Interface and thickness control of polytype heterostructures grown by molecular beam epitaxy

Applied Surface North-Holland

115

Science 40 (1989) 115-119

INTERFACE AND THICKNESS CONTROL BY MOLECULAR BEAM EPITAXY M. YANO, A. KAWAGUCHI, New Material Research Received

4 Apri

M. ASHIDA,

Center, Osaka Institute

1989; accepted

for publication

OF POLYTYPE

HETEROSTRUCTURES

GROWN

Y. IWAI and M. INOUE

of Technology, Asahi-ku,

Ohmiya,

Osaka 535, Japan

2 June 1989

Molecular beam epitaxy of a polytype heterostructure, GaSb/AlSb/InAs, has been studied. Controlled continuous growth of the polytype structu1.e has been achieved by observing intensity oscillations of reflection high energy electron diffraction (RHEED) for the first time. In order to obtain a desired artificial sharp interface, we have studied the surface stability of grown layers during the interchanging oreration of component molecular beams at the growth of polytype interfaces. From the analysis of RHEED and Auger electron :;pectroscopy, the grown heterostructure was confirmed to be well controlled as designed under the growth at relatively low temperatures.

1. Introduction The lattice constants of InAs, GaSb and AlSb are relatively close enough to one another to enable the fabrication of heterostructures out of them. We can expect a wide variety of applications for the combination of these semiconductors to realize new types of devices because there are large differenm:es of band structures between them. For example, polytype heterostructures [l] consisting of these three materials, GaSb/AlSb/InAs, are expected 1.0 show attractive electrical and optical properties because of the unique band line-up [2-51. In order tc realize specific boundary conditions as designed in the heterostructure, it is important to grow sharp interfaces on an atomic scale. Molecular beam epitaxy (MBE) is one of the promising crystal growth techniques to materialize such a contrs311ed heterostructure. However, the details of the heterointerface formation in the MBE growth process of polytype heterostructures have not been properly understood. Even in the case of III-V,JIII-V, type heterostructures where only Group V elements are interchanged, it is very difficult to prepare sharp interfaces in comparison with that of 111,-V/111,-V type heterostructures such as GaAs/AlAs. This is mainly due to the 0169-4332/89/$03.50 (North-Holland)

0 Elsevier Science Publishers

B.V.

high volatility of Group V elements, which we previously discussed in a GaSb/GaAs system [6]. In the case of polytype heterostructures, more problems are expected in achieving a reasonably sharp interface because both Group III and Group V elements are interchanged at the GaSb/InAs and AlSb/InAs interfaces. In this paper, we report on the MBE growth technique of this polytype heterostructure, focusing on the control of interface and layer thickness. We examined the growth conditions and succeeded for the first time in controlling the whole growth process continuously by an in-situ monitoring of the oscillation of specular beam spots in reflection high energy electron diffraction (RHEED) patterns. Grown structures were checked and analyzed by using Raman scattering and Auger electron spectroscopy (AES) combined with Ar ion etching.

2. Experimental The MBE system used in this study was an Anelva-620 with separate growth and sample introduction chambers. Elemental metals of Al, Ga, In, As and Sb and (lOO)-oriented GaSb substrates were used. Typical beam intensities at the sub-

M. Yam

116

ef al. / Interface

and thickness control ofpolytype

strate monitored by a nude ion gauge were about 5 X 10e5 Pa for Al, Ga and In, 5 X lop4 Pa for As and 1 X 1O-4 Pa for Sb, respectively. Under these conditions, the growth rates of the materials were 0.7-1.0 pm/h. The substrate was mounted on a molybdenum heating block by indium soldering and the temperature was monitored by a thermocouple embedded in the block. The Raman scattering measurement was performed in the backscattering configuration with an excitation of the 514.5 nm line of an Ar ion laser by using a SPEX 1403 monochromator. The AES system used for the analysis was a PHI Perkin-Elmer SAM-610. The depth profile analysis was performed by 2 keV Ar ion sputtering, which gave an etching rate of about 1.5 nm/min using a 3 keV, 3 PA electron beam as a probe.

