red radiation..sup.5 Thus,
conventional wisdom in the absorption spectroscopy field dictates
that the wavelength of the infra-red radiation generated by a
radiation source and used to detect and measure an analyted target
gas sample must match the peak absorption wavelength of the
infra-red radiation capable of being absorbed by the analyte target
sample gas. Or, the frequency of the radiation used to detect and
measure the target gas analyte, must match the frequency of the
infra-red radiation that the target gas analyte can absorb. In
other words, the wavelength or frequency of the radiation generated
by a radiation source and used to detect and measure the target
analyte gas sample must match or be "tuned to", or be "in tune"
with the wavelength or frequency of the radiation that the target
analyte gas can absorb. For example, if the target analyte gas
sought to be detected and measured is CO.sub.2, then the wavelength
of the radiation directed on the target analyte gas sample must be
tuned to 4.26.mu.; the wavelength of radiation capable of being
absorbed by CO.sub.2..sup.6 If the target analyte gas sample sought
to be detected and measured is methane (CH.sub.4), then the
wavelength of the radiation must be tuned to 3.3.mu.; the
wavelength of radiation capable of being absorbed by CH.sub.4.
.sup.5 Gibson, D; MacGregor C. A Novel Solid State Non-Dispersive
Infrared CO.sub.2 Gas Sensor Compatible with Wireless and Portable
Deployment. Sensors 2013, 13, 7079-7103..sup.6 Id.
[0079] The design of the optopair of the present invention defies
the foregoing conventional wisdom. The LED of the optopair of the
present invention is engineered to emit at least one narrow wave
band of radiation in the infra-red spectrum, the wavelength of
which does not match, does not coincide with, does not correspond
to and is NOT tuned to the wavelength or frequency of the radiation
capable of being absorbed by the target sample gas analyte. It is
"de-tuned." And it is de-tuned such that the emitted radiation's
spectral maximum wavelength or frequency can be de-tuned either to
the short wavelength/frequency of the maximum absorbance wavelength
that corresponds to the maximum absorption spectrum line of the
absorption spectral band of the analyte target gas; or to the long
wavelength/frequency. For example, if the optopair is to be used
for the detection of methane, and the LED is engineered to emit
only one narrow wave band of radiation in the infra-red spectrum,
then the wavelength of the radiation generated by the LED will not
be 3.3.mu.. Rather it may be anywhere from 3.14 to 3.25.
[0080] If the LED radiation source is engineered to emit at least
two narrow wave bands of radiation in the infra-red spectrum, then
the spectral maximum of either of or both narrow wave bands'
radiation is de-tuned from the wavelength or frequency
corresponding to the maximum absorption spectral line of the
absorption spectral band of the analyte target gas. For example, if
the LED radiation source is engineered to emit at least two narrow
wave bands of radiation to be used for the detection and
quantification of methane, then the wavelength of the two narrow
wavebands of emitted radiations may be around 3.18 and 3.3 microns
(v) respectively; or 3.18 and 3.4 microns (.mu.) respectively.
[0081] Preferably, in one embodiment of the inventive optopair,
where the LED radiation source is engineered to emit only one
narrow wave band of radiation in the IR spectrum whose wavelength
is de-tuned from the maximum absorbance wavelength of, for example
methane, i.e., 3.3.mu., such radiation source may comprise a single
Quantum Well LED, having only one emitter region ("active area")
capable of generating radiation at a single wavelength.
[0082] Such LED is designed and made by forming a layered
semiconductor crystal. The layers of the semiconductor crystals
have different band gaps but same lattice constant, such that
crystalline order is preserved throughout the entire stack of
layers. The band gaps are narrow (quantum wells) which can
accumulate injected electrons and holes, thereby allowing high
recombination efficiency and precise control of the emission
wavelength sufficient to de-tune the wavelength of the radiation
that the band gaps create from the wavelength of maximum absorbance
of the target gas. A typical band diagram of the quantum well LED
used as the radiation source in the optopair of the present
invention is presented in FIG. 5 (Prior Art). If the LED emitter
region comprises no quantum wells, then the wavelength of the
narrow band gap radiation it generates, cannot be de-tuned from the
maximum absorbance wavelength of the target gas.
[0083] The general process of growing Quantum Well LEDs used in the
optopair of the present invention is described in detail in
Suchalkin, S. et. al., GaSb based Light Emitting Diodes with.
