Y. Sun

Prospect for high brightness III–nitride electron emitter

We describe p-type gallium nitride (GaN) as a candidate for high brightness photocathodes. Experiments utilizing photoemission spectroscopy and quantum yield measurements were performed on GaN films to characterize various cesium and oxygen activations. Quantum efficiencies of 0.1%–4% were obtained in reflection for the cesiated p-type 0.5 μm thick GaN films and 25%–50% on the 0.1 μm thick GaN films. The corresponding emission currents are 142–300 nA for 0.5 μm thick films and 0.7–1.3 μA for the 0.1 μm thick films. This results in an increase of several orders of magnitude in the emission current from the starting GaN films. Furthermore, an initial desorption measurement was performed in order to evaluate the Cs binding strength to GaN relative to GaAs. We observe Cs was bound to the GaN surface (0001_) at 700 °C and completely desorbed at 450 °C for a (100) GaAs surface. Finally, an alternate barium activation on GaN is included for preliminary comparison with the various cesium activations.

HfO2 gate dielectric on (NH4)2S passivated (100) GaAs grown by atomic layer deposition

The interface between hafnium oxide grown by atomic layer deposition and (100) GaAs treated with HCl cleaning and (NH{sub 4}){sub 2}S passivation has been characterized. Synchrotron radiation photoemission core level spectra indicated successful removal of the native oxides and formation of passivating sulfides on the GaAs surface. Layer-by-layer removal of the hafnia film revealed a small amount of As{sub 2}O{sub 3} formed at the interface during the dielectric deposition. Traces of arsenic and sulfur out-diffusion into the hafnia film were observed after a 450 C post-deposition anneal, and may be the origins for the electrically active defects. Transmission electron microscopy cross section images showed thicker HfO{sub 2} films for a given precursor exposure on S-treated GaAs versus the non-treated sample. In addition, the valence-band and the conduction-band offsets at the HfO{sub 2}/GaAs interface were deduced to be 3.18 eV and a range of 0.87-0.97 eV, respectively. It appears that HCl+(NH{sub 4})2{sub S} treatments provide a superior chemical passivation for GaAs and initial surface for ALD deposition.

Synchrotron radiation photoemission spectroscopic study of band offsets and interface self-cleaning by atomic layer deposited HfO2 on In0.53Ga0.47As and In0.52Al0.48As

The Synchrotron Radiation Photoemission Spectroscopic (SRPES) study was conducted to (a) investigate the surface chemistry of In{sub 0.53}Ga{sub 0.47}As and In{sub 0.52}Al{sub 0.48}As post chemical and thermal treatments, (b) construct band diagram and (c) investigate the interface property of HfO{sub 2}/In{sub 0.53}Ga{sub 0.47}As and HfO{sub 2}/In{sub 0.52}Al{sub 0.48}As. Dilute HCl and HF etch remove native oxides on In{sub 0.53}Ga{sub 0.47}As and In{sub 0.52}Al{sub 0.47}As, whereas in-situ vacuum annealing removes surface arsenic pile-up. After the atomic layer deposition of HfO{sub 2}, native oxides were considerably reduced compared to that in as-received epi-layers, strongly suggesting the self-clean mechanism. Valence and conduction band offsets are measured to be 3.37 {+-} 0.1eV, 1.80 {+-} 0.3eV for In{sub 0.53}Ga{sub 0.47}As and 3.00 {+-} 0.1eV, 1.47 {+-} 0.3eV for In{sub 0.52}Al{sub 0.47}As, respectively.

Photoemission from the Sr/Si(001) interface

The growth of Sr on n-type Si(001) was studied in detail for coverages between 0 and 1 monolayer (ML) using core level photoemission spectroscopy. In a similar manner, the Sr saturation coverage was studied in the 600–925 °C temperature range. Data analysis was carried out by a method that allows accurate determination of the band-bending shifts. Using this method it is possible to pinpoint the formation and destruction of chemical species from bungled core level photoemission data without needing to know details of the chemical composition of the spectra. Through this analysis it was established that the interaction between Sr and Si breaks down the binding energy difference between upward and downward Si dimer atoms. In addition, it was found that the saturation coverage exhibits a clear plateau at 1 ML around 650 °C, and a slope change at 1/3 ML around 850 °C. The surface band bending suffers a discontinuous increase as the Sr coverage surpasses 12 ML and as the low energy electron diffraction symmetry...

