U.S. patent application number 14/291793 was filed with the patent office on 2015-06-25 for photodetectors based on wurtzite mgzno.
This patent application is currently assigned to University of Central Florida Research Foundation, Inc.. The applicant listed for this patent is University of Central Florida Research Foundation, Inc.. Invention is credited to R. CASEY BOUTWELL, WINSTON V. SCHOENFELD, MING WEI.
Application Number | 20150179832 14/291793 |
Document ID | / |
Family ID | 53279961 |
Filed Date | 2015-06-25 |
United States Patent
Application |
20150179832 |
Kind Code |
A1 |
WEI; MING ; et al. |
June 25, 2015 |
PHOTODETECTORS BASED ON WURTZITE MgZnO
Abstract
A photodetector (PD) includes a substrate, and a ZnO nucleation
layer on the substrate. A wurtzite Mg.sub.xZn.sub.1-xO layer is on
the ZnO nucleation layer, wherein x is a mole fraction between 0
and 0.62. A level of crystallinity of the wurtzite
Mg.sub.xZn.sub.1-xO layer characterized by x-ray diffraction with a
deconvolution of a triple-crystal .omega. rocking curve of a ZnO
(0002) peak has a narrow component with a full width at half
maximum (FWHM) less than or equal to (.ltoreq.) 20 arc/s. First and
second spaced apart electrodes are on a surface of the wurtzite
Mg.sub.xZn.sub.1-xO layer. The mole fraction x can be between 0.20
and 0.46, including between 0.37 and 0.46, and provide a PD
responsivity of at least 20 A/W at 5V in the solar blind region
from 200 nm to 290 nm.
Inventors: |
WEI; MING; (ORLANDO, FL)
; BOUTWELL; R. CASEY; (OVIEDO, FL) ; SCHOENFELD;
WINSTON V.; (OVIEDO, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Central Florida Research Foundation, Inc. |
Orlando |
FL |
US |
|
|
Assignee: |
University of Central Florida
Research Foundation, Inc.
Orlando
FL
|
Family ID: |
53279961 |
Appl. No.: |
14/291793 |
Filed: |
May 30, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61831867 |
Jun 6, 2013 |
|
|
|
Current U.S.
Class: |
257/43 ;
438/85 |
Current CPC
Class: |
C30B 29/16 20130101;
H01L 27/14 20130101; H01L 21/02554 20130101; H01L 51/42 20130101;
H01L 21/02472 20130101; H01L 31/1836 20130101; H01L 21/02565
20130101; H01L 31/0368 20130101; H01L 31/02966 20130101; C30B
23/025 20130101; H01L 21/02631 20130101; H01L 31/032 20130101; H01L
31/1832 20130101; H01L 31/1085 20130101; Y02E 10/549 20130101 |
International
Class: |
H01L 31/0296 20060101
H01L031/0296; H01L 31/0368 20060101 H01L031/0368; H01L 31/18
20060101 H01L031/18 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with Government support under
Cooperative Agreement Number W911NF-11-2-0025 awarded by the U.S.
Army Research Laboratory. The Government has certain rights in this
invention.
Claims
1. A photodetector (PD), comprising: a substrate; a ZnO nucleation
layer on said substrate; a wurtzite Mg.sub.xZn.sub.1-xO layer
epitaxial to and on said ZnO nucleation layer, wherein x is a mole
fraction between 0 and 0.62, wherein a level of crystallinity of
said wurtzite Mg.sub.xZn.sub.1-xO layer characterized by x-ray
diffraction with a deconvolution of a triple-crystal .omega.
rocking curve of a ZnO (0002) peak has a narrow component with a
full width at half maximum (FWHM) less than or equal to (.ltoreq.)
20 arc/s, and first and second spaced apart electrodes on said
wurtzite Mg.sub.xZn.sub.1-xO layer.
2. The PD of claim 1, wherein said x is between 0.20 and 0.46.
3. The PD of claim 2, wherein a peak responsivity of said PD is at
least 20 A/W with 5V bias in a wavelength range from 200 nm to 290
nm.
4. The PD of claim 1, wherein a thickness of said wurtzite
Mg.sub.xZn.sub.1-xO layer is between 0.5 .mu.m and 5 .mu.m.
5. The PD of claim 1, further comprising an epitaxial ZnO layer
between said ZnO nucleation layer and said wurtzite
Mg.sub.xZn.sub.1-xO layer, wherein said epitaxial ZnO layer has a
larger average grain size relative to a grain size of said ZnO
nucleation layer.
