Comparison of graphene films grown on 6H-SiC and 4H-SiC substrates

Abstract

Atomic force microscopy, Kelvin-probe microscopy and Raman spectroscopy have been used to examine graphene films grown by thermal decomposition of the Si face of semi-insulating substrates of 6H-SiC and 4H-SiC polytypes in the atmosphere of argon. It was demonstrated that the quality of graphene grown on substrates of various polytypes at identical technological growth regimes is about the same. A conclusion was made that the differences in crystal structure between 6H-SiC and 4H-SiC does not lead to significant dissimilarities in the mechanism of sublimation of silicon carbide components from the surface of a crystal and in that of graphene crystallization.     

Investigation of Ni/4H-SiC diodes as radiation detectors with low doped n-type 4H-SiC epilayers

The development of SiC minimum ionising particle (MIP) detectors imposes severe constrains in the electronic quality and the thickness of the material due to the relatively high value of the energy required to produce an electron–hole pair in this material by MIP against the value for Si. In this work, particle detectors were made using semiconductor epitaxial undoped n-type 4H-SiC as the detection medium. The thickness of the epilayer is on the order of 40 μm and the detectors are realised by the formation of a nickel silicide on the silicon surface of the epitaxial layer (Schottky contact) and of the ohmic contact on the back side of 4H-SiC substrate. The low doping concentration (6×10¹³ cm⁻³) of the epilayer allows the detector to be totally depleted at relatively low reverse voltages (100 V). We present experimental data on the charge collection properties by using 5.486 MeV -particles impinging on the Schottky contact. A 100% charge collection efficiency (CCE) is demonstrated for reverse voltages higher than the one needed to have a depletion region equal to the -particle extrapolated range in SiC. The diffusion contribution of the minority change carriers to CCE is pointed out. By comparing measured CCE values to the outcomes of drift–diffusion simulation, values are inferred for the hole lifetime within the neutral region of the charge carrier generation layer.     

Formation of Zr-oxynitride thin films on 4H-SiC substrate

Formation of Zr-oxynitride by simultaneous oxidation and nitridation in nitrous oxide of sputtered Zr on SiC substrate is reported. Sputtered Zr on SiC substrate and followed by oxidation and nitridation in nitrous oxide ambient for 15 min at different temperatures (400-900 °C) have been systematically investigated. By using X-ray photoelectron spectroscopy, chemical compositions, and depth profile analysis have been evaluated, as well as energy band alignment of Zr-oxynitride/interfacial layer/SiC system. Zr-oxynitride film of Zr-O, Zr-N, and/or Zr-O-N and its interfacial layer composed of mixed Zr-O, Zr-N, Zr-O-N, Zr-Si-O, Si-N, and/or C-N phases were verified. A possible model related to the oxidation and nitridation mechanisms has been proposed and explicated.     

Local characteristics of heat-radiation superconductor detectors based on high-temperature superconductor films

Relations are proposed for evaluating the local values of the thermal sensitivity, speed of response, and resolution of a heat radiation detector based on high-temperature superconductor films.Moscow Institute of Engineering Physics. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 62, No. 6, pp. 873–877.     

Characterization of anodic SiO2 films on P-type 4H-SiC

The physical and electronic properties of 100-120-nm thick anodic silicon dioxide film grown on p-type 4H-SiC wafer and annealed at different temperatures (500, 600, 700, and 800 °C) have been investigated and reported. Chemical bonding of the films has been analyzed by Fourier transform infra red spectroscopy. Smooth and defect-free film surface has been revealed under field emission scanning electron microscope. Atomic force microscope has been used to study topography and surface roughness of the films. Electronic properties of the film have been investigated by high frequency capacitance-voltage and current-voltage measurements. As the annealing temperature increased, refractive index, dielectric constant, film density, SiC surface roughness, effective oxide charge, and leakage current density have been reduced until 700 °C. An increment of these parameters has been observed after this temperature. However, a reversed trend has been demonstrated in porosity of the film and barrier height between conduction band edge of SiO₂ and SiC.     

