Matching Items (38)
Description
In this dissertation, the interface chemistry and electronic structure of plasma-enhanced atomic layer deposited (PEALD) dielectrics on GaN are investigated with x-ray and ultraviolet photoemission spectroscopy (XPS and UPS). Three interrelated issues are discussed in this study: (1) PEALD dielectric growth process optimization, (2) interface electronic structure of comparative PEALD dielectrics on GaN, and (3) interface electronic structure of PEALD dielectrics on Ga- and N-face GaN. The first study involved an in-depth case study of PEALD Al2O3 growth using dimethylaluminum isopropoxide, with a special focus on oxygen plasma effects. Saturated and self-limiting growth of Al2O3 films were obtained with an enhanced growth rate within the PEALD temperature window (25-220 ºC). The properties of Al2O3 deposited at various temperatures were characterized to better understand the relation between the growth parameters and film properties. In the second study, the interface electronic structures of PEALD dielectrics on Ga-face GaN films were measured. Five promising dielectrics (Al2O3, HfO2, SiO2, La2O3, and ZnO) with a range of band gap energies were chosen. Prior to dielectric growth, a combined wet chemical and in-situ H2/N2 plasma clean process was employed to remove the carbon contamination and prepare the surface for dielectric deposition. The surface band bending and band offsets were measured by XPS and UPS for dielectrics on GaN. The trends of the experimental band offsets on GaN were related to the dielectric band gap energies. In addition, the experimental band offsets were near the calculated values based on the charge neutrality level model. The third study focused on the effect of the polarization bound charge of the Ga- and N-face GaN on interface electronic structures. A surface pretreatment process consisting of a NH4OH wet chemical and an in-situ NH3 plasma treatment was applied to remove carbon contamination, retain monolayer oxygen coverage, and potentially passivate N-vacancy related defects. The surface band bending and polarization charge compensation of Ga- and N-face GaN were investigated. The surface band bending and band offsets were determined for Al2O3, HfO2, and SiO2 on Ga- and N-face GaN. Different dielectric thicknesses and post deposition processing were investigated to understand process related defect formation and/or reduction.
ContributorsYang, Jialing (Author) / Nemanich, Robert J (Thesis advisor) / Chen, Tingyong (Committee member) / Peng, Xihong (Committee member) / Ponce, Fernando (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2014
Description
Semiconductor nanowires have the potential to emerge as the building blocks of next generation field-effect transistors, logic gates, solar cells and light emitting diodes. Use of Gallium Nitride (GaN) and other wide bandgap materials combines the advantages of III-nitrides along with the enhanced mobility offered by 2-dimensional confinement present in nanowires. The focus of this thesis is on developing a low field mobility model for a GaN nanowire using Ensemble Monte Carlo (EMC) techniques. A 2D Schrödinger-Poisson solver and a one-dimensional Monte Carlo solver is developed for an Aluminum Gallium Nitride/Gallium Nitride Heterostructure nanowire. A GaN/AlN/AlGaN heterostructure device is designed which creates 2-dimensional potential well for electrons. The nanowire is treated as a quasi-1D system in this work. A self-consistent 2D Schrödinger-Poisson solver is designed which determines the subband energies and the corresponding wavefunctions of the confined system. Three scattering mechanisms: acoustic phonon scattering, polar optical phonon scattering and piezoelectric scattering are considered to account for the electron phonon interactions in the system. Overlap integrals and 1D scattering rate expressions are derived for all the mechanisms listed. A generic one-dimensional Monte Carlo solver is also developed. Steady state results from the 1D Monte Carlo solver are extracted to determine the low field mobility of the GaN nanowires.
