Carrier Transport mechanisms contributing to the sub-threshold current in 3C-SiC-on-Si Schottky Barrier Diodes

3C-Silicon Carbide (3C-SiC) Schottky Barrier Diodes on silicon (Si) substrates (3C-SiC-on-Si) seem not to comply with the superior wide band gap expectations in terms of excessive measured sub-threshold current. In turn, that is one of the factors which deters their commercialization. Interestingly, the forward biased part of the Current-Voltage (I-V) characteristics in these devices carries considerable information about the material quality. In this context, an advanced Technology Computer Aided Design (TCAD) model for a vertical Platinum/3C-SiC Schottky power diode is created and validated with measured data. The model includes defects originating from both the Schottky contact and the hetero-interface of 3C-SiC with Si which allows the investigation of their impact on the magnification of the sub-threshold current. For this, barrier lowering, quantum field emission and trap assisted tunneling of majority carriers need to be considered at the non-ideal Schottky interface. The simulation results and measured data allowed for the comprehensive characterization of the defects affecting the carrier transport mechanisms of the forward biased 3C-SiC on Si power rectifier for the first time

Experimental and physics based study of the Schottky Barrier Height inhomogeneity and associated traps affecting 3C-SiC-on-Si Schottky Barrier Diodes

The ability of cubic phase (3C-) Silicon Carbide (SiC) to grow heteroepitaxially on Silicon (Si) substrates (3C-SiC-on-Si) is an enabling feature for cost-effective Wide Bandgap devices and homogeneous integration with Si devices. In this paper, the authors evaluated 3C-SiC-on-Si Schottky Barrier Contacts by fabricating and testing non-freestanding lateral Schottky Barrier Diodes (LSBD). To gain a deep physical insight of the complex carrier transport phenomena that take place in this material, advanced Technology Computer Aided Design (TCAD) models were developed which allowed accurately matching of measurements with simulations. The models incorporate the device geometry, an accurate representation of the bulk material properties, and complex trapping/de-trapping and tunnelling phenomena which appear to affect the device performance. The observed non-uniformities of the Schottky Barrier Height (SBH) were successfully modelled through the incorporation of interfacial traps. The combination of TCAD with fabrication and measurements enabled the identification of trap profiles and pin their influence on the electrical performance of 3C-SiC-on-Si LSBD. The effect of temperature was studied by engaging the identified trap profiles and calculating the occupation distribution of electrons in 3C-SiC at elevated temperature. The investigation constitutes an imperative knowledge step towards the development of devices that take advantage of 3C-SiC material properties

Nanoscale characterization of electrical transport at metal/3C-SiC interfaces

In this work, the transport properties of metal/3C-SiC interfaces were monitored employing a nanoscale characterization approach in combination with conventional electrical measurements. In particular, using conductive atomic force microscopy allowed demonstrating that the stacking fault is the most pervasive, electrically active extended defect at 3C-SiC(111) surfaces, and it can be electrically passivated by an ultraviolet irradiation treatment. For the Au/3C-SiC Schottky interface, a contact area dependence of the Schottky barrier height (ΦB) was found even after this passivation, indicating that there are still some electrically active defects at the interface. Improved electrical properties were observed in the case of the Pt/3C-SiC system. In this case, annealing at 500°C resulted in a reduction of the leakage current and an increase of the Schottky barrier height (from 0.77 to 1.12 eV). A structural analysis of the reaction zone carried out by transmission electron microscopy [TEM] and X-ray diffraction showed that the improved electrical properties can be attributed to a consumption of the surface layer of SiC due to silicide (Pt2Si) formation. The degradation of Schottky characteristics at higher temperatures (up to 900°C) could be ascribed to the out-diffusion and aggregation of carbon into clusters, observed by TEM analysis

A study of temperature-related non-linearity at the metal-silicon interface

In this paper, we investigate the temperature dependencies of metal-semiconductor interfaces in an effort to better reproduce the current-voltage-temperature (IVT) characteristics of any Schottky diode, regardless of homogeneity. Four silicon Schottky diodes were fabricated for this work, each displaying different degrees of inhomogeneity; a relatively homogeneous NiV/Si diode, a Ti/Si and Cr/Si diode with double bumps at only the lowest temperatures, and a Nb/Si diode displaying extensive non-linearity. The 77–300 K IVT responses are modelled using a semi-automated implementation of Tung's electron transport model, and each of the diodes are well reproduced. However, in achieving this, it is revealed that each of the three key fitting parameters within the model display a significant temperature dependency. In analysing these dependencies, we reveal how a rise in thermal energy “activates” exponentially more interfacial patches, the activation rate being dependent on the carrier concentration at the patch saddle point (the patch's maximum barrier height), which in turn is linked to the relative homogeneity of each diode. Finally, in a review of Tung's model, problems in the divergence of the current paths at low temperature are explained to be inherent due to the simplification of an interface that will contain competing defects and inhomogeneities