Strained InGaAsSb Type I Quantum Well Active Regions. Applied
Physics Letters 2008, 93, 081107, the disclosure of which is
incorporated by reference in its totality as if more fully set
forth herein. Table I herein below discloses at least 5 different
Quantum Well LED structures which have been grown in accordance
with the process described in Suchalkin. S. et. al., GaSb based
Light Emitting Diodes with Strained InGaAsSb Type I Quantum Well
Active Regions. Applied Physics Letters 2008, 93, 081107. Two of
such Quantum Well LEDs have been refined precisely to produce
radiation whose wavelength is de-tuned from, i.e., tuned to the
short wavelength of the maximum absorbance wavelength of methane.
Incidentally, the Quantum Well LED structure identified as Device
5, in Table I, is a Quantum Well LED structure with one emitter
region that produces radiation that is tuned exactly to the maximum
absorbance wavelength of methane, i.e. 0.374 eV equivalent to
3.3.mu..
TABLE-US-00001 TABLE I The parameters of the device structures.
Number of QW width, Device Cladding Barrier QW QWs nm 1
Al.sub.0.9GaAs.sub.0.07Sb Al.sub.0.35GaAs.sub.0.03Sb
In.sub.0.55GaAs.sub.0.22Sb 10 12 2 Al.sub.0.6GaAs.sub.0.05Sb
Al.sub.0.2In.sub.0.35Ga As.sub.0.24Sb In.sub.0.54GaAs.sub.0.74Sb 4
17 3 Al.sub.0.6GaAs.sub.0.05Sb Al.sub.0.2In.sub.0.2GaAs.sub.0.2Sb
In.sub.0.54GaAs.sub.0.74Sb 4 17 4 Al.sub.0.9GaAs.sub.0.07Sb
Al.sub.0.35GaAs.sub.0.03Sb In.sub.0.55GaAs.sub.0.22Sb 5 12 5
Al.sub.0.85GaAs.sub.0.07Sb Al.sub.0.2In.sub.0.25GaAs.sub.0.74Sb
In.sub.0.56GaAs.sub.0.21Sb 3 14
[0084] In another embodiment of the inventive optopair, where the
LED radiation source is engineered to emit at least two narrow wave
bands of radiation in the IR spectrum, whose wavelengths are
de-tuned from the maximum absorbance wavelength of methane, i.e.,
3.3.mu., such LED radiation source comprises a cascaded LED, having
at least two emitter regions ("active areas") in series, each one
capable of generating radiation at a single wavelength (see FIG.
20). The wavelengths of the radiation of the two active areas in
series of the cascaded LED can exactly match, or be exactly tuned
to, the wavelength corresponding to the maximum absorption spectrum
line of the absorption spectral band of the target gas sample. Or
the wavelength of the radiation of one active area of the cascaded
LED can exactly match, or be exactly tuned to, the wavelength
corresponding to the maximum absorption spectrum line of the
absorption spectral band of the target gas sample, and the
radiation of the other active area of the cascaded LED can be
de-tuned to either the short side or the long side of the
wavelength corresponding to the maximum absorption spectrum line of
the absorption spectral band of the target gas sample. Or the
wavelengths of the radiation of both active areas are both de-tuned
to different wavelengths on the short side of the wavelength
corresponding to the maximum absorption spectrum line of the
absorption spectral band of the target gas sample. Or, the
wavelength of the radiation of one active area is de-tuned to the
short side of the wavelength corresponding to the maximum
absorption spectrum line of the absorption spectral band of the
target gas sample and the wavelength of radiation of the other
active area is de-tune to the long side of the wavelength
corresponding to the maximum absorption spectrum line of the
absorption spectral band of the target gas sample.
[0085] For example, when the optical gas sensor incorporating the
present inventive optopair was designed to be used for the analysis
of methane in temperatures that range from -40 degrees Celsius to '
50 degrees Celcius, then the first emitter region of the cascaded
LED emitted energy having a frequency at 0.36 eV (3.44.mu.); and
the second emitter region of the cascaded LED emitted energy at a
frequency of 0.39 eV (3.18.mu.). When the optical gas sensor
incorporating the present inventive optopair was designed to be
used for the analysis of methane in temperatures that range from
minus 40 degrees C. to 60 degrees C., the cascaded LED was provided
with three emitter regions arranged in series. The first emitter
region emitted energy having a frequency at 0.36 eV (3.44.mu.). The
second emitter region emitted energy at a frequency of 0.39 eV
(3.18.mu.). And the third active area emitted energy at 0.40 eV
(3.09.mu.).
