Strained Si-Si Bonds Research Articles

A first principles analysis of the effect of hydrogen concentration in hydrogenated amorphous silicon on the formation of strained Si-Si bonds and the optical and mobility gaps

In this paper, we use a model of hydrogenated amorphous silicon generated from molecular dynamics with density functional theory calculations to examine how the atomic geometry and the optical and mobility gaps are influenced by mild hydrogen oversaturation. The optical and mobility gaps show a volcano curve as the hydrogen content varies from undersaturation to mild oversaturation, with largest gaps obtained at the saturation hydrogen concentration. At the same time, mid-gap states associated with dangling bonds and strained Si-Si bonds disappear at saturation but reappear at mild oversaturation, which is consistent with the evolution of optical gap. The distribution of Si-Si bond distances provides the key to the change in electronic properties. In the undersaturation regime, the new electronic states in the gap arise from the presence of dangling bonds and strained Si-Si bonds, which are longer than the equilibrium Si-Si distance. Increasing hydrogen concentration up to saturation reduces the strained bonds and removes dangling bonds. In the case of mild oversaturation, the mid-gap states arise exclusively from an increase in the density of strained Si-Si bonds. Analysis of our structure shows that the extra hydrogen atoms form a bridge between neighbouring silicon atoms, thus increasing the Si-Si distance and increasing disorder in the sample.

Microstructure and Local Charge Distribution in Hydrogenated Nanocrystalline Silicon under Illumination Studied by Electrostatic Force Microscopy.

ABSTRACTHydrogenated nanocrystalline silicon (nc-Si:H) is a promising absorber material for photovoltaic applications. Nanoscale electrical conductivity and overall electronic quality of this material are significantly affected by film microstructure, specifically the density and dimension of grains and grain-boundaries (GB). Local charge distribution at grains and grain/GB interfaces of nc-Si:H was studied by Electrostatic Force Microscopy (EFM) in constant force mode under illumination of white LED. Bias voltage from -3V to +3V was applied on the tip. Scanning Kelvin Force (KFM) images were taken before and after illumination to study the change in surface photovoltage (SP). EFM and KFM analysis were combined with film topography to draw a correlation between surface morphology and nanoscale charge distribution in this material. After illumination, small blister like structures were observed whose size and density increase with time. Raman spectroscopy confirmed these new structures as nanocrystalline silicon. This change was assumed due to relaxation of strained Si-Si bonds as an effect of photo response. Nanocrystalline grain interiors were at lower potential and amorphous grain boundaries were at higher potential for negative bias; it was opposite for positive bias. Change in polarity in bias voltage reversed the polarity of the potential in grains and GBs indicating the dominance of negative type of defects. Further study with current sensing AFM in dark and illumination with variable bias voltages will be able to identify the type and density of defects in grains and grain/GB interfaces.

GISAXS and ATR-FTIR Studies on Stress-Induced Microstructure Evolution of a-Si:H under H2 Plasma Exposure

Microstructure evolution in the surface layer of hydrogenated amorphous silicon (a-Si:H) film exposed to H2 plasma is investigated using grazing-incidence small-angle x-ray scattering and attenuated total reflection-Fourier transform infrared spectroscopy. Molecular hydrogen generated in the microvoids through H-abstraction reaction drives the evolution of the void shape from spherical to ellipsoidal as well as increases the average void volume and total void volume fraction. High-pressure H2 in the microvoid promotes the formation of a strained structure with high compressive stress within the a-Si:H film, which favours the generation of the SiHn complex in the subsurface layer of the a-Si:H film by H insertion into strained Si-Si bonds.

NBTI Degradation in LTPS TFTs Under Mechanical Tensile Strain

This letter investigates the negative-bias temperature instability (NBTI) degradation of p-channel low-temperature polycrystalline-silicon thin-film transistors (LTPS TFTs) under mechanical tensile stress. Experimental results reveal that the interface state density Nit and grain boundary trap density Ntrap of tensile-strained LTPS TFTs are more pronounced than those of unstrained LTPS TFTs. Extracted density of states and conduction activation energy Ea both show increases due to the strained Si-Si bonds, which implies that strained Si-Si bonds are able to react with dissociated H during NBTI stress. Therefore, NBTI degradation is more significant after tensile strain than in an unstrained condition.

