We report a principle that allows writing visible light emitting semiconductor patterns of arbitrary shape down to the sub-micrometer scale. We demonstrate that porous semiconductor growth can be electrochemically initiated preferentially at surface defects created in an n-type substrate by Si++ focused ion beam bombardment. For n-type material in the dark, the electrochemical pore formation potential (Schottky barrier breakdown voltage) is significantly lower at the implanted locations than for an unimplanted surface. This difference in the threshold voltages is exploited to achieve the selectivity of the pore formation process. Visible light emitting patterns of porous Si and GaAs have been created in this way. At present, the size of the structures is limited only by the diameter of the writing ion beam, and pattern diameters in the 50–200-nm range are possible.

The optical properties of oxidized free-standing porous silicon films excited by a cw laser have been investigated. It is found that samples oxidized at 800–950 °C show a strongly superlinear light emission at an excitation intensity of ∼10 W/cm2. This emission has a peak at 900–1100 nm and shows a blueshift as the oxidation temperature is increased. These samples also show a very large induced absorption, where the transmittance is found to decrease reversibly by ≤99.7 %.The induced absorption increases linearly with increasing pump laser intensity. Both the superlinear emission and the large induced absorption are quenched when the samples are attached to materials with a higher thermal conductivity, suggesting that laser-induced thermal effects are responsible for these phenomena.

External quantum efficiencies (EQEs) of electroluminescent devices based on porous silicon (PS) reported to date are still below the minimum requirements for practical applications such as display devices (1 %) and optical interconnection (10 %). Post-anodization anodic oxidation of PS to enhance the EQE of electroluminescence from devices based on a thin transparent indium tin oxide contact mounted on either porosified n+-type silicon or p+n+-type silicon has been investigated. Enhancement of EQE by more than 2 orders of magnitude has been achieved on our devices. CW EQE of 0.51% has been obtained by using a single anodically oxidized n+-type porous layer. The device based on the p+n+ substrate yielded a CW EQE of 1.1 %, with a power efficiency of 0.08%. It is the first time that EQE greater than 1% is obtained. Furthermore, anodically oxidized devices show better stability than non-oxidized devices. The anodic oxidation proceeds in such a way that it mainly decreases the size of nonconfined silicon in PS. The dramatic enhancement in EQE can therefore be explained by preferential reduction of leakage carrier flow through non-confined silicon.

Previously we reported on integrated porous silicon (PSi) based LEDs with local bipolar drivers that would have possible applications in an active-matrix configuration for display or optical signal transmission [1]. We now report further progress in device engineering and integration that has enabled the fabrication of improved light-emitting silicon devices based on oxide-passivated nanocrystalline silicon (OPNSi) that can be integrated with standard CMOS technology. Design of experiments methodologv was used to direct device engineering experiments, providing a better understanding of how process parameters influence the resulting device performance. It was discovered that the formation of stacking fault defects in the LED region prior to anodization has a predominant effect on both carrier transport and electroluminescence (EL) capabilities of the devices. Process development work has resulted in the fabrication of OPNSi LEDs that offer many of the required attributes of a useful silicon light-emitter. The devices exhibit EL at a bias < 5V, diode-like rectifying I-V characteristics, good stability under DC bias, and uniform emission over the LED contact area. These LEDs have been combined with a new fabrication technique which enables the formation of electrically isolated integrated LEDs in the single-crystal substrate. Each diode structure is formed in its own individual well, enabling junction-isolated devices without a common substrate electrode. Such a process is required to realize integrated LEDs which can be individually addressed in an X-Y array configuration using full-rail voltage modulation. Details of the LED characteristics and device integration will be discussed.

We spatially isolate and detect the luminescence from individual porous Si nanoparticles at room temperature. Our experiments show a variety of phenomena not previously observed in the emission from porous Si including a distribution of emission wavelengths, resolved vibronic structure, random spectral wandering, luminescence intermittency and irreversible photobleaching. Our results indicate that the emission from porous Si nanoparticles originates from excitons in quantum confined Si, strongly influenced by the surface passivating layer of the Si nanocrystal.

Time resolved photoluminescence (PL) measurements are performed on oxidized and fresh porous silicon at room temperature. Comparing the evolution of the nanosecond time delayed PL in both cases, a new feature of the PL spectra is identified: the fast-red band, present as well in fresh or aged samples. The nonlinear excitation intensity dependence of this component is described by a simple model where, the Auger effect inside isolated silicon nanocrystallites plays the dominant role.

Phonon-assisted and zero-phonon radiative transitions in nanoscale silicon quantum dots are studied using a new approach which combines a full calculation of the confined electronic eigenstates and vibration modes. We predict that the confinement, combined with the indirect bandgap of bulk silicon, must have several important consequences on the luminescence of a single silicon dot: i) a large broadening of the peaks, in the range of 10s of meV for a 3 nm dot, in spite of the atomic-like electronic structure of the dot ii) a great sensitivity of the spectrum to the size and the shape of the dot. We obtain that phonon-assisted transitions always dominate, even for size below 2 nm. Finally, we show that the radiative recombination in presence of an oxygen related surface defect (Si=O) is also assisted by optical phonons.

