Enhanced light absorption in waveguide Schottky photodetector

Vol. 25, No. 9 | 1 May 2017 | OPTICS EXPRESS 10057

Enhanced light absorption in waveguide Schottky photodetector integrated with ultrathin metal/silicide stripe JINGSHU GUO, ZHIWEI WU, AND YANLI ZHAO* Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, China, 430074 * [email protected]

Abstract: We investigate the light absorption enhancement in waveguide Schottky photodetector integrated with ultrathin metal/silicide stripe, which can provide high internal quantum efficiency. By using aab0-quasi-TE hybrid modes for the first time, a high absorptance of 95.6% is achieved in 5 nm thick Au stripe with area of only 0.14 μm2, without using resonance structure. In theory, the responsivity, dark current, and 3dB bandwidth of the corresponding device are 0.146 A/W, 8.03 nA, and 88 GHz, respectively. For most silicides, the quasi-TM mode should be used in this device, and an optimized PtSi device has a responsivity of 0.71 A/W and a dark current of 35.9 μA. ©2017 Optical Society of America OCIS codes: (230.5160) Photodetectors; (240.6680) Surface plasmons; (130.3120) Integrated optics devices.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

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#285495 Journal © 2017

https://doi.org/10.1364/OE.25.010057 Received 24 Jan 2017; revised 18 Apr 2017; accepted 18 Apr 2017; published 24 Apr 2017


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1. Introduction Schottky photodetectors (PDs) [1,2] based on internal photoemission (IPE) [3] have been used for many years in area of communication [4,5] and imaging [6,7]. A Schottky barrier, which is the basic structure of a Schottky PD, can be formed in the interface between lightly doped semiconductor and metal/silicide. In IPE process, the hot carriers created by light absorption in metal/silicide have chance to be emitted over the Schottky barrier and collected in the semiconductor as photocurrent. In recent years, the IPE effect is combined with plasmonics, and can be used to harvest solar energy [8] or create sensitive photodetectors and spectrometers [9]. Since the detected photo energy can be lower than the semiconductor bandgap energy. Schottky PD provides one of the solutions [2,10] to infrared photodetection in Silicon photonics [11], which has been known as the key technology of the next-generation communications systems. At present, the reported Schottky PDs have bottlenecks on their low responsivities, which are decided by the internal quantum efficiencies (IQEs) and the optical absorptances. Several methods are proposed to improve the low IQE. By choosing materials or using image force effect [12] (from bias voltage controlling), the Schottky barrier height ΦB can be lowered to improve IQE, and that is why p-type silicon (p–Si) is often used, given that Schottky barriers are usually lower thereon than on n-type silicon (n-Si). However, the reduction of ΦB leads to a large dark current, which needs to be reduced by using cryogenic operation temperature. The embedding metal structures into semiconductor provides multiSchottky barriers, and then the hot carriers generated near the Schottky contact interfaces get more chance to be emitted [13]. Furthermore, the surrounded metallic structures with small sizes below the hot carrier attenuation length can enhance IQE efficiently by utilizing hot carrier reflections, which occurs in the multiple interfaces and provide the un-emitted hot carriers more chances to be emitted [14–16]. The metal/silicide nanoparticles (NPs) embedded in the semiconductors can achieve much higher IQE in theory, but increase the difficulties of getting high light absorptance in the meantime [16]. We argue that this concept is a promising solution with potential to realize high performance Schottky PDs in the future. The adoption of ultrathin metal/silicide (several nanometers thick), as a more practical solution for IQE enhancement, has been researched theoretically and experimentally [6,17,18], and this enhancement can also be attributed to the hot carrier reflections, which occurs in the two borders of the ultrathin film. To get high light absorptances in ultrathin films, resonance optical structures are usually used to increase the light-matter interaction lengths. For the surface-illuminated type Schottky PD, the resonance optical structures for absorptance enhancement include Fabry-Perot (F-P) cavities [19], gratings [20], and optical antennas. The F-P resonance can increase the absorptance in both the thick [21] and ultrathin [6,17] metals/silicides to 60% ~near 100% at resonance wavelength. The metallic gratings, as one-dimensional periodic metallic structures, can convert the vertical incident light to the horizontal resonance Surface Plasmon Polariton Bloch Waves (SPP-BWs) supported by the metal-semiconductor surfaces, and then light-matter interaction lengths increase strongly. Absorptances of 80%~100% at resonance wavelengths are also achieved in grating structures [20] and two-dimensional periodic metallic structures [22]. By utilizing localized surface plasmon resonance [22–24], SPP-BW [22], and standing-wave resonances of short-range SPPs [25], the optical antennas can concentrate the light at resonance wavelength in the absorption areas, and then improve the absorptances. In most researches mentioned above, the metals are not thin enough to increase the low IQEs (on the order of 1% [2]). In an exceptional device in [6], the combination of 2 nm thick PtSi film and F-P cavity provides a high responsivity of 0.25 A/W at 1500 nm, but the device should operate at 40 K. The low ΦB leads to a large dark current density, on the other hand, as bulk devices, the F-P cavity and


