We present an overview of recent results for hydrogen interactions with amorphous silicon (a-Si), based on first- principles calculations. We review the current understanding regarding molecular hydrogen, and show that H2 molecules are far less inert than previously assumed. We then discuss results for motion of hydrogen through the material, as relating to diffusion and defect formation. We present a microscopic mechanism for hydrogen-hydrogen exchange, and examine the metastable ≠ SiH2 complex formed during the exchange process. We also discuss the enhanced stability of Si-D compared to Si-H bonds, which may provide a means of suppressing light-induced defect generation.

New NMR measurements show that interstitial T site-trapped molecular hydrogen can amount to more than one third of the contained hydrogen in high quality PECVD amorphous silicon. Microvoid-contained dense molecular hydrogen is negligible in these good films. Experiments on a sequence of hydrogenated and/or deuterated a-Si films have characterized individually-trapped molecular HD and D2 in films deposited from SiD4, and from SiH4+D2. The T site-trapped molecular hydrogen fraction observed here is larger than previously reported because of recent efforts to measure very slowly relaxing molecular components and the employment of radiofrequency pulse sequences to detect ortho-D2 with nuclear spin I=2. The population of interstitially trapped molecular hydrogen increases with increasing photovoltaic quality over a range of an order of magnitude in photoresponse product ημτ. Above 200 K, hopping transport of molecular hydrogen among the amorphous equivalent of interstitial T sites occurs with an activation energy near 50 meV.

A Jeener-Broekaert three-pulse sequence is used to investigate ortho-molecular hydrogen (o-H2) in device quality amorphous silicon films prepared by plasma enhanced chemical vapor deposition (PECVD) and hot wire CVD (HWCVD). For the PECVD sample, the concentration of hydrogen molecules is ~ 11% of the total hydrogen concentration, one order of magnitude larger than that inferred from spin-lattice relaxation time measurements (~1%). Hence, most of the hydrogen molecules do not serve as effective relaxation centers. For HWCVD samples with ~3 to 4% hydrogen and very low void densities, the concentrations of hydrogen molecules are ~1% of the total hydrogen concentration. In these samples, spin-lattice relaxation measurements for bonded hydrogen indicate that the concentration of hydrogen molecules that contribute to spin-lattice relaxation is at most 0.1% of the total hydrogen concentration. Spin-lattice relaxation time (T1) measurements of ortho-molecular hydrogen indicate two very different T1's. The longer T1 is ~ 0.6 s, possibly due to an electric quadrupole-quadrupole (EQQ) interaction between o-H2 molecules and, the shorter T1 is ~ 3 ms, very close to that calculated for a two-phonon Raman process for rotating o-H2.

Long-range atomic H motion in hot-wire deposited (HW) a-Si:H is compared directly to that in glow-discharge deposited (GD) a-Si:H by monitoring the deuterium secondary ion mass spectrometry (DSLMS) profiles in [GD a-Si:H]/[GD a-Si:(H,D)]/[HW a-Si:H] multilayers vs annealing temperature and time. While the profiles in the GD layer are in excellent agreement with complementary error-function behavior and previous studies, the profiles in the HW layer suggest that the multiple-trapping motion of the H and D atoms is much slower, possibly due to an interface layer of defects. However, an exponential “tail” of D atoms extends deep into the HW layer, probably due to a long diffusion length of mobile D atoms, consistent with the established release times of H and D from the GD layer and H loss typical during growth of HW films. The results are also discussed in terms of the H exchange model and compared to previous NMIR studies of HW a-Si:H, which suggest that most of the hydrogen in the HW layer is concentrated in H-rich clusters dispersed in a network of very low H content.

Hydrogen diffusion in phosphorus and boron doped polycrystalline silicon was investigated by deuterium diffusion experiments. The presence of dopants enhances hydrogen diffusion. The effective diffusion coefficient Deff is thermally activated and the activation energy varies between 0.1 and 0.4 eV. This is accompanied by a variation of the diffusion prefactor by 12 orders of magnitude. Using the theoretical diffusion prefactor the actual energy EA was calculated from Deff.EA also depends strongly on the Fermi energy and exhibits a similar dependence as the formation energies of H+ and H- in single crystal silicon.

