STM Light Emission from p-type GaAs (110) surface
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G7.23.1
STM Light Emission from p-type GaAs (110) surface 1
M.Hoshino and N.Yamamoto Dept. of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan 1 Dept. of Materials Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8551, Japan ABSTRACT STM stimulated light emissions (STM-LE) from cleaved surfaces of GaAs(110) were investigated using a UHV-STM combined with a light detection system. Clean surfaces of Zn doped GaAs (p=5×1018) were obtained by mechanical cleavage in UHV. Light emission from the cleavage surface showed a single peak in a spectrum, which corresponds to luminescence by the conduction band to acceptor level transition in GaAs. In STM image Zn acceptors show bright triangular-shaped contrasts, while in photon map they show dark contrasts when taken with a sample bias voltage of 2.0 V and 2.2 V at 80K. This contrast can be explained from the local band bending due to the charging effect or strain effect of the Zn acceptor. INTRODUCTION Light emission from STM was first reported by Cooms et al. [1], who observed enhanced light emission from an interface region between a tip and a metal surface. STM stimulated light emission (STM-LE) enables us to study optical properties of surfaces in high spatial resolution, in addition to surface topography and local electronic state usually obtained by STM and STS. Many STM studies of semiconductor surface have been carried out for - semiconductors, because in the (110) cleavage surfaces there are no surface states inside the bulk energy band gap. So bulk effects are not screened at those surfaces. Those are suitable to examine optical character of local structures inside a crystal such as defect structures. Recently individual accepter and donor were observed in the (110) cleavage surfaces of - semiconductors by STM.[2-4] Then we carried out STM-LE measurement for Zn acceptors and surface defects (steps, vacancies) on the (110) cleavage surfaces of GaAs. We could observe the photon map of acceptors and surface defects on the GaAs(110) surface for the first time. EXPERIMENTAL We used an UHV-scanning tunneling microscope (JSTM-4610) combined with a light -8 detection system. The pressure in STM chamber is kept at 2×10 Pa. GaAs (110) clean surfaces were obtained by mechanical cleavage in UHV. A sample is a Zn doped GaAs crystal (p=5×1018), and was cooled to 80 K for increasing photon intensity, and a solid angle sustained by the mirror is 61% of 2π, the solid angle of hemisphere. A Liq.N2 cooled CCD detector is used for measurement of emission spectra and a photon multiplier tube is used for photon mapping. We used chemically etched tungsten tips cleaned by electron bombardment in UHV chember. STM image and photon map measurements were performed in the constant current mode.
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RESULTS Figure 1(a) shows an STM image of the GaAs (110) surface taken with a tunneling current of It=0.2 nA and sample bias voltage of Vs=+1.5eV. Bright triangular-shaped contrasts correspond to the Zn acceptors. There are at least five kinds o
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