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Selective area growth of InGaAsP by OMVPE
Journal of Crystal Growth 123 (1992) 69—74 North-Holland
jo~~ or
CRYSTAL GROWTH
Selective area growth of InGaAsP by OMVPE Moo-Sung Kim, Catherine Caneau, Etienne Colas and Rajaram Bhat Bellcore, 331 Newman Springs Road, Red Bank, New Jersey 07701-7040, USA Received 4 February 1992; manuscript received in final form 19 March 1992
We investigated the selective area growth of A = 1.3 ~sm InGaAsP by low pressure OMVPE, and compared it with the selective growth behavior of InP, InGaAs, and InGaP. We found that, for all three alloys, the In content increased with decreasing stripe opening width. The growth rate enhancement is similar for InGaAsP, InGaAs, and InGaP, while it is somewhat higher for InP. Growth spikes at the stripe opening edges could be seen in the case of InP and InGaAsP, while InGaAs and InGaP did not exhibit such spikes.
1. Introduction Selective area growth of compound semiconductors is an attractive technique because of its potential applications to the fabrication of optoelectronic integrated circuits. In organometallic vapor phase epitaxy (OMVPE), the growth species can desorb from a dielectric mask, which makes selective growth possible. Diffusion of the growth species from the masked areas to the stripe openings causes a growth rate enhancement, a nonuniform lateral thickness profile and, in the case of an alloy, a compositional change. A growth rate enhancement of up to about 10 times the nominal growth rate has been observed [1,21. This growth rate enhancement reportedly results fom gas phase diffusion [1,3,4], rather than from surface migration [2,51.In the case of GaAs/A1GaAs, the growth rate enhancement was used to advantage in the fabrication of tapered waveguides [61. The compositional changes of epilayers grown in stripe openings have been reported for InGaAs and InGaP [7—11],the In content increasing with decreasing stripe opening width. Also, especially in the case of In-based materials, the existence of
Permanent address: KIST, P.O. Box 131, Cheongryang, Seoul 130-560, South Korea 0022-0248/92/$05.00 © 1992
—
edge spike growth has been observed when the selective growth is limited by a low growth rate (111)B plane [2,3,111. To eliminate the edge spikes, atomic layer epitaxy [121, or the use of a polycrystalline InP or AIN mask instead of Si02
[13—151 have been tried. However, a strong shift of composition was observed for InGaAs grown in openings in a polycrystalline InP mask [14]. Early studies of selective area growth were only on GaAs and AlGaAs [4,15,16]but, more recently, extensive work has been done on InP, InGaAs, and InGaP [2,3,5,7,9,11].The mechanism of the selective area growth, however, is not fully understood. Also, selective area growth of quaternary materials such as InGaAsP has, to our knowledge, not been investigated. In this paper, we report the study of the selective area growth, by low pressure OMVPE, of A = 1.3 ~tm InGaAsP lattice matched to InP on Si02 masked substrates, and compare it with the selective growth behaviour of InP, InGaAs (lattice-matched to InP), and InGaP (lattice-matched to GaAs).
2. Experimental procedure Growths were performed in a horizontal OMVPE reactor, at 76 Torr and 625°C, using trimethylindium (TM In), trimethylgallium
Elsevier Science Publishers B.V. All rights reserved
70
Moo-Sung Kim et a1.
/ Selectice area growth of InGaAsP by OMVPE
(TMGa), arsine (AsH3) and phosphine (PH3) as group III and V sources, and H2 are carrier gas. On each 5 cm diameter (100) InP or GaAs substrate, (1 cm x 1 cm) areas were patterned with 20 to 55 ~tm wide stripes parallel to [011] opened in Si02 (period 250 ~tm), leaving a wide unmasked area for reference. The growth conditions are summarized in table 1. 0.2 ~m In076 Ga0 24As054P046 with a 680 A thick cap InP, and 0.2 i~m1n053Ga047As with a 680 A thick cap InP were grown on InP substrates; 0.2 j~m In048Ga052P was grown on a GaAs substrate. The thicknesses and compositions refer to the unmasked areas. After growth, the samples were cleaved perpendicular to the stripe direction and stain-etched for thickness measurement in a scanfling electron microscope (SEM). The compositions of the grown layers were deduced from double crystal X-ray diffractometry and (in the case of the quaternary layer) room temperature photoluminescence (PL) spectra.
