We report on the study of the effect of various surface chemical treatment processes of n-GaN template layers used for subsequent growth of light emitting diode (LED) structures. The treatment procedure included cleaning in organic solvents, organic solvents followed by 5 minutes of HCl, organic solvents and 5 minutes of HCl followed by 2 minutes and finally 10 minutes of HF treatment. Chemical, optical and electrical properties of the surfaces of GaN and InGaN-based LED structures were systematically investigated by x-ray photoemission spectroscopy (XPS), auger electron spectroscopy (AES), atomic force microscopy (AFM), photoluminescence (PL) and electroluminescence (EL) spectroscopy. GaN layers that were grown on the samples treated with HCl and HF showed dramatically different surfaces having high density of 3D structures with high roughness. As measured by AFM, growth of the LED structure on top of the GaN layer continued the 3D-growth mode. LED structures grown on the HCl and HF treated GaN template layers showed minimal to no PL and EL emission and failed after a short period. We suggest a qualitative model of the growth that could potentially explain the underlying phenomena leading to such pronounced changes in the optoelectronic properties and surface conditions of the LED structures due to the treatment of the initial template layers.

The effect of a highly Mg doped subcontact layer on top of GaN grown by metal organic vapor phase epitaxy and its interface to a Pd/Au contact layer was investigated by means of transmission electron microscopy and electrical characterization techniques. Use was made of prior investigations of the Mg doping characteristics, which showed the existence of a segregation related defect free layer even for doping levels as high as 5×10−19cm−3, which thickness depends on the Mg to Ga molar precursor flow ratio. For a given subcontact layer thickness of 15 nm a critical precursor molar flow ratio of 0.035 resulted in a smooth surface showing an interfacial layer indicating a Pd/Ga alloying. This layer resulted in a contact resistivity as low as 2×10−5 Ωcm2. For a flow ratio of 0.070 the surface was found to be rough due to defect formation resulting in a contact resistivity as high as 10−3 Ωcm2 similar to the value obtained without subcontact layer. Moreover, the metallization layer showed in all cases a texture of the {111} lattice planes with respect to the {0002} planes of the GaN.

We studied photoluminescence (PL) from deep-level defects in GaN grown under Garich conditions at relatively low temperatures (700–800°C) by molecular-beam epitaxy (MBE). The dominant features of PL spectrum are red and green bands peaking respectively at ∼1.8 and ∼2.35 eV. Both PL bands decay exponentially at low temperatures (15 – 100 K) after pulsed excitation. The characteristic lifetime for the red band decreases by almost two orders of magnitude from 110 to 2 μs with increasing temperature from 15 to 100 K, while its integrated intensity after each pulse remains nearly unchanged in the same temperature range due to an increase in the peak intensity in the time-resolved PL curve. The lifetime of the green band remained unchanged in this temperature range. We suggest that these PL bands are caused by transitions between excited and ground states of some deep defects rather than transitions involving a shallow donor, conduction or valence bands.

The optical constants and polarized optical properties of amorphous III-V nitride thin films, a-(Al, Ga, In) N, deposited by RF magnetron sputtering onto crystalline silicon, c-Si, (111) and glass substrates have been investigated over the wavelength range 300 – 1400 nm. The optical constants of a-AlN were obtained by analysis of the measured ellipsometric spectra through the Cauchy–Urbach model while the optical constants of a-(In, Ga) N were determined using the Tauc-Lorentz model. Analysis of the absorption coefficient of a-AlN (in the range 200 – 1400 nm) and a-GaN (in the range 300 – 1400 nm) show the optical bandgap to be 5.9 ± 0.05 and 3.44 ± 0.05 eV. The absorption coefficient of a-InN (in the range 300 – 1400 nm) as a function of photon energy shows the absorption edge to be about 1.74 ± 0.05 eV. From the angle dependence of the p-polarized reflectivity we deduced Brewster and principal angles of these films. Measurement of the polarized optical properties revealed a high transmissivity (70 % – 95 %) and low absorptivity (< 18 %) for all three thin films in the visible and near infrared regions.

