476 Nuclear lnst~ments and Methods in Physics Research 839 (1989) 476-479

North-Holiand, Amsterdam

A NEW TYPE OF SOLID PHASE EPITAXY OF Al,Ga I _ ,Sb IN EVAPORAmD Al/GaSb SUBSTBATE BY ELECTRON-BEAM IRRADIATION (ELECTRON-BEAM EPITAXY)

Takao WADA, Yoshinobu MAEDA and Masaya ICHIMU~

Department of Eiectricai and Computer Engineering. h’agoya Institute of Technology, G&so-cho, Showa-ku, Nagoya 466, Japan

The surface of an Al evaporated layer on the (100) GaSb substrate was bombarded with a total fluence of about (0.1-5.6) X lo’*

electrons cm-* at 7 MeV. The samples were put in a circulating water bath, which was kept at a constant temperature of 50 0 C by

using a thermoregulator. After the irradiation and removal of the Al layer, it was confirmed that a layer of Al,Ga, _.,Sb (t - 250 A) was formed between

the Al evaporated layer and the GaSb substrate. The composition x was almost constant (x = 0.26-0.28) even if the total Ruence

varied from 0.1 x 10” to 5.6 X 10” electrons cm- *. The AI,Ga, _xSb epilayers were oriented along the (100) direction on the (100)

GaSb substrate. Thus, solid phase epitaxial layers of AI,Ga, _xSb were grown by using an electron-beam epitaxy method at 50 * C.

1. introduction

The formation of silicides caused by the penetration of energetic ions through the interface between a metal-film and silicon has been recognized as one aspect of ion-beam induced reactions in thin film structures [l]. The initial mixing process follows the ion mass and energy dependencies predicted by the atomic collision processes. At temperatures where there is sufficient atomic mobility, chemical driving forces can dominate in the formation of well-defined phases.

Recently, new methods of electron-beam doping [2] and electron-beam oxidation [3] processes have been reported by one of the authors. The technique employs an impurity sheet in contact with the semiconductor surface, some water on the semiconductor surface or a vacuum evaporated layer on the semiconductor surface, which is bombarded with high energy electrons [4].

In the present paper, a study was made regarding the electron-beam epitaxy (EBE) growth of Al,Ga, _,Sb crystal. When the surface of an Al layer evaporated on a GaSb substrate was bombarded with high-energy elec- trons at SO”C, a thin heteroepitaxial layer of Al, Ga, _,Sb crystal grew on (100) GaSb substrates.

2. Experimental procedures

The substrates used in this experiment were mirror- polished (100) GaSb (Te doped) with an area of 6 X 6 mm’ (thickness t - 0.35 mm). The Al (99.999~) layer (I - 3000 A) was deposited on the GaSb substrate by using vacuum evaporation. The surface of the evaporated Al layer on the GaSb substrate was bombarded with a total fluence of about (0.1-5.6) X 10” electrons cm-*

0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

at 7 MeV from an electron linear accelerator with a pulse width of - 3.5 ,us, a 200 Hz duty cycle, and an average electron-beam current of 20 PA. The samples were put in a circulating water bath, which was kept at a constant temperature of 50 o C by using a thermoregu- lator. Measurements with an X-ray photoelectron spec- troscope (XPS), and X-ray diffractometer (XD), and a reflection high-energy electron diffractometer (RHEED) were carried out after careful removal of the Al evaporated layer by etching and without annealing.

3. Experimental results

3.1. Observation of the alloy

Fig. 1 shows the transmission electron microscope photograph for the cross section of the sample after irradiation with 0.5 X lo’* electrons cm-* (EBE2). The interface between the Al evaporated layer and the GaSb substrate was clearly observed. It is suggested that the materials do not melt due to irradiation.

The XPS spectra of the electron beam epilayer EBE2 and a liquid phase epitaxy (LPE) Al,,,,Gag,,Sb layer are shown in figs. 2(a) at a depth of 50 A from the surface and (b) at a depth of 650 A from the surface, respectively. They were measured with Al Ka radiation at a pressure of - 1 x 10v5 Pa and obtained by subse- quent observations of spectra after sputtering the surface with 2 kV Ar ions at an argon pressure of - 5 X 1Oe4 Pa. Fig. 2 displays the core-level spectra for both sam- ples over a binding energy interval that includes the Al 2p (AiSb), Ga 3d (GaSb), and Sb 4d (Al,Ga,_.,Sb) peaks in Al,Ga,_,Sb layers. The Al 2p,,, core-level binding energies for elemental Al, AlSb, and Al,O, are

T Wada et al. / A new type of solid phase epitaxy

Fig. 1. TEM micrograph for the cross section of the sample after the irradiation.

