The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Chalcogenides: Photoinduced Changes and Devices I
Chair: Spyros Yannopoulos, University of Patras
Optical Properties of Photoconductor Using Crystalline Selenium
NHK Science and Technology Research Laboratories, 1-10-11 Kinuta, Setagaya-ku, Tokyo 157-8510, Japan
Amorphous selenium has been used as a photosensitive layer in the high-gain avalanche rushing amorphous photoconductor (HARP) target, which has both ultrahigh-sensitivity by using avalanche multiplication and a low-noise property. Recently, the increased needs for high-sensitive solid-state imaging devices have led to a need for more suitable photosensitive materials that can perform like the HARP target with much lower applied voltage to stack onto a complementary metal-oxide semiconductor (CMOS) readout circuit because of its endurance voltage.
Crystalline selenium, which has a high absorption coefficient in the visible-light region, is one of the candidates. In our experiment with an image pick-up tube using crystalline selenium as a photosensitive layer, a high-resolution image was obtained for the first time. Besides, I-V curve of the tube shifted to a lower voltage than that of the tube using amorphous selenium as a photosensitive layer. These results indicate that high-resolution images similar to the HARP tube can also be obtained in solid-state devices stacked with crystalline selenium.
Test sandwich cells were fabricated to measure the I-V and quantum efficiency characteristics. To realize a stable crystalline selenium film, we converted an amorphous selenium film, which was deposited by vacuum evaporation on a metal electrode, into a crystalline selenium film by annealing at temperature of 100 to 200 degree Celsius for a few minutes in air or by reacting in an organic solvent. Prior to the annealing process, to prevent the selenium film peeling, an indium-tin-oxide (ITO) transparent electrode was deposited onto the amorphous selenium film by DC sputtering.
We observed photocurrent multiplication phenomena with very high quantum efficiency of far exceeding unity in the film using crystalline selenium as a photosensitive layer at an applied voltage of 10 V. These multiplication phenomena seem to occur as a result of injected carriers caused by a large localized electric field at the interface between the crystalline selenium film and the metal electrode, because multiplication factors differ depending on surface morphology of the metal electrode.
To apply crystalline selenium to imaging devices, the level of dark current injected from external electrodes needs to be reduced at the same time. Nickel oxide (NiO), which is a p-type wide-gap semiconducting material, was deposited by RF magnetron sputtering at room temperature as an electron-blocking layer between the metal electrode and the crystalline selenium film to suppress the injected dark current. The level of dark current of the cell using NiO was decreased to that in the cell without the NiO electron-blocking layer. Since adapting a blocking layer also decreases quantum efficiency, balancing high quantum efficiency and low dark current is desired.
We will discuss the results and details of photocurrent multiplication phenomena in the crystalline selenium film.
Keywords: crystalline selenium, photocurrent multiplication, electron-blocking, NiO, imaging devices
High Sensitivity Photodetector Made of Amorphous Selenium and Diamond Cold Cathode
1. Graduate School of Arts and Sciences, International Chrisitian University (ICU), 3-10-2 Osawa, Mitaka, Tokyo 181-8585, Japan
2. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117 576, Singapore
3. Nanotube Research Centre, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
4. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, 980-8577, Japan
5. Dept. of Clinical Laboratory, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyoku Tokyo 113-8655, Japan
Since the time of xerography, amorphous selenium (a-Se) has been applied to photo imaging devices. Advantages such as wide-area film deposition, low leakage current and wide detectable wave range are suitable for imaging device, which have led to flat-panel X-ray detector for medical use. In addition, development of HARP(High-gain Avalanche Rushing amorphous Photoconductor) have opened up the possibility of ultra high-sensitivity photo imaging device: quantum efficiency of greater than 100% is possible in this device, which is reported to be enhanced by internal carrier multiplication. The next step should be ultra high sensitivity photo imaging device that covers visible to X-ray, however, application of carrier multiplication has only been reported in HARP. In this study, demonstration of carrier multiplication has been attempted in a diode-structured photodetector.
