Showing papers by "Oliver Ambacher published in 2022"

Enhanced AlScN/GaN Heterostructures Grown with a Novel Precursor by Metal–Organic Chemical Vapor Deposition

Abstract: Growth of AlScN high‐electron‐mobility transistor (HEMT) structures by metal–organic chemical vapor deposition (MOCVD) is challenging due to the low vapor pressure of the conventionally used precursor tris‐cyclopentadienyl‐scandium (Cp3Sc). It is shown that the electrical and structural characteristics of the AlScN/GaN heterostructure improve significantly by using bis‐methylcyclopentadienyl‐scandiumchloride ((MCp)2ScCl), which has a higher vapor pressure and allows for an increased molar flow and thus higher growth rate (GR). AlScN/GaN HEMT heterostructures with superior electrical characteristics deposited at different barrier growth temperatures are presented. The sheet resistance Rsh of 172 Ω sq−1 obtained at 900 °C barrier growth temperature is among the lowest reported so far for AlScN/GaN HEMT structures. The sheet charge carrier density ns is 3.23×1013 cm−2 and the electron mobility μ is 1124 cm2 Vs−1.

Effect of V/III ratio and growth pressure on surface and crystal quality of AlN grown on sapphire by metal-organic chemical vapor deposition

Abstract: The epitaxial growth of high-quality AlN on sapphire substrates is challenging due to high lattice and thermal mismatch and low Al-adatom mobility, which cause high dislocation density and rough surface morphology. High-temperature AlN (deposited at 1130 [Formula: see text]C) was grown on low-temperature AlN nucleation layers (880 [Formula: see text]C) with different V/III ratios and reactor pressures by metal-organic chemical vapor deposition. Surface and crystal quality was optimized using high V/III ratios. Thereby, slow layer-by-layer growth at high V/III laterally overgrows the 3D nucleation layer reducing the dislocation density, twist, and tilt in the crystal. This was as effective as multistep growth with increasing V/III. At high pressure of 95 mbar, step-bunching occurred. This indicates low surface supersaturation due to parasitic reactions in the gas phase. This was suppressed by low growth pressure of 50 mbar, while the crystal quality worsened.

Manipulation of the In Situ Nitrogen‐Vacancy Doping Efficiency in CVD‐Grown Diamond

Abstract: Herein, the in situ generation of nitrogen‐vacancy (NV) centers in diamond during chemical vapor deposition (CVD) is investigated depending on the electric‐field strength at the sample position. The alteration of the electric‐field strength is induced by changing the resonance conditions within the resonator cavity while keeping the growth input variables constant. The electric‐field strength distribution is obtained by simulation results. During the growth experiments, optical‐emission spectroscopy data is collected, which shows the impact of the electric‐field strength on the radical concentrations within the plasma and the gas temperature. Through the reduction of the electric‐field strength, the synthesis of thick, high‐quality, nitrogen‐doped diamond without the formation of a polycrystalline rim around the sample edges and the twinning‐induced growth of polycrystalline grains was accomplished. Therefore, a reduced internal and more homogeneous stress distribution is achieved. Furthermore, significant influences on the in situ NV doping are discovered. In addition to a considerable gain of the in situ NV generation, also a major enhancement of the in situ incorporation efficiency of NV centers in comparison to Ns0 centers up to almost 3% is observed. Depending on the application, this makes posttreatment processes for additional NV generation dispensable.

Influence of structural properties on the ferroelectric behavior of hexagonal AlScN

Abstract: The direct impact of structural quality on the ferroelectric properties of hexagonal Al1– xSc xN with an Sc-content of x = 0.3 was investigated using dynamic hysteresis measurements, high-resolution x-ray diffraction (HRXRD), and atomic force microscopy. The films investigated were deposited on p-doped (001)-Si substrates by reactive pulsed DC magnetron sputtering under different gas mixtures to vary the structural quality and surface morphology between samples. Misoriented grains were identified as ferroelectrically inactive, as these grains resulted in an underestimation and distortion of the ferroelectric quantities. In fact, a high amount of misoriented volume was found to have a significant effect on the coercive electric field, as this is mainly determined by the crystal strain in the ferroelectric [0001]-oriented regions, independent of its origin. Furthermore, it was concluded that the crystal quality does not have a pronounced effect on the coercive field strength. Conversely, the polarization in the film is mainly determined by the crystal quality, as a difference of 1° in the HRXRD FWHM of the ω-scan resulted in a 60% loss of polarization. The amount of polarization was influenced to a lesser extent by the misoriented grains since the ferroelectric volume of the layers was only slightly overestimated. This reveals that optimizing reproducible and transferable properties, such as crystal quality and surface morphology, is more reasonable, as the film with the lowest misoriented volume and the highest degree of c-axis orientation showed the highest polarization.

