低温生长砷化镓的超快光抽运-太赫兹探测光谱

本文采用光抽运-太赫兹探测技术系统研究了低温生长砷化镓(LT-GaAs)中光生载流子的超快动力学过程.光激发LT-GaAs薄层电导率峰值随抽运光强增加而增加,最后达到饱和,其饱和功率为54μJ/cm~2.当载流子浓度增大时,电子间的库仑相互作用将部分屏蔽缺陷对电子的俘获概率,从而导致LT-GaAs的快速载流子俘获时间随抽运光强增加而变长.光激发薄层电导率的色散关系可以用Cole-Cole Drude模型很好地拟合,结果表明LT-GaAs内部载流子的散射时间随抽运光强增加和延迟时间(产生光和抽运光)变长而增加,主要来源于电子-电子散射以及电子-杂质缺陷散射共同贡献,其中电子-杂质缺陷散射的强度与光激发薄层载流子浓度密切相关,并可由散射时间分布函数α来描述.通过对光激发载流子动力学、光激发薄层电导率以及迁移率变化的研究,我们提出适当增加缺陷浓度,可以进一步降低载流子迁移率和寿命,为研制和设计优良的THz发射器提供了实验依据.

Study on ultrafast dynamics of low-temperature grown GaAs by optical pump and terahertz probe spectroscopy

Low-temperature-grown GaAs (LT-GaAs) possesses high carrier mobility, fast charge trapping, high dark resistance, and large threshold breakdown voltage, which make LT-GaAs a fundamental material for fabricating the ultrafast photoconductive switch, high efficient terahertz emitter, and high sensitive terahertz detector. Although lots of researches have been done on the optical and optoelectrical properties of LT-GaAs, the ultrafast dynamics of the photoexcitation and the relaxation mechanism are still unclear at present, especially when the photocarrier density is close to or higher than the defect density in the LT-GaAs, the dispersion of photocarriers shows a complicated pump fluence dependence. With the development of THz science and technology, the terahertz spectroscopy has become a powerful spectroscopic method, and the advantages of this method are contact-free, highly sensitive to free carriers, and sub-picosecond time resolved. In this article, by employing optical pump and terahertz probe spectroscopy, we investigate the ultrafast carrier dynamics of photogenerated carriers in LT-GaAs. The results reveal that the LT-GaAs has an ultrafast carrier capture process in contrast with that in GaAs wafer. The photoconductivity in LT-GaAs increases linearly with pump fluence at low power, and the saturation can be reached when the pump fluence is higher than 54 μJ/cm². It is also found that the fast process shows a typical relaxation time of a few ps contributed by the capture of defects in the LT-GaAs, which is strongly dependent on pump fluence: higher pump fluence shows longer relaxation time and larger carrier mobility. By employing Cole-Cole Drude model, we can reproduce the photoconductivity well. Our results reveal that photocarrier relaxation time is dominated by the carrier-carrier Coulomb interaction: under low carrier density, the carrier-carrier Coulomb interaction is too small to screen the impurity-carrier scattering, and impurity-carrier scattering plays an important role in the photocarrier relaxation process. On the other hand, under high pump fluence excitation, the carrier-carrier Coulomb interaction screens partially the impurity-carrier scattering, which leads to the reduction of impurity-carrier scattering rate. As a result, the photocarrier lifetime and mobility increase with increasing pump fluence. The experimental findings provide fundamental information for developing and designing an efficient THz emitter and detector.