Strain-induced enhancement of near-infrared absorption in Ge epitaxial layers grown on Si substrate

Abstract

Epitaxially grown Ge layers on Si substrate are shown to reveal an enhanced absorption of near-infrared light, which is effective for the photodiode application in Si-based photonics. Ge layers as thick as 1μm were grown on Si substrate by ultrahigh-vacuum chemical-vapor deposition with a low-temperature buffer layer technique. X-ray-diffraction measurements showed that the Ge layer possesses a tensile strain as large as 0.2%, which is generated during the cooling from the high growth temperature due to the thermal-expansion mismatch between Ge and Si. Photoreflectance measurements showed that the tensile strain reduces the direct band-gap energy to 0.77 eV (c.f. 0.80 eV for unstrained Ge), as expected from the theory. Reflecting the band-gap narrowing, photodiodes fabricated using the Ge layer revealed an enhanced absorption of near-infrared light with the photon energy below 0.80 eV, i.e., with the wavelength above 1.55μm. This property is effective to apply the photodiodes to the L band (1.56–1.62μm) in the optical communications as well as the C band (1.53–1.56μm). It is shown that the experimental absorption spectrum agrees with the theoretical one taking into account the splitting of light-hole and heavy-hole valence bands accompanied by the band-gap narrowing. Based on the calculation, the performance of the photodiode using the tensile-strained Ge is discussed.

Synthesis of Ge1−xSnx alloys by ion implantation and pulsed laser melting: Towards a group IV direct bandgap material

“…Although at the level of 6:2 at: %Sn, one would not expect to achieve a direct bandgap alloy, the present material is applicable for a photodiode operating at the L band wavelength (k ¼ 1:56 -1:62 lm or E photon ¼ 0:77 -0:79 eV) of the optical communication network as has been suggested in Ref. 47.…”

“…The former phenomenon has been widely studied previously on other group IV semiconductors such as strained Si, strained Ge, and SiGe alloy. For GeSn alloys, such LH and HH splitting has also been illustrated by using other optical methods such as photoreflectivity 47 and photoluminescence, 48 but not previously by spectroscopic ellipsometry. Due to this band splitting, the mobility of holes in the material is expected to be enhanced as the scattering of holes between the light and heavy hole sub-band has been essentially eliminated.…”

“…These properties of the alloy indicate the possibility of fabricating a working photodiode operating at the L band of the optical communication network. 47 …”

Ge-on-Si photonic devices for photonic-electronic integration on a Si platform

“…It is noted that a tensile strain is induced in Ge grown on Si (or SOI) wafers [22,23,24,25]. As in Fig.…”

Design of low bias voltage Ge/SiGe multiple quantum wells electro-absorption modulator at 1550 nm

“…The insertion of a buffer layer followed by an anneal process at temperature of 750-850 • C is widely adopted to reduce dislocations and surface roughness. 11,15,16,36 The anneal FIG. 6.…”