Rayleigh waves in nonpolar Al0.7Sc0.3N(11 2¯0) films with enhanced electromechanical coupling and quality factor

Abstract: This work reports on the growth of 1 µm nonpolar a-plane Al0.7Sc0.3N(11[Formula: see text]0) thin films on an r-plane sapphire Al2O3(1[Formula: see text]02) via magnetron sputter epitaxy. The electro-acoustic properties of the film structures were characterized using surface acoustic wave (SAW) resonators. Measured electrical responses were found to be strongly anisotropic in terms of the wave propagation direction. We identified a sagittal polarized Rayleigh wave mode with large coupling ([Formula: see text] 3.7%), increased phase velocity ([Formula: see text] 4825 m/s), as well as high quality factor ( Q > 1000) for SAW propagation along the c-axis [0001] and normalized thicknesses [Formula: see text]. Finite element method simulations using electro-acoustic properties of Al0.7Sc0.3N obtained from the density functional theory reproduce our experimental results.

Influence of alloying and structural transition on the directional elastic and isotropic thermodynamic properties of wurtzite and layered hexagonal ScxAl1−xN crystals

Abstract: The structural, elastic, and basic thermodynamic properties of hexagonal Sc xAl1−xN crystals are calculated and discussed over the whole range of possible random alloys, including the transition from wurtzite to the layered hexagonal structure. Based on a review of lattice and internal parameters in combination with complete datasets of stiffness coefficients published in the literature, differing in the considered alloying intervals and the predicted structural transitions, changes in the crystal lattices caused by the substitution of aluminum by scandium atoms are discussed and illustrated. Crystal properties like the mass densities, average bond angles, and bond lengths are calculated, and the compliance coefficients, Young's modulus, shear modulus, Poisson's ratio, compressibility, and sound velocities are determined depending on the alloy composition and in relation to the orientation of crystal planes and axes. Particular attention is paid to the occurring directional anisotropies and the changes in structural and elastic properties in the alloy region of the structural transition between wurtzite and layered hexagonal Sc xAl1−xN crystals. The acoustic velocities determined are used to calculate basic thermodynamic properties such as the Debye temperature, heat capacity, and minimum heat conduction, as well as to evaluate both the influence of the alloying and the structural transition on these properties.

On the origin of the turn-on voltage drop of GaN-based current aperture vertical electron transistors

Abstract: Non-linear output characteristics and the related turn-on voltages of GaN-based current aperture vertical electron transistors (CAVETs) are investigated experimentally. The resistive components are systematically analyzed in dependence of the device layout to determine the dominant resistances in the devices. Current–voltage (IV) and capacitance–voltage-characteristics (CV) are compared to a proposed planar-doped barrier diode (PDBD) model, and the influence of the bound sheet charge density and drift layer carrier concentration is discussed. The observed CV characteristics are in contrast to the PDBD model as a clearly voltage-dependent capacitance was observed and dopant-diffusion forming a p-type aperture was ruled out. Thermionic emission was verified by temperature-dependent IV characteristics indicating interface states causing a potential barrier. Transient drain current measurements revealed a single dominating trap level with an activation energy of EA = 1.086 ± 0.015 eV. This activation energy was attributed to carbon-related acceptor states present at the regrowth interface and the drift layer. Additional test structures revealed that the interface potential barrier and the space charge in the drift layer limit the initial charge transport causing a turn-on voltage in the devices. The results point out the significance of a precise control of the regrowth interface properties and the effective carrier density in the drift layer to enable efficient, high-power devices based on the CAVET technology.

Erratum: “Influence of alloying and structural transition on the directional elastic and isotropic thermodynamic properties of wurtzite and layered hexagonal ScxAl1−xN crystals” [J. Appl. Phys. 132, 175101 (2022)]

Abstract: First Page

A Cryogenic On-Chip Noise Measurement Procedure With ±1.4-K Measurement Uncertainty

Abstract: This paper reports on a cryogenic noise temperature measurement system that is capable of performing measurements directly at the reference plane of a chip. The system uses the well known cold attenuator measurement principle. Wire-bonding a monolithic microwave integrated circuit attenuator to the device under test enables precise measurements at chip level. The overall noise measurement uncertainty in the Ku-band has been estimated to be ± 1.4 K in a 3 σ confidence interval. This estimated measurement uncertainty is able to compete with state-of-the-art coaxial measurement systems, although proper matching is harder to achieve in probing systems.

C-Band Low-Noise Amplifier MMIC with an Average Noise Temperature of 44.5 K and 24.8 mW Power Consumption

Abstract: This paper reports on a 4 – 8 GHz (C-band) low-noise amplifier monolithic microwave integrated circuit in 50 nm metamorphic high electron mobility transistor technology aimed for the use in large scale systems. The two-stage design exhibits a small-signal gain of 31 dB and a room temperature average noise temperature of 44.5 K between 4 and 8 GHz with a minimum of 38.4 K. The power consumption of the amplifier at optimal noise bias is only 24.8 mW. To the best of the authors’ knowledge, the amplifier has the lowest noise temperature reported among monolithic microwave integrated circuits that do not utilize an off-chip input matching network in C-band.