Undoped Silicon Research Articles

Sample Thickness Measurement with THz-TDS: Resolution and Implications

The accuracy of any measurement with terahertz time-domain spectroscopy (THz-TDS) depends strongly on knowing and supplying the precise sample thickness when processing the raw terahertz data. Sample thickness usually is estimated using other (non-THz) metrology and invariably involves some degree of uncertainty. It turns out that the terahertz data itself typically also contains information regarding sample thickness. However, there is limited systematic work addressing the following questions: What is the best method for extracting sample thickness from THz-TDS data? And, what is the thickness resolution obtainable with THz-TDS? In this study we demonstrate how these questions can be answered in general by answering them for a specific example: undoped silicon wafers. We determine the accuracy with which the exact thickness of nominally 500 μm silicon wafers can be measured using transmission mode THz-TDS. We analyze and compare the resolution of 5 different approaches for determining sample thickness using THz-TDS data, including two new methods and three methods proposed in the literature. The quantitative results and analyses of methods we present will be useful in developing far-infrared optical metrology. Conversely the quantitative results presented can be used to relate uncertainty in sample thickness to uncertainty in the measured terahertz data both in the time domain and frequency domain (phase and amplitude). Finally, a precise understanding of the relationship between sample thickness and THz-TDS data can be used to formulate superior THz-TDS data work-up methodologies.

Electronic properties of undoped microcrystalline silicon oxide films

The electronic properties of undoped microcrystalline silicon oxide films have been investigated by transient photocurrent (TPC) density of states (DOS) spectroscopy, supported by dark conductivity, steady-state photoconductivity, and constant-photocurrent measurements (CPM). Film compositions span the range from amorphous to microcrystalline and contain up to 10% oxygen content, yielding optical bandgap values E04 (the photon energy at which the absorption depth equals one micrometre) between 1.85 and 2.11 eV. Carrier transport is consistent with multiple-trapping in a localised DOS, which depends upon film structure and oxygen content. TPC measurements indicate that both conduction band-tail energy and deep defect density increase with increasing oxygen content, accompanied by a reduction in majority carrier mobility-lifetime product. CPM measurements on amorphous films show a broadening of the Urbach tail with increasing oxygen content. Significantly higher oxygen incorporation without seriously compromising electronic quality appears possible in microcrystalline films. This suggests potential application as solar cell absorber layers offering increased optical bandgap and open-circuit voltage.

Optical interferometric approach for measuring the geometrical dimension and refractive index profiles of a double-sided polished undoped Si wafer

In this investigation, we propose a technique to obtain not only the dimensional profile of an undoped double-side polished silicon (Si) wafer but also its refractive index profile simultaneously. This technique is based on low coherence scanning interferometry using a near-infrared light around 1 µm wavelength, for which transmission is non-zero for undoped silicon and also detectable by the typical visible CCD camera. As the experimental results show, the profiles of optical thicknesses, geometrical thicknesses, group refractive indices and phase refractive indices were successfully obtained by the proposed technique, and the mean values were 1869.6 µm, 490.45 µm, 3.8121 and 3.516, respectively. Compared to the reference values of the group refractive index and the phase refractive index of the Si wafer given by the previous researches, the deviations were 0.055 and 0.04, respectively. The deviations were attributed to the position error of the scanning stage. In addition, the combined technique with spectrally-resolved interferometry was also discussed to improve the measurement accuracy of the system.

Microwave losses of undoped n-type silicon and undoped 4H-SiC single crystals at cryogenic temperatures

We investigated microwave losses of single-crystalline Si and 4H-SiC at cryogenic temperatures at 8.6–24 GHz using a method involving a dielectric resonator with high-Tc superconductor YBa2Cu3O7-δ films used to improve the measurement sensitivity. The loss tangent of our undoped n-type Si appeared to be extremely low at temperatures below 20 K with a value of 1 × 10−6 at 24 GHz at 10 K, which is more than 100 times lower than the value of 2 × 10−4 at 6.8 GHz at 10 K reported by Krupka et al. [IEEE Trans. Microw. Theory Tech. 54, 3995 (2006)] for undoped p-type Si. Meanwhile, the loss tangent of pristine 4H-SiC appeared to be very high with a value of 0.01 at 10 K at 8.6 GHz, which is 4000 times higher than that of our undoped Si. When the pristine 4H-SiC was irradiated with thermal neutrons, the loss tangent was enhanced by seven times due to the significantly reduced electrical resistivity. Our results show that, at temperatures below 20 K, the loss tangent of undoped n-type Si is low enough for various cryogenic applications and that thermal neutron irradiation could provide a useful means of reducing the electrical resistivity of SiC possibly by means of neutron transmutation doping.

