GB2433447A

GB2433447A - Bulk single crystal material and method of growth

Info

Publication number: GB2433447A
Application number: GB0526073A
Authority: GB
Inventors: Arnab Basu; Ben Cantwell; Max Robinson; Andy Brinkman
Original assignee: Durham Scientific Crystals Ltd
Current assignee: Durham Scientific Crystals Ltd
Priority date: 2005-12-21
Filing date: 2005-12-21
Publication date: 2007-06-27
Also published as: GB0526073D0

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. The method of growing the bulk single crystal material comprises providing a seed substrate of a material different from the bulk crystal to be formed, forming an interfacial layer or region of single crystal material on the substrate and forming the bulk single crystal grown on the interfacial layer or region using a vapour phase deposition method. The interfacial layer is grown using a variation in growth parameters, namely the source temperature and the substrate temperature, the differential of which is increased to increase the growth rate.

Description

I

APPARATUS AND PROCESS FOR CRYSTAL GROWTH

Field of the invention

The present invention relates to an improved method for the growth of crystals, and to crystals made by the improved method. The improved method is especially suitable for the bulk growth of single crystal materials such as cadmium telluride (CdTe), cadmium zinc telluride (CZT).

Discussion of the prior art

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. Crystals of other materials may be formed.

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.

Traditionally, single crystals have been formed using direct solidification techniques, such as by the Bridgman, travelling heater (THM), gradient freeze (GE) 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 50mm.

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. Tellurjde 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.

In European Patent No EP-B-1019568 a method of forming crystals using a 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.

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.

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. I 3

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 known 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 I 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 keV8, it is necessary for a CdTe layer to have a thickness of about 11mm. 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.

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.

Summary of the invention

According to the present invention, there is provided a method of growing a bulk single crystal material using a 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 interfacial layer or region of a single crystal material is first formed on the substrate, and then the bulk single crystal material is grown on the interfacial layer or region by an appropriate vapour phase deposition method.

The method of the present invention allows high quality bulk crystal material to be formed quickly using 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 vapour phase deposition methods as larger foreign seed substrates are available than seeds of the required crystal material. Therefore, the present invention provides the advantages associated with 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.

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.

The interfacial layer or region may be formed using a number of techniques.

The interfacial layer or region may comprise a single layer, or multiple layers of material.

In one embodiment, an interfacial layer or layers 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 for, since the interfacial layer or region is very thin, the growth rate of the layer is not of significant importance in the overall manufacturing process.

In an alternative embodiment, vapour phase deposition techniques are used to grow the thin film interfacial layer or layers on the substrate. When vapour phase deposition techniques are used for bulk 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 I and 10 microns/hour, the thin film can be grown on a foreign seed.

With each of these embodiments of the invention, the interfacial layer will be one or more defined layers between the substrate and the bulk crystal material having a similar structure as the bulk crystal material.

After formation of the interfacial layer or layers by standard thin film deposition techniques or vapour phase deposition techniques, it is desirable that the substrate be treated, for example being cleaned and/or polished, prior to deposition of the bulk crystal material.

In an alternative embodiment, an interfacial region is formed on the substrate.

In this case, the interfacial 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. 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.

The parameters that should be changed may include at least one of the source temperature (Tsour) and the substrate temperature (TSUb). A variation in the source and/or substrate temperature will result in a change of the temperature differential (ST). Typically, the minimum source temperature will be around 450 C to ensure the sublimation of the material. At temperatures lower than 450 C, no substantial sublimation will occur. The minimum substrate temperature is around 200 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 C and the sublimation temperature is around 200 to 300 C.

It is preferred that the bulk crystal material is grown using a multi-tube physical vapour phase transport method, such as that disclosed in EP-B-1019568.

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.

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.

The bulk crystal materials formed may include cadmium telluride and cadmium zinc telluride, zinc selenide (ZnSe), cadmium suiphide (CdS), cadmium telluride, CZT, zinc telluride (ZnTe), gallium nitride (GaN), silicon carbide (SiC), lead iodide (Pbl2), mercury iodide (Hg12) and zinc suiphide (ZnS). Where the material is cadmium zinc telluride, this will have the composition Cdi..ZnTe.

