Thin Silicon Oxide Layer Research Articles

Improving the Efficiency of Multicrystalline Silicon by Adding an ARC Layer in the Front Device

High efficiency multicrystalline solar cells must improve performance while replacing higher cost monocrystalline silicon with lower cost multicrystalline silicon. Composite silicon dioxide-titanium dioxide (SiO2−TiO2) films are deposited on a large area of 15.6×15.6 cm2 textured multicrystalline silicon solar cells to increase the incident light trapped within the device. This is being achieved through new cell device structures that improve light trapping and energy conversion capability. These new structures depend on passivated thick and thin layers of silicon dioxide and titanum dioxide grown via wet and dry thermal oxidation. By replacing dry oxidation with wet oxidation the temperatures process can be lowered from 1050∘C to 850∘C to reduce both cycle time and wafer damage.

Thermoelectrics from silicon nanoparticles: the influence of native oxide

Thermoelectric materials were synthesized by current-assisted sintering of doped silicon nanoparticles produced in a microwave-plasma reactor. Due to their affinity to oxygen, the nanoparticles start to oxidize when handled in air and even a thin surface layer of native silicon oxide leads to a significant increase in the oxide volume ratio. This results in a considerable incorporation of oxygen into the sintered pellets, thus affecting the thermoelectric performance. To investigate the necessity of inert handling of the raw materials, the thermoelectric transport properties of sintered nanocrystalline silicon samples were characterized with respect to their oxygen content. An innovative method allowing a quantitative silicon oxide analysis by means of electron microscopy was applied: the contrast between areas of high and low electrical conductivity was attributed to the silicon matrix and silicon oxide precipitates, respectively. Thermoelectric characterization revealed that both, electron mobility and thermal conductivity decrease with increasing silicon oxide content. A maximum figure of merit with zT = 0.45 at 950 °C was achieved for samples with a silicon oxide mass fraction of 9.5 and 21.4% while the sample with more than 25% of oxygen clearly indicates a negative impact of the oxygen on the electron mobility.

Atomic layer deposition precursor step repetition and surface plasma pretreatment influence on semiconductor–insulator–semiconductor heterojunction solar cell

Semiconductor–insulator–semiconductor heterojunction solar cells were prepared using atomic layer deposition (ALD) technique. The silicon surface was treated with oxygen and hydrogen plasma in different orders before dielectric layer deposition. A plasma-enhanced ALD process was applied to deposit dielectric Al2O3 on the plasma pretreated n-type Si(100) substrate. Aluminum doped zinc oxide (Al:ZnO or AZO) was deposited by thermal ALD and serves as transparent conductive oxide. Based on transmission electron microscopy studies the presence of thin silicon oxide (SiOx) layer was detected at the Si/Al2O3 interface. The SiOx formation depends on the initial growth behavior of Al2O3 and has significant influence on solar cell parameters. The authors demonstrate that a hydrogen plasma pretreatment and a precursor dose step repetition of a single precursor improve the initial growth behavior of Al2O3 and avoid the SiOx generation. Furthermore, it improves the solar cell performance, which indicates a change of the Si/Al2O3 interface states.

Characterization of high resolution CMOS monolithic active pixel detector in SOI technology

Novel CMOS monolithic pixel detectors designed at KEK and fabricated at Lapis Semiconductor in 0.2 μm Silicon-on-Insulator (SOI) technology are presented. A thin layer of silicon oxide separates high and low resistivity silicon layers, allowing for optimization of design of detector and readout parts. Shallow wells buried under the oxide in the detector part screen the entire pixel electronics from electrical field applied to the detector. Several integration type SOI pixel detectors have been developed with pixel sizes 8–20 μm. The general features of 14 × 14 μm2 detectors designed on different wafers (CZ-n, FZ-n and FZ-p) were measured and compared. The detector performance was studied under irradiation with visible and infra-red laser, and also X-ray ionizing source. Using X-rays from an Am-241 source the noise of readout electronics was measured at different working conditions, showing the ENC in the range of 88–120 e−. The pixel current was calculated from average DC pedestal shift while varying the pixel integration time. The operation of the detector was studied under partial and full depletion conditions. The effects of temperature and detector bias voltage on noise and leakage current were studied. Characteristics of an ADC integrated in the front-end chip are also presented.

