We report experimental investigation of laser-assisted debonding of GaN-based light emitting diodes (LEDs) grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The device was photo-electrochemically (PEC) etched to form hexagonal pyramid hillocks on the debonded surface. The luminous intensity of the debonded and roughened LEDs increased by 60% when compared to the LEDs on sapphire. This increase is attributed to the enhancement in photon extraction efficiency.

The dependence of the InGaN/GaN(0001) surface morphology on the growth parameters of metal organic vapour phase epitaxy (MOVPE) at 650°C has been investigated using scanning tunneling microscopy. After deposition of InGaN under In-rich growth conditions, a preferential nucleation of islands at defects of the GaN substrate film is found. The islands have a large size (diameter ∼ 80 nm, height ∼10 nm) and show a spiral disc-like shape with an atomically flat surface, and the wetting layer surface appears very smooth. For thicker InGaN layers, In-droplets are formed on top of the large islands and the wetting layer becomes rougher. For InGaN growth at a reduced In partial pressure, hexagonal islands with an atomically flat top surface are formed and pyramidal islands of triangular base with smooth side facet structures are observed. The effects of growth rate and V/III flux ratio on the InGaN surface morphology have also been studied and the results indicate that a slower growth reduces the large island formation probability whereas a high V-III flux ratio enhances it. All findings are discussed in terms of thermal diffusion, In-incorporation and thermal decomposition.

This paper examines the core structure and composition of threading dislocations in GaN grown by hydride vapour phase epitaxy. Transmission electron microscopy showed that screw dislocations have widely varying core structures from open cores (“nanopipes”) to closed cores, with irregular variations between the two observed along the length of many dislocations. New evidence was found demonstrating that the equilibrium structure of screw dislocations is a closed core configuration. Electron energy loss spectroscopy combined with high resolution imaging showed that 1.7±0.3 monolayers of nitrogen were substituted by oxygen at the surfaces of nanopipes. In contrast, closed core dislocations showed little evidence of oxygen segregation. It is argued that these results support a model where nanopipe formation is controlled by the segregation of oxygen to pits predominately associated with, albeit not exclusively, dislocations. The implication of the results in understanding the electronic behavior of dislocations in GaN is also discussed.

MOCVD templates grown on sapphire with a Gallium Treatment Step (GTS) instead of a low temperature AlN or GaN nucleation layer are used for HVPE growth. Four templates are used with varying times of MOCVD buffer layer growth. 25 μm GaN is grown with HVPE on these templates. The HVPE layers are studied with optical microscopy and X-ray diffraction (XRD). The results show that the thickness of the buffer layer is not important for the quality of the HVPE grown layer once sufficient nucleation sites for HVPE growth are present. The excellent quality of the templates with GTS for HVPE growth is shown with a 100 μm thick HVPE layer on a 2” template. No cracks in either sapphire or GaN are visible.

The planarization of GaN has been mainly addressed by chemo-mechanical polishing methods in the literature. Such techniques were found to be successful in planarizing the nitrogen polar (000-1) surface of GaN. However, planarization of the gallium polar (0001) surface presents a challenge due to its higher degree of chemical inertness. Moreover, studies on the planarization of the remaining crystal orientations remain a topic with sparse representation in the literature. In this paper, we report our planarization studies of GaN films using an etch-back technique. Photoresist is employed as the sacrificial layer and a chlorine inductively coupled plasma (ICP) used for etching. We demonstrate the planarization of rough C-plane and A-plane GaN films grown by hydride vapor phase epitaxy (HVPE). Specifically, an rms roughness of approximately 2.1nm and 4.1nm was realized after iterative etch-back for a representative C-plane and A-plane sample with initial rms roughness of 134nm and 414nm respectively.

A normally off-mode AlGaN/GaN heterostructure field effect transistor (HFET) using a p-type GaN gate was fabricated and their static properties were compared with those of HFET having a Schottky gate. HFET having a p-GaN gate contact shows a very low leakage current density of 18.2 μA/mm at VGS and VDS of 0 V and 20 V, respectively.