Terahertz split-ring metamaterials as transducers for chemical sensors based on conducting polymers: a feasibility study with sensing of acidic and basic gases using polyaniline chemosensitive layer

We report on the first application of terahertz metamaterials acting as transducers for chemical sensors based on conducting polymers. In our feasibility study aimed at sensing of gaseous hydrochloric and ammonia, a two-dimensional sensor metamaterial consisting of an array of split-ring resonators on the surface of undoped silicon wafer was prepared. The surface of the resonator was coated with a 150-μm layer of polyaniline. Binding of hydrogen chloride to polyaniline leads to distinct changes in the resonance frequency of the metamaterial. Measurements can be performed both in the reflection and transmission mode. A numerical simulation of the response revealed an increase of both the real and the imaginary components of the dielectric function of the polyaniline film. These changes are attributed to the transition from emaraldine base to emeraldine salt. The results demonstrate a new approach for formation of highly sensitive transducers for chemical sensors.

An Etch Stop and Sacrificial Materials Study for 3D NEMS-CMOS Co-Integration

In this paper, we compare several materials that can be used as sacrificial and etch stop layer for 3D NEMS-CMOS co-integration.During these last 10 years Very Large Scale Integration techniques, well developed for transistors, have been used for the M/NEMS world (Micro/Nano Electro-Mechanical Systems). This allowed the construction of various kinds of miniaturized nano-sensors, especially resonators that can be used for gas sensing, mass spectrometry and molecules recognition for example [1,2]. Nevertheless, the use of such nano-sensors requires a CMOS electronic circuit for the readout part and for the oscillating-loop creation with the nano-resonator. The CMOS part can be on a separated die linked to the NEMS resonator using wire bonding connections (Fig.1-a).However, in order to reduce signal transmission loss and allows the fabrication of a sensor with a high density of resonator, the best approach consists in processing both NEMS and CMOS on the same die (Fig.1-b and c) [3,4], in particular following a 3D scheme (Fig.1-c) [5,6]. The NEMS release process, commonly performed by a HF (Hydrogen Fluoride) vapor etching operation, is the final step of the integration. It implies the selection of two kinds of materials. The first one is a sacrificial layer in order to achieve a well-controlled and clean release of the mechanical structure. The second one is an etch stop layer located between the NEMS and CMOS for protecting all the area under the nano-resonator (Fig.2). In such 3D approach, etch stop and sacrificial layers can be deposited at high temperature before the molecular bonding of the mechanical part with the electronic part.Two parameters are critical for the selection of those materials; the etch rate and the presence and dimensions of the residues (see Fig.3, 4, 5 and 6) which may result from a silicon-fluorine complex precipitation during the etching process [7]. These residues must not disturb the mechanical resonator during its oscillations.This paper will present results about etch-rates and residual dimensions on HF vapor etching of materials used for the NEMS realization. It is shown in Fig.3 and Fig.6 that USG (Undoped Silicon Glass) and SiH4, usually used in back-end processes, are not suitable for the vapor HF release contrary to TEOS (tetraethyl-orthosilicate) Low Rate (LR) and boron-nitride (BN). Our study shows that the latter constitutes a good etch stop layer for HF vapor processing within a 3D integration scheme.[1] J. Arcamone et al, IEDM 2011 IEEE International, pp.29.3.1, 29.3.4, 5-7 Dec. 2011[2] Y.T. Yang et al, Nano lett., Vol. 6, No. 4, Apr. 2006[3] E.Ollier et al, MEMS 2012[4] J. Arcamone et al, Tech. Digest. IEDM 2012, pp. 15.4.1-15.4.4[5] P.Batude et al, J.Emerging Sel. Top.Circuits Syst, Vol.2, No.4, Dec. 2012[6] J-C. Souriau, IEEE Trans. Compon., Packag. Manuf. Technol. Vol. 1, No.6, June 2011[7] H. Ritala et al, ECS Trans. 2009 25(5): 355-358

