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Influence of convection on the composition profiles of thick GaAlAs layers grown by liquid phase electroepitaxy
CRYSTAL GROWTH
Journal of Crystal Growth 131 (1993) 426—43() North-Holland
Influence of convection on the composition profiles of thick GaA1As layers grown by liquid phase electroepitaxy Z.R. Zytkiewicz Institute of Physics, Polish Academy of .Scwnce,s. Al. Lotnikow 32 Received 9 December I 992~manuscript received in final
/ -JO,
02-608 Warsaw, l’oland
form 4 March 1993
Liquid phase electroepitaxv (LPEE) — the method of current controlled layer growth from a solution at constant temperature was used to grow thick GaAlAs layers on GaAs substrates. We have shown that in the presence of convection in the solution. compositionally uniform layers can he grown despite compositional nonuniformity of the source material used. Layers grown in the absence of convection had an Al content which increased with thickness. This reflects the composition profile ol the source material prepared by the cooling down of the system.
1. Introduction Many experimental techniques used widely in semiconductor physics (magnetic measurements, optical absorption, etc.) require a large volume of high-quality uniform crystal to achieve the sensitivity of the equipment used. Epitaxial layers of multicomponent Ill—V semiconductors grown by liquid phase epitaxy (LPE) cannot fulfill these demands because temperature changes as well as solution depletion during LPE growth usually lead to composition grading in the layers obtained. It has been found that the application of a DC current as a driving force for epitaxial growth at constant temperature (-liquid phase electroepitaxy LPEE) results in composition stabilization effect. Uniform GaInAs crystals up to 3 mm thick has been electroepitaxially grown on lnP substrates by using the polycrystalline ternary source material placed on the top of the melt [1]. For the GaA1As system, the preparation of such a source material is difficult: therefore, a modified version of LPEE has been developed [2,3] in order to grow thick GaAIAs layers. All the details of this method can he found in the cited papers. However, it is essential to this work to remember that the modification made was to —
cool down the solution, previously saturated at 0022-0248/93/$06.Ot) © 1993
—
850°C,to 800°C.During this process the substrate was away from the liquid, and therefore, the excess GaAIAs material was deposited only on the GaAs wafer floating on top of the solution. This GaA1As was then used as a source of solutes during LPEE growth on GaAs substrate at 800°C. Because the GaAIAs source material used in modified LPEE is grown by cooling of the system. a composition profile is expected similar to that of LPE grown GaAlAs, i.e., a decrease in Al content with increasing layer thickness (fig. Ia). As the electroepitaxial layer is grown by recrystallization of the source material, one can anticipate that its composition profile should be graded too, hut with opposite sign of the composition gradient of the source (fig. lh). Thus, growth of layers with increasing Al content towards the surface is expected. This contradicts our previous results [3]. where the same technique was used to obtain layers with a highly uniform composition profile. The explanation of the above discrepancy is the purpose of this paper.
2. Experimental results Since LPE growth of epitaxial Ga1 Al As layers with uniform composition is the most diffi-
Elsevier Science Publishers By. All rights reserved
Z.R. Zytkiewicz
a)
t
/ Influence of conr’ection on composition profiles of GaAIAs b) t
>
0
layers grown by LPEE
14
L= 14mm L=lomrn
XS GaAs
~
—
GaAs
-,
-
~source
z
~
10 8
-~
source
—
L—7mm
6
Z epitay:r~
GaAs substrate
427
~
GaAs substrate XS
XS
Fig. 1. Expected distribution of Al: (a) in the source material; (b) in the growing epilayer if the simple model of source recrystallization is valid.
10
15
20
25
growth time (hours) Fig. 3. Temporal change of the growth cell resistance R(t) for different values of the solution thicknesses.
