摘要:

We report on the photoluminescence properties and the hydrogen depth distributions of plasma treated GaAs/AlGaAs multiple quantum well (MQW) structures. Specimens grown by molecular beam epitaxy were exposed to a deuterium plasma under different temperature‐time conditions. Photoluminescence measurements were made at 4.2 K, using low and high excitation powers, on the hydrogenated samples and on untreated partners. A decrease in the linewidth of the free exciton and an increase in the peak intensity were observed in specimens following plasma treatment. In general, each of the hydrogenated MQW specimens displayed an increase in luminescence efficiency and a diminution of impurity‐related peaks after hydrogenation. Secondary ion mass spectrometry measurements yielded depth distributions for2H and Al atoms. In samples having the best luminescence, the2H was nearly constant throughout the MQW region, at about 1018cm−3.

ISSN:0021-8979    

DOI:10.1063/1.353401

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The photoluminescence (PL) spectra of Te‐doped GaSb epitaxial layers grown from Ga‐rich solutions by liquid‐phase epitaxy have been studied in the electron concentrations from 8×1015to 4×1018cm−3. The electron concentration can be accurately controlled by varying the growth temperature and adding the polycrystalline Te‐doped GaSb to replace half or all the undoped GaSb starting material in the growth solution. The dependence of line position, line intensity, spectral shape, and broadening on the doping level, power excitation, and temperature has been investigated in detail. At concentrations as low as 1×1016cm−3, the GaSb sample has become degenerate because of the small effective mass of electrons and the broad band consisting of five partially resolved line dominates the low‐temperature PL spectra. At concentrations above 1×1018cm−3, the 19 K PL spectra is mainly dominated by the sub‐band‐gap, substrate‐induced line A’at 775.8 meV which is enhanced by the scattering of light off the back surface. This line A’is direct evidence for the band‐gap shrinkage at high doping level. This is the first report to present the detailed luminescence lines in the PL spectra of the Te‐doped GaSb samples.

ISSN:0021-8979    

DOI:10.1063/1.354085

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Observation of magneto‐optical transitions in a Si center &dgr;‐doped quantum well is reported. The excitons are stabilized by the application of the field. With photoluminescence excitation spectroscopy (PLE), transitions below the Fermi level are observed. At magnetic fields ≳8 T broadened Landau levels can be seen. The features in the PLE spectra sharpen up significantly when the Larmor diameter becomes less than the average interimpurity spacing. The effect of the ionized donors in the well on the optical spectra is discussed.

ISSN:0021-8979    

DOI:10.1063/1.353402

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The infrared‐absorption spectra of commercially available SiC whiskers were measured. The spectra were dominated by the small particle resonances associated with finite ionic crystals. Analysis of these resonances allowed characterization of both morphological and electronic properties of the whiskers. The data were analyzed using Rayleigh and Mie theories as well as an effective medium model based on the Maxwell–Garnett approximation. It was possible to determine whisker radius and identify whiskers with a high free‐carrier density and a core‐shell structure. The effects of decreasing aspect ratio were also studied. The results showed that infrared‐absorption spectroscopy provides an effective and relatively simple means to characterize SiC whiskers.

ISSN:0021-8979    

DOI:10.1063/1.353403

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Conventional spectroscopic ellipsometry (SE) and synchrotron radiation spectroscopic ellipsometry (SRE) measurements were used to study SiN thin films grown with different techniques. The SiN films were chosen to have low oxygen and hydrogen content (<5%). Analysis of the dielectric function (&egr;) spectra measured by SE and SRE with the tetrahedron model and in conjunction with Rutherford backscattering spectroscopy (RBS) results shows that the dielectric function can be indicative of the stoichiometry of the materials. In addition, we apply a theoretical model to estimate major optical parameters characterizing the materials, such as the fundamental and Penn gaps, the refractive index and the thickness. Furthermore, the film thickness estimated by SE coincides within 10% with that calculated by other techniques, while the stoichiometry is found almost identical with the one estimated from RBS and systematically smaller than the one calculated by Auger sputter profiling.

