Double Layers of Ultrathin a-Si:H and SiN_x for Surface Passivation of n-type Crystalline Si Wafers

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• 作者：Xuemei Cheng^a ; ^{xuemei.cheng@ntnu.no" class="auth_mail" title="E-mail the corresponding author} ; Erik Stensrud Marstein^b ; Halvard Haug^b ; Marisa Di Sabatino^a
• 关键词：a-Si ; H/SiNx ; passivation ; C-V ; GDOES
• 刊名：Energy Procedia
• 出版年：2016
• 出版时间：August 2016
• 年：2016
• 卷：92
• 期：Complete
• 页码：347-352
• 全文大小：278 K

文摘

Surface passivation by double layers made of hydrogenated amorphous silicon (a-Si:H) and hydrogenated silicon nitride (SiN_x) on float zone n-type Si substrates was investigated. The thickness of the a-Si:H layers was varied from 0 to 4 nm and they were deposited at low temperature by plasma-enhanced chemical vapour deposition. The structure and composition of the double layers was determined by a combination of spectroscopic ellipsometry and glow discharge optical emission spectroscopy. The minority carrier effective lifetime and the uniformity of the surface passivation of the wafers were measured by calibrated photoluminescence imaging. The lifetime was observed to increase with increasing a-Si:H thickness. An excellent lifetime value of 7.3 ms was measured on the double layers with an a-Si:H layer thickness of 3.4 nm, corresponding to surface recombination velocity below 2 cm/s. Capacitance-voltage and conductance-voltage measurements were used to determine the densities of fixed charge and interface states at the interface between the Si substrate and the a-Si:H/SiN_x stacks. The fixed charge density of the initial SiN_x was positive, as expected. However, the overall fixed charge density of the double layers was observed to decrease when a layer of a-Si:H was inserted beneath the SiN_x, and eventually reaches negative values when the a-Si:H thickness exceeds 2 nm. The conductance-voltage measurements revealed that the interface state density (D_it) is significantly lowered when adding the ultrathin a-Si:H film between the substrate and the SiN_x layer, providing a significant improvement of the chemical passivation, thus reducing the overall surface recombination velocity.