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Ion-beam-induced epitaxial crystallization of amorphous GaAs on GaAs(100)
Nuclear Instruments and Methods in Physics Research B59/60 (1991) 449-453 North-Holland
Ion-beam-induced on GaAs( 100)
epitaxial crystallization
449
of amorphous GaAs
Naoto Kobayashi, Masataka Hasegawa, Hisao Kobayashi and Nobuyuki Electrotechnical
Laboratory,
Hayashi
1 -I -4 Umezono, Tsukuba, Ibaraki 305, Japan
Makoto Shinohara, Fumihiko Ohtani and Masatoshi Asari Shimadzu Corporation,
I Nishinokyo-kuwabaracho,
Nakagvo-ku,
Kyoto 604, Japan
Epitaxial crystallization of deposited amorphous GaAs layers on GaAs(100) up to the surface by bombardments with 400 keV Ar and 400 keV Rr has been successfully performed at a temperature range between 125 and 200°C. Properties of crystal growth were investigated as a function of ion species (Ar and Kr), energy (400 and 800 keV), ion dose, dose rate and substrate temperature by RBS channeling experiments. The growth rate has shown a nearly linear dependence on ion fluence. Ion bombardments below 100°C have induced further amorphization beyond the initial crystal/amorphous interface. On the scale of nuclear energy deposition density, bombardments with higher electronic excitation efficiency give a small increase of the growth rate. Ar bombardments have shown a strong dependence of the growth rate on dose rate, whereas Rr bombardments have revealed a weak dependence. An apparent activation energy of 0.13 kO.06 eV for the crystal growth was observed.
1. Introduction
Ion-beam-induced epitaxial crystallization (IBIEC), which is a kind of solid-phase epitaxial growth (SPEG) using ion beams, has recently attracted much interest because of its attractive features of the dynamic roles of defects produced by ion bombardments. The properties and mechanism of IBIEC have been investigated extensively especially in Si [l-3]. It has provided not only a new field of investigation on beam-solid interactions, but also a possibility of process application of Si due to its advantages such as processing at low temperatures, controllability and local processing capability. Crystallization of amorphous III-V compound semiconductors in thermal processing is more complex than in Si. Crystal growth at low temperatures in GaAs is normally accompanied by a high density of extended defects [4,5]. The crystal growth properties of IBIEC in III-V compound semiconductors are, moreover, less known in comparison with those in Si. We have previously investigated the growth properties of an amorphous refractory III-V compound semiconductor BP under the IBIEC process at low temperatures [6,7]. Although research works on IBIEC of GaAs were previously reported [8,9], complete crystallization up to the surface was not confirmed due to severe defect formation. The features of IBIEC in GaAs has also not yet been fully investigated. This article presents the growth properties of IBIEC of amorphous layers on GaAs(100) 0168-583X/91/$03.50
induced by energetic heavy ion bombardments. Analysis of samples has been performed by the RBS (Rutherford backscattering)-channeling experiments. Structural properties of fully grown GaAs layers by IBIEC are reported elsewhere [lo].
2. Experimental Amorphous layers of GaAs with a thickness of about 80 nm were deposited onto GaAs(100) substrates by the ICB (ionized cluster beam) method. Clusters of Ga and As were deposited on a semi-insulating wafer at a substrate temperature of 200°C with a deposition rate of 1 pm/h (sample notation; a-GaAs/GaAs(lOO)(A)). The amorphous structure was confirmed by RHEED (reflective high-energy electron diffraction) and X-ray diffraction measurements. Stoichiometry was also confirmed by Auger electron spectroscopy. Some samples were amorphized additionally up to a depth of about 100 nm beyond the initial crystal/amorphous (c/a) interface by a 120 keV As ion implantation to a fluence of 6 X lOi ions/cm2 in order to eliminate the obstruction of growth by a thin layer with contamination on the original substrate surface (sample notation; aGaAs/GaAs(lOO)(B)). The samples were subsequently bombarded with energetic heavy ions (400 keV Ar and Kr and 800 keV Ar
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
IV. CRYSTALLIZATION/AMORPHIZATION
N. Kobayashi et al. / Epitaxial crystallization of a-GaAs
450
and Kr) to fluences in the range from 6 X lOI to 2.1 x 10” ions/cm2 at temperatures between 100 and 200°C. The projected ranges in GaAs are 270, 535, 130 and 260 nm for 400 keV Ar, 800 keV Ar, 400 keV Kr and 800 keV Kr, respectively. The nuclear energy deposition v(E) at the initial c/a interface is calculated to be 0.49,0.32, 2.0 and 1.6 keV/(nm ion) for 400 keV Ar, 800 keV Ar, 400 keV Kr and 800 keV Kr, respectively. Ion current density was varied from 0.25 to 4 PA/cm2 for 400 keV Ar and from 0.063 to 1 PA/cm2 for 400 keV Kr. The ratio of dose rate of Ar ion to that of Kr ion was selected to be 4 as to keep the same nuclear energy deposition rate for two ion species bombardments. The analysis of the crystal growth and structural properties of the grown layer was performed by the RBS-channeling technique using 2 MeV He+ ions. The detector was set at 105” to the ion beam incidence direction in order to enhance the depth resolution.
