Optical Characterization of Individual Nanostructures by STM Light Emission
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ABSTRACT Visible light is emitted when electrons (holes) are injected into a sample from the tip of the scanning tunneling microscope (STM). By analyzing the spectra of the emitted light, one can not only determine the surface geometry by usual STM imaging, but also learn the electronic and optical properties of specific individual nanostructures. This technique has been applied to investigate the electronic transitions of individual protrusions of porous Si and semiconductor quantum wells of AIGaAs/GaAs. The usefulness, limitations, and future expectations of this novel technique are discussed. INTRODUCTION The development of our ability to fabricate increasingly smaller material structures requires simultaneous development of characterization techniques for such nanometer-scale objects. The
scanning tunneling microscope (STM) has played a crucial role in the development of new capabilities both in fabrication and examination of nanostructures. The discovery of light emission from the tunneling gap of STM [1] has opened a new possibility for an optical spectroscopy with an atomic spatial resolution. STM light emission spectroscopy (STM-LES) adds a new dimension to the conventional STM imaging technique. In STM-LES one analyzes the spectrum of visible light that
is emitted from the tip/sample gap region. Thereby, one can determine the energies and the nature of electronic transitions that are localized under the STM tip. In the last ten years many groups have worked with this new form of spectroscopy on different sample systems, but there seems to be no comprehensive review paper on the subject. For the convenience of the reader we list several recent papers that can be used as starting points for a literature search. [2]
EXPERIMENTAL METHOD In STM-LES one first obtains a STM image of topographic surface structures by the usual scanning method. Then the STM tip is fixed over a structure of interest, and the emission spectrum from the target structure is measured. 25 Mat. Res. Soc. Symp. Proc. Vol. 588 © 2000 Materials Research Society
The typical quantum efficiency for light emission by tunneling electrons is on the order of 10'. Thus for a tunneling current of 1 nA (-6 x 10' electrons/s), the total integrated number of photons is -6 x 10 photons/s. The usual quantum efficiency of detection is around 5 x 10.2, and a typical solid angle of collection is on the order of 10'. Hence the expected number of detected photon counts becomes about 3 x 10 photons/s over the whole spectral range of emission. Then if the spectrum is divided into 101 channels of a multichannel detector, each channel will count -3 photons/ s. This is a typical level of signal in our system. If the current is increased, the emission intensity will also increase, but the current must be kept as low as possible to avoid damage to the sample surface. To have an atomic spatial resolution at the same time, one needs to operate at a current level that is suitable for imaging, and that current level is usually around I nA. To achieve a
satisfactory
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