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Non-planar crystal growth of Ga0.5In0.5P by metalorganic chemical vapour deposition
......... C R Y S T A L OIROWTH
ELSEVIER
Journal of Crystal Growth 171 (1997) 333-340
Non-planar crystal growth of Gao.5Ino.5Pby metalorganic chemical vapour deposition M.M.G. Bongers *, P.L. Bastos, M.J. Anders, L.J. Giling Unit~ersity of Nijmegen, Facul~' of Science, Research Institute of Materials, Department of Experimental Solid State Physics IlL Taernooiceld 1, 6525 ED Nijmegen, The Netherlands
Received 11 June 1996; accepted 29 July 1996
Abstract Layers of GaosIno.sP have been grown by low pressure metalorganic vapour phase epitaxy on (100) substrates, which where patterned with normal-mesa and re-entrant grooves. The experiments show fast growth at the groove's side walls at temperatures of 680°C and below, whereas all other facets and all layers grown on planar substrates show but moderate growth rates. The large growth rates are attributed to the small preference for the incorporation of Ga over In at (11 l)A-like surfaces, which is enhanced in growth on non-planar substrates. The non-stoichiometric incorporation of Ga and In causes the development of strain and subsequently a growth mode transition from two to three-dimensional growth. The resulting rough surface of the side wall facets enables fast growth, whereby the interaction with slower growing facets through the gas phase in the non-planar epitaxy enables the supply of the necessary amount of growth species. At the higher temperatures the preference for incorporation of Ga at the side wall surfaces disappears, and the growth rates and compositions of the side wall facets becomes roughly equal to the growth rates and compositions of the non-planar (100) surfaces and the planar (111)A substrates. 1. Introduction The growth of I I I / V semiconductor heterostructures on patterned substrates has become an important tool in the manufacturing and integration of semiconductor devices [1-3]. Growth on ridges or V-grooves has proved its usefulness for lasers, waveguides and low-dimensional devices [4-7]. The efforts in embedded growth have been focused to obtain grooves with {011} side walls, which preferentially should be non-growing [8-10]. It is known that in the case of non-planar growth the thickness and composition of the layer may vary throughout the
* Corresponding author.
non-planar pattern, and that this growth behaviour is influenced by the growth conditions [14-16]. Since no overall theory has been developed yet to describe the growth on non-planar substrates, it is of practical importance to study the growth behaviour of semiconductor materials on non-planar surfaces as a function of the growth parameters [20-22]. One of the materials currently under study is GaxInl_~P, grown lattice matched to GaAs ( x = 0.52). An appealing property of GaxIn I_XP is the formation of ordered or disordered layers, which can be influenced by the growth conditions [3,11-13]. This feature, combined with growth on patterned substrates, enables vast possibilities for band gap engineering in laterally confined structures and devices [2-5].
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00703- 8
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In this work the growth of GaInP on GaAs substrates patterned with normal-mesa and re-entrant grooves is studied as a function of temperature. Attention is paid to orientation dependent growth rates and compositions, and their relation to the supply of growth species from the gas phase [18,19] and to surface incorporation processes.
2. Experimental procedure Re-entrant grooves were obtained by photolithography and wet chemical etching of (100) 2 ° (110) GaAs substrates. The mask pattern consisted of 5 100 p~m wide stripes, whose centers where positioned at a 300 p~m distance. Normal mesa grooves were etched along the [011] direction and re-entrant grooves were etched along the [011] direction with respectively 4" 1 " 5 and 1 ' 8 " 1 mixtures of H2SO4'H202 :H20. The side walls of the normal mesa grooves and the re-entrantgrooves, which were oriented along the (111)A and (122)A planes, showed a slight surface damage (Fig. l a) due to the wet chemical etching. The Ga xInl xP layers, nominally lattice matched to (100) GaAs (x = 0.52), were grown by low pressure (20 mbar) metalorganic vapour phase epitaxy (LP-MOVPE) in a horizontal Aixtron reactor. The layers were grown in one experiment on patterned substrates as well as planar (100) 2 ° toward (110), (111)A, (111 )A 2° toward (110), (! 10), ( 11 I)B and (111)B 2° toward (110) substrates. The misoriented (100) surface will be indicated by (100) throughout the rest of the paper, since both planar and non-planar growths were performed using 2°C misoriented (100) substrates. All samples were positioned within a 3 × 3 cm 2 square on the substrate holder in the reactor cell, thus preventing effects of gas phase depletion. The planar (100) layer thickness, the total flow, the total group V partial pressure and the V/III ratio were 1.8-2.4 ~m, 7 slm, 2.0 × 10 -3 and 400, respectively. The group V precursor was phosphine (PH3), and the group III precursors were trimethylgallium (TMG) and trimethylindium (TMI). The temperature was varied from 640°C to 760°C in steps of 40°C. In order to maintain the lattice matched conditions for the planar (100) substrate the gas phase concentration ratio T M G / T M I was dimin-
ished from 0.52 at 640°C via 0.50 at 680°C to 0.48 at 720°C and 760°C. The experiments were reproduced with the GalnP layer separated intermittently by four 0.05 ~m spacer layers of p-type GaInP to outline the growth. Cleaved cross sections of both planar and non-planar samples were examined by a scanning electron microscope (SEM) in order to determine layer thicknesses, surface structures and the development of facets in the grooves. The 50 ~m wide grooves have been studied in detail. The surface smoothness of planar substrates was investigated by optical microscopy. The composition of the layers has been determined by spatially resolved energy dispersive analysis of X-rays (EDAX).
