Heteroepitaxial growth of MnS on GaAs substrates

Journal of Crystal Growth 117 (1992) 810—815 North-Holland

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CRYSTAL GROWTH

Heteroepitaxial growth of MnS on GaAs substrates M. Okajima and T. Tohda Central Research Laboratories, Matsushita Electric Industrial Co., Ltd., 3-15, Yagumo-naka-machi, Moriguchi, Osaka 570, Japan

Epitaxial growth characteristics of MnS onto GaAs substrates with different orientations have been investigated by X-ray diffraction and reflection high energy electron diffraction (RHEED). Growth of MnS films has been performed by molecular beam epitaxy(MBE). Epitaxial films of wurtzite type MnS(y-MnS)in a single phase have been grown on GaAs(lll) substrates at 200°C. The epitaxial y-MnS films shows good crystallinity, where full width at half maximum (FWHM) of the X-ray rocking curve is lower than 100 arc sec. On the other hand, the films on GaAs(001) at the same growth temperature exhibit multiphase crystal structures of zinc-blende and rock-salt in early growth stages, and change the main structure to wurtzite with increasing thickness. Photoluminescence spectra of the y-MnS films at 77 K exhibit an orange emission band around 570 nm which is attributed to 2~ions, and red emission bands around 620 and 690 nm which would be attributed to Mn2~ion pairs. The band gap energy of Mn the y-MnS was estimated to be 3.9 eV from the excitation spectra.

1. Introduction MnS is known to crystallize into three forms: the rock-salt type structure (a-MnS), the zincblende type structure (/3-MnS), and the wurtzite type structure (y-MnS). Both tetrahedrally coordinated /3 and y forms are unstable in a high temperature range. They are easily transformed into octahedrally coordinated stable a form by heating [1,2]. Tetrahedrally coordinated forms of binary MnS obtained so far have been only a polycrystalline multiphase of /3 and y forms. On the other hand, ternary manganese chalchogenide compounds, named diluted magnetic semiconductors (DMSs), such as Cd 1 ~Mn~Te, Zn1~Mn~Te, Cd1~Mn5Se or Zn1~Mn~Se have been extensively studied with respect to semiconducting or magnetic properties [3]. They keep the zinc-blende or the wurtzite structure up to some extent of Mn mole fraction x. Band gaps of them increase monotonically with increasing x. But they do not have these structures at x 1. In the case of Cd1_~Mn~Sand Zn1~Mn~Ssystems, single crystals in the zinc-blende or the wurtzite structure with x up to 0.5 and 0.6, respectively, have been obtained by equilibriumgrowth methods [4,5]. Recently, it has been re=

0022-0248/92/sOS .00 © 1992

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ported that zinc-blende type epitaxial films of MnTe [61 and MnSe [71have been grown by molecular beam epitaxy (MBE), although these compounds have the hexagonal NiAs and the rock-salt type structure in bulk crystals, respectively. The band gap energies of these films with the zinc-blende structure are much wider than those of bulk crystals. In such a way, MBE allows one to fabricate epitaxial films of metastable phases that are not obtained in equilibrium-grown bulk crystals. In this work, we tried to prepare epitaxial films of tetrahedrally coordinated MnS in a single phase to obtain new wide gap semiconductor films for optoelectronic devices at short wavelength.

2. Growth procedures of MnS films MnS films were grown on GaAs substrates by molecular beam epitaxy (MBE), in which Mn metal was evaporated in cracked H2S gas. The orientations of the substrates were (001}, {1l1)B, and (111}A with no offsets. Polished surfaces of the substrates were first boiled in trichlorethylene, in acetone, and in methanol to remove organic contamination, and then chemically

Elsevier Science Publishers B.V. All rights reserved

M Okajima, T Tohda

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Heteroepila.xial growth of MnS on GaAs substrates

