Electrical characteristics and thermal stability of W, WSiN, and Nb contacts to p- and n-type GaN

A comprehensive annealing study of W, WSiN, and Nb refractory contacts to both p- and n-GaN was performed. Samples were examined by current-voltage (I-V) measurements. Both W and Nb contacts to n-GaN showed rectifying characteristics with Schottky barrier heights (qø_B) of 0.63 eV. For p-GaN, the I-V curves showed very leaky behavior. In contrast, I-V curves of WSiN/n-GaN were very leaky while those of WSiN/p-GaN were rectifying with qø_B of 0.8 eV. The degradation temperature of both W and WSiN contacts was 700°C and that of Nb contacts was 300°C.     

PECVD沉积SiO2和SiNX对p-GaN的影响

在等离子增强化学气相沉积法(PECVD)沉积SiO2和SiNX掩蔽层过程中，分解等离子体中浓度较高的H原子使Mg-受主钝化，同时在p-GaN材料表面发生反应形成浅施主特性的N■■空位。高能量离子轰击造成的材料深能级缺陷增多以及沉积形成致密的SiO2和SiNX材料，阻碍了H原子向外扩散，使H原子在Ni/Au电极与p-GaN的界面处聚集，造成p-GaN近表面附近区域Mg-H络合物密度的提高，空穴浓度急剧下降，导致Ni/Au透明电极I-V特性严重恶化。选择较低的射频功率(15 W,13.56 MHz)沉积模式，经过适当的退火，可以减小沉积SiO2过程对p-GaN的影响。     

High-Temperature Degradation of GaN LEDs Related to Passivation

This paper describes the thermally activated failure mechanisms of GaN light-emitting diode (LED)-test structures related with the presence of a hydrogen-rich SiN passivation layer. It is shown that the properties of the passivation layer can remarkably affect devices' stability during high-temperature stress: Degradation mechanisms identified consist of radiative efficiency loss, emission crowding, and forward-current decrease. The radiative efficiency degradation was found to be thermally activated, with activation energy equal to 1.3 eV. This failure mechanism of LEDs is attributed to the thermally activated indiffusion of hydrogen from the passivation layer to p-type region of the diodes, with subsequent p-doping compensation and/or worsening of the transport properties of the p-side ohmic contact and p-type semiconductor     

表面处理对p-GaN欧姆接触的影响

分别用稀盐酸、王水以及(NH4)2S溶液处理p-GaN表面,通过测试样品表面Ols的X射线光电子能谱(XPS),比较了这些溶液去除p-GaN表面氧化层的能力;在经不同溶液处理后的样品表面,以相同的条件制作Ni/Au电极,并测试其与p-GaN的比接触电阻,结果表明经稀盐酸处理后的样品表面,由于其氧含量较高,不能与Ni/Au形成良好的欧姆接触,而经王水和(NH4)2S溶液处理后的p-GaN表面,能与Ni/Au形成良好的欧姆接触;最后,通过比较样品表面的Ga/N原子浓度比,探讨了王水处理p-GaN表面能够形成良好欧姆接触的原因.     

表面处理对p-GaN欧姆接触的影响

分别用稀盐酸、王水以及(NH4)2S溶液处理p-GaN表面,通过测试样品表面Ols的X射线光电子能谱(XPS),比较了这些溶液去除p-GaN表面氧化层的能力;在经不同溶液处理后的样品表面,以相同的条件制作Ni/Au电极,并测试其与p-GaN的比接触电阻,结果表明经稀盐酸处理后的样品表面,由于其氧含量较高,不能与Ni/Au形成良好的欧姆接触,而经王水和(NH4)2S溶液处理后的p-GaN表面,能与Ni/Au形成良好的欧姆接触;最后,通过比较样品表面的Ga/N原子浓度比,探讨了王水处理p-GaN表面能够形成良好欧姆接触的原因.     

