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The p-type doping of GaN with Mg, in particular doping of p++ cap layers and its influence on the resistivity of Ni/Au contacts on semipolar (20-21) GaN, has been investigated. For this purpose, we have compared GaN:Mg grown on several semipolar and polar orientations with respect to the acceptor concentration NA measured by electrochemical capacitance voltage techniques. For the same Mg precursor flow and Mg/III ratio, we observe very similar acceptor densities NA of up to 1 × 1019 cm−3 for (0001), (20-21), (20-2-1), and (11-22) GaN:Mg. Furthermore, the impact of the II/III ratio for the p++ cap layer ( NA > 1 × 1019 cm−3) on I- V characteristics of Ni/Au (20 nm/30 nm) contacts on (20-21) oriented GaN:Mg has been investigated. Ohmic I- V characteristics were observed for Mg/III ratios >1 × 10−2. Specific contact resistivities as low as 2.4 × 10−3 Ω cm2 could be achieved. Inclusion of this p++ cap layer resulted in reduced turn on voltages in light emitting diodes with 450 nm emission wavelength. [ABSTRACT FROM AUTHOR]

Abstract: We produced Mn-doped InAsP in metal-organic vapor phase epitaxy (MOVPE) grown InAsP. Good quality samples could be only prepared by heteroepitaxy on InP(001), while layers on GaAs showed very little smoothing after the initial roughening. The samples were characterized by X-ray diffraction, photoluminescence and Hall effect measurements. By varying the As content from 0% to 70% the ionization energy of the Mn-acceptor level varied from 220 to and drops into the valence band for higher concentrations. [Copyright &y& Elsevier]

Abstract: The influence of the (In)GaN quantum barrier growth temperature on the structural properties of the active region of blue–violet laser diodes has been investigated. Therefore, multiple quantum well structures were prepared by metal-organic vapor phase epitaxy, with (In)GaN barriers grown either at the same temperature as the InGaN quantum wells, or at a 60K higher growth temperature. A 60K higher growth temperature results in smoother quantum barrier layers with fewer defects on a microscopic scale, as revealed by scanning electron microscopy and cathodoluminescence studies. On the other hand samples with a 0K offset showed a higher strain in the quantum wells determined by X-ray diffraction and a narrower photoluminescence emission spectra. Secondly, laser heterostructures with InGaN barriers were prepared for optical pumping using different growth schemes. The laser structure with the active region deposited with a 0K offset exhibits significantly lower optical threshold power densities of around 200kW/cm² compared to 700kW/cm² for the 60K offset sample. Also, transmission electron microscopy revealed quantum wells with improved lateral uniformity in the samples grown without temperature offset. Based on these results, a current-injection broad area laser diode with an active region growth scheme without temperature offset was prepared on (0001) GaN substrate. The diode emits around 420nm with a threshold current density of around 1.6kA/cm². [Copyright &y& Elsevier]

We report on metal–organic vapor phase epitaxy (MOVPE) of (0001)InN layers simultaneously grown on a-plane (112¯0) and c-plane (0001) sapphire substrates. The substrates were nitridated at temperatures from 500°C to 1050°C prior to the growth of c-plane InN layers. Nitridation determined the polarity, the crystallinity, and the surface morphology of the InN layers. Nitridation temperatures above 800°C lead to N-polar InN layers, while nitridation temperatures from 700°C to 750°C produce mixed-polar InN layers, and nitridation temperatures from 500°C to 650°C produce In-polar InN layers. The roughness and crystallinity of the InN layers are correlated with the changes of polarity. The incorporation of nitrogen into the nitridation layers at different nitridation temperatures was measured. A strong N–Al bond signal after nitridation is correlated with N-polarity layers after overgrowth. [ABSTRACT FROM AUTHOR]

Abstract: The substrate miscut orientation of c-plane sapphire has an impact on the lateral growth and coalescence of AlN during epitaxial lateral overgrowth of patterned AlN/sapphire templates. A faster and more homogeneous coalescence was observed for substrates with a miscut orientation of 0.25° toward the m-direction compared to 0.25° toward the a-direction of the sapphire. A reduction of V/III ratio during overgrowth leads to an increase of lateral growth rate and therefore to a faster coalescence. The layer thickness at coalescence was clearly determined by in-situ reflectance measurements during the overgrowth. The coalescence can be controlled by carefully choosing the parameters influencing lateral growth like substrate miscut, process temperature and V/III ratio. [Copyright &y& Elsevier]

Abstract: We report on the growth of semipolar AlGaN in the entire composition range on sapphire by metal–organic vapor phase epitaxy. Growth rates increase linearly with the metal–organic supply and have been realized up to . The Al content determined by photoluminescence and transmission measurements depends linearly on the Al/Al+Ga ratio in the gas phase. The growth rate and the Al incorporation are higher for AlGaN than for (0001) AlGaN. This is attributed to different adatom mobility and desorption. The in-plane relationship of AlGaN is and [0001], as typical for semipolar nitrides on m-plane sapphire. For Al contents up to 60% triangle-like structures dominate the surface. For Al contents above 70% it transforms into an undulation along and additional dot-like structures. The smoothest surface morphologies with a rms roughnesses of 2nm were achieved for Al mole fractions of . [Copyright &y& Elsevier]

