Donor Behaviour of Implanted Hydrogen Ions in Silicon Wafers
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H7.3.1
Donor Behaviour of Implanted Hydrogen Ions in Silicon Wafers Damien Barakel and Santo Martinuzzi TECSEN- UMR 6122 - University of Aix-Marseilles III - 13397 Marseilles – France ABSTRACT P type Czochralski (Cz) and float-zone (FZ) grown silicon wafers were investigated with doping levels of 5x1014 and 2x1015 cm-3, respectively. Hydrogen ions are implanted at a dose of 2x1016 cm-2, at energies in the range 20 to 250 keV and are accumulated at depth Rp in the range 0.2 to 2.4 µm. After implantation the wafers are annealed between 350 and 600°C for 30 min, under argon flow. It is found that a graduated n-p junction is formed, and after metallization a photovoltaic device is obtained, which works like a solar cell. SIMS analysis shows that, around Rp, hydrogen concentration achieves 1021 cm-3. I-V and C-V curves confirm the formation of a N-type layer in which the donor concentration is about 5x1017 cm-3. When the samples are annealed at temperatures higher than 550°C the counterdoping vanishes. The observed behaviour of hydrogen is irrespective of oxygen concentrations in the wafers as it occurs in Cz (oxygen rich) like in FZ (oxygen poor) wafers. If the wafers are ion implanted with helium at the same dose and energy no junction appears. It is concluded that the agglomeration of hydrogen in silicon after ion implantation at a dose exceeding 1016 cm-2 gives rise to the formation of shallow donors. INTRODUCTION Hydrogen has long been recognized as having an important role in modifying the electrical properties of semiconductors. In p-type silicon hydrogen is in a positive charge state (H+), it is a fast diffuser and its motion can be influenced by internal or external electric fields [1]. In hydrogen ion implanted silicon, it should be emphasized that hydrogen can exist in two distinct states : one of these is strongly bonded to passivated dangling bonds, while the second, more mobile, could be weakly bonded interstitial atoms, accumulated below the free surface in concentration higher than the solubility limit, up to 1020 cm-3 [2]. Such interstitial hydrogen can behave as a donor [3-6], with 26 meV ionisation energy, in proton implanted silicon wafers at doses higher than 1016 cm-2 It was demonstrated that implanted hydrogen can convert p-type oxygen rich silicon in ntype by the enhanced formation of thermal donors after interaction with interstitial oxygen atoms [6]. It is also well known that implantation of H+ ions at doses higher than 1016 cm-2 leads to the formation of gas bubbles at the projected range Rp. These bubbles are transformed into nanocavities when the gas out-diffuses during annealing at temperature higher than 550°C. In the present paper, we confirm that H+ implantations at high doses create n-type regions close to the free surface and form n-p junctions in p-type silicon crystals which behave like solar cells.
H7.3.2
EXPERIMENTAL Monocrystalline silicon samples, float-zone grown (FZ) and Czochralski grown (Cz), experienced the implantations. They were P-type boron doped to 4x1013 cm-3 (FZ s
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