PIC-integrable, uniformly tensile-strained Ge-on-insulator photodiodes enabled by recessed SiN<sub>x</sub> stressor
Yiding Lin, Danhao Ma, Kwang Hong Lee, Rui‐Tao Wen, Govindo J. Syaranamual, Lionel C. Kimerling, Chuan Seng Tan, Jürgen Michel
Abstract
Mechanical strain engineering has been promising for many integrated photonic applications. However, for the engineering of a material electronic bandgap, a trade-off exists between the strain uniformity and the integration compatibility with photonic-integrated circuits (PICs). Herein, we adopted a straightforward recess-type design of a silicon nitride ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>SiN</mml:mi> </mml:mrow> <mml:mi>x</mml:mi> </mml:msub> </mml:mrow> </mml:math> ) stressor to achieve a uniform strain with enhanced magnitude in the material of interest on PICs. Normal-incidence, uniformly 0.56% tensile strained germanium (Ge)-on-insulator (GOI) metal-semiconductor-metal photodiodes were demonstrated, using the recessed stressor with 750 MPa tensile stress. The device exhibits a responsivity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:mn>1.84</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.15</mml:mn> <mml:mtext> </mml:mtext> <mml:mi mathvariant="normal">A</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">W</mml:mi> </mml:mrow> </mml:math> at 1550 nm. The extracted Ge absorption coefficient is enhanced by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m3"> <mml:mrow> <mml:mo form="prefix">∼</mml:mo> <mml:mn>3.2</mml:mn> <mml:mo form="postfix">×</mml:mo> </mml:mrow> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m4"> <mml:mrow> <mml:mn>8340</mml:mn> <mml:mtext> </mml:mtext> <mml:msup> <mml:mrow> <mml:mi>cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> at 1612 nm and is superior to that of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m5"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>In</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.53</mml:mn> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>Ga</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.47</mml:mn> </mml:mrow> </mml:msub> <mml:mi>As</mml:mi> </mml:mrow> </mml:math> up to 1630 nm limited by the measurement spectrum. Compared with the nonrecess strained device, additional absorption coefficient improvement of 10%–20% in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m6"> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> </mml:math> -band and 40%–60% in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m7"> <mml:mrow> <mml:mi mathvariant="normal">L</mml:mi> </mml:mrow> </mml:math> -band was observed. This work facilitates the recess-strained GOI photodiodes for free-space PIC applications and paves the way for various (e.g., Ge, GeSn or III-V based) uniformly strained photonic devices on PICs.