Light particle irradiation effects in Si nanocrystals

Si nanocrystals, formed by Si ion implantation into SiO₂ layers and subsequent annealing at 1150°C, were irradiated at room temperature either with He⁺ions at energies of 30 or 130 keV, or with 400 keV electrons. Transmission electron microscopy (TEM) and photoluminescence (PL) studies were performed. TEM experiments revealed that the Si nanocrystals were ultimately amorphized (for example at ion doses ∼10¹⁶ He cm⁻²) and could not be recrystallized by annealing up to 775°C. This contrasts with previous results on bulk Si, in which electron- and very light ion-irradiation never led to amorphization. Visible photoluminescence, usually ascribed to quantum-size effects in the Si nanocrystals, was found to decrease and vanish after He⁺ ion doses as low as 3 × 10¹²–3 × 10¹³ He cm⁻² (which produce about 1 displacement per nanocrystal). This PL decrease is due to defect-induced non-radiative recombination centers, possibly situated at the Si nanocrystal/SiO₂ interface, and the pre-irradiation PL is restored by a 600°C anneal.     

Impact of Ni co-implantation on Si nanocrystals formation and luminescence

We investigate the Si nanocrystals (Si-nc) growth and photoluminescence (PL) obtained by Si and Ni co-implantation. After 1 h of annealing at 1000 °C, PL emission between 600 and 1000 nm is observed to be five times higher for samples containing 0.04–0.21 at.% Ni than for samples without Ni. For samples annealed at 1100 °C, a small increase in PL intensity is observed followed by a decrease for Ni contents above 0.3 at.%. Our results are well described by a simple model which assumes that Ni atoms act both as a nucleation center for amorphous Si (a-Si) clusters crystallisation and as a non-radiative recombination site when more than one atom is included in a Si-nc. The nucleation effect is supported by the increase in PL intensity, the increase of PL decay time and the decrease of the a-Si Raman peak intensity at 480 cm^?1 for samples annealed at 1000 °C in presence of ～0.1 at.% Ni. Time-resolved PL shows that Ni mainly affects two emission bands. The first band, around 730 nm, is related to an oxygen surface state, suggesting that Ni enhances oxygen bonding with Si-nc. The second band, around 880 nm, is associated to the crystallisation effect induced by Ni at lower annealing temperature.     

Photoluminescence induced by Si implantation into Si3N4 matrix

Up to the present, photoluminescence (PL) was obtained from near stoichiometric or amorphous Si nitride films (SiN_x) after annealing at high temperatures. As a consequence, the existence of PL bands has been reported in the 400–900 nm range. In the present contribution, we report the first PL results obtained by Si implantation into a stoichiometric 380 nm Si₃N₄ film. The Si excess is obtained by a 170 keV Si implantation at different temperatures with a fluence of Φ = 10¹⁷ Si/cm². Further, we have annealed the samples in a temperature range between 350 and 900 °C in order to form the Si precipitates. PL measurements were done using an Ar laser as an excitation source, and a broad PL band basically centered at 910 nm was obtained. We show that the best annealing condition is obtained at T_a = 475 °C for the samples implanted at 200 °C, with a PL yield 20% higher than the obtained at room temperature implantation. Finally, we have varied the implantation fluence and, consequently, the Si nanocrystals size. However, no variation was observed nor in the position neither in the intensity of the PL band. We concluded that the PL emission is due to radiative states at the matrix and the Si nanocrystals interface, as previously suggested in the literature.     

Low temperature formation of luminescent Si nanocrystals with combined process of excimer UV-light irradiation and RTA

Si ion implantation was widely used to synthesize specimens of SiO₂ containing supersaturated Si and subsequent high temperature annealing induces the formation of embedded luminescent Si nanocrystals. In this work, the potentialities of excimer UV-light (172 nm, 7.2 eV) irradiation and rapid thermal annealing (RTA) to achieve low temperature (below 1000 °C) formation of luminescent Si nanocrystals in SiO₂ have been investigated. The Si ions were introduced at acceleration energy of 180 keV to fluences of 7.5 × 10¹⁶ and 1.5 × 10¹⁷ ions/cm². The implanted samples were subsequently irradiated with an excimer-UV lamp for 2 h. After the process, the samples were rapidly thermal annealed at 1050 °C for 5 min before furnace annealing (FA) at 900 °C. Photoluminescence spectra were measured at various stages at the process. Effective visible photoluminescence is found to be observed even after FA at 900 °C, only for specimens treated with excimer-UV lamp and RTA, prior to a low temperature FA process. Based on our experimental results, we discuss the mechanism for the initial formation process of the luminescent Si nanocrystals in SiO₂, together with the effects with excimer lamp irradiation and RTA process on the luminescence.     

