E. Başaran

The effect of impurities on the phase transitions in the ferroelectric semiconductors TlInS2 and TlGaSe2

The temperature dependences of the dielectric constants of the ferroelectric semiconductors TlInS2 and TlGaSe2 have been studied following their annealing within the incommensurate phase. Unusual memory effects accompanied by both a remarkable inflection of the temperature dependence curves in the incommensurate phase and various shifts of the incommensurate (Ti) and commensurate (Tc) phase transition temperatures have been revealed in both crystals. The observed effects are explained on the basis of a defect density wave model taking into account the interaction of modulation waves with charge carriers localized at impurity states. The thermally activated population of these states during the heating or cooling processes is responsible for the changes of the phase transition temperatures.

Dielectric susceptibility behaviour in the incommensurate phase of TlInS2

Abstract Peculiarities of the temperature behaviour of the dielectric susceptibility of TlInS2 in the temperature interval of incommensurate (IC) phase have been theoretically investigated on the basis of recently suggested two-sublattice model of the phase transitions in TlInS2, which is based on the hypothesis about the coexistence of improper and proper ferroelectricity and also the presence of type I and type II incommensurations in the same compound. The temperature dependence of the dielectric constant of TlInS2 in the IC phase was calculated. An agreement between theoretical and experimental results has been obtained.

Very high two‐dimensional hole gas mobilities in strained silicon germanium

We report on the growth by solid source MBE and characterization of remote doped Si/SiGe/Si two‐dimensional hole gas structures. It has been found that by reducing the Ge composition to ≤13% and limiting the thickness of the alloy layer, growth temperatures can be increased up to 950 °C for these structures while maintaining good structural integrity and planar interfaces. Record mobilities of 19 820 cm2 V−1 s−1 at 7 K were obtained in normal structures. Our calculations suggest that alloy scattering is not important in these structures and that interface roughness and interface charge scattering limit the low temperature mobilities.

Phase transitions and metastable states in TlGaSe2

The results of measurements of the dielectric constant of TlGaSe2 in temperature range of successive phase transitions are presented. An anomaly in the temperature dependence of the real part of dielectric constant in TlGaSe2 has been observed at about 242 K in addition to anomalies at 115, 108, and also near 65 K as reported in previous publications. The presence of temperature hysteresis effects in temperature interval between 115 and 242 K allowed making a conclusion about possible existence of an incommensurate phase in the mentioned temperature range. A model of succession of the structural phase transitions in TlGaSe2 has been suggested.

Improper and proper ferroelectric phase transitions in TlInS2layered crystal with incommensurate structure

The dielectric constant of TlInS2 crystal was measured in the temperature range including incommensurate (IC1 and IC2) and commensurate (C1 and C2) phase transitions under external bias electric field. It has been revealed that maxima in the temperature dependence of dielectric constant of TlInS2 at the C1 and C2 phase transition points shift to higher temperatures. The anomaly at IC2 transition point shifts to lower temperatures on applying the external electric fields. A new complete theoretical model including the presence of two order parameters and two polar sublattices in TlInS2 has been suggested.

Elemental boron and antimony doping of MBE Si and SiGe structures grown at temperatures below 600°C

Abstract This paper considers the low temperature doping of (100) Si and SiGe structures with elemental B and Sb sources particularly with regard to obtaining very narrow delta doping spikes. B is found to be an excellent dopant at SiGe growth temperatures incorporating in an active state at concentrations up to 10%. B delta layers of 1 nm or less have also been grown. Sb is also shown to be capable of providing delta doped layers less than 2 nm wide. The B deltalayers have been incorporated into modulation doped structures yielding an order of magnitude increase in mobility at 77 K.

Electrochemical capacitance-voltage depth profiling of heavily boron-doped silicon

Abstract The electrochemical capacitance-voltage profiling (eCV) technique is employed to measure the carrier concentration in heavily boron-doped Si, grown by molecular beam epitaxy. Secondary ion mass spectrometry (SIMS) and Hall measurements are also carried out for comparison. The carrier concentrations obtained by eCV match well with the Hall measurements. The results indicate that the eCV technique is capable of probing carrier concentrations well into the 1020 cm−3 range with a good precision. The relative merits of the eCV and SIMS depth profiling are shown to be useful in the analysis of boron incorporation behaviour at doping levels above solubility limits.

Choice of electrolyte for doping profiling in Si by electrochemical C–V technique

Abstract Commonly used electrolytes (NH 4 F·HF and NaF/H 2 SO 4 ) for doping profile analysis in Si are employed to clarify their capabilities and limitations using a boron-doped staircase test structure. The carrier density profiles at various measurement voltages for these electrolytes are compared with a SIMS profile of the structure. A variety of parameters affecting a carrier density profile such as static current–voltage behaviour, dissipation factor, flat band potential and series resistance at various doping levels are explored to optimise the measurement conditions. It is found that both electrolytes can be employed for doping profiling in Si for a broad range of doping levels. However, electrolyte NaF/H 2 SO 4 allows wider measurement voltage range and suits better for profiling highly doped structures as indicated by lower series resistance and hence lower dissipation factor.

Microstructural and Optical Study of ITO Thin Films Prepared by Sol-Gel Method

Indium tin oxide (ITO), a transparent conductive oxide, is a wide band gap compound semiconductor with a cubic bixbyte structure. Sn-doped indium oxide (IO) thin films (In:Sn=90:10) were deposited on microscope glass substrates by a sol-gel spin coating technique. The precursor solution was prepared by mixing indium (III) nitrate pentahydrate (In(NO3)3 . 5H2O) and tin (IV) chloride bis(2,4-pentanedionate) (SnCl2(C5H7O2)2 dissolved in acetic acid and acetone mixture. Crystalline ITO thin films were obtained after an annealing process at temperatures between 450 o C-550 o C for one hour. The microstructure and optical properties of ITO films were investigated as function of annealing temperature. XRD analysis revealed single phase In2O3 (JCPDS 06-0416). The optical transmittance of the films in the visible range was more than %80. The direct optical band gap of ITO films was measured between 3.7-3.9 eV.

Luminescence and Optical Properties of Relaxor Ferroelectrics

The optical absorption spectrum, luminescence and photoconductivity on single crystals PbMg 1/3 Nb 2/3 O 3 and on ceramics Pb 0.91 La 0.09 (Zr 0.65 Ti 0.35 )O 3 are investigated in the wide temperature range. The luminescence spectrum correlates with the photoconductivity spectrum. The position of the maximum of the luminescent emission spectrum indicates the origin of the charge carriers emitted from the defect centers. On the basis of the data the properties of the local centers are determined, and the phenomenological approach to the relaxor theory is discussed.