Effect of implanted Si concentration on the Si nanocrystal size and emitted PL spectrum

Implantation of Si⁺ in excess into SiO₂ followed by annealing produces Si nanocrystals (Si-nc) embedded in the SiO₂ layer, which can emit a strong photoluminescence (PL) signal. Several samples have been characterized by means of ellipsometry and transmission electron microscopy (TEM). For local Si concentrations in excess of ∼2.4 × 10²² Si⁺/cm³, the Si-nc diameter ranges from ∼2 to ∼22 nm in the whole sample, the Si-nc in the middle region of the implanted layer being bigger than those near the surface or the bottom of the layer. The depth distribution of the Si-nc agrees relatively well with the SRIM simulation as well as with the depth distribution of the n and k components of the complex refractive index. For SiO₂ layers thermally grown on a Si wafer, the PL spectrum is modulated by optical interference of the pump laser and of the light emitted by the Si-nc in this layer. The good agreement between the results of the model calculations and experimental measurements indicates that for low and moderate Si concentration in excess (<8 × 10²¹ cm⁻³) the PL light emitters are localized in a layer situated at the same depth as the Si-nc depth distribution. However, for a Si concentration in excess of ∼2.3 × 10²² cm⁻³, the depth distribution of light emitters is narrow and situated mostly in the first half (relative to the surface) of the Si-nc depth distribution. This observation indicates that the recombination of the electron–hole pair at the interfaces could be responsible for the emitted PL spectrum.     

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.     

EPR study of defects produced by MeV Ag ion implantation into silicon

The damage produced by implanting (1 1 1) Si wafers with 4 MeV Ag ions at implantation temperatures of 210, 350 and 400 K has been investigated by electron paramagnetic resonance as a function of implantation fluence in the range 5 × 10¹²–2 × 10¹⁵ Ag cm⁻². For each implantation temperature, at low ion fluences the EPR spectra show the presence of the point defect centres Si-P3 (neutral 4-vacancy) and Si-P6 (di-interstitial) as well the so-called Σ defect complexes. As the implantation fluence is raised the population of P3 centres goes through a maximum while the Σ centre resonance is gradually replaced by the spectrum of the well-known Si-D centre of a-Si. For implantation at 210 K the total Σ+D centre concentration increases linearly with implantation fluence up to the point at which an amorphous layer is formed; however raising the implantation temperature causes the dependence of the Σ+D concentration on implantation fluence to become increasingly sublinear with the result that the production of a given level of damage requires a larger implantation fluence. The results are discussed in the context of a previous study of the implantation damage in the same samples by optical reflectivity depth profiling [Mat. Res. Soc. Symp. Proc. 540 (1999) 31].     

Hydrogen implantation defects in MgO

Deuterium and hydrogen ions with an energy of 15 keV have been implanted in virgin MgO (1 0 0) single crystals and in single crystals containing helium implantation generated microcavities. Doses were varied from 2 × 10¹⁵ to 2 × 10¹⁶ cm⁻². The samples were annealed from room temperature to 950 K. The defects produced by hydrogen and the trapping of hydrogen at the defects were monitored by photon absorption and positron beam analysis. With this novel technique a depth distribution of defects can be determined for implantation depths from 0 to 2000 nm. The technique is very sensitive for vacancy and vacancy clusters, i.e. sites with low electron density. After 950 K annealing microcavities were observed for the 2 × 10¹⁶ cm⁻² dose but not for the 10 times lower dose. During annealing up to 750 K point defects are mobile but the defect clusters remain small and filled with hydrogen. In samples which contain already microcavities, point defects and deuterium from the deuterium irradiation are accumulated by the microcavities.     

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.     

