Type-II behavior in wurtzite InP/InAs/InP core-multishell nanowires

Abstract

We study optical transitions from a periodic array of InP/InAs/InP core-multishell nanowires ͑CMNs͒ having a wurtzite crystal structure by using photoluminescence ͑PL͒ and PL excitation ͑PLE͒ spectroscopy. Observing a large Stokes shift between PL and PLE spectra, a blueshift of the PL peak with a cube-root dependence on the excitation power and a slow and nonexponential decay of PL with an effective decay time of 16 ns suggest a type-II band alignment. Band-offset calculation based on the "model-solid theory" of Van Nanometer-scale semiconductor heterostructures such as quantum dots, quantum wires, and quantum wells ͑QWs͒ have been interesting research targets due to their unique size-dependent electronic and optical properties associated with the lower dimensionality and quantum confinement effects. There have been revived interests in semiconductor nanowires ͑NWs͒ due to recent success in growth and fabrication of a regular array of core-shell and core-multishell NWs ͑CMNs͒.1,2 Semiconductor NWs can be used as building blocks of sophisticated nanoscale electronic and photonic devices.3 Much effort has been devoted so far to the growth and fabrication of CMNs and the periodic arrays of such structures.1,2 However, very little is known about the electronic structure and optical properties of this new class of technologically important structures.In this letter we report optical studies on the periodic array of InP/InAs/InP CMNs. Both InP and InAs have a wurtzite crystal structure in the NW form, 4-6 although bulk InP and InAs crystallize to a zinc-blende structure. Moreover, in our CMN sample a thin InAs layer is surrounded by thick InP layers from all sides and the InP layers act like a mold. As a first approximation, we may consider that the InAs lattice experiences a three-dimensional compressive strain and attains the size of the InP lattice in all directions unlike InAs/InP QWs, where the InAs lattice is compressed in the crystal growth plane and is elongated in the growth direction. There have been a few studies on ultrathin InAs/ InP QWs having zinc-blende structure. 7 These experimental results and theoretical calculations using an envelopefunction scheme with effective-mass approximation and empirical tight-binding model 8 seem to show type-I direct transitions in zinc-blende InAs/InP QWs, although some calculations also predict type-II behavior.9 In contrast, the electronic structure and optical properties of wurtzite InP and InAs are almost unknown. 5,10 Due to the differences in crystal structure and strain, the insights gathered from the studies on zinc-blende InAs/InP QWs cannot be applied directly to wurtzite InP/InAs/InP CMNs.We study the InP/InAs/InP CMNs using time-resolved ͑TR͒ and spectrally resolved ͑SR͒ photoluminescence ͑PL͒ and PL excitation ͑PLE͒ measurements. A large Stokes shift between PL and PLE spectra was observed, which along with the absence of strong PLE peak suggests type-II radiative recombination. With increasing excitation power ͑P͒ the PL peaks show a blueshift with a cu...

Exciton coherence in clean single InP/InAsP/InP nanowire quantum dots emitting in infra-red measured by Fourier spectroscopy

“…Before the saturation of PL intensity, the emission energy is slightly shifted (∼200 µeV). This energy-shift is attributed to the Stark effect due to the internal electric field induced by the dipoles at the wurtzite and zinc-blende heterointerfaces in the InP NW [18].…”

“…From the streak camera image overlapping of NW and substrate emissions (inset of Fig. 2(c)), the remaining PL signals of InP NW were observed before zero delay time (marked by circle), corresponding to type-II recombination [18]. In what follows we focus on the QD emission with recombination lifetime of ∼2 ns obtained by fitting to a single exponential function (red solid curve).…”

One- and two-dimensional spectral diffusion of type-II excitons in InP/InAs/InP core-multishell nanowires

“…1͑b͒ contains bands of type-II excitons consisting of holes in the InAs ultrathin, strained hexagonal tube and electrons in the InP core or the InP outer shell. 13 Large Stokes shift between PL and PL excitation ͑PLE͒ spectra 13 and slow decay of PL with a decay tail lasting up to 300 ns indicate a type-II band lineup, as is shown in Fig. 1͑c͒.…”

“…Model-solid theory by Van de Walle 12 explains that nearly hydrostatic strain field forms type-II band lineup. 13 Hole wave function confined in the InAs layer and its quantized energy are calculated by solving effective-mass Schrödinger equation on the finite element method with triangular array of 24 888 nodes for 1/6 section of the InAs hexagonal tube surrounded by inner and outer InP layers. The squared amplitude of the calculated hole envelope functions is displayed in Fig.…”

Formation of nanoscale spatially indirect excitons: Evolution of the type-II optical character of CdTe/CdSe heteronanocrystals

“…1,[6][7][8][9][10]13,14 However, the energetic barriers in these nanostructures are usually lower than those attainable in colloidal QDs and heteroNCs, which are typically much smaller than their MBE counterparts. Moreover, colloidal chemistry methods are cheaper and easier to upscale than MBE techniques, and offer the additional advantages of processability and easier control over size, shape, and surface.…”