Professor Ben Murdin

Superposition of orbital eigenstates is crucial to quantum technology utilising atoms, such as atomic clocks and quantum computers, and control over the interaction between atoms and their neighbours is an essential ingredient for both gating and readout. The simplest coherent wavefunction control uses a 2-eigenstate admixture, but more control over the spatial distribution of the wavefunction can be obained by increasing the number of states in the wavepacket. Here we demonstrate THz laser pulse control of Si:P orbitals using multiple orbital state admixtures, observing beat patterns produced by Zeeman splitting. The beats are an observable signature of the ability to control the path of the electron, which implies we can now control the strength and duration of the interaction of the atom with different neighbours. This could simplify surface code networks which require spatially controlled interaction between atoms, and we propose an architecture that might take advantage of this.

We have measured the near-infrared photoluminescence spectrum of phosphorus doped silicon (Si: P) and extracted the donor-bound exciton (D0X) energy at magnetic fields up to 28 T. At high field the Zeeman effect is strongly nonlinear because of the diamagnetic shift, also known as the quadratic Zeeman effect (QZE). The magnitude of the QZE is determined by the spatial extent of the wave-function. High field data allows us to extract values for the radius of the neutral donor (D0) ground state, and the light and heavy hole D0X states, all with more than an order of magnitude better precision than previous work. Good agreement was found between the experimental state radius and an effective mass model for D0. The D0X results are much more surprising, and the radius of the mJ=±3/2 heavy hole is found to be larger than that of the mJ=±1/2 light hole.

The absorption of multiple photons when there is no resonant intermediate state is a well-known nonlinear process in atomic vapours, dyes and semiconductors. The N-photon absorption (NPA) rate for donors in semiconductors scales proportionally from hydrogenic atoms in vacuum with the dielectric constant and inversely with the effective mass, factors that carry exponents 6N and 4N, respectively, suggesting that extremely large enhancements are possible. We observed 1PA, 2PA and 3PA in Si:P with a terahertz free-electron laser. The 2PA coefficient for 1s?2s at 4.25 THz was 400,000,000 GM (=4 × 10?42 cm4 s), many orders of magnitude larger than is available in other systems. Such high cross-sections allow us to enter a regime where the NPA cross-section exceeds that of 1PA?that is, when the intensity approaches the binding energy per Bohr radius squared divided by the uncertainty time (only 3.84 MW cm?2 in silicon)?and will enable new kinds of terahertz quantum control.

We present time-resolved measurements of the relaxation between the orbital states of the
shallow acceptor boron in silicon. The silicon host was enriched Si-28, which exhibits life-time
broadened absorption lines. We observed a wide range of T1 lifetimes from 6ps to 130ps
depending on the excited state and the pump intensity. The fastest transients have not been
observed previously in the time domain, and they are caused by the phonon relaxation
responsible for the small-signal frequency domain line-width. We identify the slower
components with an ionisation/recombination/cascade pathway.

Excited states of a single donor in bulk silicon have previously been studied extensively based
on effective mass theory. However, proper theoretical descriptions of the excited states of a donor
cluster are still scarce. Here we study the excitations of lines of defects within a single-valley
spherical band approximation, thus mapping the problem to a scaled hydrogen atom array. A series
of detailed full configuration-interaction, time-dependent Hartree-Fock and time-dependent hybrid
density-functional theory calculations have been performed to understand linear clusters of up to 10
donors. Our studies illustrate the generic features of their excited states, addressing the competition
between formation of inter-donor ionic states and intra-donor atomic excited states. At short interdonor
distances, excited states of donor molecules are dominant, at intermediate distances ionic
states play an important role, and at long distances the intra-donor excitations are predominant
as expected. The calculations presented here emphasise the importance of correlations between
donor electrons, and are thus complementary to other recent approaches that include effective mass
anisotropy and multi-valley effects. The exchange splittings between relevant excited states have
also been estimated for a donor pair and for three-donor arrays; the splittings are much larger than
those in the ground state in the range of donor separations between 10 and 20 nm. This establishes
a solid theoretical basis for the use of excited-state exchange interactions for controllable quantum
gate operations in silicon.

