摘要:

Photoelectrochemical conversion characteristics of semiconducting, polycrystalline BaTiO3electrodes have been investigated as a function of sintering atmosphere (air/H2) and the amount of dopant (lanthanum). The photocurrents for specimens sintered in air were greater than those of specimens sintered in hydrogen. Also, as the amount of dopant in BaTiO3sintered in air increased, the photocurrent increased up to 0.2 at. % addition and then decreased. However, for BaTiO3sintered in hydrogen, the photocurrents decreased consistently with the addition of the dopant. These results could be explained in terms of ceramic microstructure and electrical resistivity. Regardless of the amount of lanthanum, the photoresponse, as the wavelength of the induced light that varied, began to appear around 420–430 and 780–800 nm, which corresponds to the energy band of La‐doped BaTiO3and the oxygen vacancy level within the gap, respectively.

ISSN:0021-8979    

DOI:10.1063/1.345745

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We report measurements on transport perpendicular to a single quantum well located on thenside of ap‐njunction in a InP/InGaAs device. Zero‐bias admittance spectroscopy and photocapacitance transient measurements are used to characterize electron and hole emission rates. Both rates show a crossover from a thermally activated regime to a regime of weak temperature dependence. The barrier dimensions for electron and hole emission preclude direct tunneling so that, in the low‐temperature regime, transport must be defect assisted. In this regime we observe discrete fluctuations in the reverse‐bias photocapacitance and photocurrent and in the forward‐bias dark current. The results are explained with a model in which transport of electrons in or out of the well is regulated by the occupancy of a single electron trap associated with a defect.

ISSN:0021-8979    

DOI:10.1063/1.345746

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Thermoelectric power of annealed &bgr;‐Ag2Se thin films of different thicknesses has been measured both while heating and cooling by the integral method. It is found that it remains practically constant (in &bgr;‐Ag2Se phase) during heating while it is a function of temperature while cooling. The thermoelectric power in both heating and cooling cycles is a function of inverse thickness of the films. The difference in behavior between Ag2Se films during heating and cooling is attributed to the possible transformation to monoclinic phase during cooling from the original orthorhombic phase during heating. The inverse thickness dependence has been explained by the size effect theories. Important material parameters like carrier concentration, Fermi energy, effective mass of carriers, and energy dependence of the mean free path have been evaluated for the &bgr;‐Ag2Se (orthorhombic) phase.

ISSN:0021-8979    

DOI:10.1063/1.345747

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Nonalloyed Ohmic contacts of evaporated Pt/Ti top‐InGaAsP (&lgr;g=1.3 &mgr;m) with different Zn doping levels ranging 5×1018–2×1019cm−3have been fabricated by rapid thermal processing. These contacts showed Ohmic behavior prior to any heat treatment with a specific contact resistance of 4×10−3&OHgr; cm2for the lowest doping level and 1×10−4&OHgr; cm2for the highest level. A decrease in the specific resistance was achieved by supplying rapid thermal processing to the contacts, while the lowest values were observed on all the contacts as a result of heating at 450 °C for 30 sec. The lowest resistance of 1×10−6&OHgr; cm2was achieved at the contact that was formed on the 2×1019cm−3Zn‐doped InGaAsP layer. Measurements of the conduction activation energy yields a good linear dependence of the specific resistance on temperature in all the contacts as deposited and after the different heat treatments. The higher the doping level and the rapid thermal processing temperature up to 450 °C, the lower the activation energy, which may suggest an increase in the tunneling carrier transport mechanism contribution at the expense of the thermionic‐emission one. The morphology and microstructure of these contacts were not influenced by the variations in the Zn doping concentration.

ISSN:0021-8979    

DOI:10.1063/1.345748

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Using Monte Carlo simulation combined with iterative solution of the Poisson equation the real‐space electron transfer (RSET) and negative differential conductivity (NDC) in GaAs–n‐AlxGa1−xAs heterostructures with electric field applied parallel to the layer interfaces have been investigated. The original Hess RSET model has been modified by considering graded AlxGa1−xAs layers with proper spatial dependence of the Al composition. Simulation results confirm that this modification enables one to avoid undesirable effects due to space‐charge fields: (1) Graded AlxGa1−xAs layers can be depleted in thermal equilibrium at higher donor concentrations than layers without compositional grading; (2) it is sufficient to use doped, but not compensated, AlxGa1−xAs layers because electron velocity in graded layers is low mainly due to electron transfer toLandXvalleys; and (3) there is no confinement of cold electrons in graded AlxGa1−xAs layers due to space‐charge fields when the RSET occurs. The RSET‐induced NDC and characteristic time constants have been found close to 1 ps at 77 and 300 K. Similar results have been obtained in the RSET model without compositional grading; however, highly compensated AlxGa1−xAs material was needed and the total number of free electrons was ten times lower.

