L. Mantese1, P.T. Wilson1, K. Selinidis2,
D. Lim1, Y. Jiang1, J.D. Canterbury1,
J.G. Ekerdt2, M.C. Downer1
1Department of Physics, University of Texas, Austin, TX 78712
2Department of Chemical Engineering, University of Texas, Austin, TX 78712
Due to their unique sensitivity to surfaces and interfaces, nonlinear optical techniques such as second harmonic (SH) and sum frequency generation (SFG) have emerged as powerful and highly versatile spectroscopic probes. With the recent availability of commercial, tunable, femtosecond laser systems, in-situ, real-time monitoring of the optical SH responses of Si(001) and SiGe(001) systems has become possible. Such investigations have included single-wavelength SHG monitoring of CVD growth chemistry, including H coverage and desorption at Si(001) in real-time [1,2]. Spectroscopic SH information around the E1 region of Si has demonstrated adsorbate-specific sensitivity to ML coverages (Fig.1).
Figure 1. SH spectra of ML and sub ML-covered 2x1-Si(001).
The E1 peak of Si shifts with increasing Ge composition, as observed in both SH spectra  and spectroscopic ellipsometry (SE). This information can be exploited for growth control, by calibrating the dielectric function of SiGe with Ge composition and using the virtual substrate approximation (VSA) developed by Aspnes [4,5]. Real-time growth control of compositionally-graded SixGe1-x has been demonstrated using SE . A similar approach is suggested by the sensitivity of SH to Ge composition (Fig. 2). However, the SH spectra acquired in several (~10) minutes by laser tuning are unacceptable for real-time growth applications. To overcome this problem, Wilson et al.  have developed a technique for rapid (~1 sec) spectroscopic detection of broadband SH spectra generated by a ~10 fs (~90 nm bandwidth) laser pulse. Real-time measurement and growth control using spectroscopic SH is thus feasible.
Figure 2. Left panels: Linear growth profile from 15% to 35% Ge composition using SE and VSA analysis with feedback control of the disilane valve. The inset corresponds to the disilane and germane valve positions during growth. The lower left panel shows corresponding post-growth SIMS data for verification. Right panels: SH responses of SiGe with increasing Ge composition. The thin-solid lineshapes were measured using a tuned laser output (acquisition time ~10 min) and the heavy-dashed lineshapes measured using a broadband laser source (acquisition time ~1sec).
In addition, the SH responses of doped Si(001) samples have been investigated  and shown to be uniquely sensitive to bulk B doping. Upon in-situ B doping to ~5x1018/cm3, the SH signal increases by about five times and the peak blue shifts toward 3.4 eV. An issue of immediate technological concern that SH may prove able to address is Å-scale gate dielectric information. Erley and Daums  SH results from SiO2/Si surfaces show a strongly process-dependent response near 3.6-3.8 eV. A similar investigation of alternative, novel dielectrics using SFG is under investigation.
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