Characterization of Ohmic contacts on GaN/AlGaN heterostructures

The surface morphology and electrical resistance of the contacts on semiconductor devices are strongly influenced by metallization scheme and annealing conditions. In this work is presented an investigation of Ohmic contacts formed by metal-semiconductor alloying on epitaxial GaN/AlGaN heterostructures. After the deposition of metallic multi-layers (Ti, Al, Au and Pt), the process of rapid thermal annealing was carried out in nitrogen, argon and forming gas atmosphere.A series of the samples with different sequences of metallic layers and diverse thicknesses was prepared by employing electron beam evaporation and lift-off deposition techniques. The chemical composition of the samples before and after annealing was studied by means of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques combined with low energy Ar⁺ ion sputtering. The sputtering has been carried out in two different modes: by using constant (square) or gradually narrowed sputtered area. The changes in the chemical state of constituent elements and compositional profiles of the contacts after thermal annealing were revealed from the obtained results. Among the technological problems, influencing on the quality of the contact, were found to be the oxidation and nitridation of the contact surface during thermal annealing, as well as the intermediate sub-layers of Al and Ti oxides, formed during the deposition of metallic multi-layered structure.     

Effects of hydrogen treatment on ohmic contacts to p-type GaN films

This study investigated the effects of hydrogen (H₂) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current-voltage (I-V) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I-V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V_N) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H₂-treated p-GaN films lies about ∼2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.     

高反射率p-GaN欧姆接触电极

根据光学薄膜原理计算了GaN/Ti/Ag、GaN/Al和GaN/Ni/Au/Ti/Ag、GaN/Ni/Au/Al多层电极结构的反射率,得出Ag基和Al基反射电极均能在全角范围内提供较高的反射率。实验测量结果表明,反射率能高于80%的Ag基反射电极,具有低欧姆接触的电学特性。并将GaN/Ni/Au/Ti/Ag多层反射电极应用在上下电极结构的GaN基LED中。实验上采用两步合金法获得了低接触电阻、高反射率的电极结构,并引入Ni/Au覆盖层克服了Ag高温时的团聚和氧化现象。解决了Ag电极的稳定性问题,显著地提高了LED的出光效率,成功制备了具有上下电极结构的GaN基LED管芯。     

Low-resistance Cr/Al Ohmic contacts to N-polar n-type GaN for high-performance vertical light-emitting diodes

We investigated the electrical properties of Cr(30 nm)/Al(200 nm) contacts to N-polar n-type GaN for high-performance vertical light-emitting diodes and compare them with those of Ti(30 nm)/Al(200 nm) contacts. Before annealing, both the samples show ohmic behaviors with a contact resistivity of 1.9-2.3 × 10⁻⁴ Ωcm². Upon annealing at 250 °C for 1 min in N₂ ambient, the Ti/Al contacts become non-ohmic, while the Cr/Al contacts remain ohmic with a contact resistivity of 1.4 × 10⁻³ Ωcm². Based on X-ray photoemission spectroscopy and secondary ion mass spectrometry results, ohmic formation and degradation mechanisms are briefly described and discussed.     

High Resolution Investigation on the NiAu Ohmic Contact to p-AlGaN|GaN Heterostructure

Physics of the Solid State - The low-resistance ohmic contact NiAu|p-type AlGaN|GaN was carefully investigated by high resolution electron microscope (HRTEM) and X-ray photoelectron spectroscopy...     

A Novel Ni/Ag/Pt Ohmic Contact to P-Type GaN for Flip-Chip Light-Emitting Diodes

We present a high-quality Ni/Ag/Pt Ohmic contact to p-type GaN. After the sample is annealed at 500℃ in 02 ambient for 3min, a specific contact resistance as low as 2.6 × 10^-5 Ωcm^2 and an optical reflectivity of 82% at 460 nm are obtained. The Auger electron spectroscopy analysis shows that the Pt layer can improve the surface morphology and thermal reliability of the annealed Ag-based electrode, Ag plays a key role in achieving good ohmic contact due to the outdiffusion of Ga into Ag forming Ga vacancies which increase the hole concentration, while the surface contamination of p-type GaN is reduced by Ni.     

