Nanocrystalline Silicon Films Research Articles

Development of high conducting phosphorous doped nanocrystalline thin silicon films for silicon heterojunction solar cells application

We have investigated the plasma-enhanced chemical vapor deposition growth of the phosphorus-doped hydrogenated nanocrystalline silicon (n-nc-Si:H) film as an electron-selective layer in silicon heterojunction (SHJ) solar cells. The effect of power densities on the precursor gas dissociation are investigated using optical emission spectra and the crystalline fraction in n-nc-Si:H films are correlated with the dark conductivity. With the P d of 122 mW cm−2 and ∼2% phosphorus doping, we observed Raman crystallinity of 53%, high dark conductivity of 43 S cm−1, and activation energy of ∼23 meV from the ∼30 nm n-nc-Si:H film. The n-nc-Si:H layer improves the textured c-Si surface passivation by two-fold to ∼2 ms compared to the phosphorus-doped hydrogenated amorphous silicon (n-a-Si:H) layers. An enhancement in the open-circuit voltage and external quantum efficiency (from >650 nm) due to the better passivation at the rear side of the cell after integrating the n-nc-Si:H layer compared to its n-a-Si:H counterpart. An improvement in the charge carrier transport is also observed with an increase in fill factor from ∼71% to ∼75%, mainly due to a reduction in electron-selective contact resistivity from ∼271 to ∼61 mΩ-cm2. Finally, with the relatively better c-Si surface passivation and carrier selectivity, a power conversion efficiency of ∼19.90% and pseudo-efficiency of ∼21.90% have been realized from the SHJ cells.

High conductivity characteristics of phosphorus-doped nanocrystalline silicon thin films by KrF pulsed excimer laser irradiation method.

The microstructure and high conductivity properties of phosphorus-doped nanocrystalline silicon films were investigated on samples prepared by a plasma-enhanced chemical vapor deposition technique and the KrF pulsed excimer laser irradiation method. The results of Fourier transform infrared spectroscopy and Raman spectroscopy show that Si nanocrystallites with an average diameter of 2 nm to 3 nm are formed in the film. The degree of crystallinity increases with the increase of laser radiation intensity, while the content of hydrogen decreases gradually. More phosphorus atoms are substitutionally incorporated into the nc-Si dots under higher laser irradiation fluence, which is responsible for the high dark conductivity. By controlling the laser fluence at 1.0 J cm-2, the dark conductivity as high as 25.7 S cm-1 can be obtained. Based on the measurements of temperature-dependent conductivity, the carrier transport processes are discussed. The phosphorus doping and the increase of electron concentration are considered to be the reason for high dark conductivity and extremely low conductivity activation energy.

Enhanced electrical stability of thin film transistors based on nanocrystalline silicon films

In this work, nanocrystalline silicon (nc-Si) films were achieved by using normally processing method of amorphous silica thin film transistors (a-Si TFTs), plasma enhanced chemical vapor deposition (PECVD). The effects of PECVD process parameters on the crystallization rate of nc-Si films were comprehensively studied and then an optimized nc-Si film has been achieved with a crystallinity of 50.87%. The electrical properties and stability of nc-Si TFTs are further investigated and compared with a-Si TFTs. The ΔV th of the nc-Si TFTs are 0.14 V and −3.32 V under positive bias-temperature-illumination stress (60 °C, 30 V, 6000 nit) and negative bias-temperature-illumination stress (60 °C, −30 V, 6000 nit) after 1 h, which are 1.96 V and −5.51 V for a-Si TFT, respectively. The result reveals better photo stability of nc-Si TFTs than a-Si TFTs which is attributed to fewer defect states in nc-Si films.

Thermoelectric properties of nanocrystalline silicon film grown by PECVD

Thermoelectric properties of nanocrystalline silicon film grown by PECVD

Reducing electrical losses of textured monolithic perovskite/silicon tandem solar cells by tailoring nanocrystalline silicon tunneling recombination junction

