Influence of annealing on the concentration profiles of boron implantations in silicon

The influence of annealing on the concentration profiles of boron implanted into silicon with does of 10¹⁴ ions/cm² up to 10¹⁶ ions/cm² and an energy of 70 keV was studied. The concentration profiles were measured with Secondary Ion Mass Spectrometry (SIMS). The broadening of the concentration profiles during annealing can be described as a superposition of effects resulting from a relatively immobile and a mobile boron fraction. The properties of the immobile boron fraction were studied by measuring the influence of a boron implantation on the distribution of a homogeneous boron background dope. From these experiments it was concluded that the immobile boron fraction consists of boron precipitates. The properties of the mobile fraction were studied from concentration profiles that were obtained after annealing during different periods at the same temperature. It was found that during the initial stage of the annealing process a fast broadening of the profile occurs; this was assumed to be due to an interstitial type boron diffusion. After prolonged annealing the much slower substitutional type diffusion prevails, due to trapping of the interstitial boron atoms by vacancies. The reliability of the SIMS method, as applied to profile measurements, was checked for the high boron doses used in this investigation. Excessive boron precipitates, obtained after annealing of a high dose, such as 10¹⁶ ions/cm² at about 1000°C, appear to give some increase of the ion yield.     

Computer controlled spreading resistance system for measurement of carrier concentration profiles of silicon structures

A new computer controlled equipment for carrier concentration profile determination of silicon structures by a spreading resistance technique is presented. The required data and plots can easily be obtained by means of a new software package, which has been specially developed for this purpose. Measurements have been performed on different structures in order to verify the suitability of both the measuring system and the software package. The reproducibility has been found to be within 5% for the investigated structures.     

High concentration effects of ion implanted boron in silicon

The diffusion behaviour of implanted boron in silicon was investigated using the¹⁰B(n,α)⁷ Li nuclear reaction. An anomalous behavior with a strong reduction of the diffusivity above an effective solubility limit at 1.5×10¹⁹, 6×10¹⁹, and 1.1×10²⁰ cm⁻³ was found for annealing temperatures of 800, 900, and 1,000°C, respectively.     

Nuclear spin polarization in silicon nanostructures with charge carrier injection

We theoretically examine injection polarization of nuclear spins in silicon nanostructures with hyperfine interaction of nuclei with excited triplet states. We predict the possibility of the appearance of self-sustaining nuclear spin polarization, initiated by an external field. We show that if the external magnetic field is varied, we observe up to a 600-fold jump in the number of spin-polarized nuclei. A similar up to 40-fold jump also appears as the charge carrier injection rate increases. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 647–653, September–October, 2006.     

The effect of annealing ambient on carrier recombination in boron implanted silicon

The selection of either an oxidising or inert ambient during high temperature annealing is shown to affect dopant activation and electron–hole recombination in boron implanted silicon samples. Samples implanted with B at fluence between 3 × 10¹⁴ cm^–2 to 3 × 10¹⁵ cm^–2 are shown to have lower dopant activation after oxidation at 1000 °C compared to an equivalent anneal in an inert ambient. In addition, emitter recombination is shown to be up to 15 times higher after oxidation compared with an inert anneal for samples with equivalent passivation from deposited Al₂O₃ films. The observed increase in recombination for oxidised samples is attributed to the enhanced formation of boron‐interstitial defect clusters and dislocation loops under oxidising conditions. It is also shown that an inert anneal for 10 minutes at 1000 °C prior to oxidation has no significant impact on sheet resistance or recombination compared with a standard oxidation process.

