Theses, Dissertations and Student Research Papers (Physics)
http://hdl.handle.net/1957/18070
2014-12-20T05:58:07ZThe contact value approximation to the pair distribution function for an inhomogeneous hard sphere fluid
http://hdl.handle.net/1957/54549
The contact value approximation to the pair distribution function for an inhomogeneous hard sphere fluid
Lurie-Gregg, Paho
We construct the contact value approximation (CVA) for the pair distribution function,
g(²)(r₁, r₂), for an inhomogeneous hard sphere fluid. The CVA is an average of two radial
distribution functions, which each take as input the distance between the particles, |r₂ −r₁|,
and the average value of the radial distribution function at contact, gσ(r) at the locations
of each of the particles. In a recently published paper, an accurate function for gσ(r) was
developed, and it is made use of here. We then make a separable approximation to the
radial distribution function, gS(r), which we use to construct the separable contact value
approximation (CVA-S) to the pair distribution function.
We compare the CVA and CVA-S to Monte Carlo simulations that we have developed and
run as well as to two prior approximations to the pair distribution function. This comparison
is done in three main cases: When one particle is near a hard wall; when there is an external
particle the size of a sphere of the fluid; and for various integrals that illustrate typical use-cases
of the pair distribution function. We show reasonable quantitative agreement between
the CVA-S and simulation data, similar to that of the prior approximations. However, due
to its separable nature, the CVA-S can be efficiently used in density functional theory, which
is not the case of the prior approximations.
2014
2014-10-03T00:00:00ZMeasurement and Modeling of Zinc Sulfide Thin Films using Ellipsometry and Reflection Spectroscopy: A Comparison of Optical Characterization Techniques
http://hdl.handle.net/1957/53769
Measurement and Modeling of Zinc Sulfide Thin Films using Ellipsometry and Reflection Spectroscopy: A Comparison of Optical Characterization Techniques
Kratzer, Aaron
Zinc sulfide thin films on silicon wafers were analyzed for layer thickness, refractive index, and absorption using reflection spectroscopy (RS), spectroscopic ellipsometry (SE), and modeling programs. RS and refractive index values from literature were used to model film thickness based on reflection and the SCOUT modeling program was used to analyze the reflectance data and generate model RS data. SE was used to measure film thickness and complex index of refraction and a VASE32 program was used to model the layers of the thin film and generate model SE data. Goals include comparing SE and RS as possible non-destructive analysis tools and developing the most efficient SCOUT interface for analyzing available optical data.
Both RS and SE data analysis have the ability to measure the thickness d and complex refractive index n and κ of ZnS given the complex refractive index of the silicon wafer. The SCOUT optical modeling software generates a graphical user interface and can analyze RS data and can be configured to analyze SE data as well.
2014
2014-06-11T00:00:00ZGrowth and characterization of the p-type semiconductors SnS and BiCuOSe
http://hdl.handle.net/1957/39519
Growth and characterization of the p-type semiconductors SnS and BiCuOSe
Francis, Jason
BiCuOSe and SnS are layered, moderate band gap (ε[subscript G] ≈ 1 eV) semiconductors that exhibit intrinsic p type conductivity. Doping of BiCuOSe with Ca results in a slight expansion of the lattice and an increase of the hole concentration from 10¹⁸ cm⁻³ to greater than 10²⁰ cm⁻³. The large carrier density in undoped films is the result of copper vacancies. Mobility is unaffected by doping, remaining constant at 1.5 cm²V⁻¹s⁻¹ in both undoped and doped films, because the Bi-O layers serve as the source of carriers, while transport occurs within the Cu-Se layers. Bi possesses a 6s² lone pair that was expected to hybridize with the oxygen p states at the top of the valence band, resulting in high hole mobility as compared to similar materials such as LaCuOSe, which lack this lone pair. However, both LaCuOSe and BiCuOSe have similar hole mobility. X-ray absorption and emission spectroscopy, combined with density functional theory calculations, reveal that the Bi 6s states contribute deep within the valence band, forming bonding and anti-bonding states with O 2p at 11 eV and 3 eV below the valence band maximum, respectively. Hence, the Bi lone pair does not contribute at the top of the valence band and does not enhance the hole mobility. The Bi 6p states contribute at the bottom of the conduction band, resulting in a smaller band gap for BiCuOSe than LaCuOSe (1 eV vs. 3 eV). SnS is a potential photovoltaic absorber composed of weakly coupled layers stacked along the long axis. This weak coupling results in the formation of strongly oriented films on amorphous substrates. The optical band gap is 1.2 eV, in agreement with GW calculations. Absorption reaches 10⁵ cm⁻¹ within 0.5 eV of the band gap. The p type conduction arises from energetically favorable tin vacancies. Variation of growth conditions yields carrier densities of 10¹⁴ - 10¹⁶ cm⁻³ and hole mobility of 7 - 15 cm²V⁻¹s⁻¹. SnS was alloyed with rocksalt CaS, which was predicted to form a rocksalt structure when the calcium content is increased past 18%. Films of Sn[subscript 1-x]Ca[subscript x]S with x from 0.4 to 0.9 adopt the rocksalt structure with a band gap of 1.1-1.3 eV, with absorption greater than 10⁵ cm⁻¹ within about 0.7 eV of the band gap. The lattice contracts as the calcium content of the films is increased, reaching 5.7 Å when x = 0.93. Films are highly insulating, but Seebeck measurements do indicate p type conduction.
Graduation date: 2013
2013-05-30T00:00:00ZDesign and architecture of an improved microcomputer-controlled perturbed angular correlation spectrometer
http://hdl.handle.net/1957/37318
Design and architecture of an improved microcomputer-controlled perturbed angular correlation spectrometer
Stevens, Darren W.
Graduation date: 1992
1992-02-25T00:00:00Z