Two-dimensional X-ray diffraction and transmission electron microscopy study on the effect of magnetron sputtering atmosphere on GaN/SiC interface and gallium nitride thin film crystal structure

The growth mechanisms of high quality GaN thin films on 6H-SiC by sputtering were investigated by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The XRD θ-2θ scans show that high quality (0002) oriented GaN was deposited on 6H-SiC by reactive magnetron sputtering. Pole figures obtained by 2D-XRD clarify that GaN thin films are dominated by (0002) oriented wurtzite GaN and {111} oriented zinc-blende GaN. A thin amorphous silicon oxide layer on SiC surfaces observed by STEM plays a critical role in terms of the orientation information transfer from the substrate to the GaN epilayer. The addition of H2 into Ar and/or N2 during sputtering can reduce the thickness of the amorphous layer. Moreover, adding 5% H2 into Ar can facilitate a phase transformation from amorphous to crystalline in the silicon oxide layer and eliminate the unwanted {33¯02} orientation in the GaN thin film. Fiber texture GaN thin films can be grown by adding 10% H2 into N2 due to the complex reaction between H2 and N2.

Performance improvement of amorphous indium–gallium–zinc oxide ReRAM with SiO2 inserting layer

In this study, the resistive switching performance of amorphous indium–gallium–zinc oxide (a-IGZO) resistive switching random-access memory (ReRAM) was improved by inserting a thin silicon oxide layer between silver (Ag) top electrode and a-IGZO resistive switching layer. Compared with the single a-IGZO layer structure, the SiO2/a-IGZO bi-layer structure exhibits the higher On/Off resistance ratio larger than 103, and the lower operation power using a smaller SET compliance current. In addition, good endurance and excellent retention characteristics were achieved. Furthermore, multilevel resistance states are obtained through adjusting SET compliance current and RESET stop voltage, which shows a promise for high-performance nonvolatile multilevel memory application.

The formation of submonolayer thorium coatings on a silicon oxide surface by electrochemical deposition

Data are reported on the mechanisms of the formation of submonolayer coatings based on thorium oxide on the surface of Si(111) single crystal with natural oxide as a result of electrochemical deposition. It is experimentally shown that the deposition of thorium atoms from an acetone solution of Th(NO3)4 onto the surface of natural silicon oxide leads to the formation of defects in the thin silicon oxide layer, which is accompanied by the deposition of thorium nanoclusters onto a pure silicon surface. The observed effects have been qualitatively explained assuming the breakdown of natural silicon oxide due to the presence of an electrical double layer in the near-surface region of the cathode.

Silicon Photodiode for Visible Spectral Region

Photodiodes with high sensitivity in the short-wavelength portion of the visible region of the spectrum, fabricated based on high-ohmic silicon, are widely used in scintillation devices for visualization of gamma radiation and x-radiation in nuclear physics and medicine [1]. The following have been used to improve collection of minority charge carriers generated by short-wavelength photons in the immediate vicinity of the silicon surface: a shallow p-n junction with a vertical [2] or a horizontal [3, 4] structure, a graded p-n junction formed by diffusion of boron through the lattice of a thin layer of silicon oxide on the substrate surface [5], and an inversion layer photodiode [6]. The use of an inversion layer photodiode is limited by the narrow range of linearity typical of the spectral responsivity. A general disadvantage of photodiodes formed by boron diffusion into n-type silicon is the presence in the near-surface layer of silicon of a barrier layer under the oxide, preventing electrons generated by light in the p-region of the silicon from crossing into the p-n junction [7]. Photodiodes based on n-p junctions formed by diffusion of phosphorus or arsenic are more stable in the short-wavelength portion of the visible region of the spectrum [8, 9]. Their sensitivity at O 1 [12]. This allows us to ensure the minimum depth for the non-photoactive zone on the silicon surface. Arsenic is selected because of the following considerations. Its low diffusion coeffi cient makes it possible to form shallow junctions. The depth of the arsenic-doped n-region in our case is 0.2 om. Due to the relatively low atomic weight and the similar size of the ionic radii for silicon and arsenic, mechanical strains arising when doping with arsenic are relatively small, which lets us decrease the number of defects in the n-region and reduce the effect of recombination processes compared with the phosphorus-doped n + -regions [13]. The ohmic contact to the n-region is formed on the periphery of the arsenic-doped n-region. A guard ring reduces the surface component of the dark current. It is designed to be combined with the ohmic contact to the substrate. The dimensions of the photosensitive region of the photodiode are 5 × 5 mm. A two-layer SiO2/Si3N4 antirefl ective coating is formed on the photosensitive surface of the n-i-p junction.

