Influence of post-hydrogenation upon electrical, optical and structural properties of hydrogen-less sputter-deposited amorphous silicon

Abstract

a b s t r a c t a r t i c l e i n f oAmorphous silicon (a Si) is common in the production of technical devices and can be deposited by several techniques. In this study intrinsic and doped, hydrogen less amorphous silicon films are RF magnetron sputter deposited and post hydrogenated in a remote hydrogen plasma reactor at a temperature of 370°C. Secondary ion mass spectrometry of a boron doped (p) a Si layer shows that the concentration of dopants in the sputtered layer becomes the same as present in the sputter target. Improved surface passivation of phosphorous doped 5 Ω cm, FZ, (n) c Si can be achieved by post hydrogenation yielding a minority carrier lifetime of~360 μs finding an optimum for~40 nm thin films, deposited at 325°C. This relatively low minority carrier lifetime indicates high disorder of the hydrogen less sputter deposited amorphous network. Post hydrogenation leads to a decrease of the number of localized states within the band gap. Optical band gaps (Taucs gab as well as E 04 ) can be determined to~1.88 eV after post hydrogenation. High resolution transmission electron microscopy and optical Raman investigations show that the sputtered layers are amorphous and stay like this during post hydrogenation. As a consequence of the missing hydrogen during deposition, sputtered a Si forms a rough surface compared to CVD a Si. Atomic force microscopy points out that the roughness decreases by up to 25% during post hydrogenation. Nuclear resonant reaction analysis permits the investigation of hydrogen depth profiles and allows determining the diffusion coefficients of several post hydrogenated samples from of a model developed within this work. A dependency of diffusion coefficients on the duration of post hydrogenation indicates trapping diffusion as the main diffusion mechanism. Additional Fourier transform infrared spectroscopy measurements show that hardly any interstitial hydrogen exists in the post hydrogenated a Si layers. The results of this study open the way for further hydrogen diffusion experiments which require an initially unhydrogenated drain layer.

Model based prediction of the trap limited diffusion of hydrogen in post‐hydrogenated amorphous silicon

“…In order to compare the hydrogenation process of different a‐Si layer thicknesses (e.g. in related studies ), the duration ( t t ) of the post‐hydrogenation treatment is normalized to the layer thickness.…”

“…Several publications have discussed hydrogen depth profiles within a‐Si:H layers as measured by NRRA . NRR‐analyses in this study were carried out using the Dynamitron tandem accelerator located at the central unit for ion beams and radioisotopes at the University of Bochum.…”

“…In an a‐Si:H layer most of the hydrogen is trapped in Si–H x bonds and hardly any interstitial hydrogen exists . This is important because it denotes that an increase of the overall hydrogen concentration is directly correlated to an increase in the quantity of Si–H x bonds.…”

“…The hydrogen concentration gradient between an a‐Si:H layer and a crystalline silicon (c‐Si) carrier substrate would not allow to investigate the migration of hydrogen, as the hydrogen concentration within the c‐Si bulk would be below the quantification limit. Therefore, performing special hydrogen diffusion experiments for a better understanding of the hydrogen related influences on PV thin films requires a hydrogen drain layer . Such a hydrogen‐free a‐Si layer can be processed by radio frequency magnetron sputter deposition (RFSD) .…”

“…The RFSD technology uses a solid target consisting solely of the material intended to be deposited, making the deposition of hydrogen‐free a‐Si possible . To investigate the hydrogen diffusion in the a‐Si layer and its influence on structural, electrical and optical characteristics, the initially hydrogen‐free grown a‐Si is hydrogenated in a subsequent post‐hydrogenation step . This hydrogenation is carried out using a hydrogen remote plasma .…”