The growth of gallium phosphide layers of high surface quality by liquid phase epitaxy

Journal of Crystal Growth 27 (1974) 193-204 9 North-Holland Publishiny Co.

THE GROWTH OF GALLIUM PHOSPHIDE LAYERS OF HIGH SURFACE QUALITY BY LIQUID PHASE EPITAXY A C O M M E R C I A L PROCESS FOR GREEN LICHT EMI'Iq'ING DIODES A. M O T T R A M and A. R. P E A K E R

Ferranti Ltd., Chadderton, Lancashire, Enyland Received 15 April 1974; revised manuscript received 26 June 1974 Equipment has been developed for the liquid phase growth of epitaxial layers of gallium phosphide on a commercial scale. A vertical dipping system is used with substrates held in a horizontal plane in a multi-tier configuration giving effectively closed cell growth conditions. Emphasis has been placed on the economical use of the melt and on establishing simple loading and growing procedures. Although the equipment has been used for the growth of metal oxides, gallium arsenide and double epitaxy gallium phosphide, its principal application is in the growth of layers of sulphur-nitrogen doped gallium phosphide for subsequent photoprocessing; in this application surface quality is of prime importance. An analysis of over a thousand layers grown in the equipment has shown that surface quality depends primarily on substrate orientation and preparation technique, the distribution of dislocations in the substrate and nitrogen concentration in the grown layer. The growth cell and crucible geometry enable conditions to be established very simply which are well outside the constitutional supercooling regime with the result that stable growth interfaces and high surface quality can be achieved routinely on commercially available LEC substrates.

1. Introduction

Many liquid epitaxy techniques have been used for the growth of gallium phosphide. In general these fall into three main categories, the tipping process first described by Nelson1), the dipping method 2) and the sliding boat technique3). All three have been'shown to be capable of producing layers suitable for the fabrication of light emitting diodes and all three are, in principle, capable of being scaled up to cope with commercial quantities of material. Recently prototype systems have been developed with this aim using the sliding boat system with thin aliquot melts 4) and a version of the tipping system using a multi-slice rotating boatS). This paper describes a multi-slice LPE technique using a vertical dipping system with horizontal substrates currently used for commercial pilot scale production. The original growth geometry was first developed for laboratory u s e 6) in order to produce layers of high surface quality suitable for photoprocessing, but has subsequently been shown to be suitable for large scale growth of gallium phosphide while similar equipment has been used for gallium arsenide and metal oxides. Liquid phase epitaxy initially gained favour as the preferred method of growth of gallium phosphide

LED's because of the problems of incorporating oxygen (necessary to create the zinc-oxygen pair responsible for the high radiative recombination efficiency in the red in this material) in either the halide or hydride transport processes ofvapour growth. Major problems also exist in achieving high recombination efficiencies in the green vapour phase techniques and although significant advances have been made 7,s) considerably better results are consistently obtained from LPE 9-11). In addition recent work 12) can be interpreted to suggest that the equilibrium vacancy population may place a fundamental limit on the efficiencies to be achieved from vapour phase growth methods. Although there is a marked technological advantage in favour of liquid phase epitaxy for gallium phosphide, the commercial aspects are far less clear particularly in view of the well established large scale vapour growth techniques used for silicon and gallium arsenide phosphide epitaxy. The aim of this paper is to specify the commercial requirements of gallium phosphide epitaxy for green light emitting diodes and to describe a system, the horizontal spade vertical dipping system, which fulfills most of these requirements. 2. Material and gro~vth system requirements In a commercial growth system the prime aim is to 193

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A. MOTTRAM AND A. R. PEAKER

Fig. 1.

Epitaxial layers grown on LEC substrates before and after photoprocessing.

