Sputtering of the gallium-indium eutectic alloy in the liquid phase

We have measured sputtering yields and angular distributions of sputtered atoms from both the solid and liquid phases of gallium, indium, and the gallium-indium eutectic alloy. This was done by Rutherford backscattering analysis of graphite collector foils. The solid eutectic target shows a predominance of indium crystallites on its surface which have to be sputtered away before the composition of the sputtered atoms equals the bulk target composition. The size of the crystallites depends upon the conditions under which the alloy is frozen. The sputtering of the liquid eutectic alloy by 15 keV Ar⁺ results in a ratio of indium to gallium sputtering yields which is 28 times greater than would be expected from the target stoichiometry. Furthermore, the angular distribution of gallium is much more sharply peaked about the normal to the target surface than the indium distribution. When the incident Ar⁺ energy is increased to 25 keV, the gallium distribution broadens to the same shape as the indium distribution. With the exception of the sharp gallium distribution taken from the liquid eutectic at 15 keV, all angular distributions from liquid targets fit a cos² θ function. An ion-scattering-spectroscopy analysis of the liquid eutectic alloy reveals a surface layer of almost pure indium. A thermodynamic explanation for this highly segregated layer is discussed. The liquid eutectic alloy provides us with a unique target system which allows us to estimate the fraction of sputtered material which comes from the first monolayer of the surface.

New technologies driving decade-bandwidth radio astronomy : quad-ridged flared horn and compound-semiconductor LNAs

Among the branches of astronomy, radio astronomy is unique in that it spans the largest portion of the electromagnetic spectrum, e.g., from about 10 MHz to 300 GHz. On the other hand, due to scientific priorities as well as technological limitations, radio astronomy receivers have traditionally covered only about an octave bandwidth. This approach of "one specialized receiver for one primary science goal" is, however, not only becoming too expensive for next-generation radio telescopes comprising thousands of small antennas, but also is inadequate to answer some of the scientific questions of today which require simultaneous coverage of very large bandwidths.

This thesis presents significant improvements on the state of the art of two key receiver components in pursuit of decade-bandwidth radio astronomy: 1) reflector feed antennas; 2) low-noise amplifiers on compound-semiconductor technologies. The first part of this thesis introduces the quadruple-ridged flared horn, a flexible, dual linear-polarization reflector feed antenna that achieves 5:1-7:1 frequency bandwidths while maintaining near-constant beamwidth. The horn is unique in that it is the only wideband feed antenna suitable for radio astronomy that: 1) can be designed to have nominal 10 dB beamwidth between 30 and 150 degrees; 2) requires one single-ended 50 Ohm low-noise amplifier per polarization. Design, analysis, and measurements of several quad-ridged horns are presented to demonstrate its feasibility and flexibility.

The second part of the thesis focuses on modeling and measurements of discrete high-electron mobility transistors (HEMTs) and their applications in wideband, extremely low-noise amplifiers. The transistors and microwave monolithic integrated circuit low-noise amplifiers described herein have been fabricated on two state-of-the-art HEMT processes: 1) 35 nm indium phosphide; 2) 70 nm gallium arsenide. DC and microwave performance of transistors from both processes at room and cryogenic temperatures are included, as well as first-reported measurements of detailed noise characterization of the sub-micron HEMTs at both temperatures. Design and measurements of two low-noise amplifiers covering 1--20 and 8—50 GHz fabricated on both processes are also provided, which show that the 1--20 GHz amplifier improves the state of the art in cryogenic noise and bandwidth, while the 8--50 GHz amplifier achieves noise performance only slightly worse than the best published results but does so with nearly a decade bandwidth.