The bulk crystal materials formed by the method of the present invention have many applications, including applications in x-ray and gamma ray detectors, for example in medical, space observation and security applications. In addition, the bulk crystal may itself be used as a seed crystal for the formation of other bulk crystal materials.

According to a further aspect of the present invention, there is provided a device comprising a substrate and a bulk single crystal material formed on the substrate, the bulk single crystal material being a material different from the material of the substrate. An intermediate layer or region may be provided between the substrate and bulk single crystal material. The intermediate layer or region may comprise one or more discrete layers.

The device of the present invention, which may be formed using the method of the first aspect of the present invention, differs from prior art devices in that the bulk single crystal material is of a different material from the substrate on which it is formed, it being conventional to form a bulk crystal material only on a substrate of substantially the same material.

Brief description of the drawings

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a cross-section of the bulk crystal grown in accordance with a first or second embodiment of the invention; Figure 2 shows a cross-section of the bulk crystal grown in accordance with an alternative embodiment of the invention; and, Figures 3 to 5 show schematic views of apparatus for bulk crystal growth on the substrate.

Detailed description of a preferred example

In all embodiments of the present invention, a bulk crystal material is formed on a seed plate or substrate. A bulk crystal material is material that has a thickness greater than a thin film. Typically a bulk crystal material will have a thickness of at least 300 microns, although more commonly the thickness will be at least 500 microns. Typical seed plates include a silicon or gallium arsenide substrate. These are commercially and readily available in diameters of up to around 300 mm.

In a first aspect of the present invention, a thin film of the required crystal material 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 I 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.

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 bulk growth of the crystal material takes place using a physical vapour phase method.

Figure 3 shows a simple linear system for vapour phase crystal growth. A sealed quartz ampoule 31 is provided within a tubular furnace 32. Within the ampoule 31 there is provided a source 33 of material for forming the bulk crystal. The substrate 34 including the thin film of material is also provided within the ampoule 31. By control of the temperature difference between the source 33 and sink 34, a differential equilibrium vapour pressure is created, providing the driving force for growth of the semiconductor material on the sink 34. This system is particularity suited for growth of Il-VI compounds, such as ZnSe, CdS and ZnS which are able to sublime easily from the solid phase.

Figure 3a shows the temperature profile between the source 33 and sink 34 achieved with this basic system. Since the source and sink are not thermally isolated, the accurate control of the deposition with such a system may be difficult, making this unsuitable for some materials.

Figure 4 shows an alternative system for bulk deposition of semiconductor material. In this case, there is provided a pressure vessel 10 within which are provided the substrate with the thin film acting as the seed 14, a source of material for forming the semiconductor material, and a capillary transport tube 17 between the source and sink. The capillary transport tube 17 acts as a flow restrictor between the source and sink zones. In the example shown in Figure 4, there is also provided a viewing port 18 allowing a user to observe the growing crystal on the substrate 14. In the arrangement shown in Figure 4, separate heaters are provided for the source, sink and transport zones.

This allows independent temperature control of each of these zones. As shown in Figure 4a, relatively stable temperatures are achieved in each zone due to the thermal isolation between these zones. The temperature profile in particular shows a staged variation reaching a maximum in the flow restrictor, with a graduated temperature decrease across the sink zone. With this embodiment, the provision of source material to the substrate is more accurately controlled due to the temperature isolation between the zones, and by the use of the capillary transport tube. Accordingly, the growth of semiconductor material on the substrate can be controlled more accurately.

A preferred apparatus for the vapour deposition of the material on the thin film formed on the substrate is shown in Figure 5.

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 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. : 11

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.

According to a second embodiment of the present invention, apparatus such as that shown in and described with respect to Figure 5 is used to form the initial thin film at layer on the substrate. In this case, the growth parameters, such as the concentration of the source, temperature differential etc. are controlled to initially grow the crystal material on the foreign substrate at a slower rate, for example in the range I to 10 microns/hour. As with the first embodiment, after formation of the thin layer of crystal material on the substrate, the substrate is removed and treated, for example by being polished. The crystal with the thin film is then reintroduced to the apparatus for growth of the bulk single crystal material as described with respect to the first embodiment.

