-
[show abstract]
[hide abstract]
ABSTRACT: Several groups have reported on intermediate band solar cells (IBSC) fabricated with InAs/GaAs quantum dots (QD) which exhibit quantum efficiencies (QE) for sub-bandgap photon energies. However, this QE is produced by the absorption of photons only through valence band (VB) to intermediate band (IB) transitions. The absorption of photons of that energy in IB to conduction band (CB) transitions is weak and is usually replaced by carrier escape. This mechanism is incompatible with the preservation of the output voltage, and therefore, it cannot lead to the high efficiencies predicted by the IBSC model. In this work, we discuss the contribution of thermal and tunneling mechanisms to IB-CB carrier escape in current QD-IBSCs. It is experimentally demonstrated that in QD-IBSC prototypes where tunnel escape has been eliminated, the sub-bandgap QE is suppressed at sufficiently low temperatures, and when this occurs, the only limit for the open-circuit voltage (V<sub>OC</sub>) is the fundamental semiconductor bandgap, as stated by the IBSC theoretical model.
Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
-
A. Marti,
E. Antolin,
P.G. Linares,
E. Canovas,
D.F. Marron,
C. Tablero,
M. Mendes,
A. Mellor,
I. Tobias,
M.Y. Levy, [......], C.R. Stanley,
R.P. Campion,
J.L. Hall,
S.V. Novikov,
C.T. Foxon,
R. Scheer,
B. Marsen,
H.W. Schock,
M. Picault,
C. Chaix
[show abstract]
[hide abstract]
ABSTRACT: IBPOWER is a Project awarded under the 7th European Framework Programme that aims to advance research on intermediate band solar cells (IBSCs). These are solar cells conceived to absorb below bandgap energy photons by means of an electronic energy band that is located within the semiconductor bandgap, whilst producing photocurrent with output voltage still limited by the total semiconductor bandgap. IBPOWER employs two basic strategies for implementing the IBSC concept. The first is based on the use of quantum dots, the IB arising from the confined energy levels of the electrons in the dots. Quantum dots have led to devices that demonstrate the physical operation principles of the IB concept and have allowed identification of the problems to be solved to achieve actual high efficiencies. The second approach is based on the creation of bulk intermediate band materials by the insertion of an appropriate impurity into a bulk semiconductor. Under this approach it is expected that, when inserted at high densities, these impurities will find it difficult to capture electrons by producing a breathing mode and will cease behaving as non-radiative recombination centres. Towards this end the following systems are being investigated: a) Mn: In<sub>1-x</sub>Ga<sub>x</sub>N; b) transition metals in GaAs and c) thin films.
Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
-
K. Kalna,
A. Asenov,
J.S. Ayubi-Moak,
A.J. Craven,
R. Droopad,
R. Hill,
M.C. Holland,
X. Li,
A.R. Long,
P. Longo,
D. MacIntyre,
M. Passlack,
G. Paterson, C.R. Stanley,
S. Thoms,
H. Zhou,
I.G. Thayne
[show abstract]
[hide abstract]
ABSTRACT: The prospect for the introduction of III-V semiconductors into the channel of n-type MOSFETs and thus replace Si with a high mobility material for 22 nm technology generation and beyond is examined in detail. The so-called implantfree (IF) III-V MOSFET architecture option is presented showing a fabricated n-type IF demonstrator suitable for scaling. We then focus on a prediction of the potential performance of III-V MOSFETs through physically-based Monte Carlo (MC) device simulations. An implanted, n-type III-V MOSFETs based on In<sub>0.3</sub>Ga<sub>0.7</sub>As channel is investigated when scaled from a gate length of 30 nm to 20 nm and 15 nm. The impact of decisive scattering mechanisms operative at the dielectric/semiconductor interface is discussed. We also simulate the IF devices with low (In<sub>0.3</sub>Ga<sub>0.7</sub>As) and high (In<sub>0.75</sub>Ga<sub>0.25</sub>As) Indium content channel scaled to gate lengths of 30, 20 and 15 nm with equivalent layer thicknesses. The IF architecture is found to deliver a high drive current because of the highly efficient injection of electrons into the channel and because of very low access resistances. However, the low Indium content channel IF transistor is not able to further increase its drive current when scaled to the 15 nm gate length. Therefore, we examine also the performance of high indium channel transistors which delivers a steady increase in the device performance down to the 15 nm gate length.
