B. K. Wagner

Photoluminescence properties of ZnS epilayers

A comprehensive study is reported of the photoluminescence properties of ZnS thin films between 1.6 and 320 K grown by metalorganic molecular beam epitaxy and chemical beam epitaxy on GaAs substrates. Both heavy- and light-hole free excitons were observed at low temperatures with linewidths of 7.0 and 5.3 meV, respectively, as well as donor- and acceptor-bound excitons and free-to-bound recombination along with their longitudinal optical (LO) phonon replicas. The free exciton emission was observed up to 320 K, and enabled the room temperature band gap of ZnS to be unambiguously determined as 3.723 eV. The temperature dependence of the peak position, intensity, and linewidth was well described by the conventional empirical relations and by Toyozawa’s exciton line shape theory. The bound exciton peak positions were found to follow the temperature dependence of the band gap whereas the free-to-bound recombination feature was displaced by (1/2)kT above the band gap energy. Thermal quenching of the donor-bound...

Green phosphor for low-voltage cathodoluminescent applications: SrGa2S4:Eu2+

The optical and physical properties of a low-voltage phosphor, SrGa2S4:Eu2+, are reviewed. This phosphor has excellent chromaticity and high luminous efficiency at excitation voltages <3 kV and at high drive current (∼100 μA/cm2). At high current densities this phosphor was found to have superior saturation properties compared to current phosphors. This is attributed to the fast decay time of this phosphor, which is expected to enhance its resistance to saturation. Recent studies show that this phosphor has good maintenance properties.

Low voltage cathodoluminescence properties of blue emitting SrGa2S4:Ce3+ and ZnS:Ag,Cl phosphors

The low voltage properties of ZnS:Ag and SrGa2S4:Ce3+ have been investigated for applications in field emission displays. It was observed that although ZnS:Ag,Cl has slightly better chromaticity than SrGa2S4:Ce3+, the high luminous efficiency, fast decay time, and better saturation behavior of the thiogallate potentially make it a very important blue phosphor.

Photoluminescence properties of ZnGa2O4:Mn powder phosphors

The results of a systematic photoluminescence study of ZnGa2O4:Mn powder phosphor are reported. At room temperature this phosphor exhibits bright green luminescence with a spectral peak at 2.46 eV and Commission International de l’Eclairage chromaticity coordinates of x=0.073 and y=0.696. At low temperatures the luminescence was found to consist of three components assigned to the 4T1–6A1 inner transition of the 3d electrons of Mn2+ ions located on three different sites of the host crystal. Selective excitation and lifetime measurements were used to investigate the assignment of these features. The photoluminescence lifetime showed a single exponential decay of about 4 ms and at T=1.6 K an optical‐phonon‐related fine structure [Ephonon=(8.2±0.2) meV] of the main photoluminescence line was observed.

Surface stoichiometry and reaction kinetics of molecular beam epitaxially grown (001) CdTe surfaces

Reflection high‐energy electron diffraction desorption studies have been performed on epitaxial (001) CdTe surfaces. Both Cd and Te desorption from CdTe were observed to follow a simple first order rate law. Activation energies of 1.95 and 7.70 eV were found for Te and Cd, respectively, on the CdTe surface. The congruent evaporation temperature was determined to be 340 °C. Under normal growth conditions (a substrate temperature of 300 °C and growth rate of 1 μm/h) a Te‐stabilized surface of (001) CdTe was found. The implications of these findings on high‐quality crystalline growth are discussed.

Carbon-Nanotube Loaded Antenna-Based Ammonia Gas Sensor

Carbon nanotubes (CNTs) have been researched extensively for gas-sensing applications due to their unique electrical, chemical, and structural properties. Single-walled carbon nanotubes (SWNTs) have been predominantly used due to their superior electrical conductivity and higher sensitivity relative to multiwalled CNTs. This paper presents the design and characterization of a novel planar sensor fabricated on paper substrate to detect small concentrations of ammonia gas, using the shift in resonance frequency of a patch antenna as the discriminator. We have investigated three main design issues in depth. First, functionalization of the SWNTs with a polymer is studied in order to enhance the gas detection sensitivity. Second, a thin film of the functionalized SWNT is characterized to create a surface impedance model for the explanation and prediction of the resonance shift due to different gas concentrations. Finally, as a proof of concept, functionalized SWNTs are integrated into a patch antenna design and the return loss is measured in a closed-system environment to show high sensitivity for low concentrations of ammonia gas. The proposed antenna-based wireless gas sensor can be utilized in several applications, given its small form factor, light weight, and little to no power requirements.

Investigation of Ce-doped silicates for low voltage field emission displays

The feasibility of using Ce-doped silicates as an alternative to the P22 blue phosphor (ZnS:Ag) was investigated for low voltage field emission displays (FEDs). Silicates of various composition were evaluated based on their chromaticity, intrinsic efficiency, and brightness saturation behavior. The influence of silicate composition and particle morphology on the cathodoluminescence properties was also assessed. Saturation measurements indicated that the high saturation resistance of the Ce-doped samples can yield better performance than ZnS:Ag when operating at low voltages. The silicates are also attractive for their stability in a FED environment.

Thermal analysis of AlGaN-GaN power HFETs

In this paper, we present a thermal analysis of AlGaN-GaN power heterojunction field-effect transistors (HFETs). We report the dc, small-signal, large-signal, and noise performances of AlGaN-GaN HFETs at high temperatures. The temperature coefficients measured for GaN HFETs are lower than that of GaAs pseudomorphic high electron-mobility transistors, confirming the potential of GaN for high-temperature applications. In addition, the impact of thermal effects on the device dc, small-signal, and large-signal characteristics is quantified using a set of pulsed and continuous wave measurement setups. Finally, a thermal model of a GaN field-effect transistor is implemented to determine design rules to optimize the heat flow and overcome self-heating. Arguments from a device, circuit, and packaging perspective are presented.

CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imaging.

Investigations are reported on the x-ray scintillation and imaging application of CdTe quantum dots (QDs) and their polymer nanocomposites. Aqueous CdTe QDs with emissions ranging between 510 and 680 nm were prepared and incorporated into polyvinyl alcohol or polymethyl methacrylate polymer matrices. The x-ray luminescent properties were evaluated and a resolution of 5 lines∕mm was obtained from the nanocomposite films. Additionally, the fast decay time, nonafterglow, and superior spectral match to conventional charge coupled devices, show that CdTe QD nanocomposites have high promise for x-ray imaging applications.

Development of nanophosphors for light emitting diodes

We report the development of new nanophosphor structures based on the Mn-doped ZnSeS material system to enhance the color properties, luminosity and efficiency of white LEDs. These structures have been demonstrated for phosphor-based white LED applications utilizing both blue and UV LED systems. Bandgap tuning for near UV (405 nm) and blue (460 nm) excitations are reported. Using various optimization procedures, we have produced ZnSe:Mn nanoparticles with an external quantum yield greater than 80%.