APPARATUS AND PROCESS FOR CRYSTAL GROWTH

Abstract: Apparatus for bulk vapour phase crystal growth comprising: at least one source zone and at least one sink zone each associated with means for independent temperature control within the zone; and at least one passage means adapted for transport of vapour from source to sink zone; and additionally comprising means for in-situ monitoring of the sink zone; wherein means for monitoring is substantially non-intrusive in terms of temperature regulation within the sink zone; process for bulk vapour phase crystal growth employing the apparatus; method for starting up the process; method for controlling the process; use for any bulk vapour transport technique; equipment for monitoring growth using the apparatus or process; and crystal grown with the apparatus or process.

Description: APPARATUS AND PROCESS FOR CRYSTAL GROWTH The present invention relates to an improved apparatus and process for crystal growth, and crystals obtained with the apparatus or process. More particularly the invention relates to an apparatus and method for vapour phase crystal growth with non-intrusive growth monitoπng, crystals grown with the apparatus or process, m particular for use as a semiconductor or optical crystals, and the use of known and novel monitoπng equipment to monitor crystal growth with the apparatus or process of the invention. In designing an effective vapour growth system which has the potential for commercial development and the production of large, high quality single crystals of semiconducting mateπals with for example cadmium telluπde (CdTe), there are three major concerns: 1. the achievement of an adequate growth rate. Without being too specific, a rate less than 1 mm per day is becoming unacceptable. The growth rate should also be controllable as it has an important influence on crystal quality and too high a growth rate results m poor quality crystals; 2 the need to achieve high quality single crystal over a 50 mm diameter boule; and 3. the requirement for a user friendly, robust, manufacturable but flexible design. Until fairly recently conventional vapour transport has involved the use of a simple linear system with a source and sink of single crystals of II- VI compounds, such as CdS, ZnS. which sublime easily from the solid phase. These together with a seed crystal are located in a sealed quartz ampoule in a tubular furnace in an arrangement for example as descπbed in W.W. Piper and S.J. Po ch, J. Appl. Phys. 32 (1961) 1278 (see Fig. 1 below) The source and sink are at different temperatures and therefore have different equilibπum vapour pressures. This vapour pressure difference provides the dπvmg force for growth This approach results m certain fundamental problem for growth of crystals such as CdTe. The equilibπum vapour composition of CdTe is non-stoichiometπc except at one temperature, the congruent evaporation temperature which is descπbed in more detail in D de Nobel Philips Res. Repts. 14 (1959) 361. Due to the law of mass action: where [Cd] and [Te 2 ] are the concentrations of cadmium and telluπum vapour respectively and K is a constant depending on temperature, T. N. Yelhn and S. Szapiro, J. Crystal Growth 69 (1984) 555 have reported that minute deviations from stoichiometry m the bulk source mateπal result m large vaπations in the composition of the vapour making the transport and hence growth highly non-reproducible. Furthermore, this effect gives πse to non-stoichiometry m the growing crystal which has a detπmental effect on its useful properties. Attempting to overcome these problems with the use of high source / sink temperatures is very difficult and does not lead to a significant improvement m growth rate. Alternatively, control of the axial temperature gradient is also difficult m simple tubular systems and it is difficult to thermally isolate source and sink regions as radiation is an important thermal flux. Furthermore, exact determination of the parameters controlling growth (i.e. surface temperatures of source and seed, vapour pressures) is difficult. This approach may be improved by the use of a reservoir containing one of the constituent elements to control the partial pressures according to equation (1) A limitation with this approach m a typical growth system is that the exact conditions of temperature and partial pressure are not determined directly and so the optimum reservoir temperature may be uncertain requiπng analysis of grown crystals This problem is compounded, m a system without m-situ monitoπng, by any change m conditions duπng a growth run and run to run variations. Another major advance in overcoming the limitations of this technology was proposed by the NASA / University of Alabama group of Rosenberger, Banish and Duval (RBD) m F Rosenberger, M.Bamsh and W.M.B. Duval, NASA Technical Memoranduml 03786 Their design was a tubular system with a flow restπctor between source and seed continuously pumped via a second flow restπctor immediately downstream of the seed crystal (Fig. 2.). The continuous pumping m conjunction with a suitable sizing of the downstream flow restπctor removed a small proportion of the source mateπal and m particular any excess component preferentially thus maintaining the vapour phase nearer stoichiometry. The first flow restπctor acted to make the mass transport rate relatively insensitive to the temperatures of the source and sink and their difference. If not restπcted in this way, in a system operating under near stoichiometπc conditions, appropnate transport rates would require the temperature difference between source and sink to be controlled to within a small fraction of a degree which is difficult especially if the temperatures of the source and growing surfaces cannot be measured directly. This system does, however, suffer from some significant limitations: 1. Thermal coupling along the axis of the furnace prevented the desired axial temperature profile from being obtained. 2. Direct determination of the surface temperatures of source and seed was not possible, and indirect determination uncertain due to the complexity of the radiation field 3 The partial pressures of source species over source and seed were not directly measurable and uncertainties m the flow modelling of the system and its restπctors made indirect determination uncertain. 