Selective dual-side functionalization of hollow SiO2 micropillar arrays for biotechnological applications

We report a method for the fabrication and selective functionalization of the internal and external surfaces of high-aspect-ratio hollow silicon dioxide (SiO2) micropillar arrays. The strategy enables independent surface reactions with no cross-contamination. The dual-functionalization is carried out during the fabrication of the micropillars in four steps. First, a sample of macroporous silicon is prepared by electrochemical etching and then it is thermally oxidized to create a thin layer of silicon dioxide on the internal surface of the macropores. Next, the internal surface is functionalized with mercaptopropyl trimethoxysilane (MPTMS). Subsequently, the micropillars are released by etching the back side of silicon wafers using a solution of tetramethylammonium hydroxide (TMAH). Finally, the external surface of the micropillars is chemically modified in a multi-step biofunctionalization with aminopropyl triethoxysilane (APTES), glutaraldehyde (GTA) and bovine serum albumin (BSA). The resulting structures were characterized by scanning electron microscopy and the surface reactions were confirmed by Fourier-transform infrared spectroscopy. The internal and external sides were photolabeled with tetramethyl rhodamine-5-maleimide and fluorescein isothiocyanate (FITC), respectively, and analyzed using fluorescence confocal microscopy. The peculiar three-dimensional (3D) geometry of the micropillars allows these double-active surfaces to be imaged at the same time. Results demonstrated the successful dual-side chemical modification and the feasibility of imaging both internal and external sides. Due to its remarkable architecture and chemical versatility, these micropillar arrays are a promising engineered platform for a range of applications such as biosensing, 3D cell culture and drug delivery.

Investigations on the Passivation Mechanism of AlN:H and AlN:H-SiN:H Stacks

In this paper, the properties of hydrogenated aluminum nitride layers (AlN:H) as an excellent passivation layer of silicon surfaces are examined. The structural and chemical properties of the AlN:H bulk are analyzed using a great variety of measurements techniques such as GI-XRD, FTIR, SEM, HR-TEM, corona charging and a thickness dependent measurements of the surface passivation. A model for the formation of negative fixed charges at the interface to silicon is presented based on the charge state of ionized point defects in AlN.Additionally, stacks of AlN:H and SiNx:H are introduced into silicon photovoltaics and their combination with two different types of thin low temperature silicon oxide layers is studied. Excellent passivation results are presented for highly doped p-type silicon and highly doped n-type silicon using the previous described stacks. Emitter saturation current densities of textured samples show, that 75 Ω/sq. phosphorous emitter can be passivated as efficient as with the typically used SiNx:H allowing maximum open circuit voltages of 657 mV. This is supported by measurement of cell parameters of p-type solar cells using these stacks as a combined anti-reflective coating and passivation layer. Furthermore, the passivation level reached for high efficiency 70 Ω/sq. boron emitters is nearly as high as with excellent passivating PECVD Al2O3/SiNx stacks. Emitter saturation current densities down to 61 fA/cm2 are presented corresponding to a maximum open circuit voltage of 696 mV. This indicates that the invented stacks containing AlN:H can also be applied as combined anti-reflective coating and passivation layers in n-type cells.

