In this paper we concentrate on an analysis of the modulated photocurrent (MPC) experiment applied to samples of amorphous semiconductors built in coplanar geometry. Taking into account both types of photocarriers, the basic equations describing the modulation of the occupation of the localized states are derived according to the statistics of Simmons and Taylor. Generalized expressions for the phase shift and the modulus of the modulated photocurrent are obtained without any restrictive assumptions and discussed. It is shown that, if one type of carrier is predominant, the modulated photocurrent gives the density as well as the capture cross section of the localized states interacting with these carriers. The precise conditions under which the two-carrier system is reduced to a single-carrier system are given.The main features of the method are illustrated by means of a simulation, where we study the inhuence of several parameters. We show that the dominant contribution to the modulated photocurrent comes from the carrier type which presents the higher value of p/(No. ), where p is the free-carrier mobility; o, the capture cross section; and N, the density of trapping states for which the emission rate is equal to the angular frequency co at which the experiment is performed. Consequently, only the trapping states corresponding to this type of carrier can be probed. Our study underlines some possible experimental misuses of the MPC technique which could lead to erroneous results regarding the inferred density of states.

Selector devices are indispensable components of large-scale nonvolatile memory and neuromorphic array systems. Besides the conventional silicon transistor, two-terminal ovonic threshold switching device with much higher scalability is currently the most industrially favored selector technology. However, current ovonic threshold switching devices rely heavily on intricate control of material stoichiometry and generally suffer from toxic and complex dopants. Here, we report on a selector with a large drive current density of 34 MA cm−2 and a ~106 high nonlinearity, realized in an environment-friendly and earth-abundant sulfide binary semiconductor, GeS. Both experiments and first-principles calculations reveal Ge pyramid-dominated network and high density of near-valence band trap states in amorphous GeS. The high-drive current capacity is associated with the strong Ge-S covalency and the high nonlinearity could arise from the synergy of the mid-gap traps assisted electronic transition and local Ge-Ge chain growth as well as locally enhanced bond alignment under high electric field.

Germanium telluride (GeTe) is one of the most studied phase change materials. Surprisingly, only little is known about the density of states (DOS) in its band gap. In this paper, the DOS of amorphous GeTe films is investigated both experimentally and theoretically. We propose a model for this DOS as well as estimates of some of the transport parameters of this material. Thin films of amorphous GeTe have been deposited by sputtering. Their dark and photoconductivity have been measured as a function of temperature. By means of the modulated photocurrent technique their DOS was probed, while their absorption was investigated by photothermal deflection spectroscopy at room temperature. Numerical calculations were employed to reproduce the experimental results with a proper set of transport parameters and choice of DOS. These data constitute a good basis for further study on the influence of the DOS on the aging of the sample resistance (“resistance drift”).

International audienceSb2S3 is widely considered to be an attractive photovoltaic material based on abundant, nontoxic elements. However, the maximum efficiency reported for solar cells based on this semiconductor does not exceed 6.5%. We have measured light intensity-dependent J-V curves, transient microwave photoconductivity, steady-state photocurrent grating, modulated photocurrent, and photoconductivity on Sb2S3-based samples. All techniques converge toward the same observation: the main recombination route controlling the density of charge carriers in the absorber is of an order greater than one and appears to stem from an exponentially decaying density of tail states within the conduction band of the material. This conclusion has direct and drastic implications for the performance of Sb2S3-based solar cells

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder, a new type of material, named polymorphous silicon (pm-Si:H) is obtained. pmSi:H exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon (a-Si:H). The study of space-charge-limited current in n+-i-n+ structures along with the use of the modulated photocurrent technique, of the constant photocurrent method and of steady-state photoconductivity and dark conductivity measurements allows us to shed some light on the origin of these improved properties. It is shown that the midgap density of states in the samples studied here is at least ten times lower than in a-Si:H, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3–4 to account for photoconductivity results. An interesting field of theoretical research is now open in order to link these low densities of states and capture cross sections to the peculiar structure of this new material.

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Understanding the physical origin of threshold switching and resistance drift phenomena is necessary for making a breakthrough in the performance of low-cost nanoscale technologies related to nonvolatile phase-change memories. Even though both phenomena of threshold switching and resistance drift are often attributed to localized states in the band gap, the distribution of defect states in amorphous phase-change materials (PCMs) has not received so far, the level of attention that it merits. This work presents an experimental study of defects in amorphous PCMs using modulated photocurrent experiments and photothermal deflection spectroscopy. This study of electrically switching alloys involving germanium (Ge), antimony (Sb) and tellurium (Te) such as amorphous germanium telluride (a-GeTe), a-Ge 15 Te 85 and a-Ge 2 Sb 2 Te 5 demonstrates that those compositions showing a high electrical threshold field also show a high defect density. This result supports a mechanism of recombination and field-induced generation driving threshold switching in amorphous chalcogenides. Furthermore, this work provides strong experimental evidence for complex trap kinetics during resistance drift. This work reports annihilation of deep states and an increase in shallow defect density accompanied by band gap widening in aged a-GeTe thin films.

In this paper we present a complete theoretical analysis of the oscillating photocarrier grating (OPG) method, starting from the generalized equations that describe charge transport and recombination under oscillating grating illumination conditions. The solution of these equations allows us to implement a calculation reproducing the experimental OPG curves. We study both experimentally and from our calculations the dependence of the OPG curves on different external parameters, such as the applied electric field, grating period and illumination intensity. We find that the response of the sample is linked to a characteristic time of the material, which could be the dielectric relaxation time or the small signal lifetime depending on the regime at which the experiment is performed. Therefore, the OPG technique provides a simple method to estimate these parameters. In addition, we demonstrate that the small signal lifetime provides information on the density of states of the material.