Donald R. Dorman

GaN (0001)-(1×1) surfaces: Composition and electronic properties

We use low energy electron diffraction, Auger electron spectroscopy, and ultraviolet and x-ray photoemission spectroscopy to study the surface structure, stoichiometry, and electronic properties of n- and p-type GaN (0001) grown by metal-organic chemical vapor deposition. Ordered (1×1) surfaces with nearly stoichiometric composition are prepared by nitrogen sputtering and annealing. The band bending is found to be 0.75±0.1 eV up and 0.75±0.1 eV down for n- and p-type samples, respectively. The work function, electron affinity, and Ga 3d core level binding energy are also determined.

Novel technique for p‐type nitrogen doped ZnSe epitaxial layers

We report a novel technique to obtain p‐type ZnSe layers doped with nitrogen. The layers were grown in a low‐pressure metalorganic vapor phase epitaxy system using ammonia as the dopant source. A rapid thermal anneal was used to enhance the activation of the nitrogen acceptors. Net acceptor concentration values as high as 3×1016/cm3 were obtained from capacitance‐voltage measurements and the profile was uniform over the thickness of the epitaxial layers. The 7 K photoluminescence spectrum was dominated by the acceptor bound exciton peak; the donor‐acceptor pair spectra were also observed.

Biplane phased array for ultrasonic medical imaging

The authors report a transducer structure that combines two orthogonal phase arrays that enable the real-time scanning of two orthogonal sectors. This biplane phased array is formed by partially dicing the opposite faces of a composite piezoelectric plate in orthogonal directions. Individual elements of the orthogonal arrays show broad radiation patterns close to the theoretical expectations for isolated elements. Arrays formed by patterning the electrode alone, without cross dicing, show a narrower directivity pattern for a single element. This narrowing is a refractive effect due to the relatively high acoustic velocities in the composite material as compared with the sound velocity in the propagation medium. It is noted that the concept of the biplane phased array forms the basis for a new class of transducer systems that combine two transducer arrays in a single piezocomposite plate.<<ETX>>

Remanent nonlinear magnetic response in superconducting Y‐Ba‐Cu‐O

We report a ‘‘memory’’ effect in the response of sintered Y‐Ba‐Cu‐O to alternating magnetic fields. The application of a steady bias field, above a certain threshold, causes the generation of harmonic components in the response. These harmonics persist after the field is removed, exhibiting a logarithmic decay with time. We explain these phenomena in the framework of the Bean model, taking into account the granularity of the sample. The memory phenomenon offers a principle for a new recording mechanism which may allow extremely high storage capacity.

Curved annular array transducers made from composite piezoelectric materials

Spherically curved annular-array transducers have been developed using a composite piezoelectric material. The composite material allows the construction of sensitive, high-resolution arrays with elements defined by the electrode pattern alone. The array elements exhibit uniform properties and clean thickness-mode oscillations uncoupled to spurious lateral resonances. A prototype 3-MHz array exhibits a minimum insertion loss of 8 dB with a 6-dB fractional bandwidth of 57%. The -20-dB and -40-dB ringdown times in this array correspond to 3.3 and 6.2 periods, respectively.<<ETX>>

Optical Spectroscopy of a Nitrogen-Hydrogen Complex in ZnSe

We have observed two local vibrational modes related to H bonded to N acceptors in ZnSe samples. grown by metal organic vapor phase epitaxy. The modes have been seen in both infrared and Raman spectroscopy. The new mode seen at 3194 cm{sup {minus}1} is assigned to an N-H stretching vibrational mode and the mode found at 783 cm{sup {minus}1} is tentatively assigned to an N-H wagging, vibrational mode. Polarized Raman spectroscopy was used to determine that the symmetry of the defect complex is C{sub 3v}, which implies that the H atom is in either a bonding or anti-bonding position.