Hemispherical silicon quantum dots (QDs) have been self-assembled with an areal density as high as ~2−1011 cm−2 on SiO2/Si(100) and quartz substrates by controlling the early states of low pressure chemical vapor deposition (LPCVD) of pure silane. It is found that, for the thermally-oxidized Si QDs, when the mean Si dot height is decreased from 6.3 nm to 1~2 nm, the photoluminescence (PL) peak energy is increased from 1.2 to 1.4 eV at room temperature while the optical absorption edge determined by photothermal deflection spectroscopy is shifted from 1.9 to 2.5 eV. In addition to the observed Stokes shift as large as 0.7−1.1 eV, a weak temperature dependence of the broad luminescence band and non-exponential luminescence decay with a mean life time of sub-msec even at room temperature suggest that localized, radiative recombination centers existing presumably in the SiO2/Si dot interface are responsible for the efficient PL from the Si QDs. From the change in room temperature PL by SiO2 thinning and removal in a dilute HF solution, it is demonstrated that the surface passivation of Si QDs plays an important role for the efficient light emission at room temperature.

Utilizing laser ablation of Si targets, nanoparticles can be cleanly formed in rare gas. In order to fabricate nanoparticles with well-defined structures such as those whose surfaces are chemically modified, it is important to investigate the formation process of the nanoparticles. We have developed a decomposition method for measuring time-resolved spatial distributions of nanoparticles in rare gas. Applying this method, we have investigated formation processes of silicon nanoparticles in 2-Torr argon gas. The nanoparticles are found to grow from 300 Ais to 1 ms after the ablation.

Correlation between defect structures and light emission from Si-nanocrystal doped SiO2 films has been studied using electron spin resonance ( ESR ) and photoluminescence ( PL ) methods. The ESR analysis revealed the presence of three kinds of ESR centers in the film after annealing at above 900 °C in argon ( Ar ) atmosphere, i.e. Si dangling bond in amorphous Si cluster ( a-center: g=2.006 ), Si dangling bond at Si-nanocrystal/SiO2 interface ( Pb-center: g=2.003 ) and conduction electrons in Si-nanocrystal ( Pce-center: g=1.998 ). Moreover, visible light emission was observed in the annealed sample from the PL measurement. Both the PL intensity and the ESR signal intensity of the Pce-center were increased with an increase of annealing temperature. These results indicate that the Pce-center is strongly associated with the emission center.

Structural changes in amorphous silicon layers have been investigated as a process route to obtain light emitting silicon nanostructures. The optical emission of the layers was studied by cathodoluminescence (CL). Under 1013 -1014 boron ions/cm2 implantation, nanosized crystalline structures grow in the amorphous matrix. A dominant emission band, centered at 400 nm, appears in the catodoluminescence spectrum of the low dose implanted film, while spectra with a 400 nm intense band and 480–500 nm and 650 weak bands are characteristic of higher dose implanted and anodized layers. The structural changes are correlated with the emission properties in the 400–650 nm range.

The present work is concerned with Er-doped oxidized porous silicon (PS). The characteristic feature of the work is that PS doping has been realized by an electrochemical procedure followed by a high temperature treatment. 5-μm thick PS layers were formed on p-type Si of 0.3-Ohm-cm resistivity. Er incorporation was performed by a cathodic polarization of PS in a 0.1 M Er(NO3)3 aqueous solution. A high temperature treatment in an oxidizing ambient at 500-1000°C was utilized to provide either partial or total oxidation of PS:Er layers. X-ray microanalysis was used to study chemical composition of the samples. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were investigated. After the partial oxidation (in the temperature range of 600-800°C), weak Er3+-related PL at 1.53 ptm was observed. A high temperature anneal in Ar atmosphere at the temperature of 1100°C caused a significant increase in the Er3+-related PL intensity. Resonant features were observed in PLE spectra of fully oxidized PS. Five peaks at 381, 492, 523, 654 and 980 nm were revealed. The strongest excitation occurred at 381 and 523 nm. The excitation of different Er3+ energy levels, cross-relaxation interactions and emission due to the 4I13/2→4I15/2 transitions were considered. Application of the Er-doped oxidized PS for integrated optical waveguides is presented.

The photoluminescent properties of erbium doped silicon rich silicon oxide (SRSO) is investigated. The silicon content of SRSO was varied from 43 to 33 at. % and Er concentration was 0.4–0.7 at. % in all cases. We observe strong 1.54 μ m luminescence due to 4I13/2⇒4I15/2 Er3+ 4f transition, excited via energy transfer from carrier recombination in silicon nanoclusters to Er 4f shells. The luminescent lifetimes at the room temperature are found to be 4–7 msec, which is longer than that reported from Er in any semiconducting host material, and comparable to that of Er doped SiO2 and A12O3. The dependence of the Er3+ luminescent intensities and lifetimes on temperature, pump power and on background illumination shows that by using SRSO, almost all non-radiative decay paths of excited Er3+ can be effectively suppressed, and that such suppression is more important than increasing excitation rate of Er3+. A planar waveguide using Er doped SRSO is also demonstrated.