3. Results Fig. 1 shows cross sectional micrographs of the 680°C GaxIn ~ .,P deposition on 50 I~m wide normal-mesa and re-entrant grooves before (a,b) and after growth (c,d). From duplicate experiments in which intermittently grown p-type layers were used (not shown) it was found that the developed facets in both normal-mesa and re-entrant grooves all had a positive growth rate, and that during growth no other facets temporally emerged and disappeared again. At temperatures of 640 and 680°C the growth of Ga~Inl_xP on both normal-mesa and re-entrant grooves is characterized by a fast growing side wall facet (Figs. lc, ld), with orientations ( l l l ) A and (122)A, respectively. These facets are very rough (Fig. 2a, 2b), and display a surface structure that does not show any regular pattern. In contrast, the (100) surfaces, both planar and non-planar, are very smooth. On the (100) surface at the bottom of the grooves sometimes circular or wave-like patterns, which extend towards the groove's side walls, are observed (see Fig. 2b). At the higher temperatures of 720 and 760°C, the growth enhancement of the side wall facets disappears, and the facet surfaces become smoother (Figs. 2c, 2d). At 760 ° the growth follows the contours of the initial groove shape. Apart from the (11 l)A-like side wall facets, several other facets appear in the normal-mesa and re-entrant grooves at the intersection of the (11 I)Alike side wall facet and the (100) top and bottom
M.M. G. Bon gers et al. / Journal of Co,stal Growth 171 (1997)333-340
335
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Fig. 1. SEM photographs of one side of 50 izm wide grooves; (a) and (b) as-etched; (c) and (d) overgrown with Ga~In I , P layers at 680°C (cross sections); (a) and (c) normal-mesa grooves; (b) and (d) re-entrant mesa grooves.
Fig. 2. SEM photographs of one side of 50 Ixm wide grooves, overgrown with Gaxln I ~P layers; (a) and (b) growth temperature of 680°C; (c) and (d) growth temperature of 720°C; (a) and (c) normal-mesa grooves: (b) and (d) re-entrant mesa grooves.
M.M.G. Bongers et al./Journal of Co'stal Growth 171 (1997) 333-340
336
surfaces. In this work the (1 ll)B and (011) facets, which develop at the top edges of the re-entrant grooves (Figs. ld and 2b), are taken along in the description of the growth behaviour of GaxIn ] _~P in grooves. Fig. 3 shows the growth rates of GaInP grown on planar (100), ( I l l ) A , (1 ll)A 2° toward (110), (011) and (11 I)B substrates as a function of temperature. The (100) growth rate increases with an increase of the temperature, despite of the decrease of the total concentration of the group III precursors in the gas phase. The (011) and (111)A growth rates are at all temperatures 25% (or less) smaller than the (100) growth rate, with the exception of the (011) growth rate at 760°C and the growth rate of ( I l I ) A 2°C (110) at 640°C. The growth rates of the exact and off-oriented (111)A surfaces differ at 640°C, but they become equal at temperatures of 720°C and higher. The growth rates of (1 ll)B surfaces, which are equal for exact and off oriented substrates (represented by one symbol in Fig. 3), are always smaller than the growth rate of all other surfaces. No information is available of the growth rate of Ga,Inj_~P on an exact oriented (100) surface, although in other III/V material systems the growth rate of an exact oriented (100) substrate does not differ much from the growth rate of a misoriented (100) substrate. The surfaces of the planar substrates were all smooth, with the exception of the exact oriented (111)A and all (11~)B
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substrates. On these surfaces isolated growth hillocks developed. The hillock densities were low and high in the case of (111)A and (1 l~)B substrates, respectively. The growth rates of the facets encountered in the 50 p~m wide normal-mesa and re-entrant grooves are shown in Fig. 4. Especially at 640 and 680°C the growth rate of the (1 l l)A facet differs remarkably from the earlier given planar growth rate (Fig. 3). At the lower temperatures both the (111)A and (122)A growth rates are much larger than the growth rate of the (100) surface between the grooves (Fig. 4). At the higher temperatures the ( l l l ) A and (122)A growth rates are smaller than the (100) growth rate and they now show a growth rate behaviour which is similar to the planar (11 I)A growth rate behaviour (Fig_. 3). At all temperatures the growth rate of the (111)B facet remains below the growth rates of all other facets in the grooves (Fig. 4). The (01~) facet growth rate, shown in Fig. 4, is almost equal to the (100) growth rate at all temperatures. In the previous paragraphs the (100) growth rate refers to the surface between the grooves. The growth rate of this surface is the same for both normal mesa and re-entrant grooves, but it is about 15% smaller than the growth rate of planar (100) substrates. The growth rate of the (100) groove bottom is even smaller, amounting to 70-80% of the growth rate of the planar (100) surface in the case of 50-100 p~m
M.M.G. Bongers et al. / Journal of Co'stal Growth 171 (1997) 333-340
wide grooves. In Fig. 5 the growth rate of the (100) bottom of re-entrant grooves is shown as a function of groove width at the various temperatures. The growth rate of the (100) bottom facet increases with increasing groove width. At all temperatures the dependence of the growth rate on the groove width is almost the same (Fig. 5), although the absolute growth rates of the (100) bottom surface differ at each temperature. The growth rate of the (100) bottom of wide normal-mesa grooves was equal to the growth rates of the bottoms of re-entrant grooves (given in Fig. 5). For normal-mesa grooves that had a width of 20 txm or less, no (100) bottom facet was present after growth (the grooves were V-shaped), so that the growth rate of the (100) bottom surface could not be determined in these cases. The composition of the GaInP layers grown on planar surfaces is shown in Fig. 6. The (100) surfaces show a Ga content x which is slightly lower than the lattice matched value, but it is within the region where the layer is elastically strained. The layers grown on (111)A and (011) substrates show a slightly higher Ga concentration than the layers grown on (100) at 640 and 680°C, but these layers are also still within the elastically strained region. At 720 and 760°C the composition of the layers grown on (100), (110) and (111)A are the same within the measurement accuracy. The Ga concentration of lay-
