Article pubs.acs.org/JPCC
Band Gap Tuning of Twinned GaAsP Ternary Nanowires Hyung Soon Im, Chan Su Jung, Kidong Park, Dong Myung Jang, Young Rok Lim, and Jeunghee Park* Department of Chemistry, Korea University, Jochiwon 339-700, Korea S Supporting Information *
ABSTRACT: GaAs1−xPx ternary alloy nanowires have drawn much interest because their tunable band gaps, which range from the nearinfrared to visible region, are promising for advanced and integrated nanoscale optoelectronic devices. In this study, we synthesized compositionally tuned GaAs1−xPx (0 ≤ x ≤ 1) alloy nanowires with two average diameters of 60 and 120 nm by vapor transport method. The nanowires exhibit exclusively twinned superlattice structures, consisting of zinc blende phase twinned octahedral slice segments between wurtzite phase planes. Smaller diameter and higher P content (x) result in shorter periodic superlattice structures. The band gap of the smaller diameter nanowires is larger than that of the larger diameter nanowires by about 90 meV, suggesting that the twinned superlattice structure increases the band gap. The increase in band gap is ascribed to the higher band gap of the wurtzite phase than that of the zinc blende phase. twinned superlattices or polytypic structures.17,29−40 These structures can alter the optical and electronic properties, such as the quantum efficiency, carrier lifetime, and carrier mobility. The production of the WZ phase is more favorable for NWs with smaller radii. Several different theoretical explanations have been proposed for this behavior.41−44 Calculations based on an empirical nucleation model indicate that the basic driving force for WZ formation is the lower surface energy and the pertinent interfacial energies at the solid−liquid−vapor interface.41,42 Herein, we synthesized GaAs1−xPx (0 ≤ x ≤ 1) alloy NWs with tuned compositions by a simple vapor transport method. The composition tuning was successfully achieved by changing the ratio of the GaAs and GaP powders. NWs with average diameters of 60 and 120 nm could be obtained by using different growth temperatures. The crystal and electronic structures of the NW samples with the two different diameters were compared using high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, and ultraviolet (UV)−visible absorption spectroscopy to reveal the nature of their twinned superlattice structures. Furthermore, the GaAs1−xPx−gallium oxide (Ga2O3) coaxial lateral heterostructured NWs were also synthesized with negligible GaAs1−xPx twin structures. In this work, we have demonstrated that the twinned superlattice structures can increase the optical band gap of the GaAs1−xPx alloy NWs over the entire compositional range.
1. INTRODUCTION III−V semiconductor nanowires (NWs) have attracted considerable attention because they can be used as well-defined building blocks of future nanodevices with unique optical and electrical properties using bottom up approaches.1−4 There is a recent surge in multicomposition alloying, which offers advantages of band gap tunability, controlled conduction band gap offsets, and localized defect energy levels, which are critical for achieving high photoconversion efficiencies in photovoltaic cells. The controlled synthesis of alloy NWs can be expected to broaden the application range of these materials to more advanced and integrated nanoscale optoelectronic devices. However, achieving compositionally tuned homogeneous alloy NWs is challenging because of the inherent differences in precursor reaction kinetics, which require adept control of the reactivity of individual precursors. Thus far, successful ternary compositional tuning of InGaAs, InGaN, AlGaAs, InGaSb, GaAsP, InAsP, AlGaP, and InAsSb NWs has been reported.5−19 Most of these syntheses were carried out by developing metal−organic chemical vapor deposition (MOCVD) techniques. GaAs1−xPx has garnered much interest as an important ternary alloy semiconductor because its band gap (Eg) can be tuned between the near-infrared (NIR) and visible regions (1.42 ≤ E g ≤ 2.3 eV). Axial or coaxial core−shell heterostructure (e.g., GaP− or GaAs−GaAs1−xPx) NWs have been demonstrated to be useful in a wide range of potential applications as photon sources (in lasers), photodetectors, and photovoltaic devices.20−23 In contrast to bulk materials, which are usually in zinc blende (ZB) phase, the GaAs and GaP NWs crystallize in the wurtzite (WZ) phase.16,24−28 The WZ phase often manifests in the ZB phase NW by forming sequential © 2014 American Chemical Society
