溶胶凝胶法制备的 MgO 粉末通过电子束蒸发形成作为绝缘体保护体应用在 PDP 领域的 MgO 薄膜

MgO 优异的发射和保护性能会由于水吸附作用形成 Mg(OH)2而变差.为了证明这一点,实验首先采用 Sol-gel 法合成了纯净的 MgO 粉末:将硝酸镁六水合物和草酸溶解在乙醇中形成凝胶体,接着将该凝胶体在 100℃时烘 24 h以形成草酸镁,然后升温到600℃使之裂解,将其磨碎,再使用240 mesh 筛分,最后再在1000℃时烧结 2 h,形成 MgO.该法制成的MgO 在气压为 10-5 mbar的气氛中通过电子束蒸发到玻璃衬底上形成薄膜.以上方法制成的 MgO 薄膜和在450℃直接退火的 MgO 薄膜在以下几点做了对比:(a)生长过程和结构;(b)表面形貌;(c)透光度;(d)OH键.以上对比结果表明前者:(i)具有 fcc 结构,其晶格常数 a 为 4.216±0.005A,(ii)具有在＜100和＜110之间的择优取向.另外前者还有以下特征;(i)表面形貌呈锥柱状、(ii)可视光范围的高透光度( 88～92%)、(iii)无OH键.此外,如果 MgO 薄膜的晶粒取向{100}和{110}之类的中性面,那么其水合作用就可以得到有效的控制.     

Comparison of MOS capacitors on n- and p-type GaN

The electrical characteristics of both n- and p-type GaN metal-oxide semiconductor (MOS) capacitors utilizing plasma-enhanced CVD-SiO₂ as the gate dielectric were measured. Both capacitance and conductance techniques were used to obtain the MOS properties (such as interface state density). Devices annealed at 1000°C/30 min. in N₂ yielded an interface state density of 3.8×10¹⁰ cm⁻² eV⁻¹ at 0.19 eV from the conduction band edge, and it decreased to 1.1×10¹⁰ cm⁻² eV⁻¹ deeper into the band gap. A total fixed oxide charge density of 8×10¹² q cm⁻² near the valence band was estimated. Unlike the symmetric interface state density distribution in Si, an asymmetric interface state density distribution with lower density near the conduction band and higher density near the valence band was determined.     

Effects of post-annealing on the passivation interface characteristics of AlGaN/GaN high electron mobility transistors

We examined the effects of post-annealing in forming-gas ambient on the spin-on-dielectric (SOD)-buffered passivation as well as the conventional plasma-enhanced chemical vapor deposition (PECVD) Si₃N₄ passivation structure in association with the quantitative analysis of defects at the passivation interfaces of AlGaN/GaN high electron mobility transistors (HEMTs). Before the annealing, the interface state densities (D_it) of the PECVD Si₃N₄ are one-order higher (10¹²–10¹³ cm⁻² eV⁻¹) than those of the SOD SiO_x (10¹¹–10¹² cm⁻² eV⁻¹) as derived from C–V characterization. Clear reduction in D_it from the PECVD Si₃N₄ is extracted to a level of 10¹¹–10¹² cm⁻² eV⁻¹ with a stronger absorption from Si–N peak in Fourier transform infrared spectroscopy spectra after the post-annealing. On the other hand, negligible difference in D_it value is obtained from the SOD SiO_x. In this paper we propose that much lower measurement levels (~156 mA/mm) before the annealing and substantial recovery (~13% increase) after the annealing in maximum drain current density of the AlGaN/GaN HEMTs with Si₃N₄ passivations are due to the original higher density before the annealing and greater reduction in D_it of the PECVD Si₃N₄ after the annealing. Significant reduction after the annealing in gate–drain leakage current (from ~10⁻³ to ~10⁻⁵ A, 100-μm gate width) of the HEMTs with the Si₃N₄ passivation is also supposed to be attributed to the reduction of D_it.     

Acid etching for accurate determination of dislocation density in GaN

Hot phosphoric-acid etching and atomic force microscopy (AFM) were used to etch and characterize various GaN materials, including freestanding GaN grown by hydride vapor-phase epitaxy (HVPE), metal-organic chemical-vapor deposition (MOCVD) GaN films on sapphire and silicon carbide, and homoepitaxial GaN films on polished freestanding-GaN wafers. It was found that etching at optimal conditions can accurately reveal the dislocations in GaN; however, the optimal etch conditions were different for samples grown by different techniques. The as-grown HVPE samples were most easily etched, while the MOCVD homoepitaxial films were most difficult to etch. Etch-pit density (EPD) ranging from 4×10⁶ cm⁻² to 5×10⁹ cm⁻² was measured in close agreement with the respective dislocation density determined from transmission electron microscopy (TEM).     

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.     

