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The valence band order of AlxGa 1−x N is investigated experimentally by analyzing the anisotropic dielectric functions of semipolar (112¯2) AlGaN thin films grown on m-plane Al2O3. Point-by-point fitted dielectric functions are obtained by spectroscopic ellipsometry and corresponding inter-band transition energies are extracted. The known strain situation of the sample layers is used to correct for the small strain-induced energy shifts within k · p perturbation theory. It also is used to identify transitions related to the three valence bands. Transitions with E ⊥ c from the Γ9 valence band verify an inter-band bowing parameter of b=0.9 eV. The transitions with E || c allow determining the crystal field splitting energy which can be described by a linear interpolation between the values for GaN and AlN satisfactorily.

Optically pumped deep-ultraviolet (DUV) lasing with low threshold was demonstrated from AlGaN-based multiple-quantum-well (MQW) heterostructures grown on sapphire substrates. The epitaxial layers were grown pseudomorphically by metalorganic chemical vapor deposition on (0001) sapphire substrates. Stimulated emission was observed at wavelengths of 256 nm and 249 nm with thresholds of 61 kW/cm2 and 95 kW/cm2 at room temperature, respectively. The thresholds are comparable to the reported state-of-the-art AlGaN-based MQW DUV lasers grown on bulk AlN substrates emitting at 266 nm. These low thresholds are attributed to the optimization of active region and waveguide layer as well as the use of high-quality AlN/sapphire templates. The stimulated emission above threshold was dominated by transverse-electric polarization. This work demonstrates the potential candidacy of sapphire substrates for DUV diode lasers.

Multiphonon resonant Raman scattering in non-polar a-plane (112¯0) GaN epitaxial layers grown on sapphire substrate were investigated. We report longitudinal optical phonon overtones up to seventh order in a-plane GaN epilayer, which has rarely been observed earlier in GaN. However, for high quality polar c-plane (0001) GaN epilayer and semi-polar r-plane (112¯2) GaN epilayer, resonant Raman spectra were not very prominent. Strong multiphonon resonant Raman scattering process is explained by exciton-mediated multiphonon Raman scattering and defect-induced Frohlich interaction.

We studied the optical polarization of surface-emitted photoluminescence from thick semi-polar (11–22) AlxGa1−xN layers on m-plane sapphire substrates with aluminum contents x between 0.0 and 0.63 at T = 10 K. Luminescence with an electric field vector E parallel to the in-plane direction [1–100] prevails for x 0.2. In case of low aluminum content, the spectra are dominated by basal plane stacking fault emission. The degree of optical polarization for both basal plane stacking fault emission and near band edge emission is comparable.

In group-III-nitride heterostructures with semipolar or nonpolar crystal orientation, anisotropic lattice and thermal mismatch with the buffer or substrate lead to a complex distortion of the unit cells, e.g., by shearing of the lattice. This makes an accurate determination of lattice parameters, composition, and strain state under assumption of the hexagonal symmetry impossible. In this work, we present a procedure to accurately determine the lattice constants, strain state, and composition of semipolar heterostructures using high resolution X-ray diffraction. An analysis of the unit cell distortion shows that four independent lattice parameters are sufficient to describe this distortion. Assuming only small deviations from an ideal hexagonal structure, a linear expression for the interplanar distances dhkl is derived. It is used to determine the lattice parameters from high resolution X-ray diffraction 2ϑ-ω-scans of multiple on- and off-axis reflections via a weighted least-square fit. The strain and co...

Silicon doping of AlxGa1−xN layers with high aluminum mole fractions (0.8

Excitonic emission in heteroepitaxially grown aluminum nitride (AlN) with reduced defect density due to the epitaxial lateral overgrowth (ELO) of patterned AlN/sapphire templates has been investigated by photoluminescence spectroscopy and compared to AlN/sapphire and homoepitaxially grown AlN. The ELO sample exhibits small linewidths of the free exciton and two different bound exciton emission bands. The free exciton emission energy is shifted by 58.5 meV with respect to unstrained homoepitaxially grown AlN attributed to compressive strain. A donor bound exciton D0X with an exciton localization energy of 13.0–13.5 meV is dominating in the photoluminescence spectra of ELO AlN/sapphire. This D0X does not show strong phonon replica and is dominant at elevated temperatures in ELO AlN/sapphire. The optical quality of heteroepitaxial AlN is significantly improved using the ELO technique and therefore suitable for high efficiency ultraviolet light emitters.

