UMD Theses and Dissertations
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Item DFT AND RELATED MODELING OF POST-SILICON VALENCE 4 MATERIALS: SiC AND Ge(2020) Darmody, Christopher; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Though silicon (Si) is in many ways the material of choice for many electronic applications due in part to its mature processing technology, its intrinsic properties are not always suited for every challenge. Specialized high power and high temper- ature devices benefit from using semiconductors with a larger band-gap and higher thermal conductivity such as silicon carbide (SiC). Additionally, the 1.1eV bandgap of Si makes it unable to effectively absorb infrared photons so a material with a smaller bandgap, like germanium (Ge), is more suited to the task. Currently SiC power transistors are commercially available but suffer from poor channel mobility due to interface roughness which limits their performance. To predict the maximum theoretically achievable mobility for different crystallo- graphic interfaces I developed a novel technique for extracting an atomic-roughness scattering rate from an arbitrary atomic surface. The term atomic-roughness here means an interface purely due to the variation of atom species and position without the presence of a crystallographic miscut due to epitaxial growth considerations. I used Density Functional Theory (DFT) to obtain a perturbation potential from which I can calculate a scattering rate. This scattering rate can then be used in a Monte Carlo simulation to predict mobility for a given field configuration. In addition to SiC’s low channel mobility, SiC p-type dopant species also ex- hibit an abnormally large ionization energy compared to its n-type dopants and to the primary dopants in many other semiconductors. This fact can cause is- sues such as unexpectedly high resistance regions at lower operating temperatures - causing the need to dope at significantly higher concentration. To characterize the incomplete ionization fraction p/N A , I first gathered nearly all existing pub- lished data on the ionization energy of aluminum (Al) in 4H-SiC and created an empirical concentration-dependent model of this function. Then I put together a physics-based model of the entire acceptor and valence band system and used my concentration-dependent ionization energy as an input to predict p/N A . I verify my physics-based model result against a separate experimental dataset derived from nearly-exhaustive literature measurements of Hall mobility and resistivity. Finally, I transform fully temperature-dependent result of p/N A from a complex numerical computation to a more easily implementable parameterized function with the use of a genetic algorithm. The remaining part of my work was performed on Germanium which has interesting application in short-wave infrared imaging due its 0.66eV indirect and 0.85 eV direct bandgaps, which corresponds closely to the peak illumination of the “night glow” at 0.75 eV. Optical devices greatly benefit from direct gap band structures to increase photon absorption and emission efficiency. Though Ge is an indirect gap material, it can be alloyed with a direct gap material, namely tin (Sn), to transition it to a direct gap material at a certain molar fraction. Through DFT calculations I investigate the nature of this transition and determine theoretically the minimum molar fraction needed to achieve a direct bandgap.Item Modeling Key Issues in Post Silicon Semiconductors: Germanium and Gallium Nitride(2018) Xiao, Ziyang; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We are rapidly approaching the end of the semiconductor roadmap with respect to silicon. To continue its growth, the semiconductor industry is therefore looking into new materials. Two primary materials that are of interest for continuing semiconductor development are germanium (Ge) and gallium nitride (GaN). Ge is of interest as a replacement for silicon, in an effort to improve electronics performances because of its high mobility and its ability to grow a native oxide. In addition, Ge is of interest because of its potential use for economical CMOS-based short wave infrared (SWIR) imaging systems. GaN is a nascent wide bandgap semiconductor and has many potential applications in high power electronics and ultraviolet imaging systems. In this thesis, the key material properties and applications of these two ”end of the roadmap” semiconductors are explored. Ge is a semiconductor material with an indirect bandgap of 0.66eV. This bandgap value corresponds to a wavelength of 1.88μm, which lies in the infrared range. The Ge material itself is also compatible with the standard Si CMOS process technology. Because of these advantages, Ge is considered a candidate for the application of photo detecting in the SWIR range. Apart from the indirect bandgap of 0.66eV, Ge also has a direct bandgap of 0.8eV. From early research, the relatively small offset between the indirect and direct bandgaps can be inverted either by applying strain[1, 9, 10, 11] or alloying with tin. GaN is a binary direct wide bandgap material with a direct bandgap of 3.4eV. It has a high breakdown field, and relatively high saturation velocity and carrier mobility. These properties give GaN an advantage in the realm of high power application. GaN can also form a heterostructure with AlGaN, which can give rise to a 2D electron gas (2DEG) layer at the interface without intentionally doping either material. The 2DEG layer has an even higher mobility when compared to the mobility of the bulk GaN, which allows the heterostructure to be utilized for the design of high electron mobility transistors (HEMTs). The formation of the 2DEG layer also gives rise to potential well confinement at the heterostructure interface. The width of the potential well is only a few nanometers, making the interface electron gas subject to quantum confinement along the direction perpendicular to the interface. The detailed shape of the potential well is determined by the configuration of the heterostructure, as well as the applied voltage across the heterostructure. The first set of goals for this research is to investigate how the bandstructure of Ge changes: Part (1) with the applied strain, and Part (2) with alloyed tin (Sn). The empirical pseudopotential method (EPM) was utilized for the band structure calculation, together with the rules for strain translation for the investigation of Part (1). In Part (1), simulation results give the optimal orientation for different types of applied strain and also thoroughly map the influence of strain applied on any arbitrary orientations. It also reveals that for biaxial strain, there exists another orientation that is more robust against misalignment with respect to the originally desired orientation than the optimal plane, with little compromise of bandgap and slightly higher requirements for the sufficient strain. For Part (2), EPM is combined with perturbation theory for the inclusion of the influence of the Sn atoms in the Ge lattice. A new and computationally inexpensive method is developed during the research. Simulation results agree significantly when compared to reported experimental measurements, indicating the capability of the method. The second set of goals is to investigate the electron transport properties of the 2DEG layer at the interface of GaN HEMT and related power transistors. The potential well is approximated and quantified by a triangular potential well and the carrier sheet density is kept the same during the approximation. Thorough simulations are conducted by calculating the band alignment of the heterostructure with different structural configurations. A fixed correlation between the carrier sheet density and the shape of the potential well (slope of the triangular potential well and the height of the well) is revealed. This correlation is used as an input for the Monte Carlo (MC) simulation. The changes to the mobility of the electrons at the 2DEG layer with changing interface potential well shape are investigated and statis- tics of drift velocity, electron energy, and valley occupation are collected. Mobility information is also extracted and compares favorably with reported experimental measurements. The simulation results are used in the device simulations, which compares the performances of two GaN/AlGaN heterostructure based devices: a lateral HEMT and a current aperture vertical electron transistor (CAVET).