N-doped Si Research Articles

Nonlinear transfer of an intense few-cycle terahertz pulse through opaque n -doped Si

Intense few cycle terahertz pulses exhibit complex non-linear behavior under interaction with heavily n-doped Si. Fast increase in the transmission of a 700 fs pulse (central frequency 1.5 THz) through the Si sample (low field transmission of 0:02 %) saturates at 8 % for the external field of 5 MV/cm and then drops twofold at 20 MV/cm. An electro-optical sampling measurements revealed formation of a single cycle terahertz pulse at this field due to formation of a thin ionized layer by the first intense oscillation of the terahertz field.

Solar-blind ultraviolet photodetector based on graphene/vertical Ga2O3 nanowire array heterojunction

Abstract In this paper, a solar-blind ultraviolet photodetector (PD) based on the graphene/vertical Ga2O3 nanowire array heterojunction was proposed and demonstrated. To the best of our knowledge, it is the first time that vertical Ga2O3 nanowire arrays have been realized. Ga2O3 nanowires were obtained by thermally oxidizing GaN nanowires grown by molecular beam epitaxy on n-doped Si substrate. Then, a monolayer graphene film was transferred to Ga2O3 nanowires to form the graphene/vertical Ga2O3 nanowire array heterojunction and transparent electrodes. The fabricated device exhibited a responsivity (R) of 0.185 A/W and rejection ratio (R258 nm/R365 nm) of 3×104 at the bias of −5 V. Moreover, the fast response times of this PD were 9 and 8 ms for the rise and decay times under 254 nm illumination, respectively, which are attributed to the unique properties of nanowire arrays and the graphene/vertical Ga2O3 nanowire array heterojunction structure.

One-Pot Green Synthesis of Ultrabright N-Doped Fluorescent Silicon Nanoparticles for Cellular Imaging by Using Ethylenediaminetetraacetic Acid Disodium Salt as an Effective Reductant.

Because of excellent photoluminescence properties, robust chemical inertness, and low cytotoxicity of silicon nanoparticles (Si NPs), exploration of their applications in bioimaging is of great interest. Up to date, a method to synthesis Si NPs with high fluorescence quantum yield (QY) is still challenging. This situation limits the further applications of Si NPs. In this work, we report a mild, simple, and green one-pot method to synthesis N-doped fluorescent Si NPs with an ultrahigh QY up to 62%, using ethylenediaminetetraacetic acid disodium salt as an effective reductant. The obtained ultrabright Si NPs have properties such as relative small size (about 2 nm), water dispersibility, robust stability, and biocompatibility. The as-prepared Si NPs were further applied for cellular imaging with satisfactory results, indicating their great potential in bioimaging applications.

Increasing Efficiency in Single-Walled Carbon Nanotube/n-Si Photodetectors by Voltage Doping

Single-walled carbon nanotube (SWCNT) ultrathin films were deposited on n-doped Si substrates provided with three electrodes for photoconductive measurements. Without illumination, the devices show good rectifier properties and holes mobility in the range ${\text{10}}^{5}\,{\text{cm}^2/ \text{V}}$ ·s, which makes them very promising for fast switching applications. Measuring the current voltage characteristics of the SWCNT film under illumination, an increase in the device performance is observed when a voltage $V_{G}$ is applied to the third electrode. In particular, increasing $V_{G}$ toward the breakdown region, an increase of more than ten times is recorded in the photocurrent and in the external quantum efficiency with respect to the values measured at $V_{G}= {{0}}$ . The experimental data are interpreted considering a hole doping of the SWCNT film by the action of the third electrode voltage $V_G$ .

Type-I alignment in MAPbI3 based solar devices with doped-silicon nanocrystals

In this work we couple silicon nanocrystals (Si NCs) with strong quantum confinement (< 4 nm diameter) with large grained MAPbI3 films combining the perovskites excellent transport properties with the unique opto-electronic properties of quantum confined Si NCs. The electronic structures of MAPbI3 and Si NCs, ideally aligned to form a type-I heterojunction, demonstrated improved carrier collection through a higher short-circuit current density (~ 20 mA cm−2) measured in photovoltaic test devices. The addition of p- and n-doped Si NCs within the MAPbI3 films also showed unexpected and interesting device performance improvements, where the typical perovskite degradation process in ambient conditions was considerably slowed down and the overall device efficiency was observed to increase under light soaking. We attribute these improvements to the presence of Si NCs, which helped maintain the perovskite film qualities for more than 14 days in open atmosphere.

