arXiv subjects

Markus Pristovsek

Publications and source records attributed to Markus Pristovsek.

6 recordsLinked to original sources

arXiv2026

Depletion-mode N-polar AlN-based high electron mobility transistors with improved on/off ratios

We report N-polar AlN-based high-electron mobility transistors (HEMTs) with a GaN channel thickness of 5.2 nm on N-polar AlN on sapphire. The threshold voltage is around -2.4 to -3.0 V with saturation currents over 240 mA/mm and on/off ratios as high as 10,000, much higher than previously reported N-polar AlN-based HEMTs. The high on/off ratio is attributed to the use of an abrupt AlN/GaN heterostructure with a dedicated AlN transition layer, together with improved gate leakage. The high frequency properties as well as the on-resistance of ~20 Ohm mm are all limited by the 2000 Ohm/square sheet resistance of the channel layer.

cond-mat.mtrl-sci↗

arXiv2024

Nitrogen-polar growth of AlN on vicinal (0001) sapphire by MOVPE

We report about metalorganic vapour phase epitaxy of smooth nitrogen-polar AlN templates on vicinal (0001) sapphire substrates. The influence of V/III ratio, growth temperature, growth rate, as well as sapphire-nitridation time and temperature were studied. With 4° offcut sapphire, step-flow growth was possible only with V/III ratios below 2. However, optimal V/III ratio required precise adjustment, possibly dependent on reactor history and geometry. A rather narrow temperature window of less than 40°C existed for smooth surface morphology. Reducing the temperature affected adatom mobility, eventually disrupting step-flow growth; increasing the temperature favoured the formation of high-aspect-ratio defects on the epilayer. A low thermal-budget nitridation step with a short nitridation time of 15 s proved to be effective in controlling polarity without inducing excessive surface damage on the sapphire substrate. Growth rate also influenced surface morphology, with an increase in RMS roughness and step-bouncing for faster growths; however, at growth rates of 1.4 $μ$m/h or higher step-flow growth could no longer form. Finally, we developed a V/III ratio fine-tuning procedure, whereby the reactor-specific value that induces optimal growth is inferred by growth-rate variations. With this method, N polar AlN templates with sub-nanometre RMS roughness were demonstrated for both 4° and 2° offcut sapphire substrates.

cond-mat.mtrl-sci↗

arXiv2020

Limitation of simple np-n tunnel junction based LEDs grown by MOVPE

We show evidence that tunnel junctions (TJs) in GaN grown by metal-organic vapor phase epitaxy are dominated by trap-assisted (Poole-Frenkel) tunneling. This stems from observations of the careful optimized doping for the TJs. Especially the p$^{++}$ and the n$^{++}$ layers are far from ideal. The n$^{++}$ layer induces 3D growth, which can be seen by a rising oxygen signal in Secondary Ions Mass Spectroscopy (SIMS). Furthermore, Mg segregation observed by SIMS indicates a depletion region of more than 10 nm. Still, we could realize TJ based LEDs with a low penalty voltage of 1.1 V and a specific differential resistance of about 10$^{-2}$ $Ω$.cm$^2$ at 20 mA without using an InGaN interlayer.

physics.app-ph↗

arXiv2002

As-rich GaAs(001) surfaces observed during As4-irradition by scanning tunneling microscopy

As-rich GaAs (001) surfaces are successfully observed during As4-irradition by a system in which scanning tunneling microscopy (STM) and molecular beam epitaxy can be performed simultaneously. With a substrate temperature of 440 C and an As4 partial pressure of 2x10-6 torr, reflection high energy electron diffraction patterns and reflectance anisotropy spectra confirm a c(4x4) As-stabilized surface. STM images clearly show alteration of the surface reconstructions while scanning. It is postulated that continual attachment / detachment of As molecules to and from the surface produces the observed dynamic behavior.

physics.chem-ph↗

arXiv2002

Structure analysis of the Ga-stabilized GaAs(001)-c(8x2) surface at high temperatures

Structure of the Ga-stabilized GaAs(001)-c(8x2) surface has been studied using rocking-curve analysis of reflection high-energy electron diffraction (RHEED). The c(8x2) structure emerges at temperatures higher than 600C, but is unstable with respect to the change to the (2x6)/(3x6) structure at lower temperatures. Our RHEED rocking-curve analysis at high temperatures revealed that the c(8x2) surface has the structure which is basically the same as that recently proposed by Kumpf et al. [Phys. Rev. Lett. 86, 3586 (2001)]. We found that the surface atomic configurations are locally fluctuated at high temperatures without disturbing the c(8x2) periodicity.

physics.chem-ph↗

arXiv2002

Ga-rich GaAs(001) surfaces observed during high-temperature annealing by scanning tunneling microscopy

Ga-rich GaAs (001) surfaces are successfully observed during high-temperature annealing by scanning tunneling microscopy (STM). With a substrate temperature of 550 C, reflection high-energy diffraction patterns and reflectance anisotropy spectra confirm a (4x2) Ga-stabilized surface. STM images clearly show alteration of the surface reconstructions while scanning. It is postulated that detaching and attaching of Ga adatoms may be the cause of these surface dynamics. For these conditions it is determined that zeta(4x4), zeta2(4x4) and zeta(4x6) reconstructions co-exist on the surface. The zeta2(4x4) reconstruction contains a Ga tetramer cluster and in more Ga-rich conditions, the zeta2(4x6) surface has a Ga octamer cluster.

physics.atm-clus↗