minimise the cost contribution to the final product while maintaining adequate technical performance. In liquid phase epitaxy in particular this is a very different requirement to minimising the cost of the grown layer. For example an improvement in deposition efficiency resulting in a more economic use of the melt could, in some circumstances, impair the surface quality resulting in a substantial reduction in yield in subsequent processes and an overall cost increase. The process requirements for low cost green emitting gallium phosphide dice have been discussed previouslyl3). Three salient features are the requirement to use batch processing, the ability to on-slice test, and compatability with existing silicon and gallium arsenide phosphide mounting, bonding and encapsulation procedures. The only process that fulfills these requirements using existing technology is a masked zinc diffusion into a single n-type epitaxy layer. To obtain high yields in this process necessitates very high surface quality in the sense that the layers must be both smooth and flat with undulations of not more than +21am. Fig. 1 shows slices of photoprocessed and "as grown" epitaxial material. The surface flatness and overall quality can be judged from the undistorted arcs which are.cutting marks on the rear face visible through the slice. Given good surface quality reproducibility depends to a large

extent on doping level; fortunately the electroluminescent efficiency is not a very rapidly moving function of shallow impurity concentration so that although dopant uniformity in the junction plane is desirable it is by no means critical and a variation of 4- 40 7o can be tolerated. A major problem in production equipment design is the variation in slice diameter. Although effective automatic diameter control equipment will start to become available for liquid encapsulated Czochralski pullers within the next year t4) it seems unlikely that diameter control will be as good as in silicon or that close diameter tolerances will be able to be met by ingot grinding, partly because of problems of strain propagation and partly because of the high intrinsic cost of the material. Consequently any commercial LPE system must be able to deal with a range of slice sizes currently up to about two inches in diameter. In addition of course the equipment must be reliable, give reproducibl e results, be economic in its use of materials and give a high output per unit investment, i.e. the equipment cost divided by its output should be minimised assuming the labour content is low as in most crystal growth processes. 3. Gro~vthsystem The growth system has been designed for ease of operation and for optimum temperature control in the

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substrate region together with rapid thermal cycling time. Fig. 2 is a schematic representation of the growth equipment and shows simply a horizontally held substrate in a conventional vertical dipping system. This technique immediately combines the advantage~ of the ease of operation and accessibility of the vertical dip with the minimal temperature fluctuations observed with horizontally held substrates. The resistance or induction heated furnace surrounding the reactor tube is controlled to give a precise thermal gradient both through the depth and immediately above the melt. The extent and direction of this gradient affects the growth rate, the crystalline quality, the surface morphology and the incorporation of growth defects into the layer. For adequate growth rates to be obtained without the early onset of constitutional supercooling, a positive gradient (i.e. surface hotter than base) of 4 ~ c m - t is used with the melt in situ. The temperature profiles must obviously be established with the melt in situ, but for convenience a standard furnace profile can be equated to suitable thermal symmetry within the melt. A typical profile is shown in fig. 2 in relation to the reactor tube.

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Accurate control of the melt temperature is obtained with a thermocouple situated in contact with and immediately below the crucible. Control within the furnace winding is not sufficient for this type of process in which rapid heating and cooling schedules are called for. A monitor thermocouple may also be held below the melt crucible whilst continuous monitoring of the substrate temperature is possible via the substrate holder stem. The substrate holder, which can vary in design to accept a range of substrates, is held through a double compression fitting in a stirring gland. With correct alignment of the apparatus efficient stirring o f the melt can be obtained throughout the growth process. A reciprocating action is used to ensure thorough mixing and to prevent melt streamlining as occurs with unidirectional stirring. Melts up to 5 cm diameter have been used in both vitreous carbon and high purity silica crucibles. In order to eliminate uncontrolled back etching of the substrates and to provide ease of operation, the melts used have always contained a considerable excess of gallium phosphide. The solubility and crystallisation of GaP in gallium under the dynamic conditions normally used with this growth system varies from the equilibrium values determined by Hall 15) and a plot of these values is shown in fig. 3. The solubility point is taken as the temperature at which the last particles of GaP disappear from the surface of the melt during a temperature rise of 5 ~ min-1, and the first crystallisation

