Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

Figure 1

(Color online) Zinc-blende/wurtzite quantum structures. (a) Aberration corrected HAADF high resolution scanning transmission electron micrograph (HRSTEM) of a zinc-blende quantum well in a wurtzite segment. (b) An atomistic model of a wurtzite/zinc-blende/wurtzite heterostructure along with a schematics of the band diagram. The Ga and As atoms have been marked in orange and green, respectively, for the WZ domains, and in red and blue, respectively, for the double unit zinc-blende quantum well.

Figure 2

(Color) Transmission electron microscopy of GaAs nanowires presenting different crystalline structures. For illustration purposes, we have indicated each orientation of the zinc-blende phase domains with blue and green circles, and the wurtzite phase regions with red circles. (a) General view of the high beam flux nanowires (conditions $\alpha$) presenting twins in the zinc-blende structure. (b) Detail of the twin zinc-blende domains. (c) HRTEM analysis of a twin interface. (d) HRTEM detail of a nanowire synthesized under conditions $\beta$. Inset in (d) corresponds to an aberration corrected HAADF HRSTEM image of the same region. (e) Magnified Cs-corrected HAADF HRSTEM detail of the squared region in (d). A zinc-blende quantum well embedded between two wurtzite regions. Twinned planes have been marked with dashed lines. Simulation of the atomic positions has been superimposed on the measurement.

Figure 3

(Color) Transmission electron micrograph from different segments of a single GaAs nanowires type $\mathbf{\beta }$. From (a) to (d), the sections belong to different regions of the nanowire from the tip to bottom, corresponding, respectively, from the latest to initial part of the growth process. The red spots correspond to regions with wurtzite structure while the blue and green spots correspond to regions with zinc-blend structure—with the two twin orientations.

Figure 4

(Color) Transmission electron micrograph from GaAs nanowire grown under As BF of $3.5\times {10}^{-7}\text{mbar}$—type $\mathbf{\gamma }$. The small insets show closer view of the structure from the selected regions.

Figure 5

(Color online) Typical photoluminescence spectra observed from the three types of nanowires at $T=4.2\text{K}$. The spectra have been obtained from single wires under illumination at 632.8 nm with a power density of $2.5\text{W}/{\text{cm}}^2$. The synthesis conditions for samples $\mathbf{\alpha }$, $\mathbf{\beta }$, and $\mathbf{\gamma }$ can be found in Table . The emission energy shifts from 1.51 eV down to 1.43 eV depending on the growth conditions which result in different proportion of wurtzite and zinc-blende GaAs.

Figure 6

(Color online) Spatially resolved confocal photoluminescence measurements along one single nanowire. The spectra correspond to sample type (a) $\mathbf{\alpha }$, (b) $\mathbf{\beta }$, and (c) $\mathbf{\gamma }$.

Figure 7

(Color) Cathodoluminescence of wurtzite/zinc-blende heterostructures. Scanning electron microscopy and monochromatic cathodoluminescence images of nanowires synthesized under conditions $\mathbf{\alpha }$ and $\mathbf{\beta }$. A shows an SEM image of a nanowire of type $\mathbf{\beta }$ and (b)–(d) shows the corresponding CL images recorded at different energies, illustrating the different spatial origin of the various emission energies. This is even clearer in the composite image of (e). For comparison, we also show images of nanowires type $\mathbf{\alpha }$. $F$ is an SEM image and $G$ the corresponding 1.51 eV CL image, showing homogeneous emission along the length of the nanowire.

Figure 8

(Color online) (a) The theoretical band alignment of wurtzite and zinc-blende GaAs, along with the values of conduction-band and valence-band discontinuities are shown. Some selected examples of quantum heterostructures which are presented, along with the first quantized level as calculated in (b). The relative scale in energy and length has been kept. The first quantized energy levels sketched correspond to quantum wells of 7.5, 5, and 2.5 nm. Due to the type II alignment, the holes are confined in the wurtzite quantum wells while the electrons are confined in the zinc-blende quantum wells. In (b), plot of the energy positions of the first electron $\left(E_{1\text{C}}\right)$ and hole $\left(E_{1\text{V}}\right)$ levels of quantum wells as a function of the quantum well thickness.

Figure 9

Time resolved luminescence of (a) the free exciton peak at 1.515 eV sample $\mathbf{\alpha }$ and (b) the peak at 1.48 eV from sample type $\mathbf{\beta }$.

Figure 10

(Color online) Typical Raman spectra of single GaAs nanowires transferred on a silicon substrate. The spectra belong to the three types of nanowires synthesized at an arsenic beam flux of $3.4\times {10}^{-6}\text{mbar}$ $\left(\alpha \right)$, $8.8\times {10}^{-7}\text{mbar}$ $\left(\beta \right)$, and $3.5\times {10}^{-7}\text{mbar}$ $\left(\gamma \right)$. In sample $\alpha$ only the TO mode is observed, which corresponds to what is typically observed in backscattering from (110) zinc-blende GaAs surfaces. The LO phonon appears weakly for sample $\left(\beta \right)$, along with a mode at lower frequencies which corresponds to the wurtzite mode. In sample $\gamma$, the wurtzite related mode is stronger and shifted due to the existence of a larger proportion of wurtzite in the nanowires. Note that all zinc-blende related modes appear shifted to lower wave numbers due to the adaptation of the lattice parameter closer to wurtzite, when zinc blende is minoritary in the structure.