Nanopatterned Contacts to GaN

The effect of nanoscale patterning using a self-organized porous anodic alumina (PAA) mask on the electrical properties of ohmic and Schottky contacts to n-GaN was investigated with the aim of evaluating this approach as a method for reducing the specific contact resistance of ohmic contacts to GaN. The electrical characteristics of contacts to these nanopatterned GaN samples were compared with contacts to planar, chemically prepared (“as-grown”) GaN samples and reactive ion etched (RIE) GaN films without any patterning. The specific contact resistivities to unintentionally doped n-GaN using a Ti/Al bilayer metallization were determined to be 7.4 × 10⁻³ Ω cm² for the RIE sample and 7.0 × 10⁻⁴ Ω cm² for the PAA patterned sample. Schottky metal contacts with Pt and Ni were prepared on the three samples to validate the effects of RIE and nanopatterning on electrical behavior. The effective barrier height was decreased and the reverse current was increased significantly in the PAA patterned sample. The radius of curvature of the nanoscale corrugation in the patterned interface was smaller than the depletion width. The reduction of the depletion width at sharp corners enhanced the local tunneling current, reducing the specific contact resistivity and decreasing the effective barrier height. These results suggest that nanopatterning with PAA on GaN can significantly lower the contact resistance.     

Correlation of microstructure with electrical behavior of Ti/GaN schottky contacts

We correlate structural and electrical characteristics of as-deposited and low-temperature annealed Ti contacts on GaN. Temperature dependent currentvoltage measurements are used to determine the effective barrier heights of the respective contacts, while high-resolution transmission electron microscopy is utilized for structural characterization. As-deposited Ti contacts are slightly rectifying with an effective barrier height of ∼200 meV. After annealing at 230°C, the barrier height increases to values of ∼450 meV. A similar behavior of Schottky contacts with more strongly rectifying diodes upon low-temperature annealing is observed for Zr metal contacts on GaN. As-deposited Ti already forms a thin TiN layer at the GaN interface. After annealing at 230°C, the average thickness and the distribution of TiN grains remain practically unchanged, but the interface with GaN roughens. We correlate the observed barrier height changes with interface roughness and phase formation and we discuss the results in terms of interface damage and the Schottky-Mott theory.     

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.     

Study of schottky barrier heights of indium-tin-oxide on p-GaN using x-ray photoelectron spectroscopy and current-voltage measurements

In this study, the current density-voltage (J-V) characteristic of Schottky diodes of indium-tin-oxide (ITO) contacts to p-type GaN (p-GaN) has been investigated. The calculated barrier-height value of ITO/p-GaN samples using the thermionic field-emission (TFE) model is 3.2 eV, which implies that the work function of ITO is equal to 4.3 eV. The result is supported by J-V measurements of ITO/n-type GaN Schottky diodes. On the other hand, the barrier height of ITO/p-GaN was also determined from x-ray photoelectron spectroscopy (XPS) data. The analysis of the XPS spectral shifts indicated that this observed barrier-height value of ITO/p-GaN by XPS is in good agreement with the value of 3.2 eV obtained from J-V measurements.     

Mechanism investigation of NiO<Subscript>x</Subscript> in Au/Ni/p-type GaN ohmic contacts annealed in air

Recently, Au/Ni/p-type GaN ohmic contacts annealed in an air ambient have been widely investigated. However, to obtain a low specific-contact resistance, the annealing window is limited. In this study, to understand the oxidation function of metallic Ni, the Au/Ni/p-type GaN structure was annealed in an air ambient for 10 min at various temperatures. Using x-ray photoelectron spectroscopy (XPS) analysis, the metallic Ni was oxidized into NiO and NiO_1.3 compositions at annealing temperatures of 500°C and 600°C, respectively. However, metallic Ni still existed on the interface of the Ni/p-type GaN annealed at 400°C. The associated barrier heights of 0.42 eV, 0.21 eV, and 0.31 eV were obtained with p-type GaN for the Ni, NiO, and NiO_1.3 contacts, respectively. The hole concentrations of p-type NiO and p-type NiO_1.3 were 2.6×10¹⁶ cm⁻³ and 2.0×10¹⁸ cm⁻³, respectively. The lower hole concentration of the p-type NiO would lead to reducing the valence-band bending of the p-type GaN, as well as the barrier height for holes crossing from the p-type NiO to the p-type GaN. The formation of NiO was thus an important issue for lowering the specific-contact resistance of the Au/Ni/p-type GaN ohmic contacts annealed in an air ambient.     

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.     

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.     

Calculations and measurements of contact resistance of semi-transparent Ni/Pd contacts to p-GaN

Calculations of specific contact resistance as a function of doping and barrier height were performed for p-type GaN. These calculations took into account two valence bands, each with different effective masses, and show that at low doping, the heavy hole band accounts for most of the conduction, whereas at heavier doping, the light hole band dominates conduction. These calculations also indicate the barrier height for typical contacts to p-GaN is between 0.75 eV and 1 eV. Specific contact resistance measurements were made for oxidized Ni/Au, Pd, and oxidized Ni/Pd ohmic contact metal schemes to p-GaN. The Ni/Pd contact had the lowest specific contact resistance, 6×10⁻⁴ Ω cm². Auger sputter depth profile analysis showed some Ni diffused away from the GaN surface to the contact surface with the bulk of the Pd located in between two areas of Ni. Both Ni and Pd interdiffused with the GaN at the semiconductor surface. The majority of the oxygen observed was with the Ni as NiO. Angle-resolved-x-ray photoelectron spectroscopy (AR-XPS) analyses showed the formation of predominantly NiO and PdO species, with higher Ni and Pd oxides at the contact surface.     