337
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ers grown on ( I l l ) B substrates is much lower than the Ga concentration of layers grown on (100) surfaces at all temperatures. The composition of the layers grown on exact oriented and misoriented (111)A substrates are equal, and thus only one point is shown in Fig. 6. Similarly, the layers grown on (1 II)B substrates show no dependence on off-orientation. The hillocks, which are present at the ( I I 1 ) A surface, exhibit a 10% enhanced Ga incorporation as compared to the flat ( l l l ) A layer which grows between the hillocks. In the case of the ( l l l ) B
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x o f G a ~In t _ ~ P l a y e r s g r o w n at f a c e t s in 5 0 a n d r e - e n t r a n t g r o o v e s as a f u n c t i o n o f t h e
338
M.M.G. Bongers et al. / Journal of Co,stal Growth 171 (1997) 333-340
surfaces, the Ga content in the hillocks is equal to the Ga content of the layers grown on (100) substrates. Fig. 7 shows the compositions of (100), (111)A, (111)B and (122) facets, which are measured in the 50 txm wide normal-mesa and re-entrant mesa grooves. Especially at the lower temperatures, large compositional differences are found between the layers grown on planar and non-planar (111)A surfaces. At 640 and 680°C the Ga concentration at the (122) and (111)A facets is much higher than the Ga concentration at the (100) facet, but at 720 and 760°C the composition of these three facets are close to each other. The compositions of the (100) groove bottom and the (100) top surface are the same within the measurement accuracy at all temperatures.
4. Discussion The most interesting feature of the growth of GaxIn~_xP is the large growth rate of the ( l l l ) A and (122)A side walls of grooves at the lower temperatures, as compared to the (100) growth rate (Fig. 4). The fast growing ( I l l ) A and (122)A facets in grooves are found to incorporate an excess of Ga, as compared to the (100) composition (Fig. 7). The growth rates and compositions of the layers grown on the side wall facets can be explained by a local strain related three-dimensional growth mode in concurrence with an enhanced supply of growth elements through the gas phase to the side walls of the grooves. The high growth rates on the (Ill)A-like side wall facets is unique, both with respect to other surfaces on the non-planar substrates that show average growth rates and with respect to the growth rates of planar substrates: although the planar ( l l l ) A growth rate is smaller than the (100) growth rate, the non-planar ( l l l ) A growth rate is much larger than the non-planar (100) growth rate. In contrast all other major orientations show non-planar growth rates that are equal or smaller than the growth rates of equivalent planar substrates. A possible second explanation for the increase of the growth rate at the groove's side wall facets may be sought in a misorientation of these facets with
respect to (11 I)A, because planar off-oriented (11 I)A substrates show growth rates that are larger than the growth rates of the (100) and exact oriented ( l l l ) A substrates. Although this phenomenon could explain the large growth rate at the (122)A facets in grooves, since this facet can be viewed as a largely off-oriented (II1)A surface, it does not account for the even larger growth rate of the (11 I)A side wall facet, because this facet is oriented exactly along the (11 l)A plane. Moreover, the (11 l)A-like side wall facets incorporate much more gallium than indium. In the case of the compositions, the planar (111) substrates do show an increased Ga content, as compared to planar (100) substrates at the lower temperatures (Fig. 6), although this increment is small. Still the enhanced Ga incorporation at ( l l l ) A surfaces indicate different incorporation rates of the group III species (Ga and In) on the (100) and ( l l l ) A surfaces at the lower temperatures. In the case of non-planar growth such a difference is enhanced [18,19]. The side wall facets of grooves thus attract a surplus of Ga from the gas phase (Fig. 6). The gas phase area above the (1 ll)A-like side wall facets is in direct contact with the gas phase area above other facets that show a smaller incorporation rate of Ga than the (111)A surface. This enables the supply of the extra amount of Ga, which is attracted by the (11 l)A-like surfaces. The consequent non-stoichiometric incorporation of Ga and In at the side wall facets causes strain to build up rapidly. The strain causes the layer to break up, and the growth mode changes from two-dimensional (2D) layer-by-layer growth to strain induced Stranski-Krastanov (semi-three-dimensional, 3D) growth [17]. The resulting "islands" provide a high number of kink sites that speed up growth at the side wall facets and causes an excess of growth species to be attracted from the gas phase. In the gas phase this excess is provided by areas above adjacent surfaces that tend to grow at a more regulated pace. At 640°C the decreased Ga incorporation at the (100) surface between grooves (Fig. 7) and the decreased growth rate of (100) surfaces between grooves as compared to the growth rate of a planar (100) substrate (Figs. 3, 4) are clear effects of the fast growth at the groove's side walls. Both observations suggest that the amount of growth elements that reach the (100) surface between the grooves is
