C. McAleese

Improvements in a-plane GaN crystal quality by a two-step growth process

Nonpolar (112¯0) a-plane GaN films have been grown by metal-organic vapor deposition on r-plane (11¯02) sapphire. Lateral growth is favored using a low V:III ratio resulting in films with a smooth surface, while pitted films are grown at a high V:III ratio indicating preferential on-axis growth. High-resolution x-ray diffraction analysis of both film types showed a strong anisotropy in the peak width of the symmetric omega rocking curve with respect to the in-plane orientation, phi. In-plane isotropic behavior of crystallinity with overall reduced omega full width at half maximum values was achieved when the growth was initiated at a high V:III ratio before reducing the V:III ratio for film coalescence. An improvement of crystal quality through initial surface roughening was equally realized by the incorporation of partial-coverage SiNx interlayers.

The consequences of high injected carrier densities on carrier localization and efficiency droop in InGaN/GaN quantum well structures

There is a great deal of interest in the underlying causes of efficiency droop in InGaN/GaN quantum welllight emitting diodes, with several physical mechanisms being put forward to explain the phenomenon. In this paper we report on the observation of a reduction in the localization induced S-shape temperature dependence of the peak photoluminescence energy with increasing excitation power density. This S-shape dependence is a key fingerprint of carrier localization. Over the range of excitation power density where the depth of the S shape is reduced, we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e., efficiency droop. Hence, the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalization. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localization due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the S-shape temperature dependence to the saturation of the available localized states. We propose that this saturation of the localized states is a contributory factor to efficiency droop whereby nonlocalized carriers recombine non-radiatively.

Equilibrium critical thickness for misfit dislocations in III-nitrides

The critical thickness gives the transition point between fully strained and relaxed heteroepitaxial films and determines the onset of defect generation, including misfit dislocations, cracks, and V-pits. An important variable in critical thickness calculations concerning misfit dislocations is the dislocation energy. It consists of two contributions: the elastic energy of the bulk material around a dislocation and the energy of the dislocation core. The latter part is often neglected. Recent atomistic calculations have estimated this quantity together with the radius of dislocation cores in wurtzite III-nitrides. The effect of the dislocation core energy on equilibrium critical thickness values for III-nitrides is investigated theoretically and is shown to be significant. The calculated values of the critical thickness are compared with experimentally determined values of the critical thickness for misfit dislocations in the InGaN/GaN system using transmission electron microscopy and x-ray diffraction te...

GaN-based LEDs grown on 6-inch diameter Si (111) substrates by MOVPE

The issues and challenges of growing GaN-based structures on large area Si substrates have been studied. These include Si slip resulting from large temperature non-uniformities and cracking due to differential thermal expansion. Using an AlN nucleation layer in conjunction with an AlGaN buffer layer for stress management, and together with the interactive use of real time in-situ optical monitoring it was possible to realise flat, crack-free and uniform GaN and LED structures on 6-inch Si (111) substrates. The EL performance of processed LED devices was also studied on-wafer, giving good EL characteristics including a forward bias voltage of ~3.5 V at 20 mA from a 500 μm x 500 μm device.

Carrier distribution in InGaN/GaN tricolor multiple quantum well light emitting diodes

Carrier transport in InGaN light emitting diodes has been studied by comparing the electroluminescence (EL) from a set of triple quantum well structures with different indium content in each well, leading to multicolor emission. Both the sequence and width of the quantum wells have been varied. Comparison of the EL spectra reveals the current dependent carrier transport between the quantum wells, with a net carrier flow toward the deepest quantum well.

Efficiency measurement of GaN-based quantum well and light-emitting diode structures grown on silicon substrates

The optical efficiency of GaN-based multiple quantum well (MQW) and light emitting diode (LED) structures grown on Si(111) substrates by metal-organic vapor phase epitaxy was measured and compared with equivalent structures on sapphire. The crystalline quality of the LED structures was comprehensively characterized using x-ray diffraction, atomic force microscopy, and plan-view transmission electron microscopy. A room temperature photoluminescence (PL) internal quantum efficiency (IQE) as high as 58% has been achieved in an InGaN/GaN MQW on Si, emitting at 460 nm. This is the highest reported PL-IQE of a c-plane GaN-based MQW on Si, and the radiative efficiency of this sample compares well with similar structures grown on sapphire. Processed LED devices on Si also show good electroluminescence (EL) performance, including a forward bias voltage of ∼3.5 V at 20 mA and a light output power of 1 mW at 45 mA from a 500×500 μm2 planar device without the use of any additional techniques to enhance the output cou...

Optical properties of GaN/AlGaN quantum wells grown on nonpolar substrates

In this paper we report on the optical properties of a series of GaN/AlGaN multiple quantum well structures grown on a-plane (112¯0) GaN, which had been deposited on r-plane (11¯02) sapphire substrates, compared to a reference GaN template of the same orientation. The low temperature photoluminescence spectrum of the template layer is dominated by two emission bands, which we attribute to recombination involving excitons in the bulk of the layer and electrons and holes trapped at basal-plane stacking faults, designated X1 and X2, respectively. The photoluminescence spectra from the quantum well structures show similar emission bands except that both X1 and X2 shift to higher energy with decreasing quantum well thickness. The shift to higher energy is due to the effects of quantum confinement on carriers trapped at the stacking faults that intersect the quantum wells, as well as those excitons that are localized within the quantum wells. This assignment is based partly on excitation spectroscopy that revea...

Compositional inhomogeneity of a high-efficiency InxGa1−xN based multiple quantum well ultraviolet emitter studied by three dimensional atom probe

An InxGa1−xN based multiple quantum well structure emitting in the ultraviolet, which has the highest reported efficiency (67%) at its wavelength (380nm), was analyzed with the three-dimensional atom probe. The results reveal gross discontinuities and compositional variations within the quantum well layers on a 20–100nm length scale. In addition, the analysis shows the presence of indium in the AlyGa1−yN barrier layers, albeit at a very low level. By comparing with analogous epilayer samples, we suggest that the quantum well discontinuities we observe may play an important role in improving the efficiency of these structures.

Towards predictive modelling of near-edge structures in electron energy loss spectra of AlN based ternary alloys

Although electron energy loss near edge structure analysis provides a tool for experimentally probing unoccupied density of states, a detailed comparison with simulations is necessary in order to understand the origin of individual peaks. This paper presents a density functional theory based technique for predicting the N K-edge for ternary (quasi-binary) nitrogen alloys by adopting a core hole approach, a methodology that has been successful for binary nitride compounds. It is demonstrated that using the spectra of binary compounds for optimising the core hole charge (

Optical polarization anisotropy of a-plane GaN/AlGaN multiple quantum well structures grown on r-plane sapphire substrates

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