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Abstract
We have experimentally demonstrated thermal evaporated group IV Ge1-xSnx-on-Si terahertz (THz) photoconductive antennas (PCA) pumped by an Er-doped femtosecond laser for broadband THz generation. The Ge1-xSnx THz PCAs, free from material epitaxial growth methods, can offer comparable material properties in photocarrier generation, transportation, recombination, and the collection as group III-V THz PCAs. At the optical pumping power of 90 mW and a bias voltage of 40V, the Ge1-xSnx THz PCAs have achieved a broadband spectrum over 1.5 THz with a 40 dB signal-to-noise ratio (SNR). This CMOS-compatible group IV THz source can be monolithically integrated on the Si photonic platform, paving the way toward THz system-on-chip (SoC) for many on-site applications in the non-destructive evaluation, biomedical imaging, and industrial inspections.
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1. Introduction
Photoconductive antenna (PCA) has been used as broadband terahertz (THz) emitter for a great variety of unique THz applications, including non-destructive imaging, chemical identification, material characterization, and biomedical sensing [1–5]. To achieve broadband operation, the photoconductive material candidates are typically required to have ultrafast carrier recombination time to provide sub-picosecond timescale surge current feeding into the antenna. Conventional photoconductive materials are mainly based on low-temperature-grown (LT) III-V materials, such as LT-GaAs grown through molecular beam epitaxy under an ultra-high vacuum environment, requiring precise control epitaxial conditions to obtain a high-quality structure [6–10]. Therefore, the complex fabrication process and time-consuming nature lead to high manufacturing costs, preventing the THz photoconductive antenna from being mass-produced and fully commercialized through well-established CMOS compatible processes. Recently, Schneider et al. has introduced Ge-based PCAs without employing epitaxy techniques [11,12]. With the nonpolar structure nature of Ge, this type of PCA offers a broadband radiated THz spectrum over 13 THz, while the GaAs-based PCAs are mainly limited to a frequency below 8 THz due to the strong absorption of longitudinal-optical (LO) phonons. Furthermore, Ge photoconductive material can be easily integrated onto the Si photonics platform resulting in low energy consumption, compact size, and a high-speed operation system [13,14]. Although the ion-implanted process can introduce additional defects in Ge lattice for carrier lifetime reduction, the O+ and phosphorous ion implantation lead to low resistivity due to the excess carriers [15,16], while the metal ion implantation can be challenging to access [11]. These drawbacks possibly hamper the group IV material system from being a competitive candidate for THz PCAs compared with the well-established group III-V materials.
To address this limitation, we employed thermal evaporation approach to deposit group IV Ge1-xSnx material system through a low-temperature growth process, leading to picosecond (ps) level photocarrier recombination time attributed to the defects formed. As the absorption coefficient of Ge dramatically drops at wavelength 1.56 µm, which falls in the commonly used fiber communication band resulting in the degrading performance [11,17,18]. To this extent, we can control the Sn concentration in Ge1-xSnx material system to modulate the bandgap which further increases the efficiency of utilizing the incident photons at longer wavelengths. Therefore, not just the selections of optical sources for pumping THz PCAs can be widely extended, but the quantum efficiency of THz PCAs could be further enhanced. In recent years, the Ge1-xSnx system has been demonstrated to transform into a direct bandgap group IV material, leading to faster photoresponse, higher carrier mobility, and optical absorption coefficient covering the SWIR region as the Sn concentration exceeds 6% [19–22]. Possessing Sn concentration lower than this extent, Ge1-xSnx can nevertheless feature with comparable resistivity, mobility, and slightly higher optical absorption, which are the critical material parameters for high performance THz PCAs. Here, we demonstrate the Ge1-xSnx-on-Si THz PCAs (with x = 0 and 0.02) following CMOS compatible fabrication processes, featuring mass production capability, low cost, high reliability, and further integration with the Si photonics platform. To the best of our knowledge, this is the first time thermal evaporated Ge1-xSnx materials have been used as the photoconductive material of telecom-compatible THz PCA emitters without employing any epitaxial methods. The cost-effective, epitaxial-free Ge1-xSnx THz PCAs generate broadband THz radiated waves with a bandwidth of 1.5 THz and a 40 dB SNR, potentially leading it a critical component for THz system-on-chip (SoC) for applications in communication, non-invasive sensing, and imaging.
