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- All Subjects: Gallium Nitride
- Creators: Nemanich, Robert
Description
In recent years, there has been increased interest in the Indium Gallium Nitride (InGaN) material system for photovoltaic (PV) applications. The InGaN alloy system has demonstrated high performance for high frequency power devices, as well as for optical light emitters. This material system is also promising for photovoltaic applications due to broad range of bandgaps of InxGa1-xN alloys from 0.65 eV (InN) to 3.42 eV (GaN), which covers most of the electromagnetic spectrum from ultraviolet to infrared wavelengths. InGaN’s high absorption coefficient, radiation resistance and thermal stability (operating with temperature > 450 ℃) makes it a suitable PV candidate for hybrid concentrating solar thermal systems as well as other high temperature applications. This work proposed a high efficiency InGaN-based 2J tandem cell for high temperature (450 ℃) and concentration (200 X) hybrid concentrated solar thermal (CSP) application via numerical simulation. In order to address the polarization and band-offset issues for GaN/InGaN hetero-solar cells, band-engineering techniques are adopted and a simple interlayer is proposed at the hetero-interface rather than an Indium composition grading layer which is not practical in fabrication. The base absorber thickness and doping has been optimized for 1J cell performance and current matching has been achieved for 2J tandem cell design. The simulations also suggest that the issue of crystalline quality (i.e. short SRH lifetime) of the nitride material system to date is a crucial factor limiting the performance of the designed 2J cell at high temperature. Three pathways to achieve ~25% efficiency have been proposed under 450 ℃ and 200 X. An anti-reflection coating (ARC) for the InGaN solar cell optical management has been designed. Finally, effective mobility model for quantum well solar cells has been developed for efficient quasi-bulk simulation.
ContributorsFang, Yi, Ph.D (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Thesis advisor) / Ponce, Fernando (Committee member) / Nemanich, Robert (Committee member) / Arizona State University (Publisher)
Created2017
Description
Gallium Nitride (GaN), being a wide-bandgap semiconductor, shows its advantage over the conventional semiconductors like Silicon and Gallium Arsenide for high temperature applications, especially in the temperature range from 300°C to 600°C. Development of stable ohmic contacts to GaN with low contact resistivity has been identified as a prerequisite to the success of GaN high temperature electronics. The focus of this work was primarily derived from the requirement of an appropriate metal contacts to work with GaN-based hybrid solar cell operating at high temperature.
Alloyed Ti/Al/Ni/Au contact and non-alloyed Al/Au contact were developed to form low-resistivity contacts to n-GaN and their stability at high temperature were studied. The alloyed Ti/Al/Ni/Au contact offered a specific contact resistivity (ρc) of 6×10-6 Ω·cm2 at room temperature measured the same as the temperature increased to 400°C. No significant change in ρc was observed after the contacts being subjected to 400°C, 450°C, 500°C, 550°C, and 600°C, respectively, for at least 4 hours in air. Since several device technology prefer non-alloyed contacts Al/Au metal stack was applied to form the contacts to n-type GaN. An initial ρc of 3×10-4 Ω·cm2, measured after deposition, was observed to continuously reduce under thermal stress at 400°C, 450°C, 500°C, 550°C, and 600°C, respectively, finally stabilizing at 5×10-6 Ω·cm2. Both the alloyed and non-alloyed metal contacts showed exceptional capability of stable operation at temperature as high as 600°C in air with low resistivity ~10-6 Ω·cm2, with ρc lowering for the non-alloyed contacts with high temperatures.
The p-GaN contacts showed remarkably superior ohmic behavior at elevated temperatures. Both ρc and sheet resistance (Rsh) of p-GaN decreased by a factor of 10 as the ambient temperature increased from room temperature to 390°C. The annealed Ni/Au contact showed ρc of 2×10-3 Ω·cm2 at room temperature, reduced to 1.6×10-4 Ω·cm2 at 390°C. No degradation was observed after the contacts being subjected to 450°C in air for 48 hours. Indium Tin Oxide (ITO) contacts, which has been widely used as current spreading layer in GaN-base optoelectronic devices, measured an initial ρc [the resistivity of the ITO/p-GaN interface, since the metal/ITO ρc is negligible] of 1×10-2 Ω·cm2 at room temperature. No degradation was observed after the contact being subjected to 450°C in air for 8 hours.
