W. E. Carlos

Luminescence cycling and defect density measurements in porous silicon: Evidence for hydride based model

Changes in dangling bond densities in porous silicon were measured and results indicate a relatively low dangling bond density (roughly 3×1016 bonds/cm3) in as‐prepared samples, which increases by a factor of 6–7 upon quenching of the photoluminescence (PL). The electron spin resonance (ESR) data suggest the presence of significant disorder in the as‐prepared 1 Ω cm porous silicon samples, which may correlate with an enhanced PL intensity. The results of heat cycling and HF dipping experiments suggest that a continuous decrease in particle size does not result in a continuous PL blue shift, as would be expected in the quantum confinement model. These results will be discussed in terms of a hydride/polysilane luminescence mechanism.

Oxygen defect center red room temperature photoluminescence from freshly etched and oxidized porous silicon

Electron spin resonance and photoluminescence experiments have been performed on freshly etched and oxidized porous silicon. Results indicate the presence of oxygen‐related centers (nonbridging oxygen‐hole center clusters), which consist of similar core structures in as‐made and oxidized porous silicon (PSi) samples. A direct correlation exists between the presence of these centers and a red photoluminescence observed in both freshly anodized and oxidized PSi, suggesting that this emission process is the result of optical transitions in the oxygen‐hole centers.

IDENTIFICATION OF PHOSPHORUS IN DIAMOND THIN FILMS USING ELECTRON PARAMAGNETIC-RESONANCE SPECTROSCOPY

An electron paramagnetic‐resonance study of diamond films doped by implantation of phosphorus during film deposition is reported. Samples with nominal phosphorus concentration between 1016 and 1017 cm−3 exhibit two isotropic lines of equal intensity separated by 27 G. The double‐line spectrum is expected for a nuclear‐spin 1/2 species such as phosphorus. The symmetry of the center and the temperature dependence of the spin concentration suggest that phosphorus forms a substitutional shallow donor in diamond.

The EX defect center in porous silicon

We report the observation of the EX center in porous silicon by electron spin resonance (ESR). This center has been previously observed in low temperature thin oxides on crystalline Si and is believed to comprise a delocalized hole on three or four oxygen dangling bonds at a Si vacancy. In porous Si the defect is seen only in samples which have been oxidized for a very short period of time (∼1 min). The ESR intensity of the EX center correlates with the red room‐temperature photoluminescence.

The nature of donor conduction in n-GaN

Single crystal GaN thin films resulting from various deposition techniques are usually dominated by residual donors. To date, the true nature of this donor conduction is not known. Nitrogen vacancies, residual oxygen, and growth defects are cited as potential causes for the residual n‐type conduction. In this communication we present the first systematic study of near conduction band edge states in n‐type GaN samples deposited over basal plane sapphire substrates using low pressure metal organic chemical vapor deposition. Electron spin resonance, low temperature photoluminescence, and Van der Pauw–Hall measurements were used as the basis for our study. We concluded that the residual n‐type conduction in GaN results from a band of delocalized donors.

Native defects and dopants in GaN studied through photoluminescence and optically detected magnetic resonance

Native defects and dopants in GaN grown by organometallic chemical vapor deposition have been studied with photoluminescence and optically detected magnetic resonance. For undoped samples, the combined results indicate the presence of residual shallow donors and acceptors and deep donors. A model for the capture and recombination among these defects is developed. For Mg-doped samples, the experiments reveal shallow and perturbed acceptors and shallow and deep donors. Hence, shallow and deep states for the native donor or donors appear in all samples. The Mg-acceptor is perturbed from its effective-mass state by nearby point defects.

Residual impurities and native defects in 6H‐SiC bulk crystals grown by halide chemical-vapor deposition

A variety of defect-sensitive techniques have been employed to detect, identify, and quantify the residual impurities and native defects in high-purity (undoped) 6H‐SiC crystals grown by halide chemical-vapor deposition technique. The incorporation efficiencies of N and B are determined by the site-competition effect. Most notably, material with low residual N levels (∼1014cm−3) can be produced. In addition, the nitrogen concentrations obtained from Hall-effect measurements and low-temperature photoluminescence are systematically lower than those determined from secondary-ion-mass spectrometry. The difference is ascribed to nitrogen forming complexes with native defects. The energy level of this complex is approximately 0.27eV below the conduction band. Four major electron traps with activation energies of 0.4, 0.5, 0.65, and 1eV and five hole traps with activation energies of 0.3, 0.4, 0.55, 0.65, and 0.85eV were observed by deep-level transient spectroscopy. The concentration of all traps decreased stro...

Radiation-induced E' centers in H2-annealed oxide films

In oxides fabricated by thermal oxidation or ion implantation, an order‐of‐magnitude increase is observed in the concentration of radiation‐induced E’ centers when the oxide is first exposed to H2 at temperatures greater than 600 °C. To the authors’ knowledge, we report the first data indicating that preirradiation hydrogen annealing of thermal oxides causes a large increase in the concentration of radiation‐induced E’ centers. We discuss evidence suggesting that the hydrogen anneal creates a new E’ precursor, Si—H, from Si—Si bonds.

Light induced electron spin resonance in porous silicon

We report on the observation of a new metastable light induced defect center in porous silicon which has a g tensor with 〈110〉 symmetry and which is similar to the thermal donor, NL8, seen in thermally annealed Si. The optical response has an onset in the vicinity of the Si indirect band gap. The time dependence of the response has a sharp rise followed by a nearly saturated regime. Changing to a higher photon energy then results in another marked increase in the response. We suggest that this dependence of the absorption on excitation energy reflects the distribution of crystallite sizes. We observe that near ultraviolet light significantly bleaches the signal.

Microwave‐detected optical response of YBa2Cu3O7−x thin films

Microwave‐detected optical response (MDOR) of YBa2Cu3O7−x and other oxide superconductor thin films is shown to yield information complementary to that provided by trasnport photoconductivity measurements. The MDOR technique yields a superposition of response from all illuminated portions of a sample, irrespective of the existence of a resistive macroscopic percolative current path. The response is found to be bolometric at temperatures for which resistance appears in transport measurements. At low temperatures MDOR results imply a nonbolometric response which in some respects is consistent with nonequilibrium quasiparticle concentration due to radiative pair breaking.