High-purity Si Research Articles

Cellular structures in Czochralski-grown SiGe bulk crystal

A high purity Si x Ge 1 −x alloy ( x 0=0.89) 48 mm in length with a maximum diameter of 25 mm was grown by the Czochralski method. Development of the cellular structures was evaluated by observing growth striations in the crystal by means of X-ray topography and optical microscopy. Temporary and fatal cellular structures were detected. Two possible factors were distinguished as being responsible for the origin of the cell formations: an enhancement of the effective growth velocity and a gradual reduction of Si composition. Both factors lead to the occurrence of constitutional supercooling beyond the critical relation between growth velocity and alloy composition.

Microstructured semiconductor neutron detectors

Perforated semiconductor neutron detectors are compact diode detectors that operate at low power and can be fashioned to have high thermal neutron detection efficiency. Fabricated from high-purity Si wafers, the perforations are etched into the diode surface with ICP-RIE and backfilled with 6LiF neutron reactive material. The intrinsic thermal neutron detection efficiency depends upon many factors, including the perforation geometry, size, and depth. Devices were fabricated from high resistivity 10 k Ω cm n-type Si with conformal p-type shallow junction diffusions into the perforations, which demonstrate improved neutron detection performance over previous selectively diffused designs. A comparison was made to previous selectively diffused designs, and pulse height spectra show improved signal-to-noise ratio, higher neutron counting efficiency, and excellent gamma-ray discrimination. Devices with 20 ( average ) μ m wide 100 μ m deep sinusoidal trenches yielded intrinsic thermal neutron detection efficiencies of 11.94 ± 0.078 %.

Intermediate band mobility in heavily titanium-doped silicon layers

The sheet resistance and the Hall mobility of high-purity Si wafers, in whose surface Ti atoms are implanted and laser annealed reaching concentrations above 10 21 cm −3, are measured in the 90–370 K range. Below 240 K, an unconventional behavior is observed that is well explained on the basis of the appearance of an intermediate band (IB) region able to form a blocking junction with the substrate and of the appearance of an IB conduction. Explanations based on ordinary device physics fail to justify all the unconventional behavior of the characteristics observed.

Perforated diode neutron detector modules fabricated from high-purity silicon

Compact neutron detectors are being designed and tested for use as low-power real-time personnel dosimeters. The neutron detectors are pin diodes that are mass produced from high-purity Si wafers. Each detector has thousands of circular perforations etched vertically into the device. The perforations are backfilled with 6LiF to make the pin diodes sensitive to thermal neutrons. The prototype devices deliver 4.7% thermal neutron detection efficiency while operating on only 15 V, showing a 9% increase in efficiency over identical planar devices.

New Method for Production of Solar-Grade Silicon by Subhalide Reduction

In this study, a new method for producing Si, called reduction, was investigated with the purpose of producing solar-grade silicon (SOG-Si); by this method, SiCl 4 was reduced by gaseous subhalide used as the reductant. Si was produced by reacting SiCl 4 with Al subhalides, which were produced by reacting AlCl 3 with metallic Al. Fibrous Si with diameters ranging from submicrons to several tens of micrometers was deposited as a result of halidothermic reduction of SiCl 4 by an Al subchloride (AlCl x ) reductant at 1273 K. The size of Si deposits and the reaction rate were increased by simultaneously supplying AlCl 3 and SiCl 4 vapors to a reaction tube holding Al metal. The impurity level of the obtained Si was found to be lower than the detection limit of X-ray fluorescence. Halidothermic reduction is suitable for producing high-purity Si since all reactants and byproducts exist in the vapor phase. Further, this process has high productivity since the overall reaction is a highly scalable metallothermic reduction reaction.

Screening length in presence of attractive traps

The electrostatic screening length due to non-equilibrium electrons in the presence of shallow attractive traps, at low lattice temperatures, has been calculated here for a wide range of electric field. The range covers from a field corresponding to a weakly heated ensemble up to a value for the onset of impurity breakdown. Compared to the results obtained under the condition of thermodynamic equilibrium, the screening length due to non-equilibrium carriers becomes field dependent and moreover, its dependence upon the lattice temperature is now quite complicated. The numerical results for high purity Ge and Si show that the screening length assumes a maximum value at some electric field. The strength of this field again changes with the change of the lattice temperature.

