Correlation Between Surface/Interface States and the Performance of MIS Structures
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Correlation Between Surface/Interface States and the Performance of MIS Structures Hugo M. B. Águas, Elvira M.C. Fortunato, Ana M. Cabrita, Vitor Silva, Pedro M.N. Tonello and Rodrigo F.P. Martins Departamento de Ciência dos Materiais / CENIMAT, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa and Cemop-Uninova, 2825-114 Caparica, Portugal. ABSTRACT In order to understand the kinetics of formation of interface/surface states and its correlation on the final device performance, a preliminary study was performed on MIS structures, before and after surface oxidation/passivation, using different oxidation techniques and oxides: thermal (in air), chemical (in H2O2) and oxygen plasma. The devices used in this work are based on a glass/Cr/a-Si:H(n+)/a-Si:H(i)/SiOx/Pd structures, where the amorphous silicon intrinsic layer (i a-Si:H) with a photosensitivity of 107 was deposited by a modified plasma enhanced chemical vapour deposition (PECVD) triode system. The electrical properties of a-Si:H MIS structures were investigated by measuring their diode current-voltage characteristics in the dark and under illumination as well as the spectral response, as a function of the various oxidation techniques. Infrared spectroscopy and spectroscopic ellipsometry were used as a complementary tool to characterise the oxidised surface. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) has been used in a wide variety of optoelectronic applications such as solar cells, image sensors, position sensors, gas sensors and thin film transistors [1]. In all of these applications, surface and interface states play an important role in determining the final performances of the devices. In the particular case of a metal-insulatorsemiconductor (MIS) structure, the incorporation of a thin oxide layer can drastically modify the electrical properties of such interfaces[2]. The decomposition of silane using the Plasma Enhanced Chemical Vapour Deposition (PECVD) technique has been the most successful process for the growth of high quality a-Si:H to be used in device applications. A modified triode PECVD reactor configuration that allows to control the energy of the ion bombardment during the film’s growth [3] has been used to grow highly intrinsic a-Si:H films with excellent photosensitivity (S ≈ 107) and with photoconductivity σph > 10-6 (Ωcm)-1, making the MIS devices an ideal application for study the interface/surface states of these films. The thin oxide layer between the a-Si:H and the Pd plays also a main role in the device performance by providing the passivation of the a-Si:H surface defects and by providing a barrier between the metal and the semiconductor [4].
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EXPERIMENTAL DETAILS The MIS structure [Cr/a-Si:H(n+)/a-Si:H(i)/SiO2/Pd] was built on AF 40, alkaline free glass substrate. A Cr layer of 0.1 µm was initially grown on the glass substrate by thermal evaporation using an electron gun. An a-Si:H n+ doped layer of 400 Å thickness and dark conductivity σd = 6×10-3 (Ωcm)-1 was deposited in a conventional PECVD reac
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