L. For L 7 mm the Al content in the grown solid increases with its thickness, as predicted in fig. 1, reaches a maximum and then decreases. It should be mentioned here that the initial positive composition gradient in the layer is sensitive to the value of the cooling rate a used during the source material preparation step. The composition gradient increases as a increases, which is a direct result of the increase with a of the absolute value of the negative composition gradient in the source grown by LPE (see fig. 1). A similar f profile is observed for L 10 mm. This time, however, the maximum of the xs distribution is shifted towards larger thicknesses as the volume of the source material available for growth is proportional to L. If a thick enough solution is used for growth (L 14 mm), a uniform composition profile is obtained, as reported previously =
cult in the direct gap region [4,5],we have chosen to grow layers with a composition close to x~ 0.2. In this case the largest compositional grading in the source material is expected. This is of benefit when the influence of source composition nonuniformity on the subsequent uniformity of the LPEE layer is to be studied. All the details of the growth procedure and the graphite boat configuration used can be found in our previous paper [3]. The only change made was the application of an additional, 2 h long homogenization of the Al—Ga contact melt after its introduction under the substrate and prior to growth. This is in order to saturate the contact melt with As. Fig. 2 shows the composition profiles of layers grown from the solutions of different thickness =
=
=
[31. 0.30 0.25
L=7mm L—lOmm —
0.20 0 15
L=l4mrrs
0.10 0.05 0.00 distance from nterfce Fig. 2. SEM composition profiles of Ga 1~Al5Aslayers grown from the solutions of different thicknesses L.
Fig. 3 represents the temporal evolution of the growth cell resistance, R(t), measured during the first 20 h of growth of layers whose composition profiles are shown in fig. 2. It has been reported previously [6] that this kind of curve can be used for in process. situ monitoring of dR(t)/dt the electroepitaxial growth The slope is proportional to the product of the growth rate, Var, and the resistivity p(Xs) of the layer, as long as the etching of the back surface of the substrate by the contact solution can be neglected. We will argue that this condition is fulfilled in our case. The use of high Al concentration in the Ga—Al melt (10 Wt%) ensures low GaAs solubility in the contact
425
ZR. Zvtkiewicz
/ Influence
of conlectto,t on composItion profiles of (7aAlAs lover, i~rost’ptby LP[-.T
solution. Moreover, a 2 h heating of the substrafe—contact solution system prior to the growth was long enough to saturate the contact solution with As. After the DC current was switched on, the state of equilibrium at the substrate—contact solution interface was disturbed only by the small Pettier heating at this interface. As the contact solution was very thin (2 mm), the relaxation time to the new equilibrium state should be much shorter than the growth time, and this effect seems to be unimportant in our case. Thus, the etching of the back surface of the substrate can be neglected and the use of the slope of the R(t) curve as a direct measure of the growth rate is justified. Because the compositions of the layers considered here are similar, the p(x’) values should not differ substantially between them. Therefore, the increase of 1/gr with L can be easily deduced from fig. 3. It should he mentioned here that in our boat we could not use solutions thinner than 6 mm. Otherwise, the side contact surface between the solution and the graphite would become too small, which would result in a pronounced Joule effect at this surface and in a nonuniformity of thickness of the epilayer (being thicker in the middle and thinner at the periphery of the layer). The latter effect limits the range of L values available in this work.
3. Discussion By changing the thickness L of the solution, we can control the contribution of convection to the transport of solutes in liquid. Thus the results presented in figs. 2 and 3 can be explained only if the contribution of convection in the melt is included. The visible increase of ~r in the presence of pronounced convection in the solution (i.e. with the increase of L), see fig. 3, is a well-known effect in crystal growth. However, the influence of convection on the composition profiles of Gat ~Al~As layers grown by modified LPEE (fig. 2) requires a more elaborate discussion. In the GaA1As system, grown from a Ga-rich solution, the distribution coefficients of the di-
luted components Al and As are larger than unity. The solution loses these components at the solution—layer phase boundary and they have to he refilled by transport from the hulk of the solution. Under isothermal conditions, the cornposition of a growing layer is dictated by the phase diagram of the system and the present composition of the solution at the solid—liquid interface, which in turn is determined by the balance between the number of particles incorporated into the growing crystal and those delivered to the interface from the rest of the solution. In the limit of thin solution, the presence of convection is less likely. In this case electrotransport remains the most important mechanism of solutes transport towards the solution—layer interface [7]. The dissolution of a source material, which has the composition profile as shown in fig. 1, causes the Al concentration in the upper part of the solution to increase with time in comparison with the concentration of As. Because the electrotransport streams of solutes are proportional to these concentrations and the condition of solute stream conservation must be fulfilled, these changes of composition should cause the appropriate changes of the solution composition in the vicinity of the layer surface. Therefore, the solution adjacent to the layer-solution phase houndary is continuously enriched with Al. resulting in the growth of a layer with an Al content which increases with thickness, as predicted in fig. I. A different situation occurs when the growth of an epilayer is performed from a thick solution in which the presence of natural convection is expected. Then transport of solutes ts dominated by their diffusion in the thin diffusion layers adjacent to the appropriate solid—liquid interfaces [7]. The rest of the solution is a wide. very-well-mixed zone with the distribution of Al and As, which, to a first approximation, can he taken as uniform. As before, the dissolution of the source material causes the ratio of Al to As concentrations in the melt to increase. However. in contrast to the previous case, this composition change is not transmitted directly to the solution—layer phase boundary. It results predominantly in a change in the ratio of Al to As concentrations in the bulk of the solution. The
ZR. Zytkiewicz
/ Influence of convection on
I, 14mm
021
composition profiles of GaA1As layers grown by LPEE
d~
2
020 ~---a—--~—~--
H
o~to 0 12
0 08
0
st.~ndurd LPE2
~
modified I,02E
~ôo
~~áo distance
from
inter-face
(pm)
Fig. 4. SEM composition profiles of Ga1 _5Al~Aslayers grown with (modified LPEE) and without (standard LPEE) GaAIAs source from 14 mm thick melt; d1 is the theoretical thickness of the layer with rectangular composition profile (dashed line); d2 is the thickness of solid grown until the source is not exhausted (see text).