M.M.G. Bongers et a l . / Journal t~f Cr3'stal Growth 171 (1997)333-340
decreased in favour of the amount of growth elements that reach the groove's side wall, and are in support of the theory developed above. Furthermore, Ga species in the gas phase are thought to be characterized by a lower ratio of the flux by chemical reaction versus the flux by diffusional transport in the gas phase (Ncv D) than In species [22], which makes the distribution of the latter species more determined by mass transport processes in the gas phase than the former. Thus, redirection of growth elements to the fastest growing surface on the nonplanar substrate is especially efficient for the Ga species, which is in support of the large enhancement of the Ga incorporation at the side wall facets. The final supporting argument follows from the growth rate difference between the ( I l l ) A and the (122)A side wall facets. Since the (122)A facet has only partly ( I I I ) A character, its growth rate will be less enhanced than the growth rate of the ( I I I ) A side wall facets. The smaller (122)A growth rate, as compared to the ( l l l ) A growth rate, is also caused by the different positioning of the two facets. The re-entrant position of the (122)A facet in the grooves usually causes it to receive less growth elements than the more favourably inclined (11 I)A side wall facet [18,19]. Surprisingly the (122)A facet incorporates more Ga (relative to In) than the ( I I I ) A facet, but this is explained by the earlier mentioned difference of Ncv o for Ga and In, by which Ga is transported much easier through the gas phase than In, and thus can reach the re-entrant (122)A facet easier than In (as compared to the (11 I)A facet). Above 680°C the preference for incorporation of Ga on the (Ill)A-like surfaces disappears and the growth rate and composition of the (11 l)A-like side wall facets is found to be similar to the planar ( I I I ) A growth rate and composition. This equal incorporation of Ga versus In at the (100) and (111)A surfaces no longer gives rise to a build up of strain at the (Ill)A-like side wall facets in grooves. Consequently the surface of the side wall facets does not break up into islands, and no acceleration of the growth rate takes place. Indeed the surfaces of the (111)A-like side wall facets become smooth at temperatures of 720°C and above, which indicates that the growth occurs in a 2D layer-by-layer mode. Apparently there is a transition temperature between 680 and 720°C, at which the incorporation of group
339
III species at the ( l l l ) A surface changes as compared to (100) surface (see Fig. 6). Although the incorporation of Ga versus In at planar ( I l I ) B substrates differs even more from the group III incorporation at (100) substrates than the incorporation at ( I l l ) A substrates, no growth rate increase is observed at (II1)B facets in grooves. In this case it is the inherent low growth rate of the (1 I~)B surfaces which causes growth elements to diffuse away to faster growing surfaces in grooves, instead of disrupting the surface to cause a 3D growth mode. Consequently the composition of layers grown on (II~)B facets in grooves can be the same as the composition of layers grown on planar ( l l l ) B substrates. The lower Ga concentration on ( I I I ) B surfaces can be explained by the non-growing nature of these surfaces when exposed to Ga species. It is well known that GaAs does not nucleate on (II1)B surfaces [20,21], whereas InP does nucleate on these surfaces [23-25]. Assuming that the non-growing nature of GaAs on ( l l l ) B extends to GaP, it is expected that the layer that forms on (II1)B surfaces shows a Ga deficiency, which is what has been found experimentally in this work. Although at the lower temperatures the roughness and large mismatch of the (Ill)A-like side wall facets is disadvantageous with respect to use of the material in devices, the high gallium content of the side wall growth is advantageous because the side wall Ga~In~_ ~P layer now has a much higher band gap than GaAs, and any currents through the side wall will be subsequently blocked.
5. Conclusions The growth of Ga0.sIn05P on patterned (100) substrates has been investigated. The growth follows the contours of the initial profile, whereby the growth rates and compositions of layers grown at facets in the grooves are comparable to the growth rates and compositions of equivalent planar surfaces, with the exception of the (111)A-like side wall facets of the grooves. These latter facets have a fast growth rate at the lower temperatures, which is caused by a slight preference of incorporation of Ga over In at (planar) (II1)A surfaces. This preference is enhanced in
340
M.M.G. Bongers et al./Journal of Crystal Growth 171 (1997) 333-340
grooves by the interaction through the gas phase with the other growing surfaces, by which also the possibility exists for the supply of an excess of growth species to the fast growing side wall facets. In detail the side wall facets build up strain by the non-stoichiometric incorporation of Ga and In, which causes the growth mode to become three-dimensional and the growth rate to increase substantially. The observed growth rate differences of the groove's side walls between the low and high temperature region can be used to fill grooves rapidly or maintain any initial profile, respectively, as is required by the experimentalists. Together with the deviating composition of the groove's side walls, as compared to the (100) surfaces, many applications may be possible.
Acknowledgements We gratefully acknowledge P.R. Hageman and A. van Geelen for part of the MOCVD growth experiments, and H. Smits (Department of Histology, Katholieke University of Nijmegen) for assistance with the SEM observations and EDAX measurements. The authors were receiving financial support from the Dutch foundations for scientific research SON, NOVEM and FOM and the Brazilian foundation CAPES.
