摘要:

We discuss recent advances made in the theory and measurements of stresses and strains in Si‐based heterostructures containing submicron‐ and micron‐size features. Several reports on theoretical as well as experimental studies of stresses in the substrates with local oxidation of silicon structures on the surface have been published recently. With the advent of GeXSi1−Xstrained layers and stripes extensive studies of both the stripe and the substrate stresses have also been made. Unlike the previous calculations and analytical models, recent finite element (FE) calculations take into account the coupling between the film–substrate stresses without making the approximation that the interface is rigid or that there is no variation of stresses in the stripes in a direction perpendicular to the interface. The results of these calculations have been compared with the analytical models and limitations of the analytical models have been pointed out. Micro‐Raman measurements of the stresses in the stripes, quantum wires, quantum dots, and substrates have been made. The measured values of stresses in GeSi stripes and quantum structures agree well with the calculated values by the FE method. The micro‐Raman measurements showed that as the ratioR=2l/h(2lis the width andhis the thickness of the stripe) decreases, the shape of the measured normal stresses in the substrate under the stripe (plotted in a direction parallel to the interface) changes dramatically, from concave upward to convex upward. Generation of dislocations in laterally small layers is also discussed briefly. FE calculations of trench‐induced stresses which include the effect of the anisotropy of Si have also been made recently. In these calculations realistic experimental conditions were simulated to determine the oxide shape, oxide–interface stresses, and intrinsic and thermal stresses of the polysilicon fill. These values were then used as inputs for the FE calculations. Calculations of stresses induced by oxide‐filled trenches were also made assuming that Si is isotropic and that the oxide fill has the same elastic constants as Si. These calculations and results of an earlier analytical model implemented under the same assumptions gave identical results; however, the calculated stress values were in error of 20%–30%. The maximum resolved shear stress for the 60° dislocation induced by a trench is 30% more if it is aligned in ⟨110⟩ direction rather than in the ⟨100⟩ direction. This explains the common observation that the ⟨100⟩‐oriented trenches cause fewer dislocations than the ⟨110⟩ trenches. The characteristics of trench isolated as well as junction isolated bipolar transistors have been studied. The trench isolated transistors had 20% higher gain; however, the collector–base capacitance was higher by up to 50% in the trenched transistors. The increase in capacitance was caused by the anomalous diffusion of the antimony dopant from the buried collector layer induced by the stress field of the trenches. The effect could be eliminated by increasing the depth of the trench. The trenched devices also had higher emitter–collector leakage current caused by the dislocations generated by the trench induced stress field. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362678

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An extremely sensitive miniature torque magnetometer using Sip‐doped piezoresistive cantilevers is described. The magnetizationm&drarr; of very small magnetic or superconducting samples (≤1 &mgr;g) deposited on the cantilever can be measured via the torque &tgr;&drarr;=m&drarr;×B&drarr; produced on them by an applied fieldB. The high resolution in the lever deflection of the order of 0.1 A˚ corresponds to a torque sensitivity of the order of &Dgr;&tgr;&bartil;10−14Nm. In a homogeneous field of 1 T this device allows magnetic moments as small as &Dgr;m&bartil;10−14Am2to be measured, a value far smaller than that measurable by the best commercial superconducting quantum interference device magnetometers. Measurements performed on microcrystals of the high‐Tcsuperconductors Bi2Sr2Ca1Cu2O8and Hg1Ba2Ca3Cu4O10in the static and dynamic modes demonstrate the excellent performance of this device at low temperature and in magnetic fields ranging between a few mT and 5 T. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362550

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Conversion of evanescent light into propagating light (as needed in near‐field scanning optical microscopy) is analyzed by means of a photocantilever. The photocantilever is a silicon cantilever with apnjunction photodiode on its tip. The photocantilever tip converts evanescent light from a sample into propagating light. Theoretical values given by scattering and transmission models are compared with our experimental values. The scattering model gives results that are closer to the experimental values than does the transmission model. This indicates that the nonpropagating evanescent light is converted into scattered light at the photocantilever tip, and that the scattered light is collected by the photodiode. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362551

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Charge collection efficiency measurements in silicon detectors at low temperature (T<0.5 K) and low applied electric field (E=0.1–100 V/cm) were performed using a variety of high‐purity,p‐type silicon samples with room‐temperature resistivity in the range 2–40 k&OHgr; cm. Good charge collection under these conditions of low temperature and low electric field is necessary for background suppression, through the simultaneous measurement of phonons and ionization, in a very low event rate dark matter search or neutrino physics experiment. Charge loss due to trapping during drift is present in some samples, but the data suggest that another charge–loss mechanism is also important. We present results which indicate that, for 60 keV energy depositions, a significant fraction of the total charge loss by trapping occurs in the initial electron‐hole cloud near the event location which may briefly act as a shielded, field‐free region. In addition, measurements of the lateral size, transverse to the applied electric field, of the initial electron‐hole cloud indicate large transverse diffusion lengths. At the lowest fields a lateral diameter on the order of 1 mm is found in a detector ∼5 mm thick. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362552

