The adhesion of atmospheric silicon (Si) to the highly polar gallium nitride (GaN) regrowth surface leads to the formation of a parasitic, conductive channel in lateral GaN-on-GaN devices. This parasitic channel can result in high transistor off-state leakage currents, impeding the use of GaN as substrate material for certain applications. Ex-situ cleaning methods to remove the surficial Si effectively were unsuccessful, because of fast re-adsorption from the ambient atmosphere. Therefore, establishing a passivation for GaN after an in-situ cleaning step, which can be desorbed prior to GaN/AlGaN heterostructure growth, is considered to be a technical solution. The formation of a thin passivating gallium oxide layer on the GaN surface is demonstrated by oxygen plasma oxidation. While an amorphous character of the passivation layer is needed for subsequent desorption, the oxygen plasma treatment resulted in a stable crystalline phase preventing desorption. Moreover, the plasma treatment causes significant damage to the GaN surface and introduces additional impurities. Specifically, these two aspects-the introduction of additional impurities and defects at the GaN surface and the crystalline character of the oxide-prevented removal of the Si contamination at the GaN/GaN regrowth interface. Nevertheless, the experiments provide valuable insights into one possible oxidation mechanism and particularly point at the necessity of establishing an amorphous gallium oxide passivation layer, which will be the focus of future work.

While attempting to form tantalum-based shallow ohmic contacts to the two-dimensional electron gas (2DEG) confined in GaN/AlGaN heterostructures, it was noticed that a parasitic channel in a few hundred nanometers depth is contacted and exhibits ohmic behavior after contact metal stack deposition and before additional thermal annealing. This indicates significant metal diffusion into the GaN/AlGaN stack and prevents the separation of the individual contributions of the 2DEG and the parasitic channel to the lateral electrical conductivity. Post-deposition thermal treatment only slightly increases electrical conductivity, which is clearly dominated by the parasitic channel. In contrast, conductivity in a reference GaN/AlGaN stack with a 2DEG as the only conductive channel increases by a few orders of magnitude after the ohmic contact is formed upon thermal annealing. Element specific analysis confirms high concentrations and, thus, sufficient diffusion of contact stack metals to a parasitic layer several hundred nanometers into GaN. Our findings demonstrate the difficulties in shallow ohmic contact formation for GaN/AlGaN stacks, which might become of interest in GaN-on-GaN high-frequency applications.

The statistical nature of impurity dopants introduced in a semiconductor leads to variability in electrical properties, posing significant challenges to device performance. This randomness becomes especially problematic as device dimensions shrink. Alternative approaches for material modification that offer precise control over the material properties, including predictable behavior at low temperatures, are essential for largescale device integration. Modulation acceptor doping in SiO2 was introduced as a nanoscale-doping alternative to classical impurity doping for silicon, where the carriers are spatially isolated from the dopants, resulting in a drastically reduced Coulomb scattering within silicon. In this work, the stability of the new doping method in ambient conditions is assessed. Further, a comparative analysis of impurity (p-type) doped and modulation acceptor doped Hall structures is presented. The modulation doped sample series reveals a consistent resistivity and a stable carrier density. While, the impurity-doped sample series demonstrates a lower dopant activation efficiency, higher resistivity and highlights the statistical nature of the classical method. In addition, the temperature behaviour of resistivity is studied from 300 K down to 100 K.

