摘要:

ZnS:Tm and ZnS:Tm,Cl thin films were grown by metalorganic chemical vapor deposition (MOCVD), using diethylzinc, H2S, Tm(thd)3(thd=2,2,6,6-tetramethyl-3,5-heptanedione), and HCl. The ZnS:Tm did not contain oxygen which might be introduced through the thd-radical. It thus has only codopant-free Tm3+luminescent centers probably associated with native defects. The electroluminescence (EL) spectrum of the ZnS:Tm,Cl showed three satellite emission lines in addition to the original emission of the ZnS:Tm, indicating the existence of Tm–Cl complex centers. In contrast, the photoluminescence (PL) spectrum of the ZnS:Tm,Cl under host excitation showed no discernible satellite emission lines. Hence, though the Tm ions in the Tm–Cl complex centers are expected to be charge compensated by Cl or a certain Cl-induced defect, they are rather inactive in the PL excitation while active in the EL excitation. The same properties were observed for the MOCVD-grown ZnS:Sm and ZnS:Sm,Cl [A. Kato, M. Katayama, A. Mizutani, N. Ito, and T. Hattori, J. Appl. Phys.77, 4616 (1995)], and therefore they probably occur for other rare-earth luminescent centers with Cl codopant. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364078

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have investigated optical properties of ZnTe epilayers, Zn1−xMgxSeyTe1−yepilayers, and ZnTe/Zn1−xMgxSeyTe1−yquantum wells (QWs) grown on (100)-InAs substrates by molecular beam epitaxy. We observed several sharp photoluminescence lines close to the excitonic position and no detectable luminescence from deep levels in ZnTe epilayers. Bright luminescence has been obtained from Zn1−xMgxSeyTe1−yepilayers which are lattice matched with InAs. The band alignment of ZnTe/Zn1−xMgxTe QWs was found to be type I. The reduction of the band gap energy of the ZnTe layer due to a tensile strain was confirmed in this structure. Nearly lattice-matched ZnTe/Zn1−xMgxSeyTe1−yQWs have been fabricated. A type II band alignment was observed for many of these QWs. We estimated bowing parameters not only of the band gap but also of the valence band for ZnSeyTe1−y.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364079

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

ZnSe/ZnMgSSe multiple quantum wells (MQWs) were fabricated by compound-source molecular beam epitaxy on GaAs epitaxial buffer layers. X-ray diffraction and photoluminescence revealed that the MQWs had excellent crystalline quality and optical properties. The roughness of the interface is less than an atomic layer. The exciton-longitudinal-optical phonon coupling constant is the same as that of ZnSe. Binding energy of a biexciton is such large value as 20 meV due to the quantum confinement effect. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364080

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The energy distributions of electron emission from a Schottky emitter have been studied at tip temperatures from 1450 to 1800 K and angular current densities from 0.1 to 240 &mgr;A/sr. We have observed broadening of the energy distribution, with increase of angular current density and decrease of tip temperature, from 0.4 to 1.32 eV resulting from electron tunneling and electron–electron interaction. Good agreement between the experimental results and predictions from Monte Carlo simulation of the emission process is observed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364081

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Photoemission through thin coating films was studied in the scope of protection of sensitive photocathodes. The transmission of low energy(∼1 eV) electrons was measured for a large number of dielectric films (LiF, NaF, CsF, NaI, MgF2, BaF2, SiO, SiO2, Al2O3,n-C36H74), evaporated in vacuum on CsI and CuI photocathodes. Some films like CsF, NaI andn-C36H74were found to have a fairly large electron attenuation length, varying from about 20 to 100 Å at a maximum initial electron energy of 1 eV. A thin CsF layer deposited on top of CuI and Al photocathodes was found to significantly increase their quantum yield. An enhancement of the photoyield following exposure to water vapour was observed for alkali fluoride-coated photocathodes. We interpret this effect as a decrease of the electron affinity by about 0.3–0.4 eV, induced by adsorption of polarized H2O dipoles. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364082

