Dian Lei

Large-scale two-dimensional MoS 2 photodetectors by magnetron sputtering

We report on the demonstration of photodetectors based on large scale two-dimensional molybdenum disulfide (MoS2) transition metal dichalcogenides. Excellent film uniformity and precise control of the MoS2 thickness down to a monolayer (~0.75nm) were achieved by magnetron sputtering synthesis approach. In particular, the photodetectors integrated with five MoS2 monolayers exhibit a high photoresponsivity of 1.8 A/W, an external quantum efficiency exceeding 260%, and a photodetectivity of ~5 x 10(8) Jones for a wavelength of 850 nm, surpassing the performance of mechanically exfoliated based photodetectors.

Germanium-Tin on Si Avalanche Photodiode: Device Design and Technology Demonstration

We report the demonstration of a Ge_0.95Sn_0.05 on silicon (Ge_0.95Sn_0.05/Si) avalanche photodiode (APD) having a separate-absorption-charge-multiplication structure, wherein a Ge_0.95Sn_0.05 layer and a Si layer function as an absorption layer and a multiplication layer, respectively. Material characterization was performed by atomic force microscopy, X-ray diffraction, and transmission electron microscopy. The dark current I_dark of the APD is dominated by the area-dependent bulk leakage rather than the surface leakage. The temperature dependence of breakdown voltage of the Ge_0.95Sn_0.05/Si APD was characterized and a thermal coefficient of 0.05% K^-1 was obtained, achieving a lower thermal sensitivity than the conventional III-V-based APDs. In the wavelength range of 1600-1630 nm, a responsivity of ~1 A/W (bias voltage V_bias = -9.8 V) was achieved due to the internal avalanche gain.

Ge0.83Sn0.17 p-channel metal-oxide-semiconductor field-effect transistors: Impact of sulfur passivation on gate stack quality

The effect of room temperature sulfur passivation of the surface of Ge0.83Sn0.17 prior to high-k dielectric (HfO2) deposition is investigated. X-ray photoelectron spectroscopy (XPS) was used to examine the chemical bonding at the interface of HfO2 and Ge0.83Sn0.17. Sulfur passivation is found to be effective in suppressing the formation of both Ge oxides and Sn oxides. A comparison of XPS results for sulfur-passivated and non-passivated Ge0.83Sn0.17 samples shows that sulfur passivation of the GeSn surface could also suppress the surface segregation of Sn atoms. In addition, sulfur passivation reduces the interface trap density Dit at the high-k dielectric/Ge0.83Sn0.17 interface from the valence band edge to the midgap of Ge0.83Sn0.17, as compared with a non-passivated control. The impact of the improved Dit is demonstrated in Ge0.83Sn0.17 p-channel metal-oxide-semiconductor field-effect transistors (p-MOSFETs). Ge0.83Sn0.17 p-MOSFETs with sulfur passivation show improved subthreshold swing S, intrinsic t...

Suppression of dark current in germanium-tin on silicon p-i-n photodiode by a silicon surface passivation technique

We demonstrate that a complementary metal-oxide-semiconductor (CMOS) compatible silicon (Si) surface passivation technique effectively suppress the dark current originating from the mesa sidewall of the Ge(0.95)Sn(0.05) on Si (Ge(0.95)Sn(0.05)/Si) p-i-n photodiode. Current-voltage (I-V) characteristics show that the sidewall surface passivation technique could reduce the surface leakage current density (Jsurf) of the photodiode by ~100 times. A low dark current density (Jdark) of 0.073 A/cm(2) at a bias voltage of -1 V is achieved, which is among the lowest reported values for Ge(1-x)Sn(x)/Si p-i-n photodiodes. Temperature-dependent I-V measurement is performed for the Si-passivated and non-passivated photodiodes, from which the activation energies of dark current are extracted to be 0.304 eV and 0.142 eV, respectively. In addition, the optical responsivity of the Ge(0.95)Sn(0.05)/Si p-i-n photodiodes to light signals with wavelengths ranging from 1510 nm to 1877 nm is reported.

GeSn-on-insulator substrate formed by direct wafer bonding

GeSn-on-insulator (GeSnOI) on Silicon (Si) substrate was realized using direct wafer bonding technique. This process involves the growth of Ge1-xSnx layer on a first Si (001) substrate (donor wafer) followed by the deposition of SiO2 on Ge1-xSnx, the bonding of the donor wafer to a second Si (001) substrate (handle wafer), and removal of the Si donor wafer. The GeSnOI material quality is investigated using high-resolution transmission electron microscopy, high-resolution X-ray diffraction (HRXRD), atomic-force microscopy, Raman spectroscopy, and spectroscopic ellipsometry. The Ge1-xSnx layer on GeSnOI substrate has a surface roughness of 1.90 nm, which is higher than that of the original Ge1-xSnx epilayer before transfer (surface roughness is 0.528 nm). The compressive strain of the Ge1-xSnx film in the GeSnOI is as low as 0.10% as confirmed using HRXRD and Raman spectroscopy.

