Germanium (Ge) photodetectors (PDs) are promising optoelectronics device for advanced Si photonics and optical interconnection due to excellent absorption property, near-infrared wavelengths, and process compatibility with Si-based complementary metal-oxide-semiconductor (CMOS). Ge metal-semiconductor-metal (MSM) PDs have many advantages over Ge PIN PDs in terms of low capacitance, high bandwidth, and process simplicity. However, Ge MSM PDs have large dark current because of a low hole barrier height induced by severe Fermi-level pinning between the metal and Ge, which results in poor sensitivity and large power consumption of PDs.

In this work, the dark current of Ge MSM PD was tremendously reduced by introducing asymmetric metal-interlayer-semiconductor-metal (MISM) structure [Fig. 1]. As shown in Fig 2, a thin TiO₂ layer was selected as the interlayer to reduce hole dark current by enhancing the hole Schottky barrier and the proposed structure can collect the photo current efficiently due to almost zero conduction band offset between TiO₂ and Ge.

Low p-type Ge wafer was used to minimize bulk defect current. To form MISM structure, TiO₂ interlayer was deposited by atomic layer deposition (ALD) system at process temperature of 250 °C. To increase responsivity, finger-type PD was used with 10 um finger spacing and 10 um finger width. Metal-semiconductor (MS) contact region is defined by e-beam lithography and etched by 6:1 buffered HF, then metal-interlayer-semiconductor (MIS) contact region is defined by e-beam lithography. Cu contact was formed by e-beam evaporation and lift off process.

As a result, MISM structure exhibits tremendous dark current reduction by approximately 4,000 times at a bias voltage of 1 V. The dark current reduces as the TiO₂ thickness increases as shown in Fig. 3. In case of 1 nm-thick TiO₂ interlayer the dark current is hardly reduced compared to symmetric MSM structure because most of holes can flow through 1 nm-thick TiO₂ interlayer by tunneling. The hole dark current from the metal side diminishes as TiO₂ thickness increases because hole tunneling probability decreases. The dark current with 3 nm-thick and 5 nm-thick TiO₂ is reduced by 64 times and 3,970 times at 1 V, respectively, from symmetric MSM structure. As shown in Fig. 4, photo I-V measurement was carried out on asymmetric Ge MISM PD with 5 nm-thick TiO₂ interlayer with collimated infrared laser (λ=1.55 um). Measured photo current value was 0.2 mA and it was saturated at 0.6 V, and the sensitivity of the PD was 24.52 at 1 V. 

In summary, we tremendously suppress dark current by approximately 4,000 times at 1 V using MISM structure. TiO₂ interlayer of asymmetric MISM structure efficiently decrease hole dark current, and this structure can be promising structure for dark-current suppression in Ge MSM PD.