Electron-spin-resonance-scanning tunneling microscopy ͑ESR͒-͑STM͒ of iron atoms in silicon was observed: Si͑111͒ surfaces were covered with iron atoms and annealed. This gives surfaces covered with small islands of different silicide phases. We could detect an ESR-STM signal that corresponds to a g value of 2.07. The signal was split by magnetic field modulation, and phase-sensitive detection was applied. This shows that electron-spin-resonance ͑ESR͒-STM signals can be detected on a surface that is imaged with atomic resolution and that it is possible to observe a g 2 spin center. Getting absorption ͑instead of the expected derivative͒ line shapes with phase-sensitive detection is explained by the asymmetry in the line shape, which is a result of a rapid scan of the signal. Then, when the integration time of the phase sensitive detector is close to the time it takes to sweep the linewidth of the signal, absorption line shapes are observed.

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