Kr- FLUENCE lx lOI 0
1
1
2
3
ions/cm’ ) 4
5
6
Ar-FLUENCE (x10’6ions/cm2) growth thickness for a-GaAs/GaAs(lOO) (A) (Ar bombardments) and (B) (Kr bombardments) as a function of ion fluences. The fluences of Ar and Kr are scaled to give the same nuclear energy deposition density. Dashed lines indicate the surface position for samples (A) and (B).
Fig. 2. Crystal
3. Results and discussion Fig. 1 demonstrates typical RBS-channeling spectra of a-GaAs/GaAs(lOO)(A) in the process of ion-beaminduced crystallization. Aligned spectra represent the progressive planar crystal growth of the amorphous layer by successive 400 keV Ar bombardments at 150°C. After a relatively low crystal growth rate at the initial c/a interface at the early stage of the bombardment, the complete crystallization up to the surface was observed by bombardments to a fluence of 2.1 X 10” ions/cm2. Thermally induced crystallization was observed to be negligibly small at 150% Similar spectra representing the complete crystal growth were also observed by
DEPTH ( nm) lx
100
IO31
I
’
1
-
3-
Initial 3.2 x 10’6/cm2 ____ 7 2
1
0 ,
a- GaAs/GoAs (100K~) 400 keV Ar+ 150°C
4
2MeV He+ 75”
I
I
O 400
I CHANNEL NUMBER
500
Fig. 1. Aligned RBS-channeling spectra for ion-beam-induced epitaxial growth of a-GaAs/GaAs(lOO)(A) by 400 keV Ar bombardments at 15OT. Initial amorphous layers with a thickness of about 80 run were deposited by the ICB method.
bombardments with 400 keV Ar and 400 keV Kr at 200°C. Angular tilt measurements have revealed a narrowing of the critical angle (#1,2) and an increase of minimum yield (xti”) in comparison with the original GaAs substrate. The substrate has values of J/,/2 = 0.66” and xmin = 0.04, whereas the fully grown layer has and xmin = 0.17 for bombardments at ti 1,2 =0.51” 150°C. Raman scattering experiments have exhibited a slightly disordered property induced in the fully grown layers [lo]. Fig. 2 shows the crystal growth thickness in aGaAs/GaAs(lOO) as a function of ion fluence of 400 keV Ar at 150 and at 200°C and 400 keV Kr at 2OO’C. Fluences of Ar and Kr ions are scaled to give the same nuclear energy deposition values. For the sample subjected to the Kr bombardments, which was further amorphized beyond the initial c/a interface in advance (sample a-GaAs/GaAs(lOO)(B)), a pseudolinear dependence of the crystal growth on the ion fluence was observed from the early stage of the bombardments. The low growth rate at the first step of the bombardments in amorphous layer deposited samples (aGaAs/GaAs(lOO)(A)), however, reflects the retarded behavior of the growth at the initial c/a interface. Complete epitaxial growth up to the surface was accomplished by bombardments to fluences of 1.7 x 10” ions/cm2 for 400 keV Ar at 2OO”C, 2.1 X 10” ions/cm2 for 400 keV Ar at 150°C and 5.6 x 1016 ions/cm2 for 400 keV Kr at 200°C. Fig. 3 shows dependences of crystal growth thickness for a-GaAs/GaAs(lOO)(B) at 150°C on the nuclear deposition energy density (product of the nuclear energy deposition and ion fluence) for bombardments with 400 keV Ar at 4 PA/cm’ (H), 400 keV Ar at 0.25 PA/cm2 (L), 800 keV Ar at 0.38
451
N. Kobayashi et al. / Epitaxial crystallization of a-GaAs 400 keV Kr4FLUENCE (10'5/cm2)
vlE).dft'd~dt(lO~eV. cti3.s-') -2
1o-3
ITI, .\
~ 25Oq!I'
I
I r,,,,,,
II-GaAs/GaAs(lOOXBl 400keV Ar,Kr +-,,
-f-----_+
Krll50"C) -----+-
1
I1
I
I1
8
10
12
I
,
I
I
14
16
18
20 Kr(lOO"C)+
Fig. 3. Dependence of the crystal growth thickness at 15O’C for a-GaAs/GaAs(lOO)(B) on the nuclear energy deposition density by bombardments with 400 keV Ar at 4 PA/cm2 (H) and 0.25 PA/cm’ (L), 800 keV Ar at 0.38 PA/cm2 (L), 400 keV Kr at 1 PA/cm2 (H) and 0.063 PA/cm2 (L) and 800 keV Kr at 0.077 PA/cm2 (L).