Multi-layer MOS capacitor based polarization insensitive electro-optic intensity modulator.

In this study, a multi-layer metal-oxide-semiconductor capacitor (MLMOSC) polarization insensitive modulator is proposed. The design is validated by numerical simulation with commercial software LUMERICAL SOLUTION. Based on the epsilon-near-zero (ENZ) effect of indium tin oxide (ITO), the device manages to uniformly modulate both the transverse electric (TE) and the transverse magnetic (TM) modes. With a 20μm-long double-layer metal-oxide-semiconductor capacitor (DLMOSC) polarization insensitive modulator, in which two metal-oxide-semiconductor (MOS) structures are formed by the n-doped Si/HfO2/ITO/HfO2/ n-doped Si stack, the extinction ratios (ERs) of both the TE and the TM modes can be over 20dB. The polarization dependent losses of the device can be as low as 0.05dB for the "OFF" state and 0.004dB for the "ON" state. Within 1dB polarization dependent loss, the device can operate with over 20dB ERs at the S, C, and L bands. The polarization insensitive modulator offers various merits including ultra-compact size, broadband spectrum, and complementary metal oxide semiconductor (CMOS) compatibility.

Black Silicon IR Photodiode Supersaturated With Nitrogen by Femtosecond Laser Irradiation

Micro-ripple and micro-bead structures are formed on a silicon (Si) surface after irradiation with femtosecond laser pulses in nitrogen (N2) atmosphere. Simultaneously, supersaturated nitrogen (N) atoms, with a concentration above 1020 cm−3, are doped into the textured black Si layer via laser ablation. The N-doped Si exhibits strong below-bandgap infrared absorption from 1.1 to $2.5~\mu \text{m}$ , which remains nearly unchanged after annealing for 30 min at 873 K. The mechanism of this thermally stable infrared absorption is analyzed by first-principles calculations. According to the transmission electron microscopy results, multiple phases (including single crystalline, nanocrystalline, and amorphous phases) are observed in the laser-irradiated layer. Hall Effect measurements prove that N-dopants induce a low background free-carrier concentration ( $\sim 1.67\times 10^{16}\,\,\text {cm}^{-3}$ ). Finally, a Schottky-based bulk structure photodiode is made. This broadband photodiode exhibits good thermal stability and a photo-responsivity of 5.3 mA/W for $1.31~\mu \text{m}$ at a reverse bias of 10 V.

Influence of silicon doping type on the adhesion of seedless electrodeposited copper layers

The influence of silicon doping on the adhesion of copper layers electroplated directly on (100) silicon without a seed layer was investigated in this work. The adhesion of Cu layers on Si(100) was derived from scratch tests where the critical loads and the types of failures of these layers on phosphorous- and boron-doped silicon were obtained. The maximum loads supported until complete layer removals were about twice as large for Cu layers electrodeposited on p-Si(100) than those deposited on n-Si(100). The Cu layers were also visually inspected using images taken with scanning electron microscopes, their topography was obtained by atomic force microscope measurements and crystal orientations by X-ray diffraction. Secondary neutral mass spectroscopy measurements and X-ray photoelectron spectroscopy were used to quantitatively determine the atomic Cu concentration at the surface of each sample. Both measurements have detected larger Cu concentrations at the p-Si(100) surface than at n-Si(100). These measurements were used to support a model previously presented by this group to explain the adhesion difference of Cu electroplated directly on p- or n-doped Si.

Influence of the contact geometry on single-walled carbon nanotube/Si photodetector response

A systematic study of the optical response of photodetectors based on carbon nanotube/Si heterojunctions is performed by measuring the responsivity, the detectivity and the time response of the devices with different contact configurations. The sensors are obtained by dry transferring single-walled carbon nanotube films on the surface of n-doped Si substrate provided with a multifinger contact geometry. The experimental data show a consistent improvement of the photodetector parameters with the increase of the number of fingers without affecting the carbon nanotube film thickness for increase its optical transmittance as in previous experiments. The role of the electrical resistance of the carbon nanotube film is discussed. The obtained results confirm the method and suggest new perspectives in the use of nanostructured materials as part of semiconducting optical devices.

