Iain F. Crowe

Probing the phonon confinement in ultrasmall silicon nanocrystals reveals a size-dependent surface energy

We validate for the first time the phenomenological phonon confinement model (PCM) of H. Richter, Z. P. Wang, and L. Ley [Solid State Commun. 39, 625 (1981)] for silicon nanostructures on the sub-3 nm length scale. By invoking a PCM that incorporates the measured size distribution, as determined from cross-sectional transmission electron microscopy (X-TEM) images, we are able to accurately replicate the measured Raman line shape, which gives physical meaning to its evolution with high temperature annealing and removes the uncertainty in determining the confining length scale. The ability of our model to explain the presence of a background scattering spectrum implies the existence of a secondary population of extremely small (sub-nm), amorphous silicon nanoclusters which are not visible in the X-TEM images. Furthermore, the inclusion of an additional fitting parameter, which takes into account the observed peak shift, can be explained by a size-dependent interfacial stress that is minimized by the nanoclu...

Hydrogenation of Graphene by Reaction at High Pressure and High Temperature

The chemical reaction between hydrogen and purely sp(2)-bonded graphene to form graphenes purely sp(3)-bonded analogue, graphane, potentially allows the synthesis of a much wider variety of novel two-dimensional materials by opening a pathway to the application of conventional chemistry methods in graphene. Graphene is currently hydrogenated by exposure to atomic hydrogen in a vacuum, but these methods have not yielded a complete conversion of graphene to graphane, even with graphene exposed to hydrogen on both sides of the lattice. By heating graphene in molecular hydrogen under compression to modest high pressure in a diamond anvil cell (2.6-5.0 GPa), we are able to react graphene with hydrogen and propose a method whereby fully hydrogenated graphane may be synthesized for the first time.

Spatially correlated erbium and Si nanocrystals in coimplanted SiO2 after a single high temperature anneal

We present a study of silicon (Si) and erbium (Er) coimplanted silica (SiO2) in which we observe, by combining high resolution scanning transmission electron microscopy and selective electron energy loss spectroscopy (EELS), a high spatial correlation between silicon nanocrystals (Si-NCs), Er, and oxygen (O) after a single high temperature (1100 °C) anneal. The observation of a spatial overlap of the EELS chemical maps of dark field (DF) images at the Er N4,5, Si L2,3, and O K edges is concomitant with an intense room temperature infrared luminescence around 1534 nm. We suggest that these observations correspond to Er–O complexes within an amorphous silicon (a-Si) shell at the Si-NC/SiO2 interface. The presence of a crystalline phase at the Si-NC center, verified by high resolution electron micrographs and DF diffraction contrast images and the low solubility of Er in crystalline Si (c-Si) would tend to suggest a preferential Er agglomeration toward the Si-NC/SiO2 interface during formation, particularly when high concentrations of both Si and Er are obtained in a narrow region of the SiO2 after coimplantation. The absence of narrow Stark related features in the Er emission spectrum at low temperature and an inhomogeneous broadening with increasing temperature, which are characteristic of Er confined by an amorphous, rather than a crystalline host further support these hypotheses. After comparing the luminescence to that from a SiO2:Er control sample prepared in exactly the same manner but without Si-NCs, we find that, despite the observed spatial correlation, only a small fraction ( ∼ 7%) of the Er are sensitized by the Si-NCs. We ascribe this low fraction to a combination of low sensitizer (Si-NC) density and Auger-type losses arising principally from Er ion-ion interactions.

Study of erbium-doped silicon nanocrystals in silica

Er-doped SiO2 and Er-doped Si-NCs embedded in a SiO2 matrix were produced by Er and/or Si ion beam implantation of a Si (100) substrate. The composition and distribution of implanted Er varies in samples either with or without Si implants. HAADF and EELS detail in samples with Si implants, the Si and Er distribution is identical, and within a band of ~110 nm width at ~75 nm below the SiO2 surface. Intense PL emission at 1.54 μm confirms formation of ErSi2, for the majority of aggregates, is unlikely. The present investigation details most Si-NCs are surrounded by Er2O3, or possess this phase within.

Structural and compositional study of Erbium-doped silicon nanocrystals by HAADF, EELS and HRTEM techniques in an aberration corrected STEM

Er-doped SiO2 and Si nano-crystals (NCs) embedded in a SiO2 matrix were produced by ion beam implantation of Si (100) substrates. After annealing Er ions agglomerate in different positions with different compositional properties in samples with and without Si implants. HAADF and EELS show that in the sample with Si implants the Si and Er distribution is identical and within a band of ~110nm width ~75nm below theSiO2 surface whereas in the sample with no excess Si, Er forms on average much larger, amorphous aggregates, presumably an Er-oxide, in the SiO2 matrix with tendency to move towards the surface of the SiO2 layer.