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of H2S04 : H202 : H20 5: 1: 1 at room temperature for 2 mm in order to remove surface mechanical damage. After having been rinsed in deionized water, they were immediately loaded into the MBE chamber. Prior to the growth, the substrates were heated up to 600°Cfor a few minutes to remove oxide layers, and then cooled down to the growth temperatures. Metal Mn chunk and thermally cracked H2S gas were used for the sources. The purity of Mn and H2S gas were both of fournines” grade. Typical source temperature of Mn cell was around 990°C,and the cracking temperature of H2S gas was 1000°C. The flow rates of H2S were controlled by a mass flow controller. The ambient pressure of the H2S gas during the deposition was fixed to 6 x i0~ Torr. Deposition rates were monitored by a water-cooled quartz crystal oscillator during depositions, and thickness of the completed films was measured by a-step (Tencor Instruments). The deposition rate was varied from 0.3 to 0.8 A/s. During the growth of the films, reflection high energy electron diffraction (RHEED) was used to monitor the crystal structure.

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2~(degree) Fig. 1. X-ray diffraction patterns of the MnS films grown on GaAs{001) substrates at growth temperatures (Tg) of 300, 200 and 100°C.

3. Results and discussion In contrast, the MnS films at lower growth 3.1. Growth temperature dependence

temperatures 7~ 200 and 100°Chad mixed forms of /3 and a both with [0011orientations parallel to the [001] direction of the substrates. The lattice constant of the /3 form is 5.570 A. There is a thickness dependence of the forms and orientations of the MnS films grown on {001} substrates in the low growth temperature range. Fig. 2 shows RHEED patterns of the MnS films grown at 200°C with thickness of (a) 800 A (mentioned above) and (b) 2200 A, respectively. In an early growth stage, a spotty [0011 oriented fcc pattern with a [111] oriented hcp-like extra pattern was observed as shown in fig. 2a, and the RHEED pattern changes with increasing thickness as shown in fig. 2b, which indicates that the films changed the main structure from the /3 to the y form with [00W1] orientation parallel to the [ill] substrate azimuth. =

3.1.1. Growth on GaAs{OO1J Fig. 1 shows X-ray diffraction patterns of the MnS films grown on GaAs{001) substrates at different growth temperatures (I~),and deposition rates of 0.5 A/s. The thickness of the sampIes shown in the figure was 800 A. The film obtained at 300°C showed polycrystalline diffraction patterns of the rock-salt and the hexagonal wurtzite-like structure. The latter diffraction pattern without (10 0) and (11 2) diffraction lines which appear in ideal y-MnS, implies the formation of a hexagonal structure with many stacking faults along [00 1] direction. This structure is also observed in SiC or ZnS with polymorph structures [8]. =

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In contrast, it was revealed from X-ray diffraction and RHEED patterns that epitaxial films of y-MnS in a single phase have been grown in a temperature range of 7~ 150—250°C. The RHEED pattern of the epitaxial films on {lll)B substrates is shown in figs. 4a and 4b. The [00 1] growth orientation of the epitaxial films is parallel to the [1111azimuth of the substrates. Growth temperature dependence of crystallinity of the epitaxial films was evaluated by double-crystal X-ray diffraction measurement. Fig. 5 shows the dependence of full width at half maximum (FWHM) from the rocking curves of the {00~2} diffraction on the growth temperature T~.It was found that the FWHM decreases with an increase in T~,reaching 96 arc sec at 250°C. The lattice constant c of the y-MnS films is 6.430 A, which is very close to the c value obtained so far from the polycrystalline powder specimen with y form [9]. Purely geometrical considerations show =

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3.1.2. Growth on GaAs{ 111) Fig. 3 shows X-ray diffraction patterns of the MnS films grown on GaAs{111}B substrates at different T~. Gas pressure and the deposition rates were the same as those of the films shown in fig. 1. The thickness of the samples shown in the figure was 4500 A. The X-ray diffraction pattern from the film obtained at T5 300°C showed the polycrystalline a form, which was confirmed by the RHEED pattern.

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M. Okajima, T. Tohda

/ Heteroepitaxial growth

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GROWTH TEMPERATURE (°c) Fig. 5. FWHM of X-ray rocking curves for the y-MnS epitaxial films with thickness of 4500 A plotted against growth temperature,

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that this c value corresponds to the bond length (i.e., the anton—cation distance) b of 2.411 A, .