Investigation of bulk indium antimonide as heterodyne detector for the submillimeter wavelength region

Bulk n-lnSb is investigated at a heterodyne detector for the submillimeter wavelength region. Two modes or operation are investigated: (1) the Rollin or hot electron bolometer mode (zero magnetic field), and (2) the Putley mode (quantizing magnetic field). The highlight of the thesis work is the pioneering demonstration or the Putley mode mixer at several frequencies. For example, a double-sideband system noise temperature of about 510K was obtained using a 812 GHz methanol laser for the local oscillator. This performance is at least a factor or 10 more sensitive than any other performance reported to date at the same frequency. In addition, the Putley mode mixer achieved system noise temperatures of 250K at 492 GHz and 350K at 625 GHz. The 492 GHz performance is about 50% better and the 625 GHz is about 100% better than previous best performances established by the Rollin-mode mixer. To achieve these results, it was necessary to design a totally new ultra-low noise, room-temperature preamp to handle the higher source impedance imposed by the Putley mode operation. This preamp has considerably less input capacitance than comparably noisy, ambient designs.

In addition to advancing receiver technology, this thesis also presents several novel results regarding the physics of n-lnSb at low temperatures. A Fourier transform spectrometer was constructed and used to measure the submillimeter wave absorption coefficient of relatively pure material at liquid helium temperatures and in zero magnetic field. Below 4.2K, the absorption coefficient was found to decrease with frequency much faster than predicted by Drudian theory. Much better agreement with experiment was obtained using a quantum theory based on inverse-Bremmstrahlung in a solid. Also the noise of the Rollin-mode detector at 4.2K was accurately measured and compared with theory. The power spectrum is found to be well fit by a recent theory of non- equilibrium noise due to Mather. Surprisingly, when biased for optimum detector performance, high purity lnSb cooled to liquid helium temperatures generates less noise than that predicted by simple non-equilibrium Johnson noise theory alone. This explains in part the excellent performance of the Rollin-mode detector in the millimeter wavelength region.

Again using the Fourier transform spectrometer, spectra are obtained of the responsivity and direct detection NEP as a function of magnetic field in the range 20-110 cm^-1. The results show a discernable peak in the detector response at the conduction electron cyclotron resonance frequency tor magnetic fields as low as 3 KG at bath temperatures of 2.0K. The spectra also display the well-known peak due to the cyclotron resonance of electrons bound to impurity states. The magnitude of responsivity at both peaks is roughly constant with magnet1c field and is comparable to the low frequency Rollin-mode response. The NEP at the peaks is found to be much better than previous values at the same frequency and comparable to the best long wavelength results previously reported. For example, a value NEP=4.5x10^-13/Hz^1/2 is measured at 4.2K, 6 KG and 40 cm^-1. Study of the responsivity under conditions of impact ionization showed a dramatic disappearance of the impurity electron resonance while the conduction electron resonance remained constant. This observation offers the first concrete evidence that the mobility of an electron in the N=0 and N=1 Landau levels is different. Finally, these direct detection experiments indicate that the excellent heterodyne performance achieved at 812 GHz should be attainable up to frequencies of at least 1200 GHz.

Design of antenna-coupled lumped-element titanium nitride KIDs for long-wavelength multi-band continuum imaging

Many applications in cosmology and astrophysics at millimeter wavelengths including CMB polarization, studies of galaxy clusters using the Sunyaev-Zeldovich effect (SZE), and studies of star formation at high redshift and in our local universe and our galaxy, require large-format arrays of millimeter-wave detectors. Feedhorn and phased-array antenna architectures for receiving mm-wave light present numerous advantages for control of systematics, for simultaneous coverage of both polarizations and/or multiple spectral bands, and for preserving the coherent nature of the incoming light. This enables the application of many traditional "RF" structures such as hybrids, switches, and lumped-element or microstrip band-defining filters.

Simultaneously, kinetic inductance detectors (KIDs) using high-resistivity materials like titanium nitride are an attractive sensor option for large-format arrays because they are highly multiplexable and because they can have sensitivities reaching the condition of background-limited detection. A KID is a LC resonator. Its inductance includes the geometric inductance and kinetic inductance of the inductor in the superconducting phase. A photon absorbed by the superconductor breaks a Cooper pair into normal-state electrons and perturbs its kinetic inductance, rendering it a detector of light. The responsivity of KID is given by the fractional frequency shift of the LC resonator per unit optical power.