Generation and detection of dissipationless spin current in a MgO/Si bilayer

Spintronics is an analogue to electronics where the spin of the electron rather than its charge is functionally controlled for devices. The generation and detection of spin current without ferromagnetic or exotic/scarce materials are two of the biggest challenges for spintronics devices. In this study, we report a solution to the two problems of spin current generation and detection in Si. Using non-local measurement, we experimentally demonstrate the generation of helical dissipationless spin current using the spin-Hall effect. Contrary to the theoretical prediction, we observe the spin-Hall effect in both n-doped and p-doped Si. The helical spin current is attributed to the site-inversion asymmetry of the diamond cubic lattice of Si and structure inversion asymmetry in a MgO/Si bilayer. The spin to charge conversion in Si is insignificant due to weak spin–orbit coupling. For the efficient detection of spin current, we report spin to charge conversion at the MgO (1 nm)/Si (2 µm) (p-doped and n-doped) thin film interface due to Rashba spin–orbit coupling. We detected the spin current at a distance of >100 µm, which is an order of magnitude larger than the longest spin diffusion length measured using spin injection techniques. The existence of spin current in Si is verified from the coercivity reduction in a Co/Pd multilayer due to spin–orbit torque generated by spin current from Si.

Silicon based mid-IR super absorber using hyperbolic metamaterial

Perfect absorbers are indispensable components for energy harvesting applications. While many absorbers have been proposed, they encounter inevitable drawbacks including bulkiness or instability over time. The urge for CMOS compatible absorber that can be integrated for on chip applications requires further investigation. We theoretically demonstrate Silicon (Si) based mid IR super absorber with absorption (A) reaching 0.948. Our structure is composed of multilayered N-doped Si/ Si hyperbolic metamaterial (HMM) integrated with sub-hole Si grating. Our proposed structure has tunable absorption peak that can be tuned from 4.5 µm to 11 µm through changing the grating parameters. We also propose two grating designs integrated with N-doped Si/ Si HMM that can achieve wide band absorption. The first grating design is based on Si grating incorporating different holes’ height with (A) varying between 0.83 and 0.97 for wavelength from 5 µm to 7 µm. The second grating design is based on Si grating with variable holes’ diameter; the latter shows broad band absorption with the maximum (A) reaching 0.97. We also show that our structure is omnidirectional. We propose an all Si based absorber which demonstrates a good candidate for thermal harvesting application.

Highly efficient back-junction PEDOT:PSS/n-Si hybrid solar cell with omnidirectional antireflection structures

Abstract Hybrid planar solar cells based on bulk n-doped Si wafers and poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) with a back-junction geometry demonstrated an excellent power conversion efficiency of more than 11%. When a random pyramid or a Si nanowire antireflection structure was incorporated into the front of the cell surface, the power conversion efficiencies were further enhanced to 13.9% or 14.5%, respectively. Moreover, the cells with the Si nanowire antireflection structure displayed omnidirectional light-trapping characteristics over the random pyramid structures. These cells also maintained a constant power conversion efficiency even at an incident angle of 60°.

Study of the Intrinsic Limitations of the Contact Resistance of Metal/Semiconductor Interfaces through Atomistic Simulations

In this contribution, we report a fundamental study of the factors that set the contact resistivity between metals and highly doped n-type 2D and 3D semiconductors. We investigate the case of n-type doped Si contacted with amorphous TiSi combining first-principles calculations with Non-Equilibrium Green functions transport simulations. The evolution of the intrinsic contact resistivity with the doping concentration is found to saturate at ∼2 × 10−10 Ω.cm2 for the case of TiSi and imposes an intrinsic limit to the ultimate contact resistance achievable for n-doped Si|amorphous-TiSi (aTiSi). The limit arises from the intrinsic properties of the semiconductors and of the metals such as their electron effective masses and Fermi energies. We illustrate that, in this regime, contacting heavy electron effective mass metals with semiconductor helps reducing the interface intrinsic contact resistivity. This observation seems to hold true regardless of the 3D character of the semiconductor, as illustrated for the case of three 2D semiconducting materials, namely MoS2, ZrS2 and HfS2.

