Electrical transport in micro- and nanoscale devices of amorphous phase-change materials
- Elektrischer Transport in Mikro- und Nanostrukturen aus amorphen Phasenwechselmaterialien
Kaes, Matthias; Wuttig, Matthias (Thesis advisor); Waser, Rainer (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2017
Storage concepts based on phase change memory cells (PRAM) have matured in recent years. These concepts take advantage of the contrast in electrical properties between the amorphous and crystalline phase of the active phase change material (PCM). The two different phases are accessed either by current-induced melt-quenching of the crystalline state or by a breakdown of the resistance of the amorphous state under applied bias (threshold switching) and subsequent crystallization. The time-scales on which threshold switching, crystallization and amorphization can occur allow to create a non-volatile memory that can operate in the nanosecond regime.A key challenge for the competitiveness of PRAM is the structural relaxation that occurs in the amorphous phase. This relaxation leads to a continuous change of electronic properties with time and is detrimental for the implementation of multi-level storage i.e. the storage of multiple states within a single memory cell. Recent studies indicate that reading the state of PRAM devices at high voltages allows the implementation of a measure of the device state that is more resilient to drift. In this context, the aim of this work is to study and enhance the current understanding of field- and temperature-dependent charge-transport in the amorphous state - especially below room temperature and voltages close to the switching point. Four research projects (i-iv) that investigate electrical transport in the amorphous state will be presented. While (i-iii) investigate transport on devices with as-deposited amorphous PCMs (GeTe, GeSbTe and AgIn-SbTe), (iv) will deal with the melt-quenched state of AgIn-SbTe. (i) Electrical transport at low fields is investigated by studying the dark- and photoconductivity. In a temperature-range from 180 K to 300 K temperature-dependent activation energies are observed. These are indicative of a temperature-dependent density of states (DoS) and can be modeled well with a parametrization that derives from the measured temperature-dependence of the optical band-gap. Below these temperatures, a rapid decrease of activation energy - previously related to the onset of hopping conduction - is observed in GeTe and GeSbTe but is absent in AgIn-SbTe. Thus, transport models that incorporate a single mechanism and are aimed at describing GeTe and GeSbTe at high temperatures most likely fail at lower temperatures. Further, it will be investigated whether the steady-state photoconductivity σ_Ph can be used to obtain more information about the DoS by matching it against a simple analytical treatment (two-state model). In contrast to previous claims, numerical evaluation will show that this treatment fails for GeTe. For AgIn-SbTe, it will be demonstrated that the combined flux- and temperature-dependence of σ_Ph can be matched well against a DoS that solely features tail-states with exponential energy dependence in the band gap - in contrast to GeTe and GeSbTe for which previous studies also revealed distinct defects at specific energies. If the AgIn-SbTe DoS differs substantially from the GeTe DoS different transport behavior must be expected. (ii) Subsequent to an evaluation of existing transport models a new model for the field- and temperature-dependent conductivity is presented. The model is based on transport in extended states together with 3D Poole-Frenkel (PF) emission from a Coulomb potential that features two centers separated by the inter-trap distance s. With this model, transport in GeTe and GeSbTe can be described well in a range from 300 K to 220 K and up to fields of 20 V/µm whereas a good description is not possible for AgIn-SbTe. To investigate governing model parameters, infrared spectroscopy is employed and reveals a weak temperature-dependence of the dielectric constant ϵ_∞. Incorporated into the transport model, these results enable the observation of a temperature-dependence of the inter-trap distance s. A possible origin is found in the occupation of defects supporting the PF-emission process. Since this inter-trap distance affects the device current in an exponential way, accounting for the temperature-dependence of the defect occupation should greatly improve device modeling. At very high fields and lower temperatures an entirely new regime of field-dependence is discovered. Therein, the device current can be described by ln(I/I_0 )=(F/F_C )^2. By combining this contribution with that of PF emission, it is found that the slope at high fields, F_C, is independent of temperature. Here, it is argued that models based on direct tunneling or thermally assisted tunneling into the valence band cannot explain the observed behavior quantitatively but, nevertheless, provide a solid basis for further research. The existence of this regime has implications on reading the device state at high fields as any employed metric should react sensitively to such nonlinear transport behavior. (iii) Here, the link between the inter-trap distance and the occupation is investigated more closely. To this end, a numerical study of the density of occupied defects in the DoS of GeSbTe is performed. These calculations are complemented with experiments, in which the IV-curve of as-deposited GeSbTe is measured both in darkness and under light. It is found that with decreasing temperature the inter-trap distance increases in darkness whereas it decreases under illumination. While the behavior in darkness is in agreement with the DoS calculations, the calculations predict an increase also under illumination. A quantity that is ill constrained by experiment - the ratio of the capture-coefficients for electrons and holes of the deep defect in GeSbTe - is identified as a likely origin that could reverse this trend. Moreover, the previously found high-field regime is characterized also under illumination. For both as-deposited GeSbTe and AgIn-SbTe, the photoconductivity exhibits a maximum as a function of electric field. Models that describe a field-dependence of the photoconductivity with effective temperature concepts fail to describe the obtained data. These measurements add to the characteristics that a model for transport at extremely high fields must match. (iv) To evaluate whether the developed concepts and measurement techniques are also relevant for melt-quenched AgIn-SbTe, IV curves on fabricated nanostructures are presented in a temperature-range from 240 K to 120 K Here, dark- and photoconductivity exhibit similar temperature-dependence as the as-deposited state. In contrast, field-dependent transport can be described well with the transport model. The previously observed high-field regime is also apparent here and thus relevant for the description of the industrially important melt-quenched state.
- Chair of Experimental Physics I A and I. Institute of Physics 
- Department of Physics