P. T. Wilson

Frequency-domain interferometric second-harmonic spectroscopy.

We report a new spectroscopic technique to measure simultaneously the intensity and the phase of second-harmonic (SH) radiation over a broad spectral range without laser tuning. Temporally separated SH pulses from two sources, excited by the same broad-bandwidth 15-fs Ti:sapphire fundamental pulse, interfere in a spectrometer to yield frequency-domain interference fringes. We demonstrate the technique by measuring the strongly bias-dependent phase of SH radiation from a Si/SiO(2)/Cr metal-oxide-semiconductor capacitor in the spectral range of the SiE(1) critical point.

dc-electric-field-induced and low-frequency electromodulation second-harmonic generation spectroscopy of Si(001)-SiO2 interfaces

The mechanism of dc-electric-field-induced second-harmonic ~EFISH! generation at weakly nonlinear buried Si(001)-SiO2 interfaces is studied experimentally in planar Si(001)-SiO 2-Cr MOS structures by optical second-harmonic generation spectroscopy with a tunable Ti:sapphire femtosecond laser. The spectral dependence of the EFISH contribution near the direct two-photon E1 transition of silicon is extracted. A systematic phenomenological model of the EFISH phenomenon, including a detailed description of the space-charge region ~SCR! at the semiconductor-dielectric interface in accumulation, depletion, and inversion regimes, has been developed. The influence of surface quantization effects, interface states, charge traps in the oxide layer, doping concentration, and oxide thickness on nonlocal screening of the dc-electric field and on breaking of inversion symmetry in the SCR is considered. The model describes EFISH generation in the SCR using a Green’s-function formalism which takes into account all retardation and absorption effects of the fundamental and second-harmonic ~SH! waves, and multiple reflection interference in the SiO 2 layer. The optical interference between field-dependent and -independent contributions to the SH field is considered as aninternal homodyne amplifier of the EFISH effects. Good agreement between the phenomenological model and our EFISH spectroscopic results is demonstrated. Finally, low-frequency electromodulated EFISH is demonstrated as a useful differential spectroscopic technique for studies of the Si-SiO 2 interface in silicon-based metaloxide-semiconductor structures. @S0163-1829~99!01836-6#

Second-harmonic Generation from Silicon Nanocrystals Embedded in SiO2

We present observations of optical second-harmonic generation (SHG) from silicon nanocrystals embedded in SiO2. SHG sensitivity to Si/SiO2 interface states, charge on the nanocrystals, and particle density gradients is demonstrated. SHG is proven to be a powerful noncontact nondestructive diagnosis tool for characterization of Si-nanocrystal-based devices and materials.

Femtosecond carrier-induced screening of dc electric-field-induced second-harmonic generation at the Si(001) SiO(2) interface.

Carrier-induced screening of the dc electric field at the Si(001)-SiO(2) interface is observed by intensity-dependent and femtosecond-time-resolved second-harmonic spectroscopy. The screening occurs on a time scale of ~?(p)(-1) , the reciprocal plasma frequency of the generated carriers.

In situ control and monitoring of doped and compositionally graded SiGe films using spectroscopic ellipsometry and second harmonic generation

We have implemented linear and nonlinear optical spectroscopies to monitor and control the growth of SixGe1−x films. Using spectroscopic ellipsometry and the virtual substrate approximation (VSA), controlled growth of compositionally graded SiGe films deposited by chemical vapor deposition (CVD) is achieved by adjustment of disilane flow based on feedback from ellipsometric inputs. Stepped and linear growth profiles are investigated. Using spectroscopic, surface second harmonic generation (SHG) by a tunable, unamplified Ti:sapphire 100 femtosecond (fs) laser, shifts of the SH spectral feature near the Si E1 critical point with varying Ge composition are observed. A comparison is made to linear spectroscopy and related qualitatively to surface composition. Data acquisition time is then reduced to a few seconds by substituting a 10 fs laser and spectrally dispersing generated SH radiation onto an array detector, thus enabling real-time spectroscopic SHG. The reflected SH spectrum near the E1 region is also highly sensitive to bulk boron doping of SiGe. Definite trends in the peak positions and amplitudes as a function of boron incorporation are observed and interpreted qualitatively in terms of dc-electric-field-induced SHG in the depletion region. The results demonstrate the feasibility of SE and spectroscopic SHG operating as complementary, in situ sensors of SiGe CVD.