However, coupling these types of optical reception elements to KIDs is a challenge because of the impedance mismatch between the microstrip transmission line exiting these architectures and the high resistivity of titanium nitride. Mitigating direct absorption of light through free space coupling to the inductor of KID is another challenge. We present a detailed titanium nitride KID design that addresses these challenges. The KID inductor is capacitively coupled to the microstrip in such a way as to form a lossy termination without creating an impedance mismatch. A parallel plate capacitor design mitigates direct absorption, uses hydrogenated amorphous silicon, and yields acceptable noise. We show that the optimized design can yield expected sensitivities very close to the fundamental limit for a long wavelength imager (LWCam) that covers six spectral bands from 90 to 400 GHz for SZE studies.

Excess phase (frequency) noise has been observed in KID and is very likely caused by two-level systems (TLS) in dielectric materials. The TLS hypothesis is supported by the measured dependence of the noise on resonator internal power and temperature. However, there is still a lack of a unified microscopic theory which can quantitatively model the properties of the TLS noise. In this thesis we derive the noise power spectral density due to the coupling of TLS with phonon bath based on an existing model and compare the theoretical predictions about power and temperature dependences with experimental data. We discuss the limitation of such a model and propose the direction for future study.

Light-matter Interactions in Semiconductors and Metals: From Nitride Optoelectronics to Quantum Plasmonics

This thesis puts forth a theory-directed approach coupled with spectroscopy aimed at the discovery and understanding of light-matter interactions in semiconductors and metals.

The first part of the thesis presents the discovery and development of Zn-IV nitride materials.The commercial prominence in the optoelectronics industry of tunable semiconductor alloy materials based on nitride semiconductor devices, specifically InGaN, motivates the search for earth-abundant alternatives for use in efficient, high-quality optoelectronic devices. II-IV-N2 compounds, which are closely related to the wurtzite-structured III-N semiconductors, have similar electronic and optical properties to InGaN namely direct band gaps, high quantum efficiencies and large optical absorption coefficients. The choice of different group II and group IV elements provides chemical diversity that can be exploited to tune the structural and electronic properties through the series of alloys. The first theoretical and experimental investigation of the ZnSnxGe1−xN2 series as a replacement for III-nitrides is discussed here.

The second half of the thesis shows ab−initio calculations for surface plasmons and plasmonic hot carrier dynamics. Surface plasmons, electromagnetic modes confined to the surface of a conductor-dielectric interface, have sparked renewed interest because of their quantum nature and their broad range of applications. The decay of surface plasmons is usually a detriment in the field of plasmonics, but the possibility to capture the energy normally lost to heat would open new opportunities in photon sensors, energy conversion devices and switching. A theoretical understanding of plasmon-driven hot carrier generation and relaxation dynamics in the ultrafast regime is presented here. Additionally calculations for plasmon-mediated upconversion as well as an energy-dependent transport model for these non-equilibrium carriers are shown.

Finally, this thesis gives an outlook on the potential of non-equilibrium phenomena in metals and semiconductors for future light-based technologies.

Metallogenic Provinces of the Southwestern United States and Northern Mexico

Some of the metallogenic provinces of the southwestern United States and northern Mexico are defined by the geographic distribution of trace elements in the primary sulfide minerals chalcopyrite and sphalerite. The elements investigated include antimony, arsenic, bismuth, cadmium, cobalt, gallium, germanium, indium, manganese, molybdenum, nickel, silver, tellurium, thallium, and tin. Of these elements, cobalt, gallium, germanium, indium, nickel, silver, and tin exhibit the best defined geographic distribution.

The data indicate that chalcopyrite is the preferred host for tin and perhaps molybdenum; sphalerite is the preferred host for cadmium, gallium, germanium, indium, and manganese; galena is the preferred host for antimony, bismuth, silver, tellurium, and thallium; and pyrite is the preferred host for cobalt, nickel, and perhaps arsenic. With respect to the two minerals chalcopyrite and sphalerite, antimony, arsenic, molybdenum, nickel, silver, and tin prefer chalcopyrite; and bismuth, cadmium, cobalt, gallium, germanium, indium, manganese, and thallium prefer sphalerite. This distribution probably is the result of the interaction of several factors, among which are these: the various radii of the elements, the association due to chemical similarities of the major and trace elements, and the degree of ionic versus covalent and metallic character of the metal-sulfur bonds in chalcopyrite and sphalerite. The type of deposit, according to a temperature classification, appears to be of minor importance in determining the trace element content of chalcopyrite and sphalerite.