ContributorsKumar, Viswanathan Naveen (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2017
Description
Diamond and cubic boron nitride (c-BN) are ultra wide band gap semiconductors (Eg>3.4 eV) and share similar properties in various aspects, including being isoelectronic, a 1% lattice mismatch, large band gap, high thermal conductivity. Particularly, the negative electron affinity (NEA) of diamond and c-BN is an unusual property that has led to effects such as p-type surface conductivity, low temperature thermionic emission, and photon enhanced thermionic emission. In this dissertation, the interface chemistry and electronic structure of dielectrics on diamond and c-BN are investigated with X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The first study established that the surface conductive states could be established for thin Al2O3 on diamond using a post deposition H-plasma process. At each step of the atomic layer deposition (ALD) and plasma processing, the band alignment was characterized by in situ photoemission and related to interface charges. An interface layer between the diamond and dielectric layer was proposed to explain the surface conductivity. The second study further investigated the improvement of the hole mobility of surface conductive diamond. A thin layer of Al2O3 was employed as an interfacial layer between surface conductive hydrogen-terminated (H-terminated) diamond and MoO3 to increase the distance between the hole accumulation layer in diamond and negatively charged states in acceptor layer. With an interfacial layer, the ionic scattering, which was considered to limit the hole mobility, was reduced. By combining two oxides (Al2O3 and MoO3), the hole mobility and concentration were modulated by altering the thickness of the Al2O3 interfacial layer. The third study focused on the electronic structure of vanadium-oxide-terminated c-BN surfaces. The vanadium-oxide-termination was formed on c-BN by combining vanadium deposition using molecular beam deposition (MBD) and oxygen plasma treatment. After thermal annealing, a thermally stable NEA was achieved on c-BN. A model was proposed based on the deduced interface charge distribution to explain the establishment of an NEA.
ContributorsYang, Yu (Author) / Nemanich, Robert J (Thesis advisor) / McCartney, Martha (Committee member) / Ponce, Fernando (Committee member) / Qing, Quan (Committee member) / Arizona State University (Publisher)
Created2018
In-situ photoemission spectroscopy characterization of electronic states in semiconductor interfaces
Description
The electronic states of semiconductor interfaces have significant importance for semiconductor device performance, especially due to the continuing miniaturization of device technology.
The application of ultra high vacuum (UHV) enables the preparation and characterization of fresh and cleaned interfaces. In a UHV environment, photoemission spectroscopy (PES) provides a non-destructive method to measure the electronic band structure, which is a crucial component of interface properties.
In this dissertation, three semiconductor interfaces were studies to understand different effects on electronic states. The interfaces studied were freshly grown or pre-treated under UHV. Then in-situ PES measurements, including x-ray photoemission spectroscopy (XPS) and ultra-violet photoemission spectroscopy (UPS), were conducted to obtain electronic states information.
First, the CdTe/InSb (100) heterointerface was employed as a model interface for II-VI and III-V heterojunctions. It was suggested that an interface layer formed, which consisted of In-Te bonding. The non-octal bonding between In and Te atoms has donor-like behavior, which was proposed to result in an electron accumulation layer in InSb. A type-I heterointerface was observed. Second, Cu/ZnO interfaces were studied to understand the interface bonding and the role of polarization on ZnO interfaces. It was shown that on O-face ZnO (0001) and PEALD ZnO, copper contacts had ohmic behavior. However, on Zn-face ZnO (0001), a 0.3 eV Schottky barrier height was observed. The lower than expected barrier heights were attributed to oxygen vacancies introduced by Cu-O bonding during interface formation. In addition, it is suggested that the different barrier heights on two sides of ZnO (0001) are caused by the different behavior for the ZnO (0001) faces. Last, a pulse mode deposition method was applied for P-doped diamond growth on (100) diamond surfaces. Pretreatment effects were studied. It is suggested that an O/H plasma treatment or a short period of H-plasma and CH4/H2 plasma could yield a higher growth rate. PES measurements were conducted on H-terminated intrinsic diamond surface and P-doped/intrinsic diamond (100) interfaces. It was suggested that electronic states near the valence band maximum caused Fermi level pinning effects, independent of the diamond doping.
The application of ultra high vacuum (UHV) enables the preparation and characterization of fresh and cleaned interfaces. In a UHV environment, photoemission spectroscopy (PES) provides a non-destructive method to measure the electronic band structure, which is a crucial component of interface properties.
In this dissertation, three semiconductor interfaces were studies to understand different effects on electronic states. The interfaces studied were freshly grown or pre-treated under UHV. Then in-situ PES measurements, including x-ray photoemission spectroscopy (XPS) and ultra-violet photoemission spectroscopy (UPS), were conducted to obtain electronic states information.