composition gradient in the source material grown by LPE in the presence of convection is smaller than in the convection-free case [41.Thus, the enrichment of the solution with Al caused by the dissolution the source smaller than before. Moreover, of it occurs in ais large volume of the well-mixed zone and has a small influence on its average composition. Therefore, despite the cornpositional nonuniformity of the source, the bulk of the solution can be considered to be a large reservoir of solutes with constant and spatially uniform composition, as long as the GaA1As source material is not exhausted. This is the straightforward reason for the fact that the composition profiles of the epilayers grown from thick solution are so uniform. However, even if 14 mm thick melt is used for the growth, the effect of GaAlAs depletion can be noticed at 200 ~m from the interface (see fig. 2 and fig. 4 where the composition profile for L 14 mm is repeated). This can be explained taking into account that the new situation occurs in the system when all the source GaA1As is consumed during the growth. In order to discuss this point, let us consider the rectangular shape of the layer composition profile as shown in fig. 4 by a dashed line. If we assume that the total amount m 1 of Al introduced to the growth systern at the saturation temperature is uniformly distributed in the solid, the thickness d1 of such a layer can be easily calculated. Assuming a linear =
429
variation of GaA1As density with xs, this calculation estimates the value of d1 to be 450 ~.im ‘~
for xs=0.2 and L=14 mm. It is obvious that d~ is the theoretical thickness of the uniform layer and this value cannot be reached in the real growth experiment, in which the more diffused xs distribution is expected in the vicinity of d~.Next, if the GaAlAs phase diagram and the melt volume are known, the value of m3 the amount of —
Al necessary to saturate the solution prior to electroepitaxial growth, can be calculated for x 0.2 and the growth temperature of 800°C. Because m3 is that part of the total mass m1 of Al which remains in the solution after the source preparatton step, m2 m~ m3 equals the amount of Al incorporated into the GaAlAs source material during the cooling of the system when the substrate was away from the liquid. As before, m2 can be used to calculate the thickness d2 of the layer grown by LPEE, until the source is not exhausted. From these calculations the5 value 0.2 of to be 220 ~im forwell x with andd2L is estimated 14 mm. This value agrees very the epilayer thickness at which the composition profile starts to decrease (fig. 4). Taking into account the statements above, the nature of the diffusive tail of the composition profile can be explained as being caused by the much faster change of the average composition of the well mixed zone of the solution when all the GaAlAs source material is exhausted. In such a case the bulk of the solution cannot any longer be considered as a large reservoir of solutes with constant composition and the composition profile of the layer is similar to the one typical for the layer grown without the GaAlAs source by standard LPEE (see ref. [3] for growth details). The composition profile of the layer grown from the 14 mm thick melt by standard LPEE technique is included to fig. 4 to show clearly that its shape is very similar to the shape of the diffusive tail of the xs distribution observed in the layer grown by modified LPEE from the same volume of solution. Therefore, we believe that the comparison =
—
=
=
‘~
of both profiles shown in fig. 4 as well as the agreement of our experimental value of d2 with the result of calculations strongly support our interpretation of effects responsible for the xs
7. 1?. Zyjkusj-u-:
43))
/ influence of (0/Il cctioit on i-o;npo 0/eel pro filc,s of Ca—I l—ls ho en i,’roe,l
decrease which appears after the highly uniform part of GaAIAs layer. The Joule effect at the side solution—graphite contact is a problem if we consider the pttrpose of this study. since it limits the range of L values available. It has been estimated [8] that a Ga-rich solution thinner than 5 mm is required to elirninate the convective flows in its volume. Thus, we expect that the solution of L 7 mm is too thick and some residual convection is still present in our system. This convective mixing of the solution volume averages, to some extent, the tiow of solutes from the source to the surface of the layer. Therefore, the value of the positive composition gradient in the layer grown from the 7 mm thick solution (fig. 2) is niuch lower than that expected in the source. In order to obtain a one-to-one correspondence between the composition gradients in the layer and that tn the source. as predicted in fig. 1. the technique of growth from a thin solution should he applied. In the case of electroepitaxy it means that a sandwichtype geometry of the boat with the electric current passing through both the source and the substrate is required for this purpose [9]. Only then can large values of positive composition gradients in the layers he obtained. In this case. however, the layer thicknesses will he markedly reduced. =
-
.
