A model of enhanced STM current due to semiconductor optical absorption J.D. Patterson * , J. Blatt, J. Burns, J.G. Mantovan, R.P. Raffaelle 1 Physics and Space Sciences Department, Florida Institute of Technology, Melbourne, FL 32901-6988, USA Received 9 March 2001; accepted 19 April 2001 Abstract We present a simple model for the change in tunneling current between a semiconductor surface and a metal tip under spectroscopic illumination in a scanning tunneling microscope. This model predicts a sharp increase in the tunneling current due to the increase in the conduction band carrier density when the photon energy exceeds the optical band gap. The tunneling current for a large diffusion length has a more pronounced onset than for a small length. Our model should provide, when combined with experiments, a method of determining localized effective stoichiometry, and therefore provides a localized alternative to the use of optical absorption measurements. Our theoretical tunneling current versus photon energy curves are in good qualitative agreement with the existing experimentally measured curves for Si, GaAs, and InP obtained by Qian and Wessels. In addition, we have examined the effects of temperature, surface recombination velocity, and degeneracy on our theoretical results for the Hg 12x Cd x Te, Hg 12x Zn x Te, and Hg 12x Zn x Se ternary narrow gap semiconductor systems. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; C. Scanning Tunneling Microscopy STM); D. Optical properties 1. Introduction STOS scanning tunneling optical spectroscopy) was ®rst performed on Si, GaAs, InP and various heterostructures and quantum wells and discussed by Qian and Wessels [1,2]. However, until recently no detailed calculations of STOS have been presented [3]. STOS is a relatively new measurement that combines electronic and optical techni- ques to allow a study of the interaction of light with solids down to the scale of atoms. It involves the tunneling current between a semiconductor surface and a metal tip under spectroscopic illumination in a scanning tunneling micro- scope. In the case of a semiconductor, a discontinuity in the tunneling current is observed as the incident photon energy exceeds the optical bandgap. Therefore, a measurement of this type could be used to determine optical bandgaps. The compound semiconductors mercury cadmium tellur- ide or Hg 12x Cd x Te MCT) and related materials Hg 12x Zn x Te MZT) and Hg 12x Zn x Se MZS) are well-known for their use in infrared detectors and related devices [4]. However, due to the dif®culty of growing uniform crystals of these materials under the load of gravity, they are currently being test grown in microgravity. Consequently, it is becoming very important to develop methods for characterizing these materialsasgrown[5].Samplehomogeneityinthesematerials is often judged using local optical bandgap determinations [3]. The optical bandgap in these materials is sensitive to small deviations in their stoichiometry. The spatial extent to which we are able to make these optical bandgap deter- minations limits the scale with which we can determine sample homogeneity by this method. The development of STOS should prove to be a bene®t in this area as it provides a more localized method for determining bandgaps than does a traditional optical transmission measurement. We present a new simple quantitative model for the photo-assisted tunneling current in the MCT, MZT, and MZS ternary narrow gap semiconductor systems. More detailed calculations than we give here are certainly pos- sible. However, our hope is to provide a simple model that will reproduce the qualitative features of measurements of Journal of Physics and Chemistry of Solids 63 2002) 257±265 0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0022-369701)00138-X www.elsevier.com/locate/jpcs * Corresponding author. Tel.: 11-605-342-6682; fax: 11-321- 674-7482. E-mail address: jdpat@rapidnet.com J.D. Patterson). 1 Now at: Physics Department, Rochester Institute of Technology, Rochester, NY 14623-5604, USA.