Nonparabolic band effects in $\mathrm{GaAs}∕{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum dots and ultrathin quantum wells

We have investigated the optical properties of unstrained $\mathrm{GaAs}∕{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum dot/well systems with the aim of studying the influence of confinement on the effective exciton mass, as determined from the photoluminescence line shift in high magnetic fields $(⩽50\mathrm{T})$. The effective exciton mass is found to be more than twice the value for bulk GaAs. We attribute this to an enhanced nonparabolicity in the GaAs conduction band at the nanoscale.

Figure 1

Schematic drawing of the sample structure, together with a typical photoluminescence spectrum. The QDs, the ultrathin QW, and the lower barrier with varying thickness, $D$, are indicated. The QD thickness is also indicated and includes the QW thickness. The two peaks at higher energy come from the QW and QD structures, and are investigated here.

Figure 2

Field dependence of the QW and the QD PL energies in sample 2 $(D=7\mathrm{nm})$ fitted with Eq. (2). The crossover field, $B_{\mathrm{c}}$, is indicated in both cases. Note that the higher $B_c$ for the QDs indicates the presence of confinement in the plane of the sample.

Figure 3

Measured effective exciton masses, $\mu$, of the QWs and QDs compared with theoretical models for the effective electron mass, $m_e^{*}$, in QWs[10] and in QWRs.[13] An estimated maximum value for the QD effective electron mass is also plotted.