3. Results and discussion 3.1. Surface stability of grown layers For respective films of InAs, GaSb and AlSb, we obtained single crystal layers in a wide range of substrate temperatures, 350-580” C. In the successive growth of these materials (except the case of GaSb/AlSb) the streaky RHEED patterns from the growing surface once changed to spotty at the interface and again became streaky with the progression of overlayer thickness when grown at the higher end of the range such as T, 2 500 a C. In such cases, we cannot have a precise control of the heterostructure because the interface is not abrupt on an atomic scale. For the growth of abrupt

Fig. 1. RHEED

patterns

for InAs: (a) during

heterostructures

interfaces by MBE, the surface stoichiometries should be preserved during the finite times required to change over the constituent element fluxes. In order to examine the desirable conditions for beam interchanges at the interface, we studied the stability of the crystal surface for various environmental conditions such as exposure to As or Sb beams. Fig. 1 shows the RHEED patterns of an InAs surface during growth (a), after 10 min in vacuum without exposure to Sb (b), and with exposure of the surface to a Sb beam at T, = 540 o C (c). These results indicate that the InAs surface becomes rough at a relatively high temperature such as T, = 540” C by the decomposition and selective re-evaporation of arsenic, and that the decomposition can be prevented by exposing the Sb beam. At T, = 420 o C, however, the smoothness of the InAs surface was maintained even after 10 min, independent of the exposure to the Sb beam. This means that the lowering of the growth temperature or the exposure fo the Sb beam to the InAs surface during growth interruption is effective in maintaining the smooth surface of InAs and to allow smooth growth of overlayers with abrupt heterointerfaces. Exposure to the As beam also effectively prevents the decomposition of the InAs surface. However, the introduction of the As beam for this purpose causes an As contamination for the subsequent antimonide layers, GaSb or AlSb; i.e. a highly volatile As beam remains at the growth environment even after closing the As cell shutter and this causes the antimonide layers at the interface to be unintentionally alloyed layers of

growth, (b) after 10 min in vacuum without the Sb exposure at T, = 540 o C.

exposure

of the surface

to Sb, and (c) with

M. Yano et al. / Interface and thickness control of polytype heterostructures

117

Fig. 2. RHEED patterns for GaSb: (a) during growth, (b) after growth with the surface exposed to As at T, = 540 o C, and (c) with the surface exposed to As at T, = 420 o C. The pattern did not change by the exposure for the case of T, = 420 o C, while it becomes spotty for T, = 540 o C.

GaAsSb or AlAsSb [6,7]. Needless to say, in the course of antimonide growth after InAs, we can wait with the start of the antimonide deposition until the As beam decays at the substrate with the exposure of the Sb beam or with the low growth temperature in order to maintain the flat surface of the grown InAs layer. For the case of InAs on GaSb or AlSb, the remaining Sb beam at the growth environment of InAs was negligible because the cut-off of the Sb beam was sharp enough due to the relatively low volatility of Sb. On the other hand, we must take care of the stability of the antimonide surfaces during the beam interchanging operation: i.e, GaSb and AlSb surfaces at high temperatures were not stable in vacuum. For instance, we show in fig. 2 the change of RHEED pattern from the GaSb surface during growth (a), with the surface exposed to As at T,= 540” C (b), and with the surface exposed to As at T,= 420” C (c), respectively. At T,= 420” C, the pattern observed even after 10 min was not changed from the initial condition as shown in fig. 2c. For the case of T,= 540 o C, on the other hand, the RHEED pattern changed immediately from streaky to spotty as shown in fig. 2b, and after several minutes the spotty pattern elongated to streaky again. The lattice constant derived from the final elongated pattern corresponded to GaAs. Raman scattering spectra from the GaSb surface after the exposure of the As beam for 10 min at T,= 540 o C contained three major peaks: two were characteristic for TO and LO phonon modes of GaAs and the other was for TO of

