U.S. patent application number 11/493282 was filed with the patent office on 2006-11-23 for photodetectors and optically pumped emitters based on iii-nitride multiple-quantum-well structures.
This patent application is currently assigned to Research Foundation of the City University of New York. Invention is credited to Robert R. Alfano, Wubao Wang, Shengkun Zhang.
Application Number | 20060263923 11/493282 |
Document ID | / |
Family ID | 32717698 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060263923 |
Kind Code |
A1 |
Alfano; Robert R. ; et
al. |
November 23, 2006 |
Photodetectors and optically pumped emitters based on III-nitride
multiple-quantum-well structures
Abstract
The design and operation of a p-i-n device, operating in a
sequential resonant tunneling condition for use as a photodetector
and an optically pumped emitter, is disclosed. The device contains
III-nitride multiple-quantum-well (MQW) layers grown between a
III-nitride p-n junction. Transparent ohmic contacts are made on
both p and n sides. The device operates under a certain electrical
bias that makes the energy level of the first excitation state in
each well layer correspond with the energy level of the ground
state in the adjoining well layer. The device works as a
high-efficiency and high-speed photodetector with photo-generated
carriers transported through the active MQW region by sequential
resonant tunneling. In a sequential resonant tunneling condition,
the device also works as an optically pumped infrared emitter that
emits infrared photons with energy equal to the energy difference
between the first excitation state and the ground state in the
MQWs.
Inventors: |
Alfano; Robert R.; (Bronx,
NY) ; Zhang; Shengkun; (Ridgewood, NY) ; Wang;
Wubao; (Flushing, NY) |
Correspondence
Address: |
Kent H. Cheng, Esq.;Cohen, Pontani, Lieberman & Pavane
Suite 1210
551 Fifth Avenue
New York
NY
10176
US
|
Assignee: |
Research Foundation of the City
University of New York
|
Family ID: |
32717698 |
Appl. No.: |
11/493282 |
Filed: |
July 26, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10728562 |
Dec 5, 2003 |
7119359 |
|
|
11493282 |
Jul 26, 2006 |
|
|
|
60430971 |
Dec 5, 2002 |
|
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|
Current U.S.
Class: |
438/48 |
Current CPC
Class: |
B82Y 10/00 20130101;
Y02E 10/548 20130101; H01L 33/06 20130101; H01L 31/035236 20130101;
B82Y 20/00 20130101; H01L 33/0012 20130101; H01L 31/1035
20130101 |
Class at
Publication: |
438/048 |
International
Class: |
H01L 21/00 20060101
H01L021/00 |
Claims
1. A method of operating a sequential resonant tunneling p-i-n
device to perform photodetection, said sequential resonant
tunneling device comprising a substrate, a p type semiconductor
layer, an n type semiconductor layer, an i type semiconductor
layer, and a plurality of ohmic contacts comprising metal alloys
deposited on surfaces of both the p type semiconductor layer and
the n type semiconductor layer, the method comprising: operating
the p-i-n device in a sequential resonant tunneling condition to
transport electrons through a multiple quantum well region located
between the p type and the n type semiconductor layers of the p-i-n
device; applying a bias between the ohmic contacts on the surface
of the p type semiconductor layer and the ohmic contacts on the
surface of the n type semiconductor layer; measuring a
current-voltage profile of the p-i-n device to determine a working
bias; generating a peak electrical current at the determined
working bias during said sequential resonant tunneling; and
performing photodetection at the peak electrical current.
2. The method of claim 1, wherein said p type semiconductor layer
comprises one or a plurality of layers selected from the group
consisting of: a) a single semiconductor layer of III-nitride
materials; b) a single semiconductor layer of a semiconductor
material with a lattice matched to III-nitride materials; and c)
multiple layers of III-nitride materials.
3. The method of claim 2, wherein the number of layers of
III-nitride materials is from 2 to 1000.
4. The method of claim 1, wherein said n type semiconductor layer
comprises one or a plurality of layers selected from the group
consisting of: a) a single semiconductor layer of III-nitride
materials; and b) multiple layers of III-nitride materials.
5. The method of claim 4, wherein the number of layers of
III-nitride materials is from 2 to 1000.
