Silicon Nanocrystals: From Synthesis To Applications

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International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016ISSN 2229-55181498Silicon Nanocrystals: From Synthesis toApplicationsR. KarmouchJazan University, Physics Department, Jazan, P.O BOX 2097, KSAAbstract—The reduction in size to nanoscale sizes not only allows to improve the performance but also to bestownew properties to materials; this is what justifies the growing interest in this type of materials such as the siliconnanocrystals that are fully compatible with existing technologies we can expect to see an explosion of applicationsbased radically on the silicon nanocrystals in disciplines such as biology, mechanics, electronics The specificity ofnc-Si resides essentially in the influence of their size and their shape. However, the n-Si must have controlledproperties, both from the point of view of a single particle or of a set of interacting particles in an amorphous matrix.Thus, it is of great technological and scientific interest to know the physical and chemical properties of Si nanocrystals,different methods of nc-Si production, the methods of their characterization and their applications in microelectronics,optoelectronics and biophotonics.Index Terms—Silicon nanocrystals, Synthesis, properties, and applications in in microelectronics, ——— ——————————1 INTRODUCTIONThe bulk crystalline silicon is today the mostinterconnections in integrated circuits [3] and nowidelyandsolutions available by using the current technologymaterial. Indeed,especially that the growth of total length of 2010,miniaturization the IC components, the increase inwill reach the astronomical value of 2000 m /cm2their operating speed, as well as the decrease of the(and 4000 m/cm2 in 2015) one possible solution totransistors production costs and other elements ofovercome this problem is the use of the opticaltheareinterconnects. In recent years, reducing the size ofmankind.the Si nanoscale brought new capabilities thatAccording to Intel Company during the fortyallowed numerous applications in optoelectronicsrecent years, the number of integrated transistorsbased on (nc-Si) silicon nanocrystals such as thecontained in the various processors manufacturedoptical interconnection [4], non volatile memoryis increased exponentially, which mean that the[5], and Third Generation Photovoltaic Solar Cells.number of transistors per processor double[6] This research field generated a very strongapproximately every 18 months, according topassionMoore's Law, set out in 1965 by the co-founder ofyears. According to the Google ScholarTM thethe firm Intel, Gordon E. Moore [1]. Thenumber of publications related to nc-Si is 122,000assessment,Technologyarticles published between 1990 and 2016. [7] InRoadmap for Semiconductors (ITRS), updatedparticular, nanocomposite materials with Si-ncannually by many actors semiconductor industryembedded in an amorphous dielectric matrix have[2], identifies many obstacles that will face the ICbeen a significant number of publications. Threemanufacturers in coming years. In particular, theapplication areas driving these studies: ehistoryInternationalyearsofIJSER 2016http://www.ijser.orgforresearchinthelastten

International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016ISSN 2229-55181499systems that rely on the emission properties of nc-nanocrystals, the origin of luminescence of nc-Si,Si in the visible and at room temperature, non-and their application in optoelectronics.volatile memories interested in electric chargingproperties of nc -Si, and photovoltaic systems suchas multi-junction Si-nc cells based on quantumconfinement phenomena in nc-Si. [8]A remind of the basic properties of silicon will beof great importance to understand well the siliconnanocrystals properties.The manufacture of silicon nanostructures is fully2 PHYSICAL PROPERTIES OF SILICONcompatible with existing technologies. However, toSilicon is, on Earth, the most abundant elementbe successfully integrated into the opto- andafter oxygen; we estimates that the Earth's surfacemicroelectronic devices, the n-Si must haveis composed about 26% of silicon. [10] Thiscontrolled properties, both from the point of viewtetravalent metalloid, has an atomic number of 14,of a single particle as a set of interacting particlesand an atomic mass of 28.0855 u.a, and has theindiamondan amorphousmatrix. d in a simple way from its volume andcombination of two face centered cubic structuresits surface whose properties are distinct. Indeed,having an offset of a quarter of the length of thewiththediagonal from each other along the diagonal of thenanostructures, the surface/volume ratio increasescube with a lattice parameter a 5.43 Å to thedrastically and surface phenomena become moreroom temperature, each atom has four nearestpronounced. In other words, the of nc-Si are strongly influenced by theproperties physical Si. The analysis of the bandnature[9]structure of the semiconductor material reveals anUp to now the origin of light emission from nc-Si isindirect band gap (or indirect gap) width Eg equalstill a big debate between scientists. Many studiesto 1.12 eV at room temperature, as shown in Figurefocus on the impact properties of nc-Si without1. Also, the pairs of recombination electron-hole inconsidering the influence of the surroundingthe crystalline silicon requires the presence of amatrix. Nevertheless, many parameters such as thephonon, which makes it unlikely, and thechemical nature of the latter, the concentration andestimated recombination rate is about 102 s-1,type of defects, mechanical stresses imposed by thecompared to 2x107 s-1 in gallium arsenide (GaAs)networkthat na direct gap. [11] Thus the lifetime ofoptoelectronic properties of the material at theradiation corresponding to recombinations is of themicroscopic and macroscopic scale. It is thereforeorder of a millisecond for the Si and nanosecondessential to have a better understanding of theseonly for GaAs. This is reflected directly in thephenomena.luminescence efficiency, which is only 10-4 to 10-5In this spirit, this work is devoted to reviewtechniques used for the fabrication of siliconfor the crystalline Si, compared to 10-1 for a directgap semiconductor. [12]IJSER 2016