3. Results Fig. 1 shows the variation of growth rate enhancement vs stripe opening width W for InP, InGaAs, InGaP, and InGaAsP. The growth rate enhancement is similar for InGaAs, InGaP, and InGaAsP, while it is 30% higher for InP. Fig. 2 shows the dependence of the lattice mismatch between InGaAs or InGaAsP and InP, and of the PL peak wavelength of InGaAsP, upon opening width. Over the large unmasked region, the lattice mismatch L~a/awas less than 5 X iO~,and the PL peak wavelength of InGaAsP was 1.30 ~m. Using the data of fig. 2 and the curves of ref. —~
1 4
1 2
I
-
I
•\\
8
I
X
InP
0
InGaP
S
InGaAsP
\~
InGaAs
6
• 2
° I
I
I
STRIPE OPENING WIDTH, W (tim)
1 0
60
Fig. 1. Variation of the growth rate enhancement of InP InGaAs, lnGaAsP, and InGaP with stripe opening width.
[17], the compositions of InGaAs and InGaAsP were calculated and are plotted in fig. 3. The Indium content x1~ of InGaAsP and InGaAs increases with decreasing W. Fewer measurements were done for InGaP; in agreement with ref. [9], we also observe a moderate increase in x1~for InGaP grown in stripe openings, to 0.51 for W= 50 ~m instead of 0.48 for the large unmasked area. The P content of InGaAsP changes very little with opening width (by ±0.01). The morphology of the growth in stripe openings is rather poor, due to the lattice-mismatch and the high growth rate. The X-ray diffraction spectra taken in the middle of each square of stripe pattern (the length of the X-ray spot was about 1.5 mm and perpendicular to the stripe direction, the spot width was 0.1 mm) show broad peaks. This is due both to the poor quality of the matenat (as reflected by the morphology) and to the expected change in composition [7,9] within the
Table 1 Experimental conditions used for growth of InP, InGaAs, InGaP, and InGaAsP; the growth rates and In mole fractions x1,, are for growth on unmasked substrates
InP InGaAs InGaAsP InGaP
TMIn mole fraction
Input V/Ill ratio
Growth rate (pm/h)
x1,,
Edge growth?
Growth on (11l)B?
5 X iO~ 5 X iO~ 5 X iO~5 2.5x10~
480 200 380 480
2.05 3.8 2.8 2.2
1 0.53 0.76 0.48
Yes No Yes No
Little No Little Yes
Moo-Sung Kim et al. 5 I
Selective area growth of InGaAsP by OMVPE
1.5
I
~
/
InGaAs
71
.70 .~
+
.85
+
~GaAsP
0
~GaAs. 20% relaxatIon
I
0 STRIPE OPENING WIDTH, W (pm)
Fig. 2. Dependence of the lattice mismatch between InGaAs (~)and InGaAsP (.) and InP, and of the room temperature FL peak wavelength of InGaAsP (0) with stripe opening width,
width of each stripe opening. The data shown in figs. 2 and 3 were obtained using the X-ray peak maxima, and assuming no lattice relaxation. The values of x1~calculated for a 20% lattice relaxation are also shown in fig. 3 for InGaAs. Assuming the same relaxation for InGaAsP induces very little change in the calculated compositions. It is well known that strain changes the bandgap of materials. A lattice-mismatch of +4 X iO~ would shift the PL wavelength of InGaAs by —0.02 ~m [18]. Using the same shift for InGaAsP only leads to a very small change in the calculated values of x1~and YAs~
STRFE OPENI’IG WIDTH (iun)
Fig. 3. Variation of the In content of InGaAsP and InGaAs with stripe opening width. For InGaAs, the compositions were deduced from the X-ray spectra assuming either no lattice relaxation or 20% relaxation.
Fig. 4 shows the cross-sections of the growth in stripe openings for InGaAs/InP, InGaAsP/InP, and InGaP. There is no edge spike growth in the case of InGaP and InGaAs, but both InP and InGaAsP show such a feature, irrespective of the stripe opening width.
4. Discussion 4.1. Increase in In content for compounds grown in stripe openings An increase in x1~has already been reported for InGaAs grown in large windows opened in a
Fig. 4. Cross-section of the stripe opening edge for(a) InGaAsP+ cap InP, (b) InGaAs+cap InP, and (c) InGaP. Markers represent I ~m.
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Moo-Sung Kim et al.