Received: January 15, 2014 Revised: February 4, 2014 Published: February 5, 2014 4546
dx.doi.org/10.1021/jp500458j | J. Phys. Chem. C 2014, 118, 4546−4552
The Journal of Physical Chemistry C
Article
2. RESULTS AND DISCUSSION The growth of the NWs on the Au-deposited Si substrates follows the typical vapor−liquid−solid (VLS) mechanism, which makes use of the Au nanoparticles (NPs) as the catalyst. The total number of GaAs1−xPx NW samples was more than 30. NWs with two different average diameters of 60 and 120 nm were obtained. We distinguished the samples as “S-GaAs1−xPx” and “L-GaAs1−xPx”, respectively. Figure 1a shows the SEM images of the high-density NWs grown on the substrates. The TEM images reveal the uniform
= 3.264 Å and dGaP = 3.146 Å) calculated by Vegard’s rule using the lattice parameters of bulk ZB GaAs (JCPDS card no. 800016; a = 5.654 Å) and GaP (JCPDS card no. 32-0397; a = 5.450 Å). The insets show the fast Fourier-transform (FFT) images. Images in (i) and (iii) correspond to that of the individual segment and (ii) corresponds to the twin plane regions of the segments. The two zigzagged segments share the [111] and [1̅1̅1̅] spots at the [011̅] zone axis and have common (111) twin planes. The TEM grid holder was rotated to tilt the NW around the axial direction. Figure 1d shows the TEM image at a tilt angle of 30°, which exhibits single-crystalline features without any twin planes. When the zone axis was [112̅], the twinned spots could not be seen (inset). When the NW was further tilted by 30°, the smooth surface turned into a zigzag surface. Each sequential rotation of 30° led to changes in the surface morphology. The surface morphology changed from zigzag to straight, and the morphology then changed back to zigzag. The [111] growth direction remained the same for each rotation. This unique sequential changes in the morphology and FFT images caused by the sample tilting confirmed that the NW consisted of octahedral slice segments in the ZB phase with alternate orientations along the axial [111] direction, although the lengths of the segments were nonuniform. Figure 1e shows the TEM images of the typical S-GaAs1‑xPx NWs, with x being 0.2, 0.5, and 0.8, from the left. In all cases, the twinned structure developed perpendicular to the wire axis. Higher values of x consistently produced twinned superlattices with shorter periodicity. In the case of L-GaAs1−xPx (x = 0.2 and 0.5 from the left), higher values of x consistently produced twinned superlattices with shorter periodicities. Negligible segments appeared at ranges x < 0.3. The GaP (x = 1) NWs contained as many twinned segments as the alloy NWs of identical diameters with high P concentrations. Figure 1f shows the HRTEM and the corresponding FFT images of the S-GaAs0.2P0.8 NWs. The image reveals the presence of high-density segments with periods as short as ∼2 nm. New WZ phase diffraction spots (marked in green) overlapped with the ZB phase twin spots (marked in red/ yellow). The zone axis is indexed as [21̅1̅0] for the WZ phase, and also as [011] for the ZB phase. The d spacings between neighboring WZ (0002) and (011̅2) planes were 3.1 and 1.25 Å, respectively, which is consistent with the values of GaAs0.2P0.8 calculated by Vegard’s rule using the lattice parameters of bulk WZ GaAs (JCPDS card no. 80-0003; a = 3.912 Å and c = 6.441 Å) and WZ GaP (JCPDS card no. 800002; a = 3.759 Å and c = 6.174 Å). The two basic WZ [0002] (or ZB [111]) ED spots were subdivided into about six equal parts. The superlattices showed a periodicity of 1.9 nm, which coincides with six times the (0002) planes. This indicates that one (0002) plane of WZ structure and five (111) planes of ZB structure are stacked alternately in the growth direction. The EDX spectrum confirmed the composition of individual NWs (see the Supporting Information, Figure S4). EDX linescanned (along the radial and axial direction) and mapping showed that the elements distribute homogeneously in the whole NW. The full-range XRD patterns of the S- and LGaAs1−xPx NWs have been acquired, as shown in Figure S1 (in the Supporting Information). All the peaks could be assigned to the ZB phase of GaAs and GaP. The composition (x) of ternary composition NWs was determined using Vegard’s law (i.e., d = (1 − x)dGaAs + xdGaP) based on the peak position of the end
