Professor Steven Clowes

In this perspective article, we discuss the application of ion implantation to manipulate strain (by either neutralizing or inducing compressive or tensile states) in suspended thin films. Emphasizing the pressing need for a high-mobility silicon-compatible transistor or a direct bandgap group-IV semiconductor that is compatible with complementary metal-oxide-semiconductor technology, we underscore the distinctive features of different methods of ion beam-induced alteration of material morphology. The article examines the precautions needed during experimental procedures and data analysis and explores routes for potential scalable adoption by the semiconductor industry. Finally, we briefly discuss how this highly controllable strain-inducing technique can facilitate enhanced manipulation of impurity-based spin quantum bits (qubits).

We report on the observation of linear and circular magnetogyrotropic photogalvanic effects in InSb/AlInSb quantum well structures. We show that intraband (Drude-like) absorption of terahertz radiation in the heterostructures causes a dc electric current in the presence of an in-plane magnetic field. The photocurrent behavior upon variation of the magnetic field strength, temperature and wavelength is studied. We show that at moderate magnetic fields the photocurrent exhibits a typical linear field dependence. At high magnetic fields, however, it becomes nonlinear and inverses its sign. The experimental results are analyzed in terms of the microscopic models based on asymmetric relaxation of carriers in the momentum space. We demonstrate that the observed nonlinearity of the photocurrent is caused by the large Zeeman spin splitting in InSb/AlInSb structures and an interplay of the spin-related and spin-independent roots of the magnetogyrotropic photogalvanic effect.

A comprehensive study of the optical properties of PbS nanocrystals (NCs) is reported that includes the temperature dependent absorption, photoluminescence (PL) and PL lifetime in the range of 3-300 K. The absorption and PL are found to display different temperature dependent behaviour though both redshift as temperature is reduced. This results in a temperature dependent Stokes shift which increases from ∼75 meV at 300 K with reducing temperature until saturating at ∼130 meV below ∼150 K prior to a small reduction to 125 meV upon cooling from 25 to 3 K. The PL lifetime is found to be single exponential at 3 K with a lifetime of τ(1) = 6.5 μs. Above 3 K biexponential behaviour is observed with the lifetime for each process displaying a different temperature dependence. The Stokes shift is modelled using a three-level rate equation model incorporating temperature dependent parameter values obtained via fitting phenomenological relationships to the observed absorption and PL behaviour. This results in a predicted energy difference between the two emitting states of ∼6 meV which is close to the excitonic exchange energy splitting predicted theoretically for these systems.

We report on the electrical detection of spin dependent photoconductivity in 500 nm wide InSb quantum well nanowires using the optical orientation of electron spins. By applying weak magnetic fields (≈ 200 mT), we observe a spin filtering effect of classical origin caused by spin dependent back scattering of electrons from the sidewalls. Spin dependent features in the longitudinal photovoltage decay with temperature and disappears at characteristic energy (≈ 50 K) consistent with the theoretical spin splitting and the thermal level broadening. We show that the observed signal is due to the inversion asymmetry of the quantum well, with an additional Zeeman contribution. © 2012 American Institute of Physics.

We have performed a high field magneto-absorption spectroscopy on silicon doped with a variety of single and double donor species. The magnetic field provides access to an experimental magnetic length, and the quadratic Zeeman effect in particular may~be used to extract the wavefunction radius without reliance on previously determined effective mass parameters. We were therefore able to determine the limits of validity for the standard one-band anisotropic effective mass model. We also provide improved parameters and use them for an independent check on the accuracy of effective mass theory. Finally, we show that the optically accessible excited state wavefunctions have the attractive property that interactions with neighbours are far more forgiving of position errors than (say) the ground state.

