Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II

Wide bandgap (WBG) semiconductors are becoming a key enabling technology for several strategic fields, including power electronics, illumination, and sensors. This reprint collects the 23 papers covering the full spectrum of the above applications and providing contributions from the on-going research at different levels, from materials to devices and from circuits to systems

Particle-Based Modeling of Reliability for Millimeter-Wave GaN Devices for Power Amplifier Applications

abstract: In this work, an advanced simulation study of reliability in millimeter-wave (mm-wave) GaN Devices for power amplifier (PA) applications is performed by means of a particle-based full band Cellular Monte Carlo device simulator (CMC). The goal of the study is to obtain a systematic characterization of the performance of GaN devices operating in DC, small signal AC and large-signal radio-frequency (RF) conditions emphasizing on the microscopic properties that correlate to degradation of device performance such as generation of hot carriers, presence of material defects and self-heating effects. First, a review of concepts concerning GaN technology, devices, reliability mechanisms and PA design is presented in chapter 2. Then, in chapter 3 a study of non-idealities of AlGaN/GaN heterojunction diodes is performed, demonstrating that mole fraction variations and the presence of unintentional Schottky contacts are the main limiting factor for high current drive of the devices under study. Chapter 4 consists in a study of hot electron generation in GaN HEMTs, in terms of the accurate simulation of the electron energy distribution function (EDF) obtained under DC and RF operation, taking into account frequency and temperature variations. The calculated EDFs suggest that Class AB PAs operating at low frequency (10 GHz) are more robust to hot carrier effects than when operating under DC or high frequency RF (up to 40 GHz). Also, operation under Class A yields higher EDFs than Class AB indicating lower reliability. This study is followed in chapter 5 by the proposal of a novel π-Shaped gate contact for GaN HEMTs which effectively reduces the hot electron generation while preserving device performance. Finally, in chapter 6 the electro-thermal characterization of GaN-on-Si HEMTs is performed by means of an expanded CMC framework, where charge and heat transport are self-consistently coupled. After the electro-thermal model is validated to experimental data, the assessment of self-heating under lateral scaling is considered.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Advanced characterisation of novel III-nitride semiconductor based photonics and electronics on polar and non-polar substrates

Advanced characterisation has been carried out on a number of novel III-nitride based photonics and electronics, including micro-LED arrays achieved by a direct epitaxy approach, high performance c-plane HEMTs structure achieved by a novel growth method and non-polar GaN/AlGaN HEMTs. In this work, a systematic study has been conducted to understand the electrical properties of these novel devices, demonstrating their excellent properties. Furthermore, the electrical properties are directly related to epitaxial growth, which provides useful information for further improving device performance, such as 2D growth mode for GaN on a large lattice-mismatched substrate which plays an important in obtaining high breakdown and minimised leakage current for HEMTs. Micro-LEDs are the key elements for a microdisplay system, where electrical properties are extremely important. Potentially, any leakage current can trigger to turn on any neighbouring microLEDs which are supposed to be off. Instead of using conventional fabrication methods which normally enhances leakage current, our team developed a direct epitaxy approach to achieving microLED arrays. In this work, detailed I-V characteristic and capacitance measurements have been conducted on these novel microLED devices, demonstrating leakage currents as low as 14.1 nA per LED and a smooth negative capacitance curve instead of odd positive capacitance performances. Furthermore, a comparison study between our microLEDs and the microLEDs prepared using the conventional method indicates our device shows a large reduction of size-dependent inefficiency while such a behaviour is never observed on the microLEDs fabricated by the conventional methods. Unlike the classic two-step method for GaN growth on large lattice-matched sapphire, our team developed a high-temperature AlN buffer technology, where a 2D growth mode, instead of an initial 2D and then 3D growth mode that typically happens for the growth of conventional GaN growth, takes place through the whole growth process. This method allows us to achieve a breakdown electric field strength of 2.5 MV/cm, a leakage current of as low as 41.7 pA at 20 V and saturation current densities as high as 1.1 A/mm. In this work a systematic study has conducted in order to establish a relationship between the excellent device performance and material properties, where a very low screw dislocation density plays a critical role, while our 2D growth method can provide an excellent opportunity for achieving such a low screw dislocation density. This demonstrates the major advantage over the classic two-step method in the growth of power and RF devices. In our case, we have obtained an unintentional doping as low as 2×10^14 cm-3 and screw dislocation densities of 2.3×10^7 cm-2. Compared with c-plane GaN based HEMTs due to its intrinsic polarisation, non-polar GaN/AlGaN HEMTs on r-plane sapphire yields potential advantages in terms of the fabrication of normal-off devices which are particularly important for practical applications. However, it is a great challenge to achieve high quality non-polar GaN on sapphire. Some initial work has been conducted, where the detailed characterisation indicates an electron mobility of 43 cm2 V-1 s-1 has been initially obtained. Furthermore, instead of using an AlGaN/GaN heterostructure with a modulation doping, we deliberately use a quantum well structure as an electron channel, leading to a mobility of 76 cm2 V-1 s-1. Our simulations as well as measurements also provide a guideline for optimising the general epitaxial structure