GaSb. This experimental result confirmed the RHEED observation, i.e., the exposure of the As beam should turn the GaSb surface to GaAs at q = 540 o C. The thickness of the GaAs layer will be about several tens of nanometers because the skindepth of the probing light, 514.5 nm from the Ar ion laser, is about 120 nm for GaAs. Phonons from GaSb were replaced completely by those from GaAs when the surface was exposed to the As beam for more than 30 min, as shown in fig. 3, which indicates that the about 60 nm thick GaSb surface had been turned to GaAs by the exposure of the As beam. On the contrary, the surface of GaSb exposed to the As beam at T,= 420 o C for 10 min did not show any trace of GaAs or GaAsSb phonon modes. Such a reaction at the GaSb surface with the As beam indicates that the deposition of the InAs layer on GaSb should be done at relatively low substrate temperatures. The reactions at InAs, GaSb and AlSb surfaces with I

i=

v, E ii-../

GaSbLO

t

Fig. 3. Recorder GaSb surface

GaAsTOGaAsLO 1 I

n+ 250

300

RAMAN SHIFT

(cm-’ 1

trace of Raman scattering measurement exposed to As at T, = 540 o C for 30 min.

on

M. Yam et al. / Interface and thickness control of polytype heterostructures

118

Table 1 Surface stability of InAs, GaSb and AlSb crystals under various environmental conditions; growth temperatures in the region where the oscillation of RHEED intensity was observed are also shown Material

Exposed

Growth temperature ( o C) for RHEED intensity oscillation

to vacuum

without beam

with As beam

with Sb beam

ItL4S

u/s

s/s

s/s

GaSb AlSb

U/S U/S

U/S U/S

S/S S/S

U/S and S/S denote unstable/stable 540 o C/420 o C, respectively.

400-480 350-450 400-530 and

stable/stable

at

various growth conditions are summrized on the left side of table 1. These results indicate that the substantial effects of surface stability for the environmental conditions during the beam switching and for an interchanged beam are the keys to arranging the sequence of beam operation for the growth of sharp interfaces.

In fig. 4, we present the intensity oscillations of specular beam spots of RHEED during the growth of InAs/AlSb/GaSb and AlSb/GaSb/InAs at T, = 420’ C. The one oscillation cycle corresponded to the monolayer step growth. The relatively long interruption time to change InAs to GaSb (indicated by the arrow A) was chosen to wait for enough decay of the residual As molecules. We observed sharp drops of the RHEED intensity at the interfaces as shown in fig. 4, which corresponds to the interchanging operation from AlSb to InAs (arrow B) and InAs to GaSb (arrow C). On the contrary, such a decrease was not observed for the growth of GaSb/AlSb (arrow D). These drops may be due to a sharp interchange of components at the heterointerfaces. If the interfaces are sharp on an atomic scale, Al-As, In-Sb and Ga-As bonds must be formed at respective heterointerfaces of AlSb/InAs, InAs/GaSb or

3.2. RHEED oscillation and AES analysis of polytype structure In order to realize ideal polytype heterostructures as designed, the thickness of the respective layers must be controlled as well as the flatness and sharpness of interfaces. Monitoring the RHEED intensity oscillation is a useful tool to control the thickness and flatness of grown layers [S]. On the right side of table 1, we show the ranges of growth temperatures suitable for InAs, GaSb and AlSb, to observe the oscillation of RHEED intensity. For the continuous monitoring of the RHEED intensity, overlapping regions of these temperature ranges, 400-450°C should be used for the growth of polytype heterostructures. For the interface formation, we have shown in table 1 that the stoichiometry of grown layer surfaces can be preserved during the interchanging operation of component molecular beams at T, = 420 o C. Hence we deposited our polytype heterostructures at T, = 420 o C.

kGa.5ib-l

/-A1Sb-j

1-llnAs-------

I

I

1

0

10

20

TIME

Fig. 4. Oscillation patterns taxy of InAs/AlSb/GaSb

L 31

(sec.)

