Photoluminescence in silicon implanted with silicon ions at amorphizing doses

Luminescent and structural properties of n-FZ-Si and n-Cz-Si implanted with Si ions at amorphizing doses and annealed at 1100°C in a chlorine-containing atmosphere have been studied. An analysis of proton Rutherford backscattering spectra of implanted samples demonstrated that an amorphous layer is formed, and its position and thickness depend on the implantation dose. An X-ray diffraction analysis revealed that defects of the interstitial type are formed in the samples upon annealing. Photoluminescence spectra measured at 78 K and low excitation levels are dominated by the dislocation-related line D1, which is also observed at 300 K. The peak position of this line, its full width at half-maximum, and intensity depend on the conduction type of Si and implantation dose. As the luminescence excitation power is raised, a continuous band appears in the spectrum. A model is suggested that explains the fundamental aspects of the behavior of the photoluminescence spectra in relation to the experimental conditions.     

Quantum-confinement effect in silicon-on-insulator films implanted with high doses of hydrogen ions

280-nm-thick silicon-on-insulator films are implanted with high doses of hydrogen with the energy 24 keV and the dose 5 × 10¹⁷ cm^?2. Peaks corresponding to optical phonons localized in the silicon nanocrystals 1.9?C2.5 nm in size are observed in the Raman spectra. The fraction of the nanocrystal phase is ??10%. A photoluminescence band with a peak at about 1.62 eV is detected. The intensity of the 1.62 eV band nonmonotonically depends on the measurement temperature in the range from 88 to 300 K. An increase in the radiative recombination intensity at temperatures <150 K is interpreted in the context of a two-level model for the energy of strongly localized electrons and holes. The activation energy of photoluminescence enhancement is 12.4 meV and corresponds to the energy of splitting of the excited state of charge carriers localized in the silicon nanocrystals.     

Short-wavelength photoluminescence of SiO2 layers implanted with high doses of Si+, Ge+, and Ar+ ions

The short-wavelength (400–700 nm) photoluminescence (PL) spectra of SiO₂ layers implanted with Si⁺, Ge⁺, and Ar⁺ ions in the dose range 3.2×10¹⁶–1.2×10¹⁷ cm⁻² are compared. After Ar⁺ implantation an extremely weak luminescence, which vanishes completely after annealing for 30 min at 400 °C or 20 ms at 1050 °C, was observed. After implantation of group-IV elements the luminescence intensities were 1 to 2 orders of magnitude higher, and the luminescence remained not only with annealings but it could also increase. The dose and heating dependences of the luminescence show that it is due to the formation of impurity clusters and this process is more likely to be of a percolation than a diffusion character. For both group-IV impurities an intense blue band and a weaker band in the orange part of the spectrum were observed immediately after implantation. The ratio of the excitation and emission energies of the blue luminescence is characteristic of oxygen vacancies in SiO₂; its properties are determined by the direct interaction of group-IV atoms. On this basis it is believed that the centers of blue PL are chains of Si (or Ge) atoms embedded in SiO₂. The orange luminescence remained after annealings only in the case of Si⁺ implantation. This is attributed directly to the nonphase precipitates of Si in the form of strongly developed nanometer-size clusters. Fiz. Tekh. Poluprovodn. 32, 439–444 (April 1998)     

Laser annealing of GaAs implanted with low doses of selenium ions

GaAs samples were implanted with 100–400 keV, 10¹²–10¹⁴/cm²Se⁺ ions and annealed using undiffused and diffused pulsed ruby-laser beams with the samples held at various temperatures.

The resulting surface and structural defects as observed by various microscopic and spectroscopic methods are related to measured electrical properties of implanted GaAs, and the causes of the observed poor and lack of electrical activation of samples identified.

Laser irradiation of samples (T ≈ 516°C) induces surface damage in the form of regular parallel broken lines with a periodicity of about 0.69 μm, the wavelength of the ruby laser.

A diffused laser beam (0.5 J/cm²) reduces surface vaporisation and eliminates periodic surface structures. The measured electrical properties are still poor.     

