Heteroepitaxy of GaAs on a modified CaF2 surface by an extremely low energy electron beam

MATERIALS CHEMISTRYAND PHYSICS ELSEVIER

Materials Chemistry and Physics 45 (1996) 216-219

Heteroepitaxy of GaAs on a modified CaF2 surface by an extremely low energy electron beam Sang-Man Hwang, Keiichiro Miyasato, Kazuo Tsutsui Department of Applied Electronics, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Teclmology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received 19 December 1994; accepted 6 November 1995

Abstract The effects of the acceleration energy and dose of electrons on the surface modification of CaFz for growing heteroepitaxial GaAs on CaF2( I 11) were investigated, focusing especially on the use of a lower energy beam. Electron Hall mobility measurements of GaAs films revealed that the acceleration energy and the dose are not independent parameters and that there is a specific value of the product of these two parameters (VE× Dz) with which good electrical properties of GaAs films are obtained. Transmission electron microscopy and energydispersive X-ray spectroscopy observations showed that excess exposure of the CaF2 surface to the electron beam caused an increase in the dislocation density and Ca contamination in the overgrown GaAs films. The low acceleration energy (around 40 eV) is more desirable, because a wide range of electron doses leads to good GaAs films. Keywords: Heteroepitaxial GaAs; Calcium fluoride; Surface modification; Acceleration energy of electrons; Electron dose

1. Introduction

III-V semiconductor films on insulators will be very important for future possible applications in OEIC, three-dimensional integrated circuits, very high speed devices, neural devices, and so on. The alkaline earth fluorides such as CaF2, SrF2, and BaFa are very promising candidates as the crystalline insulators on top of which GaAs films are to be grown epitaxially. These insulators have special features, such as a dielectric constant lower than that of semi-insulating GaAs, a relatively high resistivity, and a changeable lattice constant (obtained by mixing these fluorides). However, it is very difficult to grow good GaAs films on the fluorides using a conventional growth method such as molecular-beam epitaxy (MBE), because the surface energy of fluorides is extremely low [ 1-3]. As a solution to this problem, surface modification of the fluorides before the growth of GaAs films was found to be very useful, that is, the so-called electron beam exposure (EBE) epitaxy method [4]. The mechanism of the surface modification of fluorides is thought to be that F ions in the top surface layer are replaced by As atoms by means of simultaneous electron beam exposure, as shown in Fig. 1. However, the quality of heteroepitaxial GaAs has not been satisfactory in previous studies, where the acceleration energy of the electrons was as high as a few keV [4,5]. Although the ideal modified surface would 0254-0584/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved

SSD10254-0584 ( 95 ) 01735 -6

As4 electron beam molecular beam e" e" e" [] As4 / As4 F - i o n desorption ] A s 4 I [] ~ [] .................. Troy

Fig. 1, Schematic illustration of surface modification of CaFz ( 11 I) by simultaneous exposure of an electron beam ,and an As., molecular beam.

be a uniform one-atom monolayer (ML) of group V elements with no defects remaining in the fluoride bulk, the real modified surface of CaF2 does not seem to be like this. Some irradiation damage, depending on the electron energy and/ or electron dose during the surface modification, was indicated, and a very low acceleration energy of the electrons is expected to be necessary in order to obtain damage-free modification [6-9]. In this paper, we discuss the optimum conditions of the surface modification, such as acceleration energy and dose of

S.-M. Hwang et aI. / Materials Chemistry and Physics 45 (1996) 216-219

electrons, from the viewpoint of the properties of the overgrown GaAs films.