First, the CdTe/InSb (100) heterointerface was employed as a model interface for II-VI and III-V heterojunctions. It was suggested that an interface layer formed, which consisted of In-Te bonding. The non-octal bonding between In and Te atoms has donor-like behavior, which was proposed to result in an electron accumulation layer in InSb. A type-I heterointerface was observed. Second, Cu/ZnO interfaces were studied to understand the interface bonding and the role of polarization on ZnO interfaces. It was shown that on O-face ZnO (0001) and PEALD ZnO, copper contacts had ohmic behavior. However, on Zn-face ZnO (0001), a 0.3 eV Schottky barrier height was observed. The lower than expected barrier heights were attributed to oxygen vacancies introduced by Cu-O bonding during interface formation. In addition, it is suggested that the different barrier heights on two sides of ZnO (0001) are caused by the different behavior for the ZnO (0001) faces. Last, a pulse mode deposition method was applied for P-doped diamond growth on (100) diamond surfaces. Pretreatment effects were studied. It is suggested that an O/H plasma treatment or a short period of H-plasma and CH4/H2 plasma could yield a higher growth rate. PES measurements were conducted on H-terminated intrinsic diamond surface and P-doped/intrinsic diamond (100) interfaces. It was suggested that electronic states near the valence band maximum caused Fermi level pinning effects, independent of the diamond doping.
ContributorsWang, Xingye (Author) / Nemanich, Robert J (Thesis advisor) / Chan, Candace (Committee member) / Ponce, Fernando (Committee member) / Holman, Zachary (Committee member) / Arizona State University (Publisher)
Created2018
Description
Gallium Nitride (GaN) based Current Aperture Vertical Electron Transistors (CAVETs) present many appealing qualities for applications in high power, high frequency devices. The wide bandgap, high carrier velocity of GaN make it ideal for withstanding high electric fields and supporting large currents. The vertical topology of the CAVET allows for more efficient die area utilization, breakdown scaling with the height of the device, and burying high electric fields in the bulk where they will not charge interface states that can lead to current collapse at higher frequency.
Though GaN CAVETs are promising new devices, they are expensive to develop due to new or exotic materials and processing steps. As a result, the accurate simulation of GaN CAVETs has become critical to the development of new devices. Using Silvaco Atlas 5.24.1.R, best practices were developed for GaN CAVET simulation by recreating the structure and results of the pGaN insulated gate CAVET presented in chapter 3 of [8].
From the results it was concluded that the best simulation setup for transfer characteristics, output characteristics, and breakdown included the following. For methods, the use of Gummel, Block, Newton, and Trap. For models, SRH, Fermi, Auger, and impact selb. For mobility, the use of GANSAT and manually specified saturation velocity and mobility (based on doping concentration). Additionally, parametric sweeps showed that, of those tested, critical CAVET parameters included channel mobility (and thus doping), channel thickness, Current Blocking Layer (CBL) doping, gate overlap, and aperture width in rectangular devices or diameter in cylindrical devices.
Though GaN CAVETs are promising new devices, they are expensive to develop due to new or exotic materials and processing steps. As a result, the accurate simulation of GaN CAVETs has become critical to the development of new devices. Using Silvaco Atlas 5.24.1.R, best practices were developed for GaN CAVET simulation by recreating the structure and results of the pGaN insulated gate CAVET presented in chapter 3 of [8].
From the results it was concluded that the best simulation setup for transfer characteristics, output characteristics, and breakdown included the following. For methods, the use of Gummel, Block, Newton, and Trap. For models, SRH, Fermi, Auger, and impact selb. For mobility, the use of GANSAT and manually specified saturation velocity and mobility (based on doping concentration). Additionally, parametric sweeps showed that, of those tested, critical CAVET parameters included channel mobility (and thus doping), channel thickness, Current Blocking Layer (CBL) doping, gate overlap, and aperture width in rectangular devices or diameter in cylindrical devices.
ContributorsWarren, Andrew (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2019
Description
Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared with traditional semiconductors such as Si. Efficient GaN based light-emitting diodes (LEDs) have increasingly displaced incandescent and fluorescent bulbs as the new major light sources for lighting and display. In addition, due to their large bandgap and high critical electrical field, WBG semiconductors are also ideal candidates for efficient power conversion.
In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters.
In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices.
In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters.
