Low-doped Silicon Research Articles

Spectral Ellipsometry as a Method of Investigation of Influence of Rapid Thermal Processing of Silicon Wafers on their Optical Characteristics

One of the possible ways of improvement of the surface properties of silicon is the solid phase recrystallization of the surface silicon layer after the chemical-mechanical polishing with application of the rapid thermal treatment with the pulses of second duration. The purpose of the given paper is investigation of influence of the rapid thermal treatment of the initial silicon wafers of the various doping level and reticular density on their optical characteristics by means of the spectral ellipsometry method.The investigation results are presented by means of the spectral ellipsometry method of the rapid thermal processing influence on the initial silicon wafers (KDB12 orientation <100>, KDB10 orientation <111> and KDB0.005 orientation <100>) of the various level of doping and reticular density influence on their optical characteristics: refraction and absorption ratios. Influence was confirmed of the silicon reticular density on its optical characteristics before and after the rapid thermal processing. It was shown, that reduction of the refraction and absorption ratios in the center of the Brillouin zone for the silicon samples with the high Boron concentration after the rapid thermal processing as compared with the low doped silicon. In the area of the maximum absorption peak, corresponding to the energy of the electron exit from the silicon surface (4.34 eV) the refraction indicator of the high doped silicon becomes higher, than of the low doped silicon, which is determined by the high concentration of the vacant charge carriers on the silicon surface in this spectral range.It was established, that the spectral area 3.59–4.67 eV, corresponding to the work of the electrons, exiting the silicon surface, the most informative way shows the difference of the 3 optical parameters of silicon of the different orientation, and for evaluation of influence of the silicon doping level on its optical characteristics the most informative is the spectral range of 3.32–4.34 eV.

Origin of the Out-of-Equilibrium Body Potential In Silicon on Insulator Devices With Metal Contacts

This work addresses the origin of the transient body potential variation in silicon on insulator (SOI) with deposited metal contacts, under back-gate bias sweep from accumulation to inversion. The phenomenon arises from the difficulty to inject carriers into the silicon film due to the Schottky contact between the metal and the low p-type doped silicon. The Schottky diode current is controlled by the back-gate voltage, which fixes the potential at the bottom of the low-doped film. When the back-gate voltage drives the silicon film towards inversion, the limited current from the Schottky contact induces a deficit of minority carriers in the film and brings the body potential in an out-of-equilibrium regime. This phenomenon will be reproduced with numerical modelling based on Poisson and continuity equations, including the quasi-Fermi potential. The transient character and its dependence on the Schottky barrier height will be evidenced. A simple and effective combination of the classic Schottky current equation and inversion current calculated from the Lambert function, as in fully depleted (FD)-SOI MOSFETs, confirms the origin of the effect.

Experimental and simulation investigation of the out-of-equilibrium phenomena on the pseudo-MOSFET configuration under transient linear voltage ramps

The pseudo-MOSFET configuration is an electrical characterization technique developed for silicon-on-insulator (SOI) wafers. The wide variety of experiments that have been performed to date have also extended recently in the study of out-of-equilibrium phenomena for bio-sensing applications. However, the lack of a full understanding of the ohmic contact behaviour between the probes and the low doped silicon film results in simulation inconsistencies. This work proposes a simulated device structure that is capable of reproducing the behaviour of the device and further extends the experiments into large-signal linear ramps which are also reproduced through simulations.

Low-Loss Si-Substrates Enhanced Using Buried PN Junctions for RF Applications

A novel method for increasing the effective resistivity in low-doped silicon substrates is presented. By creating a chain series of p-n depletion junctions beneath the insulator, the parasitic surface conduction channel is interrupted, significantly lowering substrate losses and reducing harmonic distortion in the simulated and measured CPW lines achieving performance close to the widely used trap-rich silicon substrate at RF frequencies.

Room Temperature Resonant Photocurrent in an Erbium Low-Doped Silicon Transistor at Telecom Wavelength.

An erbium-doped silicon transistor prepared by ion implantation and co-doped with oxygen is investigated by photocurrent generation in the telecommunication range. The photocurrent is explored at room temperature as a function of the wavelength by using a supercontinuum laser source working in the μW range. The 1-μm2 transistor is tuned to involve in the transport only those electrons lying in the Er-O states. The spectrally resolved photocurrent is characterized by the typical absorption line of erbium and the linear dependence of the signal over the impinging power demonstrates that the Er-doped transistor is operating far from saturation. The relatively small number of estimated photoexcited atoms (≈) makes Er-dpoed silicon potentially suitable for designing resonance-based frequency selective single photon detectors at 1550 nm.

