Yellow luminescence (YL) has been studied in GaN:Mg doped with Mg concentrations ranging from 10 19 to 10 21 cm -3 by spectral CL (T=K) and TEM and explained by suggesting that a different mechanism could be responsible for the YL in p-type GaN with respect to that acting in n-type GaN. Transitions at 2.2, 2.8, 3.27, 3.21, and 3.44 eV were found. In addition to the wurtzite phase, TEM showed a different amount of the cubic phase in the samples. Nano tubes with a density of 3×10 9 cm −2 were also observed by approaching the layer/substrate interface. Besides this, coherent inclusions were found with a diameter in the nm range and a volume fraction of about 1%. The 2.8 eV transition was correlated to a deep level at 600 meV below the conduction band (CB) due to Mg Ga -V N complexes. The 3.27 eV emission was ascribed to a shallow acceptor at about 170-190 meV above the valence band (VB) due to Mg Ga . The 2.2 eV yellow band, not present in low doped samples, increased by increasing the Mg concentration. It was ascribed to a transition between a deep donor level at 0.8-1.1 eV below the CB edge due to N Ga and the shallow acceptor due to Mg Ga . This assumption was checked by studying the role of C in Mg compensation. CL spectra from a sample with high C content showed transitions between a C-related 200 meV shallow donor and a deep donor level at about 0.9-1.1 eV below the CB due to a N Ga -V N complex. In our hypothesis this should induce a decrease of the integrated intensity in both the 2.2 and 2.8 eV bands, as actually shown by CL investigations.

Epitaxial growth on GaN single bulk crystals sets new standards in GaN material quality. The outstanding properties provide new insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) being not accessible by heteroepitaxial growth on sapphire or SiC. With MOVPE and MBE we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. Those layers revealed an exceptional optical quality as determined by a reduction of the photoluminescence linewidth from 5 to 0.1 meV and a reduced XRD rocking curve width from 400 to 20 arcsec.Only recently, progress in surface preparation allowed morphologies of the layers suitable for device applications. We report on InGaN/GaN MQW structures as well as the first GaN pn- and InGaN/GaN double heterostructure LEDs on GaN single bulk crystals. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping. Time resolved electroluminescence measurements proof that band/band recombination is the dominant emission mechanism for the InGaN/GaN LEDs.

We report on the growth of GaN in GSMBE using NH3 as nitrogen source. Special focus will be on the NH3 cracking, where we applied an On Surface Cracking technique (OSC). Using OSC we achieve photoluminescence linewidths as narrow as 5.5meV (5K) and mobilities of 220 cm2/Vs at room temperature.

We present a study of the sources of strain in GaN heteroepitaxy by in- and ex-situ measurement techniques. With an in-situ curvature measurement technique the strain development can be directly correlated to the different layers and doping in simple and device structures. We show several solutions for strain reduction and control. High-quality devices grown on Si are demonstrated.

GaN devices on Si are interesting for low-cost, high-power devices as LEDs and FETs. Until recently, most LED and FET devices suffered from cracking and low output power and additionally, from high series resistances for vertically contacted LEDs. Here, we give a brief overview on state of the art crack-free, bright LEDs with an output power up to 0.42 mW and AlGaN/GaN FETs with an output power of 2.5 W/mm at 2 GHz. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Metalorganic chemical vapor phase deposition of thick, crack-free GaN on Si can be performed either by patterning of the substrate and selective growth or by low-temperature (LT) AlN interlayers enabling very thick GaN layers. A reduction in dislocation density from 1010 to 109cm−2 is observed for LT-AlN interlayers which can be further improved using monolayer thick SixNy in situ masking and subsequent lateral overgrowth. Crack-free AlGaN/GaN transistor structures show high room temperature mobilities of 1590cm2/Vs at 6.7×1012cm−2 sheet carrier concentration. Thick crack-free light emitters have a maximum output power of 0.42mW at 498nm and 20mA.

Different substrates for gallium nitride growth are discussed. The commercially relevant substrates, silicon carbide and sapphire, and the two most promising alternatives, silicon and gallium nitride, are compared in terms of suitability for epitaxial processes and in their effects on devices. An estimation on future market success is given.

In this article, multiple-step rapid thermal annealing (RTA) processes for the activation of Mg doped GaN are compared with conventional single-step RTA processes. The investigated multiple-step processes consist of a low temperature annealing step at temperatures between 350°C and 700°C with dwell times up to 5min and a short time high temperature step. With optimized process parameters, and multiple-step processes, we achieved p-type free carrier concentrations up to 1–2×1018cm−3. The best achieved conductivity, so far, lies at 1.2Ω−1cm−1. This is a 50% improvement compared to conventional single-step process at 800°C, 10min.

