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Results of a deep-level transient spectroscopy (DLTS) study of trapping states in Ni/Au Schottky diodes made on Si-doped GaN layers grown by metal–organic vapor-phase epitaxy (MOVPE) on highly conductive n-type Ammono-GaN substrates are reported. In all as-grown samples, the DLTS signals due to traps with the activation energies for electron emission (Eem) of about 0.26 and 0.60 eV (referred to as E1 and E3 traps, respectively) have been detected. It is found that the electric field (E) significantly enhances the electron emission rate (eem) for the E1 trap, while for the E3 trap the eem(E) dependence is not very strong. The eem(E) dependence for the E1 trap is found to be characteristic of attractive traps with spherically symmetric square well potential with a radius of about 2.9 ± 0.25 nm. The eem(E) dependence for the E3 trap is accounted for by a phonon-assisted tunneling mechanism. The zero-E Eem values for both traps are determined. Correlations between the concentrations of the E1 and E3 traps and concentrations of impurities (H, C, Si, O, Mg, and Fe) determined by secondary ion mass spectrometry in the samples are investigated. Charge states and origins of the E1 and E3 traps are discussed.

We present a compact, CMOS compatible, photonic integrated circuit (PIC) based spectrometer that combines a dispersive array element of SiO2-filled scattering holes within a multimode interferometer (MMI) fabricated on the silicon-on-insulator (SOI) platform. The spectrometer has a bandwidth of 67 nm, a lower bandwidth limit of 1 nm, and a peak-to-peak resolution of 3 nm for wavelengths around 1310 nm.

Funding Information: The authors would like to thank EPSRC (UK) for funding this work via grant EP/TO25131/1. J.A.T.D.G. would like to thank the Government of the Philippines through the Department of Science and Technology (DOST) for her Ph.D. funding. J.C. is thankful for the support of the i3N projects, Refs. UIDB/50025/2020 and UIDP/50025/2020, financed by the Fundação para a Ciência e a Tecnologia in Portugal. Publisher Copyright: © 2021 The Authors. physica status solidi (a) applications and materials science published by Wiley-VCH GmbH Indium-doped silicon is considered a possible p-type material for solar cells to avoid light-induced degradation (LID), which occurs in cells made from boron-doped Czochralski (Cz) silicon. Herein, the defect reactions associated with indium-related LID are examined and a deep donor is detected, which is attributed to a negative-U defect believed to be InsO2. In the presence of minority carriers or above bandgap light, the deep donor transforms to a shallow acceptor. An analogous transformation in boron-doped material is related to the BsO2 defect that is a precursor of the center responsible for BO LID. The electronic properties of InsO2 are determined and compared to those of the BsO2 defect. Structures of the BsO2 and InsO2 defects in different charges states are found using first-principles modeling. The results of the modeling can explain both the similarities and the differences between the BsO2 and InsO2 properties.

Rare-earth related optical emission in oxide semiconductors is strongly influenced by the local co-ordination around the rare-earth centres. In this contribution, we examine the effect of aluminium co-doping on the local fine structure of neodymium-doped titanium dioxide (TiO2:Nd, TiO2:Nd-Al) thin films with single-phase anatase structure, via luminescence and X-ray absorption fine structure (XAFS) measurements. Increasing the concentration of Al in these materials leads to the co-ordination around Nd3+ ions being distorted, which in turn leads to an enhanced photoluminescence (PL) intensity. We explain this in terms of a distortion of the nearest neighbour oxygen bonds with Nd3+.

We demonstrate an on-chip silicon-on-insulator (SOI) device to generate a non-diffracting beam of ≈850 µm length from a diffractive axicon-like lens etched using a low resolution (200 nm feature size, 250 nm gap) deep-ultraviolet lithographic fabrication. The device consists of circular gratings with seven stages of 1x2 multimode interferometers. We present a technique to apodize the gratings azimuthally by breaking up the circles into arcs which successfully increased the penetration depth in the gratings from ≈5 µm to ≈60 µm. We characterize the device’s performance by coupling 1300±50 nm swept source laser in to the chip from the axicon and measuring the out-coupled light from a grating coupler. Further, we also present the implementation of balanced homodyne detection method for the spectral characterization of the device and show that the position of the output lobe of the axicon does not change significantly with wavelength.

The field of silicon photonics has experienced widespread adoption in the datacoms industry over the past decade, with a plethora of other applications emerging more recently such as light detection and ranging (LIDAR), sensing, quantum photonics, programmable photonics and artificial intelligence. As a result of this, many commercial complementary metal oxide semiconductor (CMOS) foundries have developed open access silicon photonics process lines, enabling the mass production of silicon photonics systems. On the other side of the spectrum, several research labs, typically within universities, have opened up their facilities for small scale prototyping, commonly exploiting e-beam lithography for wafer patterning. Within this ecosystem, there remains a challenge for early stage researchers to progress their novel and innovate designs from the research lab to the commercial foundries because of the lack of compatibility of the processing technologies (e-beam lithography is not an industry tool). The CORNERSTONE rapid-prototyping capability bridges this gap between research and industry by providing a rapid prototyping fabrication line based on deep-UV lithography to enable seamless scaling up of production volumes, whilst also retaining the ability for device level innovation, crucial for researchers, by offering flexibility in its process flows. This review article presents a summary of the current CORNERSTONE capabilities and an outlook for the future.