We used polarized attenuated total reflection (ATR) measurements together with Fourier transform infrared (FTIR) spectroscopy to investigate the vibrational spectra of hydrogenated amorphous silicon (a-SiHx) films 0.5-1.0 microns in thickness. We deposited the films using hot-wire or plasma-enhanced chemical vapor deposition methods (HWCVD or PECVD, respectively) on crystalline silicon and cadmium telluride substrates. Our ATR technique gave a spectral range from 2100-400 cm-1, although the Si-H wagging mode absorption band at 640 cm-1 was somewhat distorted in the a-SiHx/Si samples by impurity and lattice absorption in the silicon ATR substrates. We report the identification of a Si-O-C impurity band with maximum intensity at 1240-1230 cm-1. The assignment of this band to a Si-O-C vibration is supported by secondary-ion mass spectrometry (SIMS) measurements. Our polarized FTIR-ATR spectra of HWCVD and PECVD a-SiHx films on <111> Si ATR substrates show that the impurity dipoles are oriented strongly parallel to the film growth direction. The wagging mode absorbance band is more intense in the film plane. This trend is less pronounced for the Si-H stretching vibrations. These observations are consistent with some degree of anisotropy or medium-range order in the films. The anisotropy in the Si-H bands may be related to residual stress in the films. Our scanning electron microscopy (SEM) analyses of the samples offer additional evidence of bulk structural anisotropy in the a-SiHx/Si films. However, the Si-O-C impurity band was not observed in the polarized ATR-FTIR spectra of the a-SiHx/CdTe samples, thus indicating that the Si substrates influence formation of the impurity in the a-SiHx/Si films.

The characterization of hydrogen in hydrogenated nano-crystalline silicon (nc-Si:H) films with various crystallinity has been studied using FTIR absorption spectroscopy and gas effusion spectroscopy. In gas effusion spectrum of a nc-Si:H, three evolution peaks of H2 are found near 400, 500 and 600°C. From the deconvolution of the gas effusion spectra, the hydrogen concentration in amorphous region dose not depend on the volume fraction of crystalline. The surface density of hydrogen at the interface and in the grain boundaries of nano-crystalline grains decreases with the volume fraction of crystalline. The properties of hydrogen in nc-Si:H films are discussed with the results of FTIR absorption spectroscopy.

The elastic anomaly observed previously at the triple point of bulk molecular hydrogen in hydrogenated amorphous silicon films prepared by hot-wire chemical-vapor deposition has also been observed in deuterated films at the triple point of D2. The origin of this anomaly has now been traced to bubbles formed at the crystalline-amorphous interface. An upper limit of the pressure in these bubbles at their formation temperature, 440°C, has been estimated to be 11 MPa, and is suggested to be a measure of the bonding strength between film and substrate at that temperature. Bubble formation after heat treatment at 400°C has also been observed in films prepared by plasma-enhanced chemical-vapor deposition. The internal friction anomalies resemble those observed previously in cold-worked hydrogenated iron where they have been interpreted through plastic deformation of solid hydrogen in voids.

Small angle x-ray scattering (SAXS), IR spectroscopy, and deuterium secondary ion mass spectrometry (DSIMS) were used to study the microstructure and hydrogen dynamics of undoped and boron-doped if-sputter-deposited (RFS) and electron cyclotron resonance (ECR)-deposited hydrogenated amorphous silicon carbides (a-Si1-xCx:H) with x ≤ 19 at.%. The SAXS measurements indicated residual columnar-like features and roughly spherical nanovoids of total content CnV ≤ 1.0 vol.%. The growth of CnV with annealing was due largely to an increase in the average nanovoid radius. It was noticeably smaller than in RFS a-Si:H films. The IR spectra demonstrated H transfer by annealing from mostly bulk-like Si-H groups to C-bonds. The H diffusion and its temperature dependence in undoped films resembled those of a-Si:H and were consistent with the SAXS and IR data. Suppression of long-range motion of most of the H atoms, consistent with increased CnV was observed in B-doped ECR films. However, a small fraction of the H atoms appeared to undergo fast diffusion, reminiscent of the fast diffusion in doped a-Si:H. The results are consistent with impeded relaxation processes of the Si network, caused by the presence of C atoms, and H trapping at C-H bonds.