Integrated AlN nanocaps are used to protect gallium nitride epilayers during high temperature annealing treatments following high-energy implantation of rare earth (RE) ions. Cracks formed in thicker caps due to the lattice mismatch between AlN and GaN lead to the creation of microscopic surface defects at annealing temperatures higher than around 1200 °C. GaN dissociates locally to produce holes in the caps. Simultaneous cathodoluminescence/wavelength dispersive X-ray microanalysis in a modified electron probe microanalyzer allows study of the compositional and light emission variations near these microscopic defects. The intensity of the 5D0 – 7F2 transition related emission is enhanced and spectral changes can be observed, which indicate changes in the structure and/or composition of a very thin layer that forms the walls of holes in the caps. We also report some preliminary observations on the influence of the annealing atmosphere (nitrogen or ammonia) on cap damage.

Raman scattering experiment was carried out to study material and electronic properties of the aligned GaN nanorods grown by plasma-assisted molecular beam epitaxy. Due to the small size of the nanorods and the relatively high power density of the laser illumination used in this investigation, local heating of the sample cannot be neglected. Special care has been taken to identify the frequency shift in the optical phonon peaks induced by local heating prior to any spectral analysis. The residual stress in the GaN nanorods was estimated by analyzing the frequency shift of its E2 Raman mode. It was found that the frequency of E2 mode is shifted by only 0.1 cm-1 comparing with the stress-free frequency, indicating a negligible residual stress in nanorods. Owing to aligned geometry of the nanorods and the back-scattering scattering geometry used in the experiment, A1(LO) Raman mode was collected. The free carrier concentration as well as electron mobility of the GaN nanorods were obtained by the line shape analysis of the coupled A1(LO) phonon-plasmon mode. The electron concentration and mobility of electron obtained from line shape analysis are 3.3×1017 cm-3 and 140 cm2/Vs, respectively. We presented a general method of better determining the electronic parameters of the GaN nanostructures via Raman scattering.

Dilute magnetic semiconductors (DMS) show promise as materials that can exhibit ferromagnetism at room temperature (RT). However, the nature of ferromagnetism in this material system must be well understood in order to allow intelligent design of RT spintronic devices. This work investigates the magnetic properties of the as-grown films and the effect of Mn incorporation on crystal integrity and device performance. Ga1-xMnxN films were grown by MOCVD on c-plane sapphire substrates with varying thickness and Mn concentration. Homogenous Mn incorporation throughout the films was verified with Secondary Ion Mass

Spectroscopy (SIMS), and no macroscopic second phases (MnxNy) were detected using X-ray diffraction (XRD). Superior crystalline quality in the MOCVD-grown films relative to Mn-implanted GaN epilayers was confirmed via Raman spectroscopy. Vibrating sample magnetometry measurements showed an apparent room temperature ferromagnetic hysteresis in the as-grown epiayers. Similarly, a marked decrease in the magnetization was observed with annealing and silicon doping, as well as in post-growth annealed Mg-codoped samples. The observed decrease in muB per Mn with increasing Mn concentration is explained by Raman spectroscopy results, which show a decrease in long-range lattice ordering and an increase in nitrogen vacancy concentration with increasing Mn concentration. Magnetic and electron-spin paramagnetic resonance (EPR) data also show that the position of the Fermi level relative to the Mn2+/3+ level is the determining factor in magnetization. Light emitting diodes (LEDs) containing a Mn-doped active region have also been produced. Devices were fabricated with different Mn-doped active layer thicknesses, and I-V characteristics show that the devices become more resistive as thickness of the Mn-doped active layer increases. The magnetic and structural properties observed in this work will be used in conjunction with characteristics and magneto-optical of the Mn-containing devices to discuss the theoretical models of ferromagnetism in Ga1-xMnxN

Aluminum nitride (AlN) offers exceptional properties necessary to explore the development of large area substrates for nitride based electronics and photonics. Recent studies on AlN bulk growth using the sublimation-recondensation method developed at Crystal IS demonstrated high-quality crystals with low dislocation density and crystallographic uniformity. The diameter enlargement of these AlN boules is often associated with extensive defect generation. The goal of this study is to evaluate the origin and distribution of growth defects in AlN bulk crystals.