International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016ISSN 2229-55181500discovered the intense photoluminescent emission(PL) in the visible range produced by an etched Sisubstrate. The wavelength of this emitted lightcould sweep the spectral range of the visible bysimply changing the anodizing conditions and thatthe luminescence efficiency could reach 10% in themost favorable case. The various analyzes haveshown that the luminescence originated from theetched layer that consist of an interposed lline silicon.of "columns" of Si and pores which gave its nameto silicon porous (p-Si). In fact, at the microscopiclevel, the p-Si consists of small crystallites(typically a few nanometers in diameter) of variedshape corresponding to Si- nanocrystals. The firstpublications on the subject, suggested that whenthe crystals dimension are reduced to nanoscalesize, they then become good light emitters underthe action of an optical excitation [13, 14], orelectrical excitation (electroluminescence) [15, 16].In fact, the reduced dimensions of the crystalstructure of bulk Si to nanoscale will cause verysignificantFigure 1: Silicon band structure in the directionsX(100) and cslightwasforneverconsidered seriously. Moreover, it is certainly thesame reasons that hindered for many years thedevelopment of photonics based on silicon whilethe progress of the same material in the field ofmicroelectronicstheiroptoelectronicproperties. To better understand these effects it isuseful to recall some concepts of quantumconfined system is one in which a particle (anOne can then understand why the choice of bulk Siobtaininginmechanics for a confined system. In general a3 SILICON ts around silicon as a light source hasdrastically changed in 1990 after Canham [8]electron for example) is localized in a potentialwell, surrounded by infinite potential barriers.Confinement effects will be present when the sizeof the confinement will be of the order of thespatial extension of the wave function of theparticle. In a confined system, the energy ofelectronic states increases with the confinementcondition. The result therefore is a discretization ofthe energy forming the energy levels of systemrepresented by the following expression:IJSER 2016

International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016ISSN 2229-5518𝐸𝐸𝑛𝑛 𝑛𝑛 2 𝜋𝜋 2 ℎ 22𝑚𝑚 𝑅𝑅 hous silicon layers. [27]where m mass of the particle, R is the width ofthe well.4.1 Ion implantationThe effects of confinement for a nanocrystalsemiconductor is the widening of the bandgap andan increase in the probability of irradiativetransitions and this become significant when themagnitude of the Bohr radius of the exciton ong the fabrication methods of nc-Si we findthe ion implantation that is considered as one ofthe most promising because it’s compatible 100%with the technological process of the currentmicroelectronics. The main advantage of thistechnology compared to the etching method toobtain p-Si is the high thermal and chemicalstabilityofnanostructuressincetheyareFor the case of silicon, Xia et al. [18] and Yoffe et al.[19] found that when the radius of the nc-Siapproaches the 5 nm the effects of a strongquantum confinement of carriers begin to appear.Up to now many works are published on theelectronicstatescalculationwithinnc-Si.Asummary for these calculations was published byJohn et al. and Singh et al. in 1995 [20].4 METHODS FOR NC-SI SYNTHESISThe discovery of the visible luminescence from pSi discussed in the previous section caused aparticular interest to find other methods thanchemical etching to obtain nc-Si. Among thesemethods which may be mentioned includes: Ionimplantation of Si ions in host matrices [21], thesurrounded by a passivating host matrix.Figure 2: HRTEM image for the nc-Si embedded inSiO2 matrix. [17]chemical deposition vapor phase of layers of non-The nc-Si obtained by ion implantation process arestoichiometric oxide SiO x (1 x 2) (LPCVD) [22],of good quality as shown in the image in Figure 2the reactive sputtering of a Si substrate in anobtained by (HRTEM) of a nanocrystal immersedoxidizing atmosphere to form the layer non-in a 40 nm thick SiO 2 layer. The nc-Si diameter isstoichiometricless than 5 nm as shown by the circled area. [17,oxide(sputtering)[23],laserablation of a Si substrate [24], the attack a Si21,substrate by electrical impulses (spark method)implantation of Si in a layer of SiO 2 previously[25], the deposition of clusters by decomposition ofdeposited thermally on a substrate. The process isIJSER ion

