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Semiconductor nanowire (NW) based heterostructures are a promising material system for next generation optoelectronic devices, such as flexible solar cells and light emitting diodes [1]. Their reduced contact area and surface strain relaxation allow for epitaxial growth on lattice-mismatched substrates, a key advantage for integration of different III-V semiconductors with existing silicon-based technology.Position-controlled NWs can be grown in ordered arrays on Si to improve uniformity and device integration. This is commonly performed by using a SiO 2 thin film as a mask. Patterning of circular holes in the mask ( Fig. 1(a)) allows for site-specific NW growth in predefined patterns and positions. To date, this is performed using lithography techniques such as electron beam lithography or nanoimprint lithography [2]. Important processing parameters include oxide thickness, hole diameter and pattern pitch, requiring several steps to be optimized in order to achieve a high yield of uniform NWs [3]. Additionally, the catalytic particle is rarely centered in the hole, leading to undesirable asymmetry in the NW cross-sections [4].In this work, the parameter space for direct patterning of NW growth substrates by focused ion beam (FIB) is explored (Fig. 1). Self-catalyzed GaAsSb NWs were grown using molecular beam epitaxy (MBE) on a FIB patterned Si(111) substrate with 40 nm thermal oxide, where hole size, dose and Ga-beam overlap were systematically varied ( Fig. 1(a-c)). It is expected that a higher degree of flexibility and control can be attained using FIB compared to the conventionally used resist-based patterning techniques. In addition, patterning by FIB leads to Ga implantation in both Si and SiO 2 , which could positively affect the self-catalyzed NW growth and the properties of the NW-substrate system in a unique way.After MBE growth, three distinct growth regimes can be recognized, present in all arrays ( Fig. 1(d-e)): The smallest (10 nm pattern) diameter row features a high yield (≤ 80%) of straight NWs. As the hole diameter increases there is initially a transition to more parasitic crystal growth and finally multiple (2-5) NWs grow within each hole. As the dose increases between arrays in each column, the patterned diameter for these transitions decreases proportionally. The results demonstrate that using FIB the parameter space can be mapped out efficiently within a single growth session and that growth can be tuned between aligned single NWs, 2D parasitic crystals and multiple NWs per hole. Transmission electron microscopy and electrical testing of single NWs directly on the growth substrate [5] will be used to refine the structural analysis and study the electrical properties of these NWs. It is expected that in addition to the flexibility of FIB patterning, III-V NWs grown on FIB-patterned Si will exhibit novel properties due to the implantation of Ga and the altered NW-substrate interface.[1] Joyce, H.
Semiconductor nanowire (NW) based heterostructures are a promising material system for next generation optoelectronic devices, such as flexible solar cells and light emitting diodes [1]. Their reduced contact area and surface strain relaxation allow for epitaxial growth on lattice-mismatched substrates, a key advantage for integration of different III-V semiconductors with existing silicon-based technology.Position-controlled NWs can be grown in ordered arrays on Si to improve uniformity and device integration. This is commonly performed by using a SiO 2 thin film as a mask. Patterning of circular holes in the mask ( Fig. 1(a)) allows for site-specific NW growth in predefined patterns and positions. To date, this is performed using lithography techniques such as electron beam lithography or nanoimprint lithography [2]. Important processing parameters include oxide thickness, hole diameter and pattern pitch, requiring several steps to be optimized in order to achieve a high yield of uniform NWs [3]. Additionally, the catalytic particle is rarely centered in the hole, leading to undesirable asymmetry in the NW cross-sections [4].In this work, the parameter space for direct patterning of NW growth substrates by focused ion beam (FIB) is explored (Fig. 1). Self-catalyzed GaAsSb NWs were grown using molecular beam epitaxy (MBE) on a FIB patterned Si(111) substrate with 40 nm thermal oxide, where hole size, dose and Ga-beam overlap were systematically varied ( Fig. 1(a-c)). It is expected that a higher degree of flexibility and control can be attained using FIB compared to the conventionally used resist-based patterning techniques. In addition, patterning by FIB leads to Ga implantation in both Si and SiO 2 , which could positively affect the self-catalyzed NW growth and the properties of the NW-substrate system in a unique way.After MBE growth, three distinct growth regimes can be recognized, present in all arrays ( Fig. 1(d-e)): The smallest (10 nm pattern) diameter row features a high yield (≤ 80%) of straight NWs. As the hole diameter increases there is initially a transition to more parasitic crystal growth and finally multiple (2-5) NWs grow within each hole. As the dose increases between arrays in each column, the patterned diameter for these transitions decreases proportionally. The results demonstrate that using FIB the parameter space can be mapped out efficiently within a single growth session and that growth can be tuned between aligned single NWs, 2D parasitic crystals and multiple NWs per hole. Transmission electron microscopy and electrical testing of single NWs directly on the growth substrate [5] will be used to refine the structural analysis and study the electrical properties of these NWs. It is expected that in addition to the flexibility of FIB patterning, III-V NWs grown on FIB-patterned Si will exhibit novel properties due to the implantation of Ga and the altered NW-substrate interface.[1] Joyce, H.
