Mark Hughes - Academia.edu (original) (raw)

Papers by Mark Hughes

Carrier-type reversal to enable the formation of semiconductor p-n junctions is a prerequisite fo... more Carrier-type reversal to enable the formation of semiconductor p-n junctions is a prerequisite for many electronic applications. Chalcogenide glasses are p-type semiconductors and their applications have been limited by the extraordinary difficulty in obtaining n-type conductivity. The ability to form chalcogenide glass p-n junctions could improve the performance of phase-change memory and thermoelectric devices and allow the direct electronic control of nonlinear optical devices. Previously, carrier-type reversal has been restricted to the GeCh (Ch ¼ S, Se, Te) family of glasses, with very high Bi or Pb 'doping' concentrations (B5-11 at.%), incorporated during high-temperature glass melting. Here we report the first n-type doping of chalcogenide glasses by ion implantation of Bi into GeTe and GaLaSO amorphous films, demonstrating rectification and photocurrent in a Bi-implanted GaLaSO device. The electrical doping effect of Bi is observed at a 100 times lower concentration than for Bi melt-doped GeCh glasses.

$Research paper thumbnail of Direct laser writing of relief diffraction gratings into a bulk chalcogenide glass$

We inscribed relief diffraction gratings with periods of 6, 14, and 24 μm into the surface of Ge ... more We inscribed relief diffraction gratings with periods of 6, 14, and 24 μm into the surface of Ge 15 Ga 3 Sb 12 S 70 bulk glass by the material's ablation using a femtosecond λ 800 nm Ti:sapphire pulsed laser. The laser writing was done with sample implemented on a computer-controlled stage employing surface-to-beam alignment, laser power, and raster pattern control. Pulse energies of 1.5, 3.0, and 4.5 μJ were focused on spot diameter of 1.5 μm, resulting in channel widths, measured on the surface, of around 4, 5, and 6 μm and depths up to 1.7 μm. The first-order diffraction efficiency of the fabricated gratings was up to 10% at λ 650 nm. We have also fabricated a "composite" grating combining the three relief diffraction gratings inscribed in the same position, but with a mutual tilt. The composite grating provides complex multidirectional diffraction of the light in accordance with geometrical arrangement and grating period of all the gratings inscribed. We propose practical applications of femtosecond pulsed-laser surface patterning, for example, surface-relief diffraction microgratings integrated at the ends of multimode mid-IR chalcogenide optical waveguides or on the surfaces of bare core chalcogenide glass optical fibers used for chemical sensing.

Vanadium doped chalcogenide glass has potential as an active gain medium, particularly at telecom... more Vanadium doped chalcogenide glass has potential as an active gain medium, particularly at telecommunications wavelengths. This dopant has three spin allowed absorption transitions at 1100, 737 and 578 nm, and a spin forbidden absorption transition at 1000 nm. X-ray photoelectron spectroscopy indicated the presence of vanadium in a range of oxidation states from V + to V 5+ . Excitation of each absorption band resulted in the same characteristic emission spectrum and lifetime, indicating that only one oxidation state is optically active. Arguments based on Tanabe-Sugano analysis indicated that the configuration of the optically active vanadium ion was octahedral V 2+ . The calculated crystal field parameters (Dq/B, B and C/B) were 1.85, 485.1 and 4.55, respectively.

We report electroluminescence (EL) from a carbon nanotube field-effect transistor with split-gate... more We report electroluminescence (EL) from a carbon nanotube field-effect transistor with split-gates. EL is generated by the electrostatic doping technique. Six EL bands could be observed, with the strongest band peaking between 0.867 and 0.850 eV with a full width at half maximum (FWHM) of 64 to 120 meV, depending on the bias conditions. From the EL peak position we estimate a CNT diameter of ∼ 1.05 nm. We also estimate the power and quantum efficiency of the EL to be around 1 × 10 −6 and 1 × 10 −5 respectively. With a fixed drain voltage, increasing the opposite split gate bias caused the EL to increase monotonically from zero, indicating an ambipolar emission mechanism. With a fixed opposite split gate bias the dependence of EL on drain voltage displays a threshold. Drain current is significantly higher when using equal, rather than opposite split gate biases, which we attribute to the trapping of carries by band bending when using opposite split gate bias.

