Browsing by Author "Onen, A."
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Item Open Access Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, SalimPotential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).Item Open Access GaN: From three-to two-dimensional single-layer crystal and its multilayer van der Waals solids(American Physical Society, 2016-02) Onen, A.; Kecik, D.; Durgun, Engin; Çıracı, SalimThree-dimensional (3D) GaN is a III-V compound semiconductor with potential optoelectronic applications. In this paper, starting from 3D GaN in wurtzite and zinc-blende structures, we investigated the mechanical, electronic, and optical properties of the 2D single-layer honeycomb structure of GaN (g-GaN) and its bilayer, trilayer, and multilayer van der Waals solids using density-functional theory. Based on high-temperature ab initio molecular-dynamics calculations, we first showed that g-GaN can remain stable at high temperature. Then we performed a comparative study to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct-band-gap semiconductor, g-GaN in two dimensions has a relatively wider indirect band gap. Moreover, 2D g-GaN displays a higher Poisson ratio and slightly less charge transfer from cation to anion. In two dimensions, the optical-absorption spectra of 3D crystalline phases are modified dramatically, and their absorption onset energy is blueshifted. We also showed that the physical properties predicted for freestanding g-GaN are preserved when g-GaN is grown on metallic as well as semiconducting substrates. In particular, 3D layered blue phosphorus, being nearly lattice-matched to g-GaN, is found to be an excellent substrate for growing g-GaN. Bilayer, trilayer, and van der Waals crystals can be constructed by a special stacking sequence of g-GaN, and they can display electronic and optical properties that can be controlled by the number of g-GaN layers. In particular, their fundamental band gap decreases and changes from indirect to direct with an increasing number of g-GaN layers.Item Open Access In-plane commensurate GaN/AlN junctions: single-layer composite structures, single and multiple quantum wells and quantum dots(American Physical Society, 2017) Onen, A.; Kecik, D.; Durgun, Engin; Çıracı, SalimIn-plane composite structures constructed of the stripes or core/shells of single-layer GaN and AlN, which are joined commensurately, display a diversity of electronic properties that can be tuned by the size of their constituents. In heterostructures, the dimensionality of the electrons changes from two dimensional (2D) to one dimensional (1D) upon their confinements in wide constituent stripes, leading to the type-I band alignment and hence multiple quantum well structure in the direct space. The δ doping of one wide stripe by another narrow stripe results in local narrowing or widening of the band gap. A single quantum well structure is acquired from the finite-size AlN-GaN-AlN junctions. In a patterned array of GaN/AlN core/shells, the dimensionality of the electronic states is reduced from two dimensional to zero dimensional, forming multiple quantum dots in large GaN cores, while 2D electrons propagate in multiply connected AlN shell as if they are in a supercrystal. A consistent and detailed discussion of the effects of confinement in momentum and direct spaces is provided. As a result of confinement, the variation of the band gap in the direct space is found to be rather different from the edges of the conduction and valence bands inferred from the band edges of constituent 2D single-layer GaN and AlN. Even if all the results in this study pertain to the free-standing single-layer composite structures, the effects of the different substrates over which these composites can grow are examined in detail. This study unveils the potential of composite structures in designing novel nanomaterials. These predictions are obtained from first-principles calculations based on density functional theory on 2D GaN and AlN compound semiconductors which were synthesized recently.Item Open Access Lateral and vertical heterostructures of h-GaN/h-AlN: electron confinement, band lineup, and quantum structures(American Chemical Society, 2017-11) Onen, A.; Kecik, D.; Durgun, Engin; Çıracı, SalimLateral and vertical heterostructures constructed of two-dimensional (2D) single-layer h-GaN and h-AlN display novel electronic and optical properties and diverse quantum structures to be utilized in 2D device applications. Lateral heterostructures formed by periodically repeating narrow h-GaN and h-AlN stripes, which are joined commensurately along their armchair edges, behave as composite semiconducting materials. Direct-indirect characters of the fundamental band gaps and their values vary with the widths of these stripes. However, for relatively wider stripes, electronic states are confined in different stripes and make a semiconductor-semiconductor junction with normal band alignment. This way one-dimensinonal multiple quantum well structures can be generated with electrons and holes confined to h-GaN stripes. Vertical heterostructures formed by thin stacks of h-GaN and h-AlN are composite semiconductors with a tunable fundamental band gap. However, depending on the stacking sequence and number of constituent sheets in the stacks, the vertical heterostructure can transform into a junction, which displays staggered band alignment with electrons and holes separated in different stacks. The weak bonds between the cations and anions in adjacent layers distinguish these heterostructures from those fabricated using thin films of GaN and AlN thin films in wurtzite structure, as well as from van der Waals solids. Despite the complexities due to confinement effects and charge transfer across the interface, the band diagram of the heterostructures in the direct space and band lineup are conveniently revealed from the electronic structure projected to the atoms or layers. Prominent features in the optical spectra of the lateral composite structures are observed within the limits of those of 2D parent constituents; however, significant deviations from pristine 2D constituents are observed for vertical heterostructures. Important dimensionality effects are revealed in the lateral and vertical heterostructures.Item Open Access Onset of vertical bonds in new GaN multilayers: beyond van der Waals solids(Royal Society of Chemistry, 2018) Onen, A.; Kecik, D.; Durgun, Engin; Çıracı, SalimA suspended single layer of GaN in a honeycomb structure is stable in a planar geometry. By stacking these GaN layers one can construct bilayers or multilayers, even new three-dimensional (3D) periodic structures. In this study, we clarified how the planar layers are buckled with the onset of vertical Ga-N bonds. Among the four stable phases of bilayer GaN, only one of them maintains the planar geometry, which is bound by weak van der Waals interactions. For the remaining three phases, the layers are buckled with the onset of weak vertical bonds, and attain total energies slightly lower than that of the planar geometry. Structural phase changes, as well as direct-indirect band transitions take place under strain and electrostatic charging. The vertical bonds become shorter in multilayers, and eventually attain the bulk value. Among the stable phases of 3D periodic GaN, only one with a graphite-like structure behaves as a layered, van der Waals solid; whereby others are 3D uniform crystals beyond the van der Waals solid.