【作者】 韩涛；

【导师】 刘昌；

【作者基本信息】 武汉大学 ， 微电子学与固体电子学， 2013， 博士

【摘要】 氮化镓(GaN)是一种新型的第三代半导体材料,具有高击穿电压,高热稳定性,禁带宽度高达3.42V等特点。近年来,GaN广泛应用在高功率、高温以及高速电子器件中。很多基于GaN的电子器件被广泛的研究,比如肖特基二极管,光电子二极管,紫外二极管,发光二极管,高迁移率晶体管等等。但是,在这些结构中,稳定的肖特基接触和欧姆接触显得非常重要。所以,人们开始研究各种金属电极在GaN上的接触与应用,比如Ti/Al电极就广泛应用于n型GaN的欧姆接触,Ni/Au电极应用于p型GaN的欧姆接触。随着器件的发展,尤其是在光电半导体器件中,金属电极的透光性过于低下对光电二极管,紫外探测器等等具有较大的影响。研究表明透光性问题可以用透明导电薄膜(TCO)来解决,比如掺锡的氧化铟(ITO),掺铝的氧化锌(AZO)薄膜等等。ITO薄膜作为GaN的接触电极近年来已经被应用于各种光电器件中。相比于ITO薄膜,AZO薄膜也具有低的电阻率和在可见光波段的高透光性,而且AZO薄膜常温下异常稳定,并且不含毒素,是一种很好的电极材料,因此研究在GaN上的TCO薄膜的接触就称为最近接触工作中的热点。AZO与GaN的接触还有一个明显的优点,AZO的晶格与GaN几乎一样,因此在GaN上面沉积AZO薄膜可以很容易得到高质量的AZO薄膜,这点在器件接触上至关重要。虽然近年来AZO薄膜被广泛研究,但是关于AZO/GaN的报道很少出现。本文研究了AZO在GaN薄膜上的接触性能,主要研究总结如下：1,使用电子束蒸发技术在不同的衬底温度下在n型GaN衬底上沉积了一层致密的AZO薄膜。对AZO/n-GaN的Ⅰ-Ⅴ曲线研究显示AZO/n-GaN具有很好的肖特基接触,通过热电子发射理论定量计算了AZO/n-GaN的肖特基结的肖特基势垒高度与理想因子。然后对AZO的结构性质、电学性质及其光学性质的的研究揭示了决定AZO/n-GaN肖特基接触的肖特基势垒的影响因素主要来与AZO的电子浓度有关,AZO/n-GaN肖特基结的漏电流来源于AZO本身的漏电以及AZO结晶时产生的晶界。2,在n-GaN上沉积AZO薄膜研究的基础上,期望用AZO在p-GaN上得到欧姆接触。具体方法是在p-GaN上沉积了Ag/AZO薄膜,通过在不同退火条件下得Ⅰ-Ⅴ曲线研究得出了在600℃下退火的Ag/AZO薄膜在p-GaN上可以形成欧姆接触。通过研究可以得到Ag在Ag/AZO与p-GaN欧姆接触中起到的作用：在退火条件,Ag膜会产生分解并在降温过程中团簇,在p-GaN表面形成纳米颗粒,这种纳米颗粒造成了p-GaN表面的各种表面态,引起了p-GaN表面的费米能级“钉扎”现象,这种费米能级的钉扎在p-GaN形成了正向的极化电场,从而形成了欧姆接触。更多还原

【Abstract】 GaN is one of the most promising materials for high-temperature, high-power, and high-frequency applications as it has a high break down field, a high thermal conductivity and a wide band gap of about3.4eV at room temperature. In recent years, different types of GaN-based electronic devices have been studied, such as Schottky barrier diodes (SBDs), p-n junction photodiodes (PDs), ultraviolet photodetectors, power rectifiers, heterostructural high electron mobility transistors, and high-power metal semiconductor field effect transistors. However, a reliable Schottky or Ohmic contacts on GaN is critical for these devices. Thus, different metals have been deposited on GaN to produce high-performance Schottky contacts.It was found that the responsivity of normal metal-semiconductor-metor photodetectors (MSM) or light emitting diodes (LED) is limited by the presence of the opaque metal contact electrodes in general. Fortunately, it has been shown that the transparent conductive oxide (TCO) films could be used to solve this problem. Recently, GaN-based MSM photodetectors with TCO film electrodes, such as, indium-tin-oxide (ITO), Al-doped zinc oxide (AZO), et al., have also been demonstrated. The AZO films, which show a low electrical resistance and high transmittance in the visible range of the spectrum, have superior physical and optical properties. The minimum electrical resistivity of the AZO films reaches2.5×104Ω cm and the optical transmittance is over90%in the wavelength range from400to800nm. Thus, the AZO films are widely used as a reliable electrode material. The AZO/GaN contacts have an obvious advantage that ZnO films have almost the same lattice parameters as those of GaN films. Therefore, the AZO films could be easily deposited on GaN films. So far, there were few reports about the AZO/GaN Schottky contacts. In this work, the AZO/GaN contacts have been systematically studied, and the main results are summarized as follows:1. The AZO films were deposited on n-GaN films by using an electron beam evaporation system. The I-V measurements showed that the contacts presented a good rectifying behavior. The Schottky barrier heights were calculated out by using the thermionic emission theory. The dependence of the structural and electrical properties of the AZO thin films on different deposition temperatures has been investigated. The diversifications of the AZO/GaN contacts due to the different characters of the AZO films were found. The electric resistivity and optical transmittance of the AZO films are key factors to fabricate the AZO/GaN Schottky contacts.2. The Ag/AZO films were used to form the ohmic contacts to p-GaN. When the Ag/AZO films were heated up to300℃, the Ag films break into into isolated nano-scaled pieces, and the nano Ag will "pinch off" the Fermi-level of p-GaN, which forms an polarization electric field on the front of p-GaN, and produces the ohmic behavior of Ag/AZO to p-GaN.更多还原