Rapid annealing of the vacancy-oxygen center and the divacancy center by diffusing hydrogen in silicon

In hydrogenated high-purity Si, the vacancy-oxygen $(V\mathrm{O})$ center is shown to anneal already at temperatures below $200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and is replaced by a center, identified as a vacancy-oxygen-hydrogen complex, with an energy level $0.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction-band edge and a rather low thermal stability. At long annealing times, the process is reversed and the concentration of the latter defect is reduced, while the $V\mathrm{O}$ center partly recovers. The divacancy $({V}_{2})$ center anneals in parallel with the initial annealing of the $V\mathrm{O}$ center, and the loss in ${V}_{2}$ exhibits a one-to-one proportionality with the appearance of a hole trap $0.23\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence-band edge attributed to a divacancy-hydrogen $({V}_{2}\mathrm{H})$ center.

Room-temperature annealing of vacancy-type defect in high-purityn-type Si

Electron-irradiated ${p}^{+}\text{\ensuremath{-}}{n}^{\ensuremath{-}}\text{\ensuremath{-}}{n}^{+}$ diodes produced from low-doped high-purity Si wafers were found, by deep-level transient spectroscopy (DLTS), to have a prominent defect, labeled $E4$, with an energy level $0.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction-band edge and a concentration of $\ensuremath{\sim}\frac{1}{4}$ relative to the divacancy. The samples were kept at room temperature, and the $E4$ concentration was seen to reduce to half during five weeks. Annealing data revealed a similar peak $E5$ overlapping that of the single-negatively charged divacancy and showing a one-to-one proportionality with $E4$. $E4$ and $E5$ arise most likely from a vacancy-type defect and a tentative assignment to a planar tetravacancy is put forward.

High carrier concentrations of n- and p-doped GaN on Si(111) by nitrogen plasma-assisted molecular-beam epitaxy

High-quality doped GaN layers were grown on silicon substrates by radio frequency nitrogen plasma-assisted molecular-beam epitaxy. High-temperature-grown AlN (about 200 nm) was used as a buffer layer. In-growth doping was done using high-purity Si and Mg as n- and p-type dopants, respectively. X-ray diffraction revealed that monocrystalline GaN was obtained. This is in good agreement with the results of morphological study by atomic force microscopy. Micro-photoluminescence (PL) and micro-Raman spectroscopy were used to study the room-temperature optical properties of the doping films. No yellow-band emission was observed in the PL spectroscopy. From the Hall measurements, the resulting n-type doping concentration was measured to be (1–2) × 1019 cm−3. Fairly uniform hole concentration as high as (4–5) × 1020 cm−3 throughout the GaN crystal was achieved. In terms of the carrier concentration, it was found that the results determined from the Fourier transform infrared analysis are in good agreement with the results determined from the Hall measurements.

Si multicrystals grown by the Czochralski method with multi-seeds

Si multicrystals with several large grains have been grown by the Czochralski (CZ) method with multiple seeds using semiconductor-grade high-purity Si raw material in order to investigate the influence of defects such as dislocations and sub-grain-boundaries on the minority carrier lifetime. Two or three single-crystal Si seeds with [1 0 0], [1 1 0] and [1 1 0] orientations were used for the growth. It was found that the minority carrier lifetime in the Si multicrystals decreased considerably in the region of grains near the semicoherent interfaces with others, e.g., interfaces between [2 3 2] and [1 1 1] grains, [1 0 0] and [1 1 1] grains, and [1 1 0] and [1 1 1] grains. Numerous dislocations with densities as high as 10 6/cm 2 in these grains moved to align and generate sub-grain boundaries by the shear stress caused by the semicoherence near the interfaces, which was determined to be responsible for the decrease in the minority carrier lifetime.

자전 연소 합성법에 의한 고순도 실리콘 분말제조

High purity Si powder was prepared in the system of SiO₂-Mg combustion reaction. Various conditions of combustion reaction and leaching were investigated. As the particle size of Mg decreased and the compaction pressure increased the quantity of the unreacted powder was decreased. In the acid leaching of MgO, increasing particle size, reaction temperature, rotating speed and reaction time made leaching effect low. Final Si powder produced by combustion and leaching reaction, has a high purity of 99.9% with irregular shape.

