Annealing of radiation-induced defects in n-GaAs

Abstract

A change in the charge state of defects may give rise to a change in probability of their interactions, migration activation energy and stability of the systems, which might become evident during annealing of defects. Since the level of semiconductor doping, the type of conduction, and the presence of embedded and external electrical fields control the charge state of defects, all these would affect their thermal stability. Dislocations, clusters of basic elements in compound semiconductors, zones of disordering in semiconductors irradiated by neutrons, and devices with the p–n-transitions and Schottky barriers are sources of the space charge regions (SCRs). The presence of a neutral volume (NV) and an SCR and, hence, a different position of the Fermi level with respect to the level of “deeper” defects causes differences in their charge states in these regions. This might give rise to a situation where the formation and annealing of defects in an SCR would differ from those in an NV [1]. The purpose of this work is to study the effect of the charge state of radiation-induced defects in n-GaAs on their annealing [1]. The specimens under study were Schottky diodes manufactured from an epitaxial film grown by a gas-phase method on a highly doped substrate. The concentration of free carriers in the film was (1–5)⋅10 cm. Irradiation of the specimens by γ-quanta from a Co source and by 10 MeV protons was performed at room temperature. The flux and fluence of the γ-quanta were I = 9.4⋅10 quant/(cm⋅с), D = 2.6⋅10 quant/cm, respectively. Irradiation by the protons was performed at I = 3.5⋅10 proton/cm⋅с and D = 4⋅10 proton/cm. Defect concentration was measured by the method of deep level transient spectroscopy (DLTS). In order to study the effect of the defect charge state, comparison was made of the results of specimen annealing with (in SCR) and without (in NV) application of a back bias voltage. Shown in Fig. 1 are the curves of isothermal annealing of E3 centers in gallium arsenide irradiated by γ-quanta. It is evident that the concentration logarithm of these centers is a linear function of annealing time, which testifies to the fact that the annealing reaction order η = 1. The annealing activation energy E and the frequency factor ν, determined by the method of sectioning [2] of the isotherms from Fig. 1a, agree with the literature data [3] and are equal to (1.5 ± 0.05) eV and 10 s, respectively. The parameters of annealing of E3 centers in an SCR were found from the data shown in Fig. 1b to be as follows: E = (1.6–1.8) eV, η = 1, and ν ≈ 10 s. These results demonstrate that the E3 annealing rate in an SCR is lower that that in an NV. Presented in Fig. 2 are the curves of isochronous annealing of E2 and E3 centers in NVs and SCRs of gallium arsenide irradiated by γ-quanta. It is evident from Fig. 2a that the thermal stability of E2 centers in NVs and SCRs is the same, while that of E3 in SCRs is higher than it is in NVs. Figure 2b depicts the curves of isochronous annealing of E5 centers in NVs and SCRs, which are qualitatively similar to those of annealing of E3 enters. These data demonstrate that E5, as well as E3 centers in SCRs are annealed much slower than in NVs of gallium arsenide irradiated by γ-quanta. The parameters of annealing of E5 centers in NVs and SCRs were estimated from the curves of isothermal annealing to be as follows: E = (1.5 ± 0.05) eV, ν ≈ 10 s and E = (1.7 ± 0.1) eV, ν ≈ 10 s, respectively. Thus, the annealing kinetics for E5 coincides with that for E3 centers both in NVs and SCRs. An examination was made of the “back bias/no back bias” cycle on the character of annealing of defects in NVs and SCRs at Т = 493 K for E3 center (Fig. 3). Curve 1 was obtained at U = 0, Curve 2 was constructed with alternated application of U = 30 V (sections b, d, f) and without its application (sections a, c, e) in the course of annealing. It is evident from the figure that sections a, c, e are parallel to curve 1. This implies that the kinetics of subsequent annealing in sections a, c, e (U = 0) after the preceding annealing run at U = 30 V is reversibly returned to the annealing kinetics in NVs and is characterized by the same parameters as the process in curve 1: E = 1.5 eV, η = 1, and ν ≈ 10 s.