/
Selectite area growth of InGaAsP by OMVPE
Si02 mask [7,9], x1~ decreasing with distance away from the mask edge. This was attributed to a longer migration length of the In than of the Ga species; as argued in ref. [9], it is gas phase diffusion rather than surface diffusion which should be the main factor in the composition change. In ref. [91,it was also found that, when InGaAs is grown in 20 j~mwide open stripes, x1~, increases as the width of the adjacent masked area increases. The present study shows that, for a constant period of 250 ~tm, x1~increases as the open stripe width decreases, as can be expected from the findings of refs. [71and [9]. The extra amount of In (compared to Ga) found close to a mask edge distributes itself over the width of the stripe. It can still be expected that the alloy composition is richer in In close to the stripe edge, but the average value of x1~,,as well as its value in the middle of the stripe, will be higher for narrower stripes, The difference in migration length between the In and Ga species could be due to different sticking coefficients on the mask. As shown in ref. [19], however, the influence of the sticking coefficient is small: a sticking coefficient of 0.5 instead of 0 leads all other parameters being equal to an 8% increase in growth rate enhancement. The difference in the migration lengths of In and Ga species could also be due to a difference in gas phase diffusion of the Ga and In species, as explained in the following. Ref. [20] gives the expression for bimolecular gas diffusion: 3~2 MA + MB DAB PO~Bfl2 T 2MAMB —
—
—
‘
DAB being the diffusion coefficient of gas A in gas B, T the temperature, p the pressure, and MA and MB the molecular masses of gases A and B. Since MB (H 2 mass) is much smaller than MA (alkyl mass), the diffusion coefficient of an alkyl in H2 is nearly independent of its mass. ~2 is a collision integral, which is a function of the temperature and of the energy of attraction between A and B.alkyls The difference in diffusion between is mainly caused by 0’ABcoefficients ~ (o~~ + a being the collision diameter of a molecule. =
TMIn being slightly larger than TMGa, its diffusion coefficient will be slightly smaller. However, since TMIn decomposes at tower temperatures than TMGa, decomposition of more TMIn and TMGa into dimethyl and monomethyl species in the gas phase or on the mask is possible, and those species have higher diffusion coefficients due to their smaller sizes. Using the formulae given in ref. [201, the diffusion coefficient of monomethyl-In or -Ga is calculated to be 28% higher than those of TMIn or TMGa (the difference between D for TMIn and TMGa is only 1%). When growth is performed on unmasked substrates, the difference in diffusion between In and Ga species from the gas stream toward the substrate surface is compensated for when setting flows to get a layer of given composition. In the case of selective area growth, however, the molecules which land on the mask and then desorb spend more time in the hot zone (on or close to the substrate) while diffusing toward open areas. It is during this extra amount of time that more decomposition of TMIn vs TMGa could occur than accounted for in the case of planar substrates, resulting in an averaged higher diffusion coefficient of In than of Ga species. The lower migration length of Ga than of In species can also explain why the growth rate enhancement of compounds containing both In and Ga is lower than that of InP, as is seen in fig. 1. The absence of a change in the As/P ratio of lnGaAsP is most probably related to the high consumed to the of quantity V/Ill ratio:compared the small amount group Vsupplied species makes a possible difference in migration between the As and P species insignificant. —
—
4.2. Edge spike As seen in fig. 4, InP and InGaAs grown in stripe openings show a growth spike at the edge, whereas InGaAs and InGaP do not, irrespective of the opening width. Also seen in this figure is the fact that InGaAs and InGaAsP do notgrowth grow, 111)~facets. The or very little, on the ( rate of InP on these facets is low, but not negligible, and that of InGaP is comparatively high,
Moo-SungKim et al.