AlN crystals were grown using the sublimation-recondensation technique and then they were sliced into wafers. The defect evaluation in this study was performed using x-ray topography, differential image contrast and polarized-light optical microscopy, atomic force microscopy (AFM) and etch pit pattern delineation. Special attention was paid to crack development and propagation, grain boundary distribution, micro-scale inhomogenities as well as to the origin and density of dislocations. The major cause of growth defect appears to be non-linearity of both axial and radial temperature gradients. Growth optimization results in lower defect density and improved crystallinity of the AlN crystals.

Misfit dislocations (MDs) have been observed using transmission electron microscopy (TEM) in InGaN/GaN quantum well (MQW) structures grown under different metal-organic vapour phase epitaxy (MOVPE) regimes and with In-contents equal to or higher than 20%. These dislocations are even observed in a single quantum well 3 nm thick with an In-content of 22%. Conversely, no MDs were observed in QW structures with an In-content of 16%. The presence of MDs in the QW stack leads to strain relaxation which has been confirmed in the indium-rich structures by high resolution X-ray diffraction (HRXRD).

Incorporation of Si ion implantation to GaN metal semiconductor field effect transistor (MESFET) processing has been demonstrated. The channel and source/drain regions formed using Si ion implantation into undoped GaN on sapphire substrate. In comparison with the conventional devices without ion implanted source/drain structures, the ion implanted devices showed excellent device performance. On-state resistance reduces from 210 Ω-mm to 105 Ω-mm. Saturation drain current and maximum transconductance increase from 36 mA/mm to 78 mA/mm and from 3.8 mS/mm to 10 mS/mm, respectively.

We have demonstrated growth of large AlN single crystals using (0001)-oriented AlN seeds. Boules with a diameter of 15 mm and length up to 12 mm were obtained from 5 mm seeds. Step flow growth was observed on both Al and N-polar surfaces. N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation.

Group III-nitride compound semiconductors (e.g. AlN-GaN-InN) have generated considerable interest for use in advanced optoelectronic device structures. The fabrication of multi-tandem solar cells, high-speed optoelectronics and solid state lasers operating at higher energy wavelengths will be made possible using (Ga1-y-xAlyInx)N heterostructures due to their robustness against radiation and the wide spectral application range. To date, the growth of indium rich (In1-xGax)N films and heterostructures remains a challenge, primarily due to the large thermal decomposition pressures in indium rich group III-nitride alloys at the optimum growth temperatures. In order to control the partial pressures during the growth process of InN and related alloys, a unique high-pressure chemical vapor deposition (HPCVD) system with integrated real-time optical monitoring capabilities has been developed. We report initial results on InN layers grown at temperatures as high as ∼850°C with reactor pressures around 15 bar. Such process conditions are a major step towards the fabrication of indium rich group III-nitride heterostructures that are embedded in wide band gap group III-nitrides. Real-time optical characterization techniques are applied in order to study the gas phase kinetics and surface chemistry processes during the growth process.

For an ammonia to TMI precursor flow ratio below 500, multiple phases with sharp XRD features are observed. Structural analysis perform by Raman scattering techniques indicates that the E2 high mode improves as NH3:TMI ratio is decreased to below 500. Optical characterization of these InN layers indicates that the absorption edge shifts from down from 1.85 eV to 0.7 eV. This shift seems to be caused by a series of localized absorption centers that appear as the indium to nitrogen stoichiometry varies. This contribution will correlate the process parameters to results obtained by XRD, Raman spectroscopy and optical spectroscopy, in order to assess the InN film properties.

Gallium Nitride (GaN) is a promising wide band gap semiconductor material for many optoelectronic applications, especially in the near UV range. Over the past several years, an extensive technical effort has been focused on improving the quality of GaN films through various overgrowth techniques such as epitaxial lateral overgrowth (ELOG), facet controlled epitaxial lateral overgrowth (FACELO), and Pendeoepitaxy. ELOG has been shown to reduce the density of threading dislocations by up to five orders of magnitude [1], however a complete physical model describing lateral overgrowth is needed in order to take full advantage of the process. A lateral overgrowth model will allow for the design and fabrication of three dimensional structures that can lead to novel devices and also to efficient biosensors by integrating micro and nano channels on the same chip as the optoelectronic components.

