Noboru Ohtani

Sublimation growth of 6H- and 4H-SiC single crystals in the [11¯0 0] and [1 12¯0] directions

Abstract Sublimation growth of 6H- and 4H-SiC single crystals in the [1 1¯0 0] and [1 1 2¯ 0] directions has been carried out by the modified-Lely method. Transport properties of the crystals in parallel and perpendicular directions to the [0 0 0 1] basal plane were examined by van der Pauw and resistance bar measurements using 5° off-oriented [0 0 0 1] samples. Van der Pauw resistance asymmetry significantly varied (1 to ⩾ 10 3 ) depending on the growth direction and the polytype of grown crystals, and resistivity anomaly along the off-direction was found for 6H [1 1¯ 0 0] grown crystals. The crystals contained a high density (⩾ 10 4 cm −1 ) of stacking faults lying in the basal plane, which largely hinder electron transport, giving rise to the resistivity anomaly. An atomistic surface model for the stacking fault generation is proposed and the influence of the growth direction and polytype is discussed.

Structural defects in α-SiC single crystals grown by the modified-Lely method

Abstract Structural characterization of single crystalline α-SiC has been conducted by X-ray topography. Crystals were grown in the [0001¯] and the [11¯00] directions by the modified-Lely method, and wafers perpendicular and parallel to the growth directions sliced from the grown crystals were examined by transmission topographs of the Lang method. The crystals grown in the [0001¯] and the [11¯00] directions showed a significant difference in both types and densities of crystal defects. Each growth direction exhibited a peculiar defect formation, and the topography revealed that most of the defects were formed at the very initial stage of growth.

Mechanism of Molten KOH Etching of SiC Single Crystals : Comparative Study with Thermal Oxidation

The etching mechanism of SiC single crystals by molten KOH has been investigated. The etching process is significantly affected by the etching ambience: the etching rate is greatly reduced by a nitrogen gas purge. This result clearly suggests an essential role of dissolved oxygen in the melt. SiC{0001} surfaces show a large surface polarity dependence, where the etching rate of SiC(0001)C is about four times larger than that of SiC(0001)Si. The etching rate of SiC(0001)C exhibits an Arrhenius type temperature dependence with an activation energy of 15–20 kcal/mol. The obtained activation energy and selectivity between the (0001)C and the (0001)Si surfaces are quite similar to those for thermal oxidation, which implies that the surface oxidation process occurs during molten KOH etching of SiC and is the rate-limiting step for the etching. We have conducted a comparative study of molten KOH etching with thermal oxidation in regard to the crystal orientation, polytype and carrier concentration dependence.

Growth of large high-quality SiC single crystals

Abstract The availability of large high-quality silicon carbide (SiC) single crystals is a key issue in the development of the full potential of SiC-based device technology. In this paper, recent achievements in bulk crystal growth of SiC are reviewed. We present results on the physical vapor transport growth of SiC bulk single crystals by highlighting the crystal diameter enlargement and the quality improvement of SiC crystals. The causes and formation mechanisms of crystallographic defects, such as micropipes and low-angle grain boundaries, in SiC crystals are discussed. The results of the growth perpendicular to the c -axis are also reported, where stacking faults are defects of major concern. We present an atomistic surface model for the stacking fault generation and discuss a possible way to circumvent this problem.

Detection of stacking faults in 6H-SiC by Raman scattering

Raman spectra of 6H-SiC crystals including stacking faults have been examined for the c face using backscattering geometry. The intensity of the transverse optical phonon band at 796 cm−1, which corresponds to the phonon mode at the Γ point in 3C-SiC, is sensitive to the stacking faults. We found that the intensity of this band depends on the stacking fault density. This is explained based on the bond polarizability model. The spatial distribution of the stacking faults is studied by Raman image measurement.