Compatible with crystalline silicon industry standards, thermal evaporation of perovskite cells on fully textured wafers with noticeable light trapping has been established. Then perovskite/silicon tandem solar cells prepared by this method presented potentially high photocurrent, but their open-circuit voltage ( V oc ) and fill factor (FF) were moderate. Here, electrical losses of perovskite/silicon tandem solar cells were reduced through combining the theoretical model of tunneling recombination junction (TRJ) with the growth mechanism of nanocrystalline silicon films. Their V oc loss was compensated 38 mV (from 1.767 V to 1.805 V), and FF was increased from 64.29% to 74.15% via adjusting the transport and recombination of carriers in TRJ. The fully textured tandem solar cell exhibited an efficiency of 26.91%, which provided an approach towards tandem devices with more than 30% efficiency. • The theoretical model of TRJ and the growth mechanism of nanocrystalline silicon films were both combined to reduce electrical losses. • The V oc gain reached 38 mV (from 1.767V to 1.805V), and the FF was increased from 64.29% to 74.15% for tandem device. • The PCE of champion tandem solar cell reached 26.91%.

Thermal Properties of Nanocrystalline Silicon Nanobeams

AbstractControlling thermal energy transfer at the nanoscale and thermal properties has become critically important in many applications since it often limits device performance. In this study, the effects on thermal conductivity arising from the nanoscale structure of free‐standing nanocrystalline silicon films and the increasing surface‐to‐volume ratio when fabricated into suspended optomechanical nanobeams are studied. Thermal transport and elucidate the relative impact of different grain size distributions and geometrical dimensions on thermal conductivity are characterized. A micro time‐domain thermoreflectance method to study free‐standing nanocrystalline silicon films and find a drastic reduction in the thermal conductivity, down to values below 10 W m–1 K–1 is used, with a stronger decrease for smaller grains. In optomechanical nanostructures, this effect is smaller than in membranes due to the competition of surface scattering in decreasing thermal conductivity. Finally, a novel versatile contactless characterization technique that can be adapted to any structure supporting a thermally shifted optical resonance is introduced. The thermal conductivity data agrees quantitatively with the thermoreflectance measurements. This study opens the way to a more generalized thermal characterization of optomechanical cavities and to create hot‐spots with engineered shapes at the desired position in the structures as a means to study thermal transport in coupled photon‐phonon structures.

Versatility of Nanocrystalline Silicon Films: from Thin-Film to Perovskite/c-Si Tandem Solar Cell Applications

Doped hydrogenated nanocrystalline (nc-Si:H) and silicon oxide (nc-SiOx:H) materials grown by plasma-enhanced chemical vapor deposition have favourable optoelectronic properties originated from their two-phase structure. This unique combination of qualities, initially, led to the development of thin-film Si solar cells allowing the fabrication of multijunction devices by tailoring the material bandgap. Furthermore, nanocrystalline silicon films can offer a better carrier transport and field-effect passivation than amorphous Si layers could do, and this can improve the carrier selectivity in silicon heterojunction (SHJ) solar cells. The reduced parasitic absorption, due to the lower absorption coefficient of nc-SiOx:H films in the relevant spectral range, leads to potential gain in short circuit current. In this work, we report on development and applications of hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) from material to device level. We address the potential benefits and the challenges for a successful integration in SHJ solar cells. Finally, we prove that nc-SiOx:H demonstrated clear advantages for maximizing the infrared response of c-Si bottom cells in combination with perovskite top cells.

Large‐scale data principal component analysis of phase transition from a‐Si:H to nc‐Si:H using PECVD optical emission spectra

Phase transformation of hydrogenated nanocrystalline silicon (nc-Si:H) films was essential in the solar cells industry. Therefore, the plasma chemistry monitoring of in-situ diagnostics optical emission spectroscopy (OES) of plasma-enhanced chemical vapour deposition (PECVD) is deemed crucial for investigating the phase transformation. The proposed PC2-OES algorithm can be used to monitor the PECVD process health condition of the crystallisation rate at different hydrogen dilution ratios. Principal component analysis (PCA) was performed to distinguish the crucial role of nc-Si:H emission characteristics of plasma chemistry and the resultant crystallisation rate. Measurement results revealed that OES spectra characterisation confirmed that the crystallisation rate index (Hα*/SiH*) was highly correlated to hydrogen dilution ratio (R) and phase transformation of nc-Si:H film. The proposed PCA-based evaluation method will provide valuable information to reflect consistently the crystallisation rate of deposited films with different hydrogen dilution ratio and possibly other processing parameters of mass flow rate and applied power density.

