National Institute Of Materials Physics - Romania

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847 articles found

811. Physics of optimal resonant tunneling

Authors: Racec, PN; Stoica, T; Popescu, C; Lepsa, M; vandeRoer, TG

Published: AUG 15 1997, PHYSICAL REVIEW B, 56, 3597, DOI: 10.1103/PhysRevB.56.3595

The optimal resonant tunneling, or the complete tunneling transparence of a biased double-barrier resonant-tunneling (DBRT) structure, is discussed. It is shown that its physics does not rest on the departure from the constant potential within the barriers and well, due to the applied electric field, but on the effective symmetry of the rectangular-barrier profile, which approximates the real potential profile for the corresponding applied bias.

812. Localization and charge conversion of copper in Rb2ZnCl4:Cu crystals: An ESR and optical absorption study

Authors: Stefan, M; Nistor, SV; Grecu, NM; Schoemaker, D

Published: AUG 1997, PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 202, 1011, DOI: 10.1002/1521-3951(199708)202:2<999::AID-PSSB999>3.0.CO;2-5

Electron spin resonance and optical studies reveal the presence of both Cu2+ and Cu+ centers in Rb2ZnCl4:Cu single crystals grown from melt and their conversion by X- or gamma-irradiation. Two types of paramagnetic Cu2+ centers with different concentrations and production properties have been identified. The more abundant Cu2+(I) center consists of a Cu2+ ion substituting for Zn2+ at the center of a ZnC42- tetrahedron. The less abundant Cu2+(II) center seems to be situated at a Rb site. Production experiments strongly suggest that during the crystal growth copper enters the Rb2ZnCl4 lattice as Cu2+, mainly at Zn2+ sites, part of it being afterwards converted to Cu+ precursor centers. The presence of a neighboring-charge compensating anion vacancy and its departure during the radiolytic Cu+(I) --> Cu2+(I) conversion seems to play an essential role in the stabilization of the Cu+(I) and Cu2+(I) centers, respectively.

813. Amorphous rf-sputtered Si100-xNix thin films with O<=x<=15 at.%: Structural, optical and electrical properties

Authors: BeluMarian, A; Serbanescu, MD; Manaila, R; Stoica, T; Dragomir, A; Zavaliche, F; Tanase, M; Devenyi, A

Published: JUN 1 1997, THIN SOLID FILMS, 301, 202, DOI: 10.1016/S0040-6090(97)00007-2

The temperature dependence of the conductivity (between 15 and 300 K) and optical transmission spectra (between 0.8-3.5 mu m) have been measured on r.f.-sputtered Si100-xNix with 0 less than or equal to x less than or equal to 15. The films were characterized by Rutherford backscattering spectroscopy and X-ray diffraction. Changes in the optical sub-bandgap structure, with corresponding changes in the conduction mechanism take place by varying the Ni content. The films are amorphous, showing a main broad diffraction peak. Its position deviates, for this concentrationrange, from 2(kF) (k(F) = Fermi radius), the alloy losing the Hume-Rothery character present for 50 less than or equal to x less than or equal to 60.

814. Analysis of 6-nm AlGaAs SQW low-confinement laser structures for very high-power operation

Authors: Buda, M; vandeRoer, TG; Kaufmann, LMF; Iordache, G; Cengher, D; Diaconescu, D; PetrescuPrahova, IB; Haverkort, JEM; vanderVleuten, W; Wolter, JH

Published: APR 1997, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 3, 179, DOI: 10.1109/2944.605652

This paper reports experimental results on single quantum-well separate confinement heterostructures (SQW SCH) with low-confinement factor, designed for very high-power operation, The maximum power output for AR/HR coated 3-mm-long devices, measured in very short pulsed conditions (100 ns/1 kHz), from 10-mu m-wide stripes was as high as 6.4 W before catastrophical optical degradation, If scaled to continuous-wave (CW) conditions, this value would be 800-1100 mW, which would mean a factor of 2-2.7 times more than reported for the best devices with normal design for threshold minimization, The absorption coefficient for the symmetrical structure is as low as 1.1 cm(-1), in spite of the low trapping efficiency of carriers in the quantum well (QW), The maximum differential efficiency is 40% (both faces, uncoated devices) for symmetrical structure and 33% for the asymmetrical one (all measurements in pulsed conditions), Threshold current densities were 800 A/cm(2) for 5-mm-long devices in the symmetrical case and 2200 A/cm(2) in the asymmetrical one, The effects of inefficient carrier trapping in the QW on the threshold current densities and differential efficiency are discussed.

