Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – Fully-sealed or vacuum-maintained chamber
Reexamination Certificate
2000-02-18
2002-04-23
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
Fully-sealed or vacuum-maintained chamber
C117S201000, C117S104000, C117S105000, C117S952000, C117S953000, C118S715000
Reexamination Certificate
active
06375739
ABSTRACT:
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 monitoring, crystals grown with the apparatus or process, in particular for use as a semi-conductor or optical crystals, and the use of known and novel monitoring 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 materials with for example cadmium telluride (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 in 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 described in W. W. Piper and S. J. Polich, J. Appl. Phys. 32 (1961) 1278 (see
FIG. 1
below). The source and sink are at different temperatures and therefore have different equilibrium vapour pressures. This vapour pressure difference provides the driving force for growth.
This approach results in certain fundamental problem for growth of crystals such as CdTe:
The equilibrium vapour composition of CdTe is non-stoichiometric except at one temperature, the congruent evaporation temperature which is described in more detail in D. de Nobel Philips Res. Repts. 14 (1959) 361. Due to the law of mass action:
[Cd]Te
2
]
½
=K(T) (1)
where [Cd] and [Te
2
] are the concentrations of cadmium and tellurium vapour respectively and K is a constant depending on temperature, T. N. Yellin and S. Szapiro, J. Crystal Growth 69 (1984) 555 have reported that minute deviations from stoichiometry in the bulk source material result in large variations in the composition of the vapour making the transport and hence growth highly non-reproducible. Furthermore, this effect gives rise to non-stoichiometry in the growing crystal which has a detrimental 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 in growth rate. Alternatively, control of the axial temperature gradient is also difficult in 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 in 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 requiring analysis of grown crystals. This problem is compounded, in a system without in-situ monitoring, by any change in conditions during 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) in F. Rosenberger, M. Banish and W. M. B. Duval, NASA Technical Memorandum103786. Their design was a tubular system with a flow restrictor between source and seed continuously pumped via a second flow restrictor immediately downstream of the seed crystal (FIG.
2
.). The continuous pumping in conjunction with a suitable sizing of the downstream flow restrictor removed a small proportion of the source material and in particular any excess component preferentially thus maintaining the vapour phase nearer stoichiometry. The first flow restrictor acted to make the mass transport rate relatively insensitive to the temperatures of the source and sink and their difference. If not restricted in this way, in a system operating under near stoichiometric conditions, appropriate 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 in the flow modelling of the system and its restrictors made indirect determination uncertain.
4. The quartz ware was complex, not easy to use and vulnerable in application.
In-situ optical monitoring is known and routinely employed in other methods for crystal growth, such as low temperature and thin 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 prior 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-conducting materials with effective temperature and stoichiometry control.
We have now surprisingly found that an apparatus and method for vapour phase crystal growth may be provided which enables in-situ monitoring in non-intrusive 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 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.
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. 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 appropriate cooling and/or heating control.
Preferably means for in-situ monitoring of crystal growth comprises means providing visual and/or radiation access to the growth zone but located remote therefrom. More preferably means for direct monitoring of crystal growth comprises at least one passage for monitoring communica
Jacobson & Holman PLLC
Kunemund Robert
University of Durham
LandOfFree
Apparatus and process for crystal growth does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and process for crystal growth, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and process for crystal growth will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2842129