Minority carrier traps in Czochralski-grown (Cz) silicon crystals doped with either boron, aluminum, gallium, or indium impurity atoms have been investigated by means of deep-level transient spectroscopy and other junction-related techniques. The experimental data have suggested that minority carrier trapping effects in Cz-Si samples doped with different acceptor impurities are associated with complexes incorporating a substitutional group-III impurity atom and two oxygen atoms, which are found to be negative-U defects with close locations of E(-/+) occupancy level at about Eu + 0.32 eV. We have determined the energy barriers and frequency factors for the reversible transformations of the complexes between deep donor and shallow acceptor states. These parameters are discussed in relation to light-induced degradation behavior of solar cells on p-type Cz-Si crystals.

The optical response of a graphene oxide integrated silicon micro-ring resonator (GOMRR) to a range of vapour phase Volatile Organic Compounds (VOCs) is reported. The response of the GOMRR to all but one (hexane) of the VOCs tested is significantly higher than that of the uncoated (control) silicon MRR, for the same vapour flow rate. An iterative Finite Difference Eigenmode (FDE) simulation reveals that the sensitivity of the GO integrated device (in terms of RIU/nm) is enhanced by a factor of ~2, which is coupled with a lower limit of detection. Critically, the simulations reveal that the strength of the optical response is determined by molecular specific changes in the local refractive index probed by the evanescent field of the guided optical mode in the device. Analytical modelling of the experimental data, based on Hill-Langmuir adsorption characteristics, suggests that these changes in the local refractive index are determined by the degree of molecular cooperativity, which is enhanced for molecules with a polarity that is high, relative to their kinetic diameter. We believe this reflects a molecular dependent capillary condensation within the graphene oxide interlayers, which, when combined with highly sensitive optical detection, provides a potential route for discriminating between different vapour phase VOCs.

We present a combined photoluminescence (PL) and photomodulated reflectivity (PMR) study of three GaN/InGaN multiquantum well samples. We reported previously that the change in carrier concentration (n) induced by the pump beam can be measured by lock-in techniques using a simple Drude model to calculate n from the change in reflectivity. Here we extend the work by simultansously measuring a thermal signal from the sample, we can thus measure the internal quantum efficiency ηi of samples as a function of carrier concentration. This yields an ηi vs n curve that is strikingly different to those reported previously by PL and electroluminescent techniques (EL), with a very rapid (in n) drop off due to the droop process.

For 40 years it has been deduced that the presence of Boron and Oxygen in silicon causes the formation of a non-radiative recombination centre, without any consensus as to its exact nature. Here we report the observation, using deep level transient spectroscopy and photoluminescence of the conversion of a deep boron-di-oxygen-related donor trapping state into a shallow acceptor under the action of light or injected carriers. Using ab initio modelling, we propose structures of the B-O2 defect which match the experimental findings. Implications of the presence of this defect, particularly on its deep donor (carrier trapping) configuration, on silicon photodetectors is discussed.

We introduce a compact spectrometer based on a 1×16-multimode interference splitter partially filled with random holes. The spectral resolution for this high-bandwidth, on-chip device is found to be at least ~0.04 nm.

We describe a novel technique for measuring carrier dynamics in solid-state optical materials based on photomodulated reflectivity (PMR) and, as an example, apply it to a study of an InGaN/GaN multi-quantum-well (QW) structure grown on a c-plane sapphire substrate. The technique is a form of frequency modulation spectroscopy and relies on probing changes in refractive index induced by fluctuations in free-carrier density during optical excitation. We show that it is possible to accurately determine both carrier density and lifetime, independent of any photoluminescence (PL) measurement and with no knowledge of the incident, or fraction of absorbed, laser power, quantities that can give rise to considerable uncertainties in PL studies. We demonstrate that such uncertainties can lead to an order of magnitude underestimation of the total photogenerated carrier density and compromised accuracy in determining carrier lifetime. We determine, by a comparison of the two techniques, PMR and PL, the nonradiative Shockley-Reed-Hall (SRH), radiative (excitonic), and nonradiative Auger-related coefficients (from the standard ABC model). We find marked differences in the carrier-density-dependent lifetime, determined from PMR, translate to significant differences in the SRH and excitonic coefficients, which we believe relate to the more accurate determination of carrier densities in PMR than in PL. We also find evidence from the PMR for a change in effective mass of the photoexcited carriers with excitation intensity, which points to a complex localization/delocalization mechanism, likely facilitated by random fluctuations in indium content and QW width, consistent with previous findings by independent methods.

