摘要:

This paper is the first of a series with the common theme of comparing thermal and acoustic pulse methods of measuring charge or polarization profiles across the thickness of slab‐shaped samples that are representative of different types of materials. In this paper, thermal and pressure pulse measurements are reported of the polarization distribution in poled, ferroelectric ceramic samples. The results obtained from both methods are complementary so that there is a benefit to using both. The results also demonstrate that large deviations from uniform polarization can be induced by processing differences.

ISSN:0021-8979    

DOI:10.1063/1.351306

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A rapid thermal annealing (RTA) technique has been employed to process lead zirconate titanate (PZT) films prepared by reactive magnetron sputtering. The films were fabricated by dc sputtering a multielement metal target in an oxygen ambient at a substrate temperature of 200 °C. A subsequent postdeposition RTA at 600 °C for 5 s crystallizes the films into a perovskite‐type structure through various intermediate phases. Due to the short postdeposition processing times inherent in the RTA method, the initial nature of the as‐grown films has a critical influence on the crystallization kinetics. The reaction sequence in the formation of perovskite PZT from the films deposited at low substrate temperatures by the sputtering technique has been evaluated, and various key factors influencing the crystallization of PZT have been identified. As‐grown films are constituted of polycrystalline orthorhombic lead oxide in an amorphous matrix of titania and zirconia. During annealing lead oxide transforms into a cubic phase, and the lead oxide stoichiometry determines the processing route to PZT. In the case of lead‐rich films, intermediate compounds of lead with titania and zirconia are observed, which react during the final stage of annealing at 600 °C to form PZT. In lead‐deficient films, the formation of a pyrochlore phase has been observed, which crystallizes into perovskite at 750 °C. The Zr/Ti ratio also influences the crystallization sequence. In the case of Ti‐rich PZT, the intermediate compounds initially involve a zirconium‐rich rhombohedral PZT, with which residual titanium reacts to form tetragonal PZT. The films showed good ferroelectric and other electrical properties with a remanent polarization of 24 &mgr;C/cm2, coercive field of 32 kV/cm, &egr;’=950, tan &dgr;=0.02, and &sgr;dc(300 K)=10−12&OHgr;−1 cm−1with an activation energy between 0.9 and 1.4 eV.

ISSN:0021-8979    

DOI:10.1063/1.351307

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Vapor transport equilibration was used to prepare lithium niobate crystals of a variety of controlled off‐congruent compositions. Crystals were characterized through measurement of the ferroelectric Curie temperatureTcand measurement of the temperature for noncritical phase matchingTPMof second‐harmonic generation from both 1.064‐ and 1.32‐&mgr;m laser sources. Across the majority of the single‐phase region, bothTcandTPMwere observed to vary nearly linearly with Li/Nb ratio. The variation inTPMwith Li/Nb ratio was observed to change direction within the single‐phase region at some small but finite compositional interval away from the Li‐rich phase boundary. Crystals equilibrated to the Li‐rich phase boundary had excellent optical homogeneity. Preparation of Li‐poor crystals was hampered by extremely slow lithium diffusivity and problems with second‐phase precipitation.

ISSN:0021-8979    

DOI:10.1063/1.351308

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Aluminum‐doped zinc oxide films have been deposited on soda lime glass substrates from diethyl zinc, triethyl aluminum, and ethanol by atmospheric pressure chemical‐vapor deposition in the temperature range 367–444 °C. Film roughness was controlled by the deposition temperature and the dopant concentration. The films have resistivities as low as 3.0 × 10−4&OHgr; cm, infrared reflectances close to 90%, visible transmissions of 85%, and visible absorptions of 5.0% for a sheet resistance of 4.0 &OHgr;/&laplac;. The aluminum concentration within doped films measured by electron microprobe is between 0.3 and 1.2 at. %. The electron concentration determined from Hall coefficient measurements is between 2.0 × 1020and 8.0 × 1020cm−3, which is in agreement with the estimates from the plasma wavelength. The Hall mobility, obtained from the measured Hall coefficient and dc resistivity, is between 10.0 and 35.0 cm2/V s. Over 90% of the aluminum atoms in the film are electrically active as electron donors. Scanning electron microscopy and x‐ray diffraction show that the films are crystalline with disklike structures of diameter 100–1000 nm and height 30–60 nm. The films have the desired electrical and optical properties for applications in solar cell technology and energy efficient windows.

ISSN:0021-8979    

DOI:10.1063/1.351309

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Residual stress profiles in machined germanium have been determined by measuring the longitudinal optical phonon spectra using micro‐Raman spectroscopy. Lateral spatial resolution of 1 &mgr;m results from this direct nondestructive technique. By measuring the Raman spectra with several probe wavelengths, axial resolution of 10 nm is obtained through differential absorption profiling (DAP). The stress field in these machined brittle semiconductors is characterized by a surface layer under compression, an abrupt compressive‐to‐tensile transition, and a deeper‐lying region of tension which ultimately relaxes in the unstressed bulk. The abrupt transition is indicative of a change from plastic to elastic deformation. The DAP technique has been applied to interrupted test cuts in diamond‐turned germanium to reveal that fractured regions of the machined surface possess higher tensile stress that occurs at shallower depths than unfractured regions. A qualitative mechanism of the machining process consistent with these residual stress data is presented.

