A photoluminescence study of plasma reactive ion etching-induced damage in GaN

Ga N films with reactive ion etching(RIE) induced damage were analyzed using photoluminescence(PL).We observed band-edge as well as donor-acceptor peaks with associated phonon replicas,all in agreement with previous studies.While both the control and damaged samples have their band-edge peak location change with temperature following the Varshni formula,its intensity however decreases with damage while the D–A peak increases considerably.Nitrogen post-etch plasma was shown to improve the band edge peak and decrease the D–A peak.This suggests that the N2 plasma has helped reduce the number of trapped carriers that were participating in the D–A transition and made the D°X transition more active,which reaffirms the N2post-etch plasma treatment as a good technique to heal the Ga N surface,most likely by filling the nitrogen vacancies previously created by etch damage.     

Final polishing of Ga-polar GaN substrates using reactive ion etching

Reactive ion etching of {0001} oriented plate-like GaN single crystals has been investigated using SiCl₄:Ar:SF₆ chemistry. The reactive ion etching process is highly chemical. Large anisotropy of the etching rate and of the morphology has been established on (000 ) N-polar and (0001) Ga-polar sides of the GaN crystals, with remarkably higher rate on the N-polar side. Atomic force microscopy measurements have shown smooth surface and good polishing effect obtained on Ga-polar side, while N-polar surface exhibits an increased roughness of a factor of 10 after RIE.     

Characterization of reactive ion etching-induced damage to n-GaN surfaces using schottky diodes

Dry etch-induced damage has been investigated using Pd Schottky diodes fabricated on n-type GaN surfaces that were etched by reactive ion etching in SiCl₄ and Ar plasmas. Damage was evaluated by measuring the current-voltage, current-voltage-temperature, and capacitance-voltage characteristics of the diodes. A plasma chemistry that includes a chemical etching component (SiCl₄) was found to significantly reduce the degree of induced damage in comparison to a chemistry that uses only a physical component (Ar). The effective barrier height, ideality factor, reverse breakdown voltage, reverse leakage current, and the effective Richardson coefficient of diodes etched under various plasma conditions are presented. The degree of etch-induced damage was found to depend strongly on the plasma self-bias voltage but saturates with etch time after an initial two-minute etch period. Rapid thermal annealing was found to be effective in improving the diode characteristics of the etched GaN samples.     

Comparison between TiAl and TiAlNiAu ohmic contacts to n-type GaN

We report a comparison between Ti/Al and Ti/Al/Ni/Au ohmic contacts in terms of contact resistivity, thermal stability, depth profile, and surface morphology. The metals were deposited by conventional electron-beam evaporation, and then annealed at 900°C for 30 s in a N₂ atmosphere. The lowest value for the specific contact resistivity was obtained using Ti/Al/Ni/Au metallization. Ti/Al/Ni/Au contacts showed no increase in contact resistivity after aging for five days at 600°C in an air atmosphere. Examination of the surface morphology using atomic force microscopy revealed that the surface roughness was clearly better in the case of Ti/Al/Ni/Au contacts. X-ray photoelectron spectroscopy was also employed and gave primary results of Ti/Al/Ni/Au contact formation.     

反应离子刻蚀InSb芯片引入的损伤研究

应用CH₄/H₂/Ar作为刻蚀气源对InSb微台面阵列进行了反应离子刻蚀,并对刻蚀后引入的损伤进行了分析。实验证实利用干法刻蚀与湿法腐蚀相结合的方法能有效地减少刻蚀引入的缺陷和损伤,获得较好的电学特性,达到低损伤刻蚀InSb材料的目的。     

A study of the electrical characteristics of various metals on p-type GaN for ohmic contacts

We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts, we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers with carrier concentrations of 1.2 × 10¹⁷, 1.5 × 10¹⁷ and 4.7 × 10¹⁷ cm⁻³ are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained in this study is with Co/Au at 0.0081 ohm-cm². In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from 300 to 700°C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.     

