摘要:

AmorphousSixGeyOzalloy films of different chemical composition andSiOx/GeOymultilayers are prepared by dc magnetron sputtering. The formation of Ge nanocrystals in the alloy matrix is induced by annealing at temperatures ranging from 700 to 900 °C. In contrast, the crystallization in the Ge sublayers in the multilayer structures occurs at 500 °C. Using high resolution transmission electron microscopy, we can directly image the Ge nanoclusters in the as-prepared films as well as the Ge nanocrystals after crystallization in the annealed alloy and multilayer samples. Raman spectra of the as-prepared and annealed films demonstrate the strong similarity between the alloy and the multilayer films. In both cases, the crystallization of the Ge phase is observed by the narrowing and disappearing of the amorphous mode around 270 cm−1and the appearance and increase of the mode below 300 cm−1, typical for nanocrystallized Ge. The Ge cluster formation is stimulated by a high oxygen content of the alloy films. If the Si atoms are not saturated with oxygen, Si–Ge alloy regions are formed, resulting in an increase of the crystallization temperature of the films. A weak broad Raman mode at 390 cm−1is found as a signature of these regions. Films containing small Ge nanocrystals show visible room temperature photoluminescence around 3.03 and 2.34 eV. Quantum confinement or the presence of luminescence centers (Si20, Ge20) at the surface of the Ge nanocrystals are discussed as possible origins of the bright blue luminescence. However, a peak shift to higher energies with decreasing crystal size is not detectable, contrary to the predictions of the quantum confinement model. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364242

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Detailed properties of In0.53Ga0.47As layers grown by liquid phase epitaxy from melts containing 0.04 and 0.1 wt &percent; Er are reported. The carrier concentration in the material is reduced by almost two orders of magnitude as a result of Er doping. Low temperature photoluminescence measurements indicate that both the donor and the acceptor type impurities are gettered by Er and the full-width at half-maximum of the major peak is reduced to 4 meV for the layer with the highest Er doping. From deep level transient spectroscopy experiments on undoped layers, we confirm the presence of an electron trap with activation energy of 0.17 eV. Density of this trap is reduced by more than two orders of magnitude in the Er doped material and another electron trap with activation energy of 0.15 eV is revealed from the analysis of the experimental data. We associate the 0.17 eV trap with impurities in the material. From low temperature photoconductivity and photocapacitance experiments, we further confirm that Er creates a level located 40 meV above the valence band. Density of this center increases in the material after high temperature annealing. We suggest that the Er-related level is due to Er atoms occupying cation sites in the material and acting as an isoelectronic impurity. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364243

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We used the characteristic changes in the electronic structure of the valence band of amorphous carbon films upon the formation of graphitic clusters to monitor the temperature induced graphitization of a variety of hydrogen-containing(a-C:H)as well as hydrogen-free(a-C)films. The valence band spectra of the films, which were preparedin situby ion-beam deposition and electron beam evaporation, were determined using photoelectron spectroscopy with excitation energies in the ultraviolet regime (ultraviolet photoemission spectroscopy, HeI,h&ngr;=21.22eV and HeII,h&ngr;=40.82eV). By choosing a variety of deposition parameters we are able to illustrate that the extent of temperature induced structural changes is on one hand determined by the bulk and surface diffusion coefficients of carbon atoms, and on the other hand by the ion irradiation which inhibits the graphitization process. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364244

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Micro-Raman and corroborative photoluminescence (PL); infrared (IR) absorption; scanning tunneling microscopy/spectroscopy (STM/STS) and x-ray diffraction (XRD) studies on the intercalation of Bi2Sr2CaCu2O8+&dgr;(Bi-2212, host) with C60(guest) reveal that intercalation drastically changes vibrational, electronic, and structural characteristics of both the guest and the host species. The predominant changes in vibrational modes such as splitting (651, 668 cm−1) in the radial mode (at 709 cm−1); anomalous intensity distribution; and appearance of forbidden modes in the Raman spectra of the guest have been related to the symmetry reduction in the guest sublattice and to the induced resonance effects due to the intercalation process. Further, the changes in Raman spectra corresponding to the host, such as the appearance of a strong Raman mode at 310 cm−1(in the &ngr;Cu–Oregion), redshift in the &ngr;Bi–Omode, and the appearance of a band at 1443 cm−1in the tangential mode region of the guest reveal the formation of semiconducting islands in the host sublattice due to the guest–host electron transfer process. Raman results have been correlated with PL, IR, STM/STS, and XRD data. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364245

