U.S. patent application number 15/278656 was filed with the patent office on 2018-03-29 for ultraviolet (uv) schottky diode detector having single crystal uv radiation detector material bonded directly to a support structure with proper c-axis orientation.
This patent application is currently assigned to Raytheon Company. The applicant listed for this patent is Raytheon Company. Invention is credited to Delmar L. Barker, Jeffrey Clarke, Charles W. Hicks.
Application Number | 20180090523 15/278656 |
Document ID | / |
Family ID | 58633081 |
Filed Date | 2018-03-29 |
United States Patent
Application |
20180090523 |
Kind Code |
A1 |
Barker; Delmar L. ; et
al. |
March 29, 2018 |
ULTRAVIOLET (UV) SCHOTTKY DIODE DETECTOR HAVING SINGLE CRYSTAL UV
RADIATION DETECTOR MATERIAL BONDED DIRECTLY TO A SUPPORT STRUCTURE
WITH PROPER C-AXIS ORIENTATION
Abstract
A radiation detector for detecting ultraviolet energy having a
single crystal UV radiation detector material and an amorphous
support layer disposed directly on the single crystal UV radiation
detector material with the single crystal UV radiation detector
material having a c-axis aligned along a direction of the
ultraviolet energy being detected.
Inventors: |
Barker; Delmar L.; (Tucson,
AZ) ; Clarke; Jeffrey; (Rio Rico, AZ) ; Hicks;
Charles W.; (Tucson, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Raytheon Company |
Waltham |
MA |
US |
|
|
Assignee: |
Raytheon Company
Waltham
MA
|
Family ID: |
58633081 |
Appl. No.: |
15/278656 |
Filed: |
September 28, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/02327 20130101;
H01L 31/1013 20130101; H01L 31/108 20130101; G01J 1/429 20130101;
H01L 31/1872 20130101; H01L 21/02562 20130101; H01L 21/0256
20130101; H01L 31/0296 20130101; H01L 31/09 20130101; H01L 21/02557
20130101; H01L 31/1836 20130101; H01L 27/144 20130101; H01L
21/02691 20130101; H01L 21/02378 20130101 |
International
Class: |
H01L 27/144 20060101
H01L027/144; G01J 1/42 20060101 G01J001/42; H01L 31/0296 20060101
H01L031/0296; H01L 31/18 20060101 H01L031/18; H01L 31/09 20060101
H01L031/09; H01L 31/0232 20060101 H01L031/0232 |
Claims
1-6. (canceled)
7. A method for forming an UV light detector, comprising: providing
a single crystal seed layer having a <111> crystallographic
surface; providing an amorphous support layer adjacent to the
single crystal seed layer; the amorphous support layer having a
supporting surface perpendicular to the <111> surface of the
single crystal seed layer; depositing a UV radiation detecting
material on both the supporting surface and on a portion of the
<111> crystallographic surface of the single crystal seed
layer adjacent to the supporting surface; converting the deposited
UV radiation detecting material deposited on the supporting surface
into single crystal UV radiation detecting material having a c-axis
thereof perpendicular to the <111> crystallographic axis of
the single crystal seed layer.
8. The method recited in claim 7 wherein the amorphous support
layer retards formation of imperfections in the single crystal UV
radiation detecting material on the amorphous support layer during
successively heating and cooling during the converting.
9. A method for forming an UV light detector, comprising: providing
a single crystal seed layer having a <111> crystallographic
surface; providing an amorphous support layer adjacent to the
single crystal seed layer, the amorphous support layer having a
supporting surface perpendicular to the <111> surface of the
single crystal seed layer; depositing a UV radiation detecting
material on both the supporting surface and on a portion of the
<111> crystallographic surface of the single crystal seed
layer adjacent to the supporting surface; successively heating and
cooling the deposited UV radiation detecting material forming a
single crystal layer of the UV radiation detecting material on the
supporting surface with such UV radiation detecting material on the
single crystal seed layer being processed to extend outwardly from
the single crystal seed layer to the supporting surface to convert,
the UV radiation detecting material on the supporting surface into
UV radiation detecting material having a c-axis perpendicular to
the supporting surface.
