Employing constant photocurrent method for the study of defects in silicon thin films
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Different optical characterization techniques have been performed on a series of microcrystalline silicon thin films deposited using very high-frequency-assisted plasma-enhanced chemical vapor deposition process. The constant photocurrent method has been employed to study the defects states in density of states spectra of hydrogenated microcrystalline silicon thin films. The photocurrent measurements demonstrate anisotropy in the optoelectronic properties of the material. We have analyzed the optical absorption coefficient from UV spectroscopy and with the help of constant photocurrent method. The spectra have been analyzed in broad region and are presented for both the cases, i.e., surface and bulk light scatterings. The spectra were interpreted in terms of disorder, resulting defect density, crystalline/amorphous volume fractions and material morphology. The subgap-related parameters such as absorption coefficient, characteristic energy E0 of tail states and density of subgap defect states together with an estimate of the bandgap of silicon films prepared at various crystalline fractions have also been estimated. The density of localized tail states is found to fall exponentially toward the gap with band tail width of about 110 meV.
KeywordsMicrocrystalline silicon thin films Constant photocurrent method PECVD Optical absorption coefficient Density of states
Recently, hydrogenated micro/nanocrystalline silicon thin films have attracted worldwide interest due to their diverse applications [1, 2, 3, 4, 5]. Microcrystalline silicon when combined with amorphous silicon in micromorph silicon tandem solar cell has the potential to reach 15% stable efficiency. Also compared to hydrogenated amorphous silicon (a-Si/H), it offers higher stability against light-induced degradation and wide absorption that extends to near infrared . Higher efficiencies are achievable, which result from good passivation of defects by hydrogen in the plasma growth process of μc-Si/H and also due to enhanced optical absorption leading to efficient sunlight absorption in 1–4-μm-thick films and solar cells. The silicon dangling bond is the dominant defect (recombination center) in this material. The optical, electronic properties and the performance of the microcrystalline silicon films have been correlated with deposition parameters and structural properties . It has complex heterogeneous structure comprising nano-sized crystallites, clustered into larger micrograins; all embedded in amorphous matrix. Due to heterogeneity, it leads to difficulties in explaining electronic transport and optical properties. Therefore, the detailed knowledge of the density of states (DOS) in μc-Si/H is of great importance to understand completely the transport mechanism. It is not surprising that still there is no conclusive DOS map. The absorption in the bandgap in hydrogenated amorphous silicon and related alloys is an important parameter to determine the suitability of material for device fabrication . A number of techniques, such as constant photocurrent method (CPM), photothermal deflection spectroscopy (PDS), and dual-beam photoconductivity (DBP) have been used to determine the exact information about the sub-bandgap absorption [8, 9]. These spectroscopy techniques [10, 11, 12] have also been used to determine the DOS  in the lower energy range of the bandgap near the valence band, whereas transient photoconductivity (TPC) has been used to determine the DOS in the upper energy range of the gap, close to conduction band . It is supposed that at low photon energies, an additional absorption appears involving states near the midgap, which could be silicon dangling bonds, forming a bump in the density of states. This subgap region is highly sensitive to deposition conditions. CPM has important advantages over PDS, such as it does not measure the absorption of the substrate. In the present investigation, we have used the constant photocurrent method in the ac mode to measure the absorption coefficient spectrum in microcrystalline silicon (µc-Si/H) films deposited using very high-frequency plasma-enhanced chemical vapor deposition system. Also, we have used derivative method to convert the measured data to DOS distribution curve.
Microcrystalline silicon films were deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique. The silane was used as precursor gas (5% silane diluted in hydrogen). The excitation frequency used was 60 MHz (VHF). The films were deposited on corning 7059 glass at a substrate temperature of 270 °C. Before the deposition process, proper cleaning of the substrates was adopted using standard procedures . Chamber cleaning was also carried out by Ar flushing and Ar plasma to make the chamber environment suitable for deposition process by making it free of residual contaminants. The power applied was varied in the range 10–50 W keeping other deposition parameters constant. The gaseous mixture within the chamber was maintained at a constant pressure of 0.18 Torr during deposition.