APPLICATIONS OF PLASMONICS FOR TERAHERTZ DETECTION, MODULATION AND WAVEGUIDING

Ph.DDOCTOR OF PHILOSOPH

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

Advanced gallium nitride technology for microwave power amplifiers

Gallium nitride (GaN) based technology has been heavily researched over the past two decades due to its ability to deliver higher powers and higher frequencies that are demanded by the market for various applications. One of GaN’s main advantages lies in its ability to form heterojunctions to wider bandgap materials such as Aluminium Gallium Nitride (AlGaN) and Aluminium Nitride (AlN). The heterostructure results in the formation of the so called 2 dimensional electron gas (2DEG), which exhibits high electron densities of up to 6E13 cm−2 and high electron mobilities of up to 2000 cm2/V·s that enable the devices to support high current densities. Furthermore, it supports very high breakdown fields of 3.3 MV/cm due to its wide bandgap of 3.4 eV. The main objective of this work was to further advance the transistor technology using simple, cost effective and reliable techniques. The AlN/GaN material system exhibits higher sheet carrier concentrations compared to the conventional ternary AlGaN barrier, but introduces additional challenges due to its reduced thickness of 2-6 nm compared to 18-30 nm of AlGaN. The additional challenges of the thin AlN binary barrier include strain relaxation, high gate leakage currents and high Ohmic contact resistances due to its high bandgap of 6.2 eV. In this work, a thin (5 nm) in-situ SiNx passivation layer was employed to reduce the strain relaxation, reduce gate leakage currents and improve Ohmic contacts resistances. The optimised Ohmic contact annealing condition resulted in an Ohmic contact resistance of 0.4 Ω·mm and a sheet resistance of 300 Ω

Nanocrystalline diamond thin film integration in AlGaN/GaN high electron mobility transistors and 4H-SiC heterojunction diodes

The extremely high thermal conductivity and mechanical hardness of diamond would make it the natural choice for device substrates when large area wafer production becomes possible. Until this milestone is achieved, people could utilize nanocrystalline diamond (NCD) thin films grown by chemical vapor deposition (CVD). A topside thermal contact could be pivotal for providing stable device characteristics in the high power, high temperature, and high switching frequency device operating regime that next-generation power converter circuits will mandate. This work explores thermal and electrical benefits offered by NCD films to wide bandgap semiconductor devices. Reduction of self-heating effects by integrating NCD thin films near the device channel of AlGaN/GaN high electron mobility transistors (HEMTs) is presented. The NCD layers provide a high thermal conductivity path for the reduction of hot electron dispersion, a phenomenon caused by self-heating and detrimental to the continuous operation of GaN devices in power switching circuits. Recent advances in diamond doping have made it possible to think of this material as a very wide bandgap semiconductor (5.5 eV for ideal diamond). A few unique properties, such as negative electron affinity (χ = -0.2 eV for H-terminated diamond), make this material very interesting. Using H-terminated NCD, a heterojunction with 4H-SiC has been developed. Undoped and B-doped NCD were deposited on both n- and p- 4H-SiC epilayers. Different metals were studied to provide an Ohmic contact to the NCD layer. I-V measurements on p+ NCD / n- 4H-SiC p-n junctions indicated Schottky rectifying behavior with a turn-on voltage of around 0.2 V. The current increased over 8 orders of magnitude with an ideality factor of 1.17 at 30 °C. Ideal energy-band diagrams suggested a possible conduction mechanism for electron transport from the SiC conduction band to either the valence band or Boron acceptor level of the NCD film. Cathodoluminescence and thermally stimulated current methods were employed to study the deep level assisted conduction in this heterojunction. Applications as a simultaneous UV-transparent optical and Schottky electrical contact to 4H-SiC are discussed

