Enhanced luminescence from Si quantum dots/SiO2 multilayers by hydrogen annealing

Si quantum dots/SiO₂ multilayers were prepared by annealing a-Si:H/SiO₂ stacked structures at 1100 °C . Photo- and electro-luminescence band around 750 nm can be observed from Si QDs/SiO₂ multilayers due to the recombination of electron-hole pairs in Si QDs/SiO₂ interfaces. The electro-luminescence intensity was obviously enhanced after post hydrogen annealing at 400 °C. Electron spin resonance measurements were used to characterize the change of the defect states after hydrogen annealing. It is found that there exists a-centers (g value = 2.006), which is related to the Si dangling bonds in Si QDs in our samples. Hydrogen annealing can significantly reduce non-luminescent a-centers and enhance the electro-luminescence intensity consequently.     

Silver nanoclusters formation in ion-exchanged glasses by thermal annealing,UV-laser and X-ray irradiation

The Ag-exchanged commercial soda-lime silicate glasses were treated by three methods: thermal annealing, UV-laser irradiation, and X-ray irradiation, in order to promote the silver nanoclusters formation. Absorption spectrometry and electron spin resonance measurement results indicated that the silver ions transferred to silver atoms after the above three treatments. The silver atoms diffused and then aggregated to become nanoclusters after thermal annealing in air, or after UV-laser irradiation. However, X-ray irradiation, which induced defects and reduction of Ag⁰ atoms, would not promote the silver nanocluster formation. After annealing at 600 °C for 45 h, the spherical nanoclusters with a diameter of 3–8 nm were formed. The nanoclusters with a diameter of about 2 nm were formed after 30 min UV-laser irradiation without subsequent heating. The surface plasmon resonance peak position of silver nanoclusters changed from 411 nm after thermal annealing to 425 nm after UV-laser irradiation. The peak position shift was due to the nanoclusters size difference.     

Variation of the defect density in a-Si:H and μc-Si:H based solar cells with 2 MeV electron bombardment

Improvement of the performance of solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon requires understanding of the role of the deep defects – dangling bonds – in the bulk of the intrinsic a-Si:H or μc-Si:H absorber layers. A straightforward way to understand how these defects may affect the performance of the cells is to investigate changes in the device performance upon variation in the defect density.In the present work solar cells with a-Si:H and μc-Si:H absorber layers were exposed to 2 MeV electron bombardment. The performance of the cells after various bombardment doses and annealing steps was evaluated in view of the changes in the defect density of intrinsic layers, measured with ESR on nominally identical absorber layers irradiated in parallel with the cells.The defect density was varied over a range of 2 orders of magnitude. In the solar cells a strong degradation of performance is observed upon irradiation with the biggest effect on the short circuit current density J_SC for both types of absorber layers. In most cases both V_OC and J_SC recover after the final annealing step (at 160 °C) for both types of cells.     

Luminescence decay in hydrogenated amorphous silicon and silicon nanostructures

The stretched exponential luminescence decay observed at temperatures lower than 20 K transits to the power law decay due to the electron-hopping at localized band tail states near 60 K in the hydrogenated amorphous silicon (a-Si:H). The luminescence decay at 4.2 K in a-Si:H is quite similar to that of Si-nanoparticles in the porous Si (p-Si). It is explained from the comparison with p-Si that the slow luminescence of the life time of ~ 1 ms is due to the recombination of excitonic electron–hole pairs at the spin triplet state quantum-confined in the hydrogen-free Si nanostructure in a-Si:H. The fast luminescence of the life time of ~ 1 μs is due to the recombination of the pairs at the spin-singlet state and the life time is explained as due to the indirect optical transition.     