.
.
In i.i’LI.
the solution led to a decrease of the eomposittoti gr;tdient in the source and broke down the direct correspondence between the composition profiles of the source and epilayer. This resulted in a smoothening of Al distribution in Ihe grown (iaAlAs as long as the source (i;tAlAs W;ts piesent in the system. Otherwise, even in the piesonce of convection, the solution became gr;tduallv depleted, which resulted in a progressive decrease of the Al content in the solid grown. Ihus. depending on the absence or presence of convec(ion, layers with increasing Al content or uniform composition can he grown, respectively. The growth of thick GaAIAs layers with both these types of Cofliposition profiles is ver diffieLtlt by any other method of solution growth. Acknowledgements The author would like to thank S. Miotkowska for performing SEM measurements of the cornpositional profiles of GaAIAs layers stttdied. W. Chacinska and D. Dohosz are acknowledged for excellent technical assistance and R. Pritchard (UMIST) for careful reading of the manuscript. This work was partly supported by grant No. 2 2363 92 03 of the Committee for Scientific Research.
References 4. Summary
[I]
The modified LPEE method has been applied to grow thick Ga1 1Al As layers on GaAs sub— strates. We have shown that depending on the thickness of the solution used, highly uniform epilayers its well as layers with an Al content increasing with thickness can he obtained. This is explained by the contribution of convection to the transport of solutes in the liquid. For a thin layer of solution there should he no conveetton and the solutes were transported towards the substrate by electromigration. In this limit, the picture of direct source material recrystallization (fig. I ) is valid and layers with an increasing Al content were grown. When a thick layer of solution was used, the contribution of convection cannot he neglected. The convective mixing of the hulk of -
I. Brvskiewie,’. M. Buga~ski.B. Brvskiewici, .1. Liigowski and H.C. Gatos. ill: Gallium Arsenide and Related Cornilounds 1957. Inst. l’hys. (onf. Ser. 91. Eds. A. (hristou
,~
,nd H.S. Ruppreeht (Inst. Ph~s..london—Bristol. 988) Is. [2] JJ Danicle and A J I leblig..l AppI l’hys 52)1981)4325 [3] ZR. Zytkiewicz and S. Miotkowska. J. Crystal Growth l2! 1992 457. [4] I. Crossley and MB. Small, .1. (rvstal Growth IS (972) -
208.
[~j I. 1 erarnoto. M. Kasamura and 11. ~ Phvs. 18(1979) 151(9.
‘t arnanaka
.
.Ltpitn. .1.
[0] S. Isozurni. C.J. Herman. A. Okamoto. J. l,agowski and
Il.C. Gatos. J. Electrochent. Soc. 128 (1981) 222)). [7] L. Jastrzebski, J. Lagowski. H.C. Gatos and A.F. Witt. .1.