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as the temperature at which crystals first appear on the surface of the melt during a similar rate of fall of temperature. The difference between the "solubility" curve and that given by Hall is explained by the non-equilibrium conditions under which the figures were taken, the difference between the two increasing at higher temperatures as expected. However the crystallisation curve should lie on or below Hall's solubility curve. This is in fact the case at temperatures lower than 950 ~ but not above, where crystallisation appears to occur before the solubility curve is reached. In this case, crystallisatibn on the surface of the melt in an undersaturated solution can probably be attributed to surface cooling by radiation from the large surface area of the melt. In the normal growth process, in which the substrates are inserted into an oversaturated melt, the degree of undercooling required before either growth on the substrate or free crystallisation occurs is of the order I0-12 ~ at melt temperature of 1040 ~ this value being the temperature difference between the complete dissolution and first crystallisation curves shown in fig. 3. The degree of undercooling recluired to promote growth decreases with increase in melt temperature and hence with increase in solubility but in no instance does it become as small as the figure quoted by Crossley and Small 16) of <0.25 ~ for GaAs in gallium at 777 ~ at which temperature the solubility of GaAs in gallium is very similar to that of GaP at 1040 ~ The substrate holder can be adapted to suit the size and number ofsubstrates to be used. The types used so far have been designed to accept a variety of sizes and thicknesses during one dip, As the size and uniformity of single crystal substrates improve then the holders can obviously be modified to improve the packing density in the melt. Multi-platform designs usually have plates spaced at 2-3 mm intervals, which in effect give the closed cell conditions advocated by Minden ~7) in his tipping system. Measurements of the epitaxial layers grown within and outside the "closed ceil" during a single dip show the restricted melt depth to give more uniform thickness and improved surface quality. Substrates of 4.5 cm diameter have been accommodated on substrate holders up to three platforms deep within the present growth system. The apparatus can obviously be extended and modified to greatly increase its capacity and offers the advantages of ease of opera-

tion and rapid turn-round time over other liquid epitaxy systems. 3.1 GROWTtt PROCEDURE The majority of growths have been on (100) or (11 l)B oriented substrates, the (100) orientation being preferred for device fabrication because of its cleavage habit and non-preferential etching behaviour. Prior to growth these are polished with 1 tam diamond and then either chemically polished with 1 ~o bromine/methanol solution or etched in 50~o HCI/HNO3 (2:1) a t 50 ~ immediately prior to growth. Providing the parallelity and surface flatness of the substrates are controlled within limits of +0.002" and 20 tam respectively, no intermediate lapping stage is required. The substrates are mounted, growth surface uppermost, on the holder and held in the reaction tube above the furnace region. The melt is held on a pedestal, central in the furnace and in the appropriate region of its profile. After thorough flushing with high purity hydrogen or a hydrogen/nitrogen mixture, the melt is temperature cycled as in fig. 4. Dopants may be introduced either as solids in the melt (S, Te, Zn) or as vapour via the gas stream (N, Zn). After the melt has reached thermal equilibrium (A) the substrates are lowered slowly to within 1 cm of the melt surface and preferably within the confines of the crucible. The relatively slow rise in temperature of the substrates (~300 ~ out of the furnace zone to ~ I040 ~ at the melt surface) prevents both severe strain effects and rapid condensation of vapours on their surface. Holding the substrates in a relatively cool region of the reactor tube during melt heat up also prevents deterioration of their surface due to thermal decomposition. Immersion of the substrate occurs at B, after thermal equilibrium with the melt has been reached, this is followed by an etch back C, the controlled cool D and withdrawal at E. The substrate holder is normally 4-5 mm less in diameter than the melt. This means that upon immersion the "scum" normally associated with Ga/GaP melts is depressed into the melt by the undersurface of the spade and clean saturated Ga/GaP solution is forced round the sides of the substrate holder and over the substrates. This can be seen very effectively via the viewing port and results in uniform wetting of the substrates with freedom from particulate matter in the growth region. The cooling rate normally used is from

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1.2 ~ - t to 18 ~ min -~. Below 1.2 ~ min -~ the back etch and run to run repeatability are difficult parameters to control, whilst rates above 18 ~ min-1 result in gallium trapping and highly defected layers due to the gross effects of constitutional supercooling. The growth rate of a particular epitaxial layer is dependent on a number of factors, amongst them being the substrate morphology and orientation, thermal profile, cooling rate and the melt temperature. Whilst these can obviously lead to a great deal of variation between different liquid epitaxy systems the use of a constant growth procedure and variation of the withdrawal temperature has enabled a plot of average growth rate against melt temperature to be established for the system for cooling rates between 5-10 ~ rain- t. This is shown in fig. 3 where it can be seen that below 900 ~ there is a negligible growth rate whilst above 1000 ~ rates of the order 1 I-tm ~ are obtained. At cooling rates of 1-2 ~ rain- ~ these values are approximately doubled and growt.h continues at lower temperatures. The epitaxial layers can be withdrawn at any temperature and a.summation of the values given on the curve between the point of first crystallisation, which is normally 10-12 ~ below the immersion temperature, and the withdrawal temperature gives a reasonable approximation of the thickness grown. An interesting feature of this technique is that during withdrawal of the epilayer from the melt, the excess gallium runs away from the surface of the layer, leaving an as grown surface topped by a self supporting crust of gallium phosphide; an example of which is shown in fig. 5. This is attributed to localised surface cooling of the melt by radiation. The ability to withdraw epitaxial layers at any predetermined temperature, besides giving control over thickness, eliminates one of the

