U.S. patent application number 12/158114 was filed with the patent office on 2009-02-26 for semiconductor device and method of manufacture thereof.
This patent application is currently assigned to DURHAM SCIENTIFIC CRYSTALS LIMITED. Invention is credited to Arnab Basu, Andy Brinkman, Ben Cantwell, Max Robinson.
Application Number | 20090053453 12/158114 |
Document ID | / |
Family ID | 37907644 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090053453 |
Kind Code |
A1 |
Basu; Arnab ; et
al. |
February 26, 2009 |
SEMICONDUCTOR DEVICE AND METHOD OF MANUFACTURE THEREOF
Abstract
A structure including a substrate, an intermediate layer
provided and formed directly onto the substrate, a transition
region, and a group II-VI bulk crystal material provided and formed
as an extension of the transition region. The transition region
acts to change the structure from the underlying substrate to that
of the bulk crystal. In a method of manufacture, a similar
technique can be used for growing the transition region and the
bulk crystal layer.
Inventors: |
Basu; Arnab; (Durham,
GB) ; Robinson; Max; (Shincliffe, GB) ;
Cantwell; Ben; (County Durham, GB) ; Brinkman;
Andy; (Durham, GB) |
Correspondence
Address: |
POPOVICH, WILES & O'CONNELL, PA;650 THIRD AVENUE SOUTH
SUITE 600
MINNEAPOLIS
MN
55402
US
|
Assignee: |
DURHAM SCIENTIFIC CRYSTALS
LIMITED
County Durham
GB
|
Family ID: |
37907644 |
Appl. No.: |
12/158114 |
Filed: |
December 21, 2006 |
PCT Filed: |
December 21, 2006 |
PCT NO: |
PCT/GB2006/004864 |
371 Date: |
August 1, 2008 |
Current U.S.
Class: |
428/64.1 ;
204/192.25; 257/E21.462; 428/332; 428/337; 428/446; 428/457;
438/503 |
Current CPC
Class: |
Y10T 428/266 20150115;
H01L 21/02505 20130101; H01L 21/02502 20130101; Y10T 428/26
20150115; H01L 21/02562 20130101; H01L 21/02568 20130101; H01L
21/02395 20130101; H01L 21/02474 20130101; H01L 21/0248 20130101;
H01L 21/02381 20130101; Y10T 428/21 20150115; Y10T 428/31678
20150401; H01L 21/02477 20130101; H01L 21/02617 20130101; H01L
21/0251 20130101; C30B 11/00 20130101; H01L 21/02378 20130101; C30B
29/48 20130101 |
Class at
Publication: |
428/64.1 ;
428/446; 428/457; 428/337; 428/332; 438/503; 204/192.25;
257/E21.462 |
International
Class: |
B32B 9/04 20060101
B32B009/04; B32B 3/02 20060101 B32B003/02; H01L 21/363 20060101
H01L021/363; C23C 14/34 20060101 C23C014/34 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2005 |
GB |
0526070.8 |
Dec 21, 2005 |
GB |
0526073.2 |
Claims
1. A structure including a substrate, an interfacial layer provided
and formed directly onto the substrate, a transition region, and a
group II-VI bulk crystal material provided and formed as an
extension of the transition region.
2. A structure according to claim 1, in which the substrate
comprises a substrate of silicon, gallium arsenide, germanium, or
silicon carbide.
3. A structure according to claim 1, in which the substrate has a
thickness of at least 100 microns, preferably at least 200
microns.
4. A structure according to claim 1, in which the substrate has a
diameter greater than 25 mm.
5. A structure according to claim 1, in which the bulk crystal
material comprises cadmium telluride, cadmium zinc telluride, or
cadmium manganese telluride.
6. A structure according to claim 1, in which the bulk crystal
material has a thickness of at least 700 microns.
7. A structure according to claim 1, in which the intermediate
layer comprises or group II-VI material such as CdTe, CZT, CdS.
8. A structure according to claim 1, in which the intermediate
layer has a thickness of between 25 and 1000 microns.
9. A structure according to claim 1, in which the transition region
between the intermediate layer to the material of the bulk crystal
has a thickness of between 10 and 500 microns.
10. A method of forming a structure according to claim 1, in which
the interfacial region and the bulk crystal are deposited using the
same growth technique, using a variation in the growth parameters
during the growth cycle to form the interfacial region and the bulk
crystal.