A preliminary investigation of large single crystals of sphalerite and chalcopyrite indicates that the distribution within a single crystal of some elements such as cadmium in sphalerite and indium and silver in chalcopyrite is relatively uniform, whereas the distribution of some other elements such as cobalt and manganese in sphalerite is somewhat less uniform and the distribution of tin in sphalerite is extremely erratic. The variations in trace element content probably are due largely to variations in the composition of the fluids during the growth of the crystals, but the erratic behavior of tin in sphalerite perhaps is related to the presence of numerous cavities and inclusions in the crystal studied.

Maps of the geographic distribution of trace elements in chalcopyrite and sphalerite exhibit three main belts of greater than average trace element content, which are called the Eastern, Central, and Western belts. These belts are consistent in trend and position with a beltlike distribution of copper, gold, lead, zinc, silver, and tungsten deposits and with most of the major tectonic features. However, there appear to be no definite time relationships, for as many as four metallogenic epochs, from Precambrian to late Tertiary, are represented by ore deposits within the Central belt.

The evidence suggests that the beltlike features have a deep seated origin, perhaps in the sub-crust or outer parts of the mantle, and that the deposits within each belt might be genetically related through a beltlike compositional heterogeneity in the source regions of the ores. Hence, the belts are regarded as metallogenic provinces.

Electrical and optical investigations of beta-gallium oxide

An experimental investigation of the optical properties of β–gallium oxide has been carried out, covering the wavelength range 220-2500 nm.

The refractive index and birefringence have been determined to about ± 1% accuracy over the range 270-2500 nm, by the use of a technique based on the occurrence of fringes in the transmission of a thin sample due to multiple internal reflections in the sample (ie., the "channelled spectrum" of the sample.)

The optical absorption coefficient has been determined over the range 220 - 300 nm, which range spans the fundamental absorption edge of β – Ga₂O₃. Two techniques were used in the absorption coefficient determination: measurement of transmission of a thin sample, and measurement of photocurrent from a Schottky barrier formed on the surface of a sample. Absorption coefficient was measured over a range from 10 to greater than 10⁵, to an accuracy of better than ± 20%. The absorption edge was found to be strongly polarization-dependent.

Detailed analyses are presented of all three experimental techniques used. Experimentally determined values of the optical constants are presented in graphical form.

Earth-Abundant Zinc-IV-Nitride Semiconductors

This investigation is motivated by the need for new visible frequency direct bandgap semiconductor materials that are abundant and low-cost to meet the increasing demand for optoelectronic devices in applications such as solid state lighting and solar energy conversion. Proposed here is the utilization of zinc-IV-nitride materials, where group IV elements include silicon, germanium, and tin, as earth-abundant alternatives to the more common III-nitrides in optoelectronic devices. These compound semiconductors were synthesized under optimized conditions using reactive radio frequency magnetron sputter deposition. Single phase ZnSnN₂, having limited experimental accounts in literature, is validated by identification of the wurtzite-derived crystalline structure predicted by theory through X-ray and electron diffraction studies. With the addition of germanium, bandgap tunability of ZnSn_xGe_1-xN₂ alloys is demonstrated without observation of phase separation, giving these materials a distinct advantage over In_xGa_1-xN alloys. The accessible bandgaps range from 1.8 to 3.1 eV, which spans the majority of the visible spectrum. Electron densities, measured using the Hall effect, were found to be as high as 10²² cm⁻³ and indicate that the compounds are unintentionally degenerately doped. Given these high carrier concentrations, a Burstein-Moss shift is likely affecting the optical bandgap measurements. The discoveries made in this thesis suggest that with some improvements in material quality, zinc-IV-nitrides have the potential to enable cost-effective and scalable optoelectronic devices.