ISSN:0021-8979    

DOI:10.1063/1.351310

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Stress in local isolation structures is studied by micro‐Raman spectroscopy. The results are correlated with predictions of an analytical model for the stress distribution and with cross‐sectional transmission electron microscopy observations. The measurements are performed on structures on which the Si3N4oxidation mask is still present. The influence of the pitch of the periodic local isolation pattern, consisting of parallel lines, the thickness of the mask, and the length of the bird’s beak on the stress distribution are studied. It is found that compressive stress is present in the Si substrate under the center of the oxidation mask lines, with a magnitude dependent on the width of the lines. Large tensile stress is concentrated under the bird’s beak and is found to increase with decreasing length of the bird’s beak and with increasing thickness of the Si3N4film.

ISSN:0021-8979    

DOI:10.1063/1.351311

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Strained layer superlattices of wurtzite CdS/CdSe have been grown on (111)A GaAs substrates by metalorganic chemical vapor deposition and their optical properties studied by photoluminescence spectroscopy. It is shown that the superlattice layers contain giant strain‐induced piezoelectric fields exceeding 2×108V m−1. These fields are similar to those reported for (111) orientated III–V superlattices, but an order of magnitude greater. The recombination energies from a series of samples provide evidence for a type II conduction band offset of 0.23±0.10 eV (the electron wells being in the CdS), with the band structure heavily modified by the internal electric fields. In addition, the photoluminescence peak emission energy shows a strong dependence on the excitation power. This is interpreted as further evidence for the effect of internal fields. We conclude that this system shows new effects not previously observed in II–VI compound superlattices. The large band‐gap tunability and the space‐charge effects offer possibilities for all‐optical switching devices in the 700–1300‐nm region of the spectrum.  

ISSN:0021-8979    

DOI:10.1063/1.351312

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Calibrated optical Bragg‐scattering measurements of the fundamental steady‐state space‐charge‐field amplitude, and of two higher spatial harmonics, in photorefractivep‐type barium titanate excited by a light intensity pattern that is a simple sine function of distance are reported. These measurements are performed over the entire range (0<m<1) of intensity modulation indexmwhen the modulation period is both much larger than, and comparable to, the smallest characteristic (Debye screening) length involved in the photoconduction process. There is no externally applied electric field. The measurements are compared to numerical solutions of the standard single‐carrier band‐conduction model of photoconductivity, and good agreement is found except for large unexplained differences at high modulation index (∼0.8<m<1). Phenomenological analytic expressions are also given for the fundamental and two higher field‐grating harmonics that are useful approximations to the numerical solutions of the standard model equations for allmand for (small, intermediate, and large) grating periods.

ISSN:0021-8979    

DOI:10.1063/1.351313

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Near‐infrared optical absorption and Faraday rotation have been studied, in addition to the room‐temperature sheet resistance, in a series of magnesium‐doped bismuth substituted lutetium iron garnet (Bi:LuIG) thin films grown by liquid phase epitaxy. Faraday rotation was measured at 1150 and 1300 nm as a function of the MgO concentration in the melt. Optical absorption data indicate valence exchange as a possible mechanism for increased optical absorption at compositions different from charge compensation. Faraday rotation data display a magnesium doping dependent behavior, as do theI‐Vcharacteristics. Additionally, theI‐Vdata of unetched and etched films near compensation suggest a two‐layer structure to the films.

ISSN:0021-8979    

DOI:10.1063/1.351314

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The nonlinear optical properties of an‐i‐p‐istructure containing strained superlattice quantum wells in the intrinsic regions are studied using picosecond pump and probe pulses of the same photon energy. For pump fluences as low as 1.1 &mgr;J/cm2, a blue shift of the excitonic resonance, caused by the screening of the built‐in space‐charge field and the accompanying reduction in the quantum‐confined Stark effect, is clearly observed. At higher fluences, the onset of bleaching of the excitonic absorption is observed. The nonlinearities associated with the quantum confined Stark effect in the heteron‐i‐p‐iare directly compared to those arising from excitonic bleaching in identical strained superlattice quantum wells under flatband conditions. The picosecond time resolution allows a more accurate estimation of the carrier density in the heteron‐i‐p‐iby ignoring the density‐dependent recombination and a quantitative comparison between the strength of the nonlinearities in the two structures. Although such comparisons depend on the optical fluence and structure of the heteron‐i‐p‐i, we find that the magnitudes of the nonlinearities in these two specific structures are comparable on a per carrier basis, although their spectral signatures are quite distinct.

ISSN:0021-8979    

DOI:10.1063/1.351315

出版商:AIP    

年代:1992

数据来源: AIP