Ir-based diffusion barriers for Ohmic contacts to p-GaN

Ir-based electrical contacts to p-type GaN have been fabricated and characterized. Both GaN//Ni/Au/Ir/Au and GaN//Ni/Ir/Au contact structures were deposited, however, only the former produced Ohmic current-voltage characteristics. At an anneal temperature of 500 °C, the Ni/Au/Ir/Au contact had a specific contact resistance of ∼2 × 10⁻⁴ Ω cm², comparable or superior to conventional Ni/Au contacts that are less thermally stable. Anneal temperatures above 500 °C caused the Ir-based contact to fail. Auger electron spectroscopy was used to obtain depth profiles of both types of contacts at a variety of temperatures in order to provide insight into the mechanism of Ohmic formation as well as potential reasons for failure. A comparison to other metallization schemes on p-GaN is also given.     

Ohmic contacts to GaAs epitaxial layers

Formation of ohmic contacts onto GaAs epitaxial layers was reviewed. Because the Fermi energy of GaAs is pinned at the surface near the middle of the bandgap, it is impossible to choose a metal with the proper work function to make an ohmic contact. Instead, it is necessary to create a heavily doped surface layer and using field emission or thermionic field emission to achieve an ohmic contact. The oldest known contact metallization for GaAs is sequentially deposited thin films of Au, Ge, and Ni. It was shown that the ‘dopant diffusion model’, widely accepted to date to explain the formation of an n⁺ layer on GaAs to form the ohmic contact, is incorrect. Instead, the “solid phase regrowth model” was discussed in detail and shown to describe formation of ohmic contacts in this system. Based on this result, general rules for forming ohmic contacts to compound semiconductors with pinned Fermi levels or large values of electron affinities plus bandgap were expressed.     

Improvement of GaN light-emitting diodes with surface-treated Al-doped ZnO transparent Ohmic contacts by holographic photonic crystal

This letter presents a holographic photonic crystal (H-PhC) Al-doped ZnO (AZO) transparent Ohmic contact layer on p-GaN to increase the light output of GaN-based LEDs without destroying the p-GaN. The operating voltage of the PhC LEDs at 20 mA was almost the same as that of the typical planar AZO LEDs. While the resultant PhC LED devices exhibited significant improvements in light extraction, up to 1.22 times that of planar AZO LEDs without PhC integration. Temperature dependence of the integrated photoluminescence intensity indicates that this improvement can be attributed to the increased extraction efficiency due to the surface modification. These results demonstrate that the surface-treated AZO layer by H-PhCs is suitable for fabricating high-brightness GaN-based LEDs.     

p-type ohmic contacts to AlGaAs/GaAs heterostructures

Ohmic contacts to p-AlGaAs/GaAs heterostructures have larger resistivities compared to n-type structures because holes are heavier than electrons and the heterointerface barrier for high mobility structures is thicker. This work shows that diffusion of Zn during an alloy process of a Au/Zn/Au metallization establishes high acceptor concentrations throughout the heterostructure resulting in a thin metal-semiconductor barrier and probably a degradation of the heterointerface barrier by disordering, while the heterostructure outside the contact region remains unchanged. Contact resistivities down to 3.5ωmm at 25K could thus be achieved.     

ZrB2-based Ohmic contacts to p-GaN

The annealing temperature dependence of contact resistance and layer stability of ZrB₂/Ti/Au and Ni/Au/ZrB₂/Ti/Au Ohmic contacts on p-GaN is reported. The as-deposited contacts are rectifying and transition to Ohmic behavior for annealing at ≥750 °C, a significant improvement in thermal stability compared to the conventional Ni/Au Ohmic contact on p-GaN, which is stable only to <600 °C. A minimum specific contact resistance of ∼2 × 10⁻³ Ω cm⁻² was obtained for the ZrB₂/Ti/Au after annealing at 800 °C while for Ni/Au/ZrB₂/Ti/Au the minimum value was 10⁻⁴ Ω cm⁻² at 900 °C. Auger Electron Spectroscopy profiling showed significant Ti, Ni and Zr out diffusion at 750 °C in the Ni/Au/ZrB₂/Ti/Au while the Ti and Zr intermix at 900 °C in the ZrB₂/Ti/Au. These boride-based contacts show promise for contacts to p-GaN in high temperature applications.     

Schottky contacts to polar and nonpolar n-type GaN

Journal of the Korean Physical Society - Using the current-voltage measurements, we observed the barrier heights of c-plane GaN in Pt and Au Schottky contacts to be higher than those of a-plane...     

Proposal of a new physical model for Ohmic contacts

Ohmic contacts are crucial for both device applications and the study of fundamental physics. In this study, we propose a new physical model for Ohmic contacts based on the detailed considerations of a metal/semiconductor interface, such as charge neutrality level concept, with which it is possible to describe the real situation precisely. Our proposed model contains many defect energy levels that originate from vacancies and impurities located in the vicinity of the metal/semiconductor interface, within the energy range of the Schottky barrier height. Moreover, we calculate the current–voltage characteristics based on our model. Our calculated results show that our model reveals linear Ohmic I–V characteristics and dense defect energy level distribution in the energy range of the Schottky barrier height is crucial for obtaining Ohmic I–V characteristics.     

