摘要:

We examine in detail the photoluminescence properties of the DX center in periodically &dgr;‐doped Si‐AlxGa1−xAs (x=0.32). The DX center is shown to be band structure dependent rather than a conduction miniband resonance. New photoluminescence lines at energies below the band gap at 1.878, 1.787, and 1.695 eV have been detected, and ascribed to a radiative recombination of electrons in confined subbands or in minibands with holes located at the top of the valence band between the &dgr; layers.

ISSN:0021-8979    

DOI:10.1063/1.348417

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Raman spectroscopy measurements have been performed on GaAs:Be samples with high crystalline quality and exceptional heavy doping level ranging from 1019to 1.4×1021cm−3. The recorded spectra show a structure we assigned to a coupled LO phonon‐damped plasmon mode. A theoretical expression for the Raman scattering rate by this mode has been derived from a dielectric model and compared to the experimental data. Using a fitting procedure the doping level of the samples has been estimated in agreement with Hall measurements. Moreover, the study of the Raman intensity evolution of both unscreened‐LO and coupled phonon‐plasmon structures, provided a convenient and rapid method to determine the activated carrier density inp‐doped polar semiconductors. Disorder effects due to the dopant impurities have been also observed and analyzed using a spatial correlation model description.

ISSN:0021-8979    

DOI:10.1063/1.348957

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effects of biaxial compressive and tensile strain on the excitonic resonances and associated changes in refractive index and electro‐optic effect in quantum wells have been calculated and measured. Theoretical calculations include the important heavy‐hole–light–hole band mixing effects. It is seen that the excitonic contributions dominate near the band edge. With increasing compressive strain the linear electro‐optic effect is slightly increased, while the quadratic effect is greatly enhanced. The effects are reversed in quantum wells under tensile strain.

ISSN:0021-8979    

DOI:10.1063/1.348418

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The electronic structure of severaln‐type GaAs samples containing ‘‘&dgr;‐doping’’ layers of Si have been studied using photoreflectance (PR) spectroscopy. Well‐defined oscillatory features due to electronic transitions well above the band gap are observed at 300 K and identified as Franz–Keldysh oscillations. The energy spacing and the intensity of the oscillations decrease with decreasing temperature as a consequence, we believe, of changes in the electric field due to the surface charges. Self‐consistent energy‐band calculations support the interpretation that the oscillatory structure is due to Franz–Keldysh effects. The imposition of magnetic fields up to 15 even at room temperature has a pronounced influence on the PR spectrum. Parallel fields suppress the oscillatory structure but cause a large increase in the PR peaks near the GaAs energy gap. At 4.2 K Landau‐like spectral features are observed for fields applied perpendicular to the doping layer.

ISSN:0021-8979    

DOI:10.1063/1.348419

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Semi‐insulating undoped GaAs was plastically deformed and then investigated by positron lifetime spectroscopy. Strains between 0% and 40% and temperatures of deformation of 450, 500, and 600 °C were investigated, with detailed investigations carried out for the lowest temperature of deformation. Between 0% and 4% strain a reduction of the grown‐in vacancy response takes place simultaneously with a slight increase in vacancy cluster size to 2 or 3 vacancies. Between 4% and 6% strain a very substantial increase in vacancy production occurs but nearly all of these vacancies are clustered into voids with a radius of about 50 A˚ and density of the order of 1013–1014cm−3. The total concentration of vacancies necessary to produce these voids is 1017–1018cm−3. This clearly shows that vacancies are formed upon deformation and that they are mobile at 450 °C. The small vacancy clusters (2 or 3 vacancies) are present at a concentration of about 5×1016cm−3, the same as for the 4% strained samples. Upon further deformation to 20% strain the overall defect concentration becomes so high that all positrons become trapped for which reason no absolute defect concentrations can be deduced. The dominant defect types can nevertheless be identified as voids (with average size of 20 A˚), two‐ or three‐vacancy clusters, and shallow traps. These shallow traps have a positron binding energy of about 40 meV. Isochronal and isothermal annealing of 40% strained samples shows that heat treatment reduces the void concentration but increases the average void size, reduces the small vacancy cluster concentration, but results only in a small decrease in shallow trap concentration. The shallow traps are likely the dislocation lines themselves and the small vacancy clusters appear to be associated with the dislocation lines.

