H Evolution from Nano-Crystalline Silicon-Comparison of Simulation and Experiment
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H EVOLUTION FROM NANO-CRYSTALLINE SILICON- COMPARISON OF SIMULATION AND EXPERIMENT R. BISWAS* AND B. C. PAN*§ *Department of Physics and Astronomy, Microelectronics Research Center and Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011 §Department of Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China ABSTRACT The temperature dependent H evolution from a-Si:H provides unique information on the Hbonding and microstructure. Traditional undiluted a-Si:H films show a high temperature Hevolution peak near 600 °C. However device-quality compact nanocrystalline silicon films grown near the phase boundary of amorphous and microcrystalline growth show a new low temperature H- evolution peak near 400 °C in addition to a second high temperature peak near 600 °C. The origin of this peak cannot be attributed to microvoids or a substantial density of dihydride species typical of porous low-temperature films. We have simulated the H evolution using a molecular dynamics generated model of nanocrystalline silicon, where nano-crystallites reside in a background amorphous matrix. An excess density of H occurs at the crystallite surface. We find a low temperature evolution peak at 250-400 °C, where the H-evolution starts from the surface of the nano-crystallite. In addition there is a higher temperature peak at 700800 °C providing good agreement with H-evolution measurements. The mobile H is found to exist in both the bond-centered type of species and H2 molecules – which has implications for H-diffusion models.
INTRODUCTION H evolution from a-Si:H, nanocrystalline Si and microcrystalline silicon provides key information on the H-bonding, microstructure and H diffusion. The samples are heated at a linear temperature rate b where T(t)=T0+ bt. The amount of hydrogen evolved as a function of temperature is measured together with the temperature Tm of the evolution peak. Assuming that the evolution of H from the surface is diffusion limited, Beyer and Wagner [1] have shown that the evolution temperature Tm is related to b. In diffusion theory[1,2] the H profile c(x) satisfies ∂c ∂ 2c = D(t ) 2 ∂x ∂t t 2 ∂c ∂ c ,θ (t ) = ∫ D(t ' )dt ' = ∂θ ∂x 2 o
(1)
π 2 πx c( x,θ ) = c0 exp − θ sin d d
(3)
E D(t ) = D0 exp − D k BT
(4)
d 2b D0 ED ln ln = − 2 2 k Bπ TM ED k BTM
(2)
(5)
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Hence by plotting the ln [d2b/Tm2] vs 1/Tm, the diffusion activation energy ED can be obtained. Here d is the thickness of the sample and D0 the diffusion prefactor. Traditional undoped glow discharge a-Si:H exhibits a high temperature evolution peak at 550600 °C where the SiH bonds are broken and H molecules evolve from the surface. However the recent device quality H-diluted a-Si:H films reveal an unusual low temperature peak at T~400 °C [Table 1] in addition to the conventional higher temperature evolution peak[3]. These H diluted films are grown very near the phase boundary of amorphous and microcrystalline growth. Chara
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