Showing papers in "Physica Status Solidi (a) in 1997"

Raman Investigation of SiC Polytypes

Abstract: It has been recognized that Raman scattering spectroscopy is a powerful tool to characterize SiC crystals non-destructively. We review recent significant developments in the use of Raman scattering to study structural and electronic properties of SiC crystals. The areas to be discussed in the first part include polytype identification, evaluation of stacking disorder and ion-implantation damages, and stress evaluation. The Raman scattering by electronic transitions is discussed in the second part of this article. We concentrate on the plasmon LO-phonon coupled modes whose spectral profiles are used to evaluate the carrier concentration and mobility. Anisotropic electronic properties of α-SiC and characteristics of heavily doped crystals are discussed. Semiconductor-to-metal transition and Fano interference effect are also treated.

Intrinsic SiC/SiO2 Interface States

Abstract: The energy distribution of electron states at SiC/SiO 2 interfaces produced by oxidation of various (3C, 4H, 6H) SiC polytypes is studied by electrical analysis techniques and internal photoemission spectroscopy. A similar distribution of interface traps over the SiC bandgap is observed for different polytypes indicating a common nature of interfacial defects. Carbon clusters at the SiC/SiO 2 interface and near-interfacial defects in the SiO 2 are proposed to be responsible for the dominant portion of interface traps, while contributions caused by dopant-related defects and dangling bonds at the SiC surface are not observed.

Deep Defect Centers in Silicon Carbide Monitored with Deep Level Transient Spectroscopy

Abstract: Electrical data obtained from deep level transient spectroscopy investigations on deep defect centers in the 3C, 4H, and 6H SiC polytypes are reviewed Emphasis is put on intrinsic defect centers observed in as-grown material and subsequent to ion implantation or electron irradiation as well as on defect centers caused by doping with or implantation of transition metals (vanadium, titanium, chromium, and scandium)

Red Luminescence in Pr3+‐Doped Calcium Titanates

Abstract: A strong single red emission band (λ max = 613 nm, FWMH = 450 cm -1 ) was observed at room temperature in Pr 3+ -doped calcium titanates. The evolution of the intensity of this luminescence was analysed in compensated and uncompensated samples (using Na + , Ag + or TI + as charge compensators) and also as a function of Pr concentration. Emission, excitation and reflectivity spectral repartitions were investigated at various temperatures, as well as the luminescence decays. The origin of the red luminescence is discussed.

Doping of SiC by Implantation of Boron and Aluminum

Abstract: Experimental studies on aluminum (Al) and boron (B) implantation in 4H/6H SiC are reported; the implantation is conducted at room temperature or elevated temperatures (500 to 700 C). Both Al and B act as ``shallow`` acceptors in SiC. The ionization energy of these acceptors, the hole mobility and the compensation in the implanted layers are obtained from Hall effect investigations. The degree of electrical activity of implanted Al/B atoms is determined as a function of the annealing temperature. Energetically deep centers introduced by the Al{sup +}/B{sup +} implantation are investigated. The redistribution of implanted Al/B atoms subsequent to anneals and extended lattice defects are monitored. The generation of the B-related D-center is studied by coimplantation of Si/B and C/B, respectively. (orig.) 60 refs.

Advances in SiC MOS Technology

Abstract: Silicon carbide (SiC) is the only compound semiconductor whose native oxide is SiO 2 . This places SiC in a unique position to compete with silicon in applications involving high power, high voltages, or high temperatures. SiC MOS technology has made substantial progress in recent years. This article aims to summarize the present status of this field, including MOS analysis techniques, oxidation procedures, experimental results, reliability considerations, alternative insulators, and remaining questions. In addition, we hope to convince the reader of the following: 1. Great care must be exercized in interpreting MOS data on wide bandgap semiconductors. This is due to the extremely long response times for interface states deep in the bandgap. 2. Recent results do not support the argument that interface quality on p-type SiC is inferior to that of n-type.

Optical Process for Liftoff of Group III-Nitride Films

Abstract: Films of GaN have been separated from a sapphire growth substrate by illuminating the interface with a pulsed ultraviolet laser that induces localized thermal decomposition of the GaN. Free-standing films and devices can be produced in this way. This process is also an alternative to surface etching for patterning of films and can be used for other nitride materials and more complex film systems by choosing an appropriate illumination wavelength or by including a strategically placed sacrificial absorbing layer during the film growth. This process exploits the thermally activated decomposition of GaN that begins to occur above about 800 C, resulting in the effusion of nitrogen gas. We have recently shown that this decomposition can be induced with high spatial resolution by heating the material with a short laser pulse (less than 10 ns). The rapid generation of heat allows a high localized temperature to be reached before the heat is conducted out of the illuminated region. In this way, surface patterning was achieved at illumination intensities above about 0.2 J/cm{sup 2} with 355 nm wavelength, near the absorption threshold of GaN. (orig.)

