Impact of random dopant fluctuations on the electronic properties of (In,Ga)N/GaN axial nanowire heterostructures

The growth of thin InxGa1-xN films on GaN is limited to low In contents due to the large lattice mismatch between InN and GaN as well as due to the tendency of phase separation. Therefore, light emission from planar InxGa1-xN/GaN heterostructures is restricted to a much smaller spectral window than the one that can be potentially accessed by the variation of the band gap with In content. This limitation can in principle be overcome by incorporating thin InxGa1-xN disks in a GaN nanowire. In contrast to planar structures, the free sidewalls of nanowires permit the elastic relaxation of strained axial insertions, thus facilitating higher In contents and hence a larger spectral range. On the other hand, unintentionally incorporated donors - commonly Si and O impurities – transfer their extra electron to surface states at the free side facets of the nanowire, which creates additional electrostatic potentials. In the following, we discuss the impact of randomly distributed donors in axial InxGa1-xN/GaN nanowire heterostructures on their electronic and optical properties.

Figure 1: Electron (red) and hole (blue) ground state charge density averaged over 500 individual, random donor configurations in side (left) and top (middle) view of an In0.05Ga0.95N/GaN nanowire. Right: Histogram of ground state emission energy (top) and electron-hole overlap.

 

 

To understand the influence of randomly distributed donors in axial InxGa1-xN/GaN nanowire heterostructures, we have performed an elaborate statistical analysis of the electron and hole ground state confinement and recombination energies. Our simulations employ an eight-band k·p model to compute the electronic properties, fully taking elastic and piezoelectric properties into account. We assume all donors to be ionized and study their influence on electron and hole ground state by modelling them as individual point charges.

 

Our study reveals that randomly distributed donors induce fluctuations of the ground state transition energy of about 150 meV, almost independent of the thickness or In content of the active layer. For an ensemble, these variations translate into a broadening of the lines in photo- or electroluminescence spectra, even if all other sources of inhomogeneity such as alloy fluctuations or variations of nanowire shape and diameter or layer thickness could be eliminated. Similarly, the electron-hole ground state charge density overlap - a qualitative measure for the efficiency of light emission - shows wire-to-wire fluctuations of three orders of magnitude.

 

These significant wire-to-wire fluctuations of the transition energies result in broad emission bands for nanowire ensembles, but also have severe consequences for single-photon emitters based on III-nitride nanowires. Here, the presence of unintentionally incorporated donors will make it difficult to obtain reproducible transition energies and rates from nanowire to nanowire. Additionally, nanowire-based devices for the generation of entangled photons will suffer from a reduction of the overall symmetry of the system due to the presence of ionized donors.


1 Autor O. Marquardt , L. Geelhaar , O. Brandt
Titel

Impact of random dopant fluctuations on the electronic properties of InxGa1-xN/GaN axial nanowire heterostructures

Source Nano Lett. , 15 , 4289 ( 2015 )
DOI : 10.1021/acs.nanolett.5b00101 | 2678 Cite : Bibtex RIS
O. Marquardt, L. Geelhaar, and O. Brandt