Formation of photoluminescence centers during annealing of SiO2 layers implanted with Ge ions

Photoluminescence (PL), Raman scattering, and the Rutherford backscattering of α-particles were used to study the formation of the centers of radiative-recombination emission in the visible region of the spectrum on annealing of the SiO₂ layers implanted with Ge ions. It was found that the Ge-containing centers were formed in the as-implanted layers, whereas the stages of increase and decrease in the intensities of PL bands were observed following an increase in the annealing temperature to 800°C. The diffusion-related redistribution of Ge atoms was observed only when the annealing temperatures were as high as 1000°C and was accompanied by formation of Ge nanocrystals. However, this did not give rise to intense PL as distinct from the case of Si-enriched SiO₂ layers subjected to the same treatment. It is assumed that, prior to the onset of Ge diffusion, the formation of PL centers occurs via completion of direct bonds between the neighboring excess atoms, which gives rise to the dominant violet PL band (similar to the PL of O vacancies in SiO₂) and a low-intensity long-wavelength emission from various Ge-containing complexes. The subsequent formation of centers of PL with λ_m～570 nm as a result of anneals at temperatures below 800°C is explained by agglomeration of bonded Ge atoms with formation of compact nanocrystalline precipitates. The absence of intense PL following the high-temperature anneals is believed to be caused by irregularities in the interfaces between the formed Ge nanoc-rystals and the SiO₂ matrix.     

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)     

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.     

Modeling Si nanoprecipitate formation in SiO2 layers with excess Si atoms

Computer simulations based on the Monte Carlo method are used to analyze processes leading to the formation of luminescence centers in SiO₂ implanted with Si ions. The simulations, which take place in a two-dimensional space, mimic the growth of silicon nanoprecipitates in layers containing several at.% of excess silicon. It is assumed that percolation clusters made up of neighboring Si atoms form first. As the annealing temperature increases, these clusters grow and compactify into nano-sized inclusions of a well-defined phase. It is shown that a dose dependence arises from an abrupt enhancement of the probability of forming direct Si-Si bonds when the concentration of silicon exceeds ∼1 at. %. Under these conditions, percolation chains and clusters form even before annealing begins. The effect of the temperature of subsequent anneals up to 900 °C is modeled via the well-known temperature dependence of Si diffusion in SiO₂. It is assumed that annealing at moderate temperatures increases the mobility of Si atoms, thereby facilitating percolation and development of clusters due to an increase in the interaction radius. Intrinsic diffusion processes that occur at high temperatures transform branching clusters into nanoprecipitates with well-defined phase boundaries. The dose and temperature intervals for the formation of precipitates obtained from these simulations are in agreement with the experimental intervals of dose and temperatures corresponding to the appearance of and changes in luminescence. Fiz. Tekh. Poluprovodn. 33, 389–394 (April 1999)     

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)     

Use of incoherent light for annealing implanted Si wafers and growing single-crystal Si on SiO2

By using halogen lamps, we have annealed implanted Si wafers and recrystallised deposited poly-Si. A transient anneal was accomplished with a good uniformity on 4 in Si wafers with no redistribution of the dopant profile. By using a shaped spot, we obtained ?100? single-crystal Si, over an area 2 mm by several centimetres, on a SiO2 layer grown on a Si substrate, without seeding.     

Optical absorption studies of Si implanted SiO2

Optical absorption of Si implanted SiO₂ is characterized as a function of implant dose and energy upon annealing in N₂, H₂ and O₂ ambients. Interpretation of optical data yields information regarding the structure of defects due to excess Si. These defects are responsible for the memory effect and enhanced conductivity previously reported for Si implanted SiO₂. A correlation between E-band absorption (Si-Si ‘wrong’ bond defect) intensity and the amount of excess Si was established. Annealing of this band in O₂ is diffusion-limited with a reaction cross-section of 5.10⁻¹⁵ cm². Compressive strain-induced, oxygen diffusivity-retardation was observed. The C-band absorption (relaxed oxygen vacancy defect) observed in this study is unique in its response to heat treatment in N₂ and H₂ since it does not anneal in these ambients. C-band annealing kinetics in O₂ closely parallel those of E-band. B₂-band absorption (unrelaxed oxygen vacancy defect) produced by Si implantation is very similar in its annealing properties to the published data.     

