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The initial stage of LPE growth of InGaAsP on GaAs in the region of immiscibility
978
Journal of Crystal Growth 79 (1986) 978-983 North-Holland, Amsterdam
T H E INITIAL S T A G E OF L P E G R O W T H OF InGaAsP O N GaAs IN T H E R E G I O N OF IMMISCIBILITY Shigeyasu T A N A K A , Kazumasa H I R A M A T S U , Yoshio HABU, Nobuhiko S A W A K I and Isamu A K A S A K I Department of Electronics, School of Engineering. Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan
The LPE growth process during the initial growth stage in the immiscible region of InxGaa xAsyP1 y layers lattice-matched to GaAs was studied for several solid compositions as a function of growth time %. It was found that the LPE process in the region of immiscibilityis classified into three stages as follows: (1) an LPE layer with rather good crystal quality grows because of stabilization by the mismatch strain energy, (2) the crystal quality of the layer becomes gradually worse with increasing tg, and the interface spinodal decomposition is accelerated because the crystal defects weaken the substrate strain stabilizing effect, and then (3) it is impossible to grow the layer. The interfacial spinodal decomposition is more rapid at the solid composition near the center of the immiscible region, and it is slower on a (111)B GaAs than on a (100) GaAs substrate.
1. Introduction T h e I n x G a x _ x A s y P z y q u a t e r n a r y alloy lattice-matched to GaAs is one of the most suitable materials for visible laser diodes. Recently, several authors have reported growth conditions for liquid phase epitaxy (LPE) of In~Gal_xASy P l - y [1-11], and discussed the crystal qualities and physical properties [5,12,13]. De Cremoux et al. [14], Stringfellow [15] and Onabe [16,17] showed by thermodynamic calculation that an immiscible region exists in the quaternary alloy. In addition, it has been argued that once the I n G a A s P epitaxial layer is grown on a G a A s or InP substrate, the quaternary layer is stabilized by the substrate-induced mismatch strain energy and the decomposition into two sofid phases cannot occur [18-22]. Experimentally, however, it has been pointed out that in LPE growth in the immiscible region of I n G a A s P grown on InP [23] or on GaAs [5], it is difficult to grow thick and uniform epilayers of those alloys. Moreover, the widths of the PL spectra as well as the widths of the X-ray rocking curves were large [5,23], and also microscopic periodic compositional, modulations were observed in the I n G a A s P epilayers [24-27], if growths were performed in the immiscible region. These phe-
nomena have been interpreted as spinodal decomposition occurring at the solid-liquid interface during growth [22]. Quillec et al. called this "interfacial spinodal decomposition". This mechanism is still not well understood because the growth process in the immiscible region has not been studied in detail. It is expected that the interfacial spinodal decomposition is concerned with an initial growth stage of LPE because only thin layers ( < 0.5/zm) can be grown during the initial short time [5]. Therefore, it is important to study the initial growth stage of LPE in order to understand the mechanism of the interracial decomposition in the immiscible region. To do this, we need to investigate how the crystal quality of the alloy layer in the immiscible region deteriorates, or how the strain energy induced by the substrate changes during the initial growth stage. The purpose of this paper is to investigate the initial growth stage of LPE in the immiscible region of I n G a A s P lattice-matched to a GaAs substrate. For the first time, the crystal quality is studied as a function of the growth time for the several solid compositions inside and outside the immiscible region, by observing surface morphologies, and measuring PL spectra and X-ray rocking curves. In addition, using (111)B GaAs substrates
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
S. Tanaka et al. / Initial stage of L P E growth of InGaAsP on GaAs
for comparison, the substrate strain stabilizing effect at the initial growth stage is studied.
2. Experimental procedure The procedures of crystal growth are similar to those described in the papers published previously [1,2]. InGaAsP LPE layers were grown by a conventional horizontal sliding-boat technique in a stream of H 2 gas. (100) GaAs substrates were mainly used as substrates, but ( l l l ) B GaAs substrates were also used for comparison. Source melts were prepared from a mixture of InP, GaAs and InAs polycrystals and In metal. We used the two-phase-solution growth technique [9,10], in which more InP polycrystals were added to the source melt than the saturated concentration of P at a given temperature. After the source melt had been kept at 800°C for 60 rain, it was cooled at a rate of 0 . 5 ° C / m i n . The source melt was then applied to the substrate at 785°C. The growth time was varied from 1 s to 10 min. The surface morphology was studied by SEM images. The composition x,y of the InxGal_ x AsyPl_y layers was determined from the lattice constant and the peak photon energy of the PL spectra due to band-edge emission measured at 77 K. The lattice constant was measured by doublecrystal diffractometry and PL spectra were obtained by excitation with light (514.5 nm) from an Ar ion laser. The lattice mismatch normal to the
InAs
GQAs ,
,
I
I
,
i
785°C
08
0.2
B i
]nP
i
i
0.8
0.6
0.4
0.2
iaP
x Fig. 1, Measured solid compositions (e) of InxGa I xAsyPl_v LPE layers grown on (100) GaAs at 785°C and binodal curve and tie lines (solid lines) at 785°C.
