Partial dislocations and stacking faults in 4H-SiC PiN diodes

Using plan-view transmission electron microscopy (TEM), we have identified stacking faults (SFs) in 4H-SiC PiN diodes subjected to both light and heavy electrical bias. Our observations suggest that the widely expanded SFs seen after heavy bias are faulted dislocation loops that have expanded in response to strain of the 4H-SiC film, while faulted screw or 60° threading dislocations do not give rise to widely expanded SFs. Theoretical calculations show that the expansion of SFs depends on the Peach-Koehler (PK) forces on the partial dislocations bounding the SFs, indicating that strain plays a critical role in SF expansion.     

Drift-free 10-kV, 20-A 4H-SiC PiN diodes

As impressive as the advancement in 4H-SiC material quality has been, 4H-SiC PiN diodes continue to suffer from irreversible, forward-voltage instabilities. In this work, we describe PiN diodes designed to block 10 kV and conduct 20 A at less than 4.5 V, which were fabricated on 4H-SiC PiN epitaxial layers that were grown with an innovative epitaxial process that has been developed specifically to suppress V_F drift. The diodes fabricated on epitaxial layers that implemented this new epitaxy process showed excellent V_F stability, with 86% of the diodes drifting less than 0.1 V during forward current stressing at 10 A (50 A/cm²) for 30 min. However, these improvements in V_F drift come with a cost in blocking yield, as the surface morphology and other crystal defects imparted by the epitaxial process resulted in only 1 of 50 diodes reaching the 10-kV blocking specification. Nevertheless, the remarkable progress in V_F drift yield brings us closer to commercialization of high-power 4H-SiC PiN diodes.     

Thermal Annealing and Propagation of Shockley Stacking Faults in 4H-SiC PiN Diodes

Stacking faults within 4H-SiC PiN diodes are known to be detrimental to device operation. Here, we present electroluminescence (EL) images of 4H-SiC PiN diodes providing evidence that electrically and optically stimulated Shockley stacking fault (SSF) propagation is a reversible process at temperatures as low as 210°C. Optical beam induced current (OBIC) images taken following complete optical stressing of a PiN diode and that lead to a small number of completely propagated SSFs provide evidence that such defects propagate across the n–/p+ interface and continue to grow throughout the p+ layer. These observations bring about questions regarding the validity of the currently accepted driving force mechanism for SSF propagation.     

Some Critical Materials and Processing Issues in SiC Power Devices

There has been a rapid improvement in SiC materials and power devices during the last few years. However, the materials community has overlooked some critical issues, which may threaten the emergence of SiC power devices in the coming years. Some of these pressing materials and processing issues will be presented in this paper. The first issue deals with the possibility of process-induced bulk traps in SiC immediately under the SiC/SiO₂ interface, which may be involved in the reduction of effective inversion layer electron mobility in SiC metal–oxide–semiconductor field-effect transistor (MOSFETs). The second issue addresses the effect of recombination-induced stacking faults (SFs) in majority carrier devices such as MOSFETs, Schottky diodes, and junction field-effect transistors (JFETs). In the past it was assumed that the SFs only affect the bipolar devices such as PiN diodes and thyristors. However, most majority carrier devices have built-in p–n junction diodes, which can become forward biased during operation in a circuit. Thus, all high-voltage SiC devices are susceptible to this phenomenon.     

Planar defects and double-domain epitaxy in epitaxial YSi<Subscript>2−x</Subscript> and ErSi<Subscript>2−x</Subscript> thin films on Si substrates

Interfacial reactions of Y and Er thin films on both (111)Si and (001)Si have been studied by transmission electron microscopy (TEM). Epitaxial rare-earth (RE) silicide films were grown on (111)Si. Planar defects, identified to be stacking faults on planes with 1/6 displacement vectors, were formed as a result of the coalescence of epitaxial silicide islands. Double-domain epitaxy was found to form in RE silicides on (001)Si samples resulting from a large lattice mismatch along one direction and symmetry conditions at the silicide/(001)Si interfaces. The orientation relationships are [0001]RESi_2−x// Si, RESi_2−x//(001)Si and [0001]RESi_2−x/ Si, RESi_2−x//(001)Si. The density of staking faults in (111) samples and the domain size in (001) samples were found to decrease and increase with annealing temperature, respectively.     

High-Voltage 4H-SiC PiN Diodes With Etched Junction Termination Extension

Implantation-free mesa-etched 4H-SiC PiN diodes with a near-ideal breakdown voltage of 4.3 kV (about 80% of the theoretical value) were fabricated, measured, and analyzed by device simulation and optical imaging measurements at breakdown. The key step in achieving a high breakdown voltage is a controlled etching into the epitaxially grown p-doped anode layer to reach an optimum dopant dose of $sim!! hbox{1.2}times hbox{10}^{13} hbox{cm}^{-2}$ in the junction termination extension (JTE). Electroluminescence revealed a localized avalanche breakdown that is in good agreement with device simulation. A comparison of diodes with single- and double-zone etched JTEs shows a higher breakdown voltage and a less sensitivity to varying processing conditions for diodes with a two-zone JTE.      

