Impurity Breakdown Research Articles

Microstructure evolution from silicon core to surface in electronic-grade polycrystalline silicon

Large-size electronic-grade polycrystalline silicon is an important material in the semiconductor industry with broad application prospects. However, electronic-grade polycrystalline silicon has extremely high requirements for production technology and currently faces challenges such as carbon impurity breakdown, microstructure and composition nonuniformity and a lack of methods for preparing large-size mirror-like polycrystalline silicon samples. This paper innovatively uses physical methods such as wire cutting, mechanical grinding and ion thinning polishing to prepare large-size polycrystalline silicon samples that are clean, smooth, free from wear and have clear crystal defects. The material was characterized at both macroscopic and microscopic levels using metallographic microscopy, scanning electron microscopy (SEM) with backscattered electron diffraction (EBSD) techniques and scanning transmission electron microscopy (STEM). The crystal structure changes from single crystal silicon core to the surface of the bulk in the large-size polycrystalline silicon samples were revealed, providing a technical basis for optimizing and improving production processes.

Automated determination of early eluting oligonucleotide impurities using ion-pair reversed-phase liquid chromatography high resolution-mass spectrometry

A new method is presented for the automated determination of early eluting (E.E.) oligonucleotide impurities analyzed by IP-RP HPLC HRMS. E.E. are impurities shorter than the main drug component and are primarily formed by the mechanisms of coupling failure, and depurination. The method is based on the detection of the theoretically derived most abundant mass of an impurity in the experimental data. An exhaustive list of candidate impurities and their formulas is automatically generated using the parent sequence and the known mechanisms of impurity formation. The approach accounts for possible modifications in the individual oligonucleotide sequence moieties (e.g., linkage, sugar, and base, 3′, and 5′ ends). The detected ion signal is summed to provide four nested levels of impurity breakdown information. The approach allows for the rapid determination of relationships and trends of impurities in samples generated by different manufacturing processes and conditions. Representative examples are given to illustrate the capabilities and utility of the approach in synthesis and purification process optimization applications.

Effect of impurity impact ionization on the dynamic characteristics of 4H-SiC p+–n––n+ diodes at low temperatures (77 K)

The low temperature (77 K) transient switch-on characteristics of 4H-SiC p+–n––n+ diodes are measured in the pulse mode. Using a simple analytical model, the effect of impurity breakdown in a heavily doped p+-type emitter on the current rise dynamics after applying high-amplitude forward-bias pulses to the diode is explained.

Terahertz radiation associated with the impurity electron transition in quantum wells upon optical and electrical pumping

Radiation in the terahertz (THz) spectral range from structures with GaAs/AlGaAs doped quantum wells is investigated under conditions of the interband optical excitation of electron-hole pairs in n-type structures and impurity breakdown in a longitudinal electric field in p-type structures. The emission spectra are obtained. Emission is observed at low temperatures and shown to be determined by optical transitions between impurity states and transitions between the band and impurity states. Upon optical interband pumping, the impurity states are depopulated due to the recombination of electron-hole pairs with the involvement of impurities, while, in an electric field, the impurity states are depopulated due to impact ionization.

Non-ohmic hopping conduction of a diode base extracted from the diode current-voltage characteristics

Using the method of extracting a series resistance from a p- n diode current-voltage characteristics developed earlier, parameters of low-temperature non-ohmic conduction in bases of two types of silicon diodes have been studied. In the diode of the first type, with finite value of impurity ionization energy, low-temperature impurity breakdown in the diode base is accompanied by hysteresis of its conduction; in the diode of the second type, the impurity breakdown occurs in soft form. In the pre-breakdown region, the results obtained for the diodes of both types, agree completely with existing picture of non-ohmic hopping conduction in spite of significant space inhomogeneity of the diode structures and non-equilibrium character of the transport in them. In the post-breakdown region, the certain peculiarities are available. For the first time the activationless hopping conduction has been revealed in crystalline silicon.

Impurity-related terahertz emission from quantum well nanostructures

Terahertz emission in GaAs/AlGaAs quantum well structures doped with shallow impurities was studied in conditions of interband optical excitation for n-doped structures and impurity breakdown in the lateral electric field for p-doped structures. Emission spectra were obtained. It was shown that the observed emission is related to optical transitions of charge carriers between impurity levels and to impurity-band transitions. The depopulation of the final states under interband optical pumping was realized with recombination of non-equilibrium holes and electrons localized at neutral donors.