979
interface between the InGaAsP layer and the GaAs substrate was < 0.3%. The measured solid compositions are plotted in fig. 1. The atomic fraction of As in the solid, y, was varied from 0.01 to 0.55. An immiscible region in InGaAsP is described by a region inside a binodal curve. The calculated binodal curve and related tie lines for the InGaAsP quaternary system are drawn for 785°C, which corresponds to the growth temperature. Here, the calculation was performed by using the formulas by Onabe [17], with the thermodynamic constants estimated by the DLP model of Stringfellow [28]. Several compositions obtained here are located in the immiscible region.
3. Experimental results
3.1. Surface morphology Fig. 2 shows SEM images of the epitaxial layers grown on the (100) GaAs for various growth times of tg of 1 to 10 s at Tg of 785 ° C. Since the atomic fraction of As, y, of the layers was approximately 0.14, the solid composition was expected to be in the immiscible region, as shown at a point A in fig. 1. The layer thicknesses were 0.13, 0.17, 0.25 and 0.36/~m for tg = 1, 3, 6 and 10 s, respectively. The surfaces of the layers grown for tg < 3 s were flat and mirror-like, but for tg > 6 s, irregularly undulating patterns of about 1 ~m in size appeared on the surfaces. Then, the surface became rapidly so rough that the source melt was not wiped off when tg was longer than 15 s. The layers were also confirmed to be single crystals by a measurement of reflection high energy electron diffraction (RHEED) at t~ of 1 to 10 s. Fig. 3 shows the surface morphologies of the InxGal_xASyPl_y layers for the various tg and y. Here, t c indicates the critical growth time separating the surface morphology into two regions of rough and flat surfaces. For a growth time tg longer than to, the surfaces are rough, while for tg shorter than t c, smooth surfaces are obtained. The surfaces of the layers for y = 0.31, where the instability due to the immiscibility is expected to occur most severely, became rapidly worse at < 3
980
S. Tanaka et
aL/
Initial stage of L P E growth of InGaAsP on GaAs
Fig. 2. SEM images of InxGa 1 _ x A s y P 1 _y (y = 0.14) LPE layer surfaces grown on (100) GaAs at 7 8 5 ° C at various tg of i to 10 s: (a) 1 s; (b) 3 s; (c) 6 s; (d) 10 s. Marker represents 5 #m.
s, suggesting that interfacial spinodal decomposition occurred in an initial very short time. For y < 0.10 or y > 0.50 (outside the immiscible region), however, smooth surfaces can be obtained for tg longer than 30 s. At y < 0.01, especially, the surface was uniform for tg > 10 min [1]. Therefore, t~ depends strongly on the solid composition and it is much shorter at the solid composition near the center of the immiscible region. Thus, it was found that the interracial spinodal decomposition of the alloy layer is accelerated during the growth at around to in the LPE process in the immiscible region. For a comparison, the source melt of the atomic fraction of y = 0.14 was also used for the growth on (111)B GaAs substrates. In this growth, smooth •
~
•
,
.
,
•
,
•
,
•
mirror-like surfaces were obtained for tg < 8 min, but surfaces became rough with increasing tg and the source melt was left on the surfaces for tg > 10 min. The layer thickness was about 0.3 /~m for tg = 3 min. It was also seen that surface morphologies of epilayers at solid compositions outside the immiscible region have a tendency to become gradually worse in several minutes. The lattice mismatches of these layers on the (111)B GaAs were about 0.5% to 0.9% and it was impossible to obtain a lattice mismatch less than 0.4%. It is considered that the crystal quality of the epilayer on the (111)B substrate appears more sensitive to the effect of the large lattice mismatch than that of the immiscibility. Thus, the LPE growth on (111)B GaAs is less severely affected by the immiscibility of the solid phase, compared with that on (100) GaAs.
In×Gal-×AsyPl-y on (100)GoAs
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Fig. 3. Surface morphologies of InxGaa _ x Asy Pt -y LPE layers grown on (100) GaAs at 7 8 5 ° C for various tg and y.