10. The method recited in claim 9 wherein the amorphous support
layer retards imperfections in the single crystal UV radiation
detecting material on the amorphous support layer during the
successively heating and cooling.
11. The method recited in claim 7 the amorphous support layer is
silicon carbide.
12. The method recited in claim 7 wherein the UV radiation detector
material is Cadmium Sulfide, Cadmium Selenide or Cadmium
Telluride.
13. The method recited in claim 7 wherein the amorphous support
layer is transparent to infrared radiation.
14. The method recited in claim 13 the amorphous support layer is
silicon carbide.
15. The method recited in claim 13 wherein the UV radiation
detector material is Cadmium Sulfide, Cadmium Selenide or Cadmium
Telluride.
16. The method recited in claim 9 the amorphous support layer is
silicon carbide.
17. The method recited in claim 9 wherein the UV radiation detector
material is Cadmium Sulfide, Cadmium Selenide or Cadmium
Telluride.
18. The method recited in claim 9 wherein the amorphous support
layer is transparent to infrared radiation.
19. The method recited in claim 18 the amorphous support layer is
silicon carbide.
20. The method recited in claim 18 wherein the UV radiation
detector material is Cadmium Sulfide, Cadmium Selenide or Cadmium
Telluride.
21. A method for forming a radiation detector, comprising:
providing a single crystal seed layer having a surface with a
predetermined crystallographic orientation; providing an amorphous
support layer adjacent to the single crystal seed layer, the
amorphous support layer having a supporting surface perpendicular
to the surface of the single crystal seed layer; depositing
radiation detecting material on both the supporting surface and on
a portion of the surface of the single crystal seed layer adjacent
to the supporting surface; processing the radiation detecting
material on the single crystal seed layer to use the processed
radiation detecting material on the single crystal seed layer to
convert the deposited radiation detecting material on the
supporting surface into single crystal radiation detecting material
having a c-axis perpendicular to the supporting surface.
22. A method for forming a radiation detector, comprising:
providing a single crystal seed layer having a surface having a
predetermined crystallographic orientation; forming an amorphous
support layer, the amorphous support layer having a supporting
surface perpendicular to the surface of the single crystal seed
layer; depositing a radiation detecting material on both the
supporting surface and on a portion of the surface of the single
crystal seed layer adjacent to the supporting surface; processing
the radiation detecting material on the single crystal seed layer
to use the processed radiation detecting material on the single
crystal seed layer to convert the deposited radiation detecting
material on the supporting surface into single crystal radiation
detecting material having a surface with a crystallographic
orientation perpendicular to the crystallographic orientation of
the surface of the seed layer.
23. The method recited in claim 7 wherein the processing comprises
successively heating and cooling the deposited UV radiation
detecting material.
24. The method recited in claim 21 wherein the processing comprises
successively heating and cooling the deposited UV radiation
detecting material.
25. The method recited in claim 22 wherein the processing comprises
successively heating and cooling the deposited UV radiation
detecting material.
Description
TECHNICAL FIELD
[0001] This disclosure relates generally to ultraviolet (UV)
detectors and method for forming such detectors and more
particularly to UV detectors and methods for forming such detectors
adapted for use in dual band UV and infrared (IR) detectors.
BACKGROUND
[0002] As is known in the art, UV detectors are used in many
applications including dual band IR and UV detectors. One method
used to produce the Cadmium Sulfide (CdS), in hexagonal form, for
such UV detectors, for example, has been to grow bulk CdS material
starting with a CdS seed crystal using a vapor phase process as
described in a paper entitled "SEEDED GROWTH OF LARGE SINGLE
CRYSTALS OF CdS FROM VAPOR PHASE" by G. H. Dierssen and T, Gabor,
1978 published in Journal of Crystal Growth 43 (1978) 572-576.
Wafers are sliced from this bulk crystal material, annealed,
polished and etched to prepare them for use as Schottky diode UV
detector. More particularly, the CdS layer is then cut or sliced
into the appropriate thickness by slicing and dicing and then
re-annealed in hot sulfur atmosphere to achieve detector electrical
properties. In the case of a dual band UV and IR detector, then CdS
is then glued to the surface of an infrared filter configured to
pass the infrared portion of incident radiation. In the dual band
detector, the IR filter is disposed between an upper UV detector
and a lower IR detector, as shown in FIG. 1.
