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

Spectral diffusion of type-II excitons in InP/InAs/InP core-multishell nanowires (CMNs) and type-I excitons in an InAs/InP single quantum well (SQW) was studied by means of time-resolved and spectrally resolved photoluminescence. InP/InAs/InP CMNs in hexagonal symmetry are made of six facets and six edges which work as two-dimensional quantum wells and one-dimensional quantum wires, respectively. At 5 K type-II excitons lose their energy in two stages. In the first stage, two-dimensional spectral diffusion takes place in the type-II quantum well region in CMNs similar to spectral diffusion of type-I excitons in the InAs/InP SQW. In the second stage, slower one-dimensional spectral diffusion takes place in the quantum wire region in CMNs. Acoustic-phonon-mediated migration of excitons to lower-energy-localized states leads to the spectral diffusion in two dimensions and one dimension.

Figure 1

(Color online) (a) Structure and strain in an InP/InAs/InP CMN and an InAs/InP SQW. The absolute square of the eigenwave functions of the first and the second lowest-energy holes in a CMN are plotted in contour maps. (b) PL spectrum of CMNs and a SQW at 5 K under the excitation at 1.88 eV. PL spectrum of CMNs consists of a band of one InAs monolayer (1 ML), a band of two InAs monolayers (2 ML), and a band of three InAs monolayers (3 ML). (c) Temporal change of 2 ML PL in CMNs and a SQW at 5 K. At 820 kHz repetition rate, 2 ps pulses at the excitation photon energy of 1.18 eV and 1.16 eV selectively excite ultrathin wurtzite InAs tubes and a zincblende InAs SQW, respectively. Time-resolved PL measurement is done by means of single photon counting.

Figure 2

(Color online) (a) Contour map of time-resolved and spectrally resolved 2 ML PL in InP/InAs/InP CMNs at 5 K. Ultrathin wurtzite InAs tubes were selectively excited by 2 ps pulses at 1.377 eV and 820 kHz. Time-resolved PL measurement is done by means of single photon counting. (b) The average energy of time-resolved and spectrally resolved 2 ML PL in CMNs shown in (a) is plotted by open circles. The average energies of time-resolved and spectrally resolved 2 ML PL in CMNs and PL in a 2 ML InAs/InP single quantum well taken by a streak camera are plotted by solid lines. All the data are the five adjacent averages of the raw data.

Figure 3

(Color online) Redshift of the average energy of PL versus time in a logarithmic scale ranging (a) from 0.1 to 40 ns and (b) from 200 to 1600 ps. The average energy is given by the five adjacent averages of the raw average energies. (a) A solid line shows fitting on a model of dipole-dipole-type two-dimensional spectral diffusion and a dashed line shows fitting on a model of dipole-dipole-type one-dimensional spectral diffusion. (b) A solid line and a dashed line show fitting to redshift of the average energy of PL in the InP/InAs/InP CMN and the InAs/InP SQW, respectively, on a model of dipole-dipole-type two-dimensional spectral diffusion.