Fig. 4 shows the PL spectra of the epilayers with an As atomic fraction of 0.14 (point A in fig. 1). At a tg of 1 to 3 s, a single peak due to the band-edge emission on the higher energy side is observed, while a t lg of 6 to 10 S a new peak appears on the lower energy side. The intensity of this peak increases with increasing tg. Thus, the crystal quality becomes worse with increasing tg, which corresponds to the increase in the surface roughness. The lower energy peak may be attributed to the following; (1) growth of a composition (point B in fig. 1) on the opposite side to the
S. Tanaka et al. / Initial stage of L P E growth of InGaAsP on GaAs
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(422)REFLECTION
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981
InGoAsP/GoAs Tg=785°C <
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I
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z~ 1.6 1.7 1.8 1.9 PHOTON ENERGY (eV )
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-600
I
-800
Fig. 6. X-ray rocking curves of InxGa 1 xASyPl_y (y = 0.14) LPE layers for various tg of i to 10 s.
Fig. 4. PL spectra of I n x G a l_xAsyP1 y ( y = 0.14) LPE layers grown on (100) GaAs at 7 8 5 ° C for various tg of 1 to 10 s.
3. 3. X-ray rocking curve
lattice-matched composition (point A in fig. 1) along the tie line in the immiscible region [21,22] a n d / o r (2) deep levels due to crystal defects related to the surface roughness [5], but its origin is not well understood. Fig. 5 shows the F W H M due to the band-edge emission for various atomic fractions y of < 0.01, 0.06 and 0.14 as a function of tg. At y = 0.14 (in the immiscible region) the F W H M increases with increasing tg, whereas at y < 0.01 (outside the region) the F W H M decreases and the crystal quality becomes better with increasing %. The crystal quality in the immiscible region also varies rapidly during the first few seconds of growth.
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4. Discussion and conclusions
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We measured X-ray rocking curves (using (422) asymmetric reflection) of the layers of y = 0.14 grown on (100) substrates for various t~ of I to 10 s, as shown in fig. 6. A single peak is observed for a tg of 1 to 3 S, while for a tg of 6 to 10 s wider peaks appear on the larger angle side and the width of the peak becomes wider gradually as tg is increased. The F W H M of the rocking curve varies from 100 to 250 s in angle with increasing tg from 1 to 10 s. These results indicate that the lattice constant of InGaAsP layers varies especially at tg > 6 s, which also means that the large compositional variation occurs at tg > 6 s. Such large compositional variations could not be observed in the InxGal_xASyP 1 y layers of y < 0.01, although the compositional variation at the initial short growth time occurred at < 50 s in angle [29].
I
10
i
I
20
310
tg (s) Fig. 5. PL F W H M due to the band-edge emission of I n x G a l _ x A S y P 1 _y LPE layers grown on (100) GaAs at 7 8 5 ° C for various y of < 0.01, 0.06 and 0.14 as a function of tg.
The results obtained above indicate that the LPE process for the initial growth in the immiscible region is classified into three stages. In the first stage, an epilayer with rather good crystal quality grows on the substrate. In InxGal_xASyPl_y ( y = 0.14) on (100) GaAs this stage corresponds t o tg < 3 S. It has been argued theoretically that once the InGaAsP epilayer is grown on the substrate, the layer is stabilized by
982
S. Tanaka et al. / Initial stage of LPE growth of lnGaAsP on GaAs
the substrate-induced mismatch strain energy, and the decomposition into two solid phase cannot occur. In the first stage, therefore, because of this effect, the LPE growth is possible even in the immiscible region. In the second stage, the crystal quality of the LPE layer becomes gradually worse with increasing growth time. These phenomena were observed at tg of 3 to 10 s in I n x G a i _ x A S y P i _ y ( y = 0.14) on (100) GaAs. Here, the surface becomes nonuniform, and then a different composition from the lattice-matched composition a n d / o r deep levels related to crystal defects appear during growth. In addition, the compositional variation increases with increasing tg. These results suggest that, in the second stage, the spinodal decomposition at the solid-liquid interface starts to occur and then this decomposition causes the solid composition of the InGaAsP layers to