H. S. Venugopalan

Influence of oxygen on the activation of p-type GaN

The presence of oxygen in the annealing environment can exhibit a strong influence on the activation of p-GaN, as demonstrated by experiments described in this letter. We activated p-GaN at 600–900 °C in four environments: ultrahigh purity (UHP) N2 gettered to remove residual O2, UHP N2 without gettering, 99.5% UHP N2/0.5% UHP O2, and 90% UHP N2/10% UHP O2. The resistivity of the p-GaN was lowest when O2 was intentionally introduced during activation and was highest when extra care was taken to getter residual O2 from the annealing gas. The experiments also demonstrate that unintentionally incorporated O2 can be at high enough levels to influence the activation process.

Interfacial reactions between nickel thin films and GaN

Thin Ni films on GaN were annealed at temperatures between 400 and 900 °C in N2, Ar, and forming gas and were analyzed using glancing angle x-ray diffraction and Auger depth profiling. The first indication of an interfacial reaction was found after an anneal at 600 °C for 1 h, after which Ga was observed to be dissolved in the face-centered cubic Ni film. The extent of dissolution increased with continued annealing. After annealing at 750 °C for 1 hr in either N2 or Ar, greater intermixing occurred. The reaction product was either Ni3Ga or face-centered cubic Ni with dissolved Ga. Annealing at 900 °C resulted in the formation of the B2 phase NiGa. It was clear from Auger depth profiles that the reacted film contained significantly more Ga than N and that N2 gas was released to the annealing environment, even when the samples were annealed in N2 gas at 1 atm. Thus, a trend of increasing Ga content in the reacted films was observed with increasing temperature. The observed reactions are consistent with the ...

Kinetic Study of the Oxidation of Gallium Nitride in Dry Air

The oxidation of polycrystalline GaN powder and GaN epi layers in dry air has been investigated. Bulk 0--20 X-ray diffraction (XRD) revealed no evidence of oxide formation on the powder specimen exposed to temperatures of up to 750 C for 25 h. However, when oxidized at temperatures of 900 C or greater for 1 h or longer, an oxide was observed to form and was identified as the monoclinic {beta}-Ga{sub 2}O{sub 3}, the same oxide observed previously by the present investigators to form on epitaxial GaN films. Information regarding the oxidation kinetics was obtained in the present study by measuring the intensity of the {beta}-Ga{sub 2}O{sub 3}, ({bar 2} 1 7) XRD peak as a function of oxidation time at various temperatures, and the initial stage of the oxidation was found to be limited by the rate of an interfacial reaction with an activation energy of {approximately} 3 {times} 10{sup 5} J/mol. An interfacial reaction-controlled mechanism was also observed to limit the rate of oxidation of GaN epi layers at 900 C.

Phase Formation and Morphology in Nickel and Nickel/Gold Contacts to Gallium Nitride

Metallurgical reactions between Ni and GaN have been explored at temperatures between 400 and 900 °C in N 2 , Ar, and forming gas. A trend of increasing Ga content in the reacted films was observed with increasing temperature. The reactions are consistent with the thermodynamics of the Ni-Ga-N system. Changes in the film morphology on annealing were also examined. Metal island formation and a corresponding deep, non-uniform metal penetration into GaN were observed at high temperatures. The relevance of the observed nature of phase formation and morphology in these thin films to electrical properties of Ni/GaN and Au/Ni/GaN contacts is also considered.

Morphology of nickel and nickel/gold contacts to gallium nitride

Changes in film morphology upon annealing Ni and Ni/Au contacts to GaN were examined using scanning electron microscopy and atomic force microscopy. Atomic force microscopy was performed on the GaN surface that was exposed by etching away the metal film. This technique was demonstrated to be a convenient and effective way to evaluate the morphology of the contact/semiconductor interface, rapidly providing spatial information in three dimensions. Metal island formation and a corresponding deep, nonuniform metal penetration into GaN were observed after annealing Ni/GaN and Au/Ni/GaN contacts above 800 and 700 °C, respectively.

