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Multiple surface reconstructions have been observed on ultra-thin GaN (0001) layers of 1–10 nm thickness, covering a 3 nm thick In0.11Ga0.89N single quantum well in a GaN matrix. Low energy electron diffraction patterns show (2×2) and (√3×√3)-R30° symmetries for samples annealed in nitrogen plasma, and (2×2), (3×3), (4×4), and (6×6) symmetries for samples overgrown with an additional monolayer-thin GaN film by molecular beam epitaxy under Ga-rich growth conditions. Photoelectron spectroscopy shows that the InGaN quantum wells and capping layers are stable for growth temperatures up to 760 °C, and do not show formation of indium or gallium droplets on the surface. The photoluminescence emission from the buried InGaN SQWs remains unchanged by the preparation process, demonstrating that the SQWs do not undergo any significant modification.

InxGa1−xN/GaN multiple quantum well structures (x = 0.13 and 0.18) embedded into p–i–n diodes on ( 20 2 ¯ 1 ¯) and ( 20 2 ¯ 1) oriented GaN substrates were investigated by electroreflectance, photocurrent, and electroluminescence. Transition energies in absorption and emission experiments were measured as a function of the polarization orientation of light and applied bias voltage. The results were analyzed by a perturbation theoretical model to determine polarization fields. For the ( 20 2 ¯ 1 ¯) sample (x = 0.18), the flatband voltage is found at + 1 V corresponding to a polarization field of − 458 kV / cm. For the ( 20 2 ¯ 1 ) sample (x = 0.13), the polarization field is estimated to be ≈ + 330 kV / cm at flatband voltage higher than turn-on voltage of this light emitting diode.InxGa1−xN/GaN multiple quantum well structures (x = 0.13 and 0.18) embedded into p–i–n diodes on ( 20 2 ¯ 1 ¯) and ( 20 2 ¯ 1) oriented GaN substrates were investigated by electroreflectance, photocurrent, and electroluminescence. Transition energies in absorption and emission experiments were measured as a function of the polarization orientation of light and applied bias voltage. The results were analyzed by a perturbation theoretical model to determine polarization fields. For the ( 20 2 ¯ 1 ¯) sample (x = 0.18), the flatband voltage is found at + 1 V corresponding to a polarization field of − 458 kV / cm. For the ( 20 2 ¯ 1 ) sample (x = 0.13), the polarization field is estimated to be ≈ + 330 kV / cm at flatband voltage higher than turn-on voltage of this light emitting diode.

Due to changes in the spontaneous and piezoelectric polarization, AlGaN/GaN heterostructures exhibit strong polarization fields at heterointerfaces. For quantum wells, the polarization fields lead to a strong band bending and a redshift of the emission wavelength, known as quantum-confined Stark effect. In this paper the polarization fields of thin AlGaN layers in a GaN matrix were determined by evaluating the changes in the depletion region width in comparison to a reference sample without heterostructure using capacitance-voltage-measurements. The polarization fields for Al0.09Ga0.91N (0.6 +/- 0.7 MV cm(-1)), Al0.26Ga0.74N (2.3 +/- 0.6 MV cm(-1)), Al0.34Ga0.66N (3.1 +/- 0.6 MV cm(-1)), Al0.41Ga0.59N (4.0 +/- 0.7 MV cm(-1)) and Al0.47Ga0.53N (5.0 +/- 0.8 MV cm(-1)) heterostructures were determined. The results of the field strength and field direction of all samples are in excellent agreement with values predicted by theory and a capacitance-voltage based Poisson-carrier transport simulation approach giving experimental evidence for a nonlinear increasing polarization field with Al-concentration. (C) 2019 The Japan Society of Applied Physics

We investigate the influence of inhomogeneous broadening on the optical polarization properties of high-inclination semipolar and nonpolar ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$/GaN quantum wells. Different planar m-plane and $(20\overline{2}\overline{1})$ samples were grown (including core-shell microrods) and have been characterized by excitation-dependent polarization-resolved confocal micro-photoluminescence. The measured degree of linear polarization (DLP) is compared to theoretical predictions obtained by Fermi-Dirac statistical filling of the electronic band structure calculated by the $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ envelope function method. We show that our measured DLP at room temperature, as well as values reported by other groups, are systematically higher than the theoretical predictions. We propose to solve this discrepancy between theory and experiment by introducing inhomogeneous broadening in our calculations. Considering indium content fluctuations and the localization lengths of electrons and holes, different effective broadenings are applied to different subsets of subbands. We thereby show that inhomogeneous broadening leads to an increase of the DLP at room temperature. Furthermore, the dependence of the optical properties on the excitation density is better reproduced. Looking at the DLP as a function of the temperature gives us insight into the thermalization dynamics of charge carriers.

In the framework of k · p-theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with calculations. We observe a strong correlation between DLPs and spectral widths, which indicates that inhomogeneous broadening affects the optical polarization properties. Considering indium content fluctuations, QW thickness fluctuations, and the localization length of charge carriers, we argue that different broadenings apply to each subband and introduce a formalism using effective masses to account for inhomogeneous broadening in the calculation of the DLP. We conclude that the different DLP of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) QWs might be related to different effective broadenings of their valence subbands induced by the rougher upper QW interface in ( 20 2 ¯ 1 ), by the larger sensitivity of holes to this upper interface due to the polarization field in ( 20 2 ¯ 1 ), and/or by the different degrees of localization of holes.In the framework of k · p-theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with...

The polarization fields in wurtzite group III-nitrides strongly influence the optical properties of InAlGaN-based light emitters, e.g., the electron and hole wave function overlap in quantum wells. In this paper, we propose a new approach to determine these fields by capacitance-voltage measurements (CVM). Sheet charges generated by a change of the microscopic polarization at heterointerfaces influence the charge distribution in PIN junctions and therefore the depletion width and the capacitance. We show that it is possible to determine the strength and direction of the internal fields by comparing the depletion widths of two PIN junctions, one influenced by internal polarization fields and one without as a reference. For comparison, we conducted coupled Poisson/carrier transport simulations on the CVM of the polarization-influenced sample. We also demonstrate the feasibility and limits of the method by determining the fields in GaN/InGaN and GaN/AlGaN double heterostructures on (0001) c-plane grown by metal organic vapor phase epitaxy and compare both evaluation methods. The method yields (−0.50 ± 0.07) MV/cm for In0.08Ga0.92N/GaN, (0.90 ± 0.13) MV/cm for Al0.18Ga0.82N/GaN, and (2.0 ± 0.3) MV/cm for Al0.31Ga0.69N/GaN heterostructures.