IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 12, DECEMBER 2007
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Improving the Performance of SiGe Metal–Semiconductor–Metal Photodetectors by Using an Amorphous Silicon Passivation Layer Y. H. Chen, J. D. Hwang, C. Y. Kung, P. S. Chen, C. S. Wei, C. K. Wu, and J. C. Liu
Abstract—SiGe metal–semiconductor–metal photodetectors (MSM-PDs) with a thin amorphous silicon (a-Si:H) passivation layer have been fabricated by an ultrahigh-vacuum chemical vapor deposition (UHVCVD) system. It was found that the thin (30 nm) a-Si:H passivation layer could effectively suppress the dark current of SiGe MSM-PDs. As compared to the unpassivated devices, the dark current for devices with a-Si:H passivation layers was drastically reduced by 1.7 × 105 , and the photo-to-dark current ratio was enhanced by 1.33 × 106 . We attribute this result to the passivation effect of a-Si:H films on SiGe surfaces by hydrogen diffusion, which can compensate the dangling bonds on the SiGe surface.
TABLE I DEPOSITION CONDITIONS OF THE a-Si:H AND SiO2 FILMS
Index Terms—Amorphous silicon, metal-semiconductor-metal, passivation, photodetector, SiGe.
I. I NTRODUCTION
T
HE REQUIREMENTS of silicon-based photonic devices in the next generation of chip technologies have caused extensive studies on SiGe devices because strained SiGe exhibits several advantages, such as higher electron and hole mobility than Si, longer detection wavelength (up to 1.6 µm) for near-infrared optical detection, and the possibility for integration into an existing silicon technology [1], [2]. In the past years, various types of SiGe-based optoelectronic devices [3]–[6] have been proposed. Among these devices, the metal–semiconductor–metal photodetectors (MSM-PDs) exhibit the highest response speed, owing to their ultralow intrinsic capacitance [12], [13]. The fabrication process of MSM-PDs is also compatible with that of a field-effect transistor (FET). Thus, one can easily integrate SiGe MSM-PDs with SiGe FET-based electronics to realize optoelectronic integrated circuits. However, up to now, the reported planar-type MSM-PDs are few, to the best of our knowledge. In planartype MSM-PDs, both metals (anode and cathode) are on the surface of SiGe, which may contain many surface defect states caused by Ge atoms that were segregated during the growth of SiGe [7]. These surface defect states will play a detrimental
Manuscript received August 22, 2007. The review of this letter was arranged by Editor P. Yu. Y. H. Chen, C. Y. Kung, and J. C. Liu are with the Department of Electrical Engineering, National Chung Hsing University, Taichung 402, Taiwan, R.O.C. J. D. Hwang, C. S. Wei, and C. K. Wu are with the Department of Electrical Engineering, Da-Yeh University, Changhua 515, Taiwan, R.O.C. (e-mail:
[email protected]). P. S. Chen is with the Department of Materials Science and Engineering, Minghsin University of Science and Technology, Hsinchu 300, Taiwan, R.O.C. Digital Object Identifier 10.1109/LED.2007.909853
role in the generation of a dark current because the current flows through the top SiGe layer in planar-type MSM-PDs. In this work, an amorphous silicon (a-Si:H) layer was employed to passivate the SiGe surface, and a significant reduction in the dark current was achieved, as compared with the conventional plasma-enhanced chemical vapor deposition (PECVD) SiO2 -passivated detectors.
II. E XPERIMENTAL An n-type silicon substrate with a doping concentration below 3 × 1016 cm−3 was cleaned using a standard process. A 150-nm undoped strained p-Si0.8 Ge0.2 layer with a concentration of 3 × 1016 cm−3 was then grown on the n-Si(100) substrate, with a deposition rate of 5 nm/min, by an ultrahighvacuum chemical vapor deposition (UHVCVD) system at 550 ◦ C. After that, 30 nm of a-Si:H (sample C) and 30 nm of SiO2 (sample B) were deposited by PECVD. The deposition conditions of the a-Si:H and SiO2 films are summarized in Table I. Subsequently, contact windows were opened through the a-Si:H and SiO2 layers by using an HF + H3 NO3 or a buffered oxide etch solution, followed by a lift-off step to form Au Schottky contacts in an interdigitated pattern. The interdigitated pattern has fingers that are 2 µm wide and 100 µm long, with a 4-µm spacing in between. The HF + H3 NO3 solution was used to isolate devices by mesa etching. For the convenience of comparison, the SiGe MSM-PDs without any passivation layer (sample A) were fabricated. The inset of Fig. 1 is the structure of SiGe MSM-PDs with aSi:H passivation. The dark and photo current–voltage (I–V ) characteristics were measured by an HP-4155B semiconductor parameter analyzer with an illumination of 850 nm from a laser source.
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IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 12, DECEMBER 2007
Fig. 1. X-ray rocking curve and dynamic simulation result for the Si0.8 Ge0.2 film. Inset: Structure of sample C.
Fig. 2. Dark and photo (850 nm) I–V characteristics of the fabricated MSM-PDs.