The material and electrical properties of silicon dioxide derived insulators formed with a focused electron beam in the presence of a TEOS or TMCTS precursor are presented. Raman and energy dispersive X-ray spectroscopy were used to analyse the chemical structure and composition of the deposited dielectric. It was found that the TEOS precursor induced a purer dielectric with a reduced carbon percentage (similar to 3 %) which was independent of the electron beam energy. An investigation with TMCTS showed the importance of using an additional water vapour supply as an in-situ oxidising co-reactant, suppressing the deposited carbon content by up to a factor of six. Leakage current and voltage breakdown measurements were used to calculate the effective resistance and the dielectric strength of the insulator. The two precursors formed insulators with similar electrical properties with effective resistances on the order of G omega for < 100 nm thick depositions and dielectric strengths of 7 MV cm(-1), equivalent to those found in sputtered silicon dioxides, but allowing high-resolution maskless lithography.

We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.

This paper reports on the predicted increase in the Rashba interaction due to the incorporation of Bi in GaAs/AlGaAs heterostructures. Band structure parameters obtained from the band anti-crossing theory have been used in combination with self-consistent Schrödinger-Poisson calculations and k.p models to determine the electron spin-splitting caused by structural inversion asymmetry and increased spin-orbit interaction. A near linear seven fold increase in the strength of the Rashba interaction is predicted for a 10% concentration of Bi in a GaAsBi/AlGaAs quantum well heterostructure.

We have used time resolved spectroscopy to measure the relaxation of spin polarization in InSb/AlInSb quantum wells (QWs) as a function of temperature and mobility. The results are consistent with the D'yakonov - Perel (DP) mechanism for high mobility samples over the temperature range from 50 to 300 K. For low mobility samples at high temperature the Elliott - Yafet and DP mechanisms become comparable. We show that the mobility can in certain circumstances determine which mechanism is dominant, and that above 1 m(2) V-1 s(-1) in 20 nm wide InSb QWs it is the DP mechanism. We also give a criterion for the maximum spin lifetime in terms of mobility and temperature, and show that for our 20 nm wide QWs this corresponds to 0.5 ps at 300 K and mobility 1 m(2) V-1 s(-1).

We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.

Three terminal measurements on a carbon nanotube field effect transistor (CNTFET) were carried out in high vacuum and the ambient, and its performance compared. The on-off current ratio, ION/IOFF, were 102 and 105 for devices operated in high vacuum and in ambient air, respectively. Here, we show that the conversion of p-type to ambipolar behavior may largely be attributed to the O2 in ambient doping the single walled carbon nanotubes (SWCNTs) in the active channel which consists of bundles of SWCNTs. Switching behaviour of these devices, with respect to constituent types of SWCNTs in the bundles will be discussed.

We measure transverse magnetically focused photocurrent signals in an InSb/InAlSb quantum well device. Using optical spin orientation by modulated circularly polarized light an electron spin-dependent signal is observed due to the spin-orbit interaction. Simulations of the focusing signal are performed using a classical billiard ball model, which includes both spin precession and a spin-dependent electron energy. The simulated data suggest that a signal dependent on the helicity of the incident light is expected for a Rashba parameter α > 0.1 eVÅ and that a splitting of the focusing signal is not expected to be observed in linear polarized photocurrent and purely electrical measurements.

Characterization of the Ge concentration in a Si1-xGe x heterojunction for x varying from 5% to 40% in steps of 5% is done by beveling and wet thermal oxidation of the exposed layers. The resulting difference in oxide thickness as a function of Ge concentration is visualized due to light interference. Different Ge concentrations are seen as different colors through an optical microscope. CABOOM - Characterization of the Alloy concentration by Beveling, Oxidation and Optical Microscopy - in combination with AFM - Atomic Force Microscopy, is used as a tool to study the oxidation kinetics of unstrained, crystalline Si1-xGex by wet thermal oxidation.

An innovative, extrudable material with the ability to filter the sun’s energy has been developed for the mass manufacture of high performance swimming pool covers. Solar radiation in the visible spectrum ( nm) is absorbed by the material so that minimal visible light enters the pool water which inhibits photosynthesis to prevent algae growth. Furthermore, the material has high transmission properties in the near infrared that can be efficiently absorbed by the water allowing for a higher temperature increase compared to a standard non-selective opaque cover. We have developed a model to enable the cover efficiency to convert solar energy to heat a swimming pool, calculated based on the wavelength dependent absorption and transmission properties of the cover. We have validated this model using dedicated full-scale test-facility. Our results demonstrate that a selective transmission cover can increase the absolute heating efficiencies by approximately 12% compared to the fully opaque equivalent.