6. The method of claim 1, wherein said p type semiconductor layer
has: a) a doping level for reaching a hole concentration of at
least 1.times.10.sup.18 cm.sup.-3; and b) a thickness of from one
hundred nanometers to ten micrometers.
7. The method of claim 1, wherein said n type semiconductor layer
has: a) a doping level for reaching an electron concentration of at
least 1.times.10.sup.18 cm.sup.-3; and b) a thickness of from one
hundred nanometers to ten micrometers.
8. The method of claim 1, wherein said i type semiconductor layer
is an active region of the device.
9. The method of claim 1, wherein said i type semiconductor layer
comprises multiple quantum well layers of periodically grown units,
and wherein each unit comprises one barrier layer and one adjacent
well layer.
10. The method of claim 9, wherein said barrier layer and well
layer are thin films of III-nitride materials.
11. The device of claim 9, wherein said barrier layer has a larger
electronic band gap than said well layer.
12. The method of claim 9, wherein there are a number of units
comprising alternating layers of barriers and wells.
13. The method of claim 9, wherein all barrier layers are made of
the same III-nitride material and have the same thickness.
14. The method of claim 9, wherein all well layers are made of the
same III-nitride material.
15. The method of claim 9, wherein the thicknesses of both said
well layers and said barrier layers are from 1 nanometer to 100
nanometers.
16. The method of claim 9, wherein the III-nitride materials
comprise group III elements of a predetermined mole fraction of
group III element, such that the mole fractions of the group III
elements and the thicknesses of said well layers and said barrier
layers are specified so that conduction band offset between said
well layers and said barrier layers is sufficiently large to
produce at least two electron engine states in said well layers,
wherein energy positions of said at least two electron engine
states are theoretically determined by the effective mass equation,
[ - 2 2 m * .times. .times. .gradient. 2 .times. + V .function. ( z
) ] .times. F .function. ( z ) = E n .times. .times. F .function. (
z ) , ##EQU2## where h- is the reduced Planck constant, m* is the
electron effective mass, V(z) is the electron potential along the
material growth direction, F(z) is the effective-mass envelope
function of electrons, and E.sub.n is the energy of electrons at
the n.sup.th energy level in the quantum wells.
17. The method of claim 9, wherein said well layers and said
barrier layers are semi-insulating.
18. The method of claim 9, wherein the number of units is from 3 to
1000.
19. The method of claim 1, wherein said applying step comprises
applying the bias between the ohmic contacts on the surface of the
p type semiconductor layer and the ohmic contacts on the surface of
the n type semiconductor layer such that said bias produces an
electron potential drop from the p type semiconductor layer to the
n type semiconductor layer.
20. The method of claim 1, wherein said applying step comprises
applying the bias between the ohmic contacts on the surface of the
p type semiconductor layer and the ohmic contacts on the surface of
the n type semiconductor layer such that said bias produces a near
constant electric field in the multiple quantum well region and
raises an energy state of a ground state in each well layer to the
same level as the energy state of a first excitation state in the
adjoining well layer.
21. The method of claim 1, wherein said applying step comprises
applying the working bias between the ohmic contacts on the surface
of the p type semiconductor layer and the ohmic contacts on the
surface of the n type semiconductor layer such that said bias
produces photo-generated carriers that are transported by the
sequential resonant tunneling through the multiple quantum well
region.
22. The method of claim 1, wherein said applying step comprises
applying the bias between the ohmic contacts on the surface of the
p type semiconductor layer and the ohmic contacts on the surface of
the n type semiconductor layer so as to experimentally determined
said bias by measuring a current-voltage profile of the device
while the peak electrical current exists at the bias.
23. The method of claim 1, wherein said applying step comprises
applying the bias between the ohmic contacts on the surface of the
p type semiconductor layer and the ohmic contacts on the surface of
the n type semiconductor layer such that said bias is adjustable
under high power illumination and at different temperatures.
24. The method of claim 1, further comprising the step of
alternatively striking said device with absorbing illumination from
a back side of said device and a front side of said device.
