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Observation of phase separation in (Ge2)x(GaAs)1−x alloys grown by molecular beam epitaxy
Volume 2, number 3
OBSERVATION
MATERIALS LETTERS
OF PHASE SEPARATION
GROWN BY MOLECULAR Indrajit BANERJEE,
IN (GeZ),(GaAs)l_,
February 1984
ALLOYS
BEAM EPITAXY
Herbert KROEMER
Department of Electrical and Computer Engineering, Santa Barbara, CA 93106, USA
University of California,
and Don W. CHUNG Department of Chemical and Nuclear Engineering, Santa Barbara, CA 93106, USA
University of California,
Received I2 September 1983
(Gez),(GaAs)l_X alloys with 0 < x < 1 have been grown by molecular beam epitaxy on various Ge and GaAs substrates. The structure of these alloys has been studied using transmission electron microscopy. We observe that in all cases studied, Ge tends to phase-separate from GaAs, with the Ge domain size ranging from 100 to 300 A.
Recently there have been a number of reports [l-5] on the formation and properties of metastable singlephase single-crystalline alloys formed by mixing group IV elements with III-V compounds. These alloys are potentially useful for fabrication of optoelectronic devices in the infrared region and for making ohmic contacts. In particular, (Si,),(GaAs),, was deposited on GaAs (111) substrates by rf-sputtering [ 11, (Ge2)x(GaAs)l_, and (Ge&GaSb),_, have been grown using an ultra-high-vacuum ion-beam sputter deposition technique [3-51, in addition to (Ge2)x(GaAs)l_x being grown by MOCVD [ 21 all on GaAs (100) substrates. In these studies optical and electrical measurements were made, and the results were interpreted in terms of a single-phase alloy model with various possible structures for different compositions [2,6,7]. However, no actual detailed structural investigations (TEM, etc.) were reported. Petroff et al. [8] had attempted earlier to grow Ge/GaAs superlattices on GaAs (100) substrates by molecular beam epitaxy (MBE). Instead of obtaining alternate layers of Ge and GaAs, as initially expected, they observed a columnar epitaxial growth with a compositional segregation between adjacent columns. From
the equilibrium phase diagram of Ge-GaAs the maximum solid solubility of Ge in GaAs is ~2 mole%, and 02 mole% GaAs in Ge [9]. (Takada et al. have reported even lower solubilities [lo] .) These results suggest that the tendency towards lateral phase separation during MBE growth of this alloy is strong. In our work, presented here, we attempted to control the phase separation of Ge and GaAs with the hope of achieving a quasi-periodic stripe-like structure on the substrate, the alternate stripes being Ge- and GaAs-rich regions of =lOO A width. Such a stripe structure would confine the electrons to move in one dimension, with presumably interesting transport properties along and across the stripes. A stripe-like structure might possibly be achieved if one were successful in nucleating one of the constituents preferentially at step edges on the substrate surface, hoping that the natural tendency of the constituents to phase-separate will maintain the segregated structure as the stripes grow. For this reason we chose the highly anisotropic (110) and (211) surfaces as substrates. In this letter we report the initial results of such an investigation into the growth and structure of (Ge2),(GaAs)l_, alloys of different compositions, 189
Volume 2, number 3
MATERIALS LETTERS
grown by MBE on both GaAs and Ge substrates of different orientations and at different substrate temperatures (Z’s). The substrates were Ge (21 I), GaAs (110) and GaAs (211). On the GaAs substrates, the alloy growth proper was preceded by “priming” with either a sub-monolayer quantity or xl00 A of straight Ge, in an attempt to initiate preferred nucleation. The substrate temperature range in which the alloys were grown was 550 < T, < 610°C. The structure of the films was studied by transmission electron microscopy (TEM), using conventional two-beam bright- and darkfield techniques, and convergent-beam techniques. We observe that in all cases the group IV element tends to phase-separate from the III-V compound into domains that are of the order of 100 A wide, but which are not stripe-like. The exact domain dimensions depend on the relative composition (that is, on the value of x). Prior to inserting the substrates into the vacuum chamber, they were cleaned with organic solvents, and the surfaces were passivated by etching in a solution of NH,OH: H202: H,O (1:4: 20), followed by a rinse in de-ionized water. Once in the vacuum chamber,