Ohmic contacts formed by electrodeposition and physical vapor deposition on p-GaN

Electrodeposited Pt and sputtered Ni/Pt contacts to p-GaN (p=4.6×1017 cm−3) are reported and compared to sputtered Ni, Pt, and Ni/Au contacts and electron beam and thermally evaporated Ni contacts. Sequential rapid thermal annealing was employed with samples receiving an initial five minute heat treatment of 400 °C followed by 1 min anneals at 500, 600, and 700 °C, all under flowing N2. Plots of current versus voltage for all contacts showed nonlinearity through the origin as deposited and for all annealing conditions. Extracted values of specific contact resistance are thus determined using the measured resistance for a given value of applied current. The lowest contact resistivity was reproducibly provided by the electrodeposited Pt contacts. After a 1 min anneal at 600 °C, a contact resistivity of 1.50×10−2 Ω cm 2 was obtained using the circular transmission line method at a measurement current of 10 mA. Sputtered Ni/Pt contacts provided a contact resistivity of 1.81×10−2 Ω cm2 at 10 mA after a 1 min a...

Kinetics of directed oxidation of Al-Mg alloys in the initial and final stages of synthesis of Al2O3/Al composites

Abstract In the directed oxidation of Al-Mg alloys, MgO forms in the initial stage. The mechanism of formation of MgO from the Al-Mg alloy in the initial stage of oxidation was studied. The variables studied were the total pressure in the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and oxygen diffusivity. These results suggest that MgO forms by reaction-enhanced vaporization of Mg from the alloy followed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs. The kinetics of formation of Al2O3 in the growth stage of directed oxidation of the Al-5wt.% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of time, was practically independent of oxygen pressure and exhibited an activation energy of 361 kJ mol−1. In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.

Approaches to designing thermally stable Schottky contacts to n-GaN

The barrier heights of PdIn, Ni/Ga/Ni and Re Schottky contacts to n-GaN were investigated by current-voltage and capacitance-voltage measurements. Elemental Pd and Ni contacts were also investigated for comparison. In each case, the barrier heights were determined as a function of annealing temperature. It was shown that stoichiometric PdIn contacts were more stable than Pd-only contacts. Similarly, Ni/Ga/Ni diodes were found to be more stable than Ni diodes. Both the Ni/Ga/Ni contact and the elemental Re contact were stable on short-term annealing up to 700 ?C.

Probing the initial stage of synthesis of Al2O3/Al composites by directed oxidation of Al-Mg alloys

In the directed oxidation of Al-Mg alloys, the amount of MgO that forms in the initial stage prior to the incubation period affects the rate of oxidation of Al to Al2O3 in the composite growth stage. The mechanism of formation of MgO and the duration of the initial stage were investigated experimentally and theoretically. The variables studied were total pressure in the reaction chamber, partial pressure of oxygen, and the nature of the diluent gas which affects the diffusion coefficients of magnesium vapor and oxygen in the gas phase. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and the diffusivity of oxygen. The duration of the initial stage decreased with the increase in oxygen pressure. To understand the role of magnesium evaporation in the oxidation behavior of the alloy, the velocity, temperature, and concentration fields in the gas phase were simulated numerically. The calculated concentration profiles of magnesium vapor and oxygen as a function of time were consistent with the experimentally measured oxidation rates and confirm reaction-enhanced gaseous diffusion-limited vaporization of magnesium in the initial stage of oxidation of Al-Mg alloys. The region where the magnesium vapor is oxidized in the gas phase moved progressively closer to the alloy surface during the initial stage of oxidation. The end of the initial stage and the start of the incubation period corresponded to the arrival of the oxygen front close to the surface when the spinel formation occurred.

Growth stage kinetics in the synthesis of Al2O3/Al composites by directed oxidation of Al-Mg and Al-Mg-Si alloys

Although synthesis of ceramic matrix composites by the directed oxidation process offers significant advantages over traditional composite processing routes, the scientific basis for the process is not fully understood. This paper is addressed to understanding the mechanism of composite growth from Al-Mg and Al-Mg-Si alloys theoretically and experimentally. Analysis of the oxidation kinetics of Al-Mg and Al-Mg-Si alloys for various oxygen pressures, temperatures and durations of oxidation, obtained in this study and reported in the literature, demonstrates that the growth kinetics can be tailored by the control of alloy composition. For the Al-Mg alloys, transport of oxygen through a thin alloy layer near the surface controls the growth rate. When Si is added to the alloy, the oxidation mechanism is completely changed. The rate of oxidation of Al-Mg-Si alloys depends on the transport of electronic species through a thin MgO layer at the top surface of the composite. Apart from contributing to a more complete understanding of the growth stage, the mechanism of composite growth will serve as a basis for improving growth rates.