InGaAs Diodes for Terahertz Sensing—Effect of Molecular Beam Epitaxy Growth Conditions

InGaAs-based bow-tie diodes for the terahertz (THz) range are found to be well suited for development of compact THz imaging systems. To further optimize design for sensitive and broadband THz detection, one of the major challenges remains: to understand the noise origin, influence of growth conditions and role of defects for device operation. We present a detailed study of photoreflectance, low-frequency noise characteristics and THz sensitivity of InGaAs bow-tie diodes. The diodes are fabricated from InGaAs wafers grown by molecular beam epitaxy (MBE) on semi-insulating InP substrate under different technological conditions. Photoreflectance spectra indicated the presence of strong built-in electric fields reaching up to 49 kV/cm. It was demonstrated that the spectral density of voltage fluctuations at room temperature was found to be proportional to 1/f, while at lower temperatures, 77–200 K, Lorentzian-type spectra dominate due to random telegraph signals caused by individual capture defects. Furthermore, varying bias voltage, we considered optimal conditions for device room temperature operation in the THz range with respect to signal-to-noise ratio. The THz detectors grown with beam equivalent pressure In/Ga ratio equal to 2.04 exhibit the minimal level of the low-frequency noise, while InGaAs layers grown with beam equivalent pressure In/Ga ratio equal to 2.06 are found to be well suited for fabrication of room temperature bow-tie THz detectors enabling sensitivity of 13 V/W and noise equivalent power (NEP) of 200 pW/√Hz at 0.6 THz due to strong built-in electric field effects.

Optical Characterization of AlxGal-xsb/GaSb Epitaxial Layers

Spectroscopic ellipsometry from 1.4 to 5 eV was used to systematically characterize epitaxial heterostructures AlxGa1-xSb/GaSb for different x concentrations (x≤0.5). The structures were grown by MBE at temperatures and beam equivalent pressure ratios which optimize their lowtemperature photoluminescence properties. Complex dielectric functions of AlxGa1-xSb versus x were derived, for the first time, from the ellipsometric spectra after mathematically removing the oxide overlayer effects. The spectra were analyzed with their second energyderivatives in term of standard analytical lineshapes: in particular the El, E1+Δ1 and E2 critical point energies, broadening and amplitude parameters were derived as a function of x. On this basis we verified that the energy-shift model is appropriate to interpolate for any x≤0.5, thus allowing a nondestructive optical diagnostic of layer thickness and composition of epitaxial heterostructures based on A1xGa1-xSb.