Effects of Annealing on Chemical-Vapor Deposited PureB Layers

Abstract

Chemical-vapor-deposited pure boron (PureB) layers can be used as a source of p-type boron dopants for thermal diffusion into silicon during a drive-in anneal. In this work, the effect of thermally annealing PureB layers is investigated in terms of surface morphology and electrical properties. The presence of a few nanometer-thick PureB layer on the Si surface was found to increase the silicon oxide growth rate by several factors during annealing in an oxygen-containing atmosphere. The oxide thickness was dependent on the initial PureB layer thickness and oxygen concentration during anneal. In an oxygen-limited atmosphere, the final thickness is insensitive to the anneal temperature as the reaction is diffusion-limited and, after oxide removal, a hydrophilic boron rich layer remains on the Si surface. With a high oxygen concentration, the boron is depleted by an oxidation of the boron-doped silicon resulting in a lower surface concentration and higher sheet resistance. A reaction mechanism involving the oxidation of Si-B compounds to form B 2 O 3 is proposed to explain the experimental observations. With solar cell and other photodiode applications in mind, the sheet resistance and carrier lifetime measurements were performed and show that a one-step oxidation process can simultaneously drive-in the dopants and form an oxide passivation layer. The chemical vapor deposition (CVD) of pure boron (PureB) layers has in recent years been successfully applied for fabricating extremely shallow, less than 10-nm deep, silicon p + n junction diodes. 1This has provided impressive performance of bulk-Si photodiodes for detecting low penetration-depth beams. 2,3 Ideal low-leakage diode characteristics are achieved for deposition temperatures from 400 to 700• C. 4 In addition, the 700 • C PureB deposition can be deposited with well-controlled uniformity and reproducibility down to a thickness of 2 nm in a conformal manner with high selectivity to silicon rather than silicon oxide, 5 making this technology attractive for advanced device architectures.From studies of the quantum efficiency of VUV photodiodes 3 fabricated with PureB anode regions subjected to various drive-in procedures it has been found that high efficiencies are maintained despite the increase in junction depth from a few nanometers to the hundred-nanometer range. This is accorded to the defect-free nature of the doping process and the lack of roll-off at the surface of the resulting doping profiles. These features are also attractive for the manufacture of p + emitter layers for solar cells and for the passivation of depleted interfaces in other types of photodiode structures. A batch furnace system for the CVD of PureB is currently being developed for high-throughput industrial applications. 6 For application in standard n-type crystalline silicon solar cells, substantial drive-in is required to achieve the dopant profiles and sheet resistance necessary for a low series resistance through the p-type surface layers and associated contacts. More critically, a w...

Self-aligned two-layer metallization with low series resistance for litho-less contacting of large-area photodiodes

“…This could be related to the larger Si surface area of the textured anode, differences in substrate doping, and narrowing of the depletion width at the valley points of the V-grooves. The fact that the cavity etching did not deteriorate the quality of the diode is in agreement with an earlier publication that reported an isotropic deposition of boron on different Si surface orientations [19].…”

“…The native oxide was first removed from the silicon surface by HF dip-etching and Marangoni drying before deposition. The layer thickness was about 2-3 nm and was found to be largely independent of the orientation of the Si surface [19].…”

Boron-Doped Silicon Surfaces From B$_{\bf 2}$H $_{\bf 6}$ Passivated by ALD Al$_{\bf 2}$O$_{\bf 3}$ for Solar Cells

“…3 and 4, respectively. Note that on planar Si (1 1 1) surfaces, the results are very similar [23]. During diffusion in N 2 ambient, it should be noted that the furnace is not completely O 2 -free, although the gas flow into the furnace is N 2 only.…”

“…This was attributed to the defect-free nature of the doping process and the lack of roll-off of the resulting dopant profiles. These two features have also been shown in our previous work to be potentially attractive for manufacturing p + -doped regions in Si solar cells [23], [24].…”

“…In this study, with solar cell application as objective, a further development is presented of a recently demonstrated p + doping method by chemical vapor deposition (CVD) of boron layers (PureB) using B 2 H 6 precursors [23], [24]. Up until now, the application of PureB deposition has primarily been for the fabrication of photodiodes.…”