fluctuate. Such compositional fluctuation is probably attributed to a local composition modulation of the order of 0.1 /~m scale, as pointed out by Quillec [22]. Local strain energy due to this local composition modulation may cause the defects of the crystal and then the rough surface. Hence, since the crystal defects weaken the substrate strain stabilizing effect, the spinodal decomposition may be accelerated with increasing growth time. In this stage, thus, the interfacial spinodal decomposition is occurring during growth, while degrading the crystal quality. In the third stage, the substrate strain stabilizing effect almost disappears and then it is impossible to grow the e p i l a y e r on the substrate. In I n x G a l _ x A S y P l _ y ( y = 0.14) on (100) GaAs this stage corresponds to tg > 10 s, and the source melt is kept on the surface because of the large surface roughness. In addition, the interfacial spinodal decomposition is weaker on the ( l l l ) B substrate, compared with the (100) substrate. This may be attributed to the effect that the stabilization due to substrate strain is intensified because the strain energy induced by lattice-mismatch between the substrate and the epilayer in ( l l l ) B GaAs is about twice as large as that in (100) GaAs [30]. Furthermore, the growth rate on the (111) B GaAs was much slower than that on the (100) GaAs, suggesting that the decrease in the growth rate on the (111)B de-
creases a speed of the interfacial spinodal decomposition. However, since the relation between the growth rate and the spinodal decomposition is not clear in detail, this is the subject for a future study. In conclusion, in the LPE growth process in the immiscible region, the interfacial spinodal decomposition is accelerated by the decrease in the substrate stabilizing effect with increasing defects a n d / o r compositional variation during the LPE growth. These tendencies depend strongly on the solid composition in the immiscible region and the orientation of the substrate. The interfacial spinodal decomposition is more rapid at the solid composition near the center of the immiscible region, and it is slower on (111)B GaAs substrates than on (100) GaAs. From the results mentioned above, in order to obtain uniform epitaxial alloy layers with the solid composition in the immiscible region, we propose to repeat the short-growth-time (less than a few seconds) LPE and then to form multiple alloy layers by using the uniform layer grown in the first growth stage.
Acknowledgements The authors wish to thank Dr. Y. Toyoda of Matsushita Electric Ind. Co. for his help in carrying out the X-ray diffractometry and for valuable discussions. This work was supported in part by the Scientific Research Grant-in-Aid No. 60222025 for Special Project Research on "Alloy Semiconductor Electronics" from the Ministry of Education, Science and Culture of Japan.
References [1] K. Hiramatsu, K. Tomita, N. Sawaki and I. Akasaki, Japan. J. Appl. Phys. 23 (1984) 68. [2] K. Hiramatsu, S. Tanaka, N. Sawaki and I. Akasaki, Japan. J. Appl. Phys. 24 (1985) 822. [3] S. Mukai, M. Matsuzald and J. Shimada, Japan. J. Appl. Phys. 19 (1980) L505. [41 S. Mukai, M. Matsuzaki and J. Shimada, Japan. J. Appl. Phys. 20 (1981) 321. [5] S. Mukai, J. Appl. Phys. 54 (1983) 2635.
S. Tanaka et al. / Initial stage of L P E growth of InGaAsP on GaAs
[6] A. Suzuki, H. Kyuragi, S. Matsumura and H. Matsunami, Japan. J. Appl. Phys. 19 (1980) L207. [7] A. Suzuki, T. Murakami, Y. Kuriyama and H. Matsunami, Japan. J. Appl. Phys. 21 (1982) L363. [8] S. Kaneiwa, T. Takenaka, S. Yano and T. Hijikata, J. Crystal Growth 62 (1983) 498. [9] H. Kawanishi and T. Sizuki, Japan. J. Appl. PHys. 23 (1984) L52. [10] S. Shirakata, M. Kondo, A. Tsushi, T. Nishino, Y. Hamakawa and T. Kariya, Japan. J. Appl. Phys. 24 (1985) 524. [11] T. Kato, T. Matsumoto and T. Ishida, Japan. J. Appl. Phys. 21 (1982) L667. [12] H. Kyuragi, A. Suzuki, S. Matsumura and H. Matsunami, Appl. Phys. Letters 37 (1980) 723. [13] T. Inoshita and A. Usui, Japan. J. Appl. Phys. 23 (1984) L135. [14] B. De Cremoux, P. Hirth and J. Ricciardi, in: Proc. 8th Intern. Symp. on GaAs and Related Compounds, Vienna, 1980, Inst. Phys. Conf. Ser. 56, Ed. H.W. Thim (Inst. Phys., London-Bristol, 1981) p. 115. [15] G.B. Stringfellow, J. Crystal Growth, 58 (1982) 194. [16] K. Onabe, Japan. J. Appl. Phys. 21 (1982) 797. [17] K. Onabe, NEC Res. Develop. No. 72 (1984) 1. [18] M. Ishikawa and R. Ito, Japan. J. Appl. Phys. 23 (1984) L21.