Impurity-related terahertz emission from quantum well nanostructures

Terahertz emission in GaAs/AlGaAs quantum well structures doped with shallow impurities was studied in conditions of interband optical excitation for n-doped structures and impurity breakdown in the lateral electric field for p-doped structures. Emission spectra were obtained. It was shown that the observed emission is related to optical transitions of charge carriers between impurity levels and to impurity-band transitions. The depopulation of the final states under interband optical pumping was realized with recombination of non-equilibrium holes and electrons localized at neutral donors.

Impact ionization of nitrogen in 4H- and 6H-SiC

A natural superlattice (NSL) in silicon carbide polytypes induces a miniband structure within the conduction band along the NSL axis C. It was found that the presence of NSL leads to an anisotropy of a nitrogen impurity impact ionization in SiC. For an electric field direction F∥C, the nitrogen impurity breakdown at temperature 4.2 K has not been observed up to the field 1.6 MV/cm for the polytype 6H-SiC. However, for the polytype 4H-SiC dependence breakdown field on impurity concentration demonstrates its usual behavior, which has also been observed for other semiconductor materials. Therewith, the impurity breakdown at the electric field perpendicular to the axis in 4H- and 6H-SiC demonstrates consistency with traditional conceptions. These results are explained theoretically.

Development and validation of a rapid chromatographic method for the analysis of flunarizine and its main production impurities

A rapid selective method for the analysis of flunarizine and its associated impurities was developed and validated according to ICH guidelines. The separation was carried out using a Thermo Scientific Hypersil Gold C18 column (50mm×4.6mm i.d., 1.9μm particle size) with a gradient mobile phase of acetonitrile–ammonium acetate–tetrabutylammoniumhydrogen sulfate buffer, at a flow rate of 1.8mL/min and UV detection at 230nm. Naturally aged samples were also tested to determine sample stability. A profile of sample and impurity breakdown was also presented.

Effect of magnetic field on hysteretic characteristics of silicon diodes under conditions of low-temperature impurity breakdown

Effect of magnetic field on hysteretic characteristics of silicon diodes under conditions of low-temperature impurity breakdown

Development and validation of a rapid liquid chromatographic method for the analysis of Ketorolac Tromethamine and its related production impurities

A high performance liquid chromatographic (HPLC) method for the analysis of ketorolac tromethamine and its associated impurities was examined with the aim of reducing analysis times while maintaining good efficiency. The separation was carried out using a waters X-bridge – C8 (3mm x75mm, 2.5µm particle size) column with a mobile phase of tetrahydrofuran – ammonium dihydrogen phosphate (0.05M, pH3, 28:72 v/v) at a flow rate of 1.7mL/min and UV detection at 313nm. The method was validated according to ICH (international conference on harmonisation) guidelines with respect to precision, accuracy, linearity, specificity, robustness and limits of detection and quantification. All parameters examined were found to be well within the stated guidelines. Naturally aged samples were also tested to determine sample stability. A profile of sample and impurity breakdown was presented. A three-fold reduction in analysis time was achieved in comparison with the current approved EP (european pharmacopeia) method and the method can be immediately used for routine assay and related substance determination.

Low-temperature (77–300 K) current-voltage characteristics of 4H-SiC p +-p-n + diodes: Effect of impurity breakdown in the p-type base

The effect of impurity breakdown on the low-temperature (77–300 K) current-voltage (I–V) characteristics of 4H-SiC diodes with a p-type base has been studied. Experimental samples were fabricated from CVD-grown (chemical vapor deposition) commercial p+-p-n+ 4H-SiC structures. A high electric field in the p-type base was created by applying a forward bias to the diodes. It was found that, at temperatures of 136, 89, and 81 K, the commonly observed “diode” portion of the I–V characteristics is followed by a portion in which the current grows more rapidly due to the impact ionization of frozen-out Al acceptor atoms in the ground (unexcited) state. At temperatures of 81 and 77 K, this portion is followed by one with a negative differential resistance due to the regenerative dynistor-like switching of the diode, caused by impact ionization of aluminum atoms in the excited state.

The Determination of Miconazole and its Related Production Impurities Together with Basic Solution Stability Studies Using a Sub 2 m Chromatographic Column

A selective and sensitive method for the analysis of Miconazole and its associated impurities is developed. The separation is carried out using a Thermo Scientific Hypersil Gold C18 Column (50 mm x 4.6 mm i.d., 1.9 µm particle size) with a mobile phase of acetonitrile-methanol-ammonium acetate (1.5 w/v) (30:32:38 v/v) at a flow rate of 2.5 mL/min and UV detection at 235 nm. The method is validated according to ICH guidelines with respect to precision, accuracy, linearity, specificity, robustness, and limits of detection and quantification. All parameters examined are found to be well within the stated guidelines. Naturally aged samples are also tested to determine sample stability. A profile of sample and impurity breakdown was presented. The analysis time was more than halved from just under 20 min (the current European Pharmacopeia Method) to under 8 min (developed method) and the method is applicable for assay and related substance determination.