Analysis of Schottky Barrier Heights of Metal/SiC Contacts and Its Possible Application to High-Voltage Rectifying Devices

Abstract: Schottky contacts of metal/3C-, 6H-, and 4H-SiC systems are investigated in this review. Most Schottky contacts having large barrier heights show good characteristics with low ideality factors. The barrier height depends on the metal work function without strong Fermi-level pinning for all polytypes, and linear relationships with slopes of about 0.2 to 0.7 are observed between the barrier height and the metal work function. Based on the analysis of metal/SiC systems, the fabrication of high-voltage rectifiers has been reported, and high voltages from 400 to 1100 V have been achieved using Pt/, Ti/, and Au/6H-SiC structures. In addition, high-temperature operation at 400 °C is performed for an Au/6H-SiC structure while supporting a high reverse bias (460 V). Using Ti/4H-SiC structures, high-voltage (≈1000 V) and low-power loss characteristics are realized, which is superior to Ti/6H-SiC Schottky rectifiers. To improve the reverse bias characteristics, an edge termination technique is employed for Ti/4H-SiC Schottky rectifiers, and the devices show excellent characteristics with a higher blocking voltage up to 1750 V compared with unterminated devices.

High Temperature Sensors Based on Metal–Insulator–Silicon Carbide Devices

Abstract: High temperature gas sensors based on catalytic metal-insulator-silicon carbide (MISiC) devices are developed both as capacitors and Schottky diodes. A maximum operation temperature of 1000 degrees C is obtained for capacitors based on 4H-SiC, and all sensors work routinely for several weeks at 600 degrees C. Reducing gases like hydrocarbons and hydrogen lower the flat band voltage of the capacitor and the barrier height of the diode. The time constants for the gas response are in the order of milliseconds and because of this good performance the sensors are tested for combustion engine control. For temperatures around 600 degrees C total combustion occurs on the sensor surface and the signal is high for fuel in excess and low for air in excess. At temperatures around 400 degrees C the response is more linear. The high temperature operation causes interdiffusion of the metal and insulator layers in these devices; and this interdiffusion has been studied. At sufficiently high temperatures the inversion capacitance shows different levels for hydrogen free and hydrogen containing ambients, which is suggested to be due to a reversible hydrogen annealing effect at the insulator-silicon carbide interface.

Simulation of SiC High Power Devices

Abstract: The impact of SiC on high power devices and their applications is analysed using simulations in a very wide range of design voltages. First, a detailed presentation of the anisotropic form of the basic equations and of the physical models for 4H-SiC used in the simulations is given. Following that the application ranges of unipolar and bipolar devices in the domains of voltage and frequency are predicted in the case of IGBTs versus MOSFETs and PiN versus Schottky rectifiers based on comparisons of the on-state voltage and of the total losses. The application limit of the MOSFETs compared to IGBTs and of the Schottky rectifiers compared to PiN rectifiers is predicted to be about 4.5 and 2.5 kV, respectively, in the case of the 4H-SiC polytype. The impact of technological limitations of SiC is illustrated by the case of low channel mobility. The merits of SiC as compared to Si are illustrated by the case of a SiC rectifier operating together with a Si IGBT. Dramatically reduced turn-on losses are demonstrated. The superiority of SiC from the point of view of dynamic avalanche is predicted and illlustrated. Finally, some novel SiC switch structures are introduced in response to the reliability problems encountered in ordinary trench MOSFETs.

Intrinsic defects in cubic silicon carbide

Abstract: Irradiation of fast particles like 1 MeV electrons and 2 MeV protons was made for single crystalline cubic silicon carbide (3C-SiC) grown epitaxially on Si by chemical vapor deposition in order to introduce point defects in the material. Intrinsic point defects in 3C-SiC have been characterized by electron spin resonance (ESR), positron annihilation spectroscopy (PAS), Hall and photoluminescence (PL) techniques. The structure and annealing behavior of intrinsic defects, e.g. monovacancies at silicon and carbon sublattice sites, are described based on the results obtained by ESR and PAS. The contributions of such point defects to electrical and optical properties of 3C-SiC are discussed using the Hall and PL results, with a brief review of published work.