A cross-section of the resulting crystal formed in accordance with the first or second embodiment of the invention is shown in Figure 1. It will be seen that this includes three distinct layers. The first layer is the substrate 1, on top of which is formed the interfacial layer 2. Together, these layers form the substrate with thin film are used as the seed. The upper layer 3 is the bulk crystal material.

It will be appreciated that the interfacial layer may comprise more than one layer of material, and these layers may be similar or dissimilar to the substrate or bulk crystal material.

In a further embodiment of the present invention, the substrate is provided in an apparatus such as that shown in Figure 5. The growth parameters are initially set so as to slowly form a thin film interfacial region on top of the substrate. In this example, without removing the substrate from the chamber, the process parameters are adjusted to increase the rate of growth so that bulk material can be produced. In this case, it is possible to either abruptly change the process conditions from the thin-film process growth conditions for forming the initial interfacial region to rapid growth conditions, or gradually change these over a short distance. With the material according to this aspect of the invention, there is only one defined boundary between the materials that between the substrate and the interfacial thin film region. There is a more gradual growth between the thin-film region and the thick film material.

Claims

CLAIMS

1. 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 interfacial layer or region of single crystal material on the substrate; and, forming the bulk single crystal material is grown on the interfacial layer or region using a vapour phase deposition method.

2. The method according to Claim 1, in which an interfacial layer is formed using standard thin film deposition techniques.

3. The method according to Claim 2, in which the interfacial 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).

4. The method according to Claim 1, in which an interfacial layer is formed using vapour phase deposition techniques.

5. The method according to Claim 4, in which the interfacial layer or region is grown at a growth rate of between I and 10 microns/hour.

6. The method according to any one of Claims 2 to 5, in which after formation of the interfacial layer, the substrate with the interfacial layer are treated, for example being cleaned and/or polished, prior to deposition of the bulk crystal material.

7. The method according to Claim 1, in which an interfacial region is formed on the substrate 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.

8. The method according to Claim 7, in which the growth parameters that are varied include one of the source temperature (Tsource) and the substrate temperature (Isub).

9. The method according to Claim 8, in which the temperature differential between the substrate temperature and the source temperature is increased to increase the growth rate.

10. The method according to any one of Claims 7 to 9, in which the variation in the growth parameter is a gradual variation.

11. The method according to any one of Claims 7 to 9, in which the variation in the growth parameter is an abrupt variation.

12. The method according to any one of the preceding claims, in which the source temperature is at least 450 C.

13. The method according to any one of the preceding claims, in which the substrate temperature is at least around 200 C.

14. The method according to any one of the preceding claims, in which the substrate is a silicon or gallium arsenide substrate.

15. The method according to any one of the preceding claims, in which the substrate has a diameter greater than about 25 mm.

16. The method according to Claim 15, in which the substrate has a diameter of at least 50 mm, and most preferably at least 150 mm.

17. The method according to any one of the preceding claims, in which the bulk crystal material comprises one of zinc telluride, cadmium telluride, cadmium zinc telluride, zinc selenide, lead iodide, mercury iodide, cadmium sulphide, silicon carbide, zinc suiphide and gallium nitride.

18. The method according to Claim 17, in which the bulk crystal material has the composition Cd1..ZnTe.

19. The method according to any one of the preceding claims, in which the bulk crystal material is grown at a growth rate of between 100 and 500 microns/hour.

20. The method according to any one of the preceding claims, in which the bulk crystal material has a thickness of at least 500 microns.

21. The method according to any one of the preceding claims, in which the substrate and crystal material grown on the substrate are divided into smaller pieces after formation.

22. The method according to any one of the preceding claims, further comprising the step of forming an x-ray or gamma ray detector.

23. The method of any one of Claim Ito 21, 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.

24. The method of any one of the preceding claims, in which the interfacial layer comprises a plurality of layers of the same or different material as one or other of the substrate and bulk crystal material.

25. A device made in accordance with the method of any one of the preceding claims.

26. A device comprising a substrate and a bulk single crystal material formed on the substrate, the bulk single crystal material being a material different from the material of the substrate.