Annealing silicon (Si)-rich borophosphosilicate glass (BPSG) at a high temperature results in the growth of core-shell Si quantum dots (QDs) composed of a boron (B) and phosphorus (P) codoped crystalline Si core and an amorphous shell made from B, Si and P (B and P codoped Si QDs) in a BPSG matrix. The amorphous BxSiyPz shell is responsible for many superior properties of codoped Si QDs such as hydrophilicity, high resistance to hydrofluoric acid (HF) etching, stable luminescence in different environment, robustness of the luminescence for chemical treatments, etc. In this work, we study the growth process of the amorphous shell by Raman spectroscopy and transmission electron microscopy. We show that amorphous Si particles are first grown in a BPSG matrix within 30 s of annealing of Si-rich BPSG. After 50 s annealing, a crystalline Si core appears within an amorphous Si particle. The formation of a crystalline Si core is accompanied by the formation of an amorphous BxSiyPz shell. From the annealing time dependence of the volumes of the core and the shell, we show that supersaturated B and P are expelled to the surface of a crystalline Si core during the growth, which increases B and P concentration in an amorphous BxSiyPz shell.

It is well established that boron reacts with two oxygen atoms in Czochralski-grown silicon (Cz-Si) to form a defect, which is responsible for the dominant light-induced degradation (LID) in solar cells made from Cz-Si:B material. The detrimental effect of LID has stimulated a move by solar cell manufacturers to the use of silicon with other group-III dopants, particularly with gallium. Cz-Si:Ga is immune to the BO-type LID. The information available in the literature on the interactions of oxygen with either Al, Ga, or In impurities in Si is limited. We use ab initio modeling and junction spectroscopy techniques to study a family of defects with unusual electronic properties, which have been detected in Cz-Si samples doped with different shallow acceptor species. We have carried out detailed measurements of the temperature dependencies of hole emission rate, equilibrium occupancy, and hole capture kinetics for the traps observed in differently doped p-type Cz-Si samples. It is found from the analysis of the changes in magnitude of the deep-level-transient signals with temperature that the equilibrium occupancy function of the traps is characteristic for a defect with negative-U properties in all the samples. The positions of the E(-/+) occupancy level of the defects are very close in differently doped samples, E(-/+) = Ev + (0.31 ± 0.01) eV. It is argued that the oxygen dimer interacts with group-III atoms in silicon and these interactions result in the formation of AsO2 complexes (A is either B, Al, Ga, or In atom) with very similar electronic properties.

Results available in the literature on minority carrier trapping and light induced degradation (LID) effects in silicon materials containing boron and oxygen atoms are briefly reviewed. Special attention is paid to the phenomena associated with “deep” electron traps (J.A. Hornbeck and J.R. Haynes, Phys. Rev. 1955, 97, 311) and the recently reported results which have linked LID with the transformation of a defect consisting of a substitutional boron atom and an oxygen dimer (BsO2) from a configuration with a deep donor state into a recombination active configuration associated with a shallow acceptor state (M. Vaqueiro-Contreras et al., J. Appl. Phys. 2019, 125, 185704). The significance of the latter work is discussed and detailed experimental results on the electronic and dynamic properties of the BsO2 complex are presented. It is shown that the BsO2 complex is a defect with negative-U properties and it is responsible for minority carrier trapping and persistent photo-conductivity in non-degraded Si:B+O samples and solar cells. It is argued that the “deep” electron traps observed by Hornbeck and Haynes are the pre-cursors of the “slow” forming shallow acceptor defects, which are responsible for the dominant LID in boron-doped Cz-Si crystals. Both the deep and shallow defects are BsO2 complexes, transformations between charge states and atomic configurations of which account for the observed electron trapping and LID phenomena.

Silicon solar cells containing boron and oxygen are one of the most rapidly growing forms of electricity generation. However, they suffer from significant degradation during the initial stages of use. This problem has been studied for 40 years resulting in over 250 research publications. Despite this, there is no consensus regarding the microscopic nature of the defect reactions responsible. In this paper, we present compelling evidence of the mechanism of degradation. We observe, using deep level transient spectroscopy and photoluminescence, under the action of light or injected carriers, the conversion of a deep boron-di-oxygen-related donor state into a shallow acceptor which correlates with the change in the lifetime of minority carriers in the silicon. Using ab initio modeling, we propose structures of the BsO2 defect which match the experimental findings. We put forward the hypothesis that the dominant recombination process associated with the degradation is trap-assisted Auger recombination. This assignment is supported by the observation of above bandgap luminescence due to hot carriers resulting from the Auger process.

Silicon photonics micro-ring resonator (MRR) and Mach-Zehnder waveguide based sensors have attracted much attention in recent years because of their capacity for high sensitivity, small footprint and mass-scalable (low cost) potential. This type of sensor is based on the detection of changes in optical amplitude/phase due to small changes in local, near-field refractive index (RI) in the environment surrounding the waveguide device. Sensitivity to ever smaller changes in RI are sought, e.g. for vapour/gas based sensing, which may be realised by designing devices based around the slot waveguide. Furthermore, tailoring resonant line-shapes to generate asymmetric (or Fano-like) modes through series, parallel or ‘nested’ arrangements of coupled MRRs also demonstrates the potential for such sensitivity enhancement. This type of device is likely to be of interest, for example where sensing of volatile organic compounds (VOCs) is important, e.g. in industrial process and environmental monitoring. We demonstrate a number of such photonic sensing platforms, combining both the slot waveguide and both established and novel ‘photonic molecule’ structures, fabricated on silicon-on-insulator using standard foundry fabrication processes. Integrated TiN heaters provide the capacity for thermal tuning in order to manipulate the spectral characteristics of our devices and the sensitivity of the devices to a range of VOCs; benzene, toluene and xylene, are investigated as exemplars using a custom-made vapour delivery system. Sensor performance is established with the assistance of device modelling and comparison made with conventional single MRR devices as a reference. The potential of adding functional layers to the devices as a method for achieving chemical selectivity will also be discussed.