Fig. 5. Surface crystallisation of gallium phosphide formed on the melt, shown after withdrawal of substrates and holder. major causes of poor surface morphology in LPE, i.e. the onset of constitutional supercooling at low solution temperatures. 4. Surface morphology As the ultimate yield of devices from a slice of semiconductor is closely related to the morphology and uniformity of its surface it is obviously an extremely important criteria on which to judge the effectiveness of any growth technique. Previous workers have attributed poor surface quality to a variety of causes, the principal ones being constitutional supercooling and cellular growth18-2~ misorientation of the substrate2~'22), thermal decomposition of the slice prior to growth and poor substrate preparation4), inadequate wetting and non-uniform meltback, phase changes due to high impurity levels and degree of supersaturation~6). We have found that all these conditions in extreme can cause marked deterioration of the surface. However in the growth system described simple working tolerances can be established to ensure the reproducible growth ofepitaxial layers of gallium phosphide suitable for photoprocessing without further surface preparation, provided substrates of adequate quality are available. The majority of layers grown have had smooth surfaces of low defect density and thicknesses to _+2 ILtm over > 80 ~o of the area. The edge of the slices invaria: bly exhibits a deterioration in growth quality and change in growth thickness, this being related to the increase

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in dislocation density at the periphery of commercially available LEC pulled substrates. The substrates used have had etch pit densities of between 10'~-106 cm -2

resulting in epitaxial layer etch pit densities of -,~ 103 over the majority of the area, usually increasing to 104 in peripheral areas. Layers grown have been typically 40-60 lam, the actual thickness being a function of the many parameters mentioned previously. Whilst a large number of layers exhibit smooth surfaces, a microscopic examination of these do in fact reveal the usual ripple or tide-like pattern of liquid epitaxy processes. In addition to these a number of layers continue to exhibit severe surface faulting. Both the overall surface morphology and the more severely defected areas can generally be attributed t o three factors in the growth (i) substrate character, (ii) initial nucleation, (iii) interface instabilities associated

THE G R O W T H OF G A L L I U M P t l O S P I I I D E LAYERS

with the growth process. Optical microscopic examination and X-ray topographical analysis of the epitaxial layers have shown that the major surface defects directly attributable to the substrate can be separated into those due to misorientation of the substrate, those due to the substrate dislocation structure and those due to work damage generated during the cutting, polishing and handling stages. A series of epitaxial layers grown on a range of (100) orientated substrates were investigate d with respect to the effect of orientation on final surface quality. The angle 0, between the normal to the Surface of the (100) direction and the nearest (010) or (001) plane, in a clockwise or anticlockwise direction were measured by X-ray diffraction. The results were then compared to a visual assessment of the epitaxial layer surface, with particular attention being paid to the presence of gallium inclusions and faceting. A correlation between the overall surface quality and the angle 0 was established in which the surface deteriorated as the angle increased. Values below 0.7 ~ could be associated with smooth surfaces, wide ripple spacings and a minimum of gallium trapping at the surface. Values between 0.7 ~ and 1.2 ~ could be associated with the appearance of fairly deep undulations and some gallium inclusions across the surface with the onset of facetting at the edges of the slice. Values above 1.27 gave poor surfaces, containing both deep. gallium inclusions and severe facetting. In extreme cases where 0 > 4 ~ the surface consisted of both very poor regions and uncharacteristically smooth areas, the latter being due to facets of growth exactly on the (100) plane. Fig. 6 shows surfaces of epitaxial layers grown on substrates of varying degrees of misorientation. A cleaved section of layer, similar to that shown in fig. 6e is shown in fig. 7a illustrating the ridge and facet structure, a line parallel to the substrate epitaxial layer interface has been drawn to show the misorientation. A less detailed study of growth on the (II1)B surface has shown that while similar effects still exist, the final surface quality is not so critically dependent on orientation as the (100) plane. Growth on the (110) plane, as would be expected from these results, proved to be extremely facetted and heavily faulted with gallium trapping. Although overall reductions in etch pit density of an epitaxial layer compared with the initial substrate are always observed, the presence of regions of high defect