Impact ofn-type doping on the carrier dynamics of silicon nanowires studied using optical-pump terahertz-probe spectroscopy

The n-doped and undoped silicon nanowires grown by chemical vapor deposition on quartz substrate were characterized using optical-pump terahertz-probe transmission experiments. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial photocarrier lifetime of ∼2 ps and a longer decay time of 10 ps to a few tens of picoseconds. Based on the influence of the laser fluence, a carrier capture and recombination scenario is proposed to explain these temporal decay curves. For both samples, the capture of photocarriers by the traps present on the surface of the nanowires plays an important role in the observed photoconductivity dynamics. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz (THz) traces taken at different optical-pump THz-probe delays. These data curves are reproduced using a plasmon resonance model. The fitting procedure allows us to determine carrier scattering times of about 28±6 and 14±4 fs for the undoped and doped samples, respectively. Our results show that defects and ionized impurities introduced by n doping the silicon nanowires reduce the photocarrier mobility and lifetime.Received 2 January 2014DOI:https://doi.org/10.1103/PhysRevB.89.115316©2014 American Physical Society

The Impact of SiO$_{2}$/SiN $_{\rm x}$ Stack Thickness on Laser Doping of Silicon Solar Cell

Laser doping of semiconductors has been the subject of intense research over the past decades. Previous work indicates that the use of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> stacks instead of a single dielectric film as the anti-reflection coating and passivation layer results in laser doped lines with superior properties. In this paper, the impact of the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer thickness in the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> stacks on the properties of laser doped lines is investigated through resistance measurements of the laser doped line and the silicon-metal contact and the doping profile near the edge of the dielectric window, the latter being an important factor in determining the likelihood of high recombination or even shunting from the subsequent metallization process. Fundamentally, a problem of exposed and undoped silicon near the dielectric window is identified for most of the investigated parameter range. However, optimization of the laser parameters and dielectric film conditions is shown to be capable of preventing or at least minimizing this problem. The results indicate that for the used laser system, samples with thick dielectric stack processed using a low pulse energy and pulse distance yield the most favorable properties, such as low line resistance and low contact resistivity. Under these conditions, the laser doped regions laterally extend underneath the dielectric films, thus reducing the likelihood of high surface recombination.

Picosecond all-optical switching in hydrogenated amorphous silicon microring resonators

We utilize cross-phase modulation to observe all-optical switching in microring resonators fabricated with hydrogenated amorphous silicon (a-Si:H). Using 2.7-ps pulses from a mode-locked fiber laser in the telecom C-band, we observe optical switching of a cw telecom-band probe with full-width at half-maximum switching times of 14.8 ps, using approximately 720 fJ of energy deposited in the microring. In comparison with telecom-band optical switching in undoped crystalline silicon microrings, a-Si:H exhibits substantially higher switching speeds due to reduced impact of free-carrier processes.

Ultrathin Metal/Amorphous-Silicon/Metal Diode for Bipolar RRAM Selector Applications

We propose a novel metal/silicon/metal (MSM) selector using ultrathin undoped amorphous silicon (a-Si) for resistive-RAM selector application. The new selector behaves as a bidirectional diode, showing a high current drive (~2.2 MA/cm)2, high selectivity (~240 for 1/2 bias), fast switching speed , and excellent endurance ( at target drive current). The doping-free a-Si structure alleviates the dopant-induced variability concerns for ultrascaled devices and eliminates the need for a dopant-activation anneal. Circuit simulations show feasibility of 1-Mb array, with over 25% read margin and 70% write margin, when using the new MSM structure as a selector for a HfO2-based resistive switching memory element.