     

Low energy boron implantation profiles in silicon from junction depth measurements

Some methods have been recently developed to investigate the distribution of implanted ions in semiconductors, especially into silicon. Generally, these techniques are not valid for boron due to the absence of convenient radioactive isotopes, or to a too small sensitivity when the lower part of the distribution is of interest. This corresponds to our problem, since boron implanted nuclear particle detectors prepared with high resistivity material (up to 50,000 ω.cm) are needed. The properties of these P-N junctions depend in a certain amount on the impurity distribution existing several orders of magnitude below the top of the distribution. Therefore, only the junction location method can be employed. In this method a series of N-type silicon samples, differing each from the other by an increase in resistivity are implanted with boron. The depth of the P-N junction corresponds to the point of the profile where the concentration N_A is equal to that of the substrate N_D (i.e. this latter being well known from the resistivity of the starting material). If the location of the junction can be measured, the profile can then be constructed point by point. The junction location is visualized generally by copper staining. Roosild,⁽¹⁾ Kleinfelder,⁽²⁾ Fairfield⁽³⁾ and D. E. Davies⁽⁴⁾ have used this procedure for boron implantations at energies higher than 50 keV. There is a problem due to the small penetration of the boron ion, and, for high resistivity materials, it is difficult to know the true limits of the zones stained with copper.

In our problem, when heavy particle detectors are desired, it is necessary to implant at lower energies than those indicated previously (< 20 keV). We have developed a new technique derived from the junction depth method, which is useful even at very low implant energy (≈ 10 keV). It consists in measuring the energy loss by 100 keV protons when crossing the entrance window of the P-N junctions used as detectors.

In the first part of this paper the method is described and the possible errors are analyzed and evaluated. In the second part, the distribution of 15 keV boron ions implanted under several experimental conditions is studied. Emphasis is given to the defects resulting from the silicon bombardment.     

The free charge carrier effects on elastic properties of silicon

The exact theory of the free hole contribution to the elastic moduli of p-Si taking into account all three valence bands is developed. The fair agreement is found between this theory and experimental measurements of the concentration dependence of sound velocity made for p-Si with different boron content. For n-Si analogous measurements were made and the results agree well with the theory of Keyes.     

Measurement of boron and phosphorus concentration in silicon by low-temperature FTIR spectroscopy

The calibration factors for the determination of boron and phosphorus concentration in single crystal silicon by low temperature Fourier transform infrared spectroscopy are examined, with the aim of comparing the behaviour of float-zone and Czochralski samples. It is shown that common calibration factors, derived from a correlation with four-point probe resistivity measurement, can be applied to both material types. Moreover, no significant difference in carrier mobility is observed between FZ and CZ, as determined by Hall effect measurements, in a wide oxygen range: 2–9×10¹⁷ cm^-3, confirming that the same conversion algorithm to deduce the carrier concentration from the resistivity measurement can be applied. PACS 72.80; 78.30; 81.05     

Photoexcited charge carrier dynamics in silicon nanocrystal/SiO2 superlattices

We report in detail on the dynamics of photoexcited charge carriers in size-controlled silicon nanocrystals in silicon nanocrystal/SiO₂ superlattices. The samples were prepared using plasma enhanced chemical vapor deposition and subsequent thermally induced phase separation. This unique approach allows preparation of well-defined Si nanocrystals. Experimental techniques of time-resolved absorption and photoluminescence were used to monitor the carrier dynamics on a wide time scale from picoseconds to milliseconds for a set of samples with different parameters (nanocrystal size, hydrogen annealing). The initial fast decay (tens of picoseconds) dependent on pump intensity for excitation levels exceeding one electron–hole pair per nanocrystal can be interpreted in terms of the bimolecular recombination with constant B=(2–3)×10⁻¹⁰ cm³ s⁻¹. The slow pump intensity independent decay (microseconds) can be reproduced well by a stretched-exponential function. The dependence of stretched-exponential parameters on photoluminescence photon energy and sample properties agrees well with the picture of trapped carriers.     

Low energy boron and phosphorus implants in silicon (b) doping profiles

Silicon wafers were implanted in 〈111〉-and 〈110〉-direction with boron ions of 6 keV and phosphorus ions of 20 keV at room temperature. Doses of 10¹⁴ ions/cm² were applied. At four different temperatures, 300, 420, 600, and 900°C. a few samples of each type of implant were annealed. Standard electrical techniques combined with successive layer removals were used to determine the depth distribution of electrically active centers. Since the method of using non type inverting implanted layers was applied, the local annealing behavior over the whole penetration region could be measured.