[0003] As is also known in the art, one method used to produce the
hexagonal Cadmium Sulfide (CdS) material for IR polarizers, for
example, has been to use germanium as a substrate and epitodally
grow a relatively thick layer of CdS on the <111> surface of
the germanium substrate, as described a paper entitled "Epitaxial
Growth of cadmium Sulfide on (111) on germanium (Ge) substrates":
by Paroici et al, in Journal of Material Science, 10 (1975) pages
2117-2123. The formed CdS material grows outwardly perpendicular to
the <111>crystallographic plane of the Ge substrate; that is,
the c-axis of the grown CdS is perpendicular the <111>
surfaces of both the Ge and the gown CdS. The formed CdS is then
used as infrared polarizer by positioning the hexagonal crystal
c-axis of the formed CdS perpendicular to the Infrared (IR) light
to be polarized; see Epitaxial growth of cadmium sulphide on (111)
germanium substrates; C. PAORICI, C. PELOSI, G. BOLZON I, G.
ZUCCALLI Laboratorio MASPEC-CNR, 43100 Parma, Italy; JOURNAL OF
MATERIALS SCIENCE 10 (1975) 2117-2123.
SUMMARY
[0004] The inventors have recognized that in order to use the UV
radiation detector layer as a UV detector the UV light must be
injected parallel to the c-axis and that a new method of
epitaxially producing such UV radiation detector layer with proper
c-axis orientation relative to the direction of the injected UV
light to be detected was required.
[0005] In accordance with the present disclosure, a radiation
detector for detecting ultraviolet energy is provided. The
radiation detector includes: a single crystal UV radiation detector
material; and an amorphous support layer disposed directly on the
single crystal UV radiation detector material with the single
crystal UV radiation detector material having a c-axis aligned
along a direction of the ultraviolet energy being detected.
[0006] In one embodiment, the amorphous support layer is silicon
carbide.
[0007] In one embodiment, a Schottky contact metal is provided in
Schottky contact with a single crystal UV radiation detector
material.
[0008] In one embodiment, the single crystal UV radiation detector
material is disposed between the amorphous support layer and the
Schottky contact metal.
[0009] In one embodiment, the amorphous support layer is
transparent to infrared radiation.
[0010] In one embodiment, a method is provided for forming an UV
light detector, comprising: providing a single crystal seed layer
having a <111> crystallographic surface; forming an amorphous
support layer having a supporting surface perpendicular to the
<111> surface; depositing a UV radiation detecting material
on both the <111> crystallographic surface of the seed layer
and on a portion of the supporting surface adjacent to the single
crystal seed layer; and converting the deposited UV radiation
detecting material into single crystal UV radiation detecting
material with a c-axis thereof perpendicular to the <111>
crystallographic axis of the single crystal seed layer.
[0011] In one embodiment, the amorphous support layer retards
formation of imperfections in the single crystal UV radiation
detecting material on the amorphous support layer during the
successively heating and cooling.
[0012] In one embodiment, a method is provided for forming an UV
light detector, comprising: providing a single crystal seed layer
having a <111> crystallographic surface; forming an amorphous
support layer on a first portion of the <111> surface while
exposing a second portion of the <111>0 surface, the
amorphous support layer having a supporting surface perpendicular
to the <111> surface; depositing a UV radiation detecting
material on both the second portion of the <111>
crystallogaphic surface of the seed layer and on a portion of the
supporting surface adjacent to the a portion of the second portion
of the single crystal seed layer; and successively heating and
cooling the deposited UV radiation detecting material forming a
single crystal layer of the UV radiation detecting material with
such UV radiation detecting material being formed to extend
outwardly from the second portion of the seed layer, the UV
radiation detecting material being learned with a c-axis thereof
perpendicular to the <111> crystallographic axis of the
single crystal seed layer.
