I. M. Tiginyanu

Time resolved blue and ultraviolet photoluminescence in porous GaP

Porous GaP layers prepared by electrochemical anodization of (100)‐oriented bulk material was found to exhibit blue and ultraviolet photoluminescence when excited by a KrF excimer laser. The energy position of the UV luminescence band (3.3 eV at 300 K) is explained on the basis of charge carrier confinement in crystalline quantum wires of about 25 A in diameter. Additional evidence for quantum size effect in porous GaP was obtained by Raman scattering measurements.

Self-Induced Voltage Oscillations during Anodic Etching of n-InP and Possible Applications for Three-Dimensional Microstructures

Voltage oscillations were observed during anodic etching of 100 -oriented n-InP substrates in an aqueous solution of HCl at high constant current density. Under certain conditions, the oscillations lead to a synchronous modulation of the diameters of pores on large areas of the samples which indicates a correlation between the phases of the oscillations in the pores. These self-induced diameter oscillations may be useful for three-dimensional microstructuring of n-InP and thus for the design and fabrication of new photonic materials.

Uniform and Nonuniform Nucleation of Pores during the Anodization of Si, Ge, and III-V Semiconductors

Morphology is one of the basic characteristics of porous layers. For electrochemically grown pores, morphology is strongly dependent on the starting phase of pore growth, the so-called nucleation phase. This paper addresses uniform and nonuniform nucleation of pores on the surface and consequently the development of pores into the bulk of the following semiconductor substrates: Si, Ge, and III-V compounds (GaAs, InP, and GaP). It was found that nonuniform nucleation can cause formation of domainlike porous structures in all investigated semiconductors. However, depending on the anisotropy of the substrate, these domains show significant differences between them. The particularities of each type of domains are discussed.

Persistent photoconductivity and optical quenching of photocurrent in GaN layers under dual excitation

Persistent photoconductivity (PPC) and optical quenching (OQ) of photoconductivity (PC) were investigated in a variety of n-GaN layers characterized by different carrier concentrations, luminescence characteristics, and strains. The relation between PPC and OQ of PC was studied by exciting the samples with two beams of monochromatic radiation of various wavelengths and intensities. The PPC was found to be excited by the first beam with a threshold at 2.0 eV, while the second beam induces OQ of PC in a wide range of photon energies with a threshold at 1.0 eV. The obtained results are explained on the basis of a model combining two previously put forward schemes with electron traps playing the main role in PPC and hole traps inducing OQ of PC. The possible nature of the defects responsible for optical metastability of GaN is discussed.

Raman scattering study of pressure-induced phase transitions in AIIB2IIIC4VI defect chalcopyrites and spinels

Abstract A II B 2 III C 4 VI defect chalcopyrites (DC) and spinels were investigated by Raman scattering spectroscopy under hydrostatic pressure up to 20 GPa. All these compounds were found to undergo a phase transition to a Raman inactive defect NaCl-type structure. The phase transition is reversible for spinels and irreversible for DC. From the analysis of the pressure behavior of Raman-active modes, it was concluded that the phase transition from spinel to NaCl-type structure is direct in MnIn 2 S 4 and CdIn 2 S 4 , while it occurs via an intermediate LiVO 2 -type NaCl superstructure in MgIn 2 S 4 . The observed differences in the pressures and the paths of the pressure-induced phase transitions in A II B 2 III C 4 VI compounds are discussed.

Order-disorder phase transition in CdAl2S4 under hydrostatic pressure

Abstract CdAl 2 S 4 single crystals with the defect chalcopyrite structure have been studied by Raman spectroscopy at hydrostatic pressures up to 150 kbar. The Raman scattering spectra were found to undergo substantial changes around 60 and 100 kbar, due to an order-disorder transition in the cation sublattice, which occurs in two stages as predicted earlier. From the pressure dependence of optical phonon frequencies we obtained values for mode shift parameters. The irreversible disappearance of Raman scattering signals at pressures above 140 kbar was attributed to a phase transition from the adamantine structure to a higher symmetry rocksalt-type structure.

Photoluminescence of ZnO layers grown on opals by chemical deposition from zinc nitrate solution

The emission from ZnO layers grown on the surface of bulk opals using chemical deposition is studied under excitation by the 351.1 nm line of an Ar+ laser at different excitation power densities and temperatures. The emission spectrum exhibits narrow photoluminescence (PL) bands associated with the recombination of bound and free excitons as well a relatively broad band around 3.31 eV. The width of the excitonic lines (2–3 meV) along with their energy position are indicative of the high quality and strain-free state of the layer. The origin of the 3.31 eV PL band is discussed in connection with its dependence upon the excitation power density and temperature.

Strongly frequency dependent focusing efficiency of a concave lens based on two-dimensional photonic crystals

Results of an experimental study of a concave lens based on a two-dimensional microwave photonic crystal with neff<1 are shown. We demonstrate that the lens focuses electromagnetic radiation for transverse electric (TE) and transverse magnetic (TM) polarizations. Intensity gains as high as 5.4 for TE polarization and 6.3 for TM polarization were achieved for definite frequencies lying in the explored interval from 6to15GHz, the smallest area of the focal spot being equal to 0.24λ2 and 1.02λ2 for TE and TM polarizations respectively. The proposed lens serves as a model system that can be scaled to THz and optical frequencies.

GaN Schottky multiplier diodes prepared by electroplating: a study of passivation technology

This paper presents the results of a Pt/n-GaN Schottky contact technology development based on electrochemical metal deposition. Three different technological approaches are used to fabricate GaN varactor diodes. The effects of SiNx-surface passivation and reactive ion etching (RIE) as required to define the micrometre-size Schottky contacts are investigated using photoluminescence (PL) spectroscopy and electrical characterization of the fabricated Schottky diodes. The perspective of Pt/n-GaN Schottky varactor diodes for high-frequency multipliers is estimated on the basis of dc parameters measured for a structure with a 5 ?m electrode diameter.

The interference of additional waves of forbidden polaritons excited by allowed polaritons in CuGaS2

Abstract Additional waves of exciton polaritons are studied in thin (1.5–1.8 μm) CuGaS 2 crystals at 9 K. The reflectivity spectra show a fine structure related to the interference of Fabry–Perot and additional waves which is a consequence of the polariton spatial dispersion. The main parameters of the exciton polaritons were determined from the spectra calculations. The Γ 4 excitons of big oscillator strength are shown to excite the additional polariton waves of the Γ 5 excitons of small oscillator strength, which interfere determining the fine structure in exciton resonance optical spectra.