Impurity doping at the nanoscale for silicon is becoming less efficient with conventional techniques. Here, an alternative virtual doping method is presented for silicon that can achieve an equivalent carrier density while addressing the primary limitations of traditional doping methods. The doping for silicon is carried out by placing aluminum-induced acceptor states externally in a silicon dioxide dielectric shell. This technique can be referred to as direct modulation doping. The resistivity, carrier density, and mobility are investigated by Hall effect measurements to characterize the carrier transport using the new doping method. The results thereof are compared with carrier transport analysis of conventionally doped silicon at room-temperature, demonstrating a 100% increase in carrier mobility at equal carrier density. The sheet density of hole carriers in silicon due to modulation doping remains nearly constant, approximate to 4.7 x 1012 cm-2 over a wide temperature range from 300 down to 2 K, proving that modulation-doped devices do not undergo carrier freeze-out at cryogenic temperatures. In addition, a mobility enhancement is demonstrated with an increase from 89 cm2 Vs-1 at 300 K to 227 cm2 Vs-1 at 10 K, highlighting the benefits of the new method for creating emerging nanoscale electronic devices or peripheral cryo-electronics to quantum computing.

Inconsistencies in the concentrations of unintentional donor impurities and free charge carriers in GaN/AlGaN layer stacks hosting a two-dimensional electron gas (2DEG) can be attributed to the measurement procedure and solely depend on the way in which the free charge carrier concentration is extracted. Particularly, when the 2DEG acts as the bottom electrode in capacitance versus voltage measurements, unphysically low concentrations of free charges are calculated. This originates from the depletion of the 2DEG and the accompanying disappearance of the bottom electrode. It is shown that, for the case of a defined (non-vanishing) bottom electrode, the levels of donor impurities and resulting free charges consistently match.

Fundamental aspects of spin-orbit interaction in commercially relevant GaN/AlxGa1-xN heterostructures hosting two-dimensional electron systems are extensively studied by electron spin resonance (ESR). This unprecedentedly accurate experimental technique allows access to the fine details of coupling between the spin degree of freedom and the quantized orbital motion of an electron in the quantum Hall regime through the precise measurement of the electron g-factor. Filling-factor-dependent changes of the g-factor value in strong magnetic fields allow extraction of the Rashba spin-orbit interaction constant alpha in various GaN/AlxGa1-xN heterojunctions with electron sheet densities in the range 0.8-5.2 x 10(12) cm(-2). Despite three significantly different approaches used to tune the electron density, the extracted value of alpha is on the order of 5.3 +/- 0.4 meV angstrom for all experimental realizations. This striking finding can be explained by assuming that the spin-orbit interaction is of bulk origin. Theoretical calculations confirm this observation, as the bulk cubic in-plane wave-vector term of the spin-orbit interaction compensates the rising contribution due to the change in the quantum well shape. Finally, the value of alpha is cross-checked and confirmed by weak antilocalization measurements in the longitudinal magnetoresistance at substantially lower magnetic field values compared to the ESR approach. The presented experimental findings provide knowledge for estimating the true scales of spin-orbit interaction in GaN-based spintronic devices.

The suppression of parasitic conductivity at the substrate/MBE regrowth interface in GaN/AlGaN heterostructures by carbon delta-doping is reported. Parasitic conductivity results from silicon adhesion at the GaN substrate surface; its removal before loading the substrates into the UHV growth chamber seems to be impossible. This contamination and the resulting parasitic conductivity is particularly detrimental when growing on unintentionally doped substrates since it masks the 2D transport properties in lateral transport devices even at cryogenic temperatures. The formation of this parasitic channel can be impeded by compensating the silicon-induced charges through carbon delta-doping. In consequence, the intrinsic 2D channel properties can be studied in low-temperature magneto-transport measurements.

