Structural, electrical, and magneto-optical characterization of paramagnetic GaMnAs quantum wells

The growth of GaMnAs by molecular beam epitaxy is typically performed at low substrate temperatures $(\ensuremath{\sim}250\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and high As overpressures, leading to the incorporation of excess As and Mn interstitials, which quench optical signals such as photoluminescence (PL). We report on optical-quality ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}∕{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum wells (QWs) with $xl0.2%$ grown at a substrate temperature of $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Electrical and structural measurements demonstrate that this elevated temperature reduces As defects while allowing the substitutional incorporation of Mn into Ga sites. From a combination of Hall and secondary ion mass spectroscopy measurements we estimate that at least 70%--90% of the Mn incorporates substitutionally in all samples studied. The incorporation behavior shows both a substrate temperature and QW width dependence. The low defect density of these heterostructures, compared to typical lower-temperature-grown GaMnAs, enables the observation of both polarization-resolved PL and coherent electron spin dynamics, from which the conduction-band exchange parameter is extracted. No evidence of long-range Mn spin coupling is observed, whereas negative effective Curie temperatures indicate spin heating due to photoexcitation. Light Mn doping maximizes the electron spin lifetime, indicating the importance of the Dyakonov-Perel decoherence mechanism in these structures. PL spectra reveal a low-energy peak from shallow donors, which, because of the paramagnetic behavior of its PL polarization, we ascribe to Mn interstitials.