Hydrogen Sensors Using Nitride-Based Semiconductor Diodes: The Role of Metal/Semiconductor Interfaces

In this paper, I review my recent results in investigating hydrogen sensors using nitride-based semiconductor diodes, focusing on the interaction mechanism of hydrogen with the devices. Firstly, effects of interfacial modification in the devices on hydrogen detection sensitivity are discussed. Surface defects of GaN under Schottky electrodes do not play a critical role in hydrogen sensing characteristics. However, dielectric layers inserted in metal/semiconductor interfaces are found to cause dramatic changes in hydrogen sensing performance, implying that chemical selectivity to hydrogen could be realized. The capacitance-voltage (C–V) characteristics reveal that the work function change in the Schottky metal is not responsible mechanism for hydrogen sensitivity. The interface between the metal and the semiconductor plays a critical role in the interaction of hydrogen with semiconductor devises. Secondly, low-frequency C–V characterization is employed to investigate the interaction mechanism of hydrogen with diodes. As a result, it is suggested that the formation of a metal/semiconductor interfacial polarization could be attributed to hydrogen-related dipoles. In addition, using low-frequency C–V characterization leads to clear detection of 100 ppm hydrogen even at room temperature where it is hard to detect hydrogen by using conventional current-voltage (I–V) characterization, suggesting that low-frequency C–V method would be effective in detecting very low hydrogen concentrations

Defect Characterization of 4H-SIC by Deep Level Transient Spectroscopy (DLTS) and Influence of Defects on Device Performance

Silicon carbide (SiC) is one of the key materials for high power opto-electronic devices due to its superior material properties over conventional semiconductors (e.g., Si, Ge, GaAs, etc). SiC is also very stable and a highly suitable material for radiation detection at room temperature and above. The availability of detector grade single crystalline bulk SiC is limited by the existing crystal growth techniques which introduce extended and microscopic crystallographic defects during the growth process. Recently, SiC based high-resolution semiconductor detectors for ionizing radiation have attracted world-wide attention due to the availability of high resistive, highly crystalline epitaxial layers with very low micropipe defect content (\u3c 1 cm-2). SiC Schottky barrier radiation detectors on epitaxial layers can be operated with a high signal-to-noise ratio even above room temperature due to its wide band-gap. However, significant amount of intrinsic defects and impurities still exist in the grown SiC epilayer which may act as traps or recombination/generation centers and can lead to increased leakage current, poor carrier lifetime, and reduced carrier mobility. Unfortunately, the nature of these electrically active deep levels and their behavior are not well understood. Therefore, it is extremely important to identify these electrically active defects present in the grown epitaxial layers and to understand how they affect the detector performance in terms of leakage current and energy resolution. In this work, Schottky barrier radiation detectors were fabricated on high quality n-type 4H-SiC epitaxial layers. The epitaxial layers were grown on nitrogen doped n-type 4H-SiC (0001) substrates by a hot wall chemical vapor deposition (CVD) process. The epitaxial growth was carried out with 8o off-cut towards the [112̅0] direction. The Schottky barriers were formed on the epitaxial layers (Si-face) by depositing thin (~10 nm) circular Ni contact (area ~10 mm2) which acts as the detector window. The thickness of the detector window was decided such that there was minimal alpha energy attenuation while maintaining a reliable electrical contact. For the back contact, ~100 nm thick square (~40 mm2) Ni contact was deposited on the C-face of the 4H-SiC substrate. The junction properties of the fabricated Schottky barrier radiation detectors were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. From the fabricated devices, those with high barrier height (~ 1.6 eV) and extremely low leakage current (few pA at a reverse bias of ~ -100 V) were selected for alpha spectroscopic measurements. Alpha pulse-height spectra was obtained from the charge pulses produced by the detector irradiated with a standard 0.1 μCi 241Am source. The charge transport and collection efficiency results, obtained from the alpha particle pulse-height spectroscopy, were interpreted using a drift-diffusion charge transport model. The detector performances were evaluated in terms of the energy resolution. From alpha spectroscopy measurements the FWHM (full width at half maxima) of the fabricated Schottky barrier detectors were in the range of 0.29% - 1.8% for the main alpha peak of 241Am (5.486 MeV). Deep level transient spectroscopy (DLTS) studies were conducted in the temperature range of 80 K - 800 K to identify and characterize the electrically active defects present in the epitaxial layers. Deep level defect parameters (i.e. activation energy, capture cross-section, and density) were calculated from the Arrhenius plots which were obtained from the DLTS spectra at different rate windows. The observed defects in various epitaxial layers were identified and compared with the literature. In the 50 μm epitaxial layer, a new defect level located at Ec - 2.4 eV was observed for the first time. The differences in the performance of different detectors were correlated on the basis of the barrier properties and the deep level defect types, concentrations, and capture cross-sections. It was found that detectors, fabricated on similar wafers, can perform in a substantially different manner depending on the defect types. For 20 μm epitaxial layer Schottky barrier radiation detectors, deep levels Z1/2 (located at ~ Ec - 0.6 eV) and EH6/7 (located at ~ Ec - 1.6 eV) are related to carbon vacancies and their complexes which mostly affect the detector resolution. For 50 μm epitaxial layer Schottky barrier radiation detectors, Z1/2, EH5, and the newly identified defect located at Ec - 2.4 eV mostly affect the detector resolution. The annealing behavior of deep level defects was thoroughly investigated by systematic C-DLTS measurements before and after isochronal annealing in the temperature range of 100 ˚C - 800 ˚C. Defect parameters were calculated after each isochronal annealing. Capture cross-sections and densities for all the defects were investigated to analyze the impact of annealing. The capture cross-sections of the defects Ti (c) (located at Ec ˗ 0.17 eV) and EH5 (located at Ec ˗ 1.03 eV) were observed to decrease with annealing temperature while the densities did not change significantly. Deep level defects Z1/2 and EH6/7 were found to be stable up to the annealing temperature of 800 ˚C

