Sergio F. Almeida

Resistive Switching of SnO2 Thin Films on Glass Substrates

Resistive switching of SnO2 thin films deposited by RF magnetron reactive sputtering at room temperature was investigated. Ag/SnO2/Ti structures were fabricated on glass substrates for current-voltage characteristics evaluation. Repeatable unipolar switching was observed using a compliance current of 10 mA and limiting the reset voltage between 0.8 and 1.2 V. Different top contact area were fabricated indicating a filamentary forming mechanism. Furthermore, a retention memory analysis was performed indicating an acceptable device behavior through time. An Ohmic conduction process was found in LRS and HRS. However for HRS, Ohmic conduction was observed only at voltages lower than 0.3 V. At higher voltages, conduction is not explained well by Ohmic, Poole-Frankel, Schottky emission, or space-charge-limited conduction. This indicates that a material structural change occurs at voltages above 0.3 V which is the onset to switching from HRS to LRS.

Understanding misfit strain releasing mechanisms via molecular dynamics simulations of CdTe growth on {112}zinc-blende CdS

Molecular dynamics simulations have been used to analyse microstructures of CdTe films grown on {112} surfaces of zinc-blende CdS. Interestingly, CdTe films grow in ⟨331⟩ orientations as opposed to ⟨112⟩ epitaxial orientations. At the CdTe-{331}/CdS-{112} interface, however, there exists an axis that is parallel to the ⟨110⟩ orientation of both CdS and CdTe. It is the direction orthogonal to this ⟨110⟩ that becomes different, being ⟨116⟩ for CdTe and ⟨111⟩ for CdS, respectively. Missing CdTe-{110} planes are found along the ⟨110⟩ axis, suggesting that the misfit strain is released by the conventional misfit dislocation mechanism along this axis. In the orthogonal axis, the misfit strain is found to be more effectively released by the new grain orientation mechanism. Our finding is supported by literature experimental observations of the change of growth direction when Cd0.96Zn0.04Te films are deposited on GaAs. Analyses of energetics clearly demonstrate the cause for the formation of the new orientation, and the insights gained from our studies can help understand the grain structures experimentally observed in lattice mismatched systems.

MEMS Closed-Loop Control Incorporating a Memristor as Feedback Sensing Element

In this brief, the integration of a memristor with a microelectromechanical systems (MEMS) parallel plate capacitor coupled by an amplification stage is simulated. It is shown that the MEMS upper plate position can be controlled up to 95% of the total gap. Due to its common operation principle, the change in the MEMS plate position can be interpreted by the change in the memristor resistance or memristance. A memristance modulation of ~1 kΩ was observed. A polynomial expression representing the MEMS upper plate displacement as a function of the memristance is presented. Thereafter, a simple design for a voltage closed-loop control is presented, showing that the MEMS upper plate can be stabilized up to 95% of the total gap by using the memristor as a feedback sensing element. The memristor can play important dual roles in overcoming the limited operation range of MEMS parallel plate capacitors and in simplifying read-out circuits of those devices by representing the motion of the upper plate in the form of resistance change instead of capacitance change.

SnO 2 -based memristors and the potential synergies of integrating memristors with MEMS

Memristors, usually in the form metal/metal-oxide/metal, have attracted much attention due to their potential application for non-volatile memory. Their simple structure and ease of fabrication make them good candidates for dense memory with projections of 22 terabytes per wafer. Excellent switching times of ~10 ns, memory endurance of >109 cycles, and extrapolated retention times of >10 yrs have been reported. Interestingly, memristors use the migration of ions to change their resistance in response to charge flow, and can therefore measure and remember the amount of current that has flowed. This is similar to many MEMS devices in which the motion of mass is an operating principle of the device. Memristors are also similar to MEMS in the sense that they can both be resistant to radiation effects. Memristors are radiation tolerant since information is stored as a structural change and not as electronic charge. Functionally, a MEMS devices sensitivity to radiation is concomitant to the role that the dielectric layers play in the function of the device. This is due to radiation-induced trapped charge in the dielectrics which can alter device performance and in extreme cases cause failure. Although different material systems have been investigated for memristors, SnO2 has received little attention even though it demonstrates excellent electronic properties and a high resistance to displacement damage from radiation due to a large Frenkel defect energy (7 eV) compared its bandgap (3.6 eV). This talk discusses recent research on SnO2-based memristors and the potential synergies of integrating memristors with MEMS.

Variability Study for Low-Voltage Microelectromechanical Relay Operation

Body-biased microelectromechanical relays previously have been developed for ultralow-power digital logic applications and demonstrated to reliably switch ON/OFF with sub-50-mV gate voltage swing. Since variability in relay switching voltages can practically limit reduction in the operating voltage of a relay-based integrated circuit, the effects of process-induced variations and device operating conditions, as well as the stability of relay switching voltages, are investigated in this paper. Antistiction coating is shown to stably reduce hysteresis voltage and random variation thereof, which is beneficial for voltage scaling.

Molecular dynamics simulations of ZnTe/Cu back contacts for CdTe solar cells

Molecular dynamics (MD) simulations have been applied to study the growth of ZnTe/Cu/CdTe layers on CdTe substrates. Our studies show that Cu forms pure clusters and ejects Cd atoms within the CdTe layer out towards the films surface. Elemental concentration plots indicate that the amount of Cu added to the growth plays an important role on the intermixing between ZnTe and CdTe layers and the doping of CdTe. These results provide useful insight to the development of effective and reliable back contacts used in CdTe solar cells.

Monolayer strain by NEMS for low power application

Summary form only given. In recent years, two dimensional (2D) materials have attracted much attention due to its potential to overcome the roadblocks of electronics miniaturization and follow Moores law. These materials consist of atoms covalently bonded along one plane and weakly bonded with its neighboring planes via van der Waal forces [1]. This property allows these materials to be mechanically cleaved or exfoliated in mono- or multilayers. Graphene is one of the most studied 2D materials due to its high mobility, strength and flexibility. But the lack of a band gap of this semi-metal material has limited its application in digital electronics [2]. Fortunately the transition-metal di-chalcogenides (TMD) in the form of 2D layers have been shown to possess a band gap. One example is MoS2 which consists of one plane of Mo atoms sandwiched between two planes of S atoms. Interestingly, the band gap in MoS2 monolayers and multilayers can be modulated by mechanical strain as reported theoretically [3] and experimentally [4]. A 1.9% compressive strain increases the band gap from 1.73 eV to 1.86 eV and changes it from direct to indirect. Moreover, an 11% tensile strain causes the band gap energy to be zero; essentially making the MoS2 to transition from semiconductor to metallic [3]. The demonstrations of heterojunctions with useful electronic properties [5] and even MOSFET devices [2] have illuminated the potential of MoS2 to create electronic devices beyond CMOS. However the strain effect on the band gap of MoS2 has not been fully explored for electro-mechanical devices. For example nano-electromechanical (NEM) switches offer an avenue for low-power electronics due to their minimal or zero “off” current and steep switching. However, two significant problems in the NEMs devices are mechanical fatigue and the adhesive force between the contacts. In this work, a MoS2 monolayer is used in a NEMs cantilever to achieve a low-voltage switching device that exploits the strain effect on the band gap. Essentially the NEMs cantilever strains the MoS2 monolayer and causes its bandgap to change from semiconducting to conducting.