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Abstract
Germanium (Ge) vertical p-i-n photodetectors were demonstrated with an ultra-low dark current of 0.57 mA/cm2 at −1 V. A germanium-on-insulator (GOI) platform with a 200-mm wafer scale was realized for photodetector fabrication via direct wafer bonding and layer transfer techniques, followed by oxygen annealing in finance. A thin germanium-oxide (GeOx) layer was formed on the sidewall of photodetectors by ozone oxidation to suppress surface leakage current. The responsivity of the vertical p-i-n annealed GOI photodetectors was revealed to be 0.42 and 0.28 A/W at 1,500 and 1,550 nm at −1 V, respectively. The photodetector characteristics are investigated in comparison with photodetectors with SiO2 surface passivation. The surface leakage current is reduced by a factor of 10 for photodetectors via ozone oxidation. The 3dB bandwidth of 1.72 GHz at −1 V for GeOx surface-passivated photodetectors is enhanced by approximately 2 times compared to the one for SiO2 surface-passivated photodetectors. The 3dB bandwidth is theoretically expected to further enhance to ∼70 GHz with a 5 µm mesa diameter.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Germanium (Ge) is an attractive material of choice in the fields of electronic, spintronic, and photonics due to its superior material properties [1–3]. The excellent mobility in Ge provides a key alternative to enhance the performance of the metal-oxide-semiconductor field-effect-transistor (MOSFET), as well as increasing the speed of quantum bit computing. In addition, the considerable absorption coefficient of Ge in the near-infrared range (NIR) can be exploited in bio-sensing and data communication [4,5]. Among the key-raising Ge-based optical components [6–10], Ge photodetectors have been investigated in the past decades as the key building block for Si-based photonic integrated circuits (PIC). Although numerous efforts have been put on Ge photodetectors, III-V compound, e.g., InGaAs, photodetectors have emerged preferably for integration with Si-based chips due to high responsivity and relatively low-dark current [11]. However, the cost-effectiveness and high complementary metal-oxide-semiconductor (CMOS) compatibility with Si bring Ge back as a strong candidate for Si-based photonics.
Ge photodetectors suffer from high dark leakage current compared to InGaAs photodetectors [12]. Many works on Ge photodetectors have proposed that the generation of majority carrier in the depletion region, widely known as Shockley Read Hall (SRH) leakage, is attributed as the dominant leakage mechanism for Ge photodetectors [13,14]. This is due to high misfit and threading dislocations (TDs) originated from the 4% lattice constant mismatch of Si/Ge [15]. The amount of the defects positioned at the interface of epitaxial Ge and Si substrate acts as seeds of threading dislocations and results as generation/recombination centers, which increase the SRH leakage current [16]. In addition, these defects/dislocations increase the trap-assisted tunneling (TAT) leakage current, tunneling flow of carriers through the SRH centers in relatively high reverse bias, which has been typically observed for Ge pn junctions [17,18]. Various approaches have been proposed for the dark leakage current reduction [19,20]. Ge/Si or Ge/Si-on-insulator photodetectors have been introduced to reduce leakage current [21,22]. However, the fundamental problem related to high defective region formations at the interface of epi-Ge/Si is unsolvable. In order to reduce the TDD in Ge, an annealing process is a promising method [23,24]. Oxygen (O2)-annealed germanium-on-insulator (GOI) platforms have been demonstrated with TDD as low as ∼106 cm−2 in the Ge layer, which is highly attractive to photonic applications [25,26].