Role of interface configuration in diamond-related power devices

Durante los años transcurridos en el desarrollo de esta tesis, la generación de energía eléctrica mundial habrá crecido a un ritmo medio anual del 3.6%1, que refleja las crecientes necesidades de la sociedad en términos de suministro eléctrico (voltaje, densidad de potencia, frecuencia de uso, fiabilidad o temperatura de trabajo). Estas necesidades se están volviendo más exigentes, las pérdidas de energía deben ser reducidas y el rendimiento mejorado. El progreso de las recientes décadas en el campo de la electrónica de potencia no se debe sólo a la introducción de arquitecturas novedosas, sino también a la evolución de la composición de los dispositivos. El progreso actual está obstaculizado por las limitaciones inherentes al silicio, componente del que están fabricados la mayor parte de los dispositivos electrónicos de potencia actualmente disponibles comercialmente. Los semiconductores de ancha banda prohibida tienen propiedades particularmente atractivas para funcionar a altos voltajes y frecuencias en entornos de alta temperatura. Como semiconductores de ancha banda prohibida, los dispositivos basados en diamante semiconductor se han manifestado como un campo de investigación prometedor, no sólo por la amplia aplicabilidad en las ciencias biológicas, si no por sus extraordinarias propiedades eléctricas (elevada movilidad de portadores, alto valor de ruptura eléctrica y extraordinaria conductividad del calor). Tras casi quince años de investigación en diamante semiconductor se han resuelto gran cantidad de interrogantes, lo que ha permitido la aparición de los primeros prototipos. Es esta evolución en el conocimiento la que ha posibilitado la elección del diamante como candidato idóneo para la realización de componentes electrónicos de alta potencia, entendiendo estos dispositivos como aquellos que funcionan en condiciones de alta frecuencia de conmutación de señal. Paradójicamente, a pesar de sus numerosas ventajas y de los amplios estudios en esta materia, la explosión de las tecnologías basadas en diamante aún no ha llegado a su madurez. Esto es debido, fundamentalmente, a la mala calidad estructural en la implementación de los diseños ideados para los dispositivos electrónicos con canal activo de diamante. Adicionalmente, las limitaciones en las aplicaciones tecnológicas del diamante derivan de otras de sus propiedades extremas, como la dureza (que dificulta su clivaje) o la alta energía de activación de dopantes tipo n. Sin embargo, se han conseguido numerosos progresos en el crecimiento de estructuras de diamante para dispositivos eléctricos. En particular, estos esfuerzos han permitido minimizar la densidad de dislocaciones producidas durante el crecimiento de estructuras 1 OECD, library: http://www.oecd-ilibrary.org/economics/oecd-factbook-2013_factbook-2013-en 9 multicapa u optimizar la densidad de dopantes activos durante el crecimiento de capas dopadas (lo que ha requerido de amplios estudios sobre la incorporación del boro en la red cristalina del diamante). Paralelamente a los esfuerzos desarrollados en la comprensión y el estudio de la incorporación de dopantes en la red del diamante, se han desarrollado otros no menos loables avances en el diseño de estructuras óptimas para establecer contactos eléctricos en diamante. En la presente contribución, se evidencia el uso del diamante semiconductor como base para un dispositivo de alta potencia con canal activo de diamante, las diversas alternativas de diseño, sus técnicas de estudio y las características eléctricas de los primeros prototipos de los diferentes dispositivos.Ministerio de ciencia e innovación (becas FPI): BES-2010-039524 an