(Invited) Probing the Intrinsic Limitations of the Contact Resistance of Metal/Semiconductor Interfaces through Atomistic Simulations

In this contribution, we report a fundamental study of the factors that set the contact resistivity between metals and highly doped semiconductors. We investigate the case of n-type doped Si contacted with amorphous TiSi combining first-principles calculations with Non-Equilibrium Green functions transport simulations. The intrinsic contact resistivity is found to saturate at ~2x10-10 Ω.cm2 with the doping concentration and sets an intrinsic limit to the ultimate contact resistance achievable for n-doped Si|amorphous-TiSi. This limit arises from the intrinsic properties of the semiconductor and of the metal such as their electron effective masses and Fermi energies. We illustrate that, in this regime, contacting metals with a heavy electron effective mass helps reducing the interface intrinsic contact resistivity.

High-performing MoS2-embedded Si photodetector

Transition metal dichalcogenides (TMDs) are a new class of materials that replace advanced functional materials like graphene, CNT etc., in photovoltaics and sensors. In the present work, the 2-dimensional tri-layer MoS2 is applied for high-performing photodetector. Here, the conventional n-doped p-type Si (n-Si/p-Si) was coated with tri-layer MoS2 film using CVD method. HRTEM reveals the presence of tri-layer with highly ordered lattice planes. Characteristic peaks of Mo and S are obtained in XPS profile. Due to spin-orbit coupling, the 3d band of Mo and 2p band of S are split into two states. Raman spectrum of MoS2 film shows two peaks, corresponding to its in-plane and out-of-plane vibrational modes. The wave number difference between these modes is measured as 22.97cm−1, which ensures that there are three layers in MoS2 film. The splitting of valence band generates multiple excitons which are marked as A and B in the absorption profile. The excitonic transition corresponds to the direct band gap of MoS2 (ie., 1.9eV). The prepared MoS2/n-Si/p-Si photodetector includes two rectifying junctions with considerable built-in potentials. A high rectification ratio was measured as 51.37 to ensure the quality junction formation. The photoresponse ratio of the MoS2/n-Si/p-Si photodetector was obtained as 58.74 to confirm the quality junction formation between MoS2 and Si with high detectivity of 5.42 × 1014 Jones. Moreover, the extremely fast rise and fall times of 33 µs and 30 µs were achieved without any external bias application. The functional use of MoS2 window design would provide the high potential for the enhanced photoelectric devices, such as photodetectors and solar cells.

Nanostructured 3D porous hybrid network of N-doped carbon, graphene and Si nanoparticles as an anode material for Li-ion batteries

A facile and scalable process to prepare nanostructured 3D porous networks combining graphene, N-doped carbon and silicon nanoparticles (G@Si@C) as a promising anode material for lithium ion batteries.

Investigation of organic semiconductor interlayers in hybrid PEDOT:PSS/silicon solar cells

In the last years, hybrid organic/inorganic solar cells have attracted great interest in photovoltaic research due to their expected potential to combine the advantages of both material classes, the excellent electrical properties and stability of the inorganic and the low-cost proc- essability of the organic semiconductors. This work is focused on hybrid solar cells based on n-doped crystalline Si as the inorganic and the polymer poly(3,4-ethylenedioxythio- phene):poly(styrenesulfonate) as the organic part of the device. The hole-conducting organic semiconductors poly(3-hexylthiophene-2,5-diyl) (P3HT) and 2,2',7,7'-Tetrakis(N;N-di(4- methoxyphenyl)amino)-9,9'-spirobifluorene (Spiro-MeOTAD) are investigated as electron blocking interlayers to reduce the parasitic electron current into the metal top contact and thereby increase the efficiency of the solar cell. In this context, P3HT is identified to be insufficient as an interlayer material due to unfavorable hysteresis effects. On the other hand, for solar cells with a Spiro-MeOTAD interlayer, the power conversion efficiency (PCE) is significantly increased. This is mainly attributed to an increased short-circuit current density. For the best performing device, a PCE of 14.3% is achieved, which is one of the highest values reported for this type of hybrid solar cells so far. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10