Fundamental aspects of spin-orbit interaction in commercially relevant $\mathrm{Ga}\mathrm{N}$/${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ heterostructures hosting two-dimensional electron systems are extensively studied by electron spin resonance (ESR). This unprecedentedly accurate experimental technique allows access to the fine details of coupling between the spin degree of freedom and the quantized orbital motion of an electron in the quantum Hall regime through the precise measurement of the electron $g$-factor. Filling-factor-dependent changes of the $g$-factor value in strong magnetic fields allow extraction of the Rashba spin-orbit interaction constant $\ensuremath{\alpha}$ in various $\mathrm{Ga}\mathrm{N}$/${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ heterojunctions with electron sheet densities in the range $0.8--5.2\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Despite three significantly different approaches used to tune the electron density, the extracted value of $\ensuremath{\alpha}$ is on the order of $5.3\ifmmode\pm\else\textpm\fi{}0.4$ meV $\text{\AA{}}$ for all experimental realizations. This striking finding can be explained by assuming that the spin-orbit interaction is of bulk origin. Theoretical calculations confirm this observation, as the bulk cubic in-plane wave-vector term of the spin-orbit interaction compensates the rising contribution due to the change in the quantum well shape. Finally, the value of $\ensuremath{\alpha}$ is cross-checked and confirmed by weak antilocalization measurements in the longitudinal magnetoresistance at substantially lower magnetic field values compared to the ESR approach. The presented experimental findings provide knowledge for estimating the true scales of spin-orbit interaction in GaN-based spintronic devices.

This study focuses on the wavelength-dependent conductivity of optically-induced 2-dimensional electron gases (2DEGs) in ultra-pure GaN/AlGaN heterostructures grown by Molecular Beam Epitaxy (MBE). Our experiments show that only light with energies larger than the bandgap of GaN (3.4 eV) is able to generate a 2DEG, which is else absent under illumination with wavelengths exceeding 364 nm. The conductivity of the generated 2DEG depends on the number of incident photons with sufficient energy. Moreover, resonant absorption can be clearly observed around the bandgap energy of GaN at room temperature.

Blue and yellow emission bands in carbon-doped GaN grown by MBE were investigated in low-temperature cathodoluminescence measurements performed in a scanning transmission electron microscope (STEM-CL) with high spatial resolution. Blue luminescence at 2.85 eV and two contributions in the spectral range of the yellow emission band around 2.2 eV separated by 120 meV are observed in carbon-doped material, whereas only one distinctive yellow luminescence contribution was found in unintentionally-doped GaN.

2D electron density tuning from 5.3×1012 cm−2 to 8.8×1012 cm−2 by UV illumination has been demonstrated for an ultra-pure GaN/Al0.25Ga0.75N heterojunction. A single subband is occupied in the dark with an electron density of 5.3×1012 cm−2, while in the illuminated sample with an electron density of 8.8×1012 cm−2, characteristic beating patterns appear in magnetotransport data due to the filling of the second quantized subband. Simultaneously, this distribution is unambiguously confirmed by Landau level splitting and intensity oscillations in photoluminescence spectra. The electron densities and quantum lifetimes in each individual subband have been extracted independently, and the intersubband energy spacing amounts to 82 meV. Surprisingly, the quantum scattering time for electrons residing in the second subband is increased compared to its ground state counterpart.

The presence of a 2D electron gas (2DEG) in GaN/Al x Ga 1– x N heterostructures with low aluminum mole fraction is found to depend on the residual background impurity concentration in the GaN/AlGaN layer stack. At a residual donor level of 2 × 10 16 cm −3 , a 2DEG is absent at 300 K in dark environment. Such a 2DEG can be generated at the GaN/AlGaN interface either by illumination with ultraviolet light or by applying an electrostatic potential. The latter results in inherently normally‐off switching characteristics of lateral field‐effect transistors.

The high-frequency transport of a two-dimensional (2D) electron system was investigated by measuring the rf power transmitted through a pair of emitter/detector T-shaped antennas capacitively coupled to the 2D channel. The frequency range covered amounted to 10-100 MHz. The distinctive feature of such a setup is that neither were Ohmic contacts formed to the electron system, nor were metallic pads deposited on the sample surface. We demonstrate that in such an arrangement the microwave-induced resistance oscillations could be observed in case the sample was additionally excited by a microwave radiation of 60-100 GHz frequencies. The amplitude of the first oscillation is clearly comparable to the amplitude of the Shubnikov-de Haas oscillations resolved at relatively high magnetic fields. Furthermore, introducing Ohmic contacts to the 2D channel or pads deposited directly on the sample surface did not alter significantly the amplitude of the detected microwave-induced resistance oscillations.