We report the photoluminescence characteristics of fresh dislocations introduced by the plastic deformation in GaN bulk crystals. GaN crystals prepared from free standing wafers grown by the HVPE technique were compressed plastically at 900 to 1000°C. In the deformed crystals (a/3) (1120) dislocations on the (1110) prismatic plane, corresponding to the so-called threading dislocations, were observed in TEM. In the PL studies at 11K the intensity of the near band-edge luminescence decreased drastically in the deformed GaN by about 1/12 than that of the as-grown GaN, which seems to imply the introduction of a high density of non-radiative reco!m bination centers into GaN! during the plastic deformation. By the plastic deformation the yellow band luminescence decreased remarkably meanwhile the red band and other new luminescence developed relatively. It is found that dislocations do not originate to the yellow luminescence but the red luminescence.

The growth conditions of A-plane AlN and GaN epitaxial layers by radio-frequency plasma assisted molecular beam epitaxy on R-plane sapphire substrates were investigated. The growth temperature and V/III supply ratio dependency on structural quality and surface roughness was described. The optimum V/III ratio for A-plane GaN and AlN layers was shifted to nitrogen rich side compared to the C-plane layers. A-plane GaN/AlN superlattices (SLs) were also grown on R-plane sapphire substrates. The X-ray diffraction peaks from a primary and a 1st satellite were observed. From a comparison of low temperature photoluminescence peak wavelength between A-plane and C-plane SLs, the built-in electrostatic field originated from spontaneous and piezoelectric polarization is negligible for A-plane SLs.

InxGa1-x N layers grown with compositions with the predicted miscibility gap have been studied using Transmission Electron Microscopy (TEM), x-ray diffraction and optical measurements (photoluminescence and absorption). The samples (0.34 < × < 0.8) were deposited by Molecular Beam Epitaxy at 800°C using 200 nm AlN buffer layer grown directly on sapphire substrates. Another sample with × = 0.34 was grown on a GaN buffer layer. Dislocation densities in the InGaN layers were typically in the mid-1010 cm−2 to1011 cm−2 range. Edge dislocations were the most prevalent. For In concentration × = 0.5 compositional ordering is observed leading to extra diffraction spots in electron and x-ray diffraction. The ordering was not observed for the sample with x=0.34 grown on GaN. Based on TEM measurements the estimated period of ordering Δ was about Δ = 45 Å for × = 0.5 and Δ = 65Å for × = 0.78. The sample with × = 0.5 had the highest dislocation density. In addition to the presence of threading dislocations two types of domain boundaries on (0001) and (0110) planes were also observed in this sample. This sample has a broader photoluminescence (PL) that is redshifted compared to the absorption edge (“Stokes shift”).

We report on recent results obtained for InGaN multiple quantum well laser diodes grown by ammonia based Molecular Beam Epitaxy. The laser diodes were grown on freestanding GaN substrates and operated at room temperature under pulsed current injection conditions. For devices with improved p-type doping a threshold current density of 6.7kA.cm−2 was measured for a current pulse duration of 200ns and an operating temperature of 3.9°C. A duty cycle up to 50% with a pulsed injection current duration of 500μs was also achieved at 3.9°C.

We have studied the early stages of GaN growth to realize the growth mechanism of GaN thin films on mis-oriented sapphire substrates which affects the surface and crystal quality of GaN thin films. As the result, it was found that the larger mis-orientation angle helps the growth of the larger grain of GaN and leads to the earlier shift of growth mode from 3D to 2D. The AFM observation of closed-coalesced GaN thin films revealed the difference in the micro-step structures by the mis-orientation angle of sapphire substrate. The result of x-ray rocking curve as a function of mis-orientation angle well matched with the microstructure of GaN surface, indicating that the larger mis-orientation angle helps the column ordering of GaN crystals.

This paper presents a concept of growing near single crystal quality GaN films over large areas through self-oriented growth of GaN platelets on molten gallium. The experiments were performed by nitridation of Ga films on amorphous quartz substrates using nitrogen plasma at low pressures of few mTorr. XRD texture analysis of the free standing GaN flakes with areas over 25 mm2 exhibited an overall c-axis tilt of 2.2°, while showing primary reflections from (0002) and (0004) planes. Further more, the cross-sectional TEM micrographs showed that the resulting GaN films are free from dislocation crops inside the grains but showed diffraction contrast due to small misorientation between the grains. The twist and tilt angles between adjacent columnar grains were determined using convergent beam electron diffraction technique to be less than 8° and 1°, respectively. HRTEM micrographs of the grain boundaries showed sharp interfaces with both twisted and perfect attachments.