The structural aspect of photodegradation effect in hydrogenated amorphous silicon has been investigated by the use of the simple and sensitive detection technique, the laser optical-lever bending method, for a small expansion or extraction in thin films. The volume change induced by the thermal expansion due to the photothermal effect and the residual expansion was observed in hydrogenated amorphous silicon prepared by PECVD. The latter residual expansion was persistent after light soaking and was recovered by thermal annealing at 200°C.The time dependence of the volume expansion with light soaking shows the same time dependence of photoinduced defect density. The photoinduced volume changes normalized by the initial volume are the order of 10-5~10-5, which values are two orders smaller than chalcogenide glasses such as a-As2S3. The normalized volume change of a-Si:H with the different sample preparation conditions of PECVD such as the hydrogen dilution ratio r (r = SiH4/H2) and substrate temperature is shown. Also it is demonstrated that the photoinduced expansion is observed in hydrogenated amorphous silicon prepared by photo CVD and hot-wire CVD methods. The spatial extent related to a photoinduced defect creation in a-Si:H is estimated.

We degraded hydrogenated amorphous silicon (a-Si:H) using red light pulses of 32 microseconds to 2 milliseconds. These metastable photoconductivity degradations were compared to degradation with continuous light of the same intensity and same exposure time. For a given integrated exposure time we observe higher degraded photoconductivities (by up to 40 %) as we shorten the pulses. For example, to obtain the same amount of degradation with 120 microsecond pulses as with continuous illumination, the integrated sample exposure time must be doubled. Experiments were conducted to exclude thermal effects. Our result cannot be explained with a simple recombination-driven degradation mechanism, because electron and hole populations rise and fall to steady-state values in only a few microseconds. We conclude that carrier recombination creates a more proximate precursor to metastable degradation of a-Si:H. When illumination begins, this precursor's density rises more slowly than the density of photocarriers and therefore degradation is delayed.

In hydrogenated amorphous silicon (a-Si:H), the kinetics of the light induced production of silicon dangling bonds and long-lived band-tail electrons and holes has been measured at temperatures between 65 and 340 K using light induced electron spin resonance (LESR). Below about 150 K the measurement of Si dangling bonds is masked by the accumulation of long-lived band-tail carriers. The kinetics of the growth and decay of these long-lived, trapped band-tail carriers consists of very fast components (τ < ms) and very long components (τ > h). Optical quenching of these long-lived carriers is not efficient at quenching energies of 0.6 eV. Afler removal of these long-lived band tail carriers by annealing at about 250 K we find that the total production of silicon dangling bonds at 65 K after 10 h of illumination is about a factor of five less than at 340 K. The dangling bond production resulting from 10 h of illumination is well fit to an underlying mechanism that, if thermally activated, exhibits an activation energy of approximately 10 meV.

X-ray irradiation-induced structural changes in undoped a-Si:H have been investigated in detail by X-ray photoemission spectroscopy (XPS). The Si2s and the Si2p peaks were found to shift simultaneously to lower bonding energies, by the same amount, with X-ray irradiation. The shifts are near saturation, at about 0.1 eV, after one hour of irradiation at the intensity used; they can be reversed almost completely, seemingly with an activation energy lower than that for the metastable changes in electronic properties (Staebler-Wronski effect). The present results suggest that essentially the whole Si network structure is affected by the X-ray irradiation.

