Strain relaxation in GaN grown on vicinal 4H-SiC(0001) substrates

The strain of GaN layers grown by Metal Organic Chemical Vapor Deposition (MOCVD) on three vicinal 4H-SiC substrates (0, 3.4 and 8 offcut from [0001] towards [11-20] axis) is investigated by X-ray Diffraction (XRD), Raman Scattering and Cathodoluminescence (CL). The strain relaxation mechanisms are analyzed for each miscut angle. At a microscopic scale, the GaN layer grown on on-axis substrate has a slight and homogeneous tensile in-plane stress due to a uniform distribution of threading dislocations over the whole surface. The GaN layers grown on miscut substrates presented cracks, separating areas which have a stronger tensile in-plane stress but a more elastic strain. The plastic relaxation mechanisms involved in these layers are attributed to the step flow growth on misoriented surfaces (dislocations and stacking faults) and to the macroscopical plastic release of additional thermoelastic stress upon cooling down (cracks)

Deposition of high-quality ultra-thin NbN films at ambient temperatures

This paper discusses the possibility of growing NbN ultra-thin films on Si-substrates and AlxGa1-xN buffer-layers by means of DC magnetron sputtering without intentional substrate heating. Resistance-temperature measurements were carried out and the superconducting properties such as Tc, ΔTc and R□ were deduced while HRTEM gave insight into the crystal structure and film thickness. The adjustment of the partial pressure of argon and nitrogen was found to be critical in establishing a reliable deposition process. The quality of the interface between the NbN film and the substrate was improved by optimizing the total pressure while sputtering, and is therefore particularly valuable for phonon-cooled HEB heterodyne receivers. NbN films of 5 nm thickness were obtained and exhibited a Tc from 8K on Si-substrates, and up to 10.5 K on the GaN buffer-layers. This result is significant since the absence of a high-temperature environment permits the establishment of more complex fabrication processes for intricate thin-film structures without compromising the overall integrity of e.g. dielectric layers, or hybrid circuitries with e.g. SIS junctions

Epitaxial growth of ultra-thin NbN films on AlxGa1-xN buffer-layers

The suitability of AlxGa1-xN epi-layer to deposit onto ultra-thin NbN films has been demonstrated for the first time. High quality single-crystal films with 5 nm thickness confirmed by high-resolution transmission electron microscopy (HRTEM) have been deposited in a reproducible manner by means of reactive DC magnetron sputtering at elevated temperatures and exhibit critical temperatures (Tc) as high as 13.2 K and residual resistivity ratio (RRR) ~ 1 on hexagonal GaN epi-layer. With increasing the Al-content x in the AlxGa1-xN epi-layer above 20% a gradual deterioration of Tc down to 10 K was observed. Deposition of NbN on bare silicon substrates served as reference and comparison. Excellent spatial homogeneity of the fabricated films was confirmed by R(T) measurements of patterned micro-bridges across the entire film area. The superconducting properties of those films were further characterized by critical magnetic field and critical current measurements. It is expected that the employment of GaN material as a buffer-layer for the deposition of ultra-thin NbN films prospectively benefit terahertz electronics, particularly hot electron bolometer (HEB) mixers

Study of IF bandwidth of NbN hot electron bolometers on GaN buffer layer using a direct measurement method

In this paper, we present a reliable measurement method to study the influence of the GaN buffer layer on phonon-escape time in comparison with commonly used Si substrates and, in consequence, on the IF bandwidth of HEBs. One of the key aspects is to operate the HEB mixer at elevated bath temperatures close to the critical temperature of the NbN ultra-thin film, where contributions from electron-phonon processes and self-heating effects are relatively small, therefore IF roll-off will be governed by the phonon-escape.Two independent experiments were performed at GARD and MSPU on a similar experimental setup at frequencies of approximately 180 and 140 GHz, respectively, and have shown reproducible and consistent results. The entire IF chain was characterized by S-parameter measurements. We compared the measurement results of epitaxial NbN grown onto GaN buffer-layer with Tc of 12.5 K (4.5 nm) with high quality polycrystalline NbN films on Si substrate with Tc of 10.5 K (5 nm) and observed a strong indication of an enhancement of phonon escape to the substrate by a factor of two for the NbN/GaN material combination

Reduction of Phonon Escape Time for NbN Hot Electron Bolometers by Using GaN Buffer Layers