As defects are reduced, the surface leakage current becomes dominant especially with TDD of <∼108 cm−2 [27,28]. Various Ge surface passivation techniques have been introduced to reduce the interface defect densities in the intermediate region between the channel and gate of MOSFETs [29,30]. As an alternative, thin germanium-oxide (GeOx) is attractive, taking advantages of high-k dielectric property and low intermediate surface defect states [31]. Since GeOx is soluble in air moisture, additional Al2O3 protection layer deposition followed by GeOx formation is required. This high quality of GeOx surface layer has been demonstrated in thermal annealing, O2 plasma, and ozone (O3) oxidation [32,33]. Among them, O3 oxidation is beneficial due to i) a low-temperature process, ii) effective method to improve interface quality, and iii) excellent sidewall passivation. O3 oxidation can be performed at room temperature, which is preferable for the GOI platform for back-end-of-line (BEOL) integration.
In this paper, the vertical p-i-n Ge photodetectors were demonstrated on the annealed GOI platform. The surface leakage current is suppressed by inserting a thin GeOx surface passivation layer via the O3 oxidation process. Through the activation energy study, it is observed that the SRH leakage mechanism becomes weak so that the diffusion and tunneling leakage processes become the dominant leakage generation mechanisms. The responsivity was measured to be 0.42 and 0.28 A/W at 1,500 and 1,550 nm, respectively. The 3 dB bandwidth for GeOx surface-passivated photodetectors is enhanced compared to that of SiO2 surface-passivated photodetectors. This high-speed and ultra-low dark current Ge vertical p-i-n photodetectors provide a step forward for Si-based Group-IV photonic integrated circuits.
2. Photodetector design and fabrication
A 200-mm epi-Ge was grown on the Si substrate via a reduced pressure chemical vapor deposition (RPCVD) reactor [26]. The Boron (B) dopants were implanted in the epi-Ge layer to form p+-Ge. The SiO2 and SixNy layers are deposited on both the epi-Ge and another handle Si wafer, prepared for the bonding process by a plasma-enhanced chemical vapor deposition (PECVD). Subsequently, two wafers were brought into contact at room temperature and post-bonding annealing for the bonded wafer pair was carried out at 300°C in N2 ambient [34]. The Si donor layer on the top of the wafer pair was removed through a combination of mechanical grinding and wet etching in tetramethylammonium hydroxide (TMAH) solution to form the GOI. The additional O2 annealing process was performed for 4 h at 850°C to reduce the misfit and threading dislocations in epi-Ge further followed by a diluted HF solution etching process to remove the oxidized Ge layer. It is worth noting that the two orders of magnitude reduction in TDD was observed with the annealed-GOI as described in [25]. It should be noted that TDD significantly affects the dark leakage current for Ge photodetectors, indicating the alleviation in TDD is beneficial for the leakage current reduction. Figure 1(a) displays the cross-sectional transmission electron microscope (TEM) image of the annealed GOI. The epi-Ge layer is on the insulator platform of SiO2/SixNy/SiO2. The thickness of the Ge layer is 1.2 µm. The cross-sectional view does not show the usual misfit and threading dislocations suggesting that they are annihilated during O2 annealing. TDD was estimated using the etch-pit density (EPD) method via the secondary electron microscope (SEM) analysis in the inset in Fig. 1(a). EPD was obtained to be ∼5.6 × 106 cm−2 in the annealed GOI.
Fig. 1. (a) Cross-sectional transmission electron microscope (TEM) image of the annealed germanium-on-insulator platform. Inset represents the top-view SEM image of annealed GOI after etched in iodine solution for 1 second. (b) Spreading resistance profiling (SRP) analysis of the annealed Ge vertical p-i-n junction structure.