199

Fig. 7. Cleaved sections of (100) epitaxial layers showing growth defects. (a) Ridge and facet formation due to misorientation; the angle between the growth face and the substrate-layer boundary is 5 ~ (b) Complete break up and gallium inclusions due to severe polishing damage, similar effects are produced by dislocation clusters grown into the LEC substrate. (c) Onset of constitutional supercooling partway through growth.

density, particularly those observed towards the periphery of a slice are invariably transmitted to the epitaxial layer in the form of much earlier crystalline break-up and gallium incorporation. This is illustrated in fig. 7b which shows a cleaved and stained cross section of such an area. The gross defects attributed to cutting, polishing or severe handling which can be traced by etching in K4FeCn6/KOH/H202a) at each stage, have been observed both optically and by X-ray topographs. These have proved difficult to remove by

200

A. MOTTRAM AND A. R. PEAKER

standard polishing techniques and are invariably transmitted into the epitaxial layer in the form of poor growth regions. Diodes made in these regions exhibit poor optical and electrical characteristics due to diffusion spikes and leakage paths. A detailed examination of the relationship between surface morphology and the initiation of growth in a GaAs liquid epitaxy system has been carried out by Crossley and Small24); in this they conclude that although the first few microns of growth are critically dependent on the supersaturation conditions within the melt, a gradual smoothing over of any irregularities occurs as the growth proceeds. In the system described here for GaP, the initiation of growth occurs during a fairly rapid cool at approximately l0 ~ below the substrate insertion temperature. This effectively means that growth commences in a supercooled solution which in turn eliminates the island structure attributed to the initiation of growth in equilibrium condition. The main problems associated with the degradation of growth due to substrate surface dissociation and uniform wetting are overcome with this technique in that the substrate is both held at a reduced temperature and protected from the main vapour stream, and only clean gallium solution from the lower regions of the melt flows over the substrate upon insertion. Fig. 7c shows a cleaved section of a layer in which an unstable interface has developed at the lower growth temperatures due to constitutional supercooling conditions in the melt. The careful control maintained over temperature profiles and the ability to withdraw from the melt at elevated temperatures obviously reduce the probability of these mechanisms occurring. In summary the appearance of the ripple-like structure on the surface of liquid epitaxial layers has been attributed to a number of factors. Solomon and DeFevere 18) identified them as gallium inclusions resulting f r o m constitutional supercooling, Donohue and Minden ~9) attributed it to cellular convection in the growth solution, Andre and LeDuc 2~ reported that the irregularities were due to constitutional supercooling and could be removed by introducing specific thermal gradients across the melt-substrate interface, whilst Saul and Roccasecca21) and Peters22), as reported previously have correlated the ripples with substrat.e orientation. The observations reported here indicate that the fundamental cause of ripple structure is substrate ori-

Fig. 8. Surface deterioration due to nitrogen incorporation: (a) initial dendrite formation; (b) individual dendrite at an advanced stage of growth.

entation, but poor thermal control and excessive growth rates at low solubility levels intensify its appearance. The growth of sulphur-nitrogen doped epitaxial layers has shown the incorporation of nitrogen to be a further important factor in the control of surface morphology. Epitaxial layers grown with nitrogen levels higher than that predicted by the solid solubility curve of nitrogen in gallium phosphide by StringfellowE5), always exhibit a rapid deterioration in surface quality. This means that in a continuously cooling system, layers must be withdrawn at a sufficiently high temperature to avoid the nucleation of a secondary phase at

201

TIlE GRO~,VTH OF G A L L I U M P I I O S P I I I D E LAYERS

the growth interface. Cooling to temperatures at which the maxinaum solid solubility is exceeded results in the spurious nucleation of dendrites on the surface and ultimately in break-up of the single crystal structure. The optimum nitrogen concentration for device efficiency appears to be coincident with the onset of dendrite formation and this is illustrated in fig. 8a in which the visible point defects are illustrative of the start of dendritic growth. Fig. 8b shows a dendrite at an advanced stage of growth.