Spectroscopic second harmonic generation as a diagnostic tool in silicon materials processing

Spectroscopic second harmonic generation (SHG) has been applied to study thin layers of amorphous silicon in the second harmonic photon energy range of 2.7 - 3.5 eV. The layers were synthesized by hot-wire CVD of hydrogenated amorphous silicon (a-Si:H) and by Ar + ion bombardment of crystalline silicon (c-Si). For a-Si:H a broad feature has been observed in the SHG spectrum. It is discussed that the SHG signal originates from strained Si-Si bonds in the surface or interface region of the a-Si:H film. For H-terminated Si(100) both in spectroscopic and real-time experiments the SHG signal increases by an order of magnitude upon bombardment with 70 eV Ar + ions. We argue that the SHG signal from the amorphized Si layer is generated mainly at the buried interface with c-Si, while an additional contribution seems to originate from the amorphous Si surface region. From the combination of spectroscopic and real-time SHG studies insight into the role of strained bonds in the growth and etching processes of silicon can be gained.

Novel in situ and real-time optical probes to detect (surface) defect states of a-Si:H

AbstractThis paper describes two novel optical diagnostics that were recently introduced to the field of Si-based thin films, in particular for probing defect states present in the bulk and at the surface of a-Si:H films. It is expected that these diagnostics, when applied in situ or real time during film growth, can provide new insights into the a-Si:H film properties as well as into the fundamental surface processes during growth. The first method is cavity ringdown spectroscopy (CRDS). From ex situ measurements on a-Si:H thin films, it is shown that this method is very powerful for measuring absolute defect-related absorptions at subgap energies without the need for a calibration procedure, even for films as thin as 4 nm. It is also shown that the method can be used for measuring rare-earth dopants – here Er3+ in silicon-rich oxide – to the extent that issues about absorption cross-sections can be resolved by using thin samples instead of waveguides. Furthermore, the in situ application of the method for thin films is discussed by presenting the evanescent-wave cavity ringdown (EW-CRDS) technique. The second method is spectroscopic second harmonic generation (SHG). It has been found that this non-linear optical technique yields a photon energy dependent signal for as-deposited a-Si:H films and that this signal has a contribution from a-Si:H surface states. From a comparison with c-Si surface science studies, the possible origin of the signal from surface Si dangling bonds and strained Si-Si bonds is discussed. The application of SHG during real-time film growth is also presented.

Effect of Post-Hydrogenation on the Structural Properties of Amorphous Silicon Network

Post-hydrogenation of magnetron sputtered amorphous silicon films has been carried out with the aim to study the effect of hydrogen interaction with amorphous silicon network on its short and medium range order. Raman spectroscopy has been used to study the variations in the amorphous structure. Nuclear reaction analysis (NRA) has been used to determine the total amount and depth distribution of the penetrated hydrogen atoms. The concentration of the silicon-bonded hydrogen and the bonding configurations have been established by means of infrared (IR) transmission measurements. The values of hydrogen concentration evaluated by NRA and IR spectroscopy coincide within the measurement accuracy, suggesting that the hydrogen diffusion proceeds via interaction with the host silicon atoms. This interaction is accompanied by a rearrangement of the strained Si-Si bonds which leads to an improvement of the amorphous network.

Mechanism of hydrogen-induced crystallization of amorphous silicon.

Hydrogenated amorphous and nanocrystalline silicon films manufactured by plasma deposition techniques are used widely in electronic and optoelectronic devices. The crystalline fraction and grain size of these films determines electronic and optical properties; the nanocrystal nucleation mechanism, which dictates the final film structure, is governed by the interactions between the hydrogen atoms of the plasma and the solid silicon matrix. Fundamental understanding of these interactions is important for optimizing the film structure and properties. Here we report the mechanism of hydrogen-induced crystallization of hydrogenated amorphous silicon films during post-deposition treatment with an H(2) (or D(2)) plasma. Using molecular-dynamics simulations and infrared spectroscopy, we show that crystallization is mediated by the insertion of H atoms into strained Si-Si bonds as the atoms diffuse through the film. This chemically driven mechanism may be operative in other covalently bonded materials, where the presence of hydrogen leads to disorder-to-order transitions.

