GaN Monolayer Research Articles

Effect of strain on the structural and electronic properties of graphene-like GaN: A DFT study

We present ab initio density functional theory (DFT) calculations on the effect of in-plane equi-biaxial strain on the structural and electronic properties of graphene-like GaN monolayer (ML-GaN). For compressive strain in excess of 7.2%, ML-GaN gets buckled; buckling parameter increases quadratically with compressive strain. The 2D bulk modulus of ML-GaN was found to be smaller than that of graphene and graphene-like ML-BN, which reflects weaker bond in ML-GaN. More importantly, the bandgap and effective masses of charge carriers in ML-GaN were found to be tunable by application of in-plane equi-biaxial strain. In particular, when compressive biaxial strain of about 3% was reached, a transition from indirect to direct bandgap-phase occurred with change in the value and nature of effective masses of charge carriers; buckling and tensile strain reduced the bandgap — the bandgap reduced to 50% of its unstrained value at 6.36% tensile strain and to 0 eV at an extrapolated tensile strain of 12.72%, which is well within its predicted ultimate tensile strain limit of 16%. These predictions of strain-engineered electronic properties of highly strain sensitive ML-GaN may be exploited in future for potential applications in strain sensors and other nanodevices such as the nano-electromechanical systems (NEMS).

Room-temperature stability of excitons and transverse-electric polarized deep-ultraviolet luminescence in atomically thin GaN quantum wells

We apply first-principles calculations to study the effects of extreme quantum confinement on the electronic, excitonic, and radiative properties of atomically thin (1–4 atomic monolayers) GaN quantum wells embedded in AlN. We determine the quasiparticle bandgaps, exciton energies and wave functions, radiative lifetimes, and Mott critical densities as a function of well and barrier thickness. Our results show that quantum confinement in GaN monolayers increases the bandgap up to 5.44 eV and the exciton binding energy up to 215 meV, indicating the thermal stability of excitons at room temperature. Exciton radiative lifetimes range from 1 to 3 ns at room temperature, while the Mott critical density for exciton dissociation is approximately 1013 cm−2. The luminescence is transverse-electric polarized, which facilitates light extraction from c-plane heterostructures. We also introduce a simple approximate model for calculating the exciton radiative lifetime based on the free-carrier bimolecular radiative recombination coefficient and the exciton radius, which agrees well with our results obtained with the Bethe–Salpeter equation predictions. Our results demonstrate that atomically thin GaN quantum wells exhibit stable excitons at room temperature for potential applications in efficient light emitters in the deep ultraviolet as well as room-temperature excitonic devices.

Biaxial strain and external electric field effects on the electronic structure of hydrogenated GaN monolayer

Abstract We investigate systematically the electronic properties of fully hydrogenated GaN (HGaNH) monolayer under effects of the biaxial strain and external electric field. Results assert that the pristine GaN monolayer is an indirect semiconductor with a band gap of 1.933 eV. Band structures indicate that the N-p and Ga-p states are the main contributors to the valence and conduction bands near Fermi level, respectively. The full hydrogenation can induce an increase of this parameter to 3.019 eV, with H-s state being present in both valence and conduction bands. Interestingly, the HGaNH monolayer transforms from indirect to direct gap as a consequence of GaN hydrogenation. Based on results, the electronic band gap of HGaNH monolayer shows sensitivity to the biaxial strain and external electric field. Therefore, these factors can be used to effectively tune the electronic properties of HGaNH monolayer in order to make it more suitable for optoelectronic applications.

Research on quantum efficiency of GaN monolayer reflection-mode photocathode with atomically ultra-thin emission layer

The excitonic effects of GaN will be enhanced by quantum confinement effect when it has only an atomic layer, which can greatly enhance strength of electron–hole interaction and lead to a higher carrier concentration, so GaN monolayer can be used to prepare photocathode to achieve higher quantum efficiency. In this paper, we put forward GaN monolayer reflection-mode photocathode Considering electrons generated both in buffer layer and each GaN monolayer, we propose a revised quantum efficiency (QE) formula for ultra-thin GaN ML photocathode. And based on the quantum efficiency formula, back interface recombination rate, surface escape probability and surface reflectivity are simulated when emission layer is 1 ML GaN. Furthermore, we also simulate back interface recombination rate, recombination rate of the GaN ML/GaN ML interface and number of GaN monolayers when emission layer are few GaN monolayers. The simulated results show that the number of monolayer can affect QE of GaN ML photocathode, and QE would depend largely on back interface recombination rate and recombination rate of the GaN ML/GaN ML interface. Our simulation results are expected to provide theoretical guidance for the preparation of GaN photocathodes with a certain number of GaN ML as emission layers.