Mechanisms of current flow in metal-semiconductor ohmic contacts

Published data on the properties of metal-semiconductor ohmic contacts and mechanisms of current flow in these contacts (thermionic emission, field emission, thermal-field emission, and also current flow through metal shunts) are reviewed. Theoretical dependences of the resistance of an ohmic contact on temperature and the charge-carrier concentration in a semiconductor were compared with experimental data on ohmic contacts to II–VI semiconductors (ZnSe, ZnO), III–V semiconductors (GaN, AlN, InN, GaAs, GaP, InP), Group IV semiconductors (SiC, diamond), and alloys of these semiconductors. In ohmic contacts based on lightly doped semiconductors, the main mechanism of current flow is thermionic emission with the metal-semiconductor potential barrier height equal to 0.1–0.2 eV. In ohmic contacts based on heavily doped semiconductors, the current flow is effected owing to the field emission, while the metal-semiconductor potential barrier height is equal to 0.3–0.5 eV. In alloyed In contacts to GaP and GaN, a mechanism of current flow that is not characteristic of Schottky diodes (current flow through metal shunts formed by deposition of metal atoms onto dislocations or other imperfections in semiconductors) is observed.     

调制掺杂AlGaN/GaN异质结上的Pt肖特基接触

研究了调制掺杂 Alx Ga1 - x N/Ga N表面 Pt肖特基接触的制备工艺 ,并对其进行了 I-V测量。通过改变对样品表面的处理工艺 ,研究了表面处理对调制掺杂 Alx Ga1 - x N/Ga N表面 Pt肖特基结接触特性的影响 ,在 n型掺杂浓度为 7.5× 1 0 1 7cm- 3的 Al0 .2 2 Ga0 .78N样品表面 ,制备得到了势垒高度为 0 .94e V、理想因子为 1 .4的 Pt肖特基接触。这与国外报道的结果接近 (=1 .2 e V,n=1 .1 1 [1 ] )     

Plasma Treatment for Leakage Reduction in AlGaN/GaN and GaN Schottky Contacts

It has been found that the reverse leakage current of AlGaN/GaN Schottky contacts can be significantly reduced by a CF₄ plasma treatment prior to the Schottky metal evaporation. The data of electrical characterization suggest that the leakage reduction is related to the modification of the semiconductor surface by plasma treatment. The leakage reduction effect was also observed in GaN Schottky contacts. Capacitance-voltage characterization of the GaN Schottky contacts indicates that the Schottky barrier height was slightly increased by the plasma treatment. A two-step surface treatment procedure, consisting of a BCI₃ plasma treatment followed by a brief CF₄ plasma treatment, has been developed as an efficient approach to reduce the reverse leakage of the Schottky contacts, while avoiding side effects related to the CF₄ plasma.     

GaN紫外探测器的Ti/Al接触的电学特性

在非故意掺杂的和掺Si的GaN薄膜上蒸镀Ti(24nm)/Al(nm)薄膜,氮气环境下400～800℃范围内进行退火。实验结果表明,在非故意掺杂的样品上,随退火温度的升高,肖特基势垒高度下降,理想因子升高,表面状况逐渐变差,600℃退火形成较低接触电阻的欧姆接触,比接触电阻率为3.03×10-4Ωcm2,而载流子浓度为5.88×1018cm-3的掺Si的样品未退火就形成欧姆接触,比接触电阻可达到4.03×10-4Ωcm2。     

MOCVD生长GaN膜的光调制反射谱研究

采用光调制反射光谱（PR）研究了（0001）晶向蓝宝石村底上MOCVD方法生长的单晶六角GaN薄膜的室温光学性质。测得六角GaN薄膜的禁带宽度为3．400eV，对PR谱的调制机理进行了分析，发现信号来自缺陷作用下的表面电场调制。光吸收增和光反射谱的测量，得到3．39eV的光学吸收边和3．3eV的反射峰，证实了光调制反射光谱的结果。     

p-GaN与Ni/Pt欧姆接触的研究

本文对p-GaN与Ni/Pt形成欧姆接触及其电流传输机制进行了研究。I-V变温测试曲线是线形的,表明Ni/Pt与p-GaN之所以能形成欧姆接触,是因为Ni/Pt与p-GaN接触在空气中退火时界面处的p-GaN空穴浓度增加了,从而降低了有效势垒高度。在148K～323K范围内,单位接触电阻R_c随测试温度T的升高趋于呈指数下降,表明Ni/Pt与p-GaN欧姆接触的电流传输机制遵循热电子发射。     

边欧姆工艺对AlN/GaN 异质结场效应晶体管极化梯度库仑场散射的影响

Using the measured capacitance–voltage and current–voltage characteristics of the rectangular Al N/Ga N heterostructure field-effect transistors(HFETs) with the side-Ohmic contacts, it was found that the polarization Coulomb field scattering in the Al N/Ga N HFETs was greatly weakened after the side-Ohmic contact processing, however, it still could not be ignored. It was also found that, with side-Ohmic contacts, the polarization Coulomb field scattering was much stronger in Al N/Ga N HFETs than in Al Ga N/Al N/Ga N and In0:17Al0:83N/Al N/Ga N HFETs, which was attributed to the extremely thinner barrier layer and the stronger polarization of the Al N/Ga N heterostructure.     

