摘要:

In this paper, carrier kinetics in a type II multiple quantum well hetero-n-doped-intrinsic–p-doped-intrinsic are studied through optical measurements under continuous illumination. Both field measurement by photoreflectance and time measurement confirmed our hypothesis that the recombination time is a function not only of photocarrier density, but also of photocarrier distribution. A simple model is used for recombination time, assuming exponential dependence on the barrier height and nearest distance between recombining carriers. Quantum efficiency has also been determined. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365690

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We investigate magnetic-field-dependent velocity relaxation due to boundary-roughness scattering in quantum wires, using a two-dimensional (2D) finite-difference wave-packet propagation technique. We find that for low disorder correlation lengths and far away from density-of-states singularities associated with subband minima and saddle points, the Born approximation adequately reproduces the qualitative features of the more general 2D results. However, naive application of the Born approximation in regions where disorder-induced broadening or higher-order scattering become important can lead to substantial errors. In particular, the mobility enhancement associated with the opening of a quasigap in coupled quantum wire structures can be overestimated by as much as several orders of magnitude. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365691

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The techniques of the photoreflectance and electroreflectance (ER) were used to study the built-in electric fields and the surface Fermi levels of InP surface-intrinsic-n+(SIN+)structures. The substrates ofSIN+structures are either Fe-doped semi-insulated InP or Sn-dopedN+InP with the same doping concentrations as its buffer layer. The built-in electric field and the Fermi level were calculated from the Franz–Keldysh oscillations of the photoreflectance spectra. Our studies found that for the samples with the same doping concentration in the buffer layer and substrate, the built-in electric field increases as their top layer thickness decreases. The surface Fermi level, on the other hand, remains approximately constant. For samples with a semi-insulated substrate, the photoreflectance spectra indicate the simultaneous existence of two built-in electric fields, one in the top layer and the other at the interface region between the buffer layer and substrate. ER spectra were measured with the application of a modulation electric field across the top layer. The built-in electric field across the top layer obtained from the ER spectra increases as the top layer thickness decreases while the surface Fermi level, again, remains approximately constant. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365692

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The interfacial electric field established under different reverse bias conditions in Au and Ni on semi-insulating GaAs junctions has been studied by means of a low energy positron beam. The technique used is that of monitoring the positron drift to the interface through changes in the annihilation radiation lineshape as a function of incident positron beam energy at different reverse biases. The data show a small but clear electric field drift of positrons towards the interface that increases more rapidly at low voltages (less than 50 V) which at higher biases tends towards saturation. This confirmation of electric field saturation adds further weight to the picture of an electric field enhanced electron capture cross section for the ionized EL2 defect. Electric field values extracted from the data are compared with results from other techniques and suggest that enhanced electron capture is already occurring at the relatively low built-in fields(∼1 kV cm−1)found at the unbiased junction, with a rapid increase ofEL2+neutralization occurring for biases above 10 V. At still higher fields∼10 kV cm−1(biases>50 V),there appears to be an additional threshold for more completeEL2+neutralization adjacent to the contact. The present study clearly demonstrates the often overlooked necessity of catering for built-in electric fields in positron diffusivity studies of III–V semiconductors where surface midgap Fermi-level pinning is common. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365693

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The cyclotron resonance of two-dimensional electron gases was directly measured on a modulation-doped field-effect transistor structure by the gate voltage ratio spectroscopy. The intersubband energies between the lowest subband and the subbands with quantum index up toi=5were determined by resonant subband-Landau-level coupling (RSLC) under relatively low magnetic field. The experimental results were in good agreement with the self-consistent calculations including the depolarization shift and exciton-like effect corrections. The quantum index dependence of the level splitting due to the RSLC was discussed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365694

