[en] Schottky Au-Cd_0.99Mn_0.01Te:Ga contacts were studied by the use of the capacitance-voltage measurements and thermally stimulated capacitance technique. Capacitance was recorded within 77 K-400 K temperature range. Persistent photocapacitance was observed at temperatures as high as 260 K. Obtained results can be explained assuming the presence of DX centers in the semiconductor and the interface states at the metal-semiconductor interfacial layer. The low temperature photocapacitance has been attributed to the presence of DX centers whereas that observed at higher temperatures has been linked with the interface states. The presence of the interface states has been confirmed by the thermally stimulated capacitance studies performed at various reverse biases. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] We show that a turnover from the classical Debye to the two-power-law relaxation behavior, observed in the majority of physical systems, is associated with a new type of a coupled memory continuous-time random walk driving a fractional dynamics. We derive a general class of the two-power law relaxation responses which is able to reproduce all of the observed relaxation patterns, given by the low- and high-frequency power-law exponents falling in the range (0,1]. (authors)

[en] It was recently shown that the kinetics of persistent photoconductivity (PPC) build-up in indium doped Cd_1-xMn_xTe are non-exponential and can be described solely by the stretched-exponential function. The non-exponentiality is attributed to the indium related DX centers present in the materials. In order to explain this observation, low temperature photoconductivity build-up was studied for Cd_1-xMn_xTe:In of two different manganese contents. It was found that this type of response has its origin in the heavy-tailed distribution of the DX centers. The distribution was analyzed in terms of photon flux. Increasing photon flux leads to the more dispersive behavior. It was also confirmed that the heavy-tailed distribution is due to the different local configuration of atoms surrounding DX centers in the alloy.

[en] In this paper we clarify the relationship between the relaxation rate and relaxation time distributions underlying the Kohlrausch-Williams-Watts (KWW) photoconductivity build-ups in indium- and gallium-doped Cd_1-xMn_xTe mixed crystals. We discuss the role of asymptotic properties of the corresponding probability density functions. We show that the relaxation rate distribution, as a completely asymmetric α-stable distribution, leads to an infinite mean value of the effective relaxation rate. In contrast, the relaxation time distribution related to it leads to a finite mean value of the effective relaxation time. It follows from the experimental data analysis that for all the investigated samples the KWW exponent α decreases linearly with increasing photon flux in the range of (0.6-0.99) and its values are more spread in the case of gallium-doped material. We also observe a linear dependence of the mean relaxation time on the characteristic material time constant, which is consistent with the theoretical model.

[en] The kinetics of photoconductivity build-up in indium- and gallium-doped Cd_1-xMn_xTe mixed crystals was studied at 80 K. The photoconductivity transients are non-exponential, due to the presence of metastable DX centers, formed by both elements in these semiconductors. It has been shown for the first time that the kinetics of the observed photoconductivity transients can be properly described solely by the Kohlrausch-Williams-Watts (KWW) function. This statement has been verified by the careful analysis of the derivatives of the transients. It has been found that they obey short-time power-law behavior, regardless of the donor type and the content of manganese. This asymptotic property can be explained exclusively by the KWW approach. Such an approach results from a broad distribution of relaxation rates related to the DX centers. It is reasonable to assume that, in Cd_1-xMn_xTe alloys, the broad distribution arises from the differences in local atomic configurations in the vicinity of the DX centers

[en] Highlights: • Coexistence of chiral and perovskite phases in [NH₂NH₃][Mn(HCOO)₃](HyMn) metal-organic compound. • Dielectric two-power law, generalized Mittag-Leffler response of HyMn. • The presence of polar nanoregions in HyMn suggesting the relaxor nature of the ferroelectricity. - Abstract: We report the synthesis, thermal and dielectric measurements of [NH₂NH₃][Mn(HCOO)₃] (HyMn) compound. Above room temperature this polymeric material undergoes two phase transitions at ∼360 K and ∼298 K, as observed via DSC and BDS spectra. The first high temperature phase transition is associated with the paraelectric to ferroelectric transition of perovskite HyMn. The low temperature phase transition corresponds to the paraelectric to antiferroelectric transition of chiral HyMn. However, mechanisms hidden behind the observed two polymorphic phases are not completely clear yet. Dielectric spectroscopy measurements have revealed formation of clusters and superclusters as well as the relaxor-like behavior of this compound in wide temperature range resulting from both chiral and perovskite phases. Analysis of dielectric permittivity spectra obtained for the investigated material showed the generalized Mittag-Leffler two-power-law relaxation pattern that was interpreted by means the stochastic scenario of correlated-clusters. The proposed approach brought into light the presence of polar nanoregions in the material suggesting the relaxor nature of the existing ferroelectricity.

[en] In this study the capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements have been performed on ZnTe (p-type)-Ti/Au Schottky diodes containing one layer of CdTe self-assembled quantum dots (SAQDs). The reference sample was the ZnTe-Ti/Au diode without dots. Both samples were grown by molecular beam epitaxy technique. The dots were formed during the Stranski-Krastanov growth mode. DLTS measurements for the sample with QDs reveal the presence of two hole-related signals with thermal activation energies equal to E_H1=0.2 eV and E_H2=0.4 eV. For the reference ZnTe-Ti/Au diode only the H2 signal is observed. It may be concluded that the H1=0.2 eV level can be assigned to the hole emission from the QDs. The 0.4 eV trap is attributed to the ZnTe bulk material.

[en] Non-exponential persistent photoconductivity decay kinetics in gallium doped Cd_0.99Mn_0.01Te mixed crystals were studied at various temperatures and photon fluxes. It was found that the phototransients exhibit the stretched-exponential pattern and can be fitted by means of the Kohlrausch-Williams-Watts Φ(t)=exp(-t/τ)^α, 0<α<1 function. The stochastic approach to relaxation processes was proposed to give the physical interpretation of the stretching-exponent α and its relation to the DX centers relaxation rates distribution.

[en] We report on the experimental observation of confinement-induced new phase appearance in acetylsalicylic acid (ASA)–porous glass (PG) composites. In this study, ASA was embedded in PG host matrices of various pore widths (15–200 nm). The Raman spectra and positron annihilation lifetime measurements exhibit the existence of ASA nanocrystals in the PG matrix. The DSC data revealed that the melting temperature T_M and excess specific heat decrease with decreasing the size of embedded ASA nanocrystals. The close inspection of the T_M dependence versus diameter of filled pores has shown that the ASA crystallizes in polymorph II in confined matrix. Moreover, it was demonstrated that the ASA spatial confinement results in the appearance of new polymorphic phase in the investigated composites—highly likely form ASA III. Both the changes in ASA melting temperatures due to the volume constraints and the decrease in specific heat may be crucial for the bioavailability of the drug.

[en] In this study, we present the results of investigation on p-n GaN diodes by means of deep level transient spectroscopy (DLTS) within the temperature range of 77-350 K. Si-doped GaN layers were grown by metal-organic vapor-phase epitaxy technique (MOVPE) on the free-standing GaN substrates. Subsequently Mg-doped GaN layers were grown. To perform DLTS measurements Ni/Au contacts to p-type material and Ti/Au contacts to n-type material were processed. DLTS signal spectra revealed the presence of two majority traps of activation energies obtained from Arrhenius plots equal to E₁=0.22 eV and E₂=0.65 eV. In present work we show that the trap E₁ is linked with the extended defects whereas the trap E₂ is the point defect related. Its capture cross section is thermally activated with energy barrier for capture equal to 0.2 eV.