Ohmic contacts on n-type layers formed in GaN/AlGaN/GaN by dual-energy Si ion implantation

Electrical properties of Si-implanted n-type GaN/AlGaN/GaN layers and contact resistances of ohmic electrodes (TiAl) formed on these layers have been examined. Experimental results have clearly shown that ohmic electrodes with a low specific-contact resistance of 1.4 × 10^?7 Ω cm² can be fabricated on the n-type layer having a low sheet resistance of 145 Ω/sq, which has been formed by the dual-energy Si ion implantation (80 keV:1.01 × 10¹⁵/cm² + 30 keV:1.6 × 10¹⁴/cm²) and subsequent annealing at 1200 °C for 2 min using a Si₃N₄ layer as an encapsulant.     

Effects of BF2+ implantation on the strain-relaxation of pseudomorphic metastable Ge0.06Si0.94 alloy layers

Metastable pseudomorphic Ge_0.06Si_0.94 alloy layers grown by molecular beam epitaxy (MBE) on Si (1 0 0) substrates were implanted at room temperature by 70 keV BF₂⁺ ions with three different doses of 3 × 10¹³, 1 × 10¹⁴, and 2.5 × 10¹⁴ cm⁻². The implanted samples were subsequently annealed at 800°C and 900°C for 30 min in a vacuum tube furnace. Observed by MeV ⁴He channeling spectrometry, the sample implanted at a dose of 2.5 × 10¹⁴ BF₂⁺ cm⁻² is amorphized from surface to a depth of about 90 nm among all as-implanted samples. Crystalline degradation and strain-relaxation of post-annealed Ge_0.06Si_0.94 samples become pronounced as the dose increases. Only the samples implanted at 3 × 10¹³ cm⁻² do not visibly degrade nor relax during anneal at 800°C . In the leakage current measurements, no serious leakage is found in most of the samples except for one which is annealed at 800°C for 30 min after implantation to a dose of 2.5 × 10¹⁴ cm⁻². It is concluded that such a low dose of 3 × 10¹³ BF₂⁺ cm⁻² can be doped by implantation to conserve intrinsic strain of the pseudomorphic GeSi, while for high dose regime to meet the strain-relaxation, annealing at high temperatures over 900°C is necessary to prevent serious leakages from occuring near relaxed GeSi/Si interfaces.     

Depth-profiling of implanted 28Si by (α,α) and (α,p0) reactions

Silicon nanocrystals enclosed in thin films (Si quantum dots or Si QDs) are regarded to be the cornerstone of future developments in new memory, photovoltaic and optoelectronic products. One way to synthesize these Si QDs is ion implantation in SiO₂ layers followed by thermal annealing post-treatment.Depth-profiling of these implanted Si ions can be performed by reactions induced by α-particles on ²⁸Si. Indeed, for high incident energy, nuclear levels of ³²S and ³¹P can be reached, and cross-sections for (α,α) and (α,p₀) reactions are more intense. This can help to increase the signal for surface silicon, and therefore make distinguishing more easy between implanted Si and Si coming from the SiO₂, even for low fluences.In this work, (α,α) and (α,p₀) reactions are applied to study depth distributions of 70 keV ²⁸Si⁺ ions implanted in 200 nm SiO₂ layers with fluences of 1 × 10¹⁷ and 2 × 10¹⁷ cm^?2. Analysis is performed above E_R = 3864 keV to take advantage of resonances in both (α,α) and (α,p₀) cross-sections. We show how (α,p₀) reactions can complement results provided by resonant backscattering measurements in this complex case.     

