Matching Items (38)
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
This research has studied remote plasma enhanced atomic layer deposited Ga2O3 thin films with gallium acetylacetonate (Ga(acac)3) as Ga precursor and remote inductively coupled oxygen plasma as oxidizer. The Ga2O3 thin films were mainly considered as passivation layers on GaN. Growth conditions including Ga(acac)3 precursor pulse time, O2 plasma pulse time, N2 purge time and deposition temperature were investigated and optimized on phosphorus doped Si (100) wafer to achieve a saturated self-limiting growth. A temperature growth window was observed between 150 ℃ and 320 ℃. Ga precursor molecules can saturate on the substrate surface in 0.6 s in one cycle and the plasma power saturates at 150 W. A growth rate of 0.31 Å/cycle was observed for PEALD Ga2O3. Since the study is devoted towards Ga2O3 working as passivation layer on GaN, the band alignment of Ga2O3 on GaN were further determined with X-ray Photoemission Spectroscopy and Ultraviolet Photoemission Spectroscopy. Two models are often used to decide the band alignment of a heterojunction: the electron affinity model assumes the heterojunction aligns at the vacuum level, and the charge neutrality level model (CNL) which considers the presence of an interface dipole. The conduction band offset (CBO), valence band offset (VBO) and band bending (BB) of PEALD Ga2O3 thin films on GaN were 0.1 ±0.2 eV, 1.0±0.2 eV and 0.3 eV respectively. Type-I band alignments were determined. Further study including using PEALD Ga2O3 as passivation layer on GaN MOS gate and applying atomic layer etching to GaN was described.
ContributorsHao, Mei (Author) / Nemanich, Robert J. (Thesis advisor) / Ponce, Fernando (Committee member) / Chamberlin, Ralph (Committee member) / Chowdhury, Srabanti (Committee member) / Arizona State University (Publisher)
Created2018
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
Nanoscale semiconductors with their unique properties and potential applications have been a focus of extensive research in recent years. There are many ways in which semiconductors change the world with computers, cell phones, and solar panels, and nanoscale semiconductors having a promising potential to expand the efficiency, reduce the cost, and improve the flexibility and durability of their design. In this study, theoretical quantum mechanical simulations were performed on several different nanoscale semiconductor materials, including graphene/phosphorene nanoribbons and group III-V nanowires. First principles density functional theory (DFT) was used to study the electronic and structural properties of these nanomaterials in their fully relaxed and strained states. The electronic band gap, effective masses of charge carriers, electronic orbitals, and density of states were most commonly examined with strain, both from intrinsic and external sources. For example, armchair graphene nanoribbons (AGNR) were found to have unprecedented band gap-strain dependence. Phosphorene nanoribbons (PNRs) demonstrate a different behavior, including a chemical scissors effect, and studies revealed a strong relationship between passivation species and band gap tunability. Unlike the super mechanical flexibility of AGNRs and PNRs which can sustain incredible strain, modest yet large strain was applied to group III-V nanowires such as GaAs/InAs. The calculations showed that a direct and indirect band gap transition occurs at some critical strains and the origination of these gap transitions were explored in detail. In addition to the pure nanowires, GaAs/InAs core/shell heterostructure nanowires were also studied. Due to the lattice mismatch between GaAs and InAs, the intrinsic strain in the core/shell nanowires demonstrates an interesting behavior on tuning the electronic properties. This interesting behavior suggests a mechanical way to exert compressive strain on nanowires experimentally, and can create a finite quantum confinement effect on the core.
ContributorsCopple, Andrew (Author) / Peng, Xihong (Thesis advisor) / Chan, Candace (Committee member) / Chizmeshya, Andrew (Committee member) / Ponce, Fernando (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
Description
Nuclear proliferation concerns have resulted in a desire for radiation detectors with superior energy resolution. In this dissertation a Monte Carlo code is developed for calculating energy resolution in gamma-ray detector materials. The effects of basic material properties such as the bandgap and plasmon resonance energy are studied using a model for inelastic electron scattering based on electron energy-loss spectra. From a simplified "toy model" for a generic material, energy resolution is found to oscillate as the plasmon resonance energy is increased, and energy resolution can also depend on the valence band width. By incorporating the model developed here as an extension of the radiation transport code Penelope, photon processes are also included. The enhanced version of Penelope is used to calculate the Fano factor and average electron-hole pair energy in semiconductors silicon, gallium arsenide, zinc telluride, and scintillators cerium fluoride and lutetium oxyorthosilicate (LSO). If the effects of the valence band density-of-states and phonon scattering are removed, the calculated energy-resolution for these materials is fairly close to that for a toy model with a uniform electron energy-loss probability density function. This implies that the details of the electron cascade may in some cases have only a marginal effect on energy resolution.
