IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 7, JULY 2010 701

High-Saturation-Power and High-SpeedGe-on-SOI p-i-n Photodetectors

Hai-Yun Xue, Chun-Lai Xue, Bu-Wen Cheng, Yu-De Yu, and Qi-Ming Wang

Abstract—Ge-on-silicon-on-insulator p-i-n photodetectors werefabricated using an ultralow-temperature Ge buffer by ultrahigh-vacuum chemical vapor deposition. For a detector of 70-μmdiameter, the 1-dB small-signal compression power was about110.5 mW. The 3-dB bandwidth at 3-V reverse bias was 13.4 GHz.

Index Terms—Germanium, photodetectors, saturation power,silicon-on-insulator (SOI) technology, ultrahigh-vacuum chemicalvapor deposition (UHV/CVD).

I. INTRODUCTION

IN RECENT years, the search for optical interconnects hasbeen motivated due to the inherent bandwidth limitation

of electrical interconnects. Many research works were aboutIII–V group devices in the past decades [1]. However, silicon-based photonics, which is considered the higher performancesolution to achieving more functionality into smaller spaces,has become an exploding research field today [2]–[4]. High-power high-speed photodetectors, as the key components foranalog fiber-optic links, have applications in antenna remoting,CATV distribution, and wireless networks with fiber back-bones. In particular, high-saturation-power photodetectors cangreatly increase the RF link gain, signal-to-noise ratio, andspurious-free dynamic range. At present, Ge has been theoptimal candidate material when integrated with Si. It has beenproved that Ge/Si heterostructures could enhance the compet-itiveness of Si-based optoelectronics because Ge absorbs innear-infrared wavelengths [5]–[8].

We have dedicated to research Si-based photonics for manyyears. Last year, we gained Ge-on-Si photodetectors with6.28-GHz 3-dB bandwidth at 1.55 μm [9]. In our recent work,we tried to use a silicon-on-insulator (SOI) substrate to reducethe parasitic capacitance between the pad electrodes and theformer Si substrate. Then, Ge-on-SOI p-i-n photodetectors,which can operate at 1.31 and 1.55 μm, were fabricated us-ing an ultralow-temperature Ge buffer without cyclic ther-mal annealing by ultrahigh-vacuum chemical vapor deposition(UHV/CVD). A maximum saturation power was measured,

Manuscript received March 27, 2010; accepted April 17, 2010. Date ofpublication June 1, 2010; date of current version June 25, 2010. This work wassupported in part by the Major State Basic Research Development Program ofChina under Grant 2007CB613404, by the National High-Technology Researchand Development Program of China under Grant 2006AA03Z415, and by theNational Natural Science Foundation of China under Grant 60676005. Thereview of this letter was arranged by Editor C. Jagadish.

The authors are with the State Key Laboratory on Integrated Optoelectronics,Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083,China (e-mail: xuehy@semi.ac.cn; cbw@semi.ac.cn).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2010.2048997

Fig. 1. (a) Schematic cross section and (b) top view of the double mesastructures of the Ge p-i-n photodetector.

which was, to our knowledge, the highest value reported forthis kind of photodetectors. Furthermore, other performancesof our detector were good enough for practical application.

II. EXPERIMENT

The structure of the photodetector is shown schematically inFig. 1(a). A cold-wall UHV/CVD starts on a N+-doped SOIsubstrate. Before growing, the substrate was degassed at 300 ◦Cin the pretreatment chamber and deoxidized in the growthchamber by heating to 930 ◦C for 5 min [10]. Devices ofdouble mesa structures were fabricated by standard lithography,dry etching, and metallization processing. The top of the Gelayer was implanted with BF+

2 at a dose of 4 × 1015 cm−2 andan energy of 40 KeV. The thickness of the Ge intrinsic layerwas 0.8 μm. Ge circular mesas with diameters of 25, 30, 50,and 70 μm were then fabricated by dry etching the patternedfilms down to the top Si layer of the SOI substrate usingan inductively coupled plasma (ICP) etcher. Then, second-layer circular mesas, which were bigger, were achieved using

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702 IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 7, JULY 2010

Fig. 2. High-saturation power of 0, −0.5 V, −1 V, and −2 V at 1550 nm ofdetector with 70-μm diameter.

the same technique of dry etching down to the buried oxidelayer of SOI. The double mesa layout significantly reduced theparasitic capacitance [11]. A 0.68-μm-thick SiO2 passivationfabricated by plasma-enhanced (PE) CVD was used to insulatethe mesa sidewalls from the signal contact. Windows for theTi/Al contacts were opened by C4F8 ICP etching. Contact holeswere defined by patterning resist openings and then evaporatingand lifting off the Ti/Al metal with a total thickness of 1.05 μm.Finally, a 0.46-μm-thick Si3N4 film was deposited as a passi-vation layer. Tshe micrograph of the photodetector is shown inFig. 1(b).

III. RESULTS AND DISCUSSION

A. Saturation Optical Power

The saturation optical current (optical power) of the 70-μm-diameter device was measured at different reverse bias valuesand 1.55-μm incident light. The maximum gain of the erbium-doped fiber amplifier (EDFA) is 23 dBm, and the operationwavelength ranges from 1535 to 1665 nm. The loss of fibernearby EDFA was 1.6 dBm.