25. The method of claim 1, further comprising the step of:
operating said device as an optically pumped infrared emitter in a
sequential resonant tunneling condition, wherein at least one of
the following conditions applies with respect to said infrared
emitter: a) said infrared emitter emits infrared photons created by
relaxation of photogenerated electrons from a first excited state
to a ground state in the quantum wells, where the energy positions
from the first excited states to the ground states are
theoretically determined by the effective mass equation [ - 2 2 m *
.times. .times. .gradient. 2 .times. + V .function. ( z ) ] .times.
F .function. ( z ) = E n .times. .times. F .function. ( z ) ,
##EQU3## where h- is the reduced Planck constant, m* is the
electron effective mass, V(z) is the electron potential along the
material growth direction, F(z) is the effective-mass envelope
function of electrons, and E.sub.n is the energy of electrons at
the n.sup.th energy level in the quantum wells; b) infrared photons
emitted by said infrared emitter have an energy in the range of 20
meV to 1.3 eV, with said energy being equal to an energy difference
between the first excited state and the ground state in the
multiple quantum wells within the device; and c) said infrared
emitter emits an output of M infrared photons for each incident
photon going through N quantum wells, where M.ltoreq.N.
Description
RELATED APPLICATIONS
[0001] This is a continuation of application Ser. No. 10/728,562,
filed Dec. 5, 2003, which claims priority from U.S. Provisional
Patent Application Ser. No. 60/430,971 which was filed on Dec. 05,
2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to sequential resonant tunneling
(SRT) photodetector and emitter devices based on Ill-nitride
multiple-quantum-well (MQW) structures; to methods of their
fabrication; and to methods of their use, where III-nitride refers
to compound alloys of nitride and group III elements including
aluminum, gallium, and indium.
[0004] 2. Description of the Related Art
[0005] III-nitride-based photodetectors are used in space-to-earth
and space-to-space communication, missile plume detection,
detection of biological organisms and bacteria, combustion sensing
and control for aircraft engines, optical storage, air quality
monitoring, cancer diagnosis, and personal ultraviolet exposure
dosimetry. Alloys of III-nitride are becoming the semiconductor of
choice for photodetectors and light emitters in the wavelength
range from yellow to ultraviolet due to their direct and wide band
gaps. Because of their thermal stability and radiation hardness,
these materials are remarkably tolerant in aggressive environments.
By designing the semiconductors to have different mole fractions of
group III elements in the nitride compounds, the cutoff wavelength
of III-nitride-based detectors is adjustable in a wide wavelength
range from 630 nm to 200 nm. This approach makes selective spectral
detection realizable. As the result of the rapid progress of growth
techniques, such as metal organic chemical vapor deposition (MOCVD)
and molecular beam epitaxy (MBE), ultraviolet photodetectors based
on GaN bulk material have even been commercialized. However,
quantum efficiency and response speed of bulk-based photodetectors
are limited by their low absorption efficiency and low carrier
mobilities.
[0006] U.S. Pat. No. 6,649,943 to Shibata et al, for "Group III
Nitride Compound Semiconductor Light Emitting Element", discloses a
group III nitride compound semiconductor light-emitting element
formed of group III nitride semiconductor layers, including a
multi-layer containing light-emitting layers; a p-type
semiconductor layer; and an n-type semiconductor layer, wherein the
multi-layer includes a multiple quantum barrier-well layer
containing quantum-barrier-formation barrier layers formed from a
group III nitride semiconductor and quantum-barrier well layers
formed from a group III nitride compound semiconductor, the barrier
layers and the well layers being laminated alternately and
cyclically, and a plurality of low-energy-band-gap layers which
emit light of different wavelengths, with the multiple quantum
barrier well layer being provided between the low-energy-band-gap
layers.
[0007] The devices of Shibata et al are electrically pumped light
emitters, which are used as UV and visible light sources. The
active layers of these devices are the low-energy-band-gap layers
adjacent to the multiple quantum barrier-well layer. The devices
operate under a forward bias that creates carrier injection to the
low-energy-band-gap layers. The injected carriers recombine in the
low-energy-band-gap layers and generate UV and visible light. These
devices use inter-band carrier recombination in the
low-energy-band-gap layers. Electrons from the n-type layer and
holes from the p-type layer are injected to the low-energy-band-gap
layers and recombine there to produce UV and visible light. These
devices do not rely on the phenomenon of sequential resonant
tunneling. The measurable output signal of these devices is the
optical power of the emitted light. The quantum efficiency of these
devices is evaluated by the fraction of injected electrons and
holes that recombine in the low-energy band-gap layers and
successfully generate UV and visible photons.
