Boron-doped Nanocrystalline Silicon Research Articles

Low-temperature synthesis of conducting boron-doped nanocrystalline silicon oxide thin films as the window layer of solar cells

Low-temperature synthesis of highly transparent conducting B-doped (p-type) nc-SiOX:H films has been pursued by 13.56 MHz plasma-CVD, using a combination of SiH4, CO2 and B2H6, diluted by H2 and He. Higher substrate temperature (TS) encourages nanocrystallization in B-doped nc-SiOX:H network by reducing bonded H-content, while bonded O-content also reduces simultaneously. At optimized TS = 150 °C, p–nc-SiOX:H film having an optical band gap ~1.98 eV, high conductivity ~0.18 S cm−1, has been obtained via dopant-induced escalation of the electrically active carriers at a deposition rate ~5.3 nm/min. The p–nc-SiOX:H film appears as a promising window layer for the top sub-cell of multi-junction silicon solar cells. A single-junction nc-Si:H based p-i-n solar cell of efficiency (η) ~7.14% with a current-density (JSC) ~14.18 mA/cm2, reasonable fill-factor (FF) ~66.2% and open-circuit voltage (VOC) ~0.7606 V has been fabricated, using the optimum p-type nc-SiOX:H as the window layer deposited at TS = 150 °C.

Deposition of boron-doped nanocrystalline silicon carbide thin films using H2-Ar mixed dilution for the application on thin film solar cells

Hydrogen-argon mixed dilution has been applied for the deposition of boron-doped nanocrystalline silicon carbide (nc-SiCx) thin films. The variations of structural, compositional, electrical and optical properties with the varying H2/Ar ratio are systemically investigated through various characterizations. It is shown that by using H2-Ar mixed dilution for deposition, B-doped nc-SiCx thin film possessing both wide optical band gap (∼2.22 eV) and high conductivity (∼1.9 S cm−1) can be obtained at the H2/Ar flow ratio of 360/140. In addition, the B-doped nc-SiCx thin films are fabricated as the window layers of a-Si thin film solar cells, and the highest conversion efficiency (8.13%) is obtained when applying the window layer with the largest optical band gap energy.

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.

The effect of negative bias on the preparation conditions and structural changes in boron-doped nanocrystalline silicon thin films prepared on PET

In this work, the fabrication of boron-doped nanocrystalline silicon (nc-Si) deposited on PET substrate at room temperature are reported. Negative substrate bias is applied during deposition for improving the crystallinity of samples. The effects of negative bias on the structural, electrical and optical properties of B-doped nc-Si thin films prepared on PET are systemically studied through various characterizations. XRD and Raman results show that a phase transition from amorphous phase to crystalline phase takes place at negative bias=80V. By using moderate negative bias, the crystalline volume fraction can be effectively improved. The electrical properties have been found related to the crystalline volume fraction. At negative bias=120V, the maximum values of dark conductivity (1.94S/cm) and carrier concentration (7.4×1019cm−3) are obtained for the sample which has the largest crystalline volume fraction. Furthermore, the promotion mechanism on B-doping was illustrated in this paper.

Effect of the Annealing on the Low-Temperature Charge Transport Properties of Heavily Boron-Doped Nanocrystalline Silicon Films for Thermoelectric Applications

Abstract Silicon is the reference material of microelectronics, is readily available, relatively unexpensive, and its use may take profit of a fantastic technology. This may explain why a substantial effort has focused on improving its thermoelectric efficiency, either by top-down nanostructuring or through suitable processing. In this paper we report an analysis of the electronic transport properties of heavily boron-doped nanocrystalline silicon films. High-temperature thermal treatments are confirmed to remarkably increase its thermoelectric power factor. Electrical conductivity and Hall effect measurements were carried out over the temperature range 20–300 K along with Seebeck coefficient measurements. We provide evidence of the occurrence of low-temperature hopping conduction between impurity subbands. Dopant ionization was studied as a function of temperature. Freeze-out temperature was found to correlate with the Seebeck coefficient in agreement with Pisarenko equation. This brings to the conclusion that, while untreated samples are weakly degenerate, the thermal processing reverts them into non-degenerate semiconductors, in spite of the high doping level.