Room temperature oxidation of Si nanocrystals at dry and wet air

Oxidation of HF vapor-etched nanocrystalline silicon films, prepared by drop coating from nanocrystalline Si sol in acetonitrile, was studied. Oxidation of nanocrystalline silicon at room temperature in air with 5% and 86% relative humidity was observed by means of infrared spectroscopy for 2 days. The change in film mass after 15 h of oxidation was determined using quartz crystal microbalance. In dry air, film mass and integral intensity of bands attributed to vibrations in Si3 − x–Si–Hx and Si–O–Si groups changed linearly with time. In humid air, intensity of in Si3 − x–Si–Hx band decays exponentially and intensity of Si–O–Si band increases as a square root of oxidation time. Film mass gain after 15 h of oxidation corresponds to an average oxide layer thickness of 0.02 nm in dry air and 0.51 nm in wet air.

Thermoelectric Properties of Nanocrystalline Silicon Films Prepared by Hot-Wire and Plasma-Enhanced Chemical-Vapor Depositions

We report thermoelectric measurements over a temperature range of 80 K to 300 K of heavily boron-doped nanocrystalline silicon films prepared by hot-wire and plasma-enhanced chemical-vapor depositions. The nanocrystalline silicon films were doped by either gaseous deposition precursors or post-deposition ion implantation, resulting in boron concentrations ranging from 1–2×1020 cm−3 to 3 × 1021 cm−3. Reasonable values of the Seebeck coefficient and electrical conductivity were obtained at 300 K, comparable to many other research work. We also report thermal conductivity measurements on these films before doping, which we use to estimate their prospective thermoelectric efficiency. These measurements show values as low as 0.76 W/mK at 300 K which depend highly upon the grain sizes of the nc-Si films. We find that post-deposition doping by ion-implantation is more effective at enhancing the power factor than gaseous doping, and the power factor is only weakly dependent upon doping concentration for the films doped by ion implantation. We conclude that improvements of the thermoelectric efficiency of nc-Si films may depend more on a reduction of their thermal conductivity than doping optimization. The small grain sizes and the low thermal conductivity of the undoped nc-Si films accomplished in this work are therefore encouraging developments.

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.

Quantum size effects and tunable visible photoluminescence in a-Si:H/nc-Si:H superlattices

Quantum size effects are commonly observed in semiconductor nanocrystals and quantum dots. Here, we demonstrate unexpected quantum size effects in an unusual bulk system with multiple interfaces, consisting of alternating layers of nanocrystalline silicon (nc-Si:H) and amorphous silicon (a-Si:H) material thin films. The nc-Si:H layers consist of silicon nanocrystals embedded in an amorphous matrix, with an amorphous-crystalline interface separating the two structures. Plasma-enhanced chemical vapor deposition was utilized to grow nanocrystalline-amorphous silicon superlattices with a varying thickness of the nanocrystalline layer. Strong visible photoluminescence at room temperature was deconvoluted into individual peaks. As the nanocrystalline silicon layer thickness was increased from 5 to 20 nm, the photoluminescence spectra red-shifted with the emission wavelength varying as d2 (d is the size of the nanocrystallites), the characteristic signature underlying quantum size effects. The size d of the nanocrystals was estimated by the measured shift of the Raman peak, and could be tuned by varying the thickness of the nc-Si:H layers. High resolution transmission electron microscopy show nanocrystals with a narrow size distribution, in an amorphous matrix. We also observe long wavelength photoluminescence from interfacial states that leads to persistent photconductivity. Nanocrystalline-amorphous superlattices offer a unique pathway for synthesizing embedded nanocrystals with controlled sizes and photonic signatures.

Modelling the simultaneous increase of the conductivity and the Seebeck coefficient in highly B-doped nc-Si

A simultaneous increase of the Seebeck coefficient and the electrical conductivity resulting in a remarkable enhancement of the thermoelectric power factor was observed in heavily boron doped nanocrystalline silicon films. To understand the underlying mechanisms for this behavior, we explored the thermoelectric transport properties with a theoretical model based on Boltzmann transport equation in the relaxation time approximation. Energy filtering has been included in the model assuming an energy threshold in the transmission coefficient of holes. The relevant scattering mechanisms for holes in bulk silicon have been taken into account. It has been found that the simultaneous increase of S and σ can be interpreted by the synergy of two processes that take place upon annealing: the redistribution of ionized scatterers and the formation of energy barriers. The redistribution of ionized scatterers forms energy barriers at the grain boundaries and increases Seebeck coefficient. Moreover, it decreases the density of ionized scatterers in the grains and increases the mobility. The increase of the mobility is such that over-compensates the decrease of the conductivity due to energy filtering of carriers by the energy barriers. Hence, the conductivity increases simultaneously with the Seebeck coefficient as the annealing process progresses

Lithium Titanate Matrix-Supported Nanocrystalline Silicon Film as an Anode for Lithium-Ion Batteries.