ISSN:0021-8979    

DOI:10.1063/1.345749

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Data obtained from a set of asymmetric GaAs/AlGaAs double‐barrier resonant tunneling structures is presented. Low‐temperatureI(V) characteristics, temperature dependencies, and magnetotunneling have been investigated. The data confirm conclusions drawn in previous studies on symmetric structures, lending further support to the description of tunneling in terms of sequential processes and momentum randomization most likely induced by interface roughness scattering in the well.

ISSN:0021-8979    

DOI:10.1063/1.345750

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We propose a novel method of determining conduction‐band discontinuities &Dgr;Ecby using the relationship between the splitting energy &Dgr;Edue to monolayer fluctuation and quantum energy shiftEqin single‐quantum‐well (SQW) structures. This evaluation method is described by differentiating the eigenvalue equation for a finite square quantum well. The obtained formula indicates that &Dgr;Eis approximately proportional toE3/2qand that this evaluation technique is independent of ambiguity in estimation of well thickness. We apply this method to an InGaAs/InP SQW system. 4.2‐K photoluminescence spectra of InGaAs/InP SQWs grown by low‐pressure metalorganic chemical vapor deposition show clearly resolved doublets and in some cases triplets caused by monolayer fluctuations. As a result, we found that &Dgr;Ec=(0.25–0.30)&Dgr;Eggave the best fit for the InGaAs/InP heterointerface.

ISSN:0021-8979    

DOI:10.1063/1.345751

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We report on the calculations of energy band gaps based on the semiempirical tight‐binding model for short‐period (ZnTe)m(ZnSe)nand (ZnS)m(ZnSe)nstrained‐layer superlattices (SLSs). During the calculation, much attention has been paid to the modeling of strain effect. It is found that (ZnTe)m(ZnSe)nsuperlattices grown on InAs, InP, and GaAs substrates show very different electronic properties from each other, which is consistent with experimental results now available. Assuming that the emission observed for (ZnTe)m(ZnSe)nSLS originates from intrinsic luminescence, we obtain an unstrained valence‐band offset of 1.136±0.1 eV for this superlattice. On the other hand, the band gap of (ZnS)m(ZnSe)nsuperlattice grown coherently on GaP is found to exhibit a much stronger structure dependence than that grown coherently on GaAs. The difference of energy gap between superlattice with equal monolayers (m=n) and the corresponding alloy with equal chalcogenide composition is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.345752

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Gold powder was mixed with YBa2Cu3O7−xsuperconductors for the purpose of increasing the critical current density (Jc). The critical transition temperature at zero resistance (Tzeroc) gradually decreased by the Au addition. However, the depression ofTzerocwas as low as 2 K for the YBa2Cu3O7−x‐Au composite with Au mixing up to 40 wt.%. A maximumJcof 307 A/cm2was obtained for the Au 5‐wt. % mixed composite. TheJcwas about four times as high as that for YBa2Cu3O7−xwithout Au mixing. In addition to theJcenhancement, the weak‐link problem was appreciably improved by the Au addition.

ISSN:0021-8979    

DOI:10.1063/1.345753

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

High‐TcBi‐Sr‐Ca‐Cu‐O thin films were coated with electron‐beam‐evaporated SiO2. The room‐temperature resistivity of the coated films and their superconductivity onset and zero‐resistance temperatures were not affected by the dielectric film. High‐frequency capacitance‐voltage studies did not show any modulation of space charge in the surface layer of the superconducting film. It is observed that the high‐frequency (1 MHz) capacitance of the metal‐insulator‐superconductor structure has an appreciable temperature dependence.

ISSN:0021-8979    

DOI:10.1063/1.345725

出版商:AIP    

年代:1990

数据来源: AIP