6. The PD of claim 5, wherein said x is between 0.37 and 0.46, and
wherein a peak responsivity of said PD is at least 20 A/W with 5V
bias in a wavelength range from 200 nm to 290 nm.
7. The PD of claim 5, wherein said x is between 0.37 and 0.46, and
wherein a peak responsivity of said PD is at least 100 A/W with 5V
bias in a wavelength range from 200 nm to 290 nm.
8. The PD of claim 1, wherein said substrate comprises sapphire,
GaN or ZnO.
9. The PD of claim 1, wherein said first and second spaced apart
electrodes comprise interdigitated metal fingers having a spacing
from 2 .mu.m to 15 .mu.m.
10. The PD of claim 1, wherein a root mean square (rms) surface
roughness of said ZnO nucleation layer is <1 nm and a thickness
of said ZnO nucleation layer is from 15 nm to 40 nm.
11. A method of fabricating a photodetector (PD), comprising:
depositing a ZnO nucleation layer on a substrate at a deposition
temperature less than or equal to (.ltoreq.) 400.degree. C.,
depositing a wurtzite Mg.sub.xZn.sub.1-xO layer epitaxial to and on
said ZnO nucleation layer, wherein x is a mole fraction between 0
and 0.62, and forming first and second spaced apart electrodes on
said wurtzite Mg.sub.xZn.sub.1-xO layer, wherein a level of
crystallinity said wurtzite Mg.sub.xZn.sub.1-xO layer characterized
by x-ray diffraction with a deconvolution of a triple-crystal
.omega. rocking curve of a ZnO (0002) peak has a narrow component
with a full width at half maximum (FWHM) less than or equal to
(.ltoreq.) 20 arc/s.
12. The method of claim 11, wherein said depositing said wurtzite
Mg.sub.xZn.sub.1-xO layer comprises plasma-assisted molecular beam
epitaxy (MBE).
13. The method of claim 11, wherein said x is between 0.20 and
0.46.
14. The method of claim 13, wherein a peak responsivity of said PD
is at least 20 A/W with 5V bias in a wavelength range from 200 nm
to about 350 nm.
15. The method of claim 11, further comprising depositing an
epitaxial ZnO layer on said ZnO nucleation layer at a deposition
temperature between 450.degree. C. and 525.degree. C.
16. The method of claim 15, wherein said x is between 0.37 and
0.46, and wherein a peak responsivity of said PD is at least 20 A/W
with 5V bias in a wavelength range from 200 nm to 290 nm.
17. The method of claim 15, wherein said x is between 0.37 and
0.46, and wherein a peak responsivity of said PD is at least 100
A/W with 5V bias in a wavelength range from 200 nm to 290 nm.
18. The method of claim 11, wherein said forming said first and
second spaced apart electrodes comprises forming interdigitated
metal fingers having a spacing from 2 .mu.m to 15 .mu.m.
19. The method of claim 11, wherein a thickness of said wurtzite
Mg.sub.xZn.sub.1-xO layer is between 1 .mu.m and 5 .mu.m.
20. The method of claim 11, wherein a root mean square (rms)
surface roughness of said ZnO nucleation layer is <1 nm and a
thickness of said ZnO nucleation layer is from 15 nm to 40 nm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Provisional
Application Ser. No. 61/831,867 entitled "DEEP ULTRAVIOLET
PHOTODETECTORS BASED ON WURTZITE MgZnO WITH ZnO HOMONUCLEATION
LAYER", filed on Jun. 6, 2013, which is herein incorporated by
reference in its entirety.
FIELD
[0003] Disclosed embodiments relate to optoelectronic and
microelectronic devices and fabrication methods thereof, and to
metal oxide semiconductor layers and articles including Ultraviolet
(UV) photodetectors therefrom.
BACKGROUND
[0004] UV photodetectors and light emitters find numerous uses
including applications in the defense, commercial, and scientific
arenas. These include, for example, covert space-to-space
communications, missile threat detection, chemical and biological
threat detection and spectroscopy, UV environmental monitoring, and
germicidal cleansing. UV photodetectors and light emitters
operating in the solar blind region are of special interest. The
solar blind region corresponds to the spectral UV region where
strong upper atmospheric absorption of solar radiation occurs,
generally at wavelengths from about 200 nm to about 290 nm. This
creates a natural low background window for detection of man-made
UV sources on and proximate to the earth's surface.