Strategies for increased donor electrical activity in germanium (opto-) electronic materials: a review

ABSTRACT Germanium is one of the strongest candidate materials for next generation integrated optoelectronic devices owing to its high carrier mobilities, bandgap at the telecom wavelength of 1.55 μm, and monolithic (CMOS) integration with silicon. However, for device applications requiring very high carrier concentrations, such as solid state lasers and MOSFETs, a persistent technological hurdle is the limited electrically active concentration ∼5×1019 cm−3 observed in n-type material, regardless of the chemical concentration of incorporated donors above this. This is due to the formation of donor-vacancy clusters, which electrically compensate the material and enhance dopant diffusivity. In recent years, multiple strategies have attempted to address this, with some, albeit limited, success. Here we outline some of the more novel approaches and provide a review with particular emphasis on one of the more promising of these: the co-implantation of donors with fluorine, and discuss potential methods for optimizing this process.

Electrical observation of non-radiative recombination in Er doped Si nano-crystals during thermal quenching of intra-4f luminescence

Thermal quenching of luminescence of Er dopants in Si nano-crystals (Si-ncs) was investigated employing an impedance model for the analysis of photo-injected charges. Relaxation response indicated that Er doping forms not only optical centers but also trapping centers near the Si-ncs. The response time constant of trapped charges was dependent on temperature, with the dependence correlating to thermal quenching. These findings indicate that quenching occurs by trapping followed by consumption of charges. The complex analyses revealed that the response represents nonradiative recombination at the centers rather than release of confined charges from the Si-nc through the centers. We propose a possible energy diagram for the non-radiative recombination. � 2014 The Japan Society of Applied Physics.

Resonance Raman spectroscopy of carbon nanotubes: pressure effects on G-mode

We use 488 and 568 nm laser Raman spectroscopy under high pressure to selectively follow evolution of Raman G-mode signals of single-walled carbon nanotubes (SWCNTs) of selected diameters and chiralities ((6, 5) and (6, 4)). The G-mode pressure coefficients of tubes from our previous work are consistent with the thick-wall tube model. Here we report the observation of well-resolved G-minus peaks in the Raman spectrum of SWCNTs in a diamond-anvil cell. The pressure coefficients of these identified tubes in water, however, are unexpected, having the high value of over 9 cm−1 GPa−1 for the G-plus and the G-minus, and surprisingly the shift rates of the same tubes in hexane have clearly lower values. We also report an abrupt increase of G-minus peak width at about 4 GPa superposed on a continuous peak broadening with pressure.

Luminescence quenching of conductive Si nanocrystals via "Linkage emission?: Hopping-like propagation of infrared-excited Auger electrons

Phosphorus (P) is an n-type dopant for conductive silicon nanocrystals (Si-ncs), the electrical activation of which may be monitored through a non-radiative Auger recombination process that quenches the Si-nc luminescence. We investigated this quenching mechanism through electrical measurements of Si-ncs. Infrared-excited Auger electron emission as the non-radiative process was directly probed and the dynamics of the process are determined from a frequency response analysis. To explain the dynamics, we propose a model in which Auger electrons with a low kinetic energy establish a local inter-nanocrystal conductance and the repetition of this local conductance results in a constant photocurrent (“linkage emission”). This emission becomes significant by electron filling in the Si-ncs owing to the electrical activation of P, which is consistent with observed luminescence quenching behavior. We found that the IR photo-excited emission is distinct from the thermally induced hopping conduction and show that confined, rather than trapped, charges are the source of the Auger electrons. Thus, the process consumes both confined charges and the recombination energy for Auger emission, which explains the luminescence quenching mechanism of Si-nc:P.

Low-dimensional silicon structures for use in photonic circuits

Over the past two decades, substantial efforts have been placed toward understanding and exploiting light emission in low-dimensional silicon systems due to the technological potential of a Si-based light source for integrated microphotonics. In these low-dimensional systems, quantum confinement leads to a substantial increase in the radiative efficiency of the samples in comparison with bulk silicon, however, the luminescent characteristics of the materials are often strongly dependent on defect states within the system, which in turn often depend on details of the sample fabrication and processing. In this review we consider details of the fabrication, characterization, and applications of various forms of low-dimensional silicon. Recent work aimed at resolving the physics of (i) the nanostructure formation processes and (ii) the luminescence process in these materials are presented and potential applications of Si-nanostructures are discussed.