4. The quartz ware was complex, not easy to use and vulnerable m application. In-situ optical monitoπng is known and routinely employed in other methods for crystal growth, such as low temperature and thm film growth, where the 'efficiency' of the process is not very important. Examples of this are Molecular Beam Epitaxy (MBE) (see Fig. 3) and Metal-Organic Vapour Phase Epitaxy (MOVPE) (see Fig. 4) however these techniques are not suitable for 'bulk' crystal growth which requires enclosed transport passages for efficient source utilisation and also requires heating of the quartz passages to allow optical access while preventing condensation pπor to the growth region. Accordingly there is a need for an effective vapour growth system which allows the production of large, high quality single crystals as semi-conductmg mateπals with effective temperature and stoichiometry control. We have now surpπsingly found that an apparatus and method for vapour phase crystal growth may be provided which enables in-situ monitoπng in non-mtrusive manner and moreover allows for substantial thermal isolation of source and sink regions. In its broadest aspect there is provided according to the present invention an apparatus for bulk vapour phase crystal growth compπsmg: at least one source zone and at least one sink zone each associated with means for independent temperature control within the zone; and at least one passage means adapted for transport of vapour from source to sink zone; and additionally compπsmg means for in-situ monitoπng of the sink zone; wherem means for momtoπng is substantially non-intrusive in terms of temperature regulation within the sink zone. Means for independent temperature control are for establishing a temperature differential to enable so d-vapour-solid phase transition m the respective source, transport and sink zones. Temperature control may therefore be selected according to the phase transitions for any given crystal which it is desired to grow, for example in the range from -150°C to +2000°C, employing in each case a greater source than sink temperature with use of appropnate cooling and/or heating control. Preferably means for in-situ monitoπng of crystal growth compπses means providing visual and/or radiation access to the growth zone but located remote therefrom More preferably means for direct monitoπng of crystal growth compπses at least one passage for monitoring communication between the remote visual/radiation access means and the sink zone, wherein the at least one passage for monitoπng communication and the at least one passage for transport of vapour associated with any given smk zone are coincident for at least that portion of their length proximal to the smk zone. It is a particular advantage of the invention that the apparatus as hereinbefore defined may be operated with use of conventional or modified visual/radiation monitoπng means, located external to the passages as herembefore defined, by means of the visual/radiation access means, for example x-ray and the like may be employed to monitor crystal growth. Moreover the apparatus of the invention may be employed in any bulk vapour transport technique with associated advantages in crystal quality, thereby overcoming disruption of growth conditions which are inherent with known in-situ monitoπng means proximal to the sink zone. Reference herein to locations remote from the at least one sink zone is to locations at which the presence of access means as hereinbefore defined introducing temperature vaπation or gradient m the vapour transport passage would substantially not disrupt the conditions of temperature required for uniform growth, having regard to conditions of temperature created by means of temperature controllmg means for the at least one sink zone. In contrast reference herein to locations proximal to the at least one sink zone are to locations which would be subject to substantial disruption of conditions of temperature under these circumstances. In a preferred embodiment the present invention provides an apparatus as hereinbefore defined wherein at least one of the passage for visual/radial communication and the passage for vapour transport associated with any given smk zone deviates by an angle of at least 5° at any one or more points along the length thereof remote from the growth zone, preferably 5° - 270°, more preferably 30°- 180°, most preferably 45°- 110°, for example 60°- 95°. Accordmgly the passage for vapour transport may deviate by an angle as herembefore defined whereby means for visual/radiation access may be located in the wall of the passage for vapour transport in direct line communication with the sink zone. For example means for visual/radiation access may compnse a visual/radiation-transparent port sealed into and optionally continuous with the wall of the transport passage, located opposing to the smk surface. Alternatively the configuration of respective passages for visual/radiation access and vapour transport as hereinbefore defined may be reversed, whereby the passage for visual/radiation monitoπng may deviate by an angle as hereinbefore defined from a direct line communication of source and sink zone. In this case means for visual/radiation reflection is suitably provided in association with the passage for visual/radiation momtonng at its point of deviation, whereby virtual or reflected direct line access is provided with the sink zone. For example a reflective or transmissive means such as mirrored or pnsm quartz may be provided m association with the visual/radiation monitoπng passage at its point of deviation. It will be appreciated that the present invention m its preferred embodiment as hereinbefore defined provides a simple and efficient means to achieve the desired vapour phase crystal growth overcoming the hereinbefore defined problems in admirable manner It is a particular feature of the invention that the provision of passages for visual/radiation access and vapour transport which are substantially coincident proximal to any given sink zone enables non- trusive monitoπng as herembefore defined. Preferably the apparatus as herembefore defined compπses at least one passage for vapour transport as hereinbefore defined, which deviates by an angle of at least 5° as herembefore defined along the length thereof between source and sink zones, more preferably deviates by at least 5° at two points along the length thereof whereby both zones are adapted to compπse source and sink matenal free from constraints of gravity, i.e which are substantially provided on suitable support means and with passage means extending substantially upwardly therefrom, thereby providing for optimal transport with minimum disruption of the growth process The apparatus as hereinbefore defined may compπse a plurality of source zones, for example each associated with a passage