多层Ti/Al电极结构对Ga N/AlGaN HEMT欧姆接触特性的影响

研究了多层Ti/Al结构电极对GaN/AlGaN HEMT欧姆接触特性及表面形态的影响。采用传输线模型对各结构电极的比接触电阻率进行了测量,采用扫描电子显微镜对电极表面形态进行扫描。实验结果显示,在同样的退火条件下,随着Ti/Al层数的增加,比接触电阻率逐渐减小,表面形态趋于光滑;降低Ti/Al层的厚度会加剧Au向内扩散而增加比接触电阻率,但能稍微改善表面形态;Ti比例过高会影响Ti N的形成导致比接触电阻率增加,但能明显改善表面形态。     

Ni/Au与N掺杂p型ZnO的欧姆接触

研究了Au,In,Ni/Au三种不同金属膜与N掺杂p型ZnO的接触特性,发现Ni/Au双层膜更适合作为其欧姆电极材料,并比较了不同气氛和不同温度退火对Ni/Au电极的影响.发现在O₂中退火电极性能发生蜕变,而在N₂中退火性能得到改善.指出即使在N₂中退火,退火温度的选择也是至关重要的,本实验在400℃,氮气气氛下退火150s,得到了较好的欧姆接触特性.     

AlGaN/GaN肖特基结参数分析与电流运输机理研究

通过对AlGaN/GaN HEMT器件肖特基栅电流输运机理的研究,在变温下采用I-V法对AlGaN/GaN上的Ni/Au肖特基势垒高度和理想因子进行了计算. 通过对不同电流机理的分立研究,得到了更为准确的势垒高度值_b. 通过分析温度在300—550K之间肖特基反向泄漏电流的特性,得出结论：AlGaN材料的表面漏电不是HEMT器件反向泄漏电流的主要来源. 关键词： AlGaN/GaN异质结 肖特基结 理想因子     

The reflectivity of Mo/Ag/Au ohmic contacts on p-type GaN for flip-chip light-emitting diode (FCLED) applications

The Mo/Ag/Au contact for flip-chip light-emitting diode (FCLED) applications is examined on its contact resistance and light reflectance. A high reflectance of 90% is achieved in un-annealed contact, but a strong inter-diffusion of ohmic metals and GaN during the annealing process is found to result in poor reflectance (55% at the wavelength of 465 nm). The secondary ion mass spectrometry (SIMS) depth profiles indicate that a wide inter-diffusion region existed in the annealed contacts; thus the low reflectivity of the Mo/Ag/Au-annealed contacts can be attributed to the strong inter-diffusion of Au and Ag.     

ITON Schottky contacts for GaN based UV photodetectors

Lateral Schottky ultraviolet detectors were fabricated in GaN using indium-tin-oxynitride (ITON) as a contact metal. The GaN semiconductor material was grown on 2 in. sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The Schottky contact has been realized using ITON that has been deposited using sputter techniques. I-V characteristics have been measured with and without UV illumination. The device shows photo-to-dark current ratios of 10³ at −1 V bias. The spectral responsivity of the UV detectors has been determined. The high spectral responsivity of more than 30 A/W at 240 nm is explained by a high internal gain caused by generation-recombination centers at the ITON/GaN interface. Persistent photocurrent effect has been observed in UV light (on-off) switching operation, time constant and electron capture coefficient of the transition has been determined.     

Formation of Ohmic contacts to n-GaAs using ion beam mixing of Tellurium

Ohmic contacts were formed on n-GaAs using thin evaporated layers of Te followed by bombardment of 100 keV Ar⁺ ions. The specific contact resistance _c showed a strong dependence on the ion dose in the range 10¹⁴ to 10¹⁶ ions cm^–2, with higher doses leading to progressively lower specific contact resistance. The substrate temperature during ion bombardment was varied in the range from 25 to 200° C and was found to have only a minor effect on the resultant values of _c. Elevated temperature aging of the Ohmic contacts at 200° C resulted in a progressive increase in the specific contact resistance, independent of either the ion dose or the substrate temperature used for ion beam mixing. Rutherford backscattering studies (RBS) indicate that the Ohmic contact behaviour was due to the in-diffusion of Te and subsequent formation of a heavily doped n ⁺ layer at the Te-GaAs interface.