P. Prystawko

AlGaN/GaN high electron mobility transistors as a voltage-tunable room temperature terahertz sources

We report on room temperature terahertz generation by a submicron size AlGaN/GaN-based high electron mobility transistors. The emission peak is found to be tunable by the gate voltage between 0.75 and 2.1 THz. Radiation frequencies correspond to the lowest fundamental plasma mode in the gated region of the transistor channel. Emission appears at a certain drain bias in a thresholdlike manner. Observed emission is interpreted as a result of Dyakonov–Shur plasma wave instability in the gated two-dimensional electron gas.

GaN–AlGaN heterostructure field-effect transistors over bulk GaN substrates

We report on AlGaN/GaN heterostructures and heterostructure field-effect transistors (HFETs) fabricated on high-pressure-grown bulk GaN substrates. The 2d electron gas channel exhibits excellent electronic properties with room-temperature electron Hall mobility as high as μ=1650 cm2/V s combined with a very large electron sheet density ns≈1.4×1013 cm−2. The HFET devices demonstrated better linearity of transconductance and low gate leakage, especially at elevated temperatures. We also present the comparative study of high-current AlGaN/GaN HFETs (nsμ>2×1016 V−1 s−1) grown on bulk GaN, sapphire, and SiC substrates under the same conditions. We demonstrate that in the high-power regime, the self-heating effects, and not a dislocation density, is the dominant factor determining the device behavior.

Degradation mechanisms in InGaN laser diodes grown on bulk GaN crystals

We have investigated the aging processes in InGaN laser diodes fabricated by metal organic vapor phase epitaxy on low-dislocation-density, high-pressure-grown bulk gallium nitride crystals. The measured threshold current turned out to be a square root function of aging time, indicating the importance of diffusion for device degradation. The differential efficiency, in contrast, was roughly constant during these experiments. From these two observations we can conclude that the main reason for degradation is the diffusion-enhanced increase of nonradiative recombination within the active layer of the laser diode. Additionally, microscopic studies of the degraded structures did not reveal any new dislocations within the active area of the aged diodes, thus identifying point defects as a source of nonradiative processes.

GaN homoepitaxial layers grown by metalorganic chemical vapor deposition

GaN homoepitaxial layers were grown using metalorganic chemical vapor deposition on the highly conductive GaN bulk crystals grown at high hydrostatic pressure. The epitaxial growth process was monitored by reflectivity of red laser light. The oscillations of its intensity served for the precise evaluation of the growth rate. The layers were then investigated using far-infrared reflectivity (FIR), x-ray diffraction, and photoluminescence. The FIR spectrum gave small free-electron concentration in the layers in contrast to the substrates which had about 2.5×1019 cm−3 of free electrons. In x-ray diffraction, the peaks of the substrate and of the layer were separated due to the difference between the lattice parameters of the bulk GaN substrate (expanded by free electrons and point defects) and the layer which had a low free electron concentration. Both peaks had rocking curves with half widths of about 20 arcsec. The photoluminescence spectrum contained narrow peaks of less than 1 meV with the excitonic line...

High power blue–violet InGaN laser diodes grown on bulk GaN substrates by plasma-assisted molecular beam epitaxy

We report on the InGaN multi-quantum well laser diodes (LDs) made by RF plasma-assisted molecular beam epitaxy (PAMBE). The laser operation was demonstrated in a temperature range from 297 K up to 360 K with pulsed current injections using 50 ns pulses at 0.25% duty cycle. The threshold current density and voltage for these LDs were 9 kA cm−2 (680 mA) and 8.2 V respectively at 297 K. The slope efficiency is 0.35–0.47 W A−1. A high output power of 1.1 W was obtained during pulse operation for 3.6 A and 8.7 V. We compare parameters of laser diodes with two and five InGaN/InGaN quantum wells. The new, low temperature growth mechanism which enhances surface adatom kinetics, together with bulk GaN low dislocation density substrates allowed us to grow high quality laser diode structures. Our result indicates that there are no intrinsic limitations in the growth of nitride-based optoelectronic devices by PAMBE.