[0005] Semiconductor materials having a 25.degree. C. band gap of
about 4 eV to 6 eV have been used to sense or generate solar blind
UV radiation. Conventional approaches have used compound
semiconductor materials such as AlGaN, MgZnO, or BeZnO, which
generally have wurtzite (hexagonal) lattice structures. AlGaN is
known to suffer from various problems including lattice cracking
due to strain, generally high dislocation density, and lattice
mismatch with respect to the layer it is grown on (all such effects
are generally interrelated). High dislocation density undesirably
reduces internal quantum efficiency.
[0006] The crystal structure of Mg.sub.xZn.sub.1-xO can be cubic or
wurtzite, depending on the Mg/Zn ratios. High % Mg compositions
result in a cubic structure while low % Mg compositions result in a
Wurtzite structure. However, the crystal structure difference and
large lattice mismatch between ZnO (wurtzite, 3.25 .ANG.) and MgO
(rock salt, 4.22 .ANG.) causes phase segregation in
Mg.sub.xZn.sub.1-xO with Mg compositions between about
37%<x<62% BeZnO is generally considered a somewhat more
promising semiconductor material, but has experienced doping
difficulties, particularly difficulties in obtaining high mobility
and stable p-type doping.
[0007] Epitaxial monocrystalline ZnO on sapphire (Al.sub.2O.sub.3)
substrates is known to be a relative low cost PD option. Process
temperatures used to obtain crystalline ZnO commonly exceed
500.degree. C. There is a large in-plane lattice mismatch (18%)
between c-oriented ZnO and sapphire, typically resulting in a high
dislocation density of generally more than 10.sup.9 cm.sup.-2 for
epitaxial ZnO layers, which may lead to low responsivity for
photodiodes made from or on such layers.
SUMMARY
[0008] This Summary is provided to introduce a brief selection of
disclosed concepts in a simplified form that are further described
below in the Detailed Description including the drawings provided.
This Summary is not intended to limit the claimed subject matter's
scope.
[0009] Disclosed embodiments include methods of forming photodiodes
(PDs) including forming a crystalline ZnO nucleation layer grown at
relatively low temperature (LT), such as 300.degree. C. to
400.degree. C., on a crystalline substrate. The thickness of the
ZnO nucleation layer can be from 15 nm to 40 nm, such as from 20 nm
to 25 nm in one particular embodiment.
[0010] A single crystal wurtzite Mg.sub.xZn.sub.1-xO layer, where x
is the mole fraction between 0 and 0.62, is grown at a higher
temperature relative the ZnO nucleation layer on top of the ZnO
nucleation layer. Experimental data for an example single crystal
wurtzite Mg.sub.xZn.sub.1-xO layer (x=0.09) has shown high quality
evidenced by a level of crystallinity from x-ray diffraction
showing a full width at half maximum for .omega. of <20 arc/sec,
such as a .omega. value of 13 to 20 arc/sec. Disclosed ZnO
nucleation layers have been demonstrated to have sub-nanometer
(<1 nm) root mean square (rms) surface roughness, such as
measured by atomic force microscopy. A LT ZnO nucleation layer has
been found to be important to the interfacial region to suppress
the defects induced by lattice mismatch and produces a smooth
surface that enables high quality 2D epitaxial growth of a single
crystal wurtzite Mg.sub.xZn.sub.1-xO layer to follow. The x-ray
diffraction derived triple-crystal o rocking curve measured on the
maximum of the ZnO (0002) peak can be deconvolved into a narrow
curve (e.g., FWHM=13 to 17 arc/s) and a wide diffuse base, related
to the bottom structurally deteriorated sub-layer (ZnO nucleation
layer) with a FWHM of >60 arc/sec, such as .about.120 arc/s.
With the single crystal wurtzite Mg.sub.xZn.sub.1-xO layer with x=0
(ZnO), the FWHM of the (0002) HT ZnO peak (13 arc/s) described
below is the highest crystallinity known. No cubic phase was
observed in X-ray diffraction measurements performed on an example
wurtzite Mg.sub.xZn.sub.1-xO layer deposited on a LT ZnO nucleation
layers with x being 0.37 to as high as 0.46. This is an unexpected
result since it is known in the art that phase segregation occurs
for x between 0.36 and 0.62 so that wurtzite Mg.sub.xZn.sub.1-xO
detectors in solar blind region have not been possible before due
to phase segregation.
