348 Nuclear Instruments and Methods in Physics Research B37/38 (1989) 348-351 North-Holland. Amsterdam

ELECTRON-BEAM DOPING OF Si INTO GaAs: THE ANNEALING BEHAVIOUR OF PHOTOLUMINESCENCE

Takao WADA and Akihiro TAKEDA

Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan

Si impurities are introduced in (lOO)-oriented undoped semi-insulating GaAs at 50 o C by using an electron-beam doping method from a Si impurity sheet, which is in contact with a GaAs surface. The Si surface is irradiated with a fluence of - 5 x 10” electrons cmd2 at 7 MeV. Before and after annealing at 300°C for 20 min with a SiO,-cap layer, the photoluminescence spectra at 77 K indicate two dominant peaks which are probably attributed to silicon acceptors, Si,,, at 1.48 eV and gallium antisite defects, Ga,,, at 1.445 eV. When the annealing temperature is higher than 600°C, the band-gap transition spectra appear at 1.51 eV. The hole concentration for a sample annealed at 700 o C is measured to be 1-4 X lOI cmm3 with a nearly constant depth distribution in the range of 0.5-5 pm.

1. Introduction

The use of ion beams to modify the properties of surface layers of solids is a relatively recent innovation. It is well known that ion implantation in semiconduc- tors is accompanied by severe radiation damage intro- duced with the implantation process [l]. Electron irradi- ation avoids the complication dependent upon the gen- eration of complex damage regions presumed to occur in neutron and heavy-charged particle irradiation.

New methods of electron beam doping (EBD) [2-41,

oxidation [5] and epitaxy [6,7] have been reported by one of the authors and others. The technique employs

an impurity sheet, or water, in contact with the semi- conductor surface, or a vacuum evaporated layer on the semiconductor surface, which is bombarded with high

energy electrons. In the present paper, Si impurities are introduced

into GaAs wafers using the EBD method at 50 o C. The annealing behaviour of photoluminescences (PL) for the doped samples is investigated.

2. Experimental procedure

The GaAs wafers used in the experiments were

(lOO)-oriented undoped semi-insulating GaAs grown by the liquid encapsL!Tted Czochralski (LEC) method with

an area of 5 x 5 mm* (t = 0.5 mm). 1 represents the thickness of the substrates or the impurity sheets. The Si crystals were B-doped (100) wafers with dimensions of 5 x 5 x t (= 0.4) mm3. The surface of the Si sheet which was in contact with the GaAs wafer was irradiated with a fluence of - 5 x 10” electrons cm-* at 7 MeV from an electron linear accelerator with a pulse width of 3.5

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

ps, a 200 Hz duty cycle and an average electron beam current of 40 pA cm-*, as shown in fig. 1. During the irradiation, the samples were put in a circulating water bath, which was kept at a constant temperature of 50°C by a thermoregulator [4]. The electron-beam doped samples were annealed at temperatures from 300 to 800°C for 20 min with a SiO,-cap layer by conven- tional furnace annealing (FA) and at 600, 700 and 800 o C for 6 s by rapid thermal annealing (RTA) using halogen lamps in flowing N,. The heating and cooling

rates of RTA were fixed at 50°C/s, and 10 o C/s, respectively.

3. Experimental results

Fig. 2 shows the intensity ratios of *sSi+ ions to 69Ga+ ions in the substrate of GaAs measured with SIMS as a function of depth from the GaAs surface in the case of Si/GaAs irradiated at 40 o C with a fluence of - 5 X 10” electrons cm-* at 7 MeV. The SIMS

High energy electron

-5mmd

in running water

Fig. 1. Schematic diagram for the experiments on an array of overlayer Si/substrate GaAs.

T Wada. A. Takeda / Electron-beam doping of Si into GaAs 349

EBD- GaAs

$= 5X1017e/cm2

E = 7MeV

T=40°C

1 I I”

0 0.02 004

Depth (urn)

no

0 01 5

for the doped samples above an annealing temperature of 600° C. The solid lines indicate the experimental results, and the broken lines show each Lorentzian

shaped peak computed to get the best fit for the band- gap transition and for the emission spectra attributed to Si,,. The band-gap transition peaks at 1.51 eV were observed at annealing temperatures of 600 (fig. 4a), 700 (fig. 4b) and 800” C (fig. 4c), and these intensities increased in accordance with the increase in annealing temperature. In the case of 700 and 800°C annealing temperatures, the silicon acceptor peaks at 1.48 eV also

appeared and the samples converted to p-type conduc- tion, which was measured by an electrochemical C-V profiler. On the other hand, in the case of GaAs im-

planted with Si at 150 keV with doses from 1 x lOi to 1 X lOi cmp2, p-type conversion took place above 900°C for 20 min annealing [9]. It is notable that electron-beam doped samples indicate p-type conduc-

tion by rather lower temperature annealing. Fig. 5 shows the annealing temperature dependences of the recovery for the silicon acceptor peaks, the CuGa peaks and band-gap transition. With increasing annealing temper-

