Aj Kenyon

Recent developments in rare-earth doped materials for optoelectronics

Rare-earth doped materials are of crucial importance to optoelectronics, and are widely deployed in fibre amplifiers and solid-state lasers. This article summarises the present state of the art in this rapidly growing field. Recent developments in the areas of rare-earth doped semiconductors and insulators are discussed and new classes of materials that open up new possibilities for extended functionality and greater optoelectronic integration are described. Nanostructured materials and wide bandgap semiconductors are of particular interest, though recent developments in more traditional material systems are highlighted. Emphasis is placed on erbium-doped materials, as these are of the greatest importance for telecommunications applications, but a range of other rare-earth ions are also discussed.

The origin of photoluminescence from thin films of silicon‐rich silica

We have carried out a study of the photoluminescence properties of silicon‐rich silica. A series of films grown using plasma enhanced chemical vapor deposition over a range of growth conditions were annealed under argon at selected temperatures. Photoluminescence spectra were measured for each film at room temperature and for selected films at cryogenic temperatures. The photoluminescence spectra exhibit two bands. Fourier transform infrared and electron spin resonance spectroscopies were used to investigate bonding and defect states within the films. The data obtained strongly suggest the presence of two luminescence mechanisms which exhibit different dependencies on film growth conditions and postprocessing. We make assignments of the two mechanisms as (1) defect luminescence associated with oxygen vacancies and (2) radiative recombination of electron‐hole pairs confined within nanometer‐size silicon clusters (‘‘quantum confinement’’).

Optical properties of PECVD erbium-doped silicon-rich silica: evidence for energy transfer between silicon microclusters and erbium ions

We report the fabrication by PECVD of silicon-rich erbium-doped silica films that exhibit both 1535 nm fluorescence and visible photoluminescence. Fluorescence spectra are presented along with absorption spectra that display a strong band edge in the blue, which we ascribe to the presence of Si microclusters. We are unable to observe characteristic Er3+ absorption bands and propose that excitation of the rare earth is via an energy transfer process from Si microclusters.

MODELING THE CONTRIBUTION OF QUANTUM CONFINEMENT TO LUMINESCENCE FROM SILICON NANOCLUSTERS

We present a model for the luminescence spectrum of silicon nanoclusters. We propose that the major contribution to luminescence is from radiative recombination of confined excitons (quantum confinement). Utilizing the effective mass approximation we consider the variation in oscillator strength with cluster size and the associated change in the number of available free carriers. By varying both the mean cluster size and size distribution of silicon nanoclusters, the luminescence spectra are modeled to a good fit. We compare our model with experimental photoluminescence and electroluminescence data from this group and from others.

Erbium in silicon

The overlap of the principal luminescence band of the erbium ion with the low-loss optical transmission window of silica optical fibres, along with the drive for integration of photonics and silicon technology, has generated intense interest in doping silicon with erbium to produce a silicon-based optical source. Silicon is a poor photonic material due to its very short non-radiative lifetime and indirect band gap, but it has been hoped that the incorporation of optically active erbium ions into silicon will permit the development of silicon-based light sources that will interface with both CMOS technology and optical fibre communications. Some years into this activity, there have now been a wide range of experimental studies of material growth techniques, optical, physical and electrical properties, along with a considerable body of theoretical work dealing with the site of the erbium ion in silicon, along with activation and deactivation processes. This paper reviews the current state of what remains an active field, summarizing results from a range of studies conducted over the last few years, and points to further developments by considering the prospects for successful photonic integration of erbium and silicon.

Luminescence from erbium-doped silicon nanocrystals in silica: Excitation mechanisms

We develop a model for the excitation of erbium ions in erbium-doped silicon nanocrystals via coupling from confined excitons generated within the silicon nanoclusters. The model provides a phenomenological picture of the exchange mechanism and allows us to evaluate an effective absorption cross section for erbium of up to 7.3×10−17 cm2: four orders of magnitude higher than in stoichiometric silica. We address the origin of the 1.6 eV emission band associated with the silicon nanoclusters and determine absorption cross sections and excitonic lifetimes for nanoclusters in silica which are of the order of 1.02×10−16 cm2 and 20–100 μs, respectively.

Resistive switching in silicon suboxide films

We report a study of resistive switching in a silicon-based memristor/resistive RAM (RRAM) device in which the active layer is silicon-rich silica. The resistive switching phenomenon is an intrinsic property of the silicon-rich oxide layer and does not depend on the diffusion of metallic ions to form conductive paths. In contrast to other work in the literature, switching occurs in ambient conditions, and is not limited to the surface of the active material. We propose a switching mechanism driven by competing field-driven formation and current-driven destruction of filamentary conductive pathways. We demonstrate that conduction is dominated by trap assisted tunneling through noncontinuous conduction paths consisting of silicon nanoinclusions in a highly nonstoichiometric suboxide phase. We hypothesize that such nanoinclusions nucleate preferentially at internal grain boundaries in nanostructured films. Switching exhibits the pinched hysteresis I/V loop characteristic of memristive systems, and on/off resistance ratios of 104:1 or higher can be easily achieved. Scanning tunneling microscopy suggests that switchable conductive pathways are 10 nm in diameter or smaller. Programming currents can be as low as 2 μA, and transition times are on the nanosecond scale.

Evidence of energy coupling between Si nanocrystals and Er3+ in ion-implanted silica thin films

Silica thin films containing Si nanocrystals and Er3+ were prepared by ion implantation. Excess Si concentrations ranged from 5% to 15%; Er3+ concentration for all samples was 0.5%. Samples exhibited photoluminescence at 742 nm (attributed to Si nanocrystals), 654 nm (defects due to Er3+ implantation), and at 1.53 μm (intra-4f transitions). Photoluminescence intensity at 1.53 μm increased ten times by incorporating Si nanocrystals. Strong, broad photoluminescence at 1.53 μm was observed for λPump away from Er3+ absorption peaks, implying energy transfer from Si nanocrystals. Erbium fluorescence lifetime decreased from 4 ms to 1 ms when excess Si increased from 5% to 15%, suggesting that at high Si content Er3+ ions are primarily situated inside Si nanocrystals.

Quantum Conductance in Silicon Oxide Resistive Memory Devices

Resistive switching offers a promising route to universal electronic memory, potentially replacing current technologies that are approaching their fundamental limits. In many cases switching originates from the reversible formation and dissolution of nanometre-scale conductive filaments, which constrain the motion of electrons, leading to the quantisation of device conductance into multiples of the fundamental unit of conductance, G0. Such quantum effects appear when the constriction diameter approaches the Fermi wavelength of the electron in the medium – typically several nanometres. Here we find that the conductance of silicon-rich silica (SiOx) resistive switches is quantised in half-integer multiples of G0. In contrast to other resistive switching systems this quantisation is intrinsic to SiOx, and is not due to drift of metallic ions. Half-integer quantisation is explained in terms of the filament structure and formation mechanism, which allows us to distinguish between systems that exhibit integer and half-integer quantisation.

Towards population inversion of electrically pumped Er ions sensitized by Si nanoclusters

This study reports the estimation of the inverted Er fraction in a system of Er doped silicon oxide sensitized by Si nanoclusters, made by magnetron sputtering. Electroluminescence was obtained from the sensitized erbium, with a power efficiency of 10(-2)%. By estimating the density of Er ions that are in the first excited state, we find that up to 20% of the total Er concentration is inverted in the best device, which is one order of magnitude higher than that achieved by optical pumping of similar materials.