We report on strong Er3+ luminescence in the visible and infra-red regions at room temperature in amorphous GaN:Er thin films prepared by DC magnetron co-sputtering. The intensity of the Er3+ luminescence at 1.535 μm corresponding to 4I13/2 → 4I15/2 transitions is greatly enhanced after annealing at 750°C. In this material GaN crystallites have formed and embedded in the continuous amorphous matrix. The crystallites are 4 to 7 nm in diameter as analyzed by high resolution transmission electron microscopy. The absorption edge, extending three orders of magnitude in absorption coefficient in the spectral range from 0.5 to 3.5 eV, is superimposed on resonant absorption bands of Er3+ ions.The total photoluminescence spectrum consists of welldefined Er3+ luminescence peaks imposed on a broad band edge luminescence from the amorphous GaN host matrix.

The exact control and adjustment of the etch parameters is important for the fabrication of well defined and reproducible devices based on porous silicon (PS) multilayers. In this paper the etch parameters “electrolyte volume” and “diffusion in the electrolyte” will be discussed as specific problems during a commercial fabrication of multilayer applications. Additionally the stability of the filter characteristics of multilayer systems will be analyzed by changing the climatic environments. New perspectives of multilayer applications will be given, e. g. the electrical control of filters, spatially graded interference filters and multiple interference filters on one sample.

New possibilities for modifying the phonon spectra of III-V compounds are evidenced by micro-Raman analysis of porous layers prepared by electrochemical anodization of (111 )Aoriented n-GaP substrates. In particular, a surface-related vibrational mode along with a porosity-induced decoupling between the longitudinal optical (LO) phonon and plasmon are observed. We prove that filling in the pores with other materials (aniline as a first approach) is a promising tool for controlling the surface phonon frequency.

It is demonstrated that luminescent porous silicon (PS) exhibits an efficient thermoacoustic effect owing to its extremely low thermal conductivity. The experimental device is composed of a patterned thin Al film electrode (30 nm thick), a microporous PS layer (10–50 μm thick), and a single-crystalline Si (c-Si) wafer. The PS layer was formed by a conventional anodization technique. When an electrical input is provided to the Al electrode as a sinusoidal current followed by Joule's heating, a significant acoustic pressure is produced in front of the device as a result of an efficient heat exchange between PS and air. The output amplitude is in inverse proportion to the square root of the input frequency (0.1-100 kHz) as predicted by a theoretical analysis. The observed effect is a novel useful function of PS as a completely depleted nanocrystalline system.

This paper discusses the formation of silicon nanocrystals, nc-Si, by thermal annealing of hydrogenated amorphous thin films of SiOx, x<2 and (Si, C)Ox, x<2. Comparisons are made with SiCx films, providing additional insights into pathways for generation of nc-Si. These alloys are used as model systems for understanding chemical and structural relaxations occurring at Si-SiO2 and SiC-SiO2 interfaces during post-oxidation thermal annealing. This then provides important information for optimized processing of Si-SiO2 and SiC-SiO2 interfaces for device applications.

Porous silicon multilayer structures are easily manufactured using a periodic current density square pulse during the electrochemical dissolution process. The difference in porosity profile, corresponding to a variation in current density, is attributed to a difference in refractive index. Manipulating the difference in refractive index, high quality optical filters can be made with a maximum reflectivity peak ˜ 100%. The next logical step to further exploit these optical mirrors is to incorporate them into an LED device. The benefit of adding a multilayer mirror below a luminescent film of porous silicon is to significantly reduce the amount of light loss to the silicon substrate and increase the light output. However, oxidation is required to stabilize the as-anodized porous silicon film. This disrupts the overall index profile of the multilayer stack, causing the peak reflectance to blue shift. This phenomenon must be quantified and accounted before device implementation. We present a detailed study on the effects of oxidation temperature, gas environment, and annealing time of porous silicon multilayer structures in a device configuration.

The luminescent properties of porous silicon (PSi) films in the visible region were used to improve the photoresponse of PSi/Si-wafer and PSi/Si p-n junctions UV detector prototypes in the region below 500nm. A luminescent PSi overlayer was formed on top of the Si wafers and p-n junctions by electrochemical anodization. These overlayers have emission spectra peaking close to 690nm. In the case of the PSi/Si wafer, the PSi film was produced with a high optical transparency above 600nm and highly absorbent below this value. With such characteristics, the incident UV radiation is partially absorbed and converted into visible radiation that can be highly transmitted through the PSi film and efficiently absorbed by the wafer or the junction. The UV measurements show enhancement of the photoresponse at 366nm as compared with control prototypes without PSi. Details about the enhancement process are discussed.