Ion-implanted Amorphous Silicon Research Articles

Interdigitated Back Passivated Contact (IBPC) Solar Cells Formed by Ion Implantation

We describe work toward an interdigitated back passivated contact (IBPC) solar cell formed by patterned ion-implanted passivated contacts. Formation of electron and hole passivated contacts to n-type Cz wafers using a thin SiO2 layer and ion-implanted amorphous silicon (a-Si) is described. P and B were ion implanted into intrinsic a-Si films, forming symmetric and IBPC test structures. The recombination parameter $J_{0}$ , as measured by a Sinton lifetime tester after thermal annealing, was $J_{0}$ ∼ 2.4 fA/cm2 for Si:P and $J_{0} \sim10$ fA/cm2 for Si:B contacts. The contact resistivity for the passivated contacts was found to be 0.46 Ω·cm2 for the n-type contact and 0.04 Ω·cm2 for the p-type contact. The IBPC solar cell test structure gave 1-sun $V_{{\rm oc}}$ values of 682 mV and ${\rm pFF} = 80\%$ . The benefits of the ion-implanted IBPC cell structure are discussed.

Temperature dependent deformation mechanisms in pure amorphous silicon

High temperature nanoindentation has been performed on pure ion-implanted amorphous silicon (unrelaxed a-Si) and structurally relaxed a-Si to investigate the temperature dependence of mechanical deformation, including pressure-induced phase transformations. Along with the indentation load-depth curves, ex situ measurements such as Raman micro-spectroscopy and cross-sectional transmission electron microscopy analysis on the residual indents reveal the mode of deformation under the indenter. While unrelaxed a-Si deforms entirely via plastic flow up to 200 °C, a clear transition in the mode of deformation is observed in relaxed a-Si with increasing temperature. Up to 100 °C, pressure-induced phase transformation and the observation of either crystalline (r8/bc8) end phases or pressure-induced a-Si occurs in relaxed a-Si. However, with further increase of temperature, plastic flow rather than phase transformation is the dominant mode of deformation. It is believed that the elevated temperature and pressure together induce bond softening and “defect” formation in structurally relaxed a-Si, leading to the inhibition of phase transformation due to pressure-releasing plastic flow under the indenter.

High-Resolution Radial Distribution Function of Pure Ion-Implanted Amorphous Silicon Measured Using Tilted-Illumination Selected-Area Electron Diffraction

High-resolution radial distribution functions of as-implanted and thermally relaxed amorphous silicon created by ion implantation were measured using tilted-illumination selected area electron diffraction at room temperature. The diffracted intensities were measured out to a maximum scattering vector 2 sin(θ)/λ of 3.3-3.7 Å-1. The volume-averaged pair-correlation statistics of as-implanted and relaxed ion-implanted amorphous silicon are virtually indistinguishable with coordination numbers of 3.7 ± 0.3 and 3.9 ± 0.3 (for neighbors closer than 3 Å) and average bond angles of 109 ± 0.5° and 110 ± 0.6°, respectively. The atomic rearrangements in ion-implanted amorphous silicon due to a low temperature anneal are subtle.

Correlation of indentation-induced phase transformations with the degree of relaxation of ion-implanted amorphous silicon

Abstract

Phase stability of silicon during indentation at elevated temperature: evidence for a direct transformation from metallic Si-II to diamond cubic Si-I

Abstract

Flowing damage in ion-implanted amorphous silicon

Using molecular-dynamics simulations, we have studied the creation and evolution of damage in crystalline and amorphous silicon following the implantation of energetic keV ions. A method is proposed to identify anomalous atoms based on a weighted combination of local, atomic-scale properties, which applies to both Si phases. For crystalline Si, the passage of the ions causes compact amorphous regions to form, while no evidence for melting is observed. The relaxation of the amorphouslike regions proceeds initially by the rapid recrystallization of smaller clusters and isolated atoms, followed by a long period of steplike changes in the number of defects due to spontaneous annealing of damage pockets at the crystalline-amorphous interface. In amorphous Si, the initial stage of damage annealing (which lasts a few picoseconds) resembles closely that observed in crystalline Si; on larger time scales, however, the damage is found to ``percolate,'' or flow, through the system, inducing damage away from the collision cascade, thus causing an overall ``derelaxation'' of the material.