[0011] An optional epitaxial ZnO layer can be deposited on the ZnO
nucleation layer at 450 to 525.degree. C. and the wurtzite MgZnO
layer can be deposited thereon at 400.degree. C. to 500.degree. C.,
such as around 430.degree. C. using methods including
plasma-assisted molecular beam epitaxy (MBE). The wurtzite
.sub.Mg.sub.xZn.sub.1-xO layer can have an x-value between 0 and
0.46, such as 0.2 to 0.46. As known in the art, wurtzite is
hexagonal crystal system used by various binary compounds and
ternary compounds.
[0012] Wurtzite Mg.sub.xZn.sub.1-xO has been found to provide high
performance PDs and emitters operating in the UV spectral region,
including the solar blind region from 200 nm to 290 nm for
x>about 0.3. By changing the Mg concentration,
Mg.sub.xZn.sub.1-xO has been found to provide a tunable 25.degree.
C. bandgap energy from 3.3 eV for wurtzite ZnO (no Mg) to 7.8 eV
for rock salt MgO (no Zn). As described below, high crystal and
optical quality epitaxial single crystal wurtzite
Mg.sub.xZn.sub.1-xO layers were obtained on crystalline ZnO
nucleation layers on c-plane sapphire substrates by plasma-assisted
MBE.
[0013] By tuning the Mg/Zn flux ratio during Mg.sub.xZn.sub.1-xO
deposition, a steep optical absorption edge of the wurtzite
Mg.sub.xZn.sub.1-xO with a spectral cutoff wavelength ranging from
278 nm to 377 nm was demonstrated, corresponding to the mole
fraction x ranging from 0 (377 nm) to 0.46 (278 nm). As described
in more detail in the examples section, photoconductive and
Schottky barrier metal-semiconductor-metal (MSM) wurtzite
Mg.sub.xZn.sub.1-xO PDs with an interdigitated electrode geometry
and active surface area of 1 mm.sup.2 were fabricated. The I-V
characteristics, in dark and under UV illumination, as well as
spectral and temporal responses were characterized at zero-bias and
5V bias conditions. For Mg.sub.xZn.sub.1-xO with x=0.46 PD
responsivity was found to be as high as 200 A/W at 5 V with a
rejection ratio of two orders of magnitude demonstrated in the
solar blind spectral range from 200 nm to 290 nm with a spectral
cutoff at 278 nm (the spectral cutoff being the intercept of the
linear portion of the absorption edge on wavelength axis in a
Tauc-like plot).
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1A is a cross sectional depiction of an example MSM PD
comprising electrodes shown as interdigitated metal fingers and
oppositely laying on the surface of a wurtzite Mg.sub.xZn.sub.1-xO
layer on a ZnO nucleation layer on a substrate, according to an
example embodiment.
[0015] FIG. 1B is a cross sectional depiction of an example MSM PD
based on the PD shown in FIG. 1A modified to add an epitaxial ZnO
layer between the wurtzite Mg.sub.xZn.sub.1-xO layer and the ZnO
nucleation layer, according to an example embodiment.
[0016] FIG. 2 shows the responsivity of an example MSM PD
comprising wurtzite ZnO measured at a 5 V bias.
[0017] FIGS. 3A and 3B show responsivity data for of an example MSM
PD comprising a wurtzite Mg.sub.0.46Zn.sub.0.54O layer on a ZnO
nucleation layer on a sapphire substrate measured at 0 V bias and a
5 V bias.
[0018] FIG. 4 is a log scale plot of the responsivity of an example
MSM PD comprising wurtzite Mg.sub.xZn.sub.1-xO on a ZnO nucleation
layer on a sapphire substrate with 10 .mu.m pitch interdigitated
electrodes at biases of 0, 1 V and 5 V.
[0019] FIG. 5A shows I-V characteristics of example MSM PDs
comprising wurtzite Mg.sub.0.46Zn.sub.0.54O on a ZnO nucleation
layer on a sapphire substrate with 10 .mu.m interdigitated
electrode spacing taken in the dark and under light illumination
with .lamda.=300 nm, 22 .mu.W optical power, and FIG. 5B is a
magnified dark I-V curve for the PD.
DETAILED DESCRIPTION
[0020] Disclosed embodiments in this Disclosure are described with
reference to the attached figures, wherein like reference numerals
are used throughout the figures to designate similar or equivalent
elements. The figures are not drawn to scale and they are provided
merely to illustrate the disclosed embodiments. Several aspects are
described below with reference to example applications for
illustration.
