Assessment of NSOM resolution on III-V semiconductor thin films

Abstract

Emission-mode aperture near-field scanning optical microscopy (NSOM) is applied to semiconductor thin films for resolution assessment and artifact investigation purposes. We have used GaAs thin films deposited on GaP(111) substrates with A and B polarities by metal organic vapour phase epitaxy (MOVPE). Optical discrimination of GaAs islands on GaP(111) has been accomplished with a lateral resolution better than 20 nm (λ/35) in the transmission mode. These samples have proven valuable for inspection of contrast mechanisms, based on discrimination of materials having different refractive index and absorption coefficients, as well as investigation of topography and shadowing artifacts in optical near-field imaging. Since its invention in 1984 [1] the near-field scanning optical microscope (NSOM) has become a widely adopted tool for optical investigation in the submicron range, well beyond the diffraction limit. Nowadays, near-field microscopy is applied to a variety of issues: nanospectroscopy [2], nanowriting [3], the study of biological samples [4], polymers [5], and magnetic samples [6], among others. Different working configurations have been demonstrated [7], but the most commonly used is the emission-mode aperture NSOM, as also witnessed by commercially available instruments. In the emission mode the near-field is generated by means of a tapered, metalcoated optical fiber with an aperture on the free edge in the range from 20 to 200 nm in diameter, coupled to a light source (usually a laser beam). The light scattered by the sample is collected afterwards in the far field, by means of a microscope objective, and detected by a photomultiplier (PMT). The tip is held at a constant distance from the sample by shear-force detection and stabilization [8] in order to maintain the sensor in the near-field region and to avoid damaging it. Lateral resolution of the aperture NSOM appears to be limited by the effective size of the aperture, which is determined by the penetration depth of the metal defining the aperture itself, which is ≈ 15 nm in the visible range. With the invention of the ∗ Present address: Dipartimento di Fisica, Universita di Firenze, Largo E. Fermi 2, I-50125 Firenze, Italy apertureless NSOM [9] the resolution limit has been pushed further by another order of magnitude. A resolution of few nm has been demonstrated; atomic resolution is theoretically predicted. In this configuration the subwavelength illuminating source is reduced to extreme limits, with a few dipoles located at the apex of a very sharp AFM or STM tip. When these dipoles are irradiated by an external light source they act as scattering centers and generate the near field that is used to probe the sample surface. Thus the mechanisms responsible here for enhanced resolution are of “particle imaging particle” kind [9, 10]. This excellent resolution is reached at the expenses of a more complex and critical experimental setup, as witnessed by the fact that at present, to our knowledge, only a few apertureless NSOM prototypes are operating. In this article we illustrate aperture NSOM measurements, on semiconductor thin films, that we have used for resolution calibration purposes and for evaluation of contrast mechanisms. The remarkable finding is that even in aperture NSOM with wide apertures the resolution can be higher than the aperture diameter for some genuinely optical processes resembling apertureless scattering. 1 Materials and methods The samples used here are heterolayers of a III-V semiconductor deposited on a different III-V semiconductor monocrystal. The study of features during the epitaxial growth of semiconductors is one of the most promising application of NSOM. There is a great interest in determining the growth mechanism and in growing reproducible quantum structures. In addition to the morphological analysis, NSOM is able to perform structural analysis by traditional absorption and photoluminescence characterization at a nanoscale range. NSOM is thus becoming a powerful tool for the investigation of homogeneity properties, strain distribution, and the optical behaviour of single defects like dislocations or stacking faults. The applicability of this technique to morphological studies has already been demonstrated, although continuous research efforts are increasing image resolution and improving electronic control. As regards the structural analysis, more improvements are still necessary to achieve