摘要:

Cathodoluminescence of AlN–GaN short period superlattice films was measured at 6 K, 77 K, and room temperature. The superlattice films were deposited using a switched atomic layer metalorganic chemical vapor deposition process onto a buffer layer of either AlN or GaN, which was deposited on basal plane sapphire substrates. The individual AlN and GaN layers of the superlattice films ranged in thickness from 2.6 to 20.8 A˚. The cathodoluminescence of these samples was measured at several electron acceleration voltages to allow depth profiling of the samples. This allows the region of the sample (superlattice film, buffer layer, and substrate) from which the spectral features originate to be determined. A spectral peak in the ultraviolet region above the 3.5 eV band gap of GaN has been observed in all the superlattice samples studied to date. Our results indicate that the location of this peak is determined by quantum confinement in the GaN layers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363726

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In ion‐implanted semiconductors, details of the defects involved and annealing mechanisms which determine the final disorder structure are complicated and difficult to characterize. To investigate the residual defect distribution of the implanted layers, optical experiments, photoluminescence, photoluminescence excitation, and Raman scattering have been performed on 2 MeV Yb+‐implanted InP and subsequently annealed at 750 °C for 15 min as a function of chemical etching depth (d) down tod=4.51×Rp(Rp: projected range =410 nm). Their results were compared with those obtained from Rutherford backscattering spectrometry (RBS) channeling analysis. The above optical experiments showed that two residual defective regions are present at depths ofd=0.34–0.80×Rpand aboutd=2.56×Rp, whereas RBS channeling analysis indicated the existence of the only former region. We assign the type of the two defective regions to ‘‘clamshell’’ defects in the former region and to ‘‘end‐of‐range’’ defects in the latter region. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363073

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present in this work the results of a Raman spectroscopy study on the plasmon–phonon coupling in Ge‐dopedp‐type gallium arsenide. A series of polarized Raman scattering experiments were carried out on epitaxial films grown by liquid‐phase epitaxy on (100) GaAs substrates at 20, 100, and 300 K. The films wereptype with free hole densities varying in the range of 5×1017–1×1020cm−3. Under the scattering configurations employed, the longitudinal optical (LO) mode is forbidden for crossed polarization while the transverse optical (TO) mode is forbidden for both parallel and crossed polarizations. However, all the polarized Raman spectra showed two peaks with frequencies close to the TO and LO phonons of semi‐insulating GaAs. The appearance of such forbidden modes was accounted for with a theoretical model which considers phonon–plasmon coupled modes with wave vectors much larger than those given by the regularq≊0wave vector transferred by photons. Ionized acceptor impurities provide such additional wave vector transfer through elastic scattering of the photoexcited electrons and holes. It is demonstrated that the experimental values for position and linewidth of the peaks are well described by the theoretical calculations when Fro¨hlich‐type and deformation potential mechanisms are considered as means of interaction. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363074

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We propose a new and promising technique to study quantum‐size structures. In ultrathin monocrystalline PbTe films with thicknessesL=6, 18, and 30 nm, we have examined this technique, based on two‐photon optical excitation of the renormalized electronic structure and revealed a sharp dependence of frequencies of two‐photon resonant transitions onL. We explained this result within the framework of a model taking into account the real PbTe band structure, electron–electron and electron–phonon interactions, and interband and intra band redistribution of free carriers. We have estimated the characteristic time of interband polarization decay asT2≥300 fs and have analyzed the transformation of the nonlinear response for the caseL→∞. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363075

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We analyzed the photoluminescence (PL) mechanisms of porous silicon, and in particular, the origin of the PL high quantum efficiency (QE) at room temperature. For this we used postformation treatments, anodic oxidation, and hydrofluoric acid (HF) etching (known for their strong QE enhancement effect) correlated with a PL time resolved analysis. A third parameter was the temperature which, for heating above room temperature, gave a reversible quenching of the PL. All three parameters give asimilarevolution of the PL decay shape, which we consider to originate from the same evolution of the carrier dynamics. Porous silicon is described as an undulating wire. The high QE at room temperature is attributed to carrier localization inside minima of the fluctuating potential along the wire; these considerations are extended to another porous material: amorphous porous silicon. Anodic oxidation and HF dissolution diminish the wire size, giving a reduction of the localization length of the carriers and progressive suppression of the nonradiative recombination channel. A simple model permits one to link the changes of the PL decay shape to the QE evolution. The nonexponential PL decay shape is interpreted as being due to a distribution of nonradiative recombination rates, the value of the nonradiative recombination rate being limited by a tunneling effect. This highly simplified model explains the origin of the nonexponential decay shape, its modification and gives a good description of the QE evolution as a function of temperature, oxidation level, or porosity. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363076

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Thickp‐type porous 6H SiC layers were fabricated by anodization ofp‐type 6H SiC bulk crystals in dilute HF. Striking differences are observed in the reststrahl region room‐temperature reflectance of these porous layers compared to that of bulk 6H SiC crystals. Instead of the single broad band reflectance spectrum typically observed in bulk 6H SiC, a two‐band reflectance spectrum is observed. Several effective medium models, based on different morphologies of the component materials, 6H SiC and air, are used to obtain the frequency‐dependent dielectric function of porous SiC from which calculated reflectance spectra are generated. The best match between measured and calculated spectra is obtained for a Maxwell–Garnett model with SiC acting as the host material and air cavities acting as the inclusion material. The model reproduces the two reflectance band structure observed in the measured reflectance of the porous SiC layers. The differences in the reststrahl region reflectance spectra of the porous SiC layers, compared to bulk SiC, are associated with polarization effects introduced by the cavities combined with a mean field average of interactions among the cavities. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363046