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which is almost the same as that of the /3 form of the films on GaAs{001). This value is 1.5% smaller than that of the GaAs substrates. The c value does not change for films with thickness above 600 A. Therefore the lattice relaxation seems to occur at thickness below 600 A. Epitaxial y-MnS films have been obtained also on GaAs(111}A substrates in the same growth conditions as in the case of GaAs{l11)B. It is shown in figs. 4c and 4d that the RHEED pattern of the films on {111)A substrates is streaky, which differs from that of the films on {111}B substrates. This suggests that the surface morphology

of the epitaxial films on (111}A plane is smoother than that on (111}B plane. The MnS films grown onto Si substrates coated with 1.4 ~ thick amorphous Si02 layer show

814

M. Okajima, T Tohda

/ Heteroepitaxial growth of MnS on

the polycrystalline a form. This result suggests that MnS prefers the a form at Tg 200°Cwhen it grows on the substrates without crystal lattice periodicity, which is consistent with the result reported previously [1,2]. It is interesting to compare the epitaxial growth properties of MnS films at the same growth temperature from the viewpoint of substrate orientation. It is found that both MnS films grown on GaAs(001) and GaAs(111) substrates around 200°C prefer tetrahedral coordination, which is the same as that of the substrates. It is worth noting that the epitaxial MnS films grown on GaAs(111) substrates have the y instead of the /3 form. The atomic arrangements of GaAs(111) surface do not essentially restrict which of the /3 or y forms the MnS films have, because the atomic arrangements in the (111) plane of zincblende structure are the same as those in the (00~1) plane of wurtzite structure. Consequently, y-MnS seems to be more stable than /3-MnS in this temperature range. On the other hand, the epitaxial films grown on GaAs(001) substrates mainly have the 1~ form in early growth stage, and change it to the y form with increasing thickness. Consideration of atomic arrangements is also available to understand these epitaxial characteristics. In the wurtzite structure, there is no plane with an atomic arrangement equivalent to that of the zinc-blende (001) plane, which is different from the case of the (111) surface mentioned above. This might be the reason why they have mainly the /3 instead of the form. The stacking sequence of the (111) planes of the films seems to be restricted to that of

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ENERGY(eV)

3.2. Photoluminescence and excitation spectra Photoluminescence (FL) spectra of the epitaxial y-MnS films excited by 310 nm UV light at 77 K are shown in fig. 6. A conventional spectrofluorophotometer (Shimadzu RF-5000) was used for the measurements. The PL spectra exhibit three emission bands in the visible region. One is an

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WAVELENGTH(nm) Fig. 6. Photoluminescence (PL) spectra of the epitaxial y-MnS films at 77K.

orange emission band with the maximum around 570 nm which is attributed to Mn2~ions, and the others are red emission bands around 620 and 690 nm which would be attributed to Mn2~ion pairs. No band-edge emission was observed. Fig. 7 shows the excitation spectra of the epitaxial ‘y-MnS films with thickness of (a) 650 A and (b) 4800 A at 77 K. All FL emission bands of y-MnS have the same excitation spectra. They consist of a strong peak at 315 nm and five peaks at 385, 420, 460, 487, and 515 nm. The intensities of the five peaks from 385 to 515 nm were enENERGY(eV) 4.0 3.5 I

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Excitation Spectra (EM 620nm) (77K)

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A GaAs substrates It changes described to that of as more A, B, stable C, A, wurtzite B, C, pattern described as A, B, A, B, A, B,... with increasing thicknesses.

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WAVELENGTH(nm) Fig. 7. Excitation spectra of the epitaxial ?~-MnSfilms with thickness of (a) 650 A and (b) 4800 A at 77 K.

