Electrical Stabilization of Diamond Mis Interface and Misfets by Ultrahigh-Vacuum Process
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Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560, Japan ABSTRACT In order to obtain electrically stabilized MIS interface, a diamond metal-insulatorsemiconductor field-effect transistor (MISFET) was prepared by means of reduced-oxygen process including ultrahigh-vacuum (UHV) process, and its electrical properties were closely investigated. According to the results, observed effective mobility (eff ) was 400 cm2/Vs at room temperature, which is the highest value obtained until now in the diamond FET at room temperature. The transconductance (gin) and surface state density (Nss) of the device operation region was 5mS/mm and - 1010/cm2 eV, respectively, which is also comparable with conventional Si MOSFETs with the same gate length (Lg = 30vm). INTRODUCTION Owing to its good electrical properties such as low dielectric constant, wide band gap, and large break down field, diamond has recently attracted a lot of attention as a promising candidate for high temperature and high power device [1,21. For realization of the diamond electronic device, however, the electrical stabilization of the diamond surface is indispensable. Therefore, we have been studying about electrical stabilization of diamond surface by investigating diamond MIS interface. Our results revealed that oxygen adsorption on the diamond surface during the device process induces a number of surface states, which degraded the electrical properties of MIS interface to a great extent [3]. Our results also indicated that the surface state density depends on the device processing condition (in other words, its density can be decreased by reduction of oxygen contamination on the diamond surface and vice versa). In the present work, in order to exclude the oxygen contamination on the diamond surface as much as possible during the device process, we fabricated the diamond MISFET by reduced-oxygen process including ultrahigh-vacuum (UHV) process (in our case, vacuum level is - 10-9 Torr), and its electrical properties were investigated closely. EXPERIMENTAL
For a sample # FET-UHV-02, reduced-oxygen process including UHV annealing (--I0- Torr during annealing) was introduced in order to protect the diamond surface from
the oxygen contamination during the process. The fabrication condition is as follows. Diamond film was grown on high-pressure-synthesized (100) diamond substrates using the electron-cyclotron-resonance(ECR) microwave plasma-assisted chemical vapor deposition (CVD) method using a mixture of H2 (220 sccm) and CO (12 sccm) at 8000C. Film thickness was about I gim. In this sample, hydrogenated surface conductive layer [4,5] was employed 217
Mat. Res. Soc. Symp. Proc. Vol. 512 01998 Materials Research Society
Table I. MIS structure preparation conditions. Sample #
Structure
Thermal etching of hydrogenated diamond surface conductive layer (0.3 Torr 02 ambient, 500SC)
Employed surface conduCtivelayer
Annealing condition for ohmic contact
Gate insulator deposition condition
FET-CF-22
CaF2/i-diamond
Performed
Fluorinated surface conductive
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