B-doped Films Research Articles

Flexible Solar Cells Using Doped Crystalline Si Film Prepared by Self-Biased Sputtering Solid Doping Source in SiCl4/H2 Microwave Plasma.

We developed an innovative approach of self-biased sputtering solid doping source process to synthesize doped crystalline Si film on flexible polyimide (PI) substrate via microwave-plasma-enhanced chemical vapor deposition (MWPECVD) using SiCl4/H2 mixture. In this process, P dopants or B dopants were introduced by sputtering the solid doping target through charged-ion bombardment in situ during high-density microwave plasma deposition. A strong correlation between the number of solid doping targets and the characteristics of doped Si films was investigated in detail. The results show that both P- and B-doped crystalline Si films possessed a dense columnar structure, and the crystallinity of these structures decreased with increasing the number of solid doping targets. The films also exhibited a high growth rate (>4.0 nm/s). Under optimal conditions, the maximum conductivity and corresponding carrier concentration were, respectively, 9.48 S/cm and 1.2 × 10(20) cm(-3) for P-doped Si film and 7.83 S/cm and 1.5 × 10(20) cm(-3) for B-doped Si film. Such high values indicate that the incorporation of dopant with high doping efficiency (around 40%) into the Si films was achieved regardless of solid doping sources used. Furthermore, a flexible crystalline Si film solar cell with substrate configuration was fabricated by using the structure of PI/Mo film/n-type Si film/i-type Si film/p-type Si film/ITO film/Al grid film. The best solar cell performance was obtained with an open-circuit voltage of 0.54 V, short-circuit current density of 19.18 mA/cm(2), fill factor of 0.65, and high energy conversion of 6.75%. According to the results of bending tests, the critical radius of curvature (RC) was 12.4 mm, and the loss of efficiency was less than 1% after the cyclic bending test for 100 cycles at RC, indicating superior flexibility and bending durability. These results represent important steps toward a low-cost approach to high-performance flexible crystalline Si film-based photovoltaic devices.

Electron Microscopy Study on the Influence of B-implantation on Ni Induced Lateral Crystallization in Amorphous Si

Nickel Metal Induced Lateral Crystallization (Ni-MILC) emerged as a viable technique for crystallization of a-Si films decreasing the crystallization temperature. Boron (B) implantation on a-Si films significantly enhances the crystallization rate of the Ni-MILC process. The structural characteristics of the implanted by Boron and subsequently crystallized by MILC a-Si films are studied by Transmission Electron Microscopy (TEM) and they are compared to intrinsic a-Si films which were deposited on top, as well as beside the boron implanted a-Si film. During the annealing, spontaneous nucleation occurs in the B-doped films far from the a-c interface, revealing a shorter incubation period in the B-doped films.

Structural and physical properties of boron doped ZnO films prepared by chemical spray pyrolysis method

In this work, ZnO and boron doped ZnO (ZnO:B) thin films were produced by chemical spray pyrolysis method. ZnO and ZnO:B films were obtained onto glass substrates at 450°C by spray pyrolysis method and the physical properties of those films were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible spectrometer (UV) and four probe technique. XRD measurements show that all films have hexagonal wurtzite structure and all films grow preferentially along (002) direction. Morphologies of the films were examined by using a scanning electron microscopy and it was observed that almost all films were quite intense with a regular structure. Optical measurements showed that the band gap energies of the films increased with boron concentrations. The resistances of the B-doped ZnO films were measured by four probe method and resistances of films initially decreased to its minimum 1 at% boron doping and then it increased again with increasing B concentration. It was also observed that that boron doping increased the activation energies of the films.

B-Doped Amorphous Silicon Thin Films Deposited on Flexible PET Substrate by Magnetron Sputtering

Depositing amorphous silicon doped with different elements on flexible substrates offers a large potential for the roll-to-roll processes lowering the manufacturing cost of solar cells. Unique magnetron sputtering method was adopted to deposit B-doped amorphous silicon thin film on flexible PET substrate instead of solid substrate materials like glass, metal and silicon. Effects of different substrates were investigated, as well as the processing parameters including sputtering power and working pressure. The structure and performance of the samples were characterized with SEM, XRD, surface profile meter and UV-spectrophotometer. The comparison between PET and glass shows that the alternative of PET substrate significantly improved the deposition rate. Optimal results were obtained as pressure of 0.2 Pa and RF power of 120 W, which showed the lowest transmittance as well as a minimum optical band gap, both are of great significance to photovoltaic conversion. Theoretical justification and experimental data including the feasible magnetron sputtering way, PET flexible substrate and optimized process parameters to the mass production of cheap and high performance heterojunction thin film solar cells were achieved.