Ohmic contacts to n-type GaN using Pd/Al metallization

Ohmic contacts to n-type GaN grown by metalorganic chemical vapor deposition were formed using a Pd/Al-based metallization. Ohmic contact resistances and specific contact resistances were investigated as a function of rapid thermal annealing temperature, Pd interlayer thickness, and annealing time. As-deposited Pd/AI was found to produce rectifying contacts while the metallization exhibited ohmic characteristics after annealing at temperatures as low as 400°C. A minimum contact resistance of 0.9 ohm-mm (specific contact resistance = 1.2 × 10⁻⁵ ohm-cm²) was obtained upon annealing at 650°C for 30 s. For comparison, Al and Ti/Al contacts were also investigated. Auger electron spectroscopy, secondary ion mass spectrometry, and x-ray diffraction were used to investigate metallurgical reactions.     

Au/Ge/Ni ohmic contacts to n-Type InP

The relationship between the electrical properties and microstructure for annealed Au/Ge/Ni contacts to n-type InP, with an initial doping level of 10¹⁷ cm^-3, have been studied. Metal layers were deposited by electron beam evaporation in the following sequence: 25 nm Ni, 50 nm Ge, and 40 nm Au. Annealing was done in a nitrogen atmosphere at 250-400‡C. The onset of ohmic behavior at 325‡C corresponded to the decomposition of a ternary Ni-In-P phase at the InP surface and the subsequent formation of Ni₂P plus Au₁₀In₃, producing a lower barrier height at the InP interface. This reaction was driven by the inward diffusion of Au and outward diffusion of In. Further annealing, up to 400‡C, resulted in a decrease in contact resistance, which corresponded to the formation of NiP and Au₉ln₄ from Ni₂P and Au₁₀In₃,respectively, with some Ge doping of InP also likely. A minimum contact resistance of 10^-7 Ω-cm² was achieved with a 10 s anneal at 400‡C.     

Formation of ohmic contacts to p-ZnTe

Formation and temperature stability of sputter deposited gold ohmic contacts to molecular beam epitaxially grown p-type ZnTe (doped with nitrogen to a free hole concentration of ≈3 × 1018 cm³) have been studied using current-voltage (I-V), Auger electron spectroscopy, secondary ion mass spectrometry, and optical and scanning electron microscopy. The I-V characteristics of ≈1500Å Au/p-ZnTe contacts were measured as-deposited and after heat treatments at 150, 200, 250, and 350°C for 15 min intervals up to 90 min. As deposited, the contacts were poor Schottky contacts, but became ohmic after 15 min at all temperatures. There was an increased resistance at t>15 min for T≤250°C, and a very large resistance increase upon heat treatment for all times at 350°C. The interface between the metallization and ZnTe was initially very planar, and remained planar upon formation of the ohmic contact. Upon heating at T>250°C, Au diffused into ZnTe. The ohmic behavior of the Au/p-ZnTe contacts is attributed to this diffusion which created a highly doped near-surface region in the ZnTe. Microscopy showed that Au also migrated across the ZnTe surface forming an extended reaction zone (≈100 μm) around the dot contact at T≥250°C.     

Thermally stable Ge/Cu/Ti ohmic contacts to n-type GaN

The performance of a novel Ge/Cu/Ti metallization scheme on n-type GaN has been investigated for obtaining thermally and electrically stable low-resistance ohmic contacts. Isochronal (2 min.) anneals in the 600–740°C temperature range and isothermal (690°C) anneals for 2–10 min. duration were performed in inert atmosphere. For the 690°C isothermal schedule, ohmic behavior was observed after annealing for 3 min. or longer with a lowest contact resistivity of 9.1 × 10⁻⁵ Ωcm² after the 10 min. anneal for a net donor doping concentration of 9.2 × 10¹⁷ cm^−Ω3. Mean roughness (R_a) for anneals at 690°C was almost constant at around 5 nm, up to an annealing duration of 10 min., which indicates a good thermal stability of the contact scheme.     

