Results of the electric force microscopy (EFM) and photoluminescence (PL) measurements in a set of Si-doped GaN films are presented. The EFM measurements indicate that substantial surface states exist in GaN thin films and the density of surface states is reduced with the Si doping concentration. It is found that this result strongly correlates with the optical properties of the epifilms. We show that the ratio of near-band edge luminescence of the front-side and back-side PL spectra increases with the doping concentration. This behavior manifests the effects of the surface states on the luminescent properties of GaN. Our investigation therefore not only shows the important role of surface states on the optical properties of GaN, but it also reveals that the combination of EFM and optical techniques is a very powerful way to understand the physical properties of a thin film. GaN and related heterostructures are now well established as materials of importance to both science and technology for the development of high-temperature microelectronics, light emitters, and detectors [1,2]. Great attention was paid to the emission mechanism in the active layer of the emitting devices, both experimentally [3,4] and theoretically [5,6]. On the other hand, fewer investigations have focused on surface properties of GaN [7 -11], which are very important to the reliable performance for the device applications. The n-/p-type nature of GaN and InGaN layers, and their related band bendings in surface and interfaces have been studied by contactless electroreflectance [7]. It was found that the effect of surface layer is closely related to the optical properties of GaN upon annealing [8] or after surface treatment [9,10]. Besides, the existence of the surface state has been considered as the possible reason for the observation of persistent photoluminescence (PL) in GaN [11]. Recently, there have been several reports concerning the electric force microscopy (EFM) on GaN samples [12][13][14][15][16]. The surface potential mode of EFM is very sensitive to the polarity of the surface charge and has been used to image the inversion domains in GaN [12]. The piezoelectrically induced surface charge and potential can also be measured by EFM [13]. Accordingly, it is interesting to combine the EFM and optical techniques to study the surface properties of GaN, which are important to optical and electrical performance of GaN-based devices.In this article, we present EFM measurements and the results of the front-side and back-side PL measurements in a set of the Si-doped GaN epifilms. We demonstrate that EFM technique is able to detect the effect of Si doping on the surface properties of GaN films. Our experimental results reveal that substantial surface states exist in GaN epifilms, and they can be passivated by the Si incorporation. Our results also show that the defects near the surface are responsible for the inefficient band-edge emission observed in the PL spectra. In view of effects of surface related defects on the electrical a...

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