Nanovoid Formation and Dynamics in He+-Implanted Nanocrystalline Silicon

Helium implantation in single crystal silicon is known to lead, after a proper thermal treatment, to the formation of voids with diameters ranging between 10 nm and 30 nm. Formation of voids is governed by the coalescence of vacancies created by implantation, initially trapping helium atoms. At high temperatures (\({\ge}700^{\circ }\hbox {C}\)), helium leaves the nanobubbles and outdiffuses, while the now empty voids grow in size and eventually change their shape to form tetrakaidecahedra (Wulff construction). In this communication, we report how He+ implantation in heavily boron-doped nanocrystalline silicon shows a completely different dynamics. Annealing at \(500^{\circ }\hbox {C}\) leads to the formation of large voids, located around grain boundaries, along with a large number of nanovoids with an average diameter of 2–4 nm and an estimated density of \(3\times 10^{17}\,\hbox {cm}^{-3}\) distributed throughout the grains. Annealing at higher temperature (up to \(1000^{\circ }\hbox {C}\)) also induces a decrease of the void size with a change in their density, finally accounting to \(2\times 10^{18}\,\hbox {cm}^{-3}\). The high temperature annealing also causes vacancy evaporation down to a depth of 80–100 nm from the outer surface. The possibility of obtaining a stable, uniform distribution of nanometer-sized voids is of major relevance as a novel tool for phonon and electron engineering in thermoelectric materials.

Paradoxical Enhancement of the Power Factor of Polycrystalline Silicon as a Result of the Formation of Nanovoids

Hole-containing silicon has been regarded as a viable candidate thermoelectric material because of its low thermal conductivity. However, because voids are efficient scattering centers not just for phonons but also for charge carriers, achievable power factors (PFs) are normally too low for its most common form, i.e. porous silicon, to be of practical interest. In this communication we report that high PFs can, indeed, be achieved with nanoporous structures obtained from highly doped silicon. High PFs, up to a huge 22 mW K−2 m−1 (more than six times higher than values for the bulk material), were observed for heavily boron-doped nanocrystalline silicon films in which nanovoids (NVs) were generated by He+ ion implantation. In contrast with single-crystalline silicon in which He+ implantation leads to large voids, in polycrystalline films implantation followed by annealing at 1000°C results in homogeneous distribution of NVs with final diameters of approximately 2 nm and densities of the order of 1019 cm−3 with average spacing of 10 nm. Study of its morphology revealed silicon nanograins 50 nm in diameter coated with 5-nm precipitates of SiB x . We recently reported that PFs up to 15 mW K−2 m−1 could be achieved for silicon–boron nanocomposites (without NVs) because of a simultaneous increase of electrical conductivity and Seebeck coefficient. In that case, the high Seebeck coefficient was achieved as a result of potential barriers on the grain boundaries, and high electrical conductivity was achieved as a result of extremely high levels of doping. The additional increase in the PF observed in the presence of NVs (which also include SiB x precipitates) might have several possible explanations; these are currently under investigation. Experimental results are reported which might clarify the reason for this paradoxical effect of NVs on silicon PF.

Boron-doped nanocrystalline silicon germanium thin films for uncooled infrared bolometer applications

In this paper, boron-doped nanocrystalline Si0.78Ge0.22:H thin film is assessed for use as resistive sensing layer in uncooled infrared bolometer applications. The silicon germanium thin films were deposited by PECVD (plasma enhanced chemical vapor deposition) through decomposition of silane, germane and diborane diluted with argon at substrate temperature of 230°C. Under optimum deposition parameters, the sensing films with modulate electrical resistivity (<104Ωcm) and high temperature coefficient of resistance (TCR) (>−3%/K) were obtained at room temperature. 1/f noise character in the form of the normalized Hooge parameter was measured in the frequency range of 1–64Hz, resulting in a lower 1/f noise compared to other materials currently used for device application.

Boron-Doped Nanocrystalline Silicon Thin Films Prepared by PECVD

Boron-doped nanocrystalline silicon thin films(p-nc-Si:H) were deposited on glass substrates by plasma enhanced chemical vapour deposition (PECVD) using SiH4/ H2/ B2H6. The effects of substrate temperature, rf power and diborane flow on the microstructure, the electrical properties of nanocrystalline silicon thin films have been investigated. The results show that, increasing substrate temperature, rf power and B2H6flow can improve the conductivity of P-Si thin film. However, exceeding one value, they are not advantageous to improve the conductivity due to the decrystallization of films. Hence, appropriate process conditions are crucial for the preparation of high quality p layer. crystalline volume fraction (Xc) 26.2 %, mean grain size (d) 3.5nm and conductivity 0.374S/cm, p-nc-Si:H thin film was deposited.

