The first part of the paper shows the advantages and disadvantages of using GaN and SiC substrates with an off-cut up to 2 degrees. The following experimental observations were made: i) higher efficiency in p-doping with magnesium, ii) higher critical conditions for AlGaN cracking, iii) trigonal deformation of AlGaN and InGaN unit cells. A successful growth of AlGaN/GaN HEMT epi structure on 2 degree-off SiC substrates will be shown, what paves the way to a monolithic integration of GaN-based and SiC based devices on a common SiC substrate. In the second part of the paper, we show the experimental results on AlGaN/GaN degradation upon prolonged annealing at 600oC. In the case of epi structures on Si and sapphire, the parasitic parallel conductivity appeared, whereas in the case of SiC substrates, annealing introduced no distinct changes.

The pressure dependence of the electronic band structure of InN is investigated with emphasis on the conduction band effective mass and its dependence on free-electron concentration. Transport measurements are performed under hydrostatic pressure on three n-type samples of InN with different electron concentrations. The effective mass extracted from the electron mobility is found to increase with pressure, however with a pressure coefficient, which is lower for the samples with higher electron concentration. Calculations of the InN band structure are performed within the density functional theory by means of the linear muffin-tin-orbital method, including an empirical adjustment to reproduce known band edges at ambient pressure. The calculated variations of the highest occupied conduction band energy and the electron effective mass with free-electron concentration are compared to available experimental information. The results show a pronounced deviation from parabolic behavior of the lowest conduction band, and for k>0 this induces large differences between the values of the optical and curvature masses. Both the fundamental band gap and the electron effective mass increase with pressure, but due to the nonparabolic character of the conduction band, the pressure coefficient of the effective mass decreases with electron concentration, in agreement with the experimental observation.

Usually the multi-layer technique is applied for the growth of p-type gallium nitride (GaN) films, and the active p-type layer is deposited on an n-type GaN buffer layer. In the present paper, the electrical transport phenomena (conductivity and Hall effect) have been studied in p-GaN/GaN homo-structures grown on a sapphire substrate. Two types of GaN buffer layers were used: a silicon-doped one and another strongly compensated by magnesium. We demonstrate that in the analysis of the electrical conduction phenomena the parallel conduction in the n-type buffer layer cannot be neglected, and it manifests itself in the studies of a conduction process as a function of temperature and as a function of electric field intensity.

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 degrees with respect to ideal < 0001 > plane) were employed. An increase in the hole carrier concentration to the level above 10(18) cm(-3) and a decrease in GaN:Mg resistivity below 1 Omega 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. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3013352]

Calculations of the electronic band structure and effective mass of InN are performed within the density functional theory by means of the linear muffin‐tin‐orbital method. The results show a pronounced nonparabolicity of the conduction band. Calculated variations of the highest occupied conduction band energy and electron effective mass with free electron concentration are presented and compared to available experimental data. Pressure effects are studied. Both the fundamental band gap and the electron effective mass increase with hydrostatic pressure, but due to the nonparabolic character of the conduction band of InN the pressure coefficient of the effective mass decreases with electron concentration. Experimental verification of this behavior has been performed on three n‐type samples of InN with different electron concentrations. The measurements and calculations agree in the description of the dependence of effective mass and its pressure coefficient on electron concentration. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We discuss the influence of indium segregation-induced disorder effects in InxGa1-xN alloys. Changes of the transport mechanism between InN and InxGa1-xN with x=0.58 were demonstrated by means of temperature-dependent conductivity measurements. Furthermore, an increase of (i) full width at half maximum of photoluminescence (PL) and (ii) the Stokes shift between PL and absorption was seen for samples approaching an In content of 0.5, which can also be attributed to growing disorder. Hydrostatic pressure dependent PL measurements of In-rich InGaN alloys are diacussed. Due to the fact that PL in InGaN originates from regions with higher-than-average In content, the luminescence pressure coefficient dE(E)/dp should not be associated with the average In content, but with the In content which is in accordance with the energy of the photon emission. This correction leads to a reduction of the large bowing of dE(E)/dp (associated with the band gap) which was reported earlier. Furthermore, it is shown that the electron concentration in InN has a significant influence on the measured value of dE(E)/dp.

