摘要:

A self‐consistent, one‐dimensional, numerical solution of Schro¨dinger and Poisson equations has been obtained. To solve Schro¨dinger equation, instead of the conventional finite difference approach, we start by dividing the space in intervals of constant potential energy, in which the solution type is well known. Next we match the wave functions and their first derivatives, divided by the effective mass on each side of the potential steps. This approach is very efficient on finding the eigenvalues in structures with large regions of almost constant potential energy such as quantum well structures or heterojunctions. Validation is presented by comparing the exact solution of Schro¨dinger equation for a triangular well with that obtained by our method. Poisson equation is solved considering the deep (DX) and shallow centers assuming a donor with one ground state and two excited states. Applications to isotypen‐GaAs/AlxGa1−xAs graded heterojunctions show that the density of the two‐dimensional electron gas (2DEG) is almost independent of graduality when this is smaller than about 200 A˚ and that DX centers may lower the 2DEG concentration by as much as 20%.

ISSN:0021-8979    

DOI:10.1063/1.349389

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The photoelectrochemical conversion behavior of SnO2film electrodes deposited by spray pyrolysis has been investigated as a function of deposition parameters and the amount of additive Sb. The photocurrent increased with increasing substrate temperature up to 400 °C, and then decreased. As the concentration of the spray solution increased, the photocurrent reached a maximum value at the concentration of 0.05 M. In Sb‐doped SnO2film electrodes, the photocurrent decreased due to the reduction of mobility and depletion layer width. These results could be explained in terms of microstructure and electrical property.

ISSN:0021-8979    

DOI:10.1063/1.349390

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

a‐SiNx:H films of various compositionxwere deposited by rf glow discharge (GD). The deposition rate was analyzed for three ranges of gas flow ratioR= [NH3]/[SiH4] depending on the deposition mechanism. Properties of these films were measured by means of x‐ray photoelectron spectroscopy (XPS), infrared (IR) absorption, optical absorption, and the temperature dependence of electrical conductivity. The compositionxwas determined by XPS. For large values ofR,xwas found to be saturated at 1.7. The variation of H content was detected by IR absorption. The variation of coordinating atoms of Si with increasingxwas deduced from the variation of XPS spectra of the Si 2pcore‐level and the shift of Si‐H stretching vibration frequency in IR absorption. Based on the random bonding model and assuming bonding units to the central Si atom to be Si, N, and NH, probabilities of Si tetrahedra with various coordinating units were obtained. The results indicate that there are many Si—Si bonds for the stoichiometricx=1.33 and that the concentration of Si—Si bonds diminishes at around the saturation valuex=1.7. These results seem to imply that the presence of Si—Si bonds to some extent is a prerequisite condition for film deposition by GD. Experimental results of optical absorption analyzed by Tauc relation revealed the presence of two kinds ofxregion whose properties are quite different.Forx<1.5, its optical absorption is similar toa‐Si:H modified by the presence of N. With increasingx, the optical band gapEOincreases andBcoefficient in the Tauc plot decreases. At aboutx=1.5, Si—Si bonding effectively disappears and the optical absorption abruptly changes to that similar to &bgr;‐Si3N4. Considering these results, the change of electronic band structure withxwas deduced on the basis of the atomic structure obtained above and by the tight binding approximation. Forx<1.5, the optical band gap is due to Si—Si bonding, the energy gap of which increases, and the linear band tail becomes broad with increasingx. From the observed temperature dependence of conductivity, variations of the activation energy and pre‐exponential factor are obtained withxup tox=1.0. For the decrease of conductivity withx, the contribution from the pre‐exponential factor is much larger than that from the activation energy. This result can be understood by a transport mechanism in the electronic band structure obtained above. Finally, it is concluded that the electronic properties ofa‐SiNx:H deposited by GD ran.

ISSN:0021-8979    

DOI:10.1063/1.350352

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effect of fluorine on electron trapping in SiO2films has been studied by avalanche electron injection. Samples are prepared by 25‐keV fluorine implantation into dry oxides followed by a 1000 °C N2ambient anneal. Bulk electron traps with capture cross sections on the order of 10−17–10−19cm2, which are not due to implantation damage, are filled by avalanche electron injection. An optimum dosage of fluorine implantation to suppress the so‐called turnaround effect during avalanche injection exists. This suggests that fluorine might passivate slow interface donor states or reduce bulk hydrogen diffusion. The observation that high‐temperature (120 °C) injection can eliminate most fast and slow interface states for conventional oxides is also true for fluorinated oxides. Our results indicate an enhanced generation of fast donor states for oxides containing fluorine. These states contribute a positive charge when the Fermi level is in the lower portion of the band gap.