[0013] With such structure and method: 1) No bulk crystal growth
required. More particularly the use of a bulk crystal is very, very
time and process intensive. Cutting and polishing a crystal boule
into the detector chips may require as many as 100 steps, any of
which could result in a faulty chip or entire run of faulty chips.
2) Thin film CdS significantly improves UV detector performance as
recombination noise is reduced. 3) Thin film CdS, as part of a
layered IR transmission system, improves integrated IR filter/band
pass throughput. 4) An IR transmission system layer acts as UV
reflector increasing quantum efficiency of UV detector system.
[0014] The details of one or more embodiments of the disclosure are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the disclosure will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is a diagrammatical sketch of a cross section of a
dual band UV and Infrared (IR) radiation detector according to the
PRIOR ART;
[0016] FIG. 2 is a simplified, diagrammatical sketch of a cross
section of a dual band UV and Infrared (IR) radiation detector
according to the disclosure;
[0017] FIGS. 3A, 3B and 3C are more detailed, cross-sectional, top
plan and bottom plan views, respectively, the cross section of FIG.
3A being taken along lines 3A-3A in FIGS. 3B and 2C, of a UV
radiation detector adapted for use in a dual band Ultraviolet (UV)
and Infrared (IR) radiation detector according to the
disclosure;
[0018] FIGS. 4A, 4B, and 4C through FIGS. 16A, 16B and 16C are
cross sectional, top plan and bottom plan views, respectively of
the UV radiation detector of FIGS. 2A, 2B and 2C at various steps
in the fabrication of such UV radiation detector according to the
disclosure; and
[0019] FIGS. 17A through FIG. 24 are diagrammatical sketches useful
in understanding the process for forming a semiconductor, UV
radiation detector layer used in the UV radiation detector of FIGS.
2A-2C at various stages in the fabrication thereof according to the
disclosure; with FIGS. 19 and 21 showing diagrammatically apparatus
used such fabrication.
[0020] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0021] Referring now to FIG. 2, a diagrammatical sketch of a dual
band infrared and ultraviolet radiation detector 200 is shown
having: an infrared (IR) radiation detector 202; an infrared
radiation filter 204, disposed over infrared radiation detector
202, the infrared radiation filter 204 being transparent to
infrared radiation band and absorptive to radiation adjacent to the
infrared radiation band; and an ultraviolet (UV) radiation detector
206 disposed over the infrared filter 204. The ultraviolet
radiation detector 206 includes: a Schottky contact metal layer 208
on an upper surface of the infrared radiation filter 204; layer 210
in this example, is the semiconductor, ultraviolet radiation
detector material, here, in this example, Cadmium Sulfide (CdS)
disposed directly on, and forming a Schottky contact with, the
metal layer 208; and an ohmic contact metal 212 in ohmic contact
with another portion of the CdS layer 210. It should be understood
that the semiconductor, ultraviolet radiation detector material may
be CdTe, CdTe, or CdSe, for example. Radiation in the ultraviolet
is intercepted and detected by the ultraviolet radiation detector
206 while infrared radiation passing through the ultraviolet
radiation detector 206 and through the infrared filter 204 is
detected by the infrared detector 202. The details of the infrared
(IR) radiation detector 202 and infrared radiation filter 204 will
be described in more detail below and are indicated in FIGS. 2A, 28
and 2C as a UV detector 10.