A two-step process has been used to successfully control the geometry of overgrown GaN. Conditions have been identified which give a reduced lateral growth rate, in order to allow expansion of the {112n} plane to form vertical sidewalls and for the design of channel width. These geometries are being examined for possible application in laser diode and micro-channel fabrication for integrating bio-agent detection modules.

A self-consistent, coupled opto-electronic simulation for studying GaN nanowire lasers is presented. The model solves, simultaneously and self-consistently, the carrier transport equations and the photon rate equations. The basic physical model takes into account both bulk and surface dark recombinations, stimulated emission, the anisotropic optical gain typical of the Wurtzite GaN structure, the modified spontaneous emission, and its coupling into the lasing modes by microcavity effects. The model further incorporates band gap shrinkage effects due to band renormalization and the effects of multiple lateral and longitudinal lasing and non-lasing optical modes.

The anisotropically biaxial strain in a-plane AlGaN on GaN is investigated by X-ray diffraction analysis of the heterostructure of AlGaN and GaN grown on r-plane sapphire. The AlGaN layer with a low AlN molar fraction or small thickness is coherently grown on the GaN layer both along the m-axis and c-axis. An increase in AlN molar fraction or thickness in AlGaN, results in a slight relaxation of AlGaN only in one direction due to tensile stress along the c-axis, which is caused by the underlying GaN layer during the growth. The cause of the relaxation of AlGaN in one direction is thought to be a large anisotropically biaxial stress.

InGaN/GaN MQW samples were grown by metal organic chemical vapor deposition (MOCVD) on (10-1-1) oriented GaN templates. Effects of growth temperature and reactor pressure on the photoluminescence (PL) properties were investigated. The emission intensity improved significantly when the QWs were grown at 100 Torr, compared to higher pressure growths. The effect of well-width on the luminescence properties was investigated and an optimum well width of 40 Å was determined. Excitation dependent PL measurements revealed no shift in the PL emission wavelength suggesting the absence of electric field in the quantum wells. Furthermore, LEDs fabricated on (1011) GaN templates, emitting at 439 nm, showed no shift in the EL emission wavelength with the increase in drive current, reconfirming the absence of polarization.

In this paper, we report a study of the degradation of AlGaN-based 280 nm LEDs, which were grown on sapphire substrates using migration-enhanced metalorganic chemical vapor deposition process (MEMOCVD). Electroluminescence (EL), atomic force microscopy (AFM), cathodoluminescence (CL), and scanning electron microscopy (SEM) observations showed that the degradation of deep UV LEDs generally fell into two categories: catastrophic degradation and gradual degradation. The catastrophic degradation was found to be mostly caused by the non-uniformity of surface morphology. The gradual power reduction had two characteristic time constants indicating two possible degradation mechanisms as found from temperature and bias dependent LED power degradation measurements. The faster time constant was bias dependent and virtually constant with temperature whereas the second time constant (slower) varied exponentially with junction temperature. For this temperature dependent part, the activation energies of degradation were determined to be 0.23 eV and 0.27 eV under injected current density of 100 A/cm2 and 200 A/cm2 respectively.

Cr doped GaN was grown by rf N-plasma molecular beam epitaxy on sapphire(0001) at a sample temperature of 700 °C. Cr/Ga flux ratio was set to a value from 5% to 20%. Subsequently, scanning tunneling microscopy was performed on these surfaces. Cr incorporates on the GaN surface at 700 °C at a Cr concentration of 5% and less. By increasing the Cr/Ga flux ratio to 20% in CrGaN, linear nano structures were formed on the surface, which were not observed on the bare GaN surface. The RHEED and STM studies reveal that Cr atoms form 3×3 reconstruction when 0.1 ML of Cr was deposited at room temperature on 1×1 adlayer of Ga on GaN(000-1). Cr substitutes Ga on the surface when deposited at 700 °C on the MBE grown GaN(000-1) surface for all the experiments which we have performed provided the Cr concentration is low (∼5%).