EPR and ENDOR investigations of shallow impurities in SiC polytypes

Abstract: Investigations of nitrogen donors in 6H-, 4H- and 3C-SiC using conventional electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optical detection of EPR and ENDOR as well as optical absorption and emission spectroscopy are reviewed and critically discussed. An attempt is presented to interpret the experimentally found large differences in hyperfine interactions of the 14N nuclei on the various inequivalent sites in the different polytypes of SiC in terms of valley–orbit splittings and “central-cell corrections” in the framework of the effective mass theory (EMT). P-doping by neutron transmutation in 6H-SiC resulted in various P-related EPR spectra previously associated with shallow P donors and P–vacancy complexes. In analogy to the new interpretation of the N donor spectra in various polytypes, it is proposed that all P-related spectra found hitherto in 6H-SiC are due to isolated P donors in ground and excited EMT states. A detailed discussion is presented of the electronic structure of B acceptors, as determined by EPR and in particular by ENDOR investigations: The B atom itself has only very little unpaired hole density, while the hole resides mainly on a neighbouring relaxed C atom B acceptors have a rather “deep” character and pronounced dynamical properties. A discussion of the present understanding of the so-called deep B centre (D centre) is also given. In contrast to B, the Al acceptor behaves as expected from the effective mass theory. It shows, however, two optical absorption bands identified by optical detection of EPR which are related to an ionization transition to the valence band and another transition, probably to a V impurity.

Stacking Faults as Quantum Wells for Excitons in Wurtzite GaN

Abstract: A model of the exciton bound to stacking faults (SF) in GaN is suggested. It is shown that SFs are potential wells (depth ≈120 meV and width ≈10 A) for electrons and potential barriers (height ≈60 meV and width ≈10 A) for holes. The binding energy of excitons at SF found from variational calculation is 45 meV. The 364 nm line in GaN photoluminescence at 4 K is attributed to excitons bound to SFs.

Excitons in Semiconductor–Dielectric Nanostructures

Abstract: Confinement in nanostructures with (some of the) linear dimensions small compared to the exciton radius provides a possibility of enhancement of both the binding energy and the oscillator strength of excitons. Moreover in such structures the possibility arise of controlling the strength of Coulomb interaction, responsible for binding of electron and hole to the exciton, by combining materials for the structure with essentially different values of dielectric constants. Transformations of Wannier-Mott excitons which are dependent on both of these factors — confinement geometry and dielectric constants ratio —, their optical manifestations and similar trends in many body phenomena, including electron–hole liquid formation, will be discussed.

Optical Characterization of Silicon Carbide Polytypes

Abstract: This article is a review of recent progress in our understanding of the optical properties of the important polytypes of SiC : 3C, 4H, 6H, and 15R. We focus on experimental work but also compare results obtained by experiment with theory. The topics of emphasis are: 1. vacuum ultraviolet reflectivity, 2. free excitons, 3. spectroscopy of shallow donors and acceptors, 4. the impurity boron in SiC, 5. the rare earth impurity erbium in SiC, and 6. optical properties of porous SiC.

Nitrogen Ion Implantation into α-SiC Epitaxial Layers.

Abstract: N + implantation into p-type α-SiC (6H-SiC, 4H-SiC) epilayers at room and elevated temperatures, mainly obtained by the authors' group, has been reviewed. Since recrystallization of SiC is difficult, the implantation-induced damage should be minimal during implantation to achieve higher electrical activation. The effects of hot implantation are pronounced in high-dose (>10 15 cm -2 ) implantation. The lowest sheet resistance of 542 Ω/? was obtained by implantation at 500 to 800 °C with a 4 x 10 15 cm -2 dose. The properties of pn junctions formed by N + implantation into p-type epilayers are characterized in detail. The forward current is clearly divided into two components of diffusion and recombination currents. The diodes exhihited high breakdown voltages of 615 to 820 V, which are almost ideal values expected from device structure. The reverse leakage current can significantly be reduced by employing hot implantation at 800 °C.

The Dislocation Microstructure of Cyclically Deformed Nickel Single Crystals at Different Temperatures

Abstract: Using transmission electron microscopy a systematic study of the dislocation microstructure after cyclic deformation of nickel single crystals was undertaken in order to describe quantitatively the influence of deformation temperature on microstructural parameters. The frequency, distributions of the heights and lengths of edge dislocation dipoles were measured in the walls and channels of persistent slip bands and in the matrix bundles after deformation at 77, 293, 600 and 750 K. The mean dipole height as well as the dipole annihilation distance and the critical dipole height were found to increase with increasing temperature while the dislocation density decreased. The dipole heights turned out to be independent of the position of the dipoles in the dislocation substructure. The results are discussed with regard to the development of microstructure-based models of cyclic deformation and fatigue dislocation patterning.