App!. Phys. 49 (1975) 5909. with eriatuni in .l.Appl. l’hys. 51) (1979) P69 A. Tillei~J Crystal Growth 2 11908) 69. [9] [ Bryskiewie,. Ct. Boucher,Jr.. J. l.agowski and ll.C, Gatos. J. Crystal Growth 82 (1987) 279. [8]
Journal of Crystal Growth 131 (1993) 426—43() North-Holland
Influence of convection on the composition profiles of thick GaA1As layers grown by liquid phase electroepitaxy Z.R. Zytkiewicz Institute of Physics, Polish Academy of .Scwnce,s. Al. Lotnikow 32 Received 9 December I 992~manuscript received in final
/ -JO,
02-608 Warsaw, l’oland
form 4 March 1993
Liquid phase electroepitaxv (LPEE) — the method of current controlled layer growth from a solution at constant temperature was used to grow thick GaAlAs layers on GaAs substrates. We have shown that in the presence of convection in the solution. compositionally uniform layers can he grown despite compositional nonuniformity of the source material used. Layers grown in the absence of convection had an Al content which increased with thickness. This reflects the composition profile ol the source material prepared by the cooling down of the system.
1. Introduction Many experimental techniques used widely in semiconductor physics (magnetic measurements, optical absorption, etc.) require a large volume of high-quality uniform crystal to achieve the sensitivity of the equipment used. Epitaxial layers of multicomponent Ill—V semiconductors grown by liquid phase epitaxy (LPE) cannot fulfill these demands because temperature changes as well as solution depletion during LPE growth usually lead to composition grading in the layers obtained. It has been found that the application of a DC current as a driving force for epitaxial growth at constant temperature (-liquid phase electroepitaxy LPEE) results in composition stabilization effect. Uniform GaInAs crystals up to 3 mm thick has been electroepitaxially grown on lnP substrates by using the polycrystalline ternary source material placed on the top of the melt [1]. For the GaA1As system, the preparation of such a source material is difficult: therefore, a modified version of LPEE has been developed [2,3] in order to grow thick GaAIAs layers. All the details of this method can he found in the cited papers. However, it is essential to this work to remember that the modification made was to —
cool down the solution, previously saturated at 0022-0248/93/$06.Ot) © 1993
—
850°C,to 800°C.During this process the substrate was away from the liquid, and therefore, the excess GaAIAs material was deposited only on the GaAs wafer floating on top of the solution. This GaA1As was then used as a source of solutes during LPEE growth on GaAs substrate at 800°C. Because the GaAIAs source material used in modified LPEE is grown by cooling of the system. a composition profile is expected similar to that of LPE grown GaAlAs, i.e., a decrease in Al content with increasing layer thickness (fig. Ia). As the electroepitaxial layer is grown by recrystallization of the source material, one can anticipate that its composition profile should be graded too, hut with opposite sign of the composition gradient of the source (fig. lh). Thus, growth of layers with increasing Al content towards the surface is expected. This contradicts our previous results [3]. where the same technique was used to obtain layers with a highly uniform composition profile. The explanation of the above discrepancy is the purpose of this paper.
2. Experimental results Since LPE growth of epitaxial Ga1 Al As layers with uniform composition is the most diffi-
Elsevier Science Publishers By. All rights reserved
Z.R. Zytkiewicz
a)
t
/ Influence of conr’ection on composition profiles of GaAIAs b) t
>
0
layers grown by LPEE
14
L= 14mm L=lomrn
XS GaAs
~
—
GaAs
-,
-
~source
z
~
10 8
-~
source
—
L—7mm
6
Z epitay:r~
GaAs substrate
427
~
GaAs substrate XS
XS
Fig. 1. Expected distribution of Al: (a) in the source material; (b) in the growing epilayer if the simple model of source recrystallization is valid.
10
15
20
25
growth time (hours) Fig. 3. Temporal change of the growth cell resistance R(t) for different values of the solution thicknesses.
