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We demonstrate that a complementary metal-oxide-semiconductor (CMOS) compatible silicon (Si) surface passivation technique effectively suppress the dark current originating from the mesa sidewall of the Ge0.95Sn0.05 on Si (Ge0.95Sn0.05/Si) p-i-n photodiode. Current-voltage (I-V) characteristics show that the sidewall surface passivation technique could reduce the surface leakage current density (Jsurf) of the photodiode by ~100 times. A low dark current density (Jdark) of 0.073 A/cm2 at a bias voltage of −1 V is achieved, which is among the lowest reported values for Ge1-xSnx/Si p-i-n photodiodes. Temperature-dependent I-V measurement is performed for the Si-passivated and non-passivated photodiodes, from which the activation energies of dark current are extracted to be 0.304 eV and 0.142 eV, respectively. In addition, the optical responsivity of the Ge0.95Sn0.05/Si p-i-n photodiodes to light signals with wavelengths ranging from 1510 nm to 1877 nm is reported.
© 2015 Optical Society of America
1. Introduction
Germanium-tin (Ge1-xSnx) alloy has attracted attention for its potential applications in the large-scale monolithic photonic integrated circuits (PICs). Recently, the first demonstration of lasing using a direct bandgap Ge0.874Sn0.126 grown on Si substrates was reported [1]. Besides light emitting devices, Ge1-xSnx is also promising for realizing near-infrared (NIR) photodetectors due to its extended absorption range and the enhanced absorption coefficient compared to germanium (Ge) [2]. The Ge1-xSnx-based photoconductor [3,4], p-i-n photodiode [5–15], and avalanche photodiode (APD) [16] have been reported. Ge1-xSnx photodiode with a small Sn composition of 2% is reported to be sufficient to cover all the optical communication bands (O band ~U band) [5]. Further increase of the Sn composition could extend the cutoff wavelength of Ge1-xSnx photodiode to the mid-infrared (MIR) range (2 ~5 µm), which is very attractive for application in emerging areas such as chemical and biological sensing, gas detection, industrial process control, and medical diagnostics [17].
However, compared to the photodiodes based on Ge or III-V materials, Ge1-xSnx photodiode suffers from a much larger dark current (Idark), which may increase the power consumption and degrade the signal-to-noise-ratio (SNR) of the integrated optical receivers. In general, Idark of the photodiode has two components: one is the bulk leakage current (Ibulk) which is proportional to the diode area; the other one is the perimeter-dependent surface leakage current (Isurf) originating from the surface of mesa sidewall. It has been reported by Su et al. that Isurf contributes a large part to the Idark of Ge1-xSnx/Si p-i-n photodiode due to the existence of defects at the sidewall surface [8]. Furthermore, the surface leakage becomes more significant when the diode area scales down to the dimensions needed for focal-plane-array (FPA) applications (30 × 30 µm2 or smaller). In addition, regarding to the waveguide photodiodes, the leakage from sidewall will be a more dominant dark current component [18], which is due to the much larger perimeter-to-area ratio of the waveguide structure. Therefore, a surface passivation technique which can effectively passivate the surface defects is strongly desired to achieve low Idark.
In this paper, a complementary metal-oxide-semiconductor (CMOS) compatible Si surface passivation technique is demonstrated for suppressing the Idark of Ge0.95Sn0.05/Si p-i-n photodiode. Electrical and optical characterization for both of the Si-passivated and non-passivated photodiodes were carried out. The results show that the surface leakage current density of the detector is suppressed by ~100 times due to the Si passivation. In addition, the achieved dark current density (Jdark) of 0.073 A/cm2 at a bias voltage (Vbias) of −1 V is among the lowest reported values for Ge1-xSnx/Si p-i-n photodiodes.
2. Device design and fabrication
Figure 1(a) shows the top view scanning electron microscopy (SEM) image of the Ge0.95Sn0.05/Si p-i-n photodiode with a mesa diameter (D) of 40 μm. The aluminum (Al) top and bottom electrodes of the photodiode are isolated by silicon dioxide (SiO2). The cross-sectional schematic of the device along dash line A-A’ is shown in Fig. 1(b). A commercially available substrate with a customized n+-doped Si layer and undoped Ge buffer already formed on 4-inch Si (100) wafer is used. The n+-Si layer is in situ doped with the doping concentration of 2 × 1019 cm−3. The thicknesses of the n+-Si layer and the Ge buffer are ~1000 and 350 nm, respectively. A high-quality Si passivation layer is used to passivate the surface defects at the mesa sidewall region.
