Type 4H-SiC Research Articles

Comparison of performance in GaN-HEMTs on thin SiC substrate and sapphire substrates

We have investigated the electrical properties of GaN-based high electron mobility transistors (HEMT) on N+-type 4H SiC substrates and on sapphire substrate as a reference. The three-terminal off-state breakdown voltage (BV) as a function of gate-drain spacing (LGD) was measured. Both devices possess superior Baliga figure of merit parameters (BFOM= BV3/Ron). For GaN/Sapphire with LGD of 10 μm, Ron is 14.65 mΩ·cm2 and the corresponding BFOM yields a remarkably high value of 9.88 × 108V2·Ω−1·cm−2. Conversely, GaN/N+ SiC with 20 μm LGD exhibited an Ron of 43.23 mΩ·cm2 and a BFOM value of 1.55 × 107V2·Ω−1·cm−2. In addition, we perform AFM, XRD, SEM and TEM analysis to further explore the differences in the DC-current properties for these two distinct substrates. The XRD FWHM scales with surface roughness, moreover grain boundaries for GaN-based HEMTs grown on SiC are more distinct compared to Sapphire substrate. This study provide valuable insight into the structural and morphological disparities that may contribute to the observed variations in device performance.

Growth of 4H-SiC epitaxial layers at temperatures below 1500 °C using trichlorosilane (TCS)

TCS (SiHCl3) is widely used in the rapid growth of SiC. However, in the study of using TCS to grow 4H-SiC, the growth temperature is mainly concentrated in 1550–1650 °C, and the research on lower growth temperature is lacking. In this study, TCS was used as the silicon precursor and epitaxial layers were grown in the temperature range of 1350–1500 °C. The results show that the single crystal type 4H-SiC epitaxial layer with good quality can be obtained at the growth temperature of 1450 ℃.

Theory of shallow and deep boron defects in 4H -SiC

Abstract Despite advances toward improving the quality of $p$-type 4H-SiC substrates and layers, we still have no model capable of accounting for the multitude of boron-related optical, junction, and paramagnetic resonance experiments available in the literature. A conspicuous puzzle is the observation of two shallow boron defects with rather distinct axial orientations as found by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) data. This feature is not observed in material doped with other group-III elements. Another open issue involves conflicting conclusions from photoluminescence and EPR studies of a deeper boron center, which has been linked to rather distinct models, either based on substitutional or vacancy-related boron defects. We unlock these and other problems by means of first-principles calculations, where the temperature-dependent stability, the electronic activity, and the paramagnetic response of boron defects in 4H-SiC are investigated.

Resolving Jahn-Teller induced vibronic fine structure of silicon vacancy quantum emission in silicon carbide

Point defects in semiconductors are promising single-photon emitters (SPEs) for quantum computing, communication, and sensing applications. However, factors such as emission brightness, purity. and indistinguishability are limited by interactions between localized defect states and the surrounding environment. Therefore, it is important to map the full emission spectrum from each SPE, to understand the complex interplay between the different defect configurations, their surroundings, and external perturbations. Herein, we investigate a family of regularly spaced sharp luminescence peaks appearing in the near-infrared portion of photoluminescence (PL) spectra from $n$-type 4H-SiC samples after irradiation. This periodic emitter family, labeled the L lines, is only observed when the zero-phonon line signatures of the negatively charged Si vacancy (so-called V lines) are present. The L lines appear with $1.45\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ and $1.59\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ energy spacing after H and He irradiation and increase linearly in intensity with fluence---reminiscent of the intrinsic defect trend. Furthermore, we monitor the dependence of the L-line emission energy and intensity on heat treatments, electric field strength, and PL collection temperature, discussing these data in the context of the L lines. Based on the strong similarity between the irradiation, electric field, and thermal responses of the L and V lines, the L lines are attributed to the Si vacancy in 4H-SiC. The regular and periodic appearance of the L lines provides strong arguments for a vibronic origin explaining the oscillatory multipeak spectrum. To account for the small energy separation of the L lines, we propose a model based on rotations of distortion surrounding the Si vacancy driven by a dynamic Jahn-Teller effect.