L. For L 7 mm the Al content in the grown solid increases with its thickness, as predicted in fig. 1, reaches a maximum and then decreases. It should be mentioned here that the initial positive composition gradient in the layer is sensitive to the value of the cooling rate a used during the source material preparation step. The composition gradient increases as a increases, which is a direct result of the increase with a of the absolute value of the negative composition gradient in the source grown by LPE (see fig. 1). A similar f profile is observed for L 10 mm. This time, however, the maximum of the xs distribution is shifted towards larger thicknesses as the volume of the source material available for growth is proportional to L. If a thick enough solution is used for growth (L 14 mm), a uniform composition profile is obtained, as reported previously =
cult in the direct gap region [4,5],we have chosen to grow layers with a composition close to x~ 0.2. In this case the largest compositional grading in the source material is expected. This is of benefit when the influence of source composition nonuniformity on the subsequent uniformity of the LPEE layer is to be studied. All the details of the growth procedure and the graphite boat configuration used can be found in our previous paper [3]. The only change made was the application of an additional, 2 h long homogenization of the Al—Ga contact melt after its introduction under the substrate and prior to growth. This is in order to saturate the contact melt with As. Fig. 2 shows the composition profiles of layers grown from the solutions of different thickness =
=
=
[31. 0.30 0.25
L=7mm L—lOmm —
0.20 0 15
L=l4mrrs
0.10 0.05 0.00 distance from nterfce Fig. 2. SEM composition profiles of Ga 1~Al5Aslayers grown from the solutions of different thicknesses L.
Fig. 3 represents the temporal evolution of the growth cell resistance, R(t), measured during the first 20 h of growth of layers whose composition profiles are shown in fig. 2. It has been reported previously [6] that this kind of curve can be used for in process. situ monitoring of dR(t)/dt the electroepitaxial growth The slope is proportional to the product of the growth rate, Var, and the resistivity p(Xs) of the layer, as long as the etching of the back surface of the substrate by the contact solution can be neglected. We will argue that this condition is fulfilled in our case. The use of high Al concentration in the Ga—Al melt (10 Wt%) ensures low GaAs solubility in the contact
425
ZR. Zvtkiewicz
/ Influence
of conlectto,t on composItion profiles of (7aAlAs lover, i~rost’ptby LP[-.T
solution. Moreover, a 2 h heating of the substrafe—contact solution system prior to the growth was long enough to saturate the contact solution with As. After the DC current was switched on, the state of equilibrium at the substrate—contact solution interface was disturbed only by the small Pettier heating at this interface. As the contact solution was very thin (2 mm), the relaxation time to the new equilibrium state should be much shorter than the growth time, and this effect seems to be unimportant in our case. Thus, the etching of the back surface of the substrate can be neglected and the use of the slope of the R(t) curve as a direct measure of the growth rate is justified. Because the compositions of the layers considered here are similar, the p(x’) values should not differ substantially between them. Therefore, the increase of 1/gr with L can be easily deduced from fig. 3. It should he mentioned here that in our boat we could not use solutions thinner than 6 mm. Otherwise, the side contact surface between the solution and the graphite would become too small, which would result in a pronounced Joule effect at this surface and in a nonuniformity of thickness of the epilayer (being thicker in the middle and thinner at the periphery of the layer). The latter effect limits the range of L values available in this work.
3. Discussion By changing the thickness L of the solution, we can control the contribution of convection to the transport of solutes in liquid. Thus the results presented in figs. 2 and 3 can be explained only if the contribution of convection in the melt is included. The visible increase of ~r in the presence of pronounced convection in the solution (i.e. with the increase of L), see fig. 3, is a well-known effect in crystal growth. However, the influence of convection on the composition profiles of Gat ~Al~As layers grown by modified LPEE (fig. 2) requires a more elaborate discussion. In the GaA1As system, grown from a Ga-rich solution, the distribution coefficients of the di-
luted components Al and As are larger than unity. The solution loses these components at the solution—layer phase boundary and they have to he refilled by transport from the hulk of the solution. Under isothermal conditions, the cornposition of a growing layer is dictated by the phase diagram of the system and the present composition of the solution at the solid—liquid interface, which in turn is determined by the balance between the number of particles incorporated into the growing crystal and those delivered to the interface from the rest of the solution. In the limit of thin solution, the presence of convection is less likely. In this case electrotransport remains the most important mechanism of solutes transport towards the solution—layer interface [7]. The dissolution of a source material, which has the composition profile as shown in fig. 1, causes the Al concentration in the upper part of the solution to increase with time in comparison with the concentration of As. Because the electrotransport streams of solutes are proportional to these concentrations and the condition of solute stream conservation must be fulfilled, these changes of composition should cause the appropriate changes of the solution composition in the vicinity of the layer surface. Therefore, the solution adjacent to the layer-solution phase houndary is continuously enriched with Al. resulting in the growth of a layer with an Al content which increases with thickness, as predicted in fig. I. A different situation occurs when the growth of an epilayer is performed from a thick solution in which the presence of natural convection is expected. Then transport of solutes ts dominated by their diffusion in the thin diffusion layers adjacent to the appropriate solid—liquid interfaces [7]. The rest of the solution is a wide. very-well-mixed zone with the distribution of Al and As, which, to a first approximation, can he taken as uniform. As before, the dissolution of the source material causes the ratio of Al to As concentrations in the melt to increase. However. in contrast to the previous case, this composition change is not transmitted directly to the solution—layer phase boundary. It results predominantly in a change in the ratio of Al to As concentrations in the bulk of the solution. The
ZR. Zytkiewicz
/ Influence of convection on
I, 14mm
021
composition profiles of GaA1As layers grown by LPEE
d~
2
020 ~---a—--~—~--
H
o~to 0 12
0 08
0
st.~ndurd LPE2
~
modified I,02E
~ôo
~~áo distance
from
inter-face
(pm)
Fig. 4. SEM composition profiles of Ga1 _5Al~Aslayers grown with (modified LPEE) and without (standard LPEE) GaAIAs source from 14 mm thick melt; d1 is the theoretical thickness of the layer with rectangular composition profile (dashed line); d2 is the thickness of solid grown until the source is not exhausted (see text).