Silicon nanocrystal formation upon annealing of SiO2 layers implanted with Si ions

The formation of silicon nanocrystals in SiO₂ layers implanted with Si ions was investigated by Raman scattering, X-ray photoelectron spectroscopy, and photoluminescence. The excess Si concentration was varied between 3 and 14 at. %. It was found that Si clusters are formed immediately after implantation. As the temperature of the subsequent annealing was raised, the segregation of Si accompanied by the formation of Si-Si₄ bonds was enhanced but the scattering by clusters was reduced. This effect is attributed to the transformation of loosely packed clusters into compact, separate-phase nanoscale Si precipitates, with the Raman peak observed at 490 cm^?1 being related to surface scattering. The process of Si segregation was completed at 1000°C. Nevertheless, characteristic nanocrystal photoluminescence was observed only after annealing at 1100°C. Simultaneously, scattering in the range 495–520 cm^?1, typical of nanocrystals, appeared; however, the “surface-related” peak at 490 cm^?1 persisted. It is argued that nanocrystals are composed of an inside region and a surface layer, which is responsible for their increased formation temperature.     

Crystallization induced by thermal annealing with millisecond pulses in silicon-on-insulator films implanted with high doses of hydrogen ions

The crystallization of silicon-on-insulator films, implanted with high doses of hydrogen ions, upon annealing with millisecond pulses is studied. Immediately after hydrogen-ion implantation, the formation of a three-phase structure composed of silicon nanocrystals, amorphous silicon, and hydrogen bubbles is detected. It is shown that the nanocrystalline structure of the films is retained upon pulsed annealing at temperatures of up to ～1000°C. As the temperature of the millisecond annealing is increased, the nanocrystal dimensions increase from 2 to 5 nm and the fraction of the nanocrystalline phase increases to ～70%. From an analysis of the activation energy of crystal phase growth, it is inferred that the process of the crystallization of silicon films with a high (～50 at %) hydrogen content is limited by atomic-hydrogen diffusion.     

Photoluminescence of Si3N4 films implanted with Ge+ and Ar+ ions

The room-temperature photoluminescence emission and excitation spectra of Si₃N₄ films implanted with Ge⁺ and Ar⁺ ions were investigated as a function of the ion dose and temperature of subsequent annealing. It was established that the implantation of bond-forming Ge atoms during annealing right up to temperature T _a=1000 °C stimulates the formation of centers emitting in the green and violet regions of the spectrum. Implantation of inert Ar⁺ ions introduces predominantly nonradiative defect centers. Comparative analysis of the photoluminescence spectra, Rutherford backscattering data, and Raman scattering spectra shows that the radiative recombination is due not to quantum-well effects in Ge nanocrystals but rather recombination at the defects ≡Si-Si≡, ≡Si-Ge≡, and ≡Ge-Ge≡. Fiz. Tekh. Poluprovodn. 33, 559–566 (May 1999)     

Effect of ion dose and annealing mode on photoluminescence from SiO2 implanted with Si ions

This paper discusses the photoluminescence spectra of 500-nm-thick layers of SiO₂ implanted with Si ions at doses of 1.6×10¹⁶, 4×10¹⁶, and 1.6×10¹⁷ cm⁻² and then annealed in the steady-state region (30 min) and pulsed regime (1 s and 20 ms). Structural changes were monitored by high-resolution electron microscopy and Raman scattering. It was found that when the ion dose was decreased from 4×10¹⁶ cm⁻² to 1.6×10¹⁶ cm⁻², generation of centers that luminesce weakly in the visible ceased. Moreover, subsequent anneals no longer led to the formation of silicon nanocrystallites or centers that luminesce strongly in the infrared. Annealing after heavy ion doses affected the photoluminescence spectrum in the following ways, depending on the anneal temperature: growth (up to ∼700 °C), quenching (at 800–900 °C), and the appearance of a very intense photoluminescence band near 820 nm (at >900 °C). The last stage corresponds to the appearance of Si nanocrystallites. The dose dependence is explained by a loss of stability brought on by segregation of Si from SiO₂ and interactions between the excess Si atoms, which form percolation clusters. At low heating levels, the distinctive features of the anneals originate predominantly with the percolation Si clusters; above ∼700 °C these clusters are converted into amorphous Si-phase nanoprecipitates, which emit no photoluminescence. At temperatures above 900 °C the Si nanocrystallites that form emit in a strong luminescence band because of the quantum-well effect. The difference between the rates of percolation and conversion of the clusters into nanoprecipitates allows the precipitation of Si to be controlled by combinations of these annealings. Fiz. Tekh. Poluprovodn. 32, 1371–1377 (November 1998)     

Ion implanted Si MESFET's with high cutoff frequency

A new technology of ion-implanted silicon MESFET's on high-resistivity substrates has been developed to reduce substrate effects. Consequently, the devices show an improved static behavior concerning pinchoff and drain feedback. Static and dynamic performance will be presented, the latter showingf_{max}=14GHz calculated from scattering parameter measurements and a large signal switching time of 60 ps. The transit frequency of the intrinsic device isf_{T} sim 3.9GHz.     