ISSN:0021-8979    

DOI:10.1063/1.349360

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We report on the use of the scanning tunneling microscope (STM) for etching both single crystal and thin film, YBa2Cu3O7−x. Nanoscale features can be generated with the STM by ablation of atoms rastered by the microscope tip. The etching process can be controlled to remove layers of material which are multiples of thecaxis (12 A˚). Various geometric features have been fabricated ranging from fine lines to square etch pits. The STM has also been used to study the growth mechanism and surface topography of thin films of YBa2Cu3O7−xproduced byinsitulaser deposition (Jctypically 1×106A cm−2at 77 K). Step features equal to thecaxis of the material can be readily identified which form pinnacles or chip like morphologies at the surface. These surfaces can also be etched with the STM to reveal a more continuous substructure consistent with the highJc’s observed.

ISSN:0021-8979    

DOI:10.1063/1.350351

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effect of temperature and atmosphere on stability of the high‐Tcphase in Bi1.8Pb0.4Sr2Ca2Cu3Oxsuperconductors has been investigated using ac susceptibility, x‐ray diffraction, and Raman measurements. It is found that the high‐Tcphase becomes unstable due to the structural phase transformation at temperature above 650 °C in O2of 1 atm. The formation and the decomposition of the high‐Tcphase is found to occur through the medium of the Ca2PbOxphase and depends on oxygen partial pressure at temperature above 650 °C.

ISSN:0021-8979    

DOI:10.1063/1.349361

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Intermetallic Sm(Fe1−xCox)2compounds having the cubic MgCu2‐type structure absorb a small quantity of hydrogen from the atmosphere under normal conditions. A magnetic annealing of such samples leads to the induction of a gigantic magnetic uniaxial anisotropy, which is linked to the ordered distribution of hydrogen into interstitial sites of the lattice. In this paper the induced magnetic anisotropy and spontaneous lattice distortions are studied as a function of both composition and hydrogen content. It is shown that the induced uniaxial anisotropy which adds to the initial cubic anisotropy gives rise to the occurrence of first‐order magnetization processes (FOMP). In the temperature range where spin reorientation transitions take place a peculiar behavior of the FOMP is observed.

ISSN:0021-8979    

DOI:10.1063/1.349362

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The electro‐optic Kerr effect and its wavelength dispersion have been measured in glasses representing several compositional systems. The measured Kerr effect was found to be large for glasses having large refractive indices, including: (i) glasses containing high concentrations of the heavy metals Pb, Bi, and Tl; (ii) glasses with high Nb, Ta, and Ti content; and (iii) tellurite glasses. Comparison of the third‐order nonlinear susceptibility, &khgr;eff(&ohgr; = &ohgr; + 0 + 0), obtained from the measured electro‐optic data, to values estimated from literature values of the optical frequency value, &khgr;eff(&ohgr; = &ohgr; + &ohgr; − &ohgr;) suggest an opposition of the electronic and nuclear contributions to the low‐frequency electro‐optic effect for the glasses containing Ti, Ta, or Nb.

ISSN:0021-8979    

DOI:10.1063/1.349363

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

In previous work we have identified a near‐gap photoluminescence in Ga0.52In0.48P which exhibits a strong dependence of emission energy on excitation intensity (‘‘moving emission’’) and correlated its presence and strength to conditions of growth. In this work we extend our investigations to the rise and decay lifetimes associated with the moving and nonmoving components of the emission. The two processes proceed simultaneously at the same energy. For the moving emission, the time constants scale approximately linearly with excitation intensity. Decaying luminescence can, in most cases, be well fitted with one or two exponentials with time constants as long as milliseconds. The rising luminescence is typically slower and in some cases has a nonmonotonic first time derivative. These results are discussed in terms of existing models of the microstructure of ordered Ga0.52In0.48P.

ISSN:0021-8979    

DOI:10.1063/1.349364

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The intensity change in optical absorption at 3.8, 5.0, and 5.8 eV was measured for thermal treatment up to 1000 °C done on silica glass samples manufactured by various methods. It was found that these absorption bands change differently with heat, depending on the synthesis process of the sample. The 3.8‐eV absorption band due to peroxy linkage is found to increase with high‐temperature treatment when the sample contains dissolved oxygen molecules within the sample. For the sample with no oxygen molecules, high‐temperaturetreatment does not alter the absorption intensity at 3.8 eV. The 5.0‐eV absorption band is found to decrease by high‐temperature treatment. In the case of an oxygen‐containingsample, the reaction of oxygen molecules with oxygen vacancy sites is a cause of this decrease. The 5.1‐eV band, which was thought to have no temperature dependence, is found to be annealed at a temperature region around 950 °C.

ISSN:0021-8979    

DOI:10.1063/1.349340

出版商:AIP    

年代:1991

数据来源: AIP