Doping of silicon via phosphine exposures alternating with molecular beam epitaxy overgrowth is a
path to Si:P substrates for conventional microelectronics and quantum information technologies. The
technique also provides a new and well-controlled material for systematic studies of two-dimensional
lattices with a half-filled band. We show here that for a dense (ns = 2.8 × 1014 cm?2
) disordered
two-dimensional array of P atoms, the full field angle-dependent magnetostransport is remarkably
well described by classic weak localization theory with no corrections due to interaction effects.
The two- to three-dimensional cross-over seen upon warming can also be interpreted using scaling
concepts, developed for anistropic three-dimensional materials, which work remarkably except when
the applied fields are nearly parallel to the conducting planes.

Photoluminescence (PL) has been observed from dilute InN_xAs_1?x epilayers grown by molecular-beam epitaxy. The PL spectra unambiguously show band gap reduction with increasing N content. The variation of the PL spectra with temperature is indicative of carrier detrapping from localized to extended states as the temperature is increased. The redshift of the free exciton PL peak with increasing N content and temperature is reproduced by the band anticrossing model, implemented via a (5×5) k·p Hamiltonian.

Photoluminescence (PL) has been used as a means of unambiguously observing band gap reduction in InNAs epilayers grown by molecular beam epitaxy. The observed redshift in room temperature emission as a function of nitrogen concentration is in agreement with the predictions of the band anticrossing (BAC) model, as implemented with model parameters derived from tight-binding calculations. The temperature dependence of the emission from certain samples exhibits a signature non-Varshni-like behavior indicative of electron trapping in nitrogen-related localized states below the conduction-band edge, as predicted by the linear combination of isolated nitrogen states (LCINS) model. This non-Varshni-like behavior tends to grow more pronounced with increasing nitrogen content, but for the highest nitrogen concentration studied, the more familiar Varshni-like behavior is recovered. Although unexpected, this observation is found to be consistent with the BAC and LCINS models. With consideration given to the effects of conduction-band nonparabolicity on the emission line shapes, the BAC model parameters extracted from the measured PL transition energies are found to be in excellent agreement with the predictions of the aforementioned tight-binding calculations.

The spin relaxation in undoped InSb films grown on GaAs has been investigated in the temperature range from 77 to 290 K. Two distinct lifetime values have been extracted, 1 and 2.5 ps, dependent on film thickness. Comparison of this data with a multilayer transport analysis of the films suggests that the longer time (similar to 2.5 ps at 290 K) is associated with the central intrinsic region of the film, while the shorter time (similar to 1 ps) is related to the highly dislocated accumulation region at the film-substrate interface. Whereas previous work on InAs films grown on GaAs showed that the native surface defect resulted in an additional charge accumulation layer with high conductivity but very short spin lifetime, in InSb layers the surface states introduce a depletion region. We infer that InSb could be a more attractive candidate for spintronic applications than InAs.

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.

The structural and optoelectronic properties in GaNxSb1-x alloys (0 <= x < 0.02) grown by molecular-beam epitaxy on both GaSb substrates and AlSb buffer layers on GaAs substrates are investigated. High-resolution x-ray diffraction (XRD) and reciprocal space mapping indicate that the GaNxSb1-x epilayers are of high crystalline quality and the alloy composition is found to be independent of substrate, for identical growth conditions. The band gap of the GaNSb alloys is found to decrease with increasing nitrogen content from absorption spectroscopy. Strain-induced band-gap shifts, Moss-Burstein effects, and band renormalization were ruled out by XRD and Hall measurements. The band-gap reduction is solely due to the substitution of dilute amounts of highly electronegative nitrogen for antimony, and is greater than observed in GaNAs with the same N content. (C) 2005 American Institute of Physics.

