Polymorphous Silicon Research Articles

Influence of deposition parameters on hole mobility in polymorphous silicon

Time-of-flight transient photoconductivity measurements reveal a monotonic increase with the deposition pressure in the hole mobility in polymorphous silicon for samples deposited under hydrogen dilution. With helium dilution, a maximum mobility that matches the highest value from H-dilution samples is measured at the intermediate pressure of 1.4 Torr. The deposition rate of those samples is twice the rate for the H-dilution ones. For the samples with the best hole mobilities, the valence-band tail is comparable to the one of standard hydrogenated amorphous silicon.

Fluctuation microscopy evidence for enhanced nanoscale structural order in polymorphous silicon thin films

The nanometer-scale (medium range) structural order in hydrogenated polymorphous silicon films is analyzed using fluctuation electron microscopy. The polymorphous growth regime occurs under relatively high gas pressure during plasma-enhanced chemical vapor deposition, such that small aggregates and nanocrystals form in the gas phase and impinge on the film surface. All polymorphous samples appear completely amorphous in diffraction or Raman scattering analyses. In fluctuation microscopy, carried out in the transmission electron microscope, the statistical variance V in the dark field image intensity is acquired as a function of the scattering vector k at a chosen resolution Q. Theory shows that V is quantitatively related to the three- and four-body atomic correlation functions, and thus to the nanometer scale order, in the material. Unlike typical hydrogenated amorphous silicon, the variance V is a strong function of growth conditions and displays a maximum at a silane pressure of 1.4–1.8Torr. The images also reveal the presence of a small number of unusually bright spots, roughly 5nm in diameter, only in samples grown at 0.8 and 1.4Torr; we interpret that these correspond to nanocrystallites. The observation of enhanced structural order as revealed by the variance V is consistent with previous, but less conclusive, analyses of hydrogenated polymorphous silicon.

Analysis of field dependent steady-state photocarrier grating measurements on polymorphous and microcrystalline silicon films

The steady-state photocarrier grating (SSPG) technique has been employed to investigate the field dependence of different polymorphous and microcrystalline silicon samples prepared by plasma enhanced chemical vapor deposition technique. The field-dependent experimental data at different temperatures are analyzed using two different approaches based on the small-signal photocurrent to extract more information on the electronic properties (like small signal mobility life time product, average drift length for holes and electrons,... etc.). The quality of the fits is tested by the χ2 indicator and the best choice of the transport parameters is obtained in each approach. The values of electron and hole drift mobilities obtained from one approach are compared to those found by others. The trapped charge density can be determined, for all samples, and correlated to minority-carrier diffusion length. It is also found that the decrease in temperature in a microcrystalline sample may result in an increase of the trapped charge density. This increase in trapped carrier density may be consistent with the increase in sub-gap absorption. Such behavior is found in agreement with that reported in the literature. In addition, the polymorphous samples that have values of diffusion lengths comparable to those of microcrystalline samples exhibit higher values of trapped carrier density.

Role of hydrogen diffusion on the growth of polymorphous and microcrystalline silicon thin films

Hydrogen diffusion is monitored during plasma conditions corresponding to the growth of poly- morphous and microcrystalline silicon thin films in the temperature range from 50 to 300 ◦ C. Structural changes of the films were monitored in situ by Spectroscopic Ellipsometry measurements and interpreted with the Bruggemann Effective Medium Approximation along with the Tetrahedron Model. For both ma- terials, diffusion of hydrogen leading to the formation of a hydrogen-rich subsurface layer during the first minute of exposure to the hydrogen plasma is observed. This initial phase was followed by a steady-sate regime during which the thickness of the subsurface layer stayed relatively constant and the total film thickness decreased as a function of time. This steady-state is explained as the result of the equilibrium between hydrogen diffusion and etching. The hydrogen diffusion coefficient and etching rate were calcu- lated from these measurements and found to be higher in amorphous than in polymorphous silicon. This result is consistent with the growth dynamics of polymorphous silicon. It also provides information con- cerning the nucleation of micro-crystallites in amorphous silicon under very large atomic hydrogen flow conditions. Moreover, it also explains why polymorphous silicon films do not undergo the phase transition to microcrystalline, even if deposited under relatively similar large atomic hydrogen flow conditions. PACS. 61.43.Dq Amorphous semiconductors, metals, and alloys - 66.30.-h Diffusion in solids - 68.55.Ac Nucleation and growth: microscopic aspects

Influence of deposition parameters and post-deposition plasma treatments on the photoluminescence of polymorphous silicon carbon alloys

We have studied the photoluminescence (PL) of a nanostructured material consisting of silicon nanocrystals embedded in an amorphous matrix that we call polymorphous silicon carbon (pm-Si1−xCx:H). The presence of silicon nanocrystals in the a-Si1−xCx:H matrix is confirmed by high resolution transmission electron microscopy (HRTEM) as well as by their PL emission features, in particular a strong room temperature emission and a broad spectrum, which are characteristic of silicon nanocrystals. Moreover, the PL intensity was found to be closely related with the number of nanocrystals in the film, which depends on the deposition conditions, in particular the RF power. Possible ways to enhance the PL efficiency and a model of the recombination routes are discussed.