[0022] More particularly, referring to FIGS. 3A, 38 and 3C, the UV
detector 10 is shown in more detail and is fabricated in accordance
with a process described below in connection with FIGS. 4A, 48 and
4C through FIGS. 16A, 16B and 16C. Here, an infrared IR Filter 12,
here silicon, is used to also provide a substrate for formation of
the detector 10. The detector 10 has an IR radiation
anti-reflection coated glass layer 14, here IRG 23 having a
thickness of approximately 0.35 microns, that also serves as a UV
reflector, is supported by the IR Filter 12; a support structure,
here for example, silicon carbide (SiC) layer 16, here
approximately 40-100 Angstroms thick, is disposed on the glass
layer 14; a semiconductor, UV radiation detector layer 18, for
example CdS or Cadmium Selenide (CdSe), or Cadmium Telluride
(CdTe), for example), here, in this example CdS layer 18 having a
thickness in the order of 0.45 micron, disposed on the SiC layer
16; a Schottky contact metal 24, here platinum (Pt), here having a
thickness in a range (5-50 nm), disposed on the a semiconductor, UV
radiation detector layer 18, in Schottky contact with the
semiconductor, UV radiation detector layer 18; an ohmic contact
electrode 20, here a stack of Aluminum (Al)/Gold, in ohmic contact
with the CdS layer 18; an Schottky contact electrode 28, here
Titanium (Ti) with Au on top, the Ti being in electrically
connected to the Schottky contact metal 24 and electrically
insulated from the CdS layer 18 by a dielectric, SiO.sub.2 layer
22. Also included are upper and lower IR blocking layers 26U, 26L,
here a stack of titanium (Ti), gold, titanium; a surface protection
and anti-reflection coating layer 27, here silicon dioxide
approximately 0.72 microns thick on the Schottky contact metal 24;
anti-reflection coating layer 32, here, for example, Titanium Oxide
(TiO)/Aluminum Oxide (Al.sub.2O.sub.3)/Magnesium Fluoride (Mg),
approximately, 0.45 microns thick, 0.5 microns thick, and 0.75
micros thick respectively. Dielectric passivation layers 30U, 30L,
here silicon dioxide (SiO.sub.2) approximately 0.5 microns thick
are provided, as shown. An IR radiation anti-refection dielectric
layer 27, here for example, silicon dioxide (SiO.sub.2)
approximately 0.72 microns thick is disposed on the Schottky
contact metal 24. Also included is a dielectric layer 19, here
silicon dioxide. Wires 32, 34 are connected to the ohmic contact
electrode 20 and the Schottky contact electrode 28, respectively as
shown. The index of refraction of the IR radiation anti-reflection
dielectric layer 27 to IR radiation is less than the index of
refraction of the semiconductor, UV radiation detector layer 18 to
IR radiation; the index of refraction of the semiconductor, UV
radiation detector layer 18 to IR radiation is less than the index
of refraction of the glass layer 14 to IR radiation; and the index
of refraction of glass layer 14 to IR radiation is less than the
index of refraction of the an infrared IR Filter 12. (It should be
noted that SiC layer 16 is very thin, less than 40 nm and the
refractive index of SiC is very close to that of the glass layer 14
layer so if the thickness had to be larger one could compensate by
making the glass layer thinner.
[0023] Thus, incident radiation passes onto the upper, surface of
the structure with the UV portion of the radiation being detected
by the UV detector 10 and the IR radiation passing out of the
central portion of the structure to the infrared radiation detector
202 (FIG. 1).
[0024] Referring now to FIGS. 4A, 4B and 4C, the top and bottom
surfaces of the silicon IR Filter 12 is prepared by mechanical
polishing and passivation, here for example by immersion in a
HF/NH.sub.4F bath. The bottom surface of the an infrared (IR)
filter 12, here for example, a single crystal substrate, here for
example, silicon, has formed, using any conventional
photolithographic-etching process, on the bottom surface thereof
the lower IR blocking layer 26L, here of a stack of titanium
(Ti)/gold (Au)/Ti with a central, aperture 36 therein exposing a
central, bottom surface portion of the IR filter 12, as shown.
[0025] Next, the dielectric layer 30L, here silicon dioxide
(SiO.sub.2) is formed over the lower IR blocking layer 26L and onto
the portion of the silicon IR Filter 12 exposed by the aperture 36.
Next, the portion of the dielectric layer 30L on the exposed,
central portion of the IR filter 12 is removed using any
conventional photolithographic-etching process thereby re-exposing
the central portion of the bottom of the IR Filter 12 with aperture
36, as shown in FIGS. 5A, 5B and 5C.