M. Okajima, T. Tohda

/ Heteroepitaxial growth of MnS on

hanced with increasing film thickness. The five peaks correspond to the internal d—d optical transitions of Mn2~ions, which have almost the same peak positions as those of the polycrystalline /3/y-MnS films reported previously [1,21. They are also the same as for ZnS : Mn films [10—12].The strong peak at 315 nm (3.9 eV) is attributed to the transition to the band edge. The line shape is similar to that of ZnS slightly doped with Mn [13]. We can estimate the band gap energy of y-MnS to be about 3.9 eV from this spectrum, which is close to the value of 3.5 eV predicted for pure binary y-MnS by extrapolation of previously obtained data for y-CdixMnrS (0
GaAs substrates

815

excitation spectra. MBE is a promising method to obtain epitaxial MnS films with /3 or y form which have attractive characteristics, such as a wider energy band gap than that of a bulk form.

Acknowledgements We wish to thank Dr. K. Kanai for encouragement, and Dr. S. Nakamura and Dr. Y. Yabuuchi for double-crystal X-ray diffraction measurements and useful discussions.

References [1] 0.

Goede, W. Heimbrodt and V. Weinhold, Phys. Status

Solidi (b) 136 (1986) K49. 0. Goede, W. Heimbrodt, V. Weinhold, E. Schnürer and HG. Eberle, Phys. Status Solidi (b) 143 (1987) 511. [31J.K. Furdyna and J. Kossut, in: Semiconductors and Semimetals, Vol. 25, Eds. R.K. Willardson and AC. Beer (Academic Press, New York, 1988). [41M. Ikeda, K. Itoh and H. Sato, J. Phys. Soc. Japan 25 (1968) 455. [5] C. Sombuthawee, S.B. Bonsai and F.A. Hummel, J. Solid State Chem. 25 (1978) 391. [6] SM. Durbin, J. Han, Sungi 0, M. Kobayashi, DR. Menke, R.L. Gunshor, Q. Fu, N. Pelekanos, A.V. Nurmikko, D. Li, J. Gonsalves and N. Otsuka, AppI. Phys.

121 4. Summary We have investigated epitaxial growth characteristics of MnS onto GaAs substrates with different orientations. Growth of MnS films has been performed by molecular beam epitaxy (MBE). Epitaxial films of wurtzite type MnS (y-MnS) in a single phase, which is not obtained in a bulk form, have been grown on GaAs(111) substrates around 200°C. The epitaxial y-MnS films show good crystallinity. On the other hand, the films on GaAs(00l) in the same growth temperature range change the main structure from the zincblende to the wurtzite form with increasing thickness. In an early growth stage, the structure is influenced by the zinc-blende {001) plane of GaAs substrate. This might be the reason why they have mainly the /3 instead of the y form which is more stable in this temperature range. Photoluminescence spectra of the y-MnS films at 77 K exhibit an orange emission band around 570 nm which is attributed to Mn2~ions, and red emission bands around 620 and 690 nm which would be attributed to Mn2~ion pairs. The band gap energy of y-MnS is estimated to be 3.9 eV from the

Letters 55 (1989) 2087. Kolodziejski, R.L. Gunshor, N. Otsuka, B.P. Gu, Y. Hefetz and A.V. Nurmikko, AppI. Phys. Letters 48 (1986) 1482. [81R.W.G. Wyckoff, Crystal Structures, Vol. 1, 2nd ed. (Wiley, New York, 1963) p. 113. [9] R.W.G. Wyckoff, Crystal Structures, Vol. 1, 2nd ed. (Wiley, New York, 1963) p. 112.

[71L.A.

[101Dang Dinh Thong and 120 (1983) K145.

0. Goede, Phys. Status Solidi (b) [11] 0. Goede and Dang Dinh Thong, Phys. Status Solidi (b) 124 (1984) 343. [12] T. Kushida, Y. Tanaka and Y. Oka, J. Phys. Soc. Japan 37 (1974) 1341. [131T. Hoshina and H. Kawai, Japan. J. AppI. Phys. 19(1980) 267. [14] H. Kambara, K.I. Gondaira, T. Teranishi andTerasawa, K. Sato, J.T.Phys. C 13 (1980) 5615. [15] DR. Huffman and R.L. Wild, Phys. Rev. 156 (1967) 989.