352 Nuclear Instrnments and Methods in Physics Research B37/38 (1989) 352-356 North-Holland, Amsterdam

ELECTRON BEAM DOPING OF Si INTO GaAs IN THE OVERLAYER Si/SUBSTRATE GaAs AND THE SANDWICHED SYSTEM OF GaAs/Si/GaAs

Takao WADA and Akihiro TAKEDA

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

Si atoms were introduced in (lOO)-oriented undoped semi-insulating GaAs at 50 o C by using an electron-beam doping method from a Si sheet. A Si sheet with a thickness t of - 0.5 mm was sandwiched between two wafers of GaAs: GaAs (layer 3)/Si (layer Z)/GaAs (layer 1) (sandwiched array). The surface of layer 3 was irradiated with a fluence of 5 x 10”electrons cmm2 at 7 MeV. After annealing at 800 o C for 20 rnin with a SiO, cap, the 0.6 mm thick GaAs wafers (the layers 1 and 3) converted to p-type, and the peak carrier concentrations at the surfaces of layers 1 and 3, which were in contact with the Si surface, were - 3 X 10” and - 4 X lOI cmm3, respectively. The photoluminescence spectra at 77 K for layers 1 and 3 indicated two dominant peaks, which were attributed to the Si acceptor, Si,,, at 1.48 eV and to the band gap transition at 1.51 eV. In the case of the sandwiched array electron-beam doping was experimentally determined to be more effective than in the case of the two layer array (an overlayer Si/substrate GaAs).

1. Introduction

The physical properties of semiconductors irradiated

by high energy electrons have been studied by many

workers [l]. Ion implantation offers a number of tech-

nological advantages which are important in the fabri-

cation of electronic devices. However, it is well known

that ion implantation in semiconductors is accompanied

by radiation damage introduced by the implantation

process [2]. Electron irradiation avoids the complication

of the generation of complex damage regions presumed

to occur in neutron and heavy-charged particle irradia- tion. The enhancements of defect annealing reactions in semiconductors under conditions of minority carrier injection have been directly observed [3]. Recently new methods of electron beam doping (EBD) [4-61, oxida-

tion [7] and epitaxy [8,9] were reported by one of the authors and others. The EBD technique employs an impurity sheet, or water, in contact with the semicon- ductor surface, or a vacuum evaporated layer on the semiconductor surface which is bombarded with high energy (2-7 MeV) electrons.

In the present paper we study the electrical and

optical properties of GaAs doped with Si by the EBD technique.

2. Experimental procedures

The wafers and impurity sheets used in the experi- ments were (lOO)-oriented undoped semi-insulating GaAs grown by liquid encapsulated Czochralski (LEC) and (lOO)-oriented B-doped p-Si, respectively. The di- mensions of each sample are shown in fig. 1. Two kinds

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

of arrays are used. As shown in fig. la, the Si sheet was sandwiched between two wafers of GaAs, that is GaAs (layer 3)/Si (layer 2)/GaAs (layer 1) (A.III), and each was in contact with another surface layer. In another array, the Si sheet was in contact with a GaAs surface

(Si/GaAs, (AH)) (fig. lb). The surfaces of layer 3 for A.111 and of the Si sheet for A.11 were irradiated with a fluence of - 5 x 10” electrons cm-2 at 7 MeV from an electron linear accelerator with a pulse width of 3.5 t.ts, a 200 Hz duty cycle and an average electron-beam current of - 40 PA cmp2.

During irradiation, the samples were put in an iso- thermal circulating water bath using a thermoregulator as shown in fig. lc. After irradiation, the GaAs samples of layers 1 and 3 for A.111 and of A.11 were annealed at 800°C for 20 min with a SiO, cap in a conventional furnace. After stripping off the SiO, films, photo- luminescence (PL) measurements were performed at 77 K. A focused 800 mW, 5145 A argon laser beam was used as the excitation source.

3. Experimental results

Figs. 2a and 2b show typical PL spectra for the EBD GaAs samples of layer 1 for A.111 and of AH, respec- tively. The solid and broken lines indicate the experi- mental results, and each Lorentzian shaped peak com- puted to obtain the most suitable agreement for the EBD spectrum when the following emission peaks are assumed: a peak attributed to the band gap transition at 1.51 eV, a peak attributed to silicon acceptor with isolated Si atoms on As site Si,, at 1.48 eV [lo] and a peak attributed to the residual copper in Ga site Cu,,

T. Wada, A. Take-da / Electron beam doping of Si into Ga.As

A.IB a High energy electron

(E=7MeV ~=5X10’7e/crn2)

A.111 and A.11 are shown by the solid and broken lines, respectively, in fig. 3 as a function of depth from the front and back surfaces of the GaAs. They were mea- sured with an electrochemical C-V profiler. The solid lines show the profiles to a depth of 35 urn from the front and back surfaces of layer 1 for A.111 after anneal- ing at 800°C for 20 min. Large buildups of carrier concentrations at both the front and back surfaces of GaAs layer 1 were observed. The hole concentrations near the front and the back surfaces and in the center of layer 1 wafer for A.111 are - 4 X lo”, - 3 X 10” and - 1.5 X 10” cmm3, respectively. The broken line in- dicates the profile in a depth x range of O-5 urn from the front surface of the GaAs sample for A.11 after annealing at 800 o C for 20 min.