Homogeneously size distributed Ge nanoclusters embedded in SiO2 layers produced by ion beam synthesis

500 nm SiO₂ layers were implanted with 450 keV (F=3 × 10¹⁶ at./cm²) and 230 keV (F=1.8 × 10¹⁶ at./cm²) Ge ions at room temperature to obtain an almost constant Ge concentration of about 2.5 at.% in the insulating layer. Subsequently, the specimens were annealed at temperatures between 500°C and 1200°C for 30 min in a dry N₂ ambient atmosphere. Cross-sectional TEM analysis reveal homogeneously distributed Ge nanoclusters arranged in a broad band within the SiO₂ layer. Their mean cluster size varies between 2.0 and 6.5 nm depending on the annealing conditions. Cluster-free regions are always observed close to the surface of the specimens independent of the annealing process, whereas a narrow Ge nanocluster band appears at the SiO₂/Si interface at high annealing temperatures, e.g. ⩾1000°C. The atomic Ge redistribution due to the annealing treatment was investigated with a scanning TEM energy dispersive X-ray system and Rutherford back scattering (RBS).     

Photoluminescence induced from hot Ge implantation into SiO2

Up to the present, by using the ion implantation technique, photoluminescence (PL) from Ge nanocrystals (Ge NCs) was obtained by room temperature (RT) Ge implantation into a SiO₂ matrix followed by a high temperature anneal. In this way two PL bands were observed, one at 310 nm and the second, with much higher yield at 390 nm. In the present work we have used another experimental approach. We have performed the Si implantation at high temperature (T_i) and then, we have done a higher temperature anneal (T_a) in order to nucleate the Ge NCs. With this aim we have changed T_i between RT and 600 °C. By performing the implantation at T_i = 350 °C we found a PL yield four times higher than the one obtained from the usual RT implantation at the same fluence. Moreover, by changing the implantation fluence between Φ = 0.25 × 10¹⁶ and 2.2 × 10¹⁶ Ge/cm² we observed that Φ = 0.5 × 10¹⁶ Ge/cm² induces a PL yield three times higher as compared to the usual RT implantation fluence. In conclusion, using a hot Ge implantation plus an optimal Ge atomic concentration, we were able to gain more than one order of magnitude in the 390 nm PL yield as compared with previous ion implantation results.     

Ion beam synthesis of SiC by C implantation into SIMOX(1 1 1)

We report the conversion of a 65 nm Si(1 1 1) overlayer of a SIMOX(1 1 1) into 30–45 nm SiC by 40 keV carbon implantation into it. High temperature implantation (600 °C) through a SiO₂ cap, 1250 °C post-implantation annealing under Ar ambient (with 1% of O₂), and etching are the base for the present synthesis. Sequential C implantations (fluence steps of about 5 × 10¹⁶ cm^?2), followed by 1250 °C annealing, has allowed to estimate the minimum C fluence to reach the stoichiometric composition as ～2.3 ×  10¹⁷ cm^?2. Rutherford Backscattering Spectrometry was employed to measure layer composition evolution. A two-sublayers structure is observed in the synthesized SiC, being the superficial one richer in Si. Transmission electron microscopy has shown that a single-step implantation up to the same minimum fluence results in better structural quality. For a much higher C fluence (4 × 10¹⁷ cm^?2), a whole stoichiometric layer is obtained, with reduction of structural quality.     

Strain-driven defect evolution in Sn+ implanted Si/SiGe multilayer structure

We report on secondary defect evolution in a multilayered Si/SiGe structure after 1 MeV Sn⁺-ion implantation to a fluence of 2 × 10¹⁴ cm^?2 followed by thermal annealing in a dry nitrogen atmosphere. Formation of a buried amorphous layer is registered after ion implantation. Thermal treatment leads to formation of dislocation loops in an EOR-defect band, and a mixture of tangle dislocations and “clamshell” defects at the depth of 200–500 nm. In addition, self-assembling of voids in a near-surface SiGe layer structure is observed. The voids are of nanometer size and are preferably located in thin SiGe layers. The results are discussed in terms of the separation of the vacancy and interstitial depth profiles attributed to the preferential forward momentum of recoiling Si atoms. The compressively strained SiGe layers play the role of vacancy accumulator, prevent in-surface diffusion of vacancies and, in this way, result in self-assembling of voids inside compressively strained SiGe layers.     