pA/cm2, 400 keV Kr at 1 PA/cm’ (H), 400 keV Kr at 0.063 PA/cm2 (L) and 800 keV Kr at 0.077 pA/cm2. Current densities were selected to give the same nuclear energy deposition density rate among high dose rate bombardments (H) and low dose rate bombardments (L). These results exhibit a linear dependence of growth thickness on dose up to a depth of 20 nm and a slightly larger efficiency in growth rate by bombardments with higher energies. The values of the crystal growth rate per nuclear energy deposition density (nuclearly normalized growth rate: NNGR) for a-GaAs/GaAs(lOO)(B) are plotted as a function of the nuclear energy deposition density rate and corresponding ion beam current densities in fig. 4. The growth rate values were taken at the initial stage of crystal growth (< 20 nm). The negative values of growth rate at 100°C represent further amorphization in contrast with the crystallization. The ion-beam-induced crystallization by Ar bombardments reveals a strong dose rate dependence of the growth rate, whereas that by Kr bombardments has shown a weak dependence on the dose rate. Ar bombardments show, furthermore, higher crystal growth rates than Kr bombardments over the whole range of dose rate examined. Fig. 5 shows the temperature dependence of the growth rate in a-GaAs/GaAs(lOO)(B) for Ar bombardments at 4 PA/cm2 and for Kr bombardments at 1 PA/cm2 between 125 and 2OO’C. An apparent activation energy observed for Ar bombardments was 0.13 & 0.06 eV, which agrees within the experimental error with the previously reported values [8,9]. The growth rate by the thermal process was also shown by a dashed line [ll], which was normalized to the duration of the
-
Ar(lOO"C14
01
02
I
0.5 1 2 5 Ar CURFiEM DENSITY(pA/cm'l
10
Fig. 4. Crystal growth rate per nuclear energy deposition densities (NNGR) in a-GaAs/GaAs(B) for 400 keV Ar and 400 keV Kr bombardments as a function of the energy deposition rate and corresponding ion beam current densities. The negative values at 100°C indicate further amorphization.
lOi Ar/cm2 and 2.5 x 1014 Kr/cm’ bombardments. The contribution of thermal annealing is found to be still small up to 200°C. It is noted that the present experiments on ionbeam-induced crystallization for a-GaAs/GaAs(lOO) have revealed that the complete epitaxial growth up to
IO2
TEMPERATURE("C)
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I
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2 lo' IS w % 0 1-T z -7100, i+ I 5 2100 E -F B -B 5 z -g' $
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Fig. 5. Temperature dependence of ion-beam-induced growth rate for a-GaAs/GaAs(lOO)(B) by Ar and Kr bombardments. The dashed line is the thermal growth rate normalized to the duration required for 1 X lOi Ar/cm’ and 2.5 X lOi Kr/cm2 bombardments. IV. CRYSTALLIZATION/AMORPHIZATION
452
N. Kobayashi et al. / Epitaxial crystallization of a-GaAs
the surface could be successfully performed with pseudolinear dependences of growth rate on ion fluence. Structural properties are characterized by the existence of lattice disorder to some extent in the fully grown layers, being suggested by the degraded crystalline properties (decrease in $i,a and increase in xmin in the RBS-channeling experiments). The disorder in the lattice of the grown layer should be partly due to incorporated Ar or Kr atoms in the process of crystallization. (Average residual impurity concentration in the grown layer is 6.4 X 102’/cm2 for 400 keV Ar bombardments and 8.7 X 102’/cm3 for 400 keV Kr bombardments both at the maximum fluences at 200°C.) In the previous works for ion-beam-induced crystallization of amorphized GaAs by 1.5 MeV Ne bombardments, the interruptions of growth due to dissociation of GaAs at 85°C [8] or due to severe twin formation at 165°C [9] were reported. A great difference between the present experiments and the former results lies in the values of NNGR. (The NNGR is - 1.3 nm per 10 25 eV cme3 for 400 keV Ar at 150°C and is - 60 nm per 1O25eV cme3 