GaN Monolayer Research Articles

Point defects and composition in hexagonal group-III nitride monolayers: A first-principles calculation

In this paper, we systematically investigate the structural, electronic and magnetic properties of hexagonal group-III nitride monolayers with point defects and alloying on the basis of first-principles calculations. Six typical point defects including three vacancies and three antisites are modeled in pure AlN, GaN and InN monolayers. The defect-induced modifications of band gaps and magnetic properties are demonstrated. The vacancy of nitrogen, with lowest formation energy, metalizes the semiconducting nitride monolayers. The defects losing single group-III atom introduce net magnetic moment to the systems, while others maintain non-magnetic. For ordered alloy monolayers, the AlGaN and InGaN systems are taken into consideration. The compositional variation is achieved by atomic substitution in supercells with different sizes. We find that the lattice constant and cohesive energy follow good linear relation with concentration while a slight bowing effect is observed for the band gap. These results provide a development in defective and alloy nitride monolayers and extend the potential applications.

A comparative study on magnetic properties of Mo doped AlN, GaN and InN monolayers from first-principles

First-principles calculations are performed to comparatively study the structural, electronic structures and magnetic properties of Mo doped AlN, GaN and InN monolayers (MLs). After Mo atom doping, the semiconducting GaN and InN MLs transform to metal, while the AlN ML keeps semiconducting with a reduced gap. Total magnetic moments of 1.0 and 0.54 µB, which mainly arising from the localized Mo 4d states, are induced by doping in AlN and InN MLs, respectively, while the doped GaN ML is still nonmagnetic. Nevertheless, the excessive localization and strongly ionic character of the Mo-4d states in AlN ML directly impedes the magnetic coupling, leading to a paramagnetic ground states. A similar case is observed in Mo atoms doped InN ML. The firm N-Mo interaction prevent the impurity states permeating out the range of N-Mo pair, resulting in a quick vanishing of ferromagnetic coupling as the distance between two Mo atoms increasing. All configurations of Mo atoms doped GaN ML in this paper are room temperature ferromagnetic. Spin polarized itinerant electrons mediate the magnetic interaction between two Mo atoms. Increasing the Mo concentration may stabilize the FM state and produce a higher Curie temperature. Our calculations show that GaN nanosheets with Mo atoms doped may be a nice candidate for future spintronic devices. And we conclude that a appropriate magnitude of localization (or delocalization) is what the key point to produce room temperature ferromagnetism from this comparative study.

Novel GaN-based nanocomposites: Effective band structure and optical property tuning by tensile strain or external field

The electronic and optical properties of the GaN monolayer hybrid with ZnO, MoS2 and MoSe2 are studied by means of density functional theory. The lattice mismatches of these three heterobilayers are less than 2.5% and they are stable with the binding energies larger than the ones of bilayer graphene. Despite of the indirect band gap of GaN monolayer, the three heterostructures exhibit direct band gap characteristics and their optical spectrums are broader than those of the isolated constituent layers because of interlayer coupling. Besides, both of biaxial tensile strain and external electric field (E-field) can effectively tune the band gaps of these heterostructures. The band gaps will decrease with the increase of tensile strain or the positive E-field strength while the ones will increase with the increase of the negative E-field intensity. Meanwhile, the adsorption edges of their imaginary parts of the dielectric function display corresponding redshift or blueshift characteristics and the band gaps may undergo direct to indirect transition beyond a critical strain or E-field intensity. The results shows that these GaN heterostructures can be good candidate materials for further investigations on the photo-related applications such as light-emitting diodes (LEDs), photodetection or solar energy conversion.

