Polymorphous Silicon Films Research Articles

Comparison of the Effects of Environmental Treatments on Hydrogen Concentration and Energy Gap Variations of Hydrogenated Amorphous and Polymorphous Silicon Films Prepared by PECVD Technique

Hydrogen to silicon (Si–H) bond concentration and strength play important roles in high quality hydrogenated amorphous silicon layers prepared by PECVD techniques. In this paper, a number of amorphous and polymorphous Si layers have been deposited at different plasma conditions where a wide range of hydrogen concentration in the films are obtained. Some of the samples were stored in free air and the others in nitrogen for eight days. The layers were analyzed using AFM, FTIR, Raman, UV–Visible, and TEM immediately after deposition and after treatments. The results indicate that in the amorphous films with appreciable amount of embedded silicon nanocrystals, the variation of hydrogen content behaves differently than that of the amorphous films. It has been observed that treatment in the air increases the energy gap of the nanocrystals surrounded by oxide shells, formed around the surface nanocrystals, due to the quantum confinement effects.

Molecular hydrogen‐induced nucleation of hydrogenated silicon nanocrystals at low temperature

Highly crystallized hydrogenated silicon layers were obtained via the treatment of hydrogenated polymorphous silicon films in a molecular hydrogen ambient. This contrasts other postdeposition studies that obtained nanocrystalline silicon films but necessitated either a plasma activation or high‐temperature annealing. The structure of the samples was analyzed by Raman spectroscopy to determine the crystallite volume fraction, which was found to increase up to 80% within 1 hour of treatment. Atomic force microscopy (AFM) showed that the roughness of the surfaces was found to increase after the H2 treatment. Optical transmission and spectroscopic ellipsometry revealed the pronounced porosity of the films characterized by a static refractive index that is below three, which is a low value for hydrogenated silicon films and a void fraction that is around 15% in the bulk of the films. The effect of the hydrogen molecules on the structure of the films was discussed in terms of the compressive stress exerted by the molecules, trapped in structural inhomogeneities, on the amorphous tissue. It is suggested that for this process to take effect, the films need to be porous and that the amorphous network needs to be in a “relaxed” state.

Influence of the fabrication conditions of polymorphous silicon films on their structural, electrical and optical properties

The structural, optical, and photoelectric properties of polymorphous silicon films produced by plasma-enhanced chemical vapor deposition from a mixture of monosilane and hydrogen at high pressure are studied. Variations in the pressure of the gas mixture used for film production barely change the Raman spectra of the films, but induce changes in the photoconductivity and in the absorption spectrum obtained by the constant-photocurrent technique. The experimentally observed change in the optical and photoelectric parameters of the films is attributed to some structural changes induced in the films by variations in the deposition parameters.

Features of the structure and defect states in hydrogenated polymorphous silicon films

The structural and electronic properties of thin hydrogenated polymorphous silicon films obtained by plasma-enhanced chemical vapor deposition from hydrogen (H2) and monosilane (SiH4) gas mixture have been studied by means of transmission electron microscopy, electron paramagnetic resonance (EPR) spectroscopy, and Raman spectroscopy. It has been established that the studied films consist of the amorphous phase containing silicon nanocrystalline inclusions with the average size on the order of 4–5 nm and the volume fraction of 10%. A signal was observed in the hydrogenated polymorphous silicon films during the EPR investigation that is attributed to the electrons trapped in the conduction band tail of microcrystalline silicon. It has been shown that the introduction of a small fraction of nanocrystals into the amorphous silicon films nonadditively changes the electronic properties of the material.

Probing dusty-plasma/surface interactions with a heat flux microsensor

Heat flux microsensor—a thermopile with millisecond response time—was employed for direct measurements of total energy flux to a floated surface in a dusty rf discharge. The measurements were performed in silane-hydrogen plasmas at discharge conditions close to those used for the growth of polymorphous silicon films. At rf power between 10 and 50 W, the measured energy flux was in the range of 20-250 mW/cm2. Due to fast time response, the thermopile was also capable of tracing the dynamics of nanoparticle growth. A significant decrease (about 30%) in the energy flux was observed during the agglomeration phase.

Study of the effect of the deposition parameters on the structural, electric and optical characteristics of polymorphous silicon films prepared by low frequency PECVD

Abstract In this work we present our results on the deposition and characterization of polymorphous silicon (pm-Si:H) films prepared by low frequency plasma enhanced chemical vapor deposition (LF-PECVD). We have studied the effect of the plasma deposition parameters (as the chamber pressure and gas flow rates of SiH4 and H2) on the structural, electric, and optical characteristics of the films. The temperature dependence of conductivity (σ(T)), activation energy (Ea), optical band gap (Eg) and deposition rate (Vd) were extracted for pm-Si:H films deposited at different pressure values and different gas flow rates. We observed that the chamber pressure is an important parameter that has a significant effect on the electric characteristics, and as well on the morphology of the pm-Si:H films (deduced from atomic force microscopy). It was found an optimal pressure range, in order to produce pm-Si:H films with high Ea and room temperature conductivity, σRT, which are key parameters for thermal detection applications.