Advanced Semiconductor Devices and Microsystems, 2008. ASDAM 2008. International Conference on; 11/2008
-
I.G. Thayne,
A. Asenov,
R.J.W. Hill,
M.C. Holland,
K. Kalna,
X. Li,
D. Macintyre,
D.A.J. Moran, C.R. Stanley,
S. Thoms, [......],
J. Abrokwah,
R. Droopad,
P. Fejes,
O. Hartin,
E. Johnson,
Y.-B. Park,
K. Rajagopalan,
A. Zlotnicka,
P. Zurcher,
M. Passlack
[show abstract]
[hide abstract]
ABSTRACT: This paper summarises the current state of the art in GaAs MOSFETs, and argues that the 4 decade search for a device quality compound semiconductor oxide is over. Under suitable growth conditions, a GaO/GaGdO high-k (~20) gate dielectric has been shown to have a mid-gap density of states of 2.5 x 10<sup>11</sup> cm<sup>-2</sup> eV<sup>-1</sup> [1], and vitally, an unpinned oxide-semiconductor interface [2]. With a scalable vertical architecture, enhancement mode implant-free III-V MOSFETs [3] with an In<sub>0.25</sub>GaAs channel layer, have yielded electron transport metrics comp<sub>25</sub>GaAs channel layer, have yielded electron transport metrics comparable to GaAs pHEMTs of similar materials compositions-namely mobility of above 5000 cm<sup>2</sup>/Vs for carrier concentration above 2 x 10<sup>12</sup> cm<sup>-2</sup> [4]. From these material structures, 1 mum gate length GaAs MOSFETs with a 10 nm gate oxide have been realised with threshold voltage of +0.26 V, saturation drive current, I<sub>d,sat</sub> = 407 mA/mm,arable to GaAs pHEMTs of similar materials compositions-namely mobility of above 5000 cm<sup>2</sup>/Vs for carrier concentration above 2 x 10<sup>12</sup> cm<sup>-2</sup> [4]. From these material structures, 1 mum gate length GaAs MOSFETs with a 10 nm gate oxide have been realised with threshold voltage of +0.26 V, saturation drive current, I<sub>d,sat</sub> = 407 mA/mm, maximum extrinsic transconductance, g<sub>m</sub> = 477 mS/mm, output conductance, g<sub>d</sub> = 11 mS/mm , gate leakage current, I<sub>g</sub> = 60 pA for gate voltages up to + 2.0 V, subthreshold swing, S = 102 mV/dec, on resistance, R<sub>on</sub> = 1.920 Omegamm, and I<sub>on</sub>/I<sub>off</sub> ratio = 6.3 x 10<sup>4</sup> [4]. A gate voltage swing of +2 V can easily be accommodated with these devices, making them attractive as a single supply III-V technology. Enhancement mode operation can be sustained to 300 nm with this oxide thickness, with peak intrinsic transconduc-
-
tance increasing to 600 mS/mm, indicative of the on-set of non-equilibrium transport effects, similar to short gate length pHEMTs [5]. Initial RF data from 0.8 mum gate length III-V MOSFETs with a threshold voltage of +0.4 V, resulted in f<sub>T</sub> and f<sub>max</sub> of 14 GHz and 40 GHz respectively.
RF and Microwave IC Design, 2008 IET Seminar on; 03/2008
-
[show abstract]
[hide abstract]
ABSTRACT: The solar cell industry is witnessing an era of unprecedented growth and this trend is set to continue for the foreseeable
future. Here we describe a heat reflection pigment-coated single-junction gallium arsenide solar cell that is capable of reflecting
heat-inducing near-infrared radiation. The cell maintains its performance better than non-coated cells when exposed to infrared-rich
radiant flux. In situations where solar cells get heated mainly from incident infrared radiation, these cells exhibit superior
performance. The heat reflecting pigment, cell structure, coating process and cell performance have been described.
Applied Physics A 01/2008; 90(2):231-236. · 1.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We show the experimental realization of a 108-GHz planar Gunn diode structure fabricated in GaAs/AlGaAs. There is a considerable interest in such devices since they lend themselves to integration into millimeter-wave and terahertz integrated circuits. The material used was grown by molecular beam epitaxy, and devices were made using electron beam lithography. Since the frequency of oscillation is defined by the lithographically controlled anode-cathode distance, the technology shows great promise in fabricating single chip terahertz sources.
IEEE Electron Device Letters 11/2007; · 2.85 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The characteristics of intermediate band solar cells containing 10, 20, and 50 InAs quantum dot (QD) layers embedded in otherwise “standard” (Al,Ga)As solar cell structures have been compared. The short-circuit current densities of the cells decreased and the quantum efficiencies of the devices showed a concomitant reduction in the minority carrier lifetime in the p emitters with increasing number of QD layers. Dislocations threading up from the QDs toward the surface of the cells, and revealed by bright field scanning transmission electron microscopy, are the most likely cause of the deterioration in the electrical performance of the cells.
