摘要:

Gas‐induced conductance changes were measured on palladium‐dosed single‐crystal tin oxide (SnO2) thin films having well‐characterized surface properties. Films were fabricated using two methods: reactive sputtering and chemical vapor deposition. Film orientation and crystal structure were determined by x‐ray diffraction, while surface morphology was characterized using atomic force microscopy. Conductance changes were measured continuously on film surfaces during alternating exposure and evacuation cycles to partial pressures of H2, O2, and CO in a vacuum chamber. The conductance change was found to be proportional to the square root of the initial film conductance and was interpreted in terms of gas‐induced changes in the width of a near‐surface space‐charge layer. The variation of conductance as a function of gas pressure during alternating exposure and evacuation cycles of H2and O2is consistent with a model that involves surface reactions between coadsorbates.

ISSN:0021-8979    

DOI:10.1063/1.353404

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The structure and properties of Co‐N, Co‐Fe‐N, and Co‐Zr‐N films, prepared by rf reactive sputtering using nitrogen and argon gases, have been studied. The resistivity and coercivity of Co‐N, Co‐Fe‐N, and Co‐Zr‐N films were determined as a function of nitrogen partial pressure. It was found that the properties of Co‐N and Co‐Fe‐N films were very similar where the properties were determined mainly by the nitride phases in these films. The resistivity of both Co‐N and Co‐Fe‐N films increased with the increase of nitrogen pressure. The coercivity of both films decreased with an initial increase of nitrogen pressure, then increased with a further increase of pressure so that there is a region of nitrogen pressures where the coercivity is at its lowest value. The low coercivity is attributed to the formation of the Co4N phase in Co‐N films and both Co4N and Fe4N phases in Co‐Fe‐N films. For Co‐Zr‐N films, resistivity first decreased with an increase of nitrogen pressure, then increased with a further increase of pressure, indicating the presence of a Zr‐N phase in these films. The coercivity increased with an increase of nitrogen pressure. The Zr‐N phase was not observed in the x‐ray diffraction measurements due to the low concentration of Zr in these films.

ISSN:0021-8979    

DOI:10.1063/1.353405

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We studied the structural and electronic properties of the ZnTe/CdTe(100) interface with reflection high‐energy electron diffraction and angle‐resolved synchrotron radiation photoemission spectroscopy (ARPES). ZnTe overlayers grown at 300 °C on CdTe(100) were fully strained and pseudomorphic up to ≊16 A˚. Beyond this coverage the ZnTe film starts to gradually relax the 6.6% in‐plane lattice strain. Complete relaxation is reached at a ZnTe coverage of ∼300 A˚. A valence‐band offset of &Dgr;Ev=0.00±0.05 eV was measured with ARPES at the &Ggr; point. This propitious band lineup may allow for the use of a ZnTe intermediate layer at metal/CdTe structures to induce ohmic back contacts in CdS/CdTe heterojunction solar cells.

ISSN:0021-8979    

DOI:10.1063/1.353406

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We have measured room‐temperature band‐to‐band recombination decay kinetics in superior quality GaAs heterostructures, and have observed the longest lifetime (2.5 &mgr;s) observed for any GaAs/AlxGa1−xAs structure to date. Additionally, using a novel time‐resolved optical photoluminescence imagining technique, analogous to the Haynes–Shockley experiment, we have also measured room‐temperature minority‐carrier transport in this series of ‘‘surface‐free’’ GaAs/Al0.3Ga0.7As double heterostructures, measurements only possible in high‐quality samples with long lifetimes and intense photoluminescence. We find the transport to be diffusive with diffusion lengths of ≳100 &mgr;m. Further, we find, for thick structures, minority‐carrier transport is hole‐dominated ambipolar diffusion, as expected for high‐purityn‐type material. However, for thinner structures, we find that the minority‐carrier transport is time dependent, changing from ambipolar diffusion at early times, as in thick structures, to electron‐dominated diffusion at later times. We show that these structures becomeeffectivelyp‐typemodulationdopeddue to the relative ‘‘impurity’’ and thickness of the AlxGa1−xAs compared to the GaAs. As a result, the minority‐carrier species changes from holes to electrons for decreasing GaAs layer thicknesses. Cumulatively, we show the band‐to‐band recombination decay kinetics and carrier transport results to be in excellent qualitative and quantitative agreement. Moreover, our results are in excellent agreement with electrical transport measurements of electron and hole mobilities. Finally, with our measured room‐temperature lifetimes and minority‐carrier transport measurements versus GaAs layer thickness, we accurately calculate the interface recombination velocity for these structures, with the resultS∼40 cm/s, among the lowest ever reported for any GaAs/AlxGa1−xAs structure.