Figure 1. (a) SEM image of high-density NWs homogeneously grown on the substrate. (b) TEM image revealing the general morphology of the NWs (with an average diameter of 60 and 120 nm). (c) HRTEM image of a selected L-GaAs0.2P0.8 NW (diameter = 120 nm) revealing twinned superlattice structure at the [011] zone axis. FFT images (i)− (iii) of the separated twin segments and twin planes are shown in the inset. (d) When titled by 30°, the HRTEM and corresponding FFT (zone axis = [112̅]) images show a single-crystalline feature without twin planes. (e) TEM images of typical S-GaAs1−xPx NWs with x = 0.2, 0.5, and 0.8, and L-GaAs1−xPx NWs with x = 0.2 and 0.5 (from the left), showing the progressive development of shorter twin structures with increase in x. (f) HRTEM images of a selected S-GaAs0.2P0.8 NW (diameter = 60 nm), showing superlattice structures with an average periodicity of 2 nm. The zone axis of the ED pattern (inset) is indexed as WZ [21̅1̅0] (marked in green) and ZB [011] (marked in red/ yellow).
diameter distribution of the S- and L-GaAs1−xPx NWs, with average values of 60 ± 10 and 120 ± 20 nm, respectively (Figure 1b). The NWs exhibited straight and smooth surfaces without any amorphous outerlayers. The HRTEM image of a selected L-GaAs0.2P0.8 NW (with diameter = 120 nm) reveals its ZB phase twinned segments with lengths in the range 50−150 nm. The average length was 100 nm (Figure 1c). The HRTEM image indicates that the d spacing between the neighboring (111) planes is 3.2 Å, which is consistent with the value (dGaAs 4547
dx.doi.org/10.1021/jp500458j | J. Phys. Chem. C 2014, 118, 4546−4552
The Journal of Physical Chemistry C
Article
members. The x value was found to be consistent with the corresponding EDX data. Figures 2a and 2b display the (111) peak of the S- and LGaAs1−xPx (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1) NWs. As x
Figure 3. Raman spectra of the (a) S- and (b) L-GaAs1−xPx NWs (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1), which show the GaAs-like LO, TO, and E2H, and GaP-like LO and TO (GaPP and GaPAs) modes. The data points (○) were resolved into bands (colored lines) using a Voigt function. The black line represents the sum of the resolved bands.
longitudinal optical (LO) phonon modes at 268 and 288 cm−1, respectively. The GaP (x = 1) NWs exhibited TO and LO peaks at 365 and 397 cm−1, respectively. For the various alloy compositions, we assigned the GaAs-like TO and LO modes, in addition to distinct GaP-like doublet TO modes (GaPP-like and GaPAs-like TO modes were separated by ∼12 cm−1) and GaPlike LO mode by the following “1-bond → 2-mode percolation model” suggested by Pages et al.46 The superscripts (As and P) on the GaP-like TO modes refer to the Ga−P bond species in the As-like and P-like host region, respectively. With increase in x, the frequencies of GaAs-like and GaP-like TO phonon modes increased almost linearly, with the GaP-like mode showing a more significant change. This result is consistent with the data shown by the bulk counterparts. The peak position of the resolved bands is plotted as a function of x, as shown in Figure S2 in the Supporting Information. With increase in x, an additional band (green) appeared at the lower frequency (∼12 cm−1) region of the GaAs-like TO band. It has been reported that the E2H mode of the WZ GaAs at 255−256 cm−1 is red-shifted from the ZB GaAs TO mode by about 12 cm−1.47,48 Therefore, we assigned the new band to the E2H modes originating from the twin structures. The intensity of the E2H mode bands of the GaAs-like modes increased with x for both S- and L-GaAs1−xPx. The E2H mode of the WZ GaP is known to be located at 357 or 353 cm−1, which was red-shifted from the TO band of the ZB GaP by 8−9 cm−1.49,50 Due to the limited spectroscopic resolution, this band could not be resolved from the lower frequency side of the GaP-like TO band. In addition, the optical band gap of all the NW samples prepared in this study was estimated by using UV−visible diffuse reflectance spectrum, as shown in Figure 4a. The composition tuning enabled the band gap to show absorption in a wide range of 400−1000 nm (corresponding to an energy range of 1.2−3 eV). Based on Kubelka−Munk (K−M) transformation, the plot of [F(ν)hν]1/2 and [F(ν)hν]2 (where F(ν) is the diffuse reflectance spectrum) versus photon energy yielded the indirect and direct band gaps, respectively (see the Supporting Information, Figure S5 and Table S1). The onset
Figure 2. XRD (111) peak of (a) S- and (b) L-GaAs1−xPx (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1) NWs. (c) fwhm (2θ) of the (111) peak as a function of the P content (x).