Interaction of mechanical stress with residual defects in implanted Si

After drive-in of arsenic-implanted emitter structures dislocations are found at the edge of the masking window and in the unimplanted surrounding regions while the implanted areas are defect-free. These dislocations originate from a defect structure which exists temporarily in the emitters during drive-in heat treatment. The dislocations are attracted to the window edge and are pushed into the surrounding region by mechanical stresses which have their origin in the intrinsic stress of the masking layer. No penetration of dislocations outside the implanted area is observed when the implantation is done through a thin oxide film. A mechanical stress analysis of film edge induced stresses can account for the observed defect patterns.     

Magnetism in SiC implanted with high doses of Fe and Mn

High concentrations (0.1–5 at.%) of Mn or Fe were introduced into the near-surface region (≤2000 Å) of 6H-SiC substrates by direct implantation at ～300°C. After annealing at temperatures up to 1000°C, the structural properties were examined by transmission electron microscopy (TEM) and selected-area diffraction pattern (SADP) analysis. The magnetic properties were examined by SQUID magnetometry. While the Mn-implanted samples were paramagnetic over the entire dose range investigated, the Fe-implanted material displayed a ferromagnetic contribution present at <175 K for the highest dose conditions. No secondary phases were detected, at least not to the sensitivity of TEM or SADP.     

Modifications of the structure and electrical parameters of the films of amorphous hydrogenated silicon implanted with Si+ ions

The influence of implantation of Si⁺ ions with energies of 30, 60, and 120 keV was studied on the dark conductivity, photoconductivity, hydrogen concentration, microstructure parameter, and special features of the ultrasoft X-ray emission spectra of a-Si:H films that were deposited at T _s =300°C by the dc-MASD and rf-PECVD methods and that differed in initial structural characteristics.     

Electrical properties of silicon layers implanted with ytterbium ions

It is ascertained that implantation of 1-MeV ytterbium ions with a dose of 10¹³ cm^?2 into silicon with subsequent annealing at temperatures of 600–1100°C gives rise to donor centers. The donor-center concentration is higher in the samples implanted additionally with oxygen ions. The results show that at least two types of donor centers are formed; these centers contain either ytterbium or oxygen impurity atoms. The dependence of electron mobility on the concentration of electrically active centers in the silicon layers implanted with the ytterbium rare-earth ions is determined in the concentration range of 7×10¹⁵–10¹⁷ cm^?3.     

Special features of photoluminescence in silicon-on-insulator structures implanted with hydrogen ions

The special features of photoluminescence spectra of silicon-on-insulator structures implanted with hydrogen ions are studied. An increase in the photoluminescence intensity with increasing hydrostatic pressure P during annealing and the formation of narrow periodic photoluminescence peaks in the spectral range from ～500 to 700 nm are revealed for the structures annealed at P > 6 kbar. It is shown that the fine structure of the photoluminescence spectra correlates with the slowing-down of hydrogen effusion from the implanted samples and with the suppression of the formation of hydrogen microbubbles in the surface layer. These processes promote the formation of an optical resonator, with the mirrors formed by the “silicon-on-insulator-air” and “silicon-on-insulator-SiO₂” interfaces and with the optically active layer formed by hydrogen ion implantation and subsequent annealing.     

Photoluminescence in silicon implanted with erbium ions at an elevated temperature

Photoluminescence spectra of n-type silicon upon implantation with erbium ions at 600°C and oxygen ions at room temperature and subsequent annealings at 1100°C in a chlorine-containing atmosphere have been studied. Depending on the annealing duration, photoluminescence spectra at 80 K are dominated by lines of the Er³⁺ ion or dislocation-related luminescence. The short-wavelength shift of the dislocation-related luminescence line observed at this temperature is due to implantation of erbium ions at an elevated temperature. At room temperature, lines of erbium and dislocation-related luminescence are observed in the spectra, but lines of near-band-edge luminescence predominate.     