Hetero- and homo-epitaxial growth of 3C-SiC for MOS-FETs

Planar defects, like anti-phase boundaries (APBs) and stacking faults (SFs), are reduced by growing 3C-SiC on undulant-Si whose entire surface is covered with countered slopes oriented in the [1 1 0] and directions. During the initial 3C-SiC growth, APBs are eliminated on each slope of an undulation. Then, one kind of SF self-vanishes. However, another kind of SF remains on the 3C-SiC surface, although its density is gradually reduced with increasing SiC thickness by combining with a counter-SF. The leakage current of a pn diode fabricated homo-epitaxially on 3C-SiC is roughly proportional to the SF density before homo-epitaxial growth. The viability of 3C-SiC grown on undulant-Si for semiconductor devices is discussed by reviewing recent reports on various MOS-FETs using it as the substrate. The key issue in the fabrication of a MOS-FET as a power-switching device operated at high-voltage is to reduce the leakage-current at the pn junction, thereby eliminating SFs.     

Microstructure and enhanced morphology of planar nonpolar m-plane GaN grown by hydride vapor phase epitaxy

Nonpolar ( ) m-plane gallium nitride has been grown heteroepitaxially on (100) γ-LiAlO₂ by several groups. Previous attempts to grow m-plane GaN by hydride vapor phase epitaxy (HVPE) yielded films unsuitable for subsequent device regrowth because of the high densities of faceted voids intersecting the films’ free surfaces. We report here on the growth of planar m-plane GaN films on (100) γ-LiAlO₂ and elimination of bulk and surface defects. The morphology achieved is smooth enough to allow for fabrication of m-plane GaN templates and free-standing substrates for nonpolar device regrowth. The GaN films were grown in a horizontal HVPE reactor at 860–890°C. Growth rates ranged from 30 μm/h to 240 μm/h, yielding free-standing films up to 250-μm thickness. The m-plane GaN films were optically specular and mirror-like, with undulations having 50–200-nm peak-to-valley heights over millimeter length scales. Atomic force microscopy revealed a striated surface morphology, similar to that observed in m-plane GaN films grown by molecular beam epitaxy (MBE). Root-mean-square (RMS) roughness was 0.636 nm over 25-μm² areas. Transmission electron microscopy (TEM) was performed on the m-plane GaN films to quantify microstructural defect densities. Basal-plane stacking faults of 1×10⁵ cm⁻¹ were observed, while 4×10⁹ cm⁻² threading dislocations were observed in the g=0002 diffraction condition.     

Maskless pendeo-epitaxial growth of GaN films

High-resolution x-ray diffraction (XRD) and atomic force microscopy (AFM) of pendeo-epitaxial (PE) GaN films confirmed transmission electron microscopy (TEM) results regarding the reduction in dislocations in the wings. Wing tilt ≤0.15° was due to tensile stresses in the stripes induced by thermal expansion mismatch between the GaN and the SiC substrate. A strong D°X peak at ≈3.466 eV (full-width half-maximum (FWHM) ≤300 μeV) was measured in the wing material. Films grown at 1020°C exhibited similar vertical [0001] and lateral [11 0] growth rates. Increasing the temperature increased the latter due to the higher thermal stability of the GaN(11 0). The (11 0) surface was atomically smooth under all growth conditions with a root mean square (RMS)=0.17 nm.     

Morphology and defect structures of GaSb islands on GaAs grown by metalorganic vapor phase epitaxy

The initial nucleation of GaSb on (001) GaAs substrates by metalorganic vapor phase epitaxy has been investigated using transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). TEM results showed that the GaSb islands experience a morphological transition as the growth temperature increases. For growth at 520°C, the islands are longer along the [110] direction; at 540°C, they are nearly square, and at 560°C, they are longer along the direction. Possible mechanisms are proposed to describe such a transition. TEM and HREM examination showed that lattice misfit relaxation mechanisms depend on the growth temperature. For the sample grown at 520°C, the lattice mismatch strain was accommodated mainly by 90° dislocations; for the sample grown at 540°C, the misfit strain was relieved mostly by 90° dislocations with some of 60° dislocations, and for the sample grown at 560°C, the strain was accommodated mainly by 60° dislocations which caused a local tilt of the GaSb islands with respect to the GaAs substrate. The density of threading dislocations was also found to be dependent on the growth temperature. Mechanisms are proposed to explain these phenomena.     

