The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Chalcogenides: Electronic Structure, Defects, Metastability and Transport I
Chair: Tomas Wágner, Pardubice University
Mo-A1.1 (invited) 14:00–14:30
Expansion of the Application of Chalcogenide Glasses for Establishment of Radiation Doses through Electrical Measurements
1. Dept. of Electrical Engineering Boise State University, 1910 University Dr. Boise, ID 83725-2075, USA
2. Department of Material Science and Engineering, Boise State University, 1910 University Dr., Boise, ID 83725-2090, USA
3. School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ 85287-5706 USA
Chalcogenide glasses are a long term material of interest due to their excellent properties like fast phase crystalline/amorphous transition, which gave rise of their application for non-volatile memory and optical recording, or formation of electron hole pairs by irradiation with light from visible to very short wavelengths like x-rays, which is applied for environmental monitoring and radiology. One newly emerging field of their application is usage of these glasses for radiation sensing of γ rays. We will show the principles on which this sensing relies, based on combination of the carriers' generation and silver diffusion in the active area of the devices. Our study expands over Ge-containing chalcogenide glasses and includes study of the structural and defects generation effects as a results of γ radiation. We identified a dual character of the γ radiation induced effects and their compositional dependence and discuss this based on defects creation in the chalcogenide matrix. Data will be presented from our studies related to structural and compositional changes occurring as well as by radiation induced silver diffusion in the chalcogenide glasses. The device structure modeling with Comsol software and devices' process flow will illustrate the different device structures. The device performance will be demonstrated with electrical testing data. They are discussed related to the material's composition and structure by which the leading role of the layered glass structure of the Ge rich glasses is specifically emphasized. In conclusion different modes of application of the sensing devices will be suggested like real time radiation sensing and reversibility, i.e. reversing the device in the initial condition after cessation the γ radiation due to application of a reverse bias over the devices.
Keywords: Ge-containing chalcogenide glasses, gamma radiation, radiation sensing, sensor performance
Surface Relief Grating Formation in Amorphous As40Se15Se45 and As2S3 Films under 0.532 μm Illumination
Institute of Solid State Physics, University of Latvia, Kengaraga St. 8, Riga, Latvia, Lv-1063
Recently a number of studies on direct surface structure formation by holographic recording were carried out in thin layers of chalcogenide semiconductors. This phenomenon is due to photoinduced mass transport during comparatively high light exposures. Efficiency of recording is defined by distribution of electric field vector of resulting interference pattern inside the recording media in dependence on recording light polarization. Previously obtained results suggested that the whole illuminated volume of the film takes part in the photo induced mass motion process resulting surface relief formation process.
In this research we have studied the influence of λ=0.532 μm laser light illumination on formation and properties of surface relief gratings in amorphous As40Se15Se45 and As2S3 films. The surface relief grating formation was studied for wide range grating periods overlaying ~1 – ~80 μm.
In regard to As2S3 films the formation of surface relief during exposure from film side as well from glass substrate side was studied. Examination the relief depth -Δh dependence on grating period at constant exposures in As2S3 films shows that values of Δh grows up to periods Λ≈10–15μm for recording from film side and to Λ≈30μm at exposure from glass side. Further increasing of grating period leads to decreasing of Δh values for the same exposures. Besides, in our case for films with thickness d≈7μm for grating periods up to Λ≈25μm, the profile formatting processes for samples illuminated from film side is more advantageous (higher profile height Δh at constant exposures) for larger periods than from glass substrate side. At the same time for As40Se15Se45 films under λ=0.532 μm light illumination the monotone decrease of Δh values for periods Λ from ~1 μm to ~50μm take place and for Λ>50 μm relief formation was not observed at all.
As one of the reasons for observed difference in course of dependence on grating period for relief formation in As2S3 (non-monotone) and As40Se15Se45 (monotone) under 0.532μm illumination could be consider presence of light propagation area inside the film's volume. At wave length λ=0.532 μm in As2S3 films absorption coefficient α≈103 cm-1 and light penetration depth ρ≈11μm. So, due to transparency, the surface relief formation processes versus smaller grating periods could be more restricted.
In the case of 0.532 μm wave length illumination for As40Se15Se45 with absorption coefficient α≈x104 cm-1, that light propagation area will be be significantly small corresponding to light penetration depth ρ≈0.3 μm. Consequently, there is a no possibility for additional structure formation due to diffracted light beam interaction inside the volume of film. Through the experiments we found that under certain conditions—depending on grating period and sample thickness—it is possible create reliefs with profile high Δh comparable and even exceeding the film initial thickness.
