Complete Report - University of New South Wales

ARCPHOTOVOLTAICSCENTRE OFEXCELLENCE2010/11ANNUAL REPORTRaman spectra of the samples annealed fordifferent durations.Figure 4.5.23Raman spectra of the samples annealed atdifferent temperatures.Figure 4.5.24Room temperature PL of amultilayer film containing Ge NCs inGeO 2/SiO 2.Figure 4.5.25Intensity (arb. units)Growth Temp.III 445 o C420 o C400 o CII 380 o C350 o C320 o CI RT150 200 250 300 350 400R aman S hift (cm -1 )Raman spectra from Ge-NCs insingle layer samples grown atdifferent substrate temperatures.Figure 4.5.26from structural characterization using TEM,Raman, XRD and PL have been obtained.Figure 4.5.21 shows a high resolution TEMimage of a section of a typical samplecontaining GeRO layers between GeO 2/SiO 2. The sample was annealed at 650°C.Ge nanocrystals of about 5nm diameterare evident. Although there is a slightvariation, the NCs appear to be sphericalin shape. The clear lattice fringes observedin the TEM image give direct evidenceof the formation of the Ge NCs. It shouldbe noted that the distance between thelattice fringes is 3.3 Å, which is consistentwith the lattice spacing of the {111} planes of the Gediamond structure.Fig. 4.5.22 (a) shows Raman spectra of the asdepositedand the annealed samples, includingthose for bulk Ge as a reference. For these samples,the sputtering times for each GeRO layer andGeO 2/SiO 2layer were 8 minutes and 6 minutes,respectively, and the post depositionannealing was performed at 650°C for 40minutes. A broad hump at around 270cm -1 is observed in the spectra of theas-deposited film which is attributed tothe non-crystalline Ge phase. But in theannealed film, it is replaced by a sharppeak at 300.5 cm -1 , which is very close tothe Ge-Ge optical phonon mode for bulkGe (300.2 cm -1 ), indicating the formationof Ge NCs with good crystallinity. Peakbroadening and an asymmetric shoulderon the lower frequency side can beinterpreted by the model of the opticalphonon confinement effect in nanocrystals[4.5.33]. However, it seems that in our Ge system thehigh frequency side of the Raman peak does notshow a shift to lower phonon frequencies which isusually the case for NCs.Figure 4.5.22 (b) shows GIXRD patterns for thissample. In the as-deposited film there are noobvious peaks but only two broad bands at around2θ = 26° and 2θ = 49°. After annealing, the sampleshows three sharp peaks at 27.12°, 45.13° o and53.21°, corresponding to the groups of planes {111},{220} and {311} of crystalline Ge, respectively. Thisobservation confirms good crystallinity of the Gephase in the film and agrees well with our Ramanresults. The average size of the Ge NCs, calculatedfrom the {111} peak broadening using the Schererequation, is about 4.5 nm. This value is slightlysmaller than that estimated from the HRTEM image.The difference in Ge NCs sizes obtained from thesetwo methods may possibly be due to spatial nonuniformityof the Ge NCs size; with TEM probinga much smaller sample region compared to XRDmeasurement. In addition, the penetration depthof the incident X-rays is larger than the thicknessof our film, thus information obtained from XRD isaveraged throughout the whole film.The crystallisation of Ge NCs with annealingduration has been investigated with Ramanspectroscopy. Fig. 4.5.23 shows Raman spectrafor identical multilayer samples with each GeROand GeO 2/SiO 2layer deposited for 6 minutes. Thesamples were annealed at 685°C for differentdurations as indicated in the diagram. Thecrystallization of Ge is found to take place withinthe first few minutes of annealing. However, thenoticeable broad hump, which is attributed to smallNCs, suggests an early stage of the crystallizationprocess at this annealing duration. As annealingduration increases, Raman peaks become sharperand narrower indicating an increase in Gecrystallinity. It is also observed that the peaks shownegligible difference for annealing durations longerthan 10 minutes.The average size of NCs, as calculated from XRDdata, increases with annealing duration. This isconsistent with the Raman results and agreeswell with the growth dynamics by diffusion ofneighbouring Ge atoms. Most importantly, theincrease in the NCs size begins to level off after 15minutes of annealing in this particular case. This issimilar to the size confinement effect observed forthe growth of Si NCs in SiO 2matrix [4.5.7, 4.5.34].In our superlattice structure, Ge NCs are confinedwithin the GeRO layers between GeO 2/SiO 2layers,72