Polytype Stability and Microstructural Characterization of Silicon Carbide Epitaxial Films Grown on [ {\hbox{11}}\overline{{\hbox{2}}} {\hbox{0}} ]- and [0001]-Oriented Silicon Carbide Substrates

The polytype and surface and defect microstructure of epitaxial layers grown on 4H(), 4H(0001) on-axis, 4H(0001) 8° off-axis, and 6H(0001) on-axis substrates have been investigated. High-resolution x-ray diffraction (XRD) revealed the epitaxial layers on 4H() and 4H(0001) 8° off-axis to have the 4H-SiC (silicon carbide) polytype, while the 3C-SiC polytype was identified for epitaxial layers on 4H(0001) and 6H(0001) on-axis substrates. Cathodoluminescence (CL), Raman spectroscopy, and transmission electron microscopy (TEM) confirmed these results. The epitaxial surface of 4H() films was specular with a roughness of 0.16-nm root-mean-square (RMS), in contrast to the surfaces of the other epitaxial layer-substrate orientations, which contained curvilinear boundaries, growth pits (∼3 × 10⁴ cm⁻²), triangular defects >100 μm, and significant step bunching. Molten KOH etching revealed large defect densities within 4H() films that decreased with film thickness to ∼10⁶ cm⁻² at 2.5 μm, while cross-sectional TEM studies showed areas free of defects and an indistinguishable film-substrate interface for 4H() epitaxial layers.     

Growth of oriented diamond film on single crystalline 6H-SiC substrates

The bias-enhanced nucleation (BEN) technique in hot-filament chemical vapor deposition (HF-CVD) has been applied to single crystalline 6H-SiC substrates for the deposition of oriented diamond. The results of scanning electron microscopy (SEM) showed that on (000 ) face not only oriented diamond with relationship (111) Dia.//(000 )6H-SiC and 〈110〉Dia.//(11 0)6H-SiC, but also high nucleation density (>10⁹ cm⁻²) have been achieved. In the case of deposition on (0001) face of 6H-SiC under the same experimental conditions, although the nucleation density of diamond was enhanced, however, oriented diamond was not found. Diamond nucleation density is higher on (0001) face than that on (000 ) face. The differences in diamond oriented nucleation and nucleation density on these two faces are attributed to the difference of their specific free surface energy. The experimental results have shown that the 6H-SiC substrate surfaces are etched by the accelerated H-ions during BEN process, and many micro-triangular crystals with the faces of the kind {01 4} are formed on the substrate surface. Diamonds nucleate on the top of the micro-triangular crystals. Micro-Raman spectrum shows a strong feature of diamond crystals at 1334 cm⁻¹.     

SiC power Schottky and PiN diodes

The present state of SiC power Schottky and PiN diodes are presented in this paper. The design, fabrication, and characterization of a 130 A Schottky diode, 4.9 kV Schottky diode, and an 8.6 kV 4H-SiC PiN diode, which are considered to be significant milestones in the development of high power SiC diodes, are described in detail. Design guidelines and practical issues for the realization of high-power SiC Schottky and PiN diodes are also presented. Experimental results on edge termination techniques applied to newly developed, extremely thick (e.g., 85 and 100 μm) 4H-SiC epitaxial layers show promising results. Switching and high-temperature measurements prove that SiC power diodes offer extremely low loss alternatives to conventional technologies and show the promise of demonstrating efficient power circuits. At sufficiently high on-state current densities, the on-state voltage drop of Schottky and PiN diodes have been shown to be comparable to those offered by conventional technologies     

EPR characterization of defects in monoclinic powders of ZrO2 and HfO2

Electron paramagnetic resonance (EPR) measurements have been made on a variety of commercially available samples of the monoclinic form of the high-dielectric constant (high k) materials ZrO₂ and HfO₂ with the aim of characterizing the defects they contain. All EPR measurements were at about 9.5 GHz and at room temperature. An axially symmetric spectrum with g=1.961(2), g=1.976(2) is observed in most of the ZrO₂ samples and a similar one with g=1.940(3), g=1.970(2) is seen for most of the HfO₂ samples; they are attributed to centres involving Zr³⁺and Hf ³⁺, respectively. Their average concentration lies in the approximate range 10¹⁵–10¹⁷ cm⁻³, depending on the product specification, and, with one exception is unaffected by γ-irradiation. Grinding granules to powder and/or γ-irradiation yields further EPR spectra of defects, some of which are likely to involve oxygen, those are probably in the near surface region.     

