P-type 4H-SiC Research Articles

Surface polishing by electrochemical etching of p-type 4H SiC

Electrochemical polishing of Si- and C-face p-type 4H SiC has been performed. Polishing the Si face leads to a smoother surface compared to the C face within the range of etching conditions studied. However, the results of additional experiments on C-face samples indicate that polishing a lower doped p-type material leads to improved surface quality for this face. Differences in polishing on the two faces are interpreted in terms of preferred electrochemical etching directions in SiC. Etching conditions such as HF concentration, doping, and etching current density were varied to obtain optimized values. Current-voltage plots (voltammograms) show a maximum electrochemical current density for the process. The polishing should be performed at a constant current density near this peak value to obtain both a smooth surface and a rapid rate of removal of material. In contrast to electrochemical polishing of silicon, we suggest that the applied current should match the rate at which the electrolyte can supply ions for reaction to obtain the smoothest surface required for microelectromechanical system device fabrication. Based on the experiments, we propose that, to obtain the smoothest SiC surface using electrochemical polishing, etching should be performed on Si-face p-type 4H SiC in dilute HF solution at the optimum current density determined by the current-voltage measurements.

Mechanisms of changes of hole concentration in Al-doped 6H-SiC by electron irradiation and annealing

Abstract The temperature dependence of the hole concentration p ( T ) in Al-doped p-type 6H-SiC irradiated by 100 or 200 keV electrons is investigated. Since p ( T ) is unchanged by 100 keV electron irradiation, the threshold displacement energy in SiC is higher than 20 eV. Therefore, 200 keV electrons cannot displace substitutional Si and Al in Al-doped 6H-SiC. Using p ( T ) , two types of acceptor species are detected, and the density and energy level of each acceptor species are determined. By 200 keV electron irradiation, the density ( N Al ) of the shallow acceptor (i.e., Al acceptor) decreases monotonously with increasing fluence of electrons, whereas the density ( N DA ) of the deep acceptor initially increases and then decreases. By irradiation with the 1 × 10 16 cm - 2 fluence of 200 keV electrons, especially, the decrement of N Al is nearly equal to the increment of N DA . By annealing at 500 °C, on the other hand, the increment of N Al is close to the decrement of N DA .

SiC: A Photocathode for Water Splitting and Hydrogen Storage

A SiC way to split water: Illuminated p-type 4H-SiC, as a photocathode short-circuited to Pt, was found to split water (see picture). The hydrogen generated is stored to a considerable extent in the solid.

Detection and depth analyses of deep levels generated by ion implantation in n- and p-type 4H-SiC

The authors investigated deep levels in the whole energy range of bandgap of 4H-SiC, which are generated by low-dose N+, P+, and Al+ implantation, by deep level transient spectroscopy (DLTS). Ne+-implanted samples have been also prepared to investigate the pure implantation damage. In the n-type as-grown material, the Z1∕2 (EC−0.63eV) and EH6∕7 (EC−1.6eV) centers are dominant deep levels. At least, seven peaks (IN1, IN3–IN6, IN8, and IN9) have emerged by implantation and annealing at 1000°C in the DLTS spectra from all n-type samples, irrespective of the implanted species. After high-temperature annealing at 1700°C, however, most DLTS peaks disappeared, and two peaks, IN3 and IN9, which may be assigned to Z1∕2 and EH6∕7, respectively, survive with a high concentration over the implanted atom concentration. In the p-type as-grown material, the D (EV+0.40eV) and HK4 (EV+1.4eV) centers are dominant. Two peaks (IP1 and IP3) have emerged by implantation and annealing at 1000°C, and four traps IP2 (EV+0.39eV), IP4 (EV+0.72eV), IP7 (EV+1.3eV), and IP8 (EV+1.4eV) are dominant after annealing at 1700°C in all p-type samples. The IP2 and IP8 may be assigned to the HS1 and HK4 centers, respectively. The depth analyses have revealed that the major deep levels are generated in the much deeper region than the implant profile.

