Photoreflectance Near-Field Scanning Optical Microscopy
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*Department of Chemistry, **Department of Chemical Engineering, ***Department Electrical and Computer Engineering, University of Wisconsin, Madison, WI, 53706
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ABSTRACT A novel Near-field Scanning Optical Microscopy (NSOM) technique is used to obtain simultaneous topology, photoluminescence and photoreflectance (PR) spectra. PR spectra from GaAs surfaces were obtained and the local electric fields were calculated. Sub-wavelength resolution is expected for this technique and achieved for PL and topology measurements. Photovoltages, resulting from the high intensity of light at the NSOM tip, can limit the spatial resolution of the electric field determination. INTRODUCTION Photoreflectance (PR) spectroscopy can be used to determine surface electric field in the depletion region, the surface Fermi level (EF), doping density, doping type, and the band gap energy (Eg) [1]. These quantities are important for device optimization and their measurement with micron and submicron spatial resolution is becoming increasingly important. PR is a form of electromodulation spectroscopy. Minority and majority carriers are created in a direct band gap semiconductor by excitation with above band gap light via a pump source that is chopped at a typical frequency, v, of 100 - 1000 Hz. A second tunable light source serves as the probe and is then reflected from the sample's surface. The reflected light will contain both a DC and an AC component. The AC component of the reflected light is modulated at the pump frequency. The normalized AC component, AR/R, is obtained as the PR signal. The magnitude of the measured quantity ARIR is typically 10-2 to 10-4 . When the wavelength of the probe light is scanned, Franz-Keldysh oscillations (FKOs) can be seen in the AR/R spectra due to
electric field modulation of the reflectivity of the sample. These oscillations are strongest near the probe photon energy Eprobe = Eg, and decay in intensity as the energy is increased. Only an exponential decay in the PR spectrum is seen for energies less than Eg. The extrema in the FKO's are given by equation (1), and are related to the internal electric field, F, within the semiconductor with equation (2). Here, (h0)0 is the electrooptic energy, n is the index of the nth extrema of the oscillations, E, is the energy of the probe photon, E,, is the gap energy, (pis an arbitrary phase factor, and g is the reduced mass of an electron in the sample. Therefore, F can be determined from a plot of (4j)(Eý
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number n, which should be a straight line with slope equal to
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(h07, which is proportional to the internal electric field F.
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We present here an approach to the measurement of optical spectra from sub-diffraction-limited areas based on Near-field Scanning Optical Microscopy (NSOM) [2]. This microscopic technique uses a tarpered fiber optic for a scanning probe that allows the simultaneous measurement of surface topography and optical properties by delivering and/or
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13 Mat. Res. Soc.
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