Photosensitive Metal–Insulator–Semiconductor Devices with Stepped Insulating Layer

Abstract

A preparation procedure based on localized electrochemical oxidation unites multiple metal-insulator-semiconductor ͑MIS͒ junctions ͑also arrays͒ in a single device. The "stepped MIS" enables a comparative study of several MIS junctions of different oxide thicknesses on one silicon wafer. We present a Si-SiO x -Au four-step device with oxide thicknesses of 0, 1, 2.5, and 4 nm. The samples are characterized by internal photoemission using variable wavelengths ͑300-1100 nm͒. The "1 nm" junction shows an increased photosensitivity compared to the "0 nm" junction ͑metal-semiconductor system͒. The internal photoemission drops by 2 orders of magnitude when increasing the oxide thickness from 1 to 4 nm.Soon after the beginning of the studies on metal-semiconductor ͑MS͒ heterosystems, questions regarding the role of an interjacent oxide layer in the conduction properties of the device came up. [1][2][3][4][5][6][7] Provided that the oxide layer is characterized by a large optical bandgap, the electrical conduction can be modeled by quantum mechanical tunneling processes. 2,3 However, metal-insulatorsemiconductor ͑MIS͒ devices with an ultrathin oxide layer ͑d Ͻ 5 nm͒ are especially influenced by interface states at the semiconductor-oxide interface 8-12 and also midgap states, e.g., oxygen vacancies, which may dramatically change the electrical properties 13 of the devices. Usually, in such devices, the threshold energy ͑below which no photoemission can be observed͒ is quite high, e.g., 3-4 eV, 14-18 but it may be reduced by the mentioned interface states.In this work, we present the experimental approach we employed with the intention to improve the photo-and chemosensitivity 19,20 of MIS devices rather than to optimize their barrier properties. In particular, we studied the influence of the oxide thickness on the conduction properties of a Au-SiO x -Si system by preparing an oxide layer of variable thickness on one wafer with a localized electrochemical oxidation procedure ͑see Fig. 1͒.As a measure of the photosensitivity we use the internal photoemission yield ͑or simply photoyield͒. It is defined as the net number of electrons detected in the Si electrode per number of photons incident on the Au surface. The individual junctions of the stepped MIS device were characterized by measuring the energy as well as the bias dependence of the photoyield. Using a localized electrochemical oxidation technique, one can increase the photosensitivity of the Si-SiO x -Au device with respect to the same device with 0 nm oxide thickness ͑Si-Au device͒.The four-step Au-SiO x -Si structure sketched in Fig. 1 was fabricated on a 20 ϫ 10 mm large piece of an n-type Si͑111͒ wafer ͑7.5 ⍀ cm͒. The silicon surface was cleaned with isopropanol, HF, and Milli-Q water and oxidized in an electrolytic droplet cell 21-23 using an ammonium acetate buffer electrolyte to minimize parallel corrosion processes during the oxidation. 24 Different areas ͑typically 0.04 cm 2 ͒ of the substrate were oxidized with the droplet cell. The desired oxide thicknesse...

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