We report on the pressure dependence of the threshold current in 1.3 µm InGaAsP and 1.5 µm InGaAs quantum-well lasers measured at low temperatures ~100 K. It was found that the threshold current of both devices slowly increases with increasing pressure (i.e., increasing band gap) at ~100 K consistent with the calculated variation of the radiative current. In contrast, at room temperature we observed a reduction of the threshold current with increasing pressure. Our low-temperature, high-pressure data confirm the results of previous atmospheric pressure measurements on the same devices which indicated a transition in the dominant recombination mechanism from radiative to Auger as the device temperature is increased from ~100 to 300 K

We report investigation of the spin relaxation in InAs films grown on GaAs at a temperature range from 77 K to 290 K. InAs is known to have a surface accumulation layer and the depth profile of the concentration and mobility is strongly nonuniform. We have correlated the spin relaxation with a multilayer analysis of the transport properties and find that the surface and the interface with the GaAs substrate both have subpicosecond lifetimes (due to the high carrier concentration), whereas the central semiconducting layer has a lifetime of an order of 10 ps. Even for the thickest film studied (1 mu m), the semiconducting layer only carried 30% of the total current (with 10% through the interface layer and 60% through the surface accumulation layer). Designs for spintronic devices that utilize InAs, which is attractive due to its narrow gap and strong Rashba effect, will need to include strategies for minimizing the effects of the surface.

We model theoretically the dependence of excitonic absorption spectra of semiconductor quantum wells in intense THz electric fields on the phase and intensity of those fields, and discuss the implications of our results for experiment.

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.

The spin relaxation in undoped InSb films grown on GaAs has been investigated in the temperature range from 77 to 290 K. Two distinct lifetime values have been extracted, 1 and 2.5 ps, dependent on film thickness. Comparison of this data with a multilayer transport analysis of the films suggests that the longer time (~2.5 ps at 290 K) is associated with the central intrinsic region of the film, while the shorter time (~1 ps) is related to the highly dislocated accumulation region at the film-substrate interface. Whereas previous work on InAs films grown on GaAs showed that the native surface defect resulted in an additional charge accumulation layer with high conductivity but very short spin lifetime, in InSb layers the surface states introduce a depletion region. We infer that InSb could be a more attractive candidate for spintronic applications than InAs.

The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser (wavelength 7.9 µm). Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160 meV, a value of 550 fs was obtained.

We report on the pressure dependence of the threshold current in 1.3 mum InGaAsP and 1.5 mum InGaAs quantum-well lasers measured at low temperatures similar to100 K. It was found that the threshold current of both devices slowly increases with increasing pressure (i.e., increasing band gap) at similar to100 K consistent with the calculated variation of the radiative current. In contrast, at room temperature we observed a reduction of the threshold current with increasing pressure. Our low-temperature, high-pressure data confirm the results of previous atmospheric pressure measurements on the same devices which indicated a transition in the dominant recombination mechanism from radiative to Auger as the device temperature is increased from similar to100 to 300 K.

Optical waveguides containing high percentages of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two- and one-dimensional waveguidings in these structures are demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes together with simulations of the light propagation indicate that the losses are dominated by surface roughness. The variable stripe length method is used to determine the optical gain of 230 cm^?1 from the amplified spontaneous emission. This high value makes the authors' waveguide structures very promising for applications in amplifiers and lasers with reduced threshold powers.

Optical waveguides containing high percentages of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two- and one-dimensional waveguidings in these structures are demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes together with simulations of the light propagation indicate that the losses are dominated by surface roughness. The variable stripe length method is used to determine the optical gain of 230 cm(-1) from the amplified spontaneous emission. This high value makes the authors' waveguide structures very promising for applications in amplifiers and lasers with reduced threshold powers.

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).