Dangling bond defects are created during positive bias stress of amorphous silicon thin film transistors and there is an energy barrier between 0.9 and 1 eV for this process. We have studied how this energy barrier depends on the material parameters of the amorphous silicon, namely hydrogen content, hydrogen bonding, Urbach energy and intrinsic, deposition induced stress. We observe no dependence on the hydrogen content or hydrogen bonding type, but we do observe a clear dependence on the Urbach energy and the intrinsic stress. These measurements support a localized model for defect creation involving Si-Si bond breaking and the switching of a neighboring H atom to stabilize the broken bond. These results suggest that stable amorphous silicon TFTs can be obtained at low deposition temperatures by control of the deposition-induced, intrinsic stress.

We propose a new metastable defect associated with hydrogen atoms in amorphous silicon. A higher energy metastable state is formed when H is flipped to the backside of the Si-H bond at monohydride sites. This defect is described by a double-well potential energy and occurs in addition to metastable dangling bonds. The dipole moment of this “H-flip” defect is larger and increases the infrared absorption. This defect accounts for large structural changes observed on light soaking including larger absorption and volume dilation.

The trap-controlled model of H diffusion that underpins the H collision model of amorphous silicon metastability provides new insight into thermal and light-enhanced H diffusion in a-Si:H. The longstanding puzzle of the linear DB-dependence of diffusion coefficient with doping is resolved. Expressions for the light-induced H diffusion coefficient are derived with and without D-for-H exchange reactions. It is shown that D-for-H exchange does not affect the long-time diffusion coefficient that is measured under illumination.

Proton nuclear magnetic resonance (NMR) is applied to investigate hydrogen dynamics and microstructures in a-Si:H. In addition to the generic broad and narrow lines observed in all aSi:H, an additional narrow line (about 1 kHz wide) is observed as the temperature is raised above RT. This narrow line is shifted to the up-field by about 4 ppm with respect to the generic narrow line of a few kHz in width. Below 150°C, the change of the proton spectrum with temperature is reversible; the hydrogen associated with this additional narrow line is shown to originate from hydrogen originally associated with the broad line. The spin-lattice relaxation time T1 of this up-field shifted narrow line is about 0.4 s. This short T1, along with its small linewidth, suggests that this line is associated with molecular hydrogen, possibly trapped in sites of atomic dimensions.

We observe an increase of the low-temperature internal friction of hydrogenated amorphous silicon prepared by both hot-wire and plasma-enhanced chemical-vapor deposition after extended light-soaking at room temperature. This increase, and the associated change in sound velocity, can be explained by an increase of the density of two-level tunneling states, which serves as a measure of the lattice disorder. The amount of increase in internal friction is remarkably similar in both types of films although the amount and the microstructure of hydrogen are very different. Experiments conducted on a sample prepared by hot-wire chemical-vapor deposition show that this change anneals out gradually at room temperature in about 70 days. Possible relation of the light-induced changes in the low-temperature elastic properties to the Staebler-Wronski effect is discussed.

The degradation and annealing kinetics of both a-Si:H and a-SiGe:H single-junction solar cells were investigated under varying conditions. In every case, the kinetics associated with degradation and annealing were slower for a-SiGe:H cells than for a-Si:H cells. Since deuterium diffusion studies indicate that the hydrogen in our a-SiGe:H films diffuses more slowly than that in the a-Si:H films, hydrogen motion may play a role in determining both the degradation and annealing kinetics of the devices.

Band tail states are routinely invoked in models of a-Si:H including defect pool models and models of light induced defects. These models describe the band tail states as being localized on a single stretched bond. However to our knowledge, there is no theoretical or experimental work to justify these assumptions. In this work we use ab initio calculations to support earlier tight-binding calculations that show that the band tail states are very delocalized ---- involving large numbers of atoms as the energy is varied from miidgap into the tails. Our work also shows that the valence band tail states are statistically associated with short bonds (not long bonds) and the conduction-band states with long bonds. We have slightly modified a 512 atom model of a-Si due to Djordjevic et al. to produce a large, defect free model of a-Si:H with realistic band tails.