Structural and electronic properties of hydrogenated polymorphous silicon films deposited at high rate

Structural and electronic defect-related properties of hydrogenated polymorphous silicon (pm-Si:H) films deposited at high rates of up to 9 Å/s have been investigated. Raman spectroscopy reveals the usual peaks characteristic of the amorphous nature of the material, as well as intermediate peaks associated with the effect of nanocrystallites in the amorphous matrix. These results have been correlated with infrared measurements. Transport and defect-related properties deduced from a set of complementary techniques show that hydrogenated polymorphous silicon presents better properties compared to the standard amorphous silicon (a-Si:H). In particular, capacitance measurements on Schottky diodes reveal a very low density of states at the Fermi level. Based on experimental results, band gap states modeling has been achieved for pm-Si:H and a-Si:H by means of a numerical calculation software. Some parameters that are not directly measured with experiments, such as capture cross sections, could be deduced from simulations and differences between pm-Si:H and a-Si:H could be further revealed.

Growth mechanism of microcrystalline and polymorphous silicon film with pure silane source gas

Microcrystalline, polymorphous and amorphous silicon films were deposited in a standard radio frequency (rf) plasma enhanced chemical vapour deposition system at a high growth rate by using pure silane as source gas. The influence of thermal gradient variation on the growth model of the hydrogenated silicon film was investigated. The structure and optical properties of these silicon samples were characterized by Raman spectroscopy, spectroscopic ellipsometry and Fourier transform infrared spectrometry. The phase state at the boundary of the deposited silicon films was analysed using ion drag and plasma-induced thermophoresis on particles in a rf glow discharge. The results showed that control of the thermal gradient allows a switch from polymorphous to microcrystalline silicon growth. The crystalline volume fraction increases with increasing film thickness, and this demonstrates that there is a crystalline gradient between the surface and the bottom of the microcrystalline silicon film.

Structure and 1/f noise of boron doped polymorphous silicon films

The influence of structure variation on the1/f noise of nanometric boron doped hydrogenated polymorphous silicon (pm-Si:H)films was investigated. The films were grown by the conventional radiofrequency plasma enhanced chemical vapor deposition (PECVD) method.Raman spectroscopy was used to reveal the crystalline volume fraction(Xc) and crystal size of the pm-Si:H. The measurement of optical and structure properties wascarried out with spectroscopic ellipsometry (SE) in the Tauc–Lorentz model. AFourier transform infrared (FTIR) spectrometer was used to characterize thepresence of nanostructure-sized silicon clusters in pm-Si:H film deposited on KBrsubstrate. The electrical properties of the films were measured using evaporatedcoplanar nickel as the electrode. A semiconductor system was designed to obtain the1/f noiseof pm-Si:H film as well as that of amorphous and microcrystalline silicon films. The results demonstratethat the 1/f noise of pm-Si:H is nearly as low as that of microcrystalline silicon and much lowerthan that of amorphous silicon. The disorder to order transition mechanism ofcrystallization was used to analyze the decrease of noise compared with amorphoussilicon.

Study on crystallization mechanism of hydrogenated silicon film

In this paper, amorphous, microcrystalline and polymorphous silicon films were prepared by plasma enhanced chemical deposition. Crystalline volume fraction of microcrystalline silicon was deduced from the Raman spectrum, and this fraction was validated using Bruggeman effective medium approximation (BEMA) model in spectroscopic ellipsometry measurement. The influence of thermal gradient on the deposition mechanism of microcrystalline and polymorphous silicon was investigated using a theoretical model. The dependence of crystalline volume fraction on film thickness shows there is a crystalline gradient between bottom and surface of microcrystalline film, and there is not such a gradient in polymorphous silicon film. Polymorphous and microcrystalline silicon have similar ordered state and density, which are signifieantly higher than those of amorphous silicon.

Polymorphous silicon thin films deposited at high rate: Transport properties and density of states

Hydrogenated polymorphous silicon (pm-Si:H) films were fabricated by conventional RF plasma enhanced chemical vapour deposition at high deposition rate (~ 8 Å/s). The details of DOS distribution in the whole mobility gap and transport properties of both types of carriers were studied using a set of complementary techniques before and after light-soaking. The results show that these properties are similar to those of standard a-Si:H, or even can exceed them, at least for diffusion length in both states before and after light-soaking.

