Intraband Absorption in Self-Assembled Ge-Doped GaN/AlN Nanowire Heterostructures

Abstract

We report the observation of transverse-magnetic-polarized infrared absorption assigned to the s-p(z) intraband transition in Ge-doped GaN/AlN nanodisks (NDs) in self-assembled GaN nanowires (NWs). The s-p(z) absorption line experiences a blue shift with increasing ND Ge concentration and a red shift with increasing ND thickness. The experimental results in terms of interband and intraband spectroscopy are compared to theoretical calculations of the band diagram and electronic structure of GaN/AlN heterostructured NWs, accounting for their three-dimensional strain distribution and the presence of surface states. From the theoretical analysis, we conclude that the formation of an AlN shell during the heterostructure growth applies a uniaxial compressive strain which blue shifts the interband optical transitions but has little influence on the intraband transitions. The presence of surface states with density levels expected for m-GaN plane charge-deplete the base of the NWs but is insufficient to screen the polarization-induced internal electric field in the heterostructures. Simulations show that the free-carrier screening of the polarization-induced internal electric field in the NDs is critical to predicting the photoluminescence behavior. The intraband transitions, on the other hand, are blue-shifted due to many-body effects, namely, the exchange interaction and depolarization shift, which exceed the red shift induced by carrier screening.

Intersubband absorption in Si‐ and Ge‐doped GaN/AlN heterostructures in self‐assembled nanowire and 2D layers

“…The large, inhomogeneous broadening of the absorption peaks is due to thickness/diameter variations in the NW ensemble. The result represent nevertheless a large improvement in terms of FHWM when compared with previous reports of ISB transition in NW heterostructures . The difference in the absorption peak between the two samples could easily be attributed to monolayer thickness fluctuations from wire to wire in the ensemble.…”

“…The significant blue shift of the band‐to‐band transition from the NWs (around 3.66 eV) with respect to the emission from the planar heterostructures (around 3.36 eV) cannot be explained by thickness variations It is mostly due to the particular strain distribution in the NW heterostructures, with a uniaxial compressive strain component due to the AlN shell, as modeled in Ref. .…”

“…For future device perspectives, the feasibility of electron transport via quantized levels in the conduction band of GaN/AlN heterostructured NWs has been demonstrated by the fabrication of resonant tunneling diodes . Furthermore, transverse‐magnetic polarized infrared absorption assigned to the s–p z transition in Ge‐doped GaN/AlN nanodisks in self‐assembled GaN NWs have been recently reported . This result was attributed to the replacement of Si doping by Ge .…”

Long-lived excitons in GaN/AlN nanowire heterostructures

“…Furthermore, the GaN/AlN interfaces often present {1 − 102} facets close to the NW sidewalls, highlighted by a dashed line in Fig. 1(c), due to the plane bending phenomena related to the elastic strain relaxation [15,16,25].…”

“…The PL spectral positions present signatures of both quantum confinement and of the polarization-induced Stark effect [16,[19][20][21][22][23]. However, the spectral shift associated to the Stark effect is smaller in nanodisks than in quantum wells [16,19,23], which has been attributed to dislocations [16], to the surface band bending [24] and to the 3D strain configuration [19,25]. Studies of the PL decay times in GaN NWs show exponential or biexponential PL decays with subnanosecond characteristic times [26,27].…”