GeMoW refractory ohmic contacts to n-type GaAs with In/sub 0.5/Ga/sub 0.5/As cap layer

GeMoW is presented as a refractory ohmic contact to n-type GaAs with an In/sub 0./5Ga/sub 0.5/As cap layer. The contact exhibits ohmic behaviour over a wide annealing temperature range from 300 to 700 degrees C. A minimum contact resistance of 0.176 Omega mm was obtained following furnace annealing at 500 degrees C.<>     

The thermal stability of ohmic contact to n-type InGaAs layer

The thermal stability of ohmic contact to n-type InGaAs layer is investigated. When Ni/Ge/Au is used as the contact metal, the characteristics of the ohmic contact are degraded after thermal treatment. The specific contact resistance of (Ni/Ge/Au)-InGaAs ohmic contact after annealing at 450°C is about 15 times larger than that of as-deposited sample. This is due to the decomposition of InGaAs and the interdiffusion of Ga and Au. A new phase of Au₄ln appears after annealing at 300°C. While in the case of Ti/Pt/Au, Au does not penetrate into the InGaAs layer as revealed by secondary ion mass spectroscopy. The specific contact resistance of (Ti/Pt/Au)-InGaAs ohmic contact after annealing at 450°C is eight times larger than that of as-deposited sample. Therefore, the thermal stability of (Ti/Pt/Au)-InGaAs ohmic contact is better than that of (Ni/Ge/Au)InGaAs ohmic contact.     

Au/Pt/AuGe/Ni中Pt扩散阻挡层对n-GaAs欧姆接触的影响

The multi-layer metals of Ni/Au Ge/Pt/Au with a Pt diffusion barrier layer of ohmic contact to n-GaAs were studied. The surface morphology and ohmic contact resistivity of multi-layer metals were characterized, with and without the Pt diffusion barrier layer for comparison. The SEM and EDS measurements show the Pt diffusion barrier layer can block the interdiffusion of atoms in multi-layer metals, and improve the surface morphology.The TLM results show that the samples with a Pt diffusion barrier layer have uniform ohmic contact resistance,indicating that the Pt diffusion barrier layer can increase the repetition and uniformity of ohmic contact to n-GaAs,and improve the thermal stability and reliability of GaAs-based devices.     

一种在砷化镓上形成欧姆接触的新型六层金属系统

研究了在砷化镓上欧姆接触形成机理,并提出了一种新型的欧姆接触六层金属系统(Ni/Ge/Au/Ge/Ni/Au).在n型GaAs样品上实验了在380～460℃合金温度和50～120 s合金时间下形成欧姆接触,在400℃、60 s下典型的欧姆接触值为2.1x10-7Ω·cm2,同时合金后表面形貌光滑、平整.     

In0.2Ga0.8As/GaAs quantum well laser with C doped cladding and ohmic contact layers

Carbon doping in Al_xGa_1−xAs was achieved using different approaches. The moderate growth temperature of 650°C was employed to grow C bulk-doped Al_xGa_1−xAs with a high Al mole fraction. The hole-density was altered using different V/III ratios. The trimethylaluminum (TMAl) was used as an effective C δ-doping precursor for growth of C δ-doped pipi doping superlattices in Al_xGa_1−xAs. the average hole-density of C δ-doped pipi superlattices was greater than 2−3 × 10¹⁹ cm⁻³. Zn-free GRINSCH In_0.2Ga_0.8As/GaAs laser structures were then grown using the C bulk-doped Al_xGa_1−xAs and C δ-doped pipi superlattice as a cladding and ohmic contact layer, respectively. The ridge waveguide laser diodes were fabricated and characterized to verify flexibility of these two doping approaches for device structures.     

退火参数对p型GaAs欧姆接触性能的影响

采用磁控溅射的方法在p型GaAs衬底上沉积了Ti/Pt/Au金属薄膜,研究了退火工艺参数(温度和时间)对p-GaAs/Ti/Pt/Au欧姆接触性能的影响。结果表明:p-GaAs上制作的Ti/Pt/Au金属系统能在很短的退火时间(60 s)内形成很好的欧姆接触。过分延长退火时间,并不能改善系统的欧姆接触性能。退火温度在400~450℃时均可得到较好的欧姆接触。当退火温度为420℃,退火时间为120 s时,比接触电阻率达到最低,为1.41×10–6.cm2。     

Type I and Type II Alignment of the Light Hole Band in In0.15Ga0.85As/GaAs and in ln0.15Ga0.85As/Al0.15Ga0.85As Strained Quantum Wells

We present results of photoluminescence and cathodoluminescence measurements of strained undoped In_0.15Ga_0.85As/GaAs and In_0.15Ga_0.85As/Al_0.15Ga_0.85As quantum well structures, designed to throw light on the current controversy over light-hole band alignment at low In content. We compare these data with theoretical calculations of the confined state energies within the eight band effective mass approximation. Our analysis shows that for In_0.15Ga_0.85As/GaAs, the observed two transitions are consistent with either type I or type II alignment of the light hole band for band offset ratios within the accepted range. In the case of In_0.15Ga_0.85As/Al_0.15Ga_0.85As, however, our results clearly indicate type II alignment for the light hole band. We derive the band offset ratio Q, defined here as Q = δE_c/δE_g where δE_c is the conduction band offset and δE_g is the bandgap difference between the quantum well and the barrier in the presence of strain, for the In_0.15Ga_0.85As/Al_0.15Ga_0.85As system to be Q = 0.83 and discuss it in the context of the common anion rule.     