The saturation curves of the 70-μm-diameter device at zero-and reverse-bias values of 0.5, 1, and 2 V are shown in Fig. 2.From the figure, we can see that the optical current at a reversebias of 1 V keeps linearly increasing with the optical powerduring 0–50 mW. Similarly, at a reverse bias of 2 V, the opticalpower range is 0–75 mW. The theoretical relation of opticalcurrent and optical power was the dashed line in the figure.However, there were several physical mechanisms, such asspace-charge screening effect [12] and thermal effect [13], thatcontributed to saturation, so the curve was nonlinear in reality.

The saturation current, namely, the 1-dB small-signal com-pression current, was defined as the dc photocurrent at whichthe ac output power was compressed from the linear responseby 1 dB [14]. According to this definition, the 1-dB small-signalcompression points were 22 and 40 mA at reverse bias valuesof 1 and 2 V, corresponding optical power values of 67.5 and110.5 mW. Significantly, at a reverse bias of 1 V, the opticalcurrent kept increasing when the optical power ranged from 0to 150 mW. It proves that our detector can operate well at highoptical power with high responsivity.

Fig. 3. Dark current of detectors with diameter of 15, 25, 30, 50, and 70 μm.

Fig. 4. Photocurrent of detector under illumination of 1.31 and 1.55 μm.

B. Dark-Current Characteristic

The I–V characteristic of the photodetectors in darkness wasmeasured using a Keithley 4200-SCS semiconductor parameteranalyzer. Fig. 3 shows the dark-current curves of photodetectorsof 15, 25, 30, 50, and 70 μm. The average value of dark-current density was 16 mA/cm2. The dark current of the 30-μm-diameter device as low as 105 nA at a reverse bias of 1 V hasbeen obtained, and the corresponding current density was about14.8 mA/cm2. Many previous works demonstrated that thread-ing dislocations, as a source of leakage current, could be re-duced by annealing [15]. Thus, we might obtain a lower leakagecurrent device by adding an annealing process in future work.

C. Responsivity

The optical current was measured under illumination of 1.31and 1.55 μm and incident optical power of 0.5 mW. Fig. 4shows the good rectifying characteristic of low leakage currentand high response to a certain optical incidence. At a reversesbias of 1 V, the optical responsivities were 0.65 and 0.31 A/Wat 1.31 and 1.55 μm, respectively. The quantum efficiencies of1.31 and 1.55 μm were 62% and 25%, respectively.

D. High-Frequency Characteristics

The high-frequency characteristics of different diameter pho-todetectors were measured using normalized frequency mea-surement. The measuring system includes a laser of 1.55 μm,

XUE et al.: HIGH-SATURATION-POWER AND HIGH-SPEED Ge-on-SOI p-i-n PHOTODETECTORS 703

Fig. 5. Normalized frequency responses of the 30-μm-diameter detector at1550 nm.

an N5230A network analyzer of Agilent with a test range of300 kHz–20 GHz, a JDSU high-speed modulator, and a radio-frequency GSG probe of 40 GHz. As shown in Fig. 5, the 3-dBbandwidths of the 30-μm-diameter device were 1.69, 11.34,and 12.6 GHz at zero bias and at reverse-bias values of 1 and3 V, respectively. This was lower than the theoretical valueof 20 GHz (see the Appendix for details) when consideringthe junction capacitance of the device only and neglecting theohmic contact resistance of the pad electrodes. Furthermore, a13.4-GHz 3 dB bandwidth at 3-V reverse bias was obtainedwhen the diameter of the detector was reduced to 15 μm.

IV. CONCLUSION

The good-quality Ge material on the SOI substrate was ob-tained using an ultralow-temperature Ge buffer by UHV/CVDin our work. Then, the vertical incidence p-i-n photodetectorswere achieved successfully. The 1-dB small-signal compressionoptical currents were 22 and 40 mA of the 70-μm-diameterdevice, and the corresponding optical power values were 67.5and 110.5 mW at reverse-bias values of 1 and 2 V. This highestsaturation power detector might be a booster for the opticalcommunication system in the application area. The responsivityvalues were 0.65 and 0.31 A/W at 1.31 and 1.55 μm, respec-tively. The 3-dB bandwidth was 13.4 GHz at 1.55 μm. Thisindex of the devices might be improved in our future work.

APPENDIX

The theoretical 3-dB bandwidth of a vertical p-i-n detector islimited by RC delay and drift time. The RC bandwidth fRC canbe calculated by

fRC =1

2πRC=

12πεR

di

π(D/

2)2(1)

with the intrinsic depletion capacitance C = επ(D/2)2/di (Dis the mesa diameter, ε is the permittivity, and di is the thicknessof the intrinsic zone). Assuming a constant device area, the RCfrequency would increase with the thickness of the intrinsicregion.

The drift time of carriers in the intrinsic region is an-other limitation. The minimum time is given by the saturationdrift velocity υsat, which is a material constant (Ge : υsat =

6 × 106 cm/s). The carrier transit frequency [16] fT can becalculated by

fT =√

2π

υsat

di. (2)

The 3-dB bandwidth f3dB of a vertical p-i-n detector can becalculated by

f3dB =1√

1f2RC

+ 1f2

T

. (3)

Assuming that the ohmic resistance R equals the systemimpedance of 50 Ω, with a 0.8-μm-thickness (di) and 30-μm-diameter (D) detector, the 3-dB bandwidth is 20 GHz.

ACKNOWLEDGMENT

The authors would like to thank the concerned people fromthe Engineering Research Center for Semiconductor IntegratedTechnology, Institute of Semiconductors, Chinese Academyof Sciences, for their valuable discussions and help on thefabrication techniques.

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