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Evaporation triggered with nanosecond laser pulses in self-supporting aluminum films was imaged by high-speed transmission electron microscopy. This unconventional method provides up to three images from a single transient process with exposure times ⩾5 ns and at intervals ⩾25 ns with a spatial resolution of 100 nm. The chronological order of ablation was observed on the nanosecond time scale by successive shifting of the moment of exposure. Information was obtained about the mode and duration of evaporation. The ablation reveals a threshold behavior. Above a laser energy density of 5–6 J/cm2the irradiated film region is completely evaporated during the laser pulse. Below this threshold evaporation is marginal and the film disintegrates mainly by liquid flow. The experimental results are compared with a new model comprising two different evaporation mechanisms, surface and volume evaporation. The ablation as observed cannot be explained by surface evaporation only. The strong rise of the evaporation rates above the threshold is associated with the onset of volume evaporation. It is also shown that the vaporization enthalpy must be considered as a function of temperature for a correct modeling of evaporation by short laser pulses. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364083

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Diamonds were successfully nucleated on SiO2-coated silicon substrates using a high power microwave plasma enhanced chemical vapor deposition process. Nucleation rates on SiO2surfaces (i.e., 0.5×1010cm−2) were, however, still smaller than those on Si surfaces (i.e., 1.0×1010cm−2). The major advantage in using high power microwaves was revealed by optical emission spectroscopy to be that the atomic C and H species produced are more abundant and energetic. Therefore, the negative bias effect is enabled and the formation of sp3bonds is enhanced. The nucleation of diamonds on SiO2surface is thus made possible. The growth of diamonds behaved similarly on the prenucleated surface, regardless of the nature of the substrates. Diamonds were of single grain columnar structure with random orientation when deposited without bias and were of multi-grain columnar structure with [111] or [001] preferred orientation when deposited under −100 V dc bias. Multi-grain columnar structure was ascribed to the induction of secondary nucleation at the presence of bias voltage. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364104

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A porous Si (PS) layer with a spongy microstructure on top of a dendritic microstructure was fabricated on a moderately dopedp-type Si wafer using a two-step anodization process. This illustrates that in addition to substrate doping, anodization current density also has an effect on the porous Si microstructure. A preoxidation heat treatment of the spongy-type porous Si was found to change the porous structure significantly, making it more difficult to fully oxidize the layer at low temperatures. However, dendritic porous Si can better withstand the heat treatment without suffering noticeable changes in structure. X-ray photoelectron spectroscopy, infrared spectroscopy, and electrical breakdown tests were used to analyze the oxidized porous Si samples. The oxidation process and the resultant oxide were found to depend on several factors, including the porosity, the microstructure itself (e.g., spongy or dendritic-type), and the heat treatment history prior to oxidation. With similar porosity, dendritic PS is easier to oxidize compared to spongy PS. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364107

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Variable angle spectroscopic ellipsometry (VASE) technique is used to measure the optical constants of InGaAs/GaAs multiple quantum well (MQW) structures for the purpose of designing tunable optical modulators. The VASE measurements also include field-induced changes in index of refraction and absorption. These measured changes in MQWs are in agreement with theoretical computations. In addition, the design and simulated performance of an InGaAs/GaAs MQWs Fabry–Perot modulator using the VASE data is presented. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364125

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The current voltage relationships of AlGaAs/GaAs modulation doped field effect transistors (MODFETs) were measured as a function of applied uniaxial stress. Stresses in the [110] and [11¯0] directions on MODFETs that were grown on a (001) substrate produced threshold shifts of opposite sign. Stresses in [110] and [11¯0] directions resulted in threshold voltage pressure coefficients of −15 and 64 mV/Kbar, respectively. The asymmetric shifts in the threshold voltages are attributed to piezoelectric effects. In addition, stress induced changes in the slopes of the transconductance versus gate-to-source voltage relationships were also measured. For stresses in the [110] and [11¯0] directions, the dependencies were 0.4 and −0.7 mS/(VKbar), respectively.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364126

出版商:AIP    

年代:1997

数据来源: AIP