Crystalline Si Wafers Research Articles

Dense TiO2 films grown by sol–gel dip coating on glass, F-doped SnO2, and silicon substrates

Abstract

Optoelectronic transport property measurements of an amorphous-silicon-passivated c-silicon wafer using non-contacting methodologies

Photocarrier radiometry (PCR), microwave photoconductivity decay (μ-PCD) and transient and quasi-steady-state eddy currents were used for the characterization of a crystalline (c-) Si wafer passivated with intrinsic hydrogenated amorphous silicon films on either front or back side, or on both sides of the wafer. All three techniques are capable of measuring recombination lifetimes, however additional fundamental free photoexcited carrier transport parameters (diffusion coefficient, front- and back-surface recombination velocity) can be evaluated from the PCR frequency dependence owing to the free-carrier-density wave's diffusion-length depth-dependence on frequency. The measured recombination lifetimes are compared and differences in values among the three techniques are discussed in terms of the role of surface and interface defects. μ-PCD and eddy currents yield effective or vertically averaged lifetime measurements, as opposed to PCR which yields bulk recombination lifetimes separately from surface recombination effects.

Screen-Printed Al-Alloyed Rear Junction Solar Cell Concept Applied to Very Thin (100 μm) Large-Area n-Type Si Wafers

Reducing the thickness of crystalline Si wafers processed to solar cells returns two significant benefits. Firstly, processing cost is reduced by saving cost- and energy-intensive Si material. Secondly, the required diffusion length of minority carriers is smaller, thus, wafers with a smaller carrier lifetime (e.g. due to higher base doping) can be utilized. In this work, the industrially feasible “PhosTop” cell concept is employed by manufacturing large-area n-type rear junction solar cells with a screen-printed Al-alloyed emitter featuring a selective phosphorous front surface field and a SiO2/SiNx passivation on the front.PC1D simulations for substrates with different base doping concentrations show that the range of base resistivities utilizable for those PhosTop solar cells is extended towards higher doping concentrations with decreasing wafer thickness. PC1D forecasts a conversion efficiency of the chosen 2.8Ωcm n-type Czochralski-Si wafers of 19.2% for 100μm thickness, merely 0.1% less than for standard thickness but saving ∼25% of the Si material. The manufactured thin large-area solar cells achieve a maximum efficiency of 19.0%.

The effect of bio-conjugation on aging of the photoluminescence in CdSeTe–ZnS core–shell quantum dots

The aging of the photoluminescence (PL) in bio-conjugated and non-conjugated CdSeTe–ZnS core–shell quantum dots (QDs) is studied by the micro-PL, micro-Raman and X-ray diffraction (XRD) in the samples of buffered QD solution dried on a crystalline Si wafer and stored in the atmospheric ambience for about 2years. The aging of the PL consists in a “blue” spectral shift of the PL band, an increase in PL band half-width and the decrease in the PL intensity. These changes are more pronounced in the conjugated QD samples. The XRD analysis of the aged samples revealed that the QD core diameter is reduced by ∼1.5nm in the conjugated QDs as compared to the non-conjugated ones. The possible mechanism of PL spectrum aging is the oxidation that decreases the QD core dimension. It is concluded that the bio-conjugation promotes QD oxidation and the mechanism of the effect is proposed.

Low Temperature Phosphorus Doping in Silicon Using Catalytically Generated Radicals

In this paper, we report the possibility of forming a phosphorus (P)-doped layer on silicon (Si) at low temperatures. Using the radicals catalytically generated from phosphine (PH3), a thin n-type layer is formed on a crystalline Si (c-Si) wafer at 150 °C. The secondary ion mass spectrometry (SIMS) profile of doped P atoms indicates that P atoms exist in the vicinity of the c-Si surface, and the depth at which P atom concentration decreases to 1/10 of the surface concentration is less than 12 nm for 300 s of radical treatment. The sheet carrier density on radical-treated c-Si wafers measured using the Hall effect shows that P atoms act as donors without annealing. The sheet carrier concentration of the P-doped layer is increased by adding hydrogen (H2) to the PH3 source gas. The effect of adding H2 to PH3 suggests that the surface reaction of atomic H plays an important role in the doping process.