2. Experimental

Fig. 2 shows a block diagram of the experimental procedure. The (111) oriented Si substrates were chemically cleaned and loaded into the MBE system, which consisted of a double growth chamber. Prior to the growth of CaFz films, the Si substrates were thermally cleaned at 900 °C for 30 min to obtain a clean surface by removing thin chemical oxide. The CaF2 films were epitaxially grown on the substrates at 600 °C. The deposition rate of CaF2 films was about 7 nm min- i, and their thickness was 200 nm. Then the CaF2/Si(ll 1) samples were transferred to the chamber for GaAs growth through the gate valve, and the sample surface was exposed to an electron beam under impingement by A s 4 flux. During the modification process, the substrate temperature Zsu b w a s kept at 300 °C, and the beam flux of A s 4 o n the CaF2 surface was on the order of 10 .5 torr. Electrons generated from a tungsten filament located near the sample were accelerated to the sample surface by an external d.c. bias. The electron beam current density on the sample surface was 5-6 ~A cm -2. The process parameters, such as the electron beam acceleration energy VE and electron dose DE, were varied in this work. After the surface modification, GaAs films were grown on the modified surface of CaFa. The GaAs films were grown by means of the two-step growth method [5,10], in which a 20 nm GaAs layer was grown first at 500 °C and the following 1.0 or 1.5/~m GaAs layer was grown at 570 °C, where 0.5 ~m of the surface layer was doped by Si ( 2 × 10 I7 c m - 3 ) . These GaAs films were characterized by electron Hall mobility measurements at 300 K using the van der Pauw method. Since it was confirmed that the undoped GaAs layers grown CaFa growth [ 600°C,200nm

CaF2 Si(lll)

217

on the CaF2 showed very high resistivity, it is thought that the observed mobility values are for the top Si-doped GaAs layers, which are not affected by the undoped layers. The films were also characterized by cross-sectional observation by transmission electron microscopy (TEM) along with energy-dispersive X-ray spectroscopy (EDX).

3. Results and discussion

3. i. Effects of acceleration energy and electron dose on the electricaI properties of the GaAs film Fig. 3 shows the electron Hall mobility of 1-p,m-thick GaAs on the CaF2 film as a function of VE, where the value of Dz is 2 mC cm- 2. The electron Hall mobility was found to be strongly affected by Vz. At 40 eV, the highest Hall mobility of 1800 cm 2 V - ~s - 2, with an electron concentration of 2 × 10 I7 cm -3, was obtained. In this experiment, it seems that 40 eV is the optimum energy for obtaining a GaAs film with good electrical properties. However, previous workers have pointed out that the damage generation is related to Dz [4,7,8]. In order to clarify the optimized process conditions of the surface modification, the DE dependence of the Hall mobility was investigated for various VE values, as shown in Fig. 4. The highest Hall mobility, 2100 cm z V - 1 s - 1, was obtained for VE=40 eV and DE = 10 mC cm -z. In addition, it was found that the optimum DE value for obtaining the maximum electron Hall mobility for respective VEvalues, IXma~,changed depending on VE. Combining the previous results, where a tXm~ value of 2000 cm 2 V -~ s -a was obtained at 0.15 mC cm - z and 3 keV [5 ], it can be said that tXm~ can be obtained if the value of VE × DE is a constant equal to 400 mJ cm -2, as shown in Fig. 5. Since this relation between VE and DE is itself very interesting and a bit strange, considering that the phenomenon should be related not to the bulk but to the surface, investigation of its mechanism is now under way. 250t]

As g-CaF2 I

Si(lll)

Surface modification by electron beam and [ As4 beam

.....;)~aASi!:i?~i~:::i: GaAs growth 2-step growth CaF2 (500°C/570°C) Si(111)

Ivanerauw 1 TEM EDX

[

200{]

si(111) I

~ 150t1 =

~1000 /;

Dose:2mC/cm 2 i

10

~

r

,

i

i T r [

,

r

i

100 Electron beam energy (eV)

T

i

l i t

1000

Fig. 3. Dependenceof electron Hall mobilityat 300 K in the 2× 10Z7cm-3 Fig. 2, Schematic block diagram of the experimental procedure.

Si doped GaAs layers on VE at Tsub= 300 °C with Dn = 2 mC cm-'-.

S.-M. Hwang et al./Materials Chemistryand Physics 45 (1996) 216-219

218

3000 A t 300K

?