Structural and electrical analysis of epitaxial 2D/3D vertical heterojunctions of monolayer MoS2 on GaN

Integration of two-dimensional (2D) and conventional (3D) semiconductors can lead to the formation of vertical heterojunctions with valuable electronic and optoelectronic properties. Regardless of the growth stacking mechanism implemented so far, the quality of the formed heterojunctions is susceptible to defects and contaminations mainly due to the complication involved in the transfer process. We utilize an approach that aims to eliminate the transfer process and achieve epitaxial vertical heterojunctions with low defect interfaces necessary for efficient vertical transport. Monolayers of MoS2 of approximately 2 μm domains are grown epitaxially by powder vaporization on GaN substrates forming a vertical 2D/3D heterojunction. Cross-sectional transmission electron microscopy (XTEM) is employed to analyze the in-plane lattice constants and van der Waals (vdW) gap between the 2D and 3D semiconductor crystals. The extracted in-plane lattice mismatch between monolayer MoS2 and GaN is only 1.2% which corresponds well to the expected mismatch between bulk MoS2 and GaN. The vdW gap between MoS2 and GaN, extracted from the XTEM measurements, is consistent with the vdW gap of 3.1 Å predicted by our first principles calculations. The effect of monolayer (1L) MoS2 on the electrical characteristics of 2D/3D semiconductor heterojunctions was studied using conductive atomic force microscopy (CAFM). The electrical current across the CAFM-tip/1L-MoS2/GaN vertical junctions is dominated by the tip/GaN interface of both n- and p-doped GaN. This electronic transparency of 1L-MoS2 tells us that a 2D crystal component has to be above a certain thickness before it can serve as an independent semiconductor element in 2D/3D heterojunctions.

Phonon transport properties of bulk and monolayer GaN from first-principles calculations

The thermal conductivity of isotopically pure wurtzite GaN and the corresponding two-dimensional monolayer crystals are investigated based on first-principles calculations and phonon Boltzmann transport equation. It is found that the c-axis thermal conductivity of defect-free wurtzite GaN is 42–11% higher than that along the in-plane direction when temperature increases from 100 to 800K. Our calculation also shows that the thermal conductivity of monolayer GaN with hexagonal lattice is higher than that with haeckelite lattice by a factor of four at room temperature. The phonon dispersion relation, phonon group velocity and the phonon scattering rate are extracted to uncover the underlying physical mechanism. The thermal conductivity accumulation function with respect to the phonon mean free path and the relative contribution of each phonon branch for the two kinds of monolayer GaN are further calculated as compared to that of graphene. These investigations would provide important guidance to develop GaN-based electronic and photonic device.

Effects of strain and thickness on the electronic and optical behaviors of two-dimensional hexagonal gallium nitride

The full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory has been used to study effects of strain and thickness on the electronic and optical properties of two-dimensional GaN. The band gap of monolayer and bilayer GaN under compressive in-plane strain change from indirect to direct with bond length shortening. Also, the semiconductor to semimetal transition occurs for monolayer and bilayer GaN under in-plane tensile strain with bond length elongation. It is found that the tensile and compressive strains cause the red and blue shifts in the optical spectra, respectively, for both monolayer and bilayer GaN. Applying the perpendicular strain on the bilayer GaN by decreasing the inter layer distance leads to the shift of valence band maximum towards the Γ point in the band structure and shift of peak positions and variation of peak intensities in ε2(ω) spectrum. The results show that the n-layer GaN has an indirect band gap for n < 16. The results suggest that monolayer and multilayer GaN are good candidates for application in optoelectronics and flexible electronics.

The theory of surface-enhanced Raman scattering on semiconductor nanoparticles; toward the optimization of SERS sensors.

We present an expression for the lowest order nonzero contribution to the surface-enhanced Raman spectrum obtained from a system of a molecule adsorbed on a semiconductor nanoparticle. Herzberg-Teller vibronic coupling of the zero-order Born-Oppenheimer states results in an expression which may be regarded as an extension of the Albrecht A-, B-, and C-terms to SERS substrates. We show that the SERS enhancement is caused by combinations of several types of resonances in the combined system, namely, surface, exciton, charge-transfer, and molecular resonances. These resonances are coupled by terms in the numerator, which provide selection rules that enable various tests of the theory and predict the relative intensities of the Raman lines. Furthermore, by considering interactions of the various contributions to the SERS enhancement, we are able to develop ways to optimize the enhancement factor by tailoring the semiconductor nanostructure, thereby adjusting the locations of the various contributing resonances. This provides a procedure by which molecular sensors can be constructed and optimized. We provide several experimental examples on substrates such as monolayer MoS2 and GaN nanorods.