Self‐Limiting Growth of Ultrathin GaN/AlN Quantum Wells for Highly Efficient Deep Ultraviolet Emitters

AbstractGaN/AlN ultrathin quantum wells (QWs) emitting in the deep UV spectral range are fabricated by metalorganic vapor phase epitaxy. The GaN thickness is automatically limited to the monolayer (ML) scale due to the balance between crystallization and evaporation of Ga adatoms. This growth characteristic facilitates the fabrication of highly reproducible GaN ML QWs. The strong quantum confinement within the GaN ML QWs achieves emissions below 250 nm. The photoluminescence intensity at room temperature with respect to that at low temperature, which is closely related to the emission internal quantum efficiency, is drastically improved from 0.1% for a conventional Al0.8Ga0.2N/AlN QWs to 5% for a 1 ML GaN/AlN (0001) QW and 50% for a 2 ML GaN/AlN (102) QW under weak excitation conditions. These higher emission efficiencies are attributed to the suppressed nonradiative recombination in the GaN QWs and the enhanced radiative recombination in the (102) QW.

Monolayer GaN functionalized with alkali metal and alkaline earth metal atoms: A first-principles study

Abstract Based on first-principles calculations, we have carried out a systematic study on the geometric, electronic and magnetic properties of free-standing monolayer GaN (ML GaN) functionalized with Lithium (Li), Sodium (Na), Beryllium (Be) or Calcium (Ca) atoms. We consider three different levels of concentrations i.e. θ = 1 / 8 , 1 / 18 and 1 / 32 . Within the tested θ , metallization of ML GaN only occurs with the adsorption of Li or Na atoms. The adsorption of Be or Ca atoms preserves the semiconducting characteristics of ML GaN. The ML GaN remains non-magnetic with the adsorption of Be or Ca atom. In contrast, the total magnetization of the Li-adsorbed ML GaN decreases as Li atoms concentration increases. For the case of Na adsorption, the ML GaN exhibits ferromagnetism only at θ = 1 / 18 . In addition, we found the work function of the functionalized ML GaN can be controlled by varying the concentrations of the adatoms. Our findings here suggest that by selective adsorption of Group I and Group II element, ML GaN is a promising material for the development of spintronic and field emission devices.

Coexistence of doping and strain to tune electronic and optical properties of GaN monolayer

Abstract GaN monolayer is a prospective two-dimensional (2D) graphene-like material due to its excellent physical and chemical properties. Band gap engineering plays a crucial role for extending potential applications of GaN-based optoelectronic devices. We performed first-principles calculations to study the structural, electronic and optical properties of GaN monolayers by Al and In atoms doping with different concentrations. We found that Al atom doping can increase the band gap and result in the blue-shift with the increase of doping concentration, while In atom doping presents the opposite results. However, the band gap characteristics remains indirect which is unfavourable for the optical transition and solar energy conversion. Next, biaxial strains within magnitude range of −10% to 13% are applied in Al and In-doped GaN monolayers. Coexistence of doping and strain to tune electronic and optical properties of GaN monolayer are systematically studied for the first time. The results show the band gap increases at first then decreases under compressive strain and decreases monotonically under tensile strain. Moreover, the band gap characteristics changes from indirect to direct in the certain compressive strain. For the optical property modification, compressive strain can cause blue-shift in the whole energy range, while tensile strain can result in red-shift in the visible energy range and blue-shift at first then red-shift in the ultraviolet energy range. Our results confirm coexistence of doping and strain induced in GaN monolayer is an effective method in band gap engineering for the applications in tunable nano-optoelectronic devices.