Analysis of dielectric breakdown in CoFeB/MgO/CoFeB magnetic tunnel junction

The time-dependent dielectric breakdown has been investigated in a series of nominally identical Co–Fe–B/MgO/Co–Fe–B junctions by voltage ramp experiments. The results divulge that the breakdown voltage strongly depends on the polarity of the applied voltage, junction area, ramp speed and the annealing temperature. Magnetic tunnel junctions (MTJs) with positive bias on the top electrode show higher breakdown voltage than MTJs with negative bias. We found that there is a significant decrease in the breakdown voltage when the annealing temperature is increased above 350 °C. The experimental data can be described by different specific forms of breakdown probability functions which lead to different extrapolation of life time of junctions.     

MgO含量对微波高Q介电材料电性能与微观结构的影响

借助典型的固相反应烧结机制,研究了MgO含量对微波高品质因数瓷介材料微观结构与介电性能参数影响的规律。结果表明:高温烧结的镁钛系瓷介质中,随着MgO含量增加,Mg2TiO4晶相的含量逐渐降低、瓷体晶粒由尖晶石型演变为氯化钠型结构,对应的微波介电常数出现缓慢降低,在10.10~13.37范围连续可调。当x=16时,在1 525℃/2 h烧成,可获得性能优良的(2+x)MgO-TiO2陶瓷,其ρ=3.49 g·cm–3,εr=10.37,Q·f=118 747 GHz(f=10.47 GHz),τf=–53.35×10–6℃–1。     

GaN和p型GaN薄膜中声子模和缺陷模的变温喇曼散射

研究了MOCVD生长的GaN及掺Mg GaN薄膜从78到578K下的喇曼散射谱.在GaN和掺Mg GaN的谱中都观察到一个位于247cm-1的峰,此峰被认为是缺陷诱导的散射峰,而非电子散射和Mg的局域模.同时讨论了两个谱中E2和A1(LO)声子峰的频率和线形.在掺Mg GaN样品中观察到应力松弛现象.     

Temperature Dependent Raman Scattering of Phonon Modes and Defect Modes in GaN and p-Type GaN Films

Raman spectra of undoped GaN and Mg-doped GaN films grown by metal-organic chemical-vapor deposition on sapphire are investigated between 78 and 573K.A peak at 247cm－1 is observed in both Raman spectra of GaN and Mg-doped GaN.It is suggested that the defect-induced scattering is origin of the mode.The electronic Raman scattering mechanism and Mgrelated local vibrational mode are excluded.Furthermore,the differences of E2 and A1(LO) modes in two samples are also discussed.The stress relaxation is observed in Mg-doped GaN.     

GaNHEMT栅源与栅漏表面态与电流崩塌相关性探讨

基于实验测试数据,综合分析了栅源、栅漏串联电阻增大后电流崩塌I-V曲线变化差异。研究表明:在脉冲测试条件下,RS增大,栅下沟道开启程度减弱,漏电流ID变小;RD增大,栅下沟道漏端等效漏电压减小,膝电压VKNEE变大。该实验结果可望通过RD与RS表征研究导致GaNHEMT电流崩塌的表面态变化。     

Fe掺杂GaN中Fe相关缺陷发光特性研究

利用氢化物气相外延方法制备了 Fe掺杂GaN单晶.材料发光特性随掺杂浓度、温度和激发功率的变化规律表明,位于蓝光波段的发光与掺杂的Fe离子有关.随着Fe掺杂浓度的增加,红外波段发光强度将被抑制,而蓝光波段发光强度显著增加.在较高位错密度样品中,相比于蓝光,红外发光强度显著增加.掺杂样品的光致激发光谱进一步说明了 Fe离子相关的红外和蓝光发光能级跃迁特征及过程.结果表明GaN中的本征缺陷结构以及位错对Fe相关能级跃迁具有重要影响.     

GaN n-MOSFET反型沟道电子迁移率模型

在器件物理的基础上,提出了一种半经验的GaN n-MOSFET反型沟道电子迁移率模型.该模型考虑了位错、界面态、光学声子、离化杂质、表面粗糙、声学声子,以及高场对迁移率的影响.模拟结果表明,界面态和位错是影响沟道迁移率的主要因素,尤其是界面态,它决定了迁移率的最大值,而位错密度的增加使迁移率减小.此外,表面粗糙散射和高场散射主要影响高场下载流子迁移率.由此可见,GaN n-MOSFET沟道迁移率的提高依赖于晶体质量和界面质量的提高.     

GaN材料湿法刻蚀的研究与进展

回顾了近年来GaN材料湿法刻蚀的研究进展,着重探讨了GaN材料光辅助化学湿法刻蚀的机理、p-GaN材料湿法刻蚀的难点以及湿法刻蚀在GaN材料研究中的应用。     

GaN基HEMT电流崩塌现象相关的表面陷阱效应

采用脉冲测试方法研究了与GaN基HEMT电流崩塌相关的表面陷阱效应。对特制的无台面器件进行的实验证实了表面陷阱之间存在输运过程。数据表明,当栅应力持续时间足够长时,被充电的表面陷阱会达到某种稳态。该稳态是包含了陷阱俘获与释放过程的动态平衡态。     

Drain current enhancement and negligible current collapse in GaN MOSFETs with atomic-layer-deposited HfO2 as a gate dielectric

Accumulation-type GaN metal-oxide-semiconductor field-effect-transistors (MOSFET’s) with atomic-layer-deposited HfO₂ gate dielectrics have been fabricated; a 4 μm gate-length device with a gate dielectric of 14.8 nm in thickness (an equivalent SiO₂ thickness of 3.8 nm) gave a drain current of 230 mA/mm and a broad maximum transconductance of 31 mS/mm. Owing to a low interfacial density of states (D_it) at the HfO₂/GaN interface, more than two third of the drain currents come from accumulation, in contrast to those of Schottky-gate GaN devices. The device also showed negligible current collapse in a wide range of bias voltages, again due to the low D_it, which effectively passivate the surface states located in the gate-drain access region. Moreover, the device demonstrated a larger forward gate bias of +6 V with a much lower gate leakage current.     