[0086] Processes for growing cascaded LEDs for use in the optopair
of the present invention are described in detail in the following:
Prineas, et. al., Cascaded active regions in 2.4 .mu.m GaInAsSb
light-emitting diodes for improved current efficiency. Applied
Physics Letters 2006, 89, 211108. Crowder, J G., et. al.,
Mid-infrared gas detection using optically immersed,
room-temperature, semiconductor devices. Meas. Sci. Technol. 2002,
13, 882-884. Li, W. et. al., InGaAsSbN: A dilute nitride compound
for midinfrared optoelectronic devices. Journal of Applied Physics.
2003, Vol. 94, No. 7, 4248-4250. Ashley, T. et. al., Uncooled
InSb/In.sub.1-xAl.sub.xSb mid-infrared emitter. Applied Physics
Letters 1994, 64, 2433-2435. Shterenga, L. et. al., Type-I quantum
well cascade diode lasers emitting near 3 m. Applied Physics
Letters 2013, 103, 121108. Krier, A. et. al., The development of
room temperature LEDs and lasers for the mid-infrared spectral
range. Phys. Stat. Sol. (a), 2008, 205, No. 1, 129-143, the
disclosures of which are incorporated in the present detailed
description by reference in their totality as if more fully set
forth herein.
[0087] Using the processes set forth herein above one embodiment of
the cascaded LEDs was grown on GaSb substrates using a Veeco GEN930
MBE system. The emitting area of the cascaded LED comprised 2
cascaded emitter regions connected with a transition region. Each
cascade included four compressively (.about.4.47%) strained
In0.55Ga0.45As0.3Sb quantum wells separated by 50 nm
Al0.2In0.55Ga0.25As0.23Sb bathers and doped GaSb claddings. The
emission wavelength was controlled by adjusting the In content in
the Quantum Well material and the quantum well width. The active
region of the first cascade was sandwiched between doped GaSb
claddings. The n-cladding of the second cascade was GaSb while the
p-cladding was compositionally graded from Al0.5Ga0.5As0.43Sb to
GaSb. To prevent hole leakage, the 12 A AlSb/12 A InAs superlattice
hole blockers were inserted between n-cladding and bather in each
cascade. The GaSb substrate was n-doped with Te to
2.about.3.times.1017 cm-3, the 500 A n-GaSb buffer layer grown on
the substrate was doped to 5.times.1017 cm-3. A 10 nm thick Be
doped (1.times.1019 cm-3) p+GaSb cap was grown on top of the
structure.
[0088] The radiation detector of the inventive optopaire comprises
a Photodetector (PD) preferred embodiments of which comprise the
band energy schematic structures set forth in FIG. 6 and FIGS.
17-18.
[0089] As was discussed herein above, like LEDs, photodetectors
comprise two layers of semiconductor material. Each layer has a
different type of conductivity, either n-type conductivity or
p-type conductivity. Also like LEDs, they have a depletion region.
Unlike LEDs however, when radiation in the form of photons hits the
depletion area of the photodetectors, such photons generate
electrons and holes. The motion of the generated electrons and
holes in the depletion area creates a current whose signal can be
measured precisely. But only when the ration of current signal to
electron and hole generation noise is high.
[0090] To this end, i.e., to increase and improve the ration of
signal to noise ratio by decreasing or suppressing electron and
hole generation-recombination noise, the PD of the optopair of the
present invention, optionally comprises the band energy schematic
shown m FIG. 6. Its structure comprises n-type substrate, low
n-type absorber, wide band gap barrier and p-type contact. The
electrons and holes excited and generated or released in the low
n-type absorber are separated at the absorber boundaries. The
photo-generated holes cannot be transported into the n-type
substrate because their access to it is blocked by the built-in
electric field at the n-type substrate-absorber boundary. On the
other hand, there is nothing blocking the movement of the
photo-generated holes to the p-type contact. By comparison, direct
transport of the photo-generated electrons to the p-contact is
blocked by the wide band gap barrier. And since there is no
electric field in the absorber, its thickness can be increased
without corresponding increase in the electron holes
generation-recombination noise.
[0091] In the preferred embodiment of the inventive optopair the PD
comprises a sequence of a contact layer, a middle barrier layer and
an n-type photon absorbing layer, said contact layer having a
valence band, said n-type photon absorbing layer having a
conduction band, said middle barrier layer having an energy bandgap
significantly greater than that of the photon absorbing layer, and
the top energy of said valence band of said contact layer is not
more than the bottom energy of said conduction band of said n-type
photon absorbing layer ("nBp photodetector"). Further, if the
optopair of the present invention will be used for the detection
and analysis of methane then the said n-type photon absorbing layer
comprises Indium Arsenide.