Adsorption of toxic gas molecules on pristine and transition metal doped hexagonal GaN monolayer: A first-principles study

Using the first-principles calculations based on density functional theory (DFT-D2 method), we systematically study the structural, energetic, electronic and magnetic properties of toxic gas molecules (H2S, NH3 and SO2) adsorbed on pristine and transition metal (TM) atom (Fe, Mn) doped GaN monolayer (GaN-ML). The results show that the H2S and NH3 are physisorbed on pristine GaN-ML with small adsorption energy, charge transfer, and long adsorption distance. While chemical adsorption character of SO2 on GaN-ML can be obtained, which means that the pristine GaN-ML is sensitive to SO2. We find that the adsorption ability of pristine GaN-ML can be improved by introducing TM dopants. TM (Fe, Mn) doping can increase adsorption energy and charge transfer of the adsorbed systems, except for SO2 adsorbed Fe doped GaN-ML. The enhancing interaction between adsorbed molecules and the TM doped GaN-ML can dramatically induce electrical conductivity changes. Therefore, the TM doped GaN-ML is more suitable for gas molecules detection compared with the pristine GaN-ML. These present properties of gas molecules adsorbed on the pristine and TM doped GaN-ML will help to guide scientists to develop better two-dimensional GaN-based gas sensors in the future.

Effects of strain and surface modification on stability, electronic and optical properties of GaN monolayer

Employing density-functional theory, G0W0 approach, and solving Bethe–Salpter equation (BSE), we investigate the effects of strain and surface modification (hydrogenation or fluorination) on stability, electronic and optical properties of monolayer GaN. Monolayer GaN is predicted to be an indirect semiconductor with wide gap of 4.44 eV, whose bandgap can be effectively modulated by biaxial strain, ranging from 5.49 eV (δ = −6.5%) to 2.27 eV (δ = +8%). In particular, the compressive strain arouses the structural instability of GaN monolayer. Hydrogenation or fluorination not only eliminates its instability, but also tunes the bandgap from indirect to direct. More attractively, tensile strain can significantly redshift the optical spectra of GaN monolayer into the visible-light region, broadening light harvesting. Hydrogenation or fluorination shifts optical activity to the ultra-violet region. The possibility of tuning the optoelectronic properties via strain and surface modification opens doors to novel application of the promising material.

First-principles study of magnetic properties of alkali metals and alkaline earth metals doped two-dimensional GaN materials

We systematically study the electronic structure and magnetic properties of alkali metals (Li, Na, K and Rb) and alkaline earth metals (Be, Mg and Sr) doped two-dimensional GaN monolayers using the first-principles calculations based on density functional theory. The results show that Be atom is located in the plane of the GaN monolayer, and the other doped atoms reside slightly above the plane. It is found that doping is easier to achieve under the N-rich condition. The total magnetic moment of the alkali metals doped system and the alkaline earth metals doped system are 2μB and 1μB, respectively, which are presented mainly by the spin-polarized holes of the nearest N atoms of the impurity atoms. The band structures indicate that the four alkali metal atoms doped systems are magnetic semiconductors, and the three alkaline earth metal doped systems are all semi-metallic. For a double M-doped GaN monolayer system, there is a long-range ferromagnetic coupling in the seven elements doped systems, which are realized by the hole-mediated p-p hybrid interaction. The Heisenberg mean field model is used to estimate the Curie temperature. It is found that the long-range ferromagnetic coupling states of Li, Be, Mg and Sr are existent at higher than room temperature, indicating that the four atom-doped two-dimensional GaN monolayers are very good candidates for the room temperature ferromagnetic candidate materials. The alkali metals and alkaline earth metals doped two-dimensional GaN monolayers are expected to play an important role in the studying of spintronics.

Two-Dimensional GaN: An Excellent Electrode Material Providing Fast Ion Diffusion and High Storage Capacity for Li-Ion and Na-Ion Batteries.

