B. Schineller

Photoluminescence and Raman study of compensation effects in Mg-doped GaN epilayers

The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence and Raman spectroscopy using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30, and 850±30 meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission.

Thermal admittance spectroscopy of Mg-doped GaN Schottky diodes

Thermal admittance spectroscopy measurements at temperatures ranging from room temperature to 90 K are performed on Schottky structures based on Mg-doped GaN layers grown by metalorganic vapor phase epitaxy on sapphire. The analysis of the experimental data is made by a detailed theoretical study of the steady-state and small-signal electrical characteristics of the structures. Numerical simulations are based on the solution of the basic semiconductor equations for the structure consisting of two Schottky diodes connected back to back by a conduction channel formed by the GaN layer. The description explicitly includes the Mg-related acceptor level, with its temperature- and position-dependent incomplete occupation state, leading to a dynamic exchange with the valence band. It fully reproduces the variations with temperature of the capacitance–frequency and conductance over frequency curves, allowing to give for all temperature ranges the origin of the various contributions to the junction capacitance and of the microscopic mechanisms responsible for the capacitance–frequency cutoff. Series resistance effects are shown to be dominant at temperatures above 230 K, whereas the Mg-related acceptor level governs the electrical behavior below 230 K. The existence of a second acceptor level with an activation energy of several tens of meV is revealed from the analysis of the characteristics at low temperature. An optimized fitting procedure based on the comparison of the electrical characteristics obtained from the numerical simulations to the experimental data allows one to determine the microscopic parameters describing the structure, among which the acceptor activation energies, thermal capture cross sections, concentrations, and the Schottky contact barrier heights are the most important ones. The obtained activation energy of the Mg-acceptor level of 210 meV is by a factor of 2 larger than that obtained from a classical Arrhenius plot, showing that a complete description of Mg-doped GaN junctions requires the correct treatment of the Mg level, acting as a dopant and as deep impurity, as well as the inclusion of series resistance effects.

Luminescence and Stimulated Emission from GaN on Silicon Substrates Heterostructures

Photoluminescence (PL) and stimulated emission of GaN/Si layers grown by MOVPE with AIN and AlGaN buffers have been investigated. It has been found that thermal annealing of samples in nitrogen gas flow leads to an increase of the PL efficiency of up to 20 times, with a thermal activation energy of this process of 3.1-3.4 eV. The annealing promotes a significant decrease of the non-radiative center concentration and an increase of the number of shallow states. The laser action at λ = 377 nm under pulsed optical excitation was achieved at room temperature evidencing a high quality of the samples.

Comparison of hydrogen and nitrogen as carrier gas for MOVPE growth of GaN

Abstract We investigated the effect of the carrier gas on GaN single layers grown in an AIX 200 RF horizontal flow low pressure MOVPE system. On similarly processed c -plane sapphire substrates (with regard to preparation and nucleation) GaN single layers were grown both under N 2 and H 2 . Various measurement methods were applied to assess the layer quality. While some characteristics of layers grown in both N 2 or H 2 atmospheres are promising, both processes show inherent deficiencies that will have to be addressed for device quality epitaxy. For GaN single layers grown in N 2 ambient bad morphology and poor electrical data were found. Layers grown with H 2 as carrier gas suffer from a strong yellow band emission in low excitation photoluminescence measurements and disadvantages from the large scale device production point of view, such as higher loss of material to the reactor walls.

Blue InGaN/GaN multiple-quantum-well optically pumped lasers with emission wavelength in the spectral range of 450–470 nm

Optically pumped lasing in the wavelength range of 450–470 nm in InGaN/GaN multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy was achieved and investigated. The energy and power per pulse of the laser were 80 nJ and 10 W correspondingly for one facet at room temperature. The far-field patterns of the laser emission consisted of three light spots near the angles of +30°, −15°, and −45°. The highest operating temperature was 450 K. The photoluminescence and photoluminescence excitation spectrum structures suggest that the quantum dots inside the quantum wells are involved in the recombination mechanism.