increased, the peak position shifted continuously from GaAs to GaP, and the peak width broadened for both diameters. The peak width of the smaller diameter NWs was clearly larger than that of the larger diameter NWs. Figure 2c displays the full width at half-maximum (fwhm) of the (111) peak as a function of x. The data shows that the peak of 60 nm diameter SGaAs1−xPx NWs was about twice as broad as that of the LGaAs1−xPx NWs with a diameter of 120 nm. With increase in x from 0 to 0.9, the fwhm increased from 0.2° to 0.3° in the case of S-GaAs1−xPx NWs and from 0.1° to 0.2° in the case of LGaAs1−xPx NWs. The straight lines represent the linear fit of the data points in that region (0 ≤ x ≤ 0.9). The peak broadening is an indication of the degree of crystalline defects. This could be correlated to the nature of the twinned superlattice structures in which the twin segments with shorter periodicities produced more inhomogeneous lattice constants. Despite the twin structures of GaP (x = 1), its fwhm was much smaller than that of NWs of any other composition, indicating that the alloying induced inhomogeneous lattice constants.45 Raman spectra of the S- and L-GaAs1−xPx NWs (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1), scanned over the range 200−450 cm−1, are shown in Figure S2 (in the Supporting Information). Figures 3a and 3b show the normalized Raman peaks of S- and L-GaAs1−xPx, respectively. The peaks were resolved into two or three bands using the Voigt function. The black line represents the sum of the resolved bands (represented by colored lines). GaAs (x = 0) NWs exhibited transverse optical (TO) and 4548
dx.doi.org/10.1021/jp500458j | J. Phys. Chem. C 2014, 118, 4546−4552
The Journal of Physical Chemistry C
Article
In1−xGaxAs (x < 0.4) NWs, Koblmüler and co-workers also reported a blue shift in the emission energy of ∼30−40 meV with respect to the bulk phase, owing to the larger band gap of the WZ phase.10 The linear fit of the data in the Eg versus x plot of the S-GaAs1−xPx NWs yields Eg = 1.48 eV for GaAs, which is close to the value of WZ GaAs. With increase in x, the Eg of the S-GaAs1−xPx NWs became closer to the value of the LGaAs1−xPx NWs. This could be attributed to the lower band gap of WZ GaP than ZB GaP. Bulk GaAs1−xPx exhibits the optical bowing phenomena, which is characterized by a bowing constant b, where Eg(x) = (1 − x)Eg(GaAs) + xEg(GaP) − bx(1 − x). Most experimental results indicate that the bowing parameter is in the range 0.175−0.21 eV.53,54 Theoretical studies have predicted that the crossover of direct-to-indirect band gap occurs at x = ∼0.5 and the linear dependencies of the direct (x < 0.5) and indirect band gap (x > 0.5) on x.55,56 Our data showed the absence of any bowing or crossover phenomena, which can be attributed to the twinned superlattice structures. Furthermore, the blue shift of band gap in the case of S-GaAs1−xPx may occur because of the radial quantum confinement effect. However, this is ruled out because the NW diameter is well above the Bohr radius of bulk GaAs (∼14 nm). Recent experiments have shown that the band gap increase due to the fact that the quantum confinement effect becomes prominent only for GaAs NWs with diameters