大功率倒装结构GaN LED p电极研究

从接触电阻、反射率、电流扩展等方面对Ni/Au/Ag,ITO/Ag,Ag等多种倒装结构p电极金属体系进行分析比较，给出了实现倒装结构大功率GaN LED p电极的多种设计方案. 指出Ni/Au金属化体系在大功率LED应用中存在的热稳定性问题及Ru,Ir等新型金属体系实现GaN p电极接触的潜在优势.     

The origins of leaky characteristics of Schottky diodes on p-GaN

The possible origins of the leaky characteristics of a Schottky barrier on p-GaN have been investigated. The as-grown samples did not show any electrical activity using Hall measurements. Ni diodes made on as-activated samples, either at 950/spl deg/C for 5 s or at 750/spl deg/C for 5 min exhibited quasiohmic behavior. Upon sequential etching of the sample to remove a surface layer of 150 /spl Aring/, 1200 /spl Aring/, and 5000 /spl Aring/ from the sample, the I-V behavior became rectifying. I-V-T measurements showed that the slopes of the lnI-V curves were independent of the temperature, indicative of a prominent component of carrier tunneling across the Schottky junction. C-V measurements at each etch-depth indicated a decreasing acceptor concentration from the surface. The highly doped (>1.7 /spl times/ 10/sup 19/ cm/sup -3/) and defective surface region (within the top 150 /spl Aring/ from surface) rendered the as-activated Schottky diodes quasiohmic in their I-V characteristics. The leaky I-V characteristics, often reported in the literature, were likely to originated from the surface layer, which gives rise to carrier tunneling across the Schottky barrier. This highly doped/defective surface region, however, can play an important role in ohmic contact formation on p-GaN.     

Interface observation in Au/Ni/p-GaN studied by HREM and energy-filtering TEM

The contact resistance of Au/Ni/p-GaN ohmic contacts for different annealing conditions was measured. This was then correlated with microstructure, including phase distribution, observed by high-resolution electron microscopy combined with energy-filtering imaging. A contact resistance of 2.22 x 10(-4) ohms cm2 for Au/Ni contacts to p-GaN after annealing at 500 degrees C for 5 min in air ambient was obtained. NiO layers were identified at the interface and upper area of annealed Ni/Au/p-GaN for air ambient. In addition, an Au layer was found at the interface of p-GaN due to a reversal reaction during annealing. Identification of the observed phases is discussed, along with possible formation mechanisms for the ohmic contacts in the Au/Ni/p-GaN system.     

Electrical transport and current properties of rare-earth dysprosium Schottky electrode on p-type GaN at various annealing temperatures

The electrical and current transport properties of rapidly annealed Dy/p-GaN SBD are probed by I-V and C-V techniques. The estimated barrier heights (BH) of as-deposited and 200℃ annealed SBDs are 0.80 eV (I-V)/0.93 eV (C-V) and 0.87 eV (I-V)/1.03 eV (C-V). However, the BH rises to 0.99 eV (I-V)/ 1.18 eV(C-V) and then slightly deceases to 0.92 eV (I-V)/1.03 eV (C-V) after annealing at 300℃ and 400℃. The utmost BH is attained after annealing at 300℃ and thus the optimum annealing for SBD is 300℃. By applying Cheung''s functions, the series resistance of the SBD is estimated. The BHs estimated by I-V, Cheung''s and Ψ_S-V plot are closely matched; hence the techniques used here are consistency and validity. The interface state density of the as-deposited and annealed contacts are calculated and we found that the N_SS decreases up to 300℃ annealing and then slightly increases after annealing at 400℃. Analysis indicates that ohmic and space charge limited conduction mechanisms are found at low and higher voltages in forward-bias irrespective of annealing temperatures. Our experimental results demonstrate that the Poole-Frenkel emission is leading under the reverse bias of Dy/p-GaN SBD at all annealing temperatures.     