Transport studies of as-grown and proton-irradiated n-InN have been performed aiming at verification of the nature of localized donor states resonant with the InN conduction band. These resonant donor states (RDS) show a clear contribution to the electrical conduction in low electron concentration InN epitaxial layers. We used proton irradiation to increase the number of incorporated native point defects of donor character in InN layers. Then, the performed studies of pressure dependence of the Hall electron concentration clearly show no increase in the number of RDS in samples exposed to irradiation in spite of the increase in the conducting electron concentration.

The annealing process of magnesium-doped gallium nitride (GaN:Mg) epilayers grown by metal-organic vapor-phase epitaxy was investigated by in situ measurements of electrical transport properties. The resistivity ρ and the Hall effect were studied as functions of time and temperature in the range of 20–600°C. A time-dependent p-type conductivity was observed at temperatures as low as 350°C. Activation energy of about Eact=1.5eV was found for the magnesium acceptor (Mg) from the isothermal measurements of ρ(t) kinetics in the range of 350–550°C. This value corresponds well to the theoretical prediction for the thermal dissociation of magnesium-hydrogen complexes (Mg–H). The annealing at temperatures higher than 600°C leads obviously to the activation of Mg acceptors, but the final resistivity of the sample is higher than the result obtained after annealing at 520°C. The ionization energy of electrically active Mg acceptor level of about EA=170meV was found from the temperature dependences of the resistivity ρ(T).

We used single crystals of GaN, obtained from high-pressure synthesis, as substrates for Metalorganics Vapor Phase Epitaxy growth of violet and UV laser diodes. The use of high-quality bulk GaN leads to the decrease of the dislocation density to the low level of 10 5 cm −2 , i.e. two orders of magnitude better than typical for the Epitaxial Lateral Overgrowth laser structures fabricated on sapphire. The low density and homogeneous distribution of defects in our structures enables the realization of broad stripe laser diodes. We demonstrate that our laser diodes, having 15 μm wide stripes, are able to emit 1.3-1.9 W per facet (50% reflectivity) in 30 ns long pulses. This result, which is among the best ever reported for nitride lasers, opens the path for the development of a new generation of high power laser diodes.

GaN single crystals exhibiting the lowest dislocation density (below 100/cm 2 ) are grown at high hydrostatic N 2 pressures of 10-20 kbar. Despite small dimensions of such crystals (up to 1/2) they offer a unique chance to construct high power laser diodes and some other devices. However, in order to develop high quality epitaxial structures, a number of steps, different to GaN epitaxy on foreign substrates, must be made. These steps include: (i) surface preparation of Ga-terminated side used for epitaxy, (ii) optimization of substrate thickness, (iii) optimization of substrate miscut, (iv) N-terminated side preparation for back-side contact. This work contains the following information: (i) description how the blue laser diodes on bulk GaN crystals are made, (ii) what is their crystallographic quality (in particular, a very large bowing for unrelaxed structures will be shown- the bowing radius can be as small as 0.1 m), (iii) what are the optical (very low threshold of 2.5 kW/cm 2 for optical pumping) and (iv) electrical parameters (2.6 W (1.3 W per facet) optical power under pulsed operation).

In this paper we discuss the applicability of high-pressure grown bulk GaN crystals as substrates for device oriented MOVPE homoepitaxy. First, we fabricated light emitting diodes as a step towards realization of our target device: a blue light emitting laser diode. Our homoepitaxialy grown LEDs are characterized by excellent electrical characteristics and very satisfactory optical properties. Building on the experience gained during this first stage of our research we have been able to fabricate pulse current operated laser diodes emitting light at a wavelength between 397 and 430 nm. We believe that this fast progress clearly demonstrates the usefulness of bulk GaN substrates for optoelectronic devices, especially for high power laser diodes.

