Semi-insulating 4H Research Articles

Beyond Electron Based Metamaterials: Low-Loss Surface Phonon Polariton-Based Nano-Antenna Arrays Using Silicon Carbide

Silicon carbide is well-known as a high quality wide-band gap semiconductor for use in electronic and optoelectronic applications at high powers, temperatures and radiation environments due to its high breakdown field, wide bandgap, radiation hardness and thermal conductivity. Due to its relatively low lattice mismatch with GaN and AlN and its high thermal conductivity SiC also serves as a high quality, semi-insulating substrate for use in III-N growth for high frequency electronics and optoelectronic devices. Over the past 20 years, dramatic progress in materials and device quality have been realized leading to extremely low-defect density substrates and epilayers. During this same period, significant effort has also been focused on the field of metal-based plasmonics and its applications in enhanced spectroscopy, light emitters, waveguides, and absorbers for photodetectors. However, while metal-based plasmonic systems have attracted attention for their ability to achieve sub-diffraction confinement with large concentrations of electromagnetic fields at optical frequencies, many potentially exciting applications are severely limited by the inherent optical losses caused by the high carrier densities within metals.It has been shown previously that an alternative to plasmonic metals in the mid-infrared-THz spectral ranges are polar dielectrics such as silicon carbide, whereby such large, sub-diffraction limited, localized electromagnetic fields can be sustained through the optical stimulation of plasmon-like surface phonon polaritons (SPhPs). Unlike plasmons, these SPhP modes are due to atomic displacements in polar semiconductors, and thus do not require free carriers to interact with the electromagnetic radiation. Because of this, SPhPs exhibit exceptionally low optical losses. Initial investigations into SiC for SPhPs began in the early part of the last decade and proof-of-principle experiments demonstrated that both localized and propagating SPhP modes could be supported. Recently, our group has demonstrated the first fabricated localized SPhP nano-resonators. The nanostructure arrays are composed of SiC nanoresonators having geometries that are 10 to 40 times smaller than the resonant wavelengths, which occur in the 10.3-12.5 um spectral range. The structures used varied in height from 150-1500 nm-tall nanopillars fabricated from semi-insulating 6H- and 4H-SiC and had diameters ranging from 150-1000 nm and interpillar separations between 20-10,000 nm. The low optical losses resulted in exceptionally narrow resonance linewidths of 3-24 cm-1, corresponding to quality factors in the 40-300 range, which is almost an order of magnitude greater than what is possible with silver (Q<40). Coupled with theoretical simulations of the electromagnetic fields, we have achieved an understanding of the local electromagnetic field distributions and intensities at such extreme confinement as a function of nanostructure size and interpillar separation. This allows us to identify the optical modes involved in the resonances and their optical selection rules. Our work shows that extremely small, high-quality, optical resonators are possible when we use SPhPs instead of surface plasmons in metamaterial designs in the IR. Unlike metals that are limited to the UV-visible, a larger wavelength range is possible with these materials as the phonon modes in polar semiconductors depend on the masses of the constituent atoms and their bond strength.

Growth and analysis of modulation-doped AlGaN/GaN heterostructure on semi-insulating SiC substrate

AlGaN/GaN heterostructure was grown on semi-insulating 6H–SiC substrate. The effect of the thickness of the initial AlN buffer layer on the crystalline quality and the stress of the grown GaN layer were investigated. The semi-insulating characteristic of the undoped GaN layer, which is very important for obtaining low device leakage current, was analyzed by photoluminescence measurement. Modulation doping of Si during the growth of AlGaN barrier layer was also introduced to increase the concentration of the two-dimensional electron gas density and hence to improve the device performance. The fabricated AlGaN/GaN heterostructure field effect transistor with gate length of 0.2μm and SiO2 passiviation layer exhibited good small-signal characteristics such as current gain cut-off frequency of 47GHz and the maximum oscillation frequency of 121GHz.

Defect Levels in High Purity Semi-Insulating 4H-SiC Studied by Alpha Particle Induced Charge Transient Spectroscopy

Defect levels in high purity semi-insulating 4H silicon carbide substrates are studied by charge transient spectroscopy using 5.5 MeV alpha particles. A shallow defect level with the activation energy around 0.3 eV is found in all samples annealed at temperatures from 1400 to 1600 °C. Some other defect levels lying at deeper in the bandgap are found in samples annealed at 1400 and 1500 °C. As these deep levels are annealed out by 1600 °C, the series resistance of samples is decreased.