The influence of irradiation and subsequent annealing on Si nanocrystals formed in SiO2 layers

Luminescent Si nanocrystals formed in SiO₂ layers were irradiated with electrons and He⁺ ions with energies of 400 and 25–130 keV, respectively. The effects of irradiation and subsequent annealing at 600–1000°C were studied by the methods of photoluminescence and electron microscopy. After irradiation with low doses (～1 displacement per nanocrystal), it was found that photoluminescence of nanocrystals was quenched but the number of them increased simultaneously. After irradiation with high doses (～10³ displacements per nanocrystal), amorphization was observed, which is not characteristic of bulk Si. The observed phenomena are explained in terms of the generation of point defects and their trapping by Si-SiO₂ interfaces. Photoluminescence of nanocrystals is recovered at annealing temperatures below 800°C; however, an annealing temperature of about 1000°C is required to crystallize the precipitates. An enhancement of photoluminescence observed after annealing is explained by the fact that the intensities of photoluminescence originated from initial nanocrystals and from nanocrystals formed as a result irradiation are summed.     

Incoherent annealing of implanted layers in GaAs

Incoherent light from filament lamps focused by elliptical mirrors has been used to activate implanted layers in GaAs. 4 × 10¹⁴Si⁺cm^-2and 2 × 10¹⁴Zn⁺cm^-2implants were annealed with Si3N4deposited by CVD at 400°C providing a surface protective layer. By taking advantage of the focusing properties of elliptical mirrors, most of the emitted light could be concentrated onto the GaAs to give annealing times × 1 sec. Differential Hall measurements show peak carrier concentrations of 6.5 × 10¹⁸cm^-3and 50% activation for the n+ layers. The Zn implants were completely activated and doped to ∼ 2 × 10¹⁹cm^-3. These results, together with the short annealing times, suggest the present approach to be an attractive alternative to both laser and conventional thermal annealing.     

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.     

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.     

Electrical conduction in metal/implanted SiO2/Si structures

Direct current/voltage relationships for m.o.s. capacitors are measured as function of oxide doping. Uniform P+ doping is obtained by means of multiple ion implantation. A close relationship between oxide defects, conduction and memory switching is then emphasised.     

Characterization of excimer laser annealing of ion implanted Si

An evaluation of a XeCl excimer laser for the laser processing of Si is reported. The annealing quality of ion-implantation damage, as measured by the crystalline perfection of the regrown layer and by p-n junction characteristics, is similar to that obtained with ruby or Nd:YAG lasers. However, the wide energy window between annealing and surface damage, the flat smooth surface obtained after laser irradiation, the capability of providing deep surface melting, and other features, indicate that XeCl and perhaps other excimer lasers is nearly ideal for semiconductor processing.     

A model for the formation of donor centers in silicon layers implanted with erbium and oxygen ions

Formation of donor centers in the course of annealing of layers of single-crystal silicon FZ-Si (grown by the float-zone method) and Cz-Si (grown by the Czochralski method) implanted with Er⁺ and O⁺ ions was simulated. The diffusion-kinetics equations accounting for the formation of erbium-related donor centers of three types were solved numerically. These centers were formed with the involvement of oxygen in the substrate or implanted oxygen and also self-interstitials I produced during annealing of implantation-induced defects; i.e., the Er-I, Er-O, and Er-O-I centers were considered. The results of calculations satisfactorily describe the concentration profiles of donor centers and also the influence of oxygen in the substrate and implanted oxygen on the dependence of the donor-activation coefficient of erbium on the annealing temperature in the range of 600–1200°C.     

Influence of irradiation with swift heavy ions on multilayer Si/SiO2 heterostructures

The influence of Xe ions with an energy of 167 MeV and a dose in the range 10¹²-3 × 10¹³ cm^?2 on heterostructures consisting of six pairs of Si/SiO₂ layers with the thicknesses ～8 and ～10 nm, correspondingly, is studied. As follows from electron microscopy data, the irradiation breaks down the integrity of the layers. At the same time, Raman studies give evidence for the enhancement of scattering in amorphous silicon. In addition, a yellow-orange band inherent to small-size Si clusters released from SiO₂ appears in the photoluminescence spectra. Annealing at 800°C recovers the SiO₂ network, whereas annealing at 1100°C brings about the appearance of a more intense photoluminescence peak at ～780 nm typical of Si nanocrystals. The 780-nm-peak intensity increases, as the irradiation dose is increased. It is thought that irradiation produces nuclei, which promote Si-nanocrystal formation upon subsequent annealing. The processes occur within the tracks due to strong heating because of ionization losses of the ions.     