High Anodic-Voltage Focusing of Charge Carriers in Silicon Enables the Etching of Regularly-Arranged Submicrometer Pores at High Density and High Aspect-Ratio.

The anodic dissolution of silicon in acidic electrolytes is a well-known technology enabling the silicon machining to be accurately controlled down to the micrometer scale in low-doped n-type silicon electrodes. Attempts to scale down this technology to the submicrometer scale has shown to be challenging, though it premises to enable the fabrication of meso and nano structures/systems that would greatly impact the fields of biosensors and nanomedicine. In this work, we report on the electrochemical etching at high anodic voltages (up to 40 V) of two-dimensional regular arrays of millions pores per square centimeter (up to 30 × 106 cm−2) with sub-micrometric diameter (down to ~860 nm), high depth (up to ~40 μm), and high aspect-ratio (up to ~45) using low-doped n-type silicon electrodes (resistivity 3–8 Ω cm). The use of high anodic voltages, which are over one order of magnitude higher than that commonly used in electrochemical etching of silicon, tremendously improves hole focusing at the pore tips during the etching and enables, in turn, the control of electrochemical etching of submicrometer-sized pores when spatial period reduces below 2 μm. A theoretical model allows experimental results to be interpreted in terms of an electric-field-enhanced focusing of holes at the tip apex of the pores at high anodic voltages, with respect to the pore base, which leads to a smaller curvature radius of the tip apex and enables, in turn, the etching of pore tips to be preferentially sustained over time and space.

NMR study of optically hyperpolarized phosphorus donor nuclei in silicon

We use above-bandgap optical excitation, via a 1047 nm laser, to hyperpolarize the $^{31}$P spins in low-doped (N$_D =6\times10^{15}$ cm$^{-3}$) natural abundance silicon at 4.2 K and 6.7 T, and inductively detect the resulting NMR signal. The $30$ kHz spectral linewidth observed is dramatically larger than the 600 Hz linewidth observed from a $^{28}$Si-enriched silicon crystal. We show that the observed broadening is consistent with previous ENDOR results showing discrete isotope mass effect contributions to the donor hyperfine coupling. A secondary source of broadening is likely due to variations in the local strain, induced by the random distribution of different isotopes in natural silicon. The nuclear spin T$_1$ and the build-up time for the optically-induced $^{31}$P hyperpolarization in the natural abundance silicon sample were observed to be $178\pm47$ s and $69\pm6$ s respectively, significantly shorter than the values previously measured in $^{28}$Si-enriched samples under the same conditions. We also measured the T$_1$ and hyperpolarization build-up time for the $^{31}$P signal in natural abundance silicon at 9.4 T to be $54\pm31$ s and $13\pm2$ s respectively. The shorter build-up and nuclear spin T$_1$ times at high field are likely due to the shorter electron-spin T$_1$, which drives nuclear spin relaxation via non-secular hyperfine interactions. At 6.7 T, the phosphorus nuclear spin T$_{2}$ was measured to be $16.7\pm1.6$ ms at 4.2 K, a factor of 4 shorter than in $^{28}$Si-enriched crystals. This was observed to further shorten to $1.9\pm0.4$ ms in the presence of the infra-red laser.

From EBIC images to qualitative minority carrier diffusion length maps

A novel method is presented with the aim to perform minority carrier diffusion length map on cross-sectional samples. The method is based on one Electron-Beam Induced Current (EBIC) acquisition and on the analyze of the EBIC signal slope variation on each scanned points. This method is applied on a pinned photodiode array realized on a low doped silicon epitaxy, and the electron diffusion length map which is extracted is in good accordance with our expectation taking into account the doping distribution of the device. A TCAD simulation also confirms quantitatively the measured diffusion length map. Advantages and drawbacks of this method are discussed in this study.

Macroporous silicon formation by electrochemical anodization of n-type silicon without illumination

This work reports the electrochemical anodization of low-doped n-type silicon in aqueous hydrofluoride (HF) solution without the use of external illumination to generate macroporous silicon with a thin mesoporous transition layer. We have shown that pore formation during the electrochemical anodization of low-doped n-Si in the dark is due to the avalanche breakdown mechanism. Studies of dissolution valence revealed a competition between divalent direct and tetravalent indirect dissolution processes. The effect of pore morphology on anodization parameters such as applied potential, HF concentration, and anodization time was systematically investigated. The fabricated porous silicon has well-separated and straight macropores of pore diameters ranging from 89 ± 9 to 285 ± 28 nm and with limited branching or interconnectivity. Pore diameter uniformity is maintained throughout the porous layer. XRD and Raman spectroscopy have shown that the porous Si fabricated here is highly crystalline, retaining its original crystallinity. The fabricated porous Si presented in this work with tunable pore sizes, depths, and surface features can have potential applications in various fields of microelectronics, photonics, and sensors.