Electron paramagnetic resonance characterization of aluminum ion implantation-induced defects in 4H-SiC

Deep-level defects in silicon carbide(SiC) are critical to the control of the performance of SiC electron devices. In this paper, deep-level defects in aluminum ion-implanted 4 H-SiC after high-temperature annealing were studied using electron paramagnetic resonance(EPR) spectroscopy at temperatures of 77 K and 123 K under different illumination conditions. Results showed that the main defect in aluminum ion-implanted 4 H-SiC was the positively charged carbon vacancy( VC~+), and the higher the doping concentration was, the higher was the concentration of VC~+. It was found that the type of material defect was independent of the doping concentration,although more VC~+ defects were detected during photoexcitation and at lower temperatures. These results should be helpful in the fundamental research of p-type 4 H-SiC fabrication in accordance with functional device development.     

Dielectric films fabricated in plasma as passivation of 4H-SiC Schottky diodes

In this report, we propose Al₂O₃ and AlN films as passivation for high-voltage SiC Schottky barrier diodes. A reactive pulse plasma enhanced chemical vapour deposition method is used to form the layers. The comparison between the Schottky barrier diodes with and without passivation shows the effectiveness of Al₂O₃ layer on the reverse current decrease as a result of the surface states reduction.     

Fabrication and characterization of X-ray array detectors based on polycrystalline PbI2 thick films

Polycrystalline PbI₂ thick films were grown by using close spaced vapor deposition method on glass substrate with a conducting indium–tin-oxide coating. The morphology shows a uniquely oriented film structure with hexagonal platelets accurately being upright on substrate surface. An array detector with 12 pixels was fabricated based on this structure of the thick film. It is shown that the dark current is lower than 3 nA at bias voltage below 500 V, and the dark resistivity is as high as 10¹¹ Ω cm. A mapping of dark current density of the sensitive area of the array detector exhibits a better uniformity in the central area than the fringe area. A quick photocurrent response to X-ray excitation was obtained. The photoresponse rise time about 250 μs was obtained from the detectors and the photocurrent decays in a few seconds. The values of photocurrent are higher about two orders of magnitude than the values of dark current. The distribution of photocurrent is more uniform than that of dark current in the sensitive area of the detector.     

Hetero-epitaxy and structure characterization of Si films on 6H-SiC substrates

In order to realize the non-ultraviolet application of SiC optoelectronic devices, Si/6H-SiC heterojunctions were prepared by the low-pressure chemical vapour deposition at 850 °C. The X-ray diffraction (XRD) and the selected area electron diffraction (SAED) results indicate that Si thin films have a monocrystalline structure and were grown along the (111) crystal plane. The rationality of the (111) growth plane was also analyzed by the theoretical calculation. High-density structural defects such as stacking faults and twins were observed on Si films by the high-resolution transmission electron microscopy. This phenomenon was also validated by the SAED patterns of defect-rich regions on Si films.     

Position-sensitive detectors based on two-dimensional materials

Nano Research - Two-dimensional (2D) materials have attracted great attention in optoelectronics because of their unique structure, optical and electrical properties. Designing high-performance...     

Microwave detectors based on porous silicon

Two types of structures comprising porous silicon (por-Si) layers between metal electrodes were prepared, which possessed nonlinear (A type) and linear (B type) current-voltage characteristics. The exposure to microwave radiation leads to the appearance of an emf between electrodes. The B-type structures exhibit high voltage responsivity and can be used as microwave radiation sensors.     

Structure and magnetic properties of mono- and bi-layer graphene films on ultraprecision figured 4H-SiC(0001) surfaces

Monolayer and bilayer graphene films with a few hundred nm domain size were grown on ultraprecision figured 4H-SiC(0001) on-axis and 8 degrees -off surfaces by annealing in ultra-high vacuum. Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, reflection high-energy electron diffraction, low-energy electron diffraction (LEED), Raman spectroscopy, and scanning tunneling microscopy, we investigated the structure, number of graphene layers, and chemical bonding of the graphene surfaces. Moreover, the magnetic property of the monolayer graphene was studied using in-situ surface magneto-optic Kerr effect at 40 K. LEED spots intensity distribution and XPS spectra for monolayer and bilayer graphene films could become an obvious and accurate fingerprint for the determination of graphene film thickness on SiC surface.     

Single-photon detectors based on ultranarrow superconducting nanowires

We report efficient single-photon detection (η = 20% at 1550 nm wavelength) with ultranarrow (20 and 30 nm wide) superconducting nanowires, which were shown to be more robust to constrictions and more responsive to 1550 nm wavelength photons than standard superconducting nanowire single-photon detectors, based on 90 nm wide nanowires. We also improved our understanding of the physics of superconducting nanowire avalanche photodetectors, which we used to increase the signal-to-noise ratio of ultranarrow-nanowire detectors by a factor of 4, thus relaxing the requirements on the read-out circuitry and making the devices suitable for a broader range of applications.     