for vapour transport, which passages may converge or otherwise, thereby having a common or separate passageways proximal to a single sink zone. By this means, a plurality of source zones may be located radially about a common sink zone, or extending outwardly to one side thereof or may be located in suitable pressure or temperature communication with separate sink zones A plurality of source zones may be independently activated by means of independent temperature control means associated with each source zone, whereby vapour may be generated sequentially or simultaneously from respective source zones In a preferred embodiment a plurality of source zones may be adapted to contain a combination of different elemental or compound source material providing each element or compound respectively of a binary, ternary or other multinary compound, connected to a single or plural growth zones via flow restπctors Similarly a plurality of smk zones may be provided adapted to contain a plurality of seed crystals which may be the same or different, each associated with a passageway for visual/radiation and transport proximal thereto, each smk zone having dedicated means for visual/radiation access The apparatus of the invention may additionally compπse means for visual/radiation access to the one or more source zones, provided in similar remote manner as hereinbefore defined for access means to the sink zone, preferably such that a passage for visual/radiation access to, and passage for vapour transport from, any one source zone are common proximal to the source zone. It is a particular advantage of the apparatus of the present invention that both objects of accurate temperature control of source and sink zones and non-intrusive monitoπng of at least the smk zones can be achieved in mutually beneficial manner, whereby positioning of monitoπng access means between dedicated temperature control means prevents disruption proximal to either zone. It is a further advantage of the invention that the apparatus is ideally suited to inclusion of a flow restnctor, for example as proposed by NASA/University of Alabama RBD group above, located remote from both zones, for example upstream of sink monitoπng means, for the purpose of vapour pressure control. Preferably in-situ means for momtoπng vapour pressure is provided associated with a flow restπctor, m the form of known vapour pressure monitoπng apparatus, for example as descnbed in J. Carles, J.T Mulhns and A.W. Bπnkman, J. Crystal Growth, 174 (1997) 740, the contents of which are incorporated herein by reference Flow restπctors may be selected from any known restπctors and preferably compπses a capillary, porous sintered disc or the like The apparatus of the invention is suitably constructed of any material which is adapted for use at the temperatures envisaged for crystal growth, for example is constructed of low, ambient or high temperature resistant mateπal. Suitable matenals are known in the art and preference is given to metal oxides, and m particular quartz, refractory oxides and graphite having the required mechanical strength and mtegπty, for example being reinforced with a suitable mateπal providing mechanical strength and mtegπty. These matenals are also preferred for reason of their high punty with low nsk of contamination of crystal. Preferably the apparatus compnses a sealed or sealable structure or envelope including zones and passages as herembefore defined. The apparatus is suitably operated at reduced pressure and is encased in a vacuum jacket or the like The apparatus of the invention may be used for any bulk vapour transport techniques as hereinbefore defined. It is a particular advantage that the apparatus is adapted for growth of crystals from polycrystalline binary, ternary or other multinary compounds It is a further advantage that the apparatus of the invention is suited for use with growth of crystal from binary, ternary or other multinary compounds requiπng stoichiometry control to compensate for a degree of non-stoichiometry vapour composition of the desired crystal. The source and smk zones are adapted to compnse source mateπal and seed crystal as known in the art, for example m the form of one or more reservoirs of source matenal and a crystal of seed matenal. Preferably the one or more reservoirs of source matenal compnse matenal m solid phase supported on a glass or other suitable surface or pedestal adapted to the processing conditions to be employed, allowing convenient and efficient vaponsation. In a further aspect of the invention there is provided a process for bulk vapour phase crystal growth compπsmg: providing at least one reservoir of source mateπal and at least one sink or seed crystal, each associated with independent temperature control means; and transporting vapour phase material between source and smk or seed; and providing in-situ means for momtonng growth of crystal at each sink or seed; wherein means for momtonng is substantially non-mtrusive in terms of temperature conditions at the sink or seed. Preferably means for momtonng radiation and transport for any given smk or seed is by coincident monitoπng and transport path for at least the portion of the respective paths proximal to the smk or seed, as herembefore defined. Preferably the process is operated at reduced, ambient or elevated temperature as hereinbefore defmed. The process is moreover operated at reduced pressure, for example in the range up to 1 bar, preferably 0.01 to 100 mbar reduced pressure. The process may be started up by known means to establish a sufficient vapour pressure above source mateπal to initiate growth. In a further aspect of the invention there is therefore provided a method for starting up the process as hereinbefore defined m manner to establish a sufficient vapour pressure above the source mateπal to initiate transport. In a further aspect of the invention there is therefore provided a method for starting up the process as hereinbefore defined m manner to establish transport control and temperature control in the sink zone for controlled growth at the sink or seed. The method for starting up is suitably operated with temperature and transport rate ramping profiles. It is a particular advantage that independent temperature control means provided with the apparatus of the invention enables temperature ramping specific to growth at the smk or seed, which may also be at a temperature lower than that at the source. It is thought that this gives nse to excellent crystal quality and may even prevent an amount of precipitation or eliminate precipitation entirely