We present optical properties of low dimensional quantum dot-like GaN/AlGaN structures, which were obtained in a low-pressure MOVPE reactor. A primarily grown AlGaN layer had been exposed to SiH4. On the SiH4-treated surface GaN grew on distinctly separated island sites and was capped with a second AlGaN layer.
Photoluminescence (PL) shows two bright PL bands (P1, P2) and one weak (P3) PL band at energies of 3.48 eV, 3.44 eV and 3.35 eV, respectively. Micro-photoluminescence measurements reveal that these bands consist in fact of many sharp lines coming from small spots on the sample. Such behavior is characteristic for quantum dots or islands.
Time-resolved PL shows that the P1, P2 and P3 bands have lifetimes of about 0.1 ns, 0.5 ns and 1.3 ns, respectively. The P2 and P3 bands show significant spectral shifts with time. For example, the P2 band shifts from 3.445 eV at t = 0.1 ns down to 3.430 eV at t = 2.5 ns. As confirmed by power dependent measurements, the spectral shifts are partially due to state filling effects. State filling is usually observed in QDs due to their low density of states. During few hundreds of ps, electrons relax from higher to lower states and the average energy of the QD band decreases with time. Then PL emission from the ground states of QDs can be measured and analyzed. For the P2 band, at about 3.43 eV, a ground state decay rate of about 2.4 ns-1 has been determined.
Temperature dependent PL measurements show S-shape dependence of the PL energy vs temperature for the P2 band. On the other hand, the P1 peak shifts monotonically to lower energy in agreement with the energy band-gap shrinkage. The time-resolved measurement shows the mechanism of this effect. Up to 50 K, increase of temperature leads only to promotion of some electrons to high-energy states. But above 50 K the decays are faster due to the thermal escape of carriers. Since the thermal escape is faster in the case of high-energy states, the average PL energy decrease.