摘要:

Intersubband electric dipole transitions between subbands of the same symmetry are not allowed in symmetric quantum well structures. When a small asymmetry is introduced this selection rule is broken, and the forbidden transitions become allowed. However, we have found that in many cases where the asymmetry of the structure is not too large, the intersubband electric quadrupole transitions dominate the optical transitions. For example, we show that, when the asymmetry is induced by an external dc‐electric field, then, the electric quadrupole transitions dominate up to fields of the order of 10 kV/cm. It is also shown that for structures with a ‘‘built‐in’’ asymmetry, there is a wide range of structures where the electric quadrupole transitions dominate the optical transitions. The selection rules for these transitions differ from those of the electric dipole transitions. It is found that the maximum strength of the electric quadrupole transitions is achieved when the infrared light is linearly polarized in 45° relative to the growth direction (in the quantum well plan), and therefore, the 45° waveguide geometry is optimized for inducing these transitions.

ISSN:0021-8979    

DOI:10.1063/1.354267

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The spectral quantum efficiency in ReS2heterodiodes was measured in the wavelength region between 600 and 850 nm. Anisotropy effects in the van der Waals plane were observed by means of linearly polarized light at normal incidence to the (001) plane of ReS2single crystals. The anisotropy in quantum efficiency reaches its maximum at &lgr;=627 nm and therefore, ReS2devices are well suited for detecting the angle of polarization in experiments using a HeNe laser. At wavelengths above 750 nm the sign in polarization quantum efficiency changes due to various optical transitions.

ISSN:0021-8979    

DOI:10.1063/1.354268

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A broad peak was observed in commercially available single‐crystal 6H‐SiC material. The samples were nitrogen doped,ntype with free carrier concentration (ND−NA) of 1.3×1018cm−3that was determined from capacitance‐voltage (C‐V) measurements. The defect concentration profile showed that the defect was spatially localized and had a maximum concentration of 2.5×1014cm−3at 570 A˚ from the semiconductor‐metal interface. The activation energy varied with applied voltage fromEc−0.40 eV atVR=−7 V toEc−0.54 eV atVR=−5 V. This can be explained qualitatively in terms of the Poole–Frenkel effect. The defect was removed by the growth and subsequent removal of an oxide layer. Therefore, we conclude that the defect was caused by residual damage from the polishing process.

ISSN:0021-8979    

DOI:10.1063/1.354269

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We present time‐resolved photoluminescence studies of perpendicular transport and the subsequent capture of photoexcited carriers in II–VI semiconductor superlattices with an enlarged quantum well. The trapping dynamics are investigated as a function of the superlattice period in the range of 30–80 A˚. We observed an efficient collection of electrons and holes in an enlarged CdTe quantum well confined by a short‐period CdTe/Cd1−xMnxTe superlattice. Both the characteristic time and the efficiency of carrier collection are found to be strongly dependent on the superlattice period. A time constant less of than 25 ps is determined for the carrier collection in a 30‐A˚‐period superlattice. Our experimental findings give evidence of Bloch type perpendicular transport in electron and heavy‐hole minibands.

ISSN:0021-8979    

DOI:10.1063/1.354270

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We investigated the thermal decomposition of native oxide on Si(100) under ultrahigh vacuum using high‐resolution x‐ray photoelectron spectroscopy (XPS). The native oxide was formed by wet chemical treatment (HCl/H2O2/H2O), a widely employed procedure for preparing atomically clean surfaces. XPS measurements revealed that high temperature heating (≳700 °C) leads to a remarkable alteration in Si 2pand O 1sspectra. After heating to 700 °C, the Si3+structure increases and the O 1sfull‐width‐at‐half‐maximum decreases. After heating to 800 °C, the Si4+and O 1sintensity decreases but the Si2+intensity remains almost unchanged. We suggest that the formation of volatile SiO is related to the Si3+structure produced by high temperature annealing.

ISSN:0021-8979    

DOI:10.1063/1.354271

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Preparation, x‐ray diffraction, and ac susceptibility measurements of Y1−xCaxSr2Cu2.67Ti0.33O6+zare presented.Tcincreases from 18 K forx=0 to 22 K forx=0.1 and then decreases to 18.5 K forx=0.15. The irreversibility line derived from the susceptibility data indicates an enhancement ofx=0.05.  

ISSN:0021-8979    

DOI:10.1063/1.355309

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The patterning and characterization of a single crystal diamond substrate with a regular array of 8 micron features is described. Patterned arrays in diamond have been formed by a process consisting of the evaporation of ferrous metals through a glass channel mask onto the (100) surface of diamond. The evaporation was followed by a high temperature heat treatment to form a metal carbide. The carbide was then removed by acid etching, leaving behind a positive two‐dimensional pattern on the diamond surface. Further pattern transfers were reproducibly accomplished by pressing the etched diamond surface into gold and aluminum foils.

ISSN:0021-8979    

DOI:10.1063/1.354272

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The prospect of temperature tunability in an AgGaSe2crystal for use in infrared nonlinear optics is explored for the first time. Tunable second harmonic generation from CO2laser radiation is studied at different temperatures of the crystal. The tuning offered is small and the experimental finding is consistent with dispersion measurement.

ISSN:0021-8979    

DOI:10.1063/1.354273

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In0.2Ga0.8As single strained quantum well lasers with GaAs/Al0.45Ga0.55As short‐period superlattice barrier (SPSB)layers have been successfully prepared using molecular beam epitaxy although a part of SPSB has been grown in the forbidden temperature region for AlGaAs, where the specular smooth bulk AlGaAs cannot be grown. The averaged AlAs mole fraction of the present SPSB is 0.25, which gives the heterobarrier height larger than that of the conventional GaAs barrier layers. The threshold temperature sensitivity factorT0of the laser with SPSB has been measured to be as low as 240 K, which is much larger than that of 90 K in the laser with GaAs barrier layers. This improvement results from the reduction of carrier leakage from quantum well to barrier layers.

ISSN:0021-8979    

DOI:10.1063/1.354274

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The heterostructures Hg1−xCdxTe/PbS (x=0.2) and their electrophysical and optical characteristics are investigated. Some electrophysical parameters are determined fromI‐UandC‐Vmeasurements.    

ISSN:0021-8979    

DOI:10.1063/1.354275

出版商:AIP    

年代:1993

数据来源: AIP