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Indian Journal of Science and Technology Vol. 5 No. S3 (Mar 2012) ISSN: 0974- 6

Research article Proceedings of The First Nano Biotechnology Conference held in Damghan branch, Islamic Aazad University, Damghan, Iran during 26thMay 2011 Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Techno

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Fig 1 . Schematic of a nanocrystal memory cell

Silicon nanocrystal memories

Fakhrosadat Rastegari1*

and Foroghsadat Rastegari2

1Department of Electricity, Yazd branch, Islamic Azad University of Yazd, Yazd, Iran

2 Member of Young Researchers Club, Yazd branch, Islamic Azad University, Yazd, Iran

[email protected]*

AbstractIn this paper, we present an overview of silicon nanocrystals memories. Silicon nanocrystals are as storage nodes inthese memories. These devices show promising characteristics as candidates for future deep-submicron non-volatilememories. The structure and fabrication of these devices are explained at first. Then the size, density, and emission ofsilicon nanocrystals in nanocrystal memory device are considered. Finally, a FinFET silicon nanocrystal memory isintroduced as a practical sample.

Keywords: Silicon nanocrystal, Memory, Storage, FinFETIntroduction

In this paper, it is presented an overview of memorystructures fabricated by using silicon nanocrystals as

storage nodes. These devices show promisingcharacteristics as candidates for future deep-submicronnon-volatile memories. The finite probability of having acluster of two or three defects in the tunnel oxide,producing a huge local increase of the tunnel current(namely SILC), determines the presence of bits with ananomalous failure, which are widely recognized as themost important concern in Flash devices (Fig. 1). Theamplitude of the SILC increases when the tunnel oxide isthinned, limiting the oxide scaling.

The use of discrete-trap storage nodes inconventional Flash memory technology has been invokedas one of the key items for existing Flash memorysurvival. The very special effect of the localized trappingis the high reliability associated with it. In fact, in Discrete-Trap memories a single leakage path due to a defect(intrinsic or SILC-related) in the oxide can only dischargea single storage node (Lombardo & De Salvo, 2004).Some different type of memory-cell structures, employingdiscrete-trap type storage nodes, has been demonstratedin the literature.Silicon nanocrystal dot memory

Nonvolatile memory devices using nc-Si-dot-basedelectrically isolated charge-storage nodes in an oxide filmas a floating gate have emerged in the last decade

(Fig.1), where charge leakages through localized oxidedefects could be greatly reduced, therefore, improvingperformance reliabilities, as well as the memory-retention

time. Furthermore, a much thinner tunneling oxide film innanodot memory devices causes a major tunnelingmechanism due to direct tunneling hence enabling afaster write/erase speed compared to FowlerNordheimtunneling which is met in conventional flash type memorydevices (Oda and Huang, 2007). Charge injection andstorage in dense arrays of nc-Si dots in SiO 2 is a keyaspect of their performance. The ultimate goal for thisclass of devices is approaching the few- or single-electron storage in a small number of nc-Si elements atroom temperature, relying on the Coulomb blockadeeffects as a new electron transport principle, commonlyreferred to as single electron memory.

The high-density integration capability and low-powerconsumption enable them to be an attractive candidatefor the next generation digital nonvolatile memoryapplications. However, in spite of the successfuldemonstrations of memory operations based on single-electron transports, the charge retention time of nc-Si dotmemory devices is too short for practical nonvolatilememory applications. The improved retention time wasdemonstrated to be possible without any significant lossof programming speed based on the modifications offloating-gates by the dual memory nodes. Chargeretention characteristics reveal how charges are stored innc-Si memory nodes. A major essential issue for nc-Simemory devices in nonvolatile memory applications is the

data retention characteristics over extended period. Aftersome electrons are trapped, at a chosen reading voltage(e.g., flat-band voltage), the stored electrons have a finiteprobability to tunnel back to the drain, which could causea gradual shift of channel current or capacitance ofSiO 2 /ncSi/SiO 2 diode.

These gradual shifts reflect barrier heights/widths,internal electric fields, defect or interfacial states, and soon. Therefore, investigations of time dependences ofmemory windows as well as charging/dischargingbehaviors of nc-Si dots correlated with Coulomb blockade

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Fig 2 . Time dependence of the stored-charge from the flat- band state, after electrons were injected in floating-gates,

shows a logarithmic discharging behavior. The gate voltagewas kept at the initial flat-band voltage: -1.6V for sample A

(surface nitrided nc-Si dots) and -2.4V for sample B (withoutnitridation). The inset shows the structure of memory device.

and quantum confinement effects offer betterunderstanding of charge retention characteristics of nc-Simemory devices. Investigation of charging anddischarging in nc-Si dots, based on measurements ofcapacitancevoltage and conductancevoltagecharacteristics, shows that interface states and deepdefects correspond to charging and dischargingprocesses. However, at low defect and interfacial statedensity, electron charge/discharge is only dominated bybounding electrons in nc-Si dots. For a SiO 2 /ncSi/SiO 2 capacitor memory device, in time-dependent capacitancemeasurements, the memory-retention time was found toexceed 5 h (calculated time for the loss of about 10% ofthe original charge) at room temperature (Oda & Huang,2007).