Window layer with p doped silicon oxide for high Voc thin-film silicon n-i-p solar cells

We investigate the influence of the oxygen content in boron-doped nanocrystalline silicon oxide films (p-nc-SiOx) and introduce this material as window layer in n-i-p solar cells. The dependence of both, optical and electrical properties on the oxygen content is consistent with a bi-phase model which describes the p-nc-SiOx material as a mixture of an oxygen-rich (O-rich) phase and a silicon-rich (Si-rich) phase. We observe that increasing the oxygen content enhances the optical gap E04 while deteriorating the activation energy and the planar conductivity. These trends are ascribed to a higher volume fraction of the O-rich phase. Incorporated into n-i-p a-Si:H cells, p-nc-SiOx layers with moderate oxygen content yield open circuit voltage (Voc) up to 945 mV, which corresponds to a relative gain of 11% compared to an oxygen-free p-layer. As a similar gain is obtained on planar and on textured substrates, we attribute the increase in Voc to the higher work function of the p-nc-SiOx layer made possible by its wider band gap. These results are attained without changing the dilution ratio of the 250 nm thick intrinsic layer. We also observe an enhancement of 0.6 mA cm−2 in short circuit current density in the short wavelengths due to the higher transparency of the p-nc-SiOx layer. Finally, an initial efficiency of 9.9% for a single junction 250 nm a-Si:H n-i-p solar cell on plastic foil is achieved with the optimization of the p layer thickness, the doping ratio of the front transparent conductive oxide, and the optical properties of the back reflector.

Boron-doped nanocrystalline silicon thin films for solar cells

This article reports on the structural, electronic, and optical properties of boron-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films. The films were deposited by plasma-enhanced chemical vapour deposition (PECVD) at a substrate temperature of 150 °C. Crystalline volume fraction and dark conductivity of the films were determined as a function of trimethylboron-to-silane flow ratio. Optical constants of doped and undoped nc-Si:H were obtained from transmission and reflection spectra. By employing p + nc-Si:H as a window layer combined with a p′ a-SiC buffer layer, a-Si:H-based p– p′– i– n solar cells on ZnO:Al-coated glass substrates were fabricated. Device characteristics were obtained from current–voltage and spectral-response measurements.

Modeling and Simulation of Boron-Doped Nanocrystalline Silicon Carbide Thin Film by a Field Theory

This paper presents the application of a multiscale field theory in modeling and simulation of boron-doped nanocrystalline silicon carbide (B-SiC). The multiscale field theory was briefly introduced. Based on the field theory, numerical simulations show that intergranular glassy amorphous films (IGFs) and nano-sized pores exist in triple junctions of the grains for nanocrystalline B-SiC. Residual tensile stress in the SiC grains and compressive stress on the grain boundaries (GBs) were observed. Under tensile loading, it has been found that mechanical response of 5 wt% boron-SiC exhibits five characteristic regimes. Deformation mechanism at atomic scale has been revealed. Tensile strength and Young's modulus of nanocrystalline SiC were accurately reproduced.

The Study of Boron-Doped Nanocrystalline Silicon Film with High Conductivity

Boron-doped nanocrystalline silicon film was prepared through plasma enhanced chemical vapor deposition (PECVD) on silicon substrate and glass substrate under the high deposition pressure (332.5-399Pa) and the high deposition temperature (320-360°C). The film was investigated by Raman, electron probe microanalyser, conductivity and mobility experimenting techniques. The conductivity of the boron-doped nanocrystalline silicon film was 2.97×102Ω-1cm-1. The results showed that the interface between the film and the silicon substrate might have quantum spot and small size effect, causing the increasing of conductivity.

Impurity Doping and Its Effect on the Carrier Injection Efficiency of a Nanocrystalline Silicon Electroluminescent Device

We developed an electroluminescent (EL) device using boron-doped nanocrystalline silicon (nc-Si) particles that effectively inject a carrier into a luminous layer at a low direct current (DC) voltage of 2.0 V. The injection efficiency of the carrier strongly depends on the boron concentration. The injection quantity of a carrier in an EL device with a high boron concentration of 2.3 at.% was made almost double that of an EL device without boron doping. This was due to both the lowering of the series resistance of the EL device using the doping of boron into nc-Si particles and the coagulation of nc-Si particles using a hydrofluoric acid (HF) treatment. The injection quantity of the carrier can be considerably improved using boron doping into the nc-Si particles and the HF treatment.

