Dielectric films as a surface passivation layer or a gate insulator layer, have possibility of wide applications for GaN-based devices. However, dry processes generally used for formation of those films tend to involve the GaN surface in high energies induced by plasma, high temperature, etc. In this paper, we report on formation and characterization of thin native-oxide layer on GaN by a photoelectrochemical process. The process has advantages over dry process such as low energy reactions, low damage to surface and being performable in room temperature without vacuum environment.
We used n-type GaN layers grown on sapphire substrates by MOVPE. The electron concentration and mobility of GaN were 1 x 1016cm-3 and 200~300 cm2V-1s-1, respectively. The wet oxidation of GaN was performed in a three-electrode electrochemical cell. As an electrolyte, a mixture of 3% tartaric acid solution and propylene glycol with a ratio of 1:2 was used. The GaN surface was oxidized in constant-voltage or constant-current mode. Holes which are essential for electrochemical reactions were generated by UV illumination.
In the constant-voltage mode (5- 20 V), the SEM and AFM characterizations showed that rough oxide layer was formed on GaN surface. From the x-ray photoelectron spectroscopy analysis, we found that metallic Ga components were formed in the oxide layer or at the oxide-GaN interface. These features probably arise from nonuniform oxidation caused by a localized concentration of electric field at the GaN surface under constant voltage supply.
To reduce such an impact on the surface at the initial stage of the electrochemical reaction, we adopted the constant-current process with the bias current of 0.5 mA/cm2. From AFM observation, it was shown that the oxidized surface was smooth with RMS roughness of 1.6nm. The composition of oxide was very close to Ga2O3. The formation of such native oxide greatly enhanced the band-edge PL intensity of GaN, indicating the passivation effect on the GaN surface. Thus, the present electrochemical process with the constant-current mode is promising for formation of uniform native-oxide layer on GaN surface, applicable to an interfacial control layer or a thin insulated gate in GaN-based devices.
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