Nd-doped SiO 2 -B 2 O 3 -Na 2 O-Al 2 O 3 -CaO-ZrO 2 glasses were prepared and optical properties ... more Nd-doped SiO 2 -B 2 O 3 -Na 2 O-Al 2 O 3 -CaO-ZrO 2 glasses were prepared and optical properties such as absorption, lifetime and quantum efficiencies (QEs) of the emission were characterized. QE measurement system with natural sunlight as an excitation source was constructed. The radiative QE (η r ) obtained from the lifetime by Judd-Ofelt analysis, directly measured QEs using natural sunlight (η ns ), simulated sunlight (η ss ) and an 800 nm laser (η 800 ) were measured and were compared with each other. The QE measurements suggest that the reduction of parasitic absorption by the host glass could be a more important factor for solar pumped lasers than for laser diode pumped lasers in order to increase the efficiency.

a b s t r a c t We fabricated three series of glasses in which the Y 2 O 3 , Bi 2 O 3 and Al 2 O ... more a b s t r a c t We fabricated three series of glasses in which the Y 2 O 3 , Bi 2 O 3 and Al 2 O 3 contents of Bi-doped Y 2 O 3 :Al 2 O 3 :SiO 2 glass were varied. We optimized the content of each component based on quantum efficiency (QE) and other optical properties. The strength of the Bi absorption bands decreased with increasing Y 2 O 3 content. The optimized composition was: 63.5SiO 2 :22.5Al 2 O 3 :13Y 2 O 3 :1Bi 2 O 3 . With 808 nm excitation this glass had a QE, lifetime, emission width and peak of 20%, 375 ls, 293 and 1240 nm, respectively. The Bi absorption strength had a quadratic dependence on Bi 2 O 3 content. This indicates that the optically active Bi center is Bi 2 dimers.

We fabricated a Ti/Pd asymmetrically contacted single carbon nanotube (CNT) field-effect transist... more We fabricated a Ti/Pd asymmetrically contacted single carbon nanotube (CNT) field-effect transistor (FET) with split-gates. Transfer characteristics can be explained if the Schottky barrier for electrons is lower at the Pd contact than it is at the Ti contact. Strong rectification is observed when the gates are unbiased, and the rectification direction can be inverted with the appropriate gate bias. When operated as an FET the device has an on/off ratio of 1 × 10 7 . Under illumination, photocurrent can only be observed with opposite split-gate bias. Open circuit voltage (V OC ) and short circuit current (I SC ) increase with increasing opposite polarity split-gate bias, representing the first demonstration of the modulation of V OC and I SC in an asymmetric contact CNT FET.

Reaction order in Bi-doped oxide glasses depends on the optical basicity of the glass host. Red a... more Reaction order in Bi-doped oxide glasses depends on the optical basicity of the glass host. Red and NIR photoluminescence (PL) bands result from Bi 2+ and Bi n clusters, respectively. Very similar centers are present in Bi-and Pb-doped oxide and chalcogenide glasses. Bi-implanted and Bi melt-doped chalcogenide glasses display new PL bands, indicating that new Bi centers are formed. Bi-related PL bands have been observed in glasses with very similar compositions to those in which carrier-type reversal has been observed, indicating that these phenomena are related to the same Bi centers, which we suggest are interstitial Bi 2+ and Bi clusters.

We report the compositional dependence of absorption, emission, decay constant and quantum effici... more We report the compositional dependence of absorption, emission, decay constant and quantum efficiency (QE) of GeO 2 :Al 2 O 3 :PbO:Bi 2 O 3 glass with the aim of finding the composition most suitable for a laser gain medium. As the Bi 2 O 3 content was varied we observed a non-monotonic variation in absorption spectrum. The optimized composition was 87.4GeO 2 :6Al 2 O 3 :6PbO:0.6Bi 2 O 3 (% molar) which had a QE of 67% but low transparency. We report the dependence of QE and decay constant on melting conditions of this glass. Adding 10% ZnO improved the transparency, therefore the optimized and transparent composition was 74.4GeO 2 :10ZnO:6Al 2 O 3 :6PbO:0.6Bi 2 O 3 , which had an emission full width at half maximum of 500 nm.

The single crystal growth conditions and spectroscopic characterization of Ni-doped MgGa 2 O 4 wi... more The single crystal growth conditions and spectroscopic characterization of Ni-doped MgGa 2 O 4 with inverse-spinel structure crystal family are described. Single crystals of this material have been grown by floating zone method. Ni-doped MgGa 2 O 4 single crystals have broadband fluorescence in the 1100-1600 nm wavelength range, 1.6 ms room temperature lifetime, 56% quantum efficiency and 1:05 Â 10 À21 cm 2 stimulated emission cross section at the emission peak. This new material is very promising for tunable laser applications covering the important optical communication and eye safe wavelength region.

The authors describe the fabrication of buried waveguides in a highly nonlinear chalcogenide glas... more The authors describe the fabrication of buried waveguides in a highly nonlinear chalcogenide glass, gallium lanthanum sulfide, using focused femtosecond laser pulses. Through optical characterization of the waveguides, they have proposed a formation mechanism and provide comparisons to previous work. Tunneling has been identified as the dominant nonlinear absorption mechanism in the formation of the waveguides. Single mode guidance at 633 nm has been demonstrated. The writing parameters for the minimum propagation loss of 1.47 dB/ cm are 0.36 J pulse energy and 50 m / s scanning speed.