Effect of vacuum annealing on charge transport and trapping in a-Si1 − xCx:H/c-Si heterostructures

The processes of charge transport and trapping in amorphous Si1−xCx:H films deposited on crystalline p-type Si wafers and annealed in vacuum in the temperature range 300–650°C have been evaluated. Current–voltage (I–V), capacitance–voltage (C–V) and admittance–temperature (G–T) characteristics were measured in the temperature range 100–350K. The spectrum of thermal effusion of hydrogen was measured from room temperature up to 1000°C.C–V characteristics indicate a slight increase of the dielectric constant k and a large hysteresis after annealing at 450°C. The hysteresis is believed to be associated with mobile hydrogen effusion from the a-SiC:H film, and it is not seen after a 650°C anneal. From I–V data the maximum rectification ratio is observed after annealing at 450°C. Variable-range hopping (VRH) conduction at the Fermi level is found to dominate the forward current of the as-deposited structure. After annealing at 450°C the forward current can be described by space-charge limited (SCL) mechanisms with trapping at shallow levels with energy of about 0.12eV. After annealing at 650°C the process of VRH conduction appears again, but the density of hopping sites is much higher than in the as-grown sample. From admittance spectra, the energy position of respective traps in a-SiC:H is at (EV+0.45) eV for as-deposited material and it decreases slightly after vacuum annealing. On the basis of these results, an energy band diagram of the a-Si1−xCx:H/p-Si structure annealed at 450°C is proposed.

Substrate-dependent Internal Stress of Aluminum-doped Zinc-oxide Thin Films Deposited by DC Magnetron Sputtering

The internal stress of very thin (30 nm) aluminum-doped zinc-oxide (AZO) films deposited by DC magnetron sputtering at room temperature was found to be strongly substrate dependent. The stress of an AZO film deposited on a single-crystalline Si wafer substrate changed more steeply than that of a film deposited on an amorphous glass substrate as deposition conditions, such as the sputter working pressure and the applied power on the cathode, were varied. This result can be explained by the atomistic peening effect which occurs more intensively in a more crystalline microstructure. Thus it can be inferred that the crystallinity of the substrate influences that of the growing film. In other words, the more crystalline the substrate is, the more crystalline the growing film is. Support for this point of view was supplied by the X-ray diffraction pattern result showing that the intensity of the (002) plane diffraction peak for the AZO film grown on a Si wafer was higher than that for an AZO film grown on glass. That is, the AZO film grown on a crystalline Si wafer is more crystalline than the AZO film grown on an amorphous glass wafer. This study confirmed that the substrate influenced the internal stress of the growing film strongly.

Achieving High Efficiency Silicon-Carbon Nanotube Heterojunction Solar Cells by Acid Doping

Various approaches to improve the efficiency of solar cells have followed the integration of nanomaterials into Si-based photovoltaic devices. Here, we achieve 13.8% efficiency solar cells by combining carbon nanotubes and Si and doping with dilute HNO(3). Acid infiltration of nanotube networks significantly boost the cell efficiency by reducing the internal resistance that improves fill factor and by forming photoelectrochemical units that enhance charge separation and transport. Compared to conventional Si cells, the fabrication process is greatly simplified, simply involving the transfer of a porous semiconductor-rich nanotube film onto an n-type crystalline Si wafer followed by acid infiltration.

Xenon-ion-induced and thermal mixing of Co/Si bilayers and their interplay

Studies on ion-irradiated transition-metal/silicon bilayers demonstrate that interface mixing and silicide phase formation depend sensitively on the ion and film parameters, including the structure of the metal/Si interface. Thin Co layers e-gun evaporated to a thickness of 50 nm on Si(1 0 0) wafers were bombarded at room temperature with 400-keV Xe + ions at fluences of up to 3 × 10 16 cm −2. We used either crystalline or pre-amorphized Si wafers the latter ones prepared by 1.0-keV Ar-ion implantation. The as-deposited or Xe-ion-irradiated samples were then isochronally annealed at temperatures up to 700 °C. Changes of the bilayer structures induced by ion irradiation and/or annealing were investigated with RBS, XRD and HRTEM. The mixing rate for the Co/c-Si couples, Δ σ 2/ Φ = 3.0(4) nm 4, is higher than the value expected for ballistic mixing and about half the value typical for spike mixing. Mixing of pre-amorphized Si is much weaker relative to crystalline Si wafers, contrary to previous results obtained for Fe/Si bilayers. Annealing of irradiated samples produces very similar interdiffusion and phase formation patterns above 400 °C as in the non-irradiated Co/Si bilayers: the phase evolution follows the sequence Co 2Si → CoSi → CoSi 2.

The potential of sonicated water in the cleaning processes of silicon wafers

The potential of using distilled water in environmental friendly and non-toxic ultrasonic cleaning step in crystalline Si wafer preparation is addressed. It is found that organic particle contaminants are effectively removed during the kHz-frequency sonication of wafers over the first 40–60 min, as evidenced by the disappearance of organic-related optical absorption peaks. Subsidiary effects of the sonication on electrical and photoelectrical behaviour of Si wafers are discussed. It is observed that, when treating the wafers less than 60 min, the dangling bonds at the air/oxide and oxide/wafer interface can be activated. That causes a decrease in the subsurface resistance against dislocation displacements (observed micro-hardness decrease). It affects barriers of the free carrier migration at the interfaces (revealed in the current–voltage characteristics), and acts as recombination centers, accelerating photovoltage decays. A healing of the bonds may occur at longer cleaning times (≈60–120 min), which partially recover the interfaces, thus reversing the observed changes. Although an exact mechanism is not yet clarified, the likely scenario behind the observed changes is proposed to involve oxygen and hydrogen molecules, which are decomposed in water and subsequently trapped at the silicon surface.