The influence of the dislocation density and the carrier-confining potentials on the temperature dependent behavior of the external quantum efficiency (EQE) of near-ultraviolet light-emitting diodes (LEDs) at different current densities has been investigated. The LED efficiencies were found to be very temperature sensitive with characteristic temperatures ranging from 48 to 207 K. In the low current density region the temperature dependence of the EQE is mainly dominated by the Shockley–Read–Hall recombination and hence the dislocation density of the device. In the high current density region, carrier leakage from the quantum wells is the main factor influencing the decrease in the EQE with ambient temperature, and in this region the LEDs become less temperature sensitive with increasing quantum well barrier height.

The effect of different Al(Ga)N electron blocking heterostructures (EBH) on the emission spectra and light output power of 290 nm light emitting diodes (LEDs) has been investigated. The carrier injection and internal quantum efficiency of the LEDs is simulated and compared to electroluminescence measurements. The highest light output power has been found for LEDs with an Mg-doped AlN/Al0.7Ga0.3N EBH with an AlN layer thickness >3 nm. The output power of these LEDs was 8.5-times higher compared to LEDs without EBH. This effect is attributed to an improved carrier injection and confinement which prevents electron leakage into the p-doped region of the LED with a simultaneously enhanced hole injection into the active region.

In this work the optical waveguiding in semipolar InGaN-based laser diodes is analyzed. Different designs of the separate confinement heterostructure with AlGaN or GaN cladding layers and GaN or InGaN waveguide layers are studied. The influence of waveguide material, thickness and composition on the optical confinement factor Γ, the accumulated strain energy E and the refractive index contrast is calculated. Measurements of the threshold and the far field intensity distributions of lasers with differing waveguide design confirm the predictions from model calculations. The optimum waveguide for a 410 nm single quantum well laser is found to consist of a symmetric In0.04Ga0.96N waveguide of 2×85 nm thickness with GaN cladding layers.

We present a transmission electron microscope (TEM) and x-ray diffraction (XRD) study of metalorganic vapor phase epitaxy grown GaN layers on m-sapphire with Al(Ga)N interlayers. The interlayers are demonstrated to be beneficial in reducing the stacking fault (SF) density by more than 2.5 orders of magnitude from >2 × 106 cm−1 to

In this study, the indium incorporation efficiency and critical layer thickness for misfit dislocation formation of thick (202¯1) oriented InGaN layers were investigated. InGaN layers with an indium content between 1.7% and 16% were grown by metalorganic vapor phase epitaxy. A reduced In-incorporation efficiency was found in comparison to (0001) oriented layers at 725°C. At lower growth temperature, the difference in incorporation efficiency between the two orientations is reduced. All (202¯1) InGaN layers, strained and relaxed with an In-content up to 12%, exhibit smooth surface morphologies with a rms roughness below 1 nm. In contrast to (0001) InGaN, hardly any strain is reduced by 3D-growth. The critical layer thickness for misfit dislocation formation of InGaN (202¯1) exhibits a behavior as predicted by the Matthews and Blakeslee model [J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974)]. Deviations, however, indicate that modifications of the formula are needed.

(112¯2) GaN layers were grown by metal-organic vapor phase epitaxy on (112¯2) bulk GaN substrates and (101¯0) sapphire substrates. The growth temperature was varied between 950 and 1050 °C and the total reactor pressure between 50 and 600 mbar. The growth conditions show a strong impact on the yellow band luminescence properties, while weak impact on the threading dislocation density was observed. The layer morphologies exhibit undulations with two periods along GaN [11¯00] and one period along [112¯3¯]. The different period lengths are connected to anisotropic adatom surface diffusion lengths. Arrow like features on the surface originate from the interference of the undulations along [112¯3¯] and [11¯00].

InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0 0) and on semipolar (1 0 2), (1 1 2), (1 0 1) as well as (2 0 1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0 1) < (1 1 2) = (0 0 0 1) < (2 0 1) < (1 0 0) = (1 0 2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0 0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0 0) and the semipolar (1 0 2) and (2 0 1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1 2) and (1 0 1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

The optical gain of single quantum well laser structures on semipolar (112¯2)-GaN in dependence of the optical polarization and the resonator orientation has been studied by variable stripe length method. The c′-[1123¯] resonator shows maximum gain in TE mode, followed by the m-[11¯00]-resonator with extraordinary polarization. The anisotropic gain behaviour is explained by valence sub-band ordering and birefringence of the wurtzite crystal, resulting in a modification of the transition matrix element for stimulated emission. Measurements are accompanied by 6 × 6 k · p band structure calculations and gain analysis.