/ Selective area growth of InGaAsP by OMVPE
73
being about 1/4 of the growth rate on the (100) plane. Kayser [3] showed the role of the V/Ill ratio in the formation of growth spikes and the existence of InP growth on (111)~facets. Specifically, he observed that, in OMVPE of InP, a high
ment (fig. 4b). Under similar conditions, GaAs does not grow at all on a (111)B substrate [14] (although growth can be obtained at higher ternperatures), which points toward a very slow reaction rate of Ga species with AsHy on a (111)B surface. Growth of InGaAs (x1~= 0.53) on a pla-
V/Ill ratio prevented migration of In species from the low growth rate (111)B facets to the (100) plane, and that growth did take place on (111)B: growth spikes were prevented, since such spikes are formed when In species migrate on the (111)B surface toward the (100) plane, but migrate slowly on the (100) surface and cannot reach the center of the stripe to form a flat (100) growth front. It should be stressed that, in the case of InP, (111)B is a low growth rate plane only when adjacent to a plane such as (100): when planar substrates of different orientations are used, the growth rate of InP is similar on (100) and (111)B [14]. This is not the case for GaAs which, in our experimental condition, does not grow at all on (111)B substrates [141. In our experiments, the input V/Ill ratio is highest for InP and InGaP, and lowest for InGaAs. It is difficult to know the real V/Ill ratio at the substrate surface, since the percentages of decomposition are unknown. However, it seems unlikely that the effective V/Ill ratio would be very different for InP, which shows growth spikes, and for InGaP, which does not. Therefore, we assume that the magnitude of the V/Ill ratio does not play an important role in the growth differences observed. The V/Ill ratio used in our experiments is such that growth spikes are formed when InP is grown in stripe openings, indicating that, in the presence of PH3, In species have enough mobility to migrate from the slow growth (111)B facets to the adjacent (100) plane, but not enough mobility to reach the center of the stripe. InGaP exhibits a comparatively high growth rate (= fast growth reactions) on (111)B, and little or no migration toward (100) takes place. The high growth rate of InGaP on (111)B, when compared to the behaviour of InP, is probably due to a fast reaction rate between Ga and P species. InGaAs does not grow at all on (111)B; there is no edge spike, although there is a slight edge growth enhance-
nar (111)B substrate results in x1,~= 0.8, with a correspondingly lower growth rate [14]. The absence of growth of InGaAs on (111)B facets and the slight growth edge enhancement can be explained by fast migration of the species landing on (111)B toward the adjacent (100) plane cornbined with fast migration on (100) [3], the surface migration being larger than in the case of InP. Two reasons for this fast migration could be the lower (input) V/Ill ratio for InGaAs and the fact that, since the growth reaction rate between Ga and As species is very slow on (111)B, the Ga species present on (111)B are mostly unreacted, and can be expected to migrate on the surface faster than reacted (= bonded) species. More definitive explanations would require analyzing the alloy composition on the (111)B edge facets and as a function of the distance from these facets. The behavior of InGaAsP (x1~= 0.76, YAs = 0.54) is intermediate between those of InP and InGaAs. Growth edge spikes form, and there is little growth on (111)B.
5. Conclusion We reported the study of selective area growth of A = 1.3 im InGaAsP by low pressure OMVPE, and compared it to the selective area growth of InP, InGaAs, and InGaP. In enrichment was observed as in the case of InGaAs and InGaP. The edge growth behavior of InGaAsP is intermediate between those InP and InGaAs: there is little (but not zero) growth on (111)B facets, and an edge spike forms. A comparison between the edge growth behavior of the four compounds studied indicates that the growth reactions of In and Ga species with PH~ proceed much faster than with AsHy; the difference is especially pronounced for Ga species.
74
Moo-Sung Kim et a!.
/ Selective area growth
References [1] E. Colas, C. Caneau, MR. Frei, F.M. Clausen, Jr., W.E. Quinn and MS. Kim, AppI. Phys. Letters 59 (1991) 2019. [2] Y.D. Galeuchet, P. Roentgen and V. Graf, J. AppI. Phys. 68 (1990) 560. [3] 0. Kayser, J. Crystal Growth 107 (1991) 989. [4] T.F. Kuech, MS. Goorsky, MA. Tischler, A. Palevski, P. Solomon, R. Potemski, CS. Tsai, J.A. Lebens and K.J. Vahala, J. Crystal Growth 107 (1991) 116. [5] C. Blaauw, A. Szcaplonczay, K. Fox and B. Emmerstorfer, J. Crystal Growth 77 (1986) 326. [6] F. Colas, A. Shahar and Wi. Tomlinson, AppI. Phys. Letters 56 (1990) 955. [7] J.S.C. Chang, K.W. Carey, J.E. Turner and LA. Hodge, J. Electron. Mater. 19(1990)345. [81 Y.D. Galeuchet and P. Roentgen, J. Crystal Growth 107 (1991) 145. [9] J. Finders, J. Geurts, A. Kohl, M. Weyers, B. Opitz, 0. Kayser and P. Balk, J. Crystal Growth 107 (1991) 151. [10] M. Murata, T. Katsuyama and H. Hayashi. in: Proc. 17th Intern. Symp. on GaAs and Related Compounds, Jersey, 1990, Inst. Phys. Conf. 5cr. 112, Ed. K.E. Singer (Inst. Phys., Bristol, 1991) p. 181.