The process is suitably operated with means for in-situ mo tonng as herembefore defined according to known techniques. Preferably temperature is monitored by known means at the surface of the smk, and optionally of the source, in manner to enable adjustment as required for optimum temperature control and stoichiometry. Likewise vapour pressure is suitably monitored between zones, for example at the location of a flow restnctor and may be adapted or adjusted as required for optimum growth. Preferably the process of the invention as herembefore defined additionally compnses direct reading of process vaπables, companson with optimum values of process vaπables for a desired crystal growth, for example, with use of a process model, and on line optimisation thereof. The apparatus and process of the invention as herembefore descnbed are adapted for growth of any crystal employing bulk vapour transport techniques. In a further aspect of the invention is therefore provided a crystal grown with the apparatus or process of the invention. The invention is suited for growth of crystals compπsmg any compounds which are capable of being sublimed, having a significant vapour pressure below their melting point. Preferably crystals are selected from compounds of groups IIA, IIB, III, V, VI and VTI, or Group IV, more preferably of groups π and V or Group IV of the Penodic Table of the Elements, for example selected from Be, Mg, Zn, Cd, Hg, S, Se, Te and I or from Si and C. Particularly useful crystals grown with the apparatus and process of the invention cadmium tellunde and silicon carbide. In a further aspect of the invention there is provided the use of known momtonng equipment to monitor crystal growth with the apparatus and process of the invention. In a further aspect of the invention there is provided the use of the apparatus or process of the invention for any vapour transport technique, such as semiconductor or optical crystal growth. The invention is now illustrated m nonlimiting manner with reference to the following figures wherem: Figure 1 and Figure 2 are illustrative of pπor art bulk vapour phase crystal growth apparatus; Figure 3 and Figure 4 are illustrative of pnor art MBE and MOVPE apparatus; and Figure 5 and Figure 6 are diagrammatic schemes or apparatus of the invention. In Figure 1 is shown a simple linear system for vapour phase crystal growth compnsmg a sealed quartz ampoule (1) m a tubular furnace (2) having a source (3) and sink (4) for growth of cadmium sulphide, compπsed in a growing crucible (5). The source and smk (3 and 4) are not thermally isolated. Moreover there is no means for in-situ momtonng of temperature or vapour pressure. In Figure 2 is illustrated vapour phase crystal growth apparatus compnsmg a tubular system with a flow restnctor as designed by RBD group University of Alabama The apparatus compnses a pressure vessel (10), independent heaters (11-13) for respective smk, transport passage and source zones (14-16) having a capillary transport tube (17) as flow restnctor therebetween. A viewing port (18) located adjacent to the sink zone (14) provides optical access to the growing crystal in the smk zone (14). In the temperature profile shown in Figure 2a it is clear that relatively stable temperatures are achieved in each zone as a result of the thermal isolation, however a slight irregulanty is apparent at the level of viewing port (18) adjacent to the smk zones, which results from a break m the cladding in order to provide the viewing access adjacent the crystal. The temperature profile shows a staged vanation reaching a maximum at flow restnctor (17) with graduated temperature decrease across the smk zone (14). In Figure 3 is shown a pnor art MBE apparatus as herembefore descπbed compnsmg vacuum chamber (1) having a temperature controlled source (3) and a temperature controlled sink (4). In-situ monitoπng means are provided (6) located opposite to the smk (4). Efficient source utilisation is not a concern m this process, and much of the source matenal sticks to the cold vacuum chamber wall. In Figure 4 is shown a pnor art MOVPE apparatus compnsmg a quartz envelope (1) having at one end an inlet for a metal organic source m earner gas (7) and compnsmg a heated substrate (8) on to which the metal orgames pyrolyse. Exhaust gases exit via outlet (9). Optical access via the quartz envelope (1) allows for in-situ momtonng of the growing crystal and vapour phase conditions. However, this technique is not suitable for the growth of 'bulk' crystals as the growth rates are limited and the requisite precursor metal orgames are extremely expensive, especially as is m general the case, much is lost to the exhaust. In Figure 5 is shown the apparatus of the invention in a preferred embodiment adapted for elevated temperature bulk vapour phase crystal growth as herembefore defmed The apparatus compπses an evacuated U-tube in the form of a quartz envelope (20) encased m a vacuum jacket (21). Two separate three zone vertical tubular furnaces are provided (22 and 23) for the source zone (24) and the smk zone (25) respectively The source and sink zones are connected by passage means (26) for vapour transport compnsmg an optically heated honzontal crossmember (27). Flow restπctor (28) is provided m passage (26) The passage for vapour transport compπses two separate points of deviation in each case at an angle of 90° providing respective junctions between diverging passages for in-situ momtonng and vapour transport from source zones (29), and to sink zone (30). Access means are provided (31 and 32) compnsmg windows allowing optical access to source and smk respectively. In the apparatus as shown in-situ means for momtonng of temperature of the surface of growing crystal in the sink zone (25) are provided m the form of a pyrometer or other optical diagnostic apparatus (33) located external to the vacuum jacket and m optical communication with the surface of the growing crystal. The diagnostic apparatus is m communication with a suitable control system to vary the smk zone temperature. The apparatus compnses additionally means for in-situ momtonng of vapour pressure by further access ports (33 to 36) in the region of the flow restnctor (28), through which vapour pressure momtonng lamps and optics may be directed from a position external to the vacuum jacket with detectors located as shown at a location (35 and 36) diametπcally opposed with respect to the passage for vapour transport (26). These are suitably linked to a control system providing for process control. The source tube, growth tube and crossmember, in which transport takes place, are fabπcated 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 mateπal. Radiation shields (not shown for claπty) together with the vacuum jacket which surrounds the entire system provide thermal insulation A flow restnctor (either 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 m the growth tube with the gap between this glass block and the quartz envelope forming the downstream flow restnctor. 