A facile preparation method of a Si-based anode with excellent cycling property is urgently required in the process of preparing lithium-ion batteries (LIBs). Here, lithium titanate (LTO) matrix-supported nanocrystalline Si film is prepared by radio frequency (RF) magnetron cosputtering utilizing LTO and silicon (Si) targets as the sputtering source. LTO-supported nanocrystalline Si film electrodes revealed a repeatable specific capacity of 1200 mA h g-1 at 150 mA g-1 with a maintenance of more than 75% even after 800 cycles. The remarkable electrochemical properties of the LTO-Si composite films could be attributed to the LTO matrix, preventing the electrolyte from directly making contact with the nanocrystalline Si materials, alleviating the stress of the periodic volume change and further providing efficient and rapid pathways for lithium-ion transport. The results suggest that Si-based LTO composite films are prospective anodes for LIBs, with high capacities and long cycling stabilities.

Evolution of structural topology of forming nanocrystalline silicon film by atomic-scale-mechanism-driven model based on realistic network

To establish a description of realistic structural evolution of a growth film, we propose a local definite continuous-random-network (CRN) structure combined with a kinetic Monte Carlo (KMC) method based on an atomic-scale mechanism from first-principles density-functional-theory computations and molecular-dynamics computations. The proposed CRN-KMC method elucidates the evolution of elaborate topological structure and the transformation from amorphous phase to nanocrystalline phase of Si films, which is essentially attributed to the atomic interactive behavior of film growth. The method further predicts the realistic structural networks of a growing film at various temperatures based on various atomic-scale mechanisms competing with each other, mechanisms that not only essentially drive the radical from physisorption to chemisorption with the film surface, but also decidedly influence the film-surface chemical composition. In particular, we find the evolution of topological structure’s critical dependence on the compositions of the film surface and H-induced crystallization mechanism, which provide the important information for the strategy for determining optimized deposition conditions for local crystal formation. The results of the evolution of the structural network indicate that the structure of film is similar the CRN model’s representation at relative lower temperature, and is in full agreement with the inhomogeneous crystalline model at relative higher temperature without an abrupt phase change from polycrystalline to amorphous. Our CRN-KMC realistic structure model has significance for exploring the relation of various atomic-scale mechanisms to the phase transformation of growing films.

Efficacy of Raman mapping over ellipsometric spectroscopy and XRD for characterization of structurally heterogeneous PLD nc-Si thin films

The present report demonstrates the efficacy of Raman mapping over X-Ray diffraction (XRD) and spectroscopic ellipsometry (SE) in structural characterization of nanocrystalline Silicon (nc-Si) films having spatial heterogeneity in crystallinity and structure. The Si films were deposited via pulsed-laser deposition (PLD) technique at room temperature and elevated substrate temperature (Ts) of 400 °C and 700 °C, under vacuum (∼10−6 mbar). XRD showed that all films were polycrystalline while SE studies showed that only at 700 °C, the film was nanocrystalline in nature otherwise largely amorphous at lower Ts. This discrepancy in results is well explained by Raman maps which confirmed the presence of micron sized structures composed of nc-Si embedded in otherwise uniform matrix comprised of amorphous Silicon (a-Si) for films fabricated at room temperature and 400 °C whereas at 700 °C the uniform background matrix is dominated by nc-Si. The real and imaginary parts of pseudo dielectric function (ε1,ε2) versus energy spectra obtained for films grown at RT, 400 °C, and 700 °C were modeled and fitted to substrate/film/top SiO2 layered structure. The simulated (ε1,ε2) patterns are in excellent agreement with the recorded spectra. From the SE studies the band gap and thickness of the films were estimated. Band gap energy was found to vary from 1.35 to 1.30 eV approximately with increasing Ts from room temperature to 400 °C, then gradually increases to 1.55 eV at 700 °C. The variation of other optical constants of the films as a function of Ts also reflects the structural transition from amorphous-like to nanocrystalline-like with increasing Ts.

