A non-destructive analytic tool for nanostructured materials : Raman and photoluminescence spectroscopy

Modern materials science requires efficient processing and characterization techniques for low dimensional systems. Raman spectroscopy is an important non-destructive tool, which provides enormous information on these materials. This understanding is not only interesting in its own right from a physicist's point of view, but can also be of considerable importance in optoelectronics and device applications of these materials in nanotechnology. The commercial Raman spectrometers are quite expensive. In this article, we have presented a relatively less expensive set-up with home-built collection optics attachment. The details of the instrumentation have been described. Studies on four classes of nanostructures - Ge nanoparticles, porous silicon (nanowire), carbon nanotubes and 2D InGaAs quantum layers, demonstrate that this unit can be of use in teaching and research on nanomaterials.Comment: 32 pages, 13 figure

Perspectives on formation and properties of semiconductor interfaces

Recent progress in experimentally and theoretically understanding interfaces at the atomic level suggest that ultimate electronic systems may one day be fabricated on a single integrated chip. If such elements as Si VLSI processors, GaAs/AIAs integrated optoelectronic 10 devices, II-VI superlattice visible displays and high speed III-V processors are to be integrated, interface formation and in situ processing will be required at a level of sophistication well beyond what is available today. In this paper, we review recent developments in interface formation by both MOCVD and MBE. To illustrate the power of our diagnostic methods, the details of epitaxial interface formation on an atomic scale are reviewed for lattice matched systems (Ge/GaAs/AIAs) and epitaxial silicides (Ni/Si2/Si) as well as oxidation of silicon to form Si/SiO2 interfaces. New developments in using lattice mismatched superlattices with strained layers are discussed for CdTe/ZnTe. Additional complications of growing compound semiconductors on elemental substrates (e.g., anti-phase domains) are discussed for GaAs growth on Si(l00)

Perspectives on formation and properties of semiconductor interfaces

Recent progress in experimentally and theoretically understanding interfaces at the atomic level suggest that ultimate electronic systems may one day be fabricated on a single integrated chip. If such elements as Si VLSI processors, GaAs/AIAs integrated optoelectronic 10 devices, II-VI superlattice visible displays and high speed III-V processors are to be integrated, interface formation and in situ processing will be required at a level of sophistication well beyond what is available today. In this paper, we review recent developments in interface formation by both MOCVD and MBE. To illustrate the power of our diagnostic methods, the details of epitaxial interface formation on an atomic scale are reviewed for lattice matched systems (Ge/GaAs/AIAs) and epitaxial silicides (Ni/Si2/Si) as well as oxidation of silicon to form Si/SiO2 interfaces. New developments in using lattice mismatched superlattices with strained layers are discussed for CdTe/ZnTe. Additional complications of growing compound semiconductors on elemental substrates (e.g., anti-phase domains) are discussed for GaAs growth on Si(l00)

Tunable MEMS VCSEL on Silicon substrate

We present design, fabrication and characterization of a MEMS VCSEL which utilizes a silicon-on-insulator wafer for the microelectromechanical system and encapsulates the MEMS by direct InP wafer bonding, which improves the protection and control of the tuning element. This procedure enables a more robust fabrication, a larger free spectral range and facilitates bidirectional tuning of the MEMS element. The MEMS VCSEL device uses a high contrast grating mirror on a MEMS stage as the bottom mirror, a wafer bonded InP with quantum wells for amplification and a deposited dielectric DBR as the top mirror. A 40 nm tuning range and a mechanical resonance frequency in excess of 2 MHz are demonstrated

Molecular beam epitaxial growth of monocrystalline MgxCd1-xTe/MgyCd1-yTe (x<y) double heterostructures and solar cells

abstract: This dissertation details a study of wide-bandgap molecular beam epitaxy (MBE)-grown single-crystal MgxCd1-xTe. The motivation for this study is to open a pathway to reduced $/W solar power generation through the development of a high-efficiency 1.7-eV II-VI top cell current-matched to low-cost 1.1-eV silicon. This paper reports the demonstration of monocrystalline 1.7-eV MgxCd1-xTe/MgyCd1-yTe (y>x) double heterostructures (DHs) with a record carrier lifetime of 560 nanoseconds, along with a 1.7-eV MgxCd1-xTe/MgyCd1-yTe (y>x) single-junction solar cell with a record active-area efficiency of 15.2% and a record open-circuit voltage (VOC) of 1.176 V. A study of indium-doped n-type 1.7-eV MgxCd1-xTe with a carrier activation of up to 5 × 1017 cm-3 is presented with promise to increase device VOC. Finally, this paper reports an epitaxial lift-off (ELO) technology using water-soluble MgTe for the creation of free-standing MBE-grown II-VI single-crystal CdTe and 1.7-eV MgxCd1-xTe solar cells freed from lattice-matched InSb(001) substrates. Photoluminescence (PL) spectroscopy measurements comparing intact and free-standing films reveal the survival of optical quality in CdTe DHs after ELO. This technology opens up several possibilities to drastically increase cell conversion efficiency through improved light management and transferability into monolithic multijunction devices. Lastly, this report will present considerations for future work in each of the study areas mentioned above.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