Effect of boron doping on the rectification effect and photovoltaic performance of CdS/Si heterostructure based on Si nanoporous pillar array

A series of CdS/Si heterostructures were prepared through growing B-doped CdS thin films on silicon nanoporous pillar array (Si-NPA) by a chemical bath deposition (CBD) method. The experimental data show that B-doping concentration of CdS thin films could be tuned effectively through controlling the mole ratio of [B]/[Cd] of the initial CBD solution without causing obvious variation of the crystal phase and surface morphology of CdS/Si-NPA. Both the electrical rectification and photovoltaic parameters of CdS/Si-NPA show strong dependence upon B-doping concentration, and the optimal characteristics are achieved for the samples prepared with [B]/[Cd] = 0.01. Compared with CdS/Si-NPA solar cells without B-doping, an increment over 300 times for energy conversion efficiency is realized. The mechanism for the efficiency increment is analyzed based on the effect of B-doping on the band structure of CdS/Si-NPA. These results indicate that B-doping might be an effective path for promoting the device performance of solar cells based on CdS/Si-NPA.

Highly conductive boron doped micro/nanocrystalline silicon thin films deposited by VHF-PECVD for solar cell applications

Boron doped hydrogenated micro/nanocrystalline silicon (μc/nc-Si:H) thin films have been deposited by plasma enhanced chemical vapor deposition technique (PECVD) using silane (SiH4) diluted in argon. Diborane (B2H6) was used as the dopant gas and deposition was carried out at substrate temperature of 200°C. The diborane flow (FB) varied in the range 0.00–0.30. Here, we report the effects of B2H6 doping on electronic, optical and structural properties of hydrogenated micro/nanocrystalline silicon films. The structural properties were analyzed by atomic force microscopy (AFM) and X-ray diffraction (XRD). The doped micro/nano crystalline silicon films presented a crystallographic orientation preferentially in the (111) and (220) plane. We resolve the deposition parameters that lead to the formation of p-type micro/nanocrystalline silicon thin films with very high value of conductivity and lower optical band gap. Correlations between structural and electrical properties were also studied. Based on temperature dependent conductivity measurements, it has been observed that the room temperature dark conductivity varies in the range 1.45×10−4Ω−1cm−1 to 2.02Ω−1cm−1 for the B-doped films. Meanwhile, the corresponding value of activation energies decreased to 0.06eV for the B-doped films, which indicates the doped μc/nc-Si films with high conductivity can be achieved and these films prove to be a very good candidate for application in amorphous and micro/nano crystalline silicon solar cells as a p-type window layer.

Investigating the effects of microstructure on optical properties of different kinds of polysilicon thin films

Abstract Flat, B-doped, low-stress and non-stress polysilicon (poly-Si) thin films were deposited by low pressure chemical vapor deposition (LPCVD). X-ray diffraction (XRD), combined with atomic force microscopy (AFM) and Raman system, UV-Raman measurements and spectroscopic ellipsometry, was used to study the microstructure, the morphology and optical properties of the films. The results indicate that the surface roughness is related to the grain size and the change in the microstructure. The B-doped poly-Si shows a tensile stress, while the flat poly-Si shows a high compressive stress. The bandgap of the B-doped poly-Si is the narrowest because of its most disordered microstructure.

Effect of boron-doping on the luminescent and electrical properties of a CdS/Si heterostructure based on Si nanoporous pillar array

Using silicon nanoporous pillar array (Si-NPA) as substrates and boric acid as dopant source, a series of CdS/Si nanoheterostructures were prepared by growing B-doped CdS thin films on Si-NPA via a chemical bath deposition (CBD) method. The structural, optical and electrical properties of CdS/Si-NPA were studied as a function of the [B]/[Cd] ratio of the initial CBD solutions. Our results disclosed that B concentration could be tuned effectively through changing the ratio of [B]/[Cd], which would bring large variation on the optical and electrical properties of CdS/Si-NPA without affecting its crystal structure and surface morphology. The samples with optimal optical and electrical properties were prepared with [B]/[Cd]=0.01, in which the physical properties of relatively strong light absorption, small electrical resistivity, low turn-on voltage, small leakage current density and high breakdown voltage could be obtained. These results indicated that B-doping might be an effective path for promoting the performance of the optoelectronic devices based on CdS/Si-NPA.