The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser (wavelength 7.9 mu m). Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160 meV, a value of 550 fs was obtained. (c) 2006 American Institute of Physics.

Photoluminescence (PL) has been used as a means of unambiguously observing band gap reduction in InNAs epilayers grown by molecular beam epitaxy. The observed redshift in room temperature emission as a function of nitrogen concentration is in agreement with the predictions of the band anticrossing (BAC) model, as implemented with model parameters derived from tight-binding calculations. The temperature dependence of the emission from certain samples exhibits a signature non-Varshni-like behavior indicative of electron trapping in nitrogen-related localized states below the conduction-band edge, as predicted by the linear combination of isolated nitrogen states (LCINS) model. This non-Varshni-like behavior tends to grow more pronounced with increasing nitrogen content, but for the highest nitrogen concentration studied, the more familiar Varshni-like behavior is recovered. Although unexpected, this observation is found to be consistent with the BAC and LCINS models. With consideration given to the effects of conduction-band nonparabolicity on the emission line shapes, the BAC model parameters extracted from the measured PL transition energies are found to be in excellent agreement with the predictions of the aforementioned tight-binding calculations.

We present theoretical results on the nonlinear optics of semiconductor quantum wells in intense THz electric fields (the dynamic Franz-Keldysh effect or DFKE). The absorption spectra show a rich variety of behavior, including THz replicas of the 2p exciton and THz sidebands of the 1s exciton. We calculate the dependence of these features on the phase and intensity of the THz field using the extended semiconductor Bloch equations, and discuss the relevance of our results to future experiments. The 1s-sideband absorption feature shows a strong dependence on the phase of the THz field, and phase averages to zero. We also discuss the relative advantages and disadvantages of reflectivity and absorption spectroscopies for probing the DFKE.

The structural and optoelectronic properties in GaN_xSb_1?x alloys
(0dx_xSb_1?x epilayers are of high crystalline quality and the alloy composition is found to be independent of substrate, for identical growth conditions. The band gap of the GaNSb alloys is found to decrease with increasing nitrogen content from absorption spectroscopy. Strain-induced band-gap shifts, Moss-Burstein effects, and band renormalization were ruled out by XRD and Hall measurements. The band-gap reduction is solely due to the substitution of dilute amounts of highly electronegative nitrogen for antimony, and is greater than observed in GaNAs with the same N content.

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 delta-doped n-type InSb/In1-xAlxSb quantum wells using the established eight-band k center dot p formalism [J. Deng , 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 tau(s[1 (1) over bar0]) for spins oriented along [1 (1) over bar0] based on D'yakonov-Perel' mechanism using both the theory of Averkiev [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. tau(s[1 (1) over bar0]) 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 tau(s[1 (1) over bar0]), 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 [1 (1) over bar0] direction.

The temperature dependence of the threshold current of InGaAsSb/AlGaAsSb compressively
strained lasers is investigated by analyzing the spontaneous emission from working laser devices
through a window formed in the substrate metallization and by applying high pressures. It is found
that nonradiative recombination accounts for 80% of the threshold current at room temperature and
is responsible for the high temperature sensitivity. The authors suggest that Auger recombination
involving hot holes is suppressed in these devices because the spin-orbit splitting energy is larger
than the band gap, but other Auger processes persist and are responsible for the low T₀ values.

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.

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 5.2×10^16?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.

Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser

We report the quantitative and direct determination of hole intersubband relaxation times in a voltage biased SiGe heterostructure using density matrix calculations applied to a four-level system in order to interpret photocurrent (PC) pump-pump experiments. One consistent set of parameters allows the simulation of two kinds of experiments, namely pump-pump photocurrent experiments at a free electron laser (wavelength 7.9 mu m) and the laser-power dependence of the PC signal. This strongly confirms the high reliability of these parameter values, of which the most interesting in respect to Si based quantum cascade laser development is the extracted heavy-hole relaxation time. The simulations show that this relaxation time directly determines the experimentally observed decay of the pump-pump photocurrent signal as a function of the delay time. For a heavy hole intersubband spacing of 160 meV, a value of 550 fs was obtained. The experimental method was further applied to determine the LH1-HH1 relaxation time of a second sample with a transition energy below the optical phonon energy. The observed relaxation time of 16 ps is consistent with the value found for the same structure by transmission pump-probe experiments.

