We study the vibrational spectrum of AlN grown on Si͑111͒. The AlN was deposited using gas-source molecular beam epitaxy. Raman backscattering along the growth c axis and from a cleaved surface perpendicular to the wurtzite c direction allows us to determine the E 2 1 , E 2 2 , A 1 ͑TO͒, A 1 ͑LO͒, and E 1 ͑TO͒ phonon energies. For a 0.8-m-thick AlN layer under a biaxial tensile stress of 0.6 GPa, these are 249.0, 653.6, 607.3, 884.5, and 666.5 cm Ϫ1 , respectively. By combining the Raman and x-ray diffraction studies, the Raman stress factor of AlN is found to be Ϫ6.3Ϯ1.4 cm Ϫ1 /GPa for the E 2 2 phonon. This factor depends on published values of the elastic constants of AlN, as discussed in the text. The zero-stress E 2 2 energy is determined to be 657.4 Ϯ0.2 cm Ϫ1. Fourier-transform infrared reflectance and absorption techniques allow us to measure the E 1 ͑TO͒ and A 1 ͑LO͒ phonon energies. The film thickness ͑from 0.06 to 1.0 m͒ results in great differences in the reflectance spectra, which are well described by a model using damped Lorentzian oscillators taking into account the crystal anisotropy and the film thickness.

We describe the growth of hexagonal GaN on Si(111) by gas source molecular beam epitaxy with ammonia. The initial deposition of Al, at 1130–1190 K, resulted in a very rapid transition to a two-dimensional growth mode of AlN. The rapid transition is essential for the subsequent growth of high quality GaN and AlGaN. This procedure also resulted in complete elimination of cracking in thick (>2 μm) GaN layers. For layers thicker than 1.5 μm, the full width at half maximum of the (0002) GaN diffraction peak was less than 14 arc sec. We show that a short period superlattice of AlGaN/GaN grown on the AlN buffer can be used to block defects propagating through GaN, resulting in good crystal and luminescence quality. At room temperature, the linewidth of the GaN exciton recombination peak was less than 40 meV, typical of the best samples grown on sapphire.

We report a study of the luminescence properties of coherently strained GaAs1−xNx grown on GaAs by metalorganic molecular beam epitaxy. Well-defined photoluminescence was observed in samples with a nitrogen concentration up to 3%. Samples subjected to thermal anneals, investigated by x-ray diffraction and photoluminescence, show increased nitrogen incorporation and significant improvements in the luminescence efficiency. A band-gap reduction of more than 400 meV, compared to GaAs, is observed for a nitrogen concentration of ∼3%. For the range of nitrogen concentrations investigated here, the band gap follows predictions of the dielectric model of Van Vechten [J. A. Van Vechten and T. K. Bergstresser, Phys. Rev. B 1, 3351 (1970), and references therein].

The reduction of efficiency droop by Al0.82In0.18N/GaN superlattice electron blocking layer in (0001) oriented GaN-based light emitting diodes Ultraviolet light-emitting diodes operating at 280 nm, grown by gas source molecular-beam epitaxy with ammonia, are described. The device is composed of n-and p-type superlattices of AlN͑1.2 nm thick͒/AlGaInN͑0.5 nm thick͒ doped with Si and Mg, respectively. With these superlattices, and despite the high average Al content, we obtain hole concentrations of (0.7-1.1)ϫ10 18 cm Ϫ3 , with the mobility of 3-4 cm 2 /V s and electron concentrations of 3ϫ10 19 cm Ϫ3 , with the mobility of 10-20 cm 2 /V s, at room temperature. These carrier concentrations are sufficient to form effective p -n junctions needed in UV light sources.

Raman spectra of coherently strained layers of GaAs 1Ϫx N x grown on ͑001͒ GaAs with xϭ0 -0.05 by metalorganic molecular-beam epitaxy are reported. The optical phonons of the GaAs and GaN types, as well as disorder-activated acoustical phonons, are observed. A strongly confined GaAs optical mode at ϳ255 cm Ϫ1 , indicating the ordering of As and N atoms, is also detected. The GaAs-and GaN-type optical phonons exhibit strong diagonal components, forbidden for the zinc-blende structure. A bond polarizability analysis of the Raman selection rules shows that these components are activated by the trigonal distortion of the alloy lattice. The trigonal distortion arises from the formation of ordered ͕111͖-(GaN) m (GaAs) n clusters with nϭmϭ1.

We report direct-backscattering Raman studies of GaAs1−xNx alloys, for x⩽0.03, grown on (001) GaAs. The Raman spectra exhibit a two-mode behavior. The allowed GaAs-like longitudinal-optic phonon near 292 cm−1 is found to red shift at a rate of −136±10 cm−1/x. This is well described by the combined effects of strain and alloying. The GaN-like phonon near 470 cm−1 is observed to increase in intensity in direct proportion to x, and to systematically blue shift at a rate of 197±10 cm−1/x. This blue shift is likewise attributed to strain and alloying. The GaAs-like second-order features are also seen to broaden slightly and diminish in intensity with increasing nitrogen concentration. These results are attributed to a weak breakdown in the zincblende-crystal long-range order, possibly related to the presence of ordered domains within the random alloy.

Hexagonal AlN layers were grown on Si(111) by gas-source molecular-beam epitaxy with ammonia. The transition between the (7×7) and (1×1) silicon surface reconstructions, at 1100 K, was used for in situ calibration of the substrate temperature. The initial deposition of Al, at 1130–1190 K, produced an effective nucleation layer for the growth of AlN. The Al layer also reduced islands of SiNx that might be formed due to background NH3 on the silicon surface prior to the onset of epitaxial growth. The transition to two-dimensional growth mode, under optimum conditions, was obtained after the initial AlN thickness of ∼7 nm.

We report a systematic study of the optical and electrical properties of deep ultraviolet light emitting diodes based on digital alloy structures of AlN/Al0.08Ga0.92N grown by gas source molecular beam epitaxy with ammonia. Digital alloys are formed by short period superlattices consisting of Al0.08Ga0.92N wells, 0.50 or 0.75 nm thick, and AlN barriers, 0.75 to 1.5 nm thick. For digital alloys with effective bandgap of 5.1 eV, average AlN composition 72%, we obtain room temperature electron concentrations up to 1×1019 cm-3 and resistivity of 0.005 Ω·cm and hole concentrations of 1×1018 cm-3 with resistivity of 6 Ω·cm. Light emitting diodes based on digital alloys are demonstrated operating in the range of 250 to 290 nm.