Fig. 1 (a) Top view SEM image of the Ge0.95Sn0.05/Si p-i-n photodiode with a diameter of 40 μm. (b) Cross-sectional schematic of the photodiode along the A-A’ dash line shown in (a).
A 360 nm-thick Ge0.95Sn0.05 film was epitaxially grown on the Ge buffer by molecular beam epitaxy (MBE) at 170 °C. The high resolution x-ray diffraction (HRXRD) rocking curve of the Ge0.95Sn0.05/Si sample at (004) orientation is shown in Fig. 2(a). The three peaks (from right to left) correspond to Si substrate, Ge buffer and Ge0.95Sn0.05 epitaxial layer. The substitutional Sn composition is calculated to be 5% by the same method as reported in [16]. The Ge0.95Sn0.05 film is fully strained to the Ge buffer under a biaxial compressive strain of ~0.6%. Figure 2(b) shows the 3D atomic force microscopy (AFM) image the Ge0.95Sn0.05 surface with a scan size of 10 × 10 μm2. The root-mean-square (RMS) roughness of the sample surface is 1.76 nm. Figure 2(c) shows a high resolution transmission electron microscopy (HRTEM) image of the sample at the interface region of Ge0.95Sn0.05 and Ge. The single-crystalline Ge and Ge0.95Sn0.05 epitaxial layers as well as their high quality interface can be observed.
Fig. 2 (a) HRXRD rocking curve of the Ge0.95Sn0.05/Si sample at (004) orientation. (b) AFM image of Ge0.95Sn0.05 surface. (c) HRTEM image of the sample at Ge0.95Sn0.05/Ge interface.
The Ge0.95Sn0.05/Si sample then underwent BF2+ ion implantation with a dose of 1 × 1015 cm−2 at 32 keV, followed by rapid thermal annealing at 400 °C for 5 minutes to form a 60 nm-thick p+-Ge0.95Sn0.05 layer. After that, circular mesas with various diameters were patterned by photolithography and formed by chlorine-based reactive-ion etching (RIE). An ultra-high vacuum chemical vapor deposition (UHVCVD) system was used for the Si surface passivation. The passivation process comprises three steps as illustrated in Fig. 3. Firstly, the Ge0.95Sn0.05 wafer was cleaned by diluted hydrofluoric acid DHF (HF:H2O = 1:50), and then loaded into the UHVCVD load/unload chamber. Secondly, the sample was transferred to the native oxide removal chamber, where sulfur hexafluoride (SF6) plasma treatment was performed at 320 °C for 50 s to remove any residual native oxide. Finally, the growth of Si passivation layer was performed in the process chamber using disilane (Si2H6) with a flow rate of 10 sccm as the precursor. The substrate temperature was kept at 370 °C throughout the entire Si growth process. A high quality Si layer with several monolayers-thick was grown [19–22]. For the control device without Si passivation, the second and third steps were skipped. Right after the Si passivation process, around 300 nm-thick SiO2 was deposited for isolation and anti-reflection. The device fabrication process was finished by contact region opening and Al electrode formation.
3. Results and discussion
3.1 Electrical characteristics of the Ge0.95Sn0.05/Si p-i-n photodiodes
The Idark-Vbias characteristics of the Ge0.95Sn0.05/Si p-i-n photodiodes with and without Si passivation are shown in Figs. 4(a) and 4(b), respectively. The mesa diameter ranges from 20 to 70 μm. All the measurements were performed at room temperature. It was observed that Idark of the Si-passivated devices was suppressed by around one order of magnitude compared to those without Si passivation at Vbias = −1 V. Idark of the Si-passivated devices with the diameter of 20, 30, and 40 μm at −1 V were 0.25, 0.52, and 0.92 μA, respectively. It should be noted that Idark of less than 1 μA is generally considered as an acceptable value for a high-speed receiver design, below which the trans-impedance amplifier (TIA) noise becomes the main noise source [23]. Therefore, the low-Idark Ge0.95Sn0.05/Si p-i-n photodiode achieved in this work shows promise for the future high-speed electronic-photonic integrated chips.
Fig. 4 Idark-Vbias characteristics of the Ge0.95Sn0.05/Si p-i-n photodiodes (a) with and (b) without Si passivation. The diameter D of the photodiode ranges from 20 to 70 μm.