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5. Sulphur-nltrogen doping As has already been mentioned the doping requirements for light emitting diodes are not particularly stringent, a tolerance of ___40~o being acceptable for both nitrogen and shallow donor concentrations. Logan 26) originally determined that the donor species had a significant effect on the efficiency in double liquid epitaxy diodes and we have confirmed that this is also true for diffused devices. Consequently sulphur has been used as the donor impurity in commercial work for green emitting devices. The effective distribution coefficient, k*, for growth onto a (100) substrate from gallium solution for sulphur at 990 ~ has been measured by Sudlow et al. 17) in a comparable system to be 0.2. Assuming a growth of an area of 25 cm 2 of material per run in a 100 g melt and assu.ming no volatile species of sulphur are lost the fall in dopant level after twenty runs is only calculated to be 5 ~o and in practice the change is generally less than this. Consequently replenishment o.f sulphur is not necessary unless more than fifty runs are undertaken from a single melt. Multiple use of a melt necessitates that considerable care be exercised to avoid a build up of contaminants which could affect not only the free carrier concentration but also the deep level population. It cannot be hoped to achieve laboratory standards of equipment preparation in a production environment and this is reflected in the background impurity level which is typically an order of magnitude higher than that reported for experimental work. Fig. 9 shows the background level profiles measured with a Schottky diode technique 28) of layers grown in the apparatus and various crucible and furnace tube configurations. These are the stabilised profiles, i.e. the profiles after one or two initial temperature cycles of the melt and remain

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substantially constant for the melt life. This is achieved by the use of high purity gases for enveloping the melt at temperature, the major constituent hydrogen, being palladium diffused immediately before entering into the furnace tube. From Hall analysis the dominant background impurity appears to be silicon transported by the reaction between silica and hydrogen 29) although there is evidence from the photoluminescent spectra of significant concentrations of sulphur and carbon even in the apparatus constructed wholly from synthetic quartz. In terms of shallow donor concentration this is acceptable as the level of compensation in the undoped material is approximately 20 ~o and the required free carrier concentration is almost an order of magnitude higher than the background. Consequently sulphur can be added to the melt and will be the dominant d o n o r species in the grown layer. Fig. 10 shows the net ionised impurity profile of such a layer. The shallow donor and acceptor profiles of layers grown in a dipping system with a vertical spade have been studied in detail elsewhere27), and the present work is in agreement with previous results bearing in mind the greater influence of solutal convection as a stirring mechanism in vertical spade systems. The problem of nitrogen incorporation is a much

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more intractable one, the system being dynamic requiring nitrogen to be incorporated during growth by reacting ammonia with the gallium melt. The reaction kinetics and some experimental results have been discussed previously by Stringfellow25) and Logang), however although high nitrogen concentratio.n is desirable from the point of view of achieving a high luminous efficiency in the diode, growth quality deteriorates with increasing nitrogen concentration, a point which was discussed in the last section. Consequently it is desirable only to incorporate nitrogen non-uniformly in the layer so that the concentration is low initially but increases in the region in which diffusing carriers will be present in the final device. Fig. 10 shows a nitrogen concentration histogram of an epitaxial layer measured on the Lightowlers scale 3~ by the absorption technique. The dynamics of the non-equilibrium conditions used to control nitrogen profiles are discussed in more detail in a separate publication 3~). 6. Diode performance and economic assessment The criteria by which any commerical growth system must be judged are the yield and performance of the finished devices. Many other factors affect these parameters besides the growth but in LED manufacture the quality of material dominates the light output perfor-

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Fig. 1 !. Light output distribution o f diffused diodes driven at 20 mA. The data are taken from production runs probe tested 'on slice'. Results for two melts are shown A (I) and A (16) being the first and sixteenth growths respectively while B (2) was the second growth from a melt used five months later with the same composition and procedure as A.

mance. Fig. 11, which gives luminance distribution data for 0.018" dice structures, illustrates the consistency of results achieved from the multiple use of a single melt and the melt to melt reproducibility. The absolute performance of the devices produced by this process is good compared to those fabricated by other commercial growth techniques, from the vapour phase for example. It must, of course, be remembered that the luminance figures quoted in fig. 11 are for a low current density in an unencapsulated planar structure. The devices in this state have a typical conversion efficiency of about 0.03 Yo- Encapsulation and mesa etching increase the light extraction efficiency by a factor of four