氧化对 GaN 基LED透明电极接触特性的影响

研究了热退火对InGaN/GaN 多量子阱LED的Ni/Au-p-GaN欧姆接触的影响.发现在空气和 N2气氛中交替地进行热退火的过程中Ni/Au接触特性显示出可逆现象. Ni/Au-p-GaN接触的串联电阻在空气中随合金化时间逐渐减小,在随后的 N2 中的热退火后会使该串联电阻增加,但在空气中再次热退火能使接触特性得到恢复.同时对Ni/Au-p-GaN 接触在空气中合金化过程中的层反转的成因进行了讨论.     

Effects of intermediate semiconductor layers on carrier transport mechanisms through p-ZnSe/metals interfaces

The effects of formation of intermediate semiconductor layers at p-ZnSe/metals interfaces on carrier transport mechanisms were studied by comparing contacts prepared by the deposition and annealing (DA) technique or the molecular beam epitaxy (MBE) technique. Current density vs voltage (J-V) curves of the MBE contact with a p-ZnSe/p-ZnTe superlattice intermediate layer showed ohmic behavior. However, J-V curves of the DA contact with a ZnTe intermediate layer showed rectifying behavior. The difference of the electrical properties between these two contacts was due to existence of a highly resistive intermediate layer with highly dense defects in the DA contact and a low resistance p-type conductive intermediate layer with relatively small densities of crystalline defects in the MBE contacts. From the present results, it was concluded that formation of the highly resistive semiconductor layer with dense crystalline defects prevented the DA contact to transit from non-ohmic J-V behavior to ohmic.     

Effects of annealing in an oxygen ambient on electrical properties of ohmic contacts to p-type GaN

Effects of annealing ambient of an oxygen and nitrogen mixed gas on the electrical properties were studied for Au-based ohmic contacts (NiAu, CoAu, CuAu, PdAu, and PtAu) to p-type GaN. Addition of oxygen to the nitrogen gas reduced the specific contact resistances (ρ_c) and resitivities of the p-GaN epilayers (ρ_s) of the contacts after annealing at temperatures of 500–600°C. The microstructural analysis at the p-GaN/metal interfaces did not detect the heterostructural intermediate semiconductor layer at the GaN/metal interfaces. The reason for reduction of both the ρc and ρ_s values by the oxygen gas addition was believed to be due to formation of the p-GaN epilayer with high hole concentrations, caused by removal of hydrogen atoms which bonded with Mg atoms.     

Selective wet etching of p-GaN for efficient GaN-based light-emitting diodes

The selective wet etching of a p-GaN layer by using a solution of KOH in ethylene glycol (KE) was studied to enhance the optical and electrical performance of the GaN-based light-emitting diodes (LEDs). The surface of the p-GaN, which was selectively etched in the KE solution, showed hexagonal-shaped etch pits. The light-output power of etched LEDs was improved by 29.4% compared to that of the nonetched LED. This improvement was attributed to the increase in the probability of photons to escape due to the increased surface area of textured surface and the reduction in contact resistance of the ohmic layer resulting from the increased contact area and hole concentration on the textured p-GaN. The reverse leakage current of the LED was also greatly decreased due to the surface passivation and the removal of defective regions from the p-GaN.     

p型GaN材料的表面物理特性

运用X射线光电子能谱(XPS)和俄歇电子能谱(AES)等表面分析手段对表面状态不同的p- Ga N样品进行了分析.在样品表面制作了Ni/ Au电极并进行了I- V特性测试.实验结果表明样品表面镓氮元素化学比(Ga/ N)的减小以及C,O杂质含量的减少可以改善电极的欧姆接触性能.     

Ni基n型SiC材料的欧姆接触机理及模型研究

研究了Ni基n型SiC材料的欧姆接触的形成机理,认为合金化退火过程中形成的C空位(Vc)而导致的高载流子浓度层对欧姆接触的形成起了关键作用。给出了欧姆接触的能带结构图,提出比接触电阻ρC由ρC1和ρC2两部分构成。ρC1是Ni硅化物与其下在合金化退火过程中形成的高载流子浓度层间的比接触电阻,ρC2则由高载流子浓度层与原来SiC有源层之间载流子浓度差形成的势垒引入。该模型较好地解释了n型SiC欧姆接触的实验结果,并从衬底的掺杂水平、接触金属的选择、合金化退火的温度、时间、氛围等方面给出了工艺条件的改进建议。     

