Robert E. Thorne

Incorporation rates of gallium and aluminum on GaAs during molecular beam epitaxy at high substrate temperatures

Gallium arsenide, aluminum arsenide, and aluminum gallium arsenide epitaxial layers were grown by molecular beam epitaxy in the substrate temperature range 590–720 °C. The incorporation rates of Ga and Al in this temperature range were studied by means of thickness measurements. The growth rates of GaAs and AlxGa1−xAs were observed to be dependent on growth temperature above 640 °C while the AlAs growth rate was observed to be independent of growth temperature in the range investigated. The reduction of the GaAs growth rate at a growth temperature above 640 °C was found to be lessened by the presence of minute amounts of Al and excess As. For the fixed Ga flux and a growth temperature of 700 °C the GaAs growth rate and the Ga contribution to the growth rate of Al0.3Ga0.7As were 0.50 and 0.89 times their low temperature values, respectively, while at 680 °C these values were 0.88 and 0.99, respectively.

Effects of cryoprotectant concentration and cooling rate on vitrification of aqueous solutions

Vitrification of aqueous cryoprotectant mixtures is essential in cryopreservation of proteins and other biological samples. Systematic measurements of critical cryoprotective agent (CPA) concentrations required for vitrification during plunge-cooling from T = 295 K to T = 77 K in liquid nitrogen are reported. Measurements on fourteen common CPAs, including alcohols (glycerol, methanol, 2-propanol), sugars (sucrose, xylitol, dextrose, trehalose), polyethylene glycols (ethylene glycol, PEG 200, PEG 2000, PEG 20000), glycols [dimethyl sulfoxide (DMSO), 2-methyl-2,4-pentanediol (MPD)], and salt (NaCl), were performed for volumes ranging over four orders of magnitude from ∼1 nl to 20 µl, and covering the range of interest in protein crystallography. X-ray diffraction measurements on aqueous glycerol mixtures confirm that the polycrystalline-to-vitreous transition occurs within a span of less than 2% w/v in CPA concentration, and that the form of polycrystalline ice (hexagonal or cubic) depends on CPA concentration and cooling rate. For most of the studied cryoprotectants, the critical concentration decreases strongly with volume in the range from ∼5 µl to ∼0.1 µl, typically by a factor of two. By combining measurements of the critical concentration versus volume with cooling time versus volume, the function of greatest intrinsic physical interest is obtained: the critical CPA concentration versus cooling rate during flash-cooling. These results provide a basis for more rational design of cryoprotective protocols, and should yield insight into the physics of glass formation in aqueous mixtures.

Use of a superlattice to enhance the interface properties between two bulk heterolayers

Single interface modulation‐doped Alx Ga1−xAs /GaAs heterostructures with the binary on top of ternary were grown by molecular beam epitaxy. By incorporating a 150‐A‐thick Alx Ga1−xAs /GaAs three‐period superlattice in place of an undoped Alx Ga1−xAs spacer layer, 10‐K mobilities of up to 256 000 cm2/Vs were obtained. This value is about 6.5 times that of the previous best value. This dramatic improvement is tentatively attributed to the relief of strain caused by the small but significant lattice mismatch although impurity trapping by the superlattice may also play a role. Normal modulation‐doped structures where the ternary is grown on top of binary also showed mobility improvement (about 30%) when the undoped AlGaAs spacer layer is replaced with a three‐period superlattice of the same thickness. This concept should have a significant role in heterojunction bipolar transistors, field‐effect transistors, lasers, and other heterojunction devices.

Charge‐Density‐Wave Conductors

When metals are cooled, they often undergo a phase transition to a state exhibiting a new type of order. Metals such as iron and nickel become ferromagnetic below temperatures of several hundred degrees Celsius; electron spins order to produce a net magnetization in zero field. Other metals, such as lead and aluminum, become superconductors at cryogenic temperatures; electrons form Cooper pairs of opposite spin and momentum, leading to electrical conduction with zero resistance and to expulsion of magnetic fields.

Microfabricated mounts for high-throughput macromolecular cryocrystallography

A new approach is described for mounting microcrystals of biological macromolecules for cryocrystallography. The sample mounts are prepared by patterning thin polyimide films by standard microfabrication techniques. The patterned structures contain a small hole for the crystal connected to a larger hole via a drainage channel, allowing removal of excess liquid and easier manipulation in viscous solutions. These polyimide structures are wrapped around small metal rods. The resulting curvature increases their rigidity and allows a convenient scoop-like action in retrieving crystals. The polyimide contributes minimally to X-ray background and absorption, and can be treated to obtain desired hydrophobicity or hydrophilicity. The new mounts are fully compatible with existing automated sample-handling hardware for cryocrystallography. Their potential advantages include completely reproducible sample hole sizes to below 10 µm; accurate and reproducible sample positioning and good sample-to-mount contrast, simplifying alignment; more convenient manipulation of small crystals; easier removal of excess liquid and reduced background scatter; reduced thermal mass and more rapid flash-cooling; and easy design customization and mass production. They are especially well suited to data collection from the smaller crystals produced in high-throughput crystallization trials, and are suitable for automated crystal retrieval. They should be more generally useful for X-ray data collection from small organic and inorganic crystals of all types.

