Showing papers on "Amorphous silicon published in 2010"

Material characteristics and applications of transparent amorphous oxide semiconductors

Abstract: Transparent amorphous oxide semiconductors have unique electron transport properties, such as large electron mobility (10–50 cm2/Vs) and the absence of a Hall voltage sign anomaly, that are not seen in conventional amorphous semiconductors. This class of materials has been attracting much attention as a channel layer in thin-film transistors (TFTs) utilizing the above features along with the processing advantage that thin films can be deposited at low temperatures by conventional sputtering methods. The primary driving force for this trend is a rapidly emerging demand for backplane TFTs that can drive the next generation of flat-panel displays. This article reviews the recent advances in fundamental science of these materials and their TFT applications. Emphasis is placed on the view that high ionicity in chemical bonding and large spherical spread of unoccupied metal s orbitals in p-block metal oxides lead to the realization of electronic structures that are advantageous for n-channel TFT applications. Amorphous oxide semiconductors are compared with conventional hydrogenated amorphous silicon, which is used widely as the channel material for backplane TFTs in current liquid-crystal displays.

Trap density of states in small-molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

Abstract: We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap [trap density of states (trap DOS)] of small-molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of ``fast'' hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small-molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase in the trap DOS very close $(l0.15\text{ }\text{eV})$ to the mobility edge with a characteristic slope of 10--20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase in the trap DOS. Moreover, we show that the trap DOS in TFT's with small-molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small-molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand.

Universality of non-ohmic shunt leakage in thin-film solar cells

Abstract: We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu(In,Ga)Se2 (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias (V<∼0.4) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage (Ish), across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V=0, (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propo...

Resonant and nonresonant plasmonic nanoparticle enhancement for thin-film silicon solar cells.

Abstract: This paper investigates the influence of resonant and nonresonant plasmonic nanostructures, such as arrays of silver and aluminum nanoparticles in the forward scattering configuration, on the optical absorption in a thin-film amorphous silicon solar cell. It is demonstrated that nonresonant coupling of the incident sunlight with aluminum nanoparticles results in higher optical absorption in the photoactive region than resonant coupling with silver nanoparticle arrays. In addition, aluminum nanoparticles are shown to maintain a net positive enhancement of the optical absorption in amorphous silicon, as compared to a negative effect by silver nanoparticles, when the nanoparticles are oxidized.

Method of Manufacturing a Semiconductor Device

Abstract: At present, a forming process of a base film through an amorphous silicon film is conducted in respective film forming chambers in order to obtain satisfactory films. When continuous formation of the base film through the amorphous silicon film is performed in a single film forming chamber with the above film formation condition, crystallization is not sufficiently attained in a crystallization process. By forming the amorphous silicon film using silane gas diluted with hydrogen, crystallization is sufficiently attained in the crystallization process even with the continuous formation of the base film through the amorphous silicon film in the single film forming chamber.

Temperature Dependence of the Thermal Conductivity of Thin Silicon Nanowires

Abstract: We compute the lattice thermal conductivity (kappa) of silicon nanowires as a function of temperature by molecular dynamics simulations. In wires with amorphous surfaces kappa may reach values close to that of amorphous silicon and is nearly constant between 200 and 600 K; this behavior is determined by the presence of a majority of nonpropagating vibrational modes. We develop a parameter-free model that accounts for the temperature dependence observed in our simulations and provides a qualitative explanation of recent experiments.

Phototransistor-based optoelectronic tweezers for dynamic cell manipulation in cell culture media

Abstract: Optoelectronic tweezers (OET), based on light-induced dielectrophoresis, has been shown as a versatile tool for parallel manipulation of micro-particles and cells (P. Y. Chiou, A. T. Ohta and M. C. Wu, Nature, 2005, 436, 370–372).1 However, the conventional OET device cannot operate in cell culture media or other high-conductivity physiological buffers due to the limited photoconductivity of amorphous silicon. In this paper, we report a new phototransistor-based OET (Ph-OET). Consisting of single-crystalline bipolar junction transistors, the Ph-OET has more than 500× higher photoconductivity than amorphous silicon. Efficient cell trapping of live HeLa and Jurkat cells in Phosphate Buffered Saline (PBS) and Dulbecco's Modified Eagle's Medium (DMEM) has been demonstrated using a digital light projector, with a cell transport speed of 33 µm/sec, indicating a force of 14.5 pN. Optical concentration of cells and real-time control of individually addressable cell arrays have also been realized. Precise control of separation between two cells has also been demonstrated. We envision a new platform for single cell studies using Ph-OET.