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.

We report the direct determination of nonradiative lifetimes in Si/SiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band k center dot p model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height >= 1 A), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infrared GaAs/AlxGa1-xAs multiple quantum well systems leads us to the conclusion that the dominant role of interface roughness scattering at low temperature is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV materials.

We have made direct pump-probe measurements of spin lifetimes in intrinsic and degenerate n-InAs at 300 K. In particular, we measure remarkably long spin lifetimes (tau(s)similar to1.6 ns) for near-degenerate epilayers of n-InAs. For intrinsic material, we determine tau(s)similar to20 ps, in agreement with other workers. There are two main models that have been invoked for describing spin relaxation in narrow-gap semiconductors: the D'yakonov-Perel (DP) model and the Elliott-Yafet (EY) model. For intrinsic material, the DP model is believed to dominate in III-V materials above 77 K, in agreement with our results. We show that in the presence of strong n-type doping, the DP relaxation is suppressed both by the degeneracy condition and by electron-electron scattering, and that the EY model then dominates for the n-type material. We show that this same process is also responsible for a hitherto unexplained lengthening of tau(s) with n-type doping in our earlier measurements of n-InSb.

We report investigation of the spin relaxation in InAs films grown on GaAs at a temperature range from
77 K to 290 K. InAs is known to have a surface accumulation layer and the depth profile of the concentration
and mobility is strongly nonuniform. We have correlated the spin relaxation with a multilayer analysis of the
transport properties and find that the surface and the interface with the GaAs substrate both have subpicosecond
lifetimes (due to the high carrier concentration), whereas the central semiconducting layer has a lifetime of
an order of 10 ps. Even for the thickest film studied (1 micro-m, the semiconducting layer only carried 30% of the
total current (with 10% through the interface layer and 60% through the surface accumulation layer). Designs
for spintronic devices that utilize InAs, which is attractive due to its narrow gap and strong Rashba effect, will
need to include strategies for minimizing the effects of the surface.

We report the quantitative and direct determination of hole intersubband relaxation times in a voltage biased SiGe heterostructure using density matrix calculations applied to a four-level system in order to interpret photocurrent (PC) pump?pump experiments. One consistent set of parameters allows the simulation of two kinds of experiments, namely pump?pump photocurrent experiments at a free electron laser (wavelength 7.9 ¼m) and the laser-power dependence of the PC signal. This strongly confirms the high reliability of these parameter values, of which the most interesting in respect to Si based quantum cascade laser development is the extracted heavy-hole relaxation time. The simulations show that this relaxation time directly determines the experimentally observed decay of the pump?pump photocurrent signal as a function of the delay time. For a heavy hole intersubband spacing of 160 meV, a value of 550 fs was obtained. The experimental method was further applied to determine the LH1?HH1 relaxation time of a second sample with a transition energy below the optical phonon energy. The observed relaxation time of 16 ps is consistent with the value found for the same structure by transmission pump?probe experiments.

We report the direct determination of nonradiative lifetimes in Si/SiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band k·p model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height 1 greater than or equal to Å), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infrared GaAs/Al_xGa_1?xAs multiple quantum well systems leads us to the conclusion that the dominant role of interface roughness scattering at low temperature is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV materials.

Photogalvanic effects are observed and investigated in wurtzite (0001)-oriented GaN/AlGaN low-dimensional structures excited by terahertz radiation. The structures are shown to represent linear quantum ratchets. Experimental and theoretical analysis exhibits that the observed photocurrents are related to the lack of an inversion center in the GaN-based heterojunctions.