11. A method of growing a bulk single crystal material comprising:
providing a seed substrate of a material different from the bulk
crystal material to be formed; forming an intermediate layer on the
substrate; forming a transition region on the intermediate layer;
and forming the bulk single crystal material is grown on the
transition region using a physical vapour phase deposition
method.
12. The method according to claim 11, in which the intermediate
layer is formed using standard thin film deposition techniques.
13. The method according to claim 12, in which the intermediate
layer is formed using molecular beam epitaxy, chemical vapour
deposition, sputtering, metal organic vapour phase epitaxy, liquid
phase epitaxy and metallo organic chemical vapour deposition
(MOCVD).
14. The method according to claim 11, in which the intermediate
layer is formed using physical vapour phase deposition
techniques.
15. The method according to claim 14, in which the intermediate
layer or region is grown at a growth rate of between 1 and 10
microns/hour.
16. The method according to claim 11, in which the transition
region is formed using the same growth technique as used for the
subsequent deposition of the bulk crystal material, but with a
variation in the growth parameters during the growth cycle to
gradually accelerate the rate of growth.
17. The method of claim 16, in which the intermediate layer is
formed using the same growth technique as used for the subsequent
deposition of the bulk crystal material, but with a variation in
the growth parameters during the growth cycle to gradually
accelerate the rate of growth.
18. The method according to claim 16, in which the growth
parameters that are varied include one of the source temperature
(T.sub.source) and the substrate temperature (T.sub.sub).
19. The method according to claim 18, in which the temperature
differential between the substrate temperature and the source
temperature is increased to increase the growth rate.
20. The method according to claim 16, in which the variation in the
growth parameter is a gradual variation.
21. The method according to claim 16, in which the variation in the
growth parameter is an abrupt variation.
22. The method according to claim 11, in which the source
temperature is at least 450.degree. C.
23. The method according to claim 11, in which the substrate
temperature is at least around 200.degree. C.
24. The method according to claim 11, in which the substrate is a
silicon or gallium arsenide substrate.
25. The method according to claim 11, in which the substrate has a
diameter greater than about 25 mm.
26. The method according to claim 25, in which the substrate has a
diameter of at least 50 mm, and most preferably at least 150
mm.
27. The method according to claim 11, in which the bulk crystal
material comprises one of zinc telluride, cadmium telluride,
cadmium zinc telluride and cadmium manganese telluride.
28. The method according to claim 27, in which the bulk crystal
material has the composition Cd.sub.1-xZn.sub.xTe or
Cd.sub.1-xMn.sub.xTe.
29. The method according to claim 11, in which the bulk crystal
material is grown at a growth rate of between 100 and 500
microns/hour.
30. The method according to claim 11, in which the bulk crystal
material has a thickness of at least 500 microns.
31. The method according to claim 11, in which the substrate and
crystal material grown on the substrate are divided into smaller
pieces after formation.
32. The method according to claim 11, further comprising the step
of forming an x-ray or gamma ray detector.
33. The method according to claim 11, in which the bulk crystal may
itself be used as a seed crystal for the formation of other bulk
crystal materials in accordance with the method of any one of the
preceding claims.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a semiconductor device and
a method of manufacture therefore. In particular, the present
invention relates to a device comprising a group II-VI material
formed on a substrate of a dissimilar material, and a method for
forming such a structure.
DISCUSSION OF THE PRIOR ART
[0002] Single crystal materials have a number of important
applications. For example, bulk cadmium telluride (CdTe) and
cadmium zinc telluride (CZT) semiconductors are useful as x-ray and
gamma-ray detectors which have application in security screening,
medical imaging and space exploration amongst other things.
[0003] For many applications, it is desired to have single crystals
of large size and thickness, which can be formed rapidly with
optimum uniformity and minimum impurities.
[0004] Traditionally, single crystals have been formed using direct
solidification techniques, such as by the Bridgman, travelling
heater (THM), gradient freeze (GF) or other liquid phase or
self-seeding vapour phase crystal growth methods in which the
crystals are grown from the melt. With these conventional methods,
it has been difficult to form high quality crystals consistently,
or to form single crystals having a diameter greater than 25 mm or
50 mm. In particular, with these known methods of crystal
formation, dislocations, sub-grain boundaries and twins form
easily. For high pressure Bridgman methods, there is also the
potential problem of pipe formation. These problems are particular
problems when forming CdTe crystals. The inclusion of zinc to make
CZT reduces these problems to some extent as the zinc strengthens
the lattice, however zinc segregation at the solidification
interface may result in graded axial compositional profiles.