Optical nonlinearities in hydrogenated- amorphous silicon waveguides

Abstract: We experimentally measure the optical nonlinearities in hydrogenated-amorphous silicon (a-Si:H) waveguides through the transmission of ultra-short pulses. The measured two-photon absorption coefficient β is 4.1 cm/GW and we obtain a 3.5π nonlinear phase shift at 4.1 W coupled input power corresponding to a nonlinear refractive index n2 of 4.2∙10−13 cm2/W. The measured nonlinear coefficient γ = 2003 (W∙m)−1 is at least 5 times the value in crystalline silicon. The measured free carrier absorption coefficient σ = 1.9∙10−16 cm2 agrees with the values predicted from the Drude-Lorenz model. It is seen that a-Si:H exhibits enhanced nonlinear properties at 1550 nm and is a promising platform for nonlinear silicon photonics.

Interplay of amorphous silicon disorder and hydrogen content with interface defects in amorphous/crystalline silicon heterojunctions

Abstract: We analyze the dependence of the interface defect density Dit in amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions on the microscopic properties of ultrathin (10 nm) undoped a-Si:H passivation layers. It is shown that the hydrogen bonding and network disorder, probed by infrared- and photoelectron spectroscopy, govern the initial Dit and its behavior upon a short thermal treatment at 200 °C. While the initial Dit is determined by the local and nonequilibrated interface structure, the annealed Dit is defined by the bulk a-Si:H network strain. Thus it appears that the equilibrated a-Si:H/c-Si interface does not possess unique electronic properties but is governed by the a-Si:H bulk defects.

Mixed-phase p-type silicon oxide containing silicon nanocrystals and its role in thin-film silicon solar cells

Abstract: Lower absorption, lower refractive index, and tunable resistance are three advantages of amorphous silicon oxide containing nanocrystalline silicon grains (nc-SiOx) compared to microcrystalline silicon (μc-Si), when used as a p-type layer in μc-Si thin-film solar cells. We show that p-nc-SiOx with its particular nanostructure increases μc-Si cell efficiency by reducing reflection and parasitic absorption losses depending on the roughness of the front electrode. Furthermore, we demonstrate that the p-nc-SiOx reduces the detrimental effects of the roughness on the electrical characteristics, and significantly increases μc-Si and Micromorph cell efficiency on substrates until now considered too rough for thin-film silicon solar cells.

Resistive interlayer for improved performance of thin film silicon solar cells on highly textured substrate

Abstract: The deposition of thin-film silicon solar cells on highly textured substrates results in improved light trapping in the cell. However, the growth of silicon layers on rough substrates can often lead to undesired current drains, degrading performance and reliability of the cells. We show that the use of a silicon oxide interlayer between the active area and the back contact of the cell permits in such cases to improve the electrical properties. Relative increases of up to 7.5% of fill factor and of 6.8% of conversion efficiency are shown for amorphous silicon cells deposited on highly textured substrates, together with improved yield and low-illumination performance.

Method of forming polycrystalline silicon layer and atomic layer deposition apparatus used for the same

Abstract: A method of forming a polycrystalline silicon layer and an atomic layer deposition apparatus used for the same The method includes forming an amorphous silicon layer on a substrate, exposing the substrate having the amorphous silicon layer to a hydrophilic or hydrophobic gas atmosphere, placing a mask having at least one open and at least one closed portion over the amorphous silicon layer, irradiating UV light toward the amorphous silicon layer and the mask using a UV lamp, depositing a crystallization-inducing metal on the amorphous silicon layer, and annealing the substrate to crystallize the amorphous silicon layer into a polycrystalline silicon layer This method and apparatus provide for controlling the seed position and grain size in the formation of a polycrystalline silicon layer

Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide.

Abstract: We, for the first time, present the ultrafast optical nonlinear response of a hydrogenated amorphous silicon (a-Si:H) wire waveguide using femtosecond pulses. We show cross-phase and cross-absorption modulations measured using the heterodyne pump-probe method and estimate the optical Kerr coefficient and two-photon absorption coefficient for the amorphous silicon waveguide. The pumping energy of 0.8 eV is slightly lower than that required to achieve two-photon excitation at the band gap of a-Si:H (~1.7 eV). An ultrafast response of less than 100 fs is observed, which indicates that the free-carrier effect is suppressed by the localized states in the band gap.

