Model of Organic Solar Cell Photocurrent Including the Effect of Charge Accumulation at Interfaces and Non-Uniform Carrier Generation

open7openTorto, Lorenzo; Cester, Andrea; Rizzo, Antonio; Wrachien, Nicola; Gevorgyan, Suren A.; Corazza, Michael; Krebs, Frederik C.Torto, Lorenzo; Cester, Andrea; Rizzo, Antonio; Wrachien, Nicola; Gevorgyan, Suren A.; Corazza, Michael; Krebs, Frederik C

ADVANCED MEMORIES TO OVERCOME THE FLASH MEMORY WEAKNESSES: A RADIATION VIEWPOINT RELIABILITY STUDY

Currently the large majority of commercial Flash memories are based on the floating gate MOSFET. Over the last years, the continuous scaling of nonvolatile memories has pushed the Flash technology toward its limits, which affect both the functionality and the reliability of the memory cell. Several alternatives are currently being explored as possible replacements for floating gate memories (FGM). On one hand there are the ferroelectric memories, the phase change memories, and the magnetoresistive memories, which follow a completely new approach, integrating new materials, such as ferroelectrics, chalcogenides, and ferromagnetics. On the other hand, several efforts are being investigated to improve the scalability and the reliability of the FGM technology, by adopting the discrete storage approach. The discrete storage concept consists in replacing the monolithic floating gate of the conventional Flash with a layer of many discrete storage nodes. NROM™, SONOS, and nanocrystal memories are some examples of this novel concept. This thesis analyzes the reliability of three types of advanced nonvolatile memories (nanocrystal, phase-change, and ferroelectric memories) from a radiation tolerance viewpoint. The main results highlight that these new memory concepts bring significant improvement over the conventional floating-gate based memories.La maggior parte delle memorie non volatili attuali si basa sul transistor a gate flottante. Nel corso degli anni, la dimensione della cella elementare è stata sempre più ridotta per far fronte alle crescenti richieste in termini di densità di memoria. Tuttavia, il transistor a floating gate sta raggiungendo i suoi limiti fisici intrinseci e le dimensioni della cella non possono più essere facilmente ridotte a meno di non compromettere la funzionalità o l’affidabilità del dispositivo stesso. Per far fronte a questi problemi, diverse alternative sono in fase di studio. Tra di esse, si possono annoverare le memorie ferroelettriche, le memorie a cambiamento di fase, e le memorie a nanocristalli. Questi tre tipi di memorie sono oggetto di studio di questa tesi. In particolare, viene analizzata la robustezza alle radiazioni ionizzanti di questi nuovi concetti di memoria. I risultati evidenziano che le memorie non volatili avanzate portano significativi miglioramenti in termini di tolleranza alle radiazioni ionizzanti

Ionizing Radiation Effects on Ferroelectric Non Volatile Memories and its Dependence on the Irradiation Temperature

We investigate Ferroelectric Random Access Memories subjected to X-ray and proton irradiations. We address the radiation damage dependence on irradiation temperature, its stability during annealing and cycling, and the effects of supply voltage and packaging. The radiation damage strongly depends on the irradiation temperature. Immediately after proton or X-ray irradiation, we detect only stuck bits without data corruption, at least at doses up to 9Mrad(Si) at room temperature. The radiation damage anneals in time as long as several weeks, and the recovery rate is accelerated by either electrical cycling or high temperature annealing. The radiation tolerance is much higher if the device is irradiated unpowered. Finally, we present a degradation model that accounts for the irradiation temperature dependence

Investigation of Proton and X-Ray Irradiation Effects on Nanocrystal and Floating Gate Memory Cell Arrays

We compared the radiation tolerance of nanocrystal and floating gate memories, fabricated with the same technology. We investigated the effects of 5-MeV proton and 10-keV X-Ray irradiations, focusing on the charge loss, the permanent degradation of the electrical characteristics, and the data retention. We also presented a first order model of the charge loss and the permanent threshold voltage shift. The model and the experimental results show that nanocrystal memories feature improved radiation robustness against total ionizing dose. Nanocrystal memories can withstand a radiation dose 3 and 10 times larger than floating gate memories, in terms of charge loss and data retention, respectivel