Semiconductor nanowires (NWs) have promising properties for optoelectronic devices such as solar cells and light emitting diodes. For the development of such NW‐based devices, the correlation between structural features, composition, and optoelectronic properties of the NWs must be well understood. This task can be challenging as the growth can induce large NW‐to‐NW variations. As a statistical meaningful sampling at the required spatial resolution by multiple techniques would be very time consuming, correlated studies where the exact same NWs are characterized optically and structurally by different techniques are an alternative [1]. In this study, the same single self‐catalyzed GaAs/AlGaAs core‐shell NW is studied using micro‐photoluminescence (µ‐PL) and transmission electron microscopy (TEM). After conventional TEM, cross‐sections of two regions of the same NW were made using focused ion beam (FIB) to obtain a 3D impression of the NW. To study variations in the shell, a cross‐section was made perpendicular to the growth direction from the lower half of the NW, and to study variations in the tip a section was made perpendicular to the ‐1‐12‐direction from the top of the NW (Fig. 1(a)). The cross‐sections were studied using high‐angle annular dark‐field scanning TEM (HAADF STEM) and quantitative electron‐dispersive x‐ray spectroscopy (EDS) to reveal compositional variations in the different directions of the NW. The NWs in the growth batch are mostly defect free zinc blende (ZB), with stacking faults and a wurtzite (WZ) region towards the tip area (Fig. 1(b‐c)). µ‐PL of 17 studied NWs shows a signal at the ZB GaAs free exciton energy at 12 K. About half of the NWs also have an additional PL signal at higher energy, as can be seen in Fig. 1(e). Compositional variations in the AlGaAs shell of the NWs could possibly explain this high‐energy PL emission in the 1.6‐1.8 eV energy range [2]. In both cross‐sections (Fig. 1(d) and Fig. 2) this type of structure, with narrow Al‐rich and Al‐deficient bands parallel to the facets of the NW, is visible. Quantitative EDS maps based on the zeta‐method [3] (Fig. 3(a)) shows that the Al concentration in the Al deficient bands for the observed widths is too high to explain the sharp PL emission in the range 1.6‐1.8 eV. In addition to the shell, the tip region also depicts compositional variations. These features were only visible in the cross‐section normal to the ‐1‐12 ‐ direction (Fig. 1(d) and 2(b)) and not apparent by conventional TEM imaging (Fig. 1(c)). Quantitative EDS (Fig. 3(b)) shows that the Al concentration is varying within the tip. Correlated studies on the very same NW including µ‐PL, conventional TEM, FIB preparation in different directions and quantitative EDS are required to visualize and explain self‐induced compositional variations and peculiar optical characteristics within these GaAs/AlGaAs core‐shell NWs.
A practical method to determine the composition within ternary heterostructured semiconductor compounds using energy-dispersive X-ray spectroscopy in scanning transmission electron microscopy is presented. The method requires minimal external input factors such as user-determined or calculated sensitivity factors by incorporating a known compositional relationship, here a fixed stoichiometric ratio in III–V compound semiconductors. The method is demonstrated for three different systems; AlGaAs/GaAs, GaAsSb/GaAs, and InGaN/GaN with three different specimen geometries and compared to conventional quantification approaches. The method incorporates absorption effects influencing the composition analysis without the need to know the thickness of the specimen. Large variations in absorption conditions and assumptions regarding the reference area limit the accuracy of the developed method.
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