Observation of tunable two-dimensional electron gas in Sn/Si(111)- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2</mml:mn> <mml:msqrt> <mml:mn>3</mml:mn> </mml:msqrt> <mml:mo>×</mml:mo> <mml:mn>2</mml:mn> <mml:msqrt> <

We employ cryogenic scanning tunneling microscopy to observe a tunable two-dimensional electron gas (2DEG) on the $\mathrm{Sn}/\mathrm{Si}(111)\text{\ensuremath{-}}2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3}\ensuremath{-}R{30}^{\ensuremath{\circ}}$ surface, where its properties are modulated by the underlying Si(111) substrates. The 2DEG resides near the conduction band edge of the Sn overlayer, which features a band gap of approximately 0.65 eV. By analyzing quasiparticle interference patterns induced by the 2DEG, we determine its energy dispersion and reveal a variation in effective mass with the Si substrate doping. Moreover, quantum interference fringes are found in spatially confined regions of the 2DEG, from which consistent band parameters are deduced. This study advances our understanding of the 2DEG and associated electronic properties in heterostructures.