The dependence of quasiparticle Fermi energy on electron density is investigated by analyzing radiative recombination spectra of two-dimensional electrons with photoexcited holes bound to remote acceptors. This method enables us to measure the dependence of renormalized quasiparticle mass on the concentration of two-dimensional electrons. It is established that with decreasing electron density (increasing parameter rs up to 4.5) the density-of-states effective mass of quasiparticles increases by 35% compared to the cyclotron electron mass. It is shown that in a perpendicular magnetic field the concept of quasiparticles in a two-dimensional Fermi liquid is applicable not only near the Fermi level but also deep below the Fermi surface, down to the bottom of the size-quantization band, since the broadening of excitations appears to be much less than their energy. The effective mass and broadening of quasiparticles were found to be significantly dependent on their energy measured from the Fermi surface down to the very bottom of the size-quantization band.

The bulk properties of the bilayer quantum Hall state at total filling factor one have been intensively studied in experiment. Correlation induced phenomena such as Josephson-like tunneling and zero Hall resistance have been reported. In contrast, the edge of this bilayer state remains largely unexplored. Here, we address this edge physics by realizing quasiparticle tunneling across a quantum point contact. The tunneling manifests itself as a zero bias peak that grows with decreasing temperature. Its shape agrees quantitatively with the formula for weak quasiparticle tunneling frequently deployed in the fractional quantum Hall regime in single layer systems, consistent with theory. Interestingly, we extract a fractional charge of only a few percent of the free electron charge, which may be a signature of the theoretically predicted leakage between the chiral edge and the bulk mediated by gapless excitations.

A two-dimensional electron gas (2DEG) is absent in ultrapure GaN/Al0.06Ga0.94N heterostructures grown by molecular beam epitaxy on bulk GaN at 300 K and in the dark. However, such a 2DEG can be generated by UV illumination and persists at low temperature after blanking the light. Under steady UV illumination as well as under persistence conditions, pronounced quantum transport with Shubnikov–de Haas oscillations commencing below 2 T is observed. The low temperature 2DEG mobility amounts to only ∼20 000 cm2/V s, which is much lower than predicted for the dominant scattering mechanisms in GaN/AlGaN heterostructures grown on GaN with low threading dislocation density. A rather small ratio of the transport and quantum lifetimes τt/τq of ∼10 points at elastic scattering events limiting both the transport and quantum lifetimes.

We report and discuss the performance of an enhancement mode n-channel pseudo-vertical GaN metal oxide semiconductor field effect transistor (MOSFET). The trench gate structure of the MOSFET is uniformly covered with an Al2O3 dielectric and TiN electrode material, both deposited by atomic layer deposition (ALD). Normally-off device operation is demonstrated in the transfer characteristics. Special attention is given to the estimation of the active acceptor concentration in the Mg doped body layer of the device, which is crucial for the prediction of the threshold voltage in terms of device design. A method to estimate the electrically active dopant concentration by applying a body bias is presented. The method can be used for both pseudo-vertical and truly vertical devices. Since it does not depend on fixed charges near the channel region, this method is advantageous compared to the estimation of the active doping concentration from the absolute value of the threshold voltage.

In this computational study, the influence of GaN/AlxGa1−xN layer stack parameters, such as surface potential, aluminum mole fraction, and background donor concentration, on the two-dimensional electron gas (2DEG) density in a heterostructure is verified. At a fixed Al mole fraction, the surface potential was identified to have the largest impact on the 2DEG density. The combination of a small aluminum mole fraction (x < 0.12) and large surface potential results in the absence of a 2DEG in the investigated heterostructures, while for a small surface potential value, a 2DEG will be present. For a large aluminum mole fraction (x ≥ 0.25), a 2DEG is always present, independent of the surface potential value. In the case of an intermediate aluminum mole fraction, the background donor level is one key parameter strongly influencing the 2DEG density.