Electrical leakage phenomenon in heteroepitaxial cubic silicon carbide on silicon

Heteroepitaxial 3C-SiC films on silicon substrates are of technological interest as enablers to integrate the excellent electrical, electronic, mechanical, thermal, and epitaxial properties of bulk silicon carbide into well-established silicon technologies. One critical bottleneck of this integration is the establishment of a stable and reliable electronic junction at the heteroepitaxial interface of the n-type SiC with the silicon substrate. We have thus investigated in detail the electrical and transport properties of heteroepitaxial cubic silicon carbide films grown via different methods on low-doped and high-resistivity silicon substrates by using van der Pauw Hall and transfer length measurements as test vehicles. We have found that Si and C intermixing upon or after growth, particularly by the diffusion of carbon into the silicon matrix, creates extensive interstitial carbon traps and hampers the formation of a stable rectifying or insulating junction at the SiC/Si interface. Although a reliable p-n junction may not be realistic in the SiC/Si system, we can achieve, from a point of view of the electrical isolation of in-plane SiC structures, leakage suppression through the substrate by using a high-resistivity silicon substrate coupled with deep recess etching in between the SiC structures.

Graphene-Insulator-Semiconductor Junction for Hybrid Photodetection Modalities

A sensitive optical detector is presented based on a deeply depleted graphene-insulator-semiconducting (D2GIS) junction, which offers the possibility of simultaneously leveraging the advantages of both charge integration and localized amplification. Direct read-out and built-in amplification are accomplished via photogating of a graphene field-effect transistor (GFET) by carriers generated within a deeply depleted low-doped silicon substrate. Analogous to a depleted metal-oxide-semiconducting junction, photo-generated charge collects in the potential well that forms at the semiconductor/insulator interface and induces charges of opposite polarity within the graphene film modifying its conductivity. This device enables simultaneous photo-induced charge integration with continuous “on detector” readout through use of graphene. The resulting devices exhibit responsivities as high as 2,500 A/W (25,000 S/W) for visible wavelengths and a dynamic range of 30 dB. As both the graphene and device principles are transferrable to arbitrary semiconductor absorbers, D2GIS devices offer a high-performance paradigm for imaging across the electromagnetic spectrum.

Controlling the Electrochemical Etching of Pores with High Aspect Ratio at the Submicrometer Scale in Silicon

The controlled electrochemical etching at high anodic voltage (up to 35 V) of two-dimensional arrays of ordered pores with sub-micrometric diameter (down to 800 nm) at high depths (up to 40 µm) and high density (spacing 1.8 µm), yielding a maximum aspect ratio of 50, using low-doped (resistivity 3-8 Ωcm) n-type silicon is hereby for the first time demonstrated and discussed.

Controlling the Electrochemical Etching of Pores with High Aspect Ratio at the Submicrometer Scale in Silicon

