Microwave-assisted spectroscopy of spin defect centers in silicon carbide

To summarize this entire thesis, MW-assisted spectroscopy has been proposed as a promising approach to investigate the optical properties of VSi and VV spin defects in 4H- and 6H-SiC. The MW-assisted spectroscopy has enabled to separate the spectrally overlapped contribution of different types of defects. From a PL spectrum containing no overlapping spectral contributions of other defects, the local vibrational mode of all measured VSi and VV in 4H- and 6H-SiC has been found along with the phonon energy and DW factor. The interaction of local vibrational modes with point defects has allowed to understand the spin, optical, mechanical, and thermal properties of these defects. In the investigation of V2 in 4H-SiC, a perfect agreement between the experimental data and theoretical calculation have been obtained. The MW-assisted spectra measured at different resonant frequencies associated with the same defect have been found to reveal the same vibrational mode and DW factor. Furthermore, some new ODMR lines to certain defects have been assigned, which have never been reported before. From the investigation of the V2 VSi in 6H-SiC, it has been found that the temperature does not have a clear influence on the DW factor, but high-fluence electron irradiation has been shown to decrease the DW factor. In the polarization investigation, it has been found that in 6H-SiC, V1 possesses no polarization, V2 shows a strong E||c-axis polarization, while V3 exhibits a strong E⊥c-axis polarization. It has also been demonstrated that the temperature and the orientation of the excitation laser have no influence on the photon polarization. In short, this thesis has demonstrated that MW-assisted spectroscopy is a powerful technique to investigate a large number of spin defects in wide-bandgap semiconducting materials

Electron spin resonance in zinc selenide

Electron spin resonance techniques were applied to study crystals of ZnSe both doped and undoped. Crystals grown by sublimation at 1300ºC were found to have a mixed cubic-hexagonal structure. Annealing these crystals at 1050ºC increased the hexagonal component. This is the opposite of what is usually observed in crystals produced by the flow process. ZnSe crystals produced at 850ºC by the iodine transport method were found to have a cubic structure. No cubic → hexagonal phase transition could be achieved in these crystals. From the experimental data it is concluded that the hexagonal phase in crystals grown at 1300ºC occurred as a result of the presence of an impurity, probably oxygen. Electron spin resonance of Mn was studied in detail in cubic, in twinned and in cubic-hexagonal ZnSe crystals. Comparing the Mn concentration and distribution in crystals produced at 1300ºC and at 850ºC, it was found that the solubility of Mn was larger in crystals grown at 1300ºC. Attempts were made to dope ZnSe with Mn by diffusion, but results indicated that Mn does not diffuse into ZnSe. However, spin resonance results showed that annealing ZnSe: Mn crystals in molten Zn at 850ºC removes some of the Mn impurity. Electron spin resonance investigations were also carried out in ZnSe crystals doped with I, CI, Al, and In. In these crystals spin resonance signals characteristic of mobile donor electron were detected. These signals were all found to be affected by annealing in molten Zn, and furthermore they were all photosensitive. Addition of 0.01% Cr to ZnSe was found to prevent charge transport. Spin resonance of a sintered sample containing Cr indicated a trapping level associated with Cr at 0.51 eV below the conduction band

Post-Silicon Group IV Materials: Selected Applications of Quantum Mechanics to Device Simulation

