半绝缘GaAs的双调制反射光谱研究

半导体材料电子能带结构的确定对研究其物理性质及其在半导体器件方面的应用有重要意义.光调制反射光谱是一种无损和高灵敏度的表征半导体材料电子能带结构的光学手段.光调制反射光谱中激光调制导致的材料介电函数的变化在联合态密度奇点附近表现得更为明显.通过测量这些变化,可以得到有关材料能带结构临界点的信息.然而在传统的单调制反射光谱中,激光调制信号的光谱线型拟合和临界点数目的分析往往被瑞利散射和荧光信号所干扰.本文将双调制技术与双通道锁相放大器结合,消除了瑞利信号和荧光信号的干扰,获得了具有较高信噪比的调制反射光谱信号.双通道锁相放大器可以同时解调出反射光谱信号及其经泵浦激光调制后的细微变化量,避免了多次采集时可能存在的系统误差.利用这种技术,在可见激光(2.33 eV)泵浦下,我们测量了半绝缘GaAs体材料从近红外至紫外波段(1.1-6.0 eV)的双调制反射光谱,获得了多个能带结构临界点的信息.探测到了高于泵浦能量之上的与GaAs能带结构高阶临界点对应的特征光谱信号,说明带隙以上高阶临界点的光调制反射光谱本质是光生载流子对内建电场的调制,并不是来自该临界点附近的能带填充效应.这一结果表明双调制反射光谱能够对半导体材料能带结构带隙及其带隙以上临界点进行更准确的表征.

Dual-modulated photoreflectance spectra of semi-insulating GaAs

For a semiconductor material, the characterization of its electronic band structure is very important for analyzing its physical properties and applications in semiconductor-based devices. Photoreflectance spectroscopy is a contactless and highly sensitive method of characterizing electronic band structures of semiconductor materials. In the photoreflectance spectroscopy, the modulation of pumping laser can cause a change in material dielectric function particularly around the singularity points of joint density of states. Thus the information about the critical points in electronic band structure can be obtained by measuring these subtle changes. However, in the conventional single-modulated photoreflectance spectroscopy, Rayleigh scattering and inevitable photoluminescence signals originating from the pumping laser strongly disturb the line shape fitting of photoreflectance signal and influence the determination of critical point numbers. Thus, experimental technique of photoreflectance spectroscopy needs further optimizing. In this work, we make some improvements on the basis of traditional measurement technique of photoreflectance spectroscopy. We set an additional optical chopper for the pumping laser which can modulate the amplitude of the photoreflectance signal. We use a dual-channel lock-in amplifier to demodulate both the unmodulated reflectance signals and the subtle changes in modulated reflectance signals at the same time, which avoids the systematic errors derived from multiple measurements compared with the single-modulated photoreflectance measurement. The combination of dual-modulated technique and dual-channel lock-in amplifier can successfully eliminate the disturbances from Rayleigh scattering and photoluminescence, thus improving the signal-to-noise ratio of the system. Under a visible laser (2.33 eV) pumping, we measure the room-temperature dual-modulated photoreflectance spectrum of semi-insulating GaAs in a region from near-infrared to ultraviolet (1.1 ～6.0 eV) and obtain several optical features which correspond to certain critical points in its electronic band structure. Besides the unambiguously resolved energy level transition of E₀ and E₀+Δ₀ around the bandgap, we also obtain several high-energy optical features above the energy of pumping laser which are related to high-energy level transitions of E₁, E₁+Δ₁, E₀' and E₂ in the electronic band structure of GaAs. This is consistent with the results from ellipsometric spectroscopy and electroreflectance spectroscopy. The results demonstrate that for those high-energy optical features, the mechanism for photoreflectance is that the photon-generated carriers modulate the build-in electric field which affects the overall electronic band structures, rather than the band filling effect around those critical points. This indicates that dual-modulated photoreflectance performs better in the characterization of semiconductors electronic band structure at critical point around and above its bandgap.