Deep defects in GaN/AlGaN/SiC heterostructures

Deep level transient spectroscopy (DLTS) measurements were carried out on GaN/AlGaN/SiC heterostructures prepared by low-pressure metalorganic vapor phase epitaxy. Si-doped n-GaN layers were grown using an n-AlGaN nucleation layer (8% and 30% of aluminum) on two kinds of p-type 4H-SiC substrates. The DLTS spectra of on-axis (0001) grown samples exhibit a dominant peak of a majority carrier trap with apparent activation energy close to 0.80 eV and capture cross section of about 5×10−14 cm2 regardless of the AlGaN composition. The energy of this deep level decreases with increasing electrical field due to Poole–Frenkel effect. Carrier capture kinetics indicates interacting point defects arranged along a line, probably a threading dislocation. Two additional traps (0.52 and 0.83 eV) were found in on-axis samples with 8% AlGaN composition. For 30% Al content, only a 0.83 eV level was detected. Majority carrier trap with activation energy of 0.66 eV was observed in the off-axis grown samples. This level is probably related to an interface defect or to a defect lying near the heterojunction interface.

Observation of stacking faults formed during homoepitaxial growth of p-type 4H-SiC

Threading dislocations and their transformation into stacking faults (SFs) are observed in p-type 4H-SiC epitaxial layers by high voltage transmission electron microscope. Homoepitaxial growth and in situ aluminum doping of 4H-SiC epitaxial layers are carried out using the organosilicon precursor bistrimethylsilylmethane (C7H20Si2 and the metal-organic precursor trimethylaluminum (C3H9Al), and the free hole concentration of the most heavily aluminum-doped epitaxial layers is >1021 cm−3. Threading dislocations are formed at the interface between the epitaxial layer and the substrate. However, the density of these threading dislocations decreases toward the epitaxial layer surface with their transformations to SFs.

Si<sub>C</sub>C<sub>Si</sub>Antisite Pairs as Dominant Irradiation Induced Defects in p-Type 4H-SiC

In this work we elucidate the microscopic origin of the dominant radiation induced I-II spectra in p-type doped 4H-SiC. By calculating the electronic g-tensor from first principles in the framework of density functional theory, basal antisite pairs SiCCSi + are shown to give rise to the characteristic anisotropic g-tensors found in the electron paramagnetic resonance (EPR) measurements. Additional central hyperfine (hf) splittings of about 100 MHz due to the SiC antisite nuclei are predicted theoretically and also resolved experimentally. We have, thus, identified antisite pairs as a dominant defect in electron and proton irradiated p-type doped 4H-SiC.

Deep Levels Generated by Ion-Implantation in n- and p-Type 4H-SiC

The authors have investigated deep levels in the whole energy range of bandgap of 4H-SiC, which are generated by N+, P+, Al+ implantation, by deep level transient spectroscopy (DLTS). Ne+-implanted samples have been also prepared to investigate the pure implantation damage. In the n-type as-grown material, Z1/2 (Ec – 0.63 eV) and EH6/7 (Ec – 1.6 eV) are dominant deep levels. When the implant dose is low, seven peaks (IN1, IN3 ~ IN6, IN8, IN9) have emerged by implantation and annealing at 1000oC in the DLTS spectra from all n-type samples. After high-temperature annealing at 1700oC, however, most DLTS peaks disappeared, and two peaks, Z1/2 and EH6/7 survive. In the p-type as-grown material, D center (Ev + 0.40 eV) and HK4 (Ev + 1.4 eV) are dominant. When the implant dose is low, two peaks (IP1, IP3) have emerged by implantation and annealing at 1000oC, and four traps IP2, IP4 (Ev + 0.72 eV), IP7 (Ev + 1.3 eV), and IP8 (Ev + 1.4 eV) are dominant after annealing at 1700oC.

Effects of the Surface Condition of the Substrates on the Electrical Characteristics of 4H-SiC MOSFETs

Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) were fabricated on p-type epitaxial 4H-SiC substrates with different surface conditions and these electrical characteristics were compared. The MOSFETs on Chemical Mechanical Polished substrates showed the drain current of the order of 10-12A at a gate voltage of 0 V, and the value of the drain current increased with increasing the surface roughness of substrates. With decreasing the surface roughness of substrates, the values of the threshold voltage decreased and the quality of gate oxide became better.

