{"id":3194,"date":"2011-06-28T19:19:32","date_gmt":"2011-06-28T19:19:32","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2011\/?p=3194"},"modified":"2011-07-19T20:17:33","modified_gmt":"2011-07-19T20:17:33","slug":"the-effect-of-a-nitrogen-anneal-on-the-dark-current-of-ge-photodiodes-2","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2011\/the-effect-of-a-nitrogen-anneal-on-the-dark-current-of-ge-photodiodes-2\/","title":{"rendered":"The Effect of a Nitrogen Anneal on the Dark Current of Ge Photodiodes"},"content":{"rendered":"

Germanium is a promising candidate for use in CMOS-compatible photodiodes.\u00a0 Its strong absorption in the 1.55-\u00b5m range and relative ease of integration on silicon substrates make it suitable for high-speed electronic photonic integrated circuits as well as in infrared sensors.\u00a0 To reduce power consumption and improve the signal-to-noise ratio, the diodes must have a low leakage current in reverse bias.\u00a0 This study has investigated the leakage current of germanium photodiodes grown by low-pressure chemical vapor deposition (LPCVD) using an Applied Materials epitaxial reactor. \u00a0\u00a0This study specifically looks at the effect of a post-metallization nitrogen anneal on the dark current.<\/p>\n

To fabricate these diodes, germanium was grown epitaxially on a p+<\/em> Si substrate.\u00a0 The wafers then received an in-situ<\/em> cyclic anneal to reduce the threading dislocation density.\u00a0 The wafers were subsequently implanted with phosphorus to create a vertical pin<\/em> junction, passivated with a low temperature oxide, and contacted with metal.\u00a0 Following metallization, the wafers saw an optional anneal in nitrogen for 45 minutes at varying temperatures.\u00a0 Figure 1 shows the current v. voltage characteristics for a 10- x 10-\u03bcm square device at different temperatures.\u00a0 The dark current is reduced from 10 \u03bcA for diodes without an anneal to 8 nA for a sample with a 400\u00b0C anneal.\u00a0 To further investigate this effect, Ge-on-Si capacitors were made with LTO as their dielectric.\u00a0 They were annealed at the same temperatures as the diodes.\u00a0 Figure 2 shows the capacitance v. voltage measurements for different annealing temperatures.\u00a0 The anneal shifts the flatband voltage to the right, indicating that the anneal changes the fixed charge in the LTO.\u00a0 Without an anneal, the Ge surface is depleted, causing a high surface recombination velocity.\u00a0 After an anneal at 400\u00b0C, the surface is accumulated with holes, which reduces the recombination velocity and decreases the dark current.<\/p>\n\n\t\t