Effect of Oxygen Annealing on the Characteristics of Isotype Ga2O3/4H-SiC Heterojunction Diodes

Ga2O3/n-type 4H-SiC heterojunction diodes were fabricated by depositing Ga2O3 thin films on off-axis cut n type 4H-SiC substrates by RF magnetron sputtering. The influence of oxygen atmosphere annealing on the film quality and optical properties of Ga2O3 layers is investigated. The I–V characteristics of the diodes are acquired in the range from 25 to 175 °C with temperature step of 50 °C. The annealed diodes exhibit improved rectifying ratio (∼1 × 108 for ±2V) and an improved ideality factor (1.83) at 25 °C. Additionally, the photodiode that was annealed in the presence of an oxygen atmosphere shows an increased photocurrent, higher responsivity and an improved time dependent photo-response than as-grown devices.

Thermionic emission and conversion properties of n-type AlGaN thin film cathodes grown on 6H–SiC substrates

We observed the thermionic emission (TE) and conversion (TC) characteristics of Si-doped AlGaN thin film cathodes grown on n-type 6H-SiC substrates with Cs adsorption on the surfaces. The threshold temperatures of TE decreased as the AlN mole fraction was increased, with threshold temperatures of around 205 °C for Si–doped Al0.5Ga0.5N. TE was detected even when the applied external voltage was 0 V at 600 °C, which means that the cathodes had TC characteristics at a temperature dramatically lower than that of conventional systems with metal-based cathodes. In addition, we observed a notable enhancement of the emission current when the cathodes were irradiated with white light having energy lower than that of the bandgap of the AlGaN films at low temperatures. We suggest that the enhancement was caused by an internal photoelectron effect due to the absorption of the light in the n–type 6H–SiC substrate.

Analysis of carrier lifetimes in N + B-doped n-type 4H-SiC epilayers

Control of carrier lifetimes in nitrogen (N)-doped n-type 4H-SiC epilayers was attempted by intentional boron (B) doping. Doping concentrations of B were controlled to be within 1015–1018 cm−3 by varying the triethyl boron flow rate during epitaxial growth. Time-resolved photoluminescence measurements for the band edge luminescence of the N + B-doped epilayer showed a fast decay time of less than 30 ns under a low injection level at 300 K, while a slow decay component was observed at elevated temperatures of 423–523 K. To understand the mechanism of carrier capture and recombination in N + B-doped 4H-SiC, excess carrier decay curves were simulated by solving rate equations with a simple model. As an effective “recombination enhancing layer,” the 1.6-μm-thick N + B-doped (N: 4 × 1018 cm−3, B: 7 × 1017 cm−3) buffer layer in the PiN diode showed increased intensity of D center-related electroluminescence with increasing injected hole concentrations and prevented the formation of stacking faults from basal plane dislocations in the substrate even at the injection level of 1 × 1016 cm−3 at the interface between the drift and buffer layers.

Photoelectrochemical Properties of p- and n- Type 4H-SiC Epilayers: Doping Concentration Dependence and Dopant Selectivity

Exceptional chemical stability resulting from a large bandgap and low-lying valence band edge narrows the wet-etching based micromachining choices of the covalently bonded electronic and photonic device material, SiC, to mainly photoelectrochemical (PEC) dissolution. The PEC etching process has an anisotropic nature with the accompanying advantages of dopant, bandgap, and spatial selectivity, light intensity dependence, reduced ion damage, and the ability to undercut heterostructures. For the first time, a comprehensive photogenerated minority carrier assisted etching investigation was carried out on Si polar surface of high-quality (micropipe density <1 cm-2) p- and n- type (0001) 4H-SiC epilayers having intentional or unintentional doping densities in the 1014–1018 cm-3 range. Cyclic voltametry and chronoamperometry studies under a parallel supra-bandgap (~320 nm) light illumination with varying intensities (≤0.85 Wcm-2) in a highly basic etching medium (1 wt% KOH solution, pH ~12) revealed the dependence of etch voltage and etch rate (~30–90 nm/min) on type and concentration of doping. In addition, a mutually exclusive etch voltage window (~±0.3–0.5 V versus saturated calomel electrode) was established for both types of conductivities in the diffusion-limited regime. Results to be presented can provide key insight in design and fabrication for higher performance three dimensional SiC homojunctions for various applications including photonic crystal cavities.

