Cheng Li, Qinqing Yang, Hongjie Wang, Jinzhong Yu, Qiming Wang, Yongkang Li, Junming Zhou, Hui Huang, and Xiaoming Ren. AbstractâA back-incident Si0 ...
IEEE PHOTONICS TECHNOLOGY JOURNAL, VOL. 12, NO. 10, OCTOBER 2000
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Back-Incident SiGe-Si Multiple Quantum-Well Resonant-Cavity-Enhanced Photodetectors for 1.3-m Operation Cheng Li, Qinqing Yang, Hongjie Wang, Jinzhong Yu, Qiming Wang, Yongkang Li, Junming Zhou, Hui Huang, and Xiaoming Ren
Abstract—A back-incident Si0 65 Ge0 35 /Si multiple quantum-well resonant-cavity-enhanced photodetector operating near 1.3 m is demonstrated on a separation-by-implantation-oxygen substrate. The resonant cavity is composed of an electron-beam evaporated SiO2 –Si distributed Bragg reflector as a top mirror and the interface between the buried SiO2 and the Si substrate as a bottom mirror. We have obtained the responsivity as high as 31 mA/W at 1.305 m and the full width at half maximum of 14 nm. Index Terms—Bragg reflector, resonant-cavity-enhanced photodetector, responsivity, SiGe.
I. INTRODUCTION
R
ESONANT-CAVITY-ENHANCED (RCE) photodetectors have been demonstrated to be able to circumvent the trade-off between bandwidth and responsivity [1], [2]. The enhancement of the quantum efficiency in a RCE photodetector is derived from the increased amplitude of the optical field inside a Fabry–Perot cavity under the resonant condition [3]. The idea has recently been used to carry out the silicon resonant cavity photodetectors operating at short wavelength [4], [5]. RCE photodetectors operating at 1.3 m have been the subject of extensive research because of the low loss and low dispersion coefficients exhibited by optical fibers at this wavelength [6], [7]. Since the absorption coefficient of the intrinsic SiGe alloy at 1.3 m is low due to its indirect bandgap, it is essential to use a Fabry–Perot cavity with the high reflectivity bottom and top mirrors to gain high quantum efficiency. However, with an SiGe layer grown on an Si substrate, it is difficult to get an epitaxial distributed Bragg reflector (DBR) with a high reflectance because of the large lattice misfit between Si and Ge [8], [9]. Although an SiO –Si DBR with a high reflectance can be evaporated on an Si substrate, the intrinsic SiGe absorption layer can not be grown on it because the evaporated SiO or Si are amorphous or polycrystal [10]. Manuscript received January 28, 2000; revised June 15, 2000. This work was supported by National Science Foundation of China under Grant 69896260, Grant 69789802, and by “863” Project under Grant 863-307-15-4(03). C. Li, Q. Yang, H. Wang, J. Yu, and Q. Wang are with State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China. Y. Li and J. Zhou are with the Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. H. Huang and X. Ren are with Beijing University of Posts and Telecommunication, Beijing 100876, China. Publisher Item Identifier S 1041-1135(00)08588-8.
Fig. 1. Schematic of the Si
Ge
/Si back-incident RCE photodetector
To circumvent this problem, we propose a back incident SiGe–Si multiple quantum-well (MQW) photodetector operating at 1.3 m, in which a SiGe-Si MQW absorption region is grown on a separation-by-implantation-oxygen (SIMOX) substrate and then an SiO –Si DBR with high reflectance, as a top mirror is deposited on it. The Fabry–Perot cavity is composed of the interface of Si and the buried SiO layer as the bottom mirror and the SiO –Si DBR as the top mirror. We also report on the fabrication and characterization of this SiGe back-incident RCE photodetctor. The device combines the advantages of a high reflectance, large spectral bandwidth SiO –Si DBR mirror, and an SiGe-Si MQW absorption region. An experimental responsivity of 31 mA/W at 1.305 m was obtained.
II. DEVICE STRUCTURE AND FABRICATION The completed back-incident RCE photodetector structure is shown in Fig. 1. The samples of Si Ge /Si MQWs were grown on a SIMOX substrate by molecular beam epitaxy (MBE) at a temperature of 600 C. A 300 nm intrinsic Si buffer layer was grown on the top of a n-type SIMOX substrate. The thickness of the buried silicon dioxide (SiO ) is about 250 nm and the reflectivity of the SIMOX structure is calculated about 49% at 1.3 m by the transfer matrix model [11]. A MQW region consisting of 20 periods of Si Ge , 9.3-nm thickness, and
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IEEE PHOTONICS TECHNOLOGY JOURNAL, VOL. 12, NO. 10, OCTOBER 2000
Fig. 2.
