Near-infrared Ge Photodetectors Fabricated on Si Substrates with CMOS Technology
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Near-infrared Ge Photodetectors Fabricated on Si Substrates with CMOS Technology Douglas D. Cannon, Samerkhae Jongthammanurak, Jifeng Liu, David T. Danielson, Kazumi Wada, Jurgen Michel, and Lionel C. Kimerling Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, MA 02139 ABSTRACT We have fabricated the first CMOS process compatible high-responsivity Ge p-i-n diodes for 1.55 µm wavelengths. The thermal expansion mismatch between Ge epilayers and Si substrates was used to engineer tensile strain upon cooling from the growth temperature. This 0.2% tensile strain results in a lowering of the direct transition energy in Ge by 30 meV and extends the responsivity curve to near 1.6µm. INTRODUCTION Photoreceivers are an integral part of optical communications networks. In current networks, a signal is often transduced from optical to electrical and back to optical several times before reaching its final destination. Ge is an ideal candidate for low cost, high performance photodetectors because of its compatibility with existing silicon processing, its high carrier mobility, and its high optical absorption at telecommunications wavelengths [1]. The first low temperature UHV-CVD heteroepitaxy of SiGe on Si was demonstrated by Meyerson in 1986 [2]. Fitzgerald developed high quality SiGe graded buffers on Si by UHVCVD [3]. Luan and coworkers have since demonstrated the ability to greatly reduce the dislocation density of thick Ge films (grown directly on Si by UHV-CVD) through subsequent cyclic annealing [4]. Photodiodes fabricated from annealed films exhibit much higher responsivity than diodes made from unannealed films [5]. In this report we have demonstrated the first CMOS-compatible fabrication of Ge photodiodes from epitaxial Ge films grown directly on Si by UHV-CVD. The leakage current of the diodes is small with no isolation of the photodiodes. The responsivity of the photodiodes is shifted to longer wavelengths than for diodes fabricated from bulk Ge. This shift is due to a strain-induced band gap shrinkage in the Ge resulting from the growth process. EXPERIMENTAL DETAILS The experiments can be divided into three main subsections: growth, processing, and characterization. First, high-quality Ge films were grown epitaxially directly on Si substrates without buffer layers by means of UHV-CVD and cyclically annealed to improve film quality [4]. Boron doped p-type Si wafers with a resistivity of less than 0.002 Ω-cm were used as the substrate material. Films grown at 600 C to 800 degrees Celsius were used in this study, with thicknesses of approximately 1 µm. Cyclic annealing was performed prior to device fabrication. CMOS compatible photodiodes were fabricated from these films resulting in the diode structure shown in Figure 1 (a,b). First, an SiO2 layer was deposited on the film for Ge passivation and to act as an implant mask. Next, windows were opened to expose the underlying
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