Accelerated life testing (ALT) was performed to further evaluate the contacts stability at high temperatures quantitatively. The ALT results showed that the annealed Ni/Au to p-GaN contacts is more stable in nitrogen ambient, with a lifetime of 2,628 hours at 450°C which is approximately 12 times longer than that at 450°C in air.
Alloyed Ti/Al/Ni/Au contact and non-alloyed Al/Au contact were developed to form low-resistivity contacts to n-GaN and their stability at high temperature were studied. The alloyed Ti/Al/Ni/Au contact offered a specific contact resistivity (ρc) of 6×10-6 Ω·cm2 at room temperature measured the same as the temperature increased to 400°C. No significant change in ρc was observed after the contacts being subjected to 400°C, 450°C, 500°C, 550°C, and 600°C, respectively, for at least 4 hours in air. Since several device technology prefer non-alloyed contacts Al/Au metal stack was applied to form the contacts to n-type GaN. An initial ρc of 3×10-4 Ω·cm2, measured after deposition, was observed to continuously reduce under thermal stress at 400°C, 450°C, 500°C, 550°C, and 600°C, respectively, finally stabilizing at 5×10-6 Ω·cm2. Both the alloyed and non-alloyed metal contacts showed exceptional capability of stable operation at temperature as high as 600°C in air with low resistivity ~10-6 Ω·cm2, with ρc lowering for the non-alloyed contacts with high temperatures.
The p-GaN contacts showed remarkably superior ohmic behavior at elevated temperatures. Both ρc and sheet resistance (Rsh) of p-GaN decreased by a factor of 10 as the ambient temperature increased from room temperature to 390°C. The annealed Ni/Au contact showed ρc of 2×10-3 Ω·cm2 at room temperature, reduced to 1.6×10-4 Ω·cm2 at 390°C. No degradation was observed after the contacts being subjected to 450°C in air for 48 hours. Indium Tin Oxide (ITO) contacts, which has been widely used as current spreading layer in GaN-base optoelectronic devices, measured an initial ρc [the resistivity of the ITO/p-GaN interface, since the metal/ITO ρc is negligible] of 1×10-2 Ω·cm2 at room temperature. No degradation was observed after the contact being subjected to 450°C in air for 8 hours.
Accelerated life testing (ALT) was performed to further evaluate the contacts stability at high temperatures quantitatively. The ALT results showed that the annealed Ni/Au to p-GaN contacts is more stable in nitrogen ambient, with a lifetime of 2,628 hours at 450°C which is approximately 12 times longer than that at 450°C in air.
ContributorsZhao, Shirong (Author) / Chowdhury, Srabanti (Thesis advisor) / Goodnick, Stephen (Committee member) / Zhao, Yuji (Committee member) / Nemanich, Robert (Committee member) / Arizona State University (Publisher)
Created2016
Description
Gallium Nitride (GaN) is uniquely suited for Radio Frequency (RF) and power electronic applications due to its intrinsically high saturation velocity and high mobility compared to Silicon and Silicon Carbide (SiC). High Electron Mobility Transistors (HEMTs) have remained the primary topology for GaN transistors in RF applications. However, GaN HEMTs suffer from a variety of issues such as current crowding, lack of enhancement mode (E-Mode) operation and non-linearity. These drawbacks slow the widespread adoption of GaN devices for ultra-low voltage (ULV) applications such as voltage regulators, automotive and computing applications. E-mode operation is especially desired in low-voltage high frequency switching applications. In this context, Fin Field Effect Transistors (FinFETs) offer an alternative topology for ULV applications as opposed to conventional HEMTs. Recent advances in material processing, high aspect ratio epitaxial growth and etching methods has led to an increased interest in 3D nanostructures such as Nano-FinFETs and Nanowire FETs. A typical 3D nano-FinFET is the AlGaN/GaN Metal Insulator Semiconductor (MIS) FET wherein a layer of Al2O3 surrounds the AlGaN/GaN fin. The presence of the side gates leads to additional lateral confinement of the 2D Electron Gas (2DEG). Theoretical calculations of transport properties in confined systems such as AlGaN/GaN Finfets are scarce compared to those of their planar HEMT counterparts.
