C. E. Inglefield

Physical properties of sulfur near the polymerization transition

Acoustical measurements, electron spin resonance, and Raman spectroscopy have been employed to probe sulfur over the temperature range 80-180 degrees C, which includes the polymerization transition and the supercooled liquid state. Acoustical properties (sound velocity, absorption, and impedance) have been studied with both longitudinal and transverse waves at frequencies between 500 kHz and 22 MHz. The results confirm that polymeric sulfur is a solution of long chain molecules in monomeric solvent, and that the polymerization transition is not a second-order phase transition, as was proposed theoretically. Sulfur is a viscous liquid, but not viscoelastic, both below and above the polymerization transition temperature. It is shown that the classical Navier-Stokes theory is not applicable to the sound absorption in liquid sulfur in the highly viscous state.

Quantum wells due to ordering in GaInP

CuPt ordering results in a reduction of the band-gap energy of GaInP. Thus, heterostructures and quantum wells can be produced by simply varying the order parameter, without changing the solid composition. Changes in the order parameter can be induced by changes in growth conditions. The disordered/ordered/disordered quantum wells described here are grown by changing the PH3 flow rate. Transmission electron microscopy results show that the quantum wells produced in this way are clearly defined, with abrupt interfaces. Low-temperature photoluminescence spectra show distinct peaks from quantum wells (QWs) of different widths. The QW photoluminescence peak energy increases with decreasing well width due to quantum size effects. The difference in band-gap energy between the ordered and disordered single layers is determined from photoluminescence excitation spectroscopy to be 0.06 eV.

Heterostructures in GaInP grown using a change in V/III ratio

A natural monolayer {111} superlattice (the CuPt ordered structure) is formed spontaneously during organometallic vapor phase epitaxial (OMVPE) growth of Ga0.52In0.48P. The extent of this ordering process is found to be a strong function of the input partial pressure of the phosphorus precursor during growth due to the effect of this parameter on the surface reconstruction and step structure. Thus, heterostructures can be produced by simply changing the flow rate of the P precursor during growth. It is found, by examination of transmission electron microscope (TEM) and atomic force microscope (AFM) images, and the photoluminescence (PL) and PL excitation (PLE) spectra, that order/disorder (O/D) (really less ordered on more ordered) heterostructures formed by decreasing the partial pressure of the P precursor during the OMVPE growth cycle at a temperature of 620 °C are graded over several thousands of A when PH3 is the precursor. The ordered structure from the lower layer persists into the upper layer. Sim...

Structural Characterization of SiF4, SiH4 and H2 Hot-Wire-Grown Microcrystalline Silicon Thin Films with Large Grains

In this paper, we present a comprehensive study of microcrystalline silicon thin film samples deposited by a novel growth process intended to maximize their grain size and crystal volume fraction. Using Atomic Force Microscopy, Raman spectroscopy, and x ray diffraction the structural properties of these samples were characterized qualitatively and quantitatively. Samples were grown using a Hot-Wire Chemical Vapor Deposition process with or without a post-growth hot-wire annealing treatment. During Hot-Wire Chemical Vapor Deposition, SiF4 is used along with SiH4 and H2 to grow the thin films. After growth, some samples received an annealing treatment with only SiF4 and H2 present. These samples were compared to each other in order to determine the deposition conditions that maximize grain size. Large microcrystalline grains were found to be aggregates of much smaller crystallites whose size is nearly independent of deposition type and post-annealing treatment. Thin films deposited using the deposition process with SiF4 partial flow rate of 2 sccm and post-growth annealing treatment had the largest aggregate grains ~ .5 µm and relatively high crystal volume fraction.

Microwave modulated photoluminescence used to measure surface recombination velocities

Microwave modulated photoluminescence (MMPL) is a characterization technique in which a semiconducting sample is subjected to continuous optical pumping and chopped microwave electric fields. The signal normally detected in an MMPL experiment is the change in the photoluminescence (PL) spectrum due to the presence of the microwave electric field, which increases the kinetic energy of the free carriers. We have previously correlated the quenching of the PL signal, as induced by the microwaves, with nonradiative recombination at a surface/interface of the photoexcited volume. In this work, we determine quantitatively surface recombination velocities through a combined measurement of microwave induced changes in photoconductivity and in PL. From the change in the photoconductivity we infer a change in the diffusion constant of free carriers in the material. The change in diffusion constant, along with the change in luminescent intensity, uniquely determines the surface recombination velocity of the layer. Re...

Microwave modulated photoluminescence as a contactless probe of interface states

Microwave modulated photoluminescence (MMPL) is a developing spectroscopy in which the sample is subjected to continuous optical pumping and chopped microwave electric fields. The signal detected in an MMPL experiment is the change in the photoluminescence spectrum of the sample due to the presence of microwave electric fields, which increase the kinetic energy of the free carriers. In order to investigate the effects of interfaces on this measured quantity, two types of GaAs samples have been compared. The first type of sample was grown such that the GaAs epilayers are exposed, while in the second type the GaAs is “capped” by a layer of higher band gap material. Several pairs of such samples have been studied. The MMPL results are consistent with the following mechanism: an increase in the kinetic energy of the free carriers that results from the presence of the microwave fields allows more free carriers to reach the interface or surface of the GaAs layer before recombining. The presence of a greater num...

Characterization of unicompositional GaInP2 ordering heterostructures grown by variation of V/III ratio

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies are employed to investigate single heterostructures based on two GaInP2 layers that have the same composition but different degrees of order on the cation sublattice. Four sample configurations are studied: two complementary single heterostructures, a more ordered layer grown on a less ordered layer and vice versa, and two single layers nominally equivalent to the constituent layers of the heterostructures. The degree of order of the two layers was controlled via the V/III ratio used during organometallic vapor phase epitaxial growth. From our measurements, the difference between the band gaps of the two layers is 20–30 meV. The PLE spectra show clearly that the emission comes from both layers of the heterostructures and that the PL is excited by direct absorption of the exciting light into each layer as well as the injection of carriers from the less ordered (higher band gap) layer into the more ordered (lower band gap) layer. T...

What Constitutes Successful Undergraduate Research

Research has become a point of much greater emphasis in the undergraduate science curriculum within the past few generations of undergraduate students. However, there is not universal agreement upon what constitutes a successful undergraduate research program, and how degrees of success should be measured. A model for a successful program based principally on student ownership of research projects will be presented here. This approach is an attempt to give undergraduate students the most holistic research experience possible, by involving them in all stages of a research project in a limited amount of time. A central goal for each student is a definable product of their particular project, which is disseminated to as wide an external audience as possible. Measures of success, from student perspectives, will be discussed. These measures include data from interviews of students before and after their research experience.

An Instructional Two-Dimensional Diffraction Laboratory Using Patterns Created with Electron-Beam Lithography

An instructional laboratory in two-dimensional diffraction is discussed. The experiment is appropriate for undergraduate students in materials science, solid-state physics (as was the case with our group), modern physics, or optics. The experiment is performed using visible light from a laser incident on a 2D lattice of gold dots deposited with electron beam lithography on a glass substrate. The pattern is microscopic with a lattice constant on the same order of magnitude as the wavelength of light used. Students observe the diffraction pattern, and then quantitatively determine the positions of maxima. These data are used by the students to reconstruct the (real space) microscopic lattice. The students can simulate the experiment with software that computes reciprocal lattice and diffraction patterns for an arbitrary 2D lattice.