The thickness of films was measured using stylus profilometer (Ambios XP 200, USA). The structural properties were analyzed using Raman spectroscopy (LabRAM HR 800, Horiba JY) to estimate crystalline volume fraction (Xc) and the average crystallite size in the deposited films. The excitation of the samples was performed with an air-cooled Ar+ laser (Spectra physics) tuned at 488 nm. Measurements were carried out in the backscattered geometry using a 50-LWD microscope objective. The beam was focused at a spot size of 1.19 mm, and the power density was kept low to avoid laser-induced crystallization on the films or excessive heating on the probe region. With varied power range, the films with different crystalline volume fraction were obtained. A double-beam Hitachi UV–visible spectrophotometer was used to measure the optical absorption, and the optical bandgap (Eg) was estimated on the basis of Tauc’s plot. CPM measurements were done on the sample in the ac mode in a coplanar configuration made by evaporating aluminum electrodes with electrode spacing of 0.078 cm and a width of 1.0 cm. As the light source, we used a tungsten lamp of 250 W power and the focused light was allowed to pass through a high-intensity monochromator before falling on the sample detector. In order to keep the photocurrent constant at different energies, we varied the light intensity. Silicon detectors were used to detect the incident light intensity. The intensity was then converted to the number of photons. The field emission scanning electron microscopy (FESEM) images were obtained with a NOVA NANOSEM 450, FEI electron microscope .
Constant photocurrent method
Results and discussion
As mentioned, microcrystalline silicon thin films were deposited on corning 7059 glass using very high-frequency PECVD process. Measurements were performed on coplanar geometry using aluminum electrodes of 0.08 cm spacing.
UV–VIS absorption spectroscopy
A saturation region corresponding to direct (extended states, i.e., valence band to conduction band) transition at higher energy (above about 1.9 eV) is realized.
The second region is an exponential Urbach edge region where the absorption coefficient varies until it meets the low energy defect absorption shoulder. The exponential portion of the optical absorption is named as Urbach tail and E0 defines the characteristic energy of the band tail. This transition is due to a tail state to extended band transition.
There is a third region where the sub-bandgap tail is superimposed on the Urbach edge. This is due to photon absorption by gap states. Here, the absorption is related to transition between localized states and extended states.
It is known that owing to the large density of states below the midgap the transition from occupied gap states below the Fermi level Ev to the conduction band contributes much more to the subgap absorption than transitions from the valence band to unoccupied localized states above Ev. The excess absorption due to subgap defect states is calculated from the subgap tail and Urbach edge. From Fig. 3, we observe an exponential tail around 1.1 eV. So this tail state can be interpreted as a disorder-induced broadening of the indirect absorption edge of c-Si. The defect associated with absorption largely below 1 eV is attributed to silicon dangling bonds mainly at the grain boundaries and in the amorphous tissue.
The size of nanocrystals in silicon is important to understand quantum confinement of the electrons. Information related to crystalline fraction is important to monitor stability of films . It is already reported that grain boundary component decreases with increasing crystalline volume fraction. The best sample results where the crystallite size is obtained at sample deposited at Xc of 54% along with lower fraction of grain boundaries. The films deposited above this crystalline fraction further results in reduced crystallite size with the increased grain boundaries. This can be explained based on nonuniform film deposition at higher powers. It might be assumed that at higher powers, etching process takes place and results in the increased crystalline fraction, but with the decreased crystallite size and increased grain boundaries. The samples with increased grain boundaries can be referred to as defective structures with poor optical and electrical properties. Further, these structural results are correlated with defect measurement techniques.
Crystallite size and crystalline volume fraction in various samples
Crystallite size, L (nm)
The DOS profile of microcrystalline silicon films is an important study for defect analysis in silicon films. Although it is an estimate, it can help in visualizing the shape of localized band tail distribution near one of the band edges. It is already known that density of localized band tails is exponentially distributed in the conduction band region. In the calculation of DOS, it is generally assumed that the band tails extend to the mobility edge . High-quality a-Si/H films have a somewhat lower deep defect density than μc-Si/H, suggesting that the amorphous tissue in mixed phase μc-Si/H is of poorer quality than homogeneous a-Si/H. This could be attributed to the passivating hydrogen locked into grain boundaries and being unavailable to terminate dangling bonds.
In the present work, the optoelectronic properties of microcrystalline silicon films have been studied by using ac-CPM. The CPM measured the (photocurrent) absorption spectrum directly in absolute units (cm−1) without influence of interference fringes. The defect density in the film was found to decrease from 1019 to 1017 cm−3 with the increasing crystalline volume fraction. The Urbach energy also decreased from 200 to 115 meV. However, the defect density and Urbach energy were found to increase above Xc = 54%. The disorder seemed to decrease with increasing crystallinity in the films. It was found that the light soaking enhances the value of the characteristic energy E0 and the defect density Ns in the material and this is accompanied by an increase in the subgap absorption related to an increase in the number of defects, probably dangling bonds. In view of their good optoelectronic properties, these films appear to be suitable for optoelectronics devices.
One of the authors (SJ) thanks the Department of Science and Technology, Government of India for providing NPDF (Project No. MI01803G) at IIT Delhi.
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