Electomodulated spectroscopy [photoreflectance (PR) and contactless electroreflectance (CER)] is known as a very sensitive and nondestructive absorption-like technique to investigate the optical transitions between the ground and excited states in various semiconductor structures including bulklike layers, quantum wells (QW) and quantum dots. Due to the Franz-Keldysh effect this technique can be also used to investigate the built-in electric field as well as the Fermi-level position at semiconductor surface and/or inside semiconductor structures. In this presentation the application of PR and CER techniques to study III-nitride structures will be presented. Two kind of semiconductor heterostructures will be discussed in this presentation. The first structures are GaN-based quantum wells which are dedicated for green/blue lasers and Ga(In)N/Al(In)N QW-based intersubband modulators. For these structures the optical transitions between the ground and excited states have been measured by contactless electroreflectance and compared with theoretical calculations performed within the electron effective mass approximation. For the second kind of GaN-based heterostructures (AlGaN/GaN transistor structures and so called Van Hoof structures) Franz-Keldysh oscillations were measured by PR and CER techniques. From the period of these oscillations the value of built-in eclectic field and/or the Fermi level position have been determined as well as the type of surface band bending has been identified. Obtained results will be discussed in the context of strong polarization effects in III-nitrides and their screening by the intentional and unintentional doping.

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