Blue photo- and electroluminescence of silicon dioxide layers ion-implanted with group IV elements

The microstructural, optical and electrical properties of Si-, Ge- and Sn-implanted silicon dioxide layers were investigated. It was found, that these layers exhibit strong photoluminescence (PL) around 2.7 eV (Si) and between 3 and 3.2 eV (Ge, Sn) at room temperature (RT), which is accompanied by an UV emission around 4.3 eV. This PL is compared with that of Ar-implanted silicon dioxide and that of Si- and Ge-rich oxide made by rf magnetron sputtering. Based on PL and PL excitation (PLE) spectra we tentatively interpret the blue–violet PL as due to a T₁→S₀ transition of the neutral oxygen vacancy typical for Si-rich SiO₂ and similar Ge- or Sn-related defects in Ge- and Sn-implanted silicon dioxide. The differences between Si, Ge and Sn will be explained by means of the heavy atom effect. For Ge-implanted silicon dioxide layers a strong electroluminescence (EL) well visible with the naked eye and with a power efficiency up to 5×10^-4 was achieved. The EL spectrum correlates very well with the PL one. Whereas the EL intensity shows a linear dependence on the injection current over three orders of magnitude, the shape of the EL spectrum remains unchanged. The I-V dependence exhibiting the typical behavior of Fowler–Nordheim tunneling shows an increase of the breakdown voltage and the tunnel current in comparison to the unimplanted material. Finally, the suitability of Ge-implanted silicon dioxide layers for optoelectronic applications is briefly discussed. Received: 9 March 2000 / Published online: 30 June 2000     

Thermodynamic and kinetic stability with respect to hole trapping. of silanic bonds at the Si–SiO<Subscript>2</Subscript> interface

The thermodynamic and kinetic stabilities of the Si–H bonds at the Si–SiO₂ interface are studied on the basis of high-level quantum-mechanical calculations in the framework of density-functional theory. In the absence of an applied electric field, the silanic bond is shown to be stable with respect to both hole capture from the top of the silicon valence band and electron loss to the bottom of the silicon conduction band, but unstable with respect to hole capture from the top of the SiO₂ conduction band. The positively charged hydrogen does not shift spontaneously to protonate a neighbouring siloxanic bridge unless it contains one adsorbed water molecule at least. The protonated siloxanic site thus formed may restore the original silanic site (via simultaneous electron capture from the conduction band and hydron shift to silicon) but also evolve spontaneously to a hydrogen atom via simple electron capture. Received: 2 November 2001 / Accepted: 6 January 2002 / Published online: 20 March 2002 / Published online: 20 March 2002 RID="*" ID="*"Also at: STMicroelectronics, 20041 Agrate MI, Italy (E-mail: gianfranco.cerofolini@st.com)     

c-axis-textured LiNbO3 thin films on Si (111) substrates

We have successfully prepared highly c-axis-textured LiNbO₃ films on hydrogen-terminated Si (111) substrate using sol-gel spin-coating and rapid thermal annealing. These highly c-axis-textured films were obtained with a preheating at 300 °C for 15 min followed by a rapid thermal annealing at 500–700 °C for 120 s. The c-axis orientation of the LiNbO₃ film is due to a weak effect caused by the 3-fold symmetry match between the film and the Si (111) substrate. The c-axis orientation of LiNbO₃ films is very useful in integrated optics devices and metal–ferroelectric–semiconductor nonvolatile memory applications. Received: 15 September / Accepted: 4 December / Published online: 3 April 2001     

Silicon–germanium nanostructures for on-chip optical interconnects

Silicon–germanium epitaxially grown on silicon in the form of two-dimensional (quantum wells) and three-dimensional (quantum dots) nanostructures exhibits photoluminescence and electroluminescence in the technologically important spectral range of 1.3–1.6 μm. Until recently, the major roadblocks for practical applications of these devices were strong thermal quenching of the luminescence quantum efficiency, and a long carrier radiative lifetime. This paper summarizes recent progress in the understanding of carrier recombination in Si/SiGe nanostructures and presents a potential new route toward CMOS compatible light emitters for on-chip optical interconnects.     