Characterization of Hg0.7Cd0.3Te n- on p-type structures obtained by reactive ion etching induced p- to n conversion

This paper presents transport measurements on both vacancy doped and gold doped Hg_0.7Cd_0.3Te p-type epilayers grown by liquid phase epitaxy (LPE), with N_A=2×10¹⁶ cm⁻³, in which a thin 2 μm surface layer has been converted to n-type by a short reactive ion etching (RIE) process. Hall and resistivity measurements were performed on the n-on-p structures in van der Pauw configuration for the temperature range from 30 K to 400 K and magnetic field range up to 12 T. The experimental Hall coefficient and resistivity data has been analyzed using the quantitative mobility spectrum analysis procedure to extract the transport properties of each individual carrier contributing to the total conduction process. In both samples three distinct carrier species have been identified. For 77 K, the individual carrier species exhibited the following properties for the vacancy and Au-doped samples, respectively, holes associated with the unconverted p-type epilayer with p ≈ 2 × 10¹⁶ cm⁻³, μ ≈ 350 cm²V⁻¹s⁻¹, and p ≈ 6 × 10¹⁵ cm⁻³, μ ≈ 400 cm²V⁻¹s⁻¹; bulk electrons associated with the RIE converted region with n ≈ 3 × 10¹⁵cm⁻³, μ ≈ 4 × 10⁴ cm²V⁻¹s⁻¹, and n ≈ 1.5 × 10¹⁵ cm⁻³, μ ≈ 6 × 10⁴ cm²V⁻¹s⁻¹; and surface electrons (2D concentration) n ≈ 9 × 10¹² cm⁻² and n ≈ 1 × 10¹³ cm⁻², with mobility in the range 1.5 × 10³ cm²V⁻¹s⁻¹ to 1.5 × 10⁴ cm²V⁻¹s⁻¹ in both samples. The high mobility of bulk electrons in the RIE converted n-layer indicates that a diffusion process rather than damage induced conversion is responsible for the p-to-n conversion deep in the bulk. On the other hand, these results indicate that the surface electron mobility is affected by RIE induced damage in a very thin layer at the HgCdTe surface.     

Low resistance bilayer Nd/Al ohmic contacts on n-type GaN

A bilayer Nd/Al metallization structure has been deposited onto low pressure organometallic vapor phase epitaxy grown n-type GaN ( 1 × 10¹⁸ cm⁻³) by electron-beam evaporation. Ohmic metal contacts were patterned photolithographically for standard transmission line measurement, and then thermally annealed at temperatures ranging from 200 to 350°C and from 500 to 650°C using conventional thermal annealing (CTA) and rapid thermal annealing (RTA), respectively. The lowest values for the specify contact resistivity of 9.8 × 10⁻⁶ Ω−cm² and 8 × 10⁻⁶ Ω−cm² were obtained using Nd/Al metallization with CTA of 250°C for 5 min and RTA of 600°C for 30 s. Examination of the surface morphology using atomic force microscopy as a function of annealing temperature revealed that the surface roughness was strongly influenced by conventional thermal annealing, it was smooth in the temperature range from 550 to 650°C for rapid thermal annealing. Auger electron spectroscopy depth profiling was employed to investigate the metallurgy and interdiffusion of contact formation.     

Metal contacts to n-type GaN

Contacts consisting of various single layer metals to n-type GaN have been formed and characterized. The current-voltage characteristics were measured for 17 different metals (Sc, Hf, Zr, Ag, Al, V, Nb, Ti, Cr, W, Mo, Cu, Co, Au, Pd, Ni, and Pt) deposited on the same epitaxial growth layer. The barrier height, ideality factor, breakdown voltage, and effective Richardson coefficients were measured from those metals which exhibited strong rectifying behavior. The barrier heights for these metal contacts were measured using current-voltage-temperature and capacitance-voltage techniques. It was found that an increase in metal work function correlated with an increase in the barrier height. The surface state density of GaN was approximated to be very similar to CdS and almost a factor of ten less than GaAs.     

InSb阵列探测芯片的感应耦合等离子反应刻蚀研究

利用感应耦合等离子（ICP）反应刻蚀（RIE）进行了InSb阵列芯片台面刻蚀,并利用轮廓仪、SEM及XRD对台面形貌以及刻蚀损伤进行分析。采用优化的ICP刻蚀参数,实现的刻蚀速率为70~90 nm/min,刻蚀台阶垂直度~80°,刻蚀表面平整光滑、损伤低。与常规的湿法腐蚀相比,明显降低了侧向钻蚀。台面采用此反应刻蚀工艺,制备了具有理想I-V特性的320×256 InSb探测阵列芯片,在-500 mV到零偏压范围内,光敏元（面积23 μm×23 μm）的动态阻抗（Rd）大于100 MΩ。     

Estimated phase equilibria in the transition metal-Ga-N systems: Consequences for electrical contacts to GaN

High quality electrical contacts to GaN are required for the advancement of electronic and optoelectronic devices based on the III-V nitrides. In this study, the metallurgy of contacts to GaN and the implications for the design of electrical contacts are considered. First, phase diagrams are estimated for the transition metal-Ga-N systems. The diagrams are then used as an aid in predicting the reaction products of annealed metal/GaN contacts, to suggest materials that may be useful as thermally stable electrical contacts and to explore the role of the partial pressure of N₂ in the annealing environment on the reactions in metal/GaN contacts. It is believed that this information will be particularly useful to researchers during the early stages of contact development since very little experimental information is currently available on the GaN contact metallurgy.     