From amorphous to nanocrystalline: the effect of nanograins in an amorphous matrix on the thermal conductivity of hot-wire chemical-vapor deposited silicon films

We have measured the thermal conductivity of amorphous and nanocrystalline silicon films with varying crystalline content from 85 K to room temperature. The films were prepared by the hot-wire chemical-vapor deposition, where the crystalline volume fraction is determined by the hydrogen (H2) dilution ratio to the processing silane gas (SiH4), R = H2/SiH4. We varied R from 1 to 10, where the films transform from amorphous for R < 3 to mostly nanocrystalline for larger R. Structural analyses show that the nanograins, averaging from 2 to 9 nm in sizes with increasing R, are dispersed in the amorphous matrix. The crystalline volume fraction increases from 0 to 65% as R increases from 1 to 10. The thermal conductivities of the two amorphous silicon films are similar and consistent with the most previous reports with thicknesses no larger than a few μm deposited by a variety of techniques. The thermal conductivities of the three nanocrystalline silicon films are also similar, but are about 50–70% higher than those of their amorphous counterparts. The heat conduction in nanocrystalline silicon films can be understood as the combined contribution in both amorphous and nanocrystalline phases, where increased conduction through improved nanocrystalline percolation path outweighs increased interface scattering between silicon nanocrystals and the amorphous matrix.

Peculiarities of nanocrystalline silicon films growth on porous anodic alumina surface

Peculiarities of nanocrystalline silicon films growth on porous anodic alumina surface

Thermal conductivity of amorphous and nanocrystalline silicon films prepared by hot-wire chemical-vapor deposition

We report $3\ensuremath{\omega}$ thermal conductivity measurements of amorphous and nanocrystalline silicon thin films from 85 to 300 K prepared by hot-wire chemical-vapor deposition, where the crystallinity of the films is controlled by the hydrogen dilution during growth. The thermal conductivity of the amorphous silicon film is in agreement with several previous reports of amorphous silicon prepared by a variety of deposition techniques. The thermal conductivity of the as-grown nanocrystalline silicon film is 70% higher and increases 35% more after an anneal at $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. They all have similarly weak temperature dependence. Structural analysis shows that the as-grown nanocrystalline silicon is approximately $60%$ crystalline, nanograins and grain boundaries included. The nanograins, averaging 9.1 nm in diameter in the as-grown film, are embedded in an amorphous matrix. The grain size increases to 9.7 nm upon annealing, accompanied by the disappearance of the amorphous phase. We extend the models of grain boundary scattering of phonons with two different non-Debye dispersion relations to explain our result of nanocrystalline silicon, confirming the strong grain size dependence of heat transport for nanocrystalline materials. However, the similarity in thermal conductivity between amorphous and nanocrystalline silicon suggests the heat transport mechanisms in both structures may not be as dissimilar as we currently understand.

Effect of oxygen on the optical, electrical and structural properties of mixed-phase boron doped nanocrystalline silicon oxide thin films

Systematic investigation on the optoelectronic and structural changes occurring in the p–nc-Si network due to the inclusion of oxygen into the initial crystalline-like matrix has been done. The incorporation of O into the Si network occurs via the Si–O–Si bond which is effectively responsible for the widening of optical band gap. B incorporation takes place via the B–Si bonds as well as the B–O bonds which increases at enhanced CO2 dilution to the plasma. The continuously increased intensity of (SiH2)n wagging and Si-H2 bending mode in p–nc-SiOx films indicates poly-hydrogenation as inherent to the increasing oxygen alloying of the network. Two stage activation energies across around 340K in the temperature dependent dark conductivity identify prominent two phase structure of the p–nc-SiOx material, the Si-ncs being embedded within amorphous p–SiOx matrix. The p–nc-SiOx film grown by typical 13.56MHz PE-CVD at a low growth temperature ∼170°C and optimized with a convincingly good combination of the electrical conductivity (σD ∼6.8×10−2Scm−1), optical band gap (Eg ∼1.91eV) and volume fraction (XC ∼54%) of the Si crystallinity containing CH <6 at% and CO<1 at%, emerges highly competent for effective utilization in the window layer of nc-Si p–i–n solar cells with superstrate configuration.