摘要:

InAs/InGaAs self‐assembled quantum dots (SADs) are fabricated on (311)BGaAs by molecular‐beam epitaxy using the Stranski–Krastanow growth mode. The critical thickness for the SAD formation on (311)Bsubstrates is almost the same as that on (100) surface, characterized byinsitureflection high‐energy electron‐diffraction pattern changes. Atomic force microscopy observation on In0.5Ga0.5As SADs showed uniform nanometer scale quantum dots with a typical dot diameter of around 25 nm ±2 nm and a typical height of 13.7 nm ±2.2 nm. In photoluminescence measurements, an intense peak from the SADs is observed with a sharp luminescence from the wetting layer that exists two‐dimensionally underneath the SAD structure. A compound SAD structure is proposed where the composition of the wetting layer and the SAD layer are different. This is intended to suppress the luminescence from the wetting layer. The narrowest full widths at half‐maximum of the luminescence from the compound SADs on (311)Bare about 35 meV at 2 K and 41 meV at room temperature, respectively, which are much narrower than those measured from reference (100) SADs. The PL linewidth is not strongly affected by the measurement temperature, indicating that the linewidth is mainly governed by inhomogeneous broadening due to the dot size distribution. These narrow PL spectra demonstrate an advantage of using (311)Bsubstrates for SAD formation in molecular‐beam epitaxy. A shoulder peak distinctly observed in the SAD luminescence corresponds to the transition between the second quantum level in the SADs, as estimated from a simple calculation of the quantum confinement energy as well as excitation‐intensity‐dependent photoluminescence measurements. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363216

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The valence hole subbands, TE and TM mode optical gains, transparency carrier density, and radiative current density of the zinc‐blende GaN/Ga0.85Al0.15N strained quantum well (100 A˚ well width) have been investigated using a 6×6 Hamiltonian model including the heavy hole, light hole, and spin‐orbit split‐off bands. At thek=0 point, it is found that the light hole strongly couples with the spin‐orbit split‐off hole, resulting in the so+lh hybrid states. The heavy hole does not couple with the light hole and the spin‐orbit split‐off hole. Optical transitions between the valence subbands and the conduction subbands obey the &Dgr;n=0 selection rule. At thek≠0 points, there is strong band mixing among the heavy hole, light hole, and spin‐orbit split‐off hole. The optical transitions do not obey the &Dgr;n=0 selection rule. The compressive strain in the GaN well region increases the energy separation between theso1+lh1 energy level and the hh1 energy level. Consequently, the compressive strain enhances the TE mode optical gain, and strongly depresses the TM mode optical gain. Even when the carrier density is as large as 1019cm−3, there is no positive TM mode optical gain. The TE mode optical gain spectrum has a peak at around 3.26 eV. The transparency carrier density is 6.5×1018cm−3, which is larger than that of GaAs quantum well. The compressive strain overall reduces the transparency carrier density. TheJradis 0.53 kA/cm2for the zero optical gain. The results obtained in this work will be useful in designing quantum well GaN laser diodes and detectors. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363217

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The infrared optical constants of cubic and tetragonal zirconias were determined from the analysis of the reflectance spectra of yttria doped zirconia single crystals and undoped tetragonal zirconia powders, respectively. In the latter case, the reflectance spectrum was analyzed by an effective medium Theory, which has been proven to successfully determine the dielectric properties of small powder particles. The symmetry of the resulting modes of tetragonal zirconia was elucidated by performing theoretical calculations of the vibrational properties. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363218

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effects of the channel electron density on the interband optical transitions of strained (x=0.6 and 0.65) and lattice‐matched (x=0.53) InxGa1−xAs/In0.52Al0.48As/InP high‐electron‐mobility transistor structures have been investigated by phototransmittance at room temperature. Analysis of the ground and first excited transitions for low and high densities, respectively, enabled a separate estimation of the electron densities occupying each one of the first two subbands. It was found necessary to include the modulation of the phase‐space filling in the analysis of the spectra, especially for the samples with a high electron density, in which case this modulation mechanism becomes dominant. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363219

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The microwave properties and the crystal structure of YBa2Cu3O7−&dgr;thin films are examined as a function of film thickness. Already below the critical thickness for the formation of macroscopic fractures, we observe an increase of the residual surface resistance with thickness. Cross‐sectional transmission electron microscopy reveals that this behavior is caused by structural changes of antiphase boundaries leading to normal‐conducting regions between superconducting grains. The surface resistance is calculated within the weak‐link picture. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363220

