(Ga,Mn)As光调制反射光谱

室温下我们研究了稀磁半导体(Ga,Mn)As的光调制反射(PR)光谱,观测到来自样品的Franz-Keldysh振荡(FKO)信号。随着Mn原子浓度的增加,PR线形展宽,但是临界点E₀和E₀+Δ₀没有明显的移动。根据FKO振荡数据,计算得到样品表面电场强度随Mn原子掺杂浓度的增加而增强。测量到与Mn原子掺杂相关的杂质带,其能量位置离GaAs价带边~100 meV。根据样品的表面电场强度和表面耗尽层模型,估算样品的空穴浓度为~10¹⁷cm^-3,较低的空穴浓度可能与样品具有较低的居里温度有关,或测量的PR信号来自于样品中外延层的部分耗尽区域。

Photo-modulated Reflectance Spectra of (Ga,Mn)As

Ga_1-xMn_xAs thin films with a thickness of 200 nm and with various Mn mole concentrations(x=0,2.6%,4.2% and 6.3%) were grown on epi-ready semi-insulating GaAs(001) substrates by low temperature molecular beam epitaxy.The Curie temperature of samples,in the range of 30~50 K,was determined using a superconducting quantum interference device.For photo-modulated reflectance(PR) spectra measurements at room temperature,a halogen lamp dispersed by a monochromator was used as the light source.The He-Ne laser(632.8 nm) with a power of 1 mW was used as the modulation light and the chopper frequency was at 184 Hz.A Si detector was employed to collect the modulated signal.Franz-Keldysh oscillations(FKO) from epitaxial layers of the diluted magnetic semiconductor(Ga,Mn)As have been studied by PR at room temperature.The PR line shape is broadened with increasing Mn concentration but the critical points of E₀ and E₀+Δ₀ are not shifted obviously.Based on FKO oscillation data,the calculated electric field of Ga_1-xMn_xAs is 21,45,77 and 160 kV/cm for x=0,2.6%,4.2% and 6.3%,respectively.It suggests that the surface electric field increases with the increase of Mn concentration up to 6.3%.In addition,Mn-related impurity state is observed,which is about 100 meV above the top of the valence band of the host GaAs.Based on depletion layer approximation,we solve the equations of semiconductor statistics together with Poisson’s equation for the relationship between the potential and charge concentration.The calculated hole concentration is about 10¹⁷ cm^-3,which is two orders of magnitude less than the reported value due to the PR signal coming from the(Ga,Mn)As layer with a part of depletion of the delocalized holes,or a lower Curie temperature.Further work is needed to deplete all holes within the(Ga,Mn)As layer by applied external electric field.