[0092] In practice, one embodiment the nBp photodetector described
above is an InAs-based nBp photodetector. It was grown on n-type
(1.times.10.sup.18 cm.sup.3) InAs substrates using a Veeco GEN930
MBE system. The structure comprised undoped (weak n-type) 5 micron
thick InAs absorber; 0.2 micron thick undoped AlAs.sub.0.16Sb
barrier and 0.4 micron thick p-type In.sub.0.2GaAs.sub.0.26Sb
contact layer. The latter was. Be doped to 1.times.10.sup.19
cm.sup.-3. The composition of the contact layer was chosen to align
the top of the valence band in the contact layer with the bottom of
the conduction band of the absorber. Such alignment prevents charge
transfer between contact and absorber and formation of build-in
electric field since the latter can have negative effect on the
detector's temperature performance.
[0093] Another embodiment of the nBp photodetector described above
is a GaSb-based photodetector. It was grown on n-type
(>5.times.10.sup.17 cm.sup.3) GaSb substrates using a Veeco
GEN930 MBE system. The structure comprised 0.5 micron thick n-type
(1.times.10.sup.18 cm.sup.-3) GaSb buffer layer, undoped (weak
n-type) 3 micron thick In.sub.0.36Ga.sub.0.64As.sub.0.32Sb
absorber; 0.2 micron thick undoped Al.sub.0.4GaAs.sub.0.035Sb
barrier and 0.3 micron thick p-type GaSb contact layer. The latter
was Be doped to 1.times.10.sup.19 cm.sup.-3. The benefits of using
this detector include complete transparency, thereby eliminating
loss of photons, and control of the wavelength sensitivity of the
detector through the manipulation of the composition of the
absorber region of the detector.
[0094] Optionally, the optopair can comprise a bandpass filter
matched to the absorption band of the gas of interest. If the
optopair is to be used to detect and analyze methane, then the
bandpass filter will be matched to the methane absorption band. The
filter cuts off all the emission outside the methane absorption
band which enhances relative magnitude of the absorbed emission.
The filter is characterized by the spectral transmission function
F(.omega.). For the ideal band pass filter transmitting in the
spectral range from .omega..sub.1 to .omega..sub.2,
F(.omega.)=.THETA.(.omega.-.omega..sub.1)-.THETA.(.omega.-.omega..sub.2),
where .THETA. is the step function.
[0095] As was discussed above, the LED and the photodetector (PD)
in the inventive optopair are chosen and arranged such that the at
least one emission spectral maximum of the LED, and optionally, the
responsivity spectral maximum of the PD, are de-tuned from the
maximum absorption spectrum line of the absorption spectral band of
the target gas analyte, such that the at least one emission
spectral maximum of the LED and the responsivity spectral maximum
of the PD are jointly or separately positioned on either the short
side or the long side of the maximum absorption spectrum line of
the absorption spectral band of the analyte gas. In one embodiment,
for example, the at least one emission spectral maximum of the LED
is de-tuned from, i.e., positioned on the short wavelength side of
the maximum absorption spectrum line of the absorption band of the
gas being analyzed, and the responsivity spectral maximum of the PD
is de-tuned from, i.e., positioned on the long wavelength side of
the maximum absorption spectrum line of the absorption band of the
gas being analyzed with some overlap between the LED and PD Spectra
within the absorption band of the gas being analyzed.
[0096] This de-tuning of the LED spectrum, and optionally the PD
spectrum as well, allows: a) for the compensation of the effect of
temperature on their emission and responsivity spectra
respectively, as the optical sensor is subjected to temperature
variations, by permitting at least part of each of their emission
and responsivity spectra to remain within the target analyte gas
absorption band of interest, irrespective of the change of the
intensity and shifting of their spectral maxima as a result of the
effect of temperature (see FIGS. 7 and 19); and b) the optical
sensor in which the inventive optopair is installed, to generate a
signal that reliably, precisely and accurately provides for the
identification and quantification of the target sample gas analyte
irrespective of the temperature environment it is in.
[0097] The building of an optopair, capable of compensating for the
effect of temperature on its signal, begins with the understanding
that the temperature dependence of the optopair signal can be
expressed mathematically by the following function:
S(T).about..intg..sub.-.varies..sup..varies.E(T,.omega.)R(T,.omega.)F(.o-
mega.)d.omega. (1),
where S(T) is the optopair signal, E(T,.omega.) is the normalized
LED spectrum, R(T,.omega.) is normalized PD responsivity spectrum,
and F(co) is the spectral transmission function of the band pass
filter.