Trapping phenomena in intrinsic hydrogenated amorphous silicon like materials studied using current transient spectroscopies

Transient spectroscopies such as time analyzed transients spectroscopy (TATS) provide powerful means of comparing density of states in new forms of amorphous like materials. These spectroscopies were utilized to study hydrogenated amorphous silicon (a-Si:H) and hydrogenated polymorphous silicon (pm-Si:H) grown at different pressures using PECVD. The results reveal marked differences between the two materials. In case of a-Si:H, as expected characteristic emission from a broad density of states in the form of stretched exponentials is observed. The corresponding spectra for pm-Si:H, on the other hand are dominated by nearly exponential fast current decay processes with discrete energies between 0.25eV and 0.36eV. The spectra of pm-Si:H grown at different pressures show contributions from crystallite inclusions and the medium in varying degree.

Transport properties and defects in silicon nanoparticles and effect of embedding in amorphous silicon layers

We study the properties of silicon nanoparticles produced by CO2 laser pyrolysis of silane in a gas flow reactor, and the effect of sandwiching a thin layer of such particles between two a-Si:H layers. Contrary to the case of hydrogenated polymorphous silicon for which it is known that the electronic and stability properties can be enhanced compared to conventional hydrogenated amorphous silicon, the electronic properties of such a sandwich structure are degraded and the number of deep defects is increased.

Electron and hole transport in microcrystalline silicon solar cells studied by time-of-flight photocurrent spectroscopy

A photocurrent time-of-flight study of carrier transport in microcrystalline silicon pin diodes prepared over a range of crystallinities is presented. Electron and hole drift mobilities at a crystalline volume fraction >0.35 are typically 3.8 and 1.3cm2/(Vs) respectively at 300K and a thickness to electric field ratio of 1.8×10−7cm2/V. A factor of five enhancement in hole mobility over amorphous silicon persists at a crystalline volume fraction as low as 0.1. Current decays are dispersive and mobilities are thermally activated, although detailed field-dependence is still under investigation. Evidence for a sharp fall in the density of states at 0.13eV above the valence band edge is presented. Similarities in behaviour with certain amorphous and polymorphous silicon samples are identified.

Calorimetry of dehydrogenation and dangling-bond recombination in several hydrogenated amorphous silicon materials

Differential scanning calorimetry DSC was used to study the dehydrogenation processes that take place in three hydrogenated amorphous silicon materials: nanoparticles, polymorphous silicon, and conventional device-quality amorphous silicon. Comparison of DSC thermograms with evolved gas analysis EGA has led to the identification of four dehydrogenation processes arising from polymeric chains A, SiH groups at the surfaces of internal voids A, SiH groups at interfaces B, and in the bulk C. All of them are slightly exothermic with enthalpies below 50 meV/H atoms, indicating that, after dissociation of any SiH group, most dangling bonds recombine. The kinetics of the three low-temperature processes with DSC peak temperatures at around 320 A, 360 A, and 430 °C B exhibit a kinetic-compensation effect characterized by a linear relationship between the activation entropy and enthalpy, which constitutes their signature. Their SiuH bond-dissociation energies have been determined to be ESiuH0=3.14 A, 3.19 A, and 3.28 eV B .I n these cases it was possible to extract the formation energy EDB of the dangling bonds that recombine after SiuH bond breaking 0.97 A, 1.05 A, and 1.12 B. It is concluded that EDB increases with the degree of confinement and that EDB1.10 eV for the isolated dangling bond in the bulk. After SiuH dissociation and for the low-temperature processes, hydrogen is transported in molecular form and a low relaxation of the silicon network is promoted. This is in contrast to the high-temperature process for which the diffusion of H in atomic form induces a substantial lattice relaxation that, for the conventional amorphous sample, releases energy of around 600 meV per H atom. It is argued that the density of sites in the Si network for H trapping diminishes during atomic diffusion.

