title: ErAs:In(Al)GaAs photoconductors for 1550 nm-based Terahertz time domain spectroscopy systems
Creators: name: Nandi, Uttam
Publisher: name: UNSPECIFIED
Year: 2021

description: ErAs:In(Al)GaAs photoconductors have proven to be outstanding devices for photonic terahertz (0.1 THz-10 THz) generation and detection. These superlattices are composed of ErAs, InGaAs and InAlAs layers, grown by Molecular Beam Epitaxy. This thesis presents the so far most detailed material characterization of these photoconductor materials followed by an investigation of THz performance. The variation of the material properties as a function of the ErAs concentration and the superlattice structure is discussed for both emitter and receiver materials. Infrared spectroscopy shows an absorption coefficient in the range of 4700-6600 cm⁻¹ at 1550 nm, with shallow absorption edges towards longer wavelengths caused by absorption by ErAs precipitates. The carrier lifetime of the material was obtained using differential transmission measurements. The carrier dynamics also featured a bias-dependent bi-exponential decay, which has been described by a proposed theoretical modeling. Hall measurements show that samples with only 0.8 monolayers (ML) of compensation-doped ErAs precipitates (p-delta-doped at 5×10¹³ cm⁻²) with InAlAs spacer layer featured a carrier concentration of 3.6±0.4×10¹² cm⁻³ which is almost reaching the intrinsic carrier concentration of InGaAs. The IV characteristics featured a resistance in the range of ~10-20 MΩ and high breakdown field strengths beyond 100 kV/cm, corresponding to &gt;500 V for a 50 μm electrode gap. With a higher ErAs concentration of 1.6 ML (2.4 ML) the resistance decreases by a factor of ~40 (120) for an otherwise identical superlattice structure. We further propose a theoretical model for the calculation of the excess current generated due to heating and for estimation of the photocurrent from the total illuminated current. The THz performance has been investigated for all dedicated source material structures. TDS measurements and emitted THz power proves that In(Al)GaAs with 0.8 ML ErAs precipitates is suitable for fabricating high-performance THz devices. THz characterization shows that ErAs:InGaAs receivers are suitable for detecting high average THz power of ~14 mW with current responsivity in the range of 110±25 μA/√W, yet showing slight saturation. Investigation on the antenna performance showed that low frequency optimized 200 μm H-dipole antennas emits the maximum THz signal. Similarly, 50 μm H-dipole antennas used as receivers showed a higher THz peak-peak signal with enhancement of the low-frequency spectrum, whereas for 25 μm H-dipole receivers the spectrum was enhanced uniformly. These measurements were in line with the simulated radiation resistance of the antenna structures. The maximum THz peak-peak signal obtained was 1580 nA corresponding to approx. 113 dB peak dynamic range using source and receiver antenna combinations optimized around 0.2-1 THz where the peak of the spectrum occurs. Bandwidth-optimized antenna structures feature a bandwidth of 6.5 THz. A maximum THz power of 472±35 μW was emitted by low frequency optimized 200 μm H-dipole antenna using p-compensated 0.8 ML ErAs In(Al)GaAs photoconductor material. We finally make a comparison between the emitted THz signal for antenna emitter (AE) and large are emitter (LAE), operated under low laser power (45 mW). Antenna-coupled emitters enhance the overall emitted THz signal, with a maximum of 2.42-fold increase in the THz peak-peak signal corresponding to 12 dB higher dynamic range using 200 μm H-dipole antenna. A theoretical modelling had also been developed which supports these measured results.