Role of edge facets on stability and electronic properties of III–V nanowires
- Dmitri B Migas^{1}Email author,
- Andrew B Filonov^{1},
- Dmitri A Yatsyna^{1},
- Dr Rusli^{2} and
- Cesare Soci^{3}
DOI: 10.1186/s40580-015-0045-7
© Migas et al.; licensee Springer. 2015
Received: 29 December 2014
Accepted: 3 February 2015
Published: 8 July 2015
Abstract
Results of our ab initio calculations of 〈111〉-oriented GaP, GaAs, GaSb, InP, InAs and InSb nanowires with the zinc-blende structure indicate morphology to crucially affect their electronic properties. For these nanowires, where {011} facets characterize their hexagonal cross section, the formation of small {112} facets between the adjacent {011} ones provides a more stable structure and removes surface states from the gap region even without hydrogen passivation. Our new structural model also predicts a crossover between the indirect and direct band gap in GaP, GaAs and GaSb nanowires when increasing diameters starting from 4 nm, while InP, InAs and InSb nanowires display the direct band gap at diameters of 1.5 nm and larger. Analysis of charge distribution between atoms suggests that {011} facets are positively charged even though a (011) surface of these materials is considered to be non-polar.
Keywords
III–V nanowires Morphology Band structure1 Background
In small diameter III–V nanowires (NWs), where the surface-to-volume ratio is rather large, one can expect surface effects in addition to effects caused by quantum confinement to govern properties of these NWs. It is also important to trace a link between morphology and different properties of such NWs in order to open a way to their integration in various applications [1,2]. Even though in most cases III–V compounds have the zinc-blende structure, in the case of NWs both the zinc-blende and wurtzite structures are observed [2] because of larger density of surface dangling bonds for the zinc-blende structure as confirmed by first principles calculations [3-10]. However, high quality NWs in the zinc-blende structure mainly oriented along the 〈111〉 directions can be grown by different methods involving the vapor-liquid-solid and vapor-solid growth mechanisms [1,2,11]. It is found that their cross section is hexagonal with {011} or {112} facets [12-20], while a triangular-like morphology with {112} facets can also occur [15-17,21]. Theoretical predictions on stability of zinc-blende GaP, GaAs, InP and InAs NWs showed that they were close in total energy independently of morphology [3-10]. Moreover, investigations of electronic properties of bare GaAs, InP and InAs NWs, which had only {011} facets, indicated metallic properties because the Fermi level crossed some bands originated from states of surface atoms at edges [3,5,10,22]. However, passivation of dangling bonds by different chemical species was widely used to investigate a band-gap variation with NW diameter [7,8,22-27], while bare zinc-blende NWs with {112} facets and any bare wurtzite NW turned out to be semiconductors without passivation [3,5,9,10].
In this paper by means of ab initio calculations we show that the appearance of small {112} facets acting as edges between the adjacent {011} facets in zinc-blende GaP, GaAs, GaSb, InP, InAs and InSb NWs provides lowering in total energy and eliminates bands in the gap region leading to semiconducting properties.
2 Methods
Bulk lattice parameters ( a _{ bulk } , Å), lattice parameters along a NW axis ( a _{ ∥ } , Å), III–V interatomic distances in the bulk ( d _{ I I I − V } , Å), lengths of V–V ( d _{ V − V } , Å) and III–III ( d _{ III−III } , Å) dimers on {112} facets of a NW, charge transfer between atoms ( q _{ b u l k } , in units of the electron charge) in the bulk and in the III–III ( q _{ III } , in units of the electron charge) and V–V ( q _{ V } , in units of the electron charge) dimers on {112} facets of a NW
GaP | GaAs | GaSb | InP | InAs | InSb | |
---|---|---|---|---|---|---|
a _{ bulk } | 5.419 | 5.623 | 6.067 | 5.871 | 6.063 | 6.468 |
a _{∥} | 5.415 | 5.610 | 6.046 | 5.867 | 6.038 | 6.446 |
d _{ I I I−V } | 2.35 | 2.43 | 2.63 | 2.54 | 2.63 | 2.80 |
d _{ V−V } | 2.24 | 2.49 | 2.88 | 2.21 | 2.46 | 2.86 |
d _{ I I I−I I I } | 2.44 | 2.45 | 2.46 | 2.76 | 2.78 | 2.79 |
q _{ bulk } | 0.56 | 0.58 | 0.12 | 0.56 | 0.53 | 0.21 |
q _{ III } | 0.47 | 0.38 | 0.20 | 0.45 | 0.39 | 0.25 |
q _{ V } | 0.39 | 0.30 | 0.14 | 0.36 | 0.30 | 0.16 |
3 Results and discussion
3.1 3.1 Morphology and stability
Now it is obvious that the formation of small {112} facets in III–V NWs decreases density of dangling bonds with respect to NWs without {112} facets because some V atoms at edges with multiple dangling bonds are eliminated. This issue, in turn, provides a clear lowering in total energy for all considered here NWs as can be seen in Figure 1, the bottom panel, for GaAs NWs.
We have estimated charge distribution between atoms in the III–V bulks and NWs. For the bulks of phosphides and arsenides the III atoms donate and the V atoms accept of about 0.53 – 0.58 e ^{−} (Table 1), while for the antimonide bulks the charge transfer is less (0.12 – 0.20 e ^{−}). In the case of NWs of phosphides and arsenides the effective atomic charges are essentially the same as for the corresponding bulks, while main differences can only be spotted for atoms at surface and especially for atoms forming dimers (Table 1) and back-bonds to the dimer atoms. For antimonide NWs a larger span of charge distribution than in the corresponding bulks (by 0.2 e ^{−}) is found.
3.2 3.2 Morphology and band structure
4 Conclusions
We have suggested a new structural model of III–V NWs with the zinc-blende structure and {011} facets bounded in the hexagonal shape, which involves small {112} facets between the adjacent {011} ones acting as edges. In this case it is possible to reduce number of dangling bonds at the surface because of the formation of III–III and V–V dimers and to provide a sizable lowering in total energy. We have also predicted {011} and {112} facets to be positively and negatively charged, respectively, whereas essential charge distribution can be spotted for surface atoms located near edges. Moreover, such III–V NWs are semiconductors since there is no band associated with surface states to be crossed by the Fermi level and no hydrogen passivation is necessary to investigate semiconducting properties of these NWs. Moreover, previously published and reported here changes in direct/indirect character of the gap and band dispersion near the gap region in III–V NWs with different morphology and with or without surface passivation can be viewed as a valuable tool for band-gap engineering in order to tune electronic, optical and transport properties of such NWs targeting specific applications.
Declarations
Acknowledgements
This work has been supported by Belarusian Republican Foundation for Fundamental Research under the Grant No. F14U-001 and by the Singapore Ministry of Education (project reference MOE2013-T2-1-044). The authors thank Professor V. E. Borisenko for useful suggestions and comments on the results presented in the paper.
Authors’ Affiliations
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