Optimisation of amorphous and polymorphous thin silicon layers for the formation of the front-side of heterojunction solar cells on p-type crystalline silicon substrates

In this study, we investigate the properties of n-doped amorphous silicon layer a-Si:H(n) combined with intrinsic thin silicon film deposited on high quality p-type CZ crystalline silicon, aiming at developing high performance heterojunction solar cells. The interface characteristics are analysed using Transmission Electron Microscopy TEM and capacitance measurements versus temperature and frequency. The insertion of a thin silicon film at the interface deposited under conditions of polymorphous material drives to a different structure of the interface. Our best front side heterojunction solar cells achieve 15% efficiency on 25 cm 2 , with an open-circuit voltage of 634 mV. A maximum open-circuit voltage of 676 mV has also been obtained on polymorphous/crystalline heterojunctions with a high quality rear local BSF, indicating the excellent interface passivation achieved with the intrinsic layer deposited in conditions of polymorphous silicon. This experimental study reveals the clear potential of p-type substrates for the development of amorphous/crystalline heterojunctions solar cells.

Light-induced defect creation in hydrogenated polymorphous silicon during repeated cycles of illumination and annealing

We have examined how the intensity of the ESR signal associated with dangling bonds in hydrogenated polymorphous silicon is affected by repeating light-soaking (LS) and 100°C annealing cycles. It was found that the light-induced degradation efficiency decreases with repeated LS–100°C annealing cycles. This result is accounted for in terms of the termination of dangling bonds by mobile hydrogen, i.e. the termination is enhanced by more mobile hydrogen as a result of modification of the amorphous network by the LS–annealing cycles, and consequently the net light-induced defect creation rate is reduced. This is in contrast with previous models in which the decrease in the light-induced degradation efficiency on repeated LS–annealing cycles is attributed to an increase in the amount of hydrogen being ineffective for light-induced defect creation.

Thermally Induced Structural Transformations on Polymorphous Silicon

Polymorphous Si is a nanostructured form of hydrogenated amorphous Si that contains a small fraction of Si nanocrystals or clusters. Its thermally induced transformations such as relaxation, dehydrogenation, and crystallization have been studied by calorimetry and evolved gas analysis as a complementary technique. The observed behavior has been compared to that of conventional hydrogenated amorphous Si and amorphous Si nanoparticles. In the temperature range of our experiments (650–700 °C), crystallization takes place at almost the same temperature in polymorphous and in amorphous Si. In contrast, dehydrogenation processes reflect the presence of different hydrogen states. The calorimetry and evolved gas analysis thermograms clearly show that polymorphous Si shares hydrogen states of both amorphous Si and Si nanoparticles. Finally, the total energy of the main Si–H group present in polymorphous Si has been quantified.

Light-induced defect creation in hydrogenated polymorphous silicon

Light-induced defect creation in hydrogenated polymorphous silicon (pm-Si:H) is investigated from electron spin resonance measurements and is compared with that in hydrogenated amorphous silicon (a-Si:H). Light-induced defect creation occurs at room temperature similarly for both types of films prepared at 250 °C. Thermal annealing of light-induced defects is also investigated as a function of temperature. Different behaviours of annealing characteristics for pm-Si:H from those for a-Si:H are observed and discussed. In particular, we observed a decrease of the light-induced defect creation efficiency with repeated light-soaking–annealing cycles and discuss it with respect to the hydrogen bonding in pm-Si:H films.

Nonexponential distributions of tail states in hydrogenated amorphous silicon

The analysis of the time-of-flight (TOF) photocurrent transients that leads to the identification of exponential band tails in hydrogenated amorphous silicon ($a$-Si:H), also predicts an electric-field dependence of the carrier drift mobilities. To account for $a$-Si:H samples, prepared in different ways and deposition rates, that do not show this field dependence of the drift mobility, general analytic expressions for the trap-limited-band-transport TOF signals were examined with nonexponential tail-state distributions. It is found that the use of a Gaussian tail-state component makes it possible to match the experimental curves when a field-independent mobility is measured. For samples that are prepared at, or close to, the conditions used for the plasma-enhanced chemical vapor deposition of standard ``device-quality'' $a$-Si:H, a purely exponential distribution does afford a good description of the experimental data. However, for samples prepared at high deposition rates in an expanding thermal plasma, or for polymorphous silicon, a significant Gaussian component is resolved in the tail-state distribution. These components can be linked to the presence in the amorphous matrix of nanoscopic more ordered silicon regions.

Anomalous crystallization of hydrogenated amorphous silicon during fast heating ramps

Thermal crystallization experiments carried out using calorimetry on several a-Si:H materials with different microstructures are reported. The samples were crystallized during heating ramps at constant heating rates up to 100 K/min. Under these conditions, crystallization takes place above 700 °C and progressively deviates from the standard kinetics. In particular, two crystallization processes were detected in conventional a-Si:H, which reveal an enhancement of the crystallization rate. At 100 K/min, such enhancement is consistent with a diminution of the crystallization time by a factor of 7. In contrast, no systematic variation of the resulting grain size was observed. Similar behavior was also detected in polymorphous silicon and silicon nanoparticles, thus showing that it is characteristic of a variety of hydrogenated amorphous silicon materials.