Single crystalline β-FeSi 2 grown using high-purity FeSi 2 source

We have investigated the effect of FeSi 2 source purity on the electrical property of β-FeSi 2 grown from solution. A high-purity FeSi 2 source avoided a contamination of Cu and W metals was synthesized by melting a high-purity Fe (5N) and Si (5N-up) in a quartz ampoule. Glow discharge mass spectrometry revealed that the purity of the FeSi 2 source synthesized using 5N-Fe and a quartz-ampoule-melting process is one order of magnitude higher than that of the conventional arc-melted FeSi 2 source using 4N-Fe. The β-FeSi 2 crystals grown using the high-purity FeSi 2 and Zn solvent showed n-type conduction, whereas those grown using the arc-melted FeSi 2 showed p-type. The carrier concentration of the n-type crystals was (4.9–6.3) × 10 18 cm − 3 , which was more than 10 times higher than that of the p-type crystals (5.2 × 10 17 cm − 3 ). From the ICP-MS and SIMS analysis of the grown crystals, we found that dominant impurity concentrations (Cr, Mn, Co, Ni, Cu, Zn and W) in the p-type crystals were higher than those in the n-type ones. Therefore, the p-type conductivity of undoped crystals grown using Zn solvent results from unintentional doping by the high impurity level of the used FeSi 2 source.

High purity Si–C–N thin films with tailored composition on the tie line SiC–Si 3N 4

Si–C–N thin films with tailored stoichiometries on the tie line SiC–Si 3N 4 have been produced by severalfold ion implantation and by a combination of RF magnetron sputtering and ion implantation, respectively. These high purity thin films have been heat treated at 1250 °C under high vacuum conditions using an electron beam annealing system to enable crystallisation and/or phase formation. Fourier transform infrared spectrometry as well as X-ray photoelectron spectroscopy suggests the formation of an amorphous network of mixed Si(C,N) 4 tetrahedrons for the ternary compositions and evince the crystallisation of the binary SiC and Si 3N 4 thin films.

Development of high- Q superconducting resonators for use as kinetic inductance detectors

One of the greatest challenges in the development of future space based instruments for sub-mm astronomy is the fabrication of very sensitive and large detector arrays. Within this context we have started the development of Microwave Kinetic Inductance Detectors (MKID’s). The heart of each detector consists of a high- Q superconducting quarter wavelength microwave resonator. As a result it is easy to multiplex the readout by frequency division multiplexing. The predicted fundamental sensitivity limit of the MKID is due to quasiparticle creation-recombination noise, leading to a NEP ∼ 1 × 10 - 20 W / Hz , low enough for any envisionable application in the sub-mm, optical and X-ray wavelength ranges. We describe experiments with these resonators, made of 150 nm Ta films with a 5 nm Nb seed layer on high purity Si substrates with a resonance frequency around 3 GHz. We measure the Q factors, responsivity, noise and noise equivalent power of several resonators. We find Q factors in excess of 1 × 10 5, high enough for the multiplexing of more than 10 4 pixels. The quasiparticle lifetime in our film is measured to be 25 μs. which gives, together with the measured phase noise, a NEP of ∼ 4 × 10 - 16 W / Hz at 1 kHz. At lower frequencies the noise increases.

Direct detection of 0.1–20keV electrons with delta doped, fully depleted, high purity silicon p-i-n diode arrays

Direct detection of 0.1–20keV electrons is demonstrated using a boron delta doped high purity Si p-i-n diode array. Molecular beam epitaxy is used to grow a delta layer on the back surface of these fully depletable p-i-n diode arrays to form an electrode for detecting shallow-penetrating ionizing radiation. Device structure, processing, and characterization methods used for device testing and measurement of its response to electrons are discussed. Use of this detector for measuring the Si quantum yield over this wide energy range is also discussed.