Vertical p-i-n Ge photodetectors were demonstrated on the annealed-GOI platform. Arsenic (As) ion implantation was performed at a dose of 7 × 104 cm−2 and the energy of 40 keV to form the n-type region in the top epi-Ge layer. A vertical p-i-n Ge diode structure was realized with a p-type doping layer at the bottom and with the n-type doping layer at the top as shown in Fig. 1(b). The diameter of i-Ge mesas was varied in the range of 60 to 250 µm by Cl2-based reactive ion etching (RIE). The surface of the annealed-GOI was cleaned with buffered oxide etching (BOE) to remove a native oxide layer. After that, an Al2O3 layer with a 1 nm thickness was deposited by atomic layer deposition (ALD) at 250°C. Subsequently, the GOI was subject to the O3 oxidation using Jelight UVO Cleaner at room temperature for 20 min. After that, the SiO2 additional passivation layer with a 400 nm thickness was deposited via PECVD. In order to understand the effectiveness of the sidewall passivation methods, photodetectors are fabricated with and without ALD and O3 oxidation processes. CF4-based RIE and BOE were performed to expose metal contact holes. The contact metallurgy of Ti 20 nm/TiN 50 nm/Al 300 nm was deposited by sputtering followed by a lift-off process. The contacts were treated with rapid thermal annealing (RTA) at 400°C for 1 min. The plan view SEM image of the completed Ge vertical p-i-n photodetector with GeOx surface passivation is displayed in Fig. 2(a). Both side electrodes and a central electrode are contacted with the p-type region in the bottom epi-Ge layer and n-type region in the top Ge layer, respectively. The three-dimensional (3D) and cross-section schematic images of the demonstrated photodetector are shown in Figs. 2(b) and (c), respectively. It should be acknowledged that the thin Al2O3/GeOx layers were formed between epi-Ge and the SiO2 passivation layer.
Fig. 2. (a) Top-view SEM image of the Ge vertical p-i-n photodetector with an 80 µm diameter. (b) Three-dimensional (3D) and (c) the cross-section schematic images of the Ge vertical p-i-n photodetector.
The influence of the O3 oxidation on Ge layers was investigated via X-ray photoelectron spectroscopy (XPS) with a take-off angle of 90°. Figure 3(a) displays the Al 2p and Ge 3d XPS spectra of Al2O3/GeOx before and after O3 oxidation. Compared with the one without O3 oxidation, the intensity of binding energy at ∼32 eV increases after O3 oxidation, indicating Ge oxides are formed on the surface. The existence of a thin Al2O3 layer is observed in the binding energy of ∼74 eV. The chemical components in the GeOx layer were further characterized in Fig. 3(b). A binding energy increases by 0.80, 1.80, 2.75, and 3.40 eV for each Ge oxidation states, Ge1+, Ge2+, Ge3+, and Ge4+ relative to Ge0, respectively [35]. It is observed that the core level of Ge3+ is dominant among the Ge oxidation states. The areal intensity of Ge3+ has a strong relationship with a GeOx thickness [36,37]. The GeOx thickness is estimated to be 1.1 nm by comparing the areal intensity of GeOx and Ge.
Fig. 3. (a) The XPS spectra of Al2O3/GeOx structure before and after the O3 oxidation. (b) The XPS spectra of GeOx with the extracted spectra of Ge oxidation states using the Gaussian-Lorentzian fitting.
3. Photodetector characterizations
Figure 4(a) displays the dark current density (Jdark) of vertical p-i-n photodetectors on annealed-GOI platforms with varying mesa diameters from 60 to 250 µm. Jdark for the one with a 250 µm diameter was measured to be 0.57 mA/cm2 at −1 V. In our previous dark current study, the annealed-GOI photodetector with a 60 µm diameter with SiO2 surface passivation was demonstrated with Jdark of 3.96 mA/cm2 at −1 V, suffering from the huge surface leakage current contribution of 96% [12]. The Jdark decreases to 0.88 mA/cm2 for the photodetector with GeOx surface passivation, which is reduced by roughly ×4. The diode ideality factor ranges from 1.15 to 1.23 with varying device sizes extracted from the forward Jdark-V characteristics [38]. The ideality factor of 1.15 is close to that of an ideal junction, indicating the Ge photodetector is demonstrated on the low detect epi-Ge layer.
Fig. 4. (a) The dark current density-voltage (Jdark-V) characteristic of the annealed-GOI vertical p-i-n photodetectors with the GeOx surface passivation compared with the ones with SiO2 surface passivation in [12]. (b) The Jdark-1/D characteristic under −1 V. The inset displays the current contribution of the bulk and surface leakages.