A novel simulator is presented in this dissertation, which employs self-consistent solution of the coupled 1D Boltzmann – 2D Schrödinger – 3D Poisson problem, to yield the channel electrostatics and the low electric field transport characteristics of AlGaN/GaN MIS FinFETs. The low field electron mobility is determined by solving the Boltzmann transport equation in the Quasi-1D region using 1D Ensemble Monte Carlo method. Three electron-phonon scattering mechanisms (acoustic, piezoelectric and polar optical phonon scattering) and interface roughness scattering at the AlGaN/GaN interface are considered in this theoretical model. Simulated low-field electron mobility and its temperature dependence are in agreement with experimental data reported in the literature.
A quasi-1D version of alloy clustering model is derived and implemented and the limiting effect of alloy clustering on the low-field electron mobility is investigated for the first time for MIS FinFET device structures.
ContributorsKumar, Viswanathan Naveen (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Nemanich, Robert (Committee member) / Povolotskyi, Michael (Committee member) / Esqueda, Ivan Sanchez (Committee member) / Arizona State University (Publisher)
Created2022
Description
Wide bandgap semiconductors are of much current interest due to their superior electrical properties. This dissertation describes electron microscopy characterization of GaN-on-GaN structures for high-power vertical device applications. Unintentionally-doped (UID) GaN layers grown homoepitaxially via metal-organic chemical vapor deposition on freestanding GaN substrates, were subjected to dry etching, and layers of UID-GaN/p-GaN were over-grown. The as-grown and regrown heterostructures were examined in cross-section using transmission electron microscopy (TEM). Two different etching treatments, fast-etch-only and multiple etches with decreasing power, were employed. The fast-etch-only devices showed GaN-on-GaN interface at etched location, and low device breakdown voltages were measured (~ 45-95V). In comparison, no interfaces were visible after multiple etching steps, and the corresponding breakdown voltages were much higher (~1200-1270V). These results emphasized importance of optimizing surface etching techniques for avoiding degraded device performance.
The morphology of GaN-on-GaN devices after reverse-bias electrical stressing to breakdown was investigated. All failed devices had irreversible structural damage, showing large surface craters (~15-35 microns deep) with lengthy surface cracks. Cross-sectional TEM of failed devices showed high densities of threading dislocations (TDs) around the cracks and near crater surfaces. Progressive ion-milling across damaged devices revealed high densities of TDs and the presence of voids beneath cracks: these features were not observed in unstressed devices. The morphology of GaN substrates grown by hydride vapor-phase epitaxy (HVPE) and by ammonothermal methods were correlated with reverse-bias results. HVPE substrates showed arrays of surface features when observed by X-ray topography (XRT). All fabricated devices that overlapped with these features had typical reverse-bias voltages less than 100V at a leakage current limit of 10-6 A. In contrast, devices not overlapping with such features reached voltages greater than 300V. After etching, HVPE substrate surfaces showed defect clusters and macro-pits, whereas XRT images of ammonothermal substrate revealed no visible features. However, some devices fabricated on ammonothermal substrate failed at low voltages. Devices on HVPE and ammonothermal substrates with low breakdown voltages showed crater-like surface damage and revealed TDs (~25µm deep) and voids; such features were not observed in devices reaching higher voltages. These results should assist in developing protocols to fabricate reliable high-voltage devices.
ContributorsPeri, Prudhvi Ram (Author) / Smith, David J. (Thesis advisor) / Alford, Terry (Committee member) / Mccartney, Martha R (Committee member) / Nemanich, Robert (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2021