The development of GaInN quantum well structures with nonpolar crystal orientation for light-emitting diodes and semiconductor lasers is currently one of the main foci of III-nitride-based optoelectronics research. One of the advantages of nonpolar orientations is the absence of polarization fields perpendicular to the quantum well plane. As a consequence, radiative recombination rates are higher compared to quantum wells on polar surfaces. However, due to high densities of threading dislocations and basal plane stacking faults in the case of heteroepitaxially grown nonpolar layers, and due to band gap inhomogeneities in the GaInN quantum wells, characterization of radiative and nonradiative recombination mechanisms is a complex challenge. So far, most published data about band gap fluctuations, charge carrier localization and internal quantum efficiency in nonpolar quantum wells are ambiguous. Here, we present temperature and excitation power density-dependent photoluminescence data featuring multiple characteristics related to strong charge carrier localization in m-plane (1–100) GaInN quantum wells. Thermally activated redistribution of charge carriers between localization sites in these quantum wells is weaker than in polar c-plane ones. The localization strength increases with higher indium concentration in the quantum wells. In the heteroepitaxially grown quantum well structures, the internal quantum efficiency is reduced even at low temperatures (T = 10 K) and especially for m-plane quantum wells with high indium mole fractions.

Polarized photoluminescence of strained quantum wells grown on c–plane, semipolar (101−2), (112−2), (101−1), (202−1) planes, and nonpolar GaN substrates was studied experimentally and in theory. The observed optical polarization switching between the substrate orientations (101−2) and (112−2) is in accordance with our general model of polarization switching, based on a k→·p→ model of arbitrary substrate orientation. Spectrally resolved measurements of the polarization degree stemming from (101−2) samples show that the maximum of the polarization degree is red–shifted with respect to the maximum of the photoluminescence intensity. We ascribe this effect to an increased polarization of the transitions for higher indium content.

For the realization and the improvement of GaN-based optoelectronic devices (light emitting diodes and laser diodes) emitting from the ultraviolet to the red wavelength range GaInN quantum well structures with high internal quantum efficiency are of great importance. To determine parameters which affect the internal quantum efficiency, we have analyzed the emission intensity of GaInN quantum well structures with varied electron and hole wave function overlap by temperature and excitation power dependent and by time-resolved photoluminescence. The quantum confined Stark effect reduces the temperature dependent photoluminescence emission intensity for thick polar quantum wells at low temperature. But near room temperature, these thick polar GaInN quantum wells feature less relative intensity loss than thinner polar quantum wells. This behavior can partially be assigned to increased screening effects and higher quantum well barriers for thicker quantum wells. Additionally, excitation power dependent photoluminescence points to a transition from a radiative recombination based on excitons at 10 K to a bimolecular recombination at room temperature for thick c-plane quantum wells. This transition may also affect the intensity decrease by a changed carrier diffusivity.

A correlation between the structural and optical properties of GaN thin films grown in the [112¯0] direction has been established using transmission electron microscopy and cathodoluminescence spectroscopy. The GaN films were grown on an r-plane sapphire substrate, and epitaxial lateral overgrowth was achieved using SiO2 masks. A comparison between the properties of GaN directly grown on sapphire and GaN laterally grown over the SiO2 mask is presented. The densities and dimensions of the stacking faults vary significantly with a high density of short faults in the window region and a much lower density of longer faults in the wing region. The low-temperature luminescence spectra consist of peaks at 3.465 and 3.41eV, corresponding to emission from donor-bound excitons and basal-plane stacking faults, respectively. A correlation between the structural defects and the light emission characteristics is presented.

Ultraviolet light-emitting diodes (LEDs) suffer from a low wall-plug efficiency, which is to a large extent limited by the poor light extraction efficiency (LEE). A thin-film flip-chip (TFFC) design with a roughened N-polar AlGaN surface can substantially improve this. We here demonstrate an enabling technology to realize TFFC LEDs emitting in the UVB range (280-320 nm), which includes standard LED processing in combination with electrochemical etching to remove the substrate. The integration of the electrochemical etching is achieved by epitaxial sacrificial and etch block layers in combination with encapsulation of the LED. The LEE was enhanced by around 25% when the N-polar AlGaN side of the TFFC LEDs was chemically roughened, reaching an external quantum efficiency of 2.25%. By further optimizing the surface structure, our ray-tracing simulations predict a higher LEE from the TFFC LEDs than flip-chip LEDs and a resulting higher wall-plug efficiency.