Register allocation for write activity minimization on non-volatile main memory for embedded systems

Non-volatile memories are good candidates for DRAM replacement as main memory in embedded systems and they have many desirable characteristics. Nevertheless, the disadvantages of non-volatile memory co-exist with its advantages. First, the lifetime of some of the non-volatile memories is limited by the number of erase operations. Second, read and write operations have asymmetric speed or power consumption in non-volatile memory. This paper focuses on the embedded systems using non-volatile memory as main memory. We propose register allocation technique with re-computation to reduce the number of store instructions. When non-volatile memory is applied as the main memory, reducing store instructions will reduce write activities on non-volatile memory. To re-compute the spills effectively during register allocation, a novel potential spill selection strategy is proposed. During this process, live range splitting is utilized to split certain long live ranges such that they are more likely to be assigned into registers. In addition, techniques for re-computation overhead reduction is proposed on systems with multiple functional units. With the proposed approach, the lifetime of non-volatile memory is extended accordingly. The experimental results demonstrate that the proposed technique can efficiently reduce the number of store instructions on systems with non-volatile memory by 33% on average.     

Experimental setup for non-destructive measurement of tunneling currents in semiconductor devices

A new experimental setup used to perform non-destructive measurement of electrical quantities on semiconductor devices is described in this paper. The particular case of tunneling current measurement in n-type semiconductor–oxide–semiconductor (SOS) capacitors, whose dielectrics play a crucial role in non-volatile memories, has been investigated. When the gates of such devices are polarized with a sufficient bias voltage while the other terminals are grounded, tunnel conduction of electrons through the thin oxide layer is allowed. Typical tunneling current measurements obtained with this advanced setup are presented and compared to the results yielded by older standard experimental protocols. An application to the experimental observation of the temperature dependence of the tunneling current is proposed. Conclusions about the benefits of this kind of electrical measurements are then drawn.     

Charge storage and memory effect in graphene quantum dots – PEG600 hybrid nanocomposite

We report an easy, one step, low cost method to obtain a hybrid composite material consisting in graphene quantum dots (GQDs) embedded in a polymeric – poly(ethylene glycol) bis (carboxymethyl) ether – matrix. Optical measurements show the excitation wavelength dependent photoluminescence of the GQDs – PEG₆₀₀. In comparison with self-passivated GQDs, the composite exhibits a blue shifted photoluminescence, as well as additional emission peaks in the range of 570–600 nm. These features are explained by the presence of new electronic surface states induced by the polymeric matrix as it was demonstrated by the electrochemical measurements. The transport properties consist in a large clockwise hysteresis presenting high and low resistance states, also two distinctive regions of negative differential resistance. The photocurrent decay and the transient currents indicate a large charge storage and confirm the existence of trap charge levels. The experimental findings suggest that the leading mechanism underling the transport is Simmons Verderber. We demonstrated the switching properties of GQDs – PEG₆₀₀ for applications in non-volatile memory by performing standard sequence memory tests.     

Thin ferroelectric poly(vinylidene fluoride-chlorotrifluoro ethylene) films for thermal history independent non-volatile polymer memory

In this study, we investigated the molecular and microstructures of thin poly(vinylidene fluoride-chlorotrifluoro ethylene) (PVDF-CTFE) copolymer films with three different CTFE compositions of 10, 15, 20 wt% with respect to PVDF in relation with their ferroelectric properties. All PVDF-CTFE annealed at 130 °C showed consecutive TTTT trans conformation with β type crystals while films molten and re-crystallized from a temperature above their melting points exhibited α type crystals with characteristic TGTG conformation. Microstructures of the films treated with the two different thermal histories also supported the formation of β and α type crystals with hundreds of nanometer scale sphere caps and micron level spherulites, respectively. Interestingly, PVDF-CTFE films with both α and β type crystals gave rise to relatively high remnant polarization of approximately 4 μC/cm² in metal/ferroelectric/metal capacitors regardless of the composition of CTFE. The ferroelectric polarization of a PVDF-CTFE film independent of thermal processing history allowed a wide processing window and easy fabrication protocol, resulting in a non-volatile ferroelectric field effect transistor memory which exhibited saturated hysteresis loops with the current ON/OFF ratio of approximately 10³ at ±60 V sweep and reliable data retention.     

