E. Litwin-Staszewska

Determination of the effective mass of GaN from infrared reflectivity and Hall effect

Infrared reflectivity and Hall effect measurements were performed on highly conducting n‐type GaN (n≊6×1019 cm−3) bulk crystals grown by the high‐pressure high‐temperature method. Values of electron‐plasma frequency and free‐electron concentration were determined for each sample of the set of seven crystals. It enabled us to calculate the perpendicular effective mass of electrons in the wurtzite structure of GaN as m*=0.22±0.02 m0. Effects of nonparabolicity and a difference between parallel and perpendicular components of the effective mass are small and do not exceed the experimental error.

Temperature dependence of electrical properties of gallium-nitride bulk single crystals doped with Mg and their evolution with annealing

Comprehensive studies of the electrical properties of Mg-doped bulk GaN crystals, grown by high-pressure synthesis, were performed as a function of temperature up to 750 °C. Annealing of the samples in nitrogen ambient modifies qualitatively their resistivity values ρ and the ρ(T) variation. It was found that our material is characterized by a high concentration of oxygen-related donors and that the charge transport in the studied samples is determined by two types of states, one of shallow character (Mg-related state, EA≈0.15 eV), and the second one much more deep, E2≈0.95 eV (above the valence band). Depending on the effective concentration of either states, different resistivities ρ can be observed: lower resistivity (ρ 106 Ω cm at ambient temperature) in samples with dominant E2 states. For the first type of samples, annealing at Tann<500 °C leads to a decrease of their resistivity and is associated with a...

Optical and electrical properties of homoepitaxially grown multiquantum well InGaN/GaN light-emitting diodes

We have studied the electrical and optical properties of homoepitaxially grown InGaN/GaN multiquantum-well light-emitting diodes over a wide range of temperatures and currents. We found thermionic emission rather than tunneling transport to be the most probable mechanism responsible for radiative recombination processes. As we point out, the thermal activation energies of Si donors in the n-type GaN and Mg acceptors in the p-type GaN can explain the temperature dependence of the device series resistance. Furthermore, the presence of strong localization effects in the active InGaN layers can be deduced from the observed considerable blueshifts of the electroluminescence emission peak with increasing temperature and current. We found the values of the internal and external quantum efficiencies of homoepitaxially grown light emitting diodes to be 38% and 1.9%, respectively.

The effect of pressure on the luminescence from GaAs/AlGaAs quantum wells

Photoluminescence (PL) from GaAs quantum wells with widths from 50 AA to 300 AA and AlxGa1-xAs barriers (x=0.3 and 1) was studied under pressure up to 35 kbar at two temperatures (300 K and 77 K). We used (and compared) three types of pressure devices: gas cells, liquid cells and the diamond-anvil cell. Accurate values for the pressure variation of the PL energy were obtained. They reveal the small dependence on the parameters of the well, in agreement with the envelope-function calculation. Pressure shifts of the PL lines is the same at 77 K and at 300 K. In several samples we found the change of the pressure coefficient of the direct ( Gamma ) line at the Gamma -X crossover pressure. We interpret this as the resonance effect due to the mixing of the Gamma state in the well with the X continuum in the barriers. This means that the pressure dependence of the quantum-well lines should not be fitted with a single curve below and above the Gamma -X crossover pressure. From our results we obtain the linear pressure coefficient of the GaAs energy gap equal to 11.6 meV kbar-1. The deformation potential of the gap seems to be almost independent of pressure up to 15 kbar.

Substrate misorientation induced strong increase in the hole concentration in Mg doped GaN grown by metalorganic vapor phase epitaxy

We demonstrate that relatively small GaN substrate misorientation can strongly change hole carrier concentration in Mg doped GaN layers grown by metalorganic vapor phase epitaxy. In this work intentionally misoriented GaN substrates (up to 2° with respect to ideal ⟨0001⟩ plane) were employed. An increase in the hole carrier concentration to the level above 1018 cm−3 and a decrease in GaN:Mg resistivity below 1 Ω cm were achieved. Using secondary ion mass spectroscopy we found that Mg incorporation does not change with varying misorientation angle. This finding suggests that the compensation rate, i.e., a decrease in unintentional donor density, is responsible for the observed increase in the hole concentration. Analysis of the temperature dependence of electrical transport confirms this interpretation.