Quantum mechanics is applied to the study and simulation of two features of group IV semiconductor devices: metal/n-type 4H-SiC interfaces for SiC-based Schottky diodes and GeO2 gate dielectrics for Ge-based Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). SiC is well suited for power electronics due to its relatively wide bandgap and high breakdown field. In Schottky power diodes, one consideration in device performance is reverse saturation leakage. For metal/4H-SiC interfaces, reverse saturation leakage current is modeled with quantum transmission calculated by the Symmetrized Transfer Matrix Method (STMM). The classical thermionic emission model and quantum model are compared for multiple donor concentrations. The quantum model is then compared to experimental results for Ti/4H-SiC measurements, and the effect of Fermi pinning is included to account for the correct barrier height. Multiple donor concentrations are again modeled to best fit the bias dependence of the measured curves to find an effective doping level to reflect possible barrier thinning. Ge is considered as a possible replacement for Si in MOSFET design as device lengths continue to scale down to match Moore's Law and Si MOSFETs become increasingly difficult to fabricate. Ge is considered due to its relatively high electron and hole mobilities, and its ability to grow a native oxide like Si. However, GeO2 and the Ge/GeO2 interface suffer from high defect densities, with one such defect being the oxygen vacancy defect. For GeO2, the oxygen vacancy defect, and corresponding fluorine passivation, are modeled using Density Functional Theory (DFT) to calculate the atomic configurations and energies. Incorporation of fluorine atoms in the vicinity of the defect is modeled, as well as the incorporation of fluorine atoms within the oxide network. Hydrogen passivation is also modeled and found to not be as energetically favorable. Finally, fluorine diffusion through the oxide network is investigated by calculating the reaction pathway between fluorine incorporation sites in the network

The development of ultraviolet light emitting diodes on p-SiC substrates

Ultraviolet (UV) light emitting diodes (LEDs) are promising light sources for purification, phototherapy, and resin curing applications. Currently, commercial UV LEDs are composed of AlGaN-based n-i-p junctions grown on sapphire substrates. These devices suffer from defects in the active region, inefficient p-type doping, and poor light extraction efficiency. This dissertation addresses the development of a novel UV LED device structure, grown on p-SiC substrates. In this device structure, the AlGaN-based intrinsic (i) and n-layers are grown directly on the p-type substrate, forming a p-i-n junction. The intrinsic layer (active region) is composed of an AlN buffer layer followed by three AlN/Al0.30Ga0.70N quantum wells. After the intrinsic layer, the n-layer is formed from n-type AlGaN. This device architecture addresses the deficiencies of UV LEDs on sapphire substrates while providing a vertical device geometry, reduced fabrication complexity, and improved thermal management. The device layers were grown by molecular beam epitaxy (MBE). The material properties were optimized by considering varying growth conditions and by considering the role of the layer within the device. AlN grown at 825 C and with a Ga surfactant yielded material with screw dislocation density of 1x10^7 cm^-2 based on X-ray diffraction (XRD) analysis. AlGaN alloys grown in this work contained compositional inhomogeneity, as verified by high-resolution XRD, photoluminescence, and absorption measurements. Based on Stokes shift measurements, the degree of compositional inhomogeneity was correlated with the amount of excess Ga employed during growth. Compositional inhomogeneity yields carrier localizing potential fluctuations, which are advantages in light emitting device layers. Therefore, excess Ga growth conditions were used to grow AlN/Al0.30Ga0.70N quantum wells (designed using a wurtzite k.p model) with 35% internal quantum efficiency. Potential fluctuations limit the mobility of carriers and introduce sub-bandgap absorption, making them undesirable in the n-AlGaN layers. n-Al0.60Ga0.40N grown under stoichiometric Ga flux and an In surfactant reduced the Stokes shift (compared to n-AlGaN grown without In) by 150 meV. However, even under these growth modes, some compositional inhomogeneity persisted which is speculatively attributed to the vicinal substrate. Device epitaxial layer stacks utilizing the optimum growth conditions were fabricated into prototype vertical UV LEDs which emit from 295-320 nm. In order to increase light extraction efficiency, UV distributed Bragg reflectors (DBRs) based on compositionally graded AlGaN alloys were designed using the transfer matrix method (TMM) and grown by MBE. DBRs were formed from repeated compositionally graded AlGaN alloys. This structure utilized the polarization doping and index of refraction variation of graded composition AlGaN. DBRs with square wave, sinusoidal, triangular, and sawtooth compositional profiles were realized, with reflectivity peaks over 50%, centered at 280 nm

