The deposition temperature of hydrogenated amorphous silicon films deposited by dc glow discharge was found to affect the photoresponse (ratio of the photo to dark conductivity) after crystallization of the film. This effect depended on the crystallization technique. For crystallization by laser annealing, the photoresponse (0.15 - 1.5) increased with increasing deposition temperature (150 - 300 °C) due to the increase in SiH and SiH2 bonding, as shown by infrared spectroscopy. For crystallization by furnace annealing (e.g. 650 °C, 50 h), the photoresponse (0.08 - 0) decreased with increasing deposition temperature (150 - 300 °C) due to the decrease in grain size and crystallinity as shown by x-ray diffraction; the complete loss in hydrogen during furnace annealing made the photoresponse low and the silicon-hydrogen bonding effect immaterial. Thus, laser crystallization at the highest deposition temperature gave the highest photoresponse.

The technologies of laser crystallization and methods of SiO2 formation in remote plasma chemical vapor deposition or SiO evaporation with an oxygen ambient realize the fabrication of n-channel polycrystalline and amorphous silicon thin film transistors (poly-Si and a-Si TFTs) at a temperature lower than 300 °C. The defect density was achieved to be 2∼3×1011 cm−2eV−1 and threshold voltage was about IV for both TFTs. The maximum field effect mobility was 600 cm2/Vs for poly-Si TFTs and 2.6 cm2/Vs for a-Si TFTs. The mobility of poly-Si TFT decreased as the gate voltage increases. This is interpreted as that the electrons are confined in the narrow inversion layer and electron scattering with phonon is enhanced for higher normal electric field.

We evaluated structural and electrical characteristics of undoped poly-Si oxide films. Poly-Si films made by solid phase crystallization at 600-900°C from undoped amorphous Si films were oxidized to form oxide layers of 140nm thickness. We observed protuberances on the surface of poly-Si layers after oxidation. Poly-Si oxide layers also generated protuberances above the protuberances of poly-Si films. The number of protuberances per unit area is larger in the case of high temperature crystallization. The measurement of current through the poly-Si oxide films shows that the conductivity of poly-Si oxide films depends on crystallization temperature of poly-Si films in the case of positive gate bias. When the gate is biased negatively, current through the poly-Si oxide films remained almost constant regardless of crystallization temperature. We find that poly-Si crystallized at lower temperatures offers poly-Si oxide films of lower leakage current in the case of electron injection from undoped poly-Si layers. The lower leakage current is due to highness of energy barrier for electron at undoped poly-Si/poly-Si oxide interface.

Heterojunction solar cells were fabricated by glow discharge deposition of amorphous silicon on p-type crystalline silicon resulting in a n/i/p structure. Dark I-V-T data on the devices show that the conduction in the forward bias regime (<0.4 volts) for better devices agrees with a multi-tunnelling-capture-emission process. The photoresponse was evaluated (under 100 mW/cm2) for various a-Si thicknesses and substrate resistivities. Spectral response tests showed an increased low wavelength absorption as the a-Si thickness was decreased. The blue response of the devices have better fill-factors than the red response indicating defects at the interface. Further, I-V-T and C-V measurements also corroborate the presence of defect states which seem to prevent the spread of the depletion region in crystalline silicon. The photoresponse was found to be very sensitive to the interface defects and the fill-factors ranged from 0.42, for the sample in which the depletion region had spread, to 0.1 in those where the depletion region had been reduced in thickness by the interface states.

Advanced CVD- and MBE-epitaxy allows processing of very thin epitaxial layers with extremely steep dopant gradients. As steep dopant profiles are very sensitive to temperature treatments, the impact of thermal processing from 800°C to 1000°C on the dopant redistribution in epitaxial layers was investigated.

