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2.0 Introduction

The purpose of this review is to examine recent work in the field of light emission from silicon based material grown by different techniques. This is done in order to identify similarities in optical behaviour and the proposed luminescent mechanisms.

SiOx grown by a variety of techniques has recently begun to generate interest. Although luminescence was reported in this material some time ago [1] it is only recently that its optical properties have begun to be thoroughly studied. A number of techniques have been employed in its production, including implantation of silicon into silica [2,3,4], implantation of oxygen into silicon [5,6], Plasma Enhanced Chemical Vapour Deposition (PECVD) [6,7], Low Pressure Chemical Vapour Deposition (LPCVD) [8,9], and cosputtering of silicon and oxygen [10,11]. X-ray diffraction and Transmission Electron Microscopy (TEM) studies have confirmed the presence of clusters of silicon atoms in the Si0 2 matrix [7]. Depending on the deposition technique and post­

process annealing, these inclusions can be either amorphous or crystalline: amorphous clusters will tend to crystallise on annealing at 1050°C. Recent results have demonstrated broadband light emission from the blue to near infra-red from material containing silicon aggregates of varying sizes [8,9,10,11,12]. However, there is uncertainty about the nature of the luminescence mechanism from silicon-rich silica. There are some common features to the majority of the studies: the most important being the observed visible photoluminescence consists of one or two bands that are generally located at 1.7eV and 2.0-2.2eV

C h apter Two: L iteratu re review o f light em ission from silicon based m aterials

approximately. W ith the 2.0eV band disappearing in anneals from 500-600”C.

2.1 Ion implantation of silicon into quartz

Studying silicon-implanted silica, Shimizu-Iwayama [2,4] identified two bands in the photoluminescence spectrum; one around 2.2 eV and another at 1.7 eV. Using electron spin resonance spectroscopy (ESR), they detected the presence of a large number of defects in their samples prior to annealing. It is well known that the implantation process caused structural damage to the film and a large number of defects are formed. These defects can be associated with oxygen vacancies in silica [4]. The number of defect centres detected by ESR falls rapidly on annealing at temperatures in excess of 600 °C, and the photoluminescence band at 2.2eV appears to be strongly quenched at the same rate. Making a link between the two observations, Shimizu-Iwayama et al

proposed that defects are the source of the high-energy emission and originate from the matrix SiO^. The 1.7eV photoluminescence band does not appear until samples are annealed at temperatures above 1100 °C. At this temperature the silicon and SiÛ2 become phase separated. Shimizu-Iwayama noted that silicon

clusters are not present in the material until it is annealed at 1100°C (this is supported by TEM of the annealed films that shows nanocrystalline clusters of between 3-5nm in size). Shimizu-Iwayama concluded that the presence of the 1.7eV band must be linked to the presence of the silicon clusters but did not firmly conclude what mechanism was responsible. On increasing the anneal time (from 90minutes to 240minutes) at 1100 °C it was found that the photoluminescence intensity increased but no significant red-shift of the photoluminescence band was observed. It was expected that the silicon clusters would grow even further on longer anneals and so inducing a red-shift if the quantum-confinement was responsible. Komoda’s results [3] also indicated the presence of two bands in the luminescence spectrum of SU implanted silica, also

C h apter Two: Literature review o f light em ission fro m silicon based m aterials

citing evidence for photoluminescence assosiated with nanoclusters and defect luminescence.

2.2 RF Cosputtering

Hayashi et. al. [10] fabricated Si-rich SiOz films by the RF cosputtering of silicon and Si0 2 targets, growing films of approximately 1pm on silicon or

sapphire substrates that were heated during the film growth to approximately 100°C. One interesting aspect of this and a similar study using cosputtering [12] was that only one band that varied from 1.5-1.7eV depending on anneal conditions and initial silicon content. Hayashi used indirect evidence from Raman studies to estimate the size of clusters. Osaka [12] using a similar technique reports and red-shift on anneal and ascribed the photoluminescence to quantum-confinement in the silicon nanocrystals and quoted three sizes of 3.0, 4.1 and 7.0nm. In the Osaka study the photoluminescence was extremely weak at room temperature and only significant at 77K. For even the simple quantum- confined model these sizes are quite large with only the B.Onm showing photoluminescence at approximately 1.55eV. This point is picked up by Hayashi

et. al. who only noticed significant visible photoluminescence with cluster sizes estimated to be less than 2nm. In the Hayashi et. al. study: after anneals of 300°C, 400°C, and 500®C, no redshift is observed but the photoluminescence intensity increases and peaks at 500°C. A steady redshift was observed from approximately 1.7 to 1.45eV on annealing from 500°C through to 900°C. This is accompanied by a decrease in photoluminescent intensity and an apparent broadening the line-shape. Hayashi suggest that the photoluminescence arises from the transition across the ‘highest occupied to lowest unoccupied molecular orbitals gap’ citing predictions from a model developed by Ren et. al. [13] based on confined silicon clusters that are completely hydrogen terminated. This is essentially an earlier version of the type of model further developed by Hill [14]

C h apter Two: Literature review o f light em ission fro m silicon b a se d m aterials

and is described in some detail in chapter one.

2.3 Nanocrystalline Si:H and nanocrystalline silicon

Si:H materials are formed by the decomposition of silane generally in a chemical-vapour-deposition system. It is known that Si:H has an optical bandgap that almost linearly increases with the increase in hydrogen content. Alloying effects (the alloying principle is briefly discussed in chapter one) can explain this; however, some of the photoluminescence data observed in the literature cannot be accounted for by this explanation alone. Furukawa et. a l [15] points to the fact that PECVD grown Si:H using SiH4 has a bandgap of 2.55eV at a

hydrogen content of 40% whereas PECVD of Si2Hô has a bandgap of 2.5eV at

the same hydrogen content. On examination of structural properties of the PECVD grown SiH4 Furukawa at. a l discovered that the material consisted of

small crystalline silicon particles with a diameter of 2-3nm and surrounded by hydrogen atoms. Furukawa concluded that the widening of the optical gap could be well explained by the three-dimensional quantum confinement in the silicon nanocrystals in the film.

Free oxidised silicon nanocrystals can be prepared by PECVD [16] and crystalline sizes of 2.5-lOnm can be achieved [17]. These particle are distinct separate particle i.e. form a silicon powder. Smaller sizes than this can only be achieved by embedding the silicon nanocrystals into a matrix since this represents the lower limit of the stability of free crystalline silicon [18]. The nanocrystals are oxidised in a dry oxygen atmosphere at relatively high temperatures (1250°C). The oxidation process of the silicon particles also reduces the effective size of the silicon cluster because of the finite size of the Si- SiOx interface. Riickschloss et. al. notes a change in photoluminescence intensity as the silicon cluster size decreases but no blueshift. However, Takagi et. a l [19] does observe a blueshift as the cluster size decreases with oxidation. Riickschloss

Chapter Two: Literature review o f light emission from silicon based materials

claim s this work still supports the quantum size effect of the increase in oscillator strength with decreasing size for optical transitions. Figure 2.0 shows the p hotolum inescence intensity as a function o f crystallite diam eter (solid line shows the relationship predicted by Delley and Steigmeier [20]).

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