Current–voltage characteristics of the RCE photodetector.
Si 20-nm thickness was then grown, followed by a 200-nm intrinsic Si and a 100-nm p -Si cap layer. Five pairs of the quarterwavelength SiO –Si Bragg stacks were evaporated on the p cap layer and the reflectivity of 99.7% was obtained by simulation [11]. The RCE photodetector was fabricated using standard photolithography and reactive ion etching (RIE). Mesas (150 m 150 m) were etched down to the silicon-on-oxide by SF O . A circular electrode was made on the p cap layer to ensure that the center of the mesa area was available for the high reflectivity top mirror. The other electrode was made on the Si layer of the SIMOX substrate. The wafer was thinned from back to about 100 m and polished carefully, and then an anti-reflective coating was deposited on the polished surface.
Fig. 3. Responsivity spectra of the RCE photodetector with a resonant peak at 1.305 m and the FWHM of 14 nm.
III. EXPERIMENTAL RESULTS AND DISCUSSION The current-voltage characteristics of the RCE photodetector under forward and reverse bias are shown in Fig. 2. The device has a sharp breakdown at about 30 V and the dark current at 5-V reverse bias is about 80 nA. The relatively low dark current and the sharp breakdown shape also suggest the good crystal quality of the SiGe-Si MQWs. The quantum efficiency of a RCE photodetector at normal incidence was given by Kishino et al. [3] as
where quantum efficiency; bottom and top mirror reflectivities, respectively; absorption coefficient; equivalent length of the absorption layer; propagation constant; cavity length from the top mirror to the bottom mirror; , phase shifts introduced by the bottom and top mirrors, respectively. On the right-hand side of the equation, the term in the braces , this represents the power enhancement factor. When term becomes unity and the expression becomes ,
Fig. 4. Responsivity of the RCE photodetector versus reverse bias at the resonant wavelength 1.305 m.
for a conventional p-i-n photodetector. The responsivity is defined as the ratio of photocurrent to incident power, which can be expressed with the quantum efficiency as , where is the responsivity, is the elec[12]: represents the photon energy. In this case, tron charge, and is about 49%, is about 99%, and as a function of wavelength can be estimated from [13]. With these parameters, the responsivity at the resonant wavelength of around 1.3 m is calculated at 49 mA/W, which is enhanced above tenfold compared with that of a conventional p-i-n photodetector with the same , and the full-width at half-maximum (FWHM) is about 15 nm. Fig. 3 shows the experimental spectra of the responsivity obtained for the back incident RCE photodetector. The responsivity of the RCE photodetector was measured by using a tunable laser (Tunics-PR model 3642 CR00) at room temperature. A tunable laser was used in order to scan wavelength from 1250 to 1330 nm at 1-nm intervals with the same output power. The light was coupled into a single-mode optical fiber and then aligned to the photodetector normally from the backside. As shown in Fig. 3, the peak responsivity of 18.9 mA/W and the FWHM of 14 nm were obtained at a wavelength of 1.305 m with applied reverse bias of 5 V. The resonant wavelength
LI et al.: BACK-INCIDENT SiGe–Si MULTIPLE QUANTUM-WELL RESONANT-CAVITY-ENHANCED PHOTODETECTORS
of 1.305 m and the FWHM of 14 nm are in good agreement with the prediction. Fig. 4 shows the responsivity as a function of reverse bias at a fixed wavelength of 1.305 m. The responsivity of 31 mA/W at 25 V was obtained. It is clearly seen that at a small bias the responsivity increases quickly with increasing reverse bias, and then responsivity increases slowly with the reverse bias. This behavior is possibly due to the carrier trapping by potential barriers in the valence band of the SiGe-Si heterostructures at the low bias levels and the reduced series resistance induced by the photocurrent at the high bias levels. IV. CONCLUSION In conclusion, we have demonstrated a back-incident Si Ge –Si multiple-quantum-well resonant-cavity-enhanced photodetector operating at 1.305 m. The reflectivity of the top DBR mirror was above 99% and the large oscillation was observed in the spectra. A peak responsivity of 18.9 mA/W at 1.305 m and an FWHM of 14 nm with a reverse bias of 5 V were obtained. The maximum responsivity is measured as 31 mA/W at 25 V. This indicates that the SiO –Si and SiGe–Si material system is promising for 1.3- m RCE devices. ACKNOWLEDGMENT The authors would like to thank Prof. C. Lin and Dr. M. Zhang for their providing the SIMOX substrate.
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