Roman Dimitrov

OPTICAL CONSTANTS OF EPITAXIAL ALGAN FILMS AND THEIR TEMPERATURE DEPENDENCE

We have studied the dependence of the absorption edge and the refractive index of wurtzite AlxGa1−xN films on temperature and composition using transmission and photothermal deflection spectroscopy. The Al molar fraction of the AlxGa1−xN films grown by plasma induced molecular beam epitaxy was varied through the entire range of composition (0⩽x⩽1). We determined the absorption edges of AlxGa1−xN films and a bowing parameter of 1.3±0.2 eV. The refractive index in the photon energy range between 1 and 5.5 eV and temperatures between 7 and 295 K was deduced from the interference fringes. The static refractive index n(0) changed from 2.29 for GaN to 1.96 for AlN at room temperature. A variation of temperature from 295 to 7 K resulted in a decrease of refractive index (at photon energies close to the band gap) by 0.05±0.01 and in an energy shift of the absorption edge of about 64±5 meV independent of the Al content of the films. Using the Kramers–Kronig dispersion relation and an approximation for the dispersion coefficient for photon energies near the band gap, the refractive index could be described as a function of photon energy, Al content, and temperature.

Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm/sup 2//Ns, respectively, For electron sheet density near 1/spl times/10/sup 13//cm/sup 2/, Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f/sub t/ of 106 GHz for 0.15 /spl mu/m gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of /spl sim/4 W/mm, while large periphery devices have /spl sim/2 W/mm, both thermally limited. Performance, without and with Si/sub 3/N/sub 4/ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates.

Nitrogen Effusion and Self-Diffusion in Ga14N/Ga15N Isotope Heterostructures

Ga14N/Ga15N/Ga14N isotope heterostructures are used to study nitrogen self-diffusion by secondary-ion mass spectrometry and thermally activated decomposition. After interdiffusion of Ga14N and Ga15N layers at temperatures between 770°C and 970°C the diffusion profiles are measured. The isotope heterostructures are particularly well suited for self-diffusion studies because the diffusion takes place at the interface inside the GaN crystal, and therefore the analysis is free from perturbations such as surface electric fields, mechanical stress or chemical potential gradients. The temperature dependence of the nitrogen self-diffusion coefficient (D) in hexagonal GaN was determined to be 1600 cm-2 s-1 exp [(-4.1±0.4) eV/kBT], leading to a self-diffusion entropy SSD of about 10kB. The nitrogen flux through an isotope interface is compared with the nitrogen loss from a free GaN surface in vacuum above the decomposition temperature, to obtain information about the diffusion kinetics relevant for epitaxial growth and high temperature device applications.

Power limits of polarization-induced AlGaN/GaN HEMT's

The present and predicted limits on microwave power performance of undoped AlGaN/GaN HEMTs are presented, based on measured frequency response and drain-source breakdown voltage, both as functions of gate length. The spontaneous and piezoelectric polarization that induce the 2DEG in these HEMTs are covered. Process methods, including Si/sub 3/N/sub 4/ passivation are included. Thermal simulation results are shown for heat dissipation that limits channel temperature to 300/spl deg/C. Microwave cw power density limits of 12.5 W/mm at 10 GHz are predicted for class A operation on thick SiC substrates.