Tunable photoluminescence and electroluminescence of size-controlled silicon nanocrystals in nanocrystalline-Si/SiO2 superlattices

We present photoluminescence and electroluminescence of silicon nanocrystals deposited by plasma-enhanced chemical vapor deposition (PECVD) using nanocrystalline silicon/silicon dioxide (nc-Si/SiO₂) superlattice approach. This approach allows us to tune the nanocrystal emission wavelength by varying the thickness of the Si layers. We fabricate light emitting devices (LEDs) with transparent indium tin oxide (ITO) contacts using these superlattice materials. The current-voltage characteristics of the LEDs are measured and compared to Frenkel-Poole and Fowler-Nordheim models for conduction. The EL properties of the superlattice material are studied, and tuning, similar to that of the PL spectra, is shown for the EL spectra. Finally, we observe the output power and calculate the quantum efficiency and power conversion efficiency for each of the devices.     

纳米硅上的弯曲表面效应及其特征发光

纳米硅结构使能带的带隙展宽,并形成准直接能带带隙结构.弯曲表面上的某些键合可以在带隙中产生局域电子态,计算表明:纳米硅弯曲表面上的Si-N,Si=O和Si-O-Si键合能够分别在带隙中2.02 eV,1.78 eV和2.03 eV附近形成局域态子带,对应了实验光致荧光谱(PL)中605 nm处的LN线、693 nm处的LO1线和604 nm处的LO2线特征发光.特别是,Si-Yb键合在纳米硅弯曲表面上可以将发光波长调控到光通信窗口,在1310 nm到1600 nm范围形成LYb线特征发光.     

Enhanced light emission in Si-nanoclusters arrays

An array of silicon nanoclusters aimed at producing light emission upon injection of electrons and holes from external sources is studied by Monte Carlo simulations. The conditions for obtaining a significant charge accumulation in the emitting nanoclusters are investigated as a function of array geometry and applied electric fields. It is found that if a stationary state, reached for an applied field F₀, is suddenly perturbed by a field F₁≫F₀, a significant increase in electron-hole pairs population can be obtained with respect to the case of a single field of constant intensity F₁, leading to enhanced light emission when the conductivity of the array is above 6×10^-10 [ Ω cm] ^-1. The excess population thus created gets fully recombined on the time scale of milliseconds, suggesting a device that can produce enhanced light emission in the range of kilohertz.     

The demonstration of hybrid n-ZnO nanorod/p-polymer heterojunction light emitting diodes on glass substrates

We report a demonstration of heterojunction light emitting diode (LED) based on a hybrid n-ZnO-nanorod/p-polymer layered structure. The ZnO was grown using the aqueous chemical growth (ACG) on top of the polymer(s) which were deposited on glass. The current–voltage (I–V) behavior of the heterojunctions showed good rectifying diode characteristics. Room-temperature electroluminescence (EL) spectra of the LEDs provided a broad emission band over a wide LED color range (430–650 nm), in which both zinc and oxygen vacancy peaks are clearly detected. We present here luminescent devices based on the use of ZnO-nanorods in combination with two different blended and multi-layered p-type polymers. Electroluminescence of the first batch of devices showed that white bluish strong emission for the presently used polymers is clearly observed. We obtained a turn-on voltage of 3 V and break-down voltage equal to −6 V for PVK-TFB blended device. The corresponding values for the NPD-PFO multilayer device were 4 V and −14 V, respectively. The rectification factors were equal to 3 and 10 for the two devices, respectively. The films and devices processed were characterized by scanning electron microscopy (SEM), DEKTAK 3ST Surface Profile, Semiconductor Parameter Analyzer, photoluminescence (PL), and electroluminescence (EL).     

Transient behavior of the strong violet electroluminescence of Ge-implanted SiO2 layers

Si-based light emitters will be a key element of future optoelectronics. One of the most promising approaches is Ge implantation into thin SiO₂ films on crystalline Si. This system exhibits a strong violet electroluminescence with a power efficiency up to 0.5% [18], but the mechanism of electrical excitation is not yet fully understood. In this paper the electrical excitation of the luminescence centers is investigated by means of electrical and electroluminescence transient measurements. It is found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. Furthermore, the electroluminescence rise and decay time is estimated to be of the order of 100 μs. Received: 26 September 2001 / Published online: 29 November 2001     

Synthesis of crystalline Ge nanoclusters in PE-CVD-deposited SiO2 films

The synthesis of evenly distributed Ge nanoclusters in plasma-enhanced chemical-vapour-deposited (PE-CVD) SiO₂ thin films containing 8 at. % Ge is reported. This is of importance for the application of nanoclusters in semiconductor technology. The average diameter of the Ge nanoclusters can be controlled in the range of 3–6 nm by variation of the annealing parameters. Using a combination of transmission electron microscopy and Raman-scattering spectroscopy, the nanoclusters were shown to be crystalline. However, photoluminescence measurements showed no light emission that could be definitively correlated to the presence of the nanoclusters. PACS 61.82.Rx; 78.67.Bf; 81.07.Bc     