815. Modal behaviour of weak index guided stripes in low confinement laser diodes

Authors: Buda, M; Diaconescu, D; Iordache, G; Cengher, D

Published: 1997, CAS '97 PROCEEDINGS - 1997 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 20TH EDITION, VOLS 1 AND 2, 496, DOI:

This paper presents the optical field behaviour of relatively large stripes, long laser diodes. The devices were designed to operate in the fundamental lateral mode. The stripe width, of order of 16 mu m, involves a very low refractive index step at the ridge edges and the operation of the diode in weak index guided regime. This index profile and subsequently, the optical field distribution is very easily disturbed by carrier injection or by temperature The paper aims to characterise the influence of direct current intensity on lateral far field.

816. Electrical properties of porous silicon stabilised by storage in ambient

Authors: Ciurea, ML; Lazar, M; Lazanu, S; Pentia, E; Dragoi, V

Published: 1997, CAS '97 PROCEEDINGS - 1997 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 20TH EDITION, VOLS 1 AND 2, 180, DOI:

The electrical transport of stabilised porous silicon layers was investigated. The samples were stored in ambient for 1,5-2 years, A MIS-like C-V characteristic, a strong rectifying I-V curve and I-T dependence with two activation energies were obtained, Thermally stimulated depolarization currents have an activation energy of 0.81-0.87 eV. The electrical properties are discussed in the frame of a quantum confinement model, keeping into account the surface component.

817. Visible photoluminescence in porous silicon prepared in different conditions - Temperature dependence and decay

Authors: Ciurea, ML; Pentia, E; Manea, A; BeluMarian, A; Baltog, I

Published: JUN 1996, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 195, 645, DOI: 10.1002/pssb.2221950230

Measurements of the visible photoluminescence and the decay of the photoluminescence, performed at different temperatures on porous silicon films, prepared by the anodization process under different conditions on (100) p-type silicon wafers are reported. A separation of the broad visible band of the photoluminescence in two subbands with maxima situated at 1.54 and 1.72 eV at room temperature related to the preparation conditions is obtained. This separation is still present at low temperatures but is very weak. The PL decay process is well described by a bimolecular recombination. The decay time alpha(-1) is in the order of microseconds at room temperature and nanoseconds at liquid nitrogen temperature.

818. Electrical and structural properties of anodized porous silicon

Authors: Ciurea, ML; Pentia, E; Lazar, M; BeluMarian, A; Zavaliche, F; Manaila, R

Published: 1996, CAS '96 PROCEEDINGS - 1996 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 19TH EDITION, VOLS 1 AND 2, 236, DOI:

819. Symmetric SQW structure with low confinement factor for high power laser diodes

Authors: Buda, M; Iordache, G; Vlaicu, M; Cengher, D; Diaconescu, D

Published: 1996, CAS '96 PROCEEDINGS - 1996 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 19TH EDITION, VOLS 1 AND 2, 210, DOI:

820. INTERFACE TRAPPING STATES IN MISIM STRUCTURES, WITH ZNS-MN

Authors: PETRE, D; PINTILIE, I; CIUREA, ML; BOTILA, T

Published: MAY 1 1995, THIN SOLID FILMS, 260, 57, DOI: 10.1016/0040-6090(94)06467-9

Optical charging spectroscopy and thermally stimulated depolarization measurements were performed on metal-insulator-semiconductor-insulator-metal structures with a ZnS:Mn active layer. Two groups of trapping centres situated at every insulator-semiconductor interface were in evidence. Their depths were found to be centred at 0.55 eV and 0.875 eV respectively in the forbidden gap of a ZnS:Mn film. A partial thermal healing of these interface states was observed.