The authors have measured the output spectrum and the threshold current in 9.2 mu m wavelength GaAs/Al0.45Ga0.55As quantum-cascade lasers at 115 K as a function of hydrostatic pressure up to 7.3 kbars. By extrapolation back to ambient pressure, thermally activated escape of electrons from the upper lasing state up to delocalized states of the Gamma valley is shown to be an important contribution to the threshold current. On the other hand leakage into the X valley, although it has a very high density of states and is nearly degenerate with the Gamma band edge in the barrier, is insignificant at ambient pressure.

Photoluminescence (PL) has been observed from dilute InNxAs1-x epilayers grown by molecular-beam epitaxy. The PL spectra unambiguously show band gap reduction with increasing N content. The variation of the PL spectra with temperature is indicative of carrier detrapping from localized to extended states as the temperature is increased. The redshift of the free exciton PL peak with increasing N content and temperature is reproduced by the band anticrossing model, implemented via a (5x5) k center dot p Hamiltonian.

The temperature dependence of the threshold current of InGaAsSb/AlGaAsSb compressively strained lasers is investigated by analyzing the spontaneous emission from working laser devices through a window formed in the substrate metallization and by applying high pressures. It is found that nonradiative recombination accounts for 80% of the threshold current at room temperature and is responsible for the high temperature sensitivity. The authors suggest that Auger recombination involving hot holes is suppressed in these devices because the spin-orbit splitting energy is larger than the band gap, but other Auger processes persist and are responsible for the low T-0 values.

Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser.

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.

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.

Frequency domain spectroscopy allows an experimenter to establish optical properties of solids in a wide frequency band including the technically challenging 3-10 THz region, and in other bands enables metrological comparison between competing techniques. We advance a method for extracting the optical properties of high-index solids using only transmission-mode frequency domain spectroscopy of plane-parallel Fabry-Perot optical flats. We show that different data processing techniques yield different kinds of systematic error, and that some commonly used techniques have inherent systematic errors which are underappreciated. We use model datasets to cross-compare algorithms in isolation from experimental errors, and propose a new algorithm which has qualitatively different systematic errors to its competitors. We show that our proposal is more robust to experimental non-idealities such as noise or apodization, and extract the complex refractive index spectrum of crystalline silicon as a practical example. Finally, we advance the idea that algorithms are complementary rather than competitive, and should be used as part of a toolbox for better metrology.

Implicit summation is a technique for conversion of sums over intermediate states in multiphoton
absorption and the high-order susceptibility in hydrogen into simple integrals. Here we derive the
equivalent technique for hydrogenic impurities in multi-valley semiconductors. While the absorption
has useful applications it is primarily a loss process, conversely the non-linear susceptibility is a
crucial parameter for active photonic devices. For Si:P we predict the hyperpolarizability ranges
from X⁽³⁾Dn3D = 2:9 to 580x10^-38m⁵V² depending on the frequency even while avoiding resonance.
Using samples of reasonable density n3D and thickness L to produce third harmonic generation at
9 THz, a frequency that is difficult to produce with existing solid state sources, we predict that X⁽³⁾
should exceed that of bulk InSb and X⁽³⁾L should exceed that of graphene and resonantly enhanced
quantum wells.

We investigate the spin relaxation under conditions of optical excitation between the Rydberg orbital states of phosphorus donor impurities in silicon. Here we show that the spin relaxation is less than a few percent, even after multiple excitation/relaxation cycles. The observed high level of spin preservation may be useful for readout cycling or in quantum information schemes where coupling of neighbor qubits is via orbital excitation.

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.