February 1984
the oxides were removed by heating the substrates to 600°C for 5 min. The alloys were formed by co-deposition of Ga and Ge onto the substrate in the presence of an adequate arsenic flux. The compositions of the alloys were determined from the relative growth rates of straight GaAs or Ge films under equivalent flux conditions. Since Ge is known to form two different compounds with As, the As2 flux was deliberately kept as low as possible to prevent the formation of such compounds, but high enough to prevent the formation of Ga droplets on the surface. During the growth, the surface morphology was monitored by reflection highenergy electron diffraction (RHEED). In all the growths the RHEED patterns were very streaky, indicating the surface to be smooth and single crystalline. The surface morphology was essentially featureless, just like GaAs (100) homoepitaxy. In fact, the morphologies of the alloys were distinctly better than those of straight Ge or straight GaAsgrowth with (1 lO)and(21 l)orientations. Furthermore, the RHEED patterns indicate that the (2 11) alloy surface is much more stable against atomic-scale facetting than the free GaAs (211) sur-
Fig. 1. Two-beam bright-field electron micrograph of a sample with x = 0.25 and Ts = 550°C grown on GaAs (I 10). The operating Bragg reflection is g = [ 2201.
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Volume
2, number
3
Fig. 2. Two-beam dark-field electron Bragg reflection isg = [ 2201.
MATERIALS
micrograph
of a sample with x = 0.48 and T, = 580°C
face: the alloy shows no sign of facetting as on a free GaAs (211) surface. As mentioned earlier, the structure of the samples was studied using TEM. Planar TEM samples were prepared by either chemical thinning, or ion-milling or a combination of both. From one particular sample we prepared three different TEM samples using the three different techniques; all yielded the same conclusions. Thus what we show subsequently is the real structure and not some surface morphology arising due to sample preparation. Transmission electron diffraction (TED) patterns show all samples to be single crystals, corroborating the observations in the RHEED patterns. There was no evidence of GeAs or GeAs, formation. Furthermore, there was no evidence of any defects, such as misfit dislocations, stacking faults or anti-phase boundaries. Fig. 1 shows the two-beam (g = [220]) brightfield electron micrograph of a sample with x = 0.3, grown on a GaAs(ll0) substrate at T,=55O'C. There are domains of Ge (darker regions) surrounded by domains of GaAs-rich regions. The domain boundaries appear to be (1 lo} faces. This is the natural face for the boundaries between a polar and a non-polar surface.
February
LETTERS
grown
on GaAs (110).
1984
The operating
However, the pattern is not stripe-like; it is crosshatched instead. If the (110) planes perpendicular to the substrate had dominated, we would have obtained the parallel striped geometry. In selected cases, individual domains of an elongated-hexagon cross section caused by differently-tilted (1 IO} boundaries have been seen clearly. This means that even though the phase boundaries tend to follow the { 1 lo} planes very strongly, there is no preference for any subset of { 110) planes. Fig. 2 shows the dark-field electron micrograph of a sample grown on GaAs (110) with x = 0.47 and T,= 58O’C. The basic structure is the same as seen in fig. 1, except that the size of the Ge domains is ~300 A. It appears that this size is roughly proportional to the Ge concentration. This conclusion appears to be true for all samples studied on (110) substrates. The regularity in the appearance of the domains is lost for high-temperature growths. Growths on both Ge and GaAs (211) substrates are very similar to those on GaAs (110) surfaces, the Ge domains again having { 1 IO} faces as domain boundaries. One difference is that on (21 1) the Ge domains are larger in one direction compared to the other as seen in fig. 3 which shows the dark-field electron mi191
Volume 2, number 3
MATERIALS LETTERS
February 1984
Fig. 3. Two-beam dark-field electron micrograph of a sample grown on GaAs (211) at rs = 550°C withx = 0.48. The operating B. refle:ction isg = [222].
Fig. 4. A convergent-beam transmission-electron diffraction pattern taken from a sample with x = 0.25 grown on GaAs (110) at T, =: 550°C. The beam direction isB = [ 1101. Camera length is 5.125 m at 200 kV.