Effect of Strained Si-Si Bonds in Amorphous Silicon Incubation Layer on Microcrystalline Silicon Nucleation

ABSTRACTIn order to investigate microcrystalline silicon (μc-Si:H) nucleation from the hydrogenated amorphous silicon (a-Si:H) phase, we performed a H2-plasma treatment of a-Si:H layers deposited at different temperatures. In the H-treatment experiment, the formation process of an infrared peak at ∼1937 cm-1, assigned to SiHn (n=1∼2) complex, is studied, as the SiHn complex is proposed to be a precursor for the μc-Si:H nucleation. With increasing the a-Si:H deposition temperature, the total amount of the SiHn complex formed by the H treatment increased. Nucleation of μc-Si:H under high H2-dilution conditions showed a clear relationship with the SiHn complex formed by the H treatment. The SiHn complex formation process, in terms of strained Si-Si bond breaking by H, is discussed.

Nucleation Mechanism of Microcrystalline Silicon Studied by Real Time Spectroscopic Ellipsometry and Infrared Spectroscopy

ABSTRACTWe have characterized a-Si:H initial layers for μc-Si:H nucleation by real time spectroscopic ellipsometry (SE) and infrared attenuated total reflection spectroscopy (ATR) to investigate the μc-Si:H formation mechanism. By performing Ar plasma treatment of a-Si:H layers, we confirmed a presence of a 2 monolayer thick sub-surface in a-Si:H layers. In the a-Si:H sub-surface that leads to the μc-Si:H nucleation, an important peak at ∼1937 cm−1 assigned to the SiHn complex was found in the ATR spectra. From H2 plasma treatment experiments, we proposed that this SiHn complex is formed by H insertion into strained Si-Si bonds. The SiHn complex formed in the a-Si:H sub-surface showed a clear relationship with the μc-Si:H nucleation. From these results, we conclude that the μc-Si:H nucleation occurs by the formation of the chemically active and flexible SiHn complexes in the 2 monolayer thick a-Si:H sub-surface.

Direct insertion ofSiH3radicals into strained Si-Si surface bonds during plasma deposition of hydrogenated amorphous silicon films

We investigate the interaction of silyl $({\mathrm{SiH}}_{3})$ radicals with hydrogenated amorphous silicon $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ films using real time in situ infrared spectroscopy in a mode that can detect as little as \ensuremath{\sim}0.2 ML of Si-H bonds. The results are directly relevant to the growth of $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ by plasma-chemical vapor deposition. In this paper, a remote silane plasma source is used to generate a pure ${\mathrm{SiH}}_{3}$ beam, without any contribution of H or ${\mathrm{SiH}}_{2}.$ Deuterated $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{D})$ films are exposed to this beam and the change in the IR absorption caused by the loss of SiD groups and the creation of SiH groups is measured in real time. At the beginning of exposure to ${\mathrm{SiH}}_{3}$ radicals, a hydrogen-rich layer is deposited on top of the deuterated sample, but no release of deuterium from SiD surface groups can be observed. This is a surprising result, since it has been assumed in the literature that ${\mathrm{SiH}}_{3}$ radicals can easily abstract surface-bonded H atoms, and that abstraction must occur in order to create dangling bond sites on which ${\mathrm{SiH}}_{3}$ radicals subsequently adsorb. Our results show that the reaction mechanism with the largest rate coefficient must be the direct insertion of ${\mathrm{SiH}}_{3}$ radicals into bonding sites at the film surface, which leads to a hydrogen-rich top layer while preserving the preexisting SiD bonds. After completion of one monolayer of surface Si-H bonds, deuterium atoms from the initial surface are released simultaneously with the creation of SiH bulk groups. We propose a reaction scheme based on the direct insertion of ${\mathrm{SiH}}_{3}$ radicals into strained Si-Si bonds. This scheme also predicts that the surface-bonding configurations depend on a dynamic balance between the rates of ${\mathrm{SiH}}_{3}$ adsorption and thermal desorption, which is confirmed experimentally as a function of ${\mathrm{SiH}}_{3}$ flux. We discuss the implications of this reaction mechanism for the growth of $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ from silane discharges, and for the growth of microcrystalline Si in ${\mathrm{H}}_{2}$ diluted ${\mathrm{SiH}}_{4}$ discharges.