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present the observation of solid material luminescence excited by soft x-ray laser. The 21.2 nm photons of the soft x-ray laser of the Laboratoire d’Utilisation des Lasers Intenses (Palaiseau, France) have been used to induce ultraviolet luminescence in CsCl and CsI. The laser supplied up to 6×1012photons in 80 ps. A single laser shot was sufficient to obtain luminescence spectra with very good resolution. In the case of CsI, the use of two illumination conditions, differing by a factor 150 in intensity, showed the collapse of luminescence efficiency for very strong illumination. The quenching effect is discussed in terms of the large increase of crystal excitation mean density, altering the usual process of luminescence centers production. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364246

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report the observation of room-temperature and low-temperature visible photoluminescence from nanocrystalline silicon (nc-Si) thin films produced by plasma-enhanced chemical vapor deposition (PECVD) through a gas discharge containing SiH4diluted in Ar and H2. The nanocrystalline silicon films were characterized using transmission electron microscopy, spectroscopic ellipsometry, infrared and Raman spectroscopy, and were examined for photoluminescence. Luminescent films consisted of dense silicon nanocrystals that grew in a columnar structure with approximately 20&percent;–30&percent; void space dispersed inside the film. Aside from having small crystalline silicon regions, the structure of thenc-Si films is different than that of porous Si, another luminescent Si material generally produced by electrochemical anodization. Yet, the photoluminescence spectra of the thinnc-Si films were found to be similar to those observed from porous silicon. This similarity suggests that the same mechanism responsible for light emission from porous silicon may also be responsible for emission fromnc-Si. The photoluminescence spectra are analyzed in terms of a simple quantum confinement model. Although the mechanism of visible luminescence from porous Si is still a point of controversy, our results support the hypothesis that some of the luminescence from porous silicon andnc-Si films is due to quantum confinement of electrons and holes in crystals with dimensions 2–15 nm. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364247

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The mechanism of stain formation in the chemical etching reaction of silicon has been investigated in HF–oxidizing agent–H2O solutions. The chemical formula of the stain formed during the silicon etching reaction is K2SiF6. The concentration of holes on silicon surface increases with the increase of redox potential and the concentration of oxidizing agent used in manufacturing the etching solution. The increase in the hole concentration accelerates not only the etch rate but also the formation rate of K2SiF6. The etched silicon surfaces are covered with a K2SiF6layer when redox potential and concentration of oxidizing agent are great at low HF concentrations. This happens because the formation rate of K2SiF6is much greater than its dissolution rate by HF. Sufficiently high HF concentration in the etching solution is apparently essential to increase the etch rate without the formation ofK2SiF6. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364248

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

This article describes a technique to detect shallow levels in highly compensated ZnSe by photoinduced admittance spectroscopy (PIAS). Nitrogen doped ZnSe with a Schottky barrier on top has been investigated, and the photovoltage measurements show that the electric charge in the depletion layer becomes positive under high intensity illumination with photon energies larger than the energy gap of ZnSe. An electron trap with an activation energy of 50 meV was obtained from the PIAS measurement. The accuracy of this technique is confirmed by reconstruction of the temperature dependence of the capacitance and conductance by a computer simulation. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364249

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Polycrystalline high quality freestanding 300-&mgr;m-thick diamond films were doped by diffusion of B and Li under electric bias in order to fabricate verticalp-njunctions. Circular contacts were obtained by high dose ion implantation of B and Li. TheI–Vcharacteristics were rectifying. When illuminated by deuterium lamp, an open circuit voltage was 2.6 eV. The shape of theI–Vcharacteristic under illumination points to the existence of shunt and series resistances. The obtained structure is most probably ap-njunction with bad contacts. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364250

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effect ofex situhydrogen passivation process on the performance of visible hydrogenated amorphous silicon carbide (a-SiC:H)-basedp-i-ntype thin-film light-emitting diodes has been investigated. Anex situhydrogen passivation process dramatically improved the device performance; that is, the threshold voltage decreased by about 5 V, the electroluminescence (EL) intensity increased by a factor of about 3, and the EL peak shifted toward a short wavelength from 700 to 600 nm, resulting in an increase of the brightness from 1 cd/m2to 35 cd/m2. This improvement of device performance is caused by the passivation of interface states in thep/iandi/ninterfaces as well as midgap states in the luminescent active intrinsica-SiC:H layer by hydrogen atoms. A process time dependence of theex situhydrogen passivation effect on the device performance also has been studied. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364283

出版商:AIP    

年代:1997

数据来源: AIP