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Abstract
We present photomixers made of iron doped indium gallium arsenide (InGaAs:Fe) as broadband receivers in optoelectronic continuous wave (cw) terahertz (THz) systems. InGaAs:Fe shows higher resistivity and shorter carrier lifetimes compared to the state-of-the-art low-temperature-grown material. These improved material properties translate into an improved frequency response and lower noise level of the InGaAs:Fe photomixers. We were able to measure a bandwidth of 4.5 THz with a peak dynamic range of 112 dB at 30 mW laser excitation around 1550 nm. To the best of our knowledge, these are record high values for cw THz spectroscopy. Furthermore we achieved an increased dynamic range by up to 10 dB for frequencies above 1 THz compared to state-of-the-art photomixing receivers. These improvements enable faster and more precise spectroscopy with higher bandwidth. In industrial non-destructive testing, the measurement rate may be increased by a factor of ten posing a valuable contribution to inline process monitoring.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The use of continuous wave (cw) terahertz (THz) radiation is a promising approach towards compact and cost efficient THz systems [1]. Particularly optoelectronic systems that employ components operating in the c-band (1532 nm – 1568 nm) for THz generation and detection benefit from off-the-shelf components with small form factors. In addition, cw THz systems can be miniaturized via photonic integration technology [2,3], which may enable on-chip THz systems with high bandwidth and dynamic range in the near future. However, pulsed systems are still the prevalent technology in this area mainly due to their high bandwidth of up to 6.5 THz [4–7]. The achievable bandwidth of cw technology is mainly defined by the limits of the components used [8–10]. Hence, the improvement of photomixers as cw THz sources and detectors is a key factor for THz systems to become a competitive alternative to pulsed systems [11]. On the transmitter side the main challenges are to increase the output power and bandwidth of the devices, which we investigated in previous publications [12,13]. For receivers, enhancing the measurable THz bandwidth requires either to increase the cut-off-frequencies of the device, i.e. the carrier lifetime or RC cut-off, or to improve the dynamic range of the system by lowering the noise level [14]. Therefore, different photoconductive materials such as ErAs doped In(Al)GaAs and low-temperature-grown (LT) beryllium (Be) doped InGaAs were investigated as THz photomixers in the last decades [15]. Hitherto, the highest bandwidth of 4 THz was achieved with LT-InGaAs:Be based detectors, which is still more than 2 THz lower than the bandwidth of pulsed systems [8,10,16–19].
Another promising material suitable for the illumination in the optical c-band is iron (Fe) doped InGaAs grown lattice matched on InP. Iron is incorporated as a deep acceptor in the middle of the bandgap of InGaAs and acts as a fast recombination center [20]. For pulsed emitters and receivers iron implantation was investigated already in 2005 [21,22]. However, these THz emitters and detectors had a bandwidth of 2.5 THz and a signal-to-noise ratio of 50 dB, only. Nevertheless, these results stimulated material research on InGaAs:Fe as THz photoconductors in the last decade. Iron doped InGaAs and InGaAsP were successfully employed as photoconductors for THz emitters and detectors in pulsed THz systems as well as THz emitters in a cw system [23,24]. A significant increase in bandwidth and dynamic range of pulsed THz systems compared to the first results with iron implantation could be achieved with iron doped InGaAs grown via molecular beam epitaxy (MBE). With these photoconductive antennas (PCA) pulsed THz systems with a bandwidth of more than 6 THz were demonstrated [25]. This high bandwidth stems from the favorable material properties of MBE grown InGaAs:Fe, which are a sub-picosecond electron lifetime, high resistivity and high mobility. Despite its promising properties, InGaAs:Fe has not been demonstrated for photomixing receivers in a cw THz system so far since the growth of InGaAs:Fe with sub-picosecond carrier lifetimes is challenging.