However, higher temperatures are required for CZT growth, and this
is undesirable. Also, the process tends to form precipitates and
inclusions due to the excess tellurium in the melt. Telluride
inclusions can be tens of microns in size and this may be
significant for detector applications. Further, there will be a
dislocation cloud associated with each inclusion which will affect
the performance of detectors formed from the crystal.
[0005] In European Patent No EP-B-1019568 a method of forming
crystals using a physical vapour phase technique is disclosed. This
process is known as Multi-Tube Physical Vapour Phase Transport
(MTPVT). According to this method, a sink or seed crystal of the
material to be grown is provided. Vapour phase material is provided
to the sink or seed crystal, causing nucleation and subsequent
deposition of the material to grow the crystal onto the sink or
seed crystal. The sink or seed crystal should be similar in
material and structure to the crystal material to be grown, for
example being only a doped or minor variation of the crystal
composition. In particular, EP-B-1019568 discloses a method in
which the sink or seed crystal is provided in a sink zone which is
connected to a source zone via a passage able to transport vapour
from the source zone to the sink zone. The temperature in the
source and sink zones are controllable independently, the zones
being thermally isolated.
[0006] Whilst the Multi-Tube Physical Vapour Phase Transport
process disclosed in EP-B-1019568 is able to consistently produce
crystals of a more uniform and higher quality, a problem remains
that the size of crystals that can be grown is limited as the
crystal cannot be any larger than the seed crystal on which it is
grown.
[0007] Due to the limited size of crystals formed by the prior art
methods, it has been known to produce detectors of large size by
tiling together smaller crystals in an array. In this case, it is
necessary to use computer software to compensate for the joints
between the separate pieces of material.
[0008] It is also known to provide large substrates formed from
materials such as silicon or gallium arsenide and to deposit a thin
film of single crystal cadmium telluride or cadmium zinc telluride.
The thin films can be deposited using thin film growth techniques
such as molecular beam epitaxy, chemical vapour deposition,
sputtering, metallo organic chemical vapour deposition (MOCVD),
metal organic vapour phase epitaxy (MOVPE) and liquid phase epitaxy
(LPE). These methods enable a single crystal thin film layer to be
grown at rates of between 0.1 and 10 microns per hour, and
therefore the thickness of the layer is very limited. Typically,
the maximum thickness of such thin films is 1 to 10 microns.
Although a thin film can be formed on a substrate to give a large
area semiconductor crystal, such a film is not suitable for use as
a detector for x-rays and gamma-rays. When detecting x-rays and
gamma-rays, it is necessary to provide a sufficient thickness of
material to stop the high energy photons. In order to capture 90%
of the incident radiation at a photon energy of 100 keV.sup.8, it
is necessary for a CdTe layer to have a thickness of about 11 mm.
Using typical methods for growing thin films, this would take
around 10,000 hours. Therefore, suitable crystals cannot be grown
using thin film deposition methods.
[0009] Whilst it is known that screen printing techniques can be
used to deposit a thick layer of material on a substrate, these
layers are not single crystal layers, and therefore are unsuitable
for detection of x-rays and gamma rays. International Patent
Application No WO 2002/44443 discloses a method and apparatus for
the production of Group III metal nitride materials, for example
gallium nitride. According to the disclosure in this document, a
base substrate is provided in a sputter deposition chamber together
with a group III metal target. A highly energetic plasma-enhanced
environment is generated in the chamber to sputter the target and
produce a Group III metal source vapour. A nitrogen containing gas
is also provided in the chamber. A reactant vapour species
containing components of the group III metal and the nitrogen will
be produced in the chamber, and will be deposited onto the
substrate. In one embodiment, a buffer layer of group III metal
nitride material is formed on the substrate, and the bulk group III
metal nitride material is deposited onto the buffer layer by the
reactive sputtering method. Such a method is unsuitable for the
deposition of group II-VI materials as these cannot be physically
sputtered or provided as a reactive gas.
[0010] EP-A-01691422 discloses the use of a metal organic vapour
phase epitaxy method for the formation of a cadmium telluride or
cadmium zinc telluride layer on a silicon or gallium arsenide
substrate. This method is a chemical vapour deposition technique.