The strain in Al0.80Ga0.20N/GaN distributed Bragg reflectors (DBRs) grown by metalorganic vapour phase epitaxy (MOVPE) on Si(111) was studied using high resolution X-ray diffraction (XRD). Previous studies have shown that a 50nm Al0.80Ga0.20N layer induced a compressive strain in Al0.12Ga0.88N capping layers and prevented crack formation. A seven period Al0.80Ga0.20N/GaN DBR was grown, but this was found to be cracked at room temperature, despite compression in the GaN layers. This problem was solved by growing an identical structure with the addition of a 650nm GaN cap, and due to the compression in this layer, the structure was crack-free.

Various nanofabrication technologies are currently under investigation to realize fine patterning of III-Nitrides. Dry and wet etching techniques have been explored in the past for the fabrication of GaN-based devices. However, due to etch-induced damage, it is still a major challenge to achieve high-quality GaN-based nanostructures with high aspect ratio. In this study, GaN nanopillars were fabricated by inductively coupled plasma etching (ICP) using anodic aluminium oxide (AAO) as a mask. High-spatial resolution optical techniques were employed to characterize these pillar arrays. The average diameter and length of these pillars are about 60–70 nm and 350–400 nm, respectively. Low temperature micro-photoluminescence spectra show a red shift compared with the spectrum recorded from the as-grown GaN, indicating stress relaxation in these nanopillars. The evidence of good crystalline quality is also confirmed by micro-Raman measurement where red shift of the E2(TO) mode from GaN nanopillars suggest partial relaxation of the compressive strain.

A successful attempt to use X-ray Excited Optical Luminescence (XEOL) for the detection of Extended X-ray Absorption Fine Structure (EXAFS) in III-nitrides is reported. The samples studied were InGaN and rare-earth (RE) doped GaN epilayers. For the first time Ga K-edge EXAFS oscillations were measured by monitoring the well-known “yellow” emission of GaN at 560 nm. The analysis of Optically Detected (OD) EXAFS data confirmed the expected local structure for Ga in GaN. The intensity oscillation of the “yellow” band when X-ray energy was scanned across the Ga K-edge indicates that core excitation of Ga atom has a high probability of transfer to defects responsible for “yellow” emission.

In this study, we investigate the dependence of GaN surface morphology on the absolute strain values for thin (<10 μm) epitaxial films grown by MOCVD on c-plane sapphire substrates of various miscut angles towards the m-plane. Results indicate an excellent correlation between the surface roughness observed employing an AFM tool and epilayer strain values. An overall increase of surface roughness (decrease of atomic terrace width) is found with decreasing compressive strain (epilayer vs. bulk value). In addition, sapphire substrates with increasing miscut angle (0.30 deg) appear to relax the inherent, built-in strain differently in the vertical (growth) direction when compared to just (0.00 deg) substrates. Strain relaxation by typical V-shaped, hexagonal pits is directly imaged through the comparison of surface features inside and outside of pits in the thin GaN epilayer films.

Non (0001) GalnN QWs have been grown by low pressure MOVPE on side facets of triangular shaped selectively grown GaN stripes. By analysing low temperature photo- and cathodoluminescence and room temperature electroluminescence, we found strong indications, that both, In and Mg are less efficiently incorporated on these side facets compared to the common (0001) plane with even lower efficiency for stripes running along (1–100) compared to (11–20). Nevertheless, we observed strong light emission from these quantum wells, supposed to be at least partly caused by the reduced piezo-electric field.

