Direct correlation of crystal structure and optical properties in wurtzite/zinc-blende GaAs nanowire heterostructures

A method for the direct correlation at the nanoscale of structural and optical properties of single GaAs nanowires is reported. Nanowires consisting of 100% wurtzite and nanowires presenting zinc-blende/wurtzite polytypism are investigated by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence of wurtzite GaAs is consistent with a band gap of 1.5 eV. In the polytypic nanowires, it is shown that the regions that are predominantly composed of either zinc-blende or wurtzite phase show photoluminescence emission close to the bulk GaAs band gap, while regions composed of a nonperiodic superlattice of wurtzite and zinc-blende phases exhibit a redshift of the photoluminescence spectra as low as 1.455 eV. The dimensions of the quantum heterostructures are correlated with the light emission, allowing us to determine the band alignment between these two crystalline phases. Our first-principles electronic structure calculations within density functional theory, employing a hybrid-exchange functional, predict band offsets and effective masses in good agreement with experimental results.

Figure 1

Overview of the measurement process: (a) Optical microscopy image of a TEM copper grid with marker symbols used for the studies. (b) Detail of TEM grid showing the nanowires transferred to the holey carbon film. (c) Intensity plot of integrated PL signal at 4.2 K from the area of the TEM grid marked by the box in (b).

Figure 2

Excitation power dependence of a typical capped zinc-blende GaAs nanowire, dispersed on a holey carbon film of a copper TEM grid. No pronounced heating effects are observed for excitation power densities up to 4000 W//m${}^2$. For higher excitation powers the heating of the sample causes a change in band gap, resulting in a redshift of the PL transition. For the highest excitation of 23 000 W/cm${}^2$, the redshift corresponds to a temperature increase from 4.2 to 140 K. This clearly demonstrates that heating can be neglected for measurements performed at excitation powers in the order of 10–50 W/cm${}^2$.

Figure 3

(a) Typical HAADF STEM micrograph obtained on a Au-seeded AlGaAs/GaAs nanowire. (b) EDX spectrum taken at the tip of the nanowire confirming the presence of Au. (c) EDX spectrum at the neck of the nanowire confirming the presence of AlGaAs on the neck (the percentage of Al is low).

Figure 4

Mapping of PL spectra along the length of the nanowire shown above. The scale bar applies to both the PL mapping and the TEM. (a), (b), (d) HRTEM micrographs showing the crystalline phases at the regions of the nanowire indicated by the arrows above. Filled yellow circles mark the wurtzite GaAs phase. (c) Power spectrum of HRTEM micrograph in region c. Note that micrograph in region D corresponds to the axially grown AlGaAs/GaAs material during the radial capping forming a thin extension.

Figure 5

(Top) PL spectrum from a nanowire consisting of pure wurtzite GaAs phase. (Bottom) PL of a planar zinc-blende GaAs reference sample.

Figure 6

SEMs of the as-grown nanowire sample grown with the Ga-assisted growth mechanism. The nanowires grow perpendicular to the (1$\overline{1}$1)B substrate surface.

Figure 7

(a) Mapping of PL spectra along the length of the GaAs wurtzite/zinc-blende heterostructure nanowire shown below. The scale bar applies to both the PL mapping and the TEM. (b)–(g) HRTEM micrographs showing the crystalline phases at the positions of the nanowire indicated by the respectively colored arrows. Filled yellow circles mark the wurtzite phase while filled triangles (▾,▴) indicate the zinc-blende phase.

Figure 8

(a) PL mapping and TEM reconstruction of another wurtzite/zinc-blende heterostructure GaAs nanowire from the sample. The scale bar applies to both the PL mapping and the TEM. (b) Individual PL spectra of the five positions marked in (a). All spectra are normalized in intensity. (c) HRTEM micrograph of region E of the nanowire. (d) Power spectrum of (c).

Figure 9

HRTEM micrographs showing the occurrence of (a) $n=2$ consecutive twins in a region with $<85$ twins/$\mu$m and (b) $n=4$ in a region with $>400$ twins/$\mu$m. (c) Correlation of the lowest PL transition energy to the density of rotational twins in the corresponding part of the nanowire. The symbols show data obtained from four different nanowires. The black line shows the transition energies for wurtzite segments formed by $n=1$–4 consecutive twins.

Figure 10

(a) Schematic of the type-II homogeneous SL model with wurtzite phase fraction $\gamma$ and periodicity $a=13$ nm. (b) Lowest PL transition energy as a function of the fraction of wurtzite phase $\gamma$ in the corresponding part of the nanowire (symbols). The colored lines show results from model calculations. The dashed black lines give the prediction bounds of the model fit to the experimental data.

Figure 11

HRTEM micrograph of the largest segment of the nanowire with regions composed of equal fractions of the zinc-blende and wurtzite phases, each $~$6.5 nm in length along the nanowire axis.

Figure 12

The structures used in the calculations for the zinc-blende and wurtzite lattices, with lattice vectors ${\mathbf{a}}_1,{\mathbf{a}}_2,{\mathbf{a}}_3$. The larger (blue) and smaller (red) circles denote the two types of ions and the [111] and [11$\overline{2}$] crystallographic directions in the zinc-blende lattice, as well as the [0001] and [1$\overline{1}$00] crystallographic direction in the wurtzite lattice are indicated.

Figure 13

The band structures of the zinc-blende and wurtzite lattices along the $\mathrm{Z}\Gamma$ direction in the Brillouin zone. Open black circles are the results of the LDA calculations, red filled circles are the results of HSE06 calculations, and blue lines are quadratic fits near $\Gamma$. The values in blue indicate the effective masses in units of $m_e$ obtained from the quadratic fits.