of InGaAsP by OMVPE
[11] 0. Kayser, R. Westphalen, B. Opitz and P. Balk, J. Crystal Growth 112 (1991) 111. [12] M. Ozeki, K. Mochizuki, N. Ohtsuka and K. Kodama, Thin Solid Films 174 (1989) 63. [13] A. Wakahara, K. Pak, H. Yamada, A. Yoshida and T. Nakamura, J. Appi. Phys. 62 (1987) 1284. [14] C. Caneau, R. Bhat, MR. Frei, S.A. Schwartz, W.A. Bonner and M.A. Koza, J. Crystal Growth 114 (1991) 481. [15] R. Azoulay, N. Bouadma, J.C. Bouley and L. Dugrand, J. Crystal Growth 55 (1981) 229. [16] S.D. Hersee, E. Barbier and R. Blondeau, J. Crystal Growth 77 (1986) 310. [17] G.H. Olsen and T.J. Zamerowski, Progr. Crystal Growth Characterization 2 (1979) 346. [18] C.E. Zah, private communication. [19] D.G. Coronell and K.F. Jensen, J. Crystal Growth 114 (1991) 581. [201 A.S. Forrest, L.A. Wemzel, C.W. Clump, L. Maus and LB. Andersen, Principles of Unit Operations (Wiley, New York, 1968) p. 106. [211 CA. Larsen, NI. Buchan and G.B. Stringfellow, J. Crystal Growth 85 (1987) 148. [22] GB. Stringfellow, Organometallic Vapor Phase Epitaxy (Academic Press, New York, 1989) ch. 7.
jo~~ or
CRYSTAL GROWTH
Selective area growth of InGaAsP by OMVPE Moo-Sung Kim, Catherine Caneau, Etienne Colas and Rajaram Bhat Bellcore, 331 Newman Springs Road, Red Bank, New Jersey 07701-7040, USA Received 4 February 1992; manuscript received in final form 19 March 1992
We investigated the selective area growth of A = 1.3 ~sm InGaAsP by low pressure OMVPE, and compared it with the selective growth behavior of InP, InGaAs, and InGaP. We found that, for all three alloys, the In content increased with decreasing stripe opening width. The growth rate enhancement is similar for InGaAsP, InGaAs, and InGaP, while it is somewhat higher for InP. Growth spikes at the stripe opening edges could be seen in the case of InP and InGaAsP, while InGaAs and InGaP did not exhibit such spikes.
1. Introduction Selective area growth of compound semiconductors is an attractive technique because of its potential applications to the fabrication of optoelectronic integrated circuits. In organometallic vapor phase epitaxy (OMVPE), the growth species can desorb from a dielectric mask, which makes selective growth possible. Diffusion of the growth species from the masked areas to the stripe openings causes a growth rate enhancement, a nonuniform lateral thickness profile and, in the case of an alloy, a compositional change. A growth rate enhancement of up to about 10 times the nominal growth rate has been observed [1,21. This growth rate enhancement reportedly results fom gas phase diffusion [1,3,4], rather than from surface migration [2,51.In the case of GaAs/A1GaAs, the growth rate enhancement was used to advantage in the fabrication of tapered waveguides [61. The compositional changes of epilayers grown in stripe openings have been reported for InGaAs and InGaP [7—11],the In content increasing with decreasing stripe opening width. Also, especially in the case of In-based materials, the existence of
Permanent address: KIST, P.O. Box 131, Cheongryang, Seoul 130-560, South Korea 0022-0248/92/$05.00 © 1992
—
edge spike growth has been observed when the selective growth is limited by a low growth rate (111)B plane [2,3,111. To eliminate the edge spikes, atomic layer epitaxy [121, or the use of a polycrystalline InP or AIN mask instead of Si02
[13—151 have been tried. However, a strong shift of composition was observed for InGaAs grown in openings in a polycrystalline InP mask [14]. Early studies of selective area growth were only on GaAs and AlGaAs [4,15,16]but, more recently, extensive work has been done on InP, InGaAs, and InGaP [2,3,5,7,9,11].The mechanism of the selective area growth, however, is not fully understood. Also, selective area growth of quaternary materials such as InGaAsP has, to our knowledge, not been investigated. In this paper, we report the study of the selective area growth, by low pressure OMVPE, of A = 1.3 ~tm InGaAsP lattice matched to InP on Si02 masked substrates, and compare it with the selective growth behaviour of InP, InGaAs (lattice-matched to InP), and InGaP (lattice-matched to GaAs).