International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016ISSN 2229-55181502then followed by an annealing at temperaturesabove 900 C and for 100 minutes in a mixture of(a)nitrogen and hydrogen atmosphere [29] to ensureat the same time a good demixing of phases of thesystem Si/SiO 2 and passivation of defects createdin the matrix when implanted.The nc-Si density, their average size and itsdispersion is determined by a number of factors,and the most important are the initial level ofsupersaturation of Si in the implanted matrix andthe subsequent thermal annealing conditions.(b)While these factors can explain rather adequatechanges in the population of nanocrystals, anothervery important factor also plays a role inmodifying the nc-Si population is the annealingatmosphere. In 1996, Wendler et al. [30] publisheda study on the influence of heat treatmentconditions on silicon nanocrystals. He found thatunder oxidizing atmosphere, the oxidation frontFigure3:Comparisonbetweenimplantationadvances from the surface toward the maximumprofiles for annealing a) oxidant atmosphere andconcentration of precipitates (Figure 3a), whichb) non-oxidant one. [30]causes obviously a reduction in the concentrationof excess Si within the implanted matrix. However,In contrast, the annealing under nitrogen modifywhen the annealing is under a non-oxidizingonly the size of the nanocrystals (Ostwaldatmosphere (Figure 3b) only a slight surfaceripening) [31] without significantly modifying theiroxidation occurs of the implanted layer anddensity.accompanied with a simple redistribution of SiSeveral studies have shown that passivation withatoms in excess. Thus, according to this study, it ishydrogen at temperatures around 500 C increasesgenerally considered that the facts the annealing inthe intensity of the luminescence of nc-Si by aboutan oxidizing atmosphere after the formation ofa factor of 10. [32] The weakness of thesilicon nanocrystals will reduce the number ofluminescence before passivation is attributed toexcess silicon in the matrix and therefore reducingdefects in the form of pendant bonds in thebothnanocrystal or their interface with SiO 2 . ch defects are non-radiative recombination sites.[33] The energy is then emitted as phonons ratherthan contributing to the PL. During the passivationprocess, hydrogen diffuses into the sample andIJSER 2016

International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016ISSN 2229-55181503acts on the pendants links, preventing such signalfigure 4 a) as well as a typical SEM image of the nc-losses.SiIn summary, ion implantation allows for propernanocrystals, their size and their dispersioncontrol of size distribution of nc-Si with sometimesdepend mainly on the temperature of the depositcomparable luminescence yields to those obtainedand the silane pressure in the chamber.inb).Theconcentrationofthesiliconon p-Si.(a)4.2 LPCVDThe LPCVD technique is one way of chemicaldeposition in vapor phase and it’s among the mostused techniques for the production of devices suchas MOSFETs, thin film transistors or solar cells. In(b)1996, Nakajima et al. [22, 34] were among the firstto apply this technique for the development of ncSi. Two main methods can be identified forobtaining the silicon nanocrystals using thistechnique:4.2.1 Deposited silicon nanocrystals:This method consists in making a deposit of nc-SiFigure 4: a) sketch of the process of nc-Sion an insulating layer (SiO 2 or Si 3 N 4 ) from thedeposition b) image of nc-Si islets [35]thermal decomposition of pure silane (SiH 4 ) at atemperatureofaround600 C,the4.2.2 Precipitated silicon nanocrystals:chemicalreactions that govern the growth of siliconThis second method was widely studied bynanocrystals are described by the followingHitchman et al. [36] and consist to obtain nc-Si byequations:precipitation of Si atoms in excess in a nonstoichiometric oxide layer (SiO x , x 2). The reactionSiH 4 (g) SiH 4 (s) S(s) 2H 2 (g)of SiH 4 and N 2 O at the surface of the thermal SiO 2(2)layer causes the formation of a non-stoichiometricSiH 4 (g) SiH 2 (s) 2H 2 (g) Si (s) 2H 2 (g)oxide layer (SiOx, x 2) rich in silicon, this layer is(3)then annealed to separate the SiO x phase to Si andIn the range of low pressures, adsorption anddissociation of SiH 4 on the surface (equation 1)predominate and the role of radical SiH2 resultingfrom pyrolysis of SiH 4 (equation 2) is negligible. Adescriptive scheme of this method is shown inSiO 2 giving as a final result the formation of nc-Siby precipitation as shown in the figure 5.In this case the concentration of the nanocrystals,their size and their dispersion depend on the rationof N 2 O/SiH 4 , the temperature, and the subsequentthermal annealing. The main advantage of thisIJSER 2016