2. Material characterization and device fabrication
In this work, Ge1-xSnx evaporation targets adjusted as Sn atomic concentrations of 0% and 1% are separately melted in a vacuum quartz tube for photoconductive material deposition. Then, the amorphous Ge (Sample 1) and Ge1-xSnx alloyed (Sample 2) layers with the same thickness of 300 nm are deposited on semi-insulated (resistivity > 10000 Ω-cm) Si wafers through thermal evaporation at the vacuum level of 3×10−6 torr after the RCA cleaning process for the removal of the native oxide and contaminants. Since the Ge1-xSnx has a low eutectic temperature as low as 231 °C, we employed a deposition rate as high as 1 nm/sec with growth condition at room temperature using backside cooling under the Si substrate to prevent Sn atoms from segregating. The as-deposited Ge1-xSnx samples were annealed at 400 °C for 3 minutes through rapid thermal annealing (RTA) to increase crystallinity and carrier mobility. The fast-ramping speed of 30 °C per second can repair part of the structural defects created during the low-temperature growth while keeping the low melting point Sn (231 °C) remains in the GeSn lattice after the RTA process. Figure 1(a) shows the crystallinity of the thin films of sample 1 and 2 on Si examined by Raman spectroscopy (Southport Co. Ltd., JadeMat) with a laser at a wavelength of 532 nm focus on the sample through a 50X objective lens. The results show that Ge has a Ge-Ge vibration mode peak of 286.4 cm-1 while the Ge1-xSnx alloy has a peak at 285.1 cm-1 since the Sn incorporation affects the optical phonon vibration mode to slightly left-shift, as shown in Fig. 1(a) inset. No prominent peak for Sn-Sn at 197 cm-1 is observed, implying the successful formation of GeSn alloy through thermal deposition and RTA process without severe Sn segregation [23–25]. The actual Sn concentration determined by energy dispersive spectrometry is approximately 2%, as no evident Sn segregation occurs. We speculate the higher Sn concentration of the deposited GeSn that the evaporation target is because the Sn are more likely to vaporize from the evaporation target and deposit on the Si substrate. As a result, we have prepared Ge and Ge0.98Sn0.02 as the photoconductive material of the THz PCAs. Compared with the intense peak signal at 520 cm-1 corresponding to the Si substrate, the results suggest that the Ge1-xSnx crystal quality can be improved further.
Fig. 1. (a) Raman spectroscopy of thermal evaporated GeSn and Ge through 532 nm laser excitation under room temperature. The inset shows the vibration peak around 285 cm-1 of Ge and GeSn. (b) The optical pump/probe spectroscopy shows the decay of the photocarriers of the thermal evaporated samples as a function of delay time after pumped by the fs laser at the central wavelength 800 nm (the inset shows the decay curve within 10 ps after laser excitation). (c) The absorption coefficient of Ge (black) and GeSn (blue) compared with the intensity spectrum of the Er-doped pumping fs laser (red).
