Germanium/Silicon-Germanium Heterostructure Avalanche Photodiodes on Silicon
Authors: Miyasaka Yuji , Hiraki Tatsurou , Tsuchizawa Tai , Wada Kazumi , Ishikawa Yasuhiko 
Published: February 2020

Near-infrared photodiodes (PDs) of Ge on Si have been widely studied in Si photonics for the optical communications (1.3–1.6 μm). Ge-based avalanche PDs (APDs) have been also studied for sensitive photodetections. There are mainly two different types of Ge APDs on Si. One utilizes Ge as the multiplication layer, i.e., Ge homojunction APDs [1], while the other one utilizes Si, i.e., Ge/Si heterojunction APDs [2]. Si is one of the best materials for the multiplication layer in terms of low excess noises, although relatively large applied voltage more than 20 V, corresponding to the electric field strength more than 300 kV/cm, is required to obtain the multiplication gain due to the small ionization coefficients. In order to realize low-noise and low-voltage Ge APDs, we have recently proposed a new structure utilizing Ge/graded-SiGe heterojunction multiplication layer [3]. As in Fig. 1(a), photo-generated holes are injected via the graded SiGe to the Ge multiplication layer. The band discontinuity at the graded-SiGe/Ge interface can enhance the impact ionization due to the sudden increase of kinetic energy across the interface. This is effective to realize low-noise and low-voltage Ge APDs [3]. In this work, such Ge/SiGe heterostructure APDs are fabricated, and the multiplication gain is measured. As a result, the growth temperature in ultrahigh-vacuum chemical vapor deposition (UHV-CVD) is found to be a key parameter to enhance the multiplication gain.
UHV-CVD was used to grow Ge/graded-SiGe/Ge heterostructures for free-space APDs on Si. GeH4 and Si2H6 were used as the source gases. First, 350-nm-thick undoped Ge layer was directly grown on p+-Si using low/high-temperature two-step growth. The Ge buffer layer (~50 nm) was grown at 370°C, followed by the growth at an increased temperature. As an important parameter, we compared the growth temperature, 510°C and 580°C. Then, a 24-nm-thick SiGe layer with the graded Ge composition from 70% to 100% was grown at the same temperature of 510°C or 580°C. Finally, a 50-nm-thick Ge layer was grown, and P atoms were implanted to form the n-Ge optical absorption layer. The valence band discontinuity at the bottom Si0.3Ge0.7/Ge interface is as large as 0.1 eV, assisting holes to reach the threshold energy (~1.0 eV) required for the impact ionization. As a reference, a Ge homojunction APD without the SiGe layer was also fabricated.
Figure 1(b) shows typical hole-initiated multiplication gain as a function of applied voltage. Photo-generated holes can be selectively injected from the top n-Ge absorption layer to the bottom SiGe/Ge multiplication layer, using the free-space illumination of 532 nm laser light whose penetration depth in Ge as large as 20 nm. The use of SiGe/Ge multiplication layer is found to increase the multiplication gain in comparison with the Ge homojunction APD, being effective for the low-voltage APD operation. Furthermore, the Ge/SiGe APDs formed at the lower growth temperature of 510°C showed larger multiplication gain than that for 580°C. This is ascribed to the formation of abrupt SiGe/Ge interface at the lower growth temperature, as confirmed by the x-ray diffraction measurements. Excess noise characteristics will be also presented.
[1] L. Virot et al., Nature Commun. 5, 4957 (2014).
[2] Y. Kang et al., Nature Photon. 3, 59 (2009).
[3] Y. Miyasaka et al., Jpn. J. Appl. Phys. 55, 04EH10 (2016).

Figure 1