Lateral and Vertical Heterostructures of h-GaN/h-AlN: Electron Confinement, Band Lineup, and Quantum Structures

Abstract

Lateral 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.

Chemical and substitutional doping, and anti-site and vacancy formation in monolayer AlN and GaN

“…[2][3][4] Following the predictions of graphene-like GaN and AlN, which will be denoted as h-GaN and h-AlN in this paper, theoretical studies continued to unveil their diverse features. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Attaining outstanding optoelectronic properties of three-dimensional (3D), group III-V compound nitride semiconductors in their 2D allotropes for future applications in 2D electronics has been the major driving force for the growing research interest in h-GaN and h-AlN.…”

“…They readily form bilayers and multilayers, even h-GaN/h-AlN heterostructures, whereby the electronic and optical properties can be tuned by the number of layers. [16][17][18][19][20] Other efficient ways to modify the materials and hence to attain diverse physical properties have been the chemisorption of foreign atoms or molecules, substitution of host atoms by other atoms (referred also as doping) and creation of patterned vacancies or divacancies in these 2D structures. [24][25][26][27][28] The coverage and geometry of the chemisorbed adatoms or molecules are crucial parameters to control the physical properties.…”

Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: A first-principles perspective to recent synthesis

“…[18][19][20] Since the first time prediction of 2D SL graphitic (or honeycomb) structures of GaN and AlN, 17 which will be denoted as h-GaN and h-AlN, theoretical studies continued to unveil the diverse aspects of these materials. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] While several 2D SL structures, which were first predicted through first-principles calculations, were already synthesized in the laboratories, it took, however, a long time for the realization of ultrathin layers of GaN and AlN. Earlier, followed by the synthesis of single-walled nanotubes and thick-wall tubular forms of GaN and AlN, [38][39][40] ultrathin GaN and AlN layer(s), or nanosheets, as well as their composites' several forms were grown on specific substrates such as sapphire, Si (111), 6H-SiC(0001), InGaN, single-walled carbon nanotube/graphene, etc.…”

Electronic and hyperbolic dielectric properties of ZrS2/HfS2 heterostructures

“…Recently fabricated Zirconium disulfide (ZrS2) and Hafnium disulphide (HfS2) exhibit well-balanced carrier mobility and band gaps, and attracted interest in solar cells and field effect transistors [14][15][16]. Meanwhile, they show highly anisotropic mechanical, optical, and electrical properties [5,17]. More importantly, the layered TMDs can be naturally hyperbolic materials (HMMs) [18].…”