Helium-implanted CLAM steel and evolutionary behavior of defects investigated by positron-annihilation spectroscopy

China Low Activation Martensitic (CLAM) steel has been chosen as the primary candidate structural material for the first wall/blanket for fusion reactor. The excessive helium irradiation induced damage of CLAM steel at high temperatures and the evolution of defects were investigated in this paper. The samples were homogeneously implanted with 1e + 17 ions/cm² and 100 keV of helium at room temperature, 473, 673, and 873 K. Irradiation induced damage of CLAM steel and the annealing behavior of defects were probed by slow positron beam Doppler broadening technique. Helium implantation produced a large number of vacancy-type defects which bound with helium and formed helium–vacancy complexes. Target atoms’ displacement capacity was strengthened with rising irradiation temperatures, so the S parameter increased with increasing irradiation temperatures, and helium–vacancy complexes were main defects after helium implantation at damage layers. Helium bubbles would be unstable and the desorption of helium bubbles would promote the density of defects above 673 K. By analyzing the curves of S–W and annealing tests of irradiated specimen, it suggested that there werenot only one type of defect in damage layers. Though helium–vacancy complexes were primary defects after helium implanted, introducing excessive helium might also generated other point defects or dislocation loops in the material.     

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.     

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.     

Annealing behaviour of defects in helium implanted MgO

Virgin MgO (1 1 0) single crystals have been implanted with 30 keV ³He⁺ ions to a dose of 5 × 10¹⁵ cm^-2. After implantation the samples have been annealed under air for 30 min in a tube oven. The annealing behaviour of the defects and ³He has been monitored by three experimental techniques: positron beam Doppler broadening, neutron depth profiling (NDP) and optical absorption in the UV to near-IR region. The observations in MgO lead to the conclusion that below 1000^oC the vacancy like defects are stabilised by the implanted He atoms. Above this temperature He may dissociate from these small defects, allowing the formation of larger vacancy clusters.     

Influence of tungsten–carbon mixed layer and irradiation defects on deuterium retention behavior in tungsten

The D₂⁺ fluence dependence on deuterium (D) retention was studied to clarify the D retention mechanism in tungsten. The additional D desorption stage was observed around 660 K in the TDS spectrum for a sample implanted with D₂⁺ up to the fluence of 10²³ D⁺ m^?2, which desorption stage was not observed the D₂⁺ implanted sample with the fluence less than 10²² D⁺ m^?2. The TEM observation showed that the highly dense voids were formed in tungsten by D₂⁺ implantation with the fluence of 10²³ D⁺ m^?2, considering that the D would be trapped by voids. To understand the D trapping by voids in C⁺ implanted tungsten, C⁺–D₂⁺ sequential implantation experiments at various C⁺ implantation temperatures were performed. It was found that the amount of D desorbed around 560 K was increased by increasing the C⁺ implantation temperature. The formation of the voids was observed with increasing the C⁺ implantation temperature by TEM, indicating that the increase of D desorption around 560 K was caused by the formation of voids. However, the desorption temperature of D trapped by voids in C⁺ implanted sample was lower than that in D₂⁺ implanted one. TEM observation and XPS measurement indicated that this difference was caused by the increase of void size and/or the presence of implanted carbon.     

Formation of complex Al–N–C layer in aluminium by successive carbon and nitrogen implantation

The results of Auger electron spectroscopy and transmission electron microscopy of the surface layer of aluminium after successive implantation by carbon and nitrogen ions are presented in this work. The energy of implanted ions is 40 keV. The implantation dose varies in the range (3.3–6.5) × 10¹⁷ ions/cm². The findings show that successive implantation leads to the formation of two main layers in aluminium. The first layer is AlNC_x (0 < x < 0.5) layer with violated hcp. AlN structure, where carbon atoms form bonds with nitrogen atoms. The second layer contains disoriented Al₄C₃ precipitates and carbon atoms migrated from the first layer. The mechanism of migration is discussed.     

Measuring the generation lifetime profile modified by MeV H+ ion implantation in silicon

A silicon wedge mask with thickness varying from approximately 5 μm to a few hundred μm has been used for converting the depth distribution of defect concentration induced by 4 MeV H⁺ ion implantation in silicon to a lateral scale on the surface, i.e. the distance from the edge of the wedge mask. Thus, using proper devices fabricated on bulk Si prior to ion implantation, depth profiles of the generation lifetime of minority charge carriers and of the different defect densities can be measured by the transient capacitance method and by Deep Level Transient Spectroscopy (DLTS), respectively. The distribution of lifetime follows well that of the implantation induced vacancies calculated by the TRIM code in the applied dose range (from 1 × 10¹⁰ to 3 × 10¹¹ H⁺/cm²). The correlation between implantation dose and lifetime decrease is also discussed.     

