Session Tu-A3

Phase Change I

Chair: Tomas Wágner, Pardubice University

Tu-A3.1 14:20–14:40

On the Steady-State Photoconductivity in Amorphous Ge2Sb2Te5 Phase Change Material

N. Qamhieh, S. T. Mahmoud, and A. I. Ayesh

Department of Physics, United Arab Emirates University, P.O. Box 15551 Al Ain, United Arab Emirates

Steady-state photoconductivity measurements were carried out on dc sputtered thin film of amorphous Ge2Sb2Te5 sample in the temperature range between 100 and 300K. The dark conductivity is thermally activated with activation energy of 0.34±0.042 eV, which allocates the position of the Fermi level relative to the valance band edge. The temperature dependence of the photoconductivity ensures the presence of a maximum normally observed in chalcogenides with low- and high-temperature slopes, which predict the location of discrete sets of localized states (recombination levels) in the gap at 0.12±0.04 eV and 0.62±0.01 eV above the valence band mobility edge. The presence of these defect states close to the valence and conduction band edges leaves the quasi Fermi level shifts in a continuous distribution of gap states at high temperatures, as evidenced from the γ values of approximately 1.0 in the Lux-Ampere characteristics.

Keywords: Dark conductivity, Photoconductivity, Recombination levels, electronic states, phase change materials, Ge2Sb2Te5.

Tu-A3.2 14:40–15:00

Effect of Annealing on Carrier Density in Ge2Sb2Te5 Films

Tamihiro Gotoh

Faculty of Science and Technology, Gunma University, Maebashi 371-8510, Japan

Phase-change recording using chalcogenide semiconductors is widely used in optical data storage such as rewritable digital versatile disc (DVD-RW) and Blu-ray disc (BD-RE). Further developments for electronic non-volatile memory are intensively explored. Among several developing targets, reducing energy consumption may be the most important in the application. Switching from crystalline to amorphous state is the most energy consumption operation. Switching characteristics depend on the carrier generation and transport of phase-change materials [1]. Despite many practical results presenting the electrical properties in phase change materials, little is known about characteristics of electronic carrier in Ge2Sb2Te5 films, which is typical materials for phase-change recording. In order to obtain insights into carrier density of amorphous and crystalline Ge2Sb2Te5, the Seebck coefficients of as-prepared and thermal annealed Ge2Sb2Te5 films were evaluated from thermopower measurements.

Ge2Sb2Te5 films were deposited on quartz by dc sputtering. Film thickness was approximately 200 nm. The samples were annealed at several temperatures in Ar gas. The resistivity and Seebeck coefficient of amorphous film at room temperature are determined to be 1.0x104 Ωcm and +0.89 mV/K. The resistivity and Seebeck coefficient decreased 2.4x10–1 Ωcm and the +0.028 mV/K when the sample was annealed at 210°C. X-ray diffraction results shows that crystalline peaks appear after annealed over the crystallization temperature. We also observed the increase of sub-gap absorption by annealing from photothermal deflection spectroscopy.

The carrier density p is given by p = N exp(–eS /kT ), where S is Seebeck coefficient and N is effective density of states [2]. By using the equation, estimated carrier density of crystalline state is much greater than that of amorphous states. It is known that face-centered-cubic (fcc) Ge2Sb2Te5 films contain defective structure such as vacancy. The increase of sub-gap absorption by annealing corresponds to the increase of vacancy-related states [3]. The vacancy plays the important role of carrier generation in fcc Ge2Sb2Te5 films. It is reasonable that the vacancy-related states act as acceptor and lead to higher conductivity with improving mobility than amorphous films. This is reflected in high resistivity ratio ρac between amorphous and crystalline states in Ge2Sb2Te5 films.

[1] H. Fritzsche, J. Phys. Chem. Sol. 68, 878 (2007)

[2] D. Braga, M. Curzi, S. L. Giaffreda, F. Grepioni, L. Maini, A. Pettersen, and M. Polito, in: Organic Nanostructures, edited by J. L. Atwood, J. W. Steed (WILEY-VCH,Verlag GmbH &Co. KGaA, Weinheim, 2008), p. 271

[3] T. Gotoh, J. Non-Cryst. Solids 358, 2366 (2012)

Keywords: Ge2Sb2Te5, phase change, electrical resistivity, thermopower, carrier density

