Vahid Bahrami-Yekta

Deep level defects in n-type GaAsBi and GaAs grown at low temperatures

Deep level defects in n-type GaAs1−xBix having 0 < x < 0.012 and GaAs grown by molecular beam epitaxy (MBE) at substrate temperatures between 300 and 400 °C have been investigated by Deep Level Capacitance Spectroscopy. Incorporating Bi suppresses the formation of an electron trap with activation energy 0.40 eV, thus reducing the total trap concentration in dilute GaAsBi layers by more than a factor of 20 compared to GaAs grown under the same conditions. We find that the dominant traps in dilute GaAsBi layers are defect complexes involving AsGa, as expected for MBE growth at these temperatures.

Nanoplasmonics enhanced terahertz sources

Arrayed hexagonal metal nanostructures are used to maximize the local current density while providing effective thermal management at the nanoscale, thereby allowing for increased emission from photoconductive terahertz (THz) sources. The THz emission field amplitude was increased by 60% above that of a commercial THz photoconductive antenna, even though the hexagonal nanostructured device had 75% of the bias voltage. The arrayed hexagonal outperforms our previously investigated strip array nanoplasmonic structure by providing stronger localization of the current density near the metal surface with an operating bandwidth of 2.6 THz. This approach is promising to achieve efficient THz sources.

Bandgap and optical absorption edge of GaAs1−xBix alloys with 0 < x < 17.8%

The compositional dependence of the fundamental bandgap of pseudomorphic GaAs1−xBix layers on GaAs substrates is studied at room temperature by optical transmission and photoluminescence spectroscopies. All GaAs1−xBix films (0 ≤ x ≤ 17.8%) show direct optical bandgaps, which decrease with increasing Bi content, closely following density functional theory predictions. The smallest measured bandgap is 0.52 eV (∼2.4 μm) at 17.8% Bi. Extrapolating a fit to the data, the GaAs1−xBix bandgap is predicted to reach 0 eV at 35% Bi. Below the GaAs1−xBix bandgap, exponential absorption band tails are observed with Urbach energies 3–6 times larger than that of bulk GaAs. The Urbach parameter increases with Bi content up to 5.5% Bi, and remains constant at higher concentrations. The lattice constant and Bi content of GaAs1−xBix layers (0 < x ≤ 19.4%) are studied using high resolution x-ray diffraction and Rutherford backscattering spectroscopy. The relaxed lattice constant of hypothetical zincblende GaBi is estimated t...

MBE growth optimization for GaAs1−xBix and dependence of photoluminescence on growth temperature

The effect of growth conditions on the electronic properties of GaAs1−xBix grown on GaAs by molecular beam epitaxy has been investigated by means of temperature dependent photoluminescence (PL). When the substrate temperature during growth was reduced from 400 °C to 300 °C and all other growth conditions were fixed, the Bi concentration in the deposited films increased from 1% to 5% and the PL intensity decreased by more than a factor of 1000. Two samples were grown at different temperatures (330 °C and 375 °C) with approximately the same Bi concentration (~2%) at a stoichiometric As:Ga flux ratio. The temperature dependence of the PL shows that the sample grown at high temperature has less PL emission from sub-bandgap states and a stronger temperature dependence of the bandgap. We conclude that GaAs1−xBix samples grown at higher temperatures have a lower density of shallow and deep electronic states in the bandgap.

Molecular beam epitaxy growth and optical properties of single crystal Zn3N2 films

Single crystal Zn3N2 films with (100) orientation have been grown by plasma-assisted molecular beam epitaxy on MgO and A-plane sapphire substrates with in situ optical reflectance monitoring of the growth. The optical bandgap was found to be 1.25–1.28 eV and an electron Hall mobility as high as 395 cm2 V−1 s−1 was measured. The films were n-type with carrier concentrations in the 1018–1019 cm−3 range.

Defect energy levels in p-type GaAsBi and GaAs grown by MBE at low temperatures

Deep level defects in p-type GaAs1−x Bi x (x < 1%) and GaAs grown by molecular beam epitaxy at substrate temperatures of 330 °C and 370 °C have been characterized by deep level transient spectroscopy. We find that incorporating Bi into GaAs at 330 °C does not affect the total concentration of hole traps, which is ~4 × 1016 cm−3, comparable to the concentration of electron traps observed in Si-doped GaAsBi having a similar alloy composition. Increasing the growth temperature of the p-type GaAsBi (x = 0.8%) layer from 330 °C to 370 °C reduces the hole trap concentration by an order of magnitude. Moreover, the defects having near mid-gap energy levels that are the most efficient non-radiative recombination centers are present only in GaAsBi layers grown at the lower temperature. These new results are discussed in the context of previous measurements of n-type GaAs and GaAsBi layers grown under similar conditions.

Closed-cycle cooling of cryopanels in molecular beam epitaxy

Closed-cycle cooling of the cryoshroud in a molecular beam epitaxy (MBE) system with a dimethyl polysiloxane heat transfer fluid has reduced liquid nitrogen consumption by an order of magnitude, significantly lowering operating costs. The temperature dependence of cryopanel pumping efficacy in the MBE system has been investigated. H2O, CO, CO2, and As4 are all pumped effectively by liquid nitrogen cooled cryopanels (−196 °C) in the MBE. At −78 °C, the operating temperature of the closed-cycle chiller, H2O and As4 are pumped effectively, while CO and CO2 are not. The pumping speed for H2O is found to increase exponentially with decreasing temperature. Below ∼−40 °C and ∼−95 °C, the pumping speeds for As4 and H2O saturate, respectively. AlGaAs layers grown with the closed-cycle-cooled shroud show strong photoluminescence, expected room temperature electron mobility, and background doping levels less than 4 × 1015 cm−3.

Closed cycle chiller as a low cost alternative to liquid nitrogen in molecular beam epitaxy

The high cost of cooling the cryoshroud in a molecular beam epitaxy system has been greatly reduced by replacing liquid nitrogen (LN2) as a coolant with a silicone polymer heat transfer fluid cooled to as low as −80 °C by a closed cycle chiller. Gallium arsenide epitaxial layers have been grown with two different cooling configurations of the shroud: conventional LN2 cooling and cooling to −70 °C with the chiller. The partial pressure of water in the chamber is a factor of about 2.5 higher with the closed cycle chiller operating at −70 °C than with liquid nitrogen in the shroud. No significant difference is observed in the density of deep levels in the GaAs, as determined by deep level transient spectroscopy.

Plasmon-enhanced LT-GaAs/AlAs heterostructure photoconductive antennas for sub-bandgap terahertz generation.

Photocurrent generation in low-temperature-grown GaAs (LT-GaAs) has been significantly improved by growing a thin AlAs isolation layer between the LT-GaAs layer and semi-insulating (SI)-GaAs substrate. The AlAs layer allows greater arsenic incorporation into the LT-GaAs layer, prevents current diffusion into the GaAs substrate, and provides optical back-reflection that enhances below bandgap terahertz generation. Our plasmon-enhanced LT-GaAs/AlAs photoconductive antennas provide 4.5 THz bandwidth and 75 dB signal-to-noise ratio (SNR) under 50 mW of 1570 nm excitation, whereas the structure without the AlAs layer gives 3 THz bandwidth, 65 dB SNR for the same conditions.