2. Experimental procedure Growths were performed in a horizontal OMVPE reactor, at 76 Torr and 625°C, using trimethylindium (TM In), trimethylgallium
Elsevier Science Publishers B.V. All rights reserved
70
Moo-Sung Kim et a1.
/ Selectice area growth of InGaAsP by OMVPE
(TMGa), arsine (AsH3) and phosphine (PH3) as group III and V sources, and H2 are carrier gas. On each 5 cm diameter (100) InP or GaAs substrate, (1 cm x 1 cm) areas were patterned with 20 to 55 ~tm wide stripes parallel to [011] opened in Si02 (period 250 ~tm), leaving a wide unmasked area for reference. The growth conditions are summarized in table 1. 0.2 ~m In076 Ga0 24As054P046 with a 680 A thick cap InP, and 0.2 i~m1n053Ga047As with a 680 A thick cap InP were grown on InP substrates; 0.2 j~m In048Ga052P was grown on a GaAs substrate. The thicknesses and compositions refer to the unmasked areas. After growth, the samples were cleaved perpendicular to the stripe direction and stain-etched for thickness measurement in a scanfling electron microscope (SEM). The compositions of the grown layers were deduced from double crystal X-ray diffractometry and (in the case of the quaternary layer) room temperature photoluminescence (PL) spectra.
3. Results Fig. 1 shows the variation of growth rate enhancement vs stripe opening width W for InP, InGaAs, InGaP, and InGaAsP. The growth rate enhancement is similar for InGaAs, InGaP, and InGaAsP, while it is 30% higher for InP. Fig. 2 shows the dependence of the lattice mismatch between InGaAs or InGaAsP and InP, and of the PL peak wavelength of InGaAsP, upon opening width. Over the large unmasked region, the lattice mismatch L~a/awas less than 5 X iO~,and the PL peak wavelength of InGaAsP was 1.30 ~m. Using the data of fig. 2 and the curves of ref. —~
1 4
1 2
I
-
I
•\\
8
I
X
InP
0
InGaP
S
InGaAsP
\~
InGaAs
6
• 2
° I
I
I
STRIPE OPENING WIDTH, W (tim)
1 0
60
Fig. 1. Variation of the growth rate enhancement of InP InGaAs, lnGaAsP, and InGaP with stripe opening width.
[17], the compositions of InGaAs and InGaAsP were calculated and are plotted in fig. 3. The Indium content x1~ of InGaAsP and InGaAs increases with decreasing W. Fewer measurements were done for InGaP; in agreement with ref. [9], we also observe a moderate increase in x1~for InGaP grown in stripe openings, to 0.51 for W= 50 ~m instead of 0.48 for the large unmasked area. The P content of InGaAsP changes very little with opening width (by ±0.01). The morphology of the growth in stripe openings is rather poor, due to the lattice-mismatch and the high growth rate. The X-ray diffraction spectra taken in the middle of each square of stripe pattern (the length of the X-ray spot was about 1.5 mm and perpendicular to the stripe direction, the spot width was 0.1 mm) show broad peaks. This is due both to the poor quality of the matenat (as reflected by the morphology) and to the expected change in composition [7,9] within the
Table 1 Experimental conditions used for growth of InP, InGaAs, InGaP, and InGaAsP; the growth rates and In mole fractions x1,, are for growth on unmasked substrates
InP InGaAs InGaAsP InGaP
TMIn mole fraction
Input V/Ill ratio
Growth rate (pm/h)
x1,,
Edge growth?
Growth on (11l)B?
5 X iO~ 5 X iO~ 5 X iO~5 2.5x10~
480 200 380 480
2.05 3.8 2.8 2.2
1 0.53 0.76 0.48
Yes No Yes No
Little No Little Yes
Moo-Sung Kim et al. 5 I
Selective area growth of InGaAsP by OMVPE
1.5
I
~
/
InGaAs
71
.70 .~
+
.85
+
~GaAsP
0
~GaAs. 20% relaxatIon
I
0 STRIPE OPENING WIDTH, W (pm)
Fig. 2. Dependence of the lattice mismatch between InGaAs (~)and InGaAsP (.) and InP, and of the room temperature FL peak wavelength of InGaAsP (0) with stripe opening width,
width of each stripe opening. The data shown in figs. 2 and 3 were obtained using the X-ray peak maxima, and assuming no lattice relaxation. The values of x1~calculated for a 20% lattice relaxation are also shown in fig. 3 for InGaAs. Assuming the same relaxation for InGaAsP induces very little change in the calculated compositions. It is well known that strain changes the bandgap of materials. A lattice-mismatch of +4 X iO~ would shift the PL wavelength of InGaAs by —0.02 ~m [18]. Using the same shift for InGaAsP only leads to a very small change in the calculated values of x1~and YAs~
STRFE OPENI’IG WIDTH (iun)
Fig. 3. Variation of the In content of InGaAsP and InGaAs with stripe opening width. For InGaAs, the compositions were deduced from the X-ray spectra assuming either no lattice relaxation or 20% relaxation.