Efficient silicon light emitting diodes made by dislocation engineering

Efficient silicon-based light emitting diodes have been fabricated using the dislocation engineering method. Crucially this technique uses entirely conventional ULSI processes. The devices were fabricated by conventional low-energy boron implantation into silicon substrates followed by high-temperature annealing, and strong silicon band edge luminescence was observed. Dislocation engineering is also shown to reduce the thermal quenching for other material systems. Dislocation engineered β-FeSi₂ and Er light emitting devices were fabricated and room temperature electroluminescence at 1.5 μm was observed in both cases.     

Blue and green-blue-variable light-emitting diodes based on the blend of polymer and organic molecules

Organic light-emitting diodes based on the blend of poly (p-phenylene vinylene) (PPV) derivative and naphthyl-imine–gallium complex have been fabricated by spin-coating method. Blue emission and blue-green variation depending on the ratio of the PPV derivative to the complex and the applied voltage have been observed. The investigation on PL (photoluminescence) and EL (electroluminescence) properties demonstrates that the improvement of the luminescent efficiency is related to the injection balance between holes and electrons, and the color variation is attributed to the variation of the recombination zone. Received: 7 July 1999 / Accepted: 11 October 1999 / Published online: 8 March 2000     

Strong blue photoluminescence from aligned silica nanofibers

Photoluminescence (PL) and infrared spectra of aligned silica nanofibers are investigated. Two striking strong blue luminescence emissions have been found at room temperature. This suggests that the silica nanofibers could be a candidate material for a blue-light emitter. The intensity of the PL emission decreases after annealing, which can be interpreted as the decrease of the oxygen deficiency resulting in the reduction of radiative recombination centers. Infrared spectra provide further evidence of this conclusion, where the enhancement of Si–O absorption is observed in annealed samples. Received: 2 October 2002 / Accepted: 7 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531, E-mail: ldai@vip.sina.com     

Synchrotron investigations of electronic and atomic-structure peculiarities for silicon-oxide films’ surface layers containing silicon nanocrystals

Films obtained using molecular-beam deposition of SiO powder on c-Si (111) substrates for the purpose of SiO2 system formation with silicon nanocrystals were investigated before and after 900–1100°C annealing by photoluminescence, ultrasoft X-ray emission spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy, and X-ray diffraction. The appearance of (111)-oriented luminescent silicon nanoclusters in considerable amounts upon annealing at T = 1000–1100°C is established in the investigated films. An anomalous phenomenon of X-ray absorption quantum yield intensity reversal for the L _2,3 elementary silicon edge is detected. Models for this phenomenon are suggested.     

Design of a nanoscale silicon laser

The recent observation of optical gain from silicon nanocrystals embedded in SiO₂ opens an opportunity to develop a nanoscale silicon-based laser. However, the challenge remains to design and develop a laser architecture using CMOS-compatible materials. In this paper we present two designs for a waveguide laser in which silicon nanocrystals embedded in SiO₂ are used as the optical gain media. One design employs a SiO₂ membrane containing encapsulated Si nanocrystals. Preliminary calculations given here show that a highly resonant laser cavity can be produced in a SiO₂ membrane using sub-wavelength structures. This photonic crystal architecture, used to guide and contain the light, can be combined with a gain medium of optically active Si nanocrystals synthesized in the SiO₂ membrane using ion implantation/thermal annealing to produce a Si-based laser. The laser cavity dimensions can be matched to the near-infrared wavelengths where optical gain has been observed from Si nanocrystals. The second design utilizes silicon nanocrystals embedded in a distributed-feedback laser cavity fabricated in SiO₂. Lasing action over a broad wavelength range centered at ∼770 nm should be possible in both of these configurations. Received: 20 December 2002 / Accepted: 7 January 2003 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-434/982-2037, E-mail: supriya@virginia.edu     

Photoreduction processes and nanocluster formation induced by a CO2 laser on silicate surfaces