Electrochemical method for slicing Si blocks into wafers using platinum wire electrodes

Straight grooves were formed in single crystalline Si wafers and poly-crystalline Si blocks in an aqueous solution containing HF by an electrochemical method using Pt wires ( φ=50 μm) and a Pt foil as catalytic electrodes and a counter electrode, respectively. The grooves were formed by applying anodic potential of about 2.25 V vs. Ag/AgCl to the Pt wires, which were in contact with Si. The grooving rate and morphologies were dependent on HF concentrations. The grooving rate was also dependent on the pressure applied to the Si/Pt interface. Grooves with a width of about 55 μm were formed at the edge of a 250-μm-thick single crystalline wafer at a rate of 3.2 mm/h by processing in 15 mol dm −3 HF under a pressure of about 54×10 4 g/cm 2. This pressure was necessary to increase the grooving rate. Possibly, an insulating porous silicon layer formed between Si and Pt was destroyed under this pressure. On applying this process to a poly-crystalline Si block, a Si wafer with a size of 1×1 cm 2 was obtained by the electrochemical method. However, the slicing speed was lowered as the depth of grooves increased and it took 120 h to obtain the wafer. The slow exchange of HF solution in the groove was thought to be the main reason for the slow speed of slicing the Si block.

Extremely low surface recombination velocities on crystalline silicon wafers realized by catalytic chemical vapor deposited SiNx/a-Si stacked passivation layers

Catalytic chemical vapor deposition (Cat-CVD), also called hot-wire CVD, yields silicon-nitride/amorphous-silicon (SiNx/a-Si) stacked layers with remarkably low surface recombination velocities (SRVs) of lower than 1.5 cm/s for n-type crystalline Si (c-Si) wafers, and lower than 9.0 cm/s for p-type wafers. The temperature throughout the formation of stacked layers is lower than 250 °C. The usage of a-Si films significantly enhances the effective carrier lifetime of c-Si wafers, and SiNx films are also essential for reducing SRVs to such low levels.

Pulsed laser ablation and deposition of silicon

A KrF laser was used to ablate a polycrystalline Si target for deposition of Si on MgO and GaAs substrates at room temperature. The deposition was performed in 10−8 mbar, with two types of laser beams: a homogeneous beam being imaged onto the target (2.9 J/cm2), and a non-homogeneous which is nearly focused (2 J/cm2, 6.5 J/cm2). In both cases, the beam was scanned over an area of 1 cm2. For the homogenous beam, we observed only a limited number of droplets (<0.1 μm). A high number of micron-sized (<5 μm) droplets were observed on the film by the higher fluence nonhomogeneous laser beam. Raman spectroscopy showed that the micron-sized droplets are crystalline while the film is amorphous. The generation of the large droplets is most likely related to the cone structures formed on the ablated target. We also compared cone formation on a polycrystalline Si target and a single crystalline Si wafer, using multiple laser pulses onto a single spot.

Ultraviolet Electroluminescence Emission from n-Type ZnO/p-Type Si Crossed Nanowire Light-Emitting Diodes

The optical characteristics of an n-type ZnO/p-type Si crossed nanowire (NW) light-emitting diode (LED) were investigated in this study. N-ZnO nanowires (NWs) were synthesized by thermal chemical vapor deposition, and p-Si NWs were fabricated by etching a single crystalline Si wafer. The p–n heterojunction LED formed by the cross of the n-ZnO and p-Si NWs selected from the NWs prepared in this work exhibited the current rectifying behavior with the turn-on voltage of 1.3 V. Our investigation of the photoluminescence spectrum of the as-grown n-ZnO NWs and electroluminescence spectrum of the n-ZnO/p-Si crossed NW LED reveals that both spectra have the same position of peaks at 390 nm. This result indicates that the UV emission from the crossed NW LED is mostly attributed to the band-to-band transition of electrons in the ZnO NW.

Random pn-junctions for physical cryptography

In this paper, we report on high-rectification pn-diodes (rectification ratios up to 2×107) prepared by aluminum-induced crystallization on crystalline Si-wafers, which exhibit highly random I(V) characteristics. We argue that arrays of such diodes can be employed as physical uncloneable functions for cryptography. To resolve the structure of the active diode area, focused-ion beam imaging was used. The I(V) curves of the diodes reveal that both a smaller polycrystalline silicon film thickness and a smaller diode size lead to increasing randomness due to the increasing inhomogeneity of thinner films and due to more pronounced grain boundary effects for smaller diodes.