Photo-detection in the wavelength range 1850 to 2000 nm using evanescently-coupled germanium detectors grown on silicon waveguides is described. Devices were fabricated at a silicon photonics foundry using a process flow associated with operation in the O , C and L bands, and as such offer a solution for extended wavelength detection which is readily available. Intrinsic sensitivity is via indirect band transitions, which is enhanced by tensile strain and we postulate that it may be further enhanced by defects which arise from the thermal processes associated with Ge on Si growth. The responsivity of p-i-n detectors is 20 mA/W at 1850 nm falling to 5 mA/W at 2000 nm, for a detector length of 50 μm. Responsivity is suppressed by electrical doping in the germanium detector which provides parasitic absorption from free carriers. Modifications to the current design are suggested such that integrated germanium p-i-n detectors, directly grown on silicon waveguides would be suitable for high-bandwidth photo-detection up to at least a wavelength of 2000 nm. A Separate-Absorption-Charge-Multiplication Avalanche Photo-Detector is fabricated exploiting the same indirect transition. This detector has a responsivity of 0.31 A/W at 1850 nm and 0.08 A/W at 1970 nm, for a detector length of only 14 μm.

Co-doping Aluminium (Al) with neodymium (Nd) is shown to have a sensitizing effect on the rare earth luminescence in thin film titanium dioxide (TiO2) host materials. Whilst the Al co-doping has the effect of quenching the direct (resonantly) excited, steady state photoluminescence from Nd3+ emitting centers, for indirect (non-resonant) excitation, the emission is enhanced with increasing Al. Time-resolved PL measurements reveal a decrease in both the excitation/de-excitation rates (increases in lifetime) with increasing Al co-doping, confirming the indirect nature of excitation and implying a suppression of non-radiative de-excitation of the Nd3+ centers in TiO2. Suppression of non-radiative processes by Al is considered to be the result of reduced rare earth clustering, which is known to promote de-excitation via energy migration and thus has important implications for improved efficiency infra-red LED materials

For nominally identical GaN Schottky diodes prepared by resistive thermal evaporation and plasma sputter deposition, diodes prepared by sputter deposition were found to exhibit a clear increase in the reverse leakage current, which is about two orders of magnitude higher than diodes prepared using thermal evaporation. Defects in n-type GaN Schottky diodes fabricated by plasma sputtering of gold were investigated using deep-level transient spectroscopy and compared with those in similar structures fabricated by resistive thermal evaporation. From deep level transient spectroscopy two defects were identified in the sputtered diode with the activation energies for charge carrier emission of 0 . 26 ± 0 . 01 e V relative to the conduction band edge and 0 . 62 ± 0 . 04 e V relative to the valence band edge. Defect concentration profiles of sputtered diodes show the defect density reduces from the surface to deeper in the structure, indicating that they are introduced through sputtering. Since the sputter deposition technique is widely used in device fabrication, attention should be paid to the adverse effects due to the additional defect introduction such as the charge trapping in HEMTs and the non-radiative recombination in LEDs.

Following our report of the first near-linear lanthanide (Ln) complex, [Sm(N††)2] (1), herein we present the synthesis of [Ln(N††)2] [N†† = {N(SiiPr3)2}–; Ln = Eu (2), Tm (3) and Yb (4)], thus achieving approximate uniaxial geometries for a series of “traditional” Ln(II) ions. Experimental evidence, together with calculations performed on a model of 4, indicate that dispersion forces are important for the stabilization of the near-linear geometries of 1-4. The isolation of 3 under a dinitrogen atmosphere is noteworthy, given that “[Tm(N′′)(μ-N′′)]2” (N′′ = {N(SiMe3)2}–) has not previously been structurally authenticated and reacts rapidly with N2(g) to give [{Tm(N′′)¬2}2(μ-η2:η2-N2)]. Complexes 1-4 have been characterized as appropriate by single crystal XRD, magnetic measurements, electrochemistry, and multinuclear NMR, EPR, and electronic spectroscopy, along with computational methods for 3 and 4. The remarkable geometries of monomeric 1-4 lead to interesting physical properties, which complement and contrast with comparatively well understood dimeric [Ln(N′′)(μ-N′′)]2 complexes. EPR spectroscopy of 3 shows that the near-linear geometry stabilizes mJ states with oblate spheroid electron density distributions, validating our previous suggestions. Cyclic voltammetry experiments carried out on 1-4 did not yield Ln(II) reduction potentials, so a reactivity study of 1 was performed with selected substrates in order to benchmark the Sm(III)→Sm(II) couple. The separate reactions of 1 with TEMPO (2,2,6,6-tetramethyl-piperidinyl-1-oxy), azobenzene and benzophenone gave crystals of [Sm(N††)2(TEMPO)] (5), [Sm(N††)2(N2Ph2)] (6) and [Sm(N††){μ-OPhC(C6H5)CPh2O-κ-O,O′}]2 (7), respectively. The isolation of 5-7 shows that the Sm(II) center in 1 is still accessible despite having two bulky N†† moieties, and that the N-donor atoms are able to deviate further from linearity or ligand scrambling occurs in order to accommodate another ligand in the Sm(III) coordination spheres of the products.