Simulation and Experimental Study on the Junction Termination Structure for High-Voltage 4H-SiC PiN Diodes

Designing and fabrication of 10-kV 4H-SiC PiN diodes with an improved junction termination structure have been investigated. An improved bevel mesa structure and a single-zonejunction termination extension (JTE) have been employed to achieve a high breakdown voltage $(geq!hbox{10} hbox{kV})$ . The improved bevel mesa structure, nearly a vertical sidewall at the edge of the p-n junction and a gradual slope at the mesa bottom, has been fabricated by reactive ion etching. The effectiveness of the improved bevel mesa structure has been experimentally demonstrated. The JTE region has been optimized by device simulation, and the JTE dose dependence of the breakdown voltage has been compared with experimental results. A 4H-SiC PiN diode with a JTE dose of $hbox{1.1} times hbox{10}^{13} hbox{cm}^{-2}$ has exhibited a high blocking voltage of 10.2 kV. The locations of electric field crowding and breakdown are also discussed.      

Identification of prismatic slip bands in 4H SiC boules grown by physical vapor transport

Transmission electron microscopy (TEM) and KOH etching have been used to study the dislocation structure of 4H SiC wafers grown by physical vapor transport. A new type of threading dislocation arrays was observed. Rows of etch pits corresponding to dislocation arrays were observed in vicinity of micropipes, misoriented grains and polytypic inclusions at the periphery of the boules and extended along the directions. Plan view conventional and high resolution TEM showed that the arrays consisted of dislocations threading along the c-axis with Burgers vectors having edge components of the a/3 type. The Burgers vectors were parallel to the corresponding arrays. The dislocation arrays were interpreted as slip bands formed by dislocation glide in the prismatic slip system of hexagonal SiC during post-growth cooling.     

碳化硅MPS:新一代功率开关二极管

碳化硅MPS(Merged PiN Schottky diode)具有很好的开关特性，并具有PiN二极管高阻断电压、低漏电流和SBD小开启电压，大导通电流以及高开关速度的优点，是最有希望的新一代功率开关二极管。文章系统地介绍了碳化硅MPS的结构和性能。理论和实验分析表明，碳化硅材料的优异性能与MPS结构的优势相结合，是当今功率开关管发展的趋势。     

Optimization of the electrical properties of Al/a-SiC:H Schottky diodes by means of thermal annealing of a-SiC:H thin films

The present work reports on the optimization of the electrical properties of Al/a-SiC:H Schottky diodes by means of thermal annealing of a-SiC:H thin films. Optical transmission experiments have shown that the optical properties of the films are affected by thermal annealing when T_a>600 °C, due to emission of hydrogen bonded to silicon. Although the electrical properties of Al/a-SiC:H Schottky diodes are invariant for T_a?400 °C, for higher T_a these properties are improved with the optimum result achieved at . At this annealing temperature the linear log I-V characteristics span about eight orders of magnitude and the ideality factor is 1.09±0.04, making these diodes very interesting for many potential applications. For higher T_a (>600 °C) the electrical properties of Al/a-SiC:H Schottky diodes deteriorate with complete degradation at . For temperatures up to 600 °C this behavior is attributed to relaxation of the strain in the amorphous network which is possibly combined with weak hydrogen emission for temperatures up to 600 °C, leading to an optimum material quality. For further increase of T_a (>600 °C) the observed deterioration of the electrical properties of Al/a-SiC:H Schottky diodes is due to the intensive emission of hydrogen atoms bonded to silicon that cause voids in the amorphous network. These results are also supported by the experimental values of the room temperature apparent barrier height of the Al/a-SiC:H junction ?_bRT and its temperature coefficient γ.     

Efficient blue emitters based on 1,3,5-triazine for nondoped organic light emitting diode applications

Two novel efficient blue emitters (TTT-1, TTT-2) containing 1,3,5-triazine, thiophene and triphenylamine have been designed and synthesized. Organic light emitting diodes (OLEDs) using these new triazine derivatives as emissive layers, ITO/TAPC (60 nm)/TTT-1 (Device A) or TTT-2 (Device B) (40 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm), were fabricated and tested. The OLEDs exhibited good performances with low turn-on voltage of 3 V, maximum luminance of ca. 8990 cd/m² for TTT-1 and 15,980 cd/m² for TTT-2, and maximum luminance efficiency of 4.7 cd/A for TTT-1 and 4.0 cd/A for TTT-2, respectively.     

对拥有结终端保护的高压4H-SiC PIN二极管的研究

在对4H-SiC高压PIN二极管进行了理论分析的基础上,利用仿真软件ISE10.0对具有结终端保护的高压4H-SiC PIN二极管耐压特性进行了模拟仿真计算,并取得了很多有价值的计算结果。利用平面制造工艺,结合仿真提取的参数,试制了高压4H-SiC PIN二极管。实验测试结果表明,仿真计算的结果与实际样品测试的数据一致性较好,实测此器件击穿电压值已达到1 650V。