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. This system provides the following solutions to the difficulties inherent in previous designs as outlined above. 1 Source and growth regions are thermally decoupled making the achievement of optimum axial and radial temperature profiles m the growth region more tractable 2 It is possible to observe both the growing surface and source matenal directly duπng growth allowing, for example, optical pyrometry or spectrometnc measurements as a diagnostic for the growth process. 3 The layout provides for in-situ measurement of the vapour pressures of the source elements by means of optical absorption measurements made through the crossmember on either side of the flow restπctor If the flow properties of the flow restπctor are known, then these measurements also allow the mass transport rate to be determined directly dunng growth. 4 The glassware is relatively simple and robust and may, in principle, be extended to the growth of multinary compounds by the addition of source tubes connected to the growth tube by suitable flow restπctors (designed to minimise reverse flow of species and hence contamination of the source matenals by operating at a sufficiently high flow rate). Similarly more than one crystal may be grown simultaneously by the addition of multiple growth tubes. In Figure 6 is shown an alternative arrangement compnsmg a plurality of source zones (24 a-c) mutually or independently associated with a growth zone (25a-c). The apparatus is for example suited for growth of ternary or multinary compounds employing three different source matenals, or for growth of a plurality of crystals of binary compounds employing alternative sources. In Figure 6 (a) is shown the apparatus in elevation. The invention is now illustrated in non limiting manner with reference to the examples and Figures 7-9 Example 1 - Apparatus "Multi-tube" vapour growth apparatus descnbed above for Figure 5 was developed which separates source and growth regions of the dynamically pumped growth envelope into two independent, vertical three zone furnaces connected by a honzontal optically heated crossmember which incorporates a calibrated flow restnctor to regulate the diffusionless mass transport (as proposed in J.R. Abernathey, et al, J. Crystal Growth 47 (1979) 145, F Rosenberger, et al, technical memorandum 103786, NASA (unpublished)). Growth takes place on a seed wafer supported by a pedestal which also forms a second flow restnctor between the growth envelope and the vacuum system as m the Markov arrangement (E.V Markov et al, Inorg. Mater. (USA) 11 (1975) 1504). Use of this arrangement isolates the source and sink thermally, makes transport relatively insensitive to the source/sink temperature difference, facilitates in-situ vapour pressure measurement (J. Carles, J.T Mullins, and A.W. Bπnkman, J. Crystal Growth 174 (1997) 740) and hence mass transport measurements and allows non-mvasive optical access to the growing crystal The structural properties of a boule of cadmium tellunde grown by this technique at 700°C were profiled through the crystal in the growth direction by X-ray diffraction, defect revealing etching and photolummescence (PL). Example 2 - Crystal Growth Growth was performed on a 49 mm diameter Cdo 96 Zn 004 Te seed wafer the greater portion of which compnsed a {1 1 1} B onented gram and which had been polished by a damage free bromine / methanol / ethanediol hydroplaning technique. After idling all heaters at 200°C for two days to remove water from the system, growth was commenced by ramp g the source and seed temperatures. After 10.4 hours the seed had reached its final growth temperature of 700°C corresponding to a loss to the growth tube tail of 0.2 gh "1 while the transport rate from the source side had reached 1.5 gh "1 giving a growth rate of 118 μmh "1 This transport rate was maintained for 12.5 hours after which it was increased to 2.5 gh "1 yielding a growth rate of 208 μmh "1 for a further 24 hours followed by rampmg down at a maximum rate of 30 Kh "1 The rampmg was designed to ensure that the source flow was always greater than the growth tail loss. Flow modelling of this system has indicated that the ratio of cadmium (Cd) to tellunum (Te 2 ) pressures was 1.33 over the growing crystal. Using the data of Greenberg et al (J.H. Greenberg, V.N. Guskov, V.B. Lazarev, and O.V Shebershneva, J. Solid State Chemistry 102 (1993) 382) it is possible to estimate that this corresponds to an excess of 2 x 10 "6 at.% Te. Example 3 - Sample Preparation and Analysis After growth, the mam {1 1 1 } B onented gram was cut into both honzontal and vertical slices which were then hydroplane polished apart from the uppermost honzontal slice for which the as-grown surface was assessed. A Bede D3 Triple axis X-ray diffractometer employing CuK α radiation from a sealed tube was used to obtain rocking curves and θ - 2Θ scans from the surfaces of honzontal slices. One vertical slice was etched using FeCl 3 (as descπbed by Watson et al, Mater. Sci. Eng. B16 (1993) 113) to reveal etch pits which were recorded photographically pnor to having their positions digitised for determination of the randomness of their distπbution (as descπbed by Rose et al, J. Phys. D: Appl. Phys. 31 (1998) 1008). Photolummescence spectra were obtained at 10 K using the 457.9 nm line of an Argon ion laser and a power density of 10 mWcm "2 Figure 1 shows tnple axis rocking curves and θ - 2Θ scans taken, respectively, directly from the top (as grown) surface of the boule, 6.9 mm above the seed surface and (b) 0.8 mm above the seed wafer. The solid curves show theoretical θ - 2Θ profiles calculated using dynamical diffraction theory and assuming completely unstrained cadmium tellunde. Broadenmg of the θ - 2Θ scans is only 2 arc seconds corresponding to strains in the region of 10 "5 . On the other hand, the rocking curves exhibit substantial broadenmg attributable to a mosaic structure of essentially unstrained sub-grams. Considerable reduction m the full width half maximum values of these spectra, from 107" near the seed to 43" at the top surface, is apparent. Etch pit studies showed that the dislocation density generally decreased in the direction of advancing growth. There was also some lateral vaπation but this was not systematic. The average dislocation density the upper portion of the boule was 6 xlO 4 cm "2 with values m the range 3 - 9 x 10 4 cm "2 being measured. Figure 2(a) shows a representative distnbution of etch pits from a square of side 913 μm. Figure 2 (b) shows normalised radial dislocation density (correlation function), P(r), plot for the data of (a). Fig. 2(a) is onented so that the positive y direction corresponds to the direction of growth. 