To investigate the ultrafast photocarriers dynamic inside the thin film of both samples, we conduct the differential transmission measurement through Ti: Sapphire laser with a central wavelength of 800 nm, a pulse width of 60 fs, and a repetition rate of 5.1 MHz, which has been well-established to characterize the ultrashort carrier lifetime of Ge [11,15]. The laser was split into a pump beam (25 mW) and a probe beam (1 mW) with approximately 50 µm spot size on the samples’ surface. We estimated the thermal evaporated Ge0.98Sn0.02 carrier lifetime to be 1.2 ps and the Ge to be 1.5 ps described by the bi-exponential decay function, as shown in Fig. 1(b). The ps level carrier lifetime can be attributed to the deep trap level formed inside the amorphous group IV materials as reported in [14,26,27], which is critical to providing an ultrafast surge current to drive the THz antenna and resolving the long decay time of photocarriers in group IV materials. As a result, the majority of photocarriers can be recombined and excited again by the femtosecond (fs) pulse laser with MHz level repetition rate [14,15].
To further characterize the optical properties of the Ge and the Ge0.98Sn0.02 thin film, the ellipsometer (J. A. Woollam, M-2000) system is carried out to measure material optical absorption coefficient ranging from 1.4 µm to 1.7 µm (Fig. 1 (c)). The Ge0.98Sn0.02 thin film shows a 46% higher absorption coefficient at the wavelength of 1.56 µm compared with Ge because of the lower bandgap of Ge0.98Sn0.02, allowing more photocarriers to be excited from the valence band. This behavior forces the photocarriers to be generated even more closely to the electrodes and can potentially extend its absorption edge to 2 µm to utilize the optical power of the fs pulse laser as the Sn concentration increases [19–22]. Additionally, we employed Hall measurement under room temperature to characterize the mobility and resistance of the Ge and Ge0.98Sn0.02 samples using the Van der Pauw method and summarize the material properties comparison between both samples in Table 1.
Table 1. The material properties of thermal evaporated Ge and GeSn.
To explore the performance of Ge1-xSnx as the ultrafast photoconductor for THz generation, we then fabricate the THz PCA devices with nanoelectrodes based on the corresponding samples. Noticing that the numbers of photocarriers that arrive at the electrodes within a fraction of the THz oscillation cycle are vital to efficient THz radiation, an ideal photoconductor should feature (i) high absorption coefficient, enabling more photocarriers to be excited by the pumping laser (ii) high mobility, allowing more ultrafast photocarriers to be drifted into the electrodes for driving THz antenna (iii) high sheet resistance, avoiding severe Ohmic heating as bias voltage increases for collecting more ultrafast photocurrent. According to Table 1, the result suggests that the material properties of Ge1-xSnx are comparable with the typically feasible low-temperature grown InGaAs that satisfy the condition mentioned in [7,28] and can therefore tolerate the high enough bias voltage to drift the sufficient photocarrier in time. We then design the metallic nanoelectrodes (pitch size: 480nm; width: 240nm; height: 50nm) incorporated with the Ge1-xSnx THz PCA and 1.56 µm pulse laser excitation by the finite-element method (COMSOL Multiphysics), as shown in Fig. 2(a). The metallic nanoelectrodes can guide most incident optical pump at the central wavelength of 1.56 µm through the deep-subwavelength gap and localize photocarriers at the proximity to the nanoelectrodes [29–32]. The reduced average transit path of photocarriers to the anode nanoelectrode further enhances device quantum efficiency and radiated THz power. Additionally, each nanostructure electrode covers an active area of 20 ${\times} $ 20 µm2 with a gap size of 10 µm and is patterned through e-beam lithography (Elionix, ELS7500-EX), distributing the total laser power to allow high optical pumping excitation. After the e-gun metal deposition and the following lift-off process, 5nm/45nm Cr/Au nanoelectrodes are formed on top of the photoconductor thin film to enhance the light-matter interaction at the metal-semiconductor surface. To achieve broadband THz generation, we used bowtie antenna as THz radiating element of PCA, which are patterned through UV lithography followed by 5 nm/100 nm Cr/Au metal deposition and lift-off before employing O2 plasma treatment to eliminate the photoresist residues as shown in Fig. 2(b) and 2(c). The Ge(Sn) THz PCAs are then mounted to a hemispherical Si lens for optical alignment and THz characterization, as shown in Fig. 2(d).