Instead of being governed by deep defects, at lowdefect and interfacial state density, electroncharge/discharge is only dominated by electron boundedin nc-Si dots. A repulsive built-in electric field from nc-Si

dots to silicon substrate created and controlled by chargeloss in nc-Si dots was proposed to explain such long-termretention time. On the other hand, a longer retention timecan be achieved by introducing a certain number of deeptrapping centers in nc-Si dots with decreasing theinterfacial states at the tunneling-oxide/Si interface.Compared with pure nc-Si floating-gate memory devices,nitrided nc-Si-dot-based memory devices wereexperimentally demonstrated to be helpful to remarkablyincrease retention time by three orders of magnitude, asshown in Fig. 2, without sacrificing write/erase time, inwhich memory operations based on combinedcharge/discharge processes of nitrided nc-Si dotsystems. The stored charges in such memory nodes were

identified not only in nc-Si dots but also in defect-states ofsilicon-nitride films, corresponding to electron delocalizedand localized states, respectively.Materials and device fabrication

It should be stated that the key technology for siliconnanocrystals memories is how to fabricate the nanometerscale dots. In fact, high dot density, nanometer size, gooduniformity in size and shape, lateral isolation, planarity onthe tunnel oxide, background charge minimization are allrequired in a reliable fabrication process. Severalmethods to synthesize the Si dots have been proposedand investigated in the last years: ion implantation (Gebelet al ., 2001), aerosol (De Blauwe et al ., 2000), andchemical vapor deposition (CVD).

Nanocrystalline silicon (nc-Si) particles with size lessthan 10nm were prepared by VHF plasma-enhanceddecomposition of silane gas. Pulsed gas plasmaprocessing, in which the nucleation and the growth periodwere controlled precisely, turned out to be effective for

the preparation of monodispersed nc-Si particles.The size and density of silicon nanocrystals innanocrystal memory device

Structural characterization via transmission electronmicroscopy and atomic force microscopy of arrays ofsmall Si nanocrystals embedded in SiO 2 , important tomany device applications, is usually difficult and fails tocorrectly resolve nanocrystal size and density. It isdemonstrated that scanning tunneling microscopy (STM)imaging enables a much more accurate measurement ofthe ensemble size distribution and array density for smallSi nanocrystals in SiO 2 . The reflection high-energyelectron diffraction pattern further verifies the existence ofnanocrystallites in SiO 2 . The present STM results enablenanocrystal-charging characteristics to be more clearlyunderstood: It find the nanocrystal chargingmeasurements to be consistent with single chargestorage on individual Si nanocrystals. Both electrontunneling and hole tunneling processes are suggested toexplain the asymmetric charging/discharging processesas a function of bias (Gebel et al ., 2001; De Blauwe et al .2000).

To fully exploit their potential advantages overconventional floating gate memory, it is essential tocontrol as accurately as possible Si nanocrystal size,depth distribution, and areal density, as well asnanocrystal surfacepassivation and oxide defect density

in SiO 2 matrix, all in a process compatible with ultra-large-scale integration. Transmission electron microscopy(TEM) is the most widely used tool to characterizenanocrystal size and distribution with high resolution. Ithas used a combination of contact-mode atomic forcemicroscopy (AFM) and reflection high-energy electrondiffraction (RHEED) to identify the existence ofnanocrystals, and used an ultrahigh vacuum scanningtunneling microscope (UHV STM) to estimate nanocrystalsize and aerial density (Feng & Yu, 2005).

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Fig 3 . Structure of all silicon-based light emitting device,which consists of nc-Si active layer with quasi-direct

radioactive recombination, nc-Si high efficiency electronemitter for excitation, and three-dimensional photonic

crystal layer for optical confinement

To evaluate quantitative data like nanocrystal sizes

and density, STM is found to be a better tool. Someinvestigations have used STM to characterize CdSe andInAs quantum dots on Au (Millo et al., 2000). The STMmeasurements of Si nanocrystals fabricated through low-pressure chemical vapor deposition and nanocrystallinesilicon films obtained by boron implantation of amorphousSi layers have also been reported.Charge storage and electron/light emission properties ofsilicon nanocrystals

Monodispersed silicon nanocrystals show novelelectrical and optical characteristics of silicon quantumdots, such as single-electron tunneling, ballistic electrontransport, visible photoluminescence, and high-efficiencyelectron emission. Electrical properties of nc-Si particleswere investigated by employing nanoscale electrodes,both planar and vertical configurations, prepared byelectron-beam lithography.

Single-electron memory effects were studied using ashort-channel MOSFET having Si quantum dots as afloating gate. Storing of electrons in individual Si dots wasevaluated by Kelvin probe force microscopy (KFM). It wasproposed novel memory devices based on nano electro-mechanical systems (Tsuchiya et al., 2004).