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Photoemission is widely used to generate electron beams with an energy spread lower than by thermoemission. However, when a photocathode is illuminated by a multimode laser this feature is lost and an electron beam with several eV of energy spread is produced. We have developed an explanation for this anomalous behavior pointing out its origin in the combined effect of charge relaxation, taking place within the beam, together with the modulation of the laser power imposed by laser modes. The model permits a correct interpretation overall experimental evidences. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362553

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A cholesteric liquid crystal with negative dielectric anisotropy and with a visible wavelength selective reflection presents high memory effects when encapsulated in a polymer film. Three states at least are accessible within the same film: a reflecting state, a memory state, and a refreshed state. The last two states are stable over months. When the samples were annealed, the memory state was no longer stable. By comparing samples as they were annealed we tried to understand how the memory effect was removed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362480

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A new class of quantum cascade lasers based on type‐II quantum wells is analyzed. In these novel mid‐ and long‐wavelength IR lasers, not only can a population inversion be easily created with a nearly 100% current injection efficiency, but also the nonradiative loss from the optical phonon scattering can be greatly suppressed. A general description of how the lasing threshold current depends on the injection, radiative, and inversion efficiencies is formulated to illustrate the expected improvements over the recently reported quantum cascade laser. Also, the features that distinguish quantum cascade lasers from traditional bipolar lasers are discussed in the context of the carrier transport in equivalent circuit models to illustrate the advantages of quantum cascade lasers for high power mid‐ and long‐wavelength IR source applications. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362554

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Data are presented on the photopumped laser operation of planar AlAs–GaAs superlattice (SL) minidisk lasers. The SL minidisk (70 A˚ AlAs, 30 A˚ GaAs; 100 periods; ∼37 &mgr;m diameter) is defined by impurity‐induced layer disordering (IILD), followed by wet oxidation (N2+H2O vapor, 400 °C which surrounds the minidisk with a low‐refractive‐index AlGaAs oxide. The planar minidisks exhibit laser operation at &lgr;∼7540 A˚, with wider mode separation (&Dgr;&lgr;∼13 A˚) than disks defined by only IILD (a smaller refractive index step) and cleaved sample edges. The mode separation of &Dgr;&lgr;∼13 A˚ corresponds to disk modes that utilize the perimeter of the oxide‐defined disks. In the fabrication of the SL minidisks, IILD forms a structural and doping difference beyond the disk perimeter that acts, in effect, as ap–njunction during etching or wet oxidation. Etch profiles are shown demonstrating this behavior. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362460

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Bi12(GaxBi1−x)O19.5(BGaO) films with thickness in the range 100–1000 nm have been deposited on (100) Y‐stabilized zirconia (YSZ) and (100) Bi12GeO20(BGO) substrates using a KrF excimer pulsed laser and polycrystalline targets with compositionx=0.63–0.72. The laser power density threshold for ablation of the targets has been determined to beJ0=4.8 J/cm2. A deposition rate of 0.03 A˚/pulse was found for the substrate‐target distance (6 cm) and laser fluence (J=7 J/cm2) used. Transparent and crystalline films were obtained heating the substrate in the 450–550 °C range under 1.5×10−1mbar of oxygen pressure. Films deposited on hot substrates have a Ga stoichiometryx=0.5 but a Bi content slightly lower than that corresponding to sillenite. Films deposited on YSZ show preferential orientation. From the x‐ray diffraction results and the comparison of the ionic distributions, it has been concluded that the most likely orientation between both lattices is ⟨310⟩{130}BGaO∥⟨011⟩{100}YSZ. A {100}BGaO∥{100}BGO epitaxy has been inferred from Rutherford backscattering analysis. On both substrates the films behave as step waveguides with refractive index close to the value determined in bulk BGaO. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362461

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Absorption and luminescence saturation of confined excitons are measured using a single‐beam method for CuCl microcrystals with various radii embedded in NaCl single‐crystalline matrices. From the excitation‐intensity and microcrystal‐size dependencies of their saturation, the effective saturation density of excitons, which is inversely proportional to the magnitude of optical nonlinearity, proves to be inversely proportional to the volume of microcrystals with effective radii of less than 5 nm at 77 K. Therefore, the optical nonlinearity of these microcrystals is not a function of microcrystal radii, but of the number of excitons created in a microcrystal, indicating the characteristic nature of coherent exciton confinement. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362684

出版商:AIP    

年代:1996

数据来源: AIP