composition gradient in the source material grown by LPE in the presence of convection is smaller than in the convection-free case [41.Thus, the enrichment of the solution with Al caused by the dissolution the source smaller than before. Moreover, of it occurs in ais large volume of the well-mixed zone and has a small influence on its average composition. Therefore, despite the cornpositional nonuniformity of the source, the bulk of the solution can be considered to be a large reservoir of solutes with constant and spatially uniform composition, as long as the GaA1As source material is not exhausted. This is the straightforward reason for the fact that the composition profiles of the epilayers grown from thick solution are so uniform. However, even if 14 mm thick melt is used for the growth, the effect of GaAlAs depletion can be noticed at 200 ~m from the interface (see fig. 2 and fig. 4 where the composition profile for L 14 mm is repeated). This can be explained taking into account that the new situation occurs in the system when all the source GaA1As is consumed during the growth. In order to discuss this point, let us consider the rectangular shape of the layer composition profile as shown in fig. 4 by a dashed line. If we assume that the total amount m 1 of Al introduced to the growth systern at the saturation temperature is uniformly distributed in the solid, the thickness d1 of such a layer can be easily calculated. Assuming a linear =
429
variation of GaA1As density with xs, this calculation estimates the value of d1 to be 450 ~.im ‘~
for xs=0.2 and L=14 mm. It is obvious that d~ is the theoretical thickness of the uniform layer and this value cannot be reached in the real growth experiment, in which the more diffused xs distribution is expected in the vicinity of d~.Next, if the GaAlAs phase diagram and the melt volume are known, the value of m3 the amount of —
Al necessary to saturate the solution prior to electroepitaxial growth, can be calculated for x 0.2 and the growth temperature of 800°C. Because m3 is that part of the total mass m1 of Al which remains in the solution after the source preparatton step, m2 m~ m3 equals the amount of Al incorporated into the GaAlAs source material during the cooling of the system when the substrate was away from the liquid. As before, m2 can be used to calculate the thickness d2 of the layer grown by LPEE, until the source is not exhausted. From these calculations the5 value 0.2 of to be 220 ~im forwell x with andd2L is estimated 14 mm. This value agrees very the epilayer thickness at which the composition profile starts to decrease (fig. 4). Taking into account the statements above, the nature of the diffusive tail of the composition profile can be explained as being caused by the much faster change of the average composition of the well mixed zone of the solution when all the GaAlAs source material is exhausted. In such a case the bulk of the solution cannot any longer be considered as a large reservoir of solutes with constant composition and the composition profile of the layer is similar to the one typical for the layer grown without the GaAlAs source by standard LPEE (see ref. [3] for growth details). The composition profile of the layer grown from the 14 mm thick melt by standard LPEE technique is included to fig. 4 to show clearly that its shape is very similar to the shape of the diffusive tail of the xs distribution observed in the layer grown by modified LPEE from the same volume of solution. Therefore, we believe that the comparison =
—
=
=
‘~
of both profiles shown in fig. 4 as well as the agreement of our experimental value of d2 with the result of calculations strongly support our interpretation of effects responsible for the xs
7. 1?. Zyjkusj-u-:
43))
/ influence of (0/Il cctioit on i-o;npo 0/eel pro filc,s of Ca—I l—ls ho en i,’roe,l
decrease which appears after the highly uniform part of GaAIAs layer. The Joule effect at the side solution—graphite contact is a problem if we consider the pttrpose of this study. since it limits the range of L values available. It has been estimated [8] that a Ga-rich solution thinner than 5 mm is required to elirninate the convective flows in its volume. Thus, we expect that the solution of L 7 mm is too thick and some residual convection is still present in our system. This convective mixing of the solution volume averages, to some extent, the tiow of solutes from the source to the surface of the layer. Therefore, the value of the positive composition gradient in the layer grown from the 7 mm thick solution (fig. 2) is niuch lower than that expected in the source. In order to obtain a one-to-one correspondence between the composition gradients in the layer and that tn the source. as predicted in fig. 1. the technique of growth from a thin solution should he applied. In the case of electroepitaxy it means that a sandwichtype geometry of the boat with the electric current passing through both the source and the substrate is required for this purpose [9]. Only then can large values of positive composition gradients in the layers he obtained. In this case. however, the layer thicknesses will he markedly reduced. =
-
.
