TEM Investigation on As Precipitation in Low-Temperature Grown III-V Arsenides

• Main Authors: Mao-Nan Chang, 張茂男
• Other Authors: K. C. Hsieh
• Format: Others
• Language: en_US
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/19320131510806492664

博士 === 國立中央大學 === 電機工程研究所 === 87 === This dissertation includes a general qualitative model, involving both vacancy and As antisite defects to model the distribution of As precipitates in LT-grown III-V arsenides after annealing. During the thermal treatment, excess As tends to diffuse from a As-rich region to a vacancy-rich region, forming precipitates. In a homostructure, n-type doping enhances the vacancy density and suppresses that of the antisite defects. The influence of p-type doping on point defects is opposite to that of n-type doping. As a result, fewer As precipitates form in the n-type region than in the p-type region for a p-n junction LT-GaAs annealed at 600 oC. In contrast, more As precipitates are present in the n-type region after annealing at 700 oC or higher. In addition to the distribution of As precipitates, a redistribution of point defects induced by the vacancy concentration gradient gives rise to the depletion as well as redistribution of the precipitates. The As precipitate depletion zone width depends on the vacancy concentration gradient and the density of As antisites around the junction area. In an arsenide heterostructure, the concentration of column III vacancies and hence its gradient is determined by the bond strength of the constituents, so the distribution of As precipitates is then dominated by the gradient. Material with a large InAs mole fraction has a higher vacancy concentration. In contrast, the vacancy concentration decreases with the AlAs mole fraction in material. As a result, dense As precipitates are present in the GaAs and InGaAs region in GaAs/AlGaAs MQWs and InGaAs/GaAs/AlGaAs heterostructures, respectively, after thermal treatment. A high concentration of column III vacancies in InGaAs facilitates As precipitation and leads to the existence of larger As precipitates. Moreover, combining LT-GaAs and quantum dots, we have successfully fabricated dislocation free self-assembled In0.5Ga0.5As quantum dots embedded in low-temperature GaAs by molecular beam epitaxy. After in-situ annealing at 610 oC, larger arsenic precipitates formed due to the presence of In0.5Ga0.5As quantum dots. Eliminating the influence of the strain field contrast by selecting proper imaging conditions in TEM, we observed that each dot contains only one precipitate. A quasi-horizontal, well-positioned arsenic precipitate array can be realized using a quantum dot structure. In addition, a tiny arsenic precipitate centered in an In0.5Ga0.5As quantum dot is capable of producing a novel quantum structure, whose potential approximates spherical symmetry. In this novel quantum structure, the electron wave function is confined between the GaAs and the arsenic precipitates. However, further investigations of this structure are necessary. Furthermore, structural defects, such as dislocations, provide additional vacancy sources and also play a role in the As precipitation process. Due to the extra vacancies facilitating As precipitate formation, larger As precipitates attached to dislocation are observed by TEM. The distribution of the As precipitates is non-uniform in the region around the dislocations. Base on the size of the As precipitate, dislocation exhibits the highest influence on As precipitation compared to both doping and bond strength. We also investigated the role of excess As in low-temperature grown GaAs subjected to BCl3/Ar reactive ion etching. Results for etching rate and surface roughness as a function of doping and annealing temperature show that a higher density of charged As antisites yields a higher etching rate as a result of higher vapor pressure for AsCl3 than for GaCl3. Yet, the stronger AsGa-As bond strength associated with the As antisite defects significantly increases the structural stability during annealing, and resistance to ion-induced damage during RIE. P-type doped LT-GaAs has shown the highest damage resistance and can be used as an effective capping layer in device applications, against possible ion damage during RIE processing.