Nanostructured photoelectrode consisting of TiO2 hollow spheres for non-volatile electrolyte-based dye-sensitized solar cells

A photoelectrode consisting of titania hollow spheres for dye-sensitized solar cells (DSSCs) is prepared by a paste method and the effect of the nanostructure on the performance of DSSCs with non-volatile electrolytes is investigated. The structure of the hollow sphere (HS) electrode with a large pore size and a high porosity allows highly viscous non-volatile electrolytes to penetrate the electrode thoroughly. Furthermore, its outstanding light-harvesting efficiency and long electron diffusion length make the efficiency of the DSSCs with the HS electrode comparable with those of a conventional nanocrystalline electrode, in spite of the smaller amount of the adsorbed dye, when oligomer electrolytes are used. The results show that the structure of a photoelectrode highly improves the performance of the device and the HS electrode is an effective structure for the use of non-volatile electrolytes in DSSCs.     

Nanosecond and femtosecond mass spectroscopic analysis of a molecular beam produced by the spray-jet technique

The spray-jet molecular beam apparatus enabled us to produce a molecular beam of non-volatile molecules under high vacuum from a sprayed mist of sample solutions. The apparatus has been used in spectroscopic studies and as a means of molecular beam deposition. We analyzed the molecular beam, consisting of non-volatile, solvent, and carrier-gas molecules, by using femtosecond- and nanosecond- laser mass spectroscopy. The information thus obtained provided insight into the molecular beam produced by the spray-jet technique.     

Evaluation of E2PROM data retention by field acceleration

This work presents an improved theoretical analysis of the physical mechanisms leading to charge loss from the floating gate of electrically erasable PROMs (E²PROMs). The analysis is applied to a simplified model of a FLOTOX cell in order to evaluate data retention characteristics using the applied field as an accelerating factor. A set of experimental results (obtained on virgin as well as on aged cells) is presented and degradation of data retention due to cell aging is examined.     

A new approach to cell size scaling with a multi-dual cell and a buffer/background programming of unified RAM

New investigations are presented here on a high-density and DRAM-like high-speed non-volatile memory (NVM) application of unified RAM (URAM). For a high-density application of URAM, multiple data storage is demonstrated with a multi-dual cell (MDC). Because each NVM state can be split by programming with a one-transistor (1T) DRAM without a capacitor, the total number of memory states can be doubled. Furthermore, a high-speed DRAM-level NVM scheme is proposed for the joint operation of 1T DRAM buffer programming and NVM post-background programming. The MDC and the proposed scheme are unique URAM properties that can extend the application range of memory devices.     

Modeling of program, erase and retention characteristics of charge-trap gate all around memories

This paper describes the effect of geometry in charge-trap (CT) memory devices. We first theoretically analyze the impact of the curvature radius on the behavior of the gate current in Gate-All-Around devices, and then describe the change to make to planar model in order to fit the cylindrical devices characteristics. This model is used to simulate Nanocrystal and SONOS program, erase and retention behaviors. The dynamics enhancement during program/erase due to the bending of the active region in such cylindrical devices is explained. The scaling perspectives conclude this paper.     

Effect of molecular weight of oligomer on ionic diffusion in oligomer electrolytes and its implication for dye-sensitized solar cells

This study measures the diffusion coefficients of I⁻ and I₃⁻ in oligomer electrolytes as a function of the molecular weight of oligomers and investigates their effect on the performance of dye-sensitized solar cells (DSSCs). The high-diffusion coefficients of ions in an oligomer electrolyte with a lower molecular weight can help to promote the redox mechanism in DSSCs and thereby increase the short-circuit current density. They can also cause a decrease in the open-circuit voltage since a high-diffusion coefficient of I₃⁻ is capable of reducing the lifetime of electrons in TiO₂ electrodes. To offset these effects, N-methyl-benzimidazole is added to the oligomer electrolytes, thereby improving the open-circuit voltage and fill factor and, consequently, the overall energy-conversion efficiency, which increases to over 5%. A further test involving storage at a high temperature of 75 °C demonstrates that DSSCs employing the oligomer electrolytes show excellent thermal stability over 200 h.