Philip Derek Buckle

Photoluminescence decay time measurements from self-organized InAs/GaAs quantum dots

In this article we report the results of time integrated and time resolved photoluminescence spectroscopy and photoluminescence time decay measurements as a function of excitation density at 6 K on high quality self-organized InAs/GaAs quantum dots. To understand the form of the experimentally observed photoluminescence transients a Monte Carlo model has been developed that allows for the effects of random capture of photo-excited carriers. By comparison with the results of our model we are able to ascribe the excitation density dependence of the overall form of the decay of the emission from the quantum dot ground states and the biexponential nature of the decay of the first excited state emission as being due to the combined effects of radiative recombination, density dependent carrier scattering, and the restriction of carrier scattering due to state blocking caused by the effects of Pauli exclusion. To successfully model the form of the biexponential decay of the highest energy excited states we have ...

Zero-field spin splitting and spin-dependent broadening in high-mobility InSb/In1−xAlxSb asymmetric quantum well heterostructures

We present high field magneto-transport data from a range of 30nm wide InSb/InAlSb quantum wells. The low temperature carrier mobility of the samples studied ranged from 18.4 to 39.5 m2V-1s-1 with carrier densities between 1.5x1015 and 3.28x1015 m-2. Room temperature mobilities are reported in excess of 6 m2V-1s-1. It is found that the Landau level broadening decreases with carrier density and beating patterns are observed in the magnetoresistance with non-zero node amplitudes in samples with the narrowest broadening despite the presence of a large g-factor. The beating is attributed to Rashba splitting phenomenon and Rashba coupling parameters are extracted from the difference in spin populations for a range of samples and gate biases. The influence of Landau level broadening and spin-dependent scattering rates on the observation of beating in the Shubnikov-de Haas oscillations is investigated by simulations of the magnetoconductance. Data with non-zero beat node amplitudes are accompanied by asymmetric peaks in the Fourier transform, which are successfully reproduced by introducing a spin-dependent broadening in the simulations. It is found that the low-energy (majority) spin up state suffers more scattering than the high-energy (minority) spin down state and that the absence of beating patterns in the majority of (lower density) samples can be attributed to the same effect when the magnitude of the level broadening is large.

Zero-field spin splitting and spin lifetime in n-InSb/In1−xAlxSb asymmetric quantum well heterostructures

The spin-orbit (SO) coupling parameters for the lowest conduction subband due to structural inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) are calculated for a range of carrier densities in [001]-grown δ-doped n-type InSb∕In1−xAlxSb quantum wells using the established eight-band k⋅p formalism [ J. Deng et al. Phys. Rev. B 59 R5312 (1999)]. We present calculations for conditions of zero bias at 10 K. It is shown that both the SIA and BIA parameters scale approximately linearly with carrier density, and exhibit a marked dependence on well width when alloy composition is adjusted to allow maximum upper barrier height for a given well width. In contrast to other material systems, the BIA contribution to spin splitting is found to be of significant and comparable value to the SIA mechanism in these structures. We calculate the spin lifetime τs[11 0] for spins oriented along [11 0] based on D’yakonov–Perel’ mechanism using both the theory of Averkiev et al. J. Phys.: Condens. Matter 14 R271 (2002) and also directly the rate of precession of spins about the effective magnetic field, taking into account all three SO couplings, which show good agreement. τs[11 0] is largest in the narrowest wells over the range of moderate carrier densities considered, which is attributed to the reduced magnitude of the k-cubic BIA parameter in narrow wells. The inherently large BIA induced SO coupling in these systems is shown to have considerable effect on τs[11 0], which exhibits significant reduction in the maximum spin lifetime compared to previous studies that consider systems with relatively weak BIA induced SO coupling. The relaxation rate of spins oriented in the [001] direction is found to be dominated by the k-linear SIA and BIA coupling parameters and at least an order of magnitude greater than in the [11 0] direction.

Functional molecular wires

The properties of self-assembled molecules may be tuned by sequentially coupling components on a gold surface, the molecular electronics toolbox of chemically reactive building blocks yielding molecular wires with diode-like current-voltage (I-V) characteristics. The bias for rectification in each case is dependent upon the sequence of electron-donating and electron-accepting moieties and similar behaviour has been achieved for four different contacting techniques.