Flash-cooling and annealing of protein crystals

Flash-cooling and annealing of macromolecular crystals have been investigated using in situ X-ray imaging, diffraction-peak lineshape measurements and conventional crystallographic diffraction. The dominant mechanisms by which flash-cooling creates disorder are suggested and a fixed-temperature annealing protocol for reducing this disorder is demonstrated that should be more reliable and flexible than existing protocols. Flash-cooling tetragonal lysozyme crystals degrades diffraction resolution and broadens the distributions of lattice orientations (mosaicity) and lattice spacings. The diffraction resolution strongly correlates with the width of the lattice-spacing distribution. Annealing at fixed temperatures of 253 and 233 K consistently reduces the lattice-spacing spread and improves the resolution for annealing times up to approximately 30s. X-ray images show that this improvement arises from the formation of well ordered domains with characteristic sizes >10 microm and narrower mosaicities than the crystal as a whole. Flash-cooled triclinic crystals of lysozyme, which have a smaller water content than the tetragonal form, diffract to higher resolution with smaller mosaicities and exhibit pronounced ordered domain structure even before annealing. It is suggested that differential thermal expansion of the protein lattice and solvent may be the primary cause of flash-cooling-induced disorder. Mechanisms by which annealing at T << 273 K reduce this disorder are discussed.

Quantifying X-ray radiation damage in protein crystals at cryogenic temperatures

The dependence of radiation damage to protein crystals at cryogenic temperatures upon the X-ray absorption cross-section of the crystal has been examined. Lysozyme crystals containing varying heavy-atom concentrations were irradiated and diffraction patterns were recorded as a function of the total number of incident photons. An experimental protocol and a coefficient of sensitivity to absorbed dose, proportional to the change in relative isotropic B factor, are defined that together yield a sensitive and robust measure of damage. Radiation damage per incident photon increases linearly with the absorption coefficient of the crystal, but damage per absorbed photon is the same for all heavy-atom concentrations. Similar damage per absorbed photon is observed for crystals of three proteins with different molecular sizes and solvent contents.

Macromolecular impurities and disorder in protein crystals

The mechanisms by which macromolecular impurities degrade the diffraction properties of protein crystals have been investigated using X‐ray topography, high‐resolution diffraction line shape measurements, crystallographic data collection, chemical analysis, and two‐photon excitation fluorescence microscopy. Hen egg‐white lysozyme crystals grown from solutions containing a structurally unrelated protein (ovotransferrin) and a related protein (turkey egg‐white lysozyme) can exhibit significantly broadened mosaicity due to formation of cracks and dislocations but have overall B factors and diffraction resolutions comparable to those of crystals grown from uncontaminated lysozyme. Direct fluorescence imaging of the three‐dimensional impurity distribution shows that impurities incorporate with different densities in sectors formed by growth on different crystal faces, and that impurity densities in the crystal core and along boundaries between growth sectors can be much larger than in other parts of the crystal. These nonuniformities create stresses that drive formation of the defects responsible for the mosaic broadening. Our results provide a rationale for the use of seeding to obtain high‐quality crystals from heavily contaminated solutions and have implications for the use of crystallization for protein purification. Proteins 1999;36:270–281.

Photoconductivity effects in extremely high mobility modulation‐doped (Al,Ga)As/GaAs heterostructures

Single period modulation doped Al0.35Ga0.65 As/GaAs heterostructures were grown by molecular beam epitaxy. The mobilities and sheet carrier concentrations were measured as a function of lattice temperature in the dark and in room light. Mobilities as high as 8490, 105 000, and 221 000 cm2/Vs at 300, 78, and 10 K, respectively, were obtained for samples measured in the dark. When measured in light, these mobilities increased to 9090, 136 000, and 286 000 cm2/Vs at the same respective temperatures. In all cases the sheet carrier concentrations were between 4.5×1011 and 9.5×1011 cm−2. These values represent one of the best dark values reported to date and are significant with respect to field effect transistor applications. The 300 K mobility of 9090 is equivalent to the best mobilities obtained in ultrapure GaAs (n⩽1013 cm−3). The change in the mobility and sheet carrier concentration is the result of a persistent photoconductivity effect which is attributed to the ionization of electrons from traps in the ...

Heat transfer from protein crystals: implications for flash-cooling and X-ray beam heating.

Three problems involving heat transfer from a protein crystal to a cooling agent are analyzed: flash-cooling in a cold nitrogen- or helium-gas stream, plunge-cooling into liquid nitrogen, propane or ethane and crystal heating in a cold gas stream owing to X-ray absorption. Heat transfer occurs by conduction inside the crystal and by convection from the crystals outer surface to the cooling fluid. For flash-cooling in cold gas streams, heat transfer is limited by the rate of external convection; internal temperature gradients and crystal strains during cooling are very small. Helium gas provides only a threefold improvement in cooling rates relative to nitrogen because its much larger thermal conductivity is offset by its larger kinematic viscosity. Characteristic cooling times vary with crystal size L as L(3/2) and theoretical estimates of these times are consistent with experiments. Plunge-cooling into liquid cryogens, which can give much smaller convective thermal resistances provided that surface boiling is eliminated, can increase cooling rates by more than an order of magnitude. However, the internal conduction resistance is no longer negligible, producing much larger internal temperature gradients and strains that may damage larger crystals. Based on this analysis, factors affecting the success of flash-cooling experiments can be ordered from most to least important as follows: (1) crystal solvent content and solvent composition, (2) crystal size and shape, (3) amount of residual liquid around the crystal, (4) cooling method (liquid plunge versus gas stream), (5) choice of gas/liquid and (6) relative speed between cooling fluid and crystal. Crystal heating by X-ray absorption on present high-flux beamlines should be small. For a fixed flux and illuminated area, heating can be reduced by using crystals with areas normal to the beam that are much larger than the beam area.