300eV

40eV

20eV

~2000

lOOO

Surface modification:300°C 00.1 ~ 10

T

I

Electron dose (mC/cm2) Fig. 4. Dependence of electron Hall mobility at 300 K in the 2-3 × 10~7 cm -3 Si doped GaAs layers on DE at T~ub=300 °C for V~ values of 20, 40 and 300 eV. The total thickness of GaAs was 1.0 Ixm.

m" 10

• ~

(c)), the stacking faults disappeared, while dislocations were observed. The difference in such defect structures in the GaAs film should result from the fact that GaAs grows three-dimensionally on the unmodified CaF2 at the initial stage discussed in the previous work [ 6]. When the value of VE× DE exceeds the optimum value mentioned above, as shown in Fig. 6(c), the density of dislocations is found to increase. This is thought to be due to the degradation of the crystallinity of the CaFe near the surface by the excess electron beam exposure. The density of dislocations in the surface doped region can be evaluated as being on the order of 108 cm -2 for the case shown in Fig. 6(b). However, the electron Hall mobility of this sample was 1900 cm2 V - ~s- ~, which is still lower than the value expected due to the effect of the dislocations only. Fig. 7 shows EDX spectra taken from positions in the GaAs layer 100 nm from the interface for each sample shown in Fig. 6. An apparent trace amount of Ca, evaluated as being about 0.2 at.%, can be found in spectrum (c), which corresponds to the over-exposure case. The Ca contamination decreases below the EDX detection level for spectrum (b), but it is possible that a significant amount remains here. It was reported that Ca works as an electron trap in GaAs [ 11 ].

[VExDr=400mJ/cmz

ce

1

0.1

i

10

i

i

l

lllll

~

100

i

i

~

l

lJ,I

T

i

GaAs

i

1000

5 0 0 nm

Electron energy: VE (eV) Fig. 5. Relation between V~ and Dz to obtain maximum mobility in GaAs layers for respective VEcases.

CaFE

(b) |

From the viewpoint of the values of ~max, a strong dependence of VE was not found by choosing the appropriate DE value by the VE× DE rule described above. However, the broad window of DE values with which to obtain fairly high mobilities and a minute comparison of the values of tXm~, indicate that the lower energies (20-40 eV) are more desirable than the higher one (300 eV). The shallow penetration depth of the low-energy electrons is considered to contribute to suppression of the undesirable structural degradation of the CaF2 surface, such as deformation of the lattice and surface roughening.

ii

GaAs 500 nm

3.2. TEMandEDXobservationsof GaAson CaF2 Fig. 6 shows TEM cross-sectional photographs of 1.5 Izm GaAs films grown on the CaFa surfaces modified using various values of VE×DE. In the case of no electron dose (Fig. 6(a)), a high density of stacking faults in the GaAs film was observed from the interface of the GaAs and CaF2. However, under exposure to an electron beam (Fig. 6(b) and

GaAs ~00

t

nm

l

Fig. 6. TEM cross-sectional photographs for VmXDm values of (a) 0 (not exposed), (b) (40 eV) (6 mC cm -2) = 240 mJ cm -2, and (e) (300 eV) (6 mC cm -2) = 1800 mJ cm -°-. The total thickness of GaAs was 1,5 ixm.

S.-M. Hwang et al. / Materials Chemistry and Physics 45 (1996) 216-219

GcaaA'S ~x,-~\\\\-~ i l'00nm' F2 ~......................... Si(lll)

I As I

t)

v~

'

Ca

w:

,.5

1

conditions of the electron exposure in order to obtain good properties of the overgrown GaAs over the two-order-ofmagnitude range of VEfrom 20 eV to 3 keV. The dislocation density and Ca contamination increased when the value of VE× DE exceeded the specific value. The low VE value (2040 eV) is more desirable for obtaining good GaAs films, because the value of VEXDa for obtaining good GaAs becomes critical for high VE. This phenomenon is thought to be related to the small penetration depth of low-energy electrons into the CaF2.

(a)VrxDr:0mJ/em2

Si I

219

r

I

2

r

I

~

I

3 Energy (KEY)

r

1

f

I

4

Fig. 7. EDX spectra of GaAs taken from a position 100 nm from the interface of the GaAs and CaF2. Spectra (a)-(c) correspond to samples (a)-(c) shown in Fig. 6.

Thus it can be considered that the Ca contamination might play a role in the degradation of the electrical properties and that a large amount of instable Ca produced by excess irradiation of electrons could enhance the Ca contamination in the GaAs film.

4. Conclusions The acceleration energy and dose of electrons on the surface modification of CaF2(111) are related to each other in an inversely proportional manner. That is, a specific value of the total energy density, VE?
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