Self-Contained Frequency Trimming of Micromachined Silicon Resonators via Localized Thermal Oxidation

An electronically controllable frequency trimming technique based on localized thermal oxidation of single crystalline silicon resonators is demonstrated. Thin layers of silicon dioxide can be thermally grown on silicon surfaces of resonant structures via extreme joule heating of such when biased with relatively large bias currents in an oxygen-rich environment. Changes in structural stiffness or oxidation induced internal stress causes a shift in the resonant frequency of the structures that can be used for post-fabrication frequency trimming of silicon resonators. As an added advantage, the positive temperature coefficient of Young's modulus for the added silicon dioxide layer can partially compensate the large negative temperature coefficient of frequency (TCF) for the silicon resonators. Frequency trimming as high as ~ 3.7% and TCF as low as 0.2 ppm/°C has been demonstrated for a 54 MHz I-shaped resonator using this technique. Furthermore, it has been demonstrated that coupling of mechanical resonance to electrical resistance and consequently joule heating of the structure can lead to a cooling effect at mechanical resonant frequency of the structure that allows the localized oxidation to stop automatically as soon as the resonator frequency reaches a targeted actuation frequency applied to the structure. The viability of this concept is demonstrated by application of both off-resonance and at-resonance actuation signals to fabricated resonators showing that as opposed to the off-resonance signals, the at-resonance signals with the same intensity do not lead to further frequency shift.

Colorimetric sensors using nano-patch surface plasmon resonators

A two-dimensional array of gold nano-patches on a highly reflective mirror is proposed for refractive index sensing based on changes in the reflected colors. The grating on the mirror creates localized surface plasmon resonances resulting in a minimum in the visible reflectance spectra. The wavelength of the resonance can be tuned by changing the width of the nano-patches and is also dependent on the refractive index of the surrounding medium. The color variation due to change in the refractive index is measured and used to realize a simple low-cost sensor with a refractive index resolution better than 10−5 just using image processing. The efficacy of the proposed sensor is also demonstrated for surface sensing by depositing thin layers of silicon dioxide. The color difference due to the addition of a 3 nm thick layer of silicon dioxide is detectable by the naked eye and deposition thickness of 2 Å can be resolved using image processing.

Self-catalyzed VLS grown InAs nanowires with twinning superlattices

We report on the self-catalyzed growth of InAs nanowires by molecular beam epitaxy on GaAs substrates covered by a thin silicon oxide layer. Clear evidence is presented to demonstrate that, under our experimental conditions, the growth takes place by the vapor–liquid–solid (VLS) mechanism via an In droplet. The nanowire growth rate is controlled by the arsenic pressure while the diameter depends mainly on the In rate. The contact angle of the In droplet is smaller than that of the Ga droplet involved in the growth of GaAs nanowires, resulting in much lower growth rates. The crystal structure of the VLS grown InAs nanowires is zinc blende with regularly spaced rotational twins forming a twinning superlattice.

Enhanced Light Absorption and Minority Carrier Lifetime for Pn<sup>+</sup> Multicrystalline Silicon by Treatment with HF/H<sub>2</sub>O<sub>2</sub>-Based Solutions

A novel method was proposed to form porous silicon (PS) antireflection layers and thin SiO2 films at the same time by HF/H2O2 treatment of acid-textured pn+ multicrystalline silicon. Porous silicon structures formed inside the cavities and the porosity became large with an increase of the treated time resulting in a dramatical decrease of reflectance. The reflectance decreased to less than 5% within the wavelength range of 420-970 nm after 5 min HF/H2O2 treatment. Furthermore, the minority-carrier lifetime showed an increase of about 42% for a short treated time because of the formation of a thin silicon oxide layer resulting the reduction of dangling silicon bonds in the interface between PS/Si.

