The Effect of Hydrogen Dilution on the Deposition of Sige Alloys and the Device Stability
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THE EFFECT OF HYDROGEN DILUTION ON THE DEPOSITION OF SIGE ALLOYS AND THE DEVICE STABILITY L. YANG, L. CHEN AND A. CATALANO Solarex Corporation, Thin Film Division 826 Newtown-Yardley Road, Newtown, PA 18940
Abstract We have studied the structural and transport properties of a-SilxGe x :H alloys made by DC glow discharge of SiH 4 and GeH 4 with substantial amount of H2 dilution. The dilution ratio, i.e. H2/(SiH 4 + GeH 4 ), was varied up to 40:1. IR spectroscopy revealed that H2 dilution greatly suppresses the tendency of forming microvoids due to Ge incorporation. Photoconductivity and ambipolar diffusion length, which represent transport properties of electrons and holes respectively, are also significantly improved by using H 2 dilution. At a dilution ratio of 20:1, the electron p-r product reached 10 cm /V at a bandgap 1.55 eV which is even larger than that of a device quality a-Si:H. Solar cells using alloy i-layers made with H2 dilution were fabricated. The stability of the devices under light exposure was found to be much improved with increasing H2 dilution, suggesting a strong correlation between the stability and the density of microvoids. Introduction Amorphous multijunction solar cells have clearly emerged as the design for the next generation of commercial solar modules because they exhibit much imqroved performance and stability under light exposure as compared to single junction devices. In order to utilize the solar spectrum efficiently, a low band gap semiconductor material is required to construct the back junction of such a device. a-SiGe alloys whose bandgap can be continuously varied from 1.1 eV to 1.75 eV have been considered the best candidates for this application. However, the photoelectric properties of the alloys have been found to deteriorate rapidly with increasing Ge content, or decreasing bandgay, hampering the performance of the devices. Despite extensive research effort in this area, the understanding of the alloy growth precess under various deposition conditions and its correlation with the structural and transport properties of the alloys is still very poor. Testing new alloy materials directly in devices is even more rarely done due to many practical difficulties, making the relationship between the material properties, such as the /Jr products for electrons and holes, and the device performance a big mystery. As a result, much of the device research remains quite empirical. In this paper, we systematically investigate the effect of H-2 dilution on the deposition of a-SiGe:H alloys. While many researchers have reported that the use of I-2 dilution improves the alloy properties, the dilution ratio used is normally less than 10:1. 3-7 We have extended the range of dilution to as high as 40:1 and found further improvements in structural as well as transport properties of the alloys. Solar cells consisting of a-SiGe:H alloy i-layers made with various H2 dilution were also fabricated. The stability of the devices was found to be greatly improved with increasing H2 dilution. A strong
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