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2.4.1 Hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) can be fabricated using the PECVD process with strongly hydrogen diluted silane as the reactant gas source [26]. Liu

et. al. have used this technique to obtain a-Si:H sample that were grown on a polished quartz as well as a roughened glass substrates. The substrate temperature was set below 150°C and the sample thickness was approximately 1pm. It was noted that the as-grown photoluminescence was very sensitive to substrate temperature with the photoluminescence intensity decreasing to zero at anneal > 200°C. The peak position of the photoluminescence shifted from 1.84eV at 50°C, 1.79ev at 100°C, to 1.8eV at 150°C. The full width half maximum (FWHM) also broadens as the band moves to the shorter wavelengths from 0.2eV to 0.34eV. The photoluminescence intensity was noted to be over 2 orders of magnitude below that of porous silicon. Using Raman techniques the silicon cluster size was deduced to be 2.8nm, with those grown at the 50°C gave a size of 2.6nm. Liu et. al uses this as evidence of quantum confinement of silicon nanocrystals in the a-Si:H matrix and explains the lack of photoluminescence in the high temperature anneals (above 200°C) may be due the nanoparticles being to large for significant quantum confinement to occur. The size deduced and photoluminescence observed are in good agreement with the quantum-confined model. It is interesting to note that no photoluminescence

C h apter Two: L iterature review o f light em ission fro m silicon b a sed m aterials

band was observed from this film in the 2.0eV region. In a similar study by the same group [27] the photoluminescence was enhanced by an alternating hydrogen plasma treatment. FTIR studies indicated that the increase in the photoluminescence intensity was due to an increase in oxygen rather than hydrogen in the matrix. In effect, the hydrogen plasma treatment had promoted further oxygen bonding to the network rather than promoting silicon dihydride species. The source of oxygen is thought to be the trace oxygen contained in the hydrogen source. The FTIR measurements undertaken in this study also rules out several siloxene species (HSi-Os, HSi-0 2, and H Si-0) as contributing to the

photoluminescence from the a-Si:H :0 films.

2.4.2 Silicon oxynitride, hydrogenated silicon dioxide, and silicon-rich silica

The difference between silicon oxynitride and silicon-rich silica is often not distinguished in the literature. Most of the films studied in this field that are fabricated by the PECVD process utilise N2O as the oxygen source. This means

that films using this process incorporate usually a few percent of nitrogen into the matrix of the films (as indeed this is the case of the films grown at UCL). It is also recognised that silicon nitride can produce visible luminescence [28].

The Fisher [6] and the Augustine [29] are studies of alloying effects that may be

responsible for visible light emission. Fischer e t a i [6] have produced Si-rich

oxynitride film using the PECVD process. Two different sets of photoluminescence spectra are presented: as-grown photoluminescence of N2O

grown films with only one distinct band observed and photoluminescence o f the same film after a 30 minute treatment in a hydrogen forming gas. The spectra showed a peak position in the photoluminescence spectrum of 1.65eV but with a fivefold increase in intensity after the forming gas. However, this film does not appear to have studied further. A further study is that of a film grown with NH3

instead of N2O and the same film after the hydrogen forming gas procedure: the

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

peaks are shown at 1.9eV with the same increase in photoluminescence intensity after forming. A fast and slow component was observed in the photoluminescence, with the slow component only appearing below 80K. XRD revealed that no crystalline silicon was detected in the as-grown film or in a film annealed at 1100°C. Although significant silicon crystallinity was detected after further heat treatment (not specified) with the average microcrystal diameter of

8nm. The observed photoluminescence emission at room temperature measured

by time-resolved spectroscopy was found to be 2 ,leV after 3ns and relaxes to 1.9eV after 10'^ s. At low temperatures (15K) these components (fast and slow) contribute equally to the photoluminescence; however, at room temperature only the fast component remains. It was proposed that the increase in thermal activity at room temperature allowed the long lifetime carriers to hop to non-radiative centres and so the fast decay luminescence dominates at room temperature.

The bandgap value of 2.1eV is derived from PLE studies and is assumed to be the average bandgap of the alloying of the Si-SiOxNy interface. The overall absorption bandgap was measured to be 2.5eV. Fischer postulates that because of the high defect density (non-radiative) in the surrounding matrix, absorption in the matrix should lead mainly to non-radiative recombination. However, absorption in the low bandgap regions will contribute more effectively to the photoluminescence signal, the result being a broad emission centred at 1.9eV. Augustine et. a l [29] has also studied the room temperature photoluminescence of PECVD grown silicon oxynitride as a function of post-process annealing. As in the previous study only a photoluminescence at 2.1eV is present. Annealing is done using a rapid thermal annealing technique (RTA). Using FTIR studies Augustine concludes that the changes in structure of the film occur only in short range bonding changes and rearrangement. Also, that after short anneal periods hydrogen is evolved from the network and under longer anneal times Si-O or Si- N bonding occurs to passivate dangling bond states. An increase in photoluminescence intensity at 8 minutes anneal occurs with a six-fold increase

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

in intensity at 20 minutes anneal time. It is suggested that the increase in intensity was due to the lower defect density in the film after annealing. This paper make an interesting point concerning the role of nitrogen in a-SiOxNy:H films and argues that the three-fold coordination of the nitrogen may help prevent the phase separation of Si-SiOi in the annealing films.

Lin et. al. [30] have also fabricated hydrogenated amorphous silicon using a mix of N2O and (N2 0+SiH4) with the SiH4 in a 50% dilution with hydrogen. The gas

N , 0

ratio used ( ^ ^ ^ ^ - ) were from 0.7 to 0.95. A chamber pressure of 400mx

and substrate temperature o f 250°C was used for all the film growths. Photoluminescence was observed (for films at a ratio of 0.8) in the as-grown films at 2.0eV with an increase in photoluminescence intensity at 400 °C and 500°C anneals (anneal time 2 minutes) The photoluminescence peaks at 600°C anneal and red-shifts to 1.95eV. At 700°C anneal the photoluminescent intensity begins to decrease with the photoluminescent band further shifting to 1.9eV. The increase in anneal temperature is accompanied by a decrease in the hydrogen content of the film as detected by the continuous decrease in the Si-Hn IR peaks (2082-2240 cm'^). The Si-N stretching mode at 840 cm'* is not observed (at any temperature) or is buried in the HSi-Og and HSi-0 2Si peak at 855 cm *. It is

suggested that this indicates that the nitrogen content of the film is very low and very little of the nitrogen is back-bonded to the silicon. Lin suggests that the observed redshift is due to adjoining regions of silicon that gap between due to the effusing hydrogen that bond when the energy supplied by the annealing becomes sufficient. These regions are quantum confined by a barrier of S-H and SiOx with the accompanying increase in silicon cluster size providing the red­ shift. This work does not relate any of the photoluminescence to the presence of defects in the films.

C h apter Two: Literature review o f light em ission from silicon b a sed m aterials