Pt:Ti Diffusion Barrier, Interconnect, and Simultaneous Ohmic Contacts to n- and p-Type 4H-SiC

We report the initial results of using co-sputtered Pt:Ti 80:20 at. % composition ratio metallization as a diffusion barrier against gold (Au) and oxygen (O), as an interconnect layer, as well as forming simultaneous ohmic contacts to n-and p-type 4H-SiC. Having a single conductor with such combined multi-functional attributes would appreciably reduce the fabrication costs, processing time and complexity that are inherent in the production of SiC based devices. Auger Electron Spectroscopy, Focused Ion Beam-assisted Field Emission Scanning Electron Microscopy and Energy Dispersive Spectroscopy analyses revealed no Au and O migration to the SiC contact surface and minimal diffusion through the Pt:Ti barrier layer after 15 minutes of exposure at 800 oC in atmosphere, thus offering potential long term stability of the ohmic contacts. Specific contact resistance values of 7 x 10-5 and 7.4 x 10-4 Ω-cm2 were obtained on the n (Nd=7 x 1018 cm-3) and p (Na=2 x 1020 cm-3) -type 4H-SiC, respectively. The resistivity of 75 μΩ-cm was obtained for the Pt:Ti layer that was sandwiched between two SiO2 layers and annealed in pure O ambient up to 900 °C, which offers promise as a high temperature interconnect metallization.

Electrical characterization of defects introduced during sputter deposition of tungsten on n type 4H-SiC

We have studied the defects introduced in n-type 4H-SiC during sputter deposition of tungsten using deep-level transient spectroscopy (DLTS). Current-voltage and capacitance-voltage measurements showed a deterioration of diode thermionic emission characteristics due to the sputter deposition. Two electrically active defects E0.29 and E0.69 were introduced. Depth profiling revealed that sputter deposition increases the concentration of the native Z1 defect. A comparison with prominent irradiation and process induced defects showed that the E0.29 was unique and introduced during sputter deposition only. The E0.69 may be silicon vacancy related defect.

(Invited) P-Type 3C-SiC Photocathode for Solar to Hydrogen Energy Conversion

Solar to hydrogen conversion technology attracts attention as renewable energy generation. In general, to perform solar to hydrogen conversion, photocatalytic materials are employed. However, most of photocatalytic materials cannot absorb visible light resulting in low conversion efficiency and degrade during operation. Thus, we need to find new photocatalytic materials which have both high conversion efficiencies and resistance to corrosion. Silicon carbide (SiC) is a chemically stable and photocatalytic semiconductor material. SiC can be grown by different atomic configurations resulting in a large number of polytypes. Among SiC polytypes, 3C-SiC has the smallest band gap (2.3 eV), and thus 3C-SiC is capable to absorb significant part of solar light. In addition, conductive type of SiC can be controlled by doping. Photocatalytic materials with p-type conduction operate as cathode and reduce hydrogen in water. Cathodic operation prevents oxidation of photocatalytic materials. Therefore, p-type 3C-SiC will be an efficient and durable photocatalyst. However, bulk crystal growth of 3C-SiC is very difficult because this polytype is stable only in growth at much lower temperature than sublimation of SiC, which is a typical condition for bulk SiC growth. This difficulty makes 3C-SiC growth only by chemical vapor deposition (CVD) on Si or 4H- or 6H-SiC substrates. Such the heteroepitaxial growth induces large number of structural defects or polytype mixing owing to lattice mismatch or inheritance of the substrate polytype. For our recent 3C-SiC photocathode fabrication, we employed p+-type 4H-SiC substrates with the (0001) Si-face inclined 0.7° to <1120> direction. Growth on the 4H-SiC substrates with quasi on-axis Si-face prevents polytype mixing and p+-type conductivity of the substrate facilitates ohmic contact formation on the substrate side. We performed 3C-SiC growth on the substrates by CVD with silane and propane as source gases and in an Al acceptor doping condition. A photograph of the grown crystal is shown in Fig. 1(a). After the growth, we fabricated ohmic contact on the substrate side of the crystals. The 3C-SiC surface is active surface as the photocathode. In addition, we formed Pt or Pd cocatalysts on the 3C-SiC surface to enhance hydrogen generation by the photocathodes. The schematic of the photocathode structure is shown in Fig. 1(b). We measured photocurrent-potential characteristics for the photocathodes in 1 M H2SO4 aqueous solution using chopped solar simulator light with 100 mW/cm2. The results are shown in Fig. 1(c). For bare surface, cathodic photocurrent was observed below 0.6 V vs SCE, and it reached -1.4 A/cm2 at -1 V vs SCE. On the other hand, with cocatalysts, photocurrents were observed at higher potential, and in the case of with Pt cocatalysts, it was observed from 0.9 V vs SCE. In addition, at -1 V vs SCE, the photocurrent was -2.3 A/cm2. In solar to hydrogen conversion, the Faraday efficiencies of the photocathodes were near to 100% and the photocathodes were not degraded during experiments. Solar to hydrogen conversion efficiency for the photocathode with Pt cocatalysts was 0.52 %. Considering such a high efficiency and possibility of further improvement, SiC is a very promising material for the solar to hydrogen conversion technology. Figure 1