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[19] G.B. Stringfellow, J. Electron. Mater. I1 (1982) 903. [20] R.E. Nahory, M.A. Pollack, E.D. Beebe and J.C. De Winter, J. Electrochem. Soc. 7 (1978) 1053. [21] M. Quillec, C. Daguet, J.L. Benchimol and H. Launois, Appl. Phys. Letters 40 (1982) 325. [22] M. Quillec, H. Launois and M.C. Joncour, J. Vacuum Sci. Technol. B1 (1983) 238. [23] K. Takahei and H. Nagai, Japan. J. Appl. Phys. 20 (1981) L313. [24] P. Henoc, A. Izrael, M. Quillec and H. Launois, Appl. Phys. Letters 40 (1982) 963. [25] O. Ueda, S. Isozumi and S. Komiya, Japan. J. Appl. Phys. 23 (1984) L241. [26] S.N.G. Chu, S. Nakahara, K.E. Strege and W.D. Johnston, Jr., J. Appl. Phys. 57 (1985) 4610. [27] A.G. Norman and G.R. Booker, J. Appl. Phys. 57 (1985) 4715. [28] G.B. Stringfellow, J. Crystal Growth 27 (1974) 21. [29] K. Hiramatsu, K. Tomita, N. Sawaki and I. Akasaki, in: Proc. 16th Intern. Conf. on Solid State Devices and Materials, Kobe, 1984 (Academic Societies, Japan, Tokyo, 1984) p. 201. [30] W.A. Brantley, J. Appl. Phys. 44 (1973) 534.
Journal of Crystal Growth 79 (1986) 978-983 North-Holland, Amsterdam
T H E INITIAL S T A G E OF L P E G R O W T H OF InGaAsP O N GaAs IN T H E R E G I O N OF IMMISCIBILITY Shigeyasu T A N A K A , Kazumasa H I R A M A T S U , Yoshio HABU, Nobuhiko S A W A K I and Isamu A K A S A K I Department of Electronics, School of Engineering. Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan
The LPE growth process during the initial growth stage in the immiscible region of InxGaa xAsyP1 y layers lattice-matched to GaAs was studied for several solid compositions as a function of growth time %. It was found that the LPE process in the region of immiscibilityis classified into three stages as follows: (1) an LPE layer with rather good crystal quality grows because of stabilization by the mismatch strain energy, (2) the crystal quality of the layer becomes gradually worse with increasing tg, and the interface spinodal decomposition is accelerated because the crystal defects weaken the substrate strain stabilizing effect, and then (3) it is impossible to grow the layer. The interfacial spinodal decomposition is more rapid at the solid composition near the center of the immiscible region, and it is slower on a (111)B GaAs than on a (100) GaAs substrate.
1. Introduction T h e I n x G a x _ x A s y P z y q u a t e r n a r y alloy lattice-matched to GaAs is one of the most suitable materials for visible laser diodes. Recently, several authors have reported growth conditions for liquid phase epitaxy (LPE) of In~Gal_xASy P l - y [1-11], and discussed the crystal qualities and physical properties [5,12,13]. De Cremoux et al. [14], Stringfellow [15] and Onabe [16,17] showed by thermodynamic calculation that an immiscible region exists in the quaternary alloy. In addition, it has been argued that once the I n G a A s P epitaxial layer is grown on a G a A s or InP substrate, the quaternary layer is stabilized by the substrate-induced mismatch strain energy and the decomposition into two sofid phases cannot occur [18-22]. Experimentally, however, it has been pointed out that in LPE growth in the immiscible region of I n G a A s P grown on InP [23] or on GaAs [5], it is difficult to grow thick and uniform epilayers of those alloys. Moreover, the widths of the PL spectra as well as the widths of the X-ray rocking curves were large [5,23], and also microscopic periodic compositional, modulations were observed in the I n G a A s P epilayers [24-27], if growths were performed in the immiscible region. These phe-
nomena have been interpreted as spinodal decomposition occurring at the solid-liquid interface during growth [22]. Quillec et al. called this "interfacial spinodal decomposition". This mechanism is still not well understood because the growth process in the immiscible region has not been studied in detail. It is expected that the interfacial spinodal decomposition is concerned with an initial growth stage of LPE because only thin layers ( < 0.5/zm) can be grown during the initial short time [5]. Therefore, it is important to study the initial growth stage of LPE in order to understand the mechanism of the interracial decomposition in the immiscible region. To do this, we need to investigate how the crystal quality of the alloy layer in the immiscible region deteriorates, or how the strain energy induced by the substrate changes during the initial growth stage. The purpose of this paper is to investigate the initial growth stage of LPE in the immiscible region of I n G a A s P lattice-matched to a GaAs substrate. For the first time, the crystal quality is studied as a function of the growth time for the several solid compositions inside and outside the immiscible region, by observing surface morphologies, and measuring PL spectra and X-ray rocking curves. In addition, using (111)B GaAs substrates
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
S. Tanaka et al. / Initial stage of L P E growth of InGaAsP on GaAs
for comparison, the substrate strain stabilizing effect at the initial growth stage is studied.
