Crystal-phase quantum dots in GaN quantum wires

Abstract

We study the nature of excitons bound to I 1 basal plane stacking faults in ensembles of ultrathin GaN nanowires by continuous-wave and time-resolved photoluminescence spectroscopy. These ultrathin nanowires, obtained by the thermal decomposition of spontaneously formed GaN nanowire ensembles, are tapered and have tip diameters down to 6 nm. With decreasing nanowire diameter, we observe a strong blue shift of the transition originating from the radiative decay of stacking fault-bound excitons. Moreover, the radiative lifetime of this transition in the ultrathin nanowires is independent of temperature up to 60 K and significantly longer than that of the corresponding transition in as-grown nanowires. These findings reveal a zero-dimensional character of the confined exciton state and thus demonstrate that I 1 stacking faults in ultrathin nanowires act as genuine quantum dots.Spontaneously formed GaN nanowires are comparable in structural perfection to state-of-the-art freestanding GaN. 1 The nanowire geometry inhibits the propagation of threading dislocations along the nanowire axis, resulting in dislocation-free crystals regardless of the substrate. 2,3 In contrast to group-III arsenide and phosphide nanowires, which are synthesized by vapor-liquid-solid growth and are prone to a pronounced polytypism, 4,5 spontaneously formed GaN nanowires exclusively crystallize in the wurtzite lattice structure with only occasional I 1 basal plane stacking faults (BSFs). 6,7 Consequently, the radiative transitions related to excitons bound to I 1 BSFs [(I 1 , X)] in GaN nanowires are spectrally well resolved and distinct from other excitonic transitions in GaN. This fact has been essential for shedding light on the nature of the (I 1 , X). 8,9 In particular, for nanowires with a diameter larger than 50 nm, the (I 1 , X) was shown to exhibit a two-dimensional density of states, i. e., I 1 BSFs indeed act as quantum wells. 8,10 These so-called crystal-phase quantum structures are free of strain and alloy disorder, their interfaces are atomically abrupt. 11 In GaN nanowires, the decay of the (I 1 , X) is purely radiative up to 60 K. 8,9 BSFs thus form an exceptionally well-defined model system for fundamental studies of confined excitons. In this context, the recent fabrication of ultrathin GaN nanowires with a diameter down to 6 nm is of great interest. 12 Due to the mismatch in dielectric constants between GaN and air, excitons in these ultrathin nanowires experience a strong radial confinement, i. e., the ultrathin GaN nanowires act as quantum wires despite the fact that their diameter still exceeds at least twice the exciton Bohr radius. 12 In addition, the thermal decomposition technique used for the controlled thinning of the as-grown nanowires does not affect their high structural perfection. Finally, despite their extremely small diameter, these nanostructures exhibit a high radiative efficiency due to a rather slow surface recombination velocity at the nanowires' sidewall facets less than than 210 cm/s at 60 K. 8 In this...

Optical properties of GaN nanowires grown on chemical vapor deposited-graphene

“…Brandt, unpublished]. Since the transition energy is now sensitive to fluctuations of the NWdiameter, the emission from the NW array broadens 41,42 . Nevertheless, we could find isolated single NWs whose low temperature µPL emission is characterized by a relatively sharp (D 0 ,X A ) peak.…”

Nature of excitons bound to inversion domain boundaries: Origin of the 3.45-eV luminescence lines in spontaneously formed GaN nanowires on Si(111)

“…In analogy to the result in Ref. 53 for basal-plane stacking faults, we hence expect the density of states of the (IDB * , X) to be two-dimensional. As a result of the large number of states available, the (IDB * , X) transition should be difficult to saturate even for high excitation conditions.…”

Crystal-Phase Quantum Wires: One-Dimensional Heterostructures with Atomically Flat Interfaces

“…This phenomenon has been extensively studied for quantum wells, but has also been reported for V-groove and pyramidal quantum wire heterostructures , as well as for ultrathin nanowires surrounded by air . Crystal-phase bandgap engineering in semiconductor nanowires has opened the possibility to fabricate heterostructures free of such fluctuations. − Crystal-phase quantum wells are induced by twin boundaries and basal plane stacking faults (BSFs) intersecting nanowires axially, but also by inversion domain boundaries (IDBs) of type IDB* running along the length of the nanowires . For instance, I 1 BSFs in GaN have been shown to act as three-monolayer-thick quantum wells of zinc-blende material with atomically flat interfaces that confine excitons along the [0001] direction. , Due to these perfect interfaces, the radiative decay rate of the exciton in I 1 BSFs is enhanced by its coherent macroscopic polarization .…”

“…Crystal-phase bandgap engineering in semiconductor nanowires has opened the possibility to fabricate heterostructures free of such fluctuations. − Crystal-phase quantum wells are induced by twin boundaries and basal plane stacking faults (BSFs) intersecting nanowires axially, but also by inversion domain boundaries (IDBs) of type IDB* running along the length of the nanowires . For instance, I 1 BSFs in GaN have been shown to act as three-monolayer-thick quantum wells of zinc-blende material with atomically flat interfaces that confine excitons along the [0001] direction. , Due to these perfect interfaces, the radiative decay rate of the exciton in I 1 BSFs is enhanced by its coherent macroscopic polarization . Both, I 1 BSFs and IDB*s have been observed in GaN nanowires grown by molecular-beam epitaxy and give rise to excitonic transitions labeled ( I 1 , X ) and (IDB*, X ), respectively. , Dimitrakopulos et al reported the observation of intersecting I 1 BSFs and IDB*s in planar GaN.…”

“…12,16 Due to these perfect interfaces, the radiative decay rate of the exciton in I 1 BSFs is enhanced by its coherent macroscopic polarization. 14 Both, I 1 BSFs and IDB*s have been observed in GaN nanowires grown by molecular-beam epitaxy and give rise to excitonic transitions labeled (I 1 ,X) and (IDB*,X), respectively. 17, 18 Dimitrakopulos et al 19 reported the observation of intersecting I 1 BSFs and IDB*s in planar GaN.…”