Efficient heterojunction solar cells on p-type crystal silicon wafers

Abstract: Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance.

Optical properties of crystalline-amorphous core-shell silicon nanowires.

Abstract: The optical absorption in a nanowire heterostructure consisting of a crystalline silicon core surrounded by a conformal shell of amorphous silicon is studied. We show that they exhibit extremely high absorption of 95% at short wavelengths (λ 780 nm). These results indicate that our nanowires do not have optically active energy levels in the band gap. The absorption edge of silicon nanowires arrays is observed to shift to longer wavelengths as a function of the overall nanowire diameter. The near-infrared absorption of the nanowire array is significantly better than that of thin film amorphous silicon. These properties indicate potential use in large area optoelectronic and photovoltaic applications.

Substrate conformal imprint lithography for nanophotonics

Abstract: The field of nano-photonics studies the interaction and control of light with dielectric, semiconductor and metal structures which are comparable in size or smaller than the vacuum wavelength of light. In this thesis we present Substrate Conformal Imprint Lithography (SCIL) as a novel wafer-scale nanoimprint method with nano-scale resolution which combines the resolution and accuracy of rigid stamps with the flexibility of soft stamp methods. Chapter two describes the SCIL soft nanoimprint process and introduces a novel silica sol-gel imprint resist. A new soft rubber stamp material is described which enables sub-10 nm resolution. We demonstrate that SCIL imprinted patterns have on average less than 0.1 nm distortion and demonstrate sub–50 nm overlay alignment. Chapter 3 demonstrates 30 nm dense structures and features with aspect ratios from 1/640 up to 5. Imprinted sol-gel patterns can be transferred into underlying materials while maintaining sub-10 nm resolution. Two methods are demonstrated to pattern noble metals in particle arrays and sub-wavelength hole arrays. SCIL is applied to produce photonic crystal power InGaN LEDs which exhibit strong modification of the emission pattern. Chapter 4 demonstrates a relatively simple route towards 3D woodpile type photonic crystals. We show a four layer woodpile type structure with 70 nm features on a 240 nm pitch, which is temperature stable up to 1000 C. Chapter 5 demonstrates a novel fabrication route to large area nano hole arrays, which are interesting as angle independent color filters and for sensor applications. A solid state index matched hole array exhibits SPP mediated super resonant transmission. Chapter 6 treats single mode polarization stabilized Vertical Cavity Surface Emitting Lasers (VCSELs). The lasers produced by SCIL exhibit equal performance as devices produced by e-beam. VCSELs with SCIL imprinted sub-wavelength gratings increase the laser efficiency by 29 % compared to conventional gratings. Chapter 7 studies the improved red light absorption in thin film hydrogenated amorphous silicon (a-Si:H) solar cells with plasmonic back mirrors. Thin film a-Si:H solar cells are made on SCIL structured silver mirrors and smooth reference silver mirrors. Patterning increases the external collection efficiency to 6.2 %, an increase of 26 % compared to smooth reference cells. This increase is due to an enhanced absorption in the 600-800 nm wavelength range. Chapter 8 studies the performance of ultra thin silicon solar cells. We use SCIL to pattern substrates which a large variety of nano patterns on which thin film a-Si:H solar cells are processed with a thickness of 160 and 340 nm. A 160 nm thick silicon cell is also made on randomly textured glass. The best patterned cells with 160 nm thick silicon exhibit an external collection efficiency of 6.6 %, equal to that of the best thicker cells and 37.8% better than flat cells. Crucially, some regular patterns exhibited improved efficiency over cells made on randomly textured glass, which we attribute to coupling of non-absorbed light to waveguide modes in the silicon.

The silane depletion fraction as an indicator for the amorphous/crystalline silicon interface passivation quality

Abstract: In silicon heterojunction solar cells, thin amorphous silicon layers passivate the crystalline silicon wafer surfaces. By using in situ diagnostics during plasma-enhanced chemical vapor deposition (PECVD), the authors report how the passivation quality of such layers directly relate to the plasma conditions. Good interface passivation is obtained from highly depleted silane plasmas. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells, yielding aperture efficiencies up to 20.3% on 4 cm2 cells.