Fig. 2. (a) The photocarrier concentration distribution of the Ge0.98Sn0.02 thin film with 1.56 µm pulse laser excitation. (b) Scanning electron microscope graph (SEM) of the Ge0.98Sn0.02 THz PCA. The bowtie antenna is with a gap size of 10 µm and a 20 ${\times} $ 20 µm2 active area. (b) Metallic nanoelectrodes with 240 nm spacing and 480 nm pitch size. (d) The Ge(Sn) PCA is mounted on the THz Si lens and pumped with the laser with a beam radius of 20 µm.
3. Device measurement
A THz asynchronous optical sampling system (Menlo Systems, TERA ASOPS) setup (Fig. 3) has been established to characterize the THz time-resolved electric field signal emitted from the THz PCAs based on Ge and Ge0.98Sn0.02 ultrafast photoconductors. The Ge(Sn) THz PCAs are pumped with an Er-doped fs fiber laser A with a central wavelength of 1.56 µm, a pulse width of 60 fs, and a repetition rate of 100 MHz. The optical beam (beam diameter: 20 µm full-width half-maximum) was focused through an objective lens with 10X magnification on the active area, which is the nanostructured anode electrode of the THz PCAs. The generated broadband THz radiation is then collimated and focused by two THz lenses into an LT-InGaAs THz PCA receiver. The probe laser (fs laser B) with a slightly different repetition rate (Δf) of 50 Hz is used to trigger the THz receiver, which rapidly samples the received THz electric field with a time resolution of 50 fs without the use of mechanical moving parts. A transimpedance amplifier (TIA) then amplifies the THz induced current from the receiver and links to a data acquisition (DAQ) card for further data processing.
Fig. 3. The schematic diagram of the THz asynchronous optical sampling (ASOPS) system for THz PCA measurement. Both laser A and B have the same central wavelength of 1.56 µm and 60 fs pulse width with slightly different repetition rate frequency Δf (50 Hz) for laser B. TIA: Transimpedance amplifier.
The dark current characteristic of the Ge and GeSn PCAs measured by a source meter is shown in Fig. 4(a). It indicates that the relatively higher dark current characteristic of GeSn is subjected to the Schottky contact behavior of adhesion metal Cr and the lower resistivity attributed to the metal-like Sn incorporation. A dark current ratio as high as 71.1 for Ge and 21.3 for GeSn can be achieved under 40V bias voltage and 90 mW optical power, as shown in Fig. 4(b). Since Ge and GeSn possess the comparable optical absorbing capability to the Er-doped fs fiber laser at the central wavelength of 1.56 µm, similar quantities of photocarriers can be generated. While driving the two THz PCAs from 0 V to 40 V, the induced total photocurrent shows almost the same characteristics because of the similar material carrier mobility and carrier lifetime. As expected, the total photocurrent level saturates at a higher bias region due to the utilization of most photocarriers.
Fig. 4. (a) Dark current and (b) photocurrent characteristics of the Ge and GeSn photoconductive antennas as the function of bias voltage at the optical pump power of 30, 60, and 90 mW.