Detection of carrier information in heterojunctions of nanocrystalline/crystalline Si

Hall effect measurements on heterojunctions of nanocrystalline Si:H (nc-Si:H) film with crystalline Si wafer fabricated by plasma enhanced chemical vapor deposition technique were made as a function of temperature (20–300 K) and magnetic field (0–15 T). Magnetic field-dependent resistivity and Hall data were interpreted with the quantitative mobility spectrum analysis (QMSA) method, which successfully separated the two-dimensional electron gases (2DEGs) at the nc-Si:H/c-Si interface from 3D carriers appearing in the films and substrates. Mobilities and densities of 2DEGs for the specimens were measured. Detail analyses about influences of interface and epitaxial layer quality including doping, film thickness and mean size of nanocrystals on the mobilities and densities of 2DEGs at interfaces were carried out. The important role of the amorphous buffer layer within the junction was identified. Origin of high mobility observed in the prepared films was revealed. Forward and reverse current mechanisms as well as the dependence of breakdown voltages on temperature in the operated nc-Si:H/c-Si heterostructure were elucidated.

Macroporous Si as an absorber for thin‐film solar cells

AbstractWe separate 15 μm to 32 μm thick macroporous Si films from n‐type crystalline Si wafers by electrochemical etching under back‐side illumination. These layers are surface passivated and have openings in the oxide, for local doping and contacting, over 1% of the cell area. We measure effective lifetimes within a range of 2 to 3 μs. With an analytical model for the effective lifetime in macroporous Si we find an average surface recombination velocity of 75 cm/s, dominantly caused by the contact openings. The measured optical absorption of a 26 μm thick layer allows for a maximum short circuit current density of 37 mA/cm2. The potential efficiency with the current surface passivation quality is 17%. The material loss for separating the macroporous absorber from the wafer is 5 μm. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ultra-thin crystalline silicon films produced by plasma assisted epitaxial growth on silicon wafers and their transfer to foreign substrates

We have developed a new process to produce ultra-thin crystalline silicon films with thicknesses in the range of 0.1 − 1 μ m on flexible substrates. A crystalline silicon wafer was cleaned by SiF4 plasma exposure and without breaking vacuum, an epitaxial film was grown from SiF4 , H2 and Ar gas mixtures at low substrate temperature (T sub ≈ 200 °C) in a standard RF PECVD reactor. We found that H2 dilution is a key parameter for the growth of high quality epitaxial films and modification of the structural composition of the interface with the c-Si wafer, allowing one to switch from a smooth interface at low hydrogen flow rates to a fragile one, composed of hydrogen-rich micro-cavities, at high hydrogen flow rates. This feature can be advantageously used to separate the epitaxial film from the crystalline Si wafer. As a example demonstration, we show that by depositing a metal film followed by a spin-coated polyimide layer and applying a moderate thermal treatment to the stack, the fragile interface breaks down and allows one to obtain an ultrathin crystalline wafer on the flexible polyimide support.

Observation and model of highly ordered wavy cracks due to coupling of in-plane stress and interface debonding in silica thin films

Highly ordered wavy cracks are observed in silica films deposited on crystalline Si wafer. The wavy crack path is interpreted in terms of the coupling of the in-plane film crack and the interface debonding. A model based on the analyzing of the crack path and the stress evolution is proposed to describe the propagation of the wavy cracks. A scaling relationship between the wavelength $(\ensuremath{\lambda})$ and amplitude $(A)$ is found $(\ensuremath{\lambda}\ensuremath{\sim}{A}^{\ensuremath{\xi}})$ and the scaling factor is determined to be $\ensuremath{\xi}=0.47\ifmmode\pm\else\textpm\fi{}0.01$.

Femtosecond Laser Patterning and Anisotropic Wet Etching of Silicon Wafer for Fabrication of Periodic Structures

Laser surface modification of Si wafer is a well-known method in the semiconductor industry. It has been repoted that scratching of the surface by an AFM probe induces surface phase transition on silicon (Si) wafer. This can act as an etching mask against potassium hydroxide (KOH) solution. In this study, a combination of formation of laser-induced surface periodic structure by low fluence femtosecond laser irradiation on crystalline Si wafer and anisotropic wet etching achieved the formation of the structures with different morphology from the conventional. Femtosecond laser pulses (800nm, 120fs) were irradiated on the surface of Si (110) in order to form periodic structures with the lowest fluence possible. After the structures formation, anisotropic wet etching by KOH solution was performed. The structures played as an etching mask to form the periodic structures with the aspect ratio more than 0.6 and the separation about the irradiated wavelength.