Enhanced conductance of chlorine-terminated Si(111) surfaces: Formation of a two-dimensional hole gas via chemical modification G. P. Lopinski, B. J. Eves, O. Hul’ko, C. Mark, S. N. Patitsas, R. Boukherroub, and T. R. Ward Steacie Institute for Molecular Sciences, National Research Council, 100 Sussex Drive, Ottawa, Ontario, Canada K1A 0R6 Received 29 September 2004; published 15 March 2005 Chlorine termination of low-doped, n-type Si111 is found to lead to an increase in conductance relative to the hydrogen-terminated surface. This increase is attributed to formation of an inversion layer due to the strongly electron withdrawing character of the chemisorbed chlorine. The presence of this inversion layer is confirmed by high resolution electron energy loss spectroscopy and Hall effect measurements. Electron beam irradiation destroys the inversion layer, suggesting a route to nanoscale patterning of this 2D hole gas. DOI: 10.1103/PhysRevB.71.125308 PACS numbers: 68.43.-h, 68.47.Fg, 73.25.+i, 79.20.Uv Chemical modification can strongly modulate the conduc- tivity of semiconductor surfaces by inducing surface states of acceptor or donor character, resulting in band bending. Al- though there are many reports of adsorbate-induced band bending in the literature, 1–5 the exploitation of this effect to tailor the conductivity of surfaces has remained largely un- explored. Previous surface conductivity studies on semicon- ductors have focused on clean and adsorbate-covered sur- faces prepared under ultrahigh vacuum UHV conditions, facilitating observation of conduction through surface state bands. 6–10 Recently there has been considerable progress in the development of wet chemical methods for controllably modifying silicon surfaces in ways that cannot be achieved through UHV approaches. 11,12 The starting point for these modifications is usually the atomically flat H-terminated Si111 surface produced by a wet etching procedure and exhibiting remarkably low surface state densities. 13–15 Re- placing hydrogen with functional groups of electron donating or withdrawing character offers the possibility of systemati- cally altering band bending and hence surface conductivities. In this paper, we show that photoinduced UV chlorina- tion of H-Si111 surfaces is found to significantly enhance the surface conductance. This somewhat surprising result can be explained in terms of the formation of an inversion layer resulting from the strongly electron withdrawing nature of the chemisorbed chlorine. The presence of the minority car- rier channel is confirmed via Hall effect measurements. Fur- thermore, in situ conductivity measurements are used to ob- serve a spontaneous room temperature dark reaction of molecular chlorine with the H-terminated surface, demon- strating the utility of adsorbate-induced conductivity changes in monitoring adsorption events and reactions on semicon- ductor surfaces. Samples used in these studies were cut from nphosphorous-doped silicon wafers Virginia Semiconduc- tor with resistivities of 10– 1000  cm, corresponding to doping densities in the range of 10 13 –10 14 cm -3 . Atomically flat, hydrogen-terminated Si111 surfaces were prepared by chemical cleaning followed by etching in 40% NH 4 F as pre- viously described. 14,15 Initially, chlorine-terminated surfaces Cl/Si111 were produced by exposing the H-terminated surface H/Si111 to 20 torr of molecular chlorine 2% Cl 2 in Ar in a quartz Schlenk tube under UV irradiation in a photoreactor for 5 min. 16,17 Ultrahigh vacuum UHV scanning tunneling microscopy STM images of freshly chlorinated surfaces shown in the inset of Fig. 1 indicate that they are atomically flat with an average terrace width of 50 nm, reflecting the morphology of the initial H/Si111 surface. However, an increased den- sity of etch pits is noted, indicating that the chlorination step has caused additional etching. Atomic resolution images clearly show the expected 1  1 hexagonal structure. Auger electron spectroscopy shows substantial Cl and Si signals with no detectable oxygen or carbon peaks. From the mea- sured Cl/ Si signal ratio the Cl coverage is estimated at 0.9± 0.1 ML. High-resolution electron energy loss spectros- copy HREELS, discussed in more detail below, indicates the chlorination reaction has gone to completion, with evi- dence of a small amount of oxidation and hydrocarbon con- tamination of the surface. Surface conductivity of these Cl/ Si111 surfaces was measured via ex situ four-probe measurements in the van der Pauw geometry. Several methods of making electrical con- tact to the sample were employed including direct contact of tungsten probes as well as contact pads of InGa eutectic and FIG. 1. Sheet conductance versus time in ambient for a chlorine- terminated surface prepared by UV irradiation of H / Si111 in the photoreactor for 5 min. The solid line is a fit to an exponential decay with a time constant of 17 min. The inset shows STM images of Cl/Si111: an atomic resolution image 2.5  2.5 nm, -2.1 V, 180 pA and a wider area scan 100  100 nm, +2.9 V, 50 pA. PHYSICAL REVIEW B 71, 125308 2005 1098-0121/2005/7112/1253084/$23.00 ©2005 The American Physical Society 125308-1