of RHEED intensity and AlSb/GaSb/InAs 420 o C.

for heteroepiat T, =

M. Yano et al. / Interface and thickness control ojpolytype

heterostructures

119

quantum well structures. Hence, the apparent gradient of Al/Ga compositions at the interfaces in fig. 5 should be attributed to the edge effect of Ar ion etching used for the AES measurement. Because the cut-off sharpness at the heterointerfaces is nearly the same for the two types of heterostructures, GaSb/AlSb/InAs and GaAs/ Al,,,Ga,.,As, both interfaces must be prepared as sharply as on an atomic scale. The detailed characteristics for electrical and optical properties of the polytype heterostructure will be discussed elsewhere in conjunction with the layer thickness and the interface sharpness.

Acknowledgments

Fig. 5. AES depth profile of a polytype heterostructure InAs/AISb/GaSb of 15 nm pitch in thickness. Ga and Al profiles of GaAs/AI,,Ga,,,As of IO/22 nm MQW are also presented for comparison. The cut-off sharpness at the interface is similar for these two samples.

AlSb, and GaSb/InAs. There are fairly large differences in tetrahedral bondlength [9] between interface bondings, 0.245 nm for Al-As and Ga-As and 0.281 nm for In-Sb, and non-interface bondings, 0.263 nm for In-As, Ga-Sb and Al-Sb. These differences should form a considerable strain at the interface and cause disordered atomic arrangement, decreasing the RHEED intensity, instead of the ideal one for layer-by-layer step growth. Fig. 5 shows the result of the AES analysis on a polytype heterostructure with 15 nm pitch in thickness. As a comparison, the AES profile of a GaAs/Al,,,Ga 0,7A~ multiple quantum well (MQW) structure is shown in the same figure. We interrupted the growth procedure for 30 s at each interface of the MQW structure. According to recent studies [lO,ll] on GaAs/AlGaAs, heterointerfaces prepared by this interrupting growth procedure are abrupt on an atomic scale. It was also confirmed for our GaAs/Al,,,Ga,,,As samples by a photoluminescence measurement for single

The authors wish to thank M. Hirasaka of Research Center for Structure Analyses of Teijin Ltd. for AES measurements. Part of this work was supported by the Japan Private School Promotion Foundation on The Scientific Research Promotion Foundation and Japan Securities Scholarship Foundation.

References 111L. Esaki, L.L. Chang and E.E. Mendez, Japan.

J. Appl. Phys. 20 (1981) L529. 121E.E. Mendez, L.L. Chang and L. Esaki, Surface Sci. 113 (1982) 474. Chin-An Chang, E.E. Mendez, L.L. Chang 131 H. Takaoka, and L. Esaki, Physica B 117/118 (1983) 741. [41 T.H. Chiu and A.F.J. Levi, J. Vacuum Sci. Technol. B 6 (1988) 674. Soviet J. Low-Temp. Phys. 4 (1976) 251. 151 S.I. Shevchenko, WI M. Yano, M. Ashida, Y. Iwai and M. Inoue, in: 2nd Intern. Conf. on Formation of Semiconductor Interfaces, Takarazuka, November 1988 [Appl. Surface Sci. 41/42 (1989), to be published]. Y. Iwai and M. [71 M. Yano, M. Ashida, A. Kawaguchi, Inoue, J. Vacuum Sci. Technol. B 7 (1989) 199. H. Funabashi, K. Ohta, T. Nakagawa, N.J. PI T. Sakamoto, Kawai and T. Kojima, Japan. J. Appl. Phys. 23 (1984) L657. [91 J.A. van Vechten and J.C. Phillips, Phys. Rev. B 2 (1970) 2160. 1101 H.L. Stormer, A. Pinczuk, A.C. Gossard and W. Wiegmann, AppI. Phys. Letters 38 (1981) 691. [111 M. Tanaka, H. Sakaki and Y. Yoshino, Japan. J. Appl. Phys. 25 (1986) L155.