  One-body mechanisms of spin splitting of the energy spectrum of 2D electrons
in a one-side doped (001) GaAs/Al$_x$Ga$_{1-x}$As quantum well have been
studied theoretically and experimentally. The interfacial spin splitting has
been shown to compensate (enhance) considerably the contribution of the bulk
Dresselhaus (Bychkov-Rashba) mechanism. The theoretical approach is based on
the solution of the effective mass equation in a quasitriangular well
supplemented by a new boundary condition at a high and atomically sharp
heterobarrier. The model takes into account the spin-orbit interaction of
electrons with both bulk and interfacial crystal potential having C$_{2v}$
symmetry, as well as the lack of inversion symmetry and nonparabolicity of the
conduction band in GaAs. The effective 2D spin Hamiltonian including both bulk
and interfacial contributions to the Dresselhaus ($\alpha_{BIA}$) and Rashba
($\alpha_{SIA}$) constants has been derived. The analytical relation between
these constants and the components of the anisotropic nonlinear $g$-factor
tensor in an oblique quantizing magnetic field has been found. The experimental
approach is based, on one hand, on the detection of electron spin resonance in
the microwave range and, on the other hand, on photoluminescence measurements
of the nonparabolicity parameter. The interfacial contributions to
$\alpha_{BIA}$ and $\alpha_{SIA}$ have been found from comparison with the
theory.