Micropipes : Hollow tubes in silicon carbide

Abstract: Micropipes are hollow tubular defects penetrating SiC single crystals. Different mechanisms of formation and stabilization as well as energetic aspects are discussed.

The Flow Stress of Ultra‐High‐Purity Molybdenum Single Crystals

Abstract: The paper investigates the dependence of the flow stress σ of ultra-high-purity molybdenum single crystals on temperature T, strain rate, and crystallographic orientation of the crystal axis. The cyclic-deformation technique developed by Mughrabi and Ackermann allowed a complete set of flow-stress data (covering the temperature range 125 to 460 K at 15 different shear strain rates, varying from 5.9 × 10—7 to 1.7 × 10—3 s—1) to be obtained on one and the same specimen. The two crystals investigated, with Schmid factors μ{110} = 0.50 and μ{112} = 0.43, or μ{110} = 0.40 and μ{110} = 0.43, respectively, had residual resistivity ratios exceeding 2.5 × 105, the main impurity being W. The data are in excellent agreement with the theory of flow-stress control by kink-pair formation and kink migration. It is shown that for both orientations the elementary glide steps of the a0 〈111〉/2 screw dislocations controlling the flow stress occur on {112} planes. The formation energy of a pair of isolated kinks on a {112} plane is 1.27 eV, in perfect agreement with the activation energy of the so-called γ-relaxation as determined by internal-friction measurements. The apparent kink mass and the kink diffusivity could be determined, too. The present results leave no doubt that the “hump” observed in the σ–T relationship of high-purity b.c.c. metals is due neither to a change in the glide mechanism nor to a special form of the Peierls potential but is a natural consequence of the dependence of the kink-pair formation enthalpy on the resolved shear stress.

SiC Integrated MOSFETs

Abstract: A new design and processing concepts have been applied to develop practical SiC trench MOSFETs for high power applications The designed trench MOSFET has a MOS structure consisting of epitaxially grown n-type SiC trench sidewall layers The current flows via an accumulation mode through the channel defined in the epitaxially grown SiC sidewall layer The channel is depleted by the built-in fields of the p-type SiC base layer and the p-poly-Si gate, that control the channel conditions The simulation based investigations revealed that the formed channel can withstand up to the avalanche breakdown condition The structure of the n-type SiC trench sidewall epi-layer has been optimized to realize the blocking voltage of more than 1000 V for SiC MOSFETs with low on-state resistance Moreover, our designed structure can address most of the open issues related to the MOS interface, viz, high surface state density, low channel mobility and high electric field at the trench base of the MOS structure We have fabricated the first 2 mm square large size 6H-SiC trench MOSFET chip, in which 2380 hexagonal structural microcells of 23 μm pitch were integrated The fabricated 6H-SiC trench MOSFET on (0001-) C-face wafers feature the on-state resistance as low as 2384 m Ω cm2 with blocking voltage of more than 450 V

Fine Structure of Excitonic Levels in Semiconductor Nanostructures

Abstract: A brief survey is given on recent work on the fine structure of zero-dimensional excitons in semiconductors. The paper focuses on microscopic mechanisms of anisotropic exchange splitting of the localized-exciton radiative doublet in type-I and type-II heterostructures and on polarized photoluminescence of excitons confined in self-assembled quantum dots.

6H‐Silicon Carbide Light Emitting Diodes and UV Photodiodes

Abstract: Silicon carbide has been used to fabricate a variety of short wavelength optoelectronic devices including blue LEDs, green LEDs and UV photodiodes. As a light emitter, 6H-SiC junctions can be tailored to emit light across the visible spectrum. The most widely commercialized device is the blue LED. Over the past years, the quantum efficiency of the Cree Research blue LED has increased significantly. The devices emit light with a peak wavelength of 470 nm with a spectral halfwidth of 70 nm. The optical power output is typically between 25 and 35 μW at a forward current of 20 mA and 3.2 V. This represents an external quantum efficiency of 0.05 to 0.07%. Green LEDs have been demonstrated which emit with a peak wavelength of 530 nm. As opposed to the epitaxial junction used in the blue LED, the green devises use ion implanted junctions. The typical output power is similar to that of the blue LED. However, with respect to photometric units, the die luminous intensity is a factor of two higher than the blue LED, 1.2 mcd (millicandela) for a radiant flux output of 33 μW. In addition to short wavelength light emission, the energy bandgap of 3.0 eV allows for inherently low dark currents and high quantum efficiencies for ultraviolet photodiode detectors made in 6H-SiC, even at high temperatures. These devices typically exhibit a quantum efficiency of 80 to 100% and peak response of 250 to 280 nm. These characteristics are maintained to at least 350 °C. The dark current density at -1.0 V and 473 K is 10- 11 A/cm 2 . This corresponds to an extrapolated room temperature current density of 2 x 10 -17 A/cm 2 at -1.0 V.