Highly conductive and wide optical band gap n‐type μc‐SiC prepared by electron cyclotron resonance plasma‐enhanced chemical vapor deposition

We have investigated the gas pressure dependence of electron cyclotron resonance (ECR) plasma‐enhanced chemical vapor deposition (PECVD) and prepared n‐type μc‐SiC:H with wide optical band gap (2.1–2.5 eV) and high dark conductivity (10−3– 1 S/cm). It has been suggested from plasma diagnoses of the ECR plasma that at low gas pressure a strong etching effect of hydrogen radicals and/or ions dominates the film growth process and the hydrogen ions impinging on the growing surface make the formation of μc‐SiC:H difficult, and that at high gas pressure, for the formation of μc‐SiC:H, there are nonemissive radicals contributing to the surface coverage or a nucleus formation mechanism which has not been taken into consideration in conventional rf‐PECVD.

Influence of the Seed Face Polarity on the Sublimation Growth of α-SiC

The influence of the face polarity of the seed crystal on the α-SiC sublimation growth has been investigated. Optical and electrical measurements were carried out for undoped and nitrogen-doped crystals grown on the (000)C and the (0001)Si faces. The undoped crystal grown on the (000)C face showed n-type conduction and high optical transmittance in the visible light region. In contract, the undoped crystal grown on the (0001)Si face was highly resistive p-type. It was dark in color and showed low optical transmittance. The differences between the two crystals are explained in terms of impurity incorporation during growth, which has different kinetics on the (000)C and the (0001)Si faces.

Surface Hydrogen Effects on Ge Surface Segregation during Silicon Gas Source Molecular Beam Epitaxy

Using reflection-high-energy-electron diffraction (RHEED), the temperature dependence of Ge surface segregation during silicon gas source molecular beam epitaxy (Si-GSMBE) from hydride sources was studied, and it was found that surface hydrogen, which is produced by the dissociation of Si2H6 and GeH4 on the growing surface, may act as a growth-controlling surfactant. Comparison with results from solid source growth results suggests that hydrogen significantly suppresses the Ge segregation. Segregation kinetics were examined through simulation studies and the results indicate that the addition of surface hydrogen to the system inhibits segregation due to a lowering of the Gibbs heat of segregation.

Theoretical investigation of the formation of basal plane stacking faults in heavily nitrogen-doped 4H-SiC crystals

The formation of basal plane stacking faults in heavily nitrogen-doped 4H-SiC crystals was theoretically investigated. A novel theoretical model based on the so-called quantum well action mechanism was proposed; the model considers several factors, which were overlooked in a previously proposed model, and provides a detailed explanation of the annealing-induced formation of double layer Shockley-type stacking faults in heavily nitrogen-doped 4H-SiC crystals. We further revised the model to consider the carrier distribution in the depletion regions adjacent to the stacking fault and successfully explained the shrinkage of stacking faults during annealing at even higher temperatures. The model also succeeded in accounting for the aluminum co-doping effect in heavily nitrogen-doped 4H-SiC crystals, in that the stacking fault formation is suppressed when aluminum acceptors are co-doped in the crystals.

Impurity incorporation kinetics during modified-Lely growth of SiC

The impurity incorporation kinetics during modified-Lely growth of silicon carbide (SiC) have been studied in terms of several growth parameters. It was found that the nitrogen incorporation is well described by a Langmuir isotherm type equation, implying that dynamic equilibrium between the vapor phase and the adsorbed nitrogen is established. The polytype of grown crystal and the seed orientation influence the impurity incorporation. For growth on (0001)C, 6H-SiC crystals always incorporate more nitrogen and less boron than 4H-SiC crystals, while no clear polytypic dependence of impurity incorporation is observed for growth on (1100) and (1120). Atomic force microscope observations revealed that there is a marked difference in the growth morphology between 6H-SiC(0001)C and 4H-SiC(0001)C. The origin of the polytypic dependence of impurity incorporation during growth on (0001)C is discussed with reference to the growth surface morphology.