In order to obtain a better understanding of the Jdark, the surface leakage current density (Jsur) and bulk leakage current density (Jbulk) were extracted from the following equation,
where D is a diameter of a photodetector. The linear fitting of mesa diameters and Jdark is shown in Fig. 4(b). The intercept of the line represents Jbulk and the slope of the line indicates Jsur after dividing by 4. The extracted Jbulk and Jsur were calculated to be 0.46 mA/cm2 and 0.58 µA/cm at −1 V, respectively. For the bulk leakage with varying TDD, the Jbulk with the TDD of ∼108 cm−2 was 59.49 mA/cm2 at −1 V [12]. As TDD is reduced to ∼106 cm−2, Jbulk reduces further to below 1 mA/cm2. It should be noted that Jbulk is reduced by approximately two orders of magnitude as TDD decreases from ∼108 to ∼106 cm−2. For the surface leakage, Jsur for the Al2O3 surface passivation was observed to be ∼42.0 µA/cm at −1 V [32]. The Jsur for the PECVD-deposited SiO2 surface passivation was 5.67 µA/cm. As a photodetector surface is passivated by the GeOx thin layer, Jsur reaches to 0.58 µA/cm. The photodetector with GeOx surface passivation displays lower surface leakage current compared to those with Al2O3 and SiO2 surface passivation. The current contribution of the surface and bulk leakages is estimated based on the Jbulk and Jsur in the inset of Fig. 4(b). A Ge photodetector with a 60 µm diameter shows 54% of surface leakage contribution, in contrast to 46% of bulk leakage contribution. Since the ratio of perimeter to mesa area is low as the size of devices increases, the surface leakage contribution of the Ge photodetector with the 250 µm diameter is only 17%, while the bulk leakage contribution is 83%. The surface current contribution is suppressed significantly by forming the GeOx surface passivation layer compared with 96% surface leakage current contribution for SiO2 surface-passivated photodetectors [12].The temperature-dependent I-V characteristics of vertical p-i-n photodetectors were investigated with SiO2 and GeOx surface passivation, in the temperature range of 293 to 353 K in Fig. 5(a). The dark current of the photodetector with GeOx surface passivation was measured to be 25 nA at 293 K. As temperature increases to 353 K, it increases by ×13. The dark current of the photodetector with SiO2 surface passivation was measured to be 112 nA at 293 K and increases by ×12 at 353 K. It should be noted that the dark current is suppressed by a factor of 5 for the photodetector with a GeOx surface layer. The extracted Jsur for the photodetectors with GeOx and SiO2 surface passivation is obtained in Fig. 5(b). The extracted Jsur for the photodetector with GeOx is 0.46 µA/cm at 293 K at −1 V, in contrast to the Jsur of 5.94 µA/cm for the photodetector with SiO2. It is worth noting that the Jsur is reduced by one order of magnitude for the photodetector performed with O3 oxidation in the overall temperature range. This surface leakage current suppression via the GeOx formation results from the enhancement of the interface quality. The surface leakage current of Ge photodetectors with SiO2 surface passivation is mainly governed by the SRH and TAT leakage mechanisms, which originates from the interface trap states [12]. The O3 oxidation to form the thin GeOx layer at the interface provides lower defect states than that with SiO2 [32]. As a result, the SRH and TAT leakage processes are suppressed and the surface leakage current is reduced accordingly.
Fig. 5. (a) The elevating current characteristics of p-i-n vertical photodetectors on annealed GOI with GeOx surface passivation under varying temperatures from 293 to 353 K in the comparison with the one with SiO2 surface passivation. (b) The extracted surface current density of photodetectors with GeOx surface passivation under varying temperatures from 293 to 353 K in the comparison with the one with SiO2 surface passivation. The arrows in Figs. 4(a) and (b) represent the increase in temperature from 293 to 353 K.
