Frédéric Lanois

Influence of the Applied Cooling Rate on the Type Conversion of Platinum Diffused N-Type Silicon

In this work we show that the temperature and the applied cooling rate during a platinum diffusion process strongly influence the electrical conductivity in weakly phosphorus doped silicon. Diffusions were done in the range of 800–950°C for several hours. Spreading resistance profile analyses clearly show an n-type to p-type conversion under the surface when samples are slowly cooled (5°C/min). Only a compensation of the phosphorus donors is observed when samples are quenched. This mechanism is reversible. Five Pt related deep levels, including the acceptor level at 0.23 eV from the conduction band, are observed. One of them is located at 0.427 eV from the valence band and can be related to the acceptor state at the origin of the type conversion mechanism. This acceptor state can be associated with a complex defect based on the association of a substitutional Pt atom with interstitial atoms (Pt, O) or intrinsic point defects.

Kinetic Reaction of the Formation of the Platinum Related Complex at the Origin of the p-Type Doping Effect in Silicon

Platinum is widely used in power devices manufacturing to control minority carrier lifetime. In n-type silicon, when platinum is introduced at concentrat ion similar to that of phosphorus, we observed a p-type doping effect, which can lead to a type inversion. This doping effect can be induced during the cooling stage of any thermal pr ocess and depends on the applied cooling rate. The lower the cooling rates, the higher the p-type doping. The acceptor state at the origin of this doping effect, located at 0.42 eV from the valence band, is attributed to a Pt s related complex. A simple calculus of the equilibrium solubility of Pt i, I, V, C and Oi in silicon makes them suitable for this complex formation with platinum. To decide between the se species, we propose a kinetic study of the reaction Pt s-X ↔ Pts + X involved during the cooling stage of a platinum diffusion in silicon. The diffusion calculation is integrated by a fi nite difference method by using the numerical solver PROMIS. The Pt-X concentration profiles are adjusted accordingly to the Spreading Resistance Profiles measured on n-type Fz and Cz sam ple , Pt diffused at 950°C for 8 to 168 hours and then cooled at different cooling rates in the range of [1-14] °C/min. Simulation results evidence the oxygen as the best candidate to react with platinum, lea ding to a Pt-O complex formation. Introduction Platinum introduces deep energy levels into the silicon band gap that can be used in the technology of power semiconductor devices to control carrier lifetime. The ide ntification of substitutional platinum related deep energy levels is well established [1 -10]. Platinum in substitutional position introduces an acceptor level at 0.23 eV from the conduction band and a donor leve l at 0.32 eV from the valence band [1]. Beyond these well-known substitutional platinum rela ted deep levels, other levels have been also reported such as the mid-gap level [2-5,9,10] at 0.52 eV from the conduction band that could be attributed to a complex formation with oxygen [2]. Re lated complexes with hydrogen, instable above 600K, have been also reported by Pearton et al [3] and Sachse et al [11]. Other additional acceptor states, responsible of a n-type into p-ty pe conversion in silicon, have been also reported at EV+0.36 eV [4] and EV+0.42 eV [5-7]. This type conversion was observed when the platinum is introduced in high concentration [4]. This is in agreeme nt with Kwon’s results [2] who observed n-type to p-type conversion when platinum was diffused at temperatures higher than 1000°C. This inversion has been also reported by Hayes et al [8] and by us more recently [ 12]. We have shown in a previous work [13] that the formation of an acceptor s tate located at EV+0.43 eV strongly depends on platinum diffusion conditions, and more particularly on the applie d cooling rate. This acceptor state is due to the formation of a compl ex Pt-X involving a platinum atom in substitutional site and a still undetermined species. We have seen that the Pt-X concentration increases when lowering the cooling rate. Usually, sampl es are quenched in order to reduce any precipitation mechanisms. This can explain that the acc eptor state is not observed when platinum is diffused at low temperature mostly followed by a quenching step. Corresponding authors: wilfried.vervisch@st.com ; ventura@univ-tours.fr Solid State Phenomena Online: 2003-09-30 ISSN: 1662-9779, Vols. 95-96, pp 361-366 doi:10.4028/www.scientific.net/SSP.95-96.361