Role of hydrogen atoms in anodized porous silicon

Hydrogen atoms chemisorbed in anodized porous silicon (PS) during anodization in a HF solution have been investigated by using both experimental techniques and a semi-empirical calculation method. The results show important roles of chemisorbed H atoms in PS on anodization mechanisms and a slight expansion of Si-Si bond length, as in the case of the structural change and low-temperature oxidation process of PS films previously reported. Fine structures observed in the infrared absorption band of the Si-H stretching vibrations can be related to charge redistributions of H-chemisorbed Si atoms which were calculated for various clusters with Si_nH_m using the AM1 method. The calculation results on the Si-Si bond length in the clusters are also consistently explained in relation to slight increases in the lattice constant of PS: the origin comes from Si charges attracted by chemisorbed H atoms on the pore walls.     

Role of the hydrogen in the light-induced defects in undoped hydrogenated amorphous silicon

We have investigated the effect of the deposition temperature (i.e. the hydrogen content) on the light-induced effects in undoped hydrogenated amorphous silicon (a-Si:H). Combined junction capacitance-temperature (C-T), ESR and IR absorption measurements are carried out in both the dark annealed state (A) and the saturated light-soaked state (B), as well as after partial annealing of the samples, starting from state B. The experimental results indicate that the films deposited at the highest substrate temperature (i.e. the lowest H content) exhibit a completely different behaviour from those deposited at lower substrate temperature (i.e. with higher H concentration), when the samples are left for long times at room temperature in the dark after partial annealing. These results are discussed in detail in relation to the different models proposed to explain the light-induced effects in a-Si:H.     

States of hydrogen in crystalline silicon

The thermal transformation of deuterium in p-type crystalline silicon is studied with a variety of experimental techniques. It is found that D-atoms initially trapped at acceptor sites can be transferred by low temperature annealing to a different state tentatively ascribed to interstitial D₂ molecules. Diffusion of D out of the passivated sample only occurs at temperatures significantly higher than this transformation temperature. This fact allows us to produce Si samples with extremely high deuterium concentrations (several at%) by a suitable passivation-annealing sequence. With increasing D-concentration, a number of characteristic Si-D defect complexes have been observed by vibrational spectroscopy.     

Role of MW-ECR hydrogen plasma on dopant deactivation and open-circuit voltage in crystalline silicon solar cells

Plasma hydrogenation is an efficient method to passivate intergrain and intragrain defects of polycrystalline silicon (pc-Si) solar cells. The hydrogenation experiments were carried out in hydrogen plasma generated in an electron cyclotron resonance system controlling different operating parameters such as microwave power (P _MW), process time (t _H) and hydrogenation temperature (T _H) for a fixed hydrogen flux of 30 sccm. The hydrogenation of n⁺pp⁺ pc-Si solar cells resulted in an improvement in the open-circuit voltage. The improvement was correlated with the dopant deactivation due to the formation of boron–hydrogen bonding. This was demonstrated from the changes in the doping level after hydrogenation of n⁺p diode structures made using single crystalline silicon as a reference material. It was found that deactivation of boron was more pronounced at high microwave plasma power, in good agreement with the high open-circuit voltage values obtained on pc-Si mesa cells. On the other hand, the effect of longer hydrogenation time and higher temperature resulted in a decrease of boron deactivation, while an increase in V _oc with a tendency of saturation at high T _H was observed. Reasons for such behavior were thoroughly explained.     

Electron paramagnetic resonance of hydrogen in silicon

A review of the investigations by means of electron paramagnetic resonance (EPR) of hydrogen and hydrogen-related defects in crystalline silicon is presented. The main features of the EPR center Si-AA9 (bond-centered hydrogen), which is known as the hydrogenic analogue of the anomalous muonium (Mu^*) in silicon, are discussed. It was found that the process of annealing the AA9 center is characterized by an activation energy, E = 0.48 ± 0.04 eV with a second-order pre-exponential factor, K₀ = (1.25 ± 2.5) × 10^-7 cm³/s.

