Renata Butkutė

Structure and Optical Properties of InGaAsBi with up to 7% Bismuth

InyGa1-yAs1-xBix layers on InP:Fe (100) substrates have been grown by molecular beam epitaxy. Up to 7% of Bi incorporation has been confirmed by optical and structural analyses of grown samples. Photoluminescence signals at wavelengths up to 3 µm have been observed, implying InyGa1-yAs1-xBix to be a prospective material for mid-infrared applications. A weak band-gap temperature sensitivity of Bi-containing InGaAs has been evaluated from optical absorption measurements.

Properties of hybrid MOVPE/MBE grown GaAsBi/GaAs based near-infrared emitting quantum well lasers

A combined growth approach involving both molecular-beam epitaxy and metal-organic vapor phase epitaxy has been developed to fabricate GaAsBi/GaAs-based quantum well (QW) laser structures with a Bi composition up to 8%. Lasing operation has been demonstrated at room temperature at 1.06 μm in laser diodes containing 3QWs that in turn contain approximately 6% Bi. A 5QW device demonstrated lasing at 1.09 μm at 80 K. Using temperature- and pressure-dependent measurements of stimulated emission as well as pure spontaneous emission measurements, we show that the threshold current of the devices is limited by non-radiative defect-related recombination and an inhomogeneous carrier distribution. This is suspected to be due to inhomogeneity of the QW width as well as non-uniform Bi composition in the active region.

Influence of Field Effects on the Performance of InGaAs-Based Terahertz Radiation Detectors

A detailed electrical characterization of high-performance bow-tie InGaAs-based terahertz detectors is presented along with simulation results. The local surface potential and tunnelling current were scanned over the surfaces of the detectors by means of Kelvin probe force microscopy (KPFM) and scanning tunnelling microscopy (STM), which also enabled the determination of the Fermi level. Current-voltage curves were measured and modelled using the Synopsys Sentaurus TCAD package to gain deeper insight into the processes involved in detector operation. In addition, we performed finite-difference time-domain (FDTD) simulations to reveal features related to changes in the electric field due to the metal detector contacts. The investigation revealed that field-effect-induced conductivity modulation is a possible mechanism contributing to the high sensitivity of the studied detectors.

THz pulse emission from InAs-based epitaxial structures grown on InP substrates

Undoped InAs and InAs p-n junction epitaxial layers were grown on (100)-cut InP substrates with molecular beam epitaxy. The lattice difference between the substrate and the InAs layers was matched with a graded AlInAs buffer layer. The alloy composition, structural characteristics and carrier mobility of the structures were determined from the high-resolution x-ray diffraction, atomic force microscopy and Hall-effect measurements, respectively. The optical parameters of the layers were characterized by the emission of terahertz (THz) pulses when the samples were illuminated with femtosecond laser pulses. It has been found that the built-in electric field in the p-n junction enhances the THz emission. Registering THz signals in the quasi-reflection direction, the p-n junction emits more intense radiation in comparison to an undoped bulk InAs. At excitation wavelengths >1.8 μm the InAs p-n junction provides stronger THz pulses than those from (111)-cut p-InAs, the best surface THz emitter known to date. The epitaxial layers were also exposed to a constant magnetic field from neodymium permanent magnets, which further enhances THz emission and allows registering THz radiation in the line-of-sight terahertz time-domain-spectroscopy geometry.

Influence of laser irradiation on optical properties of GaAsBi/GaAs quantum wells

This paper is focused on investigation of the impact of laser irradiation on the structural and optical properties of bismide-based multiple quantum wells (MQWs). The MQW structures, composed of 5 GaAsBi quantum wells, 7 nm thick, separated by 10 nm-thick GaAs barriers, were grown by molecular beam epitaxy on GaAs (100)-plane oriented semi-insulating substrates at 330°C temperature. The bismuth content in as-grown GaAsBi wells evaluated from the measurements of HR-XRD rocking curves was about 6%. HR-TEM and AFM studies of the MQWs evidenced sharp interfaces between the wells and barriers, and a smooth, droplet-free surface, respectively. HR-TEM images also evidenced a homogeneous bismuth distribution in the wells. The spatially-resolved photoluminescence study of GaAsBi/GaAs MQWs revealed the enhancement of PL emission efficiency of up to 80% with no shift of the spectral position after intense laser irradiation. The obtained results were explained by improvement of the GaAsBi crystal quality.