JOURNAL OF MATERIALS SCIENCE 34 (1 9 9 9 ) 1051 – 1054 Improvement in the optoelectronic properties of a-SiO:H films DEBABRATA DAS ∗ , S. M. IFTIQUAR, DEBAJYOTI DAS, A. K. BARUA Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-700 032, India Hydrogenated amorphous silicon oxide (a-SiO:H) films prepared by rf plasma enhanced chemical vapour deposition (PECVD) method have recently proved their potential as a photovoltaic material for the fabrication of high efficiency multijunction amorphous silicon solar cells. If deposited under proper conditions, it may be a better wide band gap material than the normally used a-SiC : H. In this paper we report the improvements achieved over the previously reported results. The films have been characterized in detail in terms of their optoelectronic properties, structural characteristics, defect density and light induced degradation. C  1999 Kluwer Academic Publishers 1. Introduction It is now accepted that to achieve high efficiency a-Si solar cells it is necessary to fabricate multigap multi- junction structures. The top cell should have a high quality wide band gap active material. Mostly a-SiC:H has been used for this purpose. Wide band gap a-Si:H films have also been produced but high efficiency mul- tijunction cells have not yet been fabricated using this material. More recently a-SiO:H films have been used with reasonable success [1, 2]. Investigations on the structural properties of a-SiO:H films was done by Morimoto et al. [3]. They claimed that a-SiO:H is not inferior than a-SiC:H as wide op- tical gap material for application in a-Si solar cells. Suchaneck et al. [4] reported the presence of a clus- tered O-rich phase detected by IR absorption in a-SiO:H films having an oxygen content as low as 0.4 at % de- posited by magnetic field enhanced PECVD. Sichanu- grist et al. [5] deposited p-type and n-type amorphous silicon oxide films with a microcrystalline silicon phase by plasma CVD and found that it is easier to make these films microcrystallized than a-SiC:H films. Watanabe et al. [6] proposed a two-phase structure model for the a-SiO:H films, a silicon-rich phase and an oxygen- rich phase. They suggested that the oxygen rich phase is effective in increasing the optical gap of the films, while photogenerated carriers travel mostly through the silicon rich phase. Using p-type a-SiO:H as a window layer, Fujikake et al. [7] were able to increase the effi- ciency of multijunction a-Si solar cells. However, a-SiO:H material is not yet fully under- stood and there is scope for further improvement. It may be noted that unlike a-SiC:H, a-SiO:H is most probably a two-phase material. Clusters of a-SiO:H in a matrix of a-Si:H is a possible structure. The bonding nature of oxygen with silicon is a key factor in improving the quality of a-SiO:H film. ∗ Present address: Department of Physics, Ramakrishna Mission Residential College, Narendrapur-743508, India In a previous paper [8] we have reported the proper- ties of a-SiO:H films prepared by rf PECVD by varying various deposition parameters. By analysing the results so obtained we have tried to optimize further some of these parameters. In this paper we report the improve- ments in the quality of a-SiO:H films thus obtained. Films are characterized by dark and photoconductiv- ity measurements, optical absorption measurements in the ultraviolet and visible regions, Fourier-transform infrared (FTIR) spectroscopy, constant photocurrent measurements (CPM), electron spin resonance (ESR) measurements and light induced degradation study of photoconductivity. We have reached the conclusion that low concentra- tion of CO 2 , low chamber pressure and optimum hy- drogen dilution are conducive to the production of high quality a-SiO:H films. 2. Experiment The a-SiO:H films were deposited by radio frequency plasma enhanced chemical vapour deposition (RF- PECVD, 13.56 MHz). The anode was heated to main- tain substrate temperature at 200 ◦ C. A mixture of SiH 4 + CO 2 + H 2 was used as source gas. The gases were of high purity and supplied by Matheson, Inc., USA. The ratio of the CO 2 flow rate to SiH 4 flow rate (= r c ) was varied from 0.3 to 1.0 keeping the H 2 flow rate fixed at 85 sccm. The flow of gases was controlled by electronic mass flow controllers. The ratio of H 2 flow rate to SiH 4 flow rates (= r h ) was varied from 4.0 to 18.5 keeping the CO 2 flow rate fixed at 4.0 sccm. For all depositions, the SiH 4 flow rate was kept fixed at 10 sccm. The chamber pressure during deposition was varied between 0.16–0.35 Torr and a rf power density of 30 mW/cm 2 was used. 0022–2461 C  1999 Kluwer Academic Publishers 1051