for 1.5 MeV Ne at 165°C). This difference may result from the extremely higher electronic energy deposition than the nuclear energy deposition in 1.5 MeV Ne bombardments (the ratio between electronic and nuclear energy depositions is - 1.3 for 400 keV Ar and - 27 for 1.5 MeV Ne). This could affect the atomic migration probably due to a mechanism like efficient bond breaking and could enhance the crystal growth rate. The present complete crystallization for GaAs, on the other hand, might prohibit the swift rearrangement of constituent atoms into extended defects and stimulate rather gentle growth due to fairly lower NNGR. A slight increase in the NNGR by the higher energy bombardments (800 keV) in the present GaAs should reflect the contribution of electronic excitation to the crystal growth. The larger NNGR by Ar bombardments than that by Kr bombardments over a wide range of dose rate (fig. 5) can also suggest the contribution of higher electronic excitation efficiency. In the case of Si, such a big difference in NNGRs due to the difference of electronic energy deposition density has not been observed between 600 keV Kr [12] and 1.5 MeV Ne bombardments [13]. Furthermore, the dose rate dependence of the NNGR with various ion species bombardments on Si shows an universal dependence on the dose rate times the square of the deposition energy density [3]. Therefore, a large contribution of electronic excitation to crystal growth may be one of the unique features in ion-beam-induced crystallization for GaAs. The weak dependence of the NNGR for Kr bombardments on the dose rate in comparison with Ar bombardments in GaAs is unclear but it is probably due to the difference in the distribution of defects which are responsible for the ion-beam-induced crystallization
in GaAs. One explanation is that the defects with a relatively high density in a cascade for Kr bombardments are during their migration process less susceptible to the trapping effect by the successive defect formation, even at higher dose rate bombardments. Although extended experiments with more ion species over a wide range of bombarding energy are desirable in order to extract further characteristic properties, present experiments have revealed an aspect of novel features of ion-beam-induced crystallization in GaAs.
4. Summary and conclusions Ion-beam-induced crystallization of amorphous GaAs layers on GaAs(100) was investigated by using Ar and Kr ions with varying ion energy, ion dose, dose rate and substrate temperature. Epitaxial growth of deposited amorphous layers up to the surface has been performed by bombardments with 400 keV Ar and Kr showing pseudolinear dependence of growth rate on ion fluence during the whole crystallization process. A fairly lower growth rate per nuclear energy deposition density is thought to favor the successful crystallization. Kr bombardments have shown a lower growth rate and also have shown a feeble dependence of growth rate on dose rate in comparison with Ar bombardments. The observed apparent activation energy for the crystal growth was 0.13 _+0.06 eV. A feature that ion bombardments with higher electronic excitation efficiency may give an enhancement in growth rate was found in the ion-beam-induced crystallization in GaAs.
Acknowledgements
The authors acknowledge Dr. H. Tanoue for his help in ion bombardments. Discussion with Dr. Y. Makita is also acknowledged.
References
111R.G. Elliman, J.S. Williams, W.L. Brown, A. Lieberich,
D.M. Maher and R.V. Knoell. Nucl. Instr. and Meth. B19/20 (1987) 435. PI J. Linnros, G. Holmen and B. Svensson, Phys. Rev. B32 (1985) 2770. [31 J. Limnos and G. Holmen, J. Appl. Phys. 62 (1987) 4737. [41 S.S. Kular, B.J. Sealy, K.G. Stephens, D.K. Sadana and G.R. Booker, Solid State Electron. 23 (1980) 831. [51 K.G. Rossiter, R.G. Elliman, I.V. Mitchell, A.P. Pogany and J.S. Williams, Nucl. Instr. and Meth. 218 (1983) 639. 161 N. Kobayashi, H. Kobayashi and Y. Kumashiro, Nucl. Instr. and Meth. B40/41 (1989) 550.
