Linear and nonlinear optical spectroscopies of SiGe interfaces and Si nanocrystals
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Linear and nonlinear optical spectroscopies are used to study SiGe alloy films and Si nanocrystals (NCs). With spectroscopic ellipsometry (SE), a bulk-sensitive linear optical probe, we demonstrate in-situ monitoring and control of compositionally graded SiGe films grown on Si(001) by chemical vapor deposition. Feedback control is achieved by comparing the Ge composition of the most recently deposited layer determined from SE to the set values, then adjusting the flow of disilane gas accordingly. Second harmonic generation (SHG), a surface/interfacesensitive nonlinear optical probe, complements SE greatly in monitoring film growth. We develop a real-time SHG technique by tracking surface Ge composition with the peak of the SHG spectrum (E1 resonance) using a 15 femtosecond broad bandwidth laser. Data acquisition is much faster than traditional SHG spectroscopy, in which a 100 femtosecond narrow bandwidth laser must be tuned. Using broadband SHG and SE, we also explore the strain effect caused by adding a small amount of C into SiGe alloys. SHG studies are extended from the planar surface/interface such as SiGe/Si to the sharply curved Si/SiO2 interfaces of Si NCs embedded in SiO2. We observe SHG from 3-dimensional distributions of spherical Si NCs prepared by ionimplantation into glass, which have applications in photonic and light-emitting devices. The results suggest that SHG originates microscopically from Si/SiO2 interfaces states, which are passivated by hydrogen annealing of NC samples, and macroscopically in part from fluctuations in NC size, shape and density. We also study SHG from dense (1010 or 6×1011 cm−2 ) 2-dimensional layers of Si NC (5 or 8 nm average diameter) prepared by chemical vapor deposition of Si precursor gases onto an oxidized Si wafer, and subsequently embedded in SiO2. Such Si NC layers act as a controllable planar charge storage layer in flash-memory devices. Time-dependent SHG measures the electrostatic charging and discharging of the NC layer in real-time. By polarization-dependent and frequency-domain interferometric SHG (FDISH) spectroscopy, SHG intensity and phase spectra of Si NCs are distinguished from contributions of the Si substrate, and reveal a NC-size-dependent blue-shift of the E1 resonance, consistent with quantum confinement, that can be used as an in-situ size diagnostic. Although these results were obtained ex-situ, they show that SHG can probe key material and electrical properties of Si NCs sensitively without contacting the sample, and thus can be transferred readily to in-situ, real-time monitoring of the deposition of Si NCs.