The activation energy (Ea) study provides a comprehensive understanding of various leakage current mechanisms. It has been reported that four different mechanisms, e.g. diffusion, SRH, TAT, and band-to-band tunneling (BTBT) leakages, are usually considered to interpret the leakage current. Each leakage mechanism could be expected to be observed from the Ea study. The Ea of the electronic bandgap of Ge, 0.66 eV, represents the diffusion leakage mechanism is dominant. The Ea of half of electronic bandgap of Ge, 0.33 eV, signifies the SRH leakage process governs the total leakage current [39]. As the Ea decreases below the half of material bandgap, < 0.33 eV in the case of Ge, the tunneling leakage mechanisms (TAT, BTBT leakages) become the primary leakage current mechanisms. Figure 6(a) displays Arrhenius plot for the vertical p-i-n photodetector on the annealed-GOI with GeOx surface passivation respect to the temperature range from 293 to 353 K under the reverse bias of −0.1 to −3 V. As the reverse bias increases, the slope of the adjacent dots decreases. The relationship between the Arrhenius plot and Ea can be expressed by the following equation,
where T, k, and A are temperature, the Boltzmann constant, and a constant. The Ea is extracted based on Eq. (2) and is displayed in Fig. 6(b). The Ea of 0.23 eV at 293 K shows that the tunneling current mechanisms are contributed mainly to total leakage current generation. As the reverse bias increases to −3 V, the Ea decreases to 0.01 eV at 293 K, indicating the tunneling leakage processes are enhanced under the relatively high reverse bias. With the increase in temperature up to 353 K, the overall Ea increases compared to ones at 293 K, representing the diffusion leakage contribution is enhanced at high temperature. This is because the diffusion leakage increases with the temperature rapidly compared to other leakage mechanisms. At the temperature of 348 K, the Ea under −0.1 V shows 0.55 eV, close to the bandgap of Ge, 0.66 eV. As the reverse bias reaches −3 V, the Ea displays 0.20 eV due to the enhancement of tunneling leakages with increasing of the electric field in the depletion region [16].Fig. 6. (a) Arrhenius plot for the annealed-GOI photodetector with GeOx surface passivation with the varying reverse bias from −0.1 to −3.0 V at room temperature. (b) Activation energy of the photodetector with a 250 µm diameter in the temperature range from 298 to 348 K as a function of reverse bias from −0.1 to −3 V. The arrow represents the increase in temperature from 298 to 348 K.
The photo-response of vertical p-i-n photodetectors on annealed-GOI with GeOx surface passivation is shown in Fig. 7(a) at the 1,550 nm of wavelength. The optical responsivity measurements of photodetectors were measured using 2401 SourceMeter, a probe station, and a tunable laser (1500–1630 nm) at room temperature. The incident light was introduced by a single-mode optical fiber with a 10 µm diameter. The optical power was measured by an optical power meter. The responsivity at −1 V was measured to be 0.28 A/W. The linearity of photocurrent as a function of incidence power shows that non-linear optical effect is not observed in the measured range in the inset of Fig. 7(a). The responsivity at 0 and −3 V were measured to be 0.29 and 0.27 A/W, respectively. The decrease in responsivity with increasing reverse bias could be explained by the Franz-Keldysh effect originated from the strong electric field applied in the depletion region [40]. This suggests that the photodetectors are operated in low power consumption under low reverse bias. Figure 7(b) displays the wavelength scanning from 1,500 to 1,630 nm for the Ge photodetector under −1 V. The responsivity at 1,500 nm was obtained to be 0.42 A/W and decreases down to 0.05 A/W at 1,630 nm. Considering ∼350 nm of the thin depletion width, the responsivity of 0.42 A/W is considerable. The absorption coefficient (α) of the annealed Ge on insulator was extracted by the following equation,
where tGe, h, ν, R, and r are the i-Ge thickness, the Planck’s constant, wave frequency, responsivity, and reflectivity [41,42]. r is referred to [7]. The inset in Fig. 7(b) represents the extracted α scanning with respect to the wavelength range of 1,500 to 1,630 nm. The α of annealed epi-Ge on insulator shows a higher value than that of Ge bulk, which leads to an enhanced absorption efficiency [43]. This enhancement results from the residual tensile strain in the annealed Ge layer [25] and the optical confinement enhancement due to the insulating substrate platform.Fig. 7. (a) Photocurrent of a vertical p-i-n photodetector on the annealed-GOI platform with the GeOx surface passivation. The arrow represents the incident power increases from 0.8 to 6.3 mW. Inset displays the photo-generated current of the photodetector as a function of the incident laser power. (b) Optical responsivity spectrum of the photodetector in the wavelength range from 1500 to 1630 nm. The responsivity was obtained under −1 V on the photodetector with a 250 µm diameter. Inset shows the extracted α as a function of wavelength compared with the one for Ge bulk from [43].
