Hydrogen equilibration in polycrystalline silicon

Abstract

Hydrogen equilibration in polycrystalline silicon was investigated as a function of annealing time and temperature using electron spin resonance and hydrogen effusion measurements. During a vacuum anneal at least 1.5ϫ 10 21 cm −3 H atoms are mobile in the lattice, however, only about 3.7ϫ 10 18 cm −3 H atoms passivate Si dangling bonds. The results show that the annealing treatment can cause the vast majority of H atoms to accumulate in H stabilized platelets. Since defect passivation preferentially occurs at grain boundaries and platelet nucleation and growth are confined to the interior of single-crystal grains, H equilibration is governed by two spatially separated processes. Moreover, the data demonstrate that the hydrogen density-of-states distribution is dynamic and changes in response to experimental parameters. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2716348͔The properties of hydrogen in silicon have attracted a great deal of interest. 1 This manifests itself in the neutralization of dopant atoms and the passivation of localized defects such as Si dangling bonds. The latter attribute is commonly utilized to improve the electrical and optical properties of polycrystalline silicon ͑poly-Si͒, which contains a high concentration of Si dangling bonds at grain boundaries. 2 On the other hand, prolonged exposure of poly-Si to monatomic H can cause deleterious effects such as the formation of electrical and structural defects. 3,4 In poly-Si identical crystallites are separated by grain boundaries. These two-dimensional extended defects contain deep and shallow localized states. Posthydrogenation treatments at moderate temperatures ͑e.g., 250-450°C͒ reduce the concentration of both classes of localized states and the kinetics of the grain-boundary passivation are consistent with diffusion limited passivation. 2 In addition to the passivation of localized states it has been shown that prolonged exposure of poly-Si causes deleterious effects such as the formation of acceptor-like states that leads to electrical-type conversion 3 and the formation of hydrogen stabilized platelets. 4 The latter effect is controlled by the Fermi energy. 5 In this letter, we present experimental evidence that merely the migration of hydrogen in poly-Si is sufficient to simultaneously cause beneficial and deleterious effects. A simple vacuum anneal results in a decrease of the Si dangling-bond concentration and the formation of large platelet-like structures. While the passivation occurs preferentially at grain boundaries the formation of platelets is confined to the interior of the grains. Hydrogen equilibration involves about 30% of all incorporated H atoms.Polycrystalline silicon thin films were prepared by the following procedure. First, hydrogenated amorphous silicon thin films were grown by rf glow discharge decomposition of silane ͑SiH 4 ͒ at substrate temperatures ranging from 160 to 335°C. Then the specimens were crystallized at room temperature and in high vacuum using the 308 nm line of an excimer laser a...

Revealing the factors influencing grain boundary segregation of P, As in Si: Insights from first-principles

“…Another alternative to reduce the P and As agglomeration at Si GBs is to eliminate the deep states in the bandgap of density of states, which can be enforced with hydrogen passivation. As reported, hydrogenation is an efficient routine to reduce the density of dangling bonds in mc-Si based materials by orders of magnitude [73,[77][78][79][80], which is a major cause of the deep levels in the bandgap.…”

“…Using the electron spin resonance (ESR) technique, the presence of dangling bond in Si GBs has been identified several decades before [33,71], and the density of dangling bond in GBs of mc-Si films is estimated to be around 10 -12 cm -2 [33,72,73]. As will be demonstrated in the following part, atomic sites with dangling bonds are strongly attractive for the segregation of P and As impurities.…”

“…With increasing passivation time, hydrogenation can decrease the dangling bond density in mc-Si till a residual defect concentration level [73,80]. Nevertheless, deep levels ascribed to the extra bonds in the GB cannot be simply passivated with hydrogenation.…”

Metal-catalyzed electroless etching and nanoimprinting silicon nanowire-based solar cells: Silicon nanowire defect reduction and efficiency enhancement by two-step H2 annealing

“…The explanation of this behavior is possibly a decrease in carrier collection efficiency beneath the junction caused by thermal process at higher temperature leading to significant degradation of the bulk carrier lifetime. [40][41][42] H 2 annealed samples had a trade-off between improvement and degradation of EQE in the short-and long-wavelength range, comparison with non-annealed sample, respectively. The optimal condition of the first annealing was therefore 900 °C.…”

Diffused back surface field formation in combination with two-step H2annealing for improvement of silicon nanowire-based solar cell efficiency

“…Therefore, the optimized process for BSF formation of MCEE-SiNW solar cells was with 20-min H 2 annealing. Figure 8 [34][35][36] located inside the p-Si layers. Defect densities can be calculated by double numerical integration of ESR intensity and comparison to a known reference standard.…”