Semiconductor Nanocrystals formed in SiO 2 by Ion Implantation
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annealing. A typical SiO 2 layer was 750 nm thick, formed by thermally oxidizing a (100) Si wafer. The implanted species included Ge, Si, Si+Ge, Ga+As, and Cd+Se, with doses of (0.6 to 3)x 1017 ions/cm 2. Implant energies (200 to 500 keV) were chosen to put the peak concentration at the center of the oxide layer. For the compound semiconductors, equal doses of
each constituent were implanted. Samples were annealed isochronally for 1 hr under Ar + 4%H 2 ambient. The annealing temperatures varied from 600"C to 1100"C. The nanocrystalline structures were investigated by TEM and X-ray diffraction. All the TEM specimens were prepared in cross sections, since the concentration distribution from ion implantation is a function of depth. Depth profiles were also examined by Rutherford backscattering spectrometry. PL and Raman spectra were measured at room temperature using an Ar-ion laser as the excitation source. RESULTS AND DISCUSSION
A series of samples have been implanted with different doses of Ge (400 keV) into SiOC 2 and annealed at different temperatures to study the effects of dose and annealing temperature on the nanocrystal formation. The cross-sectional TEM pictures of three samples are shown in Fig. 1. The Ge particles are spherical in shape, as is expected for an amorphous matrix. A wide range of particle sizes, from 1 to 25 nm, is observed in the sample implanted with 3x10 17 ions/cm 2 and annealed at 1000"C (Fig. la). At the peak of the concentration profile, the largest crystallites are 25 nm in diameter. The peak of the size distribution profile over the whole implanted region is at 5 nm (Fig. 1d). It is worthy to point out that plan-view TEM may provide only part of the picture when the nanocrystal size is a function of depth. In some large precipitates, crystal defects, such as twinning, have been observed. The nanocrystallites are randomly oriented. As the dose of the implanted Ge decreases, the high end of the size distribution reduces rapidly. In a sample implanted with 1.5x10 17 ions/cm 2 and annealed at 1000"C (not shown), the sizes of Ge nanocrystals range from 1 to 12 nm. At an even lower dose of 6x10l 6/cm2, the Ge nanocrystal sizes are all smaller than 7 nm, with the peak of the size distribution profile at - 4.5 nm (Fig. lb and le). It is noticed that in a sample where the size distribution covers a large size range, there is a higher number density of small nanocrystals closer to the surface (upper side of the implanted region, see Fig. 1a). This phenomenon is much less visible in samples with narrower size distributions. Another controlling factor for the size distribution is the annealing temperature. The sample implanted with a high dose of 3x10 17/cm2, but annealed at a low temperature of 600"C, contains Ge nanocrystals with sizes between 1 nm to 9 nm (Fig. Ic and If). The sizes of the majority of the nanocrystals are smaller than 5 nm, and the peak of the size distribution profile is at - 4 nm. Although the samples shown in Figs. 1(b) and 1(c) contain smaller sizes of nanocrystals, the
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