The interaction of atomic hydrogen with very thin amorphous hydrogenated silicon films analyzed using in situ real time infrared spectroscopy: Reaction rates and the formation of hydrogen platelets

The interaction of thermal atomic hydrogen (H0) with very thin amorphous hydrogenated silicon (a-Si:H) films is investigated using real time in situ infrared spectroscopy. Hydrogen bonded in isolated and in clustered network configurations is identified from the analysis of the IR line positions and the kinetics of the hydrogen uptake in hydrogen-depleted, hydrogen-deficient and deuterated samples. The use of very thin films is important for this mode identification, since the penetration of atomic hydrogen is not diffusion limited and is therefore very uniform. The analysis yields an IR mode for the isolated SiH groups centered at ∼1985 cm−1, and a newly identified IR mode for platelet-like SiH groups at ∼2033 cm−1. On the basis of this mode identification, the relative reaction probabilities for H saturation of Si dangling bonds, the insertion into strained Si-Si bonds and the H abstraction reaction are determined to 1:0.44:0.26. The kinetic evolution of the two SiH bulk modes successfully describes the measured structural changes and etching of a-Si:H during exposure to H0.

Defect-induced dissociation ofH2in silicon

Ab initio molecular-dynamics simulations of intrinsic defects and hydrogen in crystalline silicon reveal an unexpected process with considerable implications. The vacancy $(V)$ and the self-interstitial $(I),$ both rapid diffusers in $c\ensuremath{-}\mathrm{Si},$ dissociate interstitial ${\mathrm{H}}_{2}$ molecules with a substantial gain in energy: $V+{\mathrm{H}}_{2}\ensuremath{\rightarrow}{V,\mathrm{H},\mathrm{H}}+4.0\mathrm{eV}$ and $I+{\mathrm{H}}_{2}\ensuremath{\rightarrow}{I,\mathrm{H},\mathrm{H}}+1.7\mathrm{eV}.$ The dissociation of ${\mathrm{H}}_{2}$ is caused by the lattice strain associated with the defect, and occurs whenever ${\mathrm{H}}_{2}$ molecules are in the vicinity of strained Si-Si bonds. After the dissociation, the two H's may either bind to the defect that caused the strain or diffuse away from it. The calculated Frenkel pair formation energy is 8.2 eV. The reaction ${\mathrm{H}}_{2}{+\mathrm{H}}_{2}\ensuremath{\rightarrow}{V,{\mathrm{H}}_{4}}+I$ releases less than 0.1 eV, suggesting that ${\mathrm{H}}_{2}$'s in otherwise perfect Si will not generate intrinsic defects.

Recovery Kinetics of Phosphorus Ion-Implanted a-Si:H

AbstractHydrogenated amorphous silicon was implanted with phosphorus ions to a uniform concentration of 3×1020 cm-3 and defect saturation. The implants were annealed isochronally up to 400°C in the dark or under additional illumination. This illumination had no effect on recovery. The Urbach energy remains higher than that of silicon-implants. The midgap defect density anneals to ˜ 1018 cm-3, typical of gas-phase doped samples. The dark conductvity remains lower and its thermal activation energy higher than in gas-phase doped samples. We surmise that the Si-Si network absorbs some of the donor electron-induced defect density by forming strained Si-Si bonds. These strained bonds widen the band tails, and thus reduce the effective electron mobility and pin the Fermi level.

Energetics of bond-centered hydrogen in strained Si-Si bonds.

The energetics of hydrogen incorporation into strained Si-Si bonds are examined using first-principles density-functional-pseudopotential calculations. It is found that an increase in the bond length of the initial Si-Si bond leads to a lower formation energy for the Si-H-Si configuration. A simple linear relationship, describing the change in formation energy as a function of the Si-Si bond length, is presented, which applies to bond-stretching as well as bond-bending distortions. The results are discussed in the context of hydrogen interaction with grain boundaries in polycrystalline silicon.