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.     

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.     

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.     

Ion beam mixing in uranium nitride thin films studied by Rutherford Backscattering Spectroscopy

Thickness, composition, concentration depth profile and ion irradiation effects on uranium nitride thin films deposited on fused silica have been investigated by Rutherford Backscattering Spectroscopy (RBS) using 2 MeV He⁺ ions. The films were prepared by reactive DC sputtering at the temperatures of ?200 °C, +25 °C and +300 °C. A perfect 1U:1N stoichiometry with a layer thickness of 660 nm was found for the film deposited at ?200 °C. An increase of the deposition temperature led to an enhancement of surface oxidation and an increase of the thickness of the mixed U–N–Si–O layers at the interface. The sample irradiation by 1 MeV Ar⁺ ion beam with ion fluence of about 1.2–1.7 × 10¹⁶ ions/cm² caused a large change in the layer composition and a large increase of the total film thickness for the films deposited at ?200 °C and at +25 °C, but almost no change in the film thickness was detected for the film deposited at +300 °C. An enhanced mixing effect for this film was obtained after further irradiation with ion fluence of 2.3 × 10¹⁶ ions/cm².     

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.     

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.     

Damage accumulation and annealing in 6H–SiC irradiated with Si+

Damage accumulation and annealing in 6H-silicon carbide (α-SiC) single crystals have been studied in situ using 2.0 MeV He⁺ RBS in a 〈0 0 0 1〉-axial channeling geometry (RBS/C). The damage was induced by 550 keV Si⁺ ion implantation (30° off normal) at a temperature of −110°C, and the damage recovery was investigated by subsequent isochronal annealing (20 min) over the temperature range from −110°C to 900°C. At ion fluences below 7.5 × 10¹³ Si⁺/cm² (0.04 dpa in the damage peak), only point defects appear to be created. Furthermore, the defects on the Si sublattice can be completely recovered by thermal annealing at room temperature (RT), and recovery of defects on the C sublattice is suggested. At higher fluences, amorphization occurs; however, partial damage recovery at RT is still observed, even at a fluence of 6.6 × 10¹⁴ Si⁺/cm² (0.35 dpa in the damage peak) where a buried amorphous layer is produced. At an ion fluence of 6.0 × 10¹⁵ Si⁺/cm² (−90°C), an amorphous layer is created from the surface to a depth of 0.6 μm. Because of recovery processes at the buried crystalline–amorphous interface, the apparent thickness of this amorphous layer decreases slightly (<10%) with increasing temperature over the range from −90°C to 600°C.     

Micro-Raman depth profile investigations of beveled Al+-ion implanted 6H-SiC samples

6H-SiC single crystals were implanted with 450 keV Al⁺-ions to a fluence of 3.4 × 10¹⁵ cm^?2 , and in a separate experiment subjected to multiple Al⁺ implantations with the four energies: 450, 240, 115 and 50 keV and different fluences to obtain rectangular-like depth distributions of Al in SiC. The implantations were performed along [0 0 0 1] channeling and non-channeling (“random”) directions. Subsequently, the samples were annealed for 10 min at 1650 °C in an argon atmosphere. The depth profiles of the implanted Al atoms were obtained by secondary ion mass spectrometry (SIMS). Following implantation and annealing, the samples were beveled by mechanical polishing. Confocal micro-Raman spectroscopic investigations were performed with a 532 nm wavelength laser beam of a 1 μm focus diameter. The technique was used to determine precisely the depth profiles of TO and LO phonon lines intensity in the beveled samples to a depth of about 2000 nm. Micro-Raman spectroscopy was also found to be useful in monitoring very low levels of disorder remaining in the Al⁺ implanted and annealed 6H-SiC samples. The micro-Raman technique combined with sample beveling also made it possible the determination of optical absorption coefficient profiles in implanted subsurface layers.