Spatial distribution of electron concentration and strain in bulk GaN single crystals - relation to growth mechanism

Micro-infrared reflectivity and micro-Raman scattering have been used to determine free electron density and residual strain distribution in bulk GaN crystals. As-grown samples exhibit significant variation of electron concentration and strain along their surfaces. Increase of electron concentration correlates with the growth direction. The observed inhomogeneities in the properties of bulk GaN crystals can be eliminated by mechanical polishing of the crystal and removing the surface layer or growth figures formed during the sample cooling, which follows its high-pressure, high-temperature growth. Properties of that surface layer can differ from the bulk due to different growth mechanisms and impurity distribution. They suggest that oxygen is a likely candidate for the donor impurity supplying high amount of electrons to the conduction band of GaN.

Preparation of free-standing GaN substrates from GaN layers crystallized by hydride vapor phase epitaxy on ammonothermal GaN seeds

Crystallization of GaN by hydride vapor phase epitaxy (HVPE) on ammonothermally grown GaN seed crystals is overviewed. Morphology of the crystal growing surface at the beginning of the crystallization process and at the end of it is presented. Based on these results a rough growth model is proposed. Smooth GaN layers up to 1 mm thick and of a high purity, excellent crystalline quality, without any cracks, and with a low dislocation density are grown. Preparation of the free-standing HVPE-GaN crystals by slicing as well as structural, electrical and optical qualities of the resulting wafers are reported and discussed.

Simulation of trap-assisted tunneling effect on characteristics of gallium nitride diodes

In this paper, simulations of I-V characteristics and band structures of magnesium and silicon doped gallium nitride diodes are presented. The numerical algorithm is based on the drift-diffusion semi-classic model, with the van Roosbroeck differential equation system involved. The model accounts for trap-assisted tunneling, which provides better agreement between the predicted and experimental I-V characteristics of p-n junctions in the low-bias range. We have performed one-dimensional simulations of devices. We compare the results with the results obtained from the standard drift-diffusion model. It is shown that taking the trap-assisted tunneling into account leads to good agreement with experimental data. We also demonstrate that a high doping of the p-n junctions may significantly increase the nonradiative recombination rate due to the prescribed effect.

Towards proper characterization of nonlinear metal-semiconductor contacts. Generalization of the transmission line method

Modern optoelectronic devices are often based on wide-bandgap semiconductors such as GaN. In such cases the current injecting contacts are usually nonlinear, especially for p-type materials. Using the standard transmission line method (TLM), which gives satisfactory results in linear cases, characterization of nonlinear contacts can lead to serious ambiguities. In this Letter, we derive exact formulas permitting to extract the current-voltage characteristics of the non-linear metal–semiconductor contact from measurements performed on standard TLM pattern, as well as to simulate behavior of such pattern for given model contact characteristic. The application of this generalized TLM method is illustrated on the example of Ni/Au contacts on p-GaN

Parameters of multilevel structure of the DX centre in GaAlAs from pressure studies of the Hall effect

Thermal emission from resonant DX levels in Si-doped GaAlAs was studied by measuring the temperature evolution of free electron concentration. Pressure was used to fill the levels with electrons. The absolute numbers of electrons captured on individual DX levels (related to different local environments of relaxed Si centre) were determined as a function of pressure or temperature. The analysis of the data allows the authors to determine the parameters of three components of the DX multilevel system: emission energies, positions of the levels and their pressure and temperature variations. Their data support the double donor (negative-U) hypothesis. The contribution of individual levels in thermostimulated recovery of persistent photoconductivity was determined and their role in developing the step-like structure of this process explained.