Cyclotron effective mass of a two-dimensional electron layer at the GaAs/AlxGa1-xAs heterojunction subject to in-plane magnetic fields.

We have found that Fermi contours of a two-dimensional electron gas at $\rmGaAs/Al_xGa_{1-x}As$ interface deviate from a standard circular shape under the combined influence of an approximately triangular confining potential and the strong in-plane magnetic field. The distortion of a Fermi contour manifests itself through an increase of the electron effective cyclotron mass which has been measured by the cyclotron resonance in the far-infrared transmission spectra and by the thermal damping of Shubnikov-de Haas oscillations in tilted magnetic fields with an in-plane component up to 5 T. The observed increase of the cyclotron effective mass reaches almost 5 \% of its zero field value which is in good agreement with results of a self-consistent calculation.