We present the analysis of UV (325 nm) Raman scattering spectra from silicon-germanium (SiGe) microbridges where the SiGe has been formed using the so-called "condensation technique". As opposed to the conventional condensation technique in which SiGe is grown epitaxially, we use high-dose ion implantation of Ge ions into SOI as a means to introduce the initial Ge profile. The subsequent oxidation both repairs implantation induced damage, and forms epitaxial Ge. Using Si-Si and Si-Ge optical phonon modes, as well as the ratio of integrated intensities for Ge-Ge and Si-Si, we can determine both the composition and strain of the material. We show that although the material is compressively strained following condensation, by fabricating microbridge structures we can create strain relaxed or tensile strained structures, with subsequent interest for photonic applications.

We have conducted Raman spectroscopy\ud experiments on liquid ethane (C2H6) at 300 K, obtaining a\ud large amount of data at very high resolution. This has enabled\ud the observation of Raman peaks expected but not previously\ud observed in liquid ethane and a detailed experimental study of\ud the liquid that was not previously possible. We have observed a\ud transition between rigid and nonrigid liquid states in liquid\ud ethane at ca. 250 MPa corresponding to the recently proposed\ud Frenkel line, a dynamic transition between rigid liquid\ud (liquidlike) and nonrigid liquid (gaslike) states beginning in\ud the subcritical region and extending to arbitrarily high pressure\ud and temperature. The observation of this transition in liquid\ud (subcritical) ethane allows a clear differentiation to be made\ud between the Frenkel line (beginning in the subcritical region at\ud higher density than the boiling line) and the Widom lines (emanating from the critical point and not existing in the subcritical\ud region). Furthermore, we observe a narrow transition at ca. 1000 MPa to a second rigid liquid state. We propose that this\ud corresponds to a state in which orientational order must exist to achieve the expected density and can view the transition in\ud analogy to the transition in the solid state away from the orientationally disordered phase I to the orientationally ordered phases\ud II and III.

Germanium condensation is demonstrated using a two-step wet oxidation of germanium implanted Silicon-On-Insulator (SOI). Samples of 220nm thick SOI are implanted with a nominal fluence of 5x1016cm-2 Ge+ at an energy of 33keV. Primary post-implantation wet oxidation is performed initially at 870°C for 70 minutes, with the aim of capping the sample without causing significant dose loss via Ge evaporation through the sample surface. This is followed by a secondary higher temperature wet oxidation at either 900°C, 1000°C or 1080°C. The germanium retained dose and concentration profile, and the oxide thickness is examined after primary oxidation, and various secondary oxidation times, using Rutherford backscattering analysis. A mixed SiGe oxide is observed to form during the primary oxidation followed by a pure silicon oxide after higher temperature secondary oxidation. The peak germanium concentration, which varies with secondary oxidation condition, is found to range from 43 at- % to 95 at- %, while the FWHM of the Ge profile varies from 13 to 5nm, respectively. It is also observed that both the diffusion of germanium and the rate of oxidation are enhanced at 870°C and 900°C compared to equilibrium expectations. Transmission electron microscopy of a representative sample with secondary oxidation at 1080oC for 20 minutes shows that the SiGe layer is crystalline in nature and seeded from the underlying silicon. Raman spectroscopy is used to determine residual strain in the SiGe region following secondary oxidation. The strain is compressive in nature and increases with Ge concentration to a maximum of approximately 1% in the samples probed. In order to elucidate the physical mechanisms, which govern the implantation-condensation process, we fit the experimental profiles of the samples with a model that uses a modified segregation boundary condition; a modified linear rate constant for the oxidation; and an enhanced diffusion coefficient of germanium where the enhancement is inversely proportional to the temperature and decays with increasing time. Comparison of the modelled and experimental results shows reasonable agreement and allows conclusions to be made regarding the dominant physical mechanisms, despite the semi-empirical nature of the model used.

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.

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.