MIS capacitor studies on silicon carbide single crystals

Cubic SIC metal-insulator-semiconductor (MIS) capacitors with thermally grown or chemical-vapor-deposited (CVD) insulators were characterized by capacitance-voltage (C-V), conductance-voltage (G-V), and current-voltage (I-V) measurements. The purpose of these measurements was to determine the four charge densities commonly present in an MIS capacitor (oxide fixed charge, N(f); interface trap level density, D(it); oxide trapped charge, N(ot); and mobile ionic charge, N(m)) and to determine the stability of the device properties with electric-field stress and temperature. The section headings in the report include the following: Capacitance-voltage and conductance-voltage measurements; Current-voltage measurements; Deep-level transient spectroscopy; and Conclusions (Electrical characteristics of SiC MIS capacitors)

Diamond Schottky barrier diodes

Research on wide band gap semiconductors suitable for power electronic devices has spread rapidly in the last decade. The remarkable results exhibited by silicon carbide (SiC) Schottky batTier diodes (SBDs), commercially available since 2001, showed the potential of wide band gap semiconductors for replacing silicon (Si) in the range of medium to high voltage applications, where high frequency operation is required. With superior physical and electrical properties, diamond became a potential competitor to SiC soon after Element Six reported in 2002 the successful synthesis of single crystal plasma deposited diamond with high catTier mobility. This thesis discusses the remarkable properties of diamond and introduces several device structures suitable for power electronics. The calculation of several figures of merit emphasize the advantages of diamond with respect to silicon and other wide band gap semiconductors and clearly identifies the areas where its impact would be most significant. Information regarding the first synthesis of diamond, which took place back in 1954, together with data regarding the modern technological process which leads nowadays to high-quality diamond crystals suitable for electronic devices, are reviewed. Models regarding the incomplete ionization of atomic dopants and the variation of catTier mobility with doping level and temperature have been elaborated and included in numerical simulators. The study introduces the novel diamond M-i-P Schottky diode, a version of power Schottky diode which takes advantage of the extremely high intrinsic hole mobility. The structure overcomes the drawback induced by the high activation energies of acceptor dopants in diamond which yield poor hole concentration at room temperature. The complex shape of the on-state characteristic exhibited by diamond M-i-P Schottky structures is thoroughly investigated by means of experimental results, numerical simulations and theoretical considerations. The fabrication of a ramp oxide termination on a diamond device is for the first time reported in this thesis. Both experimental and simulated results show the potential of this termination structure, previously built on Si and SiC power devices. A comprehensive comparison between the ramp oxide and two other versions of the field plate termination concept, the single step and the three-step structures, has been performed, considering aspects such as electrical performance, occupied area, complexity of technological process and cost. Based on experimental results presented in this study, together with predictions made via simulations and theoretical models, it is concluded that diamond M-i-P Schottky diodes have the ability to deliver significantly higher performance compared to that of SiC SBDs if issues such as material defects, Schottky contact formation and measurement of reliable ionization coefficients are carefully addressed in the near future