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Volume 2, number 3
MATERIALS LETTERS
crograph (g = [222]) of a sample grown on GaAs (2 11) at T, = 550°C with x = 0.48. In this sample the typical Ge domain size is loo-150 A. A sample with x = 0.48 and T, = 580°C grown on Ge (211) also shows a similar structure, but with Ge domain sizes ranging from 200 to 300 8. The larger domain size in this case, we believe, is due to the higher substrate temperature and not due to the different surface chemistries of the different substrates. Fig. 4 shows a typical convergent-beam pattern taken on the sample shown in fig. 1, grown on GaAs (110). Notice the small asymmetry between the [002] and [OOT] reflections. This small asymmetry is also seen in pure GaAs [ 1l] where it is due to the lack of inversion symmetry at the midpoint of each line connecting nearest-neighbour atoms. Efforts to see a symmetric pattern by focusing the beam on Ge-rich domains was not possible because the beam diameter is larger than the domain size. Since we are observing a few domains simultaneously, the observed asymmetry suggests that in our samples the GaAs is a single continuous structure with the Ge embedded into the GaAs. Also, this result confirms that there are no anti-phase domains, for their existence would wash out the asymmetry. This method would be most useful if one wished to study the zincblende-to-diamond order-disorder transition in single-phase Ge/GaAs alloys, the existence of which has been postulated by Alferov et al. [2] and by Newman et al. [ 71. A detailed description on the uses of this technique will be published in a later paper. In conclusion, we observe phase separation in MBE grown Ge-GaAs alloys grown on GaAs (110) and (211) and on Ge (2 11) surfaces. The alloys grown appear to be structurally perfect single crystals with essentially featureless surface morphology. On (110) surfaces the size of the Ge-rich domainsvaries from 50 to 300 A, depending on the Ge concentration. Alloys grown at high temperatures have the domains randomly dis-
February 19S4
tributed as compared to the alloys grown at low temperatures. On (211) surfaces, the domain sizes appear to be more temperature dependent. Further studies are under way on the growth and properties of these alloys on different substrates. Studies are also under way to determine the lattice parameter as a function ofx. We are greatly thankful to Dr. F.A. Ponce of Hewlett-Packard Laboratories for the use of his facilities, and to Greg Anderson and Tom Yamashita for their help in TEM sample preparation.
References [II A.J. Noreika and M.H. Francombe, J. Appl. Phys. 45 (1974) 3690. 121 Zh.1. Alferov, M.Z. Zhingarev, S.G. Konikov, 1.1. Mokan, V.P. Ulin, V.E. Umanski and B.S. Yavich, Soviet Phys. Semicond. 16 (1982) 532. 131 S.A. Barnett, M.A. Ray, A. Lastras, B. Kramer, J.E. Greene, P.M. Raccah and L.L. Abels, Electron. Letters 18 (1982) 892. 141 K.C. Cadien, A.H. Eltoukhy and J.E. Greene, Appl. Phys. Letters 38 (1981) 773. 151 K.C. Cadien, M.A. Ray, S.M. Shin, J.M. Rigsbee, S.A. Barnett and J.E. Greene, J. Vacuum Sci. Technol. 20 (1982) 370. 161 Zh.1. Alferov, R.S. Vartanyan, V.I. Korolkov, 1.1.Mokan, V.P. Ulin, B.S. Yavich and A.A. Yakovenko, Soviet Phys. Semicond. 16 (1982) 567. 171 K.E. Newman, A. Lastras-Martinez, B. Kramer, S.A. Barnett, M.A. Ray, J.D. Dow and J.E. Greene, Phys. Rev. Letters 50 (1983) 1466. [81 P.M. Petroff, A.C. Gossard, A. Savage and W. Wiegmann, J. Crystal Growth 46 (1979) 172. [91 J.E. Greene, private communication (1983). IlO1 Y. Takeda, T. Hirai and M. Hirao, J. Electrochem. Sot. 112 (1965) 363. [Ill B.F. Buxton, J.A.Eades, J.W. SteedsandG.M. Rackham, Phil. Trans. Roy. Sot. A281 (1976) 171.