Transition Metals in SiC Polytypes, as Studied by Magnetic Resonance Techniques

Abstract: A review is given on the results of magnetic resonance studies of transition metal impurities in SiC polytypes The data are presented for the elements titanium (Ti), vanadium (V), chromium (Cr), molybdenum (Mo), manganese (Mn), scandium (Sc) and copper (Cu) Most of these transition metals were found to occur in multiple charge states, underlining their role as deep level defects in SiC A compilation of relevant ESR parameters for transition metal defects in various SiC polytypes is presented in the Appendices

The Influence of R2O3 (R = Yb, Er, Dy, Tb, Gd, Sm and Ce) on the Electric and Mechanical Properties of a Nickel–Zinc Ferrite

Abstract: The lattice constant, density, electrical resistivity, activation energy, carrier concentrations mobility and Vickers hardness were studied on rare-carth-substituted nickel-zinc ferrites, with formula Ni 0.7 Zn 0.3 Fe 1.98 R 0.02 O 4 , where R =Yb, Er, Dy, Tb, Gd, Sm and Ce. We have found that all the rare-earth ions favor the occurrence of a second phase resulting in an increase of the electrical resistivity, bulk density and hardness. The log Q = f(1/T) curves show two regions of different activation energy. The difference between the two activation energies was explained by an important activation of the carrier mobility at higher temperatures and by a conduction mechanism sensitive to the microstructure.

Excitons in Narrow Quantum Wells: Disorder Localization and Luminescence Kinetics

Abstract: For narrow quantum wells, the energetic spread due to interface and alloy disorder is large enough to dominate the excitonic optical spectra. The resulting disorder potential enters the center-of-mass Schrodinger equation. We derive a kinetic equation for the exciton distribution over disorder eigenstates including acoustic phonon scattering and radiative recombination. The photoluminescence lineshape as function of temperature is studied. Both peak shift and width exhibit a nonmonotonous temperature dependence which is due to phonon-assisted thermal activation of localized excitons. A sharp low-energy drop in the photoluminescence excitation spectra can be understood in terms of a ‘relaxation mobility edge’.

Influence of La3+-Doping on Radiation Hardness and Thermoluminescence Characteristics of PbWO4

Abstract: (a) ENEA-INN/TEC, Via Anguillarese 301, S.Maria di Galeria, 00060 Roma, Italy (b) Institute of Physics, Cukrovarnicka 10, 16200 Prague, Czech Republic (c) INFN, Dept. of Physics, University of Roma 1, P. le A. Moro 2, Roma, Italy (d) SIT, Shonan Institute of Technology, Fujisawa 251, Japan (e) CRYTUR Preciosa a.s., Palackeho 175, 51119 Turnov, Czech Republic (f) KEK, National Laboratory for High Energy Physics, Tsukuba 305, Japan (g) INFM and Dept. of Physics, University of Milano, Via Celoria 16, 20133 Milano, Italy (h) Furukawa Co., Yoshima, Iwaki 970-11, Japan

Raman Phonon Modes and Ferroelectric Phase Transitions in Nanocrystalline Lead Zirconate Titanate

Abstract: Micro-Raman spectra for PbZr x Ti 1-x O 3 (PZT) with grain sizes of 60 and 40 nm have been obtained. The results show that the E(TO 1 ) phonon mode of PbTiO 3 ('soft' mode) displays a decrease in frequency and an increase in linewidth with increasing Zr concentration. A discontinuous behavior in the phonon energy for the soft mode occurs at a morphotropic phase boundary (NPB) of x 0.4 and 0.2 for grain sizes of 60 and 40 nm, respectively, and it can be attributed to a phase transition from ferroelectric tetragonal to ferroelectric rhombohedral phase. The nonzero soft mode frequency near the MPB results from a level repulsion between an additional phonon mode at 10 cm -1 and the soft mode. Raman enhanced behavior has been found for the lowest phonon mode with Zr contents in the range of 0.3 to 0.6. The dependence of Raman phonon modes for PbZr 0.3 Ti 0.7 O 3 upon grain size has been analyzed and the results indicate a grain-size-induce(I phase transition at 13 nm.