Carrier Transport in High Mobility InAs Nanowire Junctionless Transistors

Ability to understand and model the performance limits of nanowire transistors is the key to design of next generation devices. Here, we report studies on high-mobility junction-less gate-all-around nanowire field effect transistor with carrier mobility reaching 2000 cm2/V.s at room temperature. Temperature-dependent transport measurements reveal activated transport at low temperatures due to surface donors, while at room temperature the transport shows a diffusive behavior. From the conductivity data, the extracted value of sound velocity in InAs nanowires is found to be an order less than the bulk. This low sound velocity is attributed to the extended crystal defects that ubiquitously appear in these nanowires. Analyzing the temperature-dependent mobility data, we identify the key scattering mechanisms limiting the carrier transport in these nanowires. Finally, using these scattering models, we perform drift-diffusion based transport simulations of a nanowire field-effect transistor and compare the device performances with experimental measurements. Our device modeling provides insight into performance limits of InAs nanowire transistors and can be used as a predictive methodology for nanowire-based integrated circuits.Comment: 22 pages, 5 Figures, Nano Letter

Mid-IR plasmonic compound with gallium oxide toplayer formed by GaSb oxidation in water

The oxidation of GaSb in aqueous environments has gained interest by the advent of plasmonic antimonide-based compound semiconductors for molecular sensing applications. This work focuses on quantifying the GaSb–water reaction kinetics by studying a model compound system consisting of a 50 nm thick GaSb layer on a 1000 nm thick highly Si-doped epitaxial grown InAsSb layer. Tracing of phonon modes by Raman spectroscopy over 14 h of reaction time shows that within 4 h, the 50 nm of GaSb, opaque for visible light, transforms to a transparent material. Energy-dispersive x-ray spectroscopy shows that the reaction leads to antimony depletion and oxygen incorporation. The final product is a gallium oxide. The good conductivity of the highly Si-doped InAsSb and the absence of conduction states through the oxide are demonstrated by tunneling atomic force microscopy. Measuring the reflectivity of the compound layer structure from 0.3 to 20 μm and fitting of the data by the transfer-matrix method allows us to determine a refractive index value of 1.6 ± 0.1 for the gallium oxide formed in water. The investigated model system demonstrates that corrosion, i.e. antimony depletion and oxygen incorporation, transforms the narrow band gap material GaSb into a gallium oxide transparent in the range from 0.3 to 20 μm

An investigation of electrical and optical properties of reactively sputtered silicon nitride and amorphous hydrogenated silicon thin films

Thin films of silicon nitride and amorphous hydrogenated silicon were prepared by radio frequency reactive sputter deposition and their properties optimized for their use as low temperature passivation coatings for optoelectronic devices. The effect of various sputter deposition parameters on the conduction and optical properties were studied. Infrared spectrophotometry and ellipsometry were used to determined the optical properties of the films whereas the electrical properties were determined from current-voltage measurements of MIS capacitors. Typical parameters of a sputter deposition run for the best Si3N4 films were: base pressure, 1-2x10-6 torr; sputtering pressure, 5 mtorr; nitrogen partial pressure, 16.5%; cathode anode gap, 10 cm; target power density, 1.97watts/cm2; and cathode voltage, 1.0 kvolts. Films of thickness 50-120nm, refractive index 1.94-2.2, and low conductivity (resistivity of 1011 Ω-cm) were obtained. The deposition rate was in the ranged of 5-8 nm/min depending on the sputtering pressure, the appied target power, and the nitrogen partial pressure. It was concluded that the quality of the silicon nitride films is strongly dependent on the total deposition pressure, nitrogen partial pressure, applied target power voltage, and possibly cathode voltage. It was also concluded that the water vapor background was the major factor in increasing the conductivity of the best films to values about three orders of magnitude above those for the best bulk silicon nitride material. Typical sputtering parameters for depositing a-Si:H films were: base pressure, 1-2x10-6 torr; sputtering pressure, 7 mtorr; hydrogen partial pressure, 5-20%; cathode anode gap, 7.6 cm; r.f. target power density, 1.58-1.82 watts/cm2; cathode voltage, 1.8-1.9 kvolts. Films of thicknesses 78-150 nm, refractive index 3.25 - 4.0, and strong absorption at 2000 cm-1 of infrared spectra were obtained. It was concluded that stoichiometric a -Si:H films can be prepared by reactive sputtering of a silicon target in the environment of argon and hydrogen