New and improved processes for fabrication of SiCr thin film resistors are desirable to achieve precision circuits. Two major draw backs to conventional single target sputtered SiCr resistors are high temperature coefficient of resistance (TCR) and low sheet rho (ρ□). Depositing a very thin film to achieve a higher ρ□, generally degrades the film stability, TCR and manufacturability. Annealing the film at higher temperatures by means of hot air, does increase the film sheet rho but the anneal temperature is limited to below 500°C to avoid film decomposition.

A new method of increasing the sheet rho of SiCr films has been established where the deposited film with a ρ□ of 400Ω/□ is annealed to 1400Ω/□. Silicon substrates with thermally grown oxide were deposited with SiCr using rf sputtering. The wafers were ran through a single wafer rapid thermal processor with the chamber filled with dry nitrogen. Anneal temperatures were varied and it was found that the highest sheet rho was obtained at a temperature of 650°C and anneal time of 60 seconds. Resistors were manufactured using the annealed film and electrical characteristics of the resistors evaluated. When compared to hot air annealed SiCr resistors, the RTA SiCr resistors showed improved stability, TCR and sheet rho performance.

Observations of efficient room temperature photoluminescence (PL) from porous Si have generated a great deal of interest in the optical properties of nm-scale Si structures. The stochastic character of porous-Si fabrication results in a distribution of crystal sizes and shapes. We report on a scalable (to large areas) and manufacturable (to high volumes) fabrication technology for uniform, nm-linewidth Si structures providing an important testbed for controlled studies of these optical properties. Large areas ( ∼ 1 cm2) of extreme sub-μm structures (to ∼ 5 nm) are re-producibly fabricated. Both walls (1-D confinement) and wires (2-D confinement) are reported. The fabrication process includes: interferometric lithography, highly anisotropic KOH etching, and structure dependent oxidation. For the walls, nearly perfect <111> crystal planes form the sidewalls and very high width/depth aspect ratios (> 50) have been achieved. Raman scattering results on the walls demonstrate three regimes: 1) lineshapes and cross sections similar to bulk Si for line widths, W > 200 nm; 2) electromagnetic resonance enhancement of the cross section ( to - 100x) for W from 50-200 nm; and 3) highly asymmetric lineshapes and splittings from W < 30 nm. Photoluminescence is observed for the thinnest samples (W < 10 nm) and is as intense as that observed from porous Si with a spectral linewidth ∼ 50 % smaller than that of porous Si.

Formation of highly-uniform and densely-packed arrays of GaAs dots by selective epitaxy using diethylgallium-chloride and arsine is reported. The arrays of GaAs dots are imaged using atomic force microscopy (AFM). Accounting for the AFM tip radius of curvature, the smallest GaAs dots formed are 15-20 nm in base diameter and 8-10 nm in height with slow-growth crystal planes limiting individual dot growth. Completely selective GaAs growth within dielectric-mask openings at these small size-scales is also demonstrated. The uniformity of the dots within each array ranged from 6% for the larger dots to 16% for the smallest dots (normalized standard deviations of the areas of individual dots within each array).

We have investigated periodic arrays of 150 and 175 nm wide GaAs-AlAs quantum wires and quantum dots, fabricated by electron beam lithography and SiCI4/O2 reactive ion etching, by means of reciprocal space mapping using triple axis x-ray diffractometry (TAD). The reciprocal space maps reveal that after the fabrication process the lattice constant along the growth direction slightly increases for the wires and even more so for the dots.

Single crystal-silicon quantum well layers with SiO2 barriers were grown from silicon-on-insulator substrates. Photoluminescence in the red and near-infrared was observed for average layer thickness < 8 nm, with peak signal for 2-nm thickness. The luminescence spectrum was essentially independent of well width for SiO2 barriers, but the photoluminescence intensity decreased sharply after annealing in Ar. These results suggest the importance of radiation from surface states. In contrast to oxide-passivated silicon nanocrystals and to porous silicon, the room-temperature photoluminescence quantum efficiency is low (10-4-10-5), probably due to variations in layer thickness and to diffusion of photoexcited carriers to fast nonradiative recombination centers.