Photoluminescence and electron emission wereobserved from surface-oxidized nc-Si particles. Efficiency

of the no-phonon-assisted transition increases withdecreasing core Si size. Electron emission efficiency ashigh as 5% has been achieved for the Si-nanocrystalbased cold electron emitter devices. The non-Maxwellian energy distribution of emitted electronssuggests that the mechanism of electron emission is dueto ballistic transport through arrays of surface-oxidized Sinanocrystals. Combined with the ballistic electronemission, the quasi-direct light emission properties canbe used for developing Si-based lasers.Silicon photonic devices

The discovery of optical gain in nc-Si, triggeredwidespread research for silicon-based lasers. Stimulatedemission was reported for a nanostructured silicon pn

junction diode using current injection. Silicon-basedRaman lasers have been demonstrated with pulsedoptical pumping and continuous wave operation (Pavesiet al ., 2000).

Silicon-based laser operation is expected as a key

technology in realizing opto-electronic integrated circuits.Silicon based lasers operate only under very highexcitation conditions, making it difficult to incorporatethem into the CMOS circuits in which ultra low powerconsumptions are required.

A completely different approach is proposed torealize silicon-based laser operation using three-dimensional photonic crystal structures combined with nc-Si quantum dots as light and electron emitters, as shownin Fig.3. First, the three-dimensional photonic crystalstructures are introduced to increase the stimulatedemission probability caused by the standing wave at thephotonic band edge, thereby resulting in a significantincrease in the external quantum efficiency. Second, nc-Si quantum dots are used for constructing an active layer,which emits light in the visible band due to the quantumconfinement effect.

Nanocrystalline silicon was fabricated with uniformdiameter of 8 nm using pulsed gas plasma CVD method.A high-efficiency visible photoluminescence wasobserved for the surface-oxidized nc-Si indicating thequantum confinement effect and quasi-directrecombination. Another key feature of device is that thedimension of nc-Si can be controlled simply by changingthe thermal oxidation time. This is vital to control theelectron emission properties, the luminescencewavelength of nc-Si, the refractive index of the structured

nc-Si arrays, and therefore to optimize the photonic bandgap properties of the whole device.Nanocrystal Memory Devices Characterization using theCharge Pumping Technique

The results are presented here by performing the twolevel charge pumping (CP) technique on nanocrystalmemory devices. The charge pumping method allows thedetermination of the silicon dots characteristics such astheir density, their spatial distribution within the dielectricand their effective diameter (Masson & Militaru, 2002).

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Memory-cell structures employing discrete-trap typestorage nodes, thus operating with a small finite numberof electrons, have recently attracted much attention asvery promising scalable ultra-dense low power memories,capable of exceeding the performance limits ofconventional floating gate devices in terms of write/erasespeed, endurance, and refresh time. Because of its veryhigh sensitivity, the charge pumping technique is anexcellent tool to determine the characteristics of the trapswithin the dielectric of MOS transistors. Experimentalresults and simulations demonstrate that the response ofthe silicon dots does not correspond to a trap-likebehavior but it is closer to a floating gate like behavior,confirming results of previous works.

Moreover, the use of the CP technique with a squaregate pulse allows the determination of the spatialdistribution of the dots within the dielectric, the extractionof the effective surface of the dots and an estimation ofthe silicon dots density. The determination of the

distribution of the Si-dot density as a function of theeffective diameter, Ddot(Deff), can be used to evaluatethe effective shape repartition of the dots for a givenprocess.The fin field-effect transistor (FET) silicon nanocrystalfloating gate memory

The FinFET silicon nanocrystal floating gate memorywith a gate length of 100 nm was successfully fabricatedand it revealed a memory effect as well as a suppressedshort-channel effect (Soo et al ., 2006). The photo-CVDfor silicon nanocrystal formation and the plasma dopingmethod for formation of source-drain extension (SDE)were proposed for fabrication of FinFET nanocrystalfloating gate memory devices. The Si nanocrystals wereformed over all channel area, including the vertical Si finsurface by photo-CVD. The plasma doping is effective forthe 3D architecture of FinFET device and reduces thecomplexity of the ion doping process. The fabricatedFinFET nanocrystal flash memory device showed bettersub threshold swing and DIBL characteristics than bulk-Sidevices, and it revealed a memory effect as well as asuppressed short channel effect. Further optimizations oftunnel oxide and control oxide can improve theperformance of FinFET nanocrystal memory device with alarger V

th window as well as a higher integrity.

ConclusionSilicon nanocrystals with novel functions such as

charge and energy quantization and efficientphoton/electron emission are promising for futureelectronics applications. Silicon nanodot memory devicecan be a promising alternative for high-density nonvolatilememory. The significance and useful application of thisdevice in electronics need more research studies.References1. De Blauwe J, Ostraat M, Green ML, Weber G, Sorsch

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8. Pavesi L, Dal Negro L, Mazzoleni C, Franzo G andPriolo F (2000) Optical gain in silicon nanocrystals.Nature . 408, 440-444.

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