For both Si(B)-and Si(P)-implants the part of the profile beyond approx. 0.15 μm, i.e. the deeper part of the channeling and the whole supertail region, is unaffected by going from 300 to 900°C. All additional annealing, with respect to the electrical yield, takes place in the amorphous range and the adjacent part of the channeling range. After raising the temperature from 600 to 900°C both B-and P-profiles undergo diffusion controlled changes in this latter region.     

Antimony implanted silicon: A comparison between the total implanted concentration profile and the donor concentration profile

Abstract

The total concentration profiles of various doses of antimony, implanted into silicon at 100 keV, have been determined by a new technique, using Kr⁺ ions to detect selectively the antimony (as Sb-M X-rays) at the expense of the silicon. Since most of these X-rays arise from only a few tens of Angstroms below the surface of the silicon, this allows the X-ray generation to be used in conjunction with an anodic stripping technique to obtain the antimony depth distribution. These profiles are compared with others, obtained by measuring the donor concentration as a function of depth, using standard Hall effect and conductivity measurements. A significant difference between these profiles was observed, which is thought to be due to the suppression of electrical activity which occurs as the result of lattice damage. Confirmatory evidence is presented in the form of electron microscope observations of the implanted region at various depths below the silicon surface.     

On optimization of the charge carrier concentration in a cooling thermoelectric branch

Computer-aided simulation of the temperature field in a thermoelectric branch is performed under the conditions of a maximal temperature difference and a maximal refrigerating capacity. A differential equation of stationary heat conduction with temperature-dependent kinetic coefficients is used in the simulation. The optimal concentration of charge carriers in the branch is determined in terms of the classical statistics.     

Water-related charge carrier traps in thermal silicon dioxide films prepared in dry oxygen

The parameters of electron and hole traps in thermal silicon dioxide films prepared in dry oxygen have been investigated using avalanche injection of electrons and holes from silicon in combination with measurements of capacitance-voltage and current-voltage characteristics of photoinjection of electrons from junctions with variations in the conditions of oxidation, annealing, and storage of Si-SiO₂ structures. The model concepts proposed by the author for water-related charge carrier traps in such films have been confirmed. It has been found that the transport characteristics of the traps depend on the time of contact between Si-SiO₂ structures and atmospheric air of natural humidity. The results obtained can be used for the suppression of degradation processes in devices based on Si-SiO₂ structures, in the design of electroluminescent instruments based on Si-SiO₂ structures, and in the study of the transport characteristics of some molecules (for example, water), atoms, and ions in pores (structural channels) whose sizes are comparable to the sizes of water molecules.     

Defect removal after low temperature annealing of boron implantations by emitter etch‐back for silicon solar cells

Ion implantation offers new possibilities for silicon solar cell production, e.g. single side doping that can be structured in‐situ with shadow masks. While phosphorus implantations can easily be annealed at low temperature, the annealing of boron implantations is challenging. In this study, we use low energy implantations of boron (1 keV and 5 keV) with a projected range of 5.6 nm and 21.2 nm that form defects causing charge carrier recombination after a low temperature anneal (950 °C, 30 min). An ozone‐based wet chemical etching step is applied to remove this near surface damage. With increasing chemical etch‐back the electrical quality (i.e. emitter saturation current density, J_0e) improves continuously. The calculated limit for J_0e was reached with an abrasion of 35 nm for 1 keV and 85 nm for 5 keV implantations, showing that the relevant defects causing charge carrier recombination are located very close to the surface, corresponding to the as‐implanted profile depth. This emitter etch‐back allows for the fabrication of defect free boron doping profiles with good sheet resistance uniformity (standard deviation <2%). With the resulting characteristics (sheet resistance <100 Ω/sq, surface doping concentration >5 × 10¹⁹ cm^–3, J_0e < 30 fA/cm²), these boron profiles are well suited for silicon solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The concentration profiles of the recoil implanted oxygen in Si after ion implantations into SiO2-Si substrates

Abstract

The annealing of bare thermal oxide on silicon at 400–500°C in a hydrogen bearing gas results in a reduced density of states N_ss at the substrate silicon/oxide interface. Treatments of this type have played a role in MOS processing schedules for several years. However, a similar approach applied to large areas (cm²) of poly-silicon coated oxide appears to be less effective in reducing N_ss. This may be due to the polysilicon acting as a partially impermeable barrier which tends to starve the substrate/oxide interface of hydrogen.