Organic Thin-Film Transistors Based on Asymmetric Chrysene Derivatives

Organic field-effect transistors (OFETs) have received significant attention because of the potential use in flexible and/or disposable portable electronic devices. The charge carrier mobility is strongly influenced by the organic semiconductor layer. But, it has been still lower than the charge carrier mobility for inorganic MOS FETs. Therefore, great efforts have been made toward the development of various semiconducting small molecules and polymers. Among these, fused π-conjugated semiconductors such as acene, thienoacene, and thienothiophene based compounds are promising materials for high-performance OFETs. In this study we investigated the characterization of thin-film OFETs based on newly developed compound,asymmetric chrysene derivatives which regard phenyl chrysene as the main structure (Fig. 1). It is expected that the large and planar chrysene core would provide intermolecular interactions to achieve high charge carrier mobilities, and asymmetric molecular structure are promising for improvement of solubility. Thin-films are fabricated by spin-coat and vapor deposition method on a n-doped Si wafer with a 220 nm thermally grown SiO2. The spin-coated films (50 nm thick) were fabricated by 0.4 wt% toluene solution at 2000 rpm for 30 sec. The vapor deposited films were deposited on insulating SiO2 layer was treated with a thin-film (ca. 30 nm) of a polymer insulator such as polystyrene (PS) or CYTOP (Asahi Glass Co.). Molecular packing and orientation was characterized by atomic force microscope (AFM), and X-ray diffractions (XRDs) analysis. The electrical properties of the OFET devices were measured by using an Agilent 4155C semiconductor parameter analyzer under vacuum at room temperature. The solution processed asymmetric chrysene based transistors showed typical p-channel FET characteristics. The field-effect carrier mobility of the device was determined to be 5.9×10-4cm2V-1s-1 and on/off ratio of 105 for P-28CR-6. The vaper deposited P-28CR-n based OFET shows also typical p-channel FET characteristics and their mobilities are strongly correlated with the applied polymer insulator, and the best field-effect mobility (3.1 cm2V-1s-1 and on/off ratio of 104) was obtained for the CYTOP-covered device. As shown in Fig. 2(b) carrier mobility is correlated by the length of alkyl chain for P-28CR-n. According to XRD analysis, orientation angle of chrysene core of P-28CR-n to the substrate is also correlated to the alkyl chain length of P-28CR-n. Interestingly, the field-effect mobility shows a similar trend to the orientation angle of the chrysene core for the alkyl chain length. Figure 1

Electron beam induced current microscopy investigation of GaN nanowire arrays grown on Si substrates

We report on the electron beam induced current (EBIC) investigation of GaN nanowires grown on n-doped Si (111) substrates. The objective of this study is to acquire information about the modifications of the substrate properties induced by the wire growth. We show that the growth procedure using deposition of an ultra-thin AlN layer prior to the nanowire growth step leads to the formation of a p-n junction in the Si substrate with a high surface conductivity. The induced p-n junction exhibits a photoresponse over the spectral range from 360nm to 1100nm. The properties of the induced p-n junction are investigated on the cross section and in a top view configuration with EBIC microscopy. For a localized contact of the GaN nanowires, the collection range in Si extends over a few millimeters. The treatment of the surface using reactive ion etching with a CHF3 plasma leads to the inhibition of the surface conductivity and to the appearance of an S-shape in the current-voltage characteristics under illumination. The conversion efficiency of the plasma-treated sample under AM1.5G solar spectrum is estimated to be in the 2.1–2.7% range.

Giant Dirac point shift of graphene phototransistors by doped silicon substrate current

Graphene is a promising new material for photodetectors due to its excellent optical properties and high-speed response. However, graphene-based phototransistors have low responsivity due to the weak light absorption of graphene. We have observed a giant Dirac point shift upon white light illumination in graphene-based phototransistors with n-doped Si substrates, but not those with p-doped substrates. The source-drain current and substrate current were investigated with and without illumination for both p-type and n-type Si substrates. The decay time of the drain-source current indicates that the Si substrate, SiO2 layer, and metal electrode comprise a metal-oxide-semiconductor (MOS) capacitor due to the presence of defects at the interface between the Si substrate and SiO2 layer. The difference in the diffusion time of the intrinsic major carriers (electrons) and the photogenerated electron-hole pairs to the depletion layer delays the application of the gate voltage to the graphene channel. Therefore, the giant Dirac point shift is attributed to the n-type Si substrate current. This phenomenon can be exploited to realize high-performance graphene-based phototransistors.