Monolithic Si nanocrystal/crystalline Si tandem cells involving Si nanocrystals in SiC

Monolithic tandem cells involving a top cell with Si nanocrystals embedded in SiC (Si NC/SiC) and a c‐Si bottom cell have been prepared. Scanning electron microscopy shows that the intended cell architecture is achieved and that it survives the 1100 °C anneal required to form Si NCs. The cells exhibit mean open‐circuit voltages V_oc of 900–950 mV, demonstrating tandem cell functionality, with ≤580 mV arising from the c‐Si bottom cell and ≥320 mV arising from the Si NC/SiC top cell. The cells are successfully connected using a SiC/Si tunnelling recombination junction that results in very little voltage loss. The short‐circuit current densities j_sc are, at 0.8–0.9 mAcm⁻², rather low and found to be limited by current collection in the top cell. However, equivalent circuit simulations demonstrate that in current‐mismatched tandem cells such as the ones studied here, higher j_sc, when accompanied by decreased V_oc, can arise from shunts or breakdown in the limiting cell rather than improved current collection from the limiting cell. This indicates that V_oc is a better optimisation parameter than j_sc for tandem cells where the limiting cell exhibits poor junction characteristics. The high‐temperature‐stable cell architecture developed in this work, coupled with simulations highlighting potential pitfalls in tandem cell analysis, provides a suitable route for optimisation of Si NC layers for photovoltaics on a tandem cell device level. Copyright © 2016 John Wiley & Sons, Ltd.     

Rapid thermal annealing of gallium arsenide implanted with sulfur ions

Sulfur ions were implanted into semi-insulating GaAs. A SiO₂ film was deposited by either of two methods onto the implanted surface. The samples were then subjected to either rapid thermal annealing (using halogen lamps) for 10–12 s at 805°C or to conventional thermal annealing for 30 min at 800°C. The content of GaAs components in the film was determined from the spectra of Rutherford backscattering. The electron-concentration profiles were plotted using the measurements of the capacitance-voltage characteristics. It is shown that sulfur diffuses in two directions, i.e., towards the surface and into the GaAs bulk. The former process is stimulated by vacancies formed near the semiconductor surface during the deposition of SiO₂. The coefficients of the “volume” diffusion of S and of the diffusion of S towards the surface are two orders of magnitude larger upon rapid thermal annealing than upon conventional thermal annealing, with the degree of S activation also being higher.     

The role of nitrogen in the formation of luminescent silicon nanoprecipitates during heat treatment of SiO2 layers implanted with Si+ ions

Twenty-five kiloelectronvolt Si⁺ ions with doses of (1–4)×10¹⁶ cm^?2 and 13-keV N⁺ ions with doses of (0.2–2)×10¹⁶ cm^?2 were implanted into SiO₂ layers, which were then annealed at 900–1100°C to form luminescent silicon nanoprecipitates. The effect of nitrogen on this process was deduced from the behavior of the photoluminescence spectra. It was found, for a certain ratio between the concentrations of implanted silicon and nitrogen, that the photoluminescence intensity increases significantly, and its peak shifts to shorter wavelengths. It is concluded that the number of precipitation nuclei increases owing to the interaction of nitrogen with excess silicon. Eventually, this results in an increase in the number of nanocrystals and in a decrease in their average sizes. In spite of introducing additional precipitation nuclei, the minimal concentrations of excess Si on the order of 10²¹ cm^?3 and heat treatments at temperatures higher than 1000°C were still required for the formation of nanocrystals.     

Diffusion and trapping of implanted hydrogen in a Si/Si:B/Si structure

H implantation in Si/Si:B/Si structures is a promising route to improve the Smart Cut™ process and transfer thin Si layers of reduced roughness and controlled thickness onto regular Si wafers. However, the mechanisms driving this process are unknown and thus difficult to model or optimize. For this reason, we have experimentally studied the redistribution of H which takes place in such structures after implantation and during annealing using SIMS and TEM. We show that the Si:B layer already traps H during implantation and form platelets parallel to the wafer surface. During annealing, the H atoms implanted in the Si regions are slowly transferred toward the Si:B layer where they are trapped on large platelets which grow further during annealing. Routes to optimize this process go through the minimization of H precipitation in the pure Si regions. This can probably be achieved by optimizing the implantation conditions.     

Tsc measurements of coupled levels in ion implanted Si

Thermally Stimulated Current (TSC) measurements have been made on deep states in ion implanted silicon. The nature of the resulting curves and the relationship of the peak to valley transition currents clearly preclude the conventional analysis based on independent emission to the valence band. The results have been analyzed with a model based on coupled levels with interlevel transitions and the fit yields reasonable energy levels and exponential pre-factors. The measurements were made by using shallow n⁺ guard ring diodes implanted with a low dose (3×10¹³ ions/cm²) of 1 MeV B⁺ ions and annealed at 350‡C for one hour. The energy was selected so that the end of the ion track would be in the depletion region of the diodes.