According to one embodiment disclosed, a thin layer of cadmium
telluride or cadmium zinc telluride, which may be doped with
arsenic, is provided between the substrate and the cadmium
telluride or cadmium zinc telluride growth layer. There are
disadvantages of metal organic vapour phase epitaxy deposition
techniques, for example the high cost of source materials, and the
problems associated with the deposition of carbon from the metal
organic precursors which are used. Such carbon must be periodically
removed to minimise the risk of the deposits being shredded onto
the growing crystal. This reduces the thickness that may be
deposited in a single run.
SUMMARY OF THE INVENTION
[0011] According to one aspect of the present invention, there is
provided a structure including a substrate, an intermediate layer
provided and formed directly onto the substrate, a transition
region and a bulk crystal of group II-VI material provided and
formed as an extension of the transition region, the transition
region provided a transition between the material of the
intermediate layer and the bulk crystal material.
[0012] It has previously been considered that crystal mismatches
between a substrate and bulk crystal material having different
lattice structures prevent the formation of the bulk crystal
material on such substrates, or would result in unacceptable
stresses between the materials affecting the device unacceptably.
For example, it is not generally considered possible to provide a
cadmium telluride crystal material, which will have a lattice
parameter a=6.481 .ANG. directly onto a silicon substrate which
will have a lattice parameter a=5.4309 .ANG. due to the lattice
mismatch. Accordingly, this limits the bulk crystal material that
can be grown on any given substrate. However, the inventors have
found that the inclusion of an intermediate layer and transition
region between the substrate and the bulk crystal material
according to the present invention enables a gradual change in the
crystal structure between the substrate and bulk crystal that can
compensate for any mismatch in the lattice structure of the
substrate and deposited crystal material.
[0013] With the structure of the present invention, it is possible
to form bulk crystal materials which differ from the substrate on
which they are formed, and in particular which have a different
lattice structure from the underlying substrate. These composite
layered materials may have better physical or structural properties
than conventionally known materials, and therefore may have
different applications.
[0014] It is preferred that the intermediate layer should have a
lattice structure compatible with the substrate.
[0015] In accordance with the present invention, preferred examples
of substrates include silicon, gallium arsenide, and silicon
carbide substrates. The group II-VI bulk crystal materials formed
may include semiconductors such as cadmium telluride, cadmium
manganese telluride and cadmium zinc telluride.
[0016] The intermediate layer may be of the same material or a
different material from the bulk crystal material.
[0017] The transition region in which there is a transition from
the material of the intermediate layer to the bulk crystal material
may include a region of gradual change from the composition of the
intermediate layer to that of the bulk crystal material.
[0018] In a preferred example, the transition region and bulk
crystal can be deposited using the same growth technique, but with
an initial variation in the growth parameters during the growth
cycle to gradually change the composition and growth rate of the
material deposited on the substrate. During the initial transition,
the transition region is formed. After completing the change to the
material of the bulk crystal to be deposited, the growth rate can
be accelerated to rapidly deposit the bulk crystal material. In
this case, it is preferred that the apparatus includes a means for
introducing different source materials to be deposited onto the
substrate.
[0019] The intermediate layer can also be formed using the same
technique as the transition region and the bulk crystal layer.
[0020] In addition to the substrate, intermediate layer, transition
region and the bulk crystal material, additional layers may be
deposited. For example, a metal layer such as a layer of indium,
platinum, gold or aluminium may be formed for electrical contact.
Alternatively or additionally a dielectric layer may be provided.
This is especially useful where the structure is to be used as a
radiation detector as the dielectric layer may act as a filter to
block visible and near infra red light.
[0021] According to a second aspect of the present invention, there
is provided a method of growing a bulk single crystal material, in
particular a group II-VI material, using a physical vapour phase
deposition method. The method provides that the crystal material is
formed on a seed substrate of a material different from the crystal
material to be formed. To enable the crystal material to be formed
on the foreign substrate, an intermediate layer of a single crystal
material is first formed on the substrate, a transition region is
formed on the intermediate layer and the bulk single crystal
material is grown on the transition region by an appropriate vapour
phase deposition method. The intermediate layer is generally a thin
film layer.