Further analysis of Idark has been carried out by separating the current into two components: the bulk leakage current which is proportional to the diode area and the perimeter-dependent surface leakage current. The dark current density Jdark of the photodiode can be expressed as:
where Jbulk and Jsurf are the bulk and surface leakage current density, respectively. Figure 5(a) presents Jdark against 1/D for Ge0.95Sn0.05/Si p-i-n photodiodes biased at −1 V, where D varies from 20 μm to 70 μm. Jbulk and Jsurf of the passivated and non-passivated photodiodes can be extracted by linearly fitting the data in this plot. The calculated Jsurf of the photodiodes with and without Si passivation are 1.1 × 10−5 A/cm and 1.04 × 10−3 A/cm, respectively, which shows that the Jsurf of the device is suppressed by ~100 times by the Si surface passivation. This could be explained by the following mechanism. The mesa formation step by RIE leads to abrupt termination of the periodic structures of lattice. This creates substantial amount of dangling bonds at the mesa sidewall, which results in the interface traps at the Ge0.95Sn0.05/oxide or Ge/oxide interface. These traps may introduce energy levels in the forbidden gap, leading the leakage current along the mesa sidewall. Previous works on the Si passivation of metal-oxide-semiconductor field-effect transistors (MOSFETs) and MOS capacitors have shown that the introduction of a Si passivation layer helps in reducing the semiconductor/oxide interface traps [24,25]. As for the Ge0.95Sn0.05/Si p-i-n photodiode, the Si passivation layer could effectively passivate the dangling bonds at the mesa sidewall, resulting in the reduction of density of interface traps (Dit) at the Ge0.95Sn0.05/oxide or Ge/oxide interface. Therefore, Isurf of the Ge0.95Sn0.05/Si p-i-n photodiode was suppressed. As shown in Fig. 5(a), it should also be noted that Jbulk of the passivated and non-passivated devices are similar with an extracted value around 0.06 A/cm2. This could be attributed to that the sub-370 °C process temperature during the Si passivation is not sufficient to reduce the threading dislocations at the bulk region, which are considered as the main source for bulk leakage current of the photodiodes [26].Fig. 5 (a) Jdark vs. 1/D characteristics of the Ge0.95Sn0.05/Si p-i-n photodiodes biased at −1 V. (b) Plot of ln(Idark/T3/2) vs. 1/kT for the photodiodes at −1 V. The extracted activation energies of the photodiodes with and without Si passivation are 0.304 eV and 0.142 eV, respectively.
In order to gain additional insight into the leakage current, an activation energy analysis of the passivated and non-passivated Ge0.95Sn0.05/Si p-i-n photodiodes (D = 20 μm) has been performed. The Idark can be modeled using
where B is a constant, T is the temperature, Ea is the activation energy and Va is the applied voltage. Figure 5(b) shows the logarithm of the measured dark current ln(Idark/T3/2) as a function of 1/kT for the photodiodes biased at −1 V with the temperature ranges from 280 K to 335 K. The linear fitting of the data yields a gradient corresponding to the activation energy. The extracted Ea of the Si-passivated photodiode is 0.304 eV, which is around half of the bandgap (Eg) of Ge0.95Sn0.05 (Eg = 0.6 eV [27]) or Ge (Eg = 0.66 eV). This indicates that Idark of the Si-passivated photodiode is dominated by the Shockley-Reed-Hall (SRH) [28,29] process via deep-level traps in the forbidden gap of Ge0.95Sn0.05 and Ge. The studies on relaxed Ge0.3Si0.7 on Si heterostructures have revealed a linear correlation between deep-level trap density and threading dislocation density, indicating that the trap is most likely associated with the core of threading dislocations in these heterostructures [30,31]. As for the Ge0.95Sn0.05/Si heterostructure in this work, a high density of threading dislocations through the Ge buffer and Ge0.95Sn0.05 epitaxial layer is expected due to the large lattice mismatch between Ge and Si. Therefore, the obtained Ea of the Si-passivated photodiode should be associated with the threading dislocations at the bulk region due to the mismatched epitaxy. On the other hand, Ea of the non-passivated photodiode is 0.142 eV, which is much smaller than half of the bandgap of Ge0.95Sn0.05 or Ge. As can be calculated based on the parameters extracted from Fig. 5(a), Idark of the non-passivated photodiode is dominated by the surface leakage current, which indicates that the high-density surface traps, rather than the deep-level traps at the bulk region, are the dominating factor for Idark. Therefore, the observation of Ea << Eg/2 could be possibly due to the shallow traps caused by the surface defects at mesa sidewall. Similar experimental result has also been observed from the SiC photodiodes [32].Figure 6 presents the benchmarks of Jdark of Ge1-xSnx/Si p-i-n photodiodes in this work compared to other reported values [5–14] at Vbias = −1 V. Jdark of the Si-passivated Ge0.95Sn0.05/Si p-i-n photodiode with D = 40 μm is 0.073 A/cm2, which is comparable to that of the reported Ge/Si p-i-n photodiodes with typical values in the range of 0.01 ~0.10 A/cm2 [33]. It should also be noted that for Ge1-xSnx photodiodes, a higher Sn composition generally leads to a larger leakage current due to its relatively smaller bandgap. Jdark of Ge0.95Sn0.05/Si p-i-n photodiode achieved in this work is among the lowest values for Ge1-xSnx/Si photodiodes despite the fact that its Sn composition is higher than most of the other reported devices. This indicates the high quality of the materials grown and the effectiveness of the Si surface passivation technique in suppressing Jdark.
