OPTICAL INVESTIGATION OF THE ELECTRONIC-STRUCTURE OF SINGLE ULTRATHININAS LAYERS GROWN PSEUDOMORPHICALLY ON (100) AND (311)A GAAS SUBSTRATES

We investigate the electronic structure of a series of InAs-monolayer and submonolayer structures synthesized on both (100) and (311)A GaAs substrates by molecular-beam epitaxy. The chosen growth conditions ens ure excellent structural and optical quality of the samples. Consequen tly, we have the opportunity to analyze the electronic properties of t hese ultrathin InAs layers which we explore by high-resolution x-ray-d iffraction, photoluminescence (PL) and PL excitation (PLE) measurement s. InAs submonolayer structures with an InAs coverage down to 0.78 x 1 0(14) cm-2 give characteristic x-ray-interference spectra that evolve continuously with increasing In content. In contrast, the optical spec troscopies reveal a clear crossover from electronically disconnected I nAs clusters to quantum-well behavior at an InAs coverage of about 3 x 10(14) CM-2 for both orientations studied. The energies of the featur es observed in PL and PLE spectra of these structures are modeled as a function of InAs layer thickness using an eight-band k . p-type effec tive-mass theory. The valence-band anisotropy of the bulk compounds is reflected in differences between both growth directions. In addition, our results indicate that the band offset ratio at the strained InAs/ GaAs interface is orientation dependent.