N. Kobayashi et al. / Epitaxial crystallization of a-G&s [7] N. Kobayashi, H. Kobayashi, H. Tanoue, N. Hayashi and
Y. Kumashiro, Mater. Res. Sot. Proc. 157 (1990) 119. [8] S.T. Johnson, J.S. Williams, E. Nygren and R.G. Elliman, J. Appl. Phys. 64 (1988) 6567. [9] S.T. Johnson, R.G. ElIiman and J.S. Williams, Nucl. Instr. and Meth. B39 (1989) 449. [lo] N. Kobayashi, M. Hasegawa, N. Hayashi, S. Kimura and Y. Makita, in: Proc. Int. Conf. on Electronic Materials, 1990, in press.
453
(111 D. Licoppe, Y.I. Nissim and C. Meriadec, J. Appl. Phys. 58 (1985) 3094. [12] A. La Ferla, S. Cannavo, G. Ferla, S.U. Campisano, E. Rimini and M. Servidori, Nucl. Instr. and Meth. B19/20 (1987) 470. (131 J.S. Williams, R.G. Elliman, W.L. Brown and T.E. Seidel, Phys. Rev. Lett. 55 (1985) 1482.
IV. CRYSTALLIZATION/AMORPHIZATION
Ion-beam-induced on GaAs( 100)
epitaxial crystallization
449
of amorphous GaAs
Naoto Kobayashi, Masataka Hasegawa, Hisao Kobayashi and Nobuyuki Electrotechnical
Laboratory,
Hayashi
1 -I -4 Umezono, Tsukuba, Ibaraki 305, Japan
Makoto Shinohara, Fumihiko Ohtani and Masatoshi Asari Shimadzu Corporation,
I Nishinokyo-kuwabaracho,
Nakagvo-ku,
Kyoto 604, Japan
Epitaxial crystallization of deposited amorphous GaAs layers on GaAs(100) up to the surface by bombardments with 400 keV Ar and 400 keV Rr has been successfully performed at a temperature range between 125 and 200°C. Properties of crystal growth were investigated as a function of ion species (Ar and Kr), energy (400 and 800 keV), ion dose, dose rate and substrate temperature by RBS channeling experiments. The growth rate has shown a nearly linear dependence on ion fluence. Ion bombardments below 100°C have induced further amorphization beyond the initial crystal/amorphous interface. On the scale of nuclear energy deposition density, bombardments with higher electronic excitation efficiency give a small increase of the growth rate. Ar bombardments have shown a strong dependence of the growth rate on dose rate, whereas Rr bombardments have revealed a weak dependence. An apparent activation energy of 0.13 kO.06 eV for the crystal growth was observed.
1. Introduction
Ion-beam-induced epitaxial crystallization (IBIEC), which is a kind of solid-phase epitaxial growth (SPEG) using ion beams, has recently attracted much interest because of its attractive features of the dynamic roles of defects produced by ion bombardments. The properties and mechanism of IBIEC have been investigated extensively especially in Si [l-3]. It has provided not only a new field of investigation on beam-solid interactions, but also a possibility of process application of Si due to its advantages such as processing at low temperatures, controllability and local processing capability. Crystallization of amorphous III-V compound semiconductors in thermal processing is more complex than in Si. Crystal growth at low temperatures in GaAs is normally accompanied by a high density of extended defects [4,5]. The crystal growth properties of IBIEC in III-V compound semiconductors are, moreover, less known in comparison with those in Si. We have previously investigated the growth properties of an amorphous refractory III-V compound semiconductor BP under the IBIEC process at low temperatures [6,7]. Although research works on IBIEC of GaAs were previously reported [8,9], complete crystallization up to the surface was not confirmed due to severe defect formation. The features of IBIEC in GaAs has also not yet been fully investigated. This article presents the growth properties of IBIEC of amorphous layers on GaAs(100) 0168-583X/91/$03.50
induced by energetic heavy ion bombardments. Analysis of samples has been performed by the RBS (Rutherford backscattering)-channeling experiments. Structural properties of fully grown GaAs layers by IBIEC are reported elsewhere [lo].