SUMMARY OF THE INVENTION
[0008] The various features of novelty which characterize the
invention are pointed out with particularity in the claims annexed
to and forming a part of the disclosure. For a better understanding
of the invention, its operating advantages, and specific objects
attained by its use, reference should be had to the drawing and
descriptive matter in which there are illustrated and described
preferred embodiments of the invention.
[0009] We have discovered that by taking advantage of sequential
resonant tunneling of carriers in multiple quantum wells, we have
been able to design III-nitride photodetectors and optically pumped
emitters based on multiple-quantum-well structures. The devices of
the present invention contain semi-insulate MQW layers sandwiched
between a p-n junction. A device according to the present invention
is also an optically pumped infrared emitter, capable of outputting
infrared light that emits from relaxation of photogenerated
carriers. We have also recognized and taken into consideration
certain important effects induced by the large spontaneous
piezoelectric polarization that occurs in III-nitride
heterojunctions.
[0010] In contrast to the related art, the devices of the present
invention are used predominantly for the detection of external
light signals or infrared light sources, not as light sources
themselves. The devices of the present invention are not used to
emit light in the visible or ultraviolet (UV) spectrum, but can be
used as emitters of photons in the infrared (IR) spectrum. The
structure of the devices of the present invention includes a p-type
semiconductor layer, an n-type semiconductor layer, and a multiple
quantum barrier-well layer. Structures of the devices of the
present invention do not contain low-energy-band-gap layers. The
active layer of the devices of the present invention is a multiple
quantum barrier-well layer. Devices of the present invention
operate under a reversed bias without carrier injection; and
require external light excitation to generate photovoltaic signals
in the multiple quantum barrier-well layer. Devices of the present
invention utilize a photovoltaic effect that produces photovoltaic
signals and infrared light as a by-product. External light
generates electrons and holes in the multiple quantum barrier-well
layer of the devices of the present invention. The photo-excited
electrons are transported to the n-type semiconductor layer, while
the photo-excited holes are transported to the p-type semiconductor
layer in the devices of the present invention. In the devices of
the present invention, electrons are transported through the
multiple quantum barrier-well layer by sequential resonant
tunneling. The measurable output signal of the devices of the
present invention is a photocurrent and a photovoltage. The quantum
efficiency of devices of the present invention is evaluated by the
fraction of incident photons that generate carriers in the multiple
quantum barrier-well layer, which are successfully transported to
the n-type semiconductor layer and the p-type semiconductor
layer.
[0011] Accordingly, we have designed devices according to the
present invention which demonstrate the following non-limiting
advantages over devices of the related art:
[0012] i) Carrier transport efficiency is significantly increased
because of both reduced non-radiative and radiative carrier
recombination that occurs in the sequential resonant tunneling
condition;
[0013] ii) Response speed of the device is greatly increased since
photogenerated carriers are transported by sequential resonant
tunneling;
[0014] iii) Both short and long cutoff wavelengths are adjustable
by setting different structure parameters;
[0015] iv) Thermal noise is decreased due to quantum confinement of
carriers in the MQW region;
[0016] v) Quantum efficiency is greatly increased because of high
optical absorption in quantum wells comparing to bulk
materials.
[0017] Photodetectors and emitters according to the present
invention are based on III-nitride multiple quantum wells. The
devices have a p-n junction with III-nitride MQW layers sandwiched
between p type semiconductor and n type semiconductor III-nitride
layers. Ohmic contacts are provided for the surfaces of both p type
and n type semiconductor layers. Structure parameters of the device
are adjustable to achieve desired features of the photodetector.
The photodetector operates in a certain biased condition where
photogenerated carriers are transported through the MQW layers by
sequential resonant tunneling. The device can also operate as an
optically pumped infrared emitter which amplifies input optical
signal by a factor of M, where M is equal to or smaller than the
number of the quantum wells. These devices have high transport
efficiency, high absorption efficiency, high quantum efficiency and
high response speed. Devices according to the present invention
incorporate undoped or lightly doped III-nitride MQW layers
embedded in p-type and n-type III-nitride semiconductor layers. By
introducing these multiple quantum well layers as an active region,
the performance of photodetectors and emitters is greatly improved.