[0098] Ideally, the optopair signal would not depend on temperature
at all, i.e., S(T)=const(T). However, as was discussed above, the
optopair signal does vary with temperature and the temperature
sensitivity of the optopair can be characterized by the following
parameter:
K .ident. S m ax ( T ) S m i n ( T ) - 1 ( 2 ) ##EQU00001##
where S.sub.max(T) and S.sub.min(T) are absolute maximum and
minimum values of the expression (1) within the temperature range
of the optopair operation.
[0099] Thus, it is clear from the foregoing that that one can build
an optopair capable of minimizing the effect of temperature on its
signal by compensating for the temperature sensitivities of the LED
and the PD either separately or jointly. For example, in one of the
optopair embodiments of the present invention, comprising a
cascaded LED, the compensation for the temperature sensitivities of
the LED and the PD occurs at the same time, by tailoring the
E(T,.omega.), R(T,.omega.) and F(.omega.) in such a way that the
temperature dependencies of the LED and PD spectra would compensate
each other, such that the resulting temperature sensitivity of the
optopair is low and the optical sensor consistently generates a
signal sufficient to provide precise and accurate qualitative and
quantitative analysis of a gas, irrespective of temperature.
[0100] On the basis of the foregoing then, the process of building
an optopair capable of minimizing for the effect of temperature
variation on its signal comprises the following steps: a)
Identifying the target sample gas analyte; b) Establishing the
frequencies/wavelengths that define the shortest, longest, and
maximum spectral peak frequencies/wavelengths of the target sample
gas analyte maximum absorbance spectrum ("the band pass filter";
also expressed as "the gas absorption band"); c) Using such
frequencies/wavelengths to identify the material systems for the
LED and the PD; d) Determining the spectral characteristics of the
LED and PD materials and the temperature dependencies of said
spectral characteristics. This can be done either experimentally by
making and characterizing the test devices, or by referring to
outside scientific references. Since semiconductor alloys are used
to fabricate the LED and PD, the necessary information includes the
dependence of the alloy band gap on its composition and temperature
dependence of the luminescence amplitude and spectrum peak position
of the alloys; e) Identifying the de-tuned target peak emission
frequency and peak responsivity frequency of each of the LED and PD
respectively through modeling of the LED emission and the PD
responsivity spectra using the information generated by step (d);
and f) finalizing the optopair design and fabricating the optopair.
The goal of the process is the precise positioning of each of the
optopair's LED spectral emission and PD spectral responsivity
maxima, such that the temperature sensitivity of the optopair
signal is minimized
A. Identifying the Gas Absorption Band for the Sensor Operation
[0101] The process of building an optopair capable of minimizing
for the effect of temperature variation on its signal begins with
the identification of the target gas sample analyte. For example
the optopair can be built for the detection and analysis of
methane. Or, it can be built for the analysis of Carbon Monoxide,
or Carbon Dioxide, or Hydrogen Sulfide or Sulfur Dioxide, or any
other gas present in the atmosphere. Once the target analyte gas is
identified, such identification can then be used to determine the
shortest, longest, and maximum absorption frequencies/wavelengths
that define the target sample gas analyte absorption band. Such
wavelengths can be found by referring to the target sample gas
analyte maximum absorbance spectrum, which in turn can be found in
any number of public sources including laboratory testing reference
materials. The maximum absorbance spectrum will provide (i) the
frequency and wavelength corresponding to the maximum absorption
spectral line of the absorption spectral band of the target sample
gas analyte, (ii) the frequency and wavelength corresponding to at
least one spectral line on the short side of the maximum absorption
spectral line, preferably the one that is the furthest away from
and on the shortest side of the maximum absorption spectral line of
the absorption spectral band of the target sample gas analyte, and
(iii) the frequency and wavelength corresponding to at least one
spectral line on the long side of the maximum absorption spectral
line, preferably the one that is the furthest away from and on the
longest side of the maximum absorption spectral line of the
absorption spectral band of the target sample gas analyte. These
frequencies and/or wavelengths can then be converted to energy
units, i.e., electron volts, i.e., E(ev)=1.24/8(:), thereby
establishing the range of energy which is further referred to as
the gas analyte absorption band. For the successful operation of
the optopair in the gas sensor, both LED emission and PD
responsivity spectra have to overlap with this band. If the target
gas sample analyte is methane, then the energy of the shortest,
longest and maximum wavelengths of its IR absorption spectral band
is 0.362 eV, 0.383 eV, and 0.367 eV respectively.
B. Determining the Material System for the LED and PD
Structures.
