摘要:

The concept of competition‐during‐heating as a mechanism for TL supralinearity and sensitization in TLD‐100 is re‐examined. It is demonstrated that with this model it is not necessary that the competing traps be removed by radiation in order for supralinearity to be obtained. Furthermore, the arguments for and against the proposal that the precursors to the 5.5 eV centers are the required competing centers are reviewed. It is shown that those experiments which are cited as arguments against this proposal have other interpretations different from those published so far. These new interpretations are consistent with the original view that the 5.5 eV centers’ precursors are indeed competitors to the main TL process. MgOH centers are also likely competitors in samples with substantial OH contents.

ISSN:0021-8979    

DOI:10.1063/1.346805

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

By applying the HAUP (high‐accuracy universal polarimeter) method, [N(CH3)4]2CuCl4was found to be optically active in its incommensurate phase, e.g.,g23=3.9×10−5at 24 °C. The temperature dependence ofg23was obtained. By making use of the fact that the commensurate phase of [N(CH3)4]2CuCl4is ferroelastic and centrosymmetric, the temperature dependence of the soliton density in its incommensurate phase was deduced by this optical measurement. It is suggested that the HAUP method will become a new and direct method for measuring soliton densities of incommensurate states.

ISSN:0021-8979    

DOI:10.1063/1.346806

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The photoluminescence (excitation, emission, and fluorescence decay time) spectra of spinel (Ti:MgAl2O4) excited by pulsed Nd:YAG and dye lasers have been systematically investigated. Excitation into theT2g→Egcrystal‐field transition of Ti3+produces a broad infrared emission band at 805 nm which could, in principle, be used for tunable laser applications. Two different Ti3+centers with lifetimes of 4.2 and 41 &mgr;s contribute to this emission. At the same time this IR band can also be observed under high‐intensity excitation in the ultraviolet region (266 nm) via a two‐photon process.

ISSN:0021-8979    

DOI:10.1063/1.346807

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We report a study of a defect responsible for the ‘‘g’’ bound exciton line at 1.5112eV that is frequently detected in photoluminescence spectra of GaAs grown by molecular beam epitaxy (MBE). A direct correlation has been observed between this line and a transition at 1.4946eV, which is shown to result from a conduction band‐to‐acceptor recombination involving a shallow, unidentified acceptorlike defect that is labeled ‘‘A.’’ The activation energy of the defect is 24.8±0.2 meV, about 1.7 meV lower than that of CAsacceptor. Upon hydrogenation the defect is passivated more extensively than any known shallow acceptor species in GaAs. This result is analyzed in terms of a passivation model, from which it can be inferred that the A defect is not due to a simple substitutional Group II impurity on a Ga site. Incorporation of the A defect strongly affects the luminescence properties of the material. An almost complete quenching of the donor‐bound exciton lines, profound changes in the line shape and relative intensity of the free exciton recombination, and appearance of a sharp transition of unknown origin at 1.5138eV were observed with increasing defect concentration. Apparently ‘‘donorless’’ low temperature exciton recombination spectra are reported for defect‐richp‐type MBE GaAs layers with donor concentrations as high as 7×1014cm−3and compensation ratios of ∼0.3. The dependence of the defect incorporation on MBE growth parameters is discussed. The feasibility of MBE growth of high purity, nearly shallow defect‐freep‐type GaAs layers at marginally As‐stabilized surface conditions over an about 1–5 &mgr;m/h range of deposition rates is demonstrated.

ISSN:0021-8979    

DOI:10.1063/1.346779

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Room‐temperature scanning photoluminescence (SPL) measurements were performed on Fe‐doped semi‐insulating InP wafers obtained from various suppliers. It was found that defects and inhomogeneities such as short‐ and long‐range doping striations, dislocations, and subsurface extended defects are associated with specific ‘‘signatures’’ in SPL images, regardless of the origin of the samples. Because SPL measurements are fast and nondestructive, they appear to be useful for the evaluation of this material.

ISSN:0021-8979    

DOI:10.1063/1.346780

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The first detailed optical investigation of the metastable III/V semiconductor alloy GaP1−xSbxis presented. Epilayers are grown by atmospheric pressure organometallic vapor phase epitaxy throughout the entire compositional range on GaP, GaAs, and GaSb substrates. The approximately 1‐&mgr;m‐thick layers are partially strained with values of lattice mismatch as large as 1.7×10−2. Values of band gap are determined for the first time from absorption spectra measured at 10 and 300 K and corrected for strain‐induced energy shifts. The resultant values of bowing parameter for the direct and indirect band gaps arec&Ggr;=3.11±0.18 eV andcx=2.06±0.18 eV, independent of temperature, yielding a value of direct/indirect crossover composition,xc, of 0.32±0.07. Single photoluminescence (PL) peaks are observed between 10 and 300 K for all samples. For samples with 0.37≤x≤0.47, they are assigned to recombination of carriers localized in bandtail states induced by compositional fluctuations. The stretch of the tails into the gap is greatly enhanced over the random alloy limit by the metastability of the Ga1−xSbxalloys. PL peaks for samples withx≤0.32 are assigned to recombination via deep centers in the gap. The PL peak of a sample withx=0.91 is assigned to recombination involving shallow acceptors.