( layer

Si ( layer

GaAs ( layer

+6mm---*/

in running water

A.11 b

High energy electron

( E=7MeV @= 5x 1017 elcm2)

in running water

C

Llnac \

353

3)

2)

,l)

The carrier profiles could not be observed near the back surface for several samples of A.11 even after annealing at 700, 800 and 900 o C for 20 min. This may be caused by lattice imperfections induced in the dop- ing process. The hole concentration near the front surface of the sample for A.11 is - 1.5 x 10” cme3, which is lower than that ‘for the sample for A.111. The profile for A.11 contains a dip at x = 2.4 urn, which may be due to the effect of complex defects.

+ Current monitor

SaGpIe

Fig. 1. Schematic diagram for the experiments of the sand- wiched array of GaAs/Si/GaAs (A.111) (a), the array of overlayer Si/substrate GaAs (A.11) (b), and schematic diagram

of electron irradiation at 50 o C (c).

at 1.36 eV [ll]. The PL spectra for layer 1 of A.111 indicated a clear peak which is attributed to the Si acceptors (Si,,). On the other hand, in the case of A.11 the Si As peak was rather unclear, The ratio of the emission intensity for the Si-acceptor of GaAs (layer 1) for A.111 to that of GaAs for A.11 was nearly 4 to 1. The hole concentration profiles in the GaAs substrate of

EBD-&As(layerl) a

A

Anneal :BOO”C, 20min II

EBD-GaAs b

A.11

Anneal: 8oO”C, 20min

Photon Energy (eV)

Fig. 2. Typical photoluminescence spectra for the EBD GaAs samples of layers 1 for A.111 (a) and of Si/GaAs (A.11) (b)

after annealing at 800 o C for 20 min.

IV. ION IMPLANTATION

354 T. Wade, A. Takedu / Electron beam doping of% into GuAs

10ZO: ,< 1

. EBD-GaAs A.@ Ann~~:~~OC,ZOmin .

- lOI9 : - layer1 ( A.lll ) ’

%

--I-- &As ( A.ll ) :

U Back Surface

.k tote : Front Surface

P -we

1o15 . * ' ) ' ' ' ' 'is

t to 20 30 30 20 10

f Front Surface - Depth in Substrate (pm) Back Surface

(x=0) (x=t) Fig. 3. Carrier concentration profiles in layer 1 for A.111 and in the GaAs samples for A.11 as a function of depth from both the front

and back surfaces. They were measured with an electrochemical C-V profiler.

10ZO. It

1 . EBD-GaAs(layer31 A.lU Bat k Surface

GaAs (x=t 1

6 lOI : layer3

Front Surface E

IZZI

Si (x=0 1

layer2

2 GaAs layer1 Anneal:8OO”C, 20min .

4, ;;( 1018 r I_

p-type

lOI I L I I I * 1::’ ’ ’ . ’ g ’ *

I 10 20 30 30 20 10

*Depth in Substrate ( pm 1 t

Front Surface Bat k Surface (x=01 (x=t f

Fig. 4. Carrier concentration profile in layer 3 for A.111 as a function of depth from both the front and back surfaces. This was measured with an electrochemical C-V profiler.

T. Wada, A. Takeda / Electron beam doping of Si into &As 355

Fig. 4 indicates the carrier concentration profile to a depth of 35 urn from the front and back surfaces of

layer 3 for A.111. This also indicates a U-shaped carrier profile. After 800 o C annealing for 20 min, the 0.6 mm thick GaAs samples of layers 1 and 3 for A.111 con- verted to p-type. The peak carrier concentrations at the

front surface of layer 3 for A.111 was - 4 X 1017 cmp3. Reproducibility of the electrochemical C-V mea-

surements for the sample of A.11 was slightly unstable through depending on the irradiation and annealing

conditions, and this may be caused by lattice imperfec- tions induced in the doping process as will be men- tioned in the section 4. On the contrary, in the case of the sample for A.111, good reproducibility was obtained.

4. Discussion

The extrapolated ranges of electrons at 7 MeV in Si and GaAs are about 15 and 5.65 mm, respectively [12].