We investigate the possibility to selectively reflect certain wavelengths while maintaining the optical properties on other spectral ranges. This is of particular interest for transparent materials, which for specific applications may require high reflectivity at pre-determined frequencies. Although there exist currently techniques such as coatings to produce selective reflection, this work focuses on new approaches for mass production of polyethylene sheets which incorporate either additives or surface patterning for selective reflection between 8 to 13 mu m. Typical additives used to produce a greenhouse effect in plastics include particles such as clays, silica or hydroxide materials. However, the absorption of thermal radiation is less efficient than the decrease of emissivity as it can be compared with the inclusion of Lambertian materials. Photonic band gap engineering by the periodic structuring of metamaterials is known in nature for producing the vivid bright colors in certain organisms via strong wavelength-selective reflection. Research to artificially engineer such structures has mainly focused on wavelengths in the visible and near infrared. However few studies to date have been carried out to investigate the properties of metastructures in the mid infrared range even though the patterning of microstructure is easier to achieve. We present preliminary results on the diffuse reflectivity using FDTD simulations and analyze the technical feasibility of these approaches.

We have used time-resolved spectroscopy to measure the relaxation of spin polarizations in the narrow gap semiconductor material n-InAs as a function of temperature, doping, and pump wavelength. The results are consistent with the D'Yakonov-Perel mechanism for temperatures between 77 and 300 K. However, the data suggest that electron-electron scattering should be taken into account in determining the dependence of the spin lifetime on the carrier concentration in the range 5.2x10(16)-8.8x10(17) cm(-3). For a sample with doping of 1.22x10(17) cm(-3) the spin lifetime was 24 ps at room temperature. By applying a magnetic field in the sample plane we also observed coherent precession of the spins in the time domain, with a g factor g(*)=-13, also at room temperature.

Highly spin polarized Heusler alloys, NiMnSb and Co2MnSi, attract a great deal of interest as potential spin injectors for spintronic applications. Spintronic devices require control of interfacial properties at the ferromagnet:semiconductor contact. To address this issue we report a systematic study of the ordinary and anomalous Hall effect, in Ni1.15Mn0.85Sb films on silicon, as a function of film thickness. In contrast to the bulk stoichiometric material, the Hall carriers in these films become increasingly electron-like as the film thickness decreases, and as the temperature increases from 50 K toward room temperature. High field Hall measurements confirm that this is representative of the majority transport carriers. This suggests that current injected from a NiMnSb:semiconductor interface may not necessarily carry the bulk spin polarization. The films also show a low temperature upturn in the resistivity, which is linked to a discontinuity in the anomalous Hall coefficient. Overall these trends indicate that the application of Heusler alloys as spin injectors will require strictly controlled interfacial engineering, which is likely to be demanding in these ternary alloys.

The introduction of strain into semiconductors offers a well-known route to modify their band structure. Here, we show a single-step procedure for generating such strains smoothly and deterministically, over a very wide range, using a simple, easily available, highly scalable, ion implantation technique to control the degree of amorphization in and around single-crystal membranes. The amorphization controls the density of the material and thus the tension in the neighboring crystalline regions. We have demonstrated up to 3.1% biaxial tensile strain and 8.5% uniaxial strain in silicon, based on micro-Raman spectroscopy. This method achieves strain levels never previously reached in mesoscopic defect-free, crystalline silicon structures. The flexible, gently controllable, single-step process points toward very high mobility complementary metal-oxide-semiconductor devices and easy fabrication of direct-bandgap germanium for silicon-compatible optoelectronics.

Electron spin relaxation times have been measured in InSb and InAs quantum wells and epi-layers in a moderate (

The point contact Andreev reflection employing niobium tips was used to determine the degree of transport current spin polarization ( Pt ) at the free surface of bulk NiMnSb at 4.2 K . The data was analyzed within the framework of a modified version of the Blonder, Tinkham, and Klapwijk formulism taking into account the two spin polarized channels in the ferromagnet and treating the interface as a planar delta function barrier of height Z between free electron materials. We find that the measured spin polarization is rather insensitive to different surface preparations and magnetic domain structure, and the maximal value of the Pt at Z=0 is 44% , consistent with recent calculations of the surface reconstruction of NiMnSb . Saturation magnetization of the samples was found to be 3.6 ìB per formula unit indicative of a small amount of atomic disorder.