Fig. 2. SIMS intensity ratios of **Si+ to 69Ga+ in a substrate of GaAs as a function of depth from the surface of GaAs for the case of Si/GaAs irradiated with a fluence of - 5x10”

electrons cm 2 at 7 MeV and at 40 o C.

measurements were performed by using the primary-ion (0:) beam (diameter: - 1 mm) with an ion energy of 7 keV in a 1.5 x lo-’ Torr vacuum. The diffusion profile is not a complementary error function. This suggests that the diffusivity is concentration dependent. The analysis of Wei [8] is used to obtain the concentration

(c) dependence of the diffusivity, D(c). The concentra- tion co at the surface is about 9 X 102’ cmm3. The value

of D ranges from lOpi4 to lo-l8 cm2 s-i. The re- sultant plot is mainly composed of three curves. It is

suggested that three kinds of species diffuse into the substrate.

Figs. 3a and 3b show typical PL spectra for the doped samples at 77 K before and after annealing at 300” C, respectively, which are obtained with an Ar laser at 5145 A. The solid lines indicate the experimen- tal results, and the broken lines show each Lorentzian shaped peak computed to get the best fit of the EBD spectrum, when emission peaks attributed to the silicon acceptor with isolated Si atoms on an As site Si,, at

1.48 eV [9], to the gallium antisite defect GaAs at 1.445 eV [9], to the Si,,- V,, complexes at 1.40 eV [9] and to residual copper in a Ga site Cu,, at 1.36 eV [lo] are assumed. In the case of 20-500 o C annealing for 20 min of GaAs implanted with Si at 150 keV with a 5 X 1012 dose, no PL spectrum appeared. This suggests that EBD GaAs is much less severely damaged than ion-im- planted GaAs. Fig. 4 indicates the typical PL spectra

LED-GaAs 77 K

Before Annealing

ifter Annealing

(3OO’C, 20min)

\ I..,,,,,.,,,,,,,,,,,,,,,,,,

1.3 1.4 1.5 Photon Energy (eV )

Fig. 3. Typical PL spectra for the EBD samples before (a) and after annealing at 300 o C for 20 min (b).

IV. ION IMPLANTATION

350 T. Wadu, A. Takeda / Electron-beam doping of Si into GoAs

EBD-GaAs a 4

77 K

Anneal :600°C,20min

b .

Anneal:700°C,20min

n

Anneal:800°C,20min

I 1 ,’ . . \

,,,I IIIIIIII I(

1.3 11 1.5 Photon Energy (eV)

Fig. 4. Typical PL spectra for the EBD samples after furnace

annealings at 600 (a), 700 (b) and 800 o C (c) for 20 min.

ature from 600 to 800°C by FA, the band-gap transi- tion and the Si,, acceptor spectra increase, while, for RTA at 700 o C annealing, the emission intensity of the band-gap transition reaches a maximum value and the spectrum contains a weak peak attributed to the Si acceptors Si,,. Carrier concentration profiles for the samples annealed at 600, 700 and 800°C for 20 mm were measured by the same electrochemical C-V pro- filer. Fig. 6 indicates a typical carrier concentration profile from the front surface of the EBD sample, which is annealed at 700 o C for 20 min. The hole concentra- tions were measured to be 1-4 X lOI cmm3 with a nearly constant depth distribution in the range of 0.5-5

pm.

4. Discussion

4.1. Recombination enhanced diffusion

The extrapolated ranges of electrons at 7 MeV in water, Si and GaAs are about 34.7, 15 and 5.65 mm, respectively. Thus the wafers and water (f - 1-2 mm)

FA RTA + -+- Band-gap transition

+ .+- Si,

* .-- CU& Band-gap(FA1

600 700 800

Annealing Temperature?.I)

Fig. 5. PL intensities for the emissions attributed to Si,,, Cu,, and band-gap transition versus annealing temperature for FA

and RTA of EBD samples.

are sufficiently thin to allow the irradiating electrons to penetrate into the overlayer, water and substrate without a significant loss in kinetic energy. The production rates of defects in Si for 7 MeV electron irradiation are about

8 cm-’ [12]. The mechanism for the EBD of Si atoms is

not only the recoil process of Si atoms, but also en- hanced diffusion of impurity atoms in GaAs.

I

ld9r EBD-GaAs

.c . $J= 5X101’e/cm~

k . E=7MeV 0

2 IO’“. 700’C,20min Anneal

SIO~-Encapsulatmn

1 y ; j

I

I 0’5;

0 1 2 3 6 5

Depth(m)

Fig. 6. Carrier concentration profile to a depth of 5 nm from the front surface of an EBD sample after annealing at 700 o C

for 20 min.