Ohmic contacts are made to the front and back surfaces of the p and
n semiconductor layers. The mole fractions of group III elements in
the nitride compounds used in the device are adjustable from 0 to 1
according to desired wavelength cutoffs of from 630 nm (lnN) to 200
nm (AlN). The device operates in an electrically biased condition,
where sequential resonant tunneling of photogenerated carriers
occurs in the MQW region. In this condition, both radiative and
non-radiative carrier recombination are effectively decreased and
high efficient photon detection and emission is obtained.
[0018] Other objects and features of the present invention will
become apparent from the following detailed description considered
in conjunction with the accompanying drawings. It is to be
understood, however, that the drawings are designed solely for
purposes of illustration and not as a definition of the limits of
the invention, for which reference should be made to the appended
claims. It should be further understood that the drawings are not
necessarily drawn to scale and that, unless otherwise indicated,
they are merely intended to conceptually illustrate the structures
and procedures described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Other objects and features of the present invention will
become apparent from the following detailed description considered
in conjunction with the accompanying drawings. It is to be
understood, however, that the drawings are designed solely for
purposes of illustration and not as a definition of the limits of
the invention, for which reference should be made to the appended
claims. It should be further understood that the drawings are not
necessarily drawn to scale and that, unless otherwise indicated,
they are merely intended to conceptually illustrate the structures
and procedures described herein.
[0020] In the drawings:
[0021] FIG. 1 is a schematic of a layered semiconductor structure
according to the present invention, based on AlGaN materials with
AlGaN quantum wells embedded in a AlGaN p-n junction, for use as a
photodetector.
[0022] FIG. 2 is a graph of confinement energies of the ground
state EO and the first excited state El, as a function of Al mole
fraction of AlGaN barriers, for a layered semiconductor structure
according to the present invention, having GaN/AlGaN multiple
quantum wells, at a well width of 4 nm.
[0023] FIG. 3 is a schematic of a device utilizing the
semiconductor structure of FIG. 1, configured as a back-illuminated
photodetector, and showing the incident direction of light.
[0024] FIG. 4 is a schematic band diagram of the photodetector of
FIG. 3, with 12 quantum wells, and under an operation bias where
sequential resonant tunneling of electrons occur, with rightward
pointing arrows indicating carrier tunneling direction, while
downward pointing arrows indicate carrier relaxation from the first
excited state to the ground state.
[0025] FIG. 5 is a graph of simulated spectral photoresponse curves
versus wavelength, established by setting different quantum-well
thicknesses for a GaN/Al.sub.0.2Ga.sub.0.8N MQW photodetector with
a 1 micrometer Al.sub.0.15Ga.sub.0.75N window layer.
[0026] FIG. 6 is a schematic of a layered semiconductor structure
according to the present invention, for use as a photodetector,
with the semiconductor being formed by taking multiple alloyed
III-nitride layers as the p type layer of the p-n junction.
[0027] FIG. 7 is a schematic of a layered semiconductor structure
according to the present invention, for use as a photodetector,
with the semiconductor structure being formed by taking 6H--SiC
material as the p type layer of the p-n junction, and also
utilizing 6H--SiC as the substrate for the growth of III-nitride
layers, with illustrative parameter values for the semiconductor
being shown.
[0028] FIG. 8 is a schematic of a device according to the present
invention, having a configuration as a front-illuminated
photodetector, utilizing a layered semiconductor having the
structure shown in FIG. 7, where 6H--SiC material is used as the
substrate and p type layer of the p-n junction. The arrows indicate
incident direction of light.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0029] All materials of III-nitride compound semiconductors are
utilizable in the design of photodetectors and emitters according
to the present invention. AlGaN alloys are used in the following
examples as being representative.