We have used time-resolved spectroscopy to measure the relaxation of spin polarizations in the narrow gap semiconductor material n-InAs as a function of temperature, doping, and pump wavelength. The results are consistent with the D'Yakonov-Perel mechanism for temperatures between 77 and 300 K. However, the data suggest that electron-electron scattering should be taken into account in determining the dependence of the spin lifetime on the carrier concentration in the range 5.2×1016–8.8×1017 cm–3. For a sample with doping of 1.22×1017 cm–3 the spin lifetime was 24 ps at room temperature. By applying a magnetic field in the sample plane we also observed coherent precession of the spins in the time domain, with a g>/i> factor g*=–13, also at room temperature.

This book contains the invited and contributed papers which were presented at this meeting and serves to provide a broad overview of the current worldwide ...

We address the inherent high-field magnetoresistance (MR) of indium antimonide epilayers on GaAs (001), studying the modification of the MR when processed into a set of geometries. The changes produced by the geometries are quite subtle. The extraordinary MR geometry produces the highest low-field MR while the Corbino geometry produces the largest high-field magnetoresistance. We demonstrate that any material with an unsaturating linear intrinsic MR, will also have an unsaturating linear Corbino MR, and that the ideal material for linear MR sensors in conventional geometries would have a high mobility and a small, linear intrinsic MR.

— Synthesized single-walled carbon nanotubes (SWCNTs) consist of a mixture of chiralities and therefore a post-synthesis separation is essential to separate them based on electronic type i.e., metallic (m-SWCNT) or semiconducting (s-SWCNT) for device applications. A key parameter to measure the effectiveness of separation process is the enrichment composition percentage between m-SWCNT and s-SWCNT, which can be estimated via several methods based on optical characterizations. In this paper, we compare the composition percentage estimations from 3 different methods based on Raman spectroscopy and UV-Vis optical absorption spectroscopy. The estimation methods are radial breathing mode (RBM) peak analysis, optical absorption area under curve (OUA) and first derivative amplitude of the optical absorption curve (FDA). Four different SWCNT sources were used in this study, which were subjected to post-synthesis separation process via agarose gel chromatography. Raman and UV-Vis spectroscopy measurements were carried out on all samples, before and after separation. From the estimations, we observed firstly that there are some variations on the estimated enrichment compositions between different methods, although the values are comparable. Secondly, for some SWCNTs samples, only a certain estimation method showed reliable composition percentage. The results presented in this work may provide viable options for characterizations of SWCNTs as there is no direct method to quantify the absolute composition percentage of SWCNTs based on electronic type. Keywords— single-walled carbon nanotube, separation, electronic type, optical characterization, purity percentage.

Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10(5) T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.8 T by the effective mass and dielectric constant. The spectra reproduce the high-field theory for free hydrogen, with quadratic Zeeman splitting and strong mixing of spherical harmonics. They show the way for experiments on He and H(2) analogues, and for investigation of He(2), a bound molecule predicted under extreme field conditions.

Due to its exceptional electronic properties, single walled carbon nanotubes (SWCNTs) can be applied as the active channel of high-performance carbon nanotube field effect transistors (CNTFETs). The electronic properties of SWCNTs are known to be affected by its intrinsic properties, including electronic type, diameter and structural defects. Since tube defect is dependent of the growth method, it is shown that the latter can also influence the CNTFET device performance. Four SWCNT samples from different growth methods were sourced to fabricate CNTFETs. Raman analysis was carried out to quantify the tube defect level of SWCNTs by determining the G-peak to D-peak height ratio. Electrical measurement was carried out to assess the key device performance parameters that include on-off current ratio, \boldsymbol{I}_{\mathbf{ON}}/\boldsymbol{I}_{\mathbf{OFF}} transconductance, \boldsymbol{g}_{\mathbf{m}} , subthreshold slope, \boldsymbol{S}_{\mathbf{p}} and field effect mobility, \mu_{\mathbf{FE}} . Correlation between the optical analysis and electrical measurement concludes that the SWCNT growth method does influence the CNTFET performance significantly.