Fig. 4 shows the cross-sections of the growth in stripe openings for InGaAs/InP, InGaAsP/InP, and InGaP. There is no edge spike growth in the case of InGaP and InGaAs, but both InP and InGaAsP show such a feature, irrespective of the stripe opening width.
4. Discussion 4.1. Increase in In content for compounds grown in stripe openings An increase in x1~has already been reported for InGaAs grown in large windows opened in a
Fig. 4. Cross-section of the stripe opening edge for(a) InGaAsP+ cap InP, (b) InGaAs+cap InP, and (c) InGaP. Markers represent I ~m.
72
Moo-Sung Kim et al.
/
Selectite area growth of InGaAsP by OMVPE
Si02 mask [7,9], x1~ decreasing with distance away from the mask edge. This was attributed to a longer migration length of the In than of the Ga species; as argued in ref. [9], it is gas phase diffusion rather than surface diffusion which should be the main factor in the composition change. In ref. [91,it was also found that, when InGaAs is grown in 20 j~mwide open stripes, x1~, increases as the width of the adjacent masked area increases. The present study shows that, for a constant period of 250 ~tm, x1~increases as the open stripe width decreases, as can be expected from the findings of refs. [71and [9]. The extra amount of In (compared to Ga) found close to a mask edge distributes itself over the width of the stripe. It can still be expected that the alloy composition is richer in In close to the stripe edge, but the average value of x1~,,as well as its value in the middle of the stripe, will be higher for narrower stripes, The difference in migration length between the In and Ga species could be due to different sticking coefficients on the mask. As shown in ref. [19], however, the influence of the sticking coefficient is small: a sticking coefficient of 0.5 instead of 0 leads all other parameters being equal to an 8% increase in growth rate enhancement. The difference in the migration lengths of In and Ga species could also be due to a difference in gas phase diffusion of the Ga and In species, as explained in the following. Ref. [20] gives the expression for bimolecular gas diffusion: 3~2 MA + MB DAB PO~Bfl2 T 2MAMB —
—
—
‘
DAB being the diffusion coefficient of gas A in gas B, T the temperature, p the pressure, and MA and MB the molecular masses of gases A and B. Since MB (H 2 mass) is much smaller than MA (alkyl mass), the diffusion coefficient of an alkyl in H2 is nearly independent of its mass. ~2 is a collision integral, which is a function of the temperature and of the energy of attraction between A and B.alkyls The difference in diffusion between is mainly caused by 0’ABcoefficients ~ (o~~ + a being the collision diameter of a molecule. =
TMIn being slightly larger than TMGa, its diffusion coefficient will be slightly smaller. However, since TMIn decomposes at tower temperatures than TMGa, decomposition of more TMIn and TMGa into dimethyl and monomethyl species in the gas phase or on the mask is possible, and those species have higher diffusion coefficients due to their smaller sizes. Using the formulae given in ref. [201, the diffusion coefficient of monomethyl-In or -Ga is calculated to be 28% higher than those of TMIn or TMGa (the difference between D for TMIn and TMGa is only 1%). When growth is performed on unmasked substrates, the difference in diffusion between In and Ga species from the gas stream toward the substrate surface is compensated for when setting flows to get a layer of given composition. In the case of selective area growth, however, the molecules which land on the mask and then desorb spend more time in the hot zone (on or close to the substrate) while diffusing toward open areas. It is during this extra amount of time that more decomposition of TMIn vs TMGa could occur than accounted for in the case of planar substrates, resulting in an averaged higher diffusion coefficient of In than of Ga species. The lower migration length of Ga than of In species can also explain why the growth rate enhancement of compounds containing both In and Ga is lower than that of InP, as is seen in fig. 1. The absence of a change in the As/P ratio of lnGaAsP is most probably related to the high consumed to the of quantity V/Ill ratio:compared the small amount group Vsupplied species makes a possible difference in migration between the As and P species insignificant. —
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4.2. Edge spike As seen in fig. 4, InP and InGaAs grown in stripe openings show a growth spike at the edge, whereas InGaAs and InGaP do not, irrespective of the opening width. Also seen in this figure is the fact that InGaAs and InGaAsP do notgrowth grow, 111)~facets. The or very little, on the ( rate of InP on these facets is low, but not negligible, and that of InGaP is comparatively high,
Moo-SungKim et al.