Effect of nitride chemical passivation of the surface of GaAs photodiodes on their characteristics

Characteristics of GaAs photodiodes have been studied before and after the chemical nitridation of their surface in hydrazine sulfide solutions, which leads to substitution of surface As atoms with N atoms to give a GaN monolayer. The resulting nitride coatings hinder the oxidation of GaAs in air and provide a decrease in the density of surface states involved in recombination processes. The device characteristics improved by nitridation are preserved during a long time.

Design of a new two-dimensional diluted magnetic semiconductor: Mn-doped GaN monolayer

To meet the need of low-dimensional spintronic devices, we investigate the electronic structure and magnetic properties of Mn-doped GaN monolayer using first-principles method. We find the nonmagnetic GaN monolayer exhibits half-metallic ferromagnetism by Mn doping due to double-exchange mechanism. Interestingly, the ferromagnetic coupling in Mn-doped GaN monolayer is enhanced with tensile strain and weakened with compressive strain. What is more, the ferromagnetic–antiferromagnetic transformation occurs under compressive strain of −9.5%. These results provide a feasible approach for fabrication of a new GaN monolayer based diluted magnetic semiconductor.

Electronic and magnetic properties in Mn‐doped IIIA‐nitride monolayers

Based on first‐principles calculations plus Hubbard U, we have studied the electronic structure and magnetic properties of Mn‐doped IIIA‐nitride monolayers. The substitution of Mn for Al or Ga atom induces a total magnetic moment of 4.00 µB per dopant, independent of the choice of functional. The doped AlN system is half‐metallic at GGA + U (&lt;5 eV) level, but is a magnetic semiconductor at the HSE06 and GGA + U (≥5 eV) levels. The doped GaN system is a magnetic semiconductor at these two levels. The Mn‐doped InN converts into metal without magnetism, due to the robust p–d hybridization between the Mn‐3d and N‐2p states and the delocalized sd states. The Mn atoms have a clear clustering tendency with the FM state in the AlN and GaN hosts. The long‐ranged ferromagnetic coupling is observed along the ⋯N‐Al/Ga⋯ zigzag edge directions, which can be attributed to p–d hybridization mechanism. These results suggest that the Mn‐doped AlN and GaN monolayers are promising candidates for preparing spintronic devices at the nanoscale.

Improvement of p-type conductivity in Al-rich AlGaN substituted by MgGaδ-doping (AlN)m/(GaN)n (m≥n) superlattice

The challenge of p-type doping in Al-rich AlGaN is investigated based on first-principles calculations. We find that the p-type conductivity can be improved by replacing Al-rich AlGaN with MgGaδ-doping (AlN)m/(GaN)n (m ≥ n) superlattice (SL). The formation energy Ef is the lowest and acceptor activation energy EA is the smallest for Mg substituting Ga in the SL. The EA increases if the doping position moves from GaN to AlN layer. Moreover, EA decreases with increasing the number of GaN monolayers. The EA can be reduced from 0.48 eV in AlN to 0.26 eV in (AlN)5/(GaN)1, 0.25 eV in (AlN)1/(GaN)1, 0.24 eV in (AlN)4/(GaN)2 and 0.22 eV in (AlN)3/(GaN)3 SLs. This will lead to a high hole concentration in the order of 1018 cm−3 at room temperature, which is favorable for AlGaN-based deep ultraviolet optoelectronic devices.

Sub-230 nm deep-UV emission from GaN quantum disks in AlN grown by a modified Stranski–Krastanov mode

We report tunable deep-ultraviolet (DUV) emission over the 222–231 nm range from 1–2 monolayer (ML) GaN quantum disks (QDs) grown in an AlN matrix. The linewidth of the emission were as narrow as ∼10 nm at 5 K. The disks were grown in modified Stranski–Krastanov (mSK) mode. High resolution scanning transmission electron microscopy (STEM) images confirmed insertion of 1–2 MLs of GaN between 3 nm AlN barriers. The internal quantum efficiency was estimated from low temperature photoluminescence measurements for the disks, and compared with 1 and 2 ML GaN quantum wells/AlN barriers. The internal quantum efficiency (IQE) of the GaN QDs was found to be ∼35% for 222 nm emission, ∼200% higher than 1 ML GaN QWs.