In this paper, we present cw THz receivers employing Fe-doped InGaAs absorbers, which enabled us to measure a bandwidth of 4.5 THz with a peak dynamic range of 112 dB. This is a bandwidth improvement of more than 500 GHz compared to the current state-of-the-art [26]. For frequencies above 1 THz we also achieved a significant increase in dynamic range of 10 dB. These new devices can be used to improve measurement speed and accuracy in various applications such as molecular spectroscopy, biosensing and thickness measurements [6,27–30].
2. Results
For our PCA receivers we used Fe doped InGaAs (InGaAs:Fe) grown by gas source molecular beam epitaxy (GS-MBE) on a semi-insulating InP:Fe substrate. The structures were grown on a RIBER 412 MBE and the growth temperature was around 400°C. The thickness of the InGaAs:Fe was 1.2 µm and the Fe doping concentration was 3.8 × 1019 cm−3. The latter value was chosen as a trade-off between the carrier lifetime, carrier mobility and material resistivity. To a certain extent higher doping concentrations reduce the carrier lifetime and increase the resistivity which benefit a high bandwidth and good noise performance. However, the carrier mobility is negatively affected by the iron defects, which reduces sensitivity of the respective THz detectors. We chose the doping concentration according to the results in [25] where further information on the growth process and the impact of the doping concentration can be found. After growth, the electrical properties, i.e. carrier mobility, carrier concentration and resistivity, were determined via room temperature Hall measurements. The carrier lifetime was measured with wavelength degenerate differential transmission (DT) around 1550 nm excitation. Details of the DT setup can be found in [31]. Table 1 compares the material properties of our InGaAs:Fe wafer with LT-InGaAs:Be used in state-of-the-art cw THz receivers. Since iron dopants form deep defect centers in the middle of the semiconductor bandgap, shorter carrier-lifetimes and a higher resistivity are expected in comparison to the widely used LT-InGaAs:Be [23–25,32]. The results presented in Table 1 resemble these assumptions. The carrier lifetime of InGaAs:Fe is by a factor of two shorter than the lifetime of LT-InGaAs:Be, which promises a higher cut-off-frequency and consequently a higher bandwidth when operated as a photomixing receiver. In addition, the resistivity of InGaAs:Fe is by a factor of ten higher, which should lead to a lower receiver noise and thus an increased dynamic range of the terahertz spectrometer. Furthermore, the high mobility of InGaAs:Fe promises high THz amplitudes. Hence, photomixing THz receivers made of InGaAs:Fe should allow for high bandwidth, slow frequency roll-off and high dynamic range.
Table 1. Material properties of the Fe-doped and LT-grown InGaAs samples used in this work. Uncertainties were derived by consecutive measurements on the same sample.
In order to verify these assumptions deduced from the material properties in Table 1, we fabricated PCAs consisting of 3 × 3 mm2 planar bowtie antennas on the InGaAs:Fe absorber material. In the feeding point of the antenna, five interdigitated fingers were structured via E-beam lithography (see Fig. 1). The distance between two finger electrodes measures 1.5 µm and the finger width is 0.3 µm. We investigated two PCAs from each of the two above mentioned wafers. The respective chips were mounted in the center of a hyperhemispherical silicon lens with a diameter of 10 mm and packaged in a fiber-coupled module. For characterization, the modules were placed in a two-mirror-setup where THz radiation from a fiber coupled PIN-photodiode emitter was focused on the receiver via two off-axis parabolic mirrors. A schematic of the experimental setup is depicted in Fig. 2. The THz beatnote was generated by the superposition of two lasers in a 3 dB coupler. While the emission wavelength of one of these lasers was kept constant, the wavelength of the second laser could be tuned within the c-band. For phase sensitive detection, the optical beatnote illuminating the transmitter (Tx) was phase modulated from 0 to 2π with 20 kHz via a fiber coupled wavelength selective phase modulator (ϕ) [33]. At the receiver (Rx), the THz photomixer downconverted the incoming THz wave into an electrical signal with a frequency of 20 kHz. This signal was amplified by a transimpedance amplifier (TIA) with $\; {10^6}\; V/A$ gain and sampled by a National Instruments USB-6356 data acquisition card. A software lock-in amplifier with a time constant of 300 ms was used to detect amplitude and phase of the signal. Both transmitter and receiver were illuminated with 30 mW of optical power. The length of the THz path was approx. 25 cm and all measurements were conducted in ambient air.