grating structures have too large contact areas (>100 μm2) which are bad for the dark current suppression. While the surface-illuminated Schottky PDs has unique application in imaging and other areas [2], the waveguide Schottky PDs can be used in the integrated Si photonic circuits. The active areas of less than 1 μm2 have been reported in integrated Schottky PDs based on SOI nanowire waveguides [26,27]. It should be noted that the compact Schottky areas are important for lowering the dark currents and the power consumptions. In theory, the waveguide Schottky PD can absorb all the optical power of the propagating bound modes. However, the performance improvements of waveguide Schottky PDs still face challenges. Responsivities of most the reported waveguide Schottky PDs are below 0.1 A/W [4,5,26,28,29]. A recent record-high responsivity of up to 0.12 A/W at 1550 nm was measured [27]. Following the mode nomenclature of [30], the ssb0 mode (known as the longrange SPP) with low optical confinement can achieve near 100% end-fire coupling efficiency [31,32], however, its low mode power attenuation (MPA) [29] leads to a long absorption length (about 500 μm) [31,32], and then the Schottky contact area of about 56~500 μm2 is too large. sab0 mode with high optical confinement in metal can provide high MPA to reduce the absorption length, however, its high optical confinement leads to low end-fire coupling efficiency (~20%) [29], which limits its absorptance (and thus responsivity). In short, the existing devices suffer from the trade-off among the absorptance, dark current, and operation speed. When using ultrathin metals/silicides to improve IQEs, the high absorptance seems to be more difficult to realize. Since the MPAs of the propagating mode in waveguides with ultrathin silicides are too low, micro-ring resonators (MRRs) [33,34] with high Q-factor are used (to ensure long photon lifetime), but the effective bandwidth of devices is limited in the meantime, as a result, the bandwidth-efficiency product in [33] is limited to ∼10.5 GHz. Besides, given that the resonance wavelengths are very sensitive to the specific material characteristics and device geometries in fabrication, the drift of the narrow response spectrums of the high-Q MRRs remains a problem for specific wavelength operation. Zhu et.al [28]. have proposed and optimized a horizontal metal-insulator-silicon-insulator-metal nanoplasmonic slot waveguide with ultrathin silicides (to get high IQE), and an optimized TaSi2 detector was estimated to have responsivity of 0.07 A/W, speed of 60 GHz, and dark current of 66 nA at room temperature. In this work, by realizing enhanced light absorption in tiny volume [35], we propose the waveguide Schottky PD with high performances in terms of responsivity, dark current, and 3dB bandwidth simultaneously. In the proposed plasmonic waveguides, the ultrathin metals /silicides (to ensure high IQEs) are integrated to the Si nanowire waveguides, which are widely used in the integrated Si photonic circuits [11]. In Section 2, the plasmonic waveguide structure is presented and the IQEs are calculated. In this waveguide, the silicides are classified to metal-like and non-metal-like type. In Section 3, the mode characteristics of plasmonic waveguide with metals/metal-like silicides are analyzed. We present the mode hybridization between aab0 mode and quasi-TE mode for the first time, to best of our knowledge. By utilizing the aab0-quasi-TE hybrid modes (plasmonic-photonic hybrid modes), our structure can provide a high absorptance of 95.6% in 5 nm thick Au stripe with area of only 0.14 μm2, without using any resonance structure. The high absorptance and large IQE of 12.2% result in large responsivity of 0.146 A/W. Besides, the small contact area brings low dark current of 8.03 nA at room temperature and a large 3dB bandwidth of 88 GHz, which also benefits from no resonance structure adoption. In Section 4, the non-metal-like silicides are studied. Quasi-TM mode should be used in this case. The optimized responsivity of 2-nm PtSi device is estimated to be 0.71 A/W (benefits from low Schottky barrier height), but the large device area leads to a high dark current of 35.9 μA. All these structures can be fabricated by mature technologies with acceptable fabrication tolerance. The conclusion is made at last. While the Schottky PDs with two-dimensional materials induced have been reported with good performances [36], our study can promote the performance improvements