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ELSEVIER
Journal of Crystal Growth 171 (1997) 333-340
Non-planar crystal growth of Gao.5Ino.5Pby metalorganic chemical vapour deposition M.M.G. Bongers *, P.L. Bastos, M.J. Anders, L.J. Giling Unit~ersity of Nijmegen, Facul~' of Science, Research Institute of Materials, Department of Experimental Solid State Physics IlL Taernooiceld 1, 6525 ED Nijmegen, The Netherlands
Received 11 June 1996; accepted 29 July 1996
Abstract Layers of GaosIno.sP have been grown by low pressure metalorganic vapour phase epitaxy on (100) substrates, which where patterned with normal-mesa and re-entrant grooves. The experiments show fast growth at the groove's side walls at temperatures of 680°C and below, whereas all other facets and all layers grown on planar substrates show but moderate growth rates. The large growth rates are attributed to the small preference for the incorporation of Ga over In at (11 l)A-like surfaces, which is enhanced in growth on non-planar substrates. The non-stoichiometric incorporation of Ga and In causes the development of strain and subsequently a growth mode transition from two to three-dimensional growth. The resulting rough surface of the side wall facets enables fast growth, whereby the interaction with slower growing facets through the gas phase in the non-planar epitaxy enables the supply of the necessary amount of growth species. At the higher temperatures the preference for incorporation of Ga at the side wall surfaces disappears, and the growth rates and compositions of the side wall facets becomes roughly equal to the growth rates and compositions of the non-planar (100) surfaces and the planar (111)A substrates. 1. Introduction The growth of I I I / V semiconductor heterostructures on patterned substrates has become an important tool in the manufacturing and integration of semiconductor devices [1-3]. Growth on ridges or V-grooves has proved its usefulness for lasers, waveguides and low-dimensional devices [4-7]. The efforts in embedded growth have been focused to obtain grooves with {011} side walls, which preferentially should be non-growing [8-10]. It is known that in the case of non-planar growth the thickness and composition of the layer may vary throughout the
* Corresponding author.
non-planar pattern, and that this growth behaviour is influenced by the growth conditions [14-16]. Since no overall theory has been developed yet to describe the growth on non-planar substrates, it is of practical importance to study the growth behaviour of semiconductor materials on non-planar surfaces as a function of the growth parameters [20-22]. One of the materials currently under study is GaxInl_~P, grown lattice matched to GaAs ( x = 0.52). An appealing property of GaxIn I_XP is the formation of ordered or disordered layers, which can be influenced by the growth conditions [3,11-13]. This feature, combined with growth on patterned substrates, enables vast possibilities for band gap engineering in laterally confined structures and devices [2-5].
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00703- 8
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In this work the growth of GaInP on GaAs substrates patterned with normal-mesa and re-entrant grooves is studied as a function of temperature. Attention is paid to orientation dependent growth rates and compositions, and their relation to the supply of growth species from the gas phase [18,19] and to surface incorporation processes.
2. Experimental procedure Re-entrant grooves were obtained by photolithography and wet chemical etching of (100) 2 ° (110) GaAs substrates. The mask pattern consisted of 5 100 p~m wide stripes, whose centers where positioned at a 300 p~m distance. Normal mesa grooves were etched along the [011] direction and re-entrant grooves were etched along the [011] direction with respectively 4" 1 " 5 and 1 ' 8 " 1 mixtures of H2SO4'H202 :H20. The side walls of the normal mesa grooves and the re-entrantgrooves, which were oriented along the (111)A and (122)A planes, showed a slight surface damage (Fig. l a) due to the wet chemical etching. The Ga xInl xP layers, nominally lattice matched to (100) GaAs (x = 0.52), were grown by low pressure (20 mbar) metalorganic vapour phase epitaxy (LP-MOVPE) in a horizontal Aixtron reactor. The layers were grown in one experiment on patterned substrates as well as planar (100) 2 ° toward (110), (111)A, (111 )A 2° toward (110), (! 10), ( 11 I)B and (111)B 2° toward (110) substrates. The misoriented (100) surface will be indicated by (100) throughout the rest of the paper, since both planar and non-planar growths were performed using 2°C misoriented (100) substrates. All samples were positioned within a 3 × 3 cm 2 square on the substrate holder in the reactor cell, thus preventing effects of gas phase depletion. The planar (100) layer thickness, the total flow, the total group V partial pressure and the V/III ratio were 1.8-2.4 ~m, 7 slm, 2.0 × 10 -3 and 400, respectively. The group V precursor was phosphine (PH3), and the group III precursors were trimethylgallium (TMG) and trimethylindium (TMI). The temperature was varied from 640°C to 760°C in steps of 40°C. In order to maintain the lattice matched conditions for the planar (100) substrate the gas phase concentration ratio T M G / T M I was dimin-
ished from 0.52 at 640°C via 0.50 at 680°C to 0.48 at 720°C and 760°C. The experiments were reproduced with the GalnP layer separated intermittently by four 0.05 ~m spacer layers of p-type GaInP to outline the growth. Cleaved cross sections of both planar and non-planar samples were examined by a scanning electron microscope (SEM) in order to determine layer thicknesses, surface structures and the development of facets in the grooves. The 50 ~m wide grooves have been studied in detail. The surface smoothness of planar substrates was investigated by optical microscopy. The composition of the layers has been determined by spatially resolved energy dispersive analysis of X-rays (EDAX).