不同缓冲层对GaAs基In0.3Ga0.7As材料特性的影响

采用组分跳变和低温大失配缓冲层技术在GaAs衬底上外延了In0.3Ga0.7As材料。测试结果表明,采用组分跳变缓冲层生长的In0.3Ga0.7As主要依靠逐层间产生失配位错来释放应力,并导致表面形成纵横交错的Cross-hatch形貌;而采用低温大失配缓冲层技术则主要通过在低温缓冲层中形成大量缺陷来充分释放应力,并在后续外延的In0.3Ga0.7As表面没有与失配位错相关的Cross-hatch形貌出现。此外,仅需50nm厚的低温大失配缓冲层即可促使In0.3Ga0.7As中的应力完全释放,这种超薄缓冲层技术在工业批产中显得更为经济。     

Electrical characterization of self-assembled In0.5Ga0.5As/GaAs quantum dots by deep level transient spectroscopy

We have investigated electron emission from self-assembled In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown by molecular-beam epitaxy (MBE). Through detailed deep level transient spectroscopy comparisons between the QD sample and a reference sample, we determine that trap D, with an activation energy of 100 meV and an apparent capture cross section of 5.4×10⁻¹⁸ cm², is associated with an electron quantum level in the In_0.5Ga_0.5As/GaAs QDs. The other deep levels observed, M1, M3, M4, and M6, are common to GaAs grown by MBE.     

p型GaN欧姆接触的研究进展

宽带隙的GaN作为半导体领域研究的热点之一,近年来发展得很快.p型GaN的欧姆接触问题一直阻碍高温大功率GaN基器件的研制.本文讨论了金属化方案的选择、表面预处理和合金化处理等几个问题,回顾了近年来p型GaN欧姆接触的研究进展.     

Low resistance, thermally stable Au/Pt/Ti/WSiN ohmic contacts on n+-InGaAs/n-GaAs layer systems

The electrical properties of the ohmic contact systems Au/Pt/Ti/WSiN and Au/Pt/Ti to n⁺-InGaAs/GaAs layers grown by metalorganic vapor phase epitaxy were investigated and compared to each other. The thermal stability properties of these contact systems were characterized by accelerated stress tests at elevated temperatures and by complementary thin film x-ray diffraction analysis to evaluate the microstructural properties of degraded and nondegraded structures. The goal of these efforts was to develop stable, homogeneous emitter contacts for power heterojunction bipolar transistors. It was found that for both contact systems the best (specific) contact resistance R_c (ρ _c) is about 0.05 Ωmm (2 × 10⁻⁷ Ωcm²) in the as-deposited state. Au/Pt/Ti/WSiN contacts show no degradation after aging at 400°C for more than 20 h. This is in contrast to standard Au/Pt/Ti contacts which significantly degrade even after short time annealing at 400°C. The good long-time stability of the Au/Pt/Ti/WSiN system is related to the advantageous properties of the reactively sputtered WSiN barrier layer.     

金属Al与半导体Ge欧姆接触的制备和表征

基于圆形传输线模型,通过测试样品的比接 触电阻率和电流-电压(I-V)特性曲线,分析 对比了Al与Si基上外延生长的p型Ge、n型Ge和n型Si的接触特性。实验结果发现,由于金 属与Ge材料接触存在强烈的费米钉效应,导致金属与n型Ge接触有高的接触电阻,难实 现低的比接触电阻率;而Al与p型Ge在掺杂浓度为4.2×10¹⁸ cm－3时,并且经过退火,比接 触电阻率能达到4.0×10－7 Ω·cm²;Al与n型Ge和n型Si接 触电极相比,后者可形成良好的 欧姆接触,其比接触电阻率较n型Ge接触降低了1个量级,经合金化处理后的Al/n⁺Si接触 电阻率能达到5.21×10－5 Ω·cm²,达到了制作高性能Ge 光电器件的要求。     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

Electrical and structural properties of W-In based ohmic contacts to GaAs

A low resistance, nonalloyed, nongold ohmic contact to n-GaAs is fabricated by cosputtering W and In targets with one of the dopant elements like Si, Ge or Te. The as-deposited rectifying contacts become ohmic when annealed to 500°C, and show an average specific contact resistance of 5 × 10⁻⁶ cm². A contact resistance as low as 3 × 10⁻⁶ is achieved in W-In-Si/n-GaAs system. The Auger and Rutherford back scattering analysis of the interface reveals an InGaAs phase formation prior to ohmic behavior. The contacts are stable up to 500°C and surface morphology is much superior to presently used AuNiGe contacts. The contact pads can be patterned by dry plasma etching without affecting the GaAs substrate.     

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.     