74, 75.0, and 75.5 eV, respectively. The photoemission intensity profile for the Al 2p, Ga 3d, and Sb 4d peaks in the EBE grown Al,Ga,_,Sb, the interface and the

(c)EBE Binding Energy(eV)

0 250 Sputtering Depth (i)

500

Fig. 2. XPS core-level spectra in the binding-energy region of the Al 2p, Ga3d, and Sb4d levels obtained from (a) EBE2 and (b) LPE samples, and (c) the spectral intensity of each level as

a function of depth from the surface for EBEZ.

substrate are shown in fig. 2(c). For EBEZ, it is sug- gested from the constant intensity ratios of Ga 3d to Sb 4d and Al 2p spectra that the Al.Ga,_,Sb epilayer of nearly uniform composition is grown for d < 250 A. For d > 350 A, the spectra of Ga 3d and Sb 4d indicate photoemission of the GaSb substrate.

The composition x of Al,Ga,_,Sb was estimated from X-ray (600) diffraction measurement. Fig. 3 shows the typical X-ray diffraction lines for the samples of EBE3 (1.0 X 10” electrons cm-‘). The composition x obtained by assuming Vegard’s law was almost constant (x = 0.26-0.28) even if the total fluence varied from

EBE

(Alo&ao&b) “Ich95”

n

+-2B(deg)

Fig. 3. X-ray diffraction patterns of Al,Ga,_,Sb for the sample of EBE3.

V. SEMICONDUCTORS: GaAs. FIB .

478 T. Wada et al. / A new type of solid phase epitaxy

0.1 X 1O’a to 5.6 X 10” electrons cmm2 (EBEl: 0.1 X

10”; EBE4: 2.1 X 10”; EBES: 5.6 X 10” electrons cmp2).

Distribution measurements of the characteristic X- ray emission lines of Al Ka,, Ga Ka,, Sb Kq on the surface with an area of 6 X 6 mm2 of an Al,Ga,_,Sb alloy (EBE3) were found to indicate the almost constant composition of Al,Ga, _,Sb.

The results of RHEED observations of the Al, Ga,_,Sb on (100) GaSb layer for EBEl-EBES, LPE (A10.36Ga,.,,Sb), and (100) GaSb substrate are shown in figs. 4(a)-4(d), (4e) and 4(f), respectively. The inci- dent electron energy for the RHEED measurements was 200 keV. The RHEED patterns represented in figs. 4(a)-4(d) indicate that Al,Ga, _,Sb epilayers grown by EBE are oriented along the (100) direction in the GaSb substrate. In the case of fig. 4(e), the pattern is spotty. As the pattern of fig. 4(d) has shorter streaks than that of fig. 4(a), the epilayer thickness for EBES is larger than that for EBEl. The LPE Al,Ga, -,Sb epilayer was grown on Te-doped (100) GaSb at 400° C for the temperature interval of 5 o C.

(a) ERIE 1 (b) E8E 3

(c) EBE4 Cd) EBE5

(e) LPE (f ) GaSb substrate

Fig. 4. RHEED patterns of Al,Ga, _,Sb layers for the samples of (a) EBEl, (b) EBE3, (c) EBE4, (d) EBES, (e) LPE

(Ak6Gao6, Sb), and (f) (100) GaSb substrate.

e- e-

I GaSb substrate

c

A, sheet Interface I. GaSb substrate

I /

11 I I

10 5 0 5 10 15

Sputtering Time(mln) I I

0 1500 3000 4500

Sputtering Depth (A)

Fig. 5. Intensity ratios of 69Gat ions and 27A1+ ions to 69Ga+ ions in GaSb substrate, and of 27A1+ ions and 69Gaf ions to

27A1+ ions in Al sheet as a function of depth from the surface

of the substrate and the Al sheet, respectively. The intensity at

a sputtering time of 10 or 15 min is taken as one unit.

3.2. Enhanced diffusion couple

It is expected that interdiffusion may occur between an Al evaporated layer and GaSb substrate. The distri- butions of Ga and Al atoms in an Al layer and GaSb substrate respectively were measured with secondary- ion-mass spectroscopy (SIMS) for the case of an Al sheet/GaSb substrate. The intensity ratio profiles of 27A1+ ions to 69Ga+ ions in the GaSb substrate and 69Ga+ ions to *‘Al’ ions in the Al sheet are shown in fig. 5 as a function of depth d from the surface of the substrate and the Al sheet, respectively. The surfaces of the Al sheet (t - 0.3 mm) which was used instead of an Al evaporated layer in the EBE experiments, in contact with the GaSb substrate were bombarded with 7 MeV electrons to a total fluence of 5 X 10” electrons cme2 at 50°C. The SIMS measurements were performed by using the primary-ion (0:) beam (diameter 0.5 mm) with an ion energy of 7 keV in a 1.5 X lo- ’ Torr vacuum, to an accuracy of within 10%. In the region of d-c 250 A, the Al,Ga, _,Sb layer of almost uniform composition was formed (fig. 2(c)). The concentration profiles of Al atoms for the samples in the range of 4500 > d > 300 .& follow two or three exponential decay curves with increasing depth. The intensity ratio of EB-doped *‘Al+ ions to 69Ga+ ions near the surface of

T. Wada et al. /A new type ofsolidphase epitaxy 479

the substrate becomes about 0.3. Similarly, the con- centration profiles of Ga atoms into the Al sheet follow two exponential decay curves with increasing depth. This suggests that the diffusivity is concentration de-

pendent. The analysis of Boltzmann and Matano is used to obtain the concentration (c) dependence of the diffu-

sivity D(c). The values of D(c) of Al atoms into GaSb are estimated from experimental data to range from 3 x lo-‘* to 7 x lo-‘5 cm* s-t, which are much larger than those usually obtained at 50 o C.