A high performance charge plasma based lateral bipolar transistor on selective buried oxide

In this paper, we present a new structure of lateral bipolar transistor on selective buried oxide. The device does not use highly doped regions; however, it employs the concept of creating n and p type charge plasma in undoped silicon by using metal electrodes of different work functions. The proposed device is named as the selective buried oxide based bipolar charge plasma transistor (SELBOX-BCPT). An extensive 2D simulation study has revealed that the proposed SELBOX-BCPT device not only possesses all the advantages of the conventional BCPT device, but it also addresses various severe problems of the BCPT device. A significant improvement in major issues of poor cutoff frequency (fT), low breakdown voltage and thermal efficiency has been achieved. It has been observed that the fT has increased by ∼94.6%, the breakdown voltage by 23.47% and the device is much cooler than the conventional BCPT device. A large current gain is obtained in the proposed device and is on a par with the conventional BCPT device. Further, by using mixed-mode simulation feature of the Atlas simulator, inverting amplifiers based on SELBOX-BCPT and the conventional BCPT have been realized. A significant improvement of 15% in switching-on transient time and 25.8% in switching-off transient time has been achieved in the proposed device in comparison to the conventional BCPT device.

Поведінка водню при кристалізації тонких плівок кремнію, легованих оловом

The behavior of a hydrogen impurity in the course of crystallization of thin silicon films doped with tin (a-SiSn films) has been studied. It is found that the band located at about 2000–2200 cm^−1 and corresponding to the IR absorption at silicon–hydrogen bonds is absent from the spectra of as-deposited (at a temperature of 300 ◦C) a-SiSn films with Sn contents within the interval of 1–10 at.%. In undoped thin silicon films (a-Si films), the hydrogen content diminishes below the sensitivity threshold of the measurement technique only after the annealing of the specimens at 700 ◦C. The absence of hydrogen in a-SiSn films is in good agreement with the results of EPR studies and the results of Raman scattering studies of structural transformations in thermally treated films. It is shown that the formation of crystalline phases in a-SiSn occurs at lower temperatures as compared to those in a-Si, with a correlation taking place between the crystallization temperature for Si clusters and the concentration of the tin impurity. Taking into account that tin reduces the temperature of the hydrogen effusion from the film volume and, accordingly, stimulates the ordering in the specimen structure, it is possible to consider that hydrogen impurity takes part in the processes that result in a decrease of the crystallization temperature for a-SiSn.

Effect of band mismatch on minority carrier transport in heterojunction solar cells

By using transient-capacitance techniques we probe the mechanism of hole transport in amorphous/crystalline silicon heterojunction solar cells. The devices are formed by depositing undoped amorphous silicon followed by p-type amorphous silicon on n-type crystalline silicon wafers. The capacitance transients indicate that hole transport from p-type amorphous silicon to n-type crystalline silicon is hindered by hole accumulation in the depletion region of the crystalline silicon. The results are explained with a model based on electrostatic repulsion owing to hole build-up at the crystalline/amorphous interface. We apply these results to other heterojunction solar cells.

Reassessment of the Limiting Efficiency for Crystalline Silicon Solar Cells

Recently, several parameters relevant for modeling crystalline silicon solar cells were improved or revised, e.g., the international standard solar spectrum or properties of silicon such as the intrinsic recombination rate and the intrinsic carrier concentration. In this study, we analyzed the influence of these improved state-of-the-art parameters on the limiting efficiency for crystalline silicon solar cells under 1-sun illumination at 25°C, by following the narrow-base approximation to model ideal solar cells. We also considered bandgap narrowing, which was not addressed so far with respect to efficiency limitation. The new calculations that are presented in this study result in a maximum theoretical efficiency of 29.43% for a 110-μm-thick solar cell made of undoped silicon. A systematic calculation of the I-V parameters as a function of the doping concentration and the cell thickness together with an analysis of the loss current at maximum power point provides further insight into the intrinsic limitations of silicon solar cells.

A preliminary development in hybrid a-silicon/polymer solar cells

Amorphous undoped intrinsic silicon, B-doped silicon and P-doped silicon hybrid bilayer structures with poly(3-hexylthiophene) have been fabricated and their photovoltaic responses have been investigated. Open-circuit voltages and fill factors of the devices are moderate, but strongly dependent on the doping type of a-Si:H films. The highest available open-circuit voltage from the hybrid solar cells within this investigation is 1.23 V in a standard test condition. Both inorganic and organic semiconducting layers contribute to the photocurrent generation. The short-circuit currents appear to be limited by unbalanced charged carriers collected from different sides of the semiconductors, which indicates a way forward in device optimization.