In the present work hydrogenation of 2-inch diameter, polysilicon coated wafers has been accomplished by hydrogen ion implantation. H2⁺ ions of 135 kV energy were implanted (to a dose of 10¹⁵ cm^?2) through a 7000 Å polysilicon coating into an underlying 1400 Å SiO₂ layer. The polysilicon was removed after 30-min anneals carried out in pure N₂ at 300, 400 or 500°C. Aluminium dots, 1 mm in diameter were then deposited on to the oxide and high frequency (1 MHz) and quasistatic C-V curves recorded for determinations of Nss. Control anneals on unimplanted material were carried out in pure N₂ and N₂-H₂ ambients. Control samples annealed in pure N₂ with their polysilicon coating intact had mid-gap N_ss values of not less than 4 × 10¹⁰ cm^?2 eV^?1. The corresponding value after N₂-H₂ anneals on polysilicon-free wafers was 3 × 10¹⁰. H₂ ⁺ implanted samples annealed in pure N₂ with their polysilicon intact had mid-gap N_ss values of 1 × 10¹⁰ cm^?2 eV^?1.

The effectiveness of ion beam hydrogenation may depend upon confinement of the associated displacement damage to the polysilicon. This allows the implanted hydrogen to be activated within the SiO₂ at temperatures similar to those employed for normal hydrogeneous gas annealing of the substrate silicon/oxide interface.     

Lattice dilatation of boron in silicon

Recent data on laser-annealed, boron-implanted silicon are analysed, and the results compared with earlier experiments and simple theory. Each substitutional boron decreases the total volume by about 90% of the volume per silicon atom in the perfect crystal.     

Implantation site of boron in heavily doped silicon: A β-NMR study

Using β-NMR with¹²B as nuclear probes the temperature dependence of the lattice-site occupation of boron implanted into heavily doped silicon is studied. In p-type material the unperturbed substitutional fraction of¹²B increases from 10% at 300 K to ≃40 % at 950 K. In n-type material this fraction starting from 20% at 300 K approaches the saturation value of ≃80 % at 600 K already. This behaviour suggests that the site of implanted boron in silicon is controlled by the Fermi level.     

Boron implanted in silicon: Segregation at angular configurations of the silicon/silicon dioxide oxidation boundary

Based on computer simulation of the physicochemical segregation processes involving dopants implanted into a host material (silicon), the details of boron injection were investigated for four types of angular configurations (direct and inverse kinks and cavities of the “trench” and “square” types) of the “silicon/silicon dioxide” oxidation boundary. A complicated picture of the B distribution inside the Si and SiO₂ regions and at the SiO₂/Si front was obtained and analyzed in general terms.     

Calculation of profile of charge carrier concentration in modulation doped structure with a wide potential well

We investigate the equilibrium state of interacting electron system with Fermi statistics in modulation doped structure with a wide quantum well. The model is formulated for the carrier system with a sufficiently high density, such that the de Broglie wavelength of electrons is smaller than the width of the quantum well. Due to a significant interaction of electrons with electric field of the doped layer, a state with strongly‐inhomogeneous density of electrons is formed. Within the hydrodynamic approach, we set up formalism for calculating the electron density across the width of the potential well. We have obtained the exact solution of the equations, which is expressed in terms of hypergeometric functions. Based on the deduced formulas, we performed numerical computations for the profile of carriers' concentrations in a potential well in the modulation doped Si/SiGe/Si structures.