cay times for both $X^-$ and $X^0$ as a function of temperature. At $B$=14 T and $T$=0.5 K, there is a large difference between the decay times for $X^-$ and $X^0$ excitons. This difference rapidly diminishes for increasing temperature, and for $T\simeq$ 3 K the two decay times are nearly identical. Data for $B$=6 T show similar behavior but the effects are smaller. These behaviors can be explained by considering the thermal ionization of $X^-$. There are two competing channels for $X^-$ decay, namely $$\begin{aligned}
X^- & \rightarrow & h\nu_- + e
\label{direct} \\
X^- & \rightarrow & X^0 + e  \rightarrow h\nu_0 +e ,
\label{ionization}\end{aligned}$$ where $h\nu_-$ and $h\nu_0$ are the corresponding photons energies. These decay channels are illustrated in Fig. 3(b). In the direct process (\[direct\]), an electron recombines with the hole but leaves the remaining electron with excess energy. In the ionization process (\[ionization\]), $X^-$ first loses the excess electron to the electron gas by thermal excitation, followed by a rapid $X^0$ recombination. At low temperatures, the direct process is not thermally activated, and most $X^-$ recombine directly with a long decay time. At elevated temperatures, in contrast, the indirect decay channel (\[ionization\]) also becomes available for $X^-$, resulting in a rapid decrease of the $X^-$ population.

The present work points out that charged excitons in a QW are confined by a perpendicular magnetic field to QDs having a size on the order of the cyclotron radius. These QDs are magnetically-adjustable and their volume is inversely proportional to the magnetic field. The large increase in radiative lifetime of the magnetically-confined charged exciton is attributed to a decrease in its localization volume. Similar changes in radiative lifetimes are expected for other QW systems in high magnetic fields, and for nanofabricated QD structures.

We thank G. Favrot and D. Broido for discussions on the Luttinger model. This work was supported by the National Science Fo