ContributorsNarayan, Raman (Author) / Rez, Peter (Thesis advisor) / Spence, John (Committee member) / Ponce, Fernando (Committee member) / Comfort, Joseph (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2011
Description
As the world energy demand increases, semiconductor devices with high energy conversion efficiency become more and more desirable. The energy conversion consists of two distinct processes, namely energy generation and usage. In this dissertation, novel multi-junction solar cells and light emitting diodes (LEDs) are proposed and studied for high energy conversion efficiency in both processes, respectively. The first half of this dissertation discusses the practically achievable energy conversion efficiency limit of solar cells. Since the demonstration of the Si solar cell in 1954, the performance of solar cells has been improved tremendously and recently reached 41.6% energy conversion efficiency. However, it seems rather challenging to further increase the solar cell efficiency. The state-of-the-art triple junction solar cells are analyzed to help understand the limiting factors. To address these issues, the monolithically integrated II-VI and III-V material system is proposed for solar cell applications. This material system covers the entire solar spectrum with a continuous selection of energy bandgaps and can be grown lattice matched on a GaSb substrate. Moreover, six four-junction solar cells are designed for AM0 and AM1.5D solar spectra based on this material system, and new design rules are proposed. The achievable conversion efficiencies for these designs are calculated using the commercial software package Silvaco with real material parameters. The second half of this dissertation studies the semiconductor luminescence refrigeration, which corresponds to over 100% energy usage efficiency. Although cooling has been realized in rare-earth doped glass by laser pumping, semiconductor based cooling is yet to be realized. In this work, a device structure that monolithically integrates a GaAs hemisphere with an InGaAs/GaAs quantum-well thin slab LED is proposed to realize cooling in semiconductor. The device electrical and optical performance is calculated. The proposed device then is fabricated using nine times photolithography and eight masks. The critical process steps, such as photoresist reflow and dry etch, are simulated to insure successful processing. Optical testing is done with the devices at various laser injection levels and the internal quantum efficiency, external quantum efficiency and extraction efficiency are measured.
ContributorsWu, Songnan (Author) / Zhang, Yong-Hang (Thesis advisor) / Menéndez, Jose (Committee member) / Ponce, Fernando (Committee member) / Belitsky, Andrei (Committee member) / Schroder, Dieter (Committee member) / Arizona State University (Publisher)
Created2010
Description
4H-SiC has been widely used in many applications. All of these benefit from its extremely high critical electric field and good electron mobility. For example, 4H-SiC possesses a critical field ten times higher than that of Si, which allows high-voltage blocking layers composed of 4H-SiC to be approximately a tenth the thickness of a comparable Si device. This, in turn, reduces the device on-resistance and power losses while maintaining the same high blocking capability.
Unfortunately, commercial TCAD tools like Sentaurus and Silvaco Atlas are based on the effective mass approximation, while most 4H-SiC devices are not operated under low electric field, so the parabolic-like band approximation does not hold anymore. Hence, to get more accurate and reliable simulation results, full-band analysis is needed. The first step in the development of a full-band device simulator is the calculation of the band structure. In this work, the empirical pseudopotential method (EPM) is adopted. The next task in the sequence is the calculation of the scattering rates. Acoustic, non-polar optical phonon, polar optical phonon and Coulomb scattering are considered. Coulomb scattering is treated in real space using the particle-particle-particle-mesh (P3M) approach. The third task is coupling the bulk full-band solver with a 3D Poisson equation solver to generate a full-band device simulator.