3. Results Fig. 1 shows cross sectional micrographs of the 680°C GaxIn ~ .,P deposition on 50 I~m wide normal-mesa and re-entrant grooves before (a,b) and after growth (c,d). From duplicate experiments in which intermittently grown p-type layers were used (not shown) it was found that the developed facets in both normal-mesa and re-entrant grooves all had a positive growth rate, and that during growth no other facets temporally emerged and disappeared again. At temperatures of 640 and 680°C the growth of Ga~Inl_xP on both normal-mesa and re-entrant grooves is characterized by a fast growing side wall facet (Figs. lc, ld), with orientations ( l l l ) A and (122)A, respectively. These facets are very rough (Fig. 2a, 2b), and display a surface structure that does not show any regular pattern. In contrast, the (100) surfaces, both planar and non-planar, are very smooth. On the (100) surface at the bottom of the grooves sometimes circular or wave-like patterns, which extend towards the groove's side walls, are observed (see Fig. 2b). At the higher temperatures of 720 and 760°C, the growth enhancement of the side wall facets disappears, and the facet surfaces become smoother (Figs. 2c, 2d). At 760 ° the growth follows the contours of the initial groove shape. Apart from the (11 l)A-like side wall facets, several other facets appear in the normal-mesa and re-entrant grooves at the intersection of the (11 I)Alike side wall facet and the (100) top and bottom
M.M. G. Bon gers et al. / Journal of Co,stal Growth 171 (1997)333-340
335
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Fig. 1. SEM photographs of one side of 50 izm wide grooves; (a) and (b) as-etched; (c) and (d) overgrown with Ga~In I , P layers at 680°C (cross sections); (a) and (c) normal-mesa grooves; (b) and (d) re-entrant mesa grooves.
Fig. 2. SEM photographs of one side of 50 Ixm wide grooves, overgrown with Gaxln I ~P layers; (a) and (b) growth temperature of 680°C; (c) and (d) growth temperature of 720°C; (a) and (c) normal-mesa grooves: (b) and (d) re-entrant mesa grooves.
M.M.G. Bongers et al./Journal of Co'stal Growth 171 (1997) 333-340
336
surfaces. In this work the (1 ll)B and (011) facets, which develop at the top edges of the re-entrant grooves (Figs. ld and 2b), are taken along in the description of the growth behaviour of GaxIn ] _~P in grooves. Fig. 3 shows the growth rates of GaInP grown on planar (100), ( I l l ) A , (1 ll)A 2° toward (110), (011) and (11 I)B substrates as a function of temperature. The (100) growth rate increases with an increase of the temperature, despite of the decrease of the total concentration of the group III precursors in the gas phase. The (011) and (111)A growth rates are at all temperatures 25% (or less) smaller than the (100) growth rate, with the exception of the (011) growth rate at 760°C and the growth rate of ( I l I ) A 2°C (110) at 640°C. The growth rates of the exact and off-oriented (111)A surfaces differ at 640°C, but they become equal at temperatures of 720°C and higher. The growth rates of (1 ll)B surfaces, which are equal for exact and off oriented substrates (represented by one symbol in Fig. 3), are always smaller than the growth rate of all other surfaces. No information is available of the growth rate of Ga,Inj_~P on an exact oriented (100) surface, although in other III/V material systems the growth rate of an exact oriented (100) substrate does not differ much from the growth rate of a misoriented (100) substrate. The surfaces of the planar substrates were all smooth, with the exception of the exact oriented (111)A and all (11~)B
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substrates. On these surfaces isolated growth hillocks developed. The hillock densities were low and high in the case of (111)A and (1 l~)B substrates, respectively. The growth rates of the facets encountered in the 50 p~m wide normal-mesa and re-entrant grooves are shown in Fig. 4. Especially at 640 and 680°C the growth rate of the (1 l l)A facet differs remarkably from the earlier given planar growth rate (Fig. 3). At the lower temperatures both the (111)A and (122)A growth rates are much larger than the growth rate of the (100) surface between the grooves (Fig. 4). At the higher temperatures the ( l l l ) A and (122)A growth rates are smaller than the (100) growth rate and they now show a growth rate behaviour which is similar to the planar (11 I)A growth rate behaviour (Fig_. 3). At all temperatures the growth rate of the (111)B facet remains below the growth rates of all other facets in the grooves (Fig. 4). The (01~) facet growth rate, shown in Fig. 4, is almost equal to the (100) growth rate at all temperatures. In the previous paragraphs the (100) growth rate refers to the surface between the grooves. The growth rate of this surface is the same for both normal mesa and re-entrant grooves, but it is about 15% smaller than the growth rate of planar (100) substrates. The growth rate of the (100) groove bottom is even smaller, amounting to 70-80% of the growth rate of the planar (100) surface in the case of 50-100 p~m
M.M.G. Bongers et al. / Journal of Co'stal Growth 171 (1997) 333-340
wide grooves. In Fig. 5 the growth rate of the (100) bottom of re-entrant grooves is shown as a function of groove width at the various temperatures. The growth rate of the (100) bottom facet increases with increasing groove width. At all temperatures the dependence of the growth rate on the groove width is almost the same (Fig. 5), although the absolute growth rates of the (100) bottom surface differ at each temperature. The growth rate of the (100) bottom of wide normal-mesa grooves was equal to the growth rates of the bottoms of re-entrant grooves (given in Fig. 5). For normal-mesa grooves that had a width of 20 txm or less, no (100) bottom facet was present after growth (the grooves were V-shaped), so that the growth rate of the (100) bottom surface could not be determined in these cases. The composition of the GaInP layers grown on planar surfaces is shown in Fig. 6. The (100) surfaces show a Ga content x which is slightly lower than the lattice matched value, but it is within the region where the layer is elastically strained. The layers grown on (111)A and (011) substrates show a slightly higher Ga concentration than the layers grown on (100) at 640 and 680°C, but these layers are also still within the elastically strained region. At 720 and 760°C the composition of the layers grown on (100), (110) and (111)A are the same within the measurement accuracy. The Ga concentration of lay-