Aluminum-Induced Crystallization of p<sup>+</sup> Silicon Pinholes for the Formation of Rear Passivation Contact in Solar Cell

Formation of nano-crystalline p+ silicon (Si) in pinholes through a silicon dioxide layer was achieved by pinning of aluminum through the thin silicon dioxide (SiO2) layer. In addition to opening holes of SiO2 layer by aluminum (Al) pining, amorphous silicon (a-Si) was subsequent deposited on the Al layer and another heated at low temperature (500°C) to allow solid- phase epitaxial growth of p+ Si in the pinholes due to the Al induced layer exchange process. The poly-crystalline p+ Si obtains lower effective surface recombination than the Al back surface field (BSF). The technique demonstrated to result in ohmic contacts with low contact resistance. The evaluation of Al-induced crystallization of a-Si in a-Si/Al bilayer was studied by X-ray diffraction. In this paper, the influence of a-Si/Al thickness ratio on the specific conductivity value and crystalline grain size of the p+ Si thin film is discussed. The obtained results are helpful for a further design of the rear passivation contact in solar cell.

Time-Dependent Determination of HF Formation in LiPF6 -Containing Electrolytes in Different Cell Types by Spectroscopic Ellipsometry

A novel experimental technique was developed to investigate HF formation in organic carbonate based electrolytes containing LiPF6. Using spectroscopic ellipsometry, aging and thermal decomposition of the electrolyte were monitored in a standard glass cell and in a polypropylene vessel. The amount of HF generated was determined by following the etching rate of a thin silicon dioxide layer on a silicon wafer in contact with the electrolyte. The material of cell construction clearly influenced the etching rate of the SiO2 sensor and affected the rate of degradation.

Selective Functionalization of the Internal and the External Surfaces of Mesoporous Silicon by Liquid Masking

A general approach for selective, differential functionalization of the interior and exterior surfaces of mesoporous Si is reported. The method employs two immiscible liquids, one inert and the other chemically reactive with the porous Si nanostructure. First, a porous Si sample is prepared by electrochemical etch and then it is mildly oxidized, which places a thin layer of silicon oxide at the surface. The inner pore walls of the partially oxidized porous Si film are then infiltrated with an inert liquid (octane). The sample is then immersed in aqueous solution containing hydrogen fluoride (HF), which serves as the reactive liquid. The hydrophobic phase is retained in the interior of the porous nanostructure, and HF(aq) attacks only the exposed surfaces of the oxidized porous Si sample, generating a hydrophobic, hydrogen-terminated (Si-H) outer layer. The reaction is self-limiting due to the immiscibility of octane and water, and the extent of penetration of the Si-H surface into the porous layer is dependent on the time of exposure to HF(aq). The Si-H surface can then be modified by thermal hydrosilylation (1-dodecene or 10-bromo-1-decene) in a subsequent step, resulting in a bifunctional porous Si film containing hydrophobic pore entrances to hydrophilic inner pores. The hydrophobic dodecyl species at the mouths of the pores is found to form a barrier for molecular transport; it decreases the rate of leaching (into water) of a rhodamine test molecule that is preloaded into the sample by >8 fold.

Study of the composition, structure, and optical properties of a-Si1 − x C x :H〈Er〉 films erbium doped from the Er(pd)3 complex compound

Rutherford backscattering, IR spectroscopy, ellipsometry, and atomic-force microscopy are used to perform an integrated study of the composition, structure and optical properties of a-Si1 − x C x :H〈Er〉 amorphous films. The technique employed to obtain the a-Si1 − x C x :H〈Er〉 amorphous films includes the high-frequency decomposition of a mixture of gases, (SiH4) a + (CH4) b , and the simultaneous thermal evaporation of a complex compound, Er(pd)3. It is demonstrated that raising the amount of CH4 in the gas mixture results in an increase in the carbon content of the films under study and an increase in the optical gap E g opt from 1.75 to 2.2 eV. Changes in the composition of a-Si1 − x C x :H〈Er〉 amorphous films, accompanied, in turn, by changes in the optical constants, are observed in the IR spectra. The ellipsometric spectra obtained are analyzed in terms of multiple-parameter models. The conclusion is made on the basis of this analysis that the experimental and calculated spectra coincide well when variation in the composition of the amorphous films with that of the gas mixture is taken into account. The existence of a thin (6–8 nm) silicon-oxide layer on the surface of the films under study and the validity of using the double-layer model in ellipsometric calculations is confirmed by the results of structural analyses by atomic-force microscopy.

