Grating-coupled external-cavity quantum-cascade lasers were studied for temperatures from 80 to 230 K. At 80 K, a tuning range of ∼65–88 nm are obtained for 4.5 and 5.1 μm laser amplifiers, respectively. The tuning ranges for both narrowed substantially with increasing temperature, to ∼23 nm at 203 K. The threshold varied slowly versus wavelength, while the efficiency appeared to be close to optimum toward wavelengths shorter than the free running wavelength.

Homoepitaxial, GaN films on both c-plane surfaces of bulk GaN crystals were examined using reflection high-energy electron diffraction (RHEED). Differences in the RHEED pattern, time development of the RHEED intensity, and surface reconstructions were observed. The substrate surfaces were prepared either by mechanical polishing [GaN(0001)A] or by chemo-mechanically polishing [GaN(0001̄)B]. Then films were grown by molecular beam epitaxy; Ga was provide by a Knudsen cell and nitrogen from NH3. On the B surface, the Ga rich reconstructions reported by Smith and co-workers [Phys. Rev. Lett. 79, 3934 (1997)] were observed. On the A surface, a (2×2) reconstruction was observed. Both reconstructions were much sharper than those seen on GaN films grown on sapphire. RHEED measurements of the specular intensity vs time showed that two different surface terminations could be maintained on the B surface, one of which is a stable, gallided surface, while the other is a nitrided surface, which is unstable in vacuum. If the nitrided surface is heated in vacuum it changes to the gallided surface in several minutes at 800 °C. Only one termination was detected on the A surface. The results are complemented by desorption mass spectroscopy measurements, and the resulting surfaces were then investigated using atomic force microscopy and scanning tunneling microscopy. We were able to distinguish the two surface terminations on the B surface, and a unique annealing process under NH3 will be documented. Preliminary investigation of the A surface revealed decorated step edges. The results were compared to films grown on sapphire with different nucleation layers, which can be grown to yield either polarity.

GaN(0001̄) films were grown by molecular beam epitaxy using ammonia and elemental Ga. The surface reactivity and growth kinetics of GaN(0001̄) were investigated as a function of growth parameters using desorption mass spectroscopy. Growth proceeds either by island nucleation or by step flow, depending on the steady state surface coverage of Ga. Three Ga adsorption states were found on the surface, one chemisorption and two weak states. One of the weak states corresponds to Ga adsorbed on a gallided surface, while the other corresponded to an intrinsic physisorption state on a hydrogen-passivated, nitrided surface. An abrupt growth mode transition between excess Ga and excess nitrogen was found as a function of growth parameters. The transition was modeled by rate equations based on growth at step edges and the three types of adsorption states.

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Hf and HfN thin films were grown on n-type GaN(0001̄) by molecular beam epitaxy using a custom built Hf electron beam source and an ammonia leak. The films were characterized by reflection high-energy electron diffraction (RHEED) and atomic force microscopy. It was found that epitaxial growth of Hf is possible even at room temperature. GaN films varying in thickness from 0.6 to 1.8 μm were grown on c-plane sapphire, using ammonia as a precursor, to serve as substrates. Then the films were annealed in ammonia as the temperature was lowered in order to produce a N termination. Hf was then deposited on top of the GaN at temperatures between 20 and 730 °C, both with and without ammonia incident on the sample. Deposition of pure Hf at room temperature revealed an epitaxial two-dimensional (2D) (although with some three-dimensional character) RHEED pattern with sixfold symmetry. The surface is reconstructed with a (2×2) R30° structure. We propose that the pattern is rotated 30° with respect to that of the substrate GaN because of a HfN interlayer between the GaN and Hf layers. When the films were annealed in vacuum up to 730 °C, the 2D pattern became more transmission like. If Hf was deposited at substrate temperatures of 350 °C and higher, a diffraction pattern corresponding to that of a polycrystalline material was observed.