4. Discussion

We speculate that the strong enhancement of the diffusion will be due to the energy release mechanism: when a carrier is nonradiatively captured at a defect (deep level), the phonons emitted help the defect to

surmount potential barriers along the migration path. In the present experiments, excess carriers are generated by the electron beam irradiation, and simultaneously defects (e.g., vacancies and interstitial impurities) are also created. The defects will operate as traps or recom- bination centers for carriers, and their migration will be strongly activated.

The results for the concentration profiles of impurity atoms in fig. 5 suggest that the interdiffusion of Ga and Al atoms occurs across the Al/GaSb interface. Similar

SIMS results were obtained for the same structure irradiated in a vacuum. Therefore, interdiffusion is not strongly affected by water or oxide layers formed by reaction with water. In these experiments, during irradi- ation, plasma may also be produced by a full or partial ionization that includes electrons, holes and some ions at the interface of Al sheet, water and GaSb wafer [3]. Thus, recoil Al atoms in an Al sheet may be introduced into GaSb wafers and move as Al interstitials. Al atoms may occupy both substitutional (Al,,) and interstitial (Al,) sites in GaSb, and therefore an interchange

Al, ti Al, (1)

takes place during the diffusion. It has been shown for all observations of gold in silicon that the interchange is controlled by the “kick-out” mechanism [5]. In the

present case, this mechanism is given by

Al, + Al,, + Ga,. (2)

During irradiation, host Ga atoms are displaced, and Ga self-interstitial (Ga;) are created. However, Gai concentration is reduced near the surface since a surface acts as a sink of interstitials. Thus, Al,, concentration can be high near the surface according to the kick-out mechanism. On the other hand, Al,, atoms are also displaced by electron irradiation, and thus Ali are pro- duced. Al, is rather mobile and thus may diffuse deep into the substrate. When an electron fluence increases,

Al,, concentration may increase in lower fluence re-

gions, but in relatively high fluences it may saturate owing to equilibrium among reactions and migrations mentioned above. Then the Al,Ga,_,Sb layer with

almost constant composition (x = 0.26-0.28) may be obtained for irradiations with a fluence range of (0.1-5.6) x lOiselectron cm-*. More detailed studies on

these mechanisms are now in progress. As shown in fig. 2(c), the relative content of Sb in

the alloy layer is almost equal to that of the substrate. This indicates that the alloy is nearly stoichiometric, i.e.,

Al atoms substitute for Ga atoms in the host lattice. This is due to the fact that the bond between group III and V atoms is much more stable than that between two group III atoms. Al interstitial atoms will prefer a group III lattice site surrounded by four group V atoms to a group V lattice site.

The results of similar experiments for Al/Gap [6] and Al/GaAs systems would support our model of the

enhanced diffusion and the alloy formation. The amount of Al incorporated into the substrate is larger in Al/Gap than in Al/GaSb but is smaller in Al/GaAs. Those

results could be explained as follows: The band struc- ture of GaP is indirect, and thus nonradiative recombi- nation is favored over radiative recombination. On the other hand, GaAs has a direct band structure, and thus the excess carriers will easily recombine radiatively. The energy escapes out of the crystal in the form of photons.

GaSb also has a direct band structure, but the energy separation between L valley and I valley is only 80 meV. Thus, at room temperature, electrons are distrib- uted in both L and P valleys, and the rate of radiative recombination is smaller than in GaAs. Therefore, if Al incorporation is mainly due to the energy release by nonradiative recombination (or capture), one can expect that the amount of incorporated Al increases in the following order: GaAs < GaSb < Gap.

We are grateful to M. Takeda, H. Masuda, K. Yasuda, and H. Morikawa of the Government Industrial Research Institute of Nagoya for their help in connec- tion with bombardment and SIMS measurement of the

samples. This work was supported in part by the Scien- tific Research Grant-in-Aid No.61114002 for Special

Project Research on Alloy Semiconductor Physics and Electronics, from the Ministry of Education, Science

and Culture.

References

[I] J.W. Mayer, B.Y. Tsauer, S.S. Lau, and L.-S. Hung, Nucl. Instr. and Meth. 182/183 (1981) 1.

[2] T. Wada, Nucl. Instr. and Meth. 182/183 (1981) 131.

[3] T. Wada, Appl. Phys. Lett. 52 (1988) 1056. [4] T. Wada and H. Hada, Phys. Rev. B30 (1984) 3384.

[5] W. Frank, A. Seeger and U. Giisele, in: Defects in Semi-

conductors (North-Holland, Amsterdam, 1981) p. 31 [6] T. Wada and Y. Maeda, Appl. Phys. Lett. 51 (1987) 2130.

V. SEMICONDUCTORS: GaAs, FIB