Sub-$10^{-9}\ \Omega-\text{cm}^{2}$ Specific Contact Resistivity on P-type Ge and GeSn: In-situ Ga Doping with Ga Ion Implantation at 300 °C, 25 °C, and −100 °C

For the first time, Ga ion implantation (Ga I/I) on in-situ Ga-doped Ge (Ge:Ga) and GeSn (GeSn:Ga) films at various temperatures (300 °C, 25 °C, and -100 °C) was investigated. It is found that cryogenic (-100 °C) and room temperature (RT, 25°C) Ga I/I retains strain and the high quality of the GeSn layer after Ga activation while hot Ga I/I (300 °C) degrades the crystalline quality due to the implantation-induced defects. An ultra-low specific contact resistivity ρc of 8×10
^-10
Ω-cm
²
is achieved for Ti/p
⁺
-GeSn contact by in-situ Ga doping followed by cryogenic or RT Ga I/I while ρc increases to 2.3×10
^-9
Ω-cm
²
using hot Ga I/I. An ultra-low ρc of 9×10
^-10
Ω-cm
²
is also demonstrated for in-situ Ga-doped Ge followed by RT Ga I/I. This is the first realization of sub- 10
^-9
Ω-cm
²
ρc on non-laser-annealed p-type Ge. The sub· 10
^-9
Ω-cm
²
ρc is thermally stable up to an annealing temperature of 500 °C.