Abstract: We report on the growth of semipolar AlN by metal-organic vapor phase epitaxy on sapphire. High temperature nitridation and nucleation at a V/III ratio of 1050 provide single phase AlN with mirror like crack free surfaces, even for layer thicknesses larger than . Increasing reactor pressure results in lower growth rates because of gas phase particle formation. The in-plane relationship is and and therewith similar to MBE grown AlN and MOVPE and HVPE grown GaN on m-plane sapphire. The AlN reflex is anisotropically broadened in XRD. The surface shows an undulation along as typical for oriented nitride epilayers. Transmission spectroscopy measurements exhibit a sharp absorption edge at 5.95eV, which is 130meV shifted towards lower energies compared to c-plane AlN. [Copyright &y& Elsevier]

Abstract: The growth rates and aluminium contents of Al x Ga1−x N layers grown in a close-coupled showerhead reactor were investigated as a function of growth pressure and chamber height during metal-organic vapour phase epitaxy. The data show strong non-linear dependencies due to nanoparticle formation in the gas-phase. Good agreement between the experimental data and modeling results is obtained when the contribution of both Ga- and Al-containing species to the gas-phase particle formation is considered. [Copyright &y& Elsevier]

Abstract: Maskless epitaxial overgrowth of AlGaN on structured AlN templates was performed and the impact of stripe orientation on the lateral growth of AlGaN was studied. AlN/sapphire templates were patterned into stripes with wide bars and a period of . AlGaN and AlN growths were performed on the patterned templates. Two stripe orientations were investigated: stripes parallel to the and parallel to the directions. Coalescence was achieved for both stripe orientations. AlGaN layers grown on stripes oriented along the direction show a flat and closed surface in contrast to overgrown stripes oriented along the direction where the surface is facetted and rough. The Al content is strongly dependent on the growth facet and varies between 25% at the sidewalls and 50% on the c-facet. The overgrowth of stripes parallel to the direction with AlN shows coalescence and flat surfaces. The X-ray rocking curve full width at half-maximum of the reflection is reduced from 1000arcsec to 500arcsec for the overgrown AlN. Temperature dependent photoluminescence measurements of AlGaN:Si grown on these coalesced AlN templates also indicate a dislocation density reduction via this method. [Copyright &y& Elsevier]

Abstract: Epitaxial lateral overgrowth (ELOG) of a-plane GaN on r-plane sapphire with stripe orientations parallel to [0001], [], and [] has been investigated. The emission from ELOG stripes with different orientations and from different growth facets has been compared by spectrally and spatially resolved cathodoluminescence and by temperature-dependent photoluminescence measurements. A reduction in the nonradiative recombination rate by the ELOG process is clearly observed. For all stripe orientations the ELOG wing regions exhibit increased radiative emission intensities compared to the window areas, indicating a reduced dislocation density in the overgrown areas. The highest increase of the near band edge (NBE) emission is observed in the wing regions for stripes parallel to []. In this region, the density of basal plane stacking faults (BSFs) is also reduced. NBE emission is detected up to temperatures of 250K. [Copyright &y& Elsevier]

Abstract: In the present work we will report on the optical properties of an AlN film homoepitaxially grown on a high-quality large bulk AlN single crystal. The latter was grown by a sublimation-recondensation technique, while the film was grown by organometallic vapor-phase epitaxy. Cathodoluminescence measurements were performed using electron beam energies between 2 and 10keV in order to excite the sample and so to probe different sample depths, making it possible to differentiate between different features which originate in the AlN homoepitaxial film. The penetration depth has been determined through the calculation of the Bohr–Bethe maximum range of excitation using the approximation to the Everhart–Hoff expression for the energy loss within a solid. [Copyright &y& Elsevier]

Abstract: AlN layers were grown by metalorganic vapor-phase epitaxy at high temperatures up to 1500°C. Nucleation layer growth parameters and flow conditions before nucleation were changed and the effect on the AlN layer grown on top was studied. Structural analysis performed by high-resolution X-ray diffractometry and transmission electron microscopy showed that pregrowth conditions affect the material quality drastically. The best structural quality as indicated by a screw (including mixed) dislocation density of 8×108 cm−2, together with smooth surface morphology was found to result from simultaneous switching on of ammonia and TMAl at the beginning of nucleation layer growth. [Copyright &y& Elsevier]

Abstract: The critical thickness for the relaxation of InGaN layers grown on (0001)GaN on sapphire for an indium content between 10% and 20% has been determined experimentally. The layers were grown by metal-organic vapour phase epitaxy (MOVPE). The indium content was varied by changing growth temperature between 700 and 750°C. In-situ ellipsometry could identify a growth mode transition during layer growth, from relatively smooth InGaN layer to a rougher layer with higher indium content. X-ray diffraction found a completely strained layer with lower indium content and a completely relaxed layer with higher indium content. These findings were consistent with absorption and photoluminescence measurements. [Copyright &y& Elsevier]