Tu-A3.3 15:00–15:20

Strong Phonon Scattering in Phase-Change Thin Films

K. S. Siegert (1), F. R. L. Lange (1), and M. Wuttig (1,2)

1. Institute of Physics (IA), RWTH Aachen University, 52056 Aachen, Germany

2. JARA – Fundamentals of Information Technology, RWTH Aachen University, 52056 Aachen, Germany

Phase-Change Materials (PCMs), for instance compounds along the pseudo-binary line between GeTe and Sb2Te3 (GSTs), are famous for the ability to be rapidly and reversibly switched between their amorphous and the crystalline states. The two structural conditions display significant contrasts in optical reflectivity and electrical resistivity making these materials highly attractive for technological utilization in optical data storage media, such as rewritable DVDs. Current PCM research focuses in the onward development of future non-volatile, energy efficient electrical memory devices with high bit densities (PRAM). In all cases the switching of the active material is induced through heat making a detailed knowledge about thermal properties decisive for material selection and technological improvement.

Here we present thermal conductivity data acquired by 3ω experiments on sputtered PCM thin films. Several compounds along the pseudo-binary line were investigated as a function of annealing, stoichiometry and temperature (50K–300K). Within the crystalline rocksalt phase thermal transport through lattice vibrations is strongly impacted by pronounced structural disorder which originates from the random distribution of Ge, Sb and vacancies on the cation sublattice. Disordered vacancies serve as dominant phonon scatterers causing small absolute values of the thermal conductivity κ and a glasslike temperature dependence. We further show that successive reduction of disorder through annealing enhances κ and results in more 'crystal-like' behavior.

Recently, it has been demonstrated by our group that structural disorder induces electronic localization effects. Thus, varying the degree of disorder through annealing or stoichiometry opens up a pathway for further tailoring of both electronic and thermal conduction channels. With this strategy GSTs could approach a phonon glass/electron crystal like material (PGEC), which bears a high potential for thermoelectric applications. Enhanced thermoelectric efficiencies ZT for certain GeTe-rich thin film samples have already been observed.

Keywords: phase-change materials, disorder, thermal conductivity, phonon scattering, thermoelectrics

Tu-A3.4 15:20–15:40

Nature of Gap States in GeSbTe Phase Change Memory Materials

J. Robertson and X Yu

Engineering Dept, Cambridge University, Cambridge, CB2 1PZ, UK

GeSbTe phase change materials are important for optical and electrical non-volatile memory. The electrical memory depends on the change in electrical conductivity between their amorphous and crystalline phases, which in turn depends on the nature of their defects [1]. In the crystalline phase, the dominant defect is the Ge vacancy whose states pin Ef near the valence band edge, giving a high conductivity. The nature of the defects in the amorphous phase is still debated. One possibility is valence alternation pairs (VAPs), defects which have a negative U [1]. Experimentally, the amorphous phase has Ef pinned near midgap, giving a high resistivity. Photoconductivity data gives values for the gap state distribution.

Our previous work has argued that the simple VAPs have a higher formation energy that comparable bulk bonding sites [2]. Thus, it is unclear if VAPs are needed.

Here we argue that the gap states might follow Anderson's [3] original definition of negative U states. a-GeSbTe possesses a wide range of bonding possibilities, with Ge and Te sites having various possible coordination numbers. Nevertheless, the bonding arranges itself to give a well defined pseudogap. The gap states give no ESR signal. The VAP defect model [4] arose for the poorly screened case of a-Se, where defects are relatively well localized and deep. GeSbTe has quite a high dielectric constant and is in the well-screened limit [5]. The defects would be shallow and delocalised. In this case, Anderson's original model is more plausible; the bonding energy is maximized if it arranges itself to sweep most states out of the pseudogap into the bands. This is consistent with our results of molecular dynamics simulations of band states and "defect" states.

[1] A. Pirovano, et al., IEEE Trans. Electron Devices 51, 452 (2004)

[2] B. Huang and J. Robertson, J. Non-Cryst. Solids 358, 2393 (2012) (ICANS 24)

[3] P. W. Anderson, Phys. Rev. Lett. 34, 953 (1975)

[4] M. Kastner, D. Adler, and H. Fritzsche, Phys. Rev. Lett. 37, 1504 (1976)

[5] B. Huang and J. Robertson, Phys. Rev. B 81, 081204 (2010)

Keywords: phase change memory, defects, calculation/theory, electronic structure