Feature Papers in Electronic Materials Section

This book entitled "Feature Papers in Electronic Materials Section" is a collection of selected papers recently published on the journal Materials, focusing on the latest advances in electronic materials and devices in different fields (e.g., power- and high-frequency electronics, optoelectronic devices, detectors, etc.). In the first part of the book, many articles are dedicated to wide band gap semiconductors (e.g., SiC, GaN, Ga2O3, diamond), focusing on the current relevant materials and devices technology issues. The second part of the book is a miscellaneous of other electronics materials for various applications, including two-dimensional materials for optoelectronic and high-frequency devices. Finally, some recent advances in materials and flexible sensors for bioelectronics and medical applications are presented at the end of the book

Journal of Telecommunications and Information Technology, 2007, nr 2

kwartalni

Advanced Analytical Microscopy at the Nanoscale: Applications in Wide Bandgap and Solid Oxide Fuel Cell Materials

The atomic-level structure and chemistry of materials ultimately dictate their observed macroscopic properties and behavior. As such, an intimate understanding of these characteristics allows for better materials engineering and improvements in the resulting devices. In our work, two material systems were investigated using advanced electron and ion microscopy techniques, relating the measured nanoscale traits to overall device performance. First, transmission electron microscopy and electron energy loss spectroscopy (TEM-EELS) were used to analyze interfacial states at the semiconductor/oxide interface in wide bandgap SiC microelectronics. This interface contains defects that significantly diminish SiC device performance, and their fundamental nature remains generally unresolved. The impacts of various microfabrication techniques were explored, examining both current commercial and next-generation processing strategies. In further investigations, machine learning techniques were applied to the EELS data, revealing previously hidden Si, C, and O bonding states at the interface, which help explain the origins of mobility enhancement in SiC devices. Finally, the impacts of SiC bias temperature stressing on the interfacial region were explored. In the second system, focused ion beam/scanning electron microscopy (FIB/SEM) was used to reconstruct 3D models of solid oxide fuel cell (SOFC) cathodes. Since the specific degradation mechanisms of SOFC cathodes are poorly understood, FIB/SEM and TEM were used to analyze and quantify changes in the microstructure during performance degradation. Novel strategies for microstructure calculation from FIB-nanotomography data were developed and applied to LSM-YSZ and LSCF-GDC composite cathodes, aged with environmental contaminants to promote degradation. In LSM-YSZ, migration of both La and Mn cations to the grain boundaries of YSZ was observed using TEM-EELS. Few substantial changes however, were observed in the overall microstructure of the cells, correlating with a lack of performance degradation induced by the H2O. Using similar strategies, a series of LSCF-GDC cathodes were analyzed, aged in H2O, CO2, and Cr-vapor environments. FIB/SEM observation revealed considerable formation of secondary phases within these cathodes, and quantifiable modifications of the microstructure. In particular, Cr-poisoning was observed to cause substantial byproduct formation, which was correlated with drastic reductions in cell performance

Total Ionizing Dose Response of High-k Dielectrics on MOS Devices

As advanced Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) technology continues to minimize the gate oxide thickness, the exponential increase in gate leakage current poses a major challenge for silicon dioxide (SiO2) based devices. In order to reduce the gate leakage current while maintaining the same gate capacitance, alternative gate insulator materials with higher dielectric constant (high-k) became the preferred replacement of SiO2 gate dielectrics. Germanium (Ge) MOSFETs have been regarded as promising candidates for future high-speed applications because they possess higher carrier mobility when compared to silicon based devices. At present, advanced microelectronics devices and circuits are used in aerospace engineering, nuclear industry, and radiotherapy equipment. These applications are unavoidably exposed to space-like radiation, which has a relative low radiation dose rate at 10-2-10-6 rad(Si)/s. For these reasons, it is necessary to understand the low-dose-rate radiation response of high-k materials based on Si and Ge MOS devices. The radiation response of high-k materials such as radiation-induced oxide and interface trap density have been typically examined by carrying out off-site capacitance-voltage (CV) measurements. However, the conventional and off-site radiation response measurements may underestimate the degradation of MOS devices. In this study, a semi-automated laboratory-scale real-time and on-site radiation response testing system was developed to evaluate the radiationresponse. The system is capable of estimating the radiation response of MOS devices whilst the devices are continuously irradiated by -rays raysrays. Moreover, the complete CV characteristics of MOS capacitors were measured in a relatively short time. The pulse CV measurement reduces the impact of charge trapping behavior on the measurement results, when compared to conventional techniques. The total ionizing dose radiation effect on HfO2 dielectric thin films prepared by atomic layer deposition (ALD) has been investigated by the proposed measurement system. The large bidirectional ΔVFB of the irradiated HfO2 capacitor was mainly attributed to the radiation-induced oxide trapped charges, which were not readily compensated by bias-induced charges produced over the measurement timescales of less than 5 ms. Radiation response of Ge MOS capacitors with HfO2 and HfxZr1-xOy gate dielectrics was also investigated. It was found that radiation-induced interface traps were the dominant factor for Flat-band Voltage shift (ΔVFB) in HfO2 thin films, whereas the radiation response for Zr-containing dielectrics under positive bias was mainly affected by oxide traps. Under positive biased irradiation, the Zr-doped HfxZr1-xOy exhibited smaller ΔVFB than that of HfO2. This is attributed to the de-passivation of Ge-S bonds in capacitors incorporating HfO2 thin films, resulting in the build-up of interface traps. Under negative biased irradiation, ΔVFB was attributed to the combined effect of the net oxide trapped charges and the passivation of Ge dangling bonds at the Ge/high-k interface