[0021] It should be understood that numerous specific details,
relationships, and methods are set forth to provide a full
understanding of the disclosed embodiments. One having ordinary
skill in the relevant art, however, will readily recognize that the
subject matter disclosed herein can be practiced without one or
more of the specific details or with other methods. In other
instances, well-known structures or operations are not shown in
detail to avoid obscuring structures or operations that are not
well-known. This Disclosure is not limited by the illustrated
ordering of acts or events, as some acts may occur in different
orders and/or concurrently with other acts or events. Furthermore,
not all illustrated acts or events are required to implement a
methodology in accordance with this Disclosure.
[0022] Notwithstanding that the numerical ranges and parameters
setting forth the broad scope of this Disclosure are
approximations, the numerical values set forth in the specific
examples are reported as precisely as possible. Any numerical
value, however, inherently contains certain errors necessarily
resulting from the standard deviation found in their respective
testing measurements. Moreover, all ranges disclosed herein are to
be understood to encompass any and all sub-ranges subsumed therein.
For example, a range of "less than 10" can include any and all
sub-ranges between (and including) the minimum value of zero and
the maximum value of 10, that is, any and all sub-ranges having a
minimum value of equal to or greater than zero and a maximum value
of equal to or less than 10, e.g., 1 to 5.
[0023] Cubic Mg.sub.xZn.sub.1-xO thin films are known having a Mg
composition above 62%, providing deep UV applications. However,
cubic Mg.sub.xZn.sub.1-xO is recognized herein to suffer from low
crystal quality and PDs based on known cubic Mg.sub.xZn.sub.1-xO
show low responsivity typically limited to several hundred mA/W
(e.g., 300 .mu.mA/W) at 5V, or lower. Wurtize Mg.sub.xZn.sub.1-xO
can provide somewhat higher responsivity, but suffers when adding
Mg sufficient to extend into the solar blind region due to phase
segregation issues. Compared to MSM PDs from cubic
Mg.sub.xZn.sub.1-x O, disclosed MSM PDs from wurtzite
Mg.sub.xZn.sub.1-xO on ZnO nucleation layers can have a Mg
composition from 37% to as high as 46% which have been found to
provide better performance due to the high quality wurtzite
Mg.sub.xZn.sub.1-xO single crystal obtained when grown on a
disclosed ZnO nucleation layer and resulting high responsivity of
the MSM PDs therefrom of at least 20 A/W up to 100 A/W, such as
.gtoreq.200 A/W at 5V.
[0024] Disclosed embodiments provide single crystal wurtzite
Mg.sub.xZn.sub.1-xO layers with the option for a relatively high Mg
concentration (e.g., x=0.20 to 0.46, or 0.37 to 0.46, or more)
enabled by adding a LT ZnO nucleation layer on the substrate, such
as grown at a temperature at 300.degree. C. to 400.degree. C., with
350.degree. C. used in one particular embodiment. By tuning the Mg
concentration through controlling the Mg/Zn flux ratio during the
wurtzite Mg.sub.xZn.sub.1-xO deposition, a steep optical absorption
edge MSM PDs having wurtzite Mg.sub.xZn.sub.1-xO with a spectral
cutoff wavelength range can be designed from 277 nm to 377 nm,
corresponding to x ranging from 0.46 to 0.
[0025] By disclosed embodiments including a LT ZnO nucleation layer
on the substrate, the properties of the wurtzite
Mg.sub.xZn.sub.1-xO layer have been found to be improved and MSM PD
responsivity has been found to be significantly increased, with a
peak responsivity of at least 20 A/W with a 5V bias in a wavelength
range of from about 200 nm to about 350 nm, with a peak
responsivity of >20 A/W with a 5V bias, typically with a peak
responsivity of above 200 A/W with a 5V bias. High responsivity is
provided because of a high crystal quality wurtzite
Mg.sub.xZn.sub.1-xO layer, through a mole fraction x range
confirmed experimentally from 0 and 0.46. As noted above, the
ability to provide high crystal quality wurtzite
Mg.sub.xZn.sub.1-xO layer in a mole fraction (x) range from 0.37 to
0.46 is particularly unexpected as it is known in the art that
phase segregation results when x is between 0.36 and 0.62 (Mg
composition between 36% and 62%) so that one cannot make wurtzite
detectors in solar blind region.
[0026] FIG. 1A is a cross sectional depiction of an example MSM PD
100 comprising first and second spaced apart electrodes 121 and 122
shown as interdigitated electrodes laying on the surface 110b.sub.1
of stacked of metal oxide semiconductor layers including a single
crystal epitaxial wurtzite M.sub.gxZ.sub.n1-xO layer (wurtzite
M.sub.gxZn.sub.1-xO layer) 110b, according to an example
embodiment. As noted above, the mole fraction x can range from 0 to
0.62, such as 0.2 to 0.46, and the wurtzite M.sub.gxZn.sub.1-xO
layer 110b is epitaxial to the ZnO nucleation layer 110a that
provides the function of a buffer layer and nucleation layer which
is on a substrate 120. The ZnO nucleation layer 110a is typically
15 nm to 40 nm thick. The MSM PD 100 can operate as a solar-blind
PD.