Liquid-phase epitaxy of 2H-SiC film on a (0001) 4H-SiC substrate in Li-Si melt

We first applied a liquid-phase epitaxy (LPE) method to the growth of 2H-SiC to obtain an oriented 2H-SiC layer. The LPE growth was performed on a (0001) 4H-SiC substrate in a Li-Si-C melt at 850 °C. A LPE layer grew on the carbon face of the (0001) 4H-SiC substrate with the area of about 1 × 1 mm². The thickness and morphology of the LPE layer were confirmed by scanning electron microscope (SEM), and the polytype of the LPE layer was determined using a high-resolution transmission electron microscope (HR-TEM) and a Raman spectroscopy. SEM measurements revealed that the thickness of the LPE layer was 50 μm and that this layer developed with a hexagonal shape. HR-TEM observations and Raman spectral analysis showed that 2H-SiC layer grows on 4H-SiC substrate with the relationship of (0001) 2H-SiC (0001) 4H-SiC. In this study, we first succeeded to grow <0001>-axis-oriented 2H-SiC films and concluded that the LPE technique is applicable to the growth of oriented 2H-SiC films in the Li-Si melt.     

Thermal stability of LiF thin films on 6H-SiC(0001) surface

The morphology and the temperature induced changes of LiF thin layers deposited on three different kinds of 6H-SiC(0001) surfaces have been investigated by the atomic force microscopy technique. As the substrates used were: a single crystal of Si-terminated on-axis oriented additionally hydrogen-etched, off oriented 3.5° from basal plane and with an epitaxial adlayer grown on the Si- or on the C-face surface. For all the systems investigated, independently of the substrate, the LiF grows uniformly at room temperature, and even for rather thick films (estimated to be about 5 nm), terraces are still visible. However, in each case the LiF layers are not temperature stable, and when the samples are heated (starting from ∼600 K), regular grains with radius up to 40 nm and with height up to 9 nm are formed (for initial layers thickness 2.5 nm). Subsequent sample heating procedures cause LiF desorption and consequently, a decrease of the dimensions of the islands.     

Annealing behavior of thin polycrystalline silicon films damaged by silicon ion implantation in the critical amorphization range

Thin polycrystalline silicon films were amorphized by silicon ion implantation at two different angles and subsequently annealed at 525°C. The structure of the as-deposited, as-implanted and annealed films was examined using Raman spectroscopy, transmission electron microscopy and the optical appearance method. Two distinct phases were found to coexist in the as-implanted films: one phase consists of a mosaic of crystallites surrounded by amorphous silicon and the other consists of only amorphous silicon. The mosaic phase was due to ion beam channeling through properly oriented polycrystalline grains. The implantation angle was found to determine whether the film recrystallizes upon annealing by a solid phase epitaxial mechanism using the surviving crystallites as seeds.     

P型4H-SiC少数载流子寿命的研究

为了更好地了解P型4H-SiC的电学特性,评价其晶体质量,利用激光和微波技术作为非接触、非破坏性测量半导体特性的一种工具,详细描述了该测试方法和实验装置,讨论了高温退火前后晶片中少数载流子寿命的变化,并用LabVIEW对测试数据进行了拟合。结果表明:高温退火能提高载流子寿命,并且实验数据与拟合结果较符合。说明了微波光电导衰减法(μ-PCD)是一种测试少子寿命的快速、有效方法,对研究半导体材料性能具有重要意义;同时,研究高温退火条件下少子寿命的变化,对提高其材料的电性能也具有重要意义。     

Photovoltaic X-ray detectors based on epitaxial GaAs structures

A new type of the photovoltaic X-ray detector based on epitaxial p ⁺-n-n′-n ⁺ GaAs structures is proposed, which provides for a high efficiency of charge carrier collection in a nonbiased operation regime at room temperature. The GaAs structures were grown by vapor phase epitaxy on heavily doped n ⁺-GaAs substrates. The X-ray sensitivity range covers the effective energies from 8 to 120 keV. The maximum output signal in the short-circuit regime is 30 μA min/(Gy cm²). The detector response to γ-radiation from a ¹³⁷Cs[660 keV] radioactive isotope was measured.