193
OBSERVATION
MATERIALS LETTERS
OF PHASE SEPARATION
GROWN BY MOLECULAR Indrajit BANERJEE,
IN (GeZ),(GaAs)l_,
February 1984
ALLOYS
BEAM EPITAXY
Herbert KROEMER
Department of Electrical and Computer Engineering, Santa Barbara, CA 93106, USA
University of California,
and Don W. CHUNG Department of Chemical and Nuclear Engineering, Santa Barbara, CA 93106, USA
University of California,
Received I2 September 1983
(Gez),(GaAs)l_X alloys with 0 < x < 1 have been grown by molecular beam epitaxy on various Ge and GaAs substrates. The structure of these alloys has been studied using transmission electron microscopy. We observe that in all cases studied, Ge tends to phase-separate from GaAs, with the Ge domain size ranging from 100 to 300 A.
Recently there have been a number of reports [l-5] on the formation and properties of metastable singlephase single-crystalline alloys formed by mixing group IV elements with III-V compounds. These alloys are potentially useful for fabrication of optoelectronic devices in the infrared region and for making ohmic contacts. In particular, (Si,),(GaAs),, was deposited on GaAs (111) substrates by rf-sputtering [ 11, (Ge2)x(GaAs)l_, and (Ge&GaSb),_, have been grown using an ultra-high-vacuum ion-beam sputter deposition technique [3-51, in addition to (Ge2)x(GaAs)l_x being grown by MOCVD [ 21 all on GaAs (100) substrates. In these studies optical and electrical measurements were made, and the results were interpreted in terms of a single-phase alloy model with various possible structures for different compositions [2,6,7]. However, no actual detailed structural investigations (TEM, etc.) were reported. Petroff et al. [8] had attempted earlier to grow Ge/GaAs superlattices on GaAs (100) substrates by molecular beam epitaxy (MBE). Instead of obtaining alternate layers of Ge and GaAs, as initially expected, they observed a columnar epitaxial growth with a compositional segregation between adjacent columns. From
the equilibrium phase diagram of Ge-GaAs the maximum solid solubility of Ge in GaAs is ~2 mole%, and 02 mole% GaAs in Ge [9]. (Takada et al. have reported even lower solubilities [lo] .) These results suggest that the tendency towards lateral phase separation during MBE growth of this alloy is strong. In our work, presented here, we attempted to control the phase separation of Ge and GaAs with the hope of achieving a quasi-periodic stripe-like structure on the substrate, the alternate stripes being Ge- and GaAs-rich regions of =lOO A width. Such a stripe structure would confine the electrons to move in one dimension, with presumably interesting transport properties along and across the stripes. A stripe-like structure might possibly be achieved if one were successful in nucleating one of the constituents preferentially at step edges on the substrate surface, hoping that the natural tendency of the constituents to phase-separate will maintain the segregated structure as the stripes grow. For this reason we chose the highly anisotropic (110) and (211) surfaces as substrates. In this letter we report the initial results of such an investigation into the growth and structure of (Ge2),(GaAs)l_, alloys of different compositions, 189
Volume 2, number 3
MATERIALS LETTERS
grown by MBE on both GaAs and Ge substrates of different orientations and at different substrate temperatures (Z’s). The substrates were Ge (21 I), GaAs (110) and GaAs (211). On the GaAs substrates, the alloy growth proper was preceded by “priming” with either a sub-monolayer quantity or xl00 A of straight Ge, in an attempt to initiate preferred nucleation. The substrate temperature range in which the alloys were grown was 550 < T, < 610°C. The structure of the films was studied by transmission electron microscopy (TEM), using conventional two-beam bright- and darkfield techniques, and convergent-beam techniques. We observe that in all cases the group IV element tends to phase-separate from the III-V compound into domains that are of the order of 100 A wide, but which are not stripe-like. The exact domain dimensions depend on the relative composition (that is, on the value of x). Prior to inserting the substrates into the vacuum chamber, they were cleaned with organic solvents, and the surfaces were passivated by etching in a solution of NH,OH: H202: H,O (1:4: 20), followed by a rinse in de-ionized water. Once in the vacuum chamber,
