摘要
作为第三代半导体材料的代表,GaN材料具有大禁带宽度(0.75-6.2eV)、强极化效应(GaAs的4-5倍)、高临界场强(2MV/cm)、高载流子饱和速率(2×107cm/s)和高热导率(1.3W/cmK)等优异的特点。GaN材料以及基于GaN材料的各种器件在近十年中得到了系统和深入的研究。目前GaN微波功率器件已经在军用雷达和航天领域得到了初步的应用;在民用领域中,各种GaN LED和LD器件已经广泛应用于汽车电子和高端照明设备。但是GaN基器件,尤其是AlGaN/GaN多层结构的应用与开发仍然存在着较多的问题值得深入的研究。特别是AlGaN/GaN超晶格由于能够通过调节Al组份和周期长度等结构参数来人为的控制器件的能带形状,因而在高频振荡器、发光器件和探测器等众多领域有着广泛的应用前景。此外GaN基材料现在仍然面临着许多的问题,特别是GaN材料的p型掺杂始终没有很好的解决,这一问题严重的制约着HBT和LED等需要高质量p型材料器件的发展。本文正是在这一背景下,针对超晶格结构的特点在超晶格p型掺杂以及外加太赫兹电场下的稳态非线性响应两个方面进行了研究。
在超晶格p型掺杂方面,本文在分析了现有GaN p型掺杂技术所面临的困难与不足后,提出了利用超晶格结构提高GaN材料p型掺杂效果的方案。随后系统的研究了超晶格结构掺杂的机理,分析了超晶格周期长度和Al组份对掺杂效果的影响。优化了MOCVD技术生长掺Mg AlGaN/GaN超晶格的实验条件,通过Hall、PL谱、HRXRD(?)口AFM测试验证了AlGaN/GaN超晶格掺Mg方案的有效性。取得了以下关于AlGaN/GaN超晶格掺Mg的成果和结论:
1. AlGaN/GaN超晶格的空穴微带效应和界面应力是超晶格结构增强p型掺杂效果的机理。超晶格形成的空穴微带能够降低AlGaN材料中受主元素的激活能,使得受主的离化率得到提高。同时界面张应力的存在使得受主Mg更容易掺入势垒层中。
2.极化效应和界面应力引起的高导带断续对AlGaN/GaN超晶格的空穴微带有较大的影响。考虑极化效应和界面应力时AlGaN/GaN超晶格的平均空穴浓度比不考虑这些效应时高约一个量级。
3.通过实验验证了对于极化效应和界面应力作用的理论分析和计算,优化了超晶格周期长度、Al组份和反应室压力等参数。认为高的反应室压力、Al组份以及约十纳米的超晶格周期长度有利于掺杂效率的提高。
4.获得了电阻率为0.31Ω·cm,空穴浓度为4.36x1018/cm3的AlGaN/GaN超晶格样品。这一结果优于目前GaN体材料p型掺杂的结果,证明了超晶格结构在p型掺杂中的作用,这一结果对于LED和HBT (?)等器件具有一定的应用价值。
在AlGaN/GaN超晶格在外加太赫兹电场下的稳态非线性响应方面,本文进行了以下的工作:
首先,针对AlGaN/GaN异质结的特点,建立了包含界面应力和极化效应的导带断续模型。使用单个势阱中电子的基态波函数作为基,利用紧束缚模型构建了AlGaN/GaN超晶格的色散模型,并对各种条件下的色散关系进行了数值计算。
第二,改进了传统的简单余弦色散模型,并在此基础上,基于半经典理论建立了AlGaN/GaN超晶格第一微带电子在外加激励下的输运模型。
第三,采用傅立叶变换分析了在外加太赫兹激励电场下超晶格的稳态非线性响应。重点计算和分析了三次谐波稳态响应的动态电阻、相位差和相对效率与基频激励电场强度、三次谐波激励电场强度和基频频率之间的关系。
As a representative of the third generation of semiconductor material, GaN-based materials are famous for their large band gap (0.75-6.2eV), strong polarization effects (4~5times of GaAs), high critical field strength (2MV/cm), high electron saturation velocity (2×107cm/s) and thermal conductivity (1.3W/cmK) and other outstanding features. GaN materials and devices based on it have been researched systematically and widely in recent decades. GaN based microwave power devices have been present in the field of military radar and space applications. In the civilian field, many types of GaN LED and LD devices have been widely used in automotive electronics and high-end lighting equipment. However, the application and development of the GaN-based devices, particularly AlGaN/GaN multi-layer structure, still have many obstacles that worthy to be studied. The energy band shape of the devices can be modified by adjusting the Al content of the AlGaN/GaN superlattice. Thus the AlGaN/GaN superlattice structure has great application potential in the high-frequency oscillator, light-emitting devices and photo-detectors and many other fields. In addition, there are still lots of problems to be studied, especially in the aspect of p-type doping in GaN, this problem seriously limits the development of the HBT and LED devices which need high-quality p-type materials. This paper is performed under this background, focused on the application in the p typed doping of AlGaN/GaN superlattice and the steady-state nonlinear response of the AlGaN/GaN superlattice under THz electric field.
At the first part, the limits of the available methods to enhance the p-type doping were analyzed, based on this analysis a new method was devised to enhance the p-type doping efficiency by utilizing the superlattice structure. We systematically studied the mechanism of superlattice doping, calculated the relationships of superlattice period and Al content with the doping efficiency, optimized the parameters of doping experiments. Based on these researches the following results are obtained:
1. The valence miniband effect and the strain effect of the AlGaN/GaN superlattice are the key to enhace the p type doping. The formation of the valence miniband can lower the activation energy of the acceptors in AlGaN, which greatly enhanced the acceptor ionization ratio of the acceptors. Furthermore the tensile stress makes the doping of Mg to the AlGaN layer much easier.
2. The formation of the valence miniband of AlGaN/GaN superlattice is mainly due to the polarization effect and the strain effect. The difference of the averaged hole concentrations with and without polar effect and strain effect can be about one magnitude.
3. The analysis on the roles of polarization effect and strain effect are verified by the following experiments. The superlattice period, Al content and chamber pressure are optimized by experiments. High chamber pressure, Al content and a length of period around10nm are benefit to the doping efficiency.
4. A Mg doped AlGaN/GaN superlattice sample of p=4.36x1018/cm3and R=0.31Ω·cm respectively is obtained. This result is better than the available result of p-type bulk GaN. This result does prove the efficiency of superlattice structure in p type doping and find its application in the LED and HBT devices.
At the second part, we carried out the study of the steady-state nonlinear response of the AlGaN/GaN superlattice under THz electric field. The work and results can be concluded as:
First, the conduction band offset model of GaN including the strain effect and the polarization effect is established. Based on the ground electron wave function in a single AlGaN/GaN well, the first miniband energy dispersion is obtained by the tight binding approach method. Then the relationships of the energy dispersion with Al content, period length, strain effect and polarization effect were explored in detail.
Second, the conventional simple cosine dispersion model is improved. Based on this improved dispersion relation, a transport model of the miniband electron of AlGaN/GaN superlattice under THz electric field is constructed within the semi-classical theory.
Third, the steady-state nonlinear response of the AlGaN/GaN superlattice under THz electric field is studied with the Fourier transform analysis. Calculation and analysis are focused on the relationships of dynamic resistance, phase and relative efficiency of the third harmonic response with the strength and frequency of the fundamental and the third harmonic excitation field.
引文
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