[0027] Although the wurtzite Mg.sub.xZn.sub.1-xO layer 110b is
shown directly on the first ZnO layer 110a, an additional
intervening high temperature ZnO nucleation layer (or layers)
referred to herein as a second ZnO nucleation layer can be added
between the wurtzite Mg.sub.xZn.sub.1-xO layer 110b and the first
ZnO buffer layer 110a to further improve the surface morphology and
crystallinity of the wurtzite Mg.sub.xZn.sub.1-xO layer 110b, which
has been found to improve PD performance including responsivity.
FIG. 1B is a cross sectional depiction of an example photodetector
MSM PD 150 based on PD 100 modified to add an epitaxial ZnO layer
110al between the wurtzite Mg.sub.xZn.sub.1-xO layer 110b and the
ZnO nucleation layer 110a.
[0028] The epitaxial ZnO layer 110a' can comprise a high
temperature (HT) grown ZnO layer relative to the deposition
temperature used to deposit the ZnO nucleation layer 110a, such as
grown at 500.degree. C., and as noted above the first ZnO
nucleation layer 110a can comprise a LT grown ZnO layer, such as
epitaxially grown at 300.degree. C. to 400.degree. C., such as
350.degree. C. in one particular embodiment. The degree of
crystallinity will generally be less for the LT ZnO layer as
compared to the HT ZnO layer(s), including in the final PD device.
This difference in crystallinity can be evidenced by the Full Width
of Half Maximum (FWHM) of the rocking curve of the respective X-ray
diffraction peaks.
[0029] As known in the art, the narrower the FWHM, the higher the
level of crystallinity is. HT-ZnO grown on LT-ZnO has been
deconvolved into two peaks, one with very narrow FWHM attributed to
HT ZnO evidencing very high crystallinity and the other peak with a
broader FWHM and weaker peak is attributed to LT ZnO evidencing
lower crystallinity. The FWHM for ZnO (0002) peaks were measured to
be 13 arc sec and 17.8 arc sec for the HT ZnO layer and wurtzite
Mg.sub.xZn.sub.1-xO (with low Mg composition, e.g., being x=9%)
respectively, indicating good ordering in the crystal growth
direction.
[0030] If the wurtzite Mg.sub.xZn.sub.1-xO layer 110b is thin (such
as .ltoreq.300 nm), it is recognized the relatively low level of
crystallinity of the ZnO nucleation layer 110a can cause
recombination of photogenerated carriers, thus reducing the photo
responsivity of the PD. Moreover, the ZnO nucleation layer 110a may
absorb light with wavelengths above 370 nm and generate carriers,
which can be collected by the contacts to the electrodes 121, 122
and can undesirably contribute to the photoresponse of the MSM
PD.
[0031] It has been found to make deep UV MSM PDs without little
responsivity for wavelengths above 300 nm influence by the ZnO
nucleation layer 110a can be minimized by having the wurtzite
Mg.sub.xZn.sub.1-xO layer 110b 1 .mu.m thick or more (e.g., 1 .mu.m
to 5 .mu.m). This rather high relative thickness for the wurtzite
Mg.sub.xZn.sub.1-xO layer 110b has been found to provide two
significant benefits: (1) the wurtzite Mg.sub.xZn.sub.1-xO layer
110b is essentially fully relaxed and thus has less dislocations
(defects); (2) and the photogenerated carriers in the ZnO
nucleation layer 110a do not measurably contribute to the PD
response, due to the thick wurtzite Mg.sub.xZn.sub.1-xO layer 110b
thereon.
[0032] The electrodes 121 and 122 form low resistance contacts to
the wurtzite Mg.sub.xZn.sub.1-xO layer 110b. The electrodes 121 and
122 can be formed by photolithography followed by deposition (e.g.,
sputtering or E-beam deposition) and then lift-off. In a typical
embodiment where the electrodes 121, 122 are interdigitated, the
gaps between the interdigitated fingers are several microns, such
as from 2 .mu.m to 15 .mu.m. The substrate 120 can be a sapphire
substrate, or other substrates such as ZnO or GaN.