[0026] Next, a sequence of three layers of TiO, Al.sub.2O.sub.3 and
MgF making up IR anti-reflection layer 32 (FIGS. 6A, 6B, 6C) is
formed: first a layer of Titanium Oxide (TiO); then a layer of
aluminum oxide (Al.sub.2O.sub.3); then a layer of Magnesium
Fluoride (MgF) are deposited over the structure; such sequence of
layers being deposited onto the re-exposed portion of the central,
bottom of the IR Filter 12 and over the remaining dielectric layer
30L. Conventional photolithographic-etching processing is used to
remove the portions of the three layers making up layer 28 disposed
over the remaining dielectric layer 30L leaving only the portion of
the three layers making up layer 28 disposed on the central portion
of the IR Filter 12, as shown in FIGS. 6A, 6B and 6C thereby
completing processing of the bottom of the IR Filter 10.
[0027] Next, referring to FIGS. 7A,7B and 7C the anti-reflection
layer of glass 14, here IRG 23 glass having a thickness in this
example, of approximately 0.35 microns, is formed using
conventional chemical vapor deposition directly on the portion of
the upper, central surface portion of the IR Filter 12.
[0028] Next, referring to FIGS. 8A, 8B and 8C, SiC layer 16 with
the UV radiation detector layer 18 thereon are formed on the glass
layer 14 in a manner to be described in connection with FIGS.
18A-25, Next, dielectric layer 19, here silicon dioxide, is formed
over the UV radiation detector layer 18 and patterned as shown
using conventional photolithographic-etching processing to have an
aperture 21 in a central portion of the surface passivation layer
to expose the central portion of the semiconductor, UV radiation
detector layer 18, as shown.
[0029] Next, referring to FIGS. 9A-9C, a layer of aluminum followed
by a layer of gold are deposited over the patterned passivation
layer and onto a portion of the exposed semiconductor, UV radiation
detector layer 18 and patterned as shown using conventional
photolithograph-etching techniques to form ohmic contact electrode
20, described above, to the semiconductor, UV radiation detector
layer 18.
[0030] Next, referring to FIGS. 10A, 108 and 10C, the dielectric
layer spacers 22 are formed over the upper surface of then
structure as shown using conventional photolithographic-etching, as
shown.
[0031] Next, referring to FIGS, 11A, 11B and 11C, the layer of the
Schottky contact metal 24, here platinum (or other suitable metals
or alloys) having a thickness in the example, approximately 5 to 50
nm, preferably 15 nm), is deposited over the surface of the
structure, patterned as shown using conventional
photolithographic-etching techniques and processing to form a
Schottky contact with the semiconductor, UV radiation detector
layer 18, as shown.
[0032] Next, referring to FIGS. 12A, 12B and 12C, a circular,
disk-shaped dielectric layer 27, here silicon dioxide is deposited
over the Schottky contact metal 24, and patterned as shown using
conventional photolithographic-etching techniques to form a surface
protection and anti-reflection coating layer; it is noted that a
circular ring-shaped window 29 is formed in layer 27 to expose a
circular ring-shaped portion of the Schottky contact metal 24.
[0033] Next, referring to FIGS. 13A, 138 and 13C, the Schottky
contact electrode 28 is formed in Schottky contact with the exposed
circular ring-shaped portion 29 (FIGS. 12A, 12B and 12C) of the
Schottky contact metal 24 using conventional
deposition-photolithographic-etching processing.
[0034] Next, referring to FIGS. 14A, 14B and 14C, the dielectric
protection, ring-shaped layer 22 is formed, as shown, on inner
portions of the layer 19, and on outer peripheral portions of ohmic
contact electrode 20 using conventional
deposition-photolithographic-etching processing.
[0035] Next, referring to FIGS. 15A, 15B and 15C, the ring-shaped
IR blocking layer 26U is formed using conventional
deposition-photolithographic-etching processing, as shown.
[0036] Next, referring to FIGS. 16A, 16B and 16, the surface
passivation layer 30U is formed as shown using conventional
deposition-photolithographic-etching processing.
[0037] Next, the bond wires 32, 34 are attached to the ohmic
contact electrode and Schottky contact electrode 28, as shown in
FIGS. 3A, 3B and 3C.