Nanocrystal Memories: An Evolutionary Approach to Flash Memory Scaling and a Class of Radiation-Tolerant Devices

The Flash memory was conceived as an improvement of the EPROM (Erasable Programmable Read Only Memory) concept invented in 1980s from an initial idea of Frohman-Bentchkowsky [1]. The EPROM memory electrically programmed and erased by ultraviolet (UV) - irradiation became the most important non-volatile memory (NVM) application in the 1980s. The Flash, which owes its name to the fact that the whole memory array can be erased quickly in the same time, introduced the advantages of the electrical erase and the possibility to reprogram the read only memory in situ, with no need of removing it from the system [2,3]. Over the years Flash memory has widely been accepted as the NVM of choice for many applications and today the large majority of NVMs is based on Flash technology. The Flash market has grown in a very fast way due to the large diffusion of portable and low power consumption multi-media applications, which requires an extensive use of NVMs. The continuous scaling of nonvolatile memories has pushed the Flash technology toward its limits [4]. Now, several constraints, mainly due to electrical and reliability reasons, are threatening the future scaling of the Flash technology and new concepts of non-volatile memories have been proposed. NVMs based on natural traps in dielectrics (such as SONOS) or on floating nanocrystals (NCs), artificially embedded in dielectrics, offer an interesting scaling alternative to the Flash with the conventional floating gate, because of several potential advantages associated with the discrete nature of the storage [5-7]. These memories are an evolution of the Flash concept where the monolithic floating gate is supplanted by a number of discrete charge trapping nodes. Because in these discrete storage nodes devices charges are immune to the leakage caused by localized oxide defects, they can allow for a very aggressive scaling of the tunnel oxide and hence of the cell area, by keeping good performance and reliability characteristics. Today, nanocrystal and SONOS memories have found an important field of application in embedded systems where the non-volatile memory is hosted into a logic system. The high interest toward embedded applications is mainly driven by the easiness of process, due to the fact that a very thin storage layer can be implemented in place of the thick poly-silicon floating gate as well as to the possibility of using lower voltages. Some semiconductor companies have announced that they have started production of embedded memories based on nanocrystals. Nanocrystals memory has also shown a higher endurance to high temperature than its counterpart SONOS. Recently the NC memories have shown a promising route toward radiation tolerant application. Actually, as information is stored in discrete centers, they are expected to exhibit a higher tolerance to radiation effects such as total ionizing dose effects (TID) and single event effects (SEE). In the first part of this chapter we will present an overview of the nanocrystal memory technology as candidate to be an alternative to conventional Flash NVMs, by showing a comparison with the mainstream technology. The discussion will be focused on the scalability of the device and on its performances and reliability. In the second part of the chapter we will address the application of NC memories as radiation tolerant devices, such as in military applications, nuclear power stations, nuclear waste disposal sites, high-altitude avionics, medical and space applications. In particular, we will compare NC memories characteristics with the ones of Flash memories

Readout drain current dependence of programming window in nanocrystal memory cells

The different behaviour of nanocrystal memory cells in linear and subthreshold region was studied. It was found that the programming window reduces with increasing readout drain current. This peculiar behaviour derives from the presence of the discrete nanodots and it has not been observed in conventional floating gate memories

Role of Oxide/Nitride Interface Traps on the Nanocrystal Memory Characteristics

In this work we are addressing the threshold voltage instability observed in non volatile nanocrystal memories (NCMs) during the retention experiments under constant applied bias. Such instability derives from the charge motion at the oxide/nitride interface traps of the oxide/nitride/oxide stack employed as control dielectric. We also investigated the impact of temperature on the cell retention properties, showing important and original results that could be attributed to the structure of the control dielectric stack