Magnetic ordering in crystalline Si implanted with Co ions with intermediate doses

Crystalline Si implanted with the 380-keV cobalt ions with the dose Φ = 10¹⁴?10¹⁶ cm^?2 is studied. The method of Rutherford backscattering is used to determine the Si amorphization threshold (Φ = 3 × 10¹⁴ cm^?2). A quasi-resonance anisotropic line with a width of approximately 170 mT is observed at a temperature of T = 78 K in the spectrum of the electron spin resonance of silicon implanted with Co⁺ ions with Φ ≥ 3 × 10¹⁴) cm^?2. A resonance signal of paramagnetic centers in amorphous Si regions (g = 2.0057 and δB = 0.74 mT) is observed against the background of the above line. A quasi-resonance line of the electron spin resonance related to Co atoms and intrinsic Si defects was not observed at T = 300 K.     

The influence of oxygen on the formation of donor centers in silicon layers implanted with erbium and oxygen ions

A model of the formation of donor centers introduced by a combined implantation of Er⁺ and O⁺ ions into silicon with subsequent thermal annealing is developed. These centers are multiparticle erbium-oxygen complexes ErO_n with n≥4. The competing process of formation of electrically inactive oxygen clusters is taken into account. The model makes it possible to describe the dependence of the activation coefficient for the donor centers on the implantation dose of oxygen ions and, also, the effects of the oxygen ion implantation and annealing temperature on the concentration profiles of the donor centers.     

Transformations of porous layers upon high-temperature annealing: Simulation

In this work, the lattice kinetic Monte Carlo model has been used to investigate morphological transformations in porous films of different density in the process of high-temperature annealing. The characteristics of porous films of different initial densities have been compared at different time moments of the sintering process. Layers with a porosity from 20% to 50% having a cubic lattice have been studied. It is shown that closed pores are formed in the film if the porosity is less than 25%; in films with a larger porosity, percolation pores arise. It has been established that the rate of sintering depends on the annealing time nonmonotonically.     

A study of SiO layers on Si using cathodoluminescence spectra

Cathodoluminescence spectra were used to study centers in thin layers of thermally grown SiO₂ on Si. The purpose was to learn more about the centers responsible for the radiation induced space-charge build-up observed in oxide-passivated, planar devices.The spectra were observed to contain three broad peaks; peak A at ～2900Å, peak B at ～ 3700Å and peak C at ～ 4500Å. These spectra were not affected by the conductivity-type, resistivity or crystalline orientation of the silicon, and were little affected by the oxide thickness or growth temperature. Ionizing radiation, destroyed the B peak, greatly enhanced the C peak but had little effect on the A peak. Wet-oxygen-grown oxides were less affected by ionizing radiation than were dry-oxygen-grown oxides. Baking wet-oxygen-grown oxides in dry nitrogen at ～ 1100°C enhanced the B and C peaks.An etch-back experiment was used to determine the distribution of the three luminescence centers. The A center was distributed approximately uniformly throughout the bulk of the oxide. The B center was concentrated near the outer surface of the oxide layer and was undetectable close to the Si/SiO₂ interface. The C center was located principally within < 200 Å of the Si/SiO₂ interface.The A center is believed due to the presence of Na. The B center is associated with the presence of H and may be due to interstitial H. The C center is believed to be the center responsible for the radiation induced space-charge buildup near the Si/SiO₂ interface and to be due to trivalent Si. The C center is apparently the same center which produces the C absorption band (2150Å) in fused silica.     

Redistribution of Al in implanted SiC layers as a result of thermal annealing

The experimental Al concentration profiles formed on implantation of Al into SiC at room temperature with subsequent high-temperature annealing are analyzed. It is shown that, at doses above the amorphization threshold, the profiles exhibit a number of specific features: a shift of the maximum of the distribution, accumulation of dopants at the surface, and formation of box-shaped profiles. To describe quantitatively the redistribution of Al dopants in the SiC layers implanted with high doses, the segregation-diffusion model is suggested for the first time. The model takes into account segregation of dopants between the α and c phases during solid-state epitaxial crystallization followed by diffusion of dopants and their evaporation from the surface The formation of box-shaped Al profiles as a result of short-term thermal annealing is attributed to the origination of highly damaged single-crystal and polycrystalline SiC layers in the recrystallized region, with a high diffusion coefficient of dopants, and to the suppression of the enhanced transient diffusion in the remaining single-crystal part of the implanted layer.