  We investigated chemical pressure effects in electron-doped (e-doped) FeSe by
fabricating an electric-double-layer structure with single crystalline FeSe
films on LaAlO$_3$ with Se substituted by isovalent Te and S. Our method
enables transport measurements of e-doped FeSe. Electron doping by applying
gate voltage of 5 V increases $T_{\mathrm c}$ of the FeSe$_{1-x}$Te$_x$ and
FeSe$_{1-y}$S$_y$ films with $0 \leq x \leq 0.4$ and $0 \leq y \leq 0.25$,
while the e-doped $x=0.5$ film showed lower $T_{\mathrm c}$ than that of the
undoped one. Both positive and chemical pressure suppress $T_{\mathrm c}$ of
the e-doped FeSe. The obtained superconducting phase diagram in the e-doped
samples is rather different from that in undoped samples. This might suggest
that the superconductivity mechanism is different between undoped and e-doped
systems. Alternatively, this is possibly explained by the absence of the
nematic order in e-doped samples.