'Sub-grams' or ' polygonisation walls' are common m CdTe and are charactensed by rows of closely spaced dislocations. CdTe is also prone to have a non-uniform distnbutions of dislocation density (clustenng) on the scale of 500 μm. These effects can be distinguished quantitatively by using a radial density distnbution (con-elation) function, P(r) To compute P(r), for a distnbution of points in a unit square, a frequency chart is first constructed. For all radii, r, the frequency with which dislocations within radii r and r + br of one another is recorded. This data is normalised by dividing by the frequency curve for a computer generated random distnbution. The resulting curve is P(r) and for a truly random distnbution P(r) = 1 for 0 < r < V2. Values of P(r) > 1 mdicate an enhanced probability of associations at radius r. Therefore m Figure 2(b) the peak near r = 0.05 mdicates that there are lines of dislocations with this spacmg. i.e. sub-gram boundaπes. Deviations from P(r) = 1 at other radii mdicate longer range effects. Notably the decline m P(r) for r > 0.75 indicates areas of non-uniformity at those radii on the unit square, i.e. there is dislocation clustenng in these samples. Etch pitting also confirmed the general trend of reducing dislocation density in the growth direction as shown by the X-ray data. Figure 3 (a) and (b) respectively show photolummescence spectra taken near the upper surface and close to the seed. In both cases, the dommant peak is due to acceptor bound exciton recombination at 1.59 eV (as is typically observed m bulk CdTe, B. Yang et al, J. Crystal Growth 159 (1996) 171). It can be seen that the intensity of this emission mcreases in both absolute terms and relative to the donor-acceptor pair emission at 1.54 - 1.55 eV with increasing distance from the seed again mdicatmg an improvement in crystalline quality. The full width half maximum vanes from 4 meV (near seed) to 2.1 meV (top of boule) through the crystal. No donor - A centre (metal vacancy complex) related acceptor pair emission (J.W. Allen, J Crystal Growth 10 (1995) 1049) was observed around 1.4 eV. The donor bound exciton emission appears as a shoulder on the high energy side of the mam peak and there is evidence for free exciton emission at 1.596 eV. Phonon replicas of the acceptor bound and free exciton transitions are also present, down-shifted by 21 meV. Conclusion The structural properties of a boule of cadmium tellunde grown on a Cdo 96 Zno 04 Te seed at 700°C by a novel 'Multi-tube' vapour growth technique have been charactensed. As typically found in CdTe (K. Durose et al, J. Crystal Growth 86 (1988) 471), dislocations were observed to polygomse into a mosaic of sub-grams, possibly propagating from the seed. Considerable improvement m quality, as evidenced by a reduction m X-ray and photolummesecence Imewidths and etch pit densities was observed with increasing distance from the seed indicating a tendency for these defects to grow out. It may be that this process was assisted by the relatively high strength of the mateπal at this low growth temperature. Photolummesence measurements exhibited a dommant acceptor bound exciton emission. As the mateπal was grown under conditions of a slight telluπum excess, it is possible that cadmium vacancies are responsible for this (B Yang et al above referred) The relatively low growth temperature was expected to improve the structural quality of the matenal as the cntical shear stress mcreases with decreasing temperature (R. Balasubramanian et al, Mater. Sci. Eng. B16 (1993) 1). Additionally, the tendency for the growth of non-stoichiometric material is less at lower temperatures due to the narrower existence region of a single phase of non-stoichiometric CdTe and the larger pressure ratios required to saturate the material with one constituent (N. Yellin et al, J. Crystal Growth 73 (1985) 77, Greenberg et al above referred).

Claims: CLAIMS 1. Apparatus for bulk vapour phase crystal growth comprising: at least one source zone and at least one sink zone each associated with means for independent temperature control within the zone; and at least one passage means adapted for transport of vapour from source to sink zone: and additionally comprising means for in-situ monitoring of the sink zone; wherem means for monitoring is substantially non-intrusive in terms of temperature regulation within the sink zone. 2. Apparatus according to Claim 1 wherein means for independent temperature control are for establishing a temperature differential to enable solid-vapour-solid phase transition in the respective source, transport and sink zones, preferably in the range from - 150°C to +2000°C, employing in each case a greater source than sink temperature with use of appropriate cooling and/or heating control. 3. Apparatus according to Claim 1 or 2 wherein means for in-situ monitoring of crystal growth comprises means providing visual and/or radiation access to the growth zone but located remote therefrom. 4. Apparatus according to Claim 3 wherein means for direct monitoring of crystal growth comprises at least one passage for monitoring communication between the remote visual/radiation access means and the sink zone, wherein the at least one passage for monitoring communication and the at least one passage for transport of vapour associated with any given sink zone are coincident for at least that portion of their length proximal to the sink zone. 5. Apparatus according to any of Claims 1 to 4 wherein means for direct monitoring of crystal growth are located external to the passages as hereinbefore defined, by means of the visual/radiation access means. 6. Apparatus according to any of Claims 4 and 5 wherein at least one of the passage for visual/radial communication and the passage for vapour transport associated with any given sink zone deviates by an angle of at least 5° at any one or more points along the length thereof remote from the growth zone, preferably 5° - 270°, more preferably 30°- 180°, most preferably 45°- 110°, for example 60°-95°. 