3.2 Photoresponse characteristics of the Ge0.95Sn0.05/Si p-i-n photodiodes
Optical characterization of the Ge0.95Sn0.05/Si p-i-n photodiodes was performed. The light was generated by a tunable laser (1510 ~1630 nm) and two separate distributed feedback (DFB) laser diodes (1742 and 1877 nm), and then illuminated vertically on the active region of photodiode through an optical fiber. Photoresponse characteristics of the Ge0.95Sn0.05/Si p-i-n photodiodes with and without Si passivation are shown in Figs. 7(a) and 7(b), respectively. The incident light power (Pin) was fixed at 0.25 mW. Both of the two photodiodes showed obvious response to light signals from 1550 to 1877 nm. The responsivities of the Si-passivated photodiode at 1550, 1630, 1742, and 1877 nm were 178, 67, 35, and 17 mA/W, respectively at Vbias = −1 V.
Fig. 7 Optical responsivity of the Ge0.95Sn0.05/Si p-i-n photodiodes (a) with and (b) without Si passivation. The light wavelength λ ranges from 1550 to 1877 nm. The incident light power is fixed at 0.25 mW.
Optical responsivity for the Ge0.95Sn0.05/Si p-i-n photodiodes at 1510 ~1877 nm is depicted in Fig. 8. Negligible change was observed for the photodiodes with and without Si passivation. This was as expected since the Si passivation process only passivated the surface defects at the mesa sidewall and did not affect the material quality of the Ge0.95Sn0.05 bulk region where electron-hole pairs were generated and transported. Considering that Idark of the Ge0.95Sn0.05/Si p-i-n photodiode was effectively suppressed by Si passivation, the SNR of the detector could be substantially improved. In addition, as shown in Fig. 8 the detectors could respond to light signals from S band to U band, indicating the coverage of entire optical communication bands. By increasing the Sn concentration, the cutoff wavelength of the photodiode could be extended even further to the MIR range where many emerging areas in biology and sensing are envisaged. It should be noted that as the Sn concentration becomes much higher, the Sn segregation and clustering may happen during the passivation process at 370 °C. Further optimization of the Si passivation process, e.g. decreasing the process temperature and using a two-step growth method [22], will be investigated for the high Sn-concentration Ge1-xSnx devices.
5. Conclusion
Suppression of Idark of the Ge0.95Sn0.05/Si p-i-n photodiode using a Si surface passivation technique was demonstrated. The Si surface passivation effectively passivates surface defects located at the mesa sidewall. It is observed that Jsurf of the diode is reduced by ~two orders due to the sidewall surface passivation. Ge0.95Sn0.05/Si p-i-n photodiode with a Jdark of 0.073 A/cm2 at Vbias = −1 V is achieved. Temperature-dependent I-V measurement is performed for the Si-passivated and non-passivated photodiodes, from which the activation energies of Idark are extracted to be 0.304 eV and 0.142 eV, respectively. When biased at −1 V, the Si-passivated photodiode shows responsivities of 178, 67, 35, and 17 mA/W at λ = 1550, 1630, 1742, and 1877 nm, respectively. The presented Si passivation technique shows promise for the monolithically-integrated Ge1-xSnx/Si NIR/MIR optical receivers and image sensors, where low-power consumption and high-SNR are needed.
Acknowledgment
This work was supported by Singapore National Research Foundation through the Competitive Research Program (Grant No: NRF-CRP6-2010-4).
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