InxGa1-xAs ohmic contacts to n-type GaAs prepared by sputter deposition

Thermally stable, low contact resistance InAs/Ni/W contacts were previously prepared by sputter depositing InAs, Ni, and W targets in our laboratory. However, the optimum annealing temperature to provide low contact resistance (R_c) was high, resulting in rough contact surface. In the present experiment, the effects of the In concentrations of In_xGa_1-x As targets on the optimum annealing temperature to prepare low R_cand the surface morphology of the_xGa_1-x/ W contacts were studied. In addition, the electrical properties and the interfacial microstructure were correlated to search the optimum In concentration to provide the minimum R_c, where the interfacial microstructure was analyzed by x-ray diffraction and transmission electron microscopy and the contact resistances (R_cc) were measured by the transmission line method. The optimum annealing temperature to provide minimum R_c was reduced by 150°C by using the In_0.7Ga_0.3As targets instead of the previous targets. The contact resistance of 0.4 Ω-mm was obtained for the In_0.7Ga_0.3As/Ni/W contacts after annealing at temperatures of around 600°C. The R_c values did not deteriorate after annealing at 400°C for 2 h. Also, the surface of this contact was smooth and no evidence of In outdiffusion on the contact surface was seen. Finally, the effect of the In concentrations at the metal/GaAs interfaces on the electrical properties will be discussed.     

In0.5Ga0.5As/In0.5Al0.5As应变耦合量子点的形貌和光学性质

提出了利用分子束外延方法生长In0.5Ga0.5As/In0.5Al0.5As应变耦合量子点,并分析量子点的形貌和光学性质随GaAs隔离层厚度变化的特点.实验结果表明,随着耦合量子点中的GaAs隔离层厚度从2 nm增加到10 nm,In0.5Ga0.5As量子点的密度增大、均匀性提高,Al原子扩散和浸润层对量子点PL谱的影响被消除,而且InAlAs材料的宽禁带特征使其成为InGaAs量子点红外探测器中的暗电流阻挡层.由此可见,选择合适的GaAs隔离层厚度形成InGaAs/InAlAs应变耦合量子点将有益于InGaAs量子点红外探测器的研究.     

Microstructure and chemistry of Cu-Ge ohmic contact layers to GaAs

We report systematic studies of microstructure and chemistry of Cu-Ge alloyed ohmic contacts to n-GaAs with very low specific contact resistivity ((4-6) x 10^-7 Ωcm² for n∼l x 10¹⁷cm^-3). Using transmission electron microscopy, x-ray microanalysis, and secondary ions mass spectroscopy, we investigated chemistry of phase formation, crystal structure, and mechanism of ohmic contact formation in Cu-Ge alloyed layers with Ge concentration in the range of 0–40 at.%. Layers with Ge deficiency to form ζ-phase (average composition Cu₅Ge) reveal the formation of a nonuniform intermediate layer of hexagonal -Cu₃As phase which grows epitaxially on Ga₁₁₁ planes of GaAs. In this case, released Ga diffuses out and dissolves in the alloyed layer stabilizing ζ-phase, which is formed in the structures with average Ge concentration as low as 5 at.%. Unique properties of the contact layers, namely low specific contact resistivity, high thermal stability, interface sharpness, and high contact layer uniformity are related to the formation of an ordered orthorhombic ε₁ Cu₃Ge phase. In the alloyed layer with Ge concentration >25 at.%, no phases due to the chemical reactions with GaAs in the interface region were found demonstrating the chemical inertness of the ε₁Cu₃Ge ordered phase with respect to GaAs. This results in sharp interfaces and uniform chemical composition, the characteristics needed for superior contacts.     

Microstructural and electrical investigation of Ni/Au ohmic contact on p-type GaN

Electrical properties of Ni/Au ohmic contacts on p-type GaN were interpreted with the change of microstructure observed under transmission electron microscopy. The contact resistivity was decreased from 1.3×10⁻² to 6.1×10⁻⁴ Ωcm² after annealing at 600°C. The reduction is due to the dissolution of Ga atoms into Au−Ni solid solution formed during annealing, via the generation of Ga vacancies. Thus, net concentration of holes increased below the contact, resulting in the reduction of contact resistivity. At 800°C, N atoms decomposed; reacted with Ni, and forming cubic Ni₄N. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to the increase of contact resistivity to 3.8×10⁻² Ωcm².     

Carbon structural transitions and ohmic contacts on 4H-SiC

The structural properties of sputtered carbon films on SiC are investigated using X-ray photoelectron spectroscopy (XPS) and Raman scattering. The as-deposited films are amorphous with an sp²/sp³ ratio of 1. The sp² carbon structures gradually increase with increasing temperatures and consist of amorphous aromatic-like carbon, polyene-like carbon, and nano-size graphite flakes. Schottky contacts on carbon/SiC are converted to ohmic contacts after annealing. The concentration of nano-graphitic flakes relative to the aromatic-like and polyene-like carbon increases nearly linearly with annealing temperature. Stacked graphitic structures are not observed. The specific contact resistivities are at 10⁻³–10⁻⁴Ωcm² on the carbon/SiC after annealing from 1050°C to 1350°C.