The influence of Mg-concentration and carrier gas on the electrical and optical properties of GaN : Mg grown by MOVPE

Annealing was found to effuse hydrogen from the lattice, hence, activating the passivated Mg-acceptors to yield p-type conductivity. We investigated the optical and electrical properties of GaN doped over a wide range of Mg-concentrations and growth-conditions before and after annealing. In low temperature photoluminescence we observed acceptor bound excitons and donor-acceptor pair recombinations (DAP) whose relative intensities depend on the doping level. At high doping levels we observe a broad blue luminescence band which we associate with a DAP involving deep donors introduced by the high concentration of Mg. Van der Pauw room-temperature Hall measurements of selected samples yield hole concentrations of up to 7 × 10 17 cm - 3. Furthermore, a significant influence of the deposition temperature and the V/III-ratio on the percentage of ionizable Mg-acceptors is found. Hydrogen as carrier gas is judged superior to nitrogen with respect to layer morphology and doping properties.

Luminescence and lasing in InGaN∕GaN multiple quantum well heterostructures grown at different temperatures

It was found that the decrease of the InGaN∕GaN multiple quantum well (MQW) growth temperature from 865 to 810 °C leads to a MQW emission wavelength shift from the violet to the green spectral region. The lowering of the growth temperature also promotes a decrease of the MQW photoluminescence (PL) intensity at high excitation and a disappearance of the excitonic features from the low-temperature reflection and PL spectra of GaN barriers and claddings. The laser threshold dependence on Tg is not monotonic, with the lowest value of 270kW∕cm2 at Tg=830°C. High-temperature annealing (900 °C, 30 min) leads to a twofold increase of the PL efficiency only from the InGaN QWs grown at the lowest temperature. The results allow one to explain the laser threshold behavior in terms of the heterostructure quality, the defect concentration, In clusterization, and the piezoelectric field dependence on the MQW growth temperature.

Selective epitaxial growth of III-V semiconductor heterostructures on Si substrates for logic applications

We have deposited III-V alloys on 200 mm Si miscut wafers with an oxide pattern. The selective epitaxial growth (SEG) of GaAs in large windows defined by SiO2 lines on a thick strained-relaxed Ge buffer layer served as a test vehicle which allowed us to demonstrate the integration of a III-V material deposition process step in a Si manufacturing line using an industrial reactor. High quality GaAs layers with high wafer-scale thickness uniformity were achieved. In a subsequent step, SEG of InP was successfully performed on wafers with a 300 nm shallow trench isolation pattern. The seed layer morphology depended on the treatment of the Ge surface and on the growth temperature. The orientation of the trench with respect to the substrate miscut direction had an impact on the quality of the InP filling. Despite of the challenges, such an approach for the integration of III-V materials on Si substrates allowed us to obtain extended-defect-free epitaxial regions suitable for the fabrication of high-performance devices.

Effects of electron–phonon interaction and chemical shift on near-band-edge recombination in GaN

A coherent analysis of the near-band-edge luminescence spectra of undoped and Si-doped GaN layers grown by metalorganic vapor phase epitaxy on a sapphire substrate has been performed with a model including the effect of the charge carrier–longitudinal optical (LO) phonon interaction, based on Frohlich’s polaron theory. This model allows one to correlate the relative intensities of the phonon sidebands to the position of the zero-phonon line from which ionization energies are obtained. Moreover, central-cell corrections are taken into account using two different models: the quantum defect model and the Lucovsky model [Solid State Comm. 3, 299 (1965)]. The effect of the electron-LO phonon interaction has been included in these models. Comparison between theory and experiment through the Huang-Rhys factor [Proc. Roy Soc. A204, 406 (1950)] allows a precise determination of the impurity binding energies and effective radii, as well as a reliable characterization of the charge carrier–LO phonon interaction.

Multiple quantum well InGaN/GaN blue optically pumped lasers operating in the spectral range of 450-470 nm

Lasing under optical pumping by N 2 -laser radiation in InGaN/GaN multiple quantum well heterostructures grown in AIXTRON MOVPE reactors was achieved and investigated in the wavelength range of 450-470 nm. The laser operation wavelength depends most strongly on V/III ratio during quantum well barrier growth. The total energy and power per pulse of the laser were 300 nJ and 40 W, respectively, with differential quantum efficiency of 3% at room temperature. The laser threshold increases exponentially with increasing operation wavelength which is mainly due to the decreasing efficiency of the spontaneous emission and due to an increase of its spectral width.