Effect of vacuum annealing on charge transport and trapping in a-Si1 − xCx:H/c-Si heterostructures

The processes of charge transport and trapping in amorphous Si_1 ? xC_x:H films deposited on crystalline p-type Si wafers and annealed in vacuum in the temperature range 300–650 °C have been evaluated. Current–voltage (I–V), capacitance–voltage (C–V) and admittance–temperature (G–T) characteristics were measured in the temperature range 100–350 K. The spectrum of thermal effusion of hydrogen was measured from room temperature up to 1000 °C.C–V characteristics indicate a slight increase of the dielectric constant k and a large hysteresis after annealing at 450 °C. The hysteresis is believed to be associated with mobile hydrogen effusion from the a-SiC:H film, and it is not seen after a 650 °C anneal. From I–V data the maximum rectification ratio is observed after annealing at 450 °C. Variable-range hopping (VRH) conduction at the Fermi level is found to dominate the forward current of the as-deposited structure. After annealing at 450 °C the forward current can be described by space-charge limited (SCL) mechanisms with trapping at shallow levels with energy of about 0.12 eV. After annealing at 650 °C the process of VRH conduction appears again, but the density of hopping sites is much higher than in the as-grown sample. From admittance spectra, the energy position of respective traps in a-SiC:H is at (E_V + 0.45) eV for as-deposited material and it decreases slightly after vacuum annealing. On the basis of these results, an energy band diagram of the a-Si_1 ? xC_x:H/p-Si structure annealed at 450 °C is proposed.     

Low energy shifted photoluminescence of Er3+ incorporated in amorphous hydrogenated silicon–germanium alloys

Low energy shifted photoluminescence from isolated erbium ions incorporated into a-SiGe:H thin films is reported. The Er³⁺ are thermally diffused from an a-SiGe:H:Er layer to a-SiGe:H subsequently grown, both by magnetron sputtering. The photoluminescence observed is associated with transitions produced by erbium emission centers activated by the oxidation in a 1 h annealing process in air at 250 °C. The resultant Er³⁺ concentration observed from the a-SiGe:H is affected by the hydrogen concentration already present in the layer. It is observed that at higher hydrogen concentrations in a-SiGe:H the resultant amount of diffused Er³⁺ decreases. As a consequence of the resultant smaller density of erbium ions, the probability of having isolated Er³⁺ ions increases. In this last regime, a correlation with stronger photoluminescence is observed.     

Light-induced annealing of hole trap states: A new aspect of light-induced changes in hydrogenated amorphous silicon

Details of light-induced annealing of hole trap state in undoped hydrogenated amorphous silicon (a-Si:H) have been studied; it has been found that prolonged illumination significantly reduces the density of hole trap states in the energy range deeper than 0.5 eV, and subsequent thermal annealing increases the density of hole trap states and restored the sample to the initial state before the illumination. We can speculate, from the experimental results and discussion in this work, that defect conversion processes are taking place during the long exposure to light; Si dangling bonds are generated from the precursors or latent sites which manifested as hole trap states located between 0.5 and 0.7 eV from the top of the valence band.     

Optical properties of microcrystalline 3C–SiC:H films measured by resonant photothermal bending spectroscopy

Optical absorption coefficient spectra of hydrogenated microcrystalline cubic silicon carbide (μc-3C–SiC:H) films prepared by Hot-Wire CVD method have been estimated for the first time by resonant photothermal bending spectroscopy (resonant-PBS). The optical bandgap energy and its temperature coefficient of μc-3C–SiC:H film is found to be about 2.2 eV and 2.3 × 10⁻⁴ eV K⁻¹, respectively. The absorption coefficient spectra of localized states, which are related to grain boundaries, do not change by exposure of air and thermal annealing. The localized state of μc-3C–SiC:H has different properties for impurity incorporation compared with that of hydrogenated microcrystalline silicon (μc-Si:H) film.     