Relaxation of the impurity photoconductivity in p-Ge/Ge1−xSix quantum well heterostructures

In this paper we report a study on relaxation of the impurity photoresponse in strained p-Ge/GeSi heterostructures excited by pulsed THz radiation. The relaxation time is found to increase with the applied dc electric field, which is interpreted within the model of cascade carrier capture by the impurity centers. A second time scale of the impurity photoconductivity relaxation is observed near the impurity breakdown field, and in post-breakdown electric fields the relaxation time (decreasing with the field) is shown to be governed by the impact ionization rather than by the recombination.

The Strong Field Transport in 4H- and 6H-SiC at Low Temperature

A natural superlattice (NSL) in silicon carbide polytypes is shown to introduce a miniband structure into the conduction band, which leads to depressing of the electron impact zone-zone ionization at 300K with the electric field directed along the crystal axis or the NSL axis. The NSL influence can be also observed in the nitrogen impact ionization at 4.2K. In this case for the parallel field the impurity breakdown has not been fixed up to the fields 1.6 MV/cm in 6H-SiC. These results are explained by the insufficient gain in miniband for the ionization electron energy. Therewith the impurity breakdown at the electric field perpendicular to this axis correlates with a classical picture.

Peculiarities of impact ionization of Impurity Al in SiC polytypes

The specific character of impact ionization of a small radius impurities was investigated in polytypes 6H-, 4H-, and 15R-SiC. The I-V characteristics of Al breakdown in 6H-, 4H-, and 15R-SiC in electrical field were studied. It was shown that at 77 K the impurity breakdown takes place only under the large impurity concentrations (three orders larger than that in Ge). The following data were obtained: the decreasing dependence of breakdown field due to the concentration increase, the absence of the low temperature breakdown at low impurity concentration, and gigantic enhancement of breakdown field when the field is parallel to the crystal axis.

Kinetics of terahertz photoconductivity in p-Ge under impurity breakdown conditions

Relaxation times of impurity photoconductivity in p-Ge samples excited by a nanosecond narrow-band source of terahertz radiation are studied at various bias voltages. It is shown that the relaxation time in prebreakdown fields increases with the applied electric field and decreases as the impurity breakdown field is exceeded. Nonmonotonic photoconductivity kinetics is observed in the studied samples differing by acceptor concentrations and degrees of compensation when approaching the impurity breakdown field.

Concerning the nature of relaxation oscillations in silicon diodes in the cryogenic temperature region

Time-resolved measurements have been performed of periodic oscillations in the voltage drop across a silicon diode which is fed by stabilized direct current (the situation for temperature measurement) in the region of instability connected with low-temperature impurity breakdown. Analysis of this effect was made on the base of simultaneous consideration of current–voltage characteristics of the diode under the same conditions. From these characteristics, by means of a new analytical method, the voltage-dependent series resistance, the breakdown voltage of the diode base, and equivalent diode capacity responsible for the observed oscillations have been determined. For the first time, practically linear relation between the feeding current and the oscillations frequency has been explained. This relation follows from the fact of approximate constancy of the charge accumulated by the equivalent diode capacity before the breakdown (irrespective of the feeding current value).

Influence of an electric field on the operation of terahertz intracenter silicon lasers

We have investigated the influence of a pulsed electric field on the intracenter population inversion between phosphorus donor states in silicon. Terahertz-range electroluminescence resulted from the populated excited donor states. It grows linearly above the impurity breakdown field but saturates at excitation powers exceeding 100 W at 5 μs pulse length. An electric field applied to the optically pumped silicon laser reduces the efficiency of the 2p0→1s(T2) phosphorus transition already at voltages below the impurity breakdown. The appearance of a current through the laser sample results in a fast quenching of the laser emission that indicates a reduction in the inversed population between the laser states.

Emission of terahertz radiation from GaN under impact ionization of donors in an electric field

The emission of terahertz radiation from epitaxial n-GaN layers in lateral electric field was found and studied for the first time. Radiation emission was observed in fields exceeding the impurity breakdown threshold. The highest -intensity radiation lines may be associated with intracenter electron transitions between donor levels (Si and O) and electron transitions from the conduction band to donor ground levels. Compact emitters of terahertz radiation operating at temperatures from 4 to 80 K can be created on the basis of n-GaN.