PECVD grown hydrogenated polymorphous silicon studied using current transient spectroscopies in PIN Diodes

AbstractHydrogenated polymorphous silicon (pm-Si:H) has steadily emerged as a potential replacement of hydrogenated amorphous silicon. Possible changes in the density of gap states due to the presence of crystallites is of central importance in understanding steady state and dynamic characteristics of devices using these materials. We have studied a-Si:H and pm-Si:H grown by PECVD at optimized conditions through the measurement of the steady state reverse current and their transients in PIN devices. The transients are analyzed using isothermal spectroscopic techniques such as Time Analyzed Transient Spectroscopy (TATS), and high resolution Laplace DLTS as a function of temperature. In case of a-Si:H, we obtain the expected signature of emission from a broad density of states in the form of stretched exponentials. In contrast the corresponding spectra for pm-Si:H are dominated by nearly exponential fast current decay processes with discrete energies between 0.20 and 0.26 eV. It is shown that the study of the density of states by dynamic methods such as transient techniques reveal features not accessible to steady state measurements.

Polymorphous silicon thin films produced in dusty plasmas: application to solar cells

We summarize our current understanding of the optimization of PIN solar cells produced by plasma enhanced chemical vapour deposition from silane–hydrogen mixtures. To increase the deposition rate, the discharge is operated under plasma conditions close to powder formation, where silicon nanocrystals contribute to the deposition of so-called polymorphous silicon thin films. We show that the increase in deposition rate can be achieved via an accurate control of the plasma parameters. However, this also results in a highly defective interface in the solar cells due to the bombardment of the P-layer by positively charged nanocrystals during the deposition of the I-layer. We show that decreasing the ion energy by increasing the total pressure or by using silane–helium mixtures allows us to increase both the deposition rate and the solar cells efficiency, as required for cost effective thin film photovoltaics.

Luminescence of polymorphous silicon carbon alloys

Abstract Polymorphous silicon carbon alloys (pm-Si 1− x C x :H) have been prepared from the decomposition of SiH 4 –CH 4 –H 2 mixtures at 200 °C in a standard capacitively coupled RF glow discharge reactor at high pressure, under conditions where silicon clusters, nanocrystals and their agglomerates are produced in the plasma and contribute to the deposition. Visible photoluminescence (PL) was observed at room temperature in these materials in the spectral region from red to orange. The PL intensity and peak position show a strong dependence on the deposition pressure. Moreover, electroluminescent (EL) diodes with p-i-n structure were realized using the optimized materials as the i-layer. EL spectra, comparable to the PL ones, were obtained by applying a DC voltage above 8 V. EL is stable for several hours of continuous operation which makes these devices interesting for applications.

Measurement of built-in potential in polymorphous Si:H p–i–n solar cells

The performance of a solar cell, especially open-circuit voltage ( V oc), is critically related to the built-in potential ( V b). In this work, the open-circuit voltage V oc and the short-circuit current density J sc of hydrogenated polymorphous silicon (pm-Si:H) p–i–n solar cell, deposited by radio-frequency powered plasma-enhanced chemical vapour deposition (RF-PECVD), have been measured. From the measured data at different monochromatic illumination intensities and temperatures, the value of V b of cell was determined by using two different methods: activation energy and differential temperature. The results from both methods were analyzed and compared as a function of excitation wavelength. It was observed that there is a good agreement between the two methods. It was found that V b increases until about 650 nm, and then decreases with increasing wavelength. This behaviour is explained and interpreted in different ways.

Effect of Dopants on the Dynamic of Powder Formation and the Properties of Polymorphous Silicon Thin Films

Polymorphous silicon (pm-Si:H) is a nanostructured material produced by the dissociation of silane-hydrogen mixtures under plasma conditions where silicon radicals, clusters and agglomerates contribute to the growth. The dynamics of power formation in a capacitively coupled radio-frequency (RF) discharge has been studied as a function of dopant gas concentration through the analysis of the evolution of the second harmonic of RF current. The intrinsic and doped films were characterized by spectroscopic ellipsometry, dark conductivity and Raman spectroscopy measurements. We have demonstrated the possibility of obtaining doped pm-Si:H films with transport properties comparable to those of standard amorphous silicon using trimethylboron or phosphine. We have found that the addition of trimethylboron reduces cluster and agglomerate concentration while phosphine has no prominent effect on particle formation. The best ordered polymorphous films were deposited under conditions where not only 1-2 nm clusters but even larger (~10 nm) agglomerates contribute to the growth. For these conditions the deposition rate reaches 5 A/s for i- and n-type, and 7 A/s for p-type materials. The dark conductivity at room temperature was 10 �2 �10 �3 � �1 cm �1 and 10 �6 � �1 cm �1 for the n- and p-type films, respectively.