Using hydride vapor phase epitaxy the influence of growth parameters on the crack density is studied for thick epitaxially lateral overgrown (ELOG) GaN layers. Reactor pressure, growth rate, and substrate temperature are key factors to obtain crack-free thick GaN layers. The cracking mechanism is discussed and void formation on top of the SiO2 stripes is proposed to play a key role in stress relaxation and crack suppression.

Undoped GaN that has been subjected to argon plasma bombardment is shown to have enhanced yellow (YL) and blue (BL) luminescence bands that are correlated with a concomitant increase in the neutral-donor-bound-exciton emission. With cw UV excitation the BL band quenches and intensity is transferred to the YL band. Thus models of YL in GaN need to consider this metastability and links with the BL.

Making use of the polar nature of III-nitride heterostructures, a new FET device concept is proposed. The structure contains an InGaN QW channel sandwiched in between two GaN barrier layers. The charge inthis structure is mainly generated by the strain field in the InGaN layer and is an electron/hole dipole sheet charge located at the opposite InGaN/GaN interfaces. To obtain nchannel characteristics the hole charge at the rear interface (for Ga-face oriented material) is compensated by donor doping of the channel or by modulation doping from the real GaN barrier layer. Growth, processing technology and characteristics of first fabricated devices is discussed.

Two-step thermal annealing processes were investigated for electrical activation of magnesium- doped galliumnitride layers. The samples were studied by room-temperature Hall measurements and photoluminescence spectroscopy at 16 K. After an annealing process consisting of a short-term step at 960 °C followed by a 600 °C dwell step for 5 min a resistivity as low as 0.84 Ω cm is achieved for the activated sample, which improves the results achieved by standard annealing (800 °C for 10 min) by 25% in resistivity and 100% in free hole concentration. Photoluminescence shows a peak centered at 3.0 eV, which is typical for Mg-doped samples with high free hole concentrations.

The influence of the mirror reflectivity on the L–I characteristics of GaN-based lasers has been studied. A cleaved, Al-coated fiber is used as an external micro-mirror to control the reflectance of the end facets allowing for a continuous adjustment of mirror losses of a particular laser. In contrast to other methods, this eliminates all ambiguities usually arising from the comparison of different or differently coated devices. An increase in the single facet external quantum efficiency by 45% is observed for uncoated lasers and simultaneously, the threshold current is reduced by 12%. Internal losses of approximately 20–30cm−1 are derived from the differential quantum efficiency variation depending on the particular device under investigation.

Dry-etching of laser facets is commonly used for (InAl)GaN/sapphire-based structures since the epitaxial planes of the nitride layers are rotated with respect to the substrate planes making cleaving impractical. To achieve steep and smooth facets by chemically assisted ion beam etching, a 3-layer resist system is developed for patterning. Characterization by scanning electron microscopy and atomic force microscopy shows facets with root-mean-square roughnesses of 7 nm and inclination angles of 2–4°. Optically pumped lasers yield low threshold excitation densities for fully doped separate confinement heterostructure lasers.

GaN technology relies on highly mismatched heteroepitaxial growth, mainly on sapphire or SiC substrates, and therefore suffers from 10 9 to 10 10 threading dislocations per cm 2 . The origin and the deteriorating influence of the extremely high dislocation densities are analyzed with regard to the specific circumstances of GaN technology. Various attempts to cope with heteroepitaxial growth are discussed, from the use of nucleation layers to the growth on GaN single bulk crystals. Special focus is put on the impact of the approaches on the device performance.

We have measured temperature-dependent reflectance (RF) and photoluminescence (PL) spectra on a high-quality unstrained homoepitaxial GaN layer grown by MOVPE on a pre-treated GaN single crystal In the reflectance spectra, intrinsic exciton-polariton features show up related to all three valence bands. For the A and B valence band, we find exceptional low damping. A full fit to the experimental curves including spatial dispersion allows to deduce exact exciton energies for a broad range of sample temperatures. Besides the dominant donor- and acceptor-bound excitons the photoluminescence spectra show relatively strong, non-thermalized emission from exciton-polaritons in so-called bottleneck states. The analysis of these lines confirms the RF data.

The temperature distribution in InGaN-MQW light emitting diodes was examined during operation with spatially resolved micro-Raman and micro-Electroluminescence measurements. The experimental results were compared to finite element simulations. A good agreement between the different experimental and calculated data is found. Maximum operation temperatures up to 140 °C at a moderate forward currents of 30 mA are revealed by all three independent methods. Influences of substrate thickness, different substrates, and even bond-wires are shown.