Electrochemical Polishing of p-Type 4H SiC

This article discusses the electrochemical polishing of silicon-face p-type 4H SiC using diluted aqueous HF solution. Etchings on the silicon and carbon faces of SiC samples are performed and compared. The experimental results show that the RMS surface roughness of the electrochemically polished Si-face could be as low as about 2 nm. Carbon-face electrochemical polishing gives a rougher surface. Therefore, silicon-face 4H SiC is a better candidate for MEMS processing. The underlying mechanism is also discussed.

Characterization of 6H-SiC Single Crystals Grown by PVT Method Using Different Source Materials and Open or Closed Seed Backside

n- and p-type 6H-SiC single crystals grown by PVT method using different charge materials – poly-SiC sinter or fresh SiC powder – have been studied. An open or closed seed backside during the growth processes have been applied. In the former, a distinct decrease backside etching of the seed was observed. Crystals have been extensively characterized with respect to their purity, quality and electrical properties using complex experimental methods. For the n-type boule an axially and radially homogeneous resistivity ~0.11 cm at 300 K was observed. Electrical properties of the p-type crystal, i.e., high room-temperature resistivity of 239 cm, were affected by compensation effects between residual donors (nitrogen and oxygen) and acceptors (mainly boron).

Application of a time-resolved four-wave mixing technique for the determination of thermal properties of 4H–SiC crystals

We applied a time-resolved four-wave mixing technique for the determination of thermal diffusivity, sound velocity, thermo-optic and photoelastic coefficients in heavily doped n-type and p-type 4H–SiC substrates. Spatial modulation of thermal properties was achieved by intraband carrier excitation using a light-interference pattern at 1064 nm and subsequent carrier thermalization. Decay of the light-induced thermal grating at its various periods and sample temperatures was monitored by diffraction of a delayed probe pulse at 532 nm wavelength. The grating decay kinetics provided the thermal diffusivity DT = 0.94 cm2 s−1, thermo-optic coefficient dn/dT = 3.6 × 10−5 K−1 and effective photoelastic constant at 300 K. Temperature dependences of dn/dT and thermal expansion almost matched that of the heat capacity, Cp(T), and correlated well with the Eg(T) dependence.

Characterization of anodic SiO 2 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 SiO 2 and SiC.

Al 이온 주입된 p-type 4H-SiC에 형성된 Ni/Ti/Al Ohmic Contact의 전기적 특성

Ni/Ti/Al multilayer system (`/`denotes the deposition sequence) was tested for low-resistance ohmic contact formation to Al-implanted p-type 4H-SiC. Ni 30 nm / Ti 50 nm / Al 300 nm layers were sequentially deposited by e-beam evaporation on the 4H-SiC samples which were implanted with Al (norminal doping concentration

Comparison of Electrical Properties of Ohmic Contact Realized on p-Type 4H-SiC

N+ 4H-SiC commercial substrates with n-type epilayers have been used to realize bipolar diodes and TLM structures. The p-type emitter of diodes was realized by Al implantations followed by a post-implantation annealing with or without a graphite capping layer. Ohmic contacts were formed by depositing Ti/Ni on the backside and Ni/Al on the topside of the wafer. It appears that capping the sample during the annealing reduces considerably the surface roughness and the specific contact resistance. Sheet resistance and specific contact resistance as low as 2kΩ/□ and respectively 1.75×10-4 Ωcm² at 300 K have been obtained. I-V measurements as a function of temperature have been performed from ~100 to ~500 K. The variations of the series resistance vs. temperature can be explained by the freeze-out of carriers and by the variation of carrier mobility.

<i>In Situ</i> Boron and Aluminum Doping and Their Memory Effects in 4H-SiC Homoepitaxial Layers Grown by Hot-Wall LPCVD

