Mechanism for the Diffusion of Zinc in Gallium Arsenide
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MECHANISM FOR THE DIFFUSION OF ZINC IN GALLIUM ARSENIDE K. B. KAHEN Corporate Research Laboratory, Eastman Kodak Company, Rochester, NY 14650-2011
ABSTRACT The anomalous shape of the Zn diffusion profile in GaAs has been quantitatively explained. The interchange between interstitial Zn and substitutional Zn is assumed to occur via the Ga vacancies. These vacancies are proposed to be either neutral or singly ionized, depending on the position of the Fermi level. In addition, two physical phenomena are proposed. Substitutional Zn thermally generates interstitial Zn-Ga vacancy pairs and there is pairing between the donor, interstitial Zn and the acceptor, substitutional Zn. This pairing leads to the interstitial Zn diffusivity being a decreasing function of the substitutional Zn concentration. The model is found to be in good agreement with the experimental data.
I. INTRODUCTION Zn diffusion has been used for many years to form ohmic p-type contacts [1]. Recently, Zn diffusion has been employed in another processing capacity, namely, for interdiffusing multiple quantum-well layers to form laterally guided laser diodes [2]. In both of these processes, it is important that there be strict control over the depth and lateral extent of the Zn diffusion. Consequently, an understanding of the physical mechanisms which underlie the Zn diffusion process is important in order to be able to predict accurately the resulting Zn diffusion profiles. A typical kinked Zn profile is indicated by the circles in Fig. 1 (the error bars on the data and the theoretical results will be discussed below). The data were taken from the recent work of Tiwari et al. [1]. The zinc source was Zn3 As 2 and the diffusion was performed for five minutes using rapid thermal annealing (RTA). 1020 RTA, 300.Os
E
10
19
850
1018o
Fig. 1. Comparison of theoretical, solid lines, and experimental Zn profiles at
a, o0
8000c
8000Cand 850°C.
"
1017
1016 0
0.3
0.6
0.9
1.2
1.5
Depth ( Im)
Mat. Res. Soc. Symp. Proc. Vol. 163. '1990 Materials Research Society
682
II. DIFFUSION MODEL The starting basis for the model we propose is the Frank-Tumbull mechanism [3], which governs the interchange between interstitial Zn (Zni) and substitutional Zn (Zns) by means of the Ga vacancies (VGa), Zni+ +Ga V
Zns + 2h+
(1)
where h÷ indicates holes. Because of the large electrical activation of Zn in GaAs [4], the hole concentration is approximately equal to the Zns concentration, Cs. Using eq. (1), it is simple to 2 show that the effective Zn diffusivity, Dff, is proportional to C, . As discussed by Reynolds et Zn profile (such as the 800'C profile in of a kinked al. [4], the near surface-region structure Fig. 1) can best be fit assuming Deft -e C,2 , while the tail region can best be described by Deft C.. Referring to eq. (1), the coefficient for holes can be reduced to one in a couple of ways (to obtain Deff - Cs). In this work, eq. (1) is modified by the assumption that the Ga vacancies have multiple ionization states, where the dominant species are deter
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