[0022] The method of the present invention allows high quality bulk
crystal material to be formed quickly using physical vapour phase
deposition methods, enabling the required thickness of material to
be formed in an acceptable time. Due to the use of a foreign seed,
it is possible to produce crystal material having a larger size
than has conventionally been possible by physical vapour phase
deposition methods as larger foreign seed substrates are often
available than seeds of the required crystal material. Therefore,
the present invention provides the advantages associated with
physical vapour phase deposition methods in terms of the speed of
formation and quality of the crystal material, whilst allowing
larger area crystals to be formed than is conventionally the
case.
[0023] Although one advantage of the present invention is the
ability to produce large size crystal materials for use in large
detectors or the like, it is possible to divide the substrate and
crystal material grown on the substrate into smaller pieces. By
producing a single, large piece of crystal and then dividing this
up into smaller pieces, it is considered possible to produce the
required crystal material more quickly and with greater consistency
than would be the case if the smaller pieces required were formed
individually.
[0024] In one embodiment, the intermediate layer can be formed
using standard thin film deposition techniques. These include
molecular beam epitaxy, chemical vapour deposition, sputtering,
metallo organic chemical vapour deposition (MOCVD), metal organic
vapour phase epitaxy and liquid phase epitaxy. Whilst all of these
methods are relatively slow, since the intermediate layer is very
thin, the growth rate of the layer is not of significant importance
in the overall manufacturing process. In an alternative embodiment,
physical vapour phase deposition techniques are used to grow the
thin film intermediate layer on the substrate. When vapour phase
deposition techniques are used for of growth of crystal materials,
typically at a growth rate of between 100 and 500 microns/hour, it
is necessary for the growth to provide an underlying layer of the
same material as that to be deposited. However, when the conditions
are adjusted to grow a thin film at a growth rate of between 1 and
10 microns/hour, the thin film can be grown on a foreign seed.
[0025] The transition region and bulk crystal can be deposited
using the same growth technique, but with a variation in the growth
parameters during the growth cycle to gradually accelerate the rate
of growth. In particular, when the material is initially deposited
on the substrate, the growth rate will be slow, enabling the
materials to be properly nucleated and formed. After depositing
this initial material, the growth parameters can be changed to
increase the rate of formation of the crystal material. Where the
same technique is used to form the intermediate layer, there will
be an initial region where the deposition changes from the slow,
thin film type, deposition to the faster, bulk crystal, deposition.
This change may be a gradual change, or may be an abrupt
change.
[0026] The parameters that should be changed may include at least
one of the source temperature (T.sub.source) and the substrate
temperature (T.sub.sub). A variation in the source and/or substrate
temperature will result in a change of the temperature differential
(.DELTA.T). Typically, the minimum source temperature will be
around 450.degree. C. to ensure the sublimation of the material. At
temperatures lower than 450.degree. C., no substantial sublimation
will occur. The minimum substrate temperature is around 200.degree.
C. By increasing the temperature differential, for example by
increasing the source temperature, the overall growth rate may be
increased. It will be appreciated that the growth and sublimation
temperatures are dependent on the material being deposited. For
example, the growth temperature for mercury iodide is around 100 to
150.degree. C. and the sublimation temperature is around 200 to
300.degree. C.
[0027] It is preferred that the transition region and/or bulk
crystal material is grown using a multi-tube physical vapour phase
transport method, such as that disclosed in EP-B-1019568.
[0028] The seed substrate can be formed from various materials.
However, preferred materials for these substrates are silicon and
gallium arsenide. An advantage of forming crystals on a silicon and
gallium arsenide substrate is that these substrates have good
mechanical strength and commercially available at an acceptable
price. This both helps ensure that the crystal material is
consistently formed on the substrate, which may be more difficult
with a less robust substrate, and also helps maintain the integrity
of the formed material in subsequent processing, use and
transportation.
[0029] The substrate may be of any size required, depending upon
the required size of the crystal material. However, it is preferred
that the substrate has a diameter greater than 25 mm, preferably
greater than 50 mm, and most preferably at least 150 mm. The
substrate can be as large as is available at the time.
[0030] The bulk crystal materials formed may include cadmium
telluride and cadmium zinc telluride (CZT), cadmium manganese
telluride, and silicon carbide (SiC). Where the material is cadmium
zinc telluride, this will have the composition
Cd.sub.1-xZn.sub.xTe.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The present invention will now be described by way of
example with reference to the accompanying drawings, in which:
[0032] FIG. 1 shows a suitable multi-tube physical vapour phase
transport device for growing structures according to the present
invention; and,
[0033] FIG. 2 shows a cross section of a material structure
according to the present invention.
