Atomic configurations of dislocation core and twin boundaries in 3C-SiC studied by high-resolution electron microscopy

The defects in 3C-SiC film grown on (001) plane of Si substrate were studied using a 200 kV high-resolution electron microscope with point resolution of 0.2 nm. A posterior image processing technique, the image deconvolution, was utilized in combination with the image contrast analysis to distinguish atoms of Si from C distant from each other by 0.109 nm in the [110] projected image. The principle of the image processing technique utilized and the related image contrast theory is briefly presented. The procedures of transforming an experimental image that does not reflect the crystal structure intuitively into the structure map and of identifying Si and C atoms from the map are described. The atomic configurations for a 30 degrees partial dislocation and a microtwin have been derived at atomic level. It has been determined that the 30 degrees partial dislocation terminates in C atom and the segment of microtwin is sandwiched between two 180 degrees rotation twins. The corresponding stacking sequences are derived and atomic models are constructed according to the restored structure maps for both the 30 degrees partial dislocation and microtwin. Images were simulated based on the two models to affirm the above-mentioned results.

Deposition of p-type nc-SiC : H thin films with subtle carbon incorporation for applications in p-i-n solar cells

This paper presents a detailed study on the effects of carbon incorporation and substrate temperature on structural, optical, and electrical properties of p-type nanocrystalline amorphous silicon films. A p-nc-SiC: H thin film with optical gap of 1.92 eV and activation energy of 0.06 eV is obtained through optimizing the plasma parameters. By using this p-type window layer, single junction diphasic nc-SiC : H/a-Si : H solar cells have been successfully prepared with a V-oc of 0.94 eV.

1-mm gate periphery AlGaN/AIN/GaN HEMTs on SiC with output power of 9.39 W at 8 GHz

AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with high mobility GaN channel layer were grown on 50 min diameter semi-insulating (SI) 6H-SiC substrates by metalorganic chemical vapor deposition and large periphery HEMT devices were fabricated and characterized. High two-dimensional electron gas mobility of 2215 cm(2)/V s at room temperature with sheet electron concentration of 1.044 x 10(13)/cm(2) was achieved. The 50 mm diameter HEMT wafer exhibited a low average sheet resistance of 251.0 Omega/square, with the resistance uniformity of 2.02%. Atomic force microscopy measurements revealed a smooth AlGaN surface with a root-mean-square roughness of 0.27 nm for a scan area of 5 mu mi x 5 pm. The 1-mm gate width devices fabricated using the materials demonstrated a very high continuous wave output power of 9.39 W at 8 GHz, with a power added efficiency of 46.2% and power gain of 7.54 dB. A maximum drain current density of 1300 mA/mm, an extrinsic transconductance of 382 mS/mm, a current gain cutoff frequency of 31 GHz and a maximum frequency of oscillation 60 GHz were also achieved in the same devices. (C) 2007 Elsevier Ltd. All rights reserved.

Growth of a novel periodic structure of SiC/AlN multilayers by low pressure chemical vapour deposition

A novel 10-period SiC/AlN multilayered structure with a SiC cap layer is prepared by low pressure chemical vapour deposition (LPCVD). The structure with total film thickness of about 1.45 mu m is deposited on a Si (111) substrate and shows good surface morphology with a smaller rms surface roughness of 5.3 nm. According to the secondary ion mass spectroscopy results, good interface of the 10 period SiC/AlN structure and periodic changes of depth profiles of C, Si, Al, N components are obtained by controlling the growth procedure. The structure exhibits the peak reflectivity close to 30% near the wavelength of 322 nm. To the best of our knowledge, this is the first report of growth of the SiC/AlN periodic structure using the home-made LPCVD system.

AlGaN/AlN/GaN/SiC HEMT structure with high mobility GaN thin layer as channel grown by MOCVD

Enhancement of the electrical properties in an AlGaN/GaN high electron mobility transistor (HEMT) structures was demonstrated by employing the combination of a high mobility GaN channel layer and an AlN interlayer. The structures were grown on 50 mm semi-insulating (SI) 6H-SiC substrates by metalorganic chemical vapor deposition (MOCVD). The room temperature (RT) two-dimensional electron gas (2DEG) mobility was as high as 2215 cm(2)/V s, with a 2DEG concentration of 1.044 x 10(13)cm(-2). The 50 mm HEMT wafer exhibited a low average sheet resistance of 251.0 Omega/square, with a resistance uniformity of 2.02%. The 0.35 Pin gate length HEMT devices based on this material structure, exhibited a maximum drain current density of 1300 mA/mm, a maximum extrinsic transconductance of 314 mS/mm, a current gain cut-off frequency of 28 GHz and a maximum oscillation frequency of 60 GHz. The maximum output power density of 4.10 W/mm was achieved at 8 GHz, with a power gain of 6.13 dB and a power added efficiency (PAE) of 33.6%. (c) 2006 Elsevier B.V. All rights reserved.