Potential variations around grain boundaries in impurity-doped BaSi2 epitaxial films evaluated by Kelvin probe force microscopy

Potential variations around the grain boundaries (GBs) in antimony (Sb)-doped n-type and boron (B)-doped p-type BaSi2 epitaxial films on Si(111) were evaluated by Kelvin probe force microscopy. Sb-doped n-BaSi2 films exhibited positively charged GBs with a downward band bending at the GBs. The average barrier height for holes was approximately 10 meV for an electron concentration n ≈ 1017 cm−3. This downward band bending changed to upward band bending when n was increased to n = 1.8 × 1018 cm−3. In the B-doped p-BaSi2 films, the upward band bending was observed for a hole concentration p ≈ 1018 cm−3. The average barrier height for electrons decreased from approximately 25 to 15 meV when p was increased from p = 2.7 × 1018 to p = 4.0 × 1018 cm−3. These results are explained under the assumption that the position of the Fermi level Ef at GBs depends on the degree of occupancy of defect states at the GBs, while Ef approached the bottom of the conduction band or the top of the valence band in the BaSi2 grain interiors with increasing impurity concentrations. In both cases, such small barrier heights may not deteriorate the carrier transport properties. The electronic structures of impurity-doped BaSi2 are also discussed using first-principles pseudopotential method to discuss the insertion sites of impurity atoms and clarify the reason for the observed n-type conduction in the Sb-doped BaSi2 and p-type conduction in the B-doped BaSi2.

The correlation of stoichiometry between boron-rich LaB6 targets and LaB6 films

Nanostructured B-doped LaB6 films prepared by DC magnetron sputtering method were expected to obtain stoichiometric LaB6 film for stable optical performance. The proportion of each element at the interface was deduced from X-ray photoelectron spectroscopy (XPS) data using the B/La atomic ratio and decomposition of lanthanum peak. A linear relation was observed between adding amount of boron and the B/La atomic ratio of the film surface layers. The composition of prepared LaB6 thin films reached LaB5.949±0.2, nearly at the stoichiometric ratio. It was concluded that practical curve fitting procedures for various lanthanum chemical states in thin film surfaces were demonstrated and results obtained by the curve-fitting procedure were reliable. A series of techniques such as X-ray diffraction (XRD), atomic force microscope (AFM) and scanning electron microscopy (SEM) were employed to characterize the structure, surface morphology and particle size of B-doped and undoped LaB6 thin films. Combining the information from XRD and topographic images, fracture cross-sections, effects of boron on grain refinement had been discussed and confirmed by the decrease of surface roughness, height distribution, average particle size and thickness of thin films.

Enhanced Thermoelectric Efficiency of Si-Ni Nanocomposite Films Grown on SOI and SOQ Substrates

To improve the poor thermoelectric performance of Si, reduction of the high thermal conductivity k (~142 Wm–1K–1) is essential, and one of the effective methods to reduce the k is making Si nanostructures such as Si nanocrystals (NCs) [1, 2] and nanowires[3], where the k is dramatically reduced by enhancement of phonon scattering at the interfaces of the nanostructures. Recently, we developed semiconducting composite films of Si and Ni silicide NCs (Si-Ni nanocomposite film) deposited on SiO2 substrates [2]with the power factor of 1.9×10-3 W/mK2 , the thermal conductivity k of 4.4 W/mK, and the dimensionless figure of merit ZT of 0.13 for the B-doped (p-type ) nanocomposite film [2], which is much higher than that of bulk Si (ZT<0.01).In this paper, we demonstrate the further improvement of the thermoelectric performance of B doped Si-Ni nanocomposite films by growing them on silicon-on-insulator (SOI) or silicon-on-quartz (SOQ) substrates. The B doped Si-Ni nanocomposite films were synthesized from amorphous Si-Ni-B films deposited on the SOI or SOQ substrates at room temperature (RT) with Ar sputtering of the B doped Ni-Si target (Ni:Si=1:20, 2mol% B). The amorphous films were annealed at 800°C in N2gas ambient, leading to crystallization and the phase separation of the films. After this annealing , rapid annealing of 10 sec at 1200°C was carried out to activate the B atoms.In Fig.1, the ZT values are plotted against the k valuesof Si-Ni nanocomposite films on SOI, SOQ, SiO2 substrates [ 2], and Si nanocomposite by Bux et al. [1] at RT. We observed low k values (< 4 W/mK) of Si-Ni nanocomposite films on SOI and SOQ, which were similar k with Si-Ni nanocomposite film on SiO2, due to phonon scattering at interfaces on NCs. As seen, the ZTs (0.23-0.36) of the Si-Ni nanocomposite films on SOI and SOQ substrates were much higher than those of the Si-Ni nanocomposite films on SiO2 and the Si nanocomposite. We observed with cross-sectional transmission electron microscopy that Si NCs in Si-Ni composite films were epitaxially grown from thin Si crystal layers of SOI /SOQ substrates, leading to improvement of the crystal quality of Si NCs in the nanocomposite films and increase of the carrier conductivities, the Seebeck coefficients and the ZTs.This work was partly supported by ALCA-JST.[1] S. K. Bux, et al., Adv. Funct. Mater. 19, 2445 (2009). [2] N. Uchida et al., J. Appl. Phys. 114, 134311 (2013). [3] A. I. Hochbaum, et al., Nature 451,163 (2008).