Advanced AlGaN/GaN HEMT technology, design, fabrication and characterization

Nowadays, the microelectronics technology is based on the mature and very well established silicon (Si) technology. However, Si exhibits some important limitations regarding its voltage blocking capability, operation temperature and switching frequency. In this sense, Gallium Nitride (GaN)-based high electron mobility transistors (HEMTs) devices have the potential to make this change possible. The unique combination of the high-breakdown field, the high-channel electron mobility of the two dimensional electron gas (2DEG), and high-temperature of operation has attracted enormous interest from social, academia and industry and in this context this PhD dissertation has been made. This thesis has focused on improving the device performance through the advanced design, fabrication and characterization of AlGaN/GaN HEMTs, primarily grown on Si templates. The first milestone of this PhD dissertation has been the establishment of a know-how on GaN HEMT technology from several points of view: the device design, the device modeling, the process fabrication and the advanced characterization primarily using devices fabricated at Centre de Recherche sur l'Hétéro-Epitaxie (CRHEA-CNRS) (France) in the framework of a collaborative project. In this project, the main workhorse of this dissertation was the explorative analysis performed on the AlGaN/GaN HEMTs by innovative electrical and physical characterization methods. A relevant objective of this thesis was also to merge the nanotechnology approach with the conventional characterization techniques at the device scale to understand the device performance. A number of physical characterization techniques have been imaginatively used during this PhD determine the main physical parameters of our devices such as the morphology, the composition, the threading dislocations density, the nanoscale conductive pattern and others. The conductive atomic force microscopy (CAFM) tool have been widely described and used to understand the conduction mechanisms through the AlGaN/GaN Ohmic contact by performing simultaneously topography and electrical conductivity measurements. As it occurs with the most of the electronic switches, the gate stack is maybe the critical part of the device in terms of performance and longtime reliability. For this reason, how the AlGaN/GaN HEMT gate contact affects the overall HEMT behaviour by means of advanced characterization and modeling has been intensively investigated. It is worth mentioning that the high-temperature characterization is also a cornerstone of this PhD. It has been reported the elevated temperature impact on the forward and the reverse leakage currents for analogous Schottky gate HEMTs grown on different substrates: Si, sapphire and free-standing GaN (FS-GaN). The HEMT' forward-current temperature coefficients (T^a) as well as the thermal activation energies have been determined in the range of 25-300 ºC. Besides, the impact of the elevated temperature on the Ohmic and gate contacts has also been investigated. The main results of the gold-free AlGaN/GaN HEMTs high-voltage devices fabricated with a 4 inch Si CMOS compatible technology at the clean room of the CNM in the framework of the industrial contract with ON semiconductor were presented. We have shown that the fabricated devices are in the state-of-the-art (gold-free Ohmic and Schottky contacts) taking into account their power device figure-of-merit ((VB^2)/Ron) of 4.05×10^8 W/cm^2. Basically, two different families of AlGaN/GaN-on-Si MIS-HEMTs devices were fabricated on commercial 4 inch wafers: (i) using a thin ALD HfO2 (deposited on the CNM clean room) and (ii) thin in-situ grown Si3N4, as a gate insulator (grown by the vendor). The scientific impact of this PhD in terms of science indicators is of 17 journal papers (8 as first author) and 10 contributions at international conferences

