M. C. DeLong

Growth temperature and substrate orientation dependences of moving emission and ordering in Ga0.52 In0.48 P

The excitation intensity and temperature dependences of the photoluminescence (PL) from Ga0.52 In0.48 P grown by organometallic vapor phase epitaxy on GaAs substrates have been investigated as a function of the epilayer growth temperature and the substrate misorientation from (100). The energy band gap is dependent on substrate temperature during growth but is effectively independent of substrate misorientation. A strong excitation intensity dependence of the PL is found to be a function of both substrate orientation and growth temperature. We postulate that the strong dependence of PL emission energy on excitation intensity in some samples may result from the sizes, shapes, and orientations of ordered domains.

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...

ORDERED AND RANDOMLY DISORDERED ALAS/GAAS SHORT-PERIOD SUPERLATTICES

Unique optical signatures of different atomic arrangements of Al0.5Ga0.5As, deposited by molecular‐beam epitaxy and having nominally identical average composition, have been observed in steady‐state photoluminescence and photoluminescence excitation spectroscopies. Compared to the observations from a random pseudobinary alloy and a (AlAs)2(GaAs)2 ordered superlattice, intense photoluminescence emission is observed from disordered (AlAs)n(GaAs)4−n superlattices where n is randomly chosen from the sets 0≤n≤4 or 1≤n≤3. The photoluminescence peak energies of the randomly ordered superlattices are red‐shifted by 100–400 meV from the emission energy of the pseudobinary alloy, suggesting that a significant density of localized or band tail states exists at energies lower than the band gap, which are confirmed by photoluminescence excitation spectroscopy. We also measure greatly increased photoluminescence intensity from the randomly ordered superlattices at high temperatures indicating that these materials may b...