of the pure integrated Schottky PDs, moreover, we hope our mode analysis can inspire the researches of the two-dimensional materials induced devices. 2. IQE evaluation and device structure The responsivity Resp of a Schottky PD can be presented by:

Resp = ηe

e e = Aηi , hν hν

(1)

where q is the unit charge, h is Plank’s constant, ν is the optical frequency, ηe and ηi are the external quantum efficiency (EQE) and IQE, respectively. Different physical models [18,37– 40] have been proposed to describe the IPE process, and evaluate the IQEs. In the mode proposed [18] by Elabd and Kosonocky, the hot carriers are assumed to reflect between the internal metal surfaces, during when hot carriers get chance to be emitted over the single Schottky barrier, and the inelastic scattering mechanisms are taken into account via a hot carrier attenuation length (L). Scales et al. [17] extended this thin-film reflection model, deriving the escape probability through a double Schottky barrier. The thin-film reflection model can be fit to the experiment results with varied L [17], and have be used in many thinfilm waveguide Schottky PD studies [28,32,33]. It should be noted that this model is rigorously valid only for temperature of 0 K, but can also result very approximate for estimating the IQEs at room temperature. In this paper, this model is used for IQE calculation of thin film Schottky PD with single barrier at room temperature. In Table 1, L and ΦB of the common metals/silicides used in our calculations are listed. Only p-Si is considered. Table 1. Material properties of several common metals/silicides Material

Refractive Indexa

Si Doping

ΦB (ev)b

9.81

p

0.34

55

16

p

0.58

100*

6.62

p

0.415

9*

n

k

Au

0.559

Al

1.44

CoSi2

0.91

Pd2Si 3.75 5.14 p 0.337 PtSi 3.09 4.25 p 0.208 a: Refractive index data [41]: for Au and Al [28], for Silicides, wavelength: 1550 nm. b&c: Data [29]: for Au and Al [31], for CoSi2, and [17] for Pd2Si and PtSi. *: Unknown L for holes is assumed as the same value for electron.

L (nm)c

39 150

The film thickness is denoted by t. As Fig. 1 shows, the IQE is very sensitive to L and ΦB. When ΦB is not low enough, the decrease of t has little promotion on the IQE improvement, e.g., the IQE of p-Si/Al Schottky PD increase from 1.1% to 1.8% when t decreases from 50 nm to 5 nm. As for device using Au, the IQEs are 6%, 12.2%, and 16.8% for t = 25 nm, 5 nm, and 3 nm, respectively. In p-Si/PtSi PD with t = 2 nm, theoretical IQE can be as high as 60%, but the dark current density is also high due to the low ΦB.


Fig. 1. Calculated IQE versus t/L and ΦB by model in [17,18], λ = 1550 nm.

In this work, the plasmonic waveguide proposed by us is constructed by Si nanowire waveguide with ultrathin metal/silicide stripe covered, as Fig. 2 shows. The waveguide crosssections are slightly different between metals and silicides because of different fabrication methods [42] (see Fig. 2(b)-2(c)). The width and height of Si core are fixed to 600 nm and 220 nm in the latter calculations. The metal/silicide stripe consists of a linear tapered part and a rectangle part, whose lengths are respectively denoted by ls and lt. wm and hm (