Guided Growth of Horizontal ZnSe Nanowires and their Integration into High‐Performance Blue–UV Photodetectors

Abstract

In order to unleash the potential of the guided growth approach and to explore its principles and generality, it must be extended to new and interesting materials with a wider range of electrical, structural, and optical properties. Zinc selenide (ZnSe) is an important direct bandgap semiconductor with a wide 2.7 eV bandgap and a large 20 meV exciton binding energy. [ 26,27 ] It is interesting from the crystal structure standpoint due to reports of both zinc-blend (ZB) and wurtzite (WZ) NWs, and as an optoelectronic material, since it has a bandgap energy in the VIS range. [ 28,29 ] This chalcogenide holds promise for the fabrication of blue LEDs and lasers, and for diverse applications such as optical switching, photodetection, and second-harmonic generation (SHG) applications. [30][31][32][33] Here we report the guided growth of horizontal and aligned ZnSe NWs with controlled orientations on six different fl at and faceted planes of sapphire (α-Al 2 O 3 ). The concepts and realization of the guide growth are presented in Figure 1 B. Three general growth modes that dictate the reproducible alignment of the NWs were realized: a) epitaxial guided growth along specifi c lattice directions on fl at surfaces, and graphoepitaxial guided growth on nanofaceted surfaces along: b) nanosteps, and c) nanogrooves.The guided ZnSe NWs were fi rst examined with respect to their elemental composition, crystallographic orientation and growth directions. Surprisingly, some new behaviors of guided NWs were found: i) temperature-dependent directional growth of epitaxial NWs was encountered, where some growth directions were observed only at elevated temperatures while others were observed throughout the entire guided growth temperature regime; ii) crystal structure variations were observed where guided NWs on some sapphire planes presented exclusively one crystal phase, either WZ or ZB, while on other planes both ZB and WZ NWs could grow side by side.In order to examine the optoelectronic properties of the ZnSe guided NWs, we exploited the advantages of the guided growth approach and fabricated multiple photodetectors using only parallel fabrication steps. The ZnSe-based photodetectors have the lowest dark currents (below our detection limit 10 −15 A) and the best measured rise and decay times (74 ms and 0.2 s, respectively) for devices that are based on ZnSe 1D nanostructures (see Table S1, Supporting Information, for previous results). [ 31,32,[34][35][36] These results along with the simple and parallel fabrication process and the ability to control the number of NWs in a single device and thus adjust its performance, reveal the advantages of guided ZnSe NWs as building blocks for fast blue and UV-light-sensitive photodetectors.The vapor-liquid-solid (VLS) growth of guided ZnSe NWs was carried out in a two-zone tube furnace system (see the Experimental Section for details). NWs grow horizontally and aligned from the Au catalyst pattern edges or from dispersed nanoparticles. Their typical diameter varied between 10 and Single-cry...

Surface-Guided Core–Shell ZnSe@ZnTe Nanowires as Radial p–n Heterojunctions with Photovoltaic Behavior

“…This behavior resembles previous guided growth reports of GaN, 28 ZnO, 29 ZnSe, 40 ZnTe 41 and CdSe 44 nanowires on C-plan sapphire. HRTEM imaging of a cross sectional lamellae 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 interfacial energy and stain that is accumulated during the heteroepitaxial growth.…”

“…40 Many reports in the literature, including our own, demonstrate changes in epitaxial growth of nanowires due to changes in the growth parameters. 55 We believe that our recently assembled growth system, and resulting changes in the growth conditions, are responsible for the different result in this work compared to our 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 11 previous study.…”

“…In a previous work, 40 we mentioned that graphoepitaxial nanowires need to adapt simultaneously to two sapphire planes and their energetic constraints. We hypothesized that the accumulated strain is manifested in a rougher surface and lesser uniformity in growth axes.…”

“…Past studies have shown that nanowires can grow along nanosteps and nanogrooves on nanofaceted sapphire substrate. 23,24,[40][41][42] This mode of guided growth, which we refer to as graphoepitaxy, has proven to be dominant over epitaxial growth, and essentially overrules the epitaxial relations between the sapphire and the nanowire. 28,29,40,41,44 This was the case also for the core-shell guided nanowires, as can be seen in figure 3.…”

“…23,24,[40][41][42] This mode of guided growth, which we refer to as graphoepitaxy, has proven to be dominant over epitaxial growth, and essentially overrules the epitaxial relations between the sapphire and the nanowire. 28,29,40,41,44 This was the case also for the core-shell guided nanowires, as can be seen in figure 3. The six-directional growth of guided nanowires on flat C-plane is overruled by annealing a C (0001) sapphire substrate that was miscut 2 degrees towards [11 ത 00] to form an 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 array of L-shaped nanosteps due to step-bunching of the high-index.…”