[0030] Layered Semiconductor Structure of a Photodetector
[0031] Referring to FIG. 1, a layered semiconductor structure of a
photodetector based on Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN
(x<y) MQWs according to the present invention is shown, where x
and y are aluminum mole fraction in Al.sub.xGa.sub.1-xN and
Al.sub.yGa.sub.1-yN layers, respectively. x and y are in the range
from 0 to 1. From bottom to top, the semiconductor structure
consists of a substrate crystal, a AlN semiconductor nucleation
layer, heavily doped n type Al.sub.wGa.sub.1-wN material,
alternately layered Al.sub.xGa.sub.1-xN and Al.sub.yGa.sub.1-yN
layers, and heavily doped p type Al.sub.zGa.sub.1-zN material. The
substrate is selected to be lattice-matched material to
III-nitride. In the case of the device of FIG. 1, for example, bulk
AlGaN materials, zinc oxide (ZnO), sapphire (Al.sub.2O.sub.3) and
silicon carbide (SiC) are proper candidates. The thicknesses and
the mole fractions of group III elements for different
semiconductor layers vary depending on desired features of designed
photodetectors.
[0032] Structure Parameters
[0033] Structure parameters, such as mole fractions of group III
elements in alloyed layers, and thickness and dopant concentration
of different layers, are chosen to meet desired requirements of the
photodetector and emitters, e.g., desired band detection, infrared
emission wavelength, and operation bias. The photodetector is
designed to create a photocurrent in short-circuit mode and a
photovoltage in open-circuit mode in a wavelength range from a
short cutoff wavelength to a long cutoff wavelength. In the emitter
mode, the device is designed to emit infrared photons pumped by
incident light.
[0034] The Al mole fraction of the aluminum-containing group III
nitride compound of either the p semiconductor layer or the n
semiconductor layer of the window layer is specified so as to make
the band gap of the semiconductor larger than the energy difference
between the ground states of heavy holes and electrons in the MQW
region, since the window layer must be transparent to desired
photoresponse wavelengths. The window layer is responsible for the
short cutoff wavelength and should be thick enough to absorb
incident light with wavelength shorter than the desired short
cutoff wavelength. The thickness of the window layer is typically
on the order of one micrometer. The window layer must be heavily
doped to achieve a carrier concentration that is not limited, but
typically is on the order of 10.sup.18 cm.sup.-3 or higher.
[0035] The MQW layers are active layers which create detectable
photovoltaic signals (either photocurrent or photovoltage) in the
desired wavelength range. The Al mole fractions x and y are
adjustable from 0 to 1 to achieve the desired features. The number
of the MQW layers varies from several to hundred, and should be
sufficient so as to cause the incident light to be completely
absorbed by the MQW layers. The thickness of one single layer is on
the order of nanometers. The Al.sub.xGa.sub.1-xN layers are all of
the same thickness and act as quantum wells. The
Al.sub.yGa.sub.1-yN layers, which have a higher Al fraction, also
all have the same thickness, which may or may not be the same as
the thickness of the layers which act as the wells, and act as
quantum barriers. The long cutoff wavelength is attributed to the
optical absorption of heavy hole to electron transition in the
Al.sub.xGa.sub.1-xN quantum wells. In order to achieve a state of
sequential resonant tunneling of electrons, there should exist at
least two eigenstates, ground state and the first excited state,
which are separated by an energy difference much greater than the
kinetic energy of carriers. The energy positions of the two
eigenstates are determined by the thicknesses and the Al mole
fractions of the wells and the barriers in the MQW region.
Theoretical values of their energy positions are obtained by
numerically solving for different MQW parameters the effective mass
equation, [ - 2 2 m * .times. .gradient. 2 .times. + V .function. (
z ) ] .times. F .function. ( z ) = E n .times. F .function. ( z ) ,
##EQU1## where h- is the reduced Planck constant, m* is the
electron effective mass, V(z) is the electron potential along the
material growth direction, F(z) is the effective-mass envelope
function of electrons, and E.sub.n is the energy of electrons at
the n.sup.th energy level in the quantum wells.
[0036] As a reference, FIG. 2 presents the calculated energy
positions of the ground state and the first excited state of
electrons in the conduction band as a function of aluminum mole
fraction of barriers for a GaN/AlGaN MQW structure with well
thickness of 4 nm. The values on the ordinate scale represent the
energy distance from the conduction minimum of GaN. The MQW layers
are typically undoped or doped by compensation to make them
semi-insulate.
