Thomas A. Kennedy

Doping semiconductor nanocrystals

Doping—the intentional introduction of impurities into a material—is fundamental to controlling the properties of bulk semiconductors. This has stimulated similar efforts to dope semiconductor nanocrystals. Despite some successes, many of these efforts have failed, for reasons that remain unclear. For example, Mn can be incorporated into nanocrystals of CdS and ZnSe (refs 7–9), but not into CdSe (ref. 12)—despite comparable bulk solubilities of near 50u2009per cent. These difficulties, which have hindered development of new nanocrystalline materials, are often attributed to ‘self-purification’, an allegedly intrinsic mechanism whereby impurities are expelled. Here we show instead that the underlying mechanism that controls doping is the initial adsorption of impurities on the nanocrystal surface during growth. We find that adsorption—and therefore doping efficiency—is determined by three main factors: surface morphology, nanocrystal shape, and surfactants in the growth solution. Calculated Mn adsorption energies and equilibrium shapes for several nanocrystals lead to specific doping predictions. These are confirmed by measuring how the Mn concentration in ZnSe varies with nanocrystal size and shape. Finally, we use our predictions to incorporate Mn into previously undopable CdSe nanocrystals. This success establishes that earlier difficulties with doping are not intrinsic, and suggests that a variety of doped nanocrystals—for applications from solar cells to spintronics—can be anticipated.

Directing nuclear spin flips in InAs quantum dots using detuned optical pulse trains.

We demonstrate that the sign of detuning of an optical pulse train from quantum dot resonances controls the direction of nuclear spin flips. This effect can produce a narrow, precise distribution of nuclear spin polarizations.

Parametric Investigation of Si1-xGex/Si Multiple Quantum Well Growth

Si0.8Ge0.2/Si multiple quantum wells (3 nm/30 nm) have been grown by molecular beam epitaxy and have been characterized using photoluminescence (PL), secondary ion mass spectrometry, and transmission electron microscopy. A parametric investigation relating the growth conditions to the PL was carried out. The existence of phonon-resolved band-edge PL appears to be strongly related to the background impurity concentration. The connection between phonon-resolved band-edge PL and higher substrate growth temperatures is probably due to the temperature-dependent incorporation of impurities. In the as-grown samples a correlation of the broad PL with platelet density in the quantum wells was observed. The broad PL may be associated with Cr at the platelets since a high temperature ( 710° C) anneal extinguished the broad PL and caused a reduction in the Cr found in the quantum wells, but had no effect on the platelet density.

Neutron irradiation of sapphire for compressive strengthening. II. Physical properties changes

Abstract Irradiation of sapphire with fast neutrons (0.8–10 MeV) at a fluence of 1022/m2 increased the c-axis compressive strength and the c-plane biaxial flexure strength at 600 °C by a factor of ∼2.5. Both effects are attributed to inhibition of r-plane twin propagation by damage clusters resulting from neutron impact. The a-plane biaxial flexure strength and four-point flexure strength in the c- and m-directions decreased by 10–23% at 600 °C after neutron irradiation. Neutron irradiation had little or no effect on thermal conductivity, infrared absorption, elastic constants, hardness, and fracture toughness. A featureless electron paramagnetic resonance signal at g=2.02 was correlated with the strength increase: This signal grew in amplitude with increasing neutron irradiation, which also increased the compressive strength. Annealing conditions that reversed the strengthening also annihilated the g=2.02 signal. A signal associated with a paramagnetic center containing two Al nuclei was not correlated with strength. Ultraviolet and visible color centers also were not correlated with strength in that they could be removed by annealing at temperatures that were too low to reverse the compressive strengthening effect of neutron irradiation.