Effects of vacuum annealing on electrical properties of GaN contacts

To understand formation and deterioration mechanisms of Ta/Ti ohmic contacts that were previously developed for p-GaN, the electrical properties of the Ta/Ti contacts, which were deposited on undoped GaN substrates and subsequently annealed in vacuum (where a slash (/) sign indicates the deposition sequence), were studied. The Ta/Ti contacts displayed good ohmic behavior after annealing at a temperature of 800°C for 10 min in vacuum, although the undoped GaN substrates were used. However, deterioration of the present ohmic contacts was observed during room-temperature storage. These contact properties were similar to those observed in the Ta/Ti contacts prepared on p-GaN. Hall-effect measurements revealed that thin n-type conductive layers were found to form on surfaces of both the undoped GaN and p-GaN substrates after annealing at 800°C in vacuum.     

Mechanism for recoverable power drift in PHEMTs

A new degradation mechanism is proposed for output power drift under rf overdrive of pseudomorphic high electron mobility transistors (PHEMT's). Similar to the previously reported power-slump mechanism, the gradual reduction in output power is caused by a decrease in the peak drain current. However, unlike power slump that is usually associated with permanent damage, in the present case the output power recovers readily once the rf input is interrupted. A possible mechanism for power drift involves charge trapping in the surface passivation layer. The passivation charge relaxes the electrical field in the semiconductor, which in turn increases the occupation of the interface states between the passivation and semiconductor. Measurements of dc and pulsed current-voltage (I-V) characteristics, rf large signal waveforms, deep level transient spectroscopy, light and temperature effects, as well as metal-insulator-metal leakage support this supposition     

Stable Electrical Operation of 6H–SiC JFETs and ICs for Thousands of Hours at 500 $^{circ}hbox{C}$

The fabrication and testing of the first semiconductor transistors and small-scale integrated circuits (ICs) to achieve up to 3000 h of stable electrical operation at 500degC in air ambient is reported. These devices are based on an epitaxial 6H-SiC junction field-effect transistor process that successfully integrated high-temperature ohmic contacts, dielectric passivation, and ceramic packaging. Important device and circuit parameters exhibited less than 10% of change over the course of the 500degC operational testing. These results establish a new technology foundation for realizing durable 500degC ICs for combustion-engine sensing and control, deep-well drilling, and other harsh-environment applications.     

Universal error corrections for finite semiconductor resistivity in cross-Kelvin resistor test structures

The Cross-Kelvin Resistor test structure is commonly used for the extraction of the specific contact resistance of ohmic contacts. Analysis using this structure are generally based on a two-dimensional model that assumes zero voltage drop in the semiconductor layer in the direction normal to the plane of the contact. This paper uses a three-dimensional (3-D) analysis to show the magnitude of the errors introduced by this assumption, and illustrates the conditions under which a 3-D analysis should be used. This paper presents for the first time 3-D universal error correction curves that account for the vertical voltage drop due to the finite depth of the semiconductor layer.     

Defect-related degradation of Deep-UV-LEDs

With this paper we describe an extensive analysis of the electro-optical degradation of deep ultraviolet Light-Emitting Diodes emitting at 310 nm, submitted to accelerated stress test. The results obtained within this study indicate that: (i) high current stress can induce a significant decrease in the optical power emitted by the LEDs; (ii) optical power decrease is more prominent at low measuring current levels, indicating that degradation is related to an increase in the concentration of defects, with subsequent decrease in the radiative efficiency of the active layer; (iii) stress can induce a significant increase in the operating voltage of the LEDs, due to the increase in the resistivity of the ohmic contacts or p-type semiconductor; and (iv) after stress, LEDs show a significantly increased green parasitic emission. This result suggest that stress induced an increase in the defectiveness of the active layer. Information on the location of the degraded region is achieved by the analysis of the Apparent Charge Distribution curves obtained by capacitance–voltage measurements.