Abhishek Motayed

Electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au multilayer Ohmic contacts to n-type GaN

The electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au (30 nm/100 nm/30 nm/30 nm) multilayer Ohmic contacts to n-GaN (doping level 5×1017u200acm−3) were studied. The lowest contact resistivity derived from the annealed contact was ρS=3.0×10−6Ωu200acm2. The contacts were robust and showed high-thermal stability. X-ray diffraction and Auger electron spectroscopy studies were made to investigate the microstructure of the annealed contacts. The key to the success of the contact was the Ti layers placed on both sides of the Al layer. Upon annealing, there occurred both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film. The in-diffusion of this led to the formation of TiN, while the out-diffusion of this led to the formation of Ti–Al alloys. The second Ti layer also in-diffused and out-diffused during annealing. However, due to the presence of Au, the out-diffusion was marginalized, and the in-diffusion was higher than the out-diffusion. The in-diffusion led to th...

Low-resistance Ti/Al/Ti/Au multilayer ohmic contact to n-GaN

A metallization scheme has been developed for obtaining low ohmic contacts to n-GaN with a low contact resistance. The metal contact is a Ti/Al/Ti/Au composite with layers that are respectively 30, 100, 30, and 30 nm thick. Contacts with a specific contact resistivity ρs, as low as 6.0×10−7u200aΩu200acm2 for a doping level of 1.40×1020u200acm−3 were obtained after annealing the sample for 30 s at 750u200a°C in a rapid thermal annealer. The Ti placed on top of the traditional Ti/Al contact appears to have the advantage of tying up the excess Al; therefore it does not form a mottled contact. Some of the additional Ti–Al intermetallic alloys that are formed also have beneficial effects. The Ti–Au layer forms a robust upper portion of the composite, which enables the contacts to have high-temperature applications.

Electrical characteristics of AlxGa1−xN Schottky diodes prepared by a two-step surface treatment

Near-ideal Schottky barrier contacts to n-type Al0.22Ga0.78N have been developed by a two-step surface treatment technique. Plasma etching of the AlxGa1−xN surface prior to Schottky metal deposition, combined with sequential chemical treatment of the etched surface, holds promise for developing high quality low-leakage Schottky contacts for low noise applications and for recessed gate high electron mobility transistors. In this work, the effect of postetch chemical treatment of the n-type Al0.22Ga0.78N surface on the performance of the Ni∕Au based Schottky contact has been investigated. Three different types of chemical treatment: viz, reactive ion etching, reactive ion etching plus dipping in hot aqua regia, and reactive ion etching plus dipping in hot KOH, are studied. Detailed current-voltage studies of three different surface treated diodes and a comparison with as-deposited diodes reveal significant improvement in the diode characteristics. The latter surface treatment yields Ni∕Au Schottky diodes wi...

High-transparency Ni/Au bilayer contacts to n-type GaN

A unique metallization scheme has been developed for obtaining both Schottky and low-resistance Ohmic contacts to n-GaN. It has been demonstrated that the same metallization can be used to make both Schottky and Ohmic contacts to n-GaN using a Ni/Au bilayer composite with Ni in contact to GaN. Using this metallization, contacts with a specific contact resistivity, ρs, as low as 6.9×10−6 Ωu200acm2 for a doping level of 5.0×1017 cm−3 was obtained after annealing the sample for 10 s at 800u200a°C in a rapid thermal annealer. The presence of only (111)Au and (111)Ni peaks in the x-ray diffraction (XRD) pattern of as-deposited samples indicates that both metals participate to form epitaxial or highly textured layers on the basal GaN plane. When the contact layer is annealed, Au and Ni react with GaN creating interfacial phases. Both XRD and transmission electron microscopy confirm that Ni3Ga and Ni2Ga3 intermetallic phases together with Au and Ni based face-centered-cubic solid solutions, are formed during annealing. ...

Tunable Ultraviolet Photoresponse in Solution-Processed p–n Junction Photodiodes Based on Transition-Metal Oxides

Solution-processed p-n heterojunction photodiodes have been fabricated based on transition-metal oxides in which NiO and ternary Zn(1-x)Mg(x)O (x = 0-0.1) have been employed as p-type and n-type semiconductors, respectively. Composition-related structural, electrical, and optical properties are also investigated for all the films. It has been observed that the bandgap of Zn(1-x)Mg(x)O films can be tuned between 3.24 and 3.49 eV by increasing Mg content. The fabricated highly visible-blind p-n junction photodiodes show an excellent rectification ratio along with good photoresponse and quantum efficiency under ultraviolet (UV) illumination. With an applied reverse bias of 1 V and depending on the value of x, the maximum responsivity of the devices varies between 0.22 and 0.4 A/W and the detectivity varies between 0.17 × 10(12) and 2.2 × 10(12) cmu202f(Hz)(1/2)/W. The photodetectors show an excellent UV-to-visible rejection ratio. Compositional nonuniformity has been observed locally in the alloyed films with x = 0.1, which is manifested in photoresponse and X-ray analysis data. This paper demonstrates simple solution-processed, low cost, band tunable photodiodes with excellent figures of merit operated under low bias.