Advanced III-Nitride Technology for mm-Wave Applications

Within wireless communication, there is a continuously growing need for more bandwidth due to an increasing number of users and data intense services. The development within sensor systems such as radars, is largely driven by the need for increased detection range and robustness. In such systems, power amplification and generation at high frequency are of importance. High-electron mobility transistors based on gallium nitride (GaN HEMTs) offer efficient generation of high output power at high frequency. This is enabled by the unique characteristics of GaN and its heterostructures, with a large breakdown field, related to the wide bandgap, and excellent electron transport properties. Due to this, it is today used in high-performing radar, telecommunications, as well as power electronic systems. Despite substantial progress over the last decade, the GaN HEMT is still the subject of intense research to reach its full potential. \ua0Recent development within epitaxy has significantly improved the quality of III-nitride semiconductors, and enabled indium aluminum nitride (InAlN) and InAlGaN as alternatives to AlGaN in the conventional AlGaN/GaN heterostructure. The higher polarization charge in these materials allows for considerable downscaling of the barrier layer thickness with a sustained high sheet carrier density. \ua0This has opened new possibilities for optimization of the high frequency performance. \ua0\ua0In this work, HEMTs with downscaled InAl(Ga)N barrier layers have been developed with the goal to optimize the devices for power amplification in the mm-wave range. Electron trapping and short-channel effects have been addressed in the design of the epi and in the optimization of the process modules. Different surface passivation layers and deposition methods have been evaluated to mitigate electron trapping at the surface. The output power density of a HEMT increased from 1.7 to 4.1 W/mm after passivation with a SiNx layer. The deposition method for Al2O3 passivation layers showed to have a profound impact on the electron trapping. A layer deposited by plasma-assisted atomic layer deposition (ALD) exhibited superior passivation of the surface traps as compared to the layer deposited by thermal ALD, resulting in an output power at 3 GHz of 3.3, and 1.9 W/mm, respectively. The effect of the channel layer thickness (50 – 150 nm) in InAlGaN/AlN/GaN HEMTs with and AlGaN back barrier demonstrated a trade-off between short-channel effects and deep-level electron trapping in the back barrier. The maximum output power was 5.3 W/mm at 30 GHz, obtained for a GaN layer thickness of 100 nm. To further enhance the high frequency performance, the ohmic contacts were optimized by the development of a Ta-based, Au free, metal scheme. Competitive contact resistance of < 0.2 Ωmm was achieved on both AlGaN/GaN and InAlN heterostructures with a Ta/Al/Ta metal stack. The contacts are annealed at a low temperature (550 – 575 \ubaC) compared to more conventional contact schemes, resulting in a smooth morphology and good edge acuity.\ua0 The implementation of microwave monolithic integrated circuits (MMICs) based on III-nitride HEMTs facilitate the use of III-nitride HEMTs in a system where frequency and compactness are key requirements. Thin film resistors (TFRs) are one of the passive components required in MMICs. In this work, a low-resistance titanium nitride (TiN) TFR was developed as a complement to the higher resistance tantalum nitride (TaN) TFR and mesa resistor in the in-house MMIC process. The developed TiN TFR exhibits a sheet resistance of 10 Ω/□, compared to 50 and 200-300 Ω/□ of the TaN TFR and semiconductor resistor, respectively. The critical dissipated power in the TFR showed a correlation to the footprint area, indicating that Joule-heating was the main cause of failure. TiN- and TaN films exhibit different signs of the thermal coefficient of resistance. This feature was used to demonstrate a temperature compensated TFR (TCR = -60 ppm \ubaC) with application in MMICs operating in a wide temperature range