Formation of Ohmic Contact With Low Contact Resistance on n-GeSn by Fermi Level Depinning Using Plasma Treatment

Processes for Fermi level (FL) depinning on metal/n-GeSn (the doping concentration of 10
¹⁹
cm
^-3
with 4.5% Sn content) contact were investigated by plasma treatment on GeSn surface. Among CF
₄
, NH
₃
, and O
₂
plasma, O
₂
plasma treatment on GeSn surface prior to metal deposition is the most viable avenue to mitigate FL pinning in terms of ohmic-like behavior with improved reverse-biased current by a factor of 14300 as compared with the untreated samples, a low contact resistance (RC) of 3 × 10
^-7
Ω-cm
²
, and the Schottky barrier height (Φ
_BN
) of 0.12 eV for Al/n-GeSn, and it is primarily due to the formation of GeSnO
_x
, which possesses the desirable bandgap (E
_G
), conduction band offset (ΔE
_c
), and permittivity (κ) value with the capability to passivate the surface dangling bonds and consequently decreases the number of surface states. R
_C
and Φ
_BN
compare favorably with other schemes for FL depinning for Ge technology. Furthermore, the effectiveness of FL depinning is also evidenced by the S factor as high as 0.58. Besides the promising electrical properties, this plasma-based process can tolerate larger process variation and, therefore, a larger process window due to self-limiting thickness of the GeSnO
_x
. By combining these advantages, surface treatment by O
₂
plasma paves the promising path for emerging GeSn technology.