337
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ers grown on ( I l l ) B substrates is much lower than the Ga concentration of layers grown on (100) surfaces at all temperatures. The composition of the layers grown on exact oriented and misoriented (111)A substrates are equal, and thus only one point is shown in Fig. 6. Similarly, the layers grown on (1 II)B substrates show no dependence on off-orientation. The hillocks, which are present at the ( I I 1 ) A surface, exhibit a 10% enhanced Ga incorporation as compared to the flat ( l l l ) A layer which grows between the hillocks. In the case of the ( l l l ) B
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x o f G a ~In t _ ~ P l a y e r s g r o w n at f a c e t s in 5 0 a n d r e - e n t r a n t g r o o v e s as a f u n c t i o n o f t h e
338
M.M.G. Bongers et al. / Journal of Co,stal Growth 171 (1997) 333-340
surfaces, the Ga content in the hillocks is equal to the Ga content of the layers grown on (100) substrates. Fig. 7 shows the compositions of (100), (111)A, (111)B and (122) facets, which are measured in the 50 txm wide normal-mesa and re-entrant mesa grooves. Especially at the lower temperatures, large compositional differences are found between the layers grown on planar and non-planar (111)A surfaces. At 640 and 680°C the Ga concentration at the (122) and (111)A facets is much higher than the Ga concentration at the (100) facet, but at 720 and 760°C the composition of these three facets are close to each other. The compositions of the (100) groove bottom and the (100) top surface are the same within the measurement accuracy at all temperatures.
4. Discussion The most interesting feature of the growth of GaxIn~_xP is the large growth rate of the ( l l l ) A and (122)A side walls of grooves at the lower temperatures, as compared to the (100) growth rate (Fig. 4). The fast growing ( I l l ) A and (122)A facets in grooves are found to incorporate an excess of Ga, as compared to the (100) composition (Fig. 7). The growth rates and compositions of the layers grown on the side wall facets can be explained by a local strain related three-dimensional growth mode in concurrence with an enhanced supply of growth elements through the gas phase to the side walls of the grooves. The high growth rates on the (Ill)A-like side wall facets is unique, both with respect to other surfaces on the non-planar substrates that show average growth rates and with respect to the growth rates of planar substrates: although the planar ( l l l ) A growth rate is smaller than the (100) growth rate, the non-planar ( l l l ) A growth rate is much larger than the non-planar (100) growth rate. In contrast all other major orientations show non-planar growth rates that are equal or smaller than the growth rates of equivalent planar substrates. A possible second explanation for the increase of the growth rate at the groove's side wall facets may be sought in a misorientation of these facets with
respect to (11 I)A, because planar off-oriented (11 I)A substrates show growth rates that are larger than the growth rates of the (100) and exact oriented ( l l l ) A substrates. Although this phenomenon could explain the large growth rate at the (122)A facets in grooves, since this facet can be viewed as a largely off-oriented (II1)A surface, it does not account for the even larger growth rate of the (11 I)A side wall facet, because this facet is oriented exactly along the (11 l)A plane. Moreover, the (11 l)A-like side wall facets incorporate much more gallium than indium. In the case of the compositions, the planar (111) substrates do show an increased Ga content, as compared to planar (100) substrates at the lower temperatures (Fig. 6), although this increment is small. Still the enhanced Ga incorporation at ( l l l ) A surfaces indicate different incorporation rates of the group III species (Ga and In) on the (100) and ( l l l ) A surfaces at the lower temperatures. In the case of non-planar growth such a difference is enhanced [18,19]. The side wall facets of grooves thus attract a surplus of Ga from the gas phase (Fig. 6). The gas phase area above the (1 ll)A-like side wall facets is in direct contact with the gas phase area above other facets that show a smaller incorporation rate of Ga than the (111)A surface. This enables the supply of the extra amount of Ga, which is attracted by the (11 l)A-like surfaces. The consequent non-stoichiometric incorporation of Ga and In at the side wall facets causes strain to build up rapidly. The strain causes the layer to break up, and the growth mode changes from two-dimensional (2D) layer-by-layer growth to strain induced Stranski-Krastanov (semi-three-dimensional, 3D) growth [17]. The resulting "islands" provide a high number of kink sites that speed up growth at the side wall facets and causes an excess of growth species to be attracted from the gas phase. In the gas phase this excess is provided by areas above adjacent surfaces that tend to grow at a more regulated pace. At 640°C the decreased Ga incorporation at the (100) surface between grooves (Fig. 7) and the decreased growth rate of (100) surfaces between grooves as compared to the growth rate of a planar (100) substrate (Figs. 3, 4) are clear effects of the fast growth at the groove's side walls. Both observations suggest that the amount of growth elements that reach the (100) surface between the grooves is
