摘要:

Charge carrier dynamics in doped and undoped hydrogenated amorphous silicon (a‐Si:H) films is studied by contactless time‐resolved photoconductivity measurements. Subband‐gap and above band‐gap excitation are used to generate excess mobile charge carriers. In undopeda‐Si:H the electron decay at charge carrier concentrations larger than 1016cm−3is mainly due to an electron‐hole recombination which is controlled by hole dispersion.ndoping introduces hole traps which increase the effective electron lifetime drastically as they quench this electron‐hole recombination channel. At highn‐doping levels the electron decay becomes faster due to an increase of the concentration of recombination centers upon doping. In lightly dopedp‐type samples the transient photoconductivity reflects the interaction of mobile holes with states in the valence‐band tail. In heavily dopedp‐ andn‐type films the majority carriers decay by a second‐order recombination process with trapped minority charge carriers. The transport parameters deduced agree with time‐of‐flight data.

ISSN:0021-8979    

DOI:10.1063/1.341465

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

GaAs(110) faces with different preparations—ultrahigh vacuum (UHV) cleaved, polished and etched, polished and sputtered—are prepared as Schottky diodes by the deposition of Ag. Diodes based on UHV‐cleaved faces do show homogeneously distributed EL2 and EL5 states in deep level transient spectroscopy (DLTS). On polished and etched samples an additional interface state (IS) distribution with a density of 9×1011eV−1 cm−2at the DLTS maximum appears. These states can be caused by defects at the oxidic interfacial layer. Polishing and sputtering also evokes the IS distribution. The absence of a DLTS signal from metal‐induced gap states (MIGS) which pin the Fermi level at 0.49 eV above the valence‐band maximum is related to the absence of an interfacial layer in the UHV prepared Schottky diodes. The sputter process increases the electron density in a thin layer below the interface by an As excess. The corresponding smaller extent of the barrier causes an additional electron emission via tunneling processes from the IS distribution. Furthermore, a near‐interface state, EL6 (VGa‐VAs), shows up. Its concentration at the interface attainsNEL6=2.5×1016cm−3comparable to the shallow donor concentration.

ISSN:0021-8979    

DOI:10.1063/1.341466

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Thin films of TiN, covering a narrow range around stoichiometric composition, were synthesized by low‐energy ion assisted growth at deposition temperaturesTD=100, 300, 500, and 700 °C. The deposition apparatus consisted of an unbalanced dc magnetron sputter source which allowed high rate deposition from a titanium target with simultaneous bombardment of the growing film by a beam of mixed Ar+and N+2ion species at an ion‐to‐condensing atom arrival rate ratio of five. For each deposition temperature, films were prepared at various ion energies in the rangeEi=2–100 eV. The presence of reactive N+2ionsand the effects of ion bombardment facilitate increased incorporation of nitrogen and decrease the overall defect density in the structure of TiN. Electrical transport properties of films were investigated by measurements of the temperature dependence of resistivity &rgr;(T) in the rangeT=4–300 K, and superconducting transition temperatureTc. These measurements were complemented by measurement of optical reflectance, x‐ray diffraction, and scanning electron microscopy investigations to determine the structure and composition of films. Collectively the film properties have a strong dependence on ion energy and deposition temperature. Films deposited at optimum conditions (TD=500 °C andEi=30–50 eV) possess a high degree of crystalline perfection with a strong (200) texture and a high optical reflectance (82% at &lgr;=800 nm). These properties correlate with the following optimum electrical properties: room‐temperature resistivity &rgr;300∼26 &mgr;&OHgr; cm, resistivity ratio RR=2.13, temperature coefficient of resistivity TCR=2.43×10−3K−1, andTc=5.35 K. These results represent the best results yet reported for microcrystalline TiN films. The temperature dependence of resistivity has a normal‐metal behavior and it obeys Matthiessen’s rule. The phonon contribution to resistivity at room temperature, &rgr;thermal, is about 14 &mgr;&OHgr; cm and is in agreement with that of single‐crystal TiN. As the disorder in the structure of TiN increases, TCR is found to decrease and zero TCR is predicted for limiting values of resistivity &rgr;300=300–400 &mgr;&OHgr; cm.