T. Wada, A. Takeda / Electron-beam doping of Si into GaAs 351

The rate of generation G of electron-hole pairs

(EHPs) per unit time by an incident electron can be estimated as follows [13]:

G=ldE&? E dx dt ’

where c is the energy for the formation of EHP ( - 4.5 eV for GaAs) [14], dE/dx - 1.4 MeV cm-’ electron-’

is the energy loss per cm of the path by a fast electron,

and d+/dt - 2.7 X 1014 electrons cmp2 s-i is the irradiation rate. The irradiation results in G - 8.4 x lOi

EHPs cmp3 s-l. When such a number of conduction electrons and/or holes in semiconductors recombine at defects via nonradiative transitions, the mobility enhancement of impurity atoms may be caused by the

energy released in these processes [13]. Deep-level tran- sient spectroscopy studies of lattice defects intentionally introduced into GaAs diodes by l-MeV electron irradi-

ation have shown a large enhancement in the defect annealing rate under conditions of e-h recombination. Recombination annealing produces an enhancement of several orders of magnitude in the annealing rate and an activation energy of 0.98 + 0.1 eV. In the case of EBD for an evaporated Ge layer on Si, the activation energy A E of the sputtering yield from the Ge layer was

0.2 eV in the temperature range of 20-60 ’ C for 7 MeV

electron irradiations with a fluence of 3 X lOI electrons cmm2.

4.2. Annealing behaviour

EBD before annealing produces simple acceptor centers Si,, and antisite double acceptor centers Ga,,

(see fig. 3a). The concentration of these acceptors is roughly estimated to be - 3.6 X lo’* cmp3 by assuming that the integrated emission intensity, which is calcu- lated as the standard value, of the band-gap transition for the sample annealed at 800 o C for 20 min (see fig. 4c) is nearly equal to the density of states N, (= 4.7 x

10’7cm-3) in the conduction band. The radiation in- fluence of 5 x 10” electrons cme2 at 7 MeV and 50 o C may produce a uniform defect density of - 5 x lo’* cm-3, which is estimated by supposing that the produc-

tion rate n of displaced atoms is TJ = 10. Thus, this defect density is similar to the acceptor concentrations. On the other hand, the irradiation also introduces cer- tain killer centers such as As vacancies which compete with luminescent centers (151. At 300 o C annealing for 20 min, the killer centers are annealed and then the

total acceptor centers increase to the calculated value of 9 X 10” cmp3. It is indicated from the PL results of fig. 4 that the induced nonstoichiometric defects may recover considerably and the emission of band-gap transitions appear by annealing above 600°C. At 700” C anneal- ing for 20 min, the integrated emission intensity of acceptor centers Si,, is estimated to be 8 X lOI cm-3

from the PL results of fig. 4b, which is similar to a hole concentration of l-4 X 1Ou’ cm-3 obtained by the C-V

profiler measurements. At 800 o C annealing, both the PL acceptor centers (4.9 X 10” cmp3) and the hole carriers (- 10” cmm3) obtained by the C-V profiler also largely increase.

5. Summary

The annealing behaviour of PL spectra for Si impuri- ties introduced in GaAs by EBD was investigated. Before and after annealing at 300 o C for 20 min, the PL spectra attributed to two and four kinds of defects were

observed. Above 600 o C annealing band-gap transition spectra appear. At 700 and 800 o C annealing tempera- tures silicon acceptor emissions were observed and p- type conversion took place.

We are grateful to M. Takeda, H. Masuda and K. Yasuda of the Government Industrial Research In- stitute of Nagoya for their help in connection with

bombardment of the sample. This work was supported

in part by the Scientific 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

[l] J.W. Mayer and O.J. Marsh, in: Applied Solid State Science, eds. C.J. Kriessman and R. Wolf (Academic Press, New York, 1968).

[2] T. Wada, Nucl. Instr. and Meth. 182/183 (1981) 131. [3] T. Wada, Proc. 3rd Int. Conf. on Neutron-Transmutation

Doped Si (Plenum, New York, London, 1981) p. 447.

[4] T. Wada and H. Hada, Phys. Rev. B30 (1984) 3384.

[5] T. Wada, Appl. Phys. Lett. 52 (1988) 1056.

[6] T. Wada and Y. Maeda, Appl. Phys. Lett. 51 (1987) 2130.

[7] T. Wada and Y. Maeda, Appl. Phys. Lett. 52 (1988) 60.

[8] L.Y. Wei, J. Phys. Chem. Solids 18 (1961) 162.

[9] T. Hiramoto, Y. Mochizuki, T. Saito and T. Ikoma, Jpn. J.

Appl. Phys. 24 (1985) L921.

[lo] T. Itoh and M. Takeuchi, Jpn. J. Appl. Phys. 16 (1977) 227.

[ll] T. Tabata, R. Itoh and S. Okabe, Nucl. Instr. and Meth. 103 (1972) 85.

[12] T. Wada, K. Yasuda, S. Ikuta, M. Takeda and H. Masuda,

J. Appl. Phys. 48 (1977) 2145. [13] J.C. Bourgoin and J.W. Corbett, Radiat. Eff. 36 (1978)

157.

[14] E. Baldinger, W. Czaja and A.Z. Farooqi, Helv. Phys. Acta. 33 (1960) 551.

[15] M. Jeong, J. Shirafuji and Y. Inuishi, Jpn. J. Appl. Phys. 12 (1973) 109.

IV. ION IMPLANTATION