N. C. Chen

Band Offsets of InN/GaN Interface

In this paper, we report on the band discontinuities of the wurtzite-InN/GaN interface. X-ray photoemission spectroscopy studies reveal that the offset ratios of conduction bands and valence bands are approximately 80 and 20%, respectively. The valence band offset (0.5 eV) is close to the theoretical value determined on the basis of the density functional theory from first principle that was reported by Wei and Zunger [Appl. Phys. Lett. 69 (1996) 2719]. The photoluminescence signals of InN/GaN quantum wells were also studied. The luminescence of the wells showed a 60 meV quantum confinement shift from the bulk InN signal. The finite potential well model of quantum mechanics is used to show that this shift supports the above results.

Blue, Green and White InGaN Light-Emitting Diodes Grown on Si

High-brightness InGaN light-emitting diodes (LEDs) have been grown on Si substrates by metal-organic vapor phase epitaxy. Both blue and green LEDs with an output power exceeding 0.7 mW and a lifetime exceeding 500 h were achieved. Two factors were analyzed: the difference between identical LED structures grown on Si and sapphire substrates, and the effect of cracking on the performance of LEDs grown on Si. Using a Si substrate, the LED emission showed a shift toward longer wavelength compared with that from a LED grown on sapphire. The presence of cracking, unless serious, showed little effect on the brightness and performance of the LEDs grown on Si. The mechanisms involved are discussed.

Magnesium Doping of In-rich InGaN

InN and In-rich InGaN were grown by metal organic vapor phase epitaxy with magnesium doping. A set of samples were grown at 550 °C, whereas a second set of samples were grown at increasing temperature with Ga content. Upon annealing, p-InGaN was obtained from the second set up to an In content above 50%, with an acceptor concentration of ~1×1019 cm-3 and a mobility of 1–2 cm2 V-1 s-1. None of the samples grown at a constant temperature of 550 °C showed a p-behavior after heat treatment. The electrical, optical, structural and morphological characteristics of the films grown were analyzed, and the leveling off of hole concentration beyond an In content of 30% was consistent with the reported decreasing activation energy of Mg with increasing In content.

Concentration dependence of carrier localization in InN epilayers

The authors studied the concentration dependence of carrier localization in InN epilayers using time-resolved photoluminescence (PL). Based on the emission-energy dependence of the PL decays and the PL quenching in thermalization, the localization energy of carriers in InN is found to increase with carrier concentration. The dependence of carrier concentration on the localization energy of carriers in InN can be explained by a model based on the transition between free electrons in the conduction band and localized holes in the deeper tail states. They suggest that carrier localization originates from the potential fluctuations of randomly located impurities.

Electron transport in In-rich InxGa1−xN films

We have performed electrical transport measurements on metal-organic vapor phase epitaxy grown In-rich InxGa1−xN (x=1, 0.98, and 0.92) films. Within the experimental error, the electron density in InGaN films is temperature independent over a wide temperature range (4K⩽T⩽285K). Therefore, InxGa1−xN (0.92⩽x⩽1) films can be regarded as degenerate semiconductor systems. The experimental results demonstrate that electron transport in In-rich InxGa1−xN (x=1, 0.98, and 0.92) films is metalliclike. This is supported by the temperature dependence of the density, resistivity, and mobility which is similar to that of a metal. We suggest that over the whole measuring temperature range residue imperfection scattering limits the electron mobility in In-rich InxGa1−xN (x=1, 0.98, and 0.92) films.

AlGaN films grown on (0001) sapphire by a two-step method

A two-step growth method, commonly used for GaN on sapphire, was applied to grow high-quality Al0.2Ga0.8N on sapphire. Comparing to the one grown on a low-temperature grown AlN buffer layer, the decomposition, recrystallization, and islands coalescence processes of the two-step growth increased the surface flatness, the crystal quality, the electrical property, suppressed the phase separation, and released the biaxial tensile strain. A 2.0μm thick high-quality crack-free nearly GaN-free Al0.2Ga0.8N epilayer was obtained.

Schottky behavior at InN–GaN interface

In this work, GaN Schottky diodes were fabricated by depositing InN on GaN surfaces. The junction between these two materials exhibits strong rectifying behavior. The barrier heights were determined to be 1.25 eV, 1.06 eV, and 1.41 eV by current-voltage, current-voltage-temperature, and capacitance-voltage methods, respectively. These values exceed those of any other metal∕GaN Schottky barriers. Therefore, the conduction-band offset between InN and GaN should not be smaller than the barrier heights obtained here.

Application of modified transmission line model to measure p-type GaN contact

This work presented a procedure for extending the modified transmission line model to measure non-ohmic contact. This method was applied to the p-type GaN contact with the resulting sheet resistance similar to that determined by the Hall measurement. The voltage–current density (V–J) curve obtained using this procedure was also similar to that by directly analyzing the current–voltage curve of a light-emitting diode. Both results revealed the validity of this procedure. Rather than yielding a specific contact resistance for an ohmic contact, this procedure yielded a V–J curve to describe the non-ohmic contact characteristics. Similarly, this procedure could also extend the linear transmission line model to the analysis of non-ohmic contacts.

Nitride light-emitting diodes grown on Si (111) using a TiN template

Nitride light-emitting diodes (LEDs) are grown on a Si (111) substrate with a TiN template. Transmission electron microscopy and x-ray diffraction indicate that the epitaxial relation follows Si(1,1,1)‖TiN(1,1,1)‖AlN(0,0,1), Si[1,1,0]‖TiN[1,1,0], and Si[0,0,1]‖TiN[0,0,1]. The reflectance measurement and simulation results indicate that the TiN can be adopted as a reflector to mitigate the substrate absorption problem, thus increasing the extraction efficiency of nitride LEDs grown on Si.

Modified transmission line model and its application to aluminum ohmic contacts with n-type GaN

A modified transmission line model (MTLM) of ohmic contact measurement is presented. This model preserves the simplicity of the circular transmission line model but eliminates the shortcoming of the possibility of obtaining misleading results. This model was applied to n-type GaN ohmic contacts and results similar to those obtained by Hall measurement were obtained. The ohmic contact pattern used in MTLM method occasionally exists during the fabrication of several devices. In such cases, the method can be used to determine the device processing quality without the need for any other test pattern.