Laser beam induced current imaging of reactive ion etching induced n-type doping in HgCdTe

The nondestructive optical characterization technique of laser beam induced current (LBIC) has been used to illustrate the effects of reactive ion etching (RIE) of mid-wavelength infrared n-type HgCdTe. RIE may be used as a method of n-p junction formation, as a means of forming n⁺ ohmic contacts to wider bandgap HgCdTe, or for mesa isolation etching of epilayers for HgCdTe detectors and emitters. Along with experimental measurements of the LBIC phenomena, this paper introduces the simulation of LBIC signals using a commercial semiconductor device modeling package. A number of LBIC maps are presented for different wafer processing conditions, with the results being explained using the simulation software. The experimental and calculated results bring to light a number of previously unreported characteristics associated with the LBIC phenomena, including the effect of junction depth, temperature, and grading of the junction region. In addition to the LBIC technique confirming the presence of an n⁺ region after RIE processing, it also provides information regarding the depth of the n⁺ region and lateral extent of the doping.     

基于再生长欧姆接触工艺的220 GHz InAlN/GaN 场效应晶体管

本文在蓝宝石衬底上研制了具有高电流增益截止频率(f_T)的InAlN/GaN异质结场效应晶体管 (HFETs)。基于MOCVD外延n -GaN欧姆接触工艺实现了器件尺寸的缩小,有效源漏间距(L_sd)缩小至600 nm。此外,采用自对准工艺制备了50 nm直栅。由于器件尺寸的缩小,V_gs= 1 V下器件最大饱和电流(I_ds)达到2.11 A/mm,峰值跨导达到609 mS/mm。小信号测试表明,器件f_T达到220 GHz、最大振荡频率(f_max)达到48 GHz。据我们所知,该f_T值是目前国内InAlN/GaN HFETs器件报道的最高结果。     

Uniformity in HgCdTe diode arrays fabricated by reactive ion etching

Two-dimensional, midwavelength infrared (MWIR) HgCdTe detector arrays have been fabricated using reactive ion etching (RIE). Detector-to-detector uniformity has been studied in the devices fabricated with CdTe- and ZnS-passivation layers. Mapping of the doping profile, passivant/HgCdTe interface electrical properties, and diode impedance-area product (R₀A_j) in a two-dimensional array of diodes has been carried out. Temperature and perimeter/area dependence of the dark current are studied to identify the bulk and surface current components. Maximum R₀A_j=2×10⁷ Θcm² was achieved in CdTe-passivated, 200×200 μm² diode arrays. It demonstrates that CdTe-passivated, RIE-processed HgCdTe is a feasible technology.     

Effect of reactive ion etch-induced damage on the performance of 4H-SiC schottky barrier diodes

The effect of reactive ion etch (RIE) induced damage on 4H-SiC surfaces etched in fluorinated plamas has been investigated and characterized using Ni Schottky diodes and x-ray photoelectron spectroscopic surface analysis. The diodes were characterized using current-voltage, current-voltage-temperature, and capacitance-voltage measurements with near ideal forward characteristics (n=1.07) and forward current density as high as 9000 A/cm² from the control (unetched) devices. High current handling capability was observed in diodes with etched surfaces as well. Diodes with surfaces etched in CHF₃ containing plasmas showed a significant reduction in the barrier height compared to the diodes with surfaces etched in CF₄ containing plasma. Control devices exhibited high leakages when reverse biased, which is attributed to the presence of a thin (∼2 nm) oxide layer at the metal-semiconductor interface. However, under reverse bias diodes with CHF₃-etched surfaces showed improvement in leakage current compared to diodes with CF₄-etched surfaces and the control diodes.