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have applied a real time spectroellipsometry data analysis procedure developed previously [S. Kim and R. W. Collins, Appl. Phys. Lett.67, 3010 (1995)] to characterize depth profiles in the optical gap for compositionally‐graded semiconductor alloy thin films prepared by plasma enhanced chemical vapor deposition. The analysis procedure employs a two‐layer (four‐medium) optical model consisting of the ambient, a thin surface roughness layer and outer‐layer (5–15 A˚) whose properties are to be determined, and a pseudo‐substrate that contains the past history of the graded‐layer deposition. The ellipsometric spectra (2.3–4.0 eV) are analyzed to provide, not only the depth‐profile of the optical gap and alloy composition for the graded layer, but also the instantaneous deposition rate and the surface roughness layer thickness versus time or accumulated layer thickness. To apply the previous analysis approach, it was necessary to (i) parameterize the dielectric function of the alloys as a continuous function of composition over the desired alloy range and (ii) express the optical gap as an accurate function of alloy composition. As an example, we have applied the extended analysis to obtain the depth‐profile of the optical gap and alloy composition with <15 A˚ resolution for a hydrogenated amorphous silicon‐carbon alloy (a‐Si1−xCx:H) film prepared by continuously varying the gas flow ratioz=[CH4]/{[CH4]+[SiH4]}. In order to demonstrate the technological importance of such structures, the graded layer has been incorporated at thep/iinterface of widegapa‐Si1−xCx:H p‐i‐n solar cells, and improvements in open‐circuit voltage have been observed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363077

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Radiation damage centers generated by x irradiation at 80 and 300 K in the storage phosphor RbI:Tl+were investigated with optically detected magnetic resonance and optical measurements. It is shown that the intensity of the photostimulated luminescence depends on the concentration of Tl2+centers. Bleaching into any of the seven identified Tl2+absorption bands destroys the Tl2+centers and leads to a Tl+emission as well as toVKcenters. Proportionally to photodestruction of Tl2+centers, the photostimulated luminescence is decreased. The traps for information storage in RbI:Tl+areFcenters and Tl2+centers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363078

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of Dy2S3‐doped As12Ge33Se55glasses demonstrate that the broad, below‐gap PLE mechanism observed previously for rare‐earth emissions in Er‐ and Pr‐doped chalcogenide glasses is a general or universal feature of rare‐earth‐doped chalcogenide glasses, provided the transition energies of the rare earths are in the correct energy range. The PL spectrum excited in the 815 nm Dy3+absorption band shows the 1150, 1340, and 1700 nm sequence of Dy3+transitions expected for Dy‐doped chalcogenide glasses. The PLE spectra of the 1340 (6F11/2,6H9/2→6H15/2) and 1700 nm (6H11/2→6H15/2) Dy3+emissions exhibit broad excitation bands from ∼500 to 1000 nm, upon which the sharp intraF‐band absorptions of Dy3+are superimposed. These broad PLE bands are characterized by an exponentially decreasing slope with decreasing energy in the spectral range below the Urbach edge which is associated with the below‐gap, defect‐ and impurity‐induced exponential tails observed in the below‐gap absorption spectra of chalcogenide glasses. At high energy, the exponentially rising Urbach absorption edge of the host glass, which leads to competing nonradiative decay mechanisms, eventually dominates the absorption spectrum and imposes an exponentially decreasing slope on the PLE spectra. These features of the broadband PLE have been observed in the PLE spectra of the Er‐, Pr‐ and Dy‐doped chalcogenide glasses we have studied. There is a difference in the relative strengths of the broad PLE bands, with the broadband for the 1340 nm PL band being a factor of 3 stronger than that for the 1700 nm PL emission. In addition, there is a shift in the peak energy of the different PLE spectra. Qualitatively speaking, the higher the energy of the luminescence transition, the higher the energy of the corresponding broad PLE peak. Proposed mechanisms for the broadband PLE of rare‐earth emissions in chalcogenide glasses are discussed in the context of models for the below‐gap, defect‐ and impurity‐induced exponential absorption tails, including the possible role of lattice relaxation associated with charge transfer transitions and the involvement of transition metal impurities. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363079

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Amorphous carbon films, deposited via radio frequency magnetron sputtering, have been electrochemically intercalated with lithium to study their possible application as anodes for rechargeable Li microbatteries. Various concentrations of intercalated lithium have been obtained, by using galvanostatic and potentiostatic methods. After preliminary examinations by optical microscopy and by optical absorption measurements, a comparative Raman spectroscopy characterization of the films has been performed with excitation wavelengths ranging from 676.4 to 457.9 nm, to follow resonance effects. A study is made on the Raman spectral shape and its excitation wavelength dependence for the film samples with different Li concentrations. A moderate graphitization process ofa‐C films after intercalation results from this analysis. The Raman spectral changes induced by Li insertion do not reverse after further deintercalation cycles, and this fact is discussed in connection to the observed electrochemical reversibility of thea‐C:Li electrodes. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363080

出版商:AIP    

年代:1996

数据来源: AIP