In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices.
ContributorsFu, Houqiang (Author) / Zhao, Yuji (Thesis advisor) / Vasileska, Dragica (Committee member) / Goodnick, Stephen (Committee member) / Yu, Hongbin (Committee member) / Wang, Liping (Committee member) / Arizona State University (Publisher)
Created2019
Description
This paper reviews several current designs of Cube Satellite (CubeSat) Electrical Power Systems (EPS) based on Silicon FET technologies and their current deficiencies, such as radiation-incurred defects and switching power losses. A strategy to fix these is proposed by the way of using Gallium Nitride (GaN) High Electron-Mobility Transistors (HEMTs) as switching devices within Buck/Boost Converters and other regulators. This work summarizes the EPS designs of several CubeSat missions, classifies them, and outlines their efficiency. An in-depth example of an EPS is also given, explaining the process in which these systems are designed. Areas of deficiency are explained along with reasoning as to why GaN can mitigate these losses, including its wide bandgap properties such as high RDS(on) and High Breakdown Voltage. Special design considerations must be kept in mind when using GaN HEMTs in this application and an example of a CubeSat using GaN HEMTs is mentioned. Finally, challenges ahead for GaN are explored including manufacturing considerations and long-term reliability.
ContributorsWilloughby, Alexander George (Author) / Kitchen, Jennifer (Thesis director) / Zhao, Yuji (Committee member) / Electrical Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2017-05
Description
GaN and AlGaN have shown great potential in next-generation power and RF electronics. However, these devices are limited by reliability issues such as leakage current and current collapse that result from surface and interface states on GaN and AlGaN. This dissertation, therefore, examined these electronic states, focusing on the following two points:
First, the surface electronic state configuration was examined with regards to the polarization bound 1013 charges/cm2 that increases with aluminum content. This large bound charge requires compensation either externally by surface states or internally by the space charge regions as relates to band bending. In this work, band bending was measured after different surface treatments of GaN and AlGaN to determine the effects of specific surface states on the electronic state configuration. Results showed oxygen-terminated N-face GaN, Ga-face GaN, and Ga-face Al0.25Ga0.75N surface were characterized by similar band bending regardless of the polarization bound charge, suggesting a Fermi level pinning state ~0.4-0.8 eV below the conduction band minimum. On oxygen-free Ga-face GaN, Al0.15Ga0.85N, Al0.25Ga0.75N, and Al0.35Ga0.65N, band bending increased slightly with aluminum content and thus did not exhibit the same pinning behavior; however, there was still significant compensating charge on these surfaces (~1013 charges/cm2). This charge is likely related to nitrogen vacancies and/or gallium dangling bonds.
In addition, this wozrk investigated the interface electronic state configuration of dielectric/GaN and AlGaN interfaces with regards to deposition conditions and aluminum content. Specifically, oxygen plasma-enhanced atomic layer deposited (PEALD) was used to deposit SiO2. Growth temperature was shown to influence the film quality, where room temperature deposition produced the highest quality films in terms of electrical breakdown. In addition, the valence band offsets (VBOs) appeared to decrease with the deposition temperature, which likely related to an electric field across the Ga2O3 interfacial layer. VBOs were also determined with respect to aluminum content at the PEALD-SiO2/AlxGa1-xN interface, giving 3.0, 2.9, 2.9, and 2.8 eV for 0%, 15%, 25%, and 35% aluminum content, respectively—with corresponding conduction band offsets of 2.5, 2.2, 1.9, and 1.8 eV. This suggests the largest difference manifests in the conduction band, which is in agreement with the charge neutrality level model.
First, the surface electronic state configuration was examined with regards to the polarization bound 1013 charges/cm2 that increases with aluminum content. This large bound charge requires compensation either externally by surface states or internally by the space charge regions as relates to band bending. In this work, band bending was measured after different surface treatments of GaN and AlGaN to determine the effects of specific surface states on the electronic state configuration. Results showed oxygen-terminated N-face GaN, Ga-face GaN, and Ga-face Al0.25Ga0.75N surface were characterized by similar band bending regardless of the polarization bound charge, suggesting a Fermi level pinning state ~0.4-0.8 eV below the conduction band minimum. On oxygen-free Ga-face GaN, Al0.15Ga0.85N, Al0.25Ga0.75N, and Al0.35Ga0.65N, band bending increased slightly with aluminum content and thus did not exhibit the same pinning behavior; however, there was still significant compensating charge on these surfaces (~1013 charges/cm2). This charge is likely related to nitrogen vacancies and/or gallium dangling bonds.