In this paper, we investigated the influence of the GaN buffer-layer on the phonon escape time of phonon-cooled hot electron bolometers based on NbN material and compared our findings to conventionally employed Si substrate. The presented experimental setup and operation of the HEB close to the critical temperature of the NbN film allowed for the extraction of phonon escape time in a simplified manner. Two independent experiments were performed at GARD/Chalmers and MSPU on a similar experimental setup at frequencies of approximately 180 and 140 GHz, respectively, and have shown reproducible and consistent results. By fitting the normalized IF measurement data to the heat balance equations, the escape time as fitting parameter has been deduced and amounts to 45 ps for the HEB based on Si substrate as in contrast to a significantly reduced escape time of 18 ps for the HEB utilizing the GaN buffer-layer under the assumption that no additional electron diffusion has taken place. This study indicates a high phonon transmissivity of the NbN-to-GaN interface and a prospective increase of IF bandwidth for HEB made of NbN on GaN buffer layers, which is desirable for future THz HEB heterodyne receivers

Reduction of Phonon Escape Time for NbN Hot Electron Bolometers by Using GaN Buffer Layers

In this paper, we investigated the influence of the GaN buffer-layer on the phonon escape time of phonon-cooled hot electron bolometers based on NbN material and compared our findings to conventionally employed Si substrate. The presented experimental setup and operation of the HEB close to the critical temperature of the NbN film allowed for the extraction of phonon escape time in a simplified manner. Two independent experiments were performed at GARD/Chalmers and MSPU on a similar experimental setup at frequencies of approximately 180 and 140 GHz, respectively, and have shown reproducible and consistent results. By fitting the normalized IF measurement data to the heat balance equations, the escape time as fitting parameter has been deduced and amounts to 45 ps for the HEB based on Si substrate as in contrast to a significantly reduced escape time of 18 ps for the HEB utilizing the GaN buffer-layer under the assumption that no additional electron diffusion has taken place. This study indicates a high phonon transmissivity of the NbN-to-GaN interface and a prospective increase of IF bandwidth for HEB made of NbN on GaN buffer layers, which is desirable for future THz HEB heterodyne receivers

Study of IF bandwidth of NbN hot electron bolometers on GaN buffer layer using a direct measurement method

In this paper, we present a reliable measurement method to study the influence of the GaN buffer layer on phonon-escape time in comparison with commonly used Si substrates and, in consequence, on the IF bandwidth of HEBs. One of the key aspects is to operate the HEB mixer at elevated bath temperatures close to the critical temperature of the NbN ultra-thin film, where contributions from electron-phonon processes and self-heating effects are relatively small, therefore IF roll-off will be governed by the phonon-escape.Two independent experiments were performed at GARD and MSPU on a similar experimental setup at frequencies of approximately 180 and 140 GHz, respectively, and have shown reproducible and consistent results. The entire IF chain was characterized by S-parameter measurements. We compared the measurement results of epitaxial NbN grown onto GaN buffer-layer with Tc of 12.5 K (4.5 nm) with high quality polycrystalline NbN films on Si substrate with Tc of 10.5 K (5 nm) and observed a strong indication of an enhancement of phonon escape to the substrate by a factor of two for the NbN/GaN material combination

Ambient temperature growth of mono- and polycrystalline NbN nanofilms and their surface and composition analysis

This paper presents the studies of high-quality 5 nmthin NbN films deposited by means of reactive DC magnetronsputtering at room temperature. The deposition withoutsubstrate heating offers major advantages from a processingpoint of view and motivates the extensive composition- andsurface characterization and comparison of the present filmswith high quality films grown at elevated temperatures.Monocrystalline NbN films have been epitaxially grown ontohexagonal GaN buffer-layers (0002) and show a distinct, lowdefect interface as confirmed by High-Resolution TEM. Thecritical temperature Tc of films on the GaN buffer-layer reached10.4 K. Furthermore, a poly-crystalline structure was observedon films grown onto Si (100) substrates, exhibiting a Tc of 8.1 Kalbeit a narrow transition from the normal to thesuperconducting state. X-ray photoelectron spectroscopy andreflected electron energy loss spectroscopy verified that thecomposition of NbN was identical irrespectively of appliedsubstrate heating. Moreover, the native oxide layer at the surfaceof NbN has been identified as NbO2 and thus, is in contrast to theNb2O5, usually claimed to be formed at the surface of Nb whenexposed to air. These findings are of significance since it wasproven the possibility of growing epitaxial NbN onto GaN bufferlayer in the absence of high temperatures hence paving the wayto employ NbN in more advanced fabrication processes involvinga higher degree of complexity. The eased integration andemployment of lift-off techniques could, in particular, lead toimproved performance of cryogenic ultra-sensitive terahertzelectronics