Thus, the impurity sheet layer is sufficiently thin to

allow the irradiating electrons to penetrate into the substrate without a significant loss in kinetic energy. The production rate of defects in Si for 7 MeV electron

irradiation are about 8 cm -’ [13]. The mechanism of EBD is not only the recoil process of Si atoms, but also enhanced diffusion of impurity atoms in GaAs.

The rate of generation G of electron-hole pairs (EHPs) per unit time by an incident electron beam can be estimated as follows [14]:

G - 1 dE d+

EHP- z dx dt ’

where c is the energy for formation of EHP (e in Si and GaAs is - 3.23 and - 4.6 eV, respectively) [15]. The electron irradiation would result in GE,, = 3 X 102’ and = 5 x 10” EHPs cmp3 s-l. The radiation in-

fluence of 5 X 1017 electrons cme2 at 7 MeV and 50 o C may produce a uniform defect density of the order of 10” cmp3 in the sample. Thus during the irradiation, plasma may be produced by a full or partial ionization

that includes electrons, holes, Si impurities, vacancies and some ions in layers 1, 2 and 3 [7]. Since alternative charge-state changes of a defect occur when the Fermi level is near the defect level, the ionization-enhanced diffusion (IED) mechanisms can be operative under an external excitation by an irradiation that creates elec- tron-hole pairs [14].

The U-shaped diffusion profile may be explained properly by taking account of the surface diffusion and the kickout mechanism [6]. The diffusivity of the surface diffusion D, is much larger than that of the volume diffusion D,. The recoil impurities diffuse very fast on the surface from the front to the back surfaces, and thus the impurity concentration at the back surface N,_, becomes nearly equal to that of the front one, N,=,.

Then the impurities diffuse into the specimen from both surfaces through the value of 0,. This diffusion process would give rise to a U-shaped diffusion profile. In the

case of EBD, a large diffusion length as shown in figs. 3 and 4 has been observed [6].

Similar PL results were obtained for array III

(GaAs/Zn/GaAs) irradiated in vacuum (10~6-10-5 Torr). Therefore, the EBD process would not be largely affected by water or oxide layers formed by reaction

with water. It was also suggested that a mechanism similar to

that of the conventional plasma-grown oxidation on semiconductors for EBD was quite plausible [7]. In the case of the two layer array (A.II), the GaAs crystal deviates from the stoichiometry due to diffusion couples

[18] between the Si sheet and GaAs samples, which

occur during the electron irradiation. Even in the Si

sheet, the enhanced-diffusions of Ga and As atoms may

also occur. The lattice imperfection causes a degrada- tion for electrical and optical properties of GaAs. On the other hand, in the case of A.111, the irradiation may

generate not only the diffusion of Si atoms from layer 2,

but also that of As and Ga atoms from layer 3 through layer 2 and into layer 1. More effective EBD may be

accomplished by the sandwiched array, and it is thought atom migration through the Si sheet will play an im- portant role in the doping process. A study on atom migration in the Si sheet is now in progress.

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 samples. 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

111

121

[31

[41

[51

[cl

[71 PI

[91 WI

J.W. Corbett, Electron Radiation Damage in Semiconduc-

tors and Metals (Academic Press, New York, London,

1966). J.W. Mayer and O.J. Marsh, in: Applied Solid State

Science, eds. C.J. Kriessman and R. Wolf (Academic

Press, New York, 1968).

L.C. KimerIing and D.V. Lang, Inst. Phys. Conf. Ser. No.

23 (1975) p. 589. T. Wada, NucI. Instr. and Meth. 182/183 (1981) 131.

T. Wada, Proc. 3rd Int. Conf. on Neutron-Transmutation Doped Si (Plenum, New York, London, 1981) p. 447.

T. Wada and H. Hada, Phys. Rev. B30 (1984) 3384. T. Wada, Appl. Phys. Lett. 52 (1988) 1056. T. Wada and Y. Maeda, Appl. Phys. Lett. 51 (1987) 2130.

T. Wada and Y. Maeda, Appl. Phys. Lett. 52 (1988) 60. T. Hiramoto, Y. Mocbizuki, T. Saito and T. Ikoma, Jpn. J.

Appl. Phys. 24 (1985) L921.

IV. ION IMPLANTATION

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

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

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

[13] T. Wada, K. Yasuda, S. Ikuta, M. Takeda and H. Masuda, J. Appl. Phys. 48 (1977) 2145.

[14] J.C. Bourgoin and J.W. Corbett, Radiat. Eff. 36 (1978) 157.

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

[16] D.V. Lang and L.C. Kimerling, Phys. Rev. Lett. 33 (1974) 489.

[17] T. Wada, M. Takeda and K. Yasuda, J. Electron. Mater. 14 (1985) 171.

[18] T. Wada and M. Takeda, Nucl. Instr. and Meth. B21 (1987) 574.