We report a monometallic dysprosium(iii) single molecule magnet with record energy barriers and unusual spin relaxation behaviour., We report a dysprosium(iii) bis(methanediide) single molecule magnet (SMM) where stabilisation of the highly magnetic states and suppression of mixing of opposite magnetic projections is imposed by a linear arrangement of negatively-charged donor atoms supported by weak neutral donors. Treatment of [Ln(BIPMTMS)(BIPMTMSH)] [Ln = Dy, 1Dy; Y, 1Y; BIPMTMS = {C(PPh2NSiMe3)2}2–; BIPMTMSH = {HC(PPh2NSiMe3)2}–] with benzyl potassium/18-crown-6 ether (18C6) in THF afforded [Ln(BIPMTMS)2][K(18C6)(THF)2] [Ln = Dy, 2Dy; Y, 2Y]. AC magnetic measurements of 2Dy in zero DC field show temperature- and frequency-dependent SMM behaviour. Orbach relaxation dominates at high temperature, but at lower temperatures a second-order Raman process dominates. Complex 2Dy exhibits two thermally activated energy barriers (Ueff) of 721 and 813 K, the largest Ueff values for any monometallic dysprosium(iii) complex. Dilution experiments confirm the molecular origin of this phenomenon. Complex 2Dy has rich magnetic dynamics; field-cooled (FC)/zero-field cooled (ZFC) susceptibility measurements show a clear divergence at 16 K, meaning the magnetic observables are out-of-equilibrium below this temperature, however the maximum in ZFC, which conventionally defines the blocking temperature, TB, is found at 10 K. Magnetic hysteresis is also observed in 10% 2Dy@2Y at these temperatures. Ab initio calculations suggest the lowest three Kramers doublets of the ground 6H15/2 multiplet of 2Dy are essentially pure, well-isolated |±15/2, |±13/2 and |±11/2 states quantised along the C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 Dy 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 C axis. Thermal relaxation occurs via the 4th and 5th doublets, verified experimentally for the first time, and calculated Ueff values of 742 and 810 K compare very well to experimental magnetism and luminescence data. This work validates a design strategy towards realising high-temperature SMMs and produces unusual spin relaxation behaviour where the magnetic observables are out-of-equilibrium some 6 K above the formal blocking temperature.

Tunable Raman excitation spectroscopy with 1.55–1.77 eV laser energies was used to map the second van Hove singularities of semiconducting single-walled carbon nanotubes ropes in air and immersed in water. The optical transitions are assigned to different (n,m) tubes using a correlation of the diameter and radial breathing mode (RBM) in the Raman spectrum. The resonance energies are blue-shifted when the tubes are immersed in water and the shift depends on the chiral angle. In addition, the RBM frequencies are also blue-shifted upon immersion in water.

Obtaining high level active n+ carrier concentrations in germanium (Ge) has been a significant challenge for further development of Ge devices. By ion implanting phosphorus (P) and fluorine (F) into Ge and restoring crystallinity using Nd:YAG nanosecond pulsed laser melting (PLM), we demonstrate 1020 cm−3 n+ carrier concentration in tensile-strained epitaxial germanium-on-silicon. Scanning electron microscopy shows that after laser treatment, samples implanted with P have an ablated surface, whereas P + F co-implanted samples have good crystallinity and a smooth surface topography. We characterize P and F concentration depth profiles using secondary ion mass spectrometry and spreading resistance profiling. The peak carrier concentration, 1020 cm−3 at 80 nm below the surface, coincides with the peak F concentration, illustrating the key role of F in increasing donor activation. Cross-sectional transmission electron microscopy of the co-implanted sample shows that the Ge epilayer region damaged during implantation is a single crystal after PLM. High-resolution X-ray diffraction and Raman spectroscopy measurements both indicate that the as-grown epitaxial layer strain is preserved after PLM. These results demonstrate that co-implantation and PLM can achieve the combination of n+ carrier concentration and strain in Ge epilayers necessary for next-generation, high-performance Ge-on-Si devices.

Co-doping with fluorine is a potentially promising method for defect passivation to increase the donor electrical activation in highly doped n-type germanium. However, regular high dose donor-fluorine co-implants, followed by conventional thermal treatment of the germanium, typically result in a dramatic loss of the fluorine, as a result of the extremely large diffusivity at elevatedtemperatures, partly mediated by the solid phase epitaxial regrowth. To circumvent this problem, we propose and experimentally demonstrate two non-amorphizing co-implantation methods; one involving consecutive, low dose fluorine implants, intertwined with rapid thermal annealing and the second, involving heating of the target wafer during implantation. Our study confirms that the fluorine solubility in germanium is defect-mediated and we reveal the extent to which both of these strategies can be effective in retaining large fractions of both the implanted fluorine and, critically, phosphorus donors.

We present our latest work on the development of an all-optical (pump-probe) technique for estimating the quantum efficiency (QE) of silicon nanocrystals. Our experiment combines measurement of micro-photoluminescence and reflectivity when driven with an RF frequency modulated pump. By varying this frequency over a wide (5 decade) range, we are able to determine the dynamics of the free carrier concentration (multi-component in these systems) and thus retrieve the PL decay rate(s). Interestingly, we find that, in addition to the dominant (10’s-us) component (typically noted from direct measurement of the PL decay), a very slow component persists (in the ms regime), which we attribute to the recombination of ‘trapped’ charges. The technique provides a method for estimating the relative QE as a function of carrier concentration, providing clues as to the efficiency limiting processes in these materials. We present our most recent data for silicon nanocrystals formed in glass cover-slips after ion implantation and thermal annealing, and we discuss how the technique can be applied to luminescent materials more generally, e.g. to study the efficiency ‘droop’ mechanism observed at high carrier concentration in nitride based LED materials.