Evolution of Structural and Electronic Properties in Boron-doped Nanocrystalline Silicon Thin Films

AbstractWe report on the boron-doping dependence of the structural and electronic properties in nanocrystalline silicon (nc-Si:H) films directly deposited by plasma- enhanced chemical vapor deposition (PECVD). The crystallinity, micro-structure, and dark conductivity of the films were investigated by gradually varying the ratio of trimethylboron [B(CH3)3 or TMB] to silane (SiH4) from 0.1 to 2 %. It was found that the low level of boron doping (&lt; 0.2 %) first compensated the nc-Si:H material which demonstrates slightly n-type properties. As the doping increased up to 0.5 %, the maximum dark conductivity (ód) of 1.11 S/cm was obtained while high crystalline fraction (Xc) of the films (over 70 %) was maintained. However, further increase in a TMB-to-SiH4 ratio reduced ód to the order of 10-7 S/cm due to a phase transition of the films from nanocrystalline to amorphous, which was indicated by Raman spectra measurements.P-channel nc-Si:H thin film transistors (TFTs) with top gate and staggered source/drain contacts were fabricated using the developed p+ nc-Si:H layer. The fabricated TFT exhibits a threshold voltage (VTp) of -26.2 V and field effective mobility of holes (μp) of 0.24 cm2/V·s.

Charge transport in hydrogenated boron-doped nanocrystalline silicon-silicon carbide alloys

We have investigated the carrier transport mechanism of mixed-phased hydrogenated boron-doped nanocrystalline silicon–silicon carbide alloy (p-nc-Si-SiC:H) films. From temperature-dependent dark conductivity measurements, we found that the p-nc-Si-SiC:H alloys have two different carrier transport mechanisms: one is the thermally activated hopping between neighboring crystallites near the room-temperature region and the other is the band tail hopping below 150 K.

Roles of hydrogen dilution on the microstructural and optoelectronic properties of B-doped nanocrystalline Si:H thin films

We report on the roles of hydrogen dilution on the microstructural and optoelectronic properties of boron-doped nanocrystalline silicon thin films grown by plasma-enhanced chemical vapor deposition on glass substrates, through x-ray diffraction, scanning electron microscope, Raman scattering, optical transmission, temperature-dependent dark conductivity, and elastic recoil detection analysis (ERDA) measurements. The grain size, crystallinity, absorption coefficient, refractive index, and conductivity are found to decrease basically with increasing hydrogen dilution ratio ΔH. The Tauc’s optical band gap is found to increase with ΔH due to the quantum size effect. By the aid of hydrogen ion bombardment effect [J. Appl. Phys. 93, 1262 (2003)] as well as the hydrogen-induced annealing effect, the clear increase of hydrogen content with ΔH and the different hydrogen distribution from ERDA help us to elucidate the correlation of hydrogen dilution with the crystallinity and grain size. The argument has been further supported by a simple absorption model and the yielded refractive index and absorption coefficient.

Role of the deposition conditions on the properties presented by nanocrystallite silicon films produced by hot wire

The aim of this work is to study the role of hydrogen dilution and filament temperature on the properties of nanocrystalline silicon thin films (undoped and doped) produced by the hot wire technique. These deposition parameters are correlated to the film's structure, composition and electro-optical properties with special emphasis on boron doped nanocrystalline silicon carbide reported here.

Intense RT Visible Photoluminescence from Anodized Amorphous and Nanocrystalline Silicon Films

ABSTRACTHeavily Boron-doped micrometer-thick amorphous (a-Si:B:H) or nanocrystalline (nc-Si) silicon layers have been deposited on a variety of conductive substrates by the 50 KHz PECVD of SiH4 / B2H6 / H2 mixtures at 320°C. These films have been partially electro-oxidized in a HF solution and then anodically oxidized in water in a manner similar to that yielding luminescent porous monocrystalline silicon layers (PcSL). Although the anodized films are x-ray amorphous, they yield intense luminescence properties at room temperature very similar to those of anodically oxidized PcSL with a similar vibrational spectrum. In both anodically oxidized materials, aging is shown to improve the external quantum efficiency. In amorphous anodized layers, optical microscopy under UV excitation showed strongly luminescent strain-related heterogeneities (20 μ,m in diameter) connected by non luminescent channels. The incidence of our results on the current debate about the origin of visible room-temperature luminescence in porous silicon and Si:O:H systems is discussed.