Electrical conductivity enhancement by boron-doping in diamond using first principle calculations

Boron doping in diamond plays a vital role in enhancing electrical conductivity of diamond by making it a semiconductor, a conductor or even a superconductor. To elucidate this fact, partial and total density of states has been determined as a function of B-content in diamond. Moreover, the orbital charge distributions, B–C bond lengths and their population have been studied for B-doping in pristine diamond thin films by applying density functional theory (DFT). These parameters have been found to be influenced by the addition of different percentages of boron atoms in diamond. The electronic density of states, B–C bond situations as well as variations in electrical conductivities of diamond films with different boron content and determination of some relationship between these parameters were the basic tasks of this study. Diamond with high boron concentration (∼5.88% B-atoms) showed maximum splitting of energy bands (caused by acceptor impurity states) at the Fermi level which resulted in the enhancement of electron/ion conductivities. Because B atoms either substitute carbon atoms and/or assemble at grain boundaries (interstitial sites) inducing impurity levels close to the top of the valence band. At very high B-concentration, impurity states combine to form an impurity band which accesses the top of the valence band yielding metal like conductivity. Moreover, bond length and charge distributions are found to decrease with increase in boron percentage in diamond. It is noted that charge distribution decreased from +1.89 to −1.90eV whereas bond length reduced by 0.04Å with increasing boron content in diamond films. These theoretical results support our earlier experimental findings on B-doped diamond polycrystalline films which depict that the addition of boron atoms to diamond films gives a sudden fall in resistivity even up to 105Ωcm making it a good semiconductor for its applications in electrical devices.

Structural and optical properties of boron-doped amorphous carbon nitride thin films synthesized by microwave electron cyclotron resonance-plasma chemical vapor deposition

The structural and optical properties of boron (B)-doped amorphous carbon nitride (a-CNx) thin films with a B content of less than 10 at. % were investigated. The B-doped a-CNx thin films were synthesized by electron cyclotron resonance (ECR)-plasma chemical vapor deposition (CVD) by varying the substrate temperature and flow rate ratio of the source gases (BCl3/CH4/N2/Ar). An increase in the B content and a decrease in the Cl content were observed with increasing substrate temperature. The Tauc energy gap of the B-doped a-CNx thin films increased with decreasing C–N sp2 bond fraction and increasing B content. The photoluminescence (PL) peak position showed larger blue shifts and a tendency of increased integrated PL intensity was observed with increasing B content. These PL results suggest an increase in PL-active B-related sp2 C–C bond cluster density.

Raman spectroscopic study on boron-doped silicon nanoparticles

Raman analyses were performed on thin films prepared from B-doped Si nanoparticles with an average diameter of 15nm using the spin-coating method. The resulting spectrum exhibited a broad band with a peak near 520cm−1. The band was decomposed into three bands corresponding to the crystalline, grain boundary (GB), and amorphous regions by the least-squares band-fitting method based on the three Voigt bands. The fractions of the crystalline, GB, and amorphous regions were 37%, 35%, and 28%, respectively. A spherical particle exhibited an ordered crystalline core surrounded by a disordered shell in a transmission electron microscope (TEM) image. The crystalline fraction of the 15-nm B-doped Si nanoparticle film was much lower than that of the 19-nm P-doped Si nanoparticle film. This result suggested that the B-doping mechanism was different from that of P-doping. The temperature of the sample was estimated from the ratio of the peak intensities of anti-Stokes to Stokes Raman bands (IAS/IS) observed near 520cm−1. The temperature of the B-doped Si nanoparticle film upon irradiation at a power density of 4.6kW/cm2 was 298°C, whereas the temperature of the P-doped Si nanoparticle film was 92°C. The B-doped Si nanoparticle films were capable of producing light-induced heat.

