Photoelectronic and optical properties of B doped DC sputtered hydrogenated silicon nitride compounds

Abstract

We report on the optical and electronic properties of boron doped DC sputtered a-Si x N 1 − x : H films prepared in a wide range of dopant, hydrogen and nitrogen concentrations. It is shown that we can prepare intrinsic and photoconductive p-type films by varying the partial pressure of diborane in an Ar + N 2 + H 2 + B 2 H 6 gas mixture whose total pressure is P 1 . Light doping of wide gap material ( E g ~ 2.25 eV) prepared in the following conditions P t = 25 mT, P H 2 /Pt = 0.1, P N 2 / P t = 0.013, 0 ≤ r = P B 2 H 6 / P t ≤ 8 × 10 −4 was first studied. For r up to 2 × 10 −5 , it is seen that boron doping produces a decrease in dark conductivity σ d and an increase of activation energy E a towards the intrinsic value E g /2. Higher diborane pressures (2 × 10 −5 < r < 8 × 10 −4 ) lead to a continuous conductivity increase of several orders of magnitude and a decrease in the activation energy indicating that efficient p-doping is achieved. For r varying between 0 and 8 × 10 −4 parallel variations of σ d and normalized photoconductivity ημτ are observed. The most simple explanation is that we measure p type photoconductivity when dark conductivity is p-type. If, due to doping, the Fermi level position is varied from 0.2 eV above the middle E i of the gap to 0.2 eV below E i , the corresponding majority carriers mobilities and lifetimes are such that μ n ~ μ p and τ n ~ 10 τ p . These results are consistent with the existence of a distribution of defects having the following characteristics: center of the D° distribution at 1.2–1.4 eV below E c , width of the distribution 0.4 eV, positive correlation energy 0.1 eV < U < 0.6 eV. Concerning the variation of E g in the same r ranges we first observe a small increase of E g (from 2.24 to 2.28 eV) and then a decrease towards 2.15 eV whereas nitrogen content in the films remains unchanged. This is accompanied by the passage through a minimum for the Urbach energy deduced from absorption curves. These two effects may be related to a relaxation of the Si matrix upon low doping. A series of p-doped films was then grown using the following conditions: P Ar = 25, 30, 35 mT, P B 2 H 6 = 0.02 mT, P H 2 varying from 0.5 to 2 mT and P N 2 from 0.125 to 0.35 mT. Systematic study of σ d , E a , ημτ , σ ph (AM-1), E g has been performed. For every film two dark conduction regimes are observed:extended-state conduction at high temperatures and hopping conduction in the valence band tail at low temperatures. From the change in activation energy of σ d and the activation energy of ημτ , the valence band tail width E B - E V and the hopping energy W are deduced. As far as an application such as the window side junction of a p.i.n solar cell is concerned the best results correspond to P Ar = 25 mT, P H 2 = 1.75 mT, P N 2 = 0.125 mT. One then has E g ~ 1.85 eV, E a ~ 0.5 eV, σ d ~ σ ph (AM-1) ~ 10 −7 ( Ω cm) −1 , ημτ ~ 10 −7 cm 2 V −1 . Finally IR spectroscopy reveals that the integrated absorption coefficient over the 840 cm −1 Si-N band, I , first increases linearly with N content giving an oscillator strength factor of 7.5 × 10 18 cm −2 and then becomes saturated whereas the N-H bands begin to grow. Furthermore it is shown that I is systematically lower for B-doped films than for undoped ones and that the B-N band grows with P B 2 H 6 . These results on B-doped films confirm that in addition to threefold bonded boron leading to a relaxation of the Si matrix, boron is largely incorporated at silicon sites surrounded with four silicon atoms and also at the other sites where B-N bonds are created.