ISSN:0021-8979    

DOI:10.1063/1.353379

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Pb(⋅Si≡Si3) andE’(⋅Si≡O3) centers in the Si/SiO2structure under gamma‐ray radiation are studied with the electron‐spin‐resonance technique. The Si/SiO2structures of (111), (110), and (100) planes are obtained using three different oxidation methods to control the concentration of the hydrogen‐related impurities. It is first observed that the concentration ofPbcenters is decreased with increasing radiation dosage in samples with lower concentrations of impurities; the concentration dramatically increases in the samples with a higher concentration of impurities. The concentration ofE’centers also increases with accumulated dosage. When the Si/SiO2structure is fabricated under different oxidation temperatures, the saturated concentrations ofPbcenters after irradiation depend upon the oxidation temperature. The concentration ofE’centers increases with oxidation temperature after irradiation. When the Si/SiO2structure is fabricated under different cooling rates, concentrations of thePbcenters are saturated after irradiation. The concentration ofE’centers increases with increasing cooling rate.

ISSN:0021-8979    

DOI:10.1063/1.353380

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Detailed excited‐state excitation spectra are recorded in the infrared emission domains around 1.06 and 1.32 &mgr;m of the Nd3+‐doped Y3Al5O12, YLiF4, and LaMgAl11O19laser crystals. The positions of the observed lines are compared with that deduced from the energy levels of the Nd3+ion in these materials.

ISSN:0021-8979    

DOI:10.1063/1.353381

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The Stanford Linear Accelerator Center is currently operating with a photocathode electron gun (PEG) to produce polarized electrons for its experimental program. Bunch intensities of up to 1011electrons within 2 ns (8 A) are required from the electron gun. Operation of PEG has demonstrated a charge limit phenomenon, whereby the charge that can be extracted from the gun with an intense laser beam saturates at significantly less than 1011electrons (the expected space‐charge‐limited charge) when the photocathode quantum efficiency is low. Studies of this charge limit phenomenon observed with a GaAs photocathode are reported.

ISSN:0021-8979    

DOI:10.1063/1.353382

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The decrease of the Richardson constant by more than 3 orders of magnitude in the indirect energy‐gap range of composition of the GaAs/AlxGa1−xAs interface has been interpreted in the past in several ways. We present here a phenomenological model based on envelope wave functions that can describe the transmission coefficient involving two mechanisms:zero‐phonontransitionsdue to &Ggr;‐Xmixing andphononassistedtransitions. The model shows reasonable agreement with a wide range of experimental data. We conclude that a general relation for the thermionic current across heterojunctions must include phonon effects when the Al mole fraction exceeds 0.45. We also find that calculated transmission coefficients are very different from the step function used in the classical theory.

ISSN:0021-8979    

DOI:10.1063/1.353383

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The gas flow of particles laser desorbed from an elemental target into a vacuum is studied by Monte Carlo simulation. Pulsed desorption off afiniteareais modeled; this is possible by using a three‐dimensional simulation algorithm. We monitor the temporal evolution of the desorption cloud and global features of the flow, such as the number of collisions occurring in the gas, and the fraction of particles backscattered to the surface. The angle and energy distribution of the desorbed particles is investigated as a function of the number of monolayers desorbed, and the laser spot width. Our results show the formation of a desorption jet, in which fast particles are focused towards the jet axis, while slow particles leave the jet at oblique angles. Many features of the particle flux may be fitted by so‐called elliptical distributions. However, these represent the velocity distribution of particles at oblique angles only poorly. Finally, we demonstrate the differences which exist between our three‐dimensional simulation and previous, one‐dimensional treatments. These studies are of fundamental interest for the characteristics of thin films deposited from the desorbed material.

ISSN:0021-8979    

DOI:10.1063/1.353384

出版商:AIP    

年代:1993

数据来源: AIP