2. Experimental Amorphous layers of GaAs with a thickness of about 80 nm were deposited onto GaAs(100) substrates by the ICB (ionized cluster beam) method. Clusters of Ga and As were deposited on a semi-insulating wafer at a substrate temperature of 200°C with a deposition rate of 1 pm/h (sample notation; a-GaAs/GaAs(lOO)(A)). The amorphous structure was confirmed by RHEED (reflective high-energy electron diffraction) and X-ray diffraction measurements. Stoichiometry was also confirmed by Auger electron spectroscopy. Some samples were amorphized additionally up to a depth of about 100 nm beyond the initial crystal/amorphous (c/a) interface by a 120 keV As ion implantation to a fluence of 6 X lOi ions/cm2 in order to eliminate the obstruction of growth by a thin layer with contamination on the original substrate surface (sample notation; aGaAs/GaAs(lOO)(B)). The samples were subsequently bombarded with energetic heavy ions (400 keV Ar and Kr and 800 keV Ar
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
IV. CRYSTALLIZATION/AMORPHIZATION
N. Kobayashi et al. / Epitaxial crystallization of a-GaAs
450
and Kr) to fluences in the range from 6 X lOI to 2.1 x 10” ions/cm2 at temperatures between 100 and 200°C. The projected ranges in GaAs are 270, 535, 130 and 260 nm for 400 keV Ar, 800 keV Ar, 400 keV Kr and 800 keV Kr, respectively. The nuclear energy deposition v(E) at the initial c/a interface is calculated to be 0.49,0.32, 2.0 and 1.6 keV/(nm ion) for 400 keV Ar, 800 keV Ar, 400 keV Kr and 800 keV Kr, respectively. Ion current density was varied from 0.25 to 4 PA/cm2 for 400 keV Ar and from 0.063 to 1 PA/cm2 for 400 keV Kr. The ratio of dose rate of Ar ion to that of Kr ion was selected to be 4 as to keep the same nuclear energy deposition rate for two ion species bombardments. The analysis of the crystal growth and structural properties of the grown layer was performed by the RBS-channeling technique using 2 MeV He+ ions. The detector was set at 105” to the ion beam incidence direction in order to enhance the depth resolution.
Kr- FLUENCE lx lOI 0
1
1
2
3
ions/cm’ ) 4
5
6
Ar-FLUENCE (x10’6ions/cm2) growth thickness for a-GaAs/GaAs(lOO) (A) (Ar bombardments) and (B) (Kr bombardments) as a function of ion fluences. The fluences of Ar and Kr are scaled to give the same nuclear energy deposition density. Dashed lines indicate the surface position for samples (A) and (B).
Fig. 2. Crystal
3. Results and discussion Fig. 1 demonstrates typical RBS-channeling spectra of a-GaAs/GaAs(lOO)(A) in the process of ion-beaminduced crystallization. Aligned spectra represent the progressive planar crystal growth of the amorphous layer by successive 400 keV Ar bombardments at 150°C. After a relatively low crystal growth rate at the initial c/a interface at the early stage of the bombardment, the complete crystallization up to the surface was observed by bombardments to a fluence of 2.1 X 10” ions/cm2. Thermally induced crystallization was observed to be negligibly small at 150% Similar spectra representing the complete crystal growth were also observed by
DEPTH ( nm) lx
100
IO31
I
’
1
-
3-
Initial 3.2 x 10’6/cm2 ____ 7 2
1
0 ,
a- GaAs/GoAs (100K~) 400 keV Ar+ 150°C
4
2MeV He+ 75”
I
I
O 400
I CHANNEL NUMBER
500
Fig. 1. Aligned RBS-channeling spectra for ion-beam-induced epitaxial growth of a-GaAs/GaAs(lOO)(A) by 400 keV Ar bombardments at 15OT. Initial amorphous layers with a thickness of about 80 run were deposited by the ICB method.
