基于二维等离子体的太赫兹检测器非线性响应研究

The field effect transistor (FET) based on the two dimensional electron gas (2DEG) in the semiconductor heterostructure is one of the ideal solutions for the realization of low cost, compact, tunable and highly integrated terahertz (THz) detectors operating at room temperature. The photovoltage response of a FET at small signal excitations is proportional to the signal intensity. With the application of high power THz radiation sources, the nonlinear response and saturation effect of FET detectors excited by highly intense THz signals have not been fully studied. The objective of this proposal is to try to unveil the nonlinear mechanism. Firstly, the coupling efficiency between THz radiation and 2DEG is quantitatively calculated with the aid of grating gate coupler, including symmetric/asymmetric and infinite/finite gratings. The purpose is to find the coupling structure with the highest efficiency. On this basis, the general Boltzmann transport equation is employed to derive the conservation equations of electron density, macroscopic velocity and electron temperature of 2DEG in consideration of electron collisions. The dielectric function of the metallic gratings and semiconductor materials in a FET is modeled via the temperature dependent, nonlinear and dispersive Drude model. The absorption spectrum of the heterostructure is numerically calculated at intense THz signals. Finally, theoretical analysis will be conducted to determine the photoresponse of an FET THz detector based on three typical heterostructures, such as AlGaN/GaN，AlGaAs/GaAs and SiGe/Si/SiGe. The cause for the saturation response will be clarified and the optimal heterostructure with its dimensional parameters will be proposed as the appropriate candidate for high power THz detection. The study in this proposal will provide the fundamental theory for the future experiment research on the nonlinear properties of FET THz detectors with the aide of high intensity THz sources, such as the free electron laser (FEL) THz radiation source based on electron linear accelerators.

基于异质结二维电子气的场效应晶体管（FET）是实现低成本、紧凑、可调谐、集成度高的室温THz检测器的理想方案之一。小信号激励下的FET光电压与THz功率成线性关系。随着大功率THz源的应用，FET在高强度信号激励下的非线性响应机制及饱和特性研究还有待完善。本项目即针对此开展理论研究。首先，定量计算栅格对THz波耦合效率的影响，包括对称/非对称，无限/有限栅格，寻找高效率的耦合结构。在此基础上，从玻尔兹曼方程出发建立二维电子气浓度，速度与温度耦合的非线性模型，利用参数随温度变化的非线性Drude模型描述栅格与半导体材料特性，数值计算异质结对强THz信号的吸收谱。最后，计算三种典型异质结（AlGaN/GaN，AlGaAs/GaAs，SiGe/Si/SiGe）FET的光响应，阐明饱和现象的原因，提出适合大功率THz检测的FET材料及功率阈值，为未来实验研究（如借助FEL-THz源）提供理论依据。

半导体异质结内高浓度、高迁移率的二维电子气形成的二维等离子体与入射太赫兹波之间的耦合可以用于实现太赫兹波的探测，场效应晶体管太赫兹探测器即是此类可以室温工作、灵敏度较高的光电型探测器。本项目针对异质结类型太赫兹探测器的线性和非线性响应进行了系统深入研究，理论研究部分包括两个方面：第一，开发了计算异质结场效应晶体管探测器在不同频率、不同幅值入射太赫兹信号作用下的动态响应特性的数值程序，开展了程序的稳定性和收敛性测试，分析了响应电压的非线性特征，并与小信号理论模型进行对比，针对GaN、GaAs和Si三类常用异质结材料，开展了光电压响应的计算与优化研究。第二，编写了基于耦合波理论的数值计算程序，用于分离不同散射级的倏逝波，研究了对称和非对称周期金属栅格作为耦合结构的太赫兹探测器的吸收频谱、二维空间场分布以及光电流响应，对不同入射角度和不同频率范围的信号产生的光电流响应进行了计算，为器件的响应度提高提供了理论基础。在理论计算的同时，本项目还开展了针对高功率自由电子激光太赫兹源的功率、时域和频域参数测量工作，为未来开展实验研究和验证工作做了部分准备。