Determination of the defect density in thin film amorphous and microcrystalline silicon from ESR measurements: The influence of the sample preparation procedure

Accurate evaluation of the defect density (N_D) is of high relevance for the optimization of thin film silicon. The spin density (N_S) measured in ESR experiments is often used as a measure for the density of deep defects in the material, assuming that all defects are in a paramagnetic charge state. However, exposure to air, water, or acid during ESR sample preparation can potentially change the N_S in a sample and lead to misinterpretation of N_D. We have investigated how the preparation procedures of a Si thin film ESR sample may affect the properties of its ESR spectrum. Samples of different structural composition from highly crystalline μc-Si:H to a-Si:H deposited by PECVD on Mo-foil, Al-foil and ZnO:Al were studied for different states of exposure to ambient conditions and annealing. N_S measured directly after sample preparation and after air exposure was found to be higher than N_S measured in the annealed state. Particularly in highly crystalline material this discrepancy may reach one order of magnitude. On the other hand in a-Si:H and medium crystalline μc-Si:H relevant for applications, the difference in N_S between air-exposed and annealed conditions is smaller. ESR measurements performed at 40 K suggest that atmospheric exposure leads to charging of the defect states, which in turn influences the evaluated spin density.     

Electron spin resonance study of hydrogenated microcrystalline silicon–germanium alloy thin films

Paramagnetic defects of undoped hydrogenated microcrystalline silicon–germanium alloys (μc-Si_1?xGe_x:H) grown by low temperature (200 °C) plasma-enhanced chemical vapor desposition (PECVD) have been measured by electron spin resonance (ESR) and compared with those of hydrogenated amorphous silicon–germanium (a-Si_1?xGe_x:H). The spin density of μc-Si_1?xGe_x:H increases with Ge content and shows a broad maximum of ～10¹⁷ cm^?3 at x ～ 0.5, which reasonably accounts for the decreased photoconductivity. While the Ge dangling bond defects prevail in a-Si_1?xGe_x:H for Ge-rich compositions, we detected no ESR signal in μc-Si_1?xGe_x:H for x > 0.75 where an electrical change occurs from weak n- to strong p-type conduction. These results indicate that dangling bonds are charged in large densities due to the presence of the acceptor-like states in undoped μc-Si_1?xGe_x:H.     

Electron beam poling in amorphous Ge-doped H:SiO2 films

Amorphous Ge-doped H:SiO₂ films on silica, deposited by matrix-distributed electron cyclotron resonance – plasma enhanced chemical vapor deposition, were irradiated with an electron beam while varying the dose. Using the Maker fringe method, second-harmonic generation was measured in the irradiated regions of the films. With a current of 5 nA, and an acceleration voltage of 25 kV for 25 s, a Ge-doped H:SiO₂ film (3.8 at.% Ge) showed a maximum second-order nonlinearity of d₃₃ = 0.0005 pm/V. In contrast, a H:SiO₂ film with a smaller Ge content (1.0 at.% Ge), showed a large SHG: d₃₃ = 0.06 pm/V when irradiated for 15 s. The second-harmonic generation in the films is caused by a frozen-in electric field induced by charge implantation from the electron beam. The strength of the electric field is determined by two conditions: the trapping centers (numbers, depth) and the remaining conductivity under large electric field.     

Effect of annealing temperature on the optical and electrical properties of aluminum doped ZnO films

Aluminum doped ZnO thin films were successfully deposited on the silicon substrates by spin coating method. The effects of an annealing temperature on electrical and optical properties were investigated for 1.5 at.% of aluminum. Refractive index profile has been obtained for the film annealed at 350 °C using ellipsometry and it has shown minimum refractive index of 1.95 and maximum value of 2.1. Thickness profile shows quite good uniformity of the film having minimum thickness value of 30.1 nm and maximum value of 34.5 nm. Maximum conductivity value obtained was 4.63 Ω^?1-cm^?1 for the film annealed at 350 °C. Maximum carrier density of 2.20 × 10¹⁷ cm^?3 was deduced from the Hall measurement and Fourier transform infrared spectroscopy clearly reveals major peak at 407 cm^?1 in the spectra associated with the ZnO bond.     