M.M.G. Bongers et a l . / Journal t~f Cr3'stal Growth 171 (1997)333-340
decreased in favour of the amount of growth elements that reach the groove's side wall, and are in support of the theory developed above. Furthermore, Ga species in the gas phase are thought to be characterized by a lower ratio of the flux by chemical reaction versus the flux by diffusional transport in the gas phase (Ncv D) than In species [22], which makes the distribution of the latter species more determined by mass transport processes in the gas phase than the former. Thus, redirection of growth elements to the fastest growing surface on the nonplanar substrate is especially efficient for the Ga species, which is in support of the large enhancement of the Ga incorporation at the side wall facets. The final supporting argument follows from the growth rate difference between the ( I l l ) A and the (122)A side wall facets. Since the (122)A facet has only partly ( I I I ) A character, its growth rate will be less enhanced than the growth rate of the ( I I I ) A side wall facets. The smaller (122)A growth rate, as compared to the ( l l l ) A growth rate, is also caused by the different positioning of the two facets. The re-entrant position of the (122)A facet in the grooves usually causes it to receive less growth elements than the more favourably inclined (11 I)A side wall facet [18,19]. Surprisingly the (122)A facet incorporates more Ga (relative to In) than the ( I I I ) A facet, but this is explained by the earlier mentioned difference of Ncv o for Ga and In, by which Ga is transported much easier through the gas phase than In, and thus can reach the re-entrant (122)A facet easier than In (as compared to the (11 I)A facet). Above 680°C the preference for incorporation of Ga on the (Ill)A-like surfaces disappears and the growth rate and composition of the (11 l)A-like side wall facets is found to be similar to the planar ( I I I ) A growth rate and composition. This equal incorporation of Ga versus In at the (100) and (111)A surfaces no longer gives rise to a build up of strain at the (Ill)A-like side wall facets in grooves. Consequently the surface of the side wall facets does not break up into islands, and no acceleration of the growth rate takes place. Indeed the surfaces of the (111)A-like side wall facets become smooth at temperatures of 720°C and above, which indicates that the growth occurs in a 2D layer-by-layer mode. Apparently there is a transition temperature between 680 and 720°C, at which the incorporation of group
339
III species at the ( l l l ) A surface changes as compared to (100) surface (see Fig. 6). Although the incorporation of Ga versus In at planar ( I l I ) B substrates differs even more from the group III incorporation at (100) substrates than the incorporation at ( I l l ) A substrates, no growth rate increase is observed at (II1)B facets in grooves. In this case it is the inherent low growth rate of the (1 I~)B surfaces which causes growth elements to diffuse away to faster growing surfaces in grooves, instead of disrupting the surface to cause a 3D growth mode. Consequently the composition of layers grown on (II~)B facets in grooves can be the same as the composition of layers grown on planar ( l l l ) B substrates. The lower Ga concentration on ( I I I ) B surfaces can be explained by the non-growing nature of these surfaces when exposed to Ga species. It is well known that GaAs does not nucleate on (II1)B surfaces [20,21], whereas InP does nucleate on these surfaces [23-25]. Assuming that the non-growing nature of GaAs on ( l l l ) B extends to GaP, it is expected that the layer that forms on (II1)B surfaces shows a Ga deficiency, which is what has been found experimentally in this work. Although at the lower temperatures the roughness and large mismatch of the (Ill)A-like side wall facets is disadvantageous with respect to use of the material in devices, the high gallium content of the side wall growth is advantageous because the side wall Ga~In~_ ~P layer now has a much higher band gap than GaAs, and any currents through the side wall will be subsequently blocked.
5. Conclusions The growth of Ga0.sIn05P on patterned (100) substrates has been investigated. The growth follows the contours of the initial profile, whereby the growth rates and compositions of layers grown at facets in the grooves are comparable to the growth rates and compositions of equivalent planar surfaces, with the exception of the (111)A-like side wall facets of the grooves. These latter facets have a fast growth rate at the lower temperatures, which is caused by a slight preference of incorporation of Ga over In at (planar) (II1)A surfaces. This preference is enhanced in
340
M.M.G. Bongers et al./Journal of Crystal Growth 171 (1997) 333-340
grooves by the interaction through the gas phase with the other growing surfaces, by which also the possibility exists for the supply of an excess of growth species to the fast growing side wall facets. In detail the side wall facets build up strain by the non-stoichiometric incorporation of Ga and In, which causes the growth mode to become three-dimensional and the growth rate to increase substantially. The observed growth rate differences of the groove's side walls between the low and high temperature region can be used to fill grooves rapidly or maintain any initial profile, respectively, as is required by the experimentalists. Together with the deviating composition of the groove's side walls, as compared to the (100) surfaces, many applications may be possible.
Acknowledgements We gratefully acknowledge P.R. Hageman and A. van Geelen for part of the MOCVD growth experiments, and H. Smits (Department of Histology, Katholieke University of Nijmegen) for assistance with the SEM observations and EDAX measurements. The authors were receiving financial support from the Dutch foundations for scientific research SON, NOVEM and FOM and the Brazilian foundation CAPES.
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