Fig. 1. a: Micrograph of the bow-tie antenna used for our PCAs; b: Scanning electron micrograph (SEM) of the interdigitated fingers at the antenna feedpoint.
Fig. 2. Measurement setup for the characterization of the PCA receiver modules. The light of two detuned laser sources is superimposed in a 3 dB coupler to generate an optical intensity beating with a frequency in the range. This beatnote is used to excite both the emitter (Tx) and receiver (Rx). The THz radiation from the emitter is first collimated and then focused onto the Rx via two off-axis parabolic mirrors. Note that the Tx beating is phase shifted (ϕ) with 20 kHz so that the detected signal at the Rx is also modulated with 20 kHz. DAQ – data acquisition, TIA – transimpedance amplifier.
In our experiments, we compared the I-V characteristics and THz-spectra of the two Fe-doped and the two LT-grown PCA receivers. The reference PIN-Tx and LT-Rx modules are comparable to the devices used in [26]. The antenna geometry, the shape of the interdigitated finger electrodes as well as the packaging of the different receiver antennas was identical. Hence, differences in the measurements can be directly related to the properties of the photoconductive material.
Figure 3(a) and Fig. 3(b) compare the dark- and photocurrent of the measured devices as a function of the applied bias voltage, respectively. From these I-V-curves, we extracted the photo- and dark resistances shown in Table 2. The Fe-modules exhibit a dark resistance, which is more than a factor of ten higher than the resistance of the LT-receivers. Note that these results agree well with the resistivity of InGaAs:Fe determined by Hall measurements (see Table 1), which indicates that the antenna fabrication process did not impact the electrical properties of the photoconductor.
Fig. 3. a Dark current and b Photocurrent at 30 mW optical input of the PCA receivers over applied bias voltage. Although the LT-PCAs show a higher photo current, the ratio between photo- and dark current is more than twice as high for the Fe-doped receivers.
Table 2. Device properties of the Fe- and LT-Modules. Uncertainties were derived by consecutive measurements on the same sample.
Figure 4 compares the terahertz spectra recorded with the four different receivers. We plotted the dynamic range (DR), which was calculated via
where ${I_{THz}}$ is the detected receiver current and ${I_{Noise}}$ the detector noise current. The latter was determined by measuring the receiver signal with blocked THz beam and averaging the amplitude over 80 values. The novel InGaAs:Fe receivers show a significantly improved frequency response compared to the LT-Rx resulting in a record high bandwidth of 4.5 THz. In contrast, the bandwidth of the LT receivers is limited to 4.0 THz. In the inset of Fig. 4, a magnification of the frequency range from 3.7 THz to 4.9 THz is depicted. Water vapor absorption lines taken from the HITRAN database [34] are plotted as orange vertical lines. Note that the four absorption lines around 4.2 THz are clearly resolved with the two Fe receivers while the water lines at frequencies higher than 4.5 THz cannot be distinguished from the noisy background. This indicates that the InGaAs:Fe receivers feature a 4.5 THz bandwidth. To the best of our knowledge this is the highest bandwidth reported for any cw THz spectrometer. In addition to the higher bandwidth of the Fe-Rx, the dynamic range for frequencies above 1 THz is also higher compared to the LT-Rx. For frequencies below 500 GHz all devices show a similar behavior with a peak dynamic range of more than 110 dB around 100 GHz. For frequencies above 1 THz the dynamic range of the InGaAs:Fe receivers is up to 10 dB improved compared to the LT receivers. We explain this difference with the lower noise current of the Fe-Rx in combination with a higher cut-off frequency caused by the shorter electron lifetime. From Table 2 one deduces that the noise current of the Fe-doped receivers is 2.4 - 3.3 times lower than the noise of the LT modules. Through Eq. (1) this yields a 7.7 - 10.3 dB higher dynamic range. Again, this behavior can be directly related to the material characterization: Nyquist noise scales with the inverse of the square root of the resistance. As the resistance of the InGaAs:Fe antennas is more than a factor of ten higher than the resistance of the LT devices, the noise level of InGaAs:Fe should be reduced by a factor of 3.2 compared to LT receivers. This agrees well with the measured values in Table 2.Fig. 4. THz spectra recorded with a fiber coupled cw system employing receivers with InGaAs:Fe (green, blue) and LT-grown InGaAs:Be (black, red) absorbers. Measurements were taken in the setup shown in Fig. 2. We used the same fiber coupled PIN-PD device as THz emitter. Both, Tx and Rx were driven with 30 mW of optical power, and an integration time of 300 ms was applied on every frequency point. The dips in the spectrum stem from water vapour absorption. The inset shows a magnification of the detected spectrum from 3.7 THz – 4.9 THz. The blue lines indicate water vapour absorption lines taken from the HITRAN database [34].