[0037] The other p or n layer, except the window layer, is not
limited but generally set to have an Al mole fraction equal or
lower than that of the quantum well layers (Al.sub.xGa.sub.1-xN).
Its thickness is not limited but is generally set to be a few
hundred nanometers. This layer is designed to be heavily doped to
achieve a carrier concentration which is not limited, but typically
is on the order of 10.sup.18 cm.sup.-3 or higher.
[0038] Device Configuration
[0039] The photodetectors and emitters are designed for either
front or back side illumination, depending on the position of the
window layer. The window layer is set close to the substrate for
back-illuminated detection and close to the top surface for
front-illuminated detection. Referring to FIG. 3, the device
configuration of a back-illuminated photodetector based on the
layered structure of FIG. 1 is shown. To fabricate the device, the
semiconductor layers are etched down to the surface of the n type
Al.sub.xGa.sub.1-wN layer and suitable metal alloys are deposited
on the etched surfaces to form n-type ohmic contacts. Suitable
metal alloys are deposited on the top surfaces of the p type
Al.sub.zGa.sub.1-zN layer to form transparent p-type ohmic
contacts. Any geometry of the metal contacts is acceptable.
[0040] Operation Principle
[0041] The photodetectors and emitters work under a certain
external electrical bias that causes sequential resonant tunneling
of photogenerated carriers to occur. Referring to FIG. 4, a
schematic band diagram of the semiconductor layers in the working
condition for the device of FIG. 3 is shown. The upper and right
diagram of FIG. 4 shows the sequential carrier tunneling and
relaxation process in the quantum wells. The working bias is a
negative bias on the side of the p type semiconductor layer.
[0042] Under the action of this operation bias, a constant electric
field is created across the MQW region. This electric field,
combining together with the built-in electric field in the p-n
junction, lifts the energy level of the ground state (E0) of one
quantum well up to the energy position of the first excited state
(E1) of its neighboring quantum well close to the lower electron
potential direction. In this condition, photoexcited electrons in
the MQW region are able to tunnel through the barriers sequentially
on a time scale of picoseconds. This greatly reduces the amount of
carrier recombination and significantly increases carrier transport
efficiency, quantum efficiency, and response speed of the
photodetector.
[0043] The device can also work as an optically pumped infrared
emitter. At the same time of sequential resonant tunneling of
photogenerated carriers, electrons on the first excited states
relax to the ground states and release photons with an energy equal
to the energy difference E1-E0. This energy has a magnitude in the
range from a few tens meV to hundreds meV depending on designed
layered structures. The corresponding wavelength is on the order of
micrometer which covers infrared region. Note that one
photogenerated electron tunneling through N quantum wells will
totally relax N times from E1 to E0 and release M infrared photons,
where M.ltoreq.N by considering both radiative and non-radiative
loss of photogenerated carriers. This means under the resonant
tunneling condition, one incident photon will create M infrared
photons with the energy E1-E0 emitting out from the device. The
input optical signal is amplified by a factor of M. These infrared
photons can be readily counted by an available infrared
photodetector and reflect the number of incident photons by
multiplying a factor of 1/M. The energy of the infrared photons is
supplied by external electric power.
[0044] The photodetector has a designed spectral response from a
short cutoff wavelength to a long cutoff wavelength, corresponding
to a desired band detection. The short cutoff wavelength is
attributed to band-to-band absorption of the Al.sub.wGa.sub.1-wN
window layer, while the long cutoff wavelength is attributed to
heavy hole to electron absorption of the MQW layers. The short
cutoff wavelength .lamda..sub.s can be adjusted by setting
different values of Al mole fraction w, which is defined by the
following equation, .lamda..sub.s=1240/[6.2w+3.4(1-w)-bw(1-w)]
(nm), (1) where b is bowing constant, b=1.0.+-.0.3.
[0045] The long cutoff wavelength .lamda..sub.l can be adjusted by
setting different structure parameters of the MQW layers as
discussed above.