Improvement of Breakdown Characteristics in AlGaN/GaN Power HEMTs

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2013. 8. 한민구.Gallium Nitride (GaN) based high electron mobility transistor (HEMT) or heterostructure field effect transistor (HFET) are promising device for high-power switches which has to operate in electrically and environmentally harsh condition. The devices benefits from the material properties GaN offers: high critical field, high carrier mobility and a high saturation velocity of carriers. The breakdown voltage in AlGaN/GaN HEMTs is known to be triggered by gate leakage caused by the concentration of the electrical field at the drain-side edge of the gate electrode. The influence of gate leakage on blocking characteristics is alleviated by reducing the peak intensity of the electric field at the drain–side of the gate. There are two methods to reduce the peak intensity of the electric field: one is to decease the probability of tunneling of electrons into device active area the other is to relieve the crowding of electric field at the drain-side edge of gate. Nickel has been used as a gate electrode of the AlGaN/GaN HEMTs to form the Schottky contact due to its relatively high work function (5.15 eV). In this work, nickel oxide (NiOx) was inserted as the interfacial layer between main gate (Ni) and AlGaN barrier layer for improve the reliability of the AlGaN/GaN HEMTs. NiOx film was formed through the thermal oxidation in furnace. Material property of NiOx film depended on the two main factors: oxidation temperature, density of the film controlled by deposition rate. Only the NiOx film oxidized proper temperature range from 400℃ to 500℃ gave a favorable effect on the device performance. The NiOx film with high atomic density exhibited resistive switching characteristics, which can be used for GaN based memory device. Experiments to verify the effect of NiOx on reverse blocking operation were carried out. At the high temperature reverse bias (HTRB) test, it was found that work function of the NiOx was maintained. Moreover, it played an important role to improve the stable blocking operation. The result of electroluminescence (EL) analysis was consistent with the results obtained from HTRB test. Leakage current of the AlGaN/GaN HEMTs with NiOX interfacial layer measured at 200℃ was lower than that of the conventional device by 3 orders of magnitude. The breakdown voltage of the proposed device was up to 1.5 kV (1480 V). In recent years, improvements of the overall device performance were achieved by adopting metal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS) structure. At the gate region, by insulating gate electrode by means of a dielectric layer, electron injection is suppressed effectively. In this work, improvement of the blocking capability and reliability of AlGaN/GaN MIS-HEMTs employing atomic-layer-deposited (ALD) Al2O3 material was confirmed by experimental results. Mechanism responsible for the leakage current of the proposed device was investigated. Measured Leakage current of the fabricated MIS-HEMT was reduced to the range from sub pA (fA) to few pA. At the HTRB test, MIS-HEMT exhibited proved its thermal stability. Although drain leakage current (IDSS) was increased in proportion to the operational temperature, the leakage current of the proposed device was still lower than that of conventional device by 2 orders of magnitude. Breakdown voltage of the proposed device was up to 2 kV.Contents Abstract i List of Tables vii List of Figures ix 1. Introduction 1 1.1 Status quo of GaN power device 1 1.2 Research Background 26 1.3 Organization 42 2. Review of AlGaN/GaN HEMTs 45 2.1 Principle of AlGaN/GaN HEMTs 45 2.2 Structure and Polarization 55 2.3 Device fabrication 61 2.3.1 Pre-treatment 61 2.3.2 Isolation 63 2.3.3 Ohmic and Schottky contacts 67 2.4 Substrate used to AlGaN/GaN HEMTs 75 2.5 Factors limiting the HEMT performance 81 2.5.1 Current collapse 81 2.5.2 Leakage current 84 2.5.3 Breakdown mechanism and Failure 93 2.5.4 Technologies for high breakdown 102 3. AlGaN/GaN HEMT Employing NiOX Interfacial Layer 115 3.1 Overview 115 3.2 AlGaN/GaN HEMTs on SiC Substrate 117 3.2.1 Advantage of SiC substrate 117 3.2.2 Properties of HEMTs on SiC substrate 119 3.3 Device fabrication 122 3.4 Properties of the nickel oxide film 125 3.4.1 Resistive switching property of the NiOX 127 3.5 Device performance 138 3.5.1 Forward characteristics 138 3.5.2 Reverse blocking characteristics 151 3.6 Summary 173 4. AlGaN/GaN MOS-HEMT with ALD Al2O3 gate dielectric 174 4.1 AlGaN/GaN HEMT on Si Substrate 174 4.2 Atomic-Layer-Deposited Al2O3 dielectric 178 4.3 Device fabrication 186 4.4 Device performance 192 4.4.1 Forward characteristics 192 4.4.2 Reverse blocking characteristics 229 4.5 Summary 264 5. Conclusion. 265 Bibliography 268 초 록 286 감사의 글 290Docto