[0038] Referring now to FIGS. 17A through FIG. 25, the process for
forming the SiC layer 16 with the UV radiation detector layer 18
thereon are on the glass layer 14 will be described. First, a
germanium (Ge) crystal 40 (here having a thickness of from 1 mm to
10 mm) is affixed to one end of the structure shown in FIG. 7A, 7B
and 7C with any suitable high temperature (for example, greater
than 200-400 degrees C.) epoxy 42, (for example EP30 or EP30HT
(high temp) epoxy having a thickness .about.10 to 20 um (micron) by
Masterbond 154 Hobart Street, Hackensack .ltoreq.N.J. USA), as
shown in FIGS, 18A, 18B and 18C with a portion 45 of the surface of
the germanium (Ge) crystal 40 exposed as shown and with the
<111> crystallographic surface of face of the Ge 40, facing
to the right in FIG. 18A, 18B and 18C by the arrow along the
<111> crystallographic axis (the <111> axis being
perpendicular to the <111> crystallographic surface or face
of the Ge 40 This <111> crystallographic surface or face will
serve as a seed layer for the formation of the CdS layer 18.
[0039] Next, referring to FIG. 18, the portion 45 of the surface of
the germanium (Ge) crystal 40 is masked with a mask 51 and then is
placed in a Plasma Enhance Chemical Vapor Deposition (PECVD)
chamber, as shown in FIG. 19 for formation of the support layer 16
of amorphous SiC (.alpha.-SiC) (see for example "PECVD Amorphous
Silicon Carbide (.alpha.-SiC) Layers for MEMS Applications "by
Ciprian Iliescu and Daniel P. Poenar, INTECH, Physics and
Technology of Silicon Carbide Devices,
http://dx/doi.org/10.5772/51224 in a book edited by Yasuto
Hijikata, ISBN 978-953-51-0917-4, Published: Oct. 16, 2012,
http://dx.doi.org/10.5772/3428. The mask 51 prevents the
.alpha.-SiC layer from being formed on the Ge 40. A low temperature
deposition is used, for example, between 200-400 degrees Centigrade
(depending on the specifics of the machine and recipe employed for
the deposition, as well as on the details of the device's
fabrication process). The .alpha.-SIC layer 16 here is formed to
have an index of refraction, n, approximately, 2.5 and a final
melting point 2,730.degree. C. after deposition. In order to
deposit an .alpha.-SiC layer 16 that has a high refractive index
(.about.2.5) to match the AR requirements of the glass layer 14
below it) and has no epitaxial growth interference with the Ge
(<111>) crystal 40 for the next step of CdS layer 18
deposition to be described, the .alpha.-SiC layer 16 must have the
high temperature melting point of the final layer of .alpha.-SiC of
2,730 degrees Centigrade indicated above which is necessary for a
CdS layer 18 recrystallization step (to be described) and thereby
insure no damage to the layers already constructed.
[0040] Next, after formation of the .alpha.-SiC layer 16, the mask
51 of the structure 46 is removed, as shown in FIG. 20 and then the
structure is placed in a Chemical Vapor Deposition (CVD) chamber 50
(FIG. 21) to form the deposited CdS layer 18 on the exposed
<111> crystallographic surface or face portion 45 of the Ge
40 and on portion 47 of the .alpha.-SiC layer 16 adjacent to the
portion 45 of the Ge 40 as indicated in FIG. 22. Contact with the
Ge 40 will grow Hexagonal CdS crystals without crystal formation
interference from the lower surface. Because the .alpha.-SiC is not
a crystal surface, hexagonal CdS will propagate epitaxial growth
far along the .alpha.-SiC surface 16. Therefore, a
re-melting/recrystallization process using a pulse laser, to be
described below, is used to insure that a small area volume of the
CVD grown CdS is melted as the laser pulse is applied and then
cooled to re-crystallize the melted CdS when the laser pulse is
removed to thereby propagate the hexagon -CdS, a few milli-meters
from the Ge <111> face.
[0041] The CVD chamber 50 (FIG. 21) includes a pot P of CdS as
shown, and heated by high current power supplies, as indicated. The
crystal may not grow uniformly starting at the Ge <111> face
but rather initially on the exposed <111> crystallographic
surface or face portion 45 of the Ge 40 and on a portion 47 of the
.alpha.-SiC layer 16 adjacent to the portion 45 of the Ge 40 as
indicated in FIG. 22.