Photoluminescence spectroscopy and decay time measurements of polycrystalline thin film CdTe/CdS solar cells

The results of room temperature photoluminescence spectroscopy and decay time measurements carried out on CdTe/CdS solar cells are reported. The as-grown structures were annealed in air at temperatures in the range 350–550 °C. For excitation via the CdTe/CdS interface, longer photoluminescence decay times were observed as the anneal temperature was increased, this is attributed to localized passivation of nonradiative states possibly due to the effect of S interdiffusion. When the photoluminescence is excited via the CdTe free surface, the decay curves consist of a fast and slow component. The fast component (<130 ps) of the photoluminescence is attributed to nonradiative recombination at grain boundaries or the CdTe free surface. The slow component is attributed to the effects of carrier drift and diffusion and subsequent recombination at the CdTe/CdS interface. Modeling of the transport process has led to the extraction of a value of 0.20±0.03 cm2 s−1 for the minority carrier diffusion coefficient of th...

The cyclotron resonance in Heterostructures with the InSb/AlInSb quantum wells

The absorption of two-dimensional electrons in InSb-based quantum wells in the quantized magnetic fields in the terahertz spectral region are studied. A p-Ge-based cyclotron laser was used as the radiation source. The effective mass of carriers at the Fermi level equal to 0.0219m0 (m0 is the mass of a free electron) is determined from the cyclotron resonance spectra. It is shown that the electron spectrum is described by the Kane model in a wide range of magnetic fields. An anomalously pronounced splitting of the cyclotron resonance line not associated with the nonparabolicity of the conduction band of InAs is observed in low magnetic fields, which can be attributed to the effect of the spin-orbit interaction.

A surface-gated InSb quantum well single electron transistor

Single electron charging effects in a surface-gated InSb/AlInSb QW structure are reported. This material, due to its large g-factor and light effective mass, offers considerable advantages over more commonly used materials, such as GaAs, for quantum information processing devices. However, differences in material and device technology result in significant processing challenges. Simple Coulomb blockade and quantized confinement models are considered to explain the observation of conductance oscillations in these structures. The charging energy (e2/C) is found to be comparable with the energy spectrum for single particle states (ΔE).

Room temperature ballistic transport in InSb quantum well nanodevices.

We report the room temperature observation of significant ballistic electron transport in shallow etched four-terminal mesoscopic devices fabricated on an InSb/AlInSb quantum well (QW) heterostructure with a crucial partitioned growth-buffer scheme. Ballistic electron transport is evidenced by a negative bend resistance signature which is quite clearly observed at 295 K and at current densities in excess of 10(6) A/cm(2). This demonstrates unequivocally that by using effective growth and processing strategies, room temperature ballistic effects can be exploited in InSb/AlInSb QWs at practical device dimensions.

Transport effects in remote-doped InSb/AlxIn1-xSb heterostructures

Low- and high-field magnetotransport measurements on two 30nm -doped InSb/AlInSb quantum wells (QWs) with different doping densities are reported. The QW two-dimensional electron gas (2DEG) carrier densities and mobilities were extracted by analysis of the Hall and quantum Hall data, mobility spectra and Shubnikov-de Haas oscillations. 2DEG channel mobilities of up to 324000cm 2 V 1 s 1 (T = 2K) and 44000cm 2 V 1 s 1 (T = 300K) are extracted. Carrier densities and mobilities for transport parallel to the 2DEG layer are also deduced where observable. The importance of thermally generated carriers in the lower AlInSb barrier material and the role of transport within the -doping plane is considered and the total carrier population as a function of temperature of the two samples is deduced, which is in excellent agreement with experimental observation.

Charge accumulation in GaAs/AlGaAs triple barrier resonant tunneling structures

In this article we present photoluminescence and photoluminescence excitation spectroscopy data from three triple barrier resonant tunneling structures. The spectroscopic techniques are used to estimate the charge accumulation in both tunneling quantum wells of the devices as a function of bias. The charging behavior is extremely asymmetrical, with significant charge accumulation only in the quantum well adjacent to the emitter region of the device and not in the quantum well adjacent to the collector region, irrespective of the direction of bias. This asymmetry in the charging behavior is analogous to highly asymmetrical double barrier resonant tunneling structures. However, due to the two quantum wells present in the triple barrier design it provides a more flexible system to study charge density dependent effects. We also present evidence for negatively charged exciton formation in the first quantum well for both directions of applied bias.