Mostafa Masnadi-Shirazi

Growth of high Bi concentration GaAs1−xBix by molecular beam epitaxy

The incorporation of Bi is investigated in the molecular beam epitaxy growth of GaAs1−xBix. Bi content increases rapidly as the As2:Ga flux ratio is lowered to 0.5 and then saturates for lower flux ratios. Growth under Ga and Bi rich conditions shows that Bi content increases strongly with decreasing temperature. A model is proposed where Bi from a wetting layer incorporates through attachment to Ga-terminated surface sites. The weak Ga-Bi bond can be broken thermally, ejecting Bi back into the wetting layer. Highly crystalline films with up to 22% Bi were grown at temperatures as low as 200 °C.

Nanoplasmonic Terahertz Photoconductive Switch on GaAs

Low-temperature (LT) grown GaAs has a subpicosecond carrier response time that makes it favorable for terahertz photoconductive (PC) switching. However, this is obtained at the price of lower mobility and lower thermal conductivity than GaAs. Here we demonstrate subpicosecond carrier sweep-out and over an order of magnitude higher sensitivity in detection from a GaAs-based PC switch by using a nanoplasmonic structure. As compared to a conventional GaAs PC switch, we observe 40 times the peak-to-peak response from the nanoplasmonic structure on GaAs. The response is double that of a commercial, antireflection coated LT-GaAs PC switch.

Temperature dependence of hole mobility in GaAs1−xBix alloys

The Hall mobility of holes has been measured in GaAs grown at low temperatures and in GaAs1−xBix alloys for Bi concentrations x ranging from 0.94% to 5.5%. The hole mobility is found to decrease with increasing Bi content. The temperature dependence of the mobility in the 25 to 300 K range is fit with a combination of phonon scattering, ionized impurity scattering, and Bi related scattering. The hole scattering cross-section for an isolated Bi impurity is estimated to be 0.2 nm2. The temperature independent mobility at the highest Bi concentration (x=5.5%), is interpreted as being limited by scattering from Bi clusters.

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.

Bismuth-containing III–V semiconductors: Epitaxial growth and physical properties

The growth, surface, and bulk properties of GaAsBi and related III-V alloys are examined and the potential benefits of these materials are explored in terms of device applications. The methods used include molecular beam epitaxy growth, scanning tunneling microscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, deep-level transient spectroscopy, dynamic modeling, and theoretical analysis. The results show that considerable progress has been made in alloying bismuth with GaAs and that the structural, optical, and electronic quality is very good for the alloys investigated.