III. R ESULTS AND D ISCUSSION The X-ray rocking curve and dynamic simulation of the Si0.8 Ge0.2 layer are shown in Fig. 1. The presence of interference fringes indicates a high quality of Si0.8 Ge0.2 on the Si substrate. The scanned image of Si0.8 Ge0.2 /Si, which was observed by atomic force microscopy, appeared to have no cross-hatch patterns. A diluted Secco’s solution was used to reveal the density of threading dislocations [14], [15]. From the observation of Normaski optical microscopy, the etch pits density of Si0.8 Ge0.2 /Si was less than 104 cm−2 , which suggested that no misfit dislocations occurred at the interface of Si0.8 Ge0.2 /Si. These results indicate that the SiGe/Si films were fully under strain. The dark and photo I–V characteristics of fabricated MSMPDs are shown in Fig. 2. It is found that both the curves of samples A and B have the same dark current. It suggests that the PECVD SiO2 has no passivating effect on SiGe surfaces. This may be attributed to the plasma damage on the SiGe surface during PECVD SiO2 deposition. The PECVD SiO2 deposition
damaged the SiGe surface through ion bombardment, leading to more generated defect states on the SiGe surface. It has been reported that plasma irradiation involves the displacement of Si and Ge atoms from their lattice sites, which creates point defects of vacancy–interstitial pairs [8], [9]. These plasma-created defects on the SiGe surfaces were also found in our previous study [10].The lowest dark current is observed in sample C. For instance, at 6 V, the dark current is 1.27 × 10−8 A; whereas, in sample A, it is 2.14 × 10−3 A. Obviously, the dark current is dramatically reduced by a factor of 1.7 × 105 for the a-Si:Hpassivated sample due to the reduction of surface states, which was caused by Ge outdiffusion during the growth of SiGe. The dark current density of sample C of 1.56 × 10−5 A/cm2 is much smaller than that of the Cr–Ge–Cr MSM-PD (1 A/cm2 ) [17] at 1 V and is comparable with that of the Cr–Si–Cr MSM-PD [16] having PECVD SiO2 passivation. The MSMPDs [16] fabricated by pure Si without Ge doping had a dark current density of 1.32 × 10−4 A/cm2 , which was much lower than that of the unpassivated SiGe MSM-PDs (2.6 A/cm2 ) in this work. It has been pointed out that the hydrogen in a-Si:H can diffuse to the defect sites, effectively passivating the dangling bond states. The passivation that uses a-Si:H films can easily compete with that of high-temperature-grown (∼1000 ◦ C) SiO2 [11]. Moreover, in Fig. 2, sample C has a higher photocurrent than those in samples A and B. The possible reasons are addressed later in this letter. In sample C, most of the photo-generated carriers in the SiGe layer are directly swept into Au electrodes by an electric field, and some carriers are injected into an a-Si:H layer. The injected carriers are captured by the dangling bonds inside a-Si:H at a lower voltage; hence, a lower current is obtained, as compared to that of sample B. However, at a higher voltage, these captured carriers will be emitted; thus, the current increases with the applied bias voltage and, finally, becomes saturated as a result of the complete emission of the captured carriers. In our previous study [20], the a-Si1−x Gex :H layer will be formed at the interface of p-SiGe/a-Si:H due to the Ge atoms’ outdiffusion during the growth of a-Si:H. The photo-absorption of a-Si1−x Gex :H and the passivated surface of sample C are the possible reasons for such a high photocurrent. The Fourier transform infrared absorption spectra of the a-Si:H and SiO2 films are compared in Fig. 3. Two peaks at 2000 and 630 cm−1 are observed in a-Si:H, which respectively correspond to the stretching and waging modes [18] of the local vibrations of monohydrogen bonds species (Si–H). Although only one peak of the stretching mode at 1075 cm−1 appeared [19] in the SiO2 films (inset), such a result confirms that the PECVD SiO2 films in our work have no H atoms. Fig. 4 shows the photo-to-dark current ratio (PDR) of the three samples. It is found that the PDR of sample C increases with the applied voltage, whereas the PDR of samples A and B slightly decreased with the increased applied voltage. Such results demonstrate that more surface defect states exist on the SiGe surface in samples A and B, but on the other hand, surface defect states have been passivated by the a-Si:H layer in sample C. The characteristics of these samples are compared in the inset of Fig. 4. As compared to sample A, the dark current is drastically reduced in sample C by 1.7 × 105 , and the
CHEN et al.: IMPROVING THE PERFORMANCE OF SiGe METAL–SEMICONDUCTOR–METAL PHOTODETECTORS
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passivation effect to hydrogen diffusion, which compensates the dangling bonds on the SiGe surface. R EFERENCES
Fig. 3. films.
Fourier transform infrared absorption spectra of the a-Si:H and SiO2
Fig. 4. PDR of the fabricated MSM-PDs. Inset: Comparison table of the three MSM-PDs.
PDR is improved by a factor of 1.33 × 106 . Furthermore, the responsivity for sample C is 0.59 A/W, which is much larger than that of sample A (0.075 A/W). These results show that the thin a-Si:H films effectively passivate the surface defects on the SiGe surface, leading to an enhanced PDR. IV. C ONCLUSION Planar-type SiGe MSM-PDs have been fabricated by UHVCVD. In this letter, the dark current of the SiGe MSM-PDs has been effectively suppressed by adding a thin (30 nm) a-Si:H passivation layer on the surface of the SiGe films. As compared to the unpassivated device, the dark current was reduced by a factor of 1.7 × 105 in an a-Si:H-passivated device, and the PDR was increased by a factor of 1.33 × 106 . We attribute this
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