Crystal phase engineered quantum wells in ZnO nanowires

Abstract

We report the fabrication of quantum wells in ZnO nanowires (NWs) by crystal phase engineering approach. Basal plane stacking faults (BSF) in wurtzite structure can be considered as a minimal segment of zinc blende. Due to the existing band offsets at wurtzite (WZ) / zinc blende (ZB) materials interface incorporation of BSF of high density into ZnO NWs results in type II band alignment. Thus, the BSF structure acts as a quantum well for electrons and a potential barrier for holes in the valence band.We have studied the photoluminescence (PL) properties of ZnO NWs containing high concentration of BSF in comparison to high quality ZnO NWs of pure wurtzite structure. It is revealed that BSF form quantum wells in WZ ZnO nanowires, providing an additional luminescence peak at 3.329 eV at 4 K. The luminescence mechanism is explained as indirect exciton transition due to the recombination of electrons in the QW conduction band to holes localized near the BSF. The binding energy of electrons is found to be around 100 meV, while the excitons are localised with binding energy of holes ~ 5 meV, which is due to the coupling of BSF, which form QW-like structures.2

Low temperature near band edge recombination dynamics in ZnO nanorods

“…33 Also reported recently is the formation of a quantum wire at the intersection between a 3.8 nm thick (Al,Ga)N/GaN quantum well and BSFs, leading to radiative recombination of localized excitons. 34 Although the energetic position reported by Khranovskyy et al 5 is considerably higher than what has been reported by Schirra et al, 29 Thonke et al 30 and also in this work, Khranovskyy et al 5 clearly demonstrated the presence of a high density of BSFs perpendicular to the c-axis. They also studied temperature and laser power dependent PL and concluded that the line has an excitonic nature.…”

“…These planar inclusions of ZB ZnO in the normal WZ crystal have been theoretically shown to form type-II QWs with a valence/conduction band offset of 37/147 meV. 33 The insertions of such regions result from the formation of BSFs perpendicular to the c-axis, as experimentally observed by Khranovskyy et al 5 Theoretically, electrons are weakly confined in the well and the holes are located in the barriers of these structures. 33 Also reported recently is the formation of a quantum wire at the intersection between a 3.8 nm thick (Al,Ga)N/GaN quantum well and BSFs, leading to radiative recombination of localized excitons.…”

“…More recently, Khranovskyy et al 5 reported a line at 3.329 eV at 4 K in ZnO nanorods with a high concentration of BSFs and ascribed it to emission from a quantum well (QW) formed by the segregation of zinc blende (ZB) material within the wurtzite (WZ) ZnO. These planar inclusions of ZB ZnO in the normal WZ crystal have been theoretically shown to form type-II QWs with a valence/conduction band offset of 37/147 meV.…”

“…[1][2][3][4][5][6][7][8] In the context of utilizing ZnO in the optoelectronics industry, a thorough understanding of its optical and electrical properties is essential. In particular, a detailed understanding of carrier relaxation processes, which strongly affect its electronic and optoelectronic characteristics, is a prerequisite for the inclusion of ZnO in the optoelectronics sector.…”

“…3,4 Depending on the growth method, material quality, and intentional dopants used, the low temperature photoluminescence (PL) spectra of ZnO are dominated by shallow donor bound excitons (D o Xs), 1-3 free excitonic (FX), 4 and/or defect-related transitions, particularly involving basal plane stacking faults (BSFs). 5 Each of these recombination centers has different recombination/ relaxation characteristics. As a result, carrier relaxation processes in ZnO, in particular, those involving D o Xs, are still poorly understood.…”