Searching for Indirect Excitons in Coupled Double InGaN/GaN Quantum Wells
Résumé
In recent years indirect (dipolar) excitons have emerged as highly interesting objects for fundamental studies like Bose-Einstein condensation as well as a concept of excitonic devices for development of excitonic signal processing. An indirect exciton (IX) is a bound pair of an electron and a hole (e-h) in spatially separated quantum wells (QWs). Studies of IX have been concentrated almost entirely on (Al,Ga)As/GaAs coupled double QWs (CDQWs). Due to the large exciton binding energy polar heterostructures of Group III nitrides seem to be more suitable for observation of IX. In the present work we have studied IXs formed by a e-h pair in InGaN/GaN CDQWs. Very thin barrier of 1-2 atomic monolayer (ML) separating QWs enables a tunneling of electrons and holes after optical excitation to the adjacent QWs. The samples consisted of DQWs of In0.17Ga0.83N/In0.02Ga0.98N grown by MBE technique. Intended width of QWs was 2.6 nm and the barrier thickness varied between 1- 8 ML (025-2nm).
GaN crystals were used as substrates. The XRD and TEM measurements showed that the obtained structures agree well with the intended ones.
Modeling of their band structure using the NEXTNANO software revealed that the structures with the thinnest barriers (1-2 ML) formed one electron (hole) state located in the conduction (valence) band of adjacent QWs. The corresponding emission wavelength was λ≈510 nm. Simulations performed for structures with the thicker barriers demonstrated presence of e- and h-states in each QW and the emission at λ≈450nm. PL measured in these samples showed such an emission. For structures with 1-2 ML barrier we found a peak at λ≈505-520nm which we associated with IX in the system of
CDQW. To prove strongly indirect character of the long-wavelength PL-band we performed TRPL characterization of the studied structures. At T=4K, the band at λ≈510nm showed the decay time, τD, approaching msec range. Whereas the band at λ≈450nm is characterized by τD of about 100 ns.