February 1984
the oxides were removed by heating the substrates to 600°C for 5 min. The alloys were formed by co-deposition of Ga and Ge onto the substrate in the presence of an adequate arsenic flux. The compositions of the alloys were determined from the relative growth rates of straight GaAs or Ge films under equivalent flux conditions. Since Ge is known to form two different compounds with As, the As2 flux was deliberately kept as low as possible to prevent the formation of such compounds, but high enough to prevent the formation of Ga droplets on the surface. During the growth, the surface morphology was monitored by reflection highenergy electron diffraction (RHEED). In all the growths the RHEED patterns were very streaky, indicating the surface to be smooth and single crystalline. The surface morphology was essentially featureless, just like GaAs (100) homoepitaxy. In fact, the morphologies of the alloys were distinctly better than those of straight Ge or straight GaAsgrowth with (1 lO)and(21 l)orientations. Furthermore, the RHEED patterns indicate that the (2 11) alloy surface is much more stable against atomic-scale facetting than the free GaAs (211) sur-
Fig. 1. Two-beam bright-field electron micrograph of a sample with x = 0.25 and Ts = 550°C grown on GaAs (I 10). The operating Bragg reflection is g = [ 2201.
190
Volume
2, number
3
Fig. 2. Two-beam dark-field electron Bragg reflection isg = [ 2201.
MATERIALS
micrograph
of a sample with x = 0.48 and T, = 580°C
face: the alloy shows no sign of facetting as on a free GaAs (211) surface. As mentioned earlier, the structure of the samples was studied using TEM. Planar TEM samples were prepared by either chemical thinning, or ion-milling or a combination of both. From one particular sample we prepared three different TEM samples using the three different techniques; all yielded the same conclusions. Thus what we show subsequently is the real structure and not some surface morphology arising due to sample preparation. Transmission electron diffraction (TED) patterns show all samples to be single crystals, corroborating the observations in the RHEED patterns. There was no evidence of GeAs or GeAs, formation. Furthermore, there was no evidence of any defects, such as misfit dislocations, stacking faults or anti-phase boundaries. Fig. 1 shows the two-beam (g = [220]) brightfield electron micrograph of a sample with x = 0.3, grown on a GaAs(ll0) substrate at T,=55O'C. There are domains of Ge (darker regions) surrounded by domains of GaAs-rich regions. The domain boundaries appear to be (1 lo} faces. This is the natural face for the boundaries between a polar and a non-polar surface.
February
LETTERS
grown
on GaAs (110).
1984
The operating
However, the pattern is not stripe-like; it is crosshatched instead. If the (110) planes perpendicular to the substrate had dominated, we would have obtained the parallel striped geometry. In selected cases, individual domains of an elongated-hexagon cross section caused by differently-tilted (1 IO} boundaries have been seen clearly. This means that even though the phase boundaries tend to follow the { 1 lo} planes very strongly, there is no preference for any subset of { 110) planes. Fig. 2 shows the dark-field electron micrograph of a sample grown on GaAs (110) with x = 0.47 and T,= 58O’C. The basic structure is the same as seen in fig. 1, except that the size of the Ge domains is ~300 A. It appears that this size is roughly proportional to the Ge concentration. This conclusion appears to be true for all samples studied on (110) substrates. The regularity in the appearance of the domains is lost for high-temperature growths. Growths on both Ge and GaAs (211) substrates are very similar to those on GaAs (110) surfaces, the Ge domains again having { 1 IO} faces as domain boundaries. One difference is that on (21 1) the Ge domains are larger in one direction compared to the other as seen in fig. 3 which shows the dark-field electron mi191
Volume 2, number 3
MATERIALS LETTERS
February 1984
Fig. 3. Two-beam dark-field electron micrograph of a sample grown on GaAs (211) at rs = 550°C withx = 0.48. The operating B. refle:ction isg = [222].
Fig. 4. A convergent-beam transmission-electron diffraction pattern taken from a sample with x = 0.25 grown on GaAs (110) at T, =: 550°C. The beam direction isB = [ 1101. Camera length is 5.125 m at 200 kV.