Identifying applicable anode materials is a significant task for Li- and Na-ion battery (LIB and NIB) technologies. We propose the GaN monolayer (2D GaN) can be a good anode candidate. The GaN monolayer manifests stable Li/Na adsorption and inherently low theoretical voltages. Most excitingly, both high storage capacity and extremely fast Li/Na diffusion can be simultaneously realized in the GaN monolayer. For Li, the storage capacity and diffusion barrier is 938 mA h g-1 and 79 meV, respectively. And, the values for Na are 625 mA h g-1 and 22 meV. Compared with known two-dimensional (2D) anode materials with similar scale of ion diffusion barriers, the GaN monolayer almost possesses the highest Li/Na storage capacity discovered to date. Our work suggests that the 2D GaN is a prospective anode material offering fast ion diffusion and high storage capacity.

Structural and electronic properties of point defects in Haeckelite GaN monolayer

First-principles calculations are carried out to investigate the physical properties of Haeckelite gallium nitride monolayer with point defects, including three vacancies and three antisites. Optimized geometries, band structures and magnetic properties modified by defects are demonstrated. We find that the lattice constant and cohesive energy only get slight change compared with the perfect monolayer, while the band gap and magnetic moment appear observable variations. The VN (vacancy of N atom) induces a defect state which leads the system to be metallic; The VGa (vacancy of Ga atom), VGaN (vacancy of GaN pair) and one of Ga↔N (exchanging neighboring atoms) defects introduce net magnetic moment to the systems, which also present half-metallic features; NGa (replacing Ga with N atom) also produces magnetic moment but the system remains semiconductor. Among the defects considered, VN and NGa have the lowest and highest formation energies, respectively. These results provide a development in defective nitride monolayers and provide a reference for extending potential applications.

Theoretical research of diluted magnetic semiconductors: GaN monolayer doped with transition metal atoms

In order to enrich the potential of diluted semiconductor in low-dimensional spintronic devices, we implement a theoretical study of the magnetic properties of TM-substituted g-GaN. The results show that there is a change in the bond length and g-GaN nanosheet shows certain ionic properties after TM atoms doping. Notable electrons transfer takes place from TM atoms to g-GaN owing to the hybridization between the 3d or 4d states of TM atoms and the 2p states of N atoms near the Fermi energy level. Cr-, Ni-, Mo-, and Pd-substituted systems exhibit a certain degree of magnetism owing to fact that the nonbonding or antibonding states are not occupied completely. The magnetic moments are all originated from the polarization of TM 3d or 4d electrons and N 2p electrons.

Platinum Adsorption and Diffusion on Two-Dimensional Gallium Nitride

In this study, we examined the adsorption and diffusion of platinum (Pt) adatom on two-dimensional hexagonal gallium nitride (h-GaN), by using first-principles plane-wave calculations. Two different levels of platinum coverage ratio (θ=1/8 and θ=1/32) were considered and the changes in the electronic structure for high-level platinum coverage ratio (θ=1/8) were examined. Low-level coverage ratio (θ=1/32) is used to calculate the diffusion barrier energy of Pt adatom on GaN monolayer. Our theoretical calculations have shown that Pt atom strongly binds on the top of nitrogen atoms in GaN monolayer and high energy is required for its diffusion. While GaN monolayer has 2.1 eV indirect band gap (Γ→K), this band gap reduces to 1.3 eV with Pt adsorption. These results may lead to further investigations on forming Pt nanoparticles or Pt coating on GaN sheet.

Strain-mediated electronic properties of pristine and Mn-doped GaN monolayers

Graphene-like two-dimensional (2D) monolayer structures GaN has gained enormous amount of interest due to high thermal stability and inherent energy band gap for practical applications. First principles calculations are performed to investigate the electronic structure and strain-mediated electronic properties of pristine and Mn-doped GaN monolayer. Binding energy of Mn dopant at various adsorption site is found to be nearly same indicating these sites to be equally favorable for adsorption of foreign atom. Depending on the adsorption site, GaN monolayer can act as p-type or n-type magnetic semiconductor. The tensile strength of both pristine and doped GaN monolayer (∼24 GPa) at ultimate tensile strain of 34% is comparable with the tensile strength of graphene. The in-plane biaxial strain modulate the energy band gap of both pristine and doped-monolayer from direct to indirect gap semiconductor and finally retendered theme into metal at critical value of applied strain. These characteristics make GaN monolayer to be potential candidate for the future applications in tunable optoelectronics.

