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We have proposed a new approach to tune the operation wavelength of Franz-Keldysh Ge electro-absorption modulation in Si photonics by controlling the local strain environment to cover the whole range of C + L bands (1.53 – 1.62 μm). The present paper shows a proof of strain-tuning modulator concept by the shift of the Ge absorption edge using SiNx stressor films and Franz-Keldysh effect in strain-controlled Ge.
©2013 Optical Society of America
1. Introduction
Si photonics is an enabler of high-speed data communications as well as high performance computing. Optical modulators are currently one of the most critical devices and have achieved significant progress. However, there are still significant issues yet to be solved such as miniaturization of Mach-Zehnder modulator for high energy efficiency, temperature thermal stabilization of microring modulator reducing energy efficiency. On the other hand, the Franz-Keldysh (FK) electro-absorption (EA) modulators have various advantages over these modulators, such as low energy per bit, thermal stability, etc. It has recently been reported that Ge has a FK coefficient as large as that of III-V compound semiconductors [1], and Ge EA modulators have been prototyped to verify such small and low power operation characteristics [2,3]. The challenge is, however, that its operation wavelength window is narrow (0.015 μm) which requires a couple of SiGe modulators with different alloy composition is needed to cover the whole C + L bands (1.53-1.62 μm) [2]. We have proposed a new approach to tune the Ge bandgap with strain to cover the whole C + L band [4]. Since elastic deformation changes semiconductor bandgap, strained Ge should be an excellent material of FK EA modulator for working at C band as well as L band. The benefit of the present concept is Ge epitaxially grown for photodetector should be used as modulator as well. Localized SiNx stressors on Ge photodetector for redshift in responsivity has been experimentally demonstrated [5]. This paper reproduces a redshift in absorption edge of Ge tensile-strained by silicon nitride film in term of k·p theory and deformation potentials, and the shift is induced by external reverse biasing and reproduced by the FK effect.
2. Theoretical background
We carry the following theoretical calculation to verify the concept. The effect of strain on bandgap can be simulated with k·p theory and deformation potentials [6,7]. The boundary conditions are following: A Ge(001) waveguide is infinitely long along [010] and is stressed along [100]. The strain along [010] should not be changed upon such stressing, and thus the strain distribution is easily obtained in two dimensions across the cross section of the Ge waveguide on Si with stressor films side-by-side. The strained Ge as noted above introduces non-degeneracy in the valence band by splitting light-holes (LH) and of heavy-holes (HH), resulting in two bandgaps, C-LH and C-HH, where C stands for electrons in the conduction band. In addition, Ge epilayers on Si are strained biaxial-tensile in as-grown state due to thermal mismatch in cooling [8]. The built-in strain was measured to be 0.17% in our epilayers. Figure 1 shows the absorption edge dependence on [100] uniaxial stress applied to Ge strained biaxially tensile. It predicts that the uniaxial stress application to Ge ranging from + 0.2 to −0.9 GPa should cover the whole C + L bands. However, when we also consider the transition between conduction band and heavy hole band, the stress requirement could be smaller, only from + 0.2 to −0.4 GPa.
Fig. 1 The theoretical relationship of the absorption edge of Ge with applied stress along [010] on Ge(100).
3. Experimental procedures
We designed and fabricated free space pin Ge photodetectors with silicon nitride (SiNx) stressors to verify the prediction. 400 nm-thick Ge(001) epilayers were grown on p+ Si(001) substrates at 600þC and 50 nm-thick Si cap layers were on it using ultra-high vacuum chemical vapor epitaxy (UHV-CVD). 300 nm-thick SiO2 films were deposited on these epilayers by plasma sputtering as a mask of implantation and as an insulator between Ge and electrode pads. Then phosphorus ions were implanted as a peak concentration of 1019 cm−3 to make pin diodes with 500 μm wide square regions. Phosphorus ions were implanted twice at 10 and 30 keV to make a good electric contact of the Si cap and to avoid hole accumulation at the interface between the Si-cap and Ge. The latter precisely applied the electric field by external biasing [9]. The samples were annealed at 600þC for 30 min. 500 nm-thick SiNx film was finally deposited using an Electron-Cyclotron Resonance plasma-enhanced Chemical Vapor Deposition (ECR-PECVD) method with SiH4 and N2 gases, and was then patterned into 500 μm long stripes of 500 nm width with intervals of 500 nm. This film has built-in stress of −760 MPa measured from the wafer warpage. Al was deposited for an electrical contact layer and then removed on SiNx, allowing free space light illuminated only on Ge under the SiNx stripes. The cross section of the Ge diodes is schematically shown in Fig. 2(a).
