G. C. Chi

Effects of thermal annealing on the indium tin oxide Schottky contacts of n-GaN

In this work indium tin oxide (ITO) films were prepared using electron beam evaporation to form Schottky contacts on n-type GaN films. The thermal stability of ITO on n-type GaN was also investigated by annealing the samples at various temperatures. In addition, current–voltage (I–V) measurements were taken to deduce the Schottky barrier heights. Owing to the large series resistance, the Norde method was used to plot the F(V)–V curves and the effective Schottky barrier heights were determined as well. The effective Schottky barrier heights were 0.68, 0.88, 0.94, and 0.95 eV for nonannealed, 400, 500, and 600 °C annealed samples, respectively. Results presented herein indicate that an increase of the barrier heights may be attributed to the formation of an interfacial layer at the ITO/GaN interface after annealing.

The effect of thermal annealing on the Ni/Au contact of p-type GaN

In this study, the Ni/Au layers prepared by electron beam evaporation and thermal alloying were used to form Ohmic contacts on p-type GaN films. Before thermal alloying, the current–voltage (I–V) characteristic of Ni/Au contact on p-type GaN film shows non-Ohmic behavior. As the alloying temperature increases to 700 °C, the I–V curve shows a characteristic of Ohmic contact. The Schottky barrier height reduction may be attributed to the presence of Ga–Ni and Ga–Au compounds, such as Ga4Ni3, Ga3Ni2, GaAu, and GaAu2, at the metal-semiconductor interface. The diffusing behavior of both Ni and Au have been studied by using Auger electron spectroscopy and Rutherford backscattering spectrometry. In addition, x-ray diffraction measurements indicate that the Ni3N and Ga4Ni3 compounds were formed at the metal-semiconductor interface.

The dependence of the electrical characteristics of the GaN epitaxial layer on the thermal treatment of the GaN buffer layer

The GaN buffer layer was grown on the sapphire substrate by low‐pressure metalorganic chemical vapor deposition (LP‐MOCVD) at 525 °C. The following 1.3 μm epitaxial GaN growth was carried out at 1025 °C. We varied the ramping rate from 12.5 to 100 °C/min to study the quality of the epitaxial GaN. It has been found that the x‐ray peak width, photoluminescence (PL) linewidth, Hall mobilities, and carrier concentrations of GaN epitaxial layer strongly depend on the in situ thermal ramping rate. An optimum thermal ramping rate was found to be of 20 °C/min. The maximum mobility is 435 cm2/V s at carrier concentration of 1.7×1017 cm−3. The minimum full width at half maximum (FWHM) of x ray and PL were 5.5 min and 12 meV occur at a ramping rate of 20 °C/min. The decrease of the mobility at high and low ramping rate can be attributed to the thermal stress and the reevaporation of the GaN buffer layer.

Inductively coupled plasma etching of GaN using Cl2/Ar and Cl2/N2 gases

This work investigates inductively coupled plasma (ICP) etching processes of GaN. Etching behaviors are also characterized by varying the ICP power, Cl2/Ar or Cl2/N2 mixing ratio, radio-frequency (rf) power, and chamber pressure. Experimental results indicate that the etching profiles are highly anisotropic over the range of etching conditions. Maximum etching rates of 8200 A/min in Cl2/Ar plasma and 8330 A/min in Cl2/N2 plasma are obtained as well. In addition, pressure, ICP power, Cl2/Ar(N2) flow ratio and rf power significantly influence etching rate and surface morphology. In particular, dc bias heavily influences the etching rates, suggesting that the ion-bombardment effect is an important factor of these etching processes.

Low-operation voltage of InGaN/GaN light-emitting diodes by using a Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN superlattice

Low-resistivity Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN strained-layer superlattices were grown. In these superlattices, the maximum hole concentration is 3/spl times/10/sup 18//cm/sup 3/ at room temperature. Hall-effect measurements indicate high conductivity of this structure in which the high activation efficiency is attributed to the strain-induced piezoelectric fields. This work also fabricated InGaN/GaN blue LEDs that consist of a Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN SLs. Experimental results indicate that the LEDs can achieve a lower operation voltage of around 3 V, i.e., smaller than conventional devices which have an operation voltage of about 3.8 V.