[0046] As a reference, FIG. 5 presents the simulated photoresponse
curves by setting different thickness of the quantum-well layers
for a GaN/Al.sub.0.2Ga.sub.0.8N MQW photodetector with a 1
micrometer Al.sub.0.15Ga.sub.0.75N window layer.
[0047] Experimentally, the working bias can be found by measuring
current-voltage curves of the photodetector. It is the voltage
position where a current peak appears.
[0048] Note this operation bias is allowed to be shifted a little
due to i) carrier screening effect under high power illumination
and ii) temperature change.
[0049] Polarization-induced Effects
[0050] There exist giant internal fields (up to 10.sup.6 V/cm) in
the quantum wells based on III-nitride materials which are induced
by large spontaneous and piezoelectric polarization. These fields
dramatically modify optical and electrical properties of
III-nitride MQW structures, as well as the performance of the
designed photodetector. To maximize the quantum efficiency and the
transport efficiency of the photodetector, the growth process
should make the orientation of the spontaneous polarization point
to the n type layer. In this case, the internal electric fields in
the quantum wells point from the p layer to the n layer. Under the
working bias, the magnitude of these internal electric fields will
be reduced. This will greatly increase absorption efficiency of the
MQWs, and at the same time, keep a low dark current level.
[0051] Experimentally, the polarity can be selected by using
different growth process. For instance, in the AlGaN based
structure shown in FIG. 1, during growth process of molecular beam
epitaxy, introduction of the thin AlN nucleation layer makes the
structure have gallium-face polarity which leads to the spontaneous
polarization pointing from the surface to the sapphire substrate in
the c-axis (0001) orientation. This will satisfy the design
requirement for the polarity.
[0052] Other Sample Designs
[0053] P and n type semiconductor layers for photodetectors and
emitters disclosed in this patent are not limited to be a single
III-nitride layer and not limited to be III-nitride materials. For
example, p and n type semiconductor layers can be multiple
III-nitride layers and other kind of semiconductor materials with
lattice matched to III-nitride materials.
[0054] Referring to FIG. 6, a schematic diagram of a layered
semiconductor structure for a photodetector with p type layer of
Al.sub.uGa.sub.1-uN/Al.sub.vGa.sub.1-vN multiple layers, which is a
so called superlattice, is shown. The superlattice is p type doped
and plays the role of window layer. This device is for front side
illumination where incident light firstly reaches the surface of
the superlattice. Structure parameters for this device are not
limited to the values shown in FIG. 6, which are set for a
reference only. The device is designed to have the same device
configuration as shown in FIG. 3 except that incident light is from
front side. Transparent ohmic contact for the p type superlattice
is required.
[0055] Referring to FIG. 7, a schematic diagram of layered
semiconductor structure for a photodetector with p type layer of
6H--SiC material is shown. For this design, SiC is p type doped and
used as substrate. The window layer is a Al.sub.wGa.sub.1-wN layer
which is n type doped. This device is for front side illumination
where incident light firstly reaches the surface of the n type
Al.sub.wGa.sub.1-wN layer. Transparent ohmic contact for the n type
Al.sub.wGa.sub.1-wN layer is required. Structure parameters for
this device are not limited to the values shown in FIG. 7, which
are set for a reference only. The device configuration for this
device is shown in FIG. 8.
[0056] Thus, while there have shown and described and pointed out
fundamental novel features of the invention as applied to a
preferred embodiment thereof, it will be understood that various
omissions and substitutions and changes in the form and details of
the devices illustrated, and in their operation, may be made by
those skilled in the art without departing from the spirit of the
invention. For example, it is expressly intended that all
combinations of those elements and/or method steps which perform
substantially the same function in substantially the same way to
achieve the same results are within the scope of the invention.
Moreover, it should be recognized that structures and/or elements
and/or method steps shown and/or described in connection with any
disclosed form or embodiment of the invention may be incorporated
in any other disclosed or described or suggested form or embodiment
as a general matter of design choice. It is the intention,
therefore, to be limited only as indicated by the scope of the
claims appended hereto.
[0057] The invention is not limited by the embodiments described
above which are presented as examples only but can be modified in
various ways within the scope of protection defined by the appended
patent claims. All references cited herein are incorporated in
their entirety by reference.
[0058] All references cited herein are incorporated in their
entirety by reference.
* * * * *