In recent years, anodic dissolution of silicon in HF (hydrofluoric acid)-based electrolytes has been largely used for the preparation of ordered and random forms of porous silicon [1,2], from both p-type and n-type materials. In particular, randomly-distributed meso and nanopores have been prepared from p-type silicon electrochemically etched in dark at high HF concentrations [2]; ordered macropores and complex microstructures [3] have been fabricated by back-side illumination electrochemical etching of n-type silicon at low HF concentrations. Here we report for the first time on the controlled etching at high anodic voltage (up to 40V) of two-dimensional (2D) arrays of ordered pores with sub-micrometric diameter (down to 800 nm) at high depths (up to 40µm), yielding a maximum aspect ratio (AR) of 50, using low-doped (resistivity 3-8 ٟcm) n-type silicon. Back-side illumination etching of silicon at low HF concentration (5% by vol. in H2O) is used for the silicon etching, starting from a pre-patterned silicon surface featuring a 2D lattice of holes with size of 1µm and spacing of 1.8µm. The electrochemical etching of the above-described patterned silicon at low anodic voltages (<3V), which is the standard condition for back-side illumination etching, does not allow to obtain a uniform array of pores with constant diameter and same depth, according to both literature and experimental data obtained in this work. Holes generated at the back-side surface (by illumination) diffuse through bulk silicon and are collected at the silicon/electrolyte interface in correspondence of surface defects (either native or pre-patterned), which are known to act as seed-points for pore growth due to the hole-focusing effect arising from the higher electric field established at their tip. At low Vetch values (<3 V) hole-focusing effect has been already proven to be highly effective when diameter and spacing of defects/pores are greater than a few microns (>2 μm) [3]. Nonetheless, when diameter and spacing of defects/pores is below 2μm, low Vetchvalues (<3 V) lead to an unstable and non-uniform pore growth, where some of the pores stop growing and others pores continue their growth though uncontrolled both in depth and diameter. We argue that such a mechanism is due to overlapping (partially at least) of the depletion regions between adjacent pores, which leads to defocusing of the electric field lines in correspondence of surface defects and, in turn, to an uncontrolled etching. In this work we show that increasing the Vetch value (from 15 up to 40V) has beneficial effects on the controlled etching of pores with sub-micrometric diameter and spacing below 2μm. Experimental data show that as the anodic voltage is increased over 15V, electric field lines narrow more and more around the defect/pore tips, thus improving the hole-focusing effect and enabling, in turn, the uniform etching of pores with constant diameter and same depth. In fact, the number of missing pores (pores that stop their growth at a given depth) significantly reduces as the anodic voltage approaches 15V and for arrays etched at Vetch=35V no missing pores are visible. According to the literature [4], our experimental results also show that a Jetch threshold value also exists setting the minimum porosity (P) value (Pmin about 15% for this work) versus pore density over which stable growth of pores can be achieved at high anodic voltages. For a given Vetch value, as Jetch increases, the array porosity and, in turn, the pore diameter increases. On the other hand, for a given Jetch value, as the anodic voltage increases, the diameter of etched pores reduces and depth increases. For instance, etching of our patterned silicon with Jetch=13.4 mAcm-2 (P=17%) and Vetch=35V leads to the formation of ordered arrays of sub-micrometric pores with diameter of about 800nm and depth of about 40μm. Building on these results, controlled electrochemical etching of ordered meso and nanopores and structures in low-doped n-type silicon is envisaged. If successful, this would result to be game-changing in the silicon micro and nanofabrication arena, with applications (though not limited) to nanoelectronics, nanoelectromechanical systems (NEMS), and nanomedicine.

Single-molecule electrical contacts on silicon electrodes under ambient conditions

The ultimate goal in molecular electronics is to use individual molecules as the active electronic component of a real-world sturdy device. For this concept to become reality, it will require the field of single-molecule electronics to shift towards the semiconducting platform of the current microelectronics industry. Here, we report silicon-based single-molecule contacts that are mechanically and electrically stable under ambient conditions. The single-molecule contacts are prepared on silicon electrodes using the scanning tunnelling microscopy break-junction approach using a top metallic probe. The molecular wires show remarkable current–voltage reproducibility, as compared to an open silicon/nano-gap/metal junction, with current rectification ratios exceeding 4,000 when a low-doped silicon is used. The extension of the single-molecule junction approach to a silicon substrate contributes to the next level of miniaturization of electronic components and it is anticipated it will pave the way to a new class of robust single-molecule circuits.

Static and Dynamic Magnetization Investigation in Permalloy Electrodeposited onto High Resistive N-Type Silicon Substrates

The present study reports on the development of permalloy thin films obtained by electrodeposition onto low-doped n-type silicon substrates. While changing from non-percolated clusters into percolated thin films upon increasing the electrodeposition time, the static and dynamic magnetic properties of the as-obtained structures were investigated. We found the experimental magnetic results to be in very good agreement with the simulations performed by solving the Landau-Lifshitz for the dynamics of the magnetic moment. For short electrodeposition times we found the static and dynamic magnetization behavior of the as-formed nanoclusters evidencing vortex magnetization with random chirality and polarization, which is explained in terms of dipolar interaction minimization. Indeed, it is herein emphasized that recent applications of ferromagnetic materials in silicon-based spintronic devices, such as logic and bipolar magnetic transistors and magnetic memories, have revived the possible utilization of low cost and simple electrodeposition techniques for the development of these upcoming hetero-nanostructured devices.

CO_2 laser micromachining of nanocrystalline diamond films grown on doped silicon substrates

We demonstrate that nanocrystalline diamond films grown on highly doped silicon substrates can be patterned using a CO2 laser operating at a wavelength of 10.6 μm, where both low doped silicon and diamond exhibit negligible optical absorption. The patterning is initiated by free carrier absorption in the silicon substrate and further enhanced by the thermal runaway effect, which results in surface heating in the silicon substrate and subsequent thermal ablation of the diamond film in an oxygen rich atmosphere. Using this approach, micron-scale grating and dot patterns are patterned in thin film diamond. The localized heating is simulated and analyzed using concurrent optical and thermal finite element modelling. The laser patterning method described here offers a cost effective and rapid solution for micro-structuring diamond films.