Investigation of the microstructure and optical properties of hydrogenated polymorphous silicon films prepared with pure silane

The dependences of microstructure and optical properties of hydrogenated polymorphous silicon (pm-Si:H) films on total gas pressure were studied. Instead of using high diluted silane in H2, pure silane was used as the source gas. The films were grown by the radio-frequency plasma-enhanced chemical vapour deposition method. Fourier-transform infrared spectrometry was used to characterize the presence of Si m H n clusters in pm-Si:H film deposited on KBr substrate. Atomic force microscopy (AFM) analysis characterized the morphology of the pm-Si:H films and X-ray diffraction at grazing incidence angle (XRDGI) microstructure analysis also confirmed the existence of Si m H n nanocrystalline clusters in pm-Si:H. The thickness and optical constants of the films were measured by spectra ellipsometry as well as scanning electron microscopy. Derived using the Tauc relation, the dependence of optical bandgap, Eg , and coefficient, B, on the pressure during deposition process is discussed. The influence of inter-electrode distance on growth rate and surface smooth was analyzed using AFM.

Synthesis of silicon nanocrystals in silane plasmas for nanoelectronics and large area electronic devices

The synthesis of silicon nanocrystals in standard radio-frequency glow discharge systems is studied with respect to two main objectives: (i) the production of devices based on quantum size effects associated with the small dimensions of silicon nanocrystals and (ii) the synthesis of polymorphous and polycrystalline silicon films in which silicon nanocrystals are the elementary building blocks. In particular we discuss results on the mechanisms of nanocrystal formation and their transport towards the substrate. We found that silicon nanocrystals can contribute to a significant fraction of deposition (50–70%) and that they can be positively charged. This has a strong influence on their deposition because positively charged nanocrystals will be accelerated towards the substrate with energy of the order of the plasma potential. However, the important parameter with respect to the deposition of charged nanocrystals is not the accelerating voltage but the energy per atom and thus a doubling of the diameter will result in a decrease in the energy per atom by a factor of 8. To leverage this geometrical advantage we propose the use of more electronegative gases, which may have a strong effect on the size and charge distribution of the nanocrystals. This is illustrated in the case of deposition from silicon tetrafluoride plasmas in which we observe low-frequency plasma fluctuations, associated with successive generations of nanocrystals. The contribution of larger nanocrystals to deposition results in a lower energy per deposited atom and thus polycrystalline films.

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

Characterization of Polymorphous Silicon Thin Film and Solar Cells

Polymorphous silicon (pm-Si:H) films have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the pm-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. There are a blue shift for the stretching mode of IR spectra and a red shift for the wagging mode. The shifts are attributed to the variation of the microstructure. By using pm-Si:H film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100mw/cm(2)) at room temperature (T-R).

Composition, Structure and Optical Characteristics of Polymorphous Silicon Films Deposited by PECVD at 27.12 MHz

Composition, Structure and Optical Characteristics of Polymorphous Silicon Films Deposited by PECVD at 27.12 MHz

Modeling of reverse bias dark currents in pin structures using amorphous and polymorphous silicon

The reverse bias dark currents in pin solar cells having standard amorphous silicon (a-Si:H) or polymorphous silicon (pm-Si:H) as the intrinsic layer have been studied using experiments and modeling. Our electrical–optical model, takes into account high field enhancement of thermal generation via the Poole–Frenkel effect. Polymorphous silicon films have a higher hole mobility and a lower bulk density of states (DOS) than standard a-Si:H. However this does not result in lower dark leakage currents, except in thick (∼2.5 μm) pin diodes. For the latter, we have measured dark leakage currents as low as ∼3 pA cm −2 at −3 V, which to our knowledge is the lowest reported for such devices. Modeling reveals that the dark leakage current is controlled by thermal generation through the mid-gap defects in the intrinsic layer for a-Si:H cells, while it is dominated by the p/i interface defects in pm-Si:H cells. We conclude by estimating from the model, the dark leakage current possible in pm-Si:H diodes, if their p/i interface DOS were to be as in standard a-Si:H ones.

Characterization of the density of states of polymorphous silicon films produced at 13.56 and 27.12 MHz using CPM and SCLC techniques

The aim of this paper is to compare the density of bulk states (DOS) of polymorphous silicon (pm-Si:H) films produced by plasma enhanced chemical vapor deposition at 13.56 and 27.12 MHz using the constant photocurrent method and the space charge limited current (SCLC) technique. The data achieved revealed that the set of films produced present similar DOS. Apart from that, data concerning the correlation of the deposition conditions that lead to the production of pm-Si:H as well as their characteristics, such as the hydrogen content and how hydrogen is bonded, will be discussed, giving special emphasis to the set of mechanical stresses developed. By doing so we could get a better understanding of the nature of hydrogen bonding in pm-Si:H films as well as to determine the role of the excitation frequency on the film's performances, where films with amounts of hydrogen around 20 at.% can have DOS as low as 8 × 10 14 cm −3 with Urbach energies in the range of 41–50 meV.