ISSN:0021-8979    

DOI:10.1063/1.341468

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have performed measurements of carrier concentration and Hall mobility, between 4.2 K and room temperature, in InGaAsP (energy gap of 0.96 eV) epitaxial layers grown on semi‐insulating InP. It is shown that for this kind of material in the range of concentration studied (2.0×1016–1.6×1018cm−3), alloy scattering has to be taken into account as a mechanism limiting the mobility even at low temperatures. Also, in the entire range of temperature and concentration studied, Fermi‐Dirac statistics are better suited than Boltzmann statistics for theoretical calculations of Fermi energy, because even for concentrations as low as 2.0×1016cm−3, the material is degenerated at low temperatures. Following these assumptions, we have calculated the alloy scattering potential as being between 0.52 and 0.62 eV.

ISSN:0021-8979    

DOI:10.1063/1.341469

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have analyzed the characteristics of flux modulated voltages of a dc SQUID for both positive and negative bias currents. It is shown that the change in the modulated voltage caused by fluctuations of the junction critical currents is eliminated after subtraction between the modulated voltages for the two biased currents. In the flux feedback mode using an alternating bias current, a large reduction of 1/fnoise is observed for dc SQUIDs made of NbN junctions with amorphous Si tunnel barriers.

ISSN:0021-8979    

DOI:10.1063/1.341470

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have studied in detail the current‐voltage (I‐V) curves of dc SQUIDs in the presence of rf fields in the frequency range 2–18 GHz. Strong voltage locking at half‐integral as well as the usual integral Shapiro step spacing is observed when the applied dc‐magnetic flux is close to half‐integral numbers of flux quanta. Numerical simulations are in excellent agreement with the experimentalI‐Vcurves. The half‐integral steps are found to be associated with the flip‐flop between two adjacent fluxoid states of the SQUID synchronized to the rf field.

ISSN:0021-8979    

DOI:10.1063/1.341471

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A technique for preparing Si substrates in molecular‐beam epitaxy (MBE) systems devoted to III–V compound growth is described. A C‐ and O‐free Si surface is obtained on previously oxidized substrates using a HF solution etch. A (2×1) reconstructed surface with steps of monoatomic height is achieved with an atomic Si flux provided by a standard effusion cell, while the substrate is maintained at 600 °C for ∼10 min. The experimental results presented were obtained using Auger electron spectrometry and reflection high‐energy electron diffraction.

ISSN:0021-8979    

DOI:10.1063/1.341472

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Preparation and magnetic properties of ThMn12‐type phases R,Fe,Mo,(Co),R=Y, Ce, Nd, Sm, Gd, Dy, Er are described. The ternary material, without Co, exhibits saturation magnetization up to 1 T and Curie temperature up to 215 °C, the quaternary material, with Co, up to 1.2 T, and 295 °C, respectively.

ISSN:0021-8979    

DOI:10.1063/1.341473

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Computer model calculations and experimental comparison are presented for thermomagnetic writing in TbFe media. Two models are discussed which are appropriate for two observed types of writing behavior. A ‘‘bubble’’ type writing model is found to give good experimental agreement when wall motion processes dominate and the laser‐written marks exhibit single‐domain behavior. This model accurately accounts for the writing behavior of Tb0.23Fe0.77. The gradient of the domain‐wall energy is found to be an important factor in the writing process and can cause nucleation to be delayed until some cooling has occurred. This results in written domain sizes which are a strong function of the applied bias field. For compositions which exhibit multiple domain writing behavior, a ‘‘nucleation field’’ model is found to be appropriate. This model predicts the observed weak dependence of mark size on applied bias field. In addition, the nucleation field model is used to calculate the bias fields which correspond to the onset of writing (onset field) and single‐domain mark formation (saturation field).

ISSN:0021-8979    

DOI:10.1063/1.341474

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Domain characteristics (size and shape) of TbFeCo disks of several compositions are studied systematically through direct optical observation under various recording conditions. The relationships among recorded domains, magnetic properties, and recording conditions are also investigated. Magnetic films suitable for very high‐density recording are found to have a compensation temperature between room temperature and 100 °C and a Curie temperature near 200 °C. In those films, the formation of domains is stable against fluctuations in laser power and external field. These results can be explained by a simple domain formation model in which coercive force, demagnetizing field, and wall energy at the domain wall are taken into account. Teardroplike domains are observed in the case of relatively long laser pulse recording. Such domains cause detection errors during the domain edge detection. It is also found that recording noise is high under certain recording conditions. The origin of this noise is supposed to be caused by inversely magnetized regions existing in a domain and irregularities in the outline of domains.

ISSN:0021-8979    

DOI:10.1063/1.341419

出版商:AIP    

年代:1988

数据来源: AIP