bombardments with 400 keV Ar and 400 keV Kr at 200°C. Angular tilt measurements have revealed a narrowing of the critical angle (#1,2) and an increase of minimum yield (xti”) in comparison with the original GaAs substrate. The substrate has values of J/,/2 = 0.66” and xmin = 0.04, whereas the fully grown layer has and xmin = 0.17 for bombardments at ti 1,2 =0.51” 150°C. Raman scattering experiments have exhibited a slightly disordered property induced in the fully grown layers [lo]. Fig. 2 shows the crystal growth thickness in aGaAs/GaAs(lOO) as a function of ion fluence of 400 keV Ar at 150 and at 200°C and 400 keV Kr at 2OO’C. Fluences of Ar and Kr ions are scaled to give the same nuclear energy deposition values. For the sample subjected to the Kr bombardments, which was further amorphized beyond the initial c/a interface in advance (sample a-GaAs/GaAs(lOO)(B)), a pseudolinear dependence of the crystal growth on the ion fluence was observed from the early stage of the bombardments. The low growth rate at the first step of the bombardments in amorphous layer deposited samples (aGaAs/GaAs(lOO)(A)), however, reflects the retarded behavior of the growth at the initial c/a interface. Complete epitaxial growth up to the surface was accomplished by bombardments to fluences of 1.7 x 10” ions/cm2 for 400 keV Ar at 2OO”C, 2.1 X 10” ions/cm2 for 400 keV Ar at 150°C and 5.6 x 1016 ions/cm2 for 400 keV Kr at 200°C. Fig. 3 shows dependences of crystal growth thickness for a-GaAs/GaAs(lOO)(B) at 150°C on the nuclear deposition energy density (product of the nuclear energy deposition and ion fluence) for bombardments with 400 keV Ar at 4 PA/cm’ (H), 400 keV Ar at 0.25 PA/cm2 (L), 800 keV Ar at 0.38
451
N. Kobayashi et al. / Epitaxial crystallization of a-GaAs 400 keV Kr4FLUENCE (10'5/cm2)
vlE).dft'd~dt(lO~eV. cti3.s-') -2
1o-3
ITI, .\
~ 25Oq!I'
I
I r,,,,,,
II-GaAs/GaAs(lOOXBl 400keV Ar,Kr +-,,
-f-----_+
Krll50"C) -----+-
1
I1
I
I1
8
10
12
I
,
I
I
14
16
18
20 Kr(lOO"C)+
Fig. 3. Dependence of the crystal growth thickness at 15O’C for a-GaAs/GaAs(lOO)(B) on the nuclear energy deposition density by bombardments with 400 keV Ar at 4 PA/cm2 (H) and 0.25 PA/cm’ (L), 800 keV Ar at 0.38 PA/cm2 (L), 400 keV Kr at 1 PA/cm2 (H) and 0.063 PA/cm2 (L) and 800 keV Kr at 0.077 PA/cm2 (L).
pA/cm2, 400 keV Kr at 1 PA/cm’ (H), 400 keV Kr at 0.063 PA/cm2 (L) and 800 keV Kr at 0.077 pA/cm2. Current densities were selected to give the same nuclear energy deposition density rate among high dose rate bombardments (H) and low dose rate bombardments (L). These results exhibit a linear dependence of growth thickness on dose up to a depth of 20 nm and a slightly larger efficiency in growth rate by bombardments with higher energies. The values of the crystal growth rate per nuclear energy deposition density (nuclearly normalized growth rate: NNGR) for a-GaAs/GaAs(lOO)(B) are plotted as a function of the nuclear energy deposition density rate and corresponding ion beam current densities in fig. 4. The growth rate values were taken at the initial stage of crystal growth (< 20 nm). The negative values of growth rate at 100°C represent further amorphization in contrast with the crystallization. The ion-beam-induced crystallization by Ar bombardments reveals a strong dose rate dependence of the growth rate, whereas that by Kr bombardments has shown a weak dependence on the dose rate. Ar bombardments show, furthermore, higher crystal growth rates than Kr bombardments over the whole range of dose rate examined. Fig. 5 shows the temperature dependence of the growth rate in a-GaAs/GaAs(lOO)(B) for Ar bombardments at 4 PA/cm2 and for Kr bombardments at 1 PA/cm2 between 125 and 2OO’C. An apparent activation energy observed for Ar bombardments was 0.13 & 0.06 eV, which agrees within the experimental error with the previously reported values [8,9]. The growth rate by the thermal process was also shown by a dashed line [ll], which was normalized to the duration of the
-
Ar(lOO"C14
01
02
I
0.5 1 2 5 Ar CURFiEM DENSITY(pA/cm'l
10
Fig. 4. Crystal growth rate per nuclear energy deposition densities (NNGR) in a-GaAs/GaAs(B) for 400 keV Ar and 400 keV Kr bombardments as a function of the energy deposition rate and corresponding ion beam current densities. The negative values at 100°C indicate further amorphization.