Tuning magnetism of monolayer GaN by vacancy and nonmagnetic chemical doping

In view of important role of inducing and manipulating the magnetism in 2D materials for the development of low-dimensional spintronic devices, the magnetism of GaN monolayer with Ga vacancy and nonmagnetic chemical doping are investigated using first-principles calculations. It is found that pure GaN monolayer has graphene-like structure and is nonmagnetic. While, a neutral Ga vacancy can induce 3μB intrinsic magnetic moment, localized mainly on the neighboring N atoms. Interestingly, after one Mg or Si atom doping in g-GaN with Ga vacancy, the magnetic moment can be modified to 4μB or 2μB respectively due to the change in hole number. Meantime, Mg-doped g-GaN with Ga vacancy shows half-metal character. With the increasing of doping concentrations, the magnetic moment can be further tuned. The results are interesting from a theoretical point of view and may open opportunities for these 2D GaN based materials in magnetic devices.

Semiconductor-Insulator-Semiconductor Diode Consisting of Monolayer MoS2, h-BN, and GaN Heterostructure.

We propose a semiconductor-insulator-semiconductor (SIS) heterojunction diode consisting of monolayer (1-L) MoS2, hexagonal boron nitride (h-BN), and epitaxial p-GaN that can be applied to high-performance nanoscale optoelectronics. The layered materials of 1-L MoS2 and h-BN, grown by chemical vapor deposition, were vertically stacked by a wet-transfer method on a p-GaN layer. The final structure was verified by confocal photoluminescence and Raman spectroscopy. Current-voltage (I-V) measurements were conducted to compare the device performance with that of a more classical p-n structure. In both structures (the p-n and SIS heterojunction diode), clear current-rectifying characteristics were observed. In particular, a current and threshold voltage were obtained for the SIS structure that was higher compared to that of the p-n structure. This indicated that tunneling is the predominant carrier transport mechanism. In addition, the photoresponse of the SIS structure induced by the illumination of visible light was observed by photocurrent measurements.

Chemical Functionalization of GaN Monolayer by Adatom Adsorption

The effect of adatom adsorption on the magnetism and band structure of graphene-like GaN monolayer at different coverages (1/8 and 1/2) was studied by spin-polarized density functional theory calculations. Generally speaking, though bare GaN monolayer is an indirect-band gap semiconductor, it could be turned to magnetic half-metal/metal by adsorbing F or N atoms at certain coverage. The Curie temperatures for half-metallic F-adsorbed and metallic N-adsorbed GaN monolayer are estimated to be about 480 and 420 K by Monte Carlo simulations, respectively. The induced magnetism mainly comes from the spin splitting of N 2p (2pz or 2px/2py) orbitals. In the meantime, the adsorption of O atoms reduces the gap of the system. These excellent electronic and magnetic properties indicate great potential application of 2D GaN-based materials in future spintronics and optoelectronics.

Improving p-type doping efficiency in Al0.83Ga0.17N alloy substituted by nanoscale (AlN)5/(GaN)1 superlattice with MgGa-ON δ-codoping: Role of O-atom in GaN monolayer

We calculate Mg-acceptor activation energy EA and investigate the influence of O-atom, occupied the Mg nearest-neighbor, on EA in nanoscale (AlN)5/(GaN)1 superlattice (SL), a substitution for Al0.83Ga0.17N disorder alloy, using first-principles calculations. We find that the N-atom bonded with Ga-atom is more easily substituted by O-atom and nMgGa-ON (n = 1-3) complexes are favorable and stable in the SL. The O-atom plays a dominant role in reducing EA. The shorter the Mg-O bond is, the smaller the EA is. The Mg-acceptor activation energy can be reduced significantly by nMgGa-ON δ-codoping. Our calculated EA for 2MgGa-ON is 0.21 eV, and can be further reduced to 0.13 eV for 3MgGa-ON, which results in a high hole concentration in the order of 1020 cm−3 at room temperature in (AlN)5/(GaN)1 SL. Our results prove that nMgGa-ON (n = 2,3) δ-codoping in AlN/GaN SL with ultrathin GaN-layer is an effective way to improve p-type doping efficiency in Al-rich AlGaN.