Erase and Retention Improvements in Charge Trap Flash Through Engineered Charge Storage Layer

The simultaneous improvement in the erase and retention characteristics in a TANOS (TaN-Al₂O₃-Si₃N₄-SiO₂-Si) flash memory transistor by utilizing the band-engineered and compositionally graded SiN_x trap layer is demonstrated. With the process optimizations, a > 4V memory window and excellent 150 degC 24-h retention (0.1-0.5 V charge loss) for a programmed DeltaV_t = 4V with respect to the initial state are obtained. The band-engineered SiN_x charge storage layer enables flash scaling beyond the floating-gate technology with a promise for improved erase speed, retention, lower supply voltages, and multilevel cell applications.

The work function of submonolayer cesium-covered gold: A photoelectron spectroscopy study

Using visible and x-ray photoelectron spectroscopy, we measured the work function of a Au(111) surface at a well-defined submonolayer coverage of Cs. For a Cs coverage producing a photoemission maximum with a He-Ne laser, the work function is 1.61+/-0.08 eV, consistent with previous assumptions used to analyze vibrationally promoted electron emission. A discussion of possible Cs layer structures is also presented.

Reactive-ion-etched diffraction-limited unstable resonator semiconductor lasers

We present characterization and analysis of wide-stripe unstable resonator semiconductor lasers with reactive-ion-etched facets. The mirror facets have RMS roughnesses of only 3-5 mm. Laser beam quality and brightness performance are measured in terms of resonator structure and fabrication parameters. Lateral M/sup 2/ values as low as 1.25 at five times threshold are found. This data is compared to that derived from a Huygens integral-beam propagation method simulation which includes appropriate physical and process-induced aberrations, and good agreement is found.

Thermally controlled lateral beam shift and beam steering in semiconductor lasers

We demonstrate a thermally controlled, offset-gain and index-guiding (OGIG) structure, which permits continuous control of output beam position and direction from a semiconductor laser. The gain and index guiding axes in this structure are parallel but transversely offset. For parabolic gain and index profiles the resulting modes will be Hermite-Gaussian modes with complex-valued spot sizes and tilted wavefronts such that the beam emerges from the end face at a tunable angle with respect to the facet normal. An experimental demonstration of thermally controlled mode displacement and beam steering by as much as 15/spl deg/ is obtained by applying asymmetric heating to a semiconductor laser using a microstripe heater.<<ETX>>

Band offsets between amorphous LaAlO3 and In0.53Ga0.47As

The band offsets between an amorphous LaAlO3 dielectric prepared by molecular-beam deposition and a n-type In0.53Ga0.47As (001) layer have been measured using synchrotron radiation photoemission spectroscopy. The valence and conduction band offsets at the postdeposition annealed LaAlO3∕InGaAs interface are 3.1±0.1 and 2.35±0.2eV, respectively. The band gap of LaAlO3, as determined by Al 2p and O 1s core level energy loss spectra, is 6.2±0.1eV. Within the resolution of the medium energy ion scattering technique, no interfacial oxide layer is seen between the InGaAs and the 3.6nm thick amorphous LaAlO3.

The metal-insulator transition in vanadium dioxide: A view at bulk and surface contributions for thin films and the effect of annealing

Vanadium dioxide is investigated as potential oxide barrier in spin switches, and in order to incorporate VO2 layers in complex multilayer devices, it is necessary to understand the relation between bulk and surface/interface properties. Highly oriented VO2 thin films were grown on (0001) sapphire single crystal substrates with reactive bias target ion beam deposition. In the analysis of the VO2 films, bulk-sensitive methods [x-ray diffraction (XRD) and transport measurements] and surface sensitive techniques [photoelectron spectroscopy (PES) and scanning tunneling microscopy and spectroscopy] were employed. The samples were subjected to heating cycles with annealing temperatures of up to 425 and 525K. Prior to annealing the VO2 films exhibit the transition from the monoclinic to the tetragonal phase with the concurrent change in conductivity by more than a factor of 103 and their phase purity is confirmed by XRD. Annealing to 425K and thus cycling across the metal-insulator transition (MIT) temperature h...