The in-situ p-type doping of 4H-SiC grown on off-oriented (0001) 4H-SiC substrates was performed with trimethylaluminum (TMA) and/or diborane (B2H6) as the dopants. The incorporations of Al and B atoms and their memory effects and the electrical properties of p-type 4H-SiC epilayers were characterized by secondary ion mass spectroscopy (SIMS) and Hall effect measurements, respectively. Both Al- and B-doped 4H-SiC epilayers were p-type conduction. It was shown that the profiles of the incorporated boron and aluminum concentration were in agreement with the designed TMA and B2H6 flow rate diagrams. The maximum hole concentration for the Al doped 4H-SiC was 3.52×1020 cm-3 with Hall mobility of about 1 cm2/Vs and resistivity of 1.6~2.2×10-2 Wcm. The heavily boron-doped 4H-SiC samples were also obtained with B2H6 gas flow rate of 5 sccm, yielding values of 0.328 Wcm for resistivity, 5.3×1018 cm-3 for hole carrier concentration, and 7 cm2/Vs for hole mobility. The doping efficiency of Al in SiC is larger than that of B. The memory effects of Al and B were investigated in undoped 4H-SiC by using SIMS measurement after a few run of doped 4H-SiC growth. It was clearly shown that the memory effect of Al is stronger than that of B. It is suggested that p-type 4H-SiC growth should be carried out in a separate reactor, especially for Al doping, in order to avoid the join contamination on the subsequent n-type growth. 4H-SiC PiN diodes were fabricated by using heavily B doped epilayers. Preliminary results of PiN diodes with blocking voltage of 300 V and forward voltage drop of 3.0 V were obtained.

Laser Doping of Chromium and Selenium in p-Type 4H-SiC

Chromium (Cr) and selenium (Se) are laser doped in silicon carbide (4H-SiC p-type aluminium) to fabricate an embedded light emitting device and to tune the light emission. A near infrared Nd:YAG (1064 nm wavelength) laser source and an organometallic Cr compound (bis (ethyl benzene)-chromium) and organometallic Se compound (dimethyl selenide) were used to laser dope SiC. A p-n junction device structure was created using these dopants. The dopant profiles have been characterized using secondary ion mass spectrometry. Electrical properties were measured using Hall effect measurement. Enhanced diffusivity and solubility with complete activation of dopants was observed for laser doped Cr and Se. Cr and Se are unconventional dopants, which serve as a double donor and a double acceptor respectively, while aluminium (Al) behaves as single acceptor and nitrogen (N) as a single donor in SiC. The defect levels (donor and acceptor) created within the forbidden band gap of SiC due to Se, Cr and Al onsets the donor acceptor pair (DAP) recombination mechanism for luminescence observed in SiC. Electroluminescence studies showed an orange (677 nm) corresponding to Cr-Al and, red (698 nm) and white (380-900 nm) for Se-Al and pure white for Cr-Se-Al. The Cr-Se-Al white light exhibited a correlated color temperature of 4935 K, which compares well to average daylight (5500 K).

Search for Hydrogen Related Defects in p-Type 6H and 4H-SiC

Al-doped 4H and 6H epitaxial layers have been implanted with 200 keV hydrogen or irradiated with 1 MeV electrons. Heat treatments have been carried out up to 1000 °C and electrical characterization, by means of deep level transient spectroscopy (DLTS), has been performed after every annealing treatment in the 100-750 K temperature range. We have detected several deep levels and the possible involvement of hydrogen in the microscopic structure of these defects is discussed in the light of their thermal stability and previous results found in the literature. All the detected defects, except for a level located at 0.55 eV above the valence band (Ev), do not display any electric field dependence of their emission time constant.

Impact of RIE Etching on the Breakdown Voltage of 4H-SiC Mesa Diodes

This paper presents a comparison of the reverse characteristics of mesa terminated PiN diodes fabricated on n- and p-type 4H-SiC substrates. For n-type the attained breakdown voltages are higher and for p-type lower than expected. This is likely to be explained by the presence of negative charges at the interface between passivation oxide and SiC. Supported by XPS data we come to the conclusion that the RIE process creates surface charges which have an impact on the breakdown voltage of the fabricated diodes.

Characterization of Intrinsic Defects in High-Purity High-Resistivity p-Type 6H-SiC

The densities, cross sections, and energy levels of intrinsic defects in high-purity high-resistivity (approximately 106 Ω cm) p-type 6H-SiC are determined using isothermal capacitance transient spectroscopy (ICTS). Five intrinsic defects are detected ranging from 0.76 to 1.35 eV above the valence band. Since the sum of the densities of intrinsic defects detected is the same order of magnitude as the acceptor density in the p-type 6H-SiC, the intrinsic defects are found to decrease the majority-carrier concentration making its resistivity as high as approximately 106 Ω cm.