DETAILED DESCRIPTION OF A PREFERRED EXAMPLE
[0034] A preferred apparatus for the formation of a structure
according to the present invention is shown in FIG. 1. The
apparatus is suitable for forming bulk single crystal materials.
Generally bulk crystal materials will have a thickness of at least
500 microns.
[0035] The apparatus comprises an evacuated U-tube in the form of a
quartz envelope 20 encased in a vacuum jacket 21. Two separate
three zone vertical tubular furnaces are provided 22, 23 for the
source 24 and the sink zone 25 respectively. The source and sink
zones are connected by an optically heated horizontal crossmember
27 forming a passage 26. A flow restrictor 28 is provided in the
passage 26. The passage comprises two separate points of
deviation--in each case at an angle of 90.degree.--providing
respective junctions between diverging passages for in-situ
monitoring and vapour transport from the source to the sink zone.
Windows allowing optical access to source and sink respectively are
provided. The temperature of the surface of growing crystal in the
sink zone can be monitored by a pyrometer or other optical
diagnostic apparatus 33 located external to the vacuum jacket and
in optical communication with the surface of the growing crystal.
The diagnostic apparatus is in communication with a suitable
control system to vary the sink zone temperature. The apparatus
also comprises means for in-situ monitoring of vapour pressure by
access ports 33 to 36 in the region of the flow restrictor 28,
through which vapour pressure monitoring lamps and optics may be
directed from a position external to the vacuum jacket with
detectors located as shown at a location 35, 36 diametrically
opposed with respect to the passage for vapour transport 26. These
are suitably linked to a control system providing for process
control.
[0036] The source tube, growth tube and crossmember, in which
transport takes place, are fabricated from quartz and the system is
demountable with ground glass joints between the crossmember and
the two vertical tubes allowing removal of grown crystals and
replenishment of source material. Radiation shields (not shown for
clarity) together with the vacuum jacket which surrounds the entire
system provide thermal insulation. A flow restrictor such as a
capillary or a sintered quartz disc is located in the centre of the
crossmember. Growth takes place on a substrate located on a quartz
block in the growth tube with the gap between this glass block and
the quartz envelope forming the downstream flow restrictor.
Provision is made for a gas inlet to the source tube and the growth
tube may be pumped by a separate pumping system or by connection to
the vacuum jacket via a cool dump tube.
[0037] A number of additional source tubes may be provided. In this
case, the additional source tubes can include different materials
for deposition, and will include separate heaters.
[0038] The structure of the device according to the present
invention is shown in FIG. 2. As will be described in more detail
below, the structure comprises a substrate 10, an intermediate
layer 11, a transition region 12 and a bulk crystal material 14. In
a preferred example, the overall structure can be defined by the
formula a:b:y.sub.1,2,3 . . . :c where a is the substrate 10, b is
the intermediate layer 11, c is the bulk crystal material 14 and
y.sub.1,2,3 . . . is the interfacial or transition region 12.
[0039] The substrate 10 is provided in the apparatus as described
with respect to FIG. 1. The substrate can be one of a number of
different materials, including silicon, gallium arsenide,
germanium, silicon carbide and sapphire. The substrate 10 will
typically have a thickness greater than 100 microns, preferably of
at least 200 microns for mechanical stability and can have any
available size. Silicon substrates with a diameter of up to 300 mm
are currently available.
[0040] A source is provided to supply a material to be deposited
onto the substrate 10.
[0041] There are a number of factors which determine whether a
particular material can suitably be deposited on an existing layer,
or whether problems will arise from the mismatch between the
adjacent layers. A mismatch may occur where there is a mismatch
between parameters such as the lattice parameters, the thermal
expansion coefficient and/or the coefficients of elasticity.
Ideally, the parameters for the material of adjacent layers should
be as close as possible to minimise mismatches. Where there is a
large difference in the lattice parameters for adjacent layers, for
example where the difference between lattice parameters is greater
than 3%, misfit dislocations will occur as the subsequent layer is
deposited. However, these misfit dislocations will in most cases
grow out over the first few atomic layers--typically within 10
microns--so that the remainder of the material will be fully
relaxed. However, this relaxation occurs only at the temperature of
growth. Where there is a difference between the thermal expansion
coefficients of the adjacent layers, at temperatures other than the
temperature of growth, there will be thermal strain. Such strain
can be transmitted to other layers in the structure, for example to
the substrate or crystal material. Where the crystal material is
sufficiently thick, the strain will generally be located in the
substrate. For example, it has been found that when a CdTe layer,
with a thickness of about 250 microns, is formed on a 350 micron
gallium arsenide substrate at 500.degree. C., there will be
substantially no strain in the CdTe layer when the device is held
at a temperature of around 700.degree. C. during subsequent crystal
formation.
