N. Hozhabri

Vacancy‐type defects in molecular beam epitaxy low temperature grown GaAs, a positron beam lifetime study

Positron beam lifetime spectroscopy has been utilized to study the depth distribution of vacancy‐type defects in molecular beam epitaxy GaAs grown at low temperature. Lifetime spectra were measured as a function of positron energy. From the analysis of the positron lifetime in as‐grown and annealed low temperature grown GaAs, the concentrations of Ga monovacancies and voids are estimated. Our results show that in an as‐grown sample the Ga monovacancy concentration is >3×1018 cm−3. It is also known that vacancy‐cluster concentration in an annealed sample exceeds 1018 cm−3 with a nonuniform spatial distribution.

Infrared measurements in annealed molecular beam epitaxy GaAs grown at low temperature

We have utilized an infrared absorption technique to study deep level defects in molecular beam epitaxy GaAs grown at 250 °C. From an observed broad absorption band below the conduction edge, the concentration of defects is estimated to be ≊5×1019 cm−3 in an as‐grown sample. The concentration of defects decreases exponentially by one order of magnitude due to annealing of the sample at temperatures of 400–500 °C. From the temperature dependence of the defect concentration, the migration energy of defects is calculated to be 0.52±0.02 eV. The measured migration energy shows that part of the defects are arsenic interstitials.

High performance surface plasmon sensors: Simulations and measurements

Through computer simulations and surface plasmon resonance (SPR) measurements, we establish optimum parameters for the design and fabrication of SPR sensors of high sensitivity, resolution, stability, and long decay-length evanescent fields. We present simulations and experimental SPR data for variety of sensors fabricated by using bimetal (Ag/Au) and multilayer waveguide-coupled Ag/Si3N4/Au structures. The simulations were carried out by using the transfer matrix method in MATLAB environment. Results are presented as functions of the thickness of the metal (Ag or Au) and the waveguide dielectric used in Ag/Si3N4/Au structures. Excellent agreement is observed between the simulations and experiments. For optimized thickness of the Si3N4 waveguide (150 nm), the sensor exhibits very high sensitivity to changes in the refractive index of analytes, Sn≈52°/RIU, extremely high resolution (FWHM≤0.28°), and long penetration depth of evanescent fields (δ≥305nm).

Monoenergetic positron beam, positron lifetime, and Hall‐effect measurements in III–V epilayers grown at low temperatures by molecular beam epitaxy

A monoenergetic positron beam has been utilized to depth profile defects in GaAs and in low temperature GaAs buffer layers (LTBL) grown by molecular beam epitaxy. Doppler broadening spectra were measured as a function of positron implantation depth and analyzed in terms of positron diffusion and annihilations of free and trapped positrons. Positron lifetime spectra were measured for the LTBL samples for temperatures in the range 10–295 K. These spectra show two temperature‐dependent components resulting from the annihilations of free positrons and of positrons trapped in vacancy‐type shallow defects. We have performed Hall‐effect and low magnetic field temperature dependent resistivity measurements on the LTBL samples and have determined that the defect activation energy is =0.78 eV. The magnetic field dependence of the Hall data for the LTBL samples shows several interesting new features including sharp stepwise increases for every 700 G between 3.5 and 6 kG.

Defects in molecular beam epitaxial GaAs grown at low temperatures

We have utilized a variable energy positron beam and infrared transmission spectroscopy to study defects in GaAs epilayers grown at low temperatures (LT-GaAs) by molecular beam epitaxy. We have measured the Doppler broadening of the positron-electron annihilation gamma ray spectra as a function of positron implantation energy. From these measurements, we have obtained results for the depth profiles of Ga monovacancies in unannealed LT-GaAs and Ga monovacancies and arsenic cluster related defects in annealed LT-GaAs. We have also studied the effects of the Si impurities in annealed LT-GaAs. The infrared transmission measurements on unannealed LT-GaAs furnish a broad defect band, related to As antisites, centered at 0.370 eV below the conduction band.

Correlation between arsenic precipitates and vacancy-type defects in low-temperature-grown GaAs

We have utilized x-ray photoelectron and variable energy positron beam spectroscopies for depth profiling excess arsenic, arsenic precipitates, and vacancy-type defects in GaAs grown by molecular beam epitaxy at low temperatures (LT-GaAs). XPS results show about 1.3% excess arsenic in as-grown LT-GaAs and a non-uniform depth profile of arsenic concentration in annealed LT-GaAs. Doppler broadening of the positron-electron annihilation radiation (S parameter) reveals a non-uniform depth profile of defects in annealed LT-GaAs. We observe a clear correlation between the depth profile of the S parameter and As in annealed LT-GaAs.

Depth and radial profiles of defects in Czochralski-grown silicon

We have studied the depth and spatial profiles of vacancies in Czochralski‐grown silicon wafers by positron annihilation spectroscopy. By using a variable energy positron beam and γ‐ray spectroscopy, we have obtained depth profiles of defects in as‐grown, annealed, and 〈100〉 epitaxial Si wafers. We discuss these results in terms of vacancies and oxygen precipitates. The bulk position lifetime measurements, made as a function of axial displacement of a positron source, resolve vacancies, and divacancies in the wafer.

Hafnium dioxide as a dielectric for highly-sensitive waveguide-coupled surface plasmon resonance sensors

Hafnium dioxide has been recognized as an excellent dielectric for microelectronics. However, its usefulness for the surface plasmon based sensors has not yet been tested. Here we investigate its usefulness for waveguide-coupled bi-metallic surface plasmon resonance sensors. Several Ag/HfO2/Au multilayer structure sensors were fabricated and evaluated by optical measurements and computer simulations. The resulting data establish correlations between the growth parameters and sensor performance. The sensor sensitivity to refractive index of analytes is determined to be Sn=∂θSPR∂n≥470. The sensitivity data are supported by simulations, which also predict 314 nm for the evanescent field decay length in air.

Defects and arsenic distribution in low-temperature-grown GaAs

We have used X-ray photoelectron spectroscopy (XPS) and variable energy positron beam spectroscopy (VEPBS) to study the excess arsenic and related defects in GaAs grown by molecular beam epitaxy at low temperatures (LT-GaAs). XPS shows about 1.3% excess arsenic in as-grown LT-GaAs and a non-uniform depth profile of arsenic concentration in the annealed LT-GaAs. From the S parameter, we obtain a non-uniform depth profile of defects in annealed LT-GaAs. From similarity between depth profiles of the S parameter and As concentration, we conclude that the positrons are trapped in vacancy complexes associated with arsenic clusters in the annealed LT-GaAs.

Slow positron beam measurements on GaAs grown by molecular beam epitaxy at low temperatures

Variable energy positron beam spectroscopy is employed to study defects and arsenic precipitates in GaAs epilayers grown at low temperatures by Molecular Beam Epitaxy (MBE). We have measured the S‐parameter as a function of positron implantation energy. We have obtained results for the depth profiles of the Ga monovacancy defects in unannealed LT‐GaAs and Ga monovacancies and arsenic clusters related defects in annealed LT‐GaAs.