ISSN:0021-8979    

DOI:10.1063/1.346781

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Gold films that are initially isotropic in the plane of the substrate following deposition at normal incidence are shown to become anisotropic as they are sputter etched with a beam of Ar+ions.Insitumonitoring of the optical transmittance at normal incidence for two polarizations during deposition and during etching shows that the effect increases with angle of incidence of the ion beam and depends strongly on the initial post‐deposition morphology of the gold film. Films with an initial globular morphology subsequently exhibit the largest anisotropy, about 1%–2% in transmittance. Fully coalesced films show no anisotropy until they break up at very small thickness. Anisotropy versus mean transmittance deposition and etch paths are quite dissimilar and are indicative of different deposition/etch island morphologies.

ISSN:0021-8979    

DOI:10.1063/1.346782

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We study in this paper the different physical processes involved in laser‐produced plasma in confined geometry. With this technique, a laser irradiates a target at an intensity of a few GW/cm2, and the produced plasma is confined by a transparent overlay to the laser which covers this target. This configuration has appeared necessary for example for metallurgical applications where, for a given laser energy, enhanced pressures must be realized in order to achieve high shock pressures. Therefore, a physical study of this method is useful in order to optimize this technique. We have first developed an analytical model which describes the different steps involved in this process, points out the interest of this technique, and compares it to the direct ablation regime. In the first stage, during the laser heating, the generated pressure is typically 4–10 times greater than the corresponding one obtained in direct ablation. The second step begins after the switch‐off of the laser and is characterized by an adiabatic cooling of the plasma which maintains the applied pressure over a period which is about 2 times the laser‐pulse duration. Finally, the third stage concerns also the adiabatic cooling of the recombined plasma, but during this period the exerted pressure is too small to realize a plastic deformation of the material.We show that the impulse momentum given to the target is mainly generated during this step. This model allows us to also determine the velocities of thin foils accelerated with confined plasmas, and we show that very high hydrodynamic efficiencies can be achieved by this technique. Experimentally, we measured with quartz gauges, the pressures obtained in confined geometry, for 30‐, 3‐, and 0.6‐ns laser‐pulse duration. This study shows that short pulse durations are sensitive to the initial roughness of the interface, and such an effect should be suppressed by using a liquid confinement. Then, we conclude that a large fraction of the absorbed laser energy (80%–90%) is used for the ionization of the medium in these conditions of irradiation. Finally, we experimentally point out that the laser‐induced breakdown of the confining medium is the main mechanism which limits the generated pressure and show the influence of the laser‐pulse duration on this effect.

ISSN:0021-8979    

DOI:10.1063/1.346783

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Structural and electrical damage imparted to InP and In0.72Ga0.28As0.6P0.4(&lgr;g&bartil;1.3 &mgr;m) surfaces during CH4/H2reactive ion etching (RIE) have been examined. X‐ray photoelectron spectroscopy was used to monitor changes in the surface chemistry, Rutherford backscattering spectrometry was used to measure crystallographic damage, and current‐voltage and capacitance‐voltage measurements were made to examine electrically active damage and its depth. Two classes of damage are observed: crystallographic damage originating from preferential loss of P (As) and/or ion bombardment‐induced collision cascade mixing and, forp‐type material, hydrogen passivation of Zn acceptors. Etching at 13.6 MHz, 60–90 mTorr, 10% CH4/H2, and bias voltages of ∼300 V contains gross (≳1%) damage as measured by RBS to within 40 A˚ and electrically active damage to within 200 A˚ of the surface. This is a factor of 3–6 shallower than other RIE processes operated below 10 mT with comparable or higher bias voltages. Acceptor passivation of both InP and InGaAsP, arising from the association of hydrogen with Zn sites, occurs to a depth of 2000 A˚ after RIE and causes a decrease in carrier concentration in this layer. The effect is reversed, however, by rapid thermal processing at temperatures between 350 and 500 °C.

ISSN:0021-8979    

DOI:10.1063/1.346758

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper, results for dielectric oxide films deposited using downstream microwave plasma‐enhanced chemical vapor deposition in the temperature range between 250 and 400 °C are presented. The deposition of oxide using TEOS (tetraethoxysilane)+O2and TEOS+N2O chemistries are studied. In the reactor, the TEOS is injected directly into the deposition chamber without passing through the discharge. Only He, O2, or N2O are fed through the microwave cavity where the discharge is generated. In addition, no ions but chemically active species are present in the deposition chamber during the deposition. The deposition rate is found to decrease with increasing temperature. In addition, it appears that the deposition rate increases with increasing concentration of active oxygen species in the deposition chamber. These suggest that the generation of intermediate species of TEOS and adsorption/desorption of the reactant on the surface are the key steps that determine the deposition rate. The stress of the deposited oxide films is found to be tensile and less than 2×109dyn/cm2. The Si‐OH concentration in the films is found to be low and can be below the detection limit of infrared spectrometry by increasing the flow ratio of O2/TEOS during the deposition. The step coverage of the oxide films over the Al runners is found to be excellent due to the long diffusion time available for TEOS surface species before forming SiO2. The mechanisms of oxide deposition using TEOS+O2and SiH4+N2O chemistries are studied and compared. The details of oxide step coverage versus different deposition processes are also discussed.

ISSN:0021-8979    

DOI:10.1063/1.346759

出版商:AIP    

年代:1990

数据来源: AIP