Thermal annealing of a-Si/Au superlattice thin films

The superlattice films, which consist of amorphous silicon (a-Si) and amorphous gold (Au), were prepared by ultra-high vacuum evaporation system. The first layer was grown a-Si with a thickness of 4.2 nm and the second layer was grown Au with a thickness of 0.8 nm. Thermal annealing was performed at 473, 673, and 873 K, respectively. The structural properties of the films were investigated using transmission electron microscope (TEM), X-ray diffraction (XRD), and Raman scattering spectroscopy. The electrical property was assessed by the temperature dependence of electrical conductivity. A crystallization of Si and a forming of Au nanoparticles were observed in all of the annealing films. The crystalline volume fraction reached 70% by annealing time for 15 min. An average diameter of the Au nanoparticles embedded in Si matrix also increased with increasing the annealing temperature. At annealing temperature above 873 K, Au atoms migrated toward the film surface. It was observed that the electrical conductivity changed in several temperatures.     

Influence of the amorphous/crystalline silicon heterostructure properties on planar conductance measurements

We report a quasi-analytical calculation describing the heterojunction between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) at equilibrium. It has been developed and used to determine the carrier sheet density in the strongly inverted layer at the a-Si:H/ c-Si interface. The model assumes an exponential band tail for the defect distribution in a-Si:H. The effects of the different parameters involved in the calculation are investigated in detail, such as the Fermi level position in a-Si:H, the density of states and the band offsets. The calculation was used to interpret temperature dependent planar conductance measurements carried out on (n) a-Si:H/ (p) c-Si and (p) a-Si:H/(n) c-Si structures, which allowed us to confirm a previous evaluation of the conduction band offset, ?E_C = 0.18 ± 0.05 eV, and to evaluate the valence band offset: ?E_V = 0.36 ± 0.05 eV at the a-Si:H/ c-Si heterojunction. The results are placed in the frame of recent publications.     

Improving the performance of amorphous and crystalline silicon heterojunction solar cells by monitoring surface passivation

The influence of thermal annealing on the crystalline silicon surface passivating properties of selected amorphous silicon containing layer stacks (including intrinsic and doped films), as well as the correlation with silicon heterojunction solar cell performance has been investigated. All samples have been isochronally annealed for 1 h in an N₂ ambient at temperatures between 150 °C and 300 °C in incremental steps of 15 °C. For intrinsic films and intrinsic/n-type stacks, an improvement in passivation quality is observed up to 255 °C and 270 °C, respectively, and a deterioration at higher temperatures. For intrinsic/n-type a-Si:H layer stacks, a maximum minority carrier lifetime of 13.3 ms at an injection level of 10¹⁵ cm^? 3 has been measured. In contrast, for intrinsic/p-type a-Si:H layer stacks, a deterioration in passivation is observed upon annealing over the whole temperature range. Comparing the lifetime values and trends for the different layer stacks to the performance of the corresponding cells, it is inferred that the intrinsic/p-layer stack is limiting device performance. Furthermore, thermal annealing of p-type layers should be avoided entirely. We therefore propose an adapted processing sequence, leading to a substantial improvement in efficiency to 16.7%, well above the efficiency of 15.8% obtained with the ‘standard’ processing sequence.     

Insights into effects of annealing on microstructure from SnO2 thin films prepared by pulsed delivery

Tin dioxide thin films were prepared by pulsed laser deposition techniques on clean glass substrates, and the thin films were then annealed for 30 min from 50 to 550 °C with a step of 50 °C, respectively. The influence of the annealing temperature on the microstructural and morphological properties of the tin dioxide thin films was investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. The experimental results showed that the amorphous microstructure almost transformed into a polycrystalline tin dioxide phase exhibiting a preferred orientation related to the (1 1 0), (1 0 1) and (2 1 1) crystal planes with increased temperatures. The thin film annealed at 200 °C demonstrated the best crystalline properties, viz. optimum growth conditions. However, the thin film annealed at 100 °C revealed the minimum average root-mean-square roughness of 20.6 nm with average grain size of 26.6 nm. These findings indicate that the annealing temperature is very important parameter to determining the thin film quality, which involves the phase formation, microstructure and preferred orientation of the thin films.     