X-&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$\gamma$&lt;/tex&gt; &lt;/formula&gt; Ray Spectroscopy With Semi-Insulating 4H-Silicon Carbide

Radiation detectors on a semi-insulating (SI) 4H silicon carbide (SiC) wafer have been manufactured and characterized with X and γ photons in the range 8-59 keV. The detectors were 400-μm-diameter circular Ni-SiC junctions on an SI 4H-SiC wafer thinned to 70 μm. Dark current densities of 3.5 nA/cm2 at +20°C and 0.3 μA / cm2 at +104°C with an internal electric field of 7 kV/cm have been measured. X- γ ray spectra from 241Am have been acquired at room temperature with pulser line width of 756 eV FWHM. The charge collection efficiency (CCE) has been measured under different experimental conditions with a maximum CCE = 75% at room temperature. Polarization effects have been observed, and the dependence of CCE on time and temperature has been measured and analyzed. The charge trapping has been described by the Hecht model with a maximum total mean drift length of 107 μm at room temperature.

Conventional thermal annealing for a more efficient p-type doping of Al+ implanted 4H-SiC

Abstract

Status of Large Diameter SiC Single Crystals

Large-diameter SiC single crystals are grown at II-VI by the sublimation technique. 100mm substrates of semi-insulating 6H SiC and n-type 4H SiC are produced as commercial products; in development, diameter expansion to 125mm has been achieved. Over the last two years, significant improvements have been made in crystal quality. The values of FWHM of x-ray rocking curves are typically 20-40 arc-seconds for 6H SI wafers and 12-30 arc-seconds for 4H n+ wafers. Micropipe density is less than 3 cm-2, and less than 0.1 cm-2 in best substrates. Electrical resistivity of SI substrates is, typically, of 1011 Ω•cm or above. For 4H n+ substrates, the typical dislocation density is about 9×103 cm-2 and the typical BPD density is less than 1×103 cm-2.

Transport Properties of Multi-Graphene Films Grown on Semi-Insulating SiC

Multi-graphene films grown by sublimation on the surface of a semi-insulating 6H_SiC substrate have been studied. It is shown that pre-growth annealing of the substrate in a quasi-closed growth cell improves the structural quality of a multi-graphene film. Ohmic contacts to the film have been fabricated, and the Hall effect has been studied at low temperatures. It is found that a 2D electron gas exists in the films. It is concluded that the conductivity of the film is determined by defects existing within the graphene layer or at the interface between the graphene film and a SiC substrate.

Fully implanted vertical p–i–n diodes using high-purity semi-insulating 4H–SiC wafers

The fabrication of a fully ion-implanted vertical p–i–n diode using high-purity semi-insulating 4H–SiC substrate has been demonstrated for the first time. The intrinsic region is the wafer itself with a thickness of 350 µm. The anode and cathode are obtained by doping the front and back wafer surfaces with implanted Al+ and P+ ions, respectively, with concentrations of about 1020 cm−3. The electrical activation of the implanted dopants is obtained by microwave heating the samples up to 2100 °C for 30 s. At ±100 V the on and off state current ratio is in the order of 104. Forward saturation current is five orders larger than it would be if controlled by the series resistance of the thick HPSI 4H–SiC intrinsic region.

Manganese in silicon carbide

Structural disorder and relocation of implanted Mn in semi-insulating 4H–SiC has been studied. Subsequent heat treatment of Mn implanted samples has been performed in the temperature range 1400–2000 °C. The depth distribution of manganese is recorded by secondary ion mass spectrometry. Rutherford backscattering spectrometry has been employed for characterization of crystal disorder. Ocular inspection of color changes of heat-treated samples indicates that a large portion of the damage has been annealed. However, Rutherford backscattering shows that after heat treatment, most disorder from the implantation remains. Less disorder is observed in the [0 0 0 1] channel direction compared to [ 1 1 2 ¯ 3 ] channel direction. A substantial rearrangement of manganese is observed in the implanted region. No pronounced manganese diffusion deeper into the sample is recorded.

Characterization of Semi-Insulating 4H Silicon Carbide for Radiation Detectors

Radiation detectors have been fabricated on 8 mm × 8 mm substrates, ~390 μm in thickness, diced from a (0001) 4H-SiC semi-insulating (SI) wafer (≥ 1012 Ohm-cm). The crystals used for detector fabrication have been characterized by x-ray diffraction (XRD), electron beam induced current (EBIC), chemical etching, cross-polarized imaging, and Raman spectroscopy. Current-voltage (I-V) characteristics showed very low leakage current (≤ 50 pA at -800 V) and the capability of detector's operation ≥ 470 K. EBIC investigations revealed that the screw dislocations produce dark EBIC contrast indicating high leakage current in the defective regions. Thermally stimulated current (TSC) measurements and high temperature resistivity measurements revealed deep level centers with activation energies 1.1-1.2 eV, and 1.56 eV. The TSC peak at ~460 K associated with the ~1.2 eV center was much stronger than the other high temperature peaks (e.g., 370 K due to vanadium impurity, 0.95 eV below of conduction band edge), indicating that this level along with the 1.56 eV level should dominate in controlling the resistivity and carrier lifetime in the studied 4H-SiC. Based on the literature data, we associate these centers with intrinsic defects and/or V-related complex. Nuclear detection measurements on the single-element SiC detectors with 241Am X-γ ray source clearly detected 59.6 keV and other low energy x-rays.