Keywords: As-S-Se films, holographic recording, surface relief grating
A Comparison of the Phenomena of Photoluminescence and Carrier-type Reversal in Bi- and Pb-doped Glasses
1. Advanced Technology Institute, Department of Electronic Engineering, University of Surrey, Guildford, GU2 7XH, United Kingdom
2. Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, United Kingdom
3. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom
4. Toyota Technological Institute, 2-12-1, Hisakata, Tempaku, Nagoya 468-8511, Japan
5. Centre for Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice, CS. Legion's sq. 565, Pardubice 532 10, Czech Republic
We present evidence connecting the phenomena of carrier type reversal and photoluminescence (PL), which are observed in certain Bi- and Pb-doped glasses. We also report PL from Bi- and Pb-implanted glass, and that the order of the reaction which generates optically active Bi centers varies significantly between different glass hosts.
Comparing contour plots of PL spectra at various excitation wavelengths of Bi-doped chalcogenide, Bi-doped germanate and Pb-doped germanate glasses, indicates that five absorption/PL bands are in approximately the same position. This suggests that very similar active centers are present in Bi- and Pb-doped oxide and chalcogenide glasses. In 4% and 10% PbO doped germanate glass, one and two crystallization temperatures, respectively, can be observed. This could be seen as being analogous to the phase separation observed in Bi-doped GeS glasses displaying carrier-type reversal when the Bi content is increased past 11 mol%. When excited at 782 nm, Bi- and Pb- implanted gallium-lanthanum-sulphide-oxide (GaLaSO) glass thin films display PL bands centered at 820 nm and 860 nm, respectively. The intensity (I) of the 820 nm PL band has a power law dependence on Bi dose (d) of d1.4; a similar power-law dependence occurs in a Bi melt-doped oxide glass. When excited at 514 nm, Bi-implanted GaLaSO thin films display a PL band at 700 nm, which is not present in a Bi melt-doped chalcogenide glass having a similar composition to the implanted glass. This indicates that new Bi centers are formed through implantation, which are absent in the melt-doped glasses. This has important implications for Bi-doped glass lasers, in which the control of Bi centers is critical for improving performance. We report Bi-related red PL bands in Bi-implanted bulk Ge33S67 and Ga5Ge25S70 glasses, and NIR PL bands in Ge23Ga12S64Bi1 glass; all of which have very similar compositions to those in which carrier-type reversal has been observed. This indicates that Bi-related PL and carrier-type reversal may be caused by the same Bi centers.
We determined the reaction order for the generation of Bi centers in various oxide glass hosts by extracting absorption data from previously published work, and measuring the gradient (equivalent to the power-law factor) of a double-logarithmic plot of Bi-related absorption coefficient against Bi2O3 doping concentration. The reaction order in Bi-doped oxide glasses decreased with increasing optical basicity of the glass host. A sequential redox reaction involving the decomposition of Bi2O3 into BiO, then Bin clusters, can explain a reaction order dependence on optical basicity. We suggest that red and NIR PL bands result from Bi2+ and Bin clusters, respectively, and these centers are also related to carrier type reversal.
Keywords: chalcogenide, implantation, photoluminescence, carrier-type
Photo-induced Structural Changes in a-Se Triggering its Crystallization
1. Thunder Bay Regional Research Institute, 290 Munro St., Thunder Bay, ON, P7A 7T1, Canada
2. Department of Physics, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1
The first principles calculations are applied to establish a mechanism of photo-induced crystallization of amorphous Se that is performed based on study of the structural changes caused by electron excitation. In order to disclose a mechanism of crystallization, system of increased crystalline order with inclusion of the preformed helical chains acting as the nucleation embryos has been investigated. This system has been assigned to recognize the hexagonal symmetry such as following the photo-induced structural changes it is expected to proceed into the new conformation characterized not only by lower total energy but also by better ordering. We found that the ideally relaxed lattice rather supports twofold coordinated Se atoms which are arranged in long polymer chains. Because of variation in the Se-Se bond length (2.37–2.8 Å) and in orientation of the lone pairs, even two-fold conformation shows formation of the defect tails near the conduction and the valence bands. Upon excitation of single or two electrons to occur from the lone electron pairs, the system stability is impaired to be resolved through formation of the threefold sites (increase in coordination number is observed with light irradiation ). There are two ways for amorphous system to resolve the local metastability, one is relaxation to the original state that involves the carrier recombination, so-called reversible defects , while second is a relaxation to a lower energy state expected to be responsible for the observed irreversible defects . We found that for system of increased crystalline order recognizing the hexagonal symmetry, a transition to the lower energy state involves a breaking of the existing bonds and their rearrangement (the lattice relaxation energy is found to be in the range of 0.5 eV). It is directed on formation of the ideal helical coils associated with the crystalline form. A cascade of irreversible changes eventually induces a crystallization of a-Se.
 A. V. Kolobov, et al., PRB 55, 726 (1997)
 A. Reznik, et al., J. Mater. Sci.: Mater. Electron 20, S111 (2009)
Keywords: amorphous Se, photo-induced metastability, doping, reversible defects, irreversible defects