Hybrid photovoltaics based on semiconductor nanocrystals and amorphous silicon

Abstract: Semiconductor nanocrystals (NCs) are promising materials for applications in photovoltaic (PV) structures that could benefit from size-controlled tunability of absorption spectra, the ease of realization of various tandem architectures, and perhaps, increased conversion efficiency in the ultraviolet through carrier multiplication. The first practical step toward utilization of the unique properties of NCs in PV technologies could be through their integration into traditional silicon-based solar cells. Here, we demonstrate an example of such hybrid PV structures that combine colloidal NCs with amorphous silicon. In these structures, NCs and silicon are electronically coupled, and the regime of this coupling can be tuned by altering the alignment of NC states with regard to silicon band edges. For example, using wide-gap CdSe NCs we demonstrate a photoresponse which is exclusively due to the NCs. On the other hand, in devices comprising narrow-gap PbS NCs, both the NCs and silicon contribute to photocurrent, which results in PV response extending from the visible to the near-infrared. This work demonstrates the feasibility of hybrid PV devices that combine advantages of mature silicon fabrication technologies with the unique electronic properties of semiconductor NCs.

Fast and Scalable Printing of Large Area Monolayer Nanoparticles for Nanotexturing Applications

Abstract: Recently, there have been several studies demonstrating that highly ordered nanoscale texturing can dramatically increase performance of applications such as light absorption in thin-film solar cells. However, those methods used to make the nanostructures are not compatible with large-scale fabrication. Here we demonstrate that a technique currently used in roll-to-roll processing to deposit uniform thin films from solution, a wire-wound rod coating method, can be adapted to deposit close-packed monolayers or multilayers of silica nanoparticles on a variety of rigid and flexible substrates. Amorphous silicon thin films deposited on these nanoparticle monolayers exhibit 42% higher absorption over the integrated AM 1.5 spectrum than the planar controls. This simple assembly technique can be used to improve solar cells, fuel cells, light emitting diodes and other devices where ordered nanoscale texturing is critical for optimal performance.

Heterojunction III-V Photovoltaic Cell Fabrication

Abstract: A method for forming a heterojunction III-V photovoltaic (PV) cell includes performing layer transfer of a base layer from a wafer of a III-V substrate, the base layer being less than about 20 microns thick; forming an intrinsic layer on the base layer; forming an amorphous silicon layer on the intrinsic layer; and forming a transparent conducting oxide layer on the amorphous silicon layer. A heterojunction III-V photovoltaic (PV) cell includes a base layer comprising a III-V substrate, the base layer being less than about 20 microns thick; an intrinsic layer located on the base layer; an amorphous silicon layer located on the intrinsic layer; and a transparent conducting oxide layer located on the amorphous silicon layer.

Amorphous silicon–indium–zinc oxide semiconductor thin film transistors processed below 150 °C

Abstract: Amorphous silicon–indium–zinc–oxide (a-SIZO) thin film transistor (TFT) was investigated with the process temperature below 150 °C. The a-SIZO TFT exhibited a field effect mobility of 21.6 cm2/V s and an on/off ratio of 107. The stabilities of a-SIZO TFT and indium–zinc–oxide (IZO) TFT were compared, and a-SIZO TFT showed 3.7 V shift for threshold voltage (Vth) compared to 10.8 V shift in IZO TFT after bias temperature stress. Si incorporation into IZO-system as a stabilizer, which was confirmed by x-ray photoelectron spectroscopy, resulted in small shift in Vth in a-SIZO TFT without deteriorating mobility of higher than 21.6 cm2/V s.

