Development of high-speed and high-sensitivity semiconductor multiple quantum well photorefractive devices

光折变材料在非均匀光强图案的照射下，动态地改变折射率。它们有很多潜在的应用，如用于下一代信息网络系统的高速并行信号处理设备，用于振动检测的高速自适应传感器，用于生物医学应用的近红外光学器件等。本研究的目标是利用InGaAs/GaAs半导体多量子阱来开发高速、高灵敏度的光折变材料。(1)利用时间分辨四波混频技术测量了光载流子的寿命和扩散系数。质子注入样品的载流子寿命约为100ps。(2)制作了量子受限斯塔克几何结构的光折变器件。器件由夹有SiO_2绝缘层的MQW层组成。与Franz-Keldysh器件相比，其衍射效率提高了20倍。然而，空间分辨率变得很差。(3)为了提高空间分辨率，用低温生长的GaAs层代替了SiO_2层作为绝缘层。这种新结构大大提高了设备的空间分辨率。(4)制作了工作波长为1064 nm的器件，即1064 nm的激光波长。为了使激子共振与该波长相匹配，在InGaAs层中增加了In的含量。然而，这导致了InGaAs层和GaAs层之间存在较大的晶格常数失配。在适当设计的缓冲层上成功地生长了MQW层，从而释放了MQW层中的应变。

Photorefractive materials dynamically change the refractive index by the illumination of nonuniform intensity pattern. They have lots of potential applications, such as, high-speed parallel signal processing devices for next generation information network systems, high-speed adaptive sensor for vibration detection, near-infrared optical devices for biomedical applications, etc. The target of this research was to develop high-speed and high-sensitivity photorefractive materials using InGaAs/GaAs semiconductor multiple quantum wells. (1) We measured the lifetime and diffusion coefficient of photocarriers using time-resolved four wave mixing. The carrier lifetime is about 100 ps in proton-implanted samples. (2) We fabricated the photorefractive devices in quantum confined Stark geometry. The devices consist of the MQW layers sandwiched by SiO2 insulating layers. The diffraction efficiency is improved by 20 times in comparison with the Franz-Keldysh devices. However, the spatial resolution became poor. (3) In order to improve the spatial resolution, we used the low-temperature grown GaAs layers for insulating layers instead of SiO2 layers. This new structure substantially improved the spatial resolution of the devices. (4) We fabricated the devices that work at 1064 nm, that is, the wavelength of Nd : YAG laser. To fit the exciton resonance to this wavelength, the fraction of indium is increased in InGaAs quantum well layers. However, this results in large lattice constant mismatch between InGaAs and GaAs layers. The MQW layers are successfully grown on the properly designed buffer layer which releases the strain in the MQW layers.