Er+ implantation in SnO2:SiO2 layers: Structure changes and light emission

Structure changes and light emission behavior in Er⁺ implanted SnO₂:SiO₂ layers are studied, using transmission electron microscopy (TEM), Rutherford backscattering (RBS) and cathodoluminescence (CL). SnO₂:SiO₂ layers of different composition deposited with RF magnetron sputtering on Si wafers were implanted with 200 keV Er⁺ to a fluence of 3 × 10¹⁵ cm^?2 at room temperature. The implanted structures were then annealed at 600–1000 °C for 30 min, resulting in the formation of crystalline SnO₂ nanoclusters. Cross-section TEM revealed a strong reduction of the SnO₂ crystallite size down to several nanometers in the implanted area of the SnO₂:SiO₂ layer as compared to the undoped layer. In addition, a very narrow layer of SnO₂ nanocrystals appears at the SiO₂/Si interface. Several narrow CL emission peaks and wide bands were found which could be related to the decay of SnO₂ free excitons, to oxygen deficiency centers in SiO₂ and to transitions between the energy levels in the Er ions, apparently located at nanoclusters. The mechanisms of nanostructuring as well as the emission process are discussed.     

MeV Si ions bombardment effects on the thermoelectric properties of Co0.1SbxGey thin films

We have grown three different monolayer Co_0.1Sb_xGe_y (x = 2, 4, 11 and y = 15, 7, 15) thin films on silica substrates with varying thickness between 100 and 200 nm using electron beam deposition. The high-energy (in the order of 5 MeV) Si ion bombardments have been performed on samples with varying fluencies of 1 × 10¹², 1 × 10¹³, 1 × 10¹⁴ and 1 × 10¹⁵ ions/cm². The thermopower, electrical and thermal conductivity measurements were carried out before and after the bombardment on samples to calculate the figure of merit, ZT. The Si ions bombardment caused changes on the thermoelectric properties of films. The fluence and temperature dependence of cross plane thermoelectric parameters were also reported. Rutherford backscattering spectrometry (RBS) was used to analyze the elemental composition of the deposited materials and to determine the layer thickness of each film.     

Ion implantation effects in single crystal Si investigated by Raman spectroscopy

A study of the effects of Ar ion implantation on the structural transformation of single crystal Si investigated by confocal Raman spectroscopy is presented. Implantation was performed at 77 K using 150 keV Ar⁺⁺ with fluences ranging from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². The Raman spectra showed a progression from crystalline to highly disordered structure with increasing fluence. The 520 cm^?1 c-Si peak was seen to decrease in intensity, broaden and exhibit spectral shifts indicating an increase in lattice disorder and changes in the residual stress state. In addition, an amorphous Si band first appeared as a shoulder on the 520 cm^?1 peak and then shifted to lower wavenumbers as a single broadband peak with a spectral center of 465 cm^?1. Additionally, the emergence of the a-Si TA phonon band and the decrease of the c-Si 2TA and 2TO phonon bands also indicated the same structural transition from crystalline to highly disordered. The Raman results were compared to those obtained by channeling RBS.     

Nanometer-thick SGOI structures produced by Ge+ ion implantation of SiO2 films and subsequent hydrogen transfer of Si layers

Nanometer-thick silicon-germanium-on-insulator (SGOI) structures have been produced by the implantation of Ge⁺ ions into thermally grown SiO₂ layer and subsequent hydrogen transfer of silicon film on the Ge⁺ ion implanted substrate. The intermediate nanometer-thick Ge layer has been formed as a result of the germanium atom segregation at the Si/SiO₂ bonding interface during annealing at temperatures 800–1100 ^оС. From a thermodynamic analysis of Si/Ge/SiO₂ system, it has been suggested that the growth of the epitaxial Ge layer is provided by the formation of a molten layer at the Si/SiO₂ interface due to the Ge accumulation. The effect of germanium on the hole mobility in modulation-doped heterostructures grown over the 3–20 nm thick SGOI layers was studied. An increase in the Hall hole mobility in SGOI-based structures by a factor of 3–5 was obtained in comparison with that in respective Ge-free SOI structures.     