The responsivities of vertical epi-Ge photodetectors are summarized with respect to the dark current density at −1 V in Fig. 8. The reported responsivities and the dark current densities were obtained at the wavelength of 1,550 nm and at −1 V, respectively. In this work, in order to suppress Jdark below 1 mA/cm2, the annealed GOI platform with the O3 oxidation was introduced as a competitive candidate compared with photodetectors made of epi-Ge/Si and epi-Ge grown on Si substrates. Noted that the responsivity can be enhanced to 0.42 A/W at the 1,500 nm wavelength. We strongly believe that the responsivity can be further improved i) by applying the photon-trapping microstructures [51] and ii) by increasing the thickness of Ge epilayers.
Fig. 8. Benchmarking of epi-Ge vertical photodetector performance. The responsivity at 1,550 nm displays with dark current density under −1 V. The epi-Ge/Si photodetectors represent the photodetectors with heterojunction of Ge/Si structure. The i-Ge thicknesses are indicated below Refs.: [7,22,41,44–50].
The 3dB bandwidth of the vertical p-i-n photodetector on the annealed GOI with GeOx surface passivation was investigated by frequency measurements up to 15 GHz using a lightwave component analyzer (Keysight N4373D). The wavelength was selected as 1,550 nm. For the photodetector with a 60 µm diameter, the 3dB cut-off frequency was measured to be 1.72 GHz at −1 V, in contrast to 0.88 GHz for the one with SiO2 surface passivation as shown in Fig. 9(a). It should be noted that the 3 dB bandwidth is enhanced by 2 times with the insertion of the thin GeOx layer. The photodetectors with varying device sizes are summarized in Fig. 9(b). It can be observed that the 3dB bandwidth increases with smaller mesa diameter, implying that RC-limited delay is the main contributor to 3dB cut-off frequency. Through capacitance-voltage (C-V) measurements, it was revealed that the capacitance for the SiO2 surface-passivated photodetector was 1.6 pF and it was reduced to 1.1 pF by inserting the thin GeOx layer. The decrease in the capacitance can be explained by the reduction in the surface defect states. The junction capacitance of a photodetector is determined by the bulk and sidewall capacitances. The surface defect density could contribute to the sidewall capacitance variation, as was observed in a metal-oxide-semiconductor (MOS) capacitance [52]. By forming GeOx on the mesa-sidewall, the sidewall capacitance is suppressed due to the enhancement of the interface quality via O3 oxidation. In another word, the decrease in the surface defect results in low total capacitance. As a result, the photodetector capacitance formed with the O3 oxidation enhances the resistor-capacitor (RC) delay-limited bandwidth.
Fig. 9. (a) 3 dB bandwidth measurement of the vertical p-i-n photodetector with a diameter of 60 µm on annealed GOI with GeOx surface passivation at the incident wavelength of 1,550 nm under −1 V in the comparison with the one with SiO2 surface passivation. (b) 3 dB bandwidth as a function of reverse bias voltage with varying mesa sizes. The arrow indicates a decrease in the diameter of devices.