Low Temperature Activation of Mg-Doped GaN in O2 Ambient

In this study, Mg-doped GaN epitaxial layers were grown by metalorganic vapor phase epitaxy (MOVPE) and annealed in O2, air and N2. It was found that we could achieve a low-resistive p-type GaN by O2-ambient annealing at a temperature as low as 400°C. The resistivity and hole concentration of the 400°C O2-ambient annealed Mg-doped GaN was 2 Ω-cm and 3×1017 cm-3, respectively. These values are equivalent to those values obtained from Mg-doped GaN annealed in N2 ambient at 700°C.

Vertical High Quality Mirrorlike Facet of GaN-Based Device by Reactive Ion Etching

Reactive ion etching (RIE) of GaN has been performed using BCl3 and additives Ar, CH4, and N2 to BCl3 plasma. The etching rate, surface roughness and the etch profile have been investigated. When BCl3/Ar was used as the RIE plasma source with 200 W RF power and 60 mTorr pressure, the highest etching rates of 505 A/min and 448 A/min were obtained for n- and p-GaN, respectively. It was found that the addition of CH4 and N2 to the BCl3 plasma would result in significant changes of the etching results, such as the etching rate and surface morphology. It was also found that with the proper etching parameter, the mirrorlike facet of GaN can be obtained using BCl3/Ar/CH4/N2 by RIE.

X‐ray crystallographic study of GaN epitaxial films on Al2O3(0001) substrates with GaN buffer layers

Samples of GaN(0001) epitaxial films on sapphire Al2O3(0001) with different thicknesses of GaN buffer layers were characterized by x‐ray diffraction method in both in‐plane and plane‐normal directions. The results show that all the epitaxial films are of good quality with the GaN[1010] ∥Al2O3[1120] and GaN[1210]∥Al2O3[1100]. This arrangement of crystal orientation can be attributed to the chemical potential overriding the lattice spacing mismatch. The x‐ray results also indicate that the crystal coherence lengths in the in‐plane direction are smaller than those measured in the plane‐normal direction, i.e., a columnar‐like structure normal to the film is observed. The rocking curve widths in the in‐plane direction are also larger than those measured in the plane‐normal direction. In‐plane measurement of rocking curve and coherence length are essential physical quantities directly related to the electron mobility which was measured predominantly in the in‐plane direction. The best epitaxial structure is...

Improved contact performance of GaN film using Si diffusion

In this letter, we investigate a metalization process for reducing the contact resistance on undoped GaN layers. The Si metal source was diffused successfully into the GaN films by using SiOx/Si/GaN/Al2O3 structures. By using a high-temperature annealing process, we diffused and activated the Si atoms into the GaN film. This caused a heavy doped n-type GaN layer to be formed near the GaN surface. Under high temperatures, such as a diffusion process at 1000 °C, the as-deposited Ni/Al/Ti contact had good ohmic properties and a low specific contact resistivity (ρc) of 1.6×10−3 Ω cm2. Rapid thermal annealing the contact at 800 °C for 30 s caused the ρc to decrease rapidly to 5.6×10−7 Ω cm2. The Ni/Al/Ti contact characteristics on the GaN films diffused at various temperatures are also discussed.

W2B-based rectifying contacts to n-GaN

Schottky contact formation on n-GaN using a novel W2B∕Ti∕Au metallization scheme was studied using current-voltage, scanning electron microscopy and Auger electron spectroscopy measurements. A maximum barrier height of 0.55 eV was achieved on as-deposited samples, with a negative temperature coefficient of 8×10−4eV∕°C over the range 25–150 °C. There was also a negative temperature coefficient for the reverse breakdown of the Schottky diodes with W2B contacts. The barrier height was essentially independent of annealing temperature up to 500 °C and decreased thereafter due to the onset of metallurgical reactions with the GaN. The Ti began to out-diffuse to the surface at temperatures of >500°C. The reverse current magnitude was larger than predicted by thermionic emission alone.