192
Volume 2, number 3
MATERIALS LETTERS
crograph (g = [222]) of a sample grown on GaAs (2 11) at T, = 550°C with x = 0.48. In this sample the typical Ge domain size is loo-150 A. A sample with x = 0.48 and T, = 580°C grown on Ge (211) also shows a similar structure, but with Ge domain sizes ranging from 200 to 300 8. The larger domain size in this case, we believe, is due to the higher substrate temperature and not due to the different surface chemistries of the different substrates. Fig. 4 shows a typical convergent-beam pattern taken on the sample shown in fig. 1, grown on GaAs (110). Notice the small asymmetry between the [002] and [OOT] reflections. This small asymmetry is also seen in pure GaAs [ 1l] where it is due to the lack of inversion symmetry at the midpoint of each line connecting nearest-neighbour atoms. Efforts to see a symmetric pattern by focusing the beam on Ge-rich domains was not possible because the beam diameter is larger than the domain size. Since we are observing a few domains simultaneously, the observed asymmetry suggests that in our samples the GaAs is a single continuous structure with the Ge embedded into the GaAs. Also, this result confirms that there are no anti-phase domains, for their existence would wash out the asymmetry. This method would be most useful if one wished to study the zincblende-to-diamond order-disorder transition in single-phase Ge/GaAs alloys, the existence of which has been postulated by Alferov et al. [2] and by Newman et al. [ 71. A detailed description on the uses of this technique will be published in a later paper. In conclusion, we observe phase separation in MBE grown Ge-GaAs alloys grown on GaAs (110) and (211) and on Ge (2 11) surfaces. The alloys grown appear to be structurally perfect single crystals with essentially featureless surface morphology. On (110) surfaces the size of the Ge-rich domainsvaries from 50 to 300 A, depending on the Ge concentration. Alloys grown at high temperatures have the domains randomly dis-
February 19S4
tributed as compared to the alloys grown at low temperatures. On (211) surfaces, the domain sizes appear to be more temperature dependent. Further studies are under way on the growth and properties of these alloys on different substrates. Studies are also under way to determine the lattice parameter as a function ofx. We are greatly thankful to Dr. F.A. Ponce of Hewlett-Packard Laboratories for the use of his facilities, and to Greg Anderson and Tom Yamashita for their help in TEM sample preparation.
References [II A.J. Noreika and M.H. Francombe, J. Appl. Phys. 45 (1974) 3690. 121 Zh.1. Alferov, M.Z. Zhingarev, S.G. Konikov, 1.1. Mokan, V.P. Ulin, V.E. Umanski and B.S. Yavich, Soviet Phys. Semicond. 16 (1982) 532. 131 S.A. Barnett, M.A. Ray, A. Lastras, B. Kramer, J.E. Greene, P.M. Raccah and L.L. Abels, Electron. Letters 18 (1982) 892. 141 K.C. Cadien, A.H. Eltoukhy and J.E. Greene, Appl. Phys. Letters 38 (1981) 773. 151 K.C. Cadien, M.A. Ray, S.M. Shin, J.M. Rigsbee, S.A. Barnett and J.E. Greene, J. Vacuum Sci. Technol. 20 (1982) 370. 161 Zh.1. Alferov, R.S. Vartanyan, V.I. Korolkov, 1.1.Mokan, V.P. Ulin, B.S. Yavich and A.A. Yakovenko, Soviet Phys. Semicond. 16 (1982) 567. 171 K.E. Newman, A. Lastras-Martinez, B. Kramer, S.A. Barnett, M.A. Ray, J.D. Dow and J.E. Greene, Phys. Rev. Letters 50 (1983) 1466. [81 P.M. Petroff, A.C. Gossard, A. Savage and W. Wiegmann, J. Crystal Growth 46 (1979) 172. [91 J.E. Greene, private communication (1983). IlO1 Y. Takeda, T. Hirai and M. Hirao, J. Electrochem. Sot. 112 (1965) 363. [Ill B.F. Buxton, J.A.Eades, J.W. SteedsandG.M. Rackham, Phil. Trans. Roy. Sot. A281 (1976) 171.
193