Applied Physics Letters 07/2007; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A millimetre-wave Gunn triode device using an AlGaAs/GaAs quantum well structure is realised. The material used was grown by molecular beam epitaxy and devices were made using electron beam lithography. Oscillation occurs at 83 GHz for a 0.5 mum gate length device with a 1.3 mum gate-cathode separation. Experimental results are in excellent agreement with Monte Carlo calculations. Planar Gunn triodes have potential for high frequency chip integration for millimetre-wave and terahertz applications.
Electronics Letters 02/2007; · 0.96 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present intermediate-band solar cells manufactured using quantum dot technology that show for the first time the production of photocurrent when two sub-band-gap energy photons are absorbed simultaneously. One photon produces an optical transition from the intermediate-band to the conduction band while the second pumps an electron from the valence band to the intermediate-band. The detection of this two-photon absorption process is essential to verify the principles of operation of the intermediate-band solar cell. The phenomenon is the cornerstone physical principle that ultimately allows the production of photocurrent in a solar cell by below band gap photon absorption, without degradation of its output voltage.
Physical Review Letters 01/2007; 97(24):247701. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the first integration of a resonant tunneling diode and an optical communications laser operating at around 1.5 μm. We demonstrate its low-frequency bistable operation and model its electrical characteristics.
IEEE Photonics Technology Letters 08/2006; · 2.19 Impact Factor
-
R. Iwanow,
G I Stegeman,
D N Christodoulides,
R Morandotti,
D. Modotto,
A. Locatelli,
C. De Angelis,
F Pozzi, C.R. Stanley,
M Sorel,
J.S. Aitchison
[show abstract]
[hide abstract]
ABSTRACT: We report the first observation of self-phase-modulation enhancement in 500 nm wide, Kerr nonlinear AlGaAs nanowaveguides.
Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference. CLEO/QELS 2006. Conference on; 06/2006
-
[show abstract]
[hide abstract]
ABSTRACT: GaAs-based transistors with the highest f<sub>T</sub> and lowest noise figure reported to date are presented in this letter. A 50-nm T-gate In<sub>0.52</sub>Al<sub>0.48</sub>As/In<sub>0.53</sub>Ga<sub>0.47</sub>As metamorphic high-electron mobility transistors (mHEMTs) on a GaAs substrate show f<sub>T</sub> of 440 GHz, f<sub>max</sub> of 400 GHz, a minimum noise figure of 0.7 dB and an associated gain of 13 dB at 26 GHz, the latter at a drain current of 185 mA/mm and g<sub>m</sub> of 950 mS/mm. In addition, a noise figure of below 1.2 dB with 10.5 dB or higher associated gain at 26 GHz was demonstrated for drain currents in the range 40 to 470 mA/mm at a drain bias of 0.8 V. These devices are ideal for low noise and medium power applications at millimeter-wave frequencies.
IEEE Electron Device Letters 12/2005; · 2.85 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: One-dimensional (1-D) deep-etched gratings on a specially grown AlGaAs wafer were designed and fabricated. The gratings were fabricated using state-of-the-art electron beam lithography and high-aspect-ratio reactive ion etching (RIE) in order to achieve the required narrow deep air slots with good accuracy and reproducibility. Since remarkable etch depths (up to 1.5 μm), which completely cut through the waveguide core layer, have been attained, gratings composed of only five periods (and, thus, shorter than 6 μm) have a bandgap larger than 100 nm. A defect was introduced by increasing the width of the central semiconductor tooth to create microcavities that exhibit a narrow transmission peak (less than 7 nm) around the wavelength of 1530 nm. The transmission spectra between 1460 and 1580 nm have been systematically measured, and the losses have been estimated for a set of gratings, both with and without a defect, for different periods and air slot dimensions. Numerical results obtained via a bidirectional beam propagation code allowed the evaluation of transmissivity, reflectivity, and diffraction losses. By comparing experimental results with the authors' numerical findings, a clear picture of the role of the grating's geometric parameters in determining its spectral features and diffractive losses is illustrated.
Journal of Lightwave Technology 12/2005; 23(11):3832- 3842. · 2.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the first integration of a resonant tunneling diode and an optical communications laser operating at 1550 nm to form an electrically bistable device suitable for non-return to zero (NRZ) modulation.