Compressive strengthening of sapphire by neutron irradiation

Neutron irradiation of sapphire with 1 x 1022 neutrons(<EQ MeV)/m2 increases the c-axis compressive strength by a factor of 3 at 600 degree(s)C. The mechanism of strength enhancement is the retardation of r-plane twin propagation by radiation-induced defects. 1-B and Cd shielding was employed during irradiation to filter our thermal neutrons (<EQ1 eV), thereby reducing residual radioactivity in the sapphire to background levels in a month. Yellow-brown irradiated sapphire is nearly decolorized to pale yellow by annealing at 600 degree(s)C with no loss of mechanical strength. Annealing at sufficiently high temperature (such as 1200 degree(s)C for 24 h) reduces the compressive strength back to its baseline value. Neutron irradiation decreases the flexure strength of sapphire at 600 degree(s)C by 0-20% in some experiments. However, the c- plane ring-on-ring flexure strength at 600 degree(s)C is doubled by irradiation. Elastic constants of irradiated sapphire are only slightly changed by irradiation. Infrared absorption and emission and thermal conductivity of sapphire are not affected by irradiation at the neutron fluence used in this study. Defects that might be correlated with strengthening were characterized by electron paramagnetic resonance spectroscopy. Color centers observed in the ultraviolet absorption spectrum were not clearly correlated with mechanical response. No radiation-induced changes could be detected by x-ray topography or x-ray diffraction.

Spin dynamics of InAs quantum dots with uniform height

Spin g-factors and lifetimes were studied with picosecond pump-probe techniques for a set of samples of InAs quantum dots of uniform height. The samples were grown by MBE with a cap and flush sequence to produce a height of 2.5 nm. Remote doping provided electrons in the dots. Electron coherence was excited by a fast pump pulse and detected through the Faraday rotation of a probe pulse. The results show an in plane g-factor of 0.427 and lifetimes around 1 ns that shorten for increasing magnetic fields. For an undoped sample, signals from singly charged and neutral dots are observed and simulated to provide the hole g-factor and parameters for the neutral exciton. The undoped sample also exhibits signals for negative delays attributed to mode-locking of the spin coherence to the optical pulse train. This observation indicates that the true spin coherence lasts at least 12 ns.

Ultrafast optical control of electron spins in quantum wells and quantum dots

Using two-color time-resolved Faraday rotation and ellipticity, we demonstrate ultrafast optical control of electron spins in GaAs quantum wells and InAs quantum dots. In quantum wells, a magnetic-field induced electron spin polarization is manipulated by off-resonant pulses. By measuring the amplitude and phase of the spin polarization as a function of pulse detuning, we observe the two competing optical processes: real excitation, which generates a spin polarization through excitation of electron-hole pairs; and virtual excitation, which can manipulate a spin polarization through a stimulated Raman process without exciting electron-hole pairs. In InAs quantum dots, the spin coherence time is much longer, so that the effect of many repetitions of the pump pulses is important. Through real excitation, the pulse train efficiently polarizes electron spins that precess at multiples of the laser repetition frequency, leading to a mode-locking phenomenon. Through virtual excitation, the spins can be partially rotated toward the magnetic field direction, leading to a sensitive dependence of the spin orientation on the precession frequency and detuning. The electron spin dynamics strongly influence the nuclear spin dynamics as well, leading to directional control of the nuclear polarization distribution.

Directing nuclear spin flips in InAs quantum dots using detuned optical pulse trains

We demonstrate that the sign of detuning of an optical pulse train from quantum dot resonances controls the direction of nuclear spin flips. This effect can produce a narrow, precise distribution of nuclear spin polarizations.

Time-Resolved Optically Detected Magnetic Resonance of Luminescence from Si/Si1-xGex Superlattices.

Time-resolved optically detected magnetic resonance (TR-ODMR) experiments have been performed on Si/Si1-x Gex superlattices grown by molecular beam epitaxy (MBE) on (001) Si. These structures exhibit a single, broad (full width at half-maximum amplitude ~70 meV) photoluminescence (PL) band with peak energy 100 meV below the expected band gap. Fast (τ≤5 µ s) and slow (τ20 µ s) processes have been separated with the TR-ODMR. The slow resonances are new and provide more detailed information on the nature of the recombination. These features are tentatively assigned to Ge-rich regions in the SiGe layers based on the resonance parameters and the faster spin-lattice relaxation.