摘要:

Basic properties of hydrogenated amorphous silicon–germanium (a‐SiGe:H) alloys deposited by plasma‐enhanced chemical‐vapor deposition were studied. We found that there is an optimal growth temperature in the range 250–280 °C. Infrared‐absorption spectra measurements show that in this temperature range the alloys have optimal composition of hydrogen content and bonding, providing a minimum in dangling bond defect densityNsand a high photoconductivity &sgr;ph. Growth at lower temperatures results in hydrogen‐rich alloys with highNs. Hydrogen in these alloys is mainly bonded as SiH2and as clusters on internal voids. Growth at temperatures above the optimal value gives less deterioration of the properties ofa‐SiGe:H than the growth below this optimal temperature. We found two different dependencies of &sgr;phonNs: in optimized alloys &sgr;phchanges proportionally toN−1sand in low‐temperature alloys &sgr;phdecreases more steeply withN−1sdue to a decrease in electron mobility. It is shown that annealing ofa‐SiGe:H causes a sharp increase inNsstarting at annealing temperatures about 20 °C below the temperature at which the alloy was grown. We observed a new phenomenon that during annealingNsincreases much more (by two orders of magnitude) than the photoconductivity decreases (only factor of 5). The entropy‐based model was applied to explain this experimental phenomenon. We found that annealing shifted the centrum energy of theD+/0dangling bond levels to the valence‐band edge. The rate of electron recombination lowers due to this shift and the change in the effective electron capture crosssection of the defects, and therefore the decrease in &sgr;phis smaller than the increase inNs. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363640

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The reduction of SiO2by Al was studied in Al/SiO2/Si structures above 350 °C. It was found that Al displaces Si in the oxide, forming an Al–O compound with an Al:O concentration ratio between 1:1 and 1.3:1. In the reacted areas, less than 1 at. % Si is left in what was originally a pure SiO2matrix. The activation energy for the reaction in a pyrogenic oxide is approximately 2 eV. The reaction was found to be responsible for electrical failure of diodes manufactured using Al/SiO2/Si technology. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363669

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A new neural network method for reconstructing the depth profile of thermally inhomogeneous materials from a photothermal spectrum is described in detail. The possibilities and limits of this kind of photothermal depth profiling are evaluated. The influence of the network architecture and several profile and signal parameters on the reconstruction quality is discussed. Statistical predictions are given for the accuracy of the reconstructed profiles for different levels of Gaussian noise in the signal and at different depths. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363670

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Electrical transport in zinc oxide varistors is simulated using two‐dimensional Voronoi networks. The networks are assumed to contain randomly distributed grain boundaries of three electrical types: (1) high nonlinearity (i.e., ‘‘good’’) junctions; (2) poor nonlinearity (i.e., ‘‘bad’’) junctions; and (3) linear with low‐resistivity (i.e., ohmic) junctions. These type classifications are those found in experimental measurements. By varying the type concentrations, the simulated current density versus electric field (J–E) characteristics can be made to conform to the different experimentally observed characteristics of ZnO varistors. These characteristics include the sharpness of switching at the transition between ohmic and nonlinearJ–Eresponse (i.e., knee region), as well as the degree of nonlinearity. It is shown that the reduction of the nonlinearity coefficient of bulk varistors, relative to that of isolated grain boundaries, can be explained only by the presence of ‘‘bad’’ varistor junctions. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363645

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The numerical investigation of the radiation emission by a system of two magnetically coupled, long Josephson junctions is reported. Time‐dependent synchronized voltage response in the flux‐flow regime is analyzed for the case of in‐phase and out‐of‐phase oscillations in the junctions. Simulations show that Josephson junctions operating in the in‐phase flux‐flow mode may generate rf radiation power by a factor of more than 4 larger than that of a single Josephson junction. The radiation in the out‐of‐phase flux‐flow mode is characterized by nearly completely suppressed amplitudes of odd harmonics and considerably damped even harmonics as compared to that of a single barrier junction. The dependence of the radiation power on the parameter spread between the junctions is investigated. The advantages of using stacked Josephson junctions as oscillators for the sub‐mm wave band are discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363671

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Following a method proposed by Divin and Modovets [Sov. Tech. Phys. Lett.9, 108 (1983)], we have measured at millimeter waveband the intrinsic noise temperaturesTNof YBa2Cu3O7−&dgr;Josephson junctions or dc superconducting quantum interference devices (SQUIDs) fabricated on SrTiO3, yttria‐stabilized ZrO2, or Si bicrystal substrates. Over wide ranges of physical temperaturesTPand the junction’s normal resistanceRN, it was found thatTNfollowsTPpretty well. This indicates that the intrinsic noise in the devices is dominated by Johnson noise.TNwas also measured in cases where there is external magnetic field applied, or where there is another microwave radiation like the local oscillator in a mixer. The magnetic field or microwave radiation does not seem to affectTNin any appreciable way. To estimate the high frequency performance of the junctions on Si bicrystal substrates, direct irradiation by a far infrared laser at 1.81 THz is carried out and the clear first Shapiro step is observed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363672

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

ZnSe/ZnS quantum wires and quantum dots were grown on unpatterned substrates by molecular beam epitaxy. Several substrate orientations were compared. These structures exhibited a strong luminescence. A linear crystallographic dependence of the photoluminescence peak energy was observed when comparing structures grown in the same experimental conditions. All samples exhibited also strong zero‐dimensional–one‐dimensional confinement signatures. An empirical analysis is provided illuminating the issue involved by the redshift. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363673

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Optically pumped surface‐emitting stimulated emission at room temperature was observed from GaN:Ge grown by metalorganic chemical vapor deposition. The sample was optically pumped perpendicularly on the top surface while the stimulated emission was collected from the back colinearly with the pump beam. The cavity was formed by the GaN/air and GaN/sapphire interfaces without any other structure. The stimulated emission was gain guided by the pump beam. The threshold optical pump density for stimulated emission was approximately 2.8 MW/cm2and the linewidth was 2.5 nm. The emission from GaN:Ge showed a redshift as the pump density increased. The comparison between theoretical calculations and experimental results suggested that many‐body interactions can account well for the redshift. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363674

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The output facet temperature of high‐power AlGaAs/GaAs single quantum well (SQW) laser diodes was measured during operation. The front output facets were passivated with Al2O3coatings. The spectral shift of photoluminescence from the cladding layers was used to determine the temperature rise at the front facet with increasing output power. The spatial resolution of the technique allowed to look at each cladding layer individually and to study the correlation between the near‐field pattern and the temperature profile along the active layer. The local temperature on the facet at 1 W total optical power (corresponding to an average linear power density of 10 mW/&mgr;m) was found to vary between 25 and 45 K above the average active layer temperature and to exceed the heat‐sink temperature by up to 70 K. This represents a significant reduction of facet temperature in comparison to earlier reports and can be attributed to high‐quality passivation coatings. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363675

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The thermal stability of Cu/Ta/Cr/Si structure is analyzed and compared with that of Cu/Ta/Si and Cu/Ta/Cr/Ta/Si structures. The Cu/Ta/Si and Cu/Ta/Cr/Ta/Si systems retained their structures up to 600 °C without increase in resistivity but the Cu/Ta/Cr/Si structure was degraded after annealing at 400 °C. In the latter case, the degradation was dominated by the outdiffusion of free Si, probably released from the substrate in the formation of CrSi2. It is suggested that the released Si is reactive and its outdiffusion through Ta layer is facilitated by the high affinity of Si toward Ta, as expected from the large negative value of mixing enthalpy between Ta and Si. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363676

出版商:AIP    

年代:1996

数据来源: AIP