Ultrathin oxynitrides have several advantages over conventional oxides of silicon. In this paper we report the results of ultrathin (2.7 nm) oxynitride films grown on n-Si by N 2 O plasma-assisted oxidation at 200 • C. The oxynitride is characterized by fabricating Al/thin oxynitride/n-Si tunnel diodes and measuring the capacitance-voltage (C-V ) and conductance-voltage (G-V ) characteristics in the frequency range 10 kHz to 1 MHz. These tunnel diodes with 'as-grown' oxide have shown a frequency dependence in C-V characteristics, indicating a high (>10 12 cm −2 eV −1 ) interface state density (D it ). The effect of series resistance on the accumulation capacitance of the tunnel diodes is also studied. The density of interface states (D it ) is estimated from the G-V characteristics at 10 kHz. Annealing of these oxynitrides (called post-oxidation annealing (POA)) is found to have a profound effect on the interface state density (D it ) and fixed oxide charge density (Q f ). The POA is carried out in the temperature range 300-650 • C in ambient nitrogen for 20 min. The tunnel diodes with POA oxide showed little frequency dependence in the C-V characteristics; this observation is attributed to the reduction in the interface state density (∼5 × 10 11 cm −2 eV −1 ) due to POA. The fixed oxide charge (Q f ) has been evaluated from the C-V characteristics. It is observed that Q f has a minimum value (∼2.2 × 10 12 cm −2 ) when the oxynitride is annealed at 350 • C.
The growth of ultrathin oxides of silicon (<5 nm) under low thermal budgets is extremely important in the context of ultralarge scale integration. One of the important factors that influence the growth and quality of the ultrathin oxide is the surface preparation of silicon prior to oxidation. In the present paper, the surface of p-type silicon (with (100) orientation) is prepared by three methods: (i) normal cleaning, (ii) chemical polishing and (iii) sacrificial oxide growth. The ultrathin oxide (in the range 2.0 to 2.5 nm) is grown by a low temperature (600 • C), wet oxidation (0.3 atm of water vapour pressure) technique. These ultrathin oxides are characterized for their electrical properties by fabricating aluminium-thin SiO 2 -Si tunnel diodes employing conventional current-voltage (I -V ) and capacitance-voltage (C-V ) techniques. The results indicate that the ultrathin oxides grown on the samples prepared by the 'sacrificial oxide growth' method give better oxide quality and uniformity; these results may be supported from the information in the literature that the sacrificial oxide reduces the roughness of the silicon surface.
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