In order to investigate the 3dB cut-off limit of photodetectors theoretically, 3dB bandwidth (f3dB) was extracted by following equations [7]
where fRC and fT are RC delay-limited bandwidth and carrier transit-limited bandwidth, respectively, and R, vsat, and di represents resistance, the saturation drift velocity of carriers in Ge, and depletion width, respectively. R consists of load resistance (Rl = 50 Ω) and series resistance (Rs = 19 Ω at 2 V). C can be separated into junction capacitance (Cj) and parasitic capacitance (Cp). Cj is proportional to the mesa area (A), while Cp is a constant value. Thus, Cj and Cp can be obtained by extracting the slope and the intercept values from the linear fitting of C as a function of A as shown in the inset in Fig. 10(a). Extracted Cp and Cj are 0.13 and 1.06 pF for the photodetector with a 60 µm diameter at −1 V, respectively. vsat was selected as 6 × 106 cm/s for calculation. di can be extracted in Fig. 10(a) based on with the equation of di = εA/Cj. The extracted di is ∼200 nm at 0 V and increases up to ∼700 nm at −5 V since the majority carriers are pushed away from the i-Ge region, leaving behind a widening space charge region in the relatively high reverse bias voltage. fT is only limited by vsat and di, expected fT ranges from 130 to 39 GHz as reverse bias increases from 0 to 5 V. It is worth noting that fT is estimated to be above 100 GHz due to a ∼200 nm thin depletion width at 0 V. It is expected that f3dB for the photodetectors with the diameter range of 60 to 250 µm is mainly determined by fRC. Since fRC is affected by the device size, it is obvious that f3dB can be improved by further decreasing the device diameter. With the assumption that the junction capacitance is proportional to the mesa area, expected f3dB at −1 V was extracted in Fig. 10(b), excluding the impact of Cp. As can be seen, the measured f3dB matches reasonably with theoretically obtained f3dB. The underestimated experimental 3dB bandwidth value is due to the existence of parasitic capacitance. As the diameter shrinks down to 5 µm, f3dB of ∼70 GHz is expected due to the significant fT of 72 GHz under the reverse bias of −1 V.Fig. 10. (a) Extracted depletion region width as a function of reverse bias via C-V measurements. Inset represents capacitance as a function of mesa area under −1 V. (b) Expected 3 dB bandwidth with the measured f3dB at −1 V.
4. Conclusion
Vertical p-i-n Ge photodetectors were demonstrated on annealed GOI platform with GeOx surface passivation. The combination of the annealed GOI platform and O3 oxidation methods for effective sidewall passivation provides Ge photodetectors with extremely low dark leakage current density of 0.57 mA/cm2 at −1 V. The extracted bulk and surface leakage current were 0.46 mA/cm2 and 0.58 µA/cm, respectively. Through activation energy study, it is revealed that dark leakage current is not only governed by SRH leakage, but also contributed by diffusion and TAT leakage processes in varying temperature of 293 to 353 K. The responsivity of the photodetector was measured to be 0.28 A/W at 1,550 nm. It was further enhanced up to 0.42 A/W at 1,500 nm. The extracted absorption coefficient at 1,550 nm was estimated to be 3,227 cm−1, which is higher than that of Ge bulk. The 3dB bandwidth was obtained to be 1.72 GHz at −1 V. Theoretically, it can be improved up to ∼70 GHz with a 5 µm diameter. In this work, the demonstrated Ge vertical p-i-n photodetectors with the ultra-low dark current and high speed are proposed and demonstrated for Si-based photonic integrated circuits.
Funding
National Research Foundation Singapore Competitive Research Programme (NRF-CRP19-2017-01).
Acknowledgement
We thank the Silicon-Center of Excellence for the use of optical measurement equipment.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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