[0102] Once the gas analyte absorption band is established, it is
used to determine each of the material systems for the LED and the
PD respectively. The material system for each of the LED and the PD
respectively, is defined as the chemical composition of the LED or
PD, respectively. The material system further refers to the
material of the substrate, which determines the average lattice
constant of the LED or PD structure, able to emit or absorb
radiation having energy that coincides with the range of energy of
the target gas analyte's absorption spectral band established
above. Examples of LED and PD material systems capable of
generating radiation within the absorption spectral band of methane
include, but are not limited to, GaSb based material systems, or
InAs based material systems, or InP based material systems. One
embodiment of a methane photodetector can comprise a InAs absorber
whose band gap energy is fixed and coincides with the maximum
absorbance spectrum peak of methane gas.
[0103] The choice of the material systems for the LED depends on
whether the LED will be a bulk LED or a Quantum Well LED. And if it
is a Quantum Well LED, whether it will have a single emission
active area or whether it is a cascaded Quantum Well LED.
[0104] A bulk LED has an active area consisting of a single thick
material capable of emitting radiation having a wavelength
dependent strictly on the chemical composition of the material. By
comparison, a Quantum Well LED has an active area consisting of one
or more very thin layers of material capable of emitting radiation
having a wavelength or energy dependent not only on the chemical
composition of the material, but also on the layer width.
[0105] If the LED used in the optopair is a cascaded LED, then its
spectrum is a superposition of several peaks centered at different
frequencies. Each of the spectral peaks is produced by emission
from one cascade or the group which includes several identical
cascades. The number of the peaks corresponds to the number of the
groups. The peak frequencies are determined by active area
composition of each cascade while the relative intensities of the
peaks are controlled by the number of the identical cascades in the
group.
C. Determining the Spectral Characteristics of the LED and PD
Materials and the Temperature Dependencies of Such Spectral
Characteristics.
[0106] The establishing of each of the material systems for the LED
and the PD respectively, is followed by the determination of the
spectral characteristics of the chosen LED and PD materials. Such
determination can be achieved experimentally, or can be achieved by
referring to public sources and scientific literature. Spectral
characteristics include but are not limited to, the respective
dependence of the LED and PD alloy band gap on its composition and
temperature dependence of the spectral shape, luminescence
amplitude and spectrum peak position of the alloys.
[0107] The experimental determination of the spectral
characteristics of the chosen LED or PD is suggested, if
information is not publicly available, as in the case where a new
material system is identified. Such determination includes
fabrication of the LED or PD structure with their respective
emission and responsivity spectral peaks close to the gas
absorption band. The temperature dependencies of their respective
emission and responsivity spectral shapes and positions can then be
determined experimentally in the temperature range of interest.
D. Identifying the De-Tuned Target Peak Emission Frequency of Each
of the LED and PD Respectively Through Modeling of the LED Emission
and the PD Responsivity Spectra.
[0108] Once the determination of the spectral characteristics of
the LED and PD materials and the temperature dependencies of said
spectral characteristics is complete, it is used to identifying the
target peak wavelength, de-tuned from the maximum spectral
absorption peak wavelength of the target sample gas analyte for
each of the LED and Photodetector emission and responsivity spectra
respectively. Alternatively, the PD target peak wavelength can
coincide with the maximum spectral absorption peak wavelength of
the target sample gas analyte, instead of being detuned therefrom.
The identification of the target peak wavelength is achieved
through modeling of the LED emission and the Photodetector
responsivity spectra using the information generated by the
preceding steps and minimizing the temperature sensitivity of the
optopair as determined by the technical requirements of the signal
registration circuitry of the optopair.
[0109] As was discussed herein above, for all semiconductor alloys
lattice matched to GaSb, the band gap energy, luminescence
intensity and responsivity decrease with the increase of
temperature. Thus, it follows that the electroluminescence
intensity of semiconductor LEDs and responsivity of semiconductor
PDs decreases with increase of temperature as well and that such
decrease at higher temperatures can be compensated by increasing
the LED emission spectrum or PD responsivity spectrum overlap with
the gas analyte absorption band. Quantum Well LEDs have an
advantage over bulk LEDs in connection with increasing the LED
emission spectrum overlap with the gas analyte absorption band
because Quantum Well LEDs provide for precise control of the LED
spectrum position, not only by manipulating their chemical
composition, but also by varying the widths of their optically
active layers (quantum wells).
[0110] The overlap between the LED emission spectrum and/or the PD
responsivity spectrum with the gas absorption band changes with
temperature since, as the temperature changes, the spectra shift,
while the gas absorption band does not. This effect can be fully or
partially compensated by proper positioning of the LED and/or PD
spectrum peaks with respect to the gas absorption band. For
example, one can chose an LED and/or a PD with spectral maxima that
are de-tuned to the high frequency side of the gas absorption band.
