Ricardo Ascazubi

Terahertz emission by InN

We report on optically excited terahertz (THz) emission by indium nitride (InN) thin films. We have used 70 fs titanium–sapphire laser pulses with wavelengths at 800 nm to generate THz-radiation pulses. The InN thin films are deposited on sapphire substrates with GaN buffer layer by molecular-beam epitaxy. The THz-radiation emitted from the InN surface is significantly stronger than that of the GaN/InN interface. The origin of the THz emission are transient photocarrier currents. These results are in agreement with recent experimental results of InN which show that this material is a small band-gap semiconductor. The magnitude of the THz emission from the InN is strong compared to THz emission from previously investigated semiconductors.

Millimeter wave emission from GaN high electron mobility transistor

We report on millimeter wave electromagnetic radiation from a GaN high electron mobility transistor with the gate length of 1.5 μm at 8 K. The emission takes place at gate and drain voltages in the linear regime of operation but close to the saturation voltage with the principal emission peak at approximately 75 GHz, which is much higher than the device cut-off frequency. An explanation of this effect involves the “shallow water” plasma wave instability, with the frequency of the plasma waves decreased by the ungated regions of the device.

Ultrafast recombination in Si-doped InN

We report femtosecond near-infrared transient photoreflection measurements of native n-type indium nitride and silicon-doped indium nitride thin films. The overall time dependence of the ultrafast reflectivity transient is characterized by the different time scales of carrier cooling and carrier recombination. Experimental analysis demonstrates nonradiative recombination in the picosecond and subpicosecond range as the dominant recombination mechanism at room temperature even at very high carrier concentrations. Silicon-doped InN films exhibit carrier lifetimes as short as 680fs.

Terahertz emission from silicon and magnesium doped indium nitride

We report an experimental study of femtosecond optically excited emission of terahertz frequency electromagnetic radiation from as-grown n-type InN, silicon doped InN, and magnesium doped InN. We have measured the terahertz emission from these materials as function of dc Hall mobility and carrier concentrations. Terahertz emission from InN:Si and native n-type InN increases with mobility as expected for transient photocurrents as primary mechanism of terahertz emission from InN. InN:Mg exhibits enhanced terahertz emission compared to InN:Si. This is experimental evidence for Mg being electrically active as an acceptor in InN. Terahertz emission from InN:Si is less strong than terahertz emission from native n-type InN because of an increased electron concentration due to silicon being an electrically active donor in InN.

Dephasing in modulation-doped quantum structures probed by THz time-domain spectroscopy

The dephasing of intersubband transitions in semiconductor heterostructures was investigated by time-resolved THz spectroscopy. Single quantum structures show dephasing rates, which are nearly identical to scattering rates obtained by conventional Hall measurements and allow insight into the dephasing dynamics. In multiple quantum wells, inhomogeneous broadening of the density of states is the main dephasing mechanism.

Time-domain terahertz spectroscopy of artificial skin

Time-domain Terahertz (THz) spectroscopy and imaging is currently evaluated as a novel tool for medical imaging and diagnostics. The application of THz-pulse imaging of human skin tissues and related cancers has been demonstrated recently in-vitro and in-vivo. With this in mind, we present a time-domain THz-transmission study of artificial skin. The skin samples consist of a monolayer of porous matrix of fibers of cross-linked bovine tendon collagen and a glycosaminoglycan (chondroitin-6-sulfate) that is manufactured with a controlled porosity and defined degradation rate. Another set of samples consists of the collagen monolayer covered with a silicone layer. We have measured the THz-transmission and determined the index of refraction and absorption of our samples between 0.1 and 3 THz for various states of hydration in distilled water and saline solutions. The transmission of the THz-radiation through the artificial skin samples is modeled by electromagnetic wave theory. Moreover, the THz-optical properties of the artificial skin layers are compared to the THz-optical properties of freshly excised human skin samples. Based on this comparison the potential use of artificial skin samples as photo-medical phantoms for human skin is discussed.

MBE Grown InN: A Novel THz Emitter

Strong optically excited terahertz emission has been observed from InN thin films. The emission mechanism has been determined to be photocarrier acceleration. This observation implies that InN has a bandgap smaller than 1.5 eV.

Indium Nitride as novel THz-radiation source for time-domain THz-systems

We report an experimental study of femtosecond near-infrared optically excited THz-emission from InN thin films grown by molecular beam epitaxy (MBE) on sapphire substrates. THz-emission was investigated as a function of structural as well as electronic properties such as types of buffer layer, film thickness, electron mobilities, electron concentrations and doping of the InN with Si and Mg atoms. The THz-emission mechanism in InN has been analyzed. Ultrafast transient currents are identified as dominant THz-emission mechanisms. Ultrafast carrier recombination is identified as a limiting factor of THz-emission from n-type InN: Si.

Enhanced THz Emission from Te Doped GaSb

Strong enhancement of optically excited THz emission from GaSb due to compensation of native acceptors by tellurium donors has been observed. THz emission of compensated GaSb is an order of magnitude stronger than undoped GaSb.