InAlP-Capped (100) Ge nFETs with 1.06 nm EOT: Achieving record high peak mobility and first integration on 300 mm Si substrate

InAlP-capped Ge nFETs with sub-400 °C process modules were reported. Ge nFETs on Ge substrates with InAlP/Al2O3/HfO2 as gate dielectrics demonstrate the highest reported Ge (100) peak μeff for inversion mode devices. In addition, the gate stack with HfO2 directly deposited on the InAlP cap was implemented in Ge nFETs on 300 mm Si substrates for the first time. This leads to the realization of long-channel Ge nFETs with 1.06 nm EOT, high drive current, excellent S, and low gate leakage current. InAlP is a good passivation technique for Ge nFET gate stack formation, and could enable the use of Ge channel for both nFETs and pFETs in future high performance and low power logic applications.

Germanium-tin multiple quantum well on silicon avalanche photodiode for photodetection at two micron wavelength

We report the demonstration of a germanium-tin multiple quantum well (Ge0.9Sn0.1 MQW)-on-Si avalanche photodiode (APD) for light detection near the 2 μm wavelength range. The measured spectral response covers wavelengths from 1510 to 2003 nm. An optical responsivity of 0.33 A W−1 is achieved at 2003 nm due to the internal avalanche gain. In addition, a thermal coefficient of breakdown voltage is extracted to be 0.053% K−1 based on the temperature-dependent dark current measurement. As compared to the traditional 2 μm wavelength APDs, the Si-based APD is promising for its small excess noise factor, less stringent demand on temperature stability, and its compatibility with silicon technology.

Floating-base germanium-tin heterojunction phototransistor for high-efficiency photodetection in short-wave infrared range

The floating-base germanium-tin (Ge1-xSnx) heterojunction phototransistor (HPT) is designed and investigated as an efficient optical receiver in the short-wave infrared range. Simulations indicate that as the Sn content increases, the responsivity significantly increases due to a higher absorption coefficient and a larger valence band offset between Ge and Ge1-xSnx. Ge0.935Sn0.065 HPTs that incorporated high-quality Ge0.935Sn0.065 film grown by molecular beam epitaxy were fabricated, demonstrating optical response beyond wavelength of 2003 nm. At a low bias voltage of 1.0 V, optical response enhancement of ~10 times was achieved over the conventional Ge0.935Sn0.065 p-i-n photodiode. High responsivities of ~1.8 A/W at 1550 nm and ~0.043 A/W at 2003 nm were demonstrated with low dark current density of 0.147 A/cm2.

Two-micron-wavelength germanium-tin photodiodes with low dark current and gigahertz bandwidth

We report the demonstration of a germanium-tin (Ge0.9Sn0.1) multiple-quantum-well p-i-n photodiode on silicon (Si) substrate for 2 μm-wavelength light detection. Characterization of the photodetector in both direct current (DC) and radio frequency (RF) regimes was performed. At the bias voltage of -1 V, a dark current density of 0.031 A/cm2 is realized at room-temperature, which is among the lowest reported values for Ge1-xSnx-on-Si p-i-n photodiodes. In addition, for the first time, a 3 dB bandwidth (f3dB) of around 1.2 GHz is achieved in Ge1-xSnx photodetectors operating at 2 μm. It is anticipated that further device optimization would extend the f3dB to above 10 GHz.

Germanium-Tin on Silicon avalanche photodiode for short-wave infrared imaging

The first demonstration of Germanium-Tin on Silicon (Ge_1-xSn_x/Si) avalanche photodiode (APD) for short-wave infrared (SWIR) imaging is reported. The temperature dependence of breakdown voltage was characterized. An extracted thermal coefficient of 0.05% K^-1 indicates that the Ge_1-xSn_x/Si APD achieved a lower thermal sensitivity than conventional III-V-based APDs. At the wavelength λ of 1600 to 1630 nm, a responsivity of ~ 1 A/W (bias voltage V_bias = -9.7 V) was achieved due to the internal avalanche gain of Ge_1-xSn_x/Si/Si APD. The monolithic and CMOS-compatible Ge_1-xSn_x/Si APD presented here shows promise in SWIR imaging applications where low-cost and high sensitivity sensing arrays are needed.