The effects of high-energy proton irradiation on the electrical characteristics of Au/Ni/4H-SiC Schottky barrier diodes

Au/Ni (20:80) Schottky barrier diodes (SBDs) were resistively evaporated on nitrogen-doped n–type 4H-SiC. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the SDBs were investigated before and after bombardment with 1.8MeV proton irradiation at a fluence of 2.0×1012cm–2. The measurements were carried out in the temperature range 40–300K in steps of 20K. Results obtained at room temperature (300K) showed highly rectifying devices before and after bombardment. It was observed that the proton irradiation induced an increase of ideality factor from 1.05 to 1.13, a decrease in Schottky barrier height from 1.40 to 1.22eV, an increase in series resistance from 10 to 66 Ω and a noticeable increase of the saturation current from 3.0×10–21 to 6.8×10–17A. The increase in saturation current after proton irradiation was attributed to the presence of interfacial states created by irradiation-induced defects. Thermionic emission dominated the I-V characteristics in the temperature range 120–300K but the I-V characteristics deviated from thermionic emission theory at temperatures below 120K for devices both before and after irradiation. The variation of the SBDs characteristics with temperature was attributed to the presence of lateral inhomogeneities of the SBH. Modified Richardson constants were determined from a Gaussian distribution of barrier heights to be 133 and 165Acm–2K–2 before and after irradiation, respectively.

Simultaneous ohmic contacts to p- and n-type 4H-SiC by phase segregation annealing of co-sputtered Pt-Ti

A new concept, Phase Segregation Annealing (PSA), was investigated for implementing simultaneous ohmic contacts (SOCs) to p- and n-type 4H-SiC. Test structures with selected ratio compositions of co-sputtered Pt:Ti contacts were fabricated in p-type 4H-SiC epitaxial layers having aluminum acceptor concentrations, Na = 2 × 1019, 7 × 1019, and 2.5 × 1020 cm−3, and a nitrogen doped n-type epitaxial layer having donor concentration, Nd = 7 × 1018 cm−3. The ratios of the co-sputtered Pt-Ti metallization were 80:20, 50:50, and 30:70 at. %. After rapid thermal annealing (RTA) ranging between 800 and 1200 °C in vacuum and confirming SOCs by linear current-voltage (I-V) measurement, the specific contact resistance (ρc) values were extracted using the Transfer Length Measurement method. SOCs were realized with the Pt80:Ti20 composition starting from 1000 °C, and the Pt30:Ti70 composition from 1100 °C, with both exhibiting eutectic and segregated phases. The Pt50:Ti50 composition produced no SOC and eutectic and segregated phases were absent. The Pt80:Ti20 composition had the lowest pair of average ρc values of 7 × 10−5 Ω cm2 and 7.3 × 10−4 Ω cm2 on the highest doped p-type and the n-type samples after RTA at 1000 °C, respectively. Auger electron spectroscopy and focused ion beam field emission scanning electron microscopy with energy dispersive x-ray spectroscopy indicated distinct phase segregation via the eutectic-liquidus-eutectic transitions, the coalescence of likely Pt3Si and Pt2Si binary phases, and solid phases of Ti3Si, Ti5Si3, and TiC, with all the active phases maintaining intimate contact to both the p- and n-type 4H-SiC surfaces. The SOC formation was attributed to the disparate work functions of these phases, which was in good agreement with the proposed PSA model.

