Koichi Shimakawa

Advances in Amorphous Semiconductors

This book presents the current level of understanding of the structural, electronic and optical properties of amorphous semiconductors. As amorphous materials depart significantly from the crystalline counterparts, some of the basic problems associated with the validity of the effective mass approximation, whether K is a good quantum number, and concepts of phonons and excitons will be addressed in detail. A significant part of the book is devoted to present recent progress made in the understanding of light-induced degradations in amorphous semiconductors, which is regarded as the most limiting problem in device applications. The monograph presents a comprehensive review of both experimental and theoretical studies on amorphous semiconductors.

Amorphous chalcogenide semiconductors and related materials

Introduction.- Structures.- Structural properties.- Electronic properties.- Photo-electronic properties.- Light-induced phenomena.- Applications.- Future Prospects.

On the compositional dependence of the optical gap in amorphous semiconducting alloys

Abstract It is found that the optical gap EAB for amorphous A-B alloys can be determined by the energy gap EA for element A and EAB for the element B in the equation EAB(Y) = YEA + (1−Y) EB where Y is the volume fraction of element A. Calculations based on a random bond network agree with experiments for Siue5f8Ge, Sbue5f8Se, and Asue5f8Te films (class A). Calculations based on a chemically ordered bond network which tends to form microscopic molecular species gree with experimental results for the Asue5f8Se, Asue5f8S, Geue5f8Te and Sb2Se3ue5f8As2Se3 systems (class B). In contrast to the above systems, agreement with experiment is not obtained for the Teue5f8Se, As2Te3ue5f8As2Se3 and GeTe2ue5f8GeSe2 systems which contain atoms of both Te and Se (class C). The classification into three types (classes A, B and C) is consistent with the calculation based on effective medium percolation theory which interprets the compositional dependence of the conductivity of chalcogenide glasses.

Thermal Switching in Chalcogenide Glass Semiconductors

The current-voltage characteristics in off state and the switching phenomena are investigated. The results can be well explained by a thermal model based on the self joule heating of the sample. The results are summarized as follows: (1) I-V characteristics in off state can be normalized over a wide range of temperatures and voltages, and the resulting normalized current has an ohmic and an exponential regions. Calculations based on the thermal model agree very well with the experimental I-V characteristics. (2) From the dependences of the switching threshold voltage upon temperature and resistivity, it is found that the switching can be initiated by a thermal process.

Photoconduction of glasses in the TeSeSb system

Abstract The photoconductivity of bulk glasses of the Teue5f8Seue5f8Sb system is measured as a function of light intensity and photon energy. The relative sensitivity ( ΔI / I d ) has linear and square-root dependences on light intensity in low and high illumination intensities, respectively, and is nearly proportional to the square-root of the resistivity at room temperature. The spectral response of photoconductivity, which is calculated by taking into account the effect of surface recombination of carriers, agrees qualitatively with the experimental results. The experimentally determined broad spectral response suggests the presence of band tails below the conduction band and above the valence band. The large residual dark conductivity in the decay response is associated with the presence of many deep trapping centers.

On the mechanism of d.c. and a.c. transport in transition metal oxide glasses

Abstract Although it is well established that small polarons are formed in transition metal oxide (TMO) glasses, no satisfactory quantitative analysis of d.c. and a.c. transport exists. The temperature dependences of the d.c. and a.c. conductivities for TMO glasses are re-examined. The charge transport cannot be explained quantitatively by the small-polaron model (strong electron-lattice coupling). An alternative possibility—a multiphonon tunnelling process with weak coupling—is also discussed. The a.c. conductivity, which is strongly correlated to the d.c. conductivity and which has never been explained by current theories based on the pair approximation, is interpreted in terms of the continuous-time random-walk approximation.

Electrical Properties of Chalcogenide Glasses of Te-Se-Ge and Te-Se-Sb Systems

The properties of chalocogenide glass semiconductors of Te-Se-Ge and Te-Se-Sb systems are investigated by electrical conduction and differential thermal analysis. When germanium is introduced to a binary Te-Se system, it forms a strong three-dimensional network structure, and increases the glass transition temperature because it is rather easy to form covalent bonds with selenium and tellurium. Introducing antimony into the glass decreases the activation energy remarkably and increases the conductivity, and a further increase of the antimony content promotes the crystallization of the glass; whereas even by increasing the germanium content the crystallization scarcely occurs. The resistivity of the present glasses is expressed by the reiationship, ρ=ρ0exp (ΔE/kT). The pre-exponential factor, mainly determined by the mobility of the carriers, is nearly constant, and independent of the composition.

On photoinduced volume change in amorphous selenium: Quantum chemical calculation and Raman spectroscopy

Hartree–Fock ab initio Raman spectra calculations and Raman spectroscopic measurements were carried out on amorphous selenium in order to identify the characteristic vibrational mode due to sigma bonds. Variations in measured peak intensity were observed as a consequence of band gap light illumination. Based on our previous theoretical investigations we originate these intensity variations from photoinduced covalent bond breaking.

Difference of Thermal Properties between Threshold Type and Memory Type Chalcogenide Glass Semiconductors

Thermal effects on the Te65Se15Ge20 threshold glass and Te45Se50Sb5 memory glass are investigated by electrical conduction, D.T.A. and X-ray diffraction. The results are summarized as follows: 1) the crystallization by annealing proceeds from the surfaces for both types of glass. 2) Crystallites mainly formed are Te and Sb for Te65Se15Ge20 and for Te45Se50Sb5 respectively. 3) From the curve of resistivities vs. annealing time, the rate constant u for the decrease in the resistivity is estimated to be u=0.13/min and 0.25/min for Te65Se15Ge20 and Te45Se50Sb5 respectively. 4) The area of the exothermic peak is proportional to the heat of transition, and it is about three times larger for Te45Se50Sb5 than for Te65Se15Ge20. But the melting temperature Tm of Te45Se50Sb5 is lower than that of Te65Se15Ge20. 5) The fast rate of glass-fluid-crystallite transition of Te45Se50Sb5 and therefore its action as the memory type switching can be well understood.

Microwave absorption in amorphous chalcogenide semiconductors

Abstract Although the a.c. conductivity of amorphous chalcogenides in the radio frequency region has been well interpreted by the combined mechanism of correlated barrier hopping (CBH) of bipolarons and single polarons based on charged defects, it is not clear whether the CBH model can be extended to the microwave region. The validity of the CBH of bipolarons with regard to both the frequency and temperature dependencies of conductivity is examined in the microwave region. It is concluded that the observed microwave optical absorption (conductivity) is well interpreted in terms of the CBH of bipolarons for glassy Se, As2Se3, As2S3, As2Te3, (As2Te3)1-x Ge x and Tl2SeAs2Te3.