Low‐dimensional microwave interconnects have important applications for nanoscale electronics, from complementary metal–oxide‐semiconductor (CMOS) to silicon quantum technologies. Graphene is naturally nanoscale and has already demonstrated attractive electronic properties, however its application to electronics is limited by available fabrication techniques and CMOS incompatibility. Here, the characteristics of transmission lines made from silicon doped with phosphorus are investigated using phosphine monolayer doping. S‐parameter measurements are performed between 4–26 GHz from room temperature down to 4.5 K. At 20 GHz, the measured monolayer transmission line characteristics consist of an attenuation constant of 40 dB mm−1 and a characteristic impedance of 600 Ω. The results indicate that Si:P monolayers are a viable candidate for microwave transmission and that they have a.c. properties similar to graphene, with the additional benefit of extremely precise, reliable, stable, and inherently CMOS compatible fabrication. Low‐dimensional microwave interconnects have important applications for nanoscale electronics, from complementary metal–oxide‐semiconductor (CMOS) to silicon quantum technologies. In this study, monolayer transmission lines made from silicon delta‐doped with phosphorus are characterized and it is shown that they have a.c. properties similar to graphene, with the additional benefit of extremely precise, reliable, stable, and inherently CMOS compatible fabrication.

We report X-ray fluorescence (XRF) imaging of nanoscale inclusions of impurities for quantum technology. A very bright diffraction-limited focus of the X-ray beam produces very high sensitivity and resolution. We investigated gallium (Ga) dopants in silicon (Si) produced by a focused ion beam (FIB). These dopants might provide 3/2-spin qubits or p-type electrical contacts and quantum dots. We find that the ion beam spot is somewhat larger than expected, and the technique provides a useful calibration for the resolution of FIBs. Enticingly, we demonstrate that with a single shot detection of 1 second integration time, the sensitivity of the XRF would be sufficient to find amongst background a single isolated inclusion of unknown location comprising only 3000 Ga impurities (a mass of just 350 zg) without any need for specialized nm-thickness lamellae, and down from >105 atoms in previous reports of similar work. With increased integration we were able to detect 650 impurities. The results show that planned facility upgrades might achieve single atom sensitivity with a generally applicable, non-destructive technique in the near future.

This study reports the effect of an increasing ion dose on both the electrical activation yield and the characteristic properties of implanted bismuth donors in silicon. A strong dependence of implant fluence is observed on both the yield of bismuth donors and the measured impurity diffusion. This is such that higher ion concentrations result in both a decrease in activation and an enhancement in donor migration through interactions with mobile silicon lattice vacancies and interstitials. Furthermore, the effect of implant fluence on the properties of the Si:Bi donor bound exciton, D0X, is also explored using photoluminescence (PL) measurements. In the highest density sample, centers corresponding to the PL of bismuth D0Xs within both the high density region and the lower concentration diffused tail of the implanted donor profile are identifiable.

InSb is a III-V narrow-gap semiconductor with properties such as low effective mass, high mobility, and strong spin-orbit coupling making it an ideal material for applications such as spintronics mid-infrared photonics, and nanoelectronics. InSb quantum wells can be made by growing an InSb/InAlSb structure on a Ga substrate using molecular beam epitaxy. However, it is notoriously difficult to fabricate nanodevices from InSb/InAlSb quantum wells due to factors such as its low thermal budget and the production of non-volatile by-products in conventional etching processes, leading to unwanted deposition of material onto the material surface. Current wet and dry etching techniques take a long time and require expensive lithography masks to make new devices, slowing the development of optimised nanodevices.We investigate focused ion beam (FIB) lithography as a "rapid prototyping" fabrication technique to create semiconductor nanodevices from InSb quantum wells. FIB methods have the advantage of being relatively quick and "maskless", making them ideal for use in the research environment as new iterations of device design can be made quickly and different etching chemistries and electrical properties can be tested in-situ. A variety of Xe plasma FIB parameters were tested to optimise the feature resolution and etching quality of milled trenches at low temperatures. The XeF2 gas-assisted etching process was also studied as an alternative to the Cl2 chemistry that is typically employed for dry etching of InSb. Cross-sections and profiles of the trenches indicate that the XeF2 etch yields superior trench smoothness and mills material from the surface at a much higher rate. This method was also less prone to deposition of unwanted material onto the surface of the sample. This high-resolution fabrication method can be used for the rapid development and optimisation of individual nanoscale devices before mass production.