Design and experiment of 4H-SiC JBS diodes achieving a near-theoretical breakdown voltage with non-uniform floating limiting rings terminal

In this paper, a 4H-SiC Junction Barrier Schottky diode (JBS) with non-uniform floating limiting rings (FLRs) has been investigated and fabricated using n type 4H-SiC epitaxial layer with thickness of 31μm and doping concentration of 3.3×1015cm−3. According to the simulated results, the key parameters of a FLRs design to achieve a high voltage are the minimum space between two adjacent doped rings, spacing growth step and number of rings. The experimental results also show a great agreement with simulated results. Meanwhile, a near-ideal breakdown voltage of 3.7kV was achieved, which yield around 95% of the parallel-plane breakdown voltage. The forward characteristics show that the fabricated JBS diodes have a forward current density of 210A/cm2 at 3V and a specific on-resistance (Rsp-on) of 7.58mΩcm2. Different FLRs parameters have no effect on the forward device performance.

Investigation of Interface State Density with Varied SiO&lt;sub&gt;2&lt;/sub&gt; Thickness in La&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;/SiO&lt;sub&gt;2&lt;/sub&gt;/4H-SiC MOS Capacitors

SiO2 with varying thickness (0, 4.45, and 8.05 nm) were grown on n type 4H-SiC epilayer by thermal oxidation and La2O3 were stacked on them using atomic layer deposition (ALD). The La2O3/SiO2/4H-SiC metal-oxide-semiconductor (MOS) capacitors were analyzed by X-ray photoelectron spectroscopy (XPS) and capacitance-voltage (C-V) measurements. C-V curves show that introducing an ultrathin SiO2 can reduce the effect of lattice mismatch of La2O3/4H-SiC structure and then improve interface property. However, the interface quality is reduced as SiO2 was grown thicker. XPS data show that more carbon cluster remains at the interfacial between SiO2 and 4H-SiC as the oxidation time increases.

The isotype ZnO/SiC heterojunction prepared by molecular beam epitaxy--A chemical inert interface with significant band discontinuities.

ZnO/SiC heterojunctions show great potential for various optoelectronic applications (e.g., ultraviolet light emitting diodes, photodetectors, and solar cells). However, the lack of a detailed understanding of the ZnO/SiC interface prevents an efficient and rapid optimization of these devices. Here, intrinsic (but inherently n-type) ZnO were deposited via molecular beam epitaxy on n–type 6H-SiC single crystalline substrates. The chemical and electronic structure of the ZnO/SiC interfaces were characterized by ultraviolet/x-ray photoelectron spectroscopy and x-ray excited Auger electron spectroscopy. In contrast to the ZnO/SiC interface prepared by radio frequency magnetron sputtering, no willemite-like zinc silicate interface species is present at the MBE-ZnO/SiC interface. Furthermore, the valence band offset at the abrupt ZnO/SiC interface is experimentally determined to be (1.2 ± 0.3) eV, suggesting a conduction band offset of approximately 0.8 eV, thus explaining the reported excellent rectifying characteristics of isotype ZnO/SiC heterojunctions. These insights lead to a better comprehension of the ZnO/SiC interface and show that the choice of deposition route might offer a powerful means to tailor the chemical and electronic structures of the ZnO/SiC interface, which can eventually be utilized to optimize related devices.