Thermally stimulated current and high temperature resistivity measurements of 4H semi-insulating silicon carbide

High purity semi-insulating 4H SiC single crystals have potential applications for room temperature radiation detectors because of the wide band gap and radiation hardness. To control carrier lifetime, a key parameter for high performance radiation detectors, it is important to understand the nature of the deep traps in this material. For this purpose, we have successfully applied thermally stimulated current (TSC) and high temperature resistivity measurements to investigate deep level centers in semi-insulating 4H SiC samples grown by physical vapor transport. High temperature resistivity measurements showed that the resistivity at elevated temperatures is controlled by the deep level with an activation energy of 1.56 eV. The dominant traps revealed by TSC measurements were at 1.1–1.2 eV. The deep trap levels in 4H-SiC samples, the impurity and point defect nature of TSC traps peaked at ∼106 K (0.23 eV), ∼126 K (0.32 eV), ∼370 K (0.95 eV), ∼456 K (1.1–1.2 eV) are discussed.

Hole-channel conductivity in epitaxial graphene determined by terahertz optical-Hall effect and midinfrared ellipsometry

We report noncontact, optical determination of free-charge carrier mobility, sheet density, and resistivity parameters in epitaxial graphene at room temperature using terahertz and midinfrared ellipsometry and optical-Hall effect measurements. The graphene layers are grown on Si- and C-terminated semi-insulating 6H silicon carbide polar surfaces. Data analysis using classical Drude functions and multilayer modeling render the existence of a p-type channel with different sheet densities and effective mass parameters for the two polar surfaces. The optically obtained parameters are in excellent agreement with results from electrical Hall effect measurements.

The EI4 EPR centre in 6H SiC

We present the results of our recent electron paramagnetic resonance (EPR) studies of the EI4 EPR centre in electron-irradiated high-purity semi-insulating 6H SiC. Higher signal intensities and better resolution compared with previous studies have enabled a more detailed study of the hyperfine (hf) structure. Based on the observed hf structure due to the interaction with Si and C neighbours, the effective spin S=1, the C1h-symmetry and the annealing behaviour, we suggest a carbon vacancy–carbon antisite complex in the neutral charge state, VCVCCSi0, with the vacancies and the antisite in the basal plane, as a new defect model for the centre.

Deep levels affecting the resistivity in semi-insulating 6H–SiC

We characterized the temperature dependence of electrical resistivities and deep levels in semi-insulating 6H–SiC bulk wafers. Two of the three wafers characterized showed n-type conductivity while the third showed p-type conductivity. We measured the temperature dependence of the resistivity (ρ) and estimated its activation energy for all the samples. On the basis of the deep level parameters obtained from current deep level transient spectroscopy and photoinduced current level transient spectroscopy, we were able to reproduce ρ for n-type samples but not for the p-type sample. These results suggest that although the deep levels observed in this study have significant effects on ρ for semi-insulating n-type 6H–SiC, there are unobserved deep levels compensating acceptors.

Effects of single-layer Shockley stacking faults on the transport properties of high-purity semi-insulating 4H–SiC

The electrical properties of high-purity 4H–SiC epitaxial layers are investigated. The current density versus applied voltage curves shows that some specimens have space charge limited transport behavior. Lampert–Mark model for trap-controlled electron transport in semi-insulating materials demonstrated the presence of an intragap state at 0.26 eV below the conduction band. Photodeep level transient spectroscopy has confirmed the presence of a deep level at about 0.3 eV below the conduction band only in samples with non-Ohmic behavior. Cathodoluminescence imaging has revealed the presence of high luminescent areas related to stacking faults (SFs) under the devices showing non-Ohmic behavior. The presence of single-layer SFs was detected by cathodoluminescence spectroscopy due to the typical emission at 2.95 eV. Thus, we demonstrated that the SFs modified the charge transport from Ohmic conduction to space charge limited conduction of the affected samples.