As the temperature increases, the LED spectrum will shift to the
lower frequency side of the gas absorption band. This will result
in an LED spectrum which will overlap with the gas absorption band
even more, thus compensating for the drop in amplitude. This makes
possible to maintain the overlap area of the LED spectrum and the
gas absorption band within the limits which are necessary to
provide a signal sufficient for the gas detection and
quantification.
[0111] The mathematical modeling and the minimization of the
temperature sensitivity of the optopair as determined by the
technical requirements of the optopairs signal registration
circuitry permits the achievement of the foregoing.
[0112] Based on the data obtained the determination of the spectral
characteristics of the LED and PD materials and the temperature
dependencies of said spectral characteristics, the LED emission
spectrum E(.omega.,T) can be modeled at different temperatures. An
example of the fitting function is given by the following
expression:
E ( .omega. , .omega. 0 E , T ) = - T T E 0 j E j exp [ - ( n ~
.omega. - n ~ ( .omega. 0 E - .omega. Ej ) + .alpha. E T ) 2 2
.DELTA. Ej 2 ] ( 3 ) ##EQU00002##
Here T.sub.E0, .alpha..sub.E and .DELTA..sub.EJ are the material
parameters determined in step (d); peak frequency .omega..sub.0E is
determined by the active region composition, parameters
.omega..sub.Ej and E.sub.j are chosen to best fit the spectrum
shape of the LED emission, obtained during the determination of the
spectral characteristics of the LED and PD materials and the
temperature dependencies of said spectral characteristics. The
fitting of the LED spectral shape is done at a fixed temperature,
for example, room temperature. Parameters T.sub.E0, and
.alpha..sub.E allows obtaining LED spectrum at any temperature
within the range of interest.
[0113] Once the LED Emission spectrum is modeled at different
temperatures as discussed herein above, the information generated
by such modeling is used to calculate the temperature dependence of
the optopair signal and minimize the temperature sensitivity of the
optopair in the temperature range of interest for use of the
optopair. The Temperature dependence of the optopair signal can be
characterized by the expression (1). For the ideal band pass filter
transmitting in the spectral range from .omega..sub.1 to
.omega..sub.2,
F(.omega.)=.THETA.(.omega.-.omega..sub.1)-.THETA.(.omega.-.omega..sub.2),
where .THETA. is the step function. So the expression (1) can be
simplified as:
S(T)=.intg..sub..omega..sub.1.sup..omega..sup.2E(T,.omega.)R(T,.omega.)d-
.omega. (4)
[0114] The minimization can be done numerically using the
expression (2). An example of the minimization procedure which can
be used is the nonlinear least square method. The result of this
process is the magnitude .omega..sub.0E which determines the LED
design. The target magnitude of Kmin is determined by technical
requirements of the signal registration circuitry of the gas
sensor. If Kmin can be reached using a single cascaded LED, then
the optopair design can be finalized and the optopair can be
fabricated.
[0115] In case K.sub.min can not be reached using a one cascade
LED, one can use LED with active area including several cascades. A
multi cascade LED structure is two or more LED structures grown "in
series`. Schematic layout of the two cascade LED structure is shown
in FIG. 16. The LED spectrum is combination of two emission lines
from the active regions. By changing the parameters of the active
regions the position and shape of the resulting spectrum can be
further controlled. The example is given in FIG. 17. Such control
over the LED spectral shape gives another tool for compensation of
the thermal drift of the optopair signal.
[0116] Application of a multi cascade LED gives additional
minimization parameters so the compensation of the temperature
dependence of the optopair signal can be done more accurately. Now,
general expression for the fitting function is:
E(.omega.,.omega..sub.0E1,.omega..sub.0E2 . . .
.omega..sub.0EnT)=.SIGMA..sub.nN.sub.nE(.omega.-.omega..sub.0En)
(5).
[0117] Here N.sub.n is the number of identical LED cascades
emitting at frequencies .omega..sub.0En. The procedure of
minimization should be repeated iteratively until the required
K.sub.min is reached. The number of cascades is increased by one at
each iteration. The result of the procedure is the set of the
parameters N.sub.n, and .omega..sub.0En which determine the LED
design.
[0118] The PD responsivity spectrum is determined by the absorber
band gap as well. To this end, quaternary alloy InGaAsSb can be
used as the PD absorber material. Variation of the alloy content is
a way to control the alloy's band gap energy while maintaining same
lattice constant. The latter is a necessary condition for high
quality PD fabrication. A band diagram of one of the proposed photo
detectors is shown in FIG. 13. The advantages of the proposed
structure are the possibility of the band gap control through the
absorber alloy composition and the transparency of the substrate in
the spectral range of sensitivity (backside illumination
possible).