Titanium silicide (Ti 5Si 3) synthesis under shock loading

Ti–Si binary system shows exothermic reactions during the formation of several line compounds, which can be used for self-propagating high temperature synthesis (SHS) of starting powders. In this research, titanium silicide (Ti 5Si 3) was synthesized from high purity Ti and Si powders using shock waves in a 10 cm diameter and 14 m long gas gun. Influences of compact density, shock velocity, milling time and filler concentrations were studied. Mullite was used as an inert ceramic filler in different wt.% and mixed with ball-milled powders. Phase analysis, microstructural analysis and microhardness measurements were done on samples that were recovered after shock loading. Results indicate that powders with a lower compact density generated higher temperatures during shock consolidation. The change in velocity generated different compressive stresses in Cu target ranging 4.4–8.6 GPa, and found to have a significant effect on the reaction kinetics. Both increasing filler material amount and decreasing milling time reduced the reaction kinetics. Though the presence of mullite decreased the reaction kinetics, it also reduced residual porosity in the compacts via forming an in situ intermetallic-ceramic composite.

Development of high- Q superconducting resonators for use as kinetic inductance sensing elements

One of the greatest challenges in the development of very sensitive radiation detectors is the fabrication of a large detector array. Within this context we have started the development of Kinetic Inductance Detectors. The heart of each detector consists of a high- Q superconducting quarter wavelength microwave resonator. As a result, it is easy to multiplex the readout by frequency division multiplexing. We describe our initial experiments with these resonators, made on 20×4 mm high-purity Si chips using 100 nm Nb films. We measure the Q factors of several resonators using a vector network analyzer and find Q factors up to 90.000, limited by the intrinsic quality of the Nb resonator.

Studies of the composition, tribology and wetting behavior of silicon nitride films formed by pulsed reactive closed-field unbalanced magnetron sputtering

Silicon nitride films were formed by pulsed reactive closed-field unbalanced magnetron sputtering of high purity Si targets in an Ar–N2 mixture. The effects of N2 fraction on the chemical composition, and tribological and wetting behaviors were investigated. The films deposited at a high N2 fraction were consistently N-rich. The surface microstructure changed from continuous granular surrounded by tiny void regions to a homogeneous and dense microstructure, and densitied as the N2 fraction is increased. The as-deposited films have a relatively low friction coefficient and better wear resistance than 316L stainless steel under dry sliding friction and experienced only abrasive wear. The decreased surface roughness and increased nitrogen incorporation in the film give rise to increased contact angle with double-stilled water from 24° to 49.6°. To some extent, the silicon nitride films deposited are hydrophilic in nature.

Single atom Si nanoelectronics using controlled single-ion implantation

We present recent developments in controlled single-ion implantation techniques. A low energy (14 keV) ion-beam is used to produce shallow phosphorus implants in high-purity Si. Single atom control during implantation is achieved by monitoring on-chip p-i-n detectors, integrated within the device structure, while positional accuracy of 20 nm is achieved via a nanolithographic resist mask. This technique has been used to implant only two phosphorus dopant atoms for use as charge-based Si:P quantum bits (qubits). Voltages applied to precisely aligned surface electrodes control the double-donor system, and dual single-electron transistors (SETs) provide readout with spurious signal rejection. Preliminary low temperature measurements on devices implanted with less than 10 dopant atoms demonstrate isolated charge transfer events.

Variation of Width and Composition of Grain-Boundary Film in a High-Purity Silicon Nitride with Minimal Silica

Contrary to the widely accepted observation that grain-boundary amorphous films for a given Si 3 N 4 composition have common (equilibrium) widths and compositions, a significant variation for both parameters from film to film was observed in an undoped high-purity Si 3 N 4 prepared using a hot isostatic pressing method. This material previously has been reported to have an equilibrium film width of 0.6 nm, as measured using a high-resolution electron microscopy (HREM) method; this value is significantly different from that which is typical for other high-purity Si 3 N 4 ceramics (1.0 nm). A total of four boundaries were analyzed, using spatially resolved electron energy-loss spectroscopy methods, which can give the chemical width and composition for the film. Widths of these grain-boundary films were substantially different from each other; only the thinnest matches the previous HREM observations. The nitrogen content in the film decreased concurrently as the film thickened. This material had many cavities and complicated configurations at triple pockets, because of the very low total-SiO 2 content (0.55 vol%). They created locally different equilibrium conditions for grain-boundary films, in comparison with other fully densified Si 3 N 4 , causing such strong variation in both film structure and chemistry. This observation reveals the importance of triple pockets in equilibrium film structures, providing new insight in evaluating the absorption and wetting models. The thinnest film may correspond to the amorphous structure that is required to bind two randomly oriented Si 3 N 4 grains under greater local stress.