[0042] According to one example of the present invention, the
source is selected so as to initially deposit the intermediate
layer 11. The intermediate layer will have a thickness of between
about 10 and 1000 microns, preferably in the region of 100 to 700
microns. An intermediate layer of this thickness will withstand any
initial sublimation of the layer during the initial stages of bulk
crystal growth. As discussed above, a thickness of 10 microns will
be sufficient for misfit dislocations to grow out, and a thicker
layer will help ensure that any strain will be primarily located in
the substrate. Where the substrate is a silicon substrate, this
will have a lattice parameter a=5.4309 .ANG.. In this case, and the
intermediate layer may comprise an initial layer of GaP deposited
on the substrate. GaP has a lattice parameter a=5.4506 .ANG.. This
lattice parameter is sufficiently close to that of the underlying
silicon substrate 10 that any lattice mismatch is minimised.
[0043] The source material supplied to the growth chamber may be
altered so as to deposit a transition region on the GaP layer. The
transition region y will have a thickness of between about 10 and
200 microns, preferably up to about 500, to achieve lattice and
thermal matching. The region may be considered a number of layers
y.sub.1, y.sub.2, y.sub.3 etc of different materials or properties
to complete the transition from the intermediate layer 11 to the
bulk crystal material 14 or may be considered a gradual transition.
In one particular example, the transition layer may comprise a
single layer of CdSe having a lattice parameter a=6.05 .ANG..
[0044] After forming the transition layer y, the bulk crystal
material 14 can be deposited by changing the source material. A
preferred bulk crystal material is cadmium telluride which has a
lattice parameter a=6.481 .ANG.. The bulk crystal material may be
deposited to a thickness of about 700 microns or more. This is
important where the material is required to ensure effective
absorption of high energy radiation. It has been found that to
absorb 90% of x-rays at 100 KeV, a thickness of 11 mm is
required.
[0045] During the formation of the transition region, the growth
parameters are controlled such that the transitional region has a
desired thickness. Once the transition has been made to the bulk
crystal material to be deposited, the growth parameters can be
adjusted so that the bulk crystal material can be deposited at a
higher rate.
[0046] Various possible material structures can be achieved in
accordance with the present invention. The transitional region will
typically be very small compared to the substrate and bulk crystal
material, and therefore the effects are considered negligible in
the overall device.
[0047] The selection of the substrate will generally be determined
by the availability of substrates of a required size, but other
factors include the mechanical strength, thermal expansion and
elasticity coefficients required for a desired application.
Differences in the lattice parameters and elasticity and thermal
expansion coefficients between the bulk crystal material and
substrate can be compensated for in accordance with the present
invention, although it will be appreciated that if the substrate
can be chosen to minimise these differences, the overall structure
may be improved.
[0048] Examples of possible structures, giving the substrate,
intermediate layer and bulk crystal material are set out below.
TABLE-US-00001 Intermediate Layer + trace Bulk Example Substrate
elements Crystal Overall Structure 1 Si CdTe CdTe Si: CdTe: CdTe 2
Si CZT CZT Si: CZT: CZT 3 Si CZT CdTe Si: CZT: CdTe 4 Si CdTe CZT
Si: CdTe: CZT 5 GaAs CdTe CdTe GaAs: CdTe: CdTe 6 GaAs CZT CZT
GaAs: CZT: CZT 7 GaAs CZT CdTe GaAs: CZT: CdTe 8 GaAs CdTe CZT
GaAs: CdTe: CZT 9 Ge CdTe CdTe Ge: CdTe: CdTe 10 Ge CZT CZT Ge:
CZT: CZT 11 Ge CZT CdTe Ge: CZT: CdTe 12 Ge CdTe CZT Ge: CdTe: CZT
13 Silicon CdTe CdTe Silicon Carbide: CdTe: Carbide CdTe 14 Silicon
CZT CZT Silicon Carbide: CZT: Carbide CZT 15 Silicon CZT CdTe
Silicon Carbide: CZT: Carbide CdTe 16 Silicon CdTe CZT Silicon
Carbide: CdTe: Carbide CZT 17 SiC CdS CdTe SiC: CdS: CdTe 18 SiC
CdS CZT SiC: CdS: CZT
[0049] One particular advantage of devices made in accordance with
the present invention is that the different materials used to form
the substrate, intermediate layer and bulk crystal material may
provide different functions in the final apparatus. For example, in
the example of a silicon substrate, cadmium telluride bulk crystal
material, the cadmium telluride material may be used to detect
high-energy photons, whilst the silicon substrate may be able to
detect lower energy photons.