A prototype photodetector was fabricated using a-Se based photoconductive film and nitrogen(N)-doped diamond film serving as a electron emitter. n-doped diamond emitter was selected since it steadily emit electrons at applied voltage of ~400 V, which is suitable to induce carrier multiplication in a-Se. The emission current-applied voltage(I-V) characteristics of this device were compared between different lighting conditions: visible, UV and no illumination. It was found from the comparison that the light illumination leads to enhanced emission current, which proves successful photo detection. The sensitivity of the detector was evaluated in terms of quantum efficiency, which was estimated to be 100–1,000 depending on the wavelength of the light. A detailed physical model of photo detection as well as possible plans for higher sensitivity will be discussed in the presentation.
Keywords: amorphous selenium, photodetector, carrier multiplication, diamond, cold cathode
Studies of Silver Photo-diffusion Dynamics in Ag/GexS1–x (x=0.2 and 0.4) Films by Means of Neutron Reflectometry
1. Research Center for Neutron Science and Technology, Comprehensive Research Organization for Science and Society, Tokai, 319-1106, Japan
2. Japan Atomic Energy Agency, Tokai, 319-1195, Japan
3. Department of Electrical and Computer Engineering, Boise State University, Boise, ID, 83725-2075, U.S.A.
Silver photo-diffusion is one of the attractive phenomena observed in amorphous chalcogenide films with high application potential. So far the studies regarding this unique phenomenon have shown a step-like silver-concentration change as a function of the depth direction along 'a diffusion front'. These results have been conventionally obtained by combining two types of technique: the measurement of time-evolutionary changes using methods such as electrical resistivity and optical reflectivity; and, the determination of detailed concentration profiles using, Rutherford Backscattering and/or ESCA. It is desirable, to bypass the intrinsic assumptions of the above approaches and measure the depth profile "dynamically" during light illumination. Neutron and X-ray reflectivity are powerful techniques that can be used to 'capture' such transient depth profiles. Synchrotron radiation (SR) provides brilliant photon flux for time-resolved measurements. However it is well known that light and X-ray can induce silver diffusion in the chalcogenide material. Therefore, there is a strong possibility that the SR flux may decompose the sample films rapidly before useful data can be collected when using this technique. Neutrons, on the other hand, offer a safer approach by excluding the possibility of changes induced by the probe beam and the use of an intense pulsed neutron source is suitable to observe time-resolved measurements.
In this presentation, we report our recent results of neutron reflectivity measurements for Ag/a-GexS1–x (x=0.2, 0.4) films under light illumination. The neutron reflectivity measurements have been performed on a polarized neutron reflectometer (BL17, SHARAKU) at Japan Proton Accelerator Research Complex (J-PARC), Japan. By using the time-of flight instrument with intensive pulsed neutrons produced by 300 kW proton beams, time evolution of the neutron reflectivity under a light illumination has been revealed, with at least 2-min time resolution. From the detailed analysis, it was found for Ag 50 nm/Ge40S60 150 nm films under a light illumination from the Ag layer side that there are two types of diffusion processes: a fast change observed in the first 10 min. after illumination using a xenon lamp, which is then followed by a slow change observed after a 1 hour of additional light exposure. The result indicates that there is a comparatively stable (metastable) state in the Ag-doped Ge40S60 layer in terms of Ag composition, and the next silver diffusion process occurs by affecting the Ag-doped Ge40S60 layer / Ge40S60 interface. This coincides with the results of Wágner et al.  obtained for Ag/As-S films. These results are also in accord with the results reported by some of us by modeling of the Ag transport in Ge-Se glass  showing the presence of slow and fast moving Ag ions. Our result demonstrated that the idea of a two-step reaction process can be applied to Ge-chalcogenide system. We also discuss illumination-side dependence and Ge-composition dependence of reaction process.