Lasers and Electro-Optics Society, 2005. LEOS 2005. The 18th Annual Meeting of the IEEE; 11/2005
-
A.C. Walker,
S.J. Fancey,
M.P.Y. Desmulliez,
M.G. Forbes,
J.J. Casswell,
G.S. Buller,
M.R. Taghizadeh,
J.A.B. Dines, C.R. Stanley,
G. Pennelli, [......],
D. Byrne,
J. Hegarty,
S. Eitel,
H.-P. Gauggel,
K.-H. Gulden,
A. Gauthier,
P. Benabes,
J.-L. Gutzwiller,
M. Goetz,
J. Oksman
[show abstract]
[hide abstract]
ABSTRACT: The experimental operation of a terabit-per-second scale optoelectronic connection to a silicon very-large-scale-integrated circuit is described. A demonstrator system, in the form of an optoelectronic crossbar switch, has been constructed as a technology test bed. The assembly and testing of the components making up the system, including a flip-chipped InGaAs-GaAs optical interface chip, are reported. Using optical inputs to the electronic switching chip, single-channel routing of data through the system at the design rate of 250 Mb/s (without internal fan-out) was achieved. With 4000 optical inputs, this corresponds to a potential aggregate data input of a terabit per second into the single 14.6 × 15.6 mm CMOS chip. In addition 50-Mb/s data rates were switched utilizing the full internal optical fan-out included in the system to complete the required connectivity. This simultaneous input of data across the chip corresponds to an aggregate data input of 0.2 Tb/s. The experimental system also utilized optical distribution of clock signals across the CMOS chip.
IEEE Journal of Quantum Electronics 08/2005; · 1.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on W-band LNA (MMMICs) based around a 50nm InP-HEMTs with an f<sub>T</sub> of 0.550 THz. The LNA noise figure is 2.5 dB and associated gain of 7.3 dB at 90 GHz with a bandwidth of 24 GHz
Indium Phosphide and Related Materials, 2005. International Conference on; 06/2005
-
[show abstract]
[hide abstract]
ABSTRACT: We report electro-absorption modulation of light at around 1550 nm in a unipolar InGaAlAs optical waveguide containing a InGaAs/AlAs double-barrier resonant tunneling diode (DB-RTD). The RTD peak-to-valley transition increases the electric field across the waveguide, which shifts the core material absorption band-edge to longer wavelengths via the Franz-Keldysh effect, thus changing the light-guiding characteristics of the waveguide. Low-frequency characterisation of a device shows modulation up to 28 dB at 1565 nm. When dc biased close to the negative differential conductance (NDC) region, the RTD optical waveguide behaves as an electro-absorption modulator integrated with a wide bandwidth electrical amplifier, offering a potential advantage over conventional pn modulators.
Applied Physics Letters 04/2005; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report high speed optical modulation in a resonant tunneling relaxation oscillator consisting of a resonant tunneling diode (RTD) integrated with a unipolar optical waveguide and incorporated in a package with a coplanar waveguide transmission line. When appropriately biased, the RTD can provide wide-bandwidth electrical gain. For wavelengths near the material band-edge, small changes of the applied voltage give rise to large, high-speed electro-absorption modulation of the light. We have observed optical modulation at frequencies up to 14 GHz, associated with sub harmonic injection locking of the RTD oscillation at the fundamental mode of the coplanar transmission line, as well as generation of 33 ps optical pulses due to relaxation oscillation.
Applied Physics Letters 04/2005; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Embedding a double barrier resonant tunnelling diode (RTD) in an unipolar InGaAlAs optical waveguide gives rise to a very low driving voltage electroabsorption modulator (EAM) at optical wavelengths around 1550 nm. The presence of the RTD within the waveguide core introduces high non-linearity and negative differential resistance in the current-voltage (I-V) characteristic of the waveguide. This makes the electric field distribution across the waveguide core strongly dependent on the bias voltage: when the current decreases from the peak to the valley there is an increase of the electric field across the depleted core. The electric field enhancement in the core-depleted layer causes the Franz-Keldysh absorption band-edge to red shift, which is responsible for the electroabsorption effect. High frequency ac signals as low as 100 mV can induce electric field high speed switching, producing substantial light modulation (up to 15 dB) at photon energies slightly lower than the waveguide core band-gap energy. The key difference between this device and conventional p-i-n EAMs is that the tunnelling characteristics of the RTD are employed to switch the electric field across the core depleted region; the RTDEAM has in essence an integrated electronic amplifier and therefore requires considerably less switching power.
Journal of Modern Optics 04/2005; · 1.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The work describes recent progress in the design, simulation, implementation and characterisation of photonic crystal (PhC) GaAs-based quantum cascade lasers (QCLs). The benefits of applying active PhC confinement around a QCL cavity are explained, highlighting a route to reduced threshold current operation. Design of a suitable PhC has been performed using published bandgap maps; simulation results of this PhC show a wide, high reflectivity stopband. Implementation of the PhC for the device is particularly difficult, requiring a very durable metallic dry etch mask, high performance dry etching and a low damage epilayer-down device mounting technique. Preliminary shallow etched PhC QCLs demonstrated the viability of current injection through the metal etch mask and the device mounting technique. Development of the etch mask and dry etching have demonstrated a process suitable for the manufacture of deep etched PhC structures. All the necessary elements for implementing deep etched PhC QCLs have now been demonstrated, allowing for the development of high performance devices.
IEE Proceedings - Optoelectronics 01/2005; · 0.71 Impact Factor
