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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

The onset of plastic relaxation via misfit dislocation (MD) formation in In x Ga 1− x N layers grown by metal-organic chemical vapor deposition on the ( 11 2 ¯ 2 ) semipolar plane of GaN substrates is investigated using high-resolution X-ray diffraction, transmission electron microscopy and cathodoluminescence. The results of critical thickness calculations for MD formation as a function of In x Ga 1− x N alloy composition x are compared with experimental observations. MD generation is observed initially as a result of slip on the (0001) slip plane, and subsequently as a result of additional slip on inclined { 1 1 ¯ 00 } - type m -planes, which eventually leads to an increase in threading dislocation density.

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.

Growth on relaxed buffers provides a potential route to reduce difficulties caused by high strain (∼ 3%) in the In 0.3 Ga 0.7 N active regions of green laser diodes (LDs). We demonstrate a blue LD with partially relaxed n- and p-side waveguide/cladding interfaces.

While electron holography in the transmission electron microscope offers the possibility to measure maps of the electrostatic potential of semiconductors down to nanometer dimensions, these measurements are known to underestimate the absolute value of the potential, especially in GaN. We have varied the dose rates of electron irradiation over several orders of magnitude and observed strong variations of the holographically detected voltages. Overall, the results indicate that the electron beam generates electrical currents within the specimens primarily by the photovoltaic effect and due to secondary electron emission. These currents have to be considered for a quantitative interpretation of electron holographic measurements, as their negligence contributes to large parts in the observed discrepancy between the measured and expected potential values in GaN.

Long-wavelength (>540 nm) single-quantum-well light-emitting diodes were grown pseudomorphically on stress-relaxed InxGa1−xN buffer layers on the orientation. Basal plane dislocation glide led to the formation of misfit dislocations at the bottom of the InxGa1−xN buffer layer. This provided one-dimensional plastic relaxation in the film interior, including the active region. For wider quantum wells (>3.5 nm), the efficiency of devices grown on relaxed buffer layers showed significant improvement compared to reference devices grown with no relaxation, indicating that degradation of thicker active regions can be prevented with partial relaxation via basal plane slip.

Several series of (202¯1) oriented InGaN/GaN heterostructures were grown to examine the impact of Si and Mg doping on stress relaxation by misfit dislocation formation. Si doping greatly reduced m-plane slip misfit dislocation lines as observed in cathodoluminescence, as well as reducing relaxation from c-plane slip as measured using x-ray diffraction reciprocal space maps. However, samples with the same degree of relaxation still showed reduced m-plane slip for the highly Si doped case. Mg doping showed a similar effect while experiments with Si–Mg co-doping reversed the effect.

Long wavelength (525–575 nm) (112¯2) light emitting diodes were grown pseudomorphically on stress relaxed InGaN buffer layers. Basal plane dislocation glide led to the formation of misfit dislocations confined to the bottom of the InGaN buffer layer. This provided one-dimensional plastic relaxation in the film interior, including the device active region. The change of the stress state of the quantum well due to one-dimensional plastic relaxation altered the valence band structure, which produced a significant shift in polarization of emitted light. Devices grown on relaxed buffers demonstrated equivalent output power compared to those for control samples without relaxation.

We have studied primary and secondary slip systems in the relaxation of lattice mismatch stresses in (202¯1) and (303¯1¯) semipolar InxGa1−xN/GaN heterostructures by analyzing the geometry of traces associated with dislocations employing cathodoluminescence, x-ray diffraction, and transmission electron microscopy. For (202¯1) InxGa1−xN/GaN heterostructures, the primary relaxation is by dislocation glide on the c-plane 112¯0(0001) slip system and secondary relaxation is by dislocation glide on inclined planes including the m-plane 〈112¯0〉{11¯00} slip system. For (303¯1¯) grown heterostructures non-basal slip, namely dislocation glide on the m-plane slip system, is the initial stress relaxation pathway.

We demonstrate an electrically injected semipolar (112¯2) laser diode (LD) grown on an intentionally stress relaxed n-In0.09Ga0.91N waveguiding layer. Detrimental effects of misfit dislocations (MDs) in the proximity of the active region were effectively suppressed by utilizing a p/n-Al0.2Ga0.8N electron/hole blocking layer between the dislocated heterointerfaces and the active region. The threshold current density of the LD was ∼20.3 kA/cm2 with a lasing wavelength of 444.9 nm. This LD demonstrates an alternative approach in semipolar AlInGaN LD waveguide design where the thickness and composition of the waveguiding and/or cladding layers are not limited by the critical thickness for MD formation.