High-temperature and reliability performance of 4H-SiC Schottky-barrier photodiodes for UV detection

With its wide bandgap, excellent electrical properties, and relative mature crystal growth technique, 4H-SiC is an attractive candidate material for fabricating high performance ultraviolet photodetectors, which have potential to work in harsh environments. In this work, a vertical type 4H-SiC Schottky-barrier photodiode is designed and fabricated. The photodiode exhibits very low dark current and high quantum efficiency in the entire temperature range from 25 to 200 °C. A high Schottky barrier height of over 1.58 eV and an ideality factor as low as 1.074 at room temperature are deduced for the semitransparent Ni/4H-SiC Schottky metal contact. The high-temperature reliability characteristics are evaluated by high-temperature storage at 200 °C and high-temperature spike annealing up to 550 °C.

Thermal stability of Ni/Ti/Al ohmic contacts to p-type 4H-SiC

Low resistivity Ni/Ti/Al ohmic contacts on p-type 4H-SiC epilayer were developed, and their thermal stabilities were also experimentally investigated through high temperature storage at 600 °C for 100 h. The contact resistance of the Al/Ti/Ni/SiC contacts degraded in different degrees, and the contact morphology deteriorated with the increases of the average surface roughness and interface voids. X-ray spectra showed that Ni2Si and Ti3SiC2, which were formed during ohmic contact annealing and contributed to low contact resistivity, were stable under high temperature storage. The existence of the TiAl3 and NiAl3 intermetallic phases was helpful to prevent Al agglomeration on the interface and make the contacts thermally stable. Auger electron spectroscopy indicated that the incorporation of oxygen at the surface and interface led to the oxidation of Al or Ti resulting in increased contact resistance. Also, the formation of these oxides roughened the surface and interface. The temperature-dependence of the specific contact resistance indicated that a thermionic field emission mechanism dominates the current transport for contacts before and after the thermal treatment. It suggests that the Ni/Ti/Al composite ohmic contacts are promising for SiC devices to be used in high temperature applications.

X-ray Topography Studies of Relaxation during the Homo-Epitaxy of 4H-SiC

ABSTRACTA review is presented of Synchrotron X-ray Topography and KOH etching studies carried out on n type 4H-SiC offcut substrates before and after homo-epitaxial growth to study defect replication and strain relaxation processes and identify the nucleation sources of both interfacial dislocations (IDs) and half-loop arrays (HLAs) which are known to have a deleterious effect on device performance. We show that these types of defects can nucleate during epilayer growth from: (1) short segments of edge oriented basal plane dislocations (BPDs) in the substrate which are drawn by glide into the epilayer; and (2) segments of half loops of BPD that are attached to the substrate surface prior to growth which also glide into the epilayer. It is shown that the initial motion of the short edge oriented BPD segments that are drawn from the substrate into the epilayer is caused by thermal stress resulting from radial temperature gradients experienced by the wafer whilst in the epi-chamber. This same stress also causes the initial glide of the surface half-loop into the epilayer and through the advancing epilayer surface. These mobile BPD segments provide screw oriented segments that pierce the advancing epilayer surface that initially replicate as the crystal grows. Once critical thickness is reached, according to the Mathews-Blakeslee model [1], these screw segments glide sideways under the action of the mismatch stress leaving IDs and HLAs in their wake. The origin of the mismatch stress is shown to be associated with lattice parameter differences at the growth temperature, arising from the differences in doping concentration between substrate and epilayer.

Numerical analysis of reverse recovery characteristics of 4H-SiC p+–n−–n+ power diode with injection conditions

Reverse recovery behavior of semiconductor power diode is usually involved in the operation of electrical equipment, which is mainly determined by the internal factor and external circuit of the device. In the present paper, the spatial–temporal distribution of minority carriers (MCs) in base region (BR) and waveforms of current and voltage in reverse recovery of p+–n−–n+ type 4H-SiC diode were numerically probed via an analytical model with injection conditions, of which parameters such as diffusion coefficient and length depend on the concentration of injection carriers. The storage time and the recovery time were evaluated. The results indicate that the current and voltage transient characteristics in reverse recovery process are affected by the width, concentration of majority carriers and lifetime of minority carriers in base region, also by the doped levels in anode and cathode, as well as by the forward current density and reverse biased voltage at initial state. The characteristic frequency was also influenced by these factors. These obtained consequences could guide for device optimized design and application.