Enhanced ferromagnetic properties of Cu doped two-dimensional GaN monolayer

We demonstrate enhanced ferromagnetism in copper doped two-dimensional GaN monolayer ( GaN -ML). Our first principle calculation based on density functional theory predicted that nonmagnetic Cu -dopant with concentration of 6.25% to be ferromagnetic (FM) in 2D GaN layer which carries a magnetic moment of 2.0 μB per Cu atom and it is found to be long range magnetic coupling among the Cu -dopant. The Cu-dopant in 2D GaN -ML which can be explained in terms of p-d hybridization at Curie temperature and this dopant prefer the FM behavior in 2D GaN layer. Hence Cu doped 2D GaN layer shows strong magnetic properties so that it is a promising material in the field of spintronics.

Ab initio studies of early stages of nitride growth process on silicon carbide

We present results of our studies dealing with the early stages of growth of Ga–N and Al–N monolayers on the 4H-SiC {0001} substrates. These studies are based on first principles calculation in the framework of density functional theory. Some stable adsorption sites are found for Ga, N, and Al atoms on the 4H-SiC {0001} surfaces. These are in particular T4 for Ga and N atoms adsorbed on the (0001) surface and on top of the topmost surface atom for Al at the 1 ML coverage. In the case of (0001¯) surface, it is the ‘on-top’ position for Ga and Al, as well as T4 for N atoms, at the same coverage. Further, the energetics of the GaN and AlN monolayers formation on the SiC substrate is investigated. It can be stated that the best 4H-SiC substrate termination allowing the single unreconstructed GaN bilayer growth is the (0001) one. Possible charge compensation patterns of interfaces are proposed together with the discussion on their stability. They involve the mixed Ga–C/SiC{0001} or Ga-Si/SiC{0001} layers that in consequence lead to the adsorption energy gain for nitrogen.

The feasibility of tunable p-type Mg doping in a GaN monolayer nanosheet

Based on density functional theory, the electronic structures, formation energy and transition energy level of a p-type Mg-doped GaN nanosheet are investigated. Numerical results show that the transition energy level decreases monotonously with increasing Mg doping concentration in Mg-doped GaN nanosheet systems, which is lower than that of the Mg-doped bulk GaN case. Moreover, the formation energy calculations indicate that Mg-doped GaN nanosheet structures can be realized under N-rich experimental growth conditions.

Molecular dynamics investigation of the thermomechanical behavior of monolayer GaN

Molecular dynamics simulations are performed on monolayer gallium nitride to study their mechanical behavior at various temperatures in the range of 10 to 1700 K. The transition from brittle to ductile nature has been illustrated from the evolution of fracture at two different temperatures of 700 and 1300 K. Brittle to ductile transition temperatures TBDT are obtained from the plots of logarithm of yield stress and inverse temperature at different strain rates and compared qualitatively with the same system in the presence of single and diatomic vacancies. Logarithm of strain rate against inverse of TBDT thus obtained represents an Arrhenius plot, the slope of which corresponds to the activation energy of dislocation glide that is found to be approximately 2.0 ± 0.05 eV for the present case. This suggests that the brittle to ductile transition is controlled by the dislocation mobility as in the case of other semiconductors like silicon and germanium. This behavior is found to be consistent with the presented underlying models. In addition, thermal conductivities are obtained over a temperature range of 300 to 2000 K from the equilibrium Green-Kubo formulations and compared with the (25,0) nanotube that is generated from the same system of monolayer GaN. The values are found to be decreased in both the cases as compared to the bulk gallium nitride, and the reduction in the values of thermal conductivity can be attributed to the finite size effects, increased surface inelastic scattering, and change of phonon spectrum at low dimensions, respectively.