The photoreduction processes stimulated either the cw (power density 10⁵–10⁶ W/cm²) or pulsed (pulse energy 3–4 J, pulse duration 200 ns, effective laser spot diameter 1 mm) radiation of a CO₂ laser on the surfaces of fused and crystalline quartz, as well as of natural silicates (nepheline KNa₃[AlSiO₄]₄, rhodonite CaMn₄[Si₅O₁₅], and zircon ZrSiO₄), are studied. The X-ray emission analysis of irradiated surfaces showed that the laser irradiation of these materials leads to the sublimation of silicon oxides and the enrichment of surfaces with constituent metal elements. Laser radiation also stimulates the formation of silicon and metal nanoclusters on irradiated surfaces. The appearance of these nanoclusters is confirmed by both photoluminescence and X-ray emission studies of irradiated surfaces.     

Electromagnetically induced transparency mediated by phonons in strongly coupled exciton–phonon systems

It is shown theoretically that electromagnetically induced transparency (EIT) due to strong exciton–phonon coupling can occur in strongly coupled exciton–phonon systems such as polymers and organic semiconductors and lead to ultra-slow light effects. The results indicate that the strong coupling of excitons and phonons is important, but the exciton– exciton interaction plays a small role in the generation of the EIT. Numerical results for polydiacetylene–toluene sulfonate are also presented. This EIT in a solid-state medium might be utilized for efficient multiwave mixing and quantum nondemolition measurements, as well as for novel acousto-optical devices. Received: 21 August 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. E-mail: zhukadi@yahoo.com     

Influence of the ion synthesis and ion doping regimes on the effect of sensitization of erbium emission by silicon nanoclusters in silicon dioxide films

The photoluminescence spectra of erbium centers in SiO₂ films with ion-synthesized silicon nanoclusters under nonresonant excitation were investigated. Erbium was introduced into thermal SiO₂ films by ion implantation. The dependences of photoluminescence intensity on the dose, the order of ion implantation of Si and Er, the annealing temperature, and additional Ar⁺ and P⁺ ion irradiation regimes, i.e., factors determining the influence of radiation damage and doping on sensitization of erbium luminescence by silicon nanoclusters, were determined. It was found that the sensitization effect and its amplification due to doping with phosphorus are most pronounced under the conditions where nanoclusters are amorphous. The quenching of photoluminescence due to radiation damage in this case manifests itself to a lesser extent than for crystalline nanoclusters. The role of various factors in the observed regularities was discussed in the framework of the existing concepts of the mechanisms of light emission and energy exchange in the system of silicon nanoclusters and erbium centers.     

Local structure of porous silicon studied by means of X-ray emission spectroscopy

2,3 X-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si-structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of n^-- and p⁺-Si-wafers. Whereas for the P-Si processed from p⁺-Si the presence of some amorphous silicon is detected, the X-ray emission spectra of porous Si prepared from n^--Si display a higher content of SiO₂. For spark-processed Si the Si L_2,3 X-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the center of the processed area is close to that of silicon dioxide, and it has an influence on the photoluminescence energy. Specifically, green photoluminescent sp-Si shows a higher degree of oxidation than the blue luminescent specimen. However, the depth of oxidation consistently decreases in areas with weak or no PL. Possible origins of the observed photoluminescence are discussed. Accepted: 6 March 1997     

High-dose carbon implantations into silicon: fundamental studies for new technological tricks

Depending on the implantation temperature, the implantation of carbon ions into silicon at high doses results in the formation of either amorphous SiC_x or crystalline 3C-SiC precipitates. Various aspects of the precipitation behaviour observed, such as the impeded nucleation, the limited growth and the resulting sensitivity to ballistic destruction are attributed to the large interfacial energy between crystalline silicon and 3C-SiC. Periodically arranged amorphous SiC_x nanoclusters, which are formed at lower temperatures, are shown to promote amorphisation by their surrounding stress field and to represent sinks for silicon self-interstitials, which can be activated by annealing at 900 °C. By control of the depth distribution of equally sized, oriented 3C-SiC precipitates formed at higher implantation temperatures, it is possible to establish suitable starting conditions for the formation of buried homogeneous, single-crystalline 3C-SiC layers during a post-implantation anneal. The properties of these ion-beam-synthesised SiC layers are described and attempts to combine them with insulating and metallic layers are reviewed. A survey is given of the emerging applications of ion-beam-synthesised buried SiC layers and microstructures in electronic, optical and micromechanical devices and as large-area SiC pseudosubstrates. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-821/598-3425, E-mail: lindner@physik.uni-augsburg.de