Selection of post-growth treatment parameters for atomic layer deposition of structurally disordered TiO2 thin films

Routes to atomic layer-deposited TiO₂ films with decreased leakage have been studied by using electrical characterization techniques. The combination of post-deposition annealing parameters, time and temperature, which provides measurable aluminum–titanium oxide–silicon structures – i.e., having capacitance–voltage curves which show accumulation behavior – are 625 °C, 10 min for p-type substrates, and 550 °C, 10 min for n-type substrates. The best annealing conditions for p-type substrates are 625 °C with the length extended to 30 min, which produces an interfacial state density of about 5–6 × 10¹¹ cm^?2 eV^?1, and disordered-induced gap state density below our experimental limits. We have also proved that a post-deposition annealing must be applied to TiO₂/HfO₂ and HfO₂/TiO₂/HfO₂ stacked structures to obtain adequate measurability conditions.     

Cross-sectional TEM study on Ni-mediated crystallization of amorphous silicon

Structural characteristics of polycrystalline silicon (poly-Si) made by Ni-mediated crystallization of amorphous silicon (a-Si) were investigated by cross-sectional transmission electron transmission (XTEM) according to various a-Si thickness. The Ni area density of ∼10¹⁴ cm⁻² was deposited onto a-Si and it was annealed at 500 °C in the presence of an electric field of 10 V/cm. It is found that NiSi₂ precipitates form at the top and bottom interfaces of a-Si during annealing. After reaching its critical size the crystallization proceeds from the top and bottom interfaces. The growth of needle-like Si crystallites has been seen, showing a migration of NiSi₂ precipitates through the a-Si network. 1700 nm thick a-Si can be crystallized within 30 min which is longer than that (10 min) of 50 nm thick a-Si. However, the quality of 50 nm thick poly-Si is better than that of 300 nm or 1700 nm thick poly-Si.     

Electronic states in a-Si:H/c-Si heterostructures

We have investigated PECVD-deposited ultrathin intrinsic a-Si:H layers on c-Si substrates using UV-excited photoemission spectroscopy (hν = 4–8 eV) and surface photovoltage measurements. For samples deposited at 230 °C, the Urbach energy is minimal, the Fermi level closest to midgap and the interface recombination velocity has a minimum. The a-Si:H/c-Si interface density of states is comparable to that of thermally oxidized silicon interfaces. However, the measured a-Si:H dangling bond densities are generally higher than in thick films and not correlated with the Urbach energy. This is ascribed to additional disorder induced by the proximity of the a-Si:H/c-Si interface and H-rich growth in the film/substrate interface region.     

The Eγ′ center as a probe of structural properties of nanometer-sized silica particles

A Q-band electron spin resonance (ESR) study is reported of E′ type point defects observed in ～7 nm-sized fumed silica nanoparticles following 10-eV irradiation to photodissociate H from passivated defects. In a comparative study with bulk silica (suprasil), the E′ center is used as an atomic probe to get more in depth information on the network structure of the nm-sized particles. The nanoparticles were brought into contact with ‘bulk’ Si/SiO₂ entities at an elevated temperature in vacuum (T_an = 1105 °C), i.e., the presence of an Si/SiO₂ interface. As a result of this post manufacture treatment, the E′ density increased drastically (>order of magnitude), enabling us to resolve hyperfine (hf) structure of the E′ centers located in the core region of the nanoparticles. Two doublet structures are observed, one each assigned to O₂Si–H entities and the primary ²⁹Si hf structure of the E′ centers. Analysis of these hf spectra reveals interesting information on the network structure of the core region of the nanoparticles: (1) Fumed silica is found to contain relatively less hydrogen than suprasil. (2) An increased ²⁹Si hf splitting (439 ± 2 G) is observed compared to bulk silica (418 ± 2 G), indicating that the E′ centers located in the core of the nanoparticles exhibit on average a slightly more pyramidal defect structure, and moreover, providing evidence that the fumed silica particles are densified compared to standard bulk silica, possibly originating from the presence of more low-membered rings (n < 5) in the nm-sized silica network.