.
.
In i.i’LI.
the solution led to a decrease of the eomposittoti gr;tdient in the source and broke down the direct correspondence between the composition profiles of the source and epilayer. This resulted in a smoothening of Al distribution in Ihe grown (iaAlAs as long as the source (i;tAlAs W;ts piesent in the system. Otherwise, even in the piesonce of convection, the solution became gr;tduallv depleted, which resulted in a progressive decrease of the Al content in the solid grown. Ihus. depending on the absence or presence of convec(ion, layers with increasing Al content or uniform composition can he grown, respectively. The growth of thick GaAIAs layers with both these types of Cofliposition profiles is ver diffieLtlt by any other method of solution growth. Acknowledgements The author would like to thank S. Miotkowska for performing SEM measurements of the cornpositional profiles of GaAIAs layers stttdied. W. Chacinska and D. Dohosz are acknowledged for excellent technical assistance and R. Pritchard (UMIST) for careful reading of the manuscript. This work was partly supported by grant No. 2 2363 92 03 of the Committee for Scientific Research.
References 4. Summary
[I]
The modified LPEE method has been applied to grow thick Ga1 1Al As layers on GaAs sub— strates. We have shown that depending on the thickness of the solution used, highly uniform epilayers its well as layers with an Al content increasing with thickness can he obtained. This is explained by the contribution of convection to the transport of solutes in the liquid. For a thin layer of solution there should he no conveetton and the solutes were transported towards the substrate by electromigration. In this limit, the picture of direct source material recrystallization (fig. I ) is valid and layers with an increasing Al content were grown. When a thick layer of solution was used, the contribution of convection cannot he neglected. The convective mixing of the hulk of -
I. Brvskiewie,’. M. Buga~ski.B. Brvskiewici, .1. Liigowski and H.C. Gatos. ill: Gallium Arsenide and Related Cornilounds 1957. Inst. l’hys. (onf. Ser. 91. Eds. A. (hristou
,~
,nd H.S. Ruppreeht (Inst. Ph~s..london—Bristol. 988) Is. [2] JJ Danicle and A J I leblig..l AppI l’hys 52)1981)4325 [3] ZR. Zytkiewicz and S. Miotkowska. J. Crystal Growth l2! 1992 457. [4] I. Crossley and MB. Small, .1. (rvstal Growth IS (972) -
208.
[~j I. 1 erarnoto. M. Kasamura and 11. ~ Phvs. 18(1979) 151(9.
‘t arnanaka
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.Ltpitn. .1.
[0] S. Isozurni. C.J. Herman. A. Okamoto. J. l,agowski and
Il.C. Gatos. J. Electrochent. Soc. 128 (1981) 222)). [7] L. Jastrzebski, J. Lagowski. H.C. Gatos and A.F. Witt. .1.
App!. Phys. 49 (1975) 5909. with eriatuni in .l.Appl. l’hys. 51) (1979) P69 A. Tillei~J Crystal Growth 2 11908) 69. [9] [ Bryskiewie,. Ct. Boucher,Jr.. J. l.agowski and ll.C, Gatos. J. Crystal Growth 82 (1987) 279. [8]