7. Apparatus according to any of Claims 4 to 6 wherein the passage for vapour transport deviates by an angle as hereinbefore defined in Claim 6 whereby means for visual/radiation access is located in the wall of the passage for vapour transport in direct line communication with the sink zone. 8. Apparatus according to any of Claims 4 to 6 wherein the passage for visual/radiation monitoring deviates by an angle as hereinbefore defined in Claim 6 from a direct line communication of source and sink zone, and means for visual/radiation reflection is provided in association with the passage for visual/radiation monitoring at its point of deviation, whereby virtual or reflected direct line access is provided with the sink zone. 9. Apparatus according to any of Claims 1 to 8 which comprises at least one passage for vapour transport as hereinbefore defined, which deviates by an angle of at least 5° as hereinbefore defined along the length thereof between source and sink zones, more preferably deviates by at least 5° at two points along the length thereof whereby both zones are adapted to comprise source and sink material free from constraints of gravity. 10. Apparatus according to any of Claims 1 to 9 which comprises a plurality of source zones, each associated with a passage for vapour transport, which passages may converge or otherwise, thereby having a common or separate passageways proximal to a single sink zone or located in suitable pressure or temperature communication with separate sink zones. 11. Apparatus according to any of claims 1 to 10 which additionally comprises a flow restrictor, located remote from both zones, for the purpose of vapour pressure control, optionally associated with in-situ means for monitoring vapour pressure. . 12. Apparatus according to Claim 11 which is constructed of low, ambient or high temperature resistant material, preferably selected from metal oxides, and in particular quartz, refractory oxides and graphite being reinforced with a suitable material providing mechanical strength and integrity. 13. Apparatus according to any of Claims 1 to 12 which comprises one or more reservoirs adapted to comprise source material and a crystal of seed material supported on a surface or pedestal adapted to the processing conditions to be employed, allowing convenient and efficient vaporisation. 14. Process for bulk vapour phase crystal growth comprising: providing at least one reservoir of source material and at least one sink or seed crystal, each associated with independent temperature control means; and transporting vapour phase material between source and sink or seed; and providing in-situ means for monitoring growth of crystal at each sink or seed; wherein means for monitoring is substantially non-intrusive in terms of temperature conditions at the sink or seed. 15. Process according to Claim 14 wherein means for monitoring radiation and transport for any given sink or seed is by coincident monitoring and transport path for at least the portion of the respective paths proximal to the sink or seed, as hereinbefore defined. 16. Process according to Claim 14 or 15 operated at reduced, ambient or elevated temperature and at reduced pressure, preferably 0.01 to 100 mbar reduced pressure. 17. Method for starting up a process according to any of Claims 14 to 16 in manner to establish a sufficient vapour pressure above source material to initiate transport. 18. Method for starting up a process according to any of Claims 14 to 16 in manner to establish transport control and temperature control in the sink zone for controlled growth at the sink seed. 19. Method for controlling a process according to any of Claims 14 to 18 by direct reading of process variables, comparison with optimum values of process variables for a desired crystal growth, for example, with use of a process model, and on line optimisation thereof. 20. Apparatus and process according to any of Claims 1 to 19 adapted for bulk vapour transport techniques, preferably for growth of crystals employing bulk vapour transport techniques, such as semiconductor or optical crystal growth, more preferably for growth of crystals from polycrystalline binary, ternary or other multinary compounds. 21. Apparatus and process according to any of Claims 1 to 19 adapted for growth of crystals selected from compounds of groups ELA, IIB, III, V, VI and VTI and from compounds of Group TV , more preferably of groups U and VT and from Group TV of the Period Table of the Elements, for example selected from Be, Mg, Zn, Cd, Hg, S, Se, Te and I and selected from Si and C. 22. Equipment for use in monitoring crystal growth with the apparatus and process as hereinbefore defined in any of Claims 1 to 21. 23. Crystal grown with the apparatus or process as hereinbefore defined in any of Claims 1 to 21. AMENDED CLAIMS [received by the International Bureau on 27 January 1999 (27.01.99); original claims 1-23 replaced by new claims 1-25 (5 pages)] 1. Apparatus for bulk vapour phase crystal growth comprising: at least one source zone and at least one sink zone each associated with means for independent temperature control within the zone; and at least one passage means adapted for transport of vapour from source to sink zone; and additionally comprising means for in-situ monitoring of the sink zone which is substantially non-intrusive in terms of temperature regulation within the sink zone wherein the passage for transport of vapour deviates by an angle of at least 5°, preferably 5° - 270°, more preferably 30°-180°, most preferably 45°-110°, for example 60°-95° along the length thereof between source and sink zones. 2. Apparatus according to Claim 1 wherein the at least one passage for vapour transport as hereinbefore defined deviates by an angle of at least 5° as hereinbefore defined at two points along the length thereof whereby both zones are adapted to comprise source and sink material free from constraints of gravity. 3. Apparatus for bulk vapour phase crystal growth comprising: a plurality of source zones and a single sink zone each associated with means for independent temperature control within the zone; and a plurality of passage means adapted for transport of vapour from respective sources to the sink zone; and additionally comprising means for in-situ monitoring of the sink zone which is substantially non-intrusive in terms of temperature regulation within the sink zone. 4. Apparatus according to any of Claims 1 to 3 wherein means for independent temperature control are for establishing a temperature differential to enable solid-vapour- solid phase transition in the respective source, transport and sink zones, preferably in the range from -150°C to +2000°C, employing m each case a greater source than sink temperature with use of appropnate cooling and/or heating control. 