In addition, this wozrk investigated the interface electronic state configuration of dielectric/GaN and AlGaN interfaces with regards to deposition conditions and aluminum content. Specifically, oxygen plasma-enhanced atomic layer deposited (PEALD) was used to deposit SiO2. Growth temperature was shown to influence the film quality, where room temperature deposition produced the highest quality films in terms of electrical breakdown. In addition, the valence band offsets (VBOs) appeared to decrease with the deposition temperature, which likely related to an electric field across the Ga2O3 interfacial layer. VBOs were also determined with respect to aluminum content at the PEALD-SiO2/AlxGa1-xN interface, giving 3.0, 2.9, 2.9, and 2.8 eV for 0%, 15%, 25%, and 35% aluminum content, respectively—with corresponding conduction band offsets of 2.5, 2.2, 1.9, and 1.8 eV. This suggests the largest difference manifests in the conduction band, which is in agreement with the charge neutrality level model.
ContributorsEller, Brianna (Author) / Nemanich, Robert J (Thesis advisor) / Chowdhury, Srabanti (Committee member) / McCartney, Martha (Committee member) / Ponce, Fernando (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2015
Description
III-nitride InGaN light-emitting diodes (LEDs) enable wide range of applications in solid-state lighting, full-color displays, and high-speed visible-light communication. Conventional InGaN quantum well LEDs grown on polar c-plane substrate suffer from quantum confined Stark effect due to the large internal polarization-related fields, leading to a reduced radiative recombination rate and device efficiency, which limits the performance of InGaN LEDs in high-speed communication applications. To circumvent these negative effects, non-trivial-cavity designs such as flip-chip LEDs, metallic grating coated LEDs are proposed. This oral defense will show the works on the high-modulation-speed LEDs from basic ideas to applications. Fundamental principles such as rate equations for LEDs/laser diodes (LDs), plasmonic effects, Purcell effects will be briefly introduced. For applications, the modal properties of flip-chip LEDs are solved by implementing finite difference method in order to study the modulation response. The emission properties of highly polarized InGaN LEDs coated by metallic gratings are also investigated by finite difference time domain method.
ContributorsChen, Hong (Author) / Zhao, Yuji (Thesis advisor) / Yao, Yu (Committee member) / Wang, Liping (Committee member) / Arizona State University (Publisher)
Created2016
Description
Cubic boron nitride (c-BN) has potential for electronic applications as an electron emitter and serving as a base material for diodes, transistors, etc. However, there has been limited research on c-BN reported, and many of the electronic properties of c-BN and c-BN interfaces have yet to be reported. This dissertation focused on probing thin film c-BN deposited via plasma enhanced chemical vapor deposition (PECVD) with in situ photoelectron spectroscopy (PES). PES measurements were used to characterize the electronic properties of c-BN films and interfaces with vacuum and diamond. First, the interface between c-BN and vacuum were characterized with ultraviolet PES (UPS). UPS measurements indicated that as-deposited c-BN, H2 plasma treated c-BN, and annealed c-BN post H2 plasma treatment exhibited negative electron affinity surfaces. A dipole model suggested dipoles from H-terminated N surface sites were found to be responsible for the NEA surface. Then, Si was introduced into c-BN films to realize n-type doped c-BN. The valence structure and work function of c-BN:Si films were characterized with XPS and UPS measurements. Measurements were unable to confirm n-type character, and it is concluded that silicon nitride formation was the primary effect for the observations. Finally, XPS measurements were employed to measure the band offsets at the c-BN/diamond interface. Measurements indicated the valence band maximum (VBM) of c-BN was positioned ~0.8 eV above the VBM of diamond.
ContributorsShammas, Joseph (Author) / Nemanich, Robert J (Thesis advisor) / Ponce, Fernando (Committee member) / Chen, Tingyong (Committee member) / Chamberlin, Ralph (Committee member) / Arizona State University (Publisher)
Created2016