Enhanced photon lifetime in silicon nanowire arrays and increased efficiency of optical processes in them

Silicon nanowire (SiNW) arrays formed by metal-assisted chemical etching of boron low-doped crystalline silicon (c-Si) substrates were studied regarding enhanced efficiency of optical interactions in them. Along with high diffuse reflection of the structures under study, Raman and third-harmonic signals exhibited an order of magnitude growth compared to c-Si response in the near-infrared spectral region. Changes in the orientation dependencies of the third-harmonic intensity with SiNW length increase promoted the explanation of the optical peculiarities of SiNW arrays. Cross-correlation of the scattered radiation function evidenced that the effects could be explained by enhanced photon lifetime inside mesoscopic structure of SiNW arrays due to multiple scattering. Hence a correlation between the photon lifetime and the Raman and third-harmonic efficiency in SiNWs was revealed.

Porous Silicon Covered with Silver Nanoparticles as Surface-Enhanced Raman Scattering (SERS) Substrate for Ultra-Low Concentration Detection.

Microporous and macro-mesoporous silicon templates for surface-enhanced Raman scattering (SERS) substrates were produced by anodization of low doped p-type silicon wafers. By immersion plating in AgNO3, the templates were covered with silver metallic film consisting of different silver nanostructures. Scanning electron microscopy (SEM) micrographs of these SERS substrates showed diverse morphology with significant difference in an average size and size distribution of silver nanoparticles. Ultraviolet-visible-near-infrared (UV-Vis-NIR) reflection spectroscopy showed plasmonic absorption at 398 and 469 nm, which is in accordance with the SEM findings. The activity of the SERS substrates was tested using rhodamine 6G (R6G) dye molecules and 514.5 nm laser excitation. Contrary to the microporous silicon template, the SERS substrate prepared from macro-mesoporous silicon template showed significantly broader size distribution of irregular silver nanoparticles as well as localized surface plasmon resonance closer to excitation laser wavelength. Such silver morphology has high SERS sensitivity that enables ultralow concentration detection of R6G dye molecules up to 10(-15) M. To our knowledge, this is the lowest concentration detected of R6G dye molecules on porous silicon-based SERS substrates, which might even indicate possible single molecule detection.

High temperature setup for measurements of Seebeck coefficient and electrical resistivity of thin films using inductive heating.

We have developed an automated setup for simultaneous measurement of Seebeck coefficient S(T) and electrical resistivity ρ(T) of thin film samples from room temperature to ∼650 °C. S and ρ are extracted from current-voltage (I-V) measurements obtained using a semiconductor parameter analyzer and temperature measurements obtained using commercial thermocouples. The slope and the x-axis intercept of the I-V characteristics represent the sample conductance G and the Seebeck voltage, respectively. The measured G(T) can be scaled to ρ(T) by the geometry factor obtained from the room temperature resistivity measurement of the film. The setup uses resistive or inductive heating to control the temperature and temperature gradient on the sample. Inductive heating is achieved with steel plates that surround the test area and a water cooled copper pipe coil underneath that generates an AC magnetic field. The measurements can be performed using resistive heating only or inductive heating only, or a combination of both depending on the desired heating ranges. Inductive heating provides a more uniform heating of the test area, does not require contacts to the sample holder, can be used up to the Curie temperature of the particular magnetic material, and the temperature gradients can be adjusted by the relative positions of the coil and sample. Example results obtained for low doped single-crystal silicon with inductive heating only and with resistive heating only are presented.

Low Doped n-type Localized Porous Silicon Made by Hole Injection from Back-side p+/n Junction for Power Switches Application

The macropore morphology in low doped n-type silicon depending on hole supply technique is investigated. Electron-hole pair generation by illumination and hole injection through p+/n- back-side junction are compared in a highly concentrated HF electrolyte (30 wt. %). Then, the morphology similarities generated by the two techniques are verified towards anodization current density and electrolyte additives. Porous silicon layer is realized in low-doped n-type silicon as part of a device fabrication in industrial environment. Consequently, illumination technique induces implementation complexity. Then, porous silicon etching was investigated using hole injection from back-side p+/n- junction technique. In addition, implanted p+ regions were incorporated in the low-doped n-type silicon at the anodic surface. We studied these regions in the capacity of masking layers. It was shown to be ineffective to prevent porous silicon formation as a mesoporous layer was observed after anodization.