Ab initiocalculations on metastable defects in a-Si:H: The Staebler-Wronski effect

Abstract We use real-space local density functional theory to investigate the formation and stability of paramagnetic defects involving H in a-Si:H. We have considered the mechanism for dangling-bond formation by exciting an electron-hole pair at a strained Si-Si bond, using the clusters Si8H18, Si7H16 and Si26H30. We find in these clusters that a Si-Si bond strained by ∼12% can break once an electron is promoted from the bonding to the antibonding level. Calculations on the clusters Si5H12, Si12H22 and Si26H40 serve to show that metastable defects comprising Si-dangling bonds and bond-centred H atoms can form with energies of 0·0–3·0eV, depending on the initial strain in the system. These results support the argument that light induces H motion in locally strained regions and causes the Staebler-Wronski effect.

Generation annealing kinetics of interface states on oxidized silicon activated by 10.2-eV photohole injection

Analysis of the generation and annealing kinetics of U-shaped and peaked donorlike interface density of states (DOS’s) are reported. The U-shaped interface DOS appears to be related to oxygen deficiency or silicon dangling bonds formed during oxidation and it does not anneal after 50 h at 25 C but anneals rapidly (minutes) above 300 C in hydrogen containing ambients, suggesting Si-H bond formation during anneal. The U-shaped DOS reappears following a first-order kinetics when 10.2-eV photogenerated holes are injected into the oxide, suggesting that these Si-H bonds are broken by hole capture and dissolved hydrogen molecules are formed. As many as three donorlike interface DOS peaks above Si midgap are also generated by the 10.2-eV photoholes following a first-order kinetics, and they appear to be related to excess oxygen or oxygen dangling bonds formed at the interface during oxidation. These donor DOS peaks anneal out in less than 50 h at 25 C following a second-order annealing kinetics, suggesting that the oxygen dangling bonds react with H2 to form SiO-H. Band edge tail states seem also evident whose density is not affected by the photoinjected holes suggesting an association with strained Si-Si bonds.

A model of interface states and charges at the Si-SiO2 interface: Its predictions and comparison with experiments

We develop a model for states and charges at the Si-SiO2 interface based on the expected existence at this interface of threefold coordinated Si atoms, strained Si-Si bonds, etc. The associated electronic states are studied by using a generalized bond orbital approximation (supplemented in some cases by multiband cluster Bethe lattice calculations), where the various configurations ofinterest are allowed to relax in order to minimize the energy of the system. It is shown that the reconstructing character of interface states so obtained has a number of interesting experimental implications, and in particular it is found that by including reconstruction effects one is able to understand and in some cases correlate many different experimental results. Among others, these include some observed metal-oxide-semiconductor device instabilities, the dependence of the interface density of states on the conductivity type of the semiconductor substrate, and the amphoteric character of interface states.

Polaron and bipolaron states in Si-inversion layers: Possible origins and effects

Abstract We report results of a series of multiband tight-binding cluster calculations we have made to further uncover the origin and nature of the intrinsic interface states at the Si-SiO 2 interface as well as their effects on the n- and p-channel inversion layers. These results show that distortion (reconstruction) effects are important ingredients in the description of these states and this together with additional analysis indicates that threefold coordinated Si atoms at the Si-SiO 2 interface as well as strained Si-Si bonds on the oxide “side” on the interface can exist, as we have suggested earlier, in metastable configurations leading to e.g., device instabilities. A subgroup of these reconstructing interface states associated with weakly strained Si-Si bonds on the semiconductor side of the interface is identified and analyzed with emphasis on how such states can give rise to polaronic and bipolaronic effects in inversion layers and hence provide an additional basis for the Anderson-like negative U models previously employed by us in studying the low carrier density regime of this system.

Polaron and bipolaron states in Si-inversion layers: Possible origins and effects

We report results of a series of multiband tight-binding cluster calculations we have made to further uncover the origin and nature of the intrinsic interface states at the Si-SiO 2 interface as well as their effects on the n- and p-channel inversion layers. These results show that distortion (reconstruction) effects are important ingredients in the description of these states and this together with additional analysis indicates that threefold coordinated Si atoms at the Si-SiO 2 interface as well as strained Si-Si bonds on the oxide “side” on the interface can exist, as we have suggested earlier, in metastable configurations leading to e.g., device instabilities. A subgroup of these reconstructing interface states associated with weakly strained Si-Si bonds on the semiconductor side of the interface is identified and analyzed with emphasis on how such states can give rise to polaronic and bipolaronic effects in inversion layers and hence provide an additional basis for the Anderson-like negative U models previously employed by us in studying the low carrier density regime of this system.