Single crystal GaN substrates are a more reliable, efficient and low cost alternative to sapphire and SiC substrates that are currently being used for developing nitride based devices such as LEDs, high power FETs and laser diodes. GaN growth by two techniques are investigated here. In ammonothermal growth (analogous to hydrothermal growth which is commonly used for quartz), using potassium and lithium amide as mineralizers and GaN powders and vapor grown polycrystals as nutrients, thick GaN single crystal layers have been grown on both faces of (0001) HVPE GaN seeds in nickel based autoclaves. In GaN sublimation growth, Ga is transported under a temperature gradient in an ammonia atmosphere from a GaN powder source to the sapphire substrate which has a 3–6 micron GaN epilayer deposited on it. Synchrotron white beam x-ray topography (SWBXT) was initially used to image the defect structures in the layers. High resolution x-ray diffraction experiments were also carried out and reciprocal space maps were recorded from the grown GaN layers. Characterization results will be discussed with respect to the growth conditions.

A detailed modeling of the electronic bandstructure of GaInN alloys and GaInN/GaN heterostructures typically used for high efficiency light emitting diodes is of high relevance for future improvements. Here we are exploring opportunities to accurately quantify the carrier dynamics under forward and reverse voltage bias. In GaInN/GaN LED-type heterostructures we observe distinct steps in the junction capacitance as a function of bias voltage within the depletion regime. Up to three individual steps can be identified that correspond to alternating ranges of capacitive and resistive impedances. Our analysis suggests that we are quantitatively monitoring the electron concentration in each individual quantum well. The pronounced clarity of the data reveals a high level of epitaxial perfection and spatial homogeneity across the entire area of the junction.

The two-dimensional (2D) Ga-nitride/air photonic quasi-crystals (PQC) were successfully fabricated by the technique of focused Ga ion beam (FIB) milling on GaN based epitaxial wafers. The effects of the PQC on the current injected edge emitting GaN-based light emitters were investigated. The quasi-crystal structures studied in this work are based on square-triangular tiling with 8-fold or 12-fold symmetry. The air hole diameter in the different PQC patterns was varied from 95nm up to 1200nm, the filling factor of air hole was in the range of 10% to 50% and the depth of the hole was 90nm to 370nm, respectively. Among these, the nearest center-to-center distance of the holes and/or the lattice constant was reached to be 170nm. The photonic quasi-crystals on the GaN-based light emitters demonstrated a two to three factor of magnitude enhancement of surface extractive emission. The blocking of the propagation of planar-guided modes by the photonic quasi-crystals was observed. By comparison of symmetry effect among the triangular lattice PC and PQCs, it was found that the GaN-based photonic quasi-crystal (PQC) with 12-fold symmetry provided a much favorable enhancement of the extractive light emission over that of triangle lattice PC and 8-fold symmetric PQC as well.

We report on deliberate O doping of GaN epitaxial layers during MOCVD growth, using H2O and CO2 as precursors. The photoluminescence spectra show a dominant 3.27 eV emission at 2 K known to be a donor-acceptor pair (DAP) transition. In our samples the intensity of this DAP spectrum correlates with the commonly observed 3.466 eV acceptor bound exciton (ABE) peak, suggesting these spectra are related to the same acceptor. The general correlation of these acceptor spectra with O concentration (as established with SIMS data) suggest that the acceptor is O-related, most likely a VGa-O complex. The concentration was measured with positron annihilation spectroscopy and found to be in the 1016 cm−3 -1017 cm−3 range in different samples. Considering previous results the identity of this residual acceptor is suggested to be a VGa-O-H complex. Previous suggestions that this acceptor is related to Mg, Si or C are discussed and found to be less likely.

We report progress in growing non-polar a-plane III-nitride films and heterostructures over a-plane 4H-SiC. a-plane SiC is more closely lattice-matched to a-plane GaN than is r-plane sapphire. Consequently, better structural quality a-plane nitride films may result over a-plane SiC substrates. By migration enhanced metalorganic chemical vapor deposition (MEMOCVD), an atomically smooth (1120)AlN layer with RMS roughness of 0.3nm was obtained. From the results of XRD, the structural defects in the AlN layer on SiC substrates were strongly reduced compared to those grown on r-plane sapphire. Also by applying our selective area lateral epitaxy (SALE) growth procedure, we achieved high structural and optical quality a-plane GaN films on 4H-SiC with RMS roughness only 0.4nm. Therefore, non-polar III-nitride films and heterostructures on SiC substrates are promising building blocks for realizing high performance polarization-free devices.