Direct observation of above-quantum-step quasibound states in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{u}\mathrm{u}\mathrm{m}$ heterostructures

In this paper we demonstrate the existence of above-quantum-step quasibound states in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.24}{\mathrm{Ga}}_{0.76}\mathrm{A}\mathrm{s}/\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{u}\mathrm{u}\mathrm{m}$ double heterostructures using a modulated photoreflection spectroscopy technique at room temperature. A series of clearly resolved structures was found in the spectra. Their energy positions agree well with those of transitions between quasibound states in the state density of the heteorostructure. Further envelope wave-function analysis indicates that these quasibound states mainly center in the ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.24}{\mathrm{Ga}}_{0.76}\mathrm{As}$ quantum-step region, and are found to be affected by the limited length of electron coherence.