High Purity Semi-insulating 4H Research Articles

Lateral spreads of ion-implanted Al and P atoms in silicon carbide

Lateral spreads of Al and P atoms implanted (10 ∼ 700 keV) into a high-purity semi-insulating 4H silicon carbide (SiC) substrate have been experimentally investigated. Scanning capacitance microscopy observation revealed that lateral distributions are highly dependent on the implantation energies of ions. Asymmetric lateral distributions were observed along the direction, which may be attributed to the 4° off-angle of the substrate. The lateral spreads are determined to be 0.33 μm for Al atoms, 0.38 ± 0.02 μm for P atoms along the direction, and 0.28 μm/0.38 μm for Al atoms, 0.25 μm/0.40 μm for P atoms along the direction (upstream/downstream side of step flow). Comparison between the lateral spreads of Al and P atoms is discussed with Al and P stopping powers and ranges in SiC.

Current status of modelling the semi-insulating 4H–SiC transient photoconductivity for application to photoconductive switches

In this paper we present the current status of modelling the time evolution of the transient conductivity of photoexcited semi-insulating (SI) 4H–SiC taking into account the properties of defect centres. A comprehensive model that includes the presence of six, the most significant, point defects occurring in SI 4H–SiC crystals is presented. The defect centres are attributed to the two kinds of nitrogen-related shallow donors, a boron-related shallow acceptor, deep electron and hole traps, and the Z 1/2 recombination centre. We present the results of the state-of-the-art numerical simulations showing how the photoconductivity transients change in time and how these changes are affected by the properties of defect centres. The properties of defect centres assumed for modelling are compared with the results of experimental studies of deep-level defects in high purity (HP) SI 4H–SiC wafers performed by the high-resolution photoinduced transient spectroscopy (HRPITS). The simulated photoconductivity transients are also compared with the experimental photocurrent transients for the HP SI 4H–SiC wafers with different deep-level defects. It is shown that a high-temperature annealing producing the C-rich material enables the fast photocurrent transients to be achieved. The presented analysis can be useful for technology of SI 4H–SiC high-power photoconductive switches with suitable characteristics.

High temperature annealing effects on deep-level defects in a high purity semi-insulating 4H-SiC substrate

Effects of high-temperature annealing on deep-level defects in a high-purity semi-insulating 4H silicon carbide substrate have been studied by employing current-voltage, capacitance-voltage, junction spectroscopy, and chemical impurity analysis measurements. Secondary ion mass spectrometry data reveal that the substrate contains boron with concentration in the mid 1015 cm−3 range, while other impurities including nitrogen, aluminum, titanium, vanadium and chromium are below their detection limits (typically ∼1014 cm−3). Schottky barrier diodes fabricated on substrates annealed at 1400–1700 °C exhibit metal/p-type semiconductor behavior with a current rectification of up to 8 orders of magnitude at bias voltages of ±3 V. With increasing annealing temperature, the series resistance of the Schottky barrier diodes decreases, and the net acceptor concentration in the substrates increases approaching the chemical boron content. Admittance spectroscopy results unveil the presence of shallow boron acceptors and deep-level defects with levels in lower half of the bandgap. After the 1400 °C annealing, the boron acceptor still remains strongly compensated at room temperature by deep donor-like levels located close to mid-gap. However, the latter decrease in concentration with increasing annealing temperature and after 1700 °C, the boron acceptor is essentially uncompensated. Hence, the deep donors are decisive for the semi-insulating properties of the substrates, and their thermal evolution limits the thermal budget for device processing. The origin of the deep donors is not well-established, but substantial evidence supporting an assignment to carbon vacancies is presented.

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.

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

Abstract

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.

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.

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.

Observation of a spin one native defect in as-grown high-purity semi-insulating 4H SiC

Electron paramagnetic resonance measurements of high-purity semi-insulating 4H SiC reveals a spectrum characteristic of an S=1 defect, which appears only after exposure to light with a wavelength less than 690 nm. Analysis of the hyperfine structure of the spectrum suggests that the defect is an intrinsic pair or defect∕impurity complex. The center is stable in an inert ambient up to temperatures of 1200 °C, but a 1-h, 1600 °C anneal reduces the concentration by at least an order of magnitude. Because the spectrum is not affected by removal of the excitation light, it is concluded that the center is the ground state of an S=1 defect. A study of the angular dependence of the spectrum shows that g=2.0052, ∣D∣=(329±14)×10−4cm−1, and ∣E∣<19×10−4cm−1.