Low-loss amorphous silicon wire waveguide for integrated photonics: effect of fabrication process and the thermal stability

Abstract: Hydrogenated amorphous silicon (a-Si:H) wire waveguides were fabricated by plasma-enhanced chemical vapor deposition and anisotropic dry etching. With the optimized fabrication process, the propagation losses of as low as 3.2 ± 0.2 dB/cm for the TE mode and 2.3 ± 0.1 dB/cm for the TM mode were measured for the 200 nm (height) × 500 nm (width) wire waveguides at 1550 nm using the standard cutback method. The loss becomes larger at shorter wavelength (~4.4 dB/cm for TE and ~5.0 dB/cm for TM at 1520 nm) and smaller at longer wavelength (~1.9 dB/cm for TE and ~1.4 dB/cm for TM at 1620 nm). With the waveguide width shrinking from 500 nm to 300 nm, the TM mode loss keeps almost unchanged whereas the TE mode loss increases, indicating that the predominant loss contributor is the waveguide sidewall roughness, similar to the crystalline silicon waveguides. Although the a-Si:H and the upper cladding SiO2 were both deposited at 400°C, the propagation loss of the fabricated a-Si:H wire waveguides starts to increase upon furnace annealing under atmosphere at a temperature larger than 300°C: ~13-15 dB/cm after 400°C/30 min annealing and >70 dB/cm after 500°C/30 min annealing, which can be attributed to hydrogen out-diffusion. Even higher temperature (i.e., >600°C) annealing leads to the propagation loss approaching to the polycrystalline silicon counterparts (~40-50 dB/cm) due to onset of a-Si:H solid-phase crystallization.

Two-dimensional photonic crystal for absorption enhancement in hydrogenated amorphous silicon thin film solar cells

Abstract: We propose a photovoltaic solar cell design based on a 100 nm thick absorbing layer made of hydrogenated amorphous silicon and patterned as a two-dimensional planar photonic crystal (PPC). After scanning the parameters of the PPC within the patterned cell, optical simulations performed on the best configuration obtained reveal that a relative increase in the integrated absorption inside the active layer of 28% can be expected between 300 and 720 nm compared to an equivalent but nonpatterned cell under normal incidence. Besides, this integrated absorption is found to be robust toward the angle of incidence. Incident light is efficiently coupled to leaky mode resonances of the PPC provided an appropriated tuning of its parameters. The effects of the reflectance of the back contact coupled to a conductive optical spacer on the absorption are also discussed.

Absorbing one-dimensional planar photonic crystal for amorphous silicon solar cell

Abstract: We report on the absorption of a 100nm thick hydrogenated amorphous silicon layer patterned as a planar photonic crystal (PPC), using laser holography and reactive ion etching Compared to an unpatterned layer, electromagnetic simulation and optical measurements both show a 50% increase of the absorption over the 038-075micron spectral range, in the case of a one-dimensional PPC Such absorbing photonic crystals, combined with transparent and conductive layers, may be at the basis of new photovoltaic solar cells

Light trapping regimes in thin-film silicon solar cells with a photonic pattern.

Abstract: We present a theoretical study of crystalline and amorphous silicon thin-film solar cells with a periodic pattern on a sub-micron scale realized in the silicon layer and filled with silicon dioxide right below a properly designed antireflection (AR) coating. The study and optimization of the structure as a function of all the photonic lattice parameters, together with the calculation of the absorption in a single layer, allows to identify the different roles of the periodic pattern in determining an increase of the absorbance. From one side, the photonic crystal and the AR coating act as impedance matching layers, thus minimizing reflection of incident light over a particularly wide range of frequencies. Moreover a strong absorption enhancement is observed when the incident light is coupled into the quasi guided modes of the photonic slab. We found a substantial increase of the short-circuit current when the parameters are properly optimized, demonstrating the advantage of a wavelength-scale, photonic crystal based approach for patterning of thin-film silicon solar cells.

Metal‐Catalyzed Etching of Vertically Aligned Polysilicon and Amorphous Silicon Nanowire Arrays by Etching Direction Confinement

Abstract: A systematic study of metal-catalyzed etching of (100), (110), and (111) silicon substrates using gold catalysts with three varying geometrical characteristics: isolated nanoparticles, metal meshes with small hole spacings, and metal meshes with large hole spacings is carried out. It is shown that for both isolated metal catalyst nanoparticles and meshes with small hole spacings, etching proceeds in the crystallographically preferred direction. However, the etching is confined to the single direction normal to the substrate surface when a catalyst meshes with large hole spacings is used. We have also demonstrated that the metal catalyzed etching method when used with metal mesh with large hole spacings can be extended to create arrays of polycrystalline and amorphous vertically aligned silicon nanowire by confining the etching to proceed in the normal direction to the substrate surface. The ability to pattern wires from polycrystalline and amorphous silicon thin films opens the possibility of making silicon nanowire array-based devices on a much wider range of substrates.