To evaluate the materials’ properties affecting the performance of PCAs, we compared the GeSn emitter operated at the optical pump power level of 90 mW under a 40 V bias voltage with the commercial InGaAs/InAlAs superlattice (SL-InGaAs) THz PCA (Menlo systems, TERA15-TX-FC). As shown in Fig. 5(a) and 5(b), the SL-InGaAs THz PCA with 40 mW optical pump and 100 V bias voltage provides an approximately 60 times higher THz field strength, 35 dB higher dynamic range and twice radiating bandwidth than thermal evaporated GeSn PCA. Compared with SL-InGaAs PCAs [7,33], the limited performance of GeSn PCA is mainly constrained by the crystalline quality, carrier mobility, and sheet resistivity of the thermal evaporated GeSn photoconductors. At the optical pump power level of 90 mW with a 40 V bias voltage, Fig. 5 (c) and 5(d) show the THz electric field signals normalized by the peak value of the GeSn PCA radiating electric field and the corresponding THz radiating spectrum obtained from the Fourier transform (FFT) of the Ge PCA and the GeSn PCA. The radiated THz spectrum (in Fig. 5(d)) compares the Ge and GeSn THz PCAs. Both devices can achieve a 1.5 THz bandwidth and a 40 dB SNR after averaging the THz pulse traces for 10000 times. Moreover, the GeSn THz PCA generates higher THz power than the Ge THz PCA above 0.7 THz. As expected, the higher the optical absorption coefficient GeSn possesses, the more significant photocarrier density is likely to generate close to the electrode, leading to a more intense photocurrent at high THz frequencies. Still, the mobility is constrained due to low crystal quality originating from the deposition method, limiting the bandwidth of Ge and GeSn PCA to around 1.5 THz. Transforming GeSn into direct bandgap material through higher Sn concentration over 8% by methods such as metal-organic chemical vapor deposition can further resolve these issues with better crystalline quality [13,15,34,35].
Fig. 5. (a) Normalized time-domain THz electric field signal and (b) frequency-domain THz power spectrum of the GeSn THz PCA and a commercial SL-InGaAs THz PCA. (c) Normalized time-domain THz electrical field signal and (d) frequency-domain THz power spectrum of Ge and GeSn THz PCAs are measured with bias voltage 40 V and 90 mW laser excitation.
The THz electric field strength (ETHz) as a function of optical power is shown in Fig. 6(a). As laser power applied to THz PCAs increases, the more excited photocarriers can contribute to the higher THz output power until the carrier screening effect takes place, where the large numbers of separated electron-hole pairs create an opposite direction of the electric field, leading to the compensation of the bias electric field and the degradation of THz power. The trend indicates that both Ge and GeSn THz PCAs have not met the saturation and can be further improved by increasing the optical pumping power. So far, the bias voltage of 40V limited by our voltage supply has not yet drifted the photocarriers at saturation velocity limitation and thus can be further increased to boost the Ge and GeSn THz PCA performance in Fig. 6(b). The THz electric field strength overcrossing of Ge and GeSn PCA at bias voltage bias between 30V to 40V is most likely attributed to the energy barrier height difference that arises from the adhesion metal layer [36]. It can still be expected that both Ge and GeSn PCA will meet the saturation as the Ohmic heating effect comes into practice at higher current densities due to the similar resistance of the photoconductor.
Fig. 6. (a) THz electric field strength (ETHz) under different illumination power with a constant bias voltage of 40 V and (b) different bias voltage with a constant illumination power 90 mW.
4. Conclusion
In this work, we successfully demonstrated the first thermal evaporated Ge(Sn)-on-Si THz PCA, which offers broadband THz radiation with a 40 dB SNR. The telecommunication wavelength compatible group IV THz PCAs are fabricated on a Si wafer through the CMOS-compatible process, providing extraordinary integration capability with Si CMOS technology, fiber-optics industry, and Si photonics platform without employing epitaxial techniques.
Moreover, the thermal evaporated group IV Ge1-xSnx material system can serve as a potential photoconductive material candidate for THz photomixers featuring its ultrafast operation capability and highly absorptive nature in VIS/NIR bands. Nevertheless, the Ge1-xSnx material is highly compatible with the massively-producible, compact telecom lasers, making Ge(Sn) THz PCA a favorable option to be integrated into on-chip THz systems. This cost-effective and CMOS-compatible group IV THz device based on the Si photonics platform could extend applications to many fields, including non-destructive evaluation, industrial inspection, advanced material development, biomedical imaging, and remote sensing.
Funding
Ministry of Science and Technology, Taiwan (MOST-110-2636-E-007-017).
Disclosures
The authors declare no conflicts 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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