栅槽刻蚀工艺对增强型GaN HEMT器件性能的影响

基于凹槽栅增强型氮化镓高电子迁移率晶体管(GaN HEMT)研究了不同的栅槽刻蚀工艺对GaN器件性能的影响。在栅槽刻蚀方面,采用了一种感应耦合等离子体(ICP)干法刻蚀技术与高温热氧化湿法刻蚀技术相结合的两步法刻蚀技术,将AlGaN势垒层全部刻蚀掉,制备出了阈值电压超过3 V的增强型Al_2O_3/AlGaN/GaN MIS-HEMT器件。相比于传统的ICP干法刻蚀技术,两步法是一种低损伤的自停止刻蚀技术,易于控制且具有高度可重复性,能够获得更高质量的刻蚀界面,所制备的器件增强型GaN MIS-HEMT器件具有阈值电压回滞小、电流开关比(I_ON/I_OFF)高、栅极泄漏电流小、击穿电压高等特性。     

影响AlGaN/GaN HFET器件二维电子气的若干因素

基于Sentaurus Workbench(SWB)TCAD可制造性设计平台进行AlGaN/GaN器件的结构设计和仿真,并对影响二维电子气的重要参数因素进行了研究及优化,诸如AlGaN势垒层中Al组分x、AlGaN势垒层厚度h、应变弛豫度r和栅偏压Vg等因素。参数相关性的制约结果,无疑会反映在对器件物理特性的制约及影响上。研究结果表明,在一定条件下增大势垒层中Al组分和势垒层厚度可以提高器件的电流传输特性。然而随着二者的不断增大将会引起应变弛豫的发生,而应变弛豫的发生会降低器件的性能。     

Wet chemical nitridation of (100)GaAs surface: Effect on electrical parameters of surface-barrier Au-Ti/GaAs structures

The influence exerted by wet chemical nitridation of the (100)GaAs substrate surface in hydrazinesulfide solutions before fabrication of barrier contacts on the parameters of surface-barrier Au-Ti/GaAs structures has been studied. The structures fabricated on nitrided substrates are characterized by lower reverse currents and higher breakdown voltages. The barrier height in structures of this kind is 0.71±0.02 eV, and the ideality factor, 1.06±0.01. The observed improvement in the electrical parameters of the structures is due to replacement of the natural oxide layer on the substrate surface by a thin coherent GaN film.     

Structural and optical characterization of InP/GalnP islands grown by solid-source MBE

We report on the growth of InP/GalnP islands on GaAs substrates by solidsource molecular beam epitaxy. It is shown by reflection high energy electron diffraction and atomic force microscopy that a rapid change from a twodimensional to a three-dimensional growth mode occurs at about nominally 1.5 monolayers (MLs) InP. Transmission electron microscopy measurements demonstrate the coherent incorporation of InP islands in an GalnP matrix for nominally 2.5 MLs InP. The energy of the InP photoluminescence (PL) shifts to lower energies (100 meV) when the growth interruption time between the island and cap layer growth is increased from 1 to 300 s in case of nominally 3 MLs InP. Simultaneously, an increase of the PL linewidth is observed from 30 to 60 meV. Room temperature photoreflectance measurements on samples with various InP thickness have been performed. Compared to PL measurements, an additional feature in the photoreflectance spectra is observed for samples with more than 7 MLs InP, which is attributed to a transition between excited electron and hole states of the islands.     

Structural properties of alloyed Ti/Al/Ti/Au and Ti/Al/Mo/Au ohmic contacts to AlGaN/GaN

Two alloyed ohmic contact structures for AlGaN/GaN–Ti/Al/Ti/Au and Ti/Al/Mo/Au were studied. Both structures were optimized for minimum ohmic contact resistance. Structures grown on sapphire and SiC substrates were used to investigate structural properties of ohmic contacts to AlGaN/GaN. Ohmic contacts to AlGaN/GaN on SiC showed higher contact resistance values compared to contacts to AlGaN/GaN on sapphire. Ohmic contact metals were etched on samples after annealing. The alloyed interface was studied with backside illumination under an optical microscope. Alloyed inclusions associated with threading dislocations were observed on the surface. For the AlGaN/GaN on SiC sample the inclusion density was an order of magnitude lower than for the sample on sapphire. Conductive atomic force microscopy with carbon nanotube tip was used to investigate topography and conductivity profile of the surface after ohmic contact metal removal by etching.