We report the lattice site and symmetry of optically active Dy 3+ and Tm 3+ implanted Si. Local s... more We report the lattice site and symmetry of optically active Dy 3+ and Tm 3+ implanted Si. Local symmetry was determined by fitting crystal field parameters (CFPs), corresponding to various common symmetries, to the ground state splitting determined by photoluminescence measurements. These CFP values were then used to calculate the splitting of every J manifold. We find that both Dy and Tm ions are in a Si substitution site with local tetragonal symmetry. Knowledge of rare-earth ion symmetry is important in maximising the number of optically active centres and for quantum technology applications where local symmetry can be used to control decoherence. OCIS codes: (160.2540) Fluorescent and luminescent materials; (160.6000) Semiconductor materials. k n n J J f LSJM C f L S J M J J M k k n n n n J q J J J J J

Low resistance ohmic contacts on AlGaN/GaN structures using implantation and the "advancing" Al/T... more Low resistance ohmic contacts on AlGaN/GaN structures using implantation and the "advancing" Al/Ti metallization Appl.

We report emission from a bismuth doped chalcogenide glass which is flattened, has a full width a... more We report emission from a bismuth doped chalcogenide glass which is flattened, has a full width at half maximum (FWHM) of 600 nm, peaks at 1300 nm and covers the entire telecommunications window. At cryogenic temperatures the FWHM reaches 850 nm. The quantum efficiency and lifetime were as high as 32% and 175 µs, respectively. We also report two new bismuth emission bands at 2000 and 2600 nm. Absorption bands at 680, 850, 1020 and 1180 nm were observed. The 1180 nm absorption band was previously unobserved. We suggest that the origin of the emission in Bi:GLS is Bi 2 2 − dimers.

In this paper we present the fluorescence decay profiles of vanadium and titanium doped gallium l... more In this paper we present the fluorescence decay profiles of vanadium and titanium doped gallium lanthanum sulphide (GLS) glass at various doping concentrations between 0.01 and 1% (molar). We demonstrate that below a critical doping concentration the fluorescence decay profile can be fitted with the stretched exponential function: exp[-(t/τ) β ], where τ is the fluorescence lifetime and β is the stretch factor. At low concentrations the lifetime for vanadium and titanium doped GLS was 30 μs and 67 μs respectively. We validate the use of the stretched exponential model and discuss the possible microscopic phenomenon it arises from. We also demonstrate that above a critical doping concentration of around 0.1% (molar) the fluorescence decay profile can be fitted with the double exponential function: a*exp-(t/τ 1 )+ b*exp-(t/τ 2 ), where τ 1 and τ 2 are characteristic fast and slow components of the fluorescence decay profile, for vanadium the fast and slow components are 5 μs and 30 μs respectively and for titanium they are 15 μs and 67 μs respectively. We also show that the fluorescence lifetime of vanadium and titanium at low concentrations in the oxide rich host gallium lanthanum oxy-sulphide (GLSO) is 43 μs and 97 μs respectively, which is longer than that in GLS. From this we deduce that vanadium and titanium fluorescing ions preferentially substitute into high efficiency oxide sites until at a critical concentration they become saturated and low efficiency sulphide sites start to be filled.

Electroluminescence from carbon nanotubes (CNTs) may have applications in nanoscale lasers and op... more Electroluminescence from carbon nanotubes (CNTs) may have applications in nanoscale lasers and optoelectronic devices. Electrostatic doping is a novel technique which has been used to demonstrate CNT field effect transistor (FET) p-n junction diodes [1], it can also be used to modify the band structure of CNTs to facilitate electroluminescence. In this work we report electroluminescence from an electrostatically doped CNT in a FET configuration. shows the device schematic. The source, drain and gate electrodes were fabricated by electron beam lithography. The source-drain distance is 1.6 μm, the gate 1-gate 2 distance is 1 μm. The band diagram in shows how the application of opposite polarities to gates 1 and 2 can facilitate electroluminescence. shows the electroluminescence spectrum of the device fitted with a Gaussian. The source, drain, gate 1 and gate 2 bias conditions were 0V, +10 V, +45 V and -40 V, respectively. The drain current was 16 μA. The peak is at 968 meV which corresponds to a CNT diameter of ~ 0.93 nm. The width of the emission is 87 meV. This is narrower than the 150 meV width reported for the recombination of hot carriers [2]. Schematic of the device configuration, b) energy band diagram of the electroluminescence process, c) electroluminescence from a device with a 1.6 μm CNT fitted with a Gaussian.

$Research paper thumbnail of Direct laser writing of relief diffraction gratings into a bulk chalcogenide glass$