Epitaxial Graphene and its Electronic Device Applications

Department of Electrical EngineeringGraphene is a two-dimensional material in which carbon atoms are bonded in honeycomb lattice. It has a unique electronic band structure that shows zero band gap energy and linear dispersion relation near the Dirac point. Because of to its outstanding electrical and mechanical properties, graphene has been actively studied in various fields. After successfully separating graphene from highly oriented pyrolytic graphite (HOPG), a variety of methods for obtaining high quality and large area graphene have been studied. Especially, a method of growing epitaxial graphene (EG) on a SiC substrate has attracted much attention as a material for next generation electronic devices. This allows the growth of large area graphene and it is not necessary for transfer process because semi-insulating SiC wafer can be used as a substrate. However, it has disadvantages of requiring high temperature (> 1300 ??C) for high quality EG growth and the single crystalline SiC substrate are too expensive. In order to overcome these problem, we propose two effective methods for growth of EG on SiC. Firstly, the high quality EG is grown on 4H-, 6H SiC substrate by molybdenum plate (Mo-plate) capping during annealing process. Mo-plate capping causes the heat accumulation on SiC surface by preventing loss of thermal radiations from SiC surface, and increase the Si vapor pressure on SiC surface by enclosing the sublimated Si atoms. Therefore, the temperature of the SiC surface becomes higher than surrounding temperature, and the Si sublimation rate is reduced. These factors enable high quality EG growth at relatively low power assumption (chamber temperature). The quality enhancement of the grown EG with Mo-plate capping is demonstrated by Raman spectra, compared to EG without Moplate capping. Secondly, the graphene is formed on SiC thin film surface at relatively low temperature by electron beam (e-beam) irradiation with low acceleration voltage. The e-beam irradiation with low acceleration voltage induces the heat accumulation within several layers of SiC thin film surface due to its shallow penetration depth. The thermalized electrons weaken the bond strength of the Si-C atoms while staying within a few layers of SiC thin film surface, which reduce the heat energy required for sublimating Si atoms. As the electron fluency increase, the crystallinity and uniformity of grown graphene are improved, which is confirmed by Raman spectra and scanning electron microscopy (SEM) images. We propose the cleanly patterning method for graphene using Al thin film as etching mask because general patterning methods such as electron beam lithography and photolithography induce the degradation of graphene quality due to polymer residue. The properties of fabricated graphene device using Al thin film are confirmed by Hall measurement and Raman spectra, compared with graphene sample patterned with conventional photolithography. In particular, the apparent Shubnikov-de Haas (SdH) oscillation measured in graphene device patterned with Al thin film demonstrates better homogeneity and 2DEG system. The carrier density and Hall mobility in Al patterned EG device are measured to be 9.16 ?? 10^12 cm-2 and ~ 2100 cm2/Vs, respectively. The complementary logic inverter having graphene channel is fabricated by using selective doping of graphene. Ti or Al adsorbed graphene is doped n-type, because Ti or Al with lower work function than graphene induces the charge transfer from the Ti or Al to graphene. On the other hand, the SiO2 adsorbed graphene is doped to p-type by the dangling bonds of SiO2 surface. The doping concentration and type of graphene are confirmed by Raman spectra and electrical measurements. We fabricated two kinds of inverter doped with Al-SiO2 and Ti-SiO2 materials. These inverters exhibit a clear voltage inversion as function of Vin at a wide range of VDD from 0.5 V to 20 V, and the highest voltage gains are ~0.93 and ~0.86, respectively. These properties can be improved by using insulating layer of higher dielectric constant and reducing thickness of gate oxide. We propose a new structure of multifunctional capacitive sensor to surmount the limitations the previous single-capacitor sensor. The proposed dual-capacitor sensor composes of two capacitors stacking vertically in a pixel which detects strength information and surface-normal directionality of external stimuli, and clearly classifies the types of stimuli. These properties have been demonstrated by detecting and distinguishing the curvature, pressure, touch and strain stimuli through the capacitances changes of the two capacitors. We successfully fabricated a stable n-type InAs NW FET with a very simple fabrication process using pre-deposition of Al2O3 layer. This oxide layer of 10 nm thickness is uniformly formed on entire surface of NW channel by ALD. It serves not only as a gate oxide but also as a protective layer of the NW channel. The structure of completed device is demonstrated by TEM images and EDX electron mapping. The n-InAs NW FET shows good current saturation and low voltage operation, the peak transconductance (gm) is extracted to be 13.4 mS/mm, the field effect mobility (??FE) is calculated to be ~1039 cm2/Vs at VDS = 0.8 V and current on/off ratio is about ~750.ope