Silicon multiquantum wells ranging in width from 3 to 15 nm were deposited on closely lattice-matched ZnS barriers. MOCVD was used to deposit the ZnS films using diethyl zinc and hydrogen sulfide as the precursors; disilane was used to deposit silicon layers at low temperatures. Single and multiple silicon nano-layers were observed by transmission electron microscopy and secondary ion mass spectrometry. Photoluminesence studies revealed emissions peaks which were blue-shifted with respect to the edge emission from bulk silicon substrates. The observation of emission from silicon nanostructures shifted to wavelengths as short as the 800-850 nm range is consistent with the effects of quantum confinement in silicon nanostructures.

SixGe1–x heterostructures on Si substrates have been widely studied due to the maturity of Si technology. However, work on SixGe1–x heterostructures on Ge substrates has not received much attention. A SixGe1–x: layer on a Si substrate is under compressive strain while SixGe1–x on Ge is under tensile strain; thus the critical points will behave differently. In order to accurately characterize high Ge content SixGe1–x layers the energy shift algorithm, which is used to calculate alloy compositions, has been modified. These results have been used along with variable angle spectroscopic ellipsometry (VASE) measurements to characterize SixGe1–x/Ge superlattices grown on Ge substrates. The results are found to agree closely with high resolution x-ray diffraction measurements made on the same samples.

The modified energy shift algorithm also allows the VASE analysis to be upgraded in order to characterize linearly graded layers. In this work VASE has been used to characterize graded SixGe1–x layers in terms of the total thickness, and the start and end alloy composition. Results are presented for a 1 µm SixGe1–x layer linearly graded in the range 0.5 ≤ x ≤ 1.0.

Disordered thin layer GaAs/AlAs superlattices with various disorder periods have been grown using MBE. The disorder was introduced by varying the thickness of GaAs and AlAs layers in the growth direction in various specific but randomly generated finite disorder sequences. The photoluminescence spectra of these disordered superlattice samples showed a sharp peak at the high energy side and a broad peak at the low energy side. The temperature, excitation, and disorder sequence dependence of the photoluminescence spectra indicates that the sharp peak is due to the pseudo-direct exciton emission, and the broad peak is strongly related to the localized states induced by disorder. In addition, the results demonstrate that the luminescence intensity of disordered superlattices can be improved by up to two orders of magnitude over that of ordered short period superlattices. Finally, we propose a kinetic model for the state population and find that the photoluminescence spectra can be well described by this model.

Low temperature optical absorption spectra are presented for a series of InAsxP1-x/InP strained layer multiple quantum well structures (0 < x = 0.35) grown by low pressure metal organic vapor phase epitaxy (LP-MOVPE) using trimethylindium (TMIn), tertiarybutylarsine (TBAs) and phosphine as precursors. The well widths and compositions in these structures are determined from high resolution x-ray diffraction and transmission electron microscopy. The absorption spectra are then analyzed by fitting the excitonic peak energy position with transition energies determined from a solution to the Schrödinger equation. We used the envelope function formalism with the Kane bands as the basis wavefunctions and included corrections for non parabolicity. From these fits and elasticity theory, both the bandgap of unstrained InAsxP1-x and the band offsets of these heterostructures are deduced self-consistently. The conduction band offsets are found between 72% and 75% of the total strained bandgap differences.

Experimental and theoretical investigations of electronic states in a strained-layer CdTe/CdZnTe coupled double quantum well structure are presented. The optical properties of this lattice mismatched heterostructure were characterized by photoluminescence (PL), PL excitation and polarization spectroscopies. The influence of electrical field on exciton states in the strained layer CdTe/CdZnTe coupled double quantum well structure is experimentally studied. The confined electronic states were calculated in the framework of the envelope function approach, taking into account the strain effect induced by the lattice-mismatch. Experimental results are compared with the calculated transition energies.