We report significant temperature dependence of the transverse electron g∗‐factor in symmetric lead chalcogenide multi‐quantum wells (MQWs). The g∗‐factor values were extracted from the electron Larmor precessions recorded by means of a circularly polarized pump probe technique under the influence of transverse external magnetic field (Voigt geometry) in the temperature range between 10 and 150K. The reported g∗‐factor values are in good agreement with theoretical predictions and available low temperature experimental data. Although temperature tuning of lead salt laser emission wavelengths has been the method of choice in these systems for many years, we demonstrate that temperature can also be used to modulate g∗, and hence the spin lifetime in lead salt QW spintronic devices.

We have used time resolved spectroscopy to measure the relaxation of spin polarization in InSb/AlInSb quantum wells (QWs) as a function of temperature and mobility. The results are consistent with the D'yakonov–Perel (DP) mechanism for high mobility samples over the temperature range from 50 to 300 K. For low mobility samples at high temperature the Elliott–Yafet and DP mechanisms become comparable. We show that the mobility can in certain circumstances determine which mechanism is dominant, and that above 1 m2 V-1 s-1 in 20 nm wide InSb QWs it is the DP mechanism. We also give a criterion for the maximum spin lifetime in terms of mobility and temperature, and show that for our 20 nm wide QWs this corresponds to 0.5 ps at 300 K and mobility 1 m2 V-1 s-1.

Several polycrystalline samples of the half-Heusler alloy NiMnSb were grown by arc melting of stoichiometric and nonstoichiometric amounts of high-purity constituent elements. The structure and the phase-purity of the prepared samples were examined systematically by powder x-ray diffraction. The transport properties of the best sample, with saturation magnetization M-s(5 K)=4 mu(B)/formula unit, were studied by measuring electrical resistivity, thermal conductivity, and thermopower. Features in both magnetic and transport data are consistent with NiMnSb being in a half-metallic state at low temperatures, i.e., the conduction electrons are fully spin polarized. However, point-contact Andreev reflection measurements on the same sample at 4.2 K demonstrate only similar to45% spin polarization.

This study reports on high energy bismuth ion implantation into silicon with a particular emphasis on the effect that annealing conditions have on the observed hyperfine structure of the Si:Bi donor state. A suppression of donor bound exciton, D0X, photoluminescence is observed in implanted samples which have been annealed at 700 °C relating to the presence of a dense layer of lattice defects that is formed during the implantation process. Hall measurments at 10 K show that this implant damage manifests itself at low temperatures as an abundance of p‐type charge carriers, the density of which is observed to have a strong dependence on annealing temperature. Using resonant D0X photoconductivity, we are able to identify the presence of a hyperfine structure in samples annealed at a minimum temperature of 800 °C; however, higher temperatures are required to eliminate effects of implantation strain.