[0119] The information on the PD spectrum and its temperature
dependence gathered in step (d) herein above, can then be used to
model the PD responsivity spectrum at different temperatures, using
the fitting function given by the following expression:
R ( .omega. , .omega. 0 E , T ) = - T T R 0 i E i exp [ - ( n ~
.omega. - n ~ ( .omega. 0 R - .omega. Ri ) + .alpha. R T ) 2 2
.DELTA. Ri 2 ] . ( 6 ) ##EQU00003##
Here T.sub.R0, .alpha..sub.R and .DELTA..sub.Ri are the material
parameters determined in step (d); peak frequency .omega..sub.0R is
determined by the PD absorber composition, parameters
.omega..sub.Ri and R.sub.i are chosen to best fit the spectrum
shape of the PD responsivity, obtained in step (d). The fitting of
the PD spectral shape is done at a fixed temperature, for example,
room temperature. Parameters T.sub.R0, and .alpha..sub.R allows
obtaining PD responsivity spectrum at any temperature within the
range of interest.
[0120] Once the PD responsivity spectrum is modeled at different
temperatures, the information generated by such modeling is used to
calculate the temperature dependence of the optopair signal and
minimize the temperature sensitivity of the optopair using the
procedure discussed herein above in connection with the LED
minimization. The result of this process is the magnitude
.omega..sub.OR which determines the PD design.
[0121] One way is to make the PD more responsive at high
temperatures. To that end an additional potential barrier for the
photo exited holes can be 19 added (FIG. 7). There are two ways for
the holes, photo exited in the absorber. One way is to tunnel
through the barrier and get captured in the p-contact. The other
way is to recombine back with an electron in the absorber. The
latter way does not make any contribution to photocurrent. Both
ways are shown as dash green arrows in FIG. 14. The probability
ratio of these two ways is determined by temperature and the
barrier height DE. As a characteristic thermal energy kT (k is the
Boltzmann constant) is much less than DE, the photo exited carrier
with high probability will recombine in the absorber and
photocurrent will be low. At higher temperature, as kT becomes
comparable with DE, the photo exited carriers can penetrate to the
p-contact and photo current is increasing. Responsivity of the
detector will be higher at higher temperatures. This temperature
dependence will fully or partially compensates the decrease of the
LED intensity at high temperatures. The strength of the temperature
dependence of the PD can be controlled through the bather
height.
[0122] Another way to compensate for the temperature dependence of
LED intensity is the application of mercury-cadmium-telluride (MCT)
photo detectors. The bandgap energy of MCT materials increases with
temperature. This is opposite to corresponding dependence in
GaSb-based materials. The responsivity amplitude of MCT detectors
is decreasing with temperature. At low temperature, the LED
emission spectrum and PD responsivity spectrum can be shifted to
shorter- and longer wavelength sides of the filter band (FIG.
15).
[0123] It is a clear from the foregoing that the optical gas
sensor, which incorporates the inventive optopair described above
will be portable, light to carry, relatively inexpensive to
manufacture and produce, capable of consuming very little power and
equipped to deliver accurate and reliable qualitative and
quantitative analysis of gases at multiple temperatures and at low
concentrations. It will accurately and precisely detect and measure
the concentration of a gas, irrespective of temperature, as a
result of the optopair's structure and its ability to compensate
for temperature susceptibility of the optopair signal. As a result,
it will reliably and effectively detect and measure the absorbance
of gas in environments across a broad range of temperatures, at low
concentrations. More particularly, an optical gas sensor
incorporating an optopair designed and fabricated in accordance
with the foregoing for the detection and analysis of methane, will
not only be capable of identifying and measuring the absorbance of
methane, but will be able to do so at below the lower explosive
limit (LEL) of methane, i.e., 5% by volume in atmosphere. A
solution to a problem that is long overdue.
[0124] While particular embodiments of the invention have been
illustrated and described in detail herein, they are provided by
way of illustration only and should not be construed to limit the
invention. Since certain changes may be made without departing from
the scope of the present invention, it is intended that all matter
contained in the above description, or shown in the accompanying
drawings be interpreted as illustrative and not in a literal sense.
Practitioners of the art will realize that the sequence of steps
and the embodiments depicted in the figures can be altered without
departing from the scope of the present invention and that the
illustrations contained herein are singular examples of a multitude
of possible depictions of the present invention.
* * * * *