Visible blind p(+)-pi-n(-)-n(+) ultraviolet photodetectors based on 4H-SiC homoepilayers

p(+)-pi-n(-)-n(+) ultraviolet photodetectors based on 4H-SiC homoepilayers have been presented. The growth of the 4H-SiC homoepilayers was carried out in a LPCVD system. The size of the active area of the photodetectors was 300 x 300 mu m(2). The dark and illuminated I-V characteristics had been measured at reverse biases form 0 to 20 V at room temperature, and the illuminated current was at least two orders of magnitude than that of dark current below 13 V bias. The peak value zones of the photoresponse were located at 280-310 nm at different reverse biases, and the peak value located at 300 nm was 100 times greater than the cut-off response value in 380 nm at a bias of 10V, which showed the device had good visible blind performance. A small red-shift about 5 nm on the peak responsivity occurred when reverse bias increased from 5 to 15 V. (c) 2006 Elsevier Ltd. All rights reserved.

Raman scattering detection of stacking faults in free-standing cubic-SiC epilayer

We report on stacking fault (SF) detection in free-standing cubic-SiC epilayer by the Raman measurements. The epilayer with enhanced SFs is heteroepitaxially grown by low pressure chemical vapour deposition on a Si(100) substrate and is released in KOH solution by micromechanical manufacture, on which the Raman measurements are performed in a back scattering geometry. The TO line of the Raman spectra is considerably broadened and distorted. We discuss the influence of SFs on the intensity profiles of TO mode by comparing our experimental data with the simulated results based on the Raman bond polarizability (BP) model in the framework of linear-chain concept. Good agreement with respect to the linewidth and disorder-induced peak shift is found by assuming the mean distance of the SFs to be 11 angstrom in the BP model.

Morphological defects and uniformity issues of 4H-SiC homoepitaxial layers grown on off-oriented (0001)Si faces

The morphological defects and uniformity of 4H-SiC epilayers grown by hot wall CVD at 1500 degrees C on off-oriented (0001) Si faces are characterized by atomic force microscope, Nomarski optical microscopy, and Micro-Raman spectroscopy. Typical morphological defects including triangular defects, wavy steps, round pits, and groove defects are observed in mirror-like SiC epilayers. The preparation of the substrate surface is necessary for the growth of high-quality 4H-SiC epitaxial layers with low-surface defect density under optimized growth conditions. (c) 2006 Elsevier Ltd. All rights reserved.

The study of high temperature annealing of a-SiC : H films

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 A) gold film was evaporated on the half area of the aSiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100 degrees C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

Strain accommodation of 3C-SiC grown on hydrogen-implanted Si (001) substrate

The hydrogen-implanted Si substrate has been used for the fabrication of the "compliant substrate", which can accommodate the mismatch strain during the heteroepitaxy. The compliance of the substrate can be modulated by the energy and dose of implanted hydrogen. In addition, the defects caused by implantation act as the gettering center for the internal gettering of the harmful metallic impurities. Compared with SiC films growth on substrate without implantation. all the measurements indicated that the mismatch strains in the SiC films grown on this substrate have been released and the crystalline qualities have been improved. It is a practical technique used for the compliant substrate fabrication and compatible with the semiconductor industry. (C) 2003 Elsevier B.V. All rights reserved.

Stress and its effect on optical properties of GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire

The stress states in unintentionally doped GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire, and their effects on optical properties of GaN films were investigated by means of room-temperature confocal micro-Raman scattering and photoluminescence techniques. Relatively large tensile stress exists in GaN epilayers grown on Si and 6H-SiC while a small compressive stress appears in the film grown on sapphire. The latter indicates effective strain relaxation in the GaN buffer layer inserted in the GaN/sapphire sample, while the 50-nm-thick AlN buffer adopted in the GaN/Si sample remains highly strained. The analysis shows that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films. Finally, a linear coefficient of 21.1+/-3.2 meV/GPa characterizing the relationship between the luminescent bandgap and the biaxial stress of the GaN films is obtained. (C) 2003 American Institute of Physics.