Light-Response Characteristics of Platinum Doped Silicon Photodetector

The purposeof this research article is present electrical characteristic of Pt-doped silicon photodetector compare with undoped silicon photodetector. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of Pt-doped detector were investigated. In this experiment, the results of Pt-doped detector show that the dark currentisobviously decreased and the photocurrent is decreased about 9 to 10 orders. Furthermore, the capacitance characteristic isslightly increased about 0.15 pF. The effects platinum on silicon indicated the carrier in silicon have been changed.

Low cost wafer metrology using a NIR low coherence interferometry

In this investigation, a low cost Si wafer metrology system based on low coherence interferometry using NIR light is proposed and verified. The whole system consists of two low coherence interferometric principles: low coherence scanning interferometry (LCSI) for measuring surface profiles and spectrally-resolved interferometry (SRI) to obtain the nominal optical thickness of the double-sided polished Si wafer. The combination of two techniques can reduce the measurement time and give adequate dimensional information of the Si wafer. The wavelength of the optical source is around 1 μm, for which transmission is non-zero for undoped silicon and can be also detected by a typical CCD camera. Because of the typical CCD camera, the whole system can be constructed inexpensively.

Impact of the Inhomogenous Structure of Polysilicon TFT's Active Layer on the Electrostatic Potential Distribution

Recently polycrystalline silicon (pc-Si) thin film transistors (TFT’s) have emerged as the devices of choice for many applications. The TFTs made of a thin un-doped polycrystalline silicon film deposited on a glass substrate by the Low Pressure Chemical Vapor Deposition technique LPCVD have limits in the technological process to the temperature &lt; 600°C. The benefit of pc-Si is to make devices with large grain size. Unfortunately, according to the conditions during deposition, the pc-Si layers can consist of a random superposition of grains of different sizes, where grains boundaries parallels and perpendiculars appear. In this paper, the transfer characteristics IDS-VGS are simulated by solving a set of two-dimensional (2D) drift-diffusion equations together with the usual density of states (DOS: exponential band tails and Gaussian distribution of dangling bonds) localized at the grains boundaries. The impact of thickness of the active layer on the distribution of the electrostatic potential and the effect of density of intergranular traps states on the TFT’s transfer characteristics IDS-VGS have been also investigated.

XANES and XES Studies of Luminescent Silicon Carbonitride Thin Films

We investigated the photoluminescence (PL) and electronic properties of both undoped and doped silicon carbonitride (SiCN) thin films, fabricated using electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD). The PL intensity of SiCN films is highly dependent on the annealing temperature and doping condition with rare-earth metals. Notably, Ce+3- and Tb+3-related luminescence were observed from doped samples after being annealed in nitrogen ambient at 1200°C for one hour. The electronic characteristics of these doped and un-doped films were analyzed using soft X-ray absorption near edge structure (XANES) and X-ray emission spectroscopy (XES) measurements, showing significant structural re-ordering occurs as the annealing temperatures were increased.

LUMINESCENT SILICON NANOSTRUCTURES VIA TIN OXIDE DOPING

Tin oxide ( SnO2 )-doped Si nanorings of diameter in the range of 100 nm to 170 nm with an average width of 25 nm are synthesized by off-axis laser ablation (PLD) and are characterized by different techniques. The AFM observations show that the surface morphological features of films depend on the tin oxide concentration. The bandgap energies of undoped quantum dots are found to be 2.29 eV, while it decreases to 2.15 eV and 1.5 eV for 3 wt.% and 0.1 wt.% SnO2 -doped samples, respectively. The increase in the value of bandgap energy can be attributed to size reduction of particles. The Raman spectra of SnO2 -doped films are characterized by a broad Raman band with intensity maximum around 478 cm-1. Raman spectrum shows frequency shift which may be due to changes in the Si – O bond length or Si – O – Si bond angle. The activation energy at higher temperature is found to be 16.99 meV for 3 SnSi 209, 21 meV for 0.1 SnSi 209 and 18.1 meV for undoped silicon which shows that defect levels are present in all the samples, the conduction is due to the presence of holes. The synthesized films exhibit PL peak in the visible region. The PL emission peak and PL intensity depend on dopants and it is concluded that luminescence does not originate from localized states in gap but from extended states. The size and shape of nanostructures depend on the SnO2 concentration and the doping effects can be used as a significant guideline for tuning the electronic and optical properties of Si .