Single walled carbon nanotubes (SWCNTs) exhibit extraordinary electronic properties that render it as an exciting candidate to be applied as the active channel of high-performance carbon nanotube field effect transistors (CNTFETs). The electronic properties of SWCNTs have been demonstrated to be dependent on the tube intrinsic properties that includes structural defects, chirality and diameter. Structural tube defects can be affected by the synthesis method and therefore the latter should also affect the device performance. Hence, this paper aims to present the influence of SWCNTs source synthesis method towards the resulting CNTFET device characteristics. A total of four SWCNT samples were sourced from different synthesis methods in fabricating CNTFETs. The synthesis methods are arc-discharge and three different variation of chemical vapor deposition (CVD) processes, which are DIPS, HiPco and CoMoCAT, respectively. Prior to fabrication, the SWCNT samples were characterized via Raman spectroscopy to quantify the tube defect levels of each sample, which are directly proportional to the G-peak to D-peak height ratio, G/D. Electrical characterization was carried out via 3-terminal field effect I-V measurement to evaluate key device performance parameters such as on-off current ratio, I-ON/T-OFF, transconductance, g(m), subthreshold slope, S-p and field effect mobility, mu(FE). Analysis shows that G/D affects the I-OFF more significantly relative to I-ON, resulting in increasing I-O(N)/I-OFF, and hence switching performance, when G/D increases. It is shown an increase of similar to 50% to the G/D of the SWCNT source resulted in similar to 860% increase in mu(FE). Based on the correlation between the optical analysis and electrical measurement, we conclude that the SWCNT growth method can significantly affect the CNTFET device performance.

The problem of preparing high-mobility thin InSb films is revisited for magnetoresistive and spintronic sensor applications. We introduce a growth process that significantly improves the electrical properties of thin unintentionally doped InSb layers (60-300 nm) epitaxially grown on GaAs(100) substrates by reducing the density of dislocations within the interfacial layer. The epilayer properties are well described by a differential two-layer model. This model confirms that the contribution of the interface can only be donor-like. Moreover, the electrical properties of the InSb layers change continuously away from the interface up to sample thickness of the order of 1 mum.

We report a systematic study of the transport properties of pulsed-laser-deposited NiMnSb films on silicon as a function of film thickness. A low-temperature upturn is observed in the resistivity for film thicknesses of 130 nm and below. The resistivity minimum corresponds to the maximum in the positive magnetoresistance for all samples. As the film thickness decreases, the magnitude of both the resistivity upturn and the. magnetoresistance increase. There is no feature associated with the upturn in the low-field Hall resistivity, which becomes systematically more electron dominated as the film thickness decreases and the temperature increases. This has implications for the use of NiMnSb as a spin injector for spintronic applications. The positive magnetoresistance of the 5 nm sample is greater than 100% at 200 K in 8 T. Further enhancement of the magnetoresistance occurs for field parallel, rather than perpendicular, to the film surface. The magnetoresistance behavior is compared to various model systems, including the band-gap tuning found in the silver chalcogenides, disorder-induced weak localization, and the, emerging class of "bad metal" ferromagnets.

Post-synthesis separation of metallic (m-SWNTs) and semiconducting (s-SWNTs) single-wall carbon nanotubes (SWNTs) remains a challenging process. Gel agarose chromatography is emerging as an efficient and large scale separation technique. However, the full (100%) separation has not been achieved yet, mainly due to the lack of understanding of the underlying mechanism. Here, we study the temperature effect on the SWNTs separation via gel agarose chromatography, for four different SWNT sources. Exploiting a gel agarose micro-beads filtration technique we achieve up to 70% m-SWNTs and over 90% s-SWNTs, independent of the source material. The process is temperature dependent, with yields up to 95% for s-SWNT (HiPco) at 6 °C. Temperature affects the sodium dodecyl sulfate surfactant-micelle distribution along the SWNT sidewalls, thus determining the effectiveness of the SWNTs sorting by electronic type. The sorted SWNTs are then used to fabricate transistors with very low OFF-currents (∼10−13 A), high ON/OFF current ratio (>106) and charge carriers mobility ∼ 40 cm2 V−1 s−1.

The electrically detected orbital spectrum of a mesoscopic silicon device containing a small number of donors has been investigated. The device was fabricated on silicon-on-insulator with an optically active channel containing 6 x 105 substitutional bismuth centers introduced by ion implantation. The 1s(A₁) → 2p± orbital transition at the energy associated with isolated bismuth donors was detected via a change in photocurrent when illuminated by THz light from a free electron laser. The spectral dependence on bias, temperature, and laser intensity is explored to determine optimum conditions for detecting orbital transitions in smaller devices with fewer donors. These results suggest that photo-induced impact ionization can offer a route for the spectroscopic detection of few impurities providing a useful tool for the development of solid-state quantum technologies.