Formation of germanium nanoparticles in silica glass studied by optical absorption and X-ray absorption fine structure analysis

We examined the relation between the 3.1 eV emission band and local structure for Ge⁺ implanted silica glass by means of photoluminescence, optical and X-ray absorption spectroscopies. In the 2 × 10¹⁵ cm^?2 implanted sample, a new emission band around 2.7 eV was observed, the origin of which was assigned to the B_2α oxygen deficient center and/or small Si clusters in silica. When the Ge⁺ fluence exceeded 2 × 10¹⁶ cm^?2, a sharp and intense 3.1 eV emission band replaced the 2.7 eV band. We found that the intense 3.1 eV PL occurred by the prolonged X-ray irradiation onto the 2 × 10¹⁵ cm^?2 implanted sample. UV–vis absorption and XAFS spectroscopies suggested that the aggregation of atomically dispersed tetravalent (Ge(IV)) atoms into Ge(0) clusters of ～1 nm exhibit strongly correlation with the generation of the 3.1 eV PL. Such nano- and/or subnano-size Ge(0) clusters formed by the X-ray radiation were oxidized and decomposed again to the isolated Ge(IV) atoms, while those produced by the higher Ge⁺ fluence were stable against the exposure to air.     

Early stage of the crystallization in amorphous Fe–Si layers: Formation and growth of metastable α-FeSi2

Crystallization processes of amorphous Fe–Si layers have been investigated using transmission electron microscopy (TEM). Si(1 1 1) substrates were irradiated with 120 keV Fe ions at ?150 °C to a fluence of 1.0 × 10¹⁷ cm². An Fe-rich amorphous layer embedded in an amorphous Si was formed in the as-irradiated sample. Plan-view TEM observations revealed that a part of the amorphous Fe–Si layer crystallized to the metastable α-FeSi₂ after thermal annealing at 350 °C for 8 h. The lattice parameter of c-axis decreased with thermal annealing. It was considered that the change in the lattice parameter originates from the decrease of the Fe occupancy at (0, 0, 1/2) and its equivalent positions in the unit cell of the metastable α-FeSi₂.     

Heteroepitaxial Er0.49Gd0.51Si1.7 layers formed by channeled ion beam synthesis

Epitaxial ternary silicide Er_0.49Gd_0.51Si_1.7 layers with a good crystalline quality (χ_min of Er and Gd is 3.7%) have been formed by 60 keV Er and Gd ion implantation into Si(1 1 1) substrates to a total dose of 1.0 × 10¹⁷/cm² at 450°C using channeled ion beam synthesis (CIBS). The composition, the structure, the strain and the thermal stability of these layers have been studied using energy dispersive spectroscopy (EDS), Rutherford backscattering (RBS)/channeling and X-ray diffraction (XRD). It is shown that the perpendicular and parallel elastic strains of the Er_0.49Gd_0.51Si_1.7 epilayer are e^⊥=−0.46% ± 0.02% and e⁶=+0.73% ± 0.19%. The layer is stable up to 900°C. Annealing at 950°C results in a phase transformation.     

High energy Si ions bombardment effects on the properties of nano-layers of SiO2/SiO2 + Ag

We grew 50 periodic SiO₂/SiO₂ + Ag multi-layers by electron beam deposition technique. The co-deposited SiO₂ + Ag layers are 7.26 nm, SiO₂ buffer layers are 4 nm, and total thickness of film was determined as 563 nm. We measured the thickness of the layers using in situ thickness monitoring during deposition, and optical interferometry afterwards. The concentration and distribution of Ag in SiO₂ were determined using Rutherford backscattering spectrometry (RBS). In order to calculate the dimensionless figure of merit, ZT, the electrical conductivity, thermal conductivity and the Seebeck coefficient of the layered structure were measured at room temperature before and after bombardment with 5 MeV Si ions. The energy of the Si ions was chosen such that the ions are stopped deep inside the silicon substrate and only electronic energy due to ionization is deposited in the layered structure. Optical absorption (OA) spectra were taken in the range 200–900 nm to monitor the Ag nanocluster formation in the thin layers.     

High efficiency gettering of Au in Si(1 1 1) by MeV C implantation

A study of MeV C implantation induced effects on gettering of Au (2.2 × 10¹⁵ cm⁻²), implanted into Si(1 1 1), has been carried out using Rutherford backscattering spectrometry. A 2 h anneal in Ar at 850 °C has been found to result in a gettering efficiency close to 55%. It increases beyond 80% with a further 2 h anneal at 900 °C. The C dose (0.3–1.5 × 10¹⁶ cm⁻²) dependence of Au gettering is also presented and discussed.