ISSN:0021-8979    

DOI:10.1063/1.353407

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The dielectric response of an electron gas in a cylindrical quantum well is calculated when the electron motion in the angular direction is restricted. The dielectric response is shown to depend in a nontrivial way on the angle of restriction &bgr; thereby making it possible to tune the optical response to external electromagnetic radiation. The resonant absorption wavelength can be adjusted by changing &bgr; over a wide range which might be used in optoelectronic detectors.  

ISSN:0021-8979    

DOI:10.1063/1.353408

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The morphology of polycrystalline films grown by low‐pressure chemical‐vapor deposition (LPCVD) is investigated by transmission electron microscopy (TEM) as a function of the film thickness, the deposition pressure, and the level of contamination. An orientation filtering mechanism, due to the growth‐velocity competition in the early stage of growth, is responsible for the preferred orientation of the films. The size of the crystallites, the surface roughness, and the type of the structural defects are investigated by combined cross‐sectional and plane‐view TEM analysis. In polycrystalline silicon thin‐film transistors (TFTs), the influence of surface roughness scattering on the mobility is investigated by measuring the effective electron mobility under high effective normal field at 295 and 77 K. Although the surface curvature is increased when the deposition pressure is decreased, the surface roughness scattering is constant in the deposition pressure range from 40 to 0.5 mTorr. By decreasing the deposition pressure from 40 to 10 mTorr, although the grain size increases, the TFT performance degrades due to the following factors: (a) the increase of the grain‐boundary trap density which is related to the change of the mode of growth at 10 mTorr; and (b) the increase of impurity contamination in the environment of the LPCVD system with constant silane flow rate at all pressures. At a deposition pressure of 0.5 mTorr the TFT performance is improved indicating that the grain size is the prevailing key factor.

ISSN:0021-8979    

DOI:10.1063/1.353409

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electron beam induced current variations in images of strain relaxed epitaxial Si/Si(Ge)/Si wafers deliberately contaminated with nickel from the backside are reported for different contamination levels. Strong recombination contrast due to NiSi2precipitates was observed both at the top Si surface and along buried interfacial misfit dislocations. A surface conductivity inversion fromntoptype was obtained for the high level Ni contaminated sample. A theoretical analysis based on the presence of a surface potential due either to a metal‐silicon Schottky contact, or to the accumulation of charged traps is used to explain the observed effects.

ISSN:0021-8979    

DOI:10.1063/1.353410

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Using an off‐axis rf sputtering configuration, we have preparedinsituhigh‐TcYBa2Cu3O7−y(YBCO) films with a relatively large deposition rate. The sputtering gas was a mixture of Ar and O2(7:3) and the substrates were MgO(100) and SrTiO3(100). We found that the distance from the target to the substrate,d, is a key factor in the deposition rate. By decreasingdto a value of about 1.5–2.5 cm, we obtained a deposition rate as great as 2000–2500 A˚ per hour with an rf power of 120 W and at a total pressure 100–200 mTorr. The transport behaviors of the as‐grown YBCO films under magnetic fields are reported. The activation energy derived from the resistive transition in magnetic fields is thickness dependent.

ISSN:0021-8979    

DOI:10.1063/1.353411

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A systematic study of the influence of oxygen partial pressure on 110 K phase formation in the Bi‐Pb‐Sr‐Ca‐Cu‐O system has been undertaken on the optimum nominal composition to promote the Bi2Sr2Ca2Cu3Ow(2223, 110 K) phase, i.e.; Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06Ow. The relationship between 110 K phase fraction, oxygen partial pressure, sintering temperature, and time has been mapped for this composition via experimental data. The optimum conditions for 110 K phase formation were identified as 0.01≤PO2≤0.35 atm, at 820≤T≤880 °C for ≤36 h with intermediate grinding every 6 h. The single 110 K phase stability range was determined asPO2≤0.5 atm. The variation of oxygen partial pressure during the 110 K phase formation process was monitored via a Y2O3‐ZrO2oxygen sensor and a possible mechanism for its formation and the effect ofPO2thereon is discussed.

ISSN:0021-8979    

DOI:10.1063/1.353412

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In this article we present a new method for the preparation of nonrandom Au‐YBa2Cu3O7−x(YBCO) composites. We describe the structural and the morphological properties of this novel system. Gold introduction into this system causes an increase in thecaxis value of the unit cell upon sintering at 950 °C. A systematic increase of the zero resistance transition temperature, up to 96 K as measured by a dc transport method, is observed upon the introduction of gold into the YBCO lattice. Introduction of gold above ≊3% by weight causes weakening of the intergranular links of the composite material.

ISSN:0021-8979    

DOI:10.1063/1.353413

出版商:AIP    

年代:1993

数据来源: AIP