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We report the growth of green emitting InGaN quantum wells (QWs) by metal-organic vapor-phase epitaxy (MOVPE) on three-dimensional GaN templates. The {10 (1) over bar1} facets of GaN pyramids, fabricated by selective-area growth (SAG), reduce the influence of the quantum-confined Stark effect (QCSE) on the emission properties of the QW. The luminescence properties of a QW are a good indicator for the crystalline quality of the GaN layer beneath. Especially the presence of voids inside the pyramids, as well as stacking faults (SFs) and threading dislocations (TDs) in the wing region strongly influence the strain situation and the incorporation of In into the overlying InGaN layer. Thus, the crystal quality of the GaN pyramid has a strong influence on the efficiency and the emission properties of the active region. Therefore, a low-temperature nucleation on the GaN buffer in conjunction with a decreasing Ga-flux taking the decreasing c-plane growth surface of the pyramid into account was introduced. The low-temperature photoluminescence (PL) properties of the InGaN QW reveal the differences between the standard formation of the pyramid and this modified growth. Cathodoluminescence (CL) and transmission electron microscopy (TEM) measurements confirm the differences between the two growth modes in the crystal quality of the inner part of the pyramid. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The optical and crystalline properties of a c-plane GaN-based LED structure with embedded semipolar InGaN quantum wells (QW) were investigated using highly spatially resolved cathodoluminescence spectroscopy (CL) directly performed in a scanning transmission electron microscope (STEM). Direct correlation of the cross-sectional STEM image with the simultaneously recorded spatially resolved CL mapping at room-temperature reveals the most intense emission coming from the semipolar InGaN QWs. We observe an inhomogeneous wavelength distribution due to local indium fluctuations and varying QW thickness. In contrast, the donor-acceptor pair recombination (DAP) becomes the dominating luminescence process at 16 K resulting in a superposition of the DAP luminescence and the InGaN QW emission.

In GaN, the basal plane stacking fault type I1 is a two-dimensional defect characterized by a cubic inclusion within the wurtzite structure. Excitons are bound at the BSF I1 similar to the localization in a quantum well heterostructure leading to an efficient radiative recombination. In this study, we present the optical and structural properties of basal plane stacking faults occurrent in silicon doped a-plane GaN layer by means of highly spatially resolved cathodoluminescence spectroscopy performed in a scanning transmission electron microscope. Drastically reduced panchromatic intensity in the vicinity of partial dislocations terminating the stacking faults, points to their non-radiative character. Originating from intersection of two-dimensional defects, the emission at 379.6 nm could be attributed to optically active stair-rod dislocations.

GaN based laser diodes with semipolar quantum wells are typically grown on free-standing pseudo-substrates of small size. We present an approach to create a distributed-feedback (DFB) laser with semipolar quantum wells (QWs) on c-oriented templates. The templates are based on 2-inch sapphire wafers, the method could easily be adapted to larger diameters which are available commercially. GaN nanostripes with triangular cross-section are grown by selective area epitaxy (SAE) and QWs are grown on their semipolar side facets. The nanostripes are completely embedded and can be sandwiched inside a waveguide. For optical pumping, open waveguide structures with only a bottom cladding are used. Using nanoimprint lithography, stripe masks with 250 nm periodicity were fabricated over the whole wafer area. The periodicity corresponds to a 3rd order DFB structure for a laser emitting in the blue wavelength regime. These samples were analyzed structurally by high-resolution transmission electron microscopy (HRTEM), and spatio-spectrally by cathodoluminescence (CL) inside a scanning transmission electron microscope (STEM). Samples with an undoped cap are pumped optically for stimulated emission. To prove the feasibility of realizing a 2nd order DFB structure with this approach, stripes with a 170 nm periodicity are fabricated by electron beam lithography and SAE.

In this paper, we compare the effectiveness of two different methods regarding the reduction of defect densities in heteroepitaxially grown a-plane GaN by heavy Si doping. The insertion of well-established in situ Si x N y nanomasks leads to locally heavy Si δ-doped GaN. By increasing the Si x N y deposition time in the range from 0 to 300 s the full width at half maxima (FWHM) of the X-ray diffraction ω-scans at in-plane GaN(1100) and GaN(0002) Bragg reflections decreases from 0.55° to 0.24° and from 0.45° to 0.16°, respectively. When growing without any Si x Ny interlayer but instead with continuously heavy Si-doping, these values are further decreased to 0.13° and 0.15°, respectively. By measuring several higher order reflections and detailed evaluation of the ω-scan broadening in Williamson―Hall-plots (WHPs) a considerable reduction in defect densities and no hint of basal plane stacking faults (BSFs) were found for the heavy Si doped a-plane GaN sample. To verify this result the micro-structural properties of this sample were additionally investigated by transmission electron microscopy and cathodoluminescence (CL).

AlGaN layers grown by MOCVD on Si(111) substrates exhibit spontaneously formed AlN/GaN superlattices along the growth direction. The phenomenon was found by high-resolution X-ray diffraction and transmission electron microscopy. Besides the main superlattice peak X-ray diffraction spectra typically show four satellite reflections. TEM images reveal a homogeneous lateral distribution of the SL. The vertical extension is equal to the total layer thickness. The SL persists upon variation of any relevant growth parameters. The period of the SL was found to correlate with the growth rate and the position of the main SL reflection corresponds to the TMAl/TMGa-ratio in the gas phase.

Carbon-doping in the concentration range from [C] = 5 × 1017 to 1.2 × 1019 cm−3 is employed to achieve semi-insulating properties of GaN layers as required for electronic power devices. Using propane as a carbon precursor, an independent analysis of the carbon incorporation during growth and its impact on electrical properties of the layers was obtained as growth parameters for optimum GaN quality could be applied. We observe that C is within precision of measurements fully incorporated in GaN as compensating deep acceptor. In a series of Si + C co-doped samples, semi-insulating properties were obtained for [C] > [Si] and the compensation efficiency for electrons is around unity. Through the extrinsic C-doping technique previous ambiguous results on electrical and optical properties of GaN:C layers are clarified.