钝化前表面预处理对AlGaN/GaN HEMTs性能的影响

提出一种新的钝化技术--采用盐酸和氢氟酸混合预处理溶液(HF:HCI:H2O=1:4:20)对AIGaN/GaNHEMTs进行表面预处理后冉淀积Si3N4钝化,研究了新型钝化技术对AlGaN/GaN HEMTs性能的影响并分析其机理.与用常规方法钝化的器件相比,经过表面顶处理再钝化,成功地抑制了 AIGaN/GaN HEMTs肖特基特性的恶化,有效地增强抑制电流崩塌效应的能力,将GaN基HEMTs的输出功率密度提高到5.2W/mm,并展现良好的电学可靠性.通过X射线光电子谱(XPS)检测预处理前后的AIGaN表面,观察到经过预处理后的AIGaN表面氧元素的含量大幅度下降.表面氧元素的含量下降,能有效地降低表面态密度和表面电荷陷阱密度,被认为是提高AIGaN/GaN HEMTs性能的主要原因.     

介质膜性质及表面处理对GaN HEMT特性的影响

GaN HEMT器件经过钝化后,抑制电流崩塌效应明显,但同时产生其他负面效应,为了改善目前GaN HEMT钝化后漏电增加和击穿电压减小等情况,研究了钝化技术对GaN HEMT电流特性的影响,包括介质膜应力、折射率和表面预处理与器件饱和电流、电流崩塌量的关系,优化了表面预处理和钝化工艺条件.实验效果在CaN HEMT电特性上的改善明显.结果表明,采用折射率为2.1～2.2的SiN钝化膜,饱和电流密度增加到1 100 mA/mm,电流崩塌量小于10%,肖特基接触反向偏压为-20 V时泄漏电流达10-5A数量级.     

SiN钝化前的NF3等离子体处理对AlGaN/GaN HEMT性能的影响

研究了SiN钝化前利用感应耦合等离子体(ICP)对AlGaN/GaN HEMT表面进行NF3等离子体处理对器件性能的影响.结果表明,运用低能量的NF3等离子体处理钝化前的AlGaN/GaN HEMT表面能有效抑制器件电流崩塌,而器件直流及微波小信号特性则未受影响.微波功率测试表明,经过6min NF3等离子体处理的AIGaN/GaN HEMT在2GHz,30V工作电压下达到6.15W/mm的输出功率密度,而未经过处理的器件只达到1.82W/mm的输出功率密度.     

高性能A1GaN/GaN HEMT的肖特基特性

研究了AIGaN/GaN HEMT制备中相关工艺对器件肖特基特性的影响,并对工艺进行了优化.首先研究了表面处理对器件肖特基势垒特性的影响,对不同的表面处理方法进行了比较,发现采用氧等离子体处理,并用V(HF):V(H2O)=1:5溶液清洗刻蚀后的表面,可以有效减小表面态密度,未经处理的样品肖特基接触理想因子为2.6,处理后理想因子减小到1.8.对SiN钝化膜的折射率与肖特基特性的关系进行了研究,发现SiN钝化膜的折射率为2.3～2.4时,钝化对肖特基特性的影响较小,但反向泄漏电流较大.     

等离子体刻蚀凹栅槽影响AlGaN/GaN HEMT栅电流的机理

对等离子体干法刻蚀形成的凹栅槽结构AlGaN/GaN HEMTs肖特基电流增加的机理进行了研究.实验表明,凹栅槽结构AIGaN/GaN HEMTs肖特基栅电流增加一个数量级以上,击穿电压有一定程度的下降.利用AFM和XPS的方法分析AlGaN表面,等离子体干法刻蚀增加了AlGaN表面粗糙度,甚至出现部分尖峰状突起,增大了栅金属与AlGaN的接触面积;另一方面,等离子体轰击使AlGaN表面出现一定量的N空位,相当于栅金属与AlGaN接触界面处出现n型掺杂层,使肖特基结的隧道效应加强,降低了肖特基势垒.由此表明,AlGaN表面粗糙度的增加以及一定量的N空位出现是引起栅电流急剧增大的根本原因.     

等离子体刻蚀凹栅槽影响AlGaN/GaN HEMT栅电流的机理

对等离子体干法刻蚀形成的凹栅槽结构AlGaN/GaN HEMTs肖特基电流增加的机理进行了研究.实验表明,凹栅槽结构AIGaN/GaN HEMTs肖特基栅电流增加一个数量级以上,击穿电压有一定程度的下降.利用AFM和XPS的方法分析AlGaN表面,等离子体干法刻蚀增加了AlGaN表面粗糙度,甚至出现部分尖峰状突起,增大了栅金属与AlGaN的接触面积;另一方面,等离子体轰击使AlGaN表面出现一定量的N空位,相当于栅金属与AlGaN接触界面处出现n型掺杂层,使肖特基结的隧道效应加强,降低了肖特基势垒.由此表明,AlGaN表面粗糙度的增加以及一定量的N空位出现是引起栅电流急剧增大的根本原因.     