2. Experimental procedure The procedures of crystal growth are similar to those described in the papers published previously [1,2]. InGaAsP LPE layers were grown by a conventional horizontal sliding-boat technique in a stream of H 2 gas. (100) GaAs substrates were mainly used as substrates, but ( l l l ) B GaAs substrates were also used for comparison. Source melts were prepared from a mixture of InP, GaAs and InAs polycrystals and In metal. We used the two-phase-solution growth technique [9,10], in which more InP polycrystals were added to the source melt than the saturated concentration of P at a given temperature. After the source melt had been kept at 800°C for 60 rain, it was cooled at a rate of 0 . 5 ° C / m i n . The source melt was then applied to the substrate at 785°C. The growth time was varied from 1 s to 10 min. The surface morphology was studied by SEM images. The composition x,y of the InxGal_ x AsyPl_y layers was determined from the lattice constant and the peak photon energy of the PL spectra due to band-edge emission measured at 77 K. The lattice constant was measured by doublecrystal diffractometry and PL spectra were obtained by excitation with light (514.5 nm) from an Ar ion laser. The lattice mismatch normal to the
InAs
GQAs ,
,
I
I
,
i
785°C
08
0.2
B i
]nP
i
i
0.8
0.6
0.4
0.2
iaP
x Fig. 1, Measured solid compositions (e) of InxGa I xAsyPl_v LPE layers grown on (100) GaAs at 785°C and binodal curve and tie lines (solid lines) at 785°C.
979
interface between the InGaAsP layer and the GaAs substrate was < 0.3%. The measured solid compositions are plotted in fig. 1. The atomic fraction of As in the solid, y, was varied from 0.01 to 0.55. An immiscible region in InGaAsP is described by a region inside a binodal curve. The calculated binodal curve and related tie lines for the InGaAsP quaternary system are drawn for 785°C, which corresponds to the growth temperature. Here, the calculation was performed by using the formulas by Onabe [17], with the thermodynamic constants estimated by the DLP model of Stringfellow [28]. Several compositions obtained here are located in the immiscible region.
3. Experimental results
3.1. Surface morphology Fig. 2 shows SEM images of the epitaxial layers grown on the (100) GaAs for various growth times of tg of 1 to 10 s at Tg of 785 ° C. Since the atomic fraction of As, y, of the layers was approximately 0.14, the solid composition was expected to be in the immiscible region, as shown at a point A in fig. 1. The layer thicknesses were 0.13, 0.17, 0.25 and 0.36/~m for tg = 1, 3, 6 and 10 s, respectively. The surfaces of the layers grown for tg < 3 s were flat and mirror-like, but for tg > 6 s, irregularly undulating patterns of about 1 ~m in size appeared on the surfaces. Then, the surface became rapidly so rough that the source melt was not wiped off when tg was longer than 15 s. The layers were also confirmed to be single crystals by a measurement of reflection high energy electron diffraction (RHEED) at t~ of 1 to 10 s. Fig. 3 shows the surface morphologies of the InxGal_xASyPl_y layers for the various tg and y. Here, t c indicates the critical growth time separating the surface morphology into two regions of rough and flat surfaces. For a growth time tg longer than to, the surfaces are rough, while for tg shorter than t c, smooth surfaces are obtained. The surfaces of the layers for y = 0.31, where the instability due to the immiscibility is expected to occur most severely, became rapidly worse at < 3
980
S. Tanaka et
aL/
Initial stage of L P E growth of InGaAsP on GaAs
Fig. 2. SEM images of InxGa 1 _ x A s y P 1 _y (y = 0.14) LPE layer surfaces grown on (100) GaAs at 7 8 5 ° C at various tg of i to 10 s: (a) 1 s; (b) 3 s; (c) 6 s; (d) 10 s. Marker represents 5 #m.
s, suggesting that interfacial spinodal decomposition occurred in an initial very short time. For y < 0.10 or y > 0.50 (outside the immiscible region), however, smooth surfaces can be obtained for tg longer than 30 s. At y < 0.01, especially, the surface was uniform for tg > 10 min [1]. Therefore, t~ depends strongly on the solid composition and it is much shorter at the solid composition near the center of the immiscible region. Thus, it was found that the interracial spinodal decomposition of the alloy layer is accelerated during the growth at around to in the LPE process in the immiscible region. For a comparison, the source melt of the atomic fraction of y = 0.14 was also used for the growth on (111)B GaAs substrates. In this growth, smooth •
~
•
,
.