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The role of high-lying minibands in superlattice vertical transport is investigated using a nonparabolic balance-equation approach extended to include multi-miniband occupation. We find that the existence of high-lying minibands results in a decrease of the electron temperature, a reduction of the peak drift velocity and a slowdown of the velocity-drop rate in the negative differential mobility (NDM) regime, in comparison with those predicted by a single miniband model. Numerical calculations of steady-state transport in typical GaAs-based superlattices at room temperature, show that these effects become significant when the strength of the electric field gets close to or falls in the NDM regime. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365695

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Using an ensemble Monte Carlo simulation, we study the electron transport properties in tensile strained Si grown on aSi1−xGexsubstrate and in ann-channelSi/Si1−xGexmodulation-doped field-effect transistor (MODFET). Owing to the strain-induced modification of the conduction-band structure, the in-plane drift mobility in undoped material reaches3250 cm2/V sat 300 K (forx⩾0.2) and 31000cm2/V sat 77 K (forx⩾0.05). The two-dimensional Monte Carlo modeling of 0.18 &mgr;m gate Si/SiGe MODFETs reveals a high velocity overshoot of2.5×107 cm/srelated to a significant ballistic transport through the gated part of the strained Si channel. For a 0.18 &mgr;m gate and a Ge mole fractionx=0.3,intrinsic transconductance and transition frequency as high as 460 mS/mm and 85 GHz are obtained. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365696

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Silicon dioxide (SiO2)films on indium antimonide (InSb) were prepared using remote plasma enhanced chemical vapor deposition. The structural characteristics of the SiO2films and the electrical characteristics of the metal-insulator-semiconductor (MIS) structures were examined. The SiO2films, as evaluated by Auger electron spectroscopy, showed that the atomic ratio of silicon to oxygen was 0.5 and the composition was uniform throughout the oxide film. The leakage current density at 1 MV/cm was about 22 nA/cm2, the breakdown electric field of the MIS capacitor using SiO2film deposited at 105 °C was about 3.5 MV/cm, and the interface-state density at midband gap extracted from the 1 MHz capacitance-voltage measurement was about2×1011cm−2 eV−1. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365697

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

YBa2Cu3O7−xbicrystal dc-superconducting quantum interference devices (SQUIDs) and single layer magnetometers working at 77 K have been fabricated and analyzed. Samples have been madein situby cylindrical magnetron dc sputtering on [001]SrTiO3bicrystal substrates. Magnetic flux noise levels and magnetic field sensitivities by standard flux-locked-loop electronics have been measured out on dc-SQUIDs and magnetometers, respectively. Flux noise levels as low as a few&mgr;&Fgr;0Hz−1/2in the white noise regime have been measured in SQUIDs, whereas the magnetic field sensitivity of the magnetometers was influenced by external sources. A comparison of experimental data with analytical expressions modelling the behavior of highTcSQUIDs has been carried out, with special reference to the flux-to-voltage transfer parameter and noise characteristics. Finally, a reduction of1/fnoise of the SQUIDs has been achieved by a wide band bias reversal electronics. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365698

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Anisotropic and isotropicSm2Co17-type permanent magnets have been prepared by using the conventional sintering technique. Transmission electron microscopy was used to characterize the cellular structure within theSm2(Fe,Co)17grains. Hysteresis loops(M∼H)were measured in the temperature range from 4.2 to about 1000 K by using a pulsed-field magnetometer with a maximum field strength up to 24 MA/m. The magnetocrystalline anisotropy fieldHAhas been measured up to 1000 K by using the singular point detection technique on anisotropic samples with external fields applied perpendicular to the magnetic alignment direction. The saturation magnetizationMshas been measured on anisotropic samples with external fields applied parallel to the magnetic alignment direction. From studies of the coercivity mechanism by using a micromagnetic analysis of the temperature dependence of the coercivity field, it follows that the coercivity ofSm(Co,Fe,Cu,Zr)zis controlled at elevated temperature (above 520 K) by a nucleation process of reversal domains. ©1997 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.365699

出版商:AIP    

年代:1997

数据来源: AIP