We report a study of novel processing approaches for the formation of silicon nanocrystals for photonic devices. A silicon rich oxide was formed using ion implantation into thermally grown oxides on Silicon and transparent sapphire substrates. These layers were then treated with rapid thermal processing to observe the formation of silicon nanocrystals on a one second to ten minute timescale. Transmission electron microscopy and Raman scattering were used to follow the evolution of the nanocrystal mean diameter with anneal time. Identical samples co-implanted with Erbium demonstrated the widely reported non-resonant energy transfer mechanism from the nanocrystals to the internal energy levels of the erbium. We quantified the sensitization efficiency, IEr/Inc as a function of the Si-NC size by correcting the relative visible and IR luminescence intensities for variations in the nanocrystal density and observed photoluminescence lifetime. We find that the sensitizing efficiency increases exponentially with decreasing Si-NC mean diameter.

Luminescent Si-based materials are of significant interest due to their potential applications in the fields of photonics and solid state lighting. The use of rare earth dopants incorporated into Si-based materials allows the attainment of specific wavelengths of emission for color tunability. Details of the RE luminescence and even the ability to obtain luminescence are strongly dependent on the composition and structure of the Si-based host material. Through the use of X-ray absorption spectroscopy details of the atomic bonding environment of the films has been obtained. X-ray excited optical luminescence, measured after excitation at the absorption edges of the constituent atoms within these materials, reveals that excitation of the rare earth ions occurs primarily at energies related to oxygen states.

We describe the impact of non-radiative de-excitation mechanisms on the optical emission from silicon nanocrystals formed in SiO 2 . Auger excitation via free carriers deliberately introduced through phosphorus ion implantation, shows a monotonic increase with increasing phosphorus concentration which can be modelled adequately using a simple statistical approach. We also report a reduction in nanocrystal luminescence intensity with increasing exposure to UV radiation and suggest this phenomenon results from the introduction of non-radiative defects in the Si/SiO 2 network. The effect of UV radiation varies significantly depending on the sample preparation.

The chemical reaction between hydrogen and purely sp2-bonded graphene to form graphene’s purely sp3-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.

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.

We present an all-optical technique for measuring the quantum efficiency (QE) of luminescent materials; and, although generally applicable, we apply it as an example to silicon nanocrystals. The concept of internal quantum efficiency (IQE) for a light emission process; the fraction of electron-hole pairs that generate photons inside a material is quite a simple one, and yet in practice this is very difficult to measure. In fact, all existing methods rely on an estimation of IQE from the externalquantum efficiency (EQE); the ratio of photons emitted to those absorbed, typically with the assumption that IQE tends to unity at low temperature. This is an over-simplification and requires further assumptions about the dielectric materials that the photons traverse. An alternative approach was proposed earlier [1], in which both the light and heat, generated by the excitation process, are measured simultaneouslyas functions of a third variable, e.g. excitation laser power. However, experimental limitations such as reabsorption of light in the sample and recovery of a non-local thermal signal mean that only the EQE is ever really determined, albeit accurately [2]. In this contribution, we describe an all-optical (pump-probe) technique in which we balance the light leaving a local (μm-scale) region of excitation (by measuring μ-PL) with the heat generated at the focussed laser spot (by measuring changes in intensity of a near-IR reflected probe). Critically, our optical arrangement (three confocal signals in one microscope objective) combined with an advanced DSP ‘lock-in’ technique, permits measurement of ΔR/R to within 1 part in 106, corresponding to temperature changes on the mK scale. We present initial data on silicon nanocrystals, formed in glass cover-slips after ion implantation and thermal annealing, and discuss how the technique relates to a true measure of IQE with limitations and how it can be applied more generally to alternative material systems. References [1] D J Dunstan, ‘On the measurement of absolute radiative and non-radiative recombination efficiencies in semiconductor lasers’, J. Phys. D: Appl. Phys. 25 (1992) 1825 – 1828 [2] K R Catchpole et al, ‘High external quantum efficiency of planar semiconductor strucutres’, Semicond. Sci. Technol. 19 (2004) 1232 – 1235

A series of Bismuth (Bi) doped silicon oxide layers were prepared by ion-implantation. All the samples exhibit strong room temperature near-infrared photoluminescence in the range 1.0µm–1.3µm which we assign to Bi related centres in the oxide matrix, similar to that reported previously for Bi doped oxides fabricated by alternative methods. The activation and sensitization of these luminescent centres was studied as a function of anneal temperature and co-doping with silicon (Si) and aluminium (Al). Comparision with Erbium doped films prepared in a similar way reveals comparable emission intensity from the Bi doped films. The even wider Bi-related luminescence makes this system very promising for use in on-chip, broadband lasers and amplifiers, particularly for use in telecommunications.