Furthermore, one observes in Fig. 4 that the difference of the dynamic range between the two types of receivers increases towards higher frequencies. This indicates, that the InGaAs:Fe receivers have a higher cut-off frequency, which can be explained with the shorter carrier lifetime of InGaAs:Fe (0.15 ps) compared to the LT-material (0.32 ps).
In order to investigate the interplay between noise and THz amplitude in more detail, Fig. 5 shows the dependence of the noise level (a) and the detected THz current at 0.5 THz (b), 1.0 THz (c) and 2.0 THz (d) as a function of the optical power incident on the receiver. Here, we observe that the THz amplitude increases linearly with the optical power without noticeable saturation effects for both InGaAs:Fe and LT devices. However, the noise level of the LT-Rx devices increases from 5 pA at 2.5 mW to more than 20 pA at 30 mW while the noise level of the Fe-Rx remains below 10 pA at 30 mW. Although the LT receivers show a slightly higher amplitude at 500 GHz and 1 THz the lower noise of the InGaAs:Fe receivers leads to a higher dynamic range as shown in Fig. 4. In Fig. 5 d) one observes that the amplitude of the InGaAs:Fe and LT receivers is almost identical at 2 THz. This behavior supports the previous assumption that the shorter carrier lifetime of InGaAs:Fe leads to a higher cut-off frequency, which manifests itself in a higher dynamic range of these receivers for higher THz frequencies (see Fig. 4). Hence, the results depicted in Fig. 5 underline the aforementioned advantages of the novel Fe-doped receivers in terms of detector noise and frequency roll-off.
Fig. 5. Noise level a) and THz signal amplitude of the Fe and LT receivers at 0.5 THz b), 1.0 THz c) and 2.0 THz d) as a function of the optical power. Note that the noise level is independent of the THz frequency as it mainly stems from Nyquist noise of the photo resistance around the modulation frequency of 20 kHz and the amplifier noise.
3. Summary and outlook
We presented PCAs with Fe-doped InGaAs absorbers as cw THz detectors, achieving a record high bandwidth of 4.5 THz with a peak dynamic range of 112 dB. This is a bandwidth increase of 500 GHz compared to previously reported values for photoconductive THz detectors. Furthermore, the novel receivers show an increase in dynamic range by 10 dB above 1 THz. We demonstrated that this improved frequency response stems from the higher resistivity in combination with a shorter carrier lifetime of the Fe-doped photoconductors compared to the often used LT-grown InGaAs:Be. This superior performance has significant impact for the application of cw THz spectroscopy in scientific and industrial applications. The increased dynamic range can be used to either improve measurement precision or to reduce measurement time in applications such as thickness determination or molecule spectroscopy. Therefore, this work is another cornerstone towards precision-/real-time spectroscopy or inline process monitoring with cw THz systems. Further performance improvements may be achieved by using other dopants such as rhodium, which have shown very promising results when employed as emitters and receiver in pulsed THz systems.
Funding
Bundesministerium für Bildung und Forschung (03VP07267).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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