/ Selective area growth of InGaAsP by OMVPE
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being about 1/4 of the growth rate on the (100) plane. Kayser [3] showed the role of the V/Ill ratio in the formation of growth spikes and the existence of InP growth on (111)~facets. Specifically, he observed that, in OMVPE of InP, a high
ment (fig. 4b). Under similar conditions, GaAs does not grow at all on a (111)B substrate [14] (although growth can be obtained at higher ternperatures), which points toward a very slow reaction rate of Ga species with AsHy on a (111)B surface. Growth of InGaAs (x1~= 0.53) on a pla-
V/Ill ratio prevented migration of In species from the low growth rate (111)B facets to the (100) plane, and that growth did take place on (111)B: growth spikes were prevented, since such spikes are formed when In species migrate on the (111)B surface toward the (100) plane, but migrate slowly on the (100) surface and cannot reach the center of the stripe to form a flat (100) growth front. It should be stressed that, in the case of InP, (111)B is a low growth rate plane only when adjacent to a plane such as (100): when planar substrates of different orientations are used, the growth rate of InP is similar on (100) and (111)B [14]. This is not the case for GaAs which, in our experimental condition, does not grow at all on (111)B substrates [141. In our experiments, the input V/Ill ratio is highest for InP and InGaP, and lowest for InGaAs. It is difficult to know the real V/Ill ratio at the substrate surface, since the percentages of decomposition are unknown. However, it seems unlikely that the effective V/Ill ratio would be very different for InP, which shows growth spikes, and for InGaP, which does not. Therefore, we assume that the magnitude of the V/Ill ratio does not play an important role in the growth differences observed. The V/Ill ratio used in our experiments is such that growth spikes are formed when InP is grown in stripe openings, indicating that, in the presence of PH3, In species have enough mobility to migrate from the slow growth (111)B facets to the adjacent (100) plane, but not enough mobility to reach the center of the stripe. InGaP exhibits a comparatively high growth rate (= fast growth reactions) on (111)B, and little or no migration toward (100) takes place. The high growth rate of InGaP on (111)B, when compared to the behaviour of InP, is probably due to a fast reaction rate between Ga and P species. InGaAs does not grow at all on (111)B; there is no edge spike, although there is a slight edge growth enhance-
nar (111)B substrate results in x1,~= 0.8, with a correspondingly lower growth rate [14]. The absence of growth of InGaAs on (111)B facets and the slight growth edge enhancement can be explained by fast migration of the species landing on (111)B toward the adjacent (100) plane cornbined with fast migration on (100) [3], the surface migration being larger than in the case of InP. Two reasons for this fast migration could be the lower (input) V/Ill ratio for InGaAs and the fact that, since the growth reaction rate between Ga and As species is very slow on (111)B, the Ga species present on (111)B are mostly unreacted, and can be expected to migrate on the surface faster than reacted (= bonded) species. More definitive explanations would require analyzing the alloy composition on the (111)B edge facets and as a function of the distance from these facets. The behavior of InGaAsP (x1~= 0.76, YAs = 0.54) is intermediate between those of InP and InGaAs. Growth edge spikes form, and there is little growth on (111)B.
5. Conclusion We reported the study of selective area growth of A = 1.3 im InGaAsP by low pressure OMVPE, and compared it to the selective area growth of InP, InGaAs, and InGaP. In enrichment was observed as in the case of InGaAs and InGaP. The edge growth behavior of InGaAsP is intermediate between those InP and InGaAs: there is little (but not zero) growth on (111)B facets, and an edge spike forms. A comparison between the edge growth behavior of the four compounds studied indicates that the growth reactions of In and Ga species with PH~ proceed much faster than with AsHy; the difference is especially pronounced for Ga species.
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/ Selective area growth
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of InGaAsP by OMVPE
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