基于不同AlN钝化层的极性GaN MOS器件界面特性

界面质量是影响GaN MOS器件性能以及可靠性的主要因素之一,Al203栅介质与极性GaN界面间插入超薄非晶AlN作为钝化层可以有效改善GaN栅界面特性,针对AIN钝化层生长方式研究了GaN界面优化特性.通过GaN MOS电容的C-V和J-V特性,结合透射电子显微镜(TEM)表征分析,对比了不同生长条件的AlN插入层对GaN MOS电容的界面特性的影响.相比常规热生长AlN钝化层制备的样品,以等离子体NH3为N源在300℃下生长AlN钝化层制备的GaN MOS电容的频散和滞回特性均得到显著改善,界面态密度也略有改善.分析认为,经过等离子体NH3的轰击作用有效地抑制了GaN表面上Ga-O键的形成,在GaN表面直接生长AlN,从而改善了界面特性.     

大电流密度高性能AlGaN/GaN HEMT器件研制

在SiC衬底上制备得到了表面钝化氮化硅(SiN)的AlGaN/Ga N高电子迁移率场效应晶体管(HEMTs)。采用高电子浓度的AlGaN/GaN异质结材料、90 nm T型栅、100 nm SiN表面钝化、低欧姆接触以及较短漏源间距等方法来提升器件性能。在Vgs=4 V时器件的最大漏源饱和电流密度为1.72 A/mm,最大跨导为305 m S/mm,此结果为国内报道的AlGaN/GaN HEMT器件最大漏源饱和电流密度。此外,栅长为90 nm T型栅器件的电流增益截止频率(fT)为109GHz,最大振荡频率(fmax)为144 GHz。     

SiN钝化前的NF3等离子体处理对AlGaN/GaN HEMT性能的影响

研究了SiN钝化前利用感应耦合等离子体（ICP）对AlGaN/GaN HEMT表面进行NF3等离子体处理对器件性能的影响. 结果表明,运用低能量的NF3等离子体处理钝化前的AlGaN/GaN HEMT表面能有效抑制器件电流崩塌,而器件直流及微波小信号特性则未受影响. 微波功率测试表明,经过6min NF3等离子体处理的AlGaN/GaN HEMT在2GHz, 30V工作电压下达到6.15W/mm的输出功率密度,而未经过处理的器件只达到1.82W/mm的输出功率密度.     

SiN钝化对W波段AlGaN/GaN HEMT射频性能的影响

在SiC衬底上制备了栅长为110 nm、漏源间距为2μm的W波段AlGaN/GaN高电子迁移率场效应晶体管(HEMT),分析了SiN钝化对器件直流和射频特性的影响.研究发现:100 nm SiN钝化可显著提升器件的漏源饱和电流及峰值跨导,漏源饱和电流从1.27 A/mm增加至1.45 A/mm (Vgs=1 V),器件峰值跨导从300 mS/mm提升至370 mS/mm,这是由于SiN钝化显著提高了AlGaN/GaN异质结材料沟道电子浓度.此外,SiN钝化可有效抑制器件电流崩塌,显著改善器件直流回扫特性.然而,由于沟道电子浓度增大,钝化后器件中短沟效应增强,器件夹断特性变差.此外,SiN钝化后W波段AlGaN/GaN HEMTs的射频特性得到显著改善,器件的电流增益截止频率从钝化前的33 GHz提升至107 GHz,最高振荡频率从钝化前的65 GHz提升至156 GHz.     

Growth of GaN Nanowires on Epitaxial GaN

We report experiments on the formation of GaN nanowires on epitaxial GaN using thin layers of Ni. GaN covered with Ni shows roughening that is strongly dependent on the thickness of the Ni layer and the annealing conditions. With the initial Ni thickness of 0.8 nm we observe formation of Ni-filled antidots. These act as nucleation sites in the growth of GaN nanowires, allowing for the preparation of nanowires with an average diameter as small as 30 nm. Dense and well-oriented nanowires are formed by pulsed metallorganic chemical vapor deposition at 750°C. The size of the Ni features determines the diameter of the GaN nanowires, resulting in good control over the formation process.     

GaN MESFET

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GaN MOS Capacitors and FETs on Plasma-Etched GaN Surfaces

The electrical characteristics of gallium nitride (GaN) metal-oxide-semiconductor (MOS) capacitors and field-effect transistors (FETs) made on as-grown surfaces, dry-etched surfaces using reactive-ion etching (RIE), and wet-etch treated surfaces after the dry etch were measured. Capacitance and conductance techniques were used to obtain the MOS properties for capacitors. Devices with only an RIE plasma dry-etch process have poor yield and noisy capacitance in the low-frequency accumulation region. Those on dry/wet-etch treated samples have more negative ultraviolet (UV) assistant capacitance-voltage (CV) shift, and higher interface-state densities than those on as-grown samples, but have similar surface potential fluctuation. Threshold voltages of 2 V for an as-grown GaN MOSFET and 1 V for a dry/wet-etched MOSFET were measured. Maximum field-effect mobility for long-channel (L _ch = 100 μm) MOSFETs on the as-grown GaN wafer and the dry/wet-etched GaN wafer were obtained as 167 cm² V⁻¹ s⁻¹ and 119 cm² V⁻¹ s⁻¹, respectively. The higher interface trap density and lower field-effect mobility indicate that post-plasma-etch wet etching can only partially remove the damages from RIE.     