[0033] The electrodes 121 and 122 can comprise a multi-layer metal
stack including a low work function metal (defined as having a
25.degree. C. work function <4 eV, such as Mg) between an
adhesion layer (e.g., 2 to 5 nm Ni or Ti layer) and an oxidation
resistant metal capping layer (e.g., Au or other noble metal). The
low work function metal layer can be polycrystalline or single
crystalline, and can be 10 nm to 30 nm thick in one embodiment. The
oxidation resistant metal capping layer can be 60 nm thicker or
higher
EXAMPLES
[0034] Disclosed embodiments are further illustrated by the
following specific Examples, which should not be construed as
limiting the scope or content of this Disclosure in any way.
[0035] The deposition of wurtzite Mg.sub.xZn.sub.1-xO layer on a
ZnO nucleation layer on a c-plane sapphire substrate was monitored
in situ by reflection high-energy electron diffraction (RHEED).
High quality single crystal wurtzite Mg.sub.xZn.sub.1-xO layers
having an active surface area of 1 mm.sup.2 was found to be
obtained by epitaxially depositing on a LT ZnO nucleation layer on
a substrate, shown as ZnO nucleation layer 110a described
above.
[0036] Elemental Zn (6N) and Mg (3N8) were evaporated from standard
hot-lipped Knudsen cell and O (6N) fluxes were supplied by a
radio-frequency (RF) radical cell (f.sub.0=13.56 MHz). Base
pressure in the growth chamber was .about.2.times.10.sup.-10 Torr.
During the growth of ZnO layers, the pressure in the chamber was
maintained around 3.times.10.sup.-6 to 5.times.10.sup.-6 Torr. To
increase the homogeneity of heat conduction during the growth, 1
.mu.m of Titanium was deposited on the backside of the sapphire
substrate with a Temscal E-beam evaporation system (model FC-2000).
The sample was thereafter degreased with iso-propanol and dried
with a nitrogen gun. After this cleaning procedure, the substrate
was immediately loaded on an indium free molybdenum holder and
thermally cleaned at 600.degree. C. in a buffer chamber (10.sup.-9
Torr) for 4 hours before transferred to the growth chamber.
[0037] A low temperature (LT) ZnO homo-nucleation layer was first
grown at 350.degree. C. on the sapphire substrate. The oxygen
plasma power was fixed at 350 W. Zinc cell temperature was
controlled at 360.degree. C., and Oxygen flux was controlled 1.5
sccm to keep oxygen rich conditions. A growth rate around 0.21
.mu.m/h was achieved for nucleation layer as monitored by Laser
reflectometry.
[0038] It was recognized because the sapphire substrate is oxygen
terminated Zn can arrive at the surface and combine with the
surface oxygen atoms with better migration ability. The Zn shutter
was opened first for 3 seconds, allowing for about one monolayer of
deposition followed by opening of the oxygen shutter. This sequence
reliably generated a terrace-step morphology. After 7 minutes of
growth, a 25 nm thick ZnO nucleation layer was epitaxially formed
on the sapphire substrate.
[0039] The RHEED pattern of the ZnO nucleation layer was initially
spotty, indicating a roughening of the growth surface. The pattern
gradually transformed into a sharp streaky pattern after optional
annealing at 600.degree. C. for 30 minutes at pressure of
10.sup.-10 to 10.sup.-9 Torr indicating the ZnO nucleation layer
was clean and flat after the annealing process. Annealing at a
temperature high than 600.degree. C. will not generally improve the
RHEED pattern further. Using MBE a subsequent HT epitaxial ZnO
layer can be optionally grown on the ZnO nucleation layer at
500.degree. C. shown as epitaxial ZnO layer 110al described above
relative to FIG. 1B.
[0040] The Zn to oxygen ratio was maintained for oxygen rich
conditions at low flux to assure a slow growth rate for HT
epitaxial ZnO layer, in this case, 0.15 .mu.m/h. The Zn to oxygen
ratio was verified by varying Zn source temperatures to find the
stoichiometric conditions where Zn source is 370.degree. C. at
oxygen flux of 1.3 sccm. This HT regime allowed for homogeneous
diffusion of the atoms and prevention of 3D growth. The RHEED
pattern taken of a 940 nm HT epitaxial ZnO layer was found to be
clearly streaky, indicating 2D growth.