The Poisson distribution of event-to-ith-nearest-event radial distances is well known for homogeneous processes that do not depend on location or time. Here we investigate the case of a non-homogeneous point process where the event probability (and hence the neighbour configuration) depends on location within the event space. The particular non-homogeneous scenario of interest to us is ion implantation into a semiconductor for the purposes of studying interactions between the implanted impurities. We calculate the probability of a simple cluster based on nearest neighbour distances, and specialise to a particular two-species cluster of interest for qubit gates. We show that if the
two species are implanted at different depths there is a maximum in the cluster probability and an optimum density
profile.

Solotronic devices formed from group V dopants in silicon are a prospective option as a qubit system for quantum technologies. A spin-based silicon based quantum computer is highly promising with the longest qubit coherence times found to date, and an existing compatibility with the CMOS industry. The electron spin states of silicon solotronic devices are controlled through resonance absorption of microwave frequencies. Due to the
small dimensions of the dopants, integration of the microwaves to ensure individual qubit addressability is a important step on the path to producing a commercial quantum computer. Within this thesis, we investigate two different pathways to potentially optimise
this process. Mesoscopic interconnects could be used to deliver microwaves to individual qubits within
a qubit array. Conventional metals are not suitable as the resistivity increases as the dimensions approach the atomic scale and have immature nanoscale fabrication techniques. Highly doped metallic phosphorous delta-doped monolayers in silicon could be a viable material for mesoscopic transmission lines. Si:P delta-doped nanowires can be fabricated with atomic precision and have been shown to maintain ohmic behaviour down to wire widths of 1.5nm. The microwave characterisation of Si:P delta-doped layers was completed validating that it is a suitable material for microwave propagation. The transmission
parameters are extracted and matched to a circuit model and complementary electromagnetic simulations. A universal nanoscale transmission model showed that Si:P
nanowires have transmission parameters equivalent to graphene nanoribbons and have optimal behaviour compared to copper nanowires below 5nm. This investigation has
further reaching applications than silicon quantum technologies as conventional microelectronics
also require mesoscopic interconnects as Moore's law progresses. The application of an external magnetic field modulates the electron spin splitting within
a group V donor and has a linear relationship with the resonant microwave frequency under high magnetic fields. Within the literature, external magnetic fields approaching 10T are being used to optimise the operating conditions of the qubit and to simplify the experimental parameters. Knowledge of the behaviour and mechanisms of the spin lattice relaxation is unknown at these fields. Electron spin resonance in phosphorous doped silicon was demonstrated at magnetic fields between 10-14T using electrically detected donor bound exciton spectroscopy. To accompany this investigation, the first donor
bound exciton spectroscopy model under the influence of magnetic field was developed and analysed.

This thesis is concerned with the generation of terahertz (THz) light in the frequency range of 5?15 THz. This range is difficult to produce at high intensities, and no high-power table-top sources exist for it. The reasons for the interest in this frequency range are broad, and include communications, chemical and biomedical spectroscopy, etc. Different applications require highly specific technological characteristics, such as room temperature operation or very high stability, and satisfying all possible requirements in one generation mechanism is difficult. In this study, we sought to develop a solution to the problem of optical pumping of donor impurities in silicon, to control the orbit and spin of the donor electron in quantum computer gates. It is obvious that great advantages would be obtained if this control were possible with a bench-top source, and this is the key to enabling such high efficiency control to spread to the wider community of physicists and engineers working on spin qubits.

To achieve this purpose, we explore the possibility of use a cheap material like single doped silicon and germanium to triple THz radiation from 1.7 to 5 THz (which is available from Quantum Cascade Laser diodes). The presence of the dopants modifies substantially the optical properties of the semiconductors, generating large nonlinear coefficients at THz frequencies. Samples of bismuth doped silicon and phosphorus, arsenic and antimony doped germanium has been characterised using frequency-domain and time-domain spectroscopy, and finally the optical nonlinear coefficient of Si:Bi has been measure by a Four Wave Mixing (FWM) experiment. We have found that the nonlinear susceptibility $\chi^{(3)}$ of Si:Bi at resonance is the highest ever reported for a bulk material.