lOi Ar/cm2 and 2.5 x 1014 Kr/cm’ bombardments. The contribution of thermal annealing is found to be still small up to 200°C. It is noted that the present experiments on ionbeam-induced crystallization for a-GaAs/GaAs(lOO) have revealed that the complete epitaxial growth up to
IO2
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3
,,,,,,,,
25
l/T IxIO-~)(K-')
? d
g
1 30
Fig. 5. Temperature dependence of ion-beam-induced growth rate for a-GaAs/GaAs(lOO)(B) by Ar and Kr bombardments. The dashed line is the thermal growth rate normalized to the duration required for 1 X lOi Ar/cm’ and 2.5 X lOi Kr/cm2 bombardments. IV. CRYSTALLIZATION/AMORPHIZATION
452
N. Kobayashi et al. / Epitaxial crystallization of a-GaAs
the surface could be successfully performed with pseudolinear dependences of growth rate on ion fluence. Structural properties are characterized by the existence of lattice disorder to some extent in the fully grown layers, being suggested by the degraded crystalline properties (decrease in $i,a and increase in xmin in the RBS-channeling experiments). The disorder in the lattice of the grown layer should be partly due to incorporated Ar or Kr atoms in the process of crystallization. (Average residual impurity concentration in the grown layer is 6.4 X 102’/cm2 for 400 keV Ar bombardments and 8.7 X 102’/cm3 for 400 keV Kr bombardments both at the maximum fluences at 200°C.) In the previous works for ion-beam-induced crystallization of amorphized GaAs by 1.5 MeV Ne bombardments, the interruptions of growth due to dissociation of GaAs at 85°C [8] or due to severe twin formation at 165°C [9] were reported. A great difference between the present experiments and the former results lies in the values of NNGR. (The NNGR is - 1.3 nm per 10 25 eV cme3 for 400 keV Ar at 150°C and is - 60 nm per 1O25eV cme3 for 1.5 MeV Ne at 165°C). This difference may result from the extremely higher electronic energy deposition than the nuclear energy deposition in 1.5 MeV Ne bombardments (the ratio between electronic and nuclear energy depositions is - 1.3 for 400 keV Ar and - 27 for 1.5 MeV Ne). This could affect the atomic migration probably due to a mechanism like efficient bond breaking and could enhance the crystal growth rate. The present complete crystallization for GaAs, on the other hand, might prohibit the swift rearrangement of constituent atoms into extended defects and stimulate rather gentle growth due to fairly lower NNGR. A slight increase in the NNGR by the higher energy bombardments (800 keV) in the present GaAs should reflect the contribution of electronic excitation to the crystal growth. The larger NNGR by Ar bombardments than that by Kr bombardments over a wide range of dose rate (fig. 5) can also suggest the contribution of higher electronic excitation efficiency. In the case of Si, such a big difference in NNGRs due to the difference of electronic energy deposition density has not been observed between 600 keV Kr [12] and 1.5 MeV Ne bombardments [13]. Furthermore, the dose rate dependence of the NNGR with various ion species bombardments on Si shows an universal dependence on the dose rate times the square of the deposition energy density [3]. Therefore, a large contribution of electronic excitation to crystal growth may be one of the unique features in ion-beam-induced crystallization for GaAs. The weak dependence of the NNGR for Kr bombardments on the dose rate in comparison with Ar bombardments in GaAs is unclear but it is probably due to the difference in the distribution of defects which are responsible for the ion-beam-induced crystallization
in GaAs. One explanation is that the defects with a relatively high density in a cascade for Kr bombardments are during their migration process less susceptible to the trapping effect by the successive defect formation, even at higher dose rate bombardments. Although extended experiments with more ion species over a wide range of bombarding energy are desirable in order to extract further characteristic properties, present experiments have revealed an aspect of novel features of ion-beam-induced crystallization in GaAs.
4. Summary and conclusions Ion-beam-induced crystallization of amorphous GaAs layers on GaAs(100) was investigated by using Ar and Kr ions with varying ion energy, ion dose, dose rate and substrate temperature. Epitaxial growth of deposited amorphous layers up to the surface has been performed by bombardments with 400 keV Ar and Kr showing pseudolinear dependence of growth rate on ion fluence during the whole crystallization process. A fairly lower growth rate per nuclear energy deposition density is thought to favor the successful crystallization. Kr bombardments have shown a lower growth rate and also have shown a feeble dependence of growth rate on dose rate in comparison with Ar bombardments. The observed apparent activation energy for the crystal growth was 0.13 _+0.06 eV. A feature that ion bombardments with higher electronic excitation efficiency may give an enhancement in growth rate was found in the ion-beam-induced crystallization in GaAs.
Acknowledgements
The authors acknowledge Dr. H. Tanoue for his help in ion bombardments. Discussion with Dr. Y. Makita is also acknowledged.
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IV. CRYSTALLIZATION/AMORPHIZATION