A detailed investigation by EPR of the defect (Si-AA1), which we identify as the hydrogen-related shallow donor in a positive charge state, is also presented. In particular it is shown that the H-related shallow donor is a helium-like center and its wave function has C_2v symmetry. Moreover, the main features of the series of EPR spectra in silicon characteristic for the implantation of hydrogen are presented.     

Structure-dependent vibrational lifetimes of hydrogen in silicon

The lifetimes of the Si-H vibrational stretch modes of the H(*)(2) ( 2062 cm(-1)) and HV.VH((110)) ( 2072.5 cm(-1)) defects in crystalline Si are measured directly by transient bleaching spectroscopy from 10 K to room temperature. The interstitial-type defect H(*)(2) has a lifetime of 4.2 ps at 10 K, whereas the lifetime of the vacancy-type complex HV.VH((110)) is 2 orders of magnitude longer, 295 ps. The temperature dependence of the lifetime of H(*)(2) is governed by TA phonons, while HV.VH((110)) is governed by LA phonons. This behavior is attributed to the distinctly different local structure of these defects and the accompanying local vibrational modes.     

Dynamics of hydrogen in hydrogenated amorphous silicon

The problem of hydrogen diffusion in hydrogenated amorphous silicon (a-Si:H) is studied semiclassically. It is found that the local hydrogen concentration fluctuations-induced extra potential wells, if intense enough, lead to the localized electronic states in a-Si:H. These localized states are metastable. The trapping of electrons and holes in these states leads to the electrical degradation of the material. These states also act as recombination centers for photo-generated carriers (electrons and holes) which in turn may excite a hydrogen atom from a nearby Si-H bond and breaks the weak (strained) Si-Si bond thereby apparently enhancing the hydrogen diffusion and increasing the light-induced dangling bonds.     

Semi-empirical calculations of hydrogen defects in silicon

Calculations for hydrogen defect(s) in a monovacancy silicon cluster yield a stable position for this defect which: (a) does not saturate any of the silicon dangling bonds; and (b) contributes defect level(s) in the gap whose implications remain to be understood.     

Hydrogen-induced-intrinsic transport in undoped microcrystalline silicon

It is demonstrated that microcrystalline silicon (μc-Si:H) of intrinsic character can be produced by post-growth atomic hydrogen treatment. Undoped μc-Si:H films with a dark-conductivity activation energy (E _a) of about 0.20 eV were grown by plasma-enhanced chemical vapor deposition, and then subsequently exposed to an atomic hydrogen plasma. The hydrogen treatment is shown to result in a gradual increase in the E _a with increasing treatment time, followed by saturation at about 0.57 eV, a value observed for truly intrinsic μc-Si:H films. In the saturated state, the dark conductivity is on the order of 10⁻⁷ S/cm. The dark conductivity prefactor is found to follow the Meyer–Neldel rule. It is proposed that charge transport takes place in amorphous-like tissue surrounding the crystalline grains. The results are attributed to the Fermi level shift due to a change in the gap state distribution.     

Phonon heat transport in silicon nanowires

Thermal conductivity of silicon and porous silicon nanowires based on the equation of phonon radiative transport is theoretically evaluated. The thermal conductivities of silicon nanowires with square cross-sections are found to match molecular dynamics simulation results reasonably well. It is shown that the results of meso-porous silicon nanowires are about two orders of magnitude lower than that of silicon nanowires in a wide range of temperature (50 K-300 K). Received 24 April 2001 and Received in final form 23 December 2001     

Simulation of fast electron transport in silicon

A model for estimating the number of electrons produced as a result of the fast-ion-surface interaction is proposed. The dependence of the number of electrons emitted by the surface in collisions between fast protons and silicon as a function of the angle of incidence and as a function of the layer depth where the electron was produced is analyzed.     

Positron-trap centers in neutron-irradiated silicon containing hydrogen

The defect evolution as a function of the annealing temperature has been studied in monocrystalline silicon grown in a hydrogen atmosphere and irradiated with 3.6×10¹⁷ neutrons/cm². Positron lifetime spectroscopy has been used and the results compared with infrated absorption measurements. Vacancy-H, vacancy-2H, vacancy-O–H and divacancy complexes withm hydrogen atoms (m<6) have been identified for the first time as possible positron traps.