L. Dmowski

Search for free holes in InN:Mg-interplay between surface layer and Mg-acceptor doped interior

We measured lateral ac transport (up to 20 MHz), thermopower, as well as resistivity and Hall effect in InN:Mg samples with various Mg content. The sign of the Hall effect for all the samples was negative (electrons), however, the thermopower (α) measurements have shown the p-type sign of α for moderate Mg content—in the window centered around 1×1019 cm−3. Further overdoping with Mg yields donor type of defects and the change of thermoelectric power sign. The ac measurements performed as a function of frequency revealed that in both samples exhibiting and nonexhibiting p-type sign of thermopower, the n-type inversion layer at the surface does not prevent the electric contact to the bulk layer. Therefore we conclude that the n-type Hall effect invariably reported for all the Mg-doped samples originates from electron domination in mobility-weighted contributions of both types of carriers.

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.

Blue‐Laser Structures Grown on Bulk GaN Crystals

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 μm 2 . The mirror facets were cleaved and coated with reflection dielectric layers.

Hole carrier concentration and photoluminescence in magnesium doped InGaN and GaN grown on sapphire and GaN misoriented substrates

Systematic studies of InxGa1−xN layers (0≤x<0.13) doped with Mg were performed. Samples were grown by metal organic vapor phase epitaxy. Intermediate Mg doping in the range of 2×1019 cm−3 was chosen to achieve a maximum hole carrier concentration, pH (as measured by Hall effect) of 4×1018 cm−3 in samples with high x. We confirmed reports on decreasing resistivity in InxGa1−xN:Mg epitaxial layers observed with increasing x. This finding is very important for applications. In the performed research we attempted to separate contributions to pH increase resulting from increase in In-content and an associated decrease in growth temperature, Tgr (necessary to obtain high x). For this purpose In-content increase was achieved by means of either (i) lowering the growth temperature (from 1020 to 830 °C) or by (ii) varying an intended GaN substrate miscut. We demonstrated that the increase in pH in InxGa1−xN:Mg is caused by higher In concentration while a drop in Tgr plays a secondary role. Studies of photoluminesce...

Band-to-band character of photoluminescence from InN and In-rich InGaN revealed by hydrostatic pressure studies

The authors studied the hydrostatic pressure dependence of photoluminescence (PL) from In-rich InxGa1−xN with In contents x between 0.58 and 1.00. The observed PL pressure coefficients of 20–25meV∕GPa agree well with previously reported experimental and theoretical values of the band gap pressure coefficient, from which they conclude that band-to-band recombination is responsible for PL emission. This contrasts with earlier reports, where relatively low PL pressure coefficients were interpreted as evidence of the involvement of strongly localized states in the PL emission. The reported observation of band-to-band recombination in In-rich InGaN is encouraging from the point of view of the construction of light emitters, since band-to-band recombination is more efficient than recombination via localized states. Furthermore, significant bowing of the band gap pressure coefficient in In-rich InxGa1−xN, as predicted by theory, is confirmed.

High pressure investigation of ferroelectric phase transition in PbSnTe

Abstract The anomalies in electrical transport properties at the displacive phase transition in Pb1−xSnxTe (x=0.18−0.86) has been investigated. The results of applying pressure point out the multiphonon scattering mechanism caused by the soft TO phonons as responsible for the resistivity anomaly.

Coexistence of free holes and electrons in InN:Mg with In- and N-growth polarities

The coexistence of two types of carriers (free electrons and free holes) in InN:Mg and their competition is demonstrated by the temperature and magnetic-field-induced change of the sign of thermopower (α) as well as the maximum entropy mobility spectrum analysis. The results confirm the existence of alternative carrier channels in addition to the n-type surface inversion layer and p-type bulk. They also show that In-polarity can be propitious for occurrence of p-type conductivity.

Low dislocation density, high power InGaN laser diodes

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.

Deformation potential in high electron mobility GaAs/GaAsAs heterostructures

We have studied scattering mechanisms in high electron mobility GaAs/Ga0.7Al0.3 As heterostructures. We have performed measurements of two-dimensional electron gas concentration and mobility as functions of temperature and hydrostatic pressure. We focused our attention on two-dimensional scattering by acoustic phonons and we found that the magnitude of the deformation potential D for GaAs in the considered heterostructures is -12±1.0 eV. Contrary to the common assumptions about the pressure independence of D we have found that its absolute value decreases with applied hydrostatic pressure by about 10%/GPa.

Kinetics of low-temperature activation of acceptors in magnesium-doped gallium nitride epilayers grown by metal-organic vapor-phase epitaxy

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...