[0050] Where the material is to be used for detection of radiation,
the required thickness of the material will be dependent upon the
energy to be absorbed. For cadmium telluride, cadmium zinc
telluride and cadmium manganese telluride, the thickness of
material required for absorption of radiation of various energies
is as set out below:
TABLE-US-00002 Thickness required for 50% Photon Energy absorption
30 keV 0.007 cm 100 keV 0.07 cm 200 keV 0.35 cm 500 keV 1.2 cm 750
keV 1.7 cm 1-10 MeV 2.3-3.5 cm
[0051] In one embodiment of the present invention, a bulk cadmium
zinc telluride layer is formed on a silicon substrate. In this
case, the silicon substrate is first treated to remove any oxides.
This treatment may include chemical etching or heating the
substrate to a high temperature in an ultra high vacuum. The
silicon substrate is provided in the growth chamber, with separate
sources of zinc telluride and cadmium telluride.
[0052] The preferred temperature for the growth of the crystal
material is around 700.degree. C., and accordingly the temperature
of the silicon substrate is increased to this temperature. The
temperature of the zinc telluride and cadmium telluride sources is
then increased at a rate of about 2.degree. C. per minute until the
temperature of these reaches the same temperature as that of the
substrate. Thereafter, the temperature of the cadmium telluride
source is maintained at this level, whilst the temperature of the
zinc telluride source is increased at the same rate to a
temperature of around 870.degree. C. When the zinc telluride source
reaches a temperature of around 870.degree. C., the temperatures of
the substrate and source materials are maintained for around 5
hours. This causes the growth of an intermediate layer of zinc
telluride to a thickness of around 50 microns on the substrate.
Thereafter, the temperature of the substrate is maintained at
around 700.degree. C. and the temperature of the zinc telluride
source is maintained at around 870.degree. C. whilst the
temperature of the cadmium telluride source is increased to the
same temperature as the zinc telluride source material at a rate of
around 2.degree. C. per minute. As the cadmium telluride material
is heated, the material layer grown on the substrate will gradually
change composition from the zinc telluride material of the
intermediate layer to a cadmium zinc telluride material with about
4% zinc. The resulting transition region will have a thickness of
around 100 microns. The transition region could be reduced in
thickness by increasing the rate of temperature increase of the
cadmium telluride source, or could be made thicker by decreasing
the rate of temperature increase. Thereafter, bulk crystal cadmium
zinc telluride material will be deposited whilst the temperatures
of the source materials are held at a higher temperature than the
substrate. The precise composition of the deposited bulk crystal
material can be controlled by varying the relative temperature of
the two source materials.
[0053] In an alternative example, the intermediate layer is
deposited on the upper surface of the seed plate by a conventional
thin film deposition method. Suitable methods include molecular
beam epitaxy, chemical vapour deposition, sputtering, metallo
organic chemical vapour deposition (MOCVD), metal organic vapour
phase epitaxy and liquid phase epitaxy methods. The thin film layer
of the required crystal material is deposited or grown on the
substrate at a typical rate of between 0.1 and 10 micron per hour,
although could be greater. However, only a very thin layer is
required to be formed on the upper surface of the substrate,
typically having a thickness of between about 1 and 10 microns,
although could be greater. The film thickness should be at least 1
micron to ensure that the layer is fully relaxed. The maximum
thickness of the layer is preferably 10 microns so that the layer
can be formed within an acceptable time.
[0054] After forming the thin film on the upper surface of the
substrate, the substrate is removed from the growth chamber, and is
treated, for example being cleaned and polished. The substrate is
then provided for the growth of the transition region and the bulk
crystal material using a physical vapour phase method.
* * * * *