[0041] A Mg.sub.xZn.sub.1-xO epilayer was grown directly on the LT
ZnO nucleation layer using a similar PA-MBE method as described
above for the growth of ZnO nucleation layer. The growth
temperature for Mg.sub.xZn.sub.1-xO used was 430.degree. C., lower
than that of the epitaxial ZnO layer. The Mg cell temperature
varied from 350.degree. C. to 420.degree. C., corresponding to a Mg
composition x change from 0% to 46%.
[0042] A Metal-semiconductor-metal (MSM) interdigital electrode
geometry with Schottky interdigitated metal electrode fingers with
a finger spacing ranging from 2 .mu.m to 15 .mu.m was then
fabricated from disclosed epitaxial articles. A Ni/Au contact (20
nm/130 nm) was chosen for the Schottky contact to the wurtzite
Mg.sub.xZn.sub.1-xO layer. To characterize the photoresponse
properties of the PDs, a 300 W Xe lamp was used as the excitation
source and a monochrometer was used for scanning the photo response
spectrum.
[0043] FIG. 2 shows the response spectrum of an example MSM PD
including wurtzite ZnO at a 5 V bias and 25.degree. C. The peak
responsivity (R) is seen to be around 2.times.10.sup.4 A/W below
380 nm, at about 300 nm. The rejection ratio of
R.sub.300nm/R.sub.400nm=526, and R.sub.300nm/R.sub.500nm=86,000.
This is comparable to the highest reported responsivity for a ZnO
PD. However, disclosed PDs had a smaller bias, which means the
responsivity should be slightly higher. The quantum efficiency (i)
and gain (g) of the PD can be expressed in the following
formula:
.eta. g = Rhv q ( 1 ) ##EQU00001##
where R is the responsivity depending on voltage, q is the electron
charge and .nu. is the light frequency. The .eta.g for the example
MSM PD was estimated to be 6.6.times.10.sup.4 at 5 V bias.
[0044] FIGS. 3A and 3B show the photoresponse of an example MSM PD
including wurtzite Mg.sub.0.46Zn.sub.0.54O on a ZnO nucleation
layer at a 0V bias and 5V bias, respectively. The log scale is
shown in FIG. 4 where the main optical cutoff in the 280-300 nm
range corresponds to the fundamental optical absorption band in the
Mg.sub.0.46Zn.sub.0.54O layer. The small shoulder that appears near
310 nm to 375 nm is believed to be due to the photocurrent
contribution from the thin ZnO buffer/nucleation layer.
[0045] Schottky PD devices including wurtzite
Mg.sub.0.46Zn.sub.0.54O on a LT ZnO nucleation layer show
responsivity at 265 nm of 0.01 A/W with dark current of 23 pA, and
rejection ratios R.sub.265 nm/R.sub.400nm=180 and
R.sub.265nm/R.sub.500nm=505. Photoconductive devices show
responsivity at 260 nm of 200 A/W, resulting in a photoconductive
gain of 1.times.10.sup.3 with dark current at 5V of 2.7 .mu.A, and
rejection ratio (RR) R.sub.265nm/R.sub.400nm=140 and RR
R.sub.265nm/R.sub.500nm=392. These results evidence the highest
photoresponse in the solar blind region believed to ever have been
reported.
[0046] To explain the high responsivity of disclosed MSM PDs
including wurtzite Mg.sub.xZn.sub.1-xO on a LT ZnO nucleation
layer, I-V curves were generated that are shown in FIG. 5A for
disclosed MSM PDs including wurtzite Mg.sub.xZn.sub.1-xO with a
relatively high Mg composition (x=0.46) having 10 .mu.m
interdigital electrode finger spacing measured in the dark and
under light illumination. The high gain can likely be attributed to
the carrier trapping process which was identified by the asymmetric
barrier height at the interdigital electrodes. In FIG. 5B, the dark
I-V curve shows clear asymmetric Schottky behavior in the positive
and negative region, implying carrier trapping at the metal
wurtzite Mg.sub.xZn.sub.1-xO interface.
[0047] While various disclosed embodiments have been described
above, it should be understood that they have been presented by way
of example only, and not limitation. Numerous changes to the
subject matter disclosed herein can be made in accordance with this
Disclosure without departing from the spirit or scope of this
Disclosure, such as coating the surface of device with an
antireflective layer. In addition, while a particular feature may
have been disclosed with respect to only one of several
implementations, such feature may be combined with one or more
other features of the other implementations as may be desired and
advantageous for any given or particular application.
[0048] Thus, the breadth and scope of the subject matter provided
in this Disclosure should not be limited by any of the above
explicitly described embodiments. Rather, the scope of this
Disclosure should be defined in accordance with the following
claims and their equivalents.
* * * * *