Control of Electric Conductivity and Electrochemical Activity of Hydrogenated Amorphous Carbon by Incorporating Boron Atoms

Highly conductive hydrogenated amorphous carbon with higher resistance to electrochemically induced corrosion was successfully synthesized by incorporating boron atoms and regulating the amount of sp2-bonded carbon with plasma-enhanced CVD method. The structure, electrical properties, and electrochemical reactivity of boron-doped amorphous carbon (B-doped DLC) were investigated. Boron atoms in amorphous carbon acted as an accepter because the conduction of B-doped DLC was p-type. Volume resistivity was 6.36 × 10−2 Ω cm at highly B-doped DLC (B = 1.3 atom%). The highly B-doped DLC films exhibited a wide working potential range over 3 V in an aqueous solution. The kinetics of hydrogen evolution, which determines the working potential range, could be suppressed by decreasing the number of boron atoms incorporated in DLC because B atoms act as an electrochemical catalyst for H2 evolution. A double-layer capacitance of the highly B-doped films was as low as boron-doped diamond. This is due to the absence of ionizable surface functionalities like –NH3 and reversible electron transfer kinetics for inorganic redox analytes (Fe(CN)63−/4− and Ru(NH3)62+/3+), which were on the same level as those of boron-doped diamond. The lower kinetics of O2 evolution at B-doped DLC enables to observe the redox reaction of Ce3+/4+ with a standard potential higher than that of H2O/O2. Mn2+ ions with a standard potential lower than that of H2 evolution could also be electrochemically analyzed in the range from 20 μM to 2.5 mM at moderately B-doped DLC with lower kinetics of H2 evolution.

Atomic layer deposition of B2O3/SiO2 thin films and their application in an efficient diffusion doping process

We investigated atomic layer deposition (ALD) of B2O3 and SiO2 thin films using trimethylborate (TMB) and bis-(diethylamino)silane (SAM-24) precursors, focusing on growth characteristics and film properties. For both cases, ALD processes using O3 and O2 plasma as reactants exhibited well-defined growth saturation and linear growth behavior without any incubation cycles, and produced highly pure, stoichiometric films. In the case of B2O3 films, however, SiO2 layer passivation is required onto the B2O3 due to a spontaneous decomposition caused by moisture in air. On the basis of electrical characterization, the detailed dielectric properties of SiO2 and B2O3/passivation SiO2 films were extensively discussed including the k-value, flat band voltage, and leakage currents. Then, boron-doped SiO2 films with different B/(B + Si) compositions were prepared by controlling B2O3 and SiO2 growth cycles, followed by drive-in annealing and a subsequent wet removal process. Based on both theoretical estimation and SIMS depth profile results, we demonstrated that the surface doping concentration is effectively modulated with controllable B doping contents in the B-doped SiO2 films.

B（C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;5&lt;/sub&gt;）&lt;sub&gt;3&lt;/sub&gt;を用いて合成したB添加導電性DLC膜の機械的特性評価

B（C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;5&lt;/sub&gt;）&lt;sub&gt;3&lt;/sub&gt;を用いて合成したB添加導電性DLC膜の機械的特性評価

Influences of deposition temperature on characteristics of B-doped ZnO films deposited by metal–organic chemical vapor deposition

Boron-doped zinc oxide films were fabricated by metal–organic chemical vapor deposition at deposition temperatures (Td) from 150 to 210°C. The deposition rate increases abruptly and monotonically with increasing Td. The resistivity also varies drastically, and a minimum resistivity of 1.6×10−3Ωcm is obtained at Td=175°C. The crystal orientation and surface texture show Td dependence. These characteristics correlate with each other. The dependence of these characteristics on Td is caused by the reactivity of the source materials.

Epitaxial Growth of Heavily B-Doped Si and Ge Films on Si(100) by Low-Energy ECR Ar Plasma CVD Without Substrate Heating

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Epitaxial growth of B-doped Si on Si(100) by electron-cyclotron-resonance Ar plasma chemical vapor deposition in a SiH4–B2H6–H2 gas mixture without substrate heating

Characteristics of B-doped Si epitaxial growth on Si(100) by using electron-cyclotron-resonance Ar plasma enhanced chemical vapor deposition without substrate heating in a SiH4–B2H6–H2–Ar gas mixture were investigated. B concentration in the deposited films increases with decreasing microwave power for plasma generation. At the microwave power of 125W, the B concentration increases up to 5×1021cm−3. Deposition rate of the B-doped Si tends to be enhanced at the higher B2H6 partial pressure. Resistivity of the B-doped Si film tends to increase with decreasing the microwave power. Referring Irvin curve, in the case of 200W, the carrier concentration is estimated to be at least about 1017cm−3 at the B concentration of 1021cm−3. After heat treatment in N2 atmosphere at 200°C and 300°C for 2h, the resistivity drastically decreases to the value which corresponds to carrier concentration of around 1019cm−3. From Fourier transform infrared spectroscopy measurement, it is found that hydrogen incorporated in the as-deposited film desorbed by the heat treatment.