Chemicophysical surface treatment and the experimental demonstration of Schottky-Mott rules for metal/semiconductor heterostructure interfaces.

Metal/semiconductor (MS) heterostructure is of wide interest in a number of areas including physics, chemistry, materials science, materials engineering, chemical engineering, and electrical engineering. It is an important element of modern technology. The present investigation describes a novel experimental technique to address the influence of interfacial chemical passivation on the Schottky-Mott [Naturwiss. 26, 843 (1938); Cambridge Philos. Soc. 34, 568 (1938)] rules for MS heterostructure, and to try to establish these rules. The success of the experiment derives from three remarkable findings: First, a semiconductor (Al(x)Ga(1-x)N), which is robust and relatively less susceptible to an easy reaction with foreign chemicals, is needed for the demonstration. Second, reactive ion etching together with wet chemical etching by certain selected chemical (such as KOH), but not by others (for example, H(3)PO(4) or aqua regia), can clean the semiconductor surface well, and remove/passivate the dangling chemical bonds from this surface. Third, a judicious selection of deposition parameters for the deposition of metal(s) preferably on a certain selected semiconductor can lead to metal deposition on the semiconductor surface by van der Waals type of epitaxy. Transmission electron microscopy and x-ray diffraction indicate that MS heterostructures, thus prepared, are very different from others; they appear to provide convincing experimental verification of the Schottky-Mott rules, and to establish these rules without any ambiguity. Others fail to do it.

Two-step surface treatment technique: Realization of nonalloyed low-resistance Ti/Al/Ti/Au ohmic contact to n-GaN

A novel two-step surface treatment method has been developed to realize low resistance nonalloyed ohmic contact to n-type GaN doped with Si to 6×1017u200acm−3. The removal of native oxide (oxides and hydroxides) formed on GaN surface is crucial for successful creation of nonalloyed low resistance ohmic contact. In the case of GaN, plasma etching of the material surface prior to metal deposition holds promise for developing such nonalloyed ohmic contacts. In this article, the effects of the postetch chemical treatment of the n-type GaN surface on the Ti based nonalloyed contact performance have been investigated. Contacts on samples without reactive ion etching (RIE) showed Schottky behavior. However, contacts on samples with 15 s of RIE using Cl2 showed ohmic behavior. The contact resistivity of this contact reached to ρs=1.2×10−3u200aΩu200acm2. Treating the RIE etched sample in boiling aqua regia for 5 min yielded a contact resistivity on the order of 3.6×10−4u200aΩu200acm2. Dramatic improvement in current–voltage character...

Formation of large-area GaN nanostructures with controlled geometry and morphology using top-down fabrication scheme

This paper details the fabrication of GaN nanoscale structures using deep ultraviolet lithography and inductively coupled plasma (ICP) etching techniques. The authors controlled the geometry (dimensions and shape) and surface morphology of such nanoscale structures through selection of etching parameters. The authors compared seven different chlorine-based etch chemistries: Cl2, Ar, Cl2/N2, Cl2/Ar, Cl2/N2/Ar, Cl2/H2/Ar, and Cl2/He/Ar. The authors found that nitrogen plays a significant role in fabricating high quality etched GaN nanostructures. This paper presents the effects of varying the etch parameters, including gas chemistry, gas flow rate, ICP power, rf power, chamber pressure, and substrate temperature, on the etch characteristics, including etch rate, sidewall angle, anisotropy, mask erosion, and surface roughness. Dominant etch mechanisms in relation to the observed characteristics of the etched features are discussed. Utilizing such methods, the authors demonstrated the fabrication of nanoscale...

Electrical, microstructural, and thermal stability characteristics of Ta'Ti'Ni'Au contacts to n-GaN

A metallization technique has been developed for obtaining low resistance Ohmic contact to n-GaN. The metallization technique involves the deposition of a metal layer combination Ta/Ti/Ni/Au on an n-GaN epilayer. It is observed that annealing at 750u200a°C for 45 s leads to low contact resistivity. Corresponding to a doping level of 5×1017u200acm−3, the contact resistivity of the contact ρS=5.0×10−6u200aΩu200acm2. The physical mechanisms underlying the realization of low contact resistivity is investigated using current–voltage characteristics, x-ray diffraction, Auger electron spectroscopy, transmission electron microscopy, and energy dispersive x-ray spectrometry.

Large-area GaN n-core/p-shell arrays fabricated using top-down etching and selective epitaxial overgrowth

We present large-area, vertically aligned GaN n-core and p-shell structures on silicon substrates. The GaN pillars were formed by inductively coupled plasma etching of lithographically patterned n-GaN epitaxial layer. Mg-doped p-GaN shells were formed using selective overgrowth by halide vapor phase epitaxy. The diameter of the cores ranged from 250u2009nm to 10u2009μm with varying pitch. The p-shells formed truncated hexagonal pyramids with {11¯01} side-facets. Room-temperature photoluminescence and Raman scattering measurements indicate strain-relaxation in the etched pillars and shells. Cross-sectional transmission electron microscopy revealed dislocation bending by 90° at the core-shell interface and reduction in their density in the shells.