In this paper, we report on observations of the operation of secondary slip systems to relieve lattice mismatch stress in semipolar InGaN/GaN heterostructures. Two-dimensional arrays of misfit dislocations were observed. Consistent with previous reports, primary relaxation occurred along the projected c direction via primary slip on the (0001) basal plane. In addition, evidence for secondary relaxation was detected in cathodoluminescence spectroscopy, high resolution x-ray diffraction, and transmission electron microscopy (TEM) studies. The secondary misfit dislocations were determined by TEM to have a-type Burgers vectors a/3 〈2¯110〉 and line directions along 〈42¯2¯3¯〉, consistent with prismatic slip on one of the m-type planes inclined with respect to the (112¯2) growth surface. Evidence of an additional slip system with approximate misfit line direction of type 〈202¯3¯〉 is also given.

Low-temperature aqueous solution deposition has been used for the first time to produce epitaxial ZnO layers on the semipolar (1011) surface of bulk GaN substrates and LEDs. Although the ZnO films have single in-plane and out-of-plane orientations, which are nominally the same as those of the (1011) GaN substrate, the ZnO lattice is observed to be slightly tilted with respect to that of the substrate. A (1011) light-emitting diode using an epitaxial ZnO film as a transparent current-spreading layer achieved a high external quantum efficiency of 48%.

The droop property of blue GaN light emitting diodes (LEDs) has been improved by creating a 9 quantum well (QW) LED on a patterned sapphire substrate (PSS). The droop ratio was improved from 45.9 to 7.6%. At a wavelength of 447 nm, and with standard on-header packaging, the 9QW PSS-LED had an output power of 27.6 mW and an EQE of 49.7% at a current of 20 mA. The output power of the 9QW PSS-LED remains linear with increasing drive current, even up to relatively high current density, and the EQE is almost constant.

The first 30-mW-class semipolar blue light-emitting diode (LED) on a free-standing (1011) GaN substrate has been demonstrated by using microscale periodic backside structures. The light extraction efficiency and corresponding output power were greatly enhanced, by up to 2.8-fold (bare chip) compare with conventional devices. At a driving current of 20 mA, the LED showed an output power of 31.1 mW and an external quantum efficiency of 54.7%. Semipolar GaN LED technology is now comparable to commercial c-plane blue LED technology, not only in terms of internal material properties but also in terms of chip processing techniques.

Using a transparent ZnO vertical stand as a submount, a novel Light-emitting diode architecture, which is similar to conventional lighting bulbs, was proposed. The emission power of a blue LED based on c-plane (0001) bulk GaN was increased by 14.2 and 5.1% compared with those of conventional and suspended die packages, respectively. The output power and external quantum efficiency of LEDs respectively reached 31.7 mW and 57.1% at a forward current of 20 mA under direct current conditions. The high thermal conductivity and refractive index of the transparent submount simultaneously resulted in high current operation and high external efficiency.

High power and high efficiency semipolar (3031) nitride light emitting diodes (LEDs), fabricated on low extended defect bulk GaN substrates, are reported for the first time. The LEDs were grown by metal organic chemical vapor deposition (MOCVD) at atmospheric pressure. The peak wavelength was 452 nm, and a minimal redshift of c-plane LEDs. The output power and external quantum efficiency (EQE) of the packaged 200 ×500 µm2 was 14.48 mW and 26.5%, respectively, at 20 mA.

The optical characteristics of InGaN/GaN light emitting diodes (LEDs) grown on (2021) bulk GaN substrates with wavelengths 469, 487, 510, and 528 nm were investigated. From 2 to 100 mA, the peak emission wavelength for green LEDs blue-shifted by 6.2 nm. Emission spectra widths were almost independent of the injection current. These results suggest that (2021) LEDs have a smaller polarization field and smaller indium fluctuations in the quantum wells. Optical polarization ratios for the LEDs varied between 0.4 and 0.5, which are larger than reported values. A weak dependence of the polarization ratio on the indium composition was observed.

The device structures of semipolar (1011) GaN blue light emitting diodes (LEDs) were optimized to achieve high power and high efficiency via metalorganic chemical vapor deposition (MOCVD). The quantum well (QW) width, barrier thickness and last barrier (LB) thickness were varied in order to optimize device performances and achieve the best growth conditions. Additional optimization methods such as Mg doping for the LB and p+ contact layers were also investigated. This study resulted in a LED with an output power of 22.75 mW and an external quantum efficiency (EQE) of 39.5% at a driving current of 20 mA, which is a significant improvement over previous results.