 T. Wágner, G. Dale, P. J. S. Ewen, A. E. Owen and V. Perina, J. Appl. Phys. 87 (2000) 7758
 De Nyago Tafen, D.A. Drabold, M. Mitkova, Phys. Rev. B 72 (2005) 054206
Keywords: amorphous chalcogenide, silver photo-diffusion, neutron reflectivity
Enhanced Mid Infrared Emission in Chalcogenide Glass-ceramics
1. Centre for Ultra-high bandwidth Devices for Optical Systems, Laser Physics Centre, The Australian National University
2. College of Applied Sciences, Beijing University of Technology, 100 Pingleyuan Chaoyang District, Beijing, 100124, China
3. College of Information Science and Engineering, Ningbo University, Ningbo 315211, PR China
High quality (GeSe2)80(Ga2Se3)20 chalcogenide glasses doped with 0.1% Dy have been successfully prepared and subsequently thermal processed with different durations at a temperature that is 30°C higher than the glass transition temperature. It was found that, with the prolonged thermal-processing time, the sharp diffraction peaks that are associated with the crystallization of the glasses began to appear, and the transmission edge of the glasses became broad due to the Mie-scattering induced by the nanocrystals in glass ceramics. Mid-infrared emission at 2.86 μm was found to be significantly enhanced with increasing thermal processing time. We further mapped the distribution of the elements during thermal-processing using electronic energy loss spectroscopy, and built up the correlation between the diffusion of the element and the enhanced mid infrared emission.
Keywords: chalcogenide, glass-ceramics, rare-earth doping, mid infrared emission
Meyer-Neldel Rule and Poole-Frenkel Effect in Chalcogenide Glasses
1. Department of Physics, Faculty of Science, University of Aswan, Aswan, Egypt
2. Department of Engineering Physics and Réseau Québécois de Matériaux de Pointe, école Polytechnique, P.O. Box 6079, Station C-V, Montréal, Québec, Canada H3C 3A7
It is now well known that the low field electric conductivity of many disordered semiconductors obeys the Meyer Neldel rule (MNR) and that their high field conductivity exhibits Poole-Frenkel (PF) behavior. It has not been recognized until recently that there may exist a close relation between the two effects. If the activation energy, ΔE, for conductivity, σ, is varied, MNR tells us that 
The prefactor to the thermal activation term contains ΔE and a characteristic temperature, TMN. It has been found empirically  that PF behavior is described by
In 2008, Emin  showed that adiabatic polaron hopping should lead to the presence of both MNR and PF effect, and predicted that TMN = T0. This relation has recently been observed for fullerene films . There is considerable evidence that conductivity in chalcogenide glasses is polaronic, and it has been shown [5,6] that the correlated barrier hopping (CBH) model, assuming that the hopping time, τ, obeys MNR, describes both DC and AC low field conductivity in these glasses. We find that CBH also predicts that the characteristic temperatures associated with MNR and PF should be the same. Further, it also predicts that the extrapolated conductivity prefactors, σ00, will be the same in the two cases. Experimental evidence from the literature suggests, but does not demonstrate conclusively, that the two predictions are satisfied for chalcogenide glasses. We interpret the result in terms of the multi-excitation entropy model for MNR . Other chalcogenides are under study, and we raise the question of the behavior of amorphous semiconductors which are not polaronic.
 A. Yelon, B. Movaghar, and R. S. Crandall, Rep. Prog. Phys. 69, 1145 (2006)
 L. B. Schein, A. Peled, and D. Glatz, J. Appl. Phys. 66, 686 (1989)
 D. Emin Phys. Rev Lett. 100, 166602 (2008)
 A. Pivrikas, M. Ullah, C. Simbrunner, H. Sitter, H. Neugebauer, and N. S. Sariciftci, Phys. Stat. Sol. B 248, 2656 (2011)
 F. Abdel-Wahab, J. Appl. Phys. 91, 265 (2002)
 F. Abdel-Wahab, Phil. Mag. B 82, 1327 (2002)
Keywords: chalcogenide, glass, Meyer-Neldel, Poole-Frenkel, polarons