Degradation Mechanism in GaAs Led
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DEGRADATION MECHANISM IN GaAs LED Zhou Jicheng, Fu Zhiping, Zhan Qianbao, Feng Shuifu Shanghai 200050, Shanghai Institute of Metallurgy, Academia Sinica,
China
ABSTRACT A series of experiments have been performed to verify the nature of hole trap A and B in GaAs proposed by Zou and revised by Zhou (one of the present authors). The relative concentration of these two Craps is responsible for the degradation behavior of the diodes. Accordingly, the degradation mechanism in GaAs LED can be reasonably deduced based on thermodynamic consideration.
It is well known that the deep levels in compound semiconductor materials play an important role in characteristics of the devices performed from them. Hence, a number of efforts have been made by the scientists all over the world since the 1970's. For gallium arsenide, two kinds of deep levels have been investigated since that time, one is electron traps and the other hole traps. The representative of the former is EL2 and those of the later are A and B as known. Among them, EL2 is a major factor in preparation of semi-insulating GaAs. Little attention was paid to the hole traps A and B as compared with EL2, although some valuable investigations have been made in these years [1,2,8]. This may be partially due to the negligence of the possible role played by them in improving the quality of the materials. A series of investigations have been taken in our laboratory [3-71, which include the proposal of the nature of hole traps A and B in GaAs and the verifications. The purpose of our work is to improve the characteristics of devices performed on these materials and to elucidate the mechanism concerned. Preliminary results are very exciting, which were based on thermodynamic consideration in addition to-the physical measurement, although much work still needed to attain abovementioned goal. These results gave us a firm confidence that the physico-chemical method is also an important tool in investigating the deep levels in semiconductor materials other than the physical method, and the former is better than the later in preparation the desired materials. A brief review on the nature of A and B is necessary to introduce the historical situation of our previous work. In 1974 [3], a model was first proposed on the nature of two unknown hole traps in GaAs by'analyzing the data of a number of GaAs samples. It was suggested that reversible reaction (1) would be occurred during liquid phase epitaxy of GaAs: GaAsAsCa + (VGGa
+ e
GaAsVGa
+ AsGaVGa
(i)
where GaAsVGa and ASGaVGa represent the nature of hole traps A and B respectively. According to reaction (1), it is obvious that hole traps A and B in GaAs should be same in concentration but different in nature. One year later, Lang and Logan [8] also found two main hole traps in GaAs with the feature as mentioned above and nominated them as A (Ev + 0.41 eV) and B (Ev + 0.71 eV). For the sake of verifying the nature proposed, a series of experiments have been taken in our laboratory. The results are: 1. The effect of growth rate on
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