Fig. 2 (a) The schematics of cross-section of the strained Ge photodetector and the reference photodetector. (b) The stress profile of the strained Ge photodetector.
4. Strain induced absorption edge shift
The stress distributions in the Ge diodes were measured by scanning μ-Raman spectroscopy. We performed mapping measurement in the perpendicular direction to the SiNx stripe with 0.5 μm intervals. 457 nm laser was used with the penetration depth in Ge of 18.7 nm. Although Ge layer is 400 nm thick, we demonstrated that the depth profile is constant [4]. Tensile stress is generated under the SiNx stripes, and compressive stress between stripes as in Fig. 2(b) [10]. Typically, SiNx with tensile built-in stress would apply a uniaxial compressive strain to the underlying Ge [11]. It should be marked here that our SiNx film has compressive built-in stress; therefore, the Ge layer under the SiNx stripes was strained tensile. We have found 200 MPa of tensile stress under SiNx stressors, which predicts about 0.02 μm redshift in the Ge absorption edge, corresponding to the L band edge 1.62 μm as in Fig. 1.
The responsivity spectra of the Ge photodetectors were measured with and without the SiNx stressor stripes in the wavelength range from 1400 to 1700 nm with the constant laser power of 0.815 mW to prevent band flattening. Figure 3 shows typical responsivity spectra of the Ge photodetectors with and without the stressors. The experimental redshift of Ge photodetector with the SiNx stressor reproduces the theory prediction.
5. Franz-Keldysh absorption change in strained Ge
Figure 4 shows the responsivity spectra of the Ge photodetector with the SiNx stressors under different applied reverse bias and indicates the occurrence of an electro-absorption effect.
To analyze these data with the generalized Franz-Keldysh formalism [12], we have simulated the strength of the electric field from the profile by implanted P and p + Si using the one-dimensional finite-difference simulator (PC1D). The depth profile of the electric field strength is shown in Fig. 5(a). Since we implanted phosphorus at the Si cap and Ge interface, no electric field changes in the heavily P implanted region upon external biasing. Thus, no change in absorption should occur in the heavily P doped region by external biasing. In other words, the external bias only changes the electric field in the i-region located deeper from the diode surface. Taking the effect into account, we should modify the equation to express figure of merit (FOM), i.e., Δα/α, where Δα is the difference between the absorption coefficients with and without reverse bias and α is the absorption coefficient without reverse bias. FOM should be expressed as:
where ti is the thickness of the i-region and t is the thickness of the whole Ge layer. Δα/α(exp) is the FOM calculated directly from photocurrent spectra and Δα/α(real) is the one after calibration by Eq. (2). One should notice here is that in the electric filed profile, we still have some electric field slope in the region between the heavily doped region and i-region. For further improving precision of our simulation, we have divided it into many step regions shown in Fig. 5(a), and calculated the change of the absorption coefficient for each step. Then, we have plotted Δα/α(real) as a function of the applied reverse bias as w/ steps in Fig. 5(b). Here, we have considered a built-in electric field of the detector when no bias is applied. As in Fig. 5(b), while the theory without the consideration of many step regions and the experiment data are not it good agreement at high bias, theory reproduces the experimental data very well with the consideration of the electric field slope, confirming that the Franz-Keldysh effect dominates the electro-absorption of the intentionally strained Ge photodetectors.Fig. 5 (a) The electric field profile of the Ge photodetector with stressors. The depth profile is calculated using P and B profiles of the Ge epilayer. (b) Figure of merit Δα/α at wavelength of 1640 nm as a function of applied reverse bias.
6. Conclusions
The strain-tuning concept has been proven to show a redshift in the Ge absorption edge and the Frank-Keldysh formalism reproduced the experimental result. The stress-tuning concept we proposed for Ge EA modulators to cover the whole C + L bands. It should be marked that the thickness and width of the SiNx stripes will change the amount of strain in Ge and can be controlled in the back end process line.
Acknowledgments
A part of this research is granted by JSPS through FIRST Program initiated by CSTP. The samples were fabricated using an EB writer F5112 + VD01 in VLSI Design and Education Center (VDEC), the University of Tokyo, donated by ADVANTEST Corporation with the collaboration with Cadence Corporation.
References and links
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