Room temperature (RT) pulsed operation of blue (402-427 nm) InGaN multi-quantum well (MQW) structure laser diodes (LDs) grown on bulk GaN crystals have been demonstrated. Substrate crystals were grown with the high nitrogen pressure and high temperature solution method. After substrate preparation the laser structures were grown by the low pressure MOVPE process. The LD structures are fully strained on the whole area of GaN crystals, without mismatch-related defects, being confirmed by X-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Stripe geometry cavity has a contact area of 10 x 500 mum(2). The mirror facets were cleaved and coated with reflection dielectric layers.

In our previous paper [1] a high pressure technique for monitoring pressure up to 700 MPa and temperature from m 40 °C to +100 °C in several pressure vessels simultaneously was reported. This technique, applied in Unipress High Pressure Multivessel Apparatus for studies of biological materials, revealed some limitations. In this paper we propose a new solution which allows to overcome them. In this solution two different pressure media are used, separated from each other: one suitable for biological studies and the other proper for electric sensors. A new integrated pressure/temperature probe is presented in which manganin pressure gauge is confined in a metal bellows separating the two pressure transmitting media. The bellows can be easily assembled or disassembled, allowing promptly to refill pressure medium or to replace the pressure gauge. Temperature is measured by constantan/copper thermocouple. The probe is linked to data acquisition system. Taking into account temperature dependence of the manganin pressure gauge, simultaneous measurements of the resistance of pressure gauge and thermocouple voltage allow to compute pressure at any temperature. The new probe is integrated with the bottom closure of the pressure vessel which also incorporates capillary inlet. Such a design leaves free access from the top of the vessel, allowing easy mounting the studied samples as well as other additional probes.

We used high-pressure grown GaN single crystal substrates to fabricate dislocation free optoelectronic devices like light emitting diodes and laser diodes structures. The latter ones demonstrated laser action under optical pumping condition with the threshold of about 200 kW/cm2 at room temperature. In the present paper we would focus on the specific aspects of the homoepitaxial growth by MOVPE method including epi-ready substrate preparation and surface polarity choice. We believe that our results demonstrate clearly the feasibility of device fabrication based on high-pressure grown GaN bulk crystals.

We have shown that the increase of the buffer layer thickness in low-dimensional CdTe/Cd1-xMgxTe structures leads to significant improvement of the sample quality and stability. In such hetrostructures high quality 2DEG in the quantum well is formed without any illumination and without parallel conduction. Moreover, the process of pressure-induced degradation is much less essential than for samples having thin buffer layer.

We have shown that in CdTe/Cd1-xMgxTe heterostructures revealing parallel transport, hydrostatic pressure induces a decrease in both 2D conduction in the quantum well and in low mobility parallel channel. This behaviour, contrary to III-V heterostructures, is due to persistent decrease of the mobility. For samples having thick buffer layer the contribution of the parallel conduction is practically eliminated at high pressure with 2D conduction remaining dominant, while for samples with thin buffer layer the situation with pure 2D conduction is not achieved.

Resistivity and Hall concentration in AlGaAs layers have been measured as a function of pressure up to 25 kbar and of temperature from 77 K to 400 K. The layers were grown by LPE on GaAs and doped with Te and Sn. We first studied the properties of AlxGa1-xAs with Al composition x from 0.1 to 0.3. Then we measured the graded-gap layers with x varying e.g., from 10% to 40%. Such graded-gap layers can be considered as the parallel connection of layers with uniform composition. They reveal strong pressure sensitivity and hat temperature variation of resistance. Tin-doped layers are good candidates for "laboratory" pressure sensors operating up to 30kbar in the 77-300K temperature range. Tellurium-doped layers reveal metastable properties below 120K but they can be useful for pressure sensing in the high-temperature range. The properties of graded-gap AlGaAs layers are superior to those of manganin and InSb sensors.

DX centers have been detected in vertical transport experiments of GaAs-AlAs superlattices. We studied miniband conduction properties of such semiconductor structures in presence of high hydrostatic pressures and controlled temperature. Hystheresis effect in the current-voltage characteristics was observed. We show that miniband transport properties are dependent on the path of the pressure cycle imposed to the sample. It is clear from our results that DX centers are present in the active superlattice region. We propose that the energy associated with DX states in superlattice results from a "hybridization" of DX centers of both GaAs and AlAs bulk materials.