Si-diffused GaN for enhancement-mode GaN mosfet on si applications

Si diffusion into GaN was studied as a function of encapsulant type (SiO₂ or SiN_x) and diffusion temperature. Using a SiO₂ encapsulant, the Si diffusion exhibited an activation energy of 0.57 eV with a prefactor of 2.07×10⁻⁴ cm² sec⁻¹ in the temperature range 800–1,000°C. An enhancement-mode MgO/GaN-on-Si metal-oxide semiconductor field-effect transistor (MOSFET) was fabricated utilizing Si-diffused regions under the source and drain to provide an accumulated channel. The gate leakage through the undoped GaN was low enough for us to achieve good saturation behavior in the drain-current-voltage characteristics. The devices showed improved transconductance and drain current relative to previous devices with Si-implanted source/drain regions.     

Growth of GaN on porous SiC and GaN substrates

We have studied the growth of GaN on porous SiC and GaN substrates, employing both plasma-assisted molecular-beam epitaxy (PAMBE) and metal-organic chemical-vapor deposition (MOCVD). For growth on porous SiC, transmission electron microscopy (TEM) observations indicate that the epitaxial-GaN growth initiates primarily from surface areas between pores, and the exposed surface pores tend to extend into GaN as open tubes and trap Ga droplets. The dislocation density in the GaN layers is similar to, or slightly less than, that observed in layers grown on nonporous substrates. For the case of GaN growth on porous GaN, the overgrown layer replicates the underlying dislocation structure (although considerable dislocation reduction can occur as this overgrowth proceeds, independent of the presence of the porous layer). The GaN layers grown on a porous SiC substrate were found to be mechanically more relaxed than those grown on nonporous substrates; electron-diffraction patterns indicate that the former are free of misfit strain or are even in tension after cooling to room temperature.     

Photoluminescence characteristics of GaN/lnGaN/GaN quantum wells

Photoluminescence (PL) characteristics of GaN/lnGaN/GaN single quantum wells (QWs) and an InGaN/GaN single heterojunction were studied using continuous wave (CW) and pulsed photoluminescence in both edge and surface emitting configurations. Samples were grown on c-plane sapphire substrates by atmospheric pressure metalorganic chemical vapor deposition (MOCVD). Room temperature and 77K PL measurements were performed using a CW Ar-ion laser (305 nm) and a frequency tripled (280 nm), pulsed, mode-locked Ti: sapphire laser. CW PL emission spectra from the quantum wells (24, 30, 80Å) were all blue shifted with respect to the reference sample. The difference (i. e., the blue shift) between the measured value of peak emission energy from the QW and the band-edge emission from the reference sample was attributed to quantum size effects, and to strain arising due to a significant lattice mismatch between InGaN and GaN. In addition, stimulated emission was observed from an InGaN/GaN single heterojunction in the edge and surface emitting configu-ration at 77K. The narrowing of emission spectra, the nonlinear dependence of output emission intensity on input power density, and the observation of a strongly polarized output are presented.     

Geiger-Mode Operation of GaN Avalanche Photodiodes Grown on GaN Substrates

Ultraviolet (UV) GaN p-i-n avalanche photodiodes (APDs) on low dislocation density free-standing GaN substrates were grown and fabricated. The GaN APD showed a stable avalanche multiplication gain in a linear mode, using UV illumination. In Geiger-mode operation at room temperature with gated quenching, no after-pulsing effect was observed up to 100 kHz. The single-photon detection efficiency and dark-count probability were measured to be ${sim}$1% and ${sim} 3times 10^{- 2}$ at 265 nm, respectively.      

Theoretical study of RF-breakdown in bulk GaN and GaN MESFETs

RF-breakdown was studied in bulk GaN and in GaN MESFETs using a full band Monte Carlo simulator. It was found that in bulk materials, increasing the frequency of an applied RF field would result in a lower overall impact ionization rate and consequently lead to higher breakdown fields. It was also found that the RF-breakdown voltage of devices increases with increasing frequency of the applied large signal RF excitation. The frequency dependence of RF-breakdown and the difference between RF and dc-breakdown is explained based on the time response of the particle energy to the change in the applied RF excitation     

GaN HEMT reliability

This paper reviews the experimental evidence behind a new failure mechanism recently identified in GaN high-electron mobility transistors subject to electrical stress. Under high voltage, it has been found that electrically active defects are generated in the AlGaN barrier or at its surface in the vicinity of the gate edge. These defects reduce the drain current, increase the parasitic resistance and provide a path for excess gate current. There is mounting evidence for the role of the inverse piezoelectric effect in introducing mechanical stress in the AlGaN barrier layer and eventually producing these defects. The key signature of this mechanism is a sudden and non-reversible increase in the gate leakage current of several orders of magnitude. This degradation mechanism is voltage driven and characterized by a critical voltage below which degradation does not occur. This hypothesis suggests several paths to enhance the electrical reliability of GaN HEMTs which are borne out by experiments.