Effect of hydrogen on nanoindentation-induced phase transformations in amorphous silicon

The effect of local hydrogen concentration on nanoindentation-induced phase transformations has been investigated in ion-implanted amorphous silicon (a-Si). Elevated concentrations of H ranging from 5×1018 to 5×1020 cm−3, over the depth of indentation-induced phase transformed zones have been formed in the a-Si by H ion-implantation. Indentation has been performed under conditions that result in phase transformed zones composed totally of Si-III/Si-XII in the “H-free” samples. Deformation during indentation and determination of phase transformation behavior has been examined by analysis of load/unload curves, Raman microspectroscopy, and cross-sectional transmission electron microscopy (XTEM). With increasing H content, the probability of forming Si-III/Si-XII and the volume fraction of Si-III/Si-XII decrease. XTEM shows that these reduced volumes are randomly distributed within the phase transformed zone and are surrounded by indentation-induced a-Si. For a H concentration of 5×1020 cm−3, the probability of forming Si-III/Si-XII is reduced to 0.5 compared to 1 in “H-free” material and for those indents that exhibit the Si-III/Si-XII end phase the volume fraction is approximately 60 %. We suggest that the monohydride bonding configuration of Si and H in a-Si reduces the formation of the high pressure crystalline phases by retarding growth of the crystallites through a similar mechanism to that of hydrogen-retarded solid phase crystallization of a-Si to diamond cubic crystalline Si-I phase.

Effect of oxygen concentration on nanoindentation-induced phase transformations in ion-implanted amorphous silicon

The effect of the local oxygen concentration in ion-implanted amorphous Si (a-Si) on nanoindentation-induced phase transformations has been investigated. Implantation of oxygen into the a-Si films has been used to controllably introduce an approximately constant concentration of oxygen, ranging from ∼1018 to ∼1021 cm−3, over the depth range of the phase transformed zones. Nanoindentation was performed under conditions that ensure a phase transformed zone composed completely of Si-III/XII in the nominally oxygen-free a-Si. The effect of the local oxygen concentration has been investigated by analysis of the unloading curves, Raman microspectroscopy, and cross-sectional transmission electron microscopy (XTEM). The formation of Si-III/XII is suppressed with increasing oxygen concentration, favoring a greater volume of a-Si within the zones. The Raman microspectroscopy and XTEM verify that the volume of Si-III/XII decreases with increasing O concentration. With the smaller volumes of Si-III/XII, the pop-out normally observed on load versus penetration depth curves during unloading decreases in magnitude, becoming more kinklike and is barely discernable at high concentrations of oxygen. The probability of forming any high pressure phases is reduced from 1 to ∼0.1 for a concentration of 1021 cm−3. We suggest that the bonding of O with Si reduces the formation of Si-III/XII during unloading through a similar mechanism to that of oxygen-retarded solid phase crystallization of a-Si.

Phase transformations induced by spherical indentation in ion-implanted amorphous silicon

The deformation behavior of ion-implanted (unrelaxed) and annealed ion-implanted (relaxed) amorphous silicon (a-Si) under spherical indentation at room temperature has been investigated. It has been found that the mode of deformation depends critically on both the preparation of the amorphous film and the scale of the mechanical deformation. Ex situ measurements, such as Raman microspectroscopy and cross-sectional transmission electron microscopy, as well as in situ electrical measurements reveal the occurrence of phase transformations in all relaxed a-Si films. The preferred deformation mode of unrelaxed a-Si is plastic flow, only under certain high load conditions can this state of a-Si be forced to transform. In situ electrical measurements have revealed more detail of the transformation process during both loading and unloading. We have used ELASTICA simulations to obtain estimates of the depth of the metallic phase as a function of load, and good agreement is found with the experiment. On unloading, a clear change in electrical conductivity is observed to correlate with a “pop-out” event on load versus penetration curves.

Comparison of the structures of evaporated and ion-implanted amorphous siliconsamples

The experimental structure factors of evaporated and ion-implanted amorphous silicon havebeen modelled by reverse Monte Carlo modelling. A detailed comparison, in terms of thepair correlation function and the distribution (of the cosines) of bond angles, is reportedhere for the two materials. It is found that for an acceptable reproduction of the measuredstructure factors the evaporated models must contain more ‘small’ bond angles, of the order of75°, than their necessary abundance in the implanted models.

Simple energy spike model for paracrystalline silicon in ion implantation

A simple energy spike model is used to interpret the experimental results of medium-range order in ion-implanted amorphous silicon. This model can justify the liquid state, which is speculated to be the initial state of paracrystalline silicon in ion implantation. According to this model, we hypothesize that a crystallite is produced from the hottest spot of a liquid state. As solidification to an amorphous state proceeds from the boundary, the central spot undergoes the maximum undercooling. Such a condition can induce homogeneous crystallization, but it is not possible to form a perfect crystal therein. One can envisage that the spike finally becomes a paracrystallite. In support of the theory, we provide the key equations of the model in this article. But this model still needs to be verified by computer simulations in the future.

Auto-correlation function analysis of phase formation in iron ion-implanted amorphous silicon layers

High-resolution transmission electron microscopy (HRTEM) in conjunction with autocorrelation function (ACF) analysis have been applied to investigate the evolution of structural order in iron ion-implanted amorphous silicon layers. β-FeSi 2 nanocrystallites as small as 5 nm in size were detected in 600 °C annealed for 60 min a-Si layers. The embedded nanocrystalline β-FeSi 2 was found to grow in the interlayer with annealing temperature.