,
•
,
•
,
•
mirror-like surfaces were obtained for tg < 8 min, but surfaces became rough with increasing tg and the source melt was left on the surfaces for tg > 10 min. The layer thickness was about 0.3 /~m for tg = 3 min. It was also seen that surface morphologies of epilayers at solid compositions outside the immiscible region have a tendency to become gradually worse in several minutes. The lattice mismatches of these layers on the (111)B GaAs were about 0.5% to 0.9% and it was impossible to obtain a lattice mismatch less than 0.4%. It is considered that the crystal quality of the epilayer on the (111)B substrate appears more sensitive to the effect of the large lattice mismatch than that of the immiscibility. Thus, the LPE growth on (111)B GaAs is less severely affected by the immiscibility of the solid phase, compared with that on (100) GaAs.
In×Gal-×AsyPl-y on (100)GoAs
100
fiat surface rough surface
0
•
Tg=78~c
50
3.2. Photoluminescence spectrum ,0
0
o
%
3O
/
•
O ~ o
10
~
0
/ ,
\
O
/"
/ o
/
~ •
,o
/
5 3
•
O
/
O
O
/ O ~ i
1
.
0
.
.
01
.
.
.
.
.
0.2
~
0.3 Y
0.4
"
'
"
0.5 0.6
Fig. 3. Surface morphologies of InxGaa _ x Asy Pt -y LPE layers grown on (100) GaAs at 7 8 5 ° C for various tg and y.
Fig. 4 shows the PL spectra of the epilayers with an As atomic fraction of 0.14 (point A in fig. 1). At a tg of 1 to 3 s, a single peak due to the band-edge emission on the higher energy side is observed, while a t lg of 6 to 10 S a new peak appears on the lower energy side. The intensity of this peak increases with increasing tg. Thus, the crystal quality becomes worse with increasing tg, which corresponds to the increase in the surface roughness. The lower energy peak may be attributed to the following; (1) growth of a composition (point B in fig. 1) on the opposite side to the
S. Tanaka et al. / Initial stage of L P E growth of InGaAsP on GaAs
•I
(422)REFLECTION
A
PL 77K
981
InGoAsP/GoAs Tg=785°C <
~_
6s
5
[
200
I
0
I
I
I
-200
z~ 1.6 1.7 1.8 1.9 PHOTON ENERGY (eV )
I
-400 (s)
I
I
-600
I
-800
Fig. 6. X-ray rocking curves of InxGa 1 xASyPl_y (y = 0.14) LPE layers for various tg of i to 10 s.
Fig. 4. PL spectra of I n x G a l_xAsyP1 y ( y = 0.14) LPE layers grown on (100) GaAs at 7 8 5 ° C for various tg of 1 to 10 s.
3. 3. X-ray rocking curve
lattice-matched composition (point A in fig. 1) along the tie line in the immiscible region [21,22] a n d / o r (2) deep levels due to crystal defects related to the surface roughness [5], but its origin is not well understood. Fig. 5 shows the F W H M due to the band-edge emission for various atomic fractions y of < 0.01, 0.06 and 0.14 as a function of tg. At y = 0.14 (in the immiscible region) the F W H M increases with increasing tg, whereas at y < 0.01 (outside the region) the F W H M decreases and the crystal quality becomes better with increasing %. The crystal quality in the immiscible region also varies rapidly during the first few seconds of growth.
'
~50
u" Y=0.14
E v40
,"
~.
U_
j
A
-$ ............
20
i
InxGol-xAsyPl-y/GoAs
Tg=785°C
zx
30
n
i
..n . . . . . ~"
-x . . . . . .
4. Discussion and conclusions
7-: 6:d~-2 ..............