5. Apparatus according to any of Claims 1 to 4 wherem means for in-situ momtonng of crystal growth compnses means providing visual and/or radiation access to the sink zone but located remote therefrom. 6 Apparatus according to Claim 5 wherem means for direct momtonng of crystal growth compnses at least one passage for momtonng communication between the remote visual/radiation access means and the smk zone, wherein the at least one passage for momtonng communication and the at least one passage for transport of vapour associated with any given sink zone are coincident for at least that portion of their length proximal to the sink zone. 7. Apparatus according to any of Claims 1 to 6 wherem means for direct momtonng of crystal growth are located external to the passages as herembefore defined, by means of the visual/radiation access means 8. Apparatus according to any of Claims 6 and 7 wherem the passage for visual/radial communication associated with any given sink zone deviates by an angle of at least 5° at any one or more points along the length thereof remote from the smk zone, preferably 5° - 270°. more preferably 30°-180°, most preferably 45°-l 10°, for example 60°-95°. 9. Apparatus according to any of Claims 1 to 8 wherem the passage for vapour transport deviates by an angle as hereinbefore defined whereby means for visual/radiation access is located m the wall of the passage for vapour transport m direct line communication with the sink zone. 10. Apparatus according to any of Claims 6 to 8 wherem the passage for visual/radiation momtonng deviates by an angle as herembefore defined from a direct line communication of source and sink zone, and means for visual/radiation reflection is provided in association with the passage for visual/radiation monitoring at its point of deviation, v/hereby virtual or reflected direct line access is provided with the sink zone. 11. Apparatus according to any of Claims 1 to 10 wherein source and sink zones are adapted to comprise source and sink material free from constraints of gravity. 12. Apparatus according to any of claims 1 to 11 which additionally comprises a flow restnctor, located remote from and between source and sink zones, for the purpose of vapour pressure control, optionally associated with in-situ means for monitoring vapour pressure. 13. Apparatus according to Claim 12 which is constructed of low, ambient or high temperature resistant material, preferably selected from metal oxides, and in particular quartz, refractory oxides and graphite being reinforced with a suitable material providing mechanical strength and integrity. 14. Apparatus according to any of Claims 1 to 13 which comprises one or more reservoirs adapted to comprise source material and a crystal of seed material supported on a surface or pedestal adapted to the processing conditions to be employed, allowing convenient and efficient vaporisation. 15. Process for bulk vapour phase crystal growth comprising: providing at least one reservoir of source material and at least one sink or seed crystal, each associated with independent temperature control means; and transporting vapour phase material between source and sink or seed; and providing in-situ means for monitoring growth of crystal, at each sink or seed which is substantially non-intrusive in terms of temperature conditions at the sink or seed wherein vapour is transported through a path which deviates by an angle of at least 5 , preferably 5° - 270°, more preferably 30°- 180°, most preferably 45°- 110°, for example 60°- 95° along the length thereof between source and sink crystals. 16. Process for bulk vapour phase crystal growth comprising: providing a plurality of reservoirs of same or different elemental or compound source material(s) and a single sink or seed crystal, each associated with independent temperature control means; and transporting vapour phase material between each source and the sink or seed; and providing in-situ means for monitoring growth of crystal at the sink or seed which is substantially non-intrusive in terms of temperature conditions at the sink or seed. 17. Process according to Claim 14 or Claim 15 wherein means for monitoring radiation and transport for any given sink or seed is by coincident monitoring and transport path for at least the portion of the respective paths proximal to the sink or seed, as hereinbefore defined. 18. Process according to Claim 16 or 17 operated at reduced, ambient or elevated temperature and at reduced pressure, preferably 0.01 to 100 mbar reduced pressure. 19. Method for starting up a process according to any of Claims 15 to 18 in manner to establish a sufficient vapour pressure above source material to initiate transport. 20. Method for starting up a process according to any of Claims 15 to 18 in manner to establish transport control and temperature control in the sink zone for controlled growth at the sink seed. 21. Method for controlling a process or method according to any of Claims 15 to 20 by direct reading of process variables, comparison with optimum values of process variables for a desired crystal growth, for example, with use of a process model, and on line optimisation thereof. 22. Apparatus, process or method according to any of Claims 1 to 21 adapted for bulk vapour transport techniques, preferably for growth of crystals employing bulk vapour transport techniques, such as semiconductor or optical crystal growth, more preferably for growth of crystals from polycrystalline binary, ternary or other multinary compounds. 23. Apparatus, process or method according to any of Claims 1 to 21 adapted for growth of crystals selected from compounds of groups HA, EB, ILL V, VI and VTJ and from compoimds of Group TV , more preferably of groups II and VI and from Group IV of the Period Table of the Elements, for example selected from Be, Mg, Zn, Cd, Hg, S, Se, Te and I and selected from Si and C. 24. Equipment for use in monitoring crystal growth with the apparatus, process or method as hereinbefore defined in any of Claims 1 to 21. 25. Crystal grown with the apparatus, process or method as hereinbefore defmed in any of Claims 1 to 21.

Inventors: MULLINS JOHN TOMLINSON, MULLINS, JOHN, TOMLINSON
Applicants: MULLINS JOHN TOMLINSON, UNIV DURHAM, MULLINS, JOHN, TOMLINSON, UNIVERSITY OF DURHAM
Application year: 1998
Main classification: C30B23
IPC codes: C30B23