摘要:

A composite hardness model based on the function of depth weight factor is presented. The model can be applied to determine the characteristic hardness of surface coatings which are too thin for the hardness to be directly measured. Its application requires only composite hardness data obtained from coated specimens by conventional microhardness measurement. DLC, TiN, and Cu films on substrates of glass, silicon, and AISI 52100 steel were used to verify this composite hardness model. It proved valid for a variety of cases. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117595

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The dynamic pyroelectric response in the smectic‐C*(SmC*) phase of a novel class of polysiloxane terpolymers is reported. In these terpolymers methyl groups and two distinct types of mesogenic side chains are attached to the siloxane backbone. Our investigations reveal that the magnitude and temperature dependence of the pyroelectric signal, as well as the temperature range of the SmC*phase, can be controlled by varying the relative side‐chain composition. This permits optimization of the pyroelectric properties over a temperature range extending to subambient. Specifically, the pyroelectric coefficientpcan be made very nearly temperature independent, and the ratiop/&egr;′ extremely high, making these materials suitable for uncooled infrared detectors. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117596

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Thin ZnSe films were grown by molecular beam epitaxy on Zn exposed (2×4) As‐stabilized surfaces of GaAs epilayers under varied beam flux ratios. A very low density of faulted defects in the range of ∼ 104/cm2was generated in samples grown under a condition with a mixture of both (2×1) and weakc(2×2) surface reconstructions at the initial stages of growth. However, an asymmetric distribution on the densities of extrinsic cation‐ and anion‐terminated Shockley‐type stacking faults were generated, respectively, in samples grown under Zn‐ and Se‐rich surface stoichiometries. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117597

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report on the hot‐electrons induced degradation in AlGaAs/GaAs high electron mobility transistors (HEMTs), consisting of a decrease in the drain current and an increase in the parasitic drain resistance. The amount of the degradation is proportional to the impact‐ionization rate which is related to the electron energy. Transconductance dispersion measurements and drain current deep level transient spectroscopy (DLTS) have been used to identify interface traps which are located at the AlGaAs/GaAs interface in the gate‐drain access region and are the causes of the observed degradation. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117598

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Time‐resolved photoluminescence (PL) spectroscopy has been performed at 18 K on excitonic‐related emissions in an unintentionally doped hexagonal GaN epitaxial layer grown by two‐flow metalorganic chemical vapor deposition (TF‐MOCVD). Under low excitation condition, radiative recombination ofAfree exciton (EXA: 3.4921 eV) and neutral shallow‐acceptor bound exciton [(A0s,X): 3.4805 eV] dominated the spectrum. Decay time of (A0s,X) emission is relatively long (585 ps) indicating a small number of non‐radiative centers in the layer. At higher excitation densities, new PL peak appeared at 3.4864 eV and grew superlinearly with excitation densities. These features, combined with their transient behavior, suggest that this new emission band is due to the biexciton recombination. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117599

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The mechanism of the negative persistent photoconductivity (NPPC) in InAs/AlSb single quantum wells is discussed. The molecular beam epitaxy grown single InAs quantum well sample is made into Hall bars with the substrate as the backgate. Using the newly designed buffer, the gate bias can deplete or enhance the two‐dimensional electrons in the InAs quantum well without substantial gate leakage current. Based on the 4.2 K magnetoresistance data, and the fact that the trapped electrons can be redistributed by gate bias, we conclude that the NPPC effect at low temperatures is a result of the capture of photoexcited electrons by ordinary, deep donors in AlSb. Numerical modeling using physical assumptions can quantitatively explain our experimental observation, and the calculated AlSb donor energy is 0.41±0.05 eV above the AlSb valence band maximum, with 4×1016/cm3to 1017/cm3in concentration. The previously discussedDX‐center‐like characteristic of deep levels in AlSb, i.e., lattice‐relaxation with a relatively high activation energy, is not evidenced in this work. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117600

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Electrical characteristics of high performance GaN modulation‐doped field‐effect transistors (MODFETs) grown by reactive molecular beam epitaxy method are studied experimentally both in dark and under white light illumination. The maximum measured drain‐source current is 626 mA/mm in dark and 695 mA/mm under illumination, which saturates at a relatively low drain‐source voltageVDS. The transconductance increases with decreasing gate length reaching a value of 210 mS/mm in dark and 222 mS/mm under illumination for devices with a gate length ofLG=1.5&mgr;m. Breakdown voltages of about 90 V have also been exhibited by devices with gate lengthLG=1.5 &mgr;m and drain‐gate distanceLDG=1 &mgr;m. To our knowledge, these are the best values obtained so far from GaN MODFETs which we attribute to the suppression of leakage currents and improved materials structure of the devices. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117601

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We use a theoretical model of semiconductor lasers to calculate threshold current density (Jth), optical gain (g), and spontaneous emission rate (rsp) as a function of the two heterojunction band offsets between Zn1−uCduSe(well)/ZnSzSe1−z(guiding) layers and ZnSzSe1−z(guiding)/ Zn1−xMgxSySe1−y(cladding) layers. We find that for the conduction‐band offset between the well and the guiding layers ranging from ∼0.55&Dgr;Egto ∼0.75&Dgr;Eg(&Dgr;Egbeing the energy band‐gap difference), there is very little change inJth,g,rsp. Furthermore, these three quantities are very weakly dependent on the conduction‐band offset between the guiding and the cladding layers ranging from 0.2&Dgr;Egto 0.5&Dgr;Eg. However, if the offset between the well and the guiding layers is increased from ∼0.75&Dgr;Egto ∼0.8&Dgr;Eg, the calculations indicate a sharp drop inJth,g, andrsp. We explain this behavior as due to the unbinding of the ground‐state light‐hole subband as the valence band offset is reduced. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117602

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Electron cyclotron resonance plasma etch rates for GaN, InN, InAlN, AlN, and InGaN were measured for a new plasma chemistry, ICl/Ar. The effects of gas chemistry, microwave and rf power on the etch rates for these materials were examined. InN proved to be the most sensitive to the plasma composition and ion density. The GaN, InN, and InGaN etch rates reached ∼13 000, 11 500, and ∼7000 A˚/min, respectively, at 250 W rf (−275 V dc) and 1000 W microwave power. The etched surface of GaN was found to be smooth, with no significant preferential loss of N from the surface at low rf powers, and no significant residue on the surface after etching. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117603

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A new sulfur passivation technique, the sulfur vapor glow discharge, has been developed to form a thick sulfide layer on GaAs surface. By using Auger electron spectroscopy and x‐ray photoelectron spectroscopy measurements, the main composition of the passivation layer is found to be gallium sulfide without the existence of unstable As–S bonds. The stability of the passivation effect is demonstrated by the nondecaying behavior of the photoluminescence intensity of the GaAs passivation surface under the illumination of the laser beam with very high intensity. ©1996 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.117604

出版商:AIP    

年代:1996

数据来源: AIP