Comment on “Quantitative analysis of annealing-induced structure disordering in ion-implanted amorphous silicon,” by Ju-Yin Chenget al., J. Vacuum Science and Technology A20, 1855 (2002)

Comment on “Quantitative analysis of annealing-induced structure disordering in ion-implanted amorphous silicon,” by Ju-Yin Cheng<i>et al.</i>, J. Vacuum Science and Technology A<b>20</b>, 1855 (2002)

Quantitative analysis of annealing-induced structure disordering in ion-implanted amorphous silicon

We use fluctuation electron microscopy to characterize medium-range order in ion-implanted amorphous silicon. In fluctuation microscopy, intensity fluctuation in a dark-field image contains the information of high-order atomic correlations in the length scale of 1–3 nm. In this study, we heated as-implanted silicon at 500, 550, and 580 °C for various times. Our results indicate that in the beginning amorphous silicon is a disordered phase with robust medium-range order. Thermal annealing leads to disordering of the structure. Furthermore, we find that the activation energy of the disordering is about 2.7 eV, close to the activation energy for thermal relaxation (about 2.2 eV). Our finding suggests a strong correlation between structure disordering and thermal relaxation.

Evolution of structural order in germanium ion-implanted amorphous silicon layers

High-resolution transmission electron microscopy in conjunction with autocorrelation function analysis have been applied to investigate the evolution of structural order in germanium ion-implanted amorphous silicon (a-Si) layers. A high density of Si nanocrystallites as small as 1 nm in size was detected in as-implanted a-Si layers. The density of embedded nanocrystalline Si was found to diminish in a-Si layers with annealing temperature first then increase. The results are discussed in the context of free energy change with annealing temperature.

Electron Irradiation on Amorphous Silicon

ABSTRACTFor the first time we use fluctuation electron microscopy to study the effect of electron irradiation on amorphous silicon. Before showing the results, we compare two variable coherence methods, hollow cone and dark field. These techniques have been successfully implemented in the Philips CM12, JEOL 4000 and Hitachi 9000 microscopes. The image fluctuations for ion-implanted and vacuum-deposited amorphous silicon can be reliably measured under the conditions used in these microscopes.

Observations of structural order in ion-implanted amorphous silicon

Medium-range order in ion-implanted amorphous silicon has been observed using fluctuation electron microscopy. In fluctuation electron microscopy, variance of dark-field image intensity contains the information of high-order atomic correlations, primarily in medium-range order length scale (1–3 nm). Thermal annealing greatly reduces the order and leaves a random network. It appears that the free energy change previously observed on relaxation may therefore be associated with randomization of the network. In this paper, we discuss the origin of the medium-range order during implantation, which can be interpreted as a paracrystalline state, that is, a disordered network enclosing compacts of highly topologically ordered grains on the length scale of 1–3 nm with significant strain fields.

A Damage Model for Disordered Structures in Ion Irradiated Silicon

AbstractMedium-range order has been observed in ion-implanted amorphous silicon, suggesting a paracrystalline structure for this material. The origin of a paracrystalline structure may be due to an energy spike phenomenon. To evaluate the influence of energy spikes on a particular process, we have attempted to calculate the characteristic energy in a spike. However, the observed depth dependence of amorphous structures in as-implanted silicon is puzzling. To explain this, we simulated the depth distribution of cascade events in a particular energy range. We found a great increase of point defect concentration and cascade events as the depth increases. This result could explain the experimental depth dependence.

A Damage Model for Disordered Structures in Ion Irradiated Silicon

ABSTRACTMedium-range order has been observed in ion-implanted amorphous silicon, suggesting a paracrystalline structure for this material. The origin of a paracrystalline structure may be due to an energy spike phenomenon. To evaluate the influence of energy spikes on a particular process, we have attempted to calculate the characteristic energy in a spike. However, the observed depth dependence of amorphous structures in as-implanted silicon is puzzling. To explain this, we simulated the depth distribution of cascade events in a particular energy range. We found a great increase of point defect concentration and cascade events as the depth increases. This result could explain the experimental depth dependence.

Ion-Implanted Amorphous Silicon Studied by Variable Coherence TEM

AbstractAmorphous silicon formed by ion-implantation of crystalline silicon is investigated with the use of VC-TEM (variable-coherence transmission electron microscopy). This technique is sensitive to medium-range-order structures. The results from high-energy Si implanted samples showed a striking similarity to sputtered amorphous silicon. We found that both ion-implanted and sputtered samples have paracrystalline structures, rather than the expected continuous random network (CRN). We also observed the structural relaxation of the ion-implanted amorphous silicon after ex-situ thermal annealing towards the random network. The more disordered structures are favored and a large heat of relaxation is released as the temperature increases. Finally, we show some preliminary results on the structural variation with the sample depth.