Study on the reverse characteristics of Ti/6H-SiC Schottky contacts

The reverse I(V) measurement and analytic calculation of the electron transport across a Ti/6H-SiC Schottky barrier are presented. Based on the consideration of the barrier fluctuations and the barrier height shift caused by image charge and the applied voltage drop across Ti/SiC interfical layer, a comprehensive analytical model for the reverse tunneling current is developed using a WKB calculation of the tunneling probability through a reverse biased Schottky barrier. This model takes into account the main reverse conduction mechanism, such as field emission, thermionic field emission and thermionic emission. The fact that the simulated results are in good agreement with the experimental data indicates that the barrier height shift and barrier fluctuation can lead to reverse current densities orders of magnitude higher than that obtained from a simple theory. It is shown that the field and thermionic field emission processes, in which carries can tunnel through the barrier but cannot surmount it with insufficient thermal energy, dominate the reverse characteristics of a SiC Schottky contacts in a normal working condition.

Epitaxial growth and characterization of SiC on C-plane sapphire substrates by ammonia nitridation

The growth of SiC epilayers on C-face (0 0 0 1) sapphire (alpha-Al2O3) has been performed using CVD method. We found that the quality of SiC epilayers has been improved through the nitridation of substrates by exposing them to ammonia ambient, as compared to growth on bare sapphire substrates. The single crystallinity of these layers was verified by XRD and double crystal XRD measurements. Atomic force microscopy was used to evaluate the surface morphology. Infrared reflectivity and Raman scattering measurement were carried out to investigate the phonon modes in the grown SiC. Detailed Raman analysis identified the 6H nature of the as-grown SiC films. (C) 2002 Elsevier Science B.V. All rights reserved.

Studies of 6H-SiC devices

Silicon carbide (SiC) is recently receiving increased attention due to its unique electrical and thermal properties. It has been regarded as the most appropriate semiconductor material for high power, high frequency, high temperature, and radiation hard microelectronic devices. The fabrication processes and characterization of basic device on 6H-SiC were systematically studied. The main works are summarized as follows:The homoepitaxial growth on the commercially available single-crystal 6H-SiC wafers was performed in a modified gas source molecular beam epitaxy system. The mesa structured p(+)n junction diodes on the material were fabricated and characterized. The diodes showed a high breakdown voltage of 800 V at room temperature. They operated with good rectification characteristics from room temperature to 673 K.Using thermal evaporation, Ti/6H-SiC Schottky barrier diodes were fabricated. They showed good rectification characteristics from room temperature to 473 K. Using neon implantation to form the edge termination, the breakdown voltage was improved to be 800 V.n-Type 6H-SiC MOS capacitors were fabricated and characterized. Under the same growing conditions, the quality of polysilicon gate capacitors was better than Al. In addition, SiC MOS capacitors had good tolerance to gamma rays. (C) 2002 Published by Elsevier Science B.V.

Study of GaN thin films grown on vicinal SiC (0001) substrates by molecular beam epitaxy

A detailed study of the characteristics of undoped GaN films, grown on either vicinal or nominal flat SiC (0001) substrates by molecular beam epitaxy, has been carried out using photoluminescence and Raman scattering techniques. The I I K photoluminescence spectra of the GaN film grown on the vicinal SiC (0001) substrate show a strong and sharp near-bandgap peak (full width at half maximum (FWHM) similar to 16 meV). This feature contrasts with that of the GaN film grown on the nominal flat SiC (0001) substrate where the I I K photoluminescence spectra exhibit the near-bandgap peak (FWHM similar to 25 meV) and the intensity is approximately seven times weaker than that of the vicinal film sample. The redshift of the near-bandgap peak associated with excitons bound to shallow donors is related to the stress caused by both the lattice mismatch and the thermal expansion coefficient difference between GaN and SiC substrates. The measured thermal activation energy of the shallow donor of 33.4 meV is determined by using an Arrhenius plot of the near-bandgap luminescence versus I IT from the slope of the graph at high temperature. The temperature dependence of the FWHM of the near-bandgap luminescence has also been studied. The Raman scattering measurements from the vicinal film reveal that the E-2 phonon peak is strengthened and the A(1)(LO) phonon peak is shifted towards the low-frequency side with enhanced intensity, in comparison to that from the nominal flat film, suggesting a reduction in the density of defects and a lower free carrier concentration in the vicinal GaN film.