Optical Second Harmonic Spectroscopy of Silicon Surfaces, Interfaces and Nanocrystals

Unlike most surface science tools, optical surface techniques are compatible with nonUHV ambients, and provide access to interfaces buried beneath transparent overlayers. Within the family of optical probes, second-order nonlinear optical processes such as second harmonic generation (SHG) possess additional unique strengths for surface/interface analysis. A dipolar SHG response exists at any surface/interface discontinuity. Thus, unlike reflectance-anisotropy spectroscopy (RAS), SHG is not restricted to anisotropic surfaces of isotropic substrates. Moreover, the second-order nonlinear susceptibility c(2) vanishes in the bulk of centrosymmetric media such as elemental semiconductors. In such materials SHG becomes extremely specific to surfaces/interfaces, as well as to bulk regions pervaded by electric fields, strain gradients, and other centrosymmetrybreaking features [1–3]. On the other hand, efficient surface SHG from such materials requires intense pulsed laser sources with limited tuning range. Thus, in contrast to linear optical surface probes, SHG has most often been implemented as a single-wavelength probe. Measurement of SHG phase is particularly cumbersome. In addition microscopic theory of surface nonlinear responses is much less advanced than the theory of linear optical properties. In this article, we review recent advances in the spectroscopic implementation and theoretical interpretation of SHG from surfaces and interfaces of centrosymmetric semiconductors.

Second harmonic phase spectroscopy: frequency vs. time domain

Summary form only given. We quantitatively compare conventional vs. frequency-domain interferometric second harmonic (FDISH) phase spectroscopy for the first time. A Si/SiO/sub 2//Cr MOS capacitor with applied bias -9 to 10 V provided a common sample. Since the space-charge field (SCF) changes sign at the flat-band voltage (-2.45 V), the phase of electric-field-induced SH (EFISH) shifts by /spl sim//spl pi/, providing a convenient point of comparison. FDISH phase spectra acquired in seconds, agree generally with conventional phase spectra acquired over hours. Both show strong bias-dependent phase shifts near the E/sub 1/ resonance (3.3 to 3.4 eV), where EFISH generation is resonantly enhanced, and relatively featureless, bias-independent phase for 2h/spl nu/ < 3.2 eV. However, FDISH phase shifts abruptly by /spl sim//spl pi/ at V/sub flat-band/, as expected, whereas conventionally measured phase shifts more gradually. Apparatus drifts during the longer measurement partly explain the discrepancy. In addition, screening of the SCF by photo-excited carriers selectively influences the conventional measurement, because electrons and holes separate more during its 100 fs pulse than during the FDISH 15 fs pulse, thus weakening the bias dependence. Thus, in this case, FDISH clearly yields the more accurate /spl Delta//spl psi/(/spl omega/) measurement.

Second harmonic generation from silicon nanocrystals embedded in SiO/sub 2/

We present observations of optical second harmonic generation (SHG) from silicon nanocrystals embedded in SiO/sub 2/. SHG sensitivity to Si/SiO/sub 2/ interface states, charge on the nanocrystals, and particle density gradients is demonstrated.

Electric-field-induced second-harmonic generation microscopy

Summary form only given. Second harmonic generation occurs in semiconducting materials via the second-order polarizability, which is governed by the second-order nonlinear susceptibility tensor /spl chi//sup (3)/ applying a DC-electric-field across the material in the region of incident light forces an additional interaction that also produces second harmonic radiation through a third order /spl chi//sup (3)/ the second harmonic polarizability. In the present work, we use EFISH in conjunction with high-resolution microscope collection optics to image Schottky barrier fields with submicron resolution. With proper contact calibration, this system, as well as being useful for characterizing electrical contacts in submicron semiconductor device structures, could be useful as a dopant concentration measurement device since the spatial extent of the depletion region depends on dopant concentration. We believe we present the first EFISH images of Schottky barrier fields.

Size effects in second harmonic generation from Si(001)-SiO2 interface: microscopic interface effects and optical Casimir nonlocality

Size effects in optical second harmonic generation (SHG) from Si(001)-SiO2 planar micro-cavity structures are studied for fundamental wavelengths from 700 nm to 850 nm. The observed dependence of SHG intensity on the thickness of oxide layer is explained, in part, by an optical Casimir nonlocality arising from interference of zero-point fluctuations in micro-cavity, which is distinguished from microscopic interface effects. Detailed theoretical analysis of the Casimir nonlocal contribution to the quadratic susceptibility using a diagrammatic technique is performed for visible and IR regions. The difference in the length scale of the Casimir contribution at 800 nm and 1064 nm fundamental wavelength is discussed.