International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016ISSN 2229-55181504technique is the high chemical and thermalinvolves the use of two targets (SiO2 substrate Sistability obtained by the passivation provided bysubstrate) placed in the same room with a Sithe surrounding matrix (SiO 2 ). The LPCVDtechnique allows a better control of the sizedistribution of the crystallites similar to the ies are significantly lower than thoseobserved on the p-Si or nc-Si obtained by ionimplantation.substrate under argon atmosphere. The secondmethod (reactive sputtering) involves the use of asingle target (Si substrate) placed in the same roomwith the substrate under Ar/O 2 . In both cases weobtain a non-stoichiometric oxide SiO x (1 x 2)layer on the Si substrate, which is then annealed toprecipitate the silicon stribution of nc-Si is rather poor and theluminescence efficiency is also significantly lowerFigure5:Diagramforobtainingnc-Sibythan that of p-Si.precipitation in oxide layer by LPCVD. TEM imageFigure 6 - Operation Principle of the spray plan view to determine the size and the[23]distribution of nc-Si. [36]4.4 Laser ablation (PLD)4.3 SputteringThistechniqueThe silicon nanocrystals are obtained throughconsiststouseamagnetcondensation of the material resulting from laser(Magnetron) placed behind the deposition sourceablation in a jet of high gas pressure. [24, 37] The(target) that is negatively biased. The magneticsize distribution depends on the pressure, thefield generated by the magnetron will cause thenature of the gas and the power of the laserformation of a closed annular field lines at theexcitation. A classic picture of the PLD depositiontarget surface thereby increasing the product of thechamber is illustrated in the figure 7.ionization probability of the sputtering reaction inthe confinement zone as illustrated in Figure 6. [23]To obtain the nc-Si using this technique, one canuse two methods. The first method (co-sputtering)IJSER 2016

International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016ISSN 2229-55181505annealing (RTA) at a temperature between 800 and900 C for coalescing the nc-Si. [27, 38] Anotherannealing is necessary in oven with a constanttemperature ramp of 10K/min between 600 andFigure 7: Schematic of PLD systemThe process begins with a laser that focus a spotwith an angle of 45 on a rotatable target, and thenthe deposit is collected on a substrate previouslyplaced of preference perpendicularly to the targetwith a typical distance of 30 mm. The duration ofthe deposition is controlled by the number of thelaser pulses. This technique allows a better controlof the size distribution of the nc-Si but the yield ofluminescence is lower than the obtained one withp-Si.1050 C to relax the stresses between the twophasesnc-Si/SiO 2andtoimprovetheagglomerated silicon nanocrystals. Just to mentionthat the amorphous layer recrystallization happensin a typically 60s or lower with the RTA srecrystallizationfactorssuchasisthetemperatures of annealing by RTA and thetemperature ramp speed of the onsrelatively easy to control but is limited on theminimum size of nc-Si ( 2.5 nm) due to the stress.4.5 Spark processThe obtained luminescence yields

A remind of the basic properties of silicon will be of great importance to understand well the silicon nanocrystals properties. 2 PHYSICAL PROPERTIES OF SILICON Silicon is, on Earth, the most abundant element after oxygen; weestimates that the Earth's surface is composed about 26% of silicon. [10] This

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