[0042] The structure shown in FIG. 22 is next removed from the
Chemical Vapor Deposition (CVD) chamber 50 (FIG. 21). It is first
noted that the crystallographic structure of the CVD CdS on the Ge
is at this stage ambiguous. Thus a process is used re-anneal
ambiguous CdS into a crystal form. Here, the re-anneal process is
described in connection with FIGS. 23A and 23B. More particularly,
a pulsed laser beam, here having a 2 mm square scan area (which is
larger than the surface area of then CdS being scanned) begins a
singe scan of the initially CVD deposited CdS at the corner of the
Ge <111> crystallographic surface and the SiC layer 16, as
shown in FIG. 23A. (Here the laser is a Q-switched. ruby having a
pulse 25 nanosecond pulse duration and fluence ranging from 0.1 to
1 Joules per cm.sup.2).
[0043] The heat from the laser beam melts a portion of the
deposited CdS at that Ge <111> crystallographic surface
(REGION A) and as the beam rotates along an arc, indicated by the
curved arrow, the melted portion of the CdS in REGION A solidifies
into a single crystal, hexagonal CdS portion, indicated as REGION
A' (FIG. 23B). Now the laser beam melts the portion of the CdS
adjacent to REGION A' to form a REGION B (FIG. 23B) of melted CdS
and here again, as the laser beam scans away from REGION B, the
melted CdS in REGION B cools on the single crystal CdS to form a
single crystal, hexagonal CdS portion on the CdS in REGION B, as
shown in FIG. 23C. Thus, the process continues and the single
crystal, hexagonal CdS layer 18 grows upon itself and forms the
layer 18 of CdS on the SiC layer, as indicated in FIG. 23D. It
should be understood that the discussion above is for understanding
the process and that the process is not a discrete process but
rather a continuous process that produces a continuous single
crystal, hexagonal CdS layer 18. A process used to re-anneal
ambiguous CdS into a crystal form of the CdS with the pulsed laser
is discussed in "PECVD Amorphous Silicon Carbide (.alpha.-SiC)
Layers for MEMS Applications" referenced above and "Fast Melting of
Amorphous Silicon Carbide Induced by Nanosecond Laser Pulse", P.
Baeri, C. Spinella, and R. Reitano); International Journal of
Thennophysies, Vol. 20, No. 4, 1999.
[0044] Thus, it is noted that re-melting and re crystallization is
done in a single step serially away from the <111> Ge surface
to generate a long hexagonal CdS crystal. The re-crystallization of
the CdS layer 18 with the pulsed laser is such that the hexagonal
CdS layer 18 is grown on the <111> face of the Ge crystal 40
with the c-axis perpendicular to the surface of the CdS layer 18
being formed (that is, parallel to the parallel to a direction of
the ultraviolet energy being detected using the re-melting
technique discussed in the paper referenced above. This fast pulsed
laser method does not over heat the layers around it such as the Ge
40 and glass Layer 14. It is noted that the SiC it is amorphous and
won't impact the CdS crystal; it can take the heat of the re-melt
step; and it has an index of refraction which matches for that
layer position in the stack for the IR "transmission. Thus the
amorphous SiC support layer retards formation of imperfections in
the single crystal UV radiation detecting material on the amorphous
support layer dining the successively heating and cooling.
[0045] Next, here the epoxy 42, Ge crystal 40, and any small amount
of CdS layer 18 formed on the epoxy 42, as indicated in FIG. 25, is
removed using mechanical polishing or any convenient solvent to
produce the structure shown in FIG. 24; however, it should be noted
that the epoxy, Ge 30, and small amount of CdS may remain with it
being used in the processing described above in connection with
FIGS. 3A through 16C.
[0046] A number of embodiments of the disclosure have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the disclosure. For example, other Group IV materials may
be used in place of Ge, for example silicon wherein the CdS is
grown on the <111> face of the silicon. Accordingly, other
embodiments are within the scope of the following claims.
* * * * *
References