203

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to five and it is in this condition that the diodes should be compared with the best laboratory figures 1~ which are around 0.3 ~ at comparable current densities (the output is superlinear with current). The light output data is read out from an automatic test machine in a way which grades light output after the devices have passed a number of other tests, reverse leakage, forward leakage and forward voltage drop for example. It is interesting to note that of the devices that pass these tests only something like 2~o have a light output less than 600 cd m -2. The other mode of failure which can be attributed primarily to the LPE growth quality is the reverse leakage characteristic. Failures for this parameter are caused by dendrites, pits and growth lips but amount normally in total to less than 5 ~o. By far the greatest contribution to the cost of LPE processes for the III-V compounds comes from the starting materials. In the case of gallium phosphide the cost of single crystal slices is the most significant. The only related parameters in the LPE process to reduce this cost item are the overall yield which, as discussed earlier, is a function of the surface quality and the ability to use slices of varying sizes. The second largest cost is the melt which contains gallium and polycrystalline gallium phosphide. Previous workers have expressed the usage of gallium as the weight of melt employed and the usage of gallium phosphide in terms of the deposition efficiency t11, defined as the ratio of gallium phosphide deposited on the substrate to the amount of gallium phosp.hide released from the melt calculated from the solubility curve over the appropriate temperature range. Neither of these parameters provide adequate data for economic assessment especially if there is multiple usage of the melt. More appropriate terms are the melt deposition efficiency tlz defined as the ratio o f the total weight o f epitaxial layer grown to the total weight of gallium phosphide used in the melt. This is an important point as the most difficult and time consuming operation in the whole process is the weighing and loading of the melt together with its dopants. The design of the substrate holder permits only liquid free from surface particulate matter to flow into the substrate holder cells and so it is of considerable advantage to add sufficient excess of gallium phosphide to be adequate for the total number of runs. This in itself effects a significant improvement in

production yields as it reduces the required accuracy of weighing compared to the exactly saturated melts often used in LPE. Consequently whilst Ill is low, because o f slow diffusion of phosphorus in gallium and consequent nucleation elsewhere in the melt particularly on those parts of the quartz holder which are both cooler than the substrate and also wetted by the melt I1z increases with the number of runs for which each melt is used, provided an adequate excess of gallium phosphide is available initially and some sulphur is added every fifty runs or so. The theoretical maximum efficiency is given by: t12 max = A d p n / ( A d p n + x W ) ,

where A is the slice area per run, d is the growth thickness, p the density of gallium phosphide, n the number of runs, x the solubility of gallium phosphide in gallium at the maximum growth temperature and W the melt weight. Using Hall's Is) value for x at 1040 ~ and our experimental value for d at the same starting temperature (401am) and assuming that it is intended to grow 25 cm 2 per run in a 150 g melt then: 112,=~x = 1/[1 +(15.7/n)'l; as n --* co, ~lzm~ ~ 1 but in practice n is limited by factors such as contamination and gallium loss on removal of the substrate holder to a maximum of about 50 runs. Fig. 12 shows the deposition efficiencies I1, 70--

~2 6O

/ / /

-

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5O

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i-

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/+

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t

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NUHBEA OF GROWTH RUNS

Fig. 12. Deposition efficiencies in the system, thelines represent predicted performance, the data points measured values (e)qa,

(+),l,-

204

A. MOTTRAM AND A. R. PEAKER

a n d 112 plotted for a melt intended for 30 runs each of 25 cm 2 of material. Ih is a function o f substrate area and because the layer thickness is almost i n d e p e n d e n t o f grown area in the cell type substrate holder, ilt cc A, hence the low value for the first growth which is a test run using only 2 cm 2. G a l l i u m usage is also low, a m o u n t i n g to an average o f 0.25 g per cm 2 o f g r o w n layer, considerably less t h a n in previously described systems. In a d d i t i o n the discarded melt is sufficiently pure to be reclaimed by filtering a n d a simple recrystallisation method o f purification.