Analysis of donor–acceptor pairs and titanium related luminescence in different compensated 6H–SiC single crystals

Abstract The photoluminescence of three kinds of compensated 6H–SiC single crystals, the vanadium heavily compensated, vanadium slightly compensated, and non-vanadium compensated, has been investigated. Nitrogen (N), aluminum (Al), vanadium (V), and titanium (Ti) are the main impurities. For the vanadium heavily compensated and non-vanadium compensated semi-insulating 6H–SiC, photoluminescence of the donor–acceptor pairs (DAPs) are inconspicuous because of the rare shallow centers. For vanadium slightly compensated 6H–SiC, DAPs and Ti-related emissions are obvious. As an effective luminescence center, the Ti-related recombinations cover a broad range from 1.70 eV to 2.86 eV.

Growth of Large Diameter 6H SI and 4H n+ SiC Single Crystals

AbstractSiC single crystals are grown at II-VI by the seeded sublimation technique. The process has been scaled up and optimized to support commercial production of high-quality 100 mm diameter, Semi-Insulating (SI) 6H substrates and 100 mm 4H n+ substrates. The growth process incorporates special elements aimed at achieving uniform sublimation of the source, steady growth rate, uniform doping and reduced presence of background impurities.Semi-insulating 6H substrates are produced using precise vanadium compensation. Vanadium doping is optimized to yield SI material with very high resistivity and low capacitance.Crystal quality of the substrates is evaluated using a wide variety of techniques. Specific defects, their interaction and evolution during growth are described with emphasis on micropipes and dislocations. The current quality of the 6H SI and 4H n+ crystals grown at II-VI is summarized.

Surface electronic property of SiC correlated with NO2 adsorption

Correlations between surface electronic properties of SiC and NO2 adsorption were investigated using electrostatic force potentiometry. It was observed that surface work function (SWF) of both 6H and 3C–SiC changes significantly with NO2 adsorption. Measurements on semi-insulating 6H–SiC revealed that the Si face has higher sensitivity toward NO2 molecules than C face producing more change in SWF due to NO2 adsorption, which can be related with the difference in their surface free energies. For an n+-doped 6H–SiC, the SWF of the C face was found to increase much more than the Si face, showing correspondingly higher NO2 sensitivity. Upon exposure to superbandgap ultraviolet (UV) illumination, the surface band bending of both the faces was found to increase for undoped 6H–SiC, which resulted in enhanced sensitivity to NO2 adsorption. Measurements on doped SiC also supported similar correlations, although the surface band bending initially decreased under UV illumination. Our results indicate that adsorption of NO2 on 6H–SiC surfaces strongly depends on the surface band bending, with lower band bending resulting in decreased sensitivity, and vice versa. Faster desorption of NO2 molecules through UV exposure was observed for undoped 6H–SiC but not for the doped one. An adsorption model for NO2 has been proposed to explain the experimental observations.

The correlation of the results of capacitance mapping and of sheet resistance mapping in semi-insulating 6H–SiC

A combination of complex surface capacitance mapping and sheet resistance mapping is applied to establish the origin of resistance variations on semi-insulating (SI) 6H–SiC substrates. The direct correlation between the capacitance quadrature and the sheet resistance is found in vanadium-doped SI samples. Regions with low capacitance quadrature show high sheet resistance. This indicates, associated with the nonhomogeneity of sheet resistance on the substrate, that the quality of crystallization is not good enough, which also leads to resistivity nonhomogeneity when comparing with different types of deep defects. According to the capacitance mapping, the region with bad crystallization quality has a high radio absorption coefficient. Another correlation is established between the capacitance in-phase and sheet resistance for the vanadium-doped sample. In this sample, the capacitance in-phase map shows not only the surface topography, but also the same distribution trend as the sheet resistance, namely, regions of high capacitance in-phase reveal high sheet resistance.

An internally-matched GaN HEMTs device with 45.2 W at 8 GHz for X-band application

Optimized AlGaN/AlN/GaN high electron mobility transistor (HEMT) with high mobility GaN channel layer structures were grown on 2-in. diameter semi-insulating 6H–SiC substrates by MOCVD. The 2-in. diameter GaN HEMT wafer exhibited a low average sheet resistance of 261.9 Ω/square, with the resistance un-uniformity as low as 2.23%. Atomic force microscopy measurements revealed a smooth AlGaN surface whose root-mean-square roughness is 0.281 nm for a scan area of 5 × 5 μm. For the single-cell HEMTs device of 2.5-mm gate width fabricated using the materials, a maximum drain current density of 1.31 A/mm, an extrinsic transconductance of 450 mS/mm, a current gain cutoff frequency of 24 GHz and a maximum frequency of oscillation 54 GHz were achieved. The four-cell internally-matched GaN HEMTs device with 10-mm total gate width demonstrated a very high output power of 45.2 W at 8 GHz under the condition of continuous-wave (CW), with a power added efficiency of 32.0% and power gain of 6.2 dB. To our best knowledge, the achieved output power of internally-matched devices are the state-of-the-art result ever reported for X-band GaN-based HEMTs.