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Deep level transient spectroscopy (DLTS) and single shot capacitance techniques were used to investigate Si doped AlxGa1−xAs with Al compositions ranging from 0.30 to 0.59. Under illumination, an additional DLTS peak was observed in samples with Al compositions close tox=0.38. This peak has been shown to originate from a localized state with an energy level shallower than theDXground state and energy barriers for both electron emission and capture processes. Persistent photoconductivity effects related to this state were observed below 50 K. Evidence is provided that this state is a metastable state of the Si donor. Its origin is discussed in terms of the intermediate state of a negativeUDXcenter and a metastable negativeUconfiguration of the Si donor. A further photoinduced DLTS peak was observed within a narrow Al composition range that has been identified as the shallowestDXlevel of the four distinctDXlevels originating from different local environments of the donors. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363637

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Low‐temperature photoluminescence (PL) spectroscopy is used for determination of the nitrogen doping concentration in noncompensated 4H– and 6H–SiC by comparing the intensity of nitrogen‐bound exciton (BE) lines to that of the free exciton (FE), the latter being used as an internal reference. The results are compared with a previous work performed for the case of 6H–SiC only. A line‐fitting procedure with the proper line shapes is used to determine the contribution of the BE and FE lines in the PL spectrum. The ratio of the BE zero‐phonon lines (R0andS0in 6H,Q0in 4H) to the FE most intensive phonon replica around 77 meV exhibits very well a direct proportional dependence on the doping as determined by capacitance–voltage (C–V) measurements for both polytypes. The use of fitting procedure which takes into account the real line shapes, the influence of the spectrometer transfer function, and the structure of the PL spectrum in the vicinity of the FE replica allows us determination of the N‐doping concentration by PL for doping levels in the region 1014cm−3–3×1016cm−3for 4H– and 1014cm−3–1017cm−3for 6H–SiC. Above these levels the free‐exciton related emission is not observable. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363221

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Low‐temperature photoluminescence (PL) spectra of detector grade Cd1−xZnxTe (x=0.1) have been measured to obtain information about shallow level defect concentration introduced during mechanical polishing and chemical etching processes. We present here a comparative PL study of Cd0.9Zn0.1Te crystals treated by different chemical solutions used for nuclear detector surface treatment. The results show that the 5% Br–MeOH+2%Br–20% lactic acid in ethylene glycol treatment combines the advantages of bromine and lactic acid for chemical etching and results in the best surface condition, as evidenced by the largestI(D0,X)/Idefintensity ratio and the narrowest full width at half‐maximum of the main peak (D0,X). Changes in the surface morphology were also analyzed by atomic force microscopy and correlated with the PL results. Current–voltage (I–V) curves and the room‐temperature55Fe spectral response of the sample etched by the best treatment are also presented and discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363222

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Visible photoluminescence (PL) bands around 2 eV were studied in60Co &ggr;‐irradiated (dose<1 MGy) oxygen‐deficient‐type amorphous SiO2(a‐SiO2) excited by 2–4 eV photons. In addition to the well‐known 1.9 eV PL band due to nonbridging oxygen hole centers, another PL band was observed at 2.2 eV when excited by 3.8 eV photons. The intensity of the 2.2 eV band increases with decreasing oxygen partial pressure during the sample preparation. Electron‐spin‐resonance measurements show that the intensity of the 2.2 eV band is correlated with the concentration of theE&dgr;′center, a paramagnetic state of a cluster of silicons. After much higher &ggr; irradiation with a dose up to 10 MGy, a new PL band was induced at 1.75 eV under excitation by 2.5 eV photons, as well as the 1.9 and 2.2 eV PL bands. By comparing its spectral shape and excitation energy with known PL band in Si‐implanteda‐SiO2, it is suggested that the 1.75 eV band is associated with Si nanocrystals formed from Si clusters ina‐SiO2by the high‐dose &ggr; irradiation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363223

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The controversial relationship between the intrinsic 1.9 eV photoluminescence (theRband), the 2.0 and 4.8 eV excitation/optical absorption bands is examined for 11 specimens with different OH, Cl, and O2concentrations and with different irradiation histories. The direct relation is found between the intensities of theR‐band luminescence measured with the 4.0 eV (IUV) or resonance (1.94 eV) excitation (Ires), although it is slightly superlinear,Ires∝(IUV). The deviation from the exactly constant ratio between the band intensities in different glass samples is attributed mainly to the inhomogeneous broadening effects. Also, nonzero polarization degree,P=−1.5±0.5%, in theR‐band maximum region is reconfirmed upon linearly polarized 4.8 eV excitation, substantiating a close relation between the 4.8 eV absorption band and theRband. These results suggest strongly that both of the two absorption bands at 2.0 and 4.8 eV are originated from a single center, and quantitatively agree with the recently proposed optical transition scheme in the nonbridging oxygen hole center. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363224

出版商:AIP    

年代:1996

数据来源: AIP