Decay of stimulated and spontaneous emission in highly excited homoepitaxial GaN

The high-density effects in the recombination of electron–hole plasma in photoexcited homoepitaxial GaN epilayers were studied by means of transient photoluminescence at room temperature. Owing to the “backward” and “lateral” photoluminescence measurement geometries employed, the influence of stimulated transitions on the decay of degenerate nonthermalized plasma was revealed. The lateral stimulated emission was demonstrated to cause a remarkable increase in the recombination rate on the early stage of the luminescence transient. A delayed enhancement of the stimulated emission due to the cooling of plasma from the initial temperature of 1100 K was observed. After completion of the thermalization process and exhaustion of the stimulated emission, the spontaneous-luminescence decay exhibited an exponential slope that relates to the nonradiative recombination of the carriers. The homoepitaxially grown GaN layer featured a luminescence decay time of 445 ps that implies a room-temperature free-carrier lifetim...

Fully-screened polarization-induced electric fields in blue∕violet InGaN/GaN light-emitting devices grown on bulk GaN

Photocurrent spectroscopy and hydrostatic-pressure-dependent electroluminescence are used to show that heavy 1×1019cm−3 Si doping of quantum barriers is sufficient to achieve full screening of polarization-induced electric fields (PIEFs) in nitride light emitting diodes (LEDs) and laser diodes (LDs) with InGaN quantum wells. Furthermore, it is shown that at currents close to lasing threshold in nitride LDs injected charge alone is sufficient to achieve full screening of PIEFs. In contrast, full screening at low currents can only be accomplished via Si doping of quantum barriers.

Electro-structural evolution and Schottky barrier height in annealed Au/Ni contacts onto p-GaN

This paper reports on the evolution of the structural and electrical proprieties of Au/Ni contacts to p-type gallium nitride (GaN) upon thermal treatments. Annealing of the metals in an oxidizing atmosphere (N2/O2) led to better electrical characteristics, in terms of specific contact resistance (ρc), with respect to non-reacting ambient (Ar). The evolution of the metal structures and the formation of a NiO layer both at the sample surface and at the interface with p-GaN were monitored by several techniques such as transmission electron microscopy (TEM), x-ray diffraction (XRD), and atomic force microscopy (AFM). Furthermore, a reduction of the Schottky barrier height from 1.07 eV (Ar annealing) to 0.71 eV (N2/O2 annealing) was determined by the temperature dependence of the ρc. Local current maps by conductive AFM demonstrate the role of the interface in the conduction mechanism. These electrical results were correlated with the interfacial microstructure of the annealed contacts considering different tr...

Acoustic phonon scattering of two-dimensional electrons in GaN/AlGaN heterostructures

We have measured the temperature dependence of the mobility of the two-dimensional electron gas in AlGaN/GaN heterostructures grown on bulk GaN substrates. The linear dependence of the inverse mobility on temperature at temperatures below 50 K indicates the importance of acoustic phonon scattering in these high mobility heterostructures. Using the temperature dependence of the mobility at a range of carrier densities, we determined the GaN conduction band deformation potential to be a(c)=9.1+/-0.7 eV. This result provides a crucial parameter for accurate calculations of intrinsic mobility limits in AlGaN/GaN heterostructures

DOPING OF HOMOEPITAXIAL GAN LAYERS

Gallium nitride doped with oxygen (unintentionally), silicon and magnesium was grown by metalorganic chemical vapor deposition on the conductive single crystals of GaN grown at high hydrostatic pressure. The layers were examined using X-ray diffraction, photoluminescence and far-infrared reflectivity. It was found that the incorporation of silicon depends on the side used for deposition. For the two Si-doped layers grown in the same run, the one grown on the (00.1) side (gallium-terminated) had always smaller free electron concentration with respect to the (00._1) side (nitrogen-terminated). This conclusion could be drawn from the lattice expansion by free electrons, the photoluminescence peak shift by Burstein-Moss effect and the position of plasma edge in far-infrared reflectivity.