Surface pre-treatment for barrier coatings on polyethylene terephthalate

Polymers have favourable properties such as light weight, flexibility and transparency. Consequently, this makes them suitable for food packaging, organic light-emitting diodes and flexible solar cells. Nonetheless, raw plastics do not possess sufficient barrier functionality against oxygen and water vapour, which is of paramount importance for most applications. A widespread solution is to deposit thin silicon oxide layers using plasma processes. However, silicon oxide layers do not always fulfil the requirements concerning adhesion and barrier performance when deposited on films. Thus, plasma pre-treatment is often necessary. To analyse the influence of a plasma-based pre-treatment on barrier performance, different plasma pre-treatments on three reactor setups were applied to a very smooth polyethylene terephthalate film before depositing a silicon oxide barrier layer. In this paper, the influence of oxygen and argon plasma pre-treatments towards the barrier performance is discussed examining the chemical and topological change of the film.It was observed that a short one-to-ten-second plasma treatment can reduce the oxygen transmission rate by a factor of five. The surface chemistry and the surface topography change significantly for these short treatment times, leading to an increased surface energy. The surface roughness rises slowly due to the development of small spots in the nanometre range. For very long treatment times, surface roughness of the order of the barrier layer's thickness results in a complete loss of barrier properties. During plasma pre-treatment, the trade-off between surface activation and roughening of the surface has to be carefully considered.

Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer

Nitric acid in air is formed by atmospheric reactions of oxides of nitrogen and is removed primarily through deposition to surfaces, either as the gas or after conversion to particulate nitrate. Many of the surfaces and particles have organic coatings, but relatively little is known about the interaction of nitric acid with organic films. We report here studies of the interaction of gaseous HNO(3) with a self-assembled monolayer (SAM) formed by reacting 7-octenyltrichlorosilane [H(2)C=CH(CH(2))(6)SiCl(3)] with the surface of a germanium infrared-transmitting attenuated total reflectance (ATR) crystal that was coated with a thin layer of silicon oxide (SiO(x)). The SAM was exposed at 298 ± 2 K to dry HNO(3) in a flow of N(2), followed by HNO(3) in humid N(2) at a controlled relative humidity (RH) between 20-90%. For comparison, similar studies were carried out using a similar crystal without the SAM coating. Changes in the surface were followed using Fourier transform infared spectroscopy (FTIR). In the case of the SAM-coated crystal, molecular HNO(3) and smaller amounts of NO(3)(-) ions were observed on the surface upon exposure to dry HNO(3). Addition of water vapor led to less molecular HNO(3) and more H(3)O(+) and NO(3)(-) complexed to water, but surprisingly, molecular HNO(3) was still evident in the spectra up to 70% RH. This suggests that part of the HNO(3) observed was initially trapped in pockets within the SAM and shielded from water vapor. After increasing the RH to 90% and then exposing the film to a flow of dry N(2), molecular nitric acid was regenerated, as expected from recombination of protons and nitrate ions as water evaporated. The nitric acid ultimately evaporated from the film. On the other hand, exposure of the SAM to HNO(3) and H(2)O simultaneously gave only hydronium and nitrate ions. Molecular dynamics simulations of defective SAMs in the presence of HNO(3) and water predict that nitric acid intercalates in defects as a complex with a single water molecule that is protected by alkyl chains from interacting with additional water molecules. These studies are consistent with the recently proposed hydrophobic nature of HNO(3). Under atmospheric conditions, if HNO(3) is formed in organic layers on surfaces in the boundary layer, e.g. through NO(3) or N(2)O(5) reactions, it may exist to a significant extent in its molecular form rather than fully dissociated to nitrate ions.