"-
"-o-....................... ~__ y <0.01
10 0
0
We measured X-ray rocking curves (using (422) asymmetric reflection) of the layers of y = 0.14 grown on (100) substrates for various t~ of I to 10 s, as shown in fig. 6. A single peak is observed for a tg of 1 to 3 S, while for a tg of 6 to 10 s wider peaks appear on the larger angle side and the width of the peak becomes wider gradually as tg is increased. The F W H M of the rocking curve varies from 100 to 250 s in angle with increasing tg from 1 to 10 s. These results indicate that the lattice constant of InGaAsP layers varies especially at tg > 6 s, which also means that the large compositional variation occurs at tg > 6 s. Such large compositional variations could not be observed in the InxGal_xASyP 1 y layers of y < 0.01, although the compositional variation at the initial short growth time occurred at < 50 s in angle [29].
I
10
i
I
20
310
tg (s) Fig. 5. PL F W H M due to the band-edge emission of I n x G a l _ x A S y P 1 _y LPE layers grown on (100) GaAs at 7 8 5 ° C for various y of < 0.01, 0.06 and 0.14 as a function of tg.
The results obtained above indicate that the LPE process for the initial growth in the immiscible region is classified into three stages. In the first stage, an epilayer with rather good crystal quality grows on the substrate. In InxGal_xASyPl_y ( y = 0.14) on (100) GaAs this stage corresponds t o tg < 3 S. It has been argued theoretically that once the InGaAsP epilayer is grown on the substrate, the layer is stabilized by
982
S. Tanaka et al. / Initial stage of LPE growth of lnGaAsP on GaAs
the substrate-induced mismatch strain energy, and the decomposition into two solid phase cannot occur. In the first stage, therefore, because of this effect, the LPE growth is possible even in the immiscible region. In the second stage, the crystal quality of the LPE layer becomes gradually worse with increasing growth time. These phenomena were observed at tg of 3 to 10 s in I n x G a i _ x A S y P i _ y ( y = 0.14) on (100) GaAs. Here, the surface becomes nonuniform, and then a different composition from the lattice-matched composition a n d / o r deep levels related to crystal defects appear during growth. In addition, the compositional variation increases with increasing tg. These results suggest that, in the second stage, the spinodal decomposition at the solid-liquid interface starts to occur and then this decomposition causes the solid composition of the InGaAsP layers to fluctuate. Such compositional fluctuation is probably attributed to a local composition modulation of the order of 0.1 /~m scale, as pointed out by Quillec [22]. Local strain energy due to this local composition modulation may cause the defects of the crystal and then the rough surface. Hence, since the crystal defects weaken the substrate strain stabilizing effect, the spinodal decomposition may be accelerated with increasing growth time. In this stage, thus, the interfacial spinodal decomposition is occurring during growth, while degrading the crystal quality. In the third stage, the substrate strain stabilizing effect almost disappears and then it is impossible to grow the e p i l a y e r on the substrate. In I n x G a l _ x A S y P l _ y ( y = 0.14) on (100) GaAs this stage corresponds to tg > 10 s, and the source melt is kept on the surface because of the large surface roughness. In addition, the interfacial spinodal decomposition is weaker on the ( l l l ) B substrate, compared with the (100) substrate. This may be attributed to the effect that the stabilization due to substrate strain is intensified because the strain energy induced by lattice-mismatch between the substrate and the epilayer in ( l l l ) B GaAs is about twice as large as that in (100) GaAs [30]. Furthermore, the growth rate on the (111) B GaAs was much slower than that on the (100) GaAs, suggesting that the decrease in the growth rate on the (111)B de-
creases a speed of the interfacial spinodal decomposition. However, since the relation between the growth rate and the spinodal decomposition is not clear in detail, this is the subject for a future study. In conclusion, in the LPE growth process in the immiscible region, the interfacial spinodal decomposition is accelerated by the decrease in the substrate stabilizing effect with increasing defects a n d / o r compositional variation during the LPE growth. These tendencies depend strongly on the solid composition in the immiscible region and the orientation of the substrate. The interfacial spinodal decomposition is more rapid at the solid composition near the center of the immiscible region, and it is slower on (111)B GaAs substrates than on (100) GaAs. From the results mentioned above, in order to obtain uniform epitaxial alloy layers with the solid composition in the immiscible region, we propose to repeat the short-growth-time (less than a few seconds) LPE and then to form multiple alloy layers by using the uniform layer grown in the first growth stage.
Acknowledgements The authors wish to thank Dr. Y. Toyoda of Matsushita Electric Ind. Co. for his help in carrying out the X-ray diffractometry and for valuable discussions. This work was supported in part by the Scientific Research Grant-in-Aid No. 60222025 for Special Project Research on "Alloy Semiconductor Electronics" from the Ministry of Education, Science and Culture of Japan.
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S. Tanaka et al. / Initial stage of L P E growth of InGaAsP on GaAs
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