For proof-of-concept of the methodology adopted here, a 3D resistor is simulated first. From the resistor simulations, the low-field electron mobility dependence upon Coulomb scattering in 4H-SiC devices is extracted. The simulated mobility results agree very well with available experimental data. Next, a 3D VDMOS is simulated. The nature of the physical processes occurring in both steady-state and transient conditions are revealed for the two generations of 3D VDMOS devices being considered in the study.
Due to its comprehensive nature, the developed tool serves as a basis for future investigation of 4H-SiC power devices.
Unfortunately, commercial TCAD tools like Sentaurus and Silvaco Atlas are based on the effective mass approximation, while most 4H-SiC devices are not operated under low electric field, so the parabolic-like band approximation does not hold anymore. Hence, to get more accurate and reliable simulation results, full-band analysis is needed. The first step in the development of a full-band device simulator is the calculation of the band structure. In this work, the empirical pseudopotential method (EPM) is adopted. The next task in the sequence is the calculation of the scattering rates. Acoustic, non-polar optical phonon, polar optical phonon and Coulomb scattering are considered. Coulomb scattering is treated in real space using the particle-particle-particle-mesh (P3M) approach. The third task is coupling the bulk full-band solver with a 3D Poisson equation solver to generate a full-band device simulator.
For proof-of-concept of the methodology adopted here, a 3D resistor is simulated first. From the resistor simulations, the low-field electron mobility dependence upon Coulomb scattering in 4H-SiC devices is extracted. The simulated mobility results agree very well with available experimental data. Next, a 3D VDMOS is simulated. The nature of the physical processes occurring in both steady-state and transient conditions are revealed for the two generations of 3D VDMOS devices being considered in the study.
Due to its comprehensive nature, the developed tool serves as a basis for future investigation of 4H-SiC power devices.
ContributorsCheng, Chi-Yin (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen M (Thesis advisor) / Ponce, Fernando (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2020
Description
Rare-earth tritellurides (RTe3) are two-dimensional materials with unique quantum properties, ideal for investigating quantum phenomena and applications in supercapacitors, spintronics, and twistronics. This dissertation examines the electronic, magnetic, and phononic properties of the RTe3 family, exploring how these can be controlled using chemical pressure, cationic alloying, and external pressure.The impact of chemical pressure on RTe3 phononic properties was investigated through noninvasive micro-Raman spectroscopy, demonstrating the potential of optical measurements for determining charge density wave (CDW) transition temperatures. Cationic alloying studies showed seamless tuning of CDW transition temperatures by modifying lattice constants and revealed complex magnetism in alloyed RTe3 with multiple magnetic transitions.
A comprehensive external pressure study examined the influence of spacing between RTe3 layers on phononic and CDW properties across the RTe3 family. Comparisons between different RTe3 materials showed LaTe3, with the largest thermodynamic equilibrium interlayer spacing (smallest chemical pressure), has the most stable CDW phases at high pressures. Conversely, CDW phases in late RTe3 systems with larger internal chemical pressures were more easily suppressed by applied pressure.
The dissertation also investigated Schottky barrier realignment at RTe3/semiconductor interfaces induced by CDW transitions, revealing changes in Schottky barrier height and ideality factor around the CDW transition temperature. This indicates that chemical potential changes of RTe3 below the CDW transition temperature influence Schottky junction properties, enabling CDW state probing through interface property measurements.
A detailed experimental and theoretical analysis of the oxidation process of RTe3 compounds was performed, which revealed faster degradation in late RTe3 systems. Electronic property changes, like CDW transition temperature and chemical potential, are observed as degradation progresses. Quantum mechanical simulations suggested that degradation primarily results from strong oxidizing reactions with O2 molecules, while humidity (H2O) plays a negligible role unless Te vacancies exist.
Lastly, the dissertation establishes a large-area thin film deposition at relatively low temperatures using a soft sputtering technique. While focused on MoTe2 deposition, this technique may also apply to RTe3 thin film deposition.
Overall, this dissertation expands the understanding of the fundamental properties of RTe3 materials and lays the groundwork for potential device applications.
ContributorsYumigeta, Kentaro (Author) / Tongay, Sefaattin (Thesis advisor) / Ponce, Fernando (Committee member) / Drucker, Jeffery (Committee member) / Erten, Onur (Committee member) / Arizona State University (Publisher)
Created2023