We report the first observation of magnetophonon resonances in the two-dimensional electronic system in single interface GaAs-AkGa^As heterojunctions and heterojunction superlattices. They result from inelastic scattering between Landau levels with resonant absorption of LO phonons of GaAs and yield a polaron mass m p^~ (0.077 ±0.004)^ .We report the first observation of magnetophonon resonances in the quasi two-dimensional (2D) electronic system, realized at the semiconductor-semiconductor interface of GaAs-Al xGa^jAs heterojunctions and GaAs-Al^Ga^As superlattices. The resonances, detected in the magnetic field dependence of the sample resistance, result from scattering of the 2D electrons with resonant absorption of phonons under the resonance condition 1where co 0 is the phonon frequency, B is the magnetic field, and m p * is the polaron mass. They reflect directly the interaction between the twodimensionally confined electrons and the lattice vibrations of the GaAs-Al^Ga^As heterostructures. Our results show that the dominant electron-phonon interaction in this system is that with the polar mode of GaAs and the strength is consistent with that expected for polar coupling of 2D electrons to the longitudinal optical (LO) phonons of bulk GaAs. The GaAs-Al 3C Ga 1 _ x As heterostructures were grown by molecular-beam epitaxy 2 on Cr-doped, semi-insulating GaAs substrates. The single interface hetero junction consists of a 6-/xm-thick, unintentionally doped, p-type GaAs layer (p~10 15 / cm 3 ) and a 0,3-jum-thick rc-type Al^Ga^^As layer, doped with Si to a concentration of ~5xl0 17 /cm 3 . The modulation-doped superlattice 3 consists of 150 layers of 320-A-thick Si-doped w-type Al xGa^^As. In both cases, the mole fraction of Al is x »0.25 and the conduction band discontinuity at the interface is A# c~3 00 meV. 4 The free carriers, resulting from ionized donors in the Al xGa 1 . x As layers, are confined along the GaAsAl^Gaj-j-As interface by the surface potential well, illustrated in Fig 0 1(a) for the single interface heterojunction, and to the GaAs layers by the potential well, illustrated in Fig. 1(b) for the superlattice. In either case, the electronic motion perpendicular to the layer is quantized and the electrons constitute a 2D system. 5,6 The samples were cut into standard "Hall bridges" and Ohmic contacts were made by alloying indium metal into the epilayers. The resistance R of the samples was measured as a function B at 4.2 K, when scattering is dominated by impurities, surface irregularities, and other interfacial defects, and at higher temperatures (-150 K), when phonon scattering dominates. In order to resolve the magnetophonon resonances, B is swept at a constant rate to allow the use of a simple RC differentiating circuit to obtain the derivatives d'R/dB and d 2 R/dB 2 . This method introduces a phase shift in the observed magnetooscillations, which can easily be corrected by measuring the derivatives in both the down and the up sweep of B. The high-magnetic-field measurements were carried out at ...