Raman excitation and emission spectra for the radial breathing mode (RBM) are reported, together with a preliminary analysis. From the position of the peaks on the two-dimensional plot of excitation resonance energy against Raman shift, the chiral indices (m, n) for each peak are identified. Peaks shift from their positions in air when different pressure media are added - water, hexane, sulphuric acid - and when the nanotubes are unbundled in water with surfactant and sonication. The shift is about 2 - 3 cm-1 in RBM frequency, but unexpectedly large in resonance energy, being spread over up to 100meV for a given peak. This contrasts with the effect of pressure. The shift of the peaks of semiconducting nanotubes in water under pressure is orthogonal to the shift from air to water. This permits the separation of the effects of the pressure medium and the pressure, and will enable the true pressure coefficients of the RBM and the other Raman peaks for each (m, n) to be established unambiguously., 6 pages, 3 Figures, Proceedings of EHPRG 2011 (Paris)

We describe preliminary results from studies of the formation of silicon nano-crystals (Si-ncs) embedded in stoichiometric, thermally grown SiO 2 using Variable Energy Positron Annihilation Spectroscopy (VEPAS). We show that the VEPAS technique is able to monitor the introduction of structural damage. In SiO2 through the high dose Si+ ion implantation required to introduce excess silicon as a precursor to Si-nc formation. VEPAS is also able to characterize the rate of the removal of this damage with high temperature annealing, showing strong correlation with photoluminescence. Finally, VEPAS is shown to be able to selectively probe the interface between Si-ncs and the host oxide. Introduction of hydrogen at these interfaces suppresses the trapping of positrons at the interfaces.

We present the results of an optical investigation of a series of Er doped silicon nanocrystal (Si-NC) samples which were fabricated via ion implantation into SiO2 on sapphire substrates, followed by a range of rapid thermal processing. The photoluminescence spectra of the Si-NC emission revealed an increase in luminescence intensity and a red-shift of the peak wavelength as a function of annealing conditions. We attribute the former effect to the reduction of implantation induced defects with increasing annealing temperature/duration. Measurements of the rate of decay of photoluminescence intensity at room temperature show a corresponding increase in the carrier lifetimes which is also an indication of a reduced contribution from non-radiative centers. The red-shift of the peak Si-NC intensity is ascribed to an increasing mean Si-NC size as a function of the annealing conditions. Also presented is an estimation of the relative Er sensitization which reveals that the smallest Si-NC size distribution leads to the greatest sensitization ratio. Further investigation in the form of excitation spectroscopy was used to show that Er ions are sensitized not only by energy transfer from the Si-NCs, but also, crucially, from defect states in the SiO2.

We present a combined analysis using cross-sectional transmission electron microscopy (X-TEM) and Raman spectroscopy to study the early formation dynamics of Si-nanocrystals, formed in SiO2 thin films after Si+ implantation and rapid thermal processing (RTP). We obtained values for the diffusion coefficient of Si in thermally grown SiO2 and the activation energy to precipitate formation in the first 100 seconds of high temperature annealing. These values indicate that the formation of Si-nanocrystals in implanted oxides proceeds much more efficiently than purely via a self diffusion process. We propose that the nanocrystal formation is assisted by the presence of both oxygen vacancies and SiO molecular species, presumably generated by the ion irradiation. Microscopy images reveal the ensemble nanocrystal population to be most accurately represented by a lognormal distribution function with characteristic values for the mean particle diameter, d and variance, σ. The evolution of the silicon nanocrystals with annealing was also investigated by measuring the Raman scattering signal associated with the TO phonon mode arising from Si-Si bonds in Si-rich oxides grown on transparent (Al2O3) substrates. This greatly simplifies the experimental observation of the Raman spectra from Si-nanocrystals as compared to previous studies of nanocrystals in oxide films on silicon substrates.

We present a combined analysis of scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELs) of silicon-rich-silicon-oxide (SRSO) thin film on silicon, grown by plasma enhanced chemical vapour deposition (PECVD). For un-doped samples, strong room temperature luminescence at ~1.6 eV (780 nm) is observed, which we ascribe, by way of plasmon intensity mapping and dasiachemical fingerprintingpsila to phase segregated, highly crystalline, silicon-rich nano-clusters embedded in an amorphous-silicon dioxide (a-SiO2) matrix. For samples doped with increasing concentrations of Er, a quenching of the 1.6 eV line, concurrent with the emergence of a second emission with increasing intensity at ~0.8 eV (1535 nm) is observed. This is attributed to a rapid and efficient, indirect nano-crystal mediated excitation of the Er.

Phosphorus (P) is an n-type dopant for conductive silicon nanocrystals (Si-nc's), 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-nc's. 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-nc's 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.

We examine the near-IR light-matter interaction for graphene integrated cavity ring resonators based on silicon-on-insulator (SOI) race-track waveguides. Fitting of the cavity resonances from quasi-TE mode transmission spectra reveal the real part of the effective refractive index for graphene, neff = 2.23 ± 0.02 and linear absorption coefficient, αgTE = 0.11 ± 0.01dBµm-1. The evanescent nature of the guided mode coupling to graphene at resonance depends strongly on the height of the graphene above the cavity, which places limits on the cavity length for optical sensing applications.