Photoconductive Terahertz Emitters and Detectors for the Operation with 1550 nm Pulsed Fiber Lasers
In this thesis, photoconductive terahertz (THz) emitters and detectors suitable for the excitation with femtosecond laser pulses centered on 1550 nm are investigated. The motivation for this study is the development of cost-efficient, flexible and rapid THz time-domain-spectroscopy (TDS) systems...
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|Summary:||In this thesis, photoconductive terahertz (THz) emitters and detectors suitable for the excitation
with femtosecond laser pulses centered on 1550 nm are investigated. The motivation for this
study is the development of cost-efficient, flexible and rapid THz time-domain-spectroscopy
(TDS) systems for the application in growing fields like non-destructive testing (NDT) and inline
process monitoring. In order to achieve this goal, the physics of the generation and
detection of THz radiation in photoconductors is investigated. The combination of experimental
data with the analytic modeling of the carrier dynamics in THz photoconductors allows for a
detailed understanding of the interplay between the growth conditions of the photoconductor
and the properties of the fabricated THz device.
In this work, three different photoconductive materials were studied as THz emitters and
detectors. All these photoconductors contain layers of the ternary semiconductor indium
gallium arsenide (InGaAs). When InGaAs is grown lattice matched to an indium phosphide
(InP) substrate, the material can be excited by erbium doped femtosecond fiber lasers with a
central wavelength around 1550 nm. Therefore, InGaAs is a predestinated absorber in
photoconductive THz emitters and detectors.
Aside from the common InGaAs layers, the photoconductors investigated in this thesis feature
essentially different electrical and optical properties. The reason is that theoretical models and
experimental results obtained within the last two decades revealed different demands on
photoconductors for THz emitters and detectors. On the detector side, a sub-picosecond electron
lifetime is required for the detection of broadband THz radiation with high dynamic range. In
contrast, photoconductive materials for THz emitters require high breakdown fields and carrier
mobility, whereas the electron lifetime is of minor importance. Therefore, the first part of this
work is dedicated to the development of InGaAs-based photoconductors for THz emitters and
Photoconductors with sub-picosecond electron lifetimes were obtained by low-temperature
growth of InGaAs with molecular beam epitaxy (MBE). At temperatures below 300 °C the
growth is non-stoichiometric and arsenic antisites are incorporated as point defects into the
lattice. When these antisites are ionized they serve as fast trapping and recombination centers.
In this work, it is shown that the concentration of the (ionized) antisites can be controlled by
the growth temperature, by using an additional p-dopant (beryllium), and by the temperature
and the duration of a post-growth annealing step. Electron lifetimes as short as 140 fs were
obtained. The precise adjustment of all these parameters allowed for the design and the
fabrication of THz receivers with a spectral bandwidth of up to 6 THz and a peak dynamic
range exceeding 95 dB.
For THz emitters, a high mobility, which is generally equivalent to a low defect density, is
required in order to enable the efficient acceleration of the photoexcited carriers in the electric
field applied to the emitter. Due to the high density of defects, low-temperature-grown (LTG)
InGaAs based photoconductors are not the material of choice for THz emitters. Instead, a
material comprising almost defect free layers of InGaAs surrounded by InAlAs barriers
containing a high density of deep defects was used. These properties were achieved at growth
temperatures close to 400 °C in a MBE system. At those temperatures, alloying forms deep
defects inside the InAlAs layers, whereas InGaAs grows almost defect free. A THz-power of
up to 112 μW ± 7 μW was measured for emitters fabricated from this photoconductor, which
is an increase by a factor of 100 compared to emitters made of the LTG material.
By combining the optimized photoconductive emitters and receivers compact THz-TDS
systems with up to 6 THz bandwidth and 90 dB peak dynamic range were realized. In addition,
an all fiber-coupled THz spectrometer with kHz measurement rate as well as a fully fibercoupled
near-field imaging system with a lateral resolution of 100 μm was demonstrated with
these optimized photoconductive devices.
However, a critical disadvantage of individual THz emitter and detector devices appears when
THz-TDS measurements are performed in reflection geometry. Since many applications in
NDT and in-line process monitoring allow only one side access to the sample under test,
reflection measurements are the common use-case of THz-TDS in these fields. In this thesis, a
fiber-coupled, monolithically integrated THz transceiver was developed, which combines the
emitter and the receiver on a single photoconductive chip. As the photoconductor, Be-doped
LTG-InGaAs/InAlAs with 0.5 ps electron lifetime was used in order to enable a broadband
detection. The optical coupling of the transceiver was realized with the help of a polymer
waveguide chip. With a bandwidth of 4.5 THz and a peak dynamic range larger than 70 dB this
THz transceiver showed a significant performance increase compared to previous transceiver
concepts (2 THz bandwidth and 50 dB peak dynamic range).
In order to further increase the performance of THz transceivers a novel photoconductor had to
be developed, which combines the required properties of THz emitters and detectors in the same
material. For this purpose, iron (Fe) doped InGaAs grown by MBE was investigated. At growth
temperatures close to 400 °C iron could be incorporated homogenously up to concentrations of
5 × 1020 cm-3. The resulting material combined sub-picosecond electron lifetime with high
breakdown fields and high mobility. Applied as a photoconductive emitter, 75 μW ± 5 μW of
radiated THz power were measured. As a detector, THz pulses with a bandwidth of up to 6 THz
and a peak dynamic range of 95 dB were obtained. Hence, Fe-doped InGaAs has not only the
potential to replace the relatively complex state-of-the art photoconductors, it also bears great
potential for future integrated THz devices.
In conclusion, the systematic study of the electrical properties and the carrier dynamics in
InGaAs-based photoconductive materials led to significant improvements of individual THz
emitter and detector devices. The detectable bandwidth was increased by 50 % from below
4 THz to 6 THz and the emitted THz power was enhanced by a factor of 100. Further, the
knowledge from these studies was exploited for the fabrication of a fiber-coupled,
monolithically integrated THz transceiver with a 4.5 THz bandwidth and 70 dB peak dynamic
range. These results are a significant increase in THz performance compared to previous
transceiver concepts (2 THz bandwidth and 50 dB dynamic range). In order to allow for further
improvements of THz transceivers and integrated THz devices, Fe-doped InGaAs was
investigated as a photoconductive emitter and detector. Due to the unique combination of subpicosecond electron lifetime, high resistivity (> 2 Ω cm) and high mobility (> 900 cm2V-1s-1)
Fe-doped InGaAs showed a performance comparable to the optimized THz photoconductors.
Hence, the results presented in this work pave the way for compact and integrated THz devices
for applications in industrial environments.|
|Physical Description:||169 Pages|