We have performed photoluminescence (PL) measurements at liquid nitrogen temperature under high pressure up to 5.5 GPa and in the temperature range 10-300 K at atmospheric pressure on {(ZnSe)30(ZnSe0.92Te0.08)30(ZnSe)30[(CdSe)1(ZnSe)2]9}x5 multiple quantum wells. The PL peaks, EB, E1 and Ew corresponding to the band edge luminescence in ZnSe barrier layer, the transitions from the first conduction subband to the heavy-hole subband in ZnSe0.92Te0.08 layers and [(CdSe)1(ZnSe)2]9 ultra short period superlattice quantum well (SPSLQW) layers have been observed. Experimental results show that ZnSe0.92Te0.08/ZnSe forms a type-I superlattice (SL) in contrast to the type-II ZnSe/ZnTe SL. The pressure coefficients of the EB, E1 and Ew exciton peaks have been determined as 67, 63 and 56 meV/GPa, respectively. With increasing temperature (or pressure), the E1 peak-intensity drastically decreases which is attributed to the thermal effect (or the appearance of many defects in ZnSe0.92Te0.08 under higher pressure).

The microstructural characteristics of wide band gap semiconductor, hexagonal A1N thin films on Si(100), (111), and sapphire (0001) and (10ī2) were studied by transmission electron microscopy (TEM) and x-ray diffraction. The films were grown by MOCVD from TMA1 + NH3 + N2 gas mixtures. Different degrees of film crystallinity were observed for films grown on α-A12O3 and Si substrates in different orientations. The epitaxial growth of high quality single crystalline A1N film on (0001) α-Al2O3 was demonstrated with a dislocation density of about 2*10 10cm−2 . The films on Si(111) and Si(100) substrates were textured with the c-axis of A1N being perpendicular to the substrate surface.

Spectroscopic ellipsometry has been used to characterize thin oxide/nitride/oxide (ONO) layer structures. Films were deposited on top of the Si/oxide/polysilicon filmstack using a low-pressure chemical vapor deposition (LPCVD) technique. Different approaches to modeling this structure are compared to select an adequate representation. Analysis of the data for the three-layer-with-abrupt-interfaces ONO model using an effective-medium or Cauchy approximation shows strong correlation of the parameters and ambiguity of the results. A more robust and physically plausible model of the ONO structure as an inhomogeneous oxynitride film with Gaussian distribution of the silicon nitride component is suggested. This interpretation of the ONO structure is supported by the results of Auger electron spectroscopy (AES) and transmission electron microscopy (XTEM).

The thermoelectric figure of merit (ZT) of a material is a measure the usefulness of the material in a thermoelectric device. Presently, the materials with the highest ZT are Bi2Te3 alloys, with ZT ≃ 1. There has been little improvement in ZT for over 30 years. So far, all the materials used in thermoelectric applications have been in bulk form. Recently, however, calculations have shown that it may be possible to increase ZT of some materials through the use of quantum-well superlattices. We have made preliminary measurements on the Bi/PbTe superlattice system using transport and optical techniques to determine whether it is possible to achieve such an increase in ZT.

Bulk EuTe is a type II Heisenberg antiferromagnet (AF2) with a single magnetic phase transition temperature at 9.6 K. However, for several isolated EuTe (111) monolayers (MLs) as can be achieved in a superlattice (SL) structure, both ferrimagnetic-like and antiferromagnetic-like phase transitions can take place, depending on the SLs configuration. The temperature-dependent magnetization M(T) of such SLs has been studied near the transition temperature (Tc) by SQUID magnetometry. The functional forms of M(T) at T ≤ Tc can be described by mean-field theory for SLs with 3, 4 and 5 EuTe MLs per SL cell. The magnetic transition temperatures obtained by mean-field analysis, using bulk exchange coupling values, are in close agreement with observed Tc values for SLs with 2, 3, 4 and 5 EuTe MLs. The qualitative behavior of the surface specific heat can be deduced from M(T) data for SLs with three EuTe monolayers.