ISSN:0021-8979    

DOI:10.1063/1.348420

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Changes in the ablation threshold of polyimide accompanying two rapid sequential pulses of 308‐nm‐XeCl excimer less radiation have been studied. Assuming that subthreshold radiation couples into the polymer thermally, the heat conduction equation for the system is solved in order to estimate resulting surface and subsurface temperatures. A ‘‘threshold’’ temperature for etching is suggested below which no surface modification is observed but where thermal degradation of the surface may occur. For pairs of pulses whose combined fluence is near the single pulse fluence threshold for etching, separation of the pulses in time resulted in the initiation, cessation, and reinitiation of soot deposition on the polymer surface. It is suggested that this temporally dependent preferential etching is caused by the formation of an atmosphere above the target that is rich in molecules such as CO, CN, C2H2, and C2. This atmosphere then undergoes dissociation through multiphoton and photopyrolytic processes upon receipt of the second pulse resulting in the deposition of a carbonaceous residue on the surface. The residue then acts as sites for the initiation of conical structures.

ISSN:0021-8979    

DOI:10.1063/1.348421

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Boron has been deposited successfully on Si(111) from the synchrotron‐radiation‐induced decomposition of decaborane (14), i.e., B10H14. The rate of deposition is limited by the adsorption rate of decaborane (14) on the surface. In addition there is some indication that there is an activation barrier to dissociative adsorption. The synchrotron‐radiation‐ induced growth rate of boron thin films from decaborane (14) is linear with coverage for a large range of thickness, suggesting a constant sticking coefficient for decaborane adsorption at room temperature.

ISSN:0021-8979    

DOI:10.1063/1.348422

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have used a Monte Carlo method to simulate laser induced chemical vapor deposition processes, assuming that the laser beam photodissociates the parent organometallic molecules in the gas phase. Metal containing fragments formed during the organometallic decomposition may diffuse to the substrate and are assumed to be instantly adsorbed on the surface at the point they hit. The effects of the total gas pressure in the system, the nature of the buffer gas, and the intensity of the laser beam on the shape of the deposited layers and on the deposition kinetics were studied. This model is also applicable to the simulation of electron beam chemical vapor deposition when low pressures are considered.

ISSN:0021-8979    

DOI:10.1063/1.348423

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effects of two‐step annealing, rapid thermal annealing (RTA) followed by furnace annealing (FA), on the crystalline and electrical properties of buriedn‐type layers formed in (100) Si by As implantation at an incident energy of 1 MeV have been investigated. The crystalline properties have been examined by Rutherford backscattering measurements and by transmission electron microscopic observations. The electrical properties have been studied by differential Hall measurements. A comparison between the annealing process of two‐step annealing and that of RTA or FA alone is made. The experimental results obtained from this work clearly show that buriedn+layers without residual defects can be formed by the use of RTA at 1050 °C followed by FA at 1000 °C, and that they are difficult to be formed by one‐step annealing of FA or of RTA.

ISSN:0021-8979    

DOI:10.1063/1.348424

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The roles ofDXcenters and surface states associated with then‐AlGaAs layer of &dgr;‐doped AlGaAs/GaAs high‐electron‐mobility transistors have been investigated by employing a two‐dimensional, self‐consistent ensemble Monte Carlo simulation. It is found that bothDXcenters and surface states degrade device performance, particularly as gate‐to‐source voltage increases. This degradation is manifested largely through reduced channel electron concentration, increased intervalley scattering, and enhanced real‐space transfer. Of the two defect states,DXcenters have more detrimental influence on drain current and transconductance.

ISSN:0021-8979    

DOI:10.1063/1.348425

出版商:AIP    

年代:1991

数据来源: AIP