X波段AlGaN/GaN HEMTs厂栅场板结构研究

基于SiC衬底成功研制X波段0.25um栅长带有г栅场板结构的AlGaN/GaN HEMT,对比T型栅结构器件,研究了г栅场板引入对器件直流、小信号及微波功率特性的影响.结果表明,г栅场板结构减小器件截止频率及振荡频率,但明显改善器件膝点电压和输出功率密度.针对场板长度分别为0.4、0.7、0.9、1.1 um,得出一定范围内增加场板长度,器件输出功率大幅度提高,并结合器件小信号模型提参结果分析原因.在频率8 GHz下,总栅宽1 mm,场板长度0.9um的器件,连续波输出功率密度7.11 W/mm,功率附加效率(PAE)35.31%,相应线性增益10.25 dB.     

14.2 W/mm internally-matched AlGaN/GaN HEMT for X-band applications

High-performance X-band AlGaN/GaN high electron mobility transistor (HEMT) has been achieved by Γ-gate process in combination with source-connected field plate. Both its Schottky breakdown voltage and pinch-off breakdown voltage are higher than 100 V. Beside, excellent superimposition of direct current (DC) I-V characteristics in different V_ds sweep range indicates that our GaN HEMT device is almost current collapse free. As a result, both outstanding breakdown characteristics and reduction of current collapse effect guarantee high microwave power performances. Based upon it, we have developed an internally-matched GaN HEMT amplifier with single chip of 2.5 mm gate periphery, which exhibits power density of 14.2 W/mm with 45.5 dBm (35.5 W) output power and a power added efficiency (PAE) of 48% under V_ds = 48 V pulse operating condition at 8 GHz. To the best of our knowledge, it is the highest power density at this power level.     

质子交换MgO：Linbo_3波导及其镁含量对波导性能影响的研究

报道了不同镁含量质子交换ＭｇＯ：ＬｉＮｂＯ３波导的制备与表征。采用多个测量波长，测量并计算了不同镁含量的Ｚ切质子交换ＭｇＯ：ＬｉＮｂＯ３波导的阶跃式折射率分布，结果表明多波长测量结果一致。给出了波导层折射率增量的波长色散，发现了波导层折射率增量和同一温度下波导的扩散系数随晶体中镁含量的增加而减小，最后讨论了镁含量对波导性能影响的机制。     

激光处理GaN薄膜的研究

通过激光损伤实验,报道了GaN薄膜10.6μm CO2激光的损伤阈值是64 J/cm2;为了改善GaN薄膜质量,对其进行了10.6μm CO2激光辐照处理,结果表明,处理后GaN薄膜的缺陷密度明显降低.并对机理进行了分析.     

Temperature and frequency dependent electrical and dielectric properties of Al/SiO2/p-Si (MOS) structure

The electrical and dielectric properties of Al/SiO₂/p-Si (MOS) structures were studied in the frequency range 10 kHz-10 MHz and in the temperature range 295-400 K. The interfacial oxide layer thickness of 320 Å between metal and semiconductor was calculated from the measurement of the oxide capacitance in the strong accumulation region. The frequency and temperature dependence of dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tan δ) and the ac electrical conductivity (σ_ac) are studied for Al/SiO₂/p-Si (MOS) structure. The electrical and dielectric properties of MOS structure were calculated from C-V and G-V measurements. Experimental results show that the ε′ and ε″ are found to decrease with increasing frequency while σ_ac is increased, and ε′, ε″, tan δ and σ_ac increase with increasing temperature. The values of ε′, ε″ and tan δ at 100 kHz were found to be 2.76, 0.17 and 0.06, respectively. The interfacial polarization can be more easily occurred at low frequencies, and the number of interface state density between Si/SiO₂ interface, consequently, contributes to the improvement of dielectric properties of Al/SiO₂/p-Si (MOS) structure. Also, the effects of interface state density (N_ss) and series resistance (R_s) of the sample on C-V characteristics are investigated. It was found that both capacitance C and conductance G were quite sensitive to temperature and frequency at relatively high temperatures and low frequencies, and the N_ss and R_s decreased with increasing temperature. This is behavior attributed to the thermal restructuring and reordering of the interface. The C-V and G/ω-V characteristics confirmed that the N_ss, R_s and thickness of insulator layer (δ) are important parameters that strongly influence both the electrical and dielectric parameters and conductivity in MOS structures.