7. Conclusion

The system described has been used for several thousand growth runs, it has provedits elf to be reliable and reproducible on a pilot production scale and work is in hand to extend the capacity of the apparatus and reduce the temperature cycle times in order to produce a full scale p r o d u c t i o n unit. The system provides excellent surface quality while m a i n t a i n i n g good e c o n o m y in the use o f starting materials with small l a b o u r a n d e q u i p m e n t costs. A l t h o u g h considerable i m p r o v e m e n t s in t h r o u g h p u t are feasible the system is in o u r experience already competitive with existing v a p o u r growth systems for gallium phosphide green emitting material. Acknowledgements We would like to t h a n k o u r colleagues at F e r r a n t i for the very considerable assistance they have given us in this work. In particular we would like to t h a n k A. Lees for his c o n t r i b u t i o n to the d e v e l o p m e n t o f the e q u i p m e n t a n d T. Hayes for his work on nitrogen incorporation.

References !) H. Nelson, RCA Rev. 24 (1963) 603. 2) H. Rupprecht: in: Galliton Arsenide, Proc. lntern. Syrup., Reading, 1966 (IPPS, London, 1967) p. 57.

3) M. B. Panish, 1. Hayashi and S. Sumski, J. Quantum Elec tron. 5 (1969) 310. 4) A. A. Berg, R. H. Saul and C. R. Paola, J. Electrochem Soe. 120 (1973) 1558. 5) O.G. Lorimor, R. It. Saul, L. R. Dawson and C. R. Paola, Solid State Electron. 16 (1973) 1289. 6) A. R. Peaker, A. Mottram and P. D. Sudlow, British Government Report on CVD Contract CP1708 Project RPI-64 (1970). 7) R. Nicklin, C. D. Molsby, G. Lidgard and P. B. Hart, J. Phys. C 4 (1971) 16. 8) M. G. Crawford and W. O. Groves, Proe. IEEE 61 (1973) 862. 9 9) R. A. Logan, H. G. White and W. Wiegmann, Solid State Electron. 14 (1971) 55. 10) I. Ladany and H. Kressel, Proc. IEEE 60 (1972) 1101. 1I) O. G. Lorimer, W. H. Hacl. ett Jr. and R. Z. Bachrach, J. Electrochem. Soc. 119 (1972~ 303C. 12) A. S. Jordan. R. Caruso, A. R. Von Neida and M. E. Weiner, J. Electrochem. Soc. 121 (1974) 153. 13) A. R. Peaker and V. Pastore, Digest of Meeting of the Society for Information Display New York (1973). 14) A. J. Fletcher and G. Sturgesr Metals Research Private Communication. 15) R. N. Hall, J. Electrochem. Soc. 110 (1963) 386. 16) I. Crossley and M. B. Small, J. Crystal Growth 15 (1972) 275. 17) H.T. Minden, J. Crystal Growth 6 (1970) 228. 18) R. Solomon and D. DeFevere, J. Electron. Mater. 1 (1972) 16. 19) J.A. Donohue and H. T. Minden, J. Crystal Growth 7 (1970) 222. 20) E. Andre and J. M. LeDuc, Mater. Res. Bull. 3 (1968) 1. 21) R. H. Saul and D. D. Roccasecca, J. Appl. Phys.44(1973) 5. 22) R. C. Peters, in: Proc. GaAs Col~, Colorado, 1972 (Institute of Physics, London) p. 55. 23) M. I. Val'kovskaya and Y. S. Bayarskaya, Soviet Phys.Semiconductors 8 (1967) 1976. 24) I. Crossley, M. B. Small, J. Crystal Growth 19 (1973) 160. 25) G. B. Stringfellow, J. Electrochem. Soc. 119 (1972) 12. 26) R. A. Logan, H. G. White and W. Wiegmann, Appl. Phys. Letters 13 (1968) 139. 27) P. D. Sudlow, A. Mottram and A. R. Peaker, J. Mater Sci. 7 (1972) 168. 28) A. R. Peaker and B. L. Smith, Solid State Electron. 13 (1970) 1407. 29) H. G. B. Hicks and P. D. Greene, in: Gallittm Arsenide and Related Compottnds (Conference Series Institute of Physics, London, 1970) p. 92. 30) E. Lightowlers, J. Electron. Mater. 1 (1972) 39. 31) T. Hayes, A. Mottram and A. R. Peaker, to be published.