Adsorption of 3d transition metal atoms on graphene-like gallium nitride monolayer: A first-principles study

We study the structural, electronic and magnetic properties of 3d transition metal (TM) atoms (Cr, Mn, Fe, Co, Ni and Cu) adsorbed GaN monolayer (GaN-ML) using first-principles calculations. The results show that, for 6 different TM adatoms, the most stable adsorption sites are the same. The adsorption of TM atoms results in significant lattice distortions. A covalent chemical bonding character between TM adatom and GaN-ML is found in TM adsorbed systems. Except for Ni adsorbed system, all TM adsorbed systems show spin polarization implying that the adsorption of TM induces magnetization. The magnetic moments of the adsorbed systems are concentrated on the TM adatoms and the nearest-neighbor N atoms of the adsorption site contributed slightly. Our analysis shows that the GaN-ML properties can be effectively modulated by TM adsorption, and exhibit various electronic and magnetic properties, such as magnetic metals (Fe adsorption), half-metal (Co adsorption), and spin gapless semiconductor (Cu adsorption). These present properties of TM adsorbed GaN-ML may be of value in electronics and spintronics applications.

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

We investigated the effects of chemical/substitutional doping, hydrogenation, and anti-site and vacancy defects on the atomic, optoelectronic and magnetic properties of AlN and GaN monolayers. Upon doping of selected atoms, AlN and GaN monolayers can acquire magnetic properties, and their fundamental band gaps are modified by the localized gap states. Spin-polarized gap states broaden into bands at patterned coverage of adatoms, whereby half-metallic or magnetic semiconducting properties can be attained. Specific adatoms adsorbed to Ga atoms break the nearest vertical Ga-N bonds in the GaN bilayer in the heackelite structure and result in changes in the electronic and atomic structure. While adjacent and distant pairs of anion + cation vacancies induce spin polarization with filled and empty gap states, anti-site defects remain nonmagnetic; but both defects induce dramatic changes in the band gap. Fully hydrogenated monolayers are stable only for specific buckled geometries, where one geometry can also lead to an indirect to direct band gap transition. Also, optical activity shifts to the ultra-violet region upon hydrogenation of the monolayers. While H2 and O2 molecules are readily physisorbed on the surfaces of the monolayers with weak van der Waals attraction, they can be dissociated into constituent atoms at the vacancy site of the cation. Our study performed within density functional theory shows that the electronic, magnetic and optical properties of AlN and GaN monolayers can be tuned by doping and point defect formation in order to acquire diverse functionalities.

234 nm and 246 nm AlN-Delta-GaN quantum well deep ultraviolet light-emitting diodes

Deep ultraviolet (DUV) AlN-delta-GaN quantum well (QW) light-emitting diodes (LEDs) with emission wavelengths of 234 nm and 246 nm are proposed and demonstrated in this work. Our results reveal that the use of AlN-delta-GaN QW with ∼1–3 monolayer GaN delta-layer can achieve a large transverse electric (TE)-polarized spontaneous emission rate instead of transverse magnetic-polarized emission, contrary to what is observed in conventional AlGaN QW in the 230–250 nm wavelength regime. The switching of light polarization in the proposed AlN-delta-GaN QW active region is attributed to the rearrangement of the valence subbands near the Γ-point. The light radiation patterns obtained from angle-dependent electroluminescence measurements for the Molecular Beam Epitaxy (MBE)-grown 234 nm and 246 nm AlN-delta-GaN QW LEDs show that the photons are mainly emitted towards the surface rather than the edge, consistent with the simulated patterns achieved by the finite-difference time-domain modeling. The results demonstrate that the proposed AlN-delta-GaN QWs would potentially lead to high-efficiency TE-polarized surface-emitting DUV LEDs.

Strain effect on the electronic properties of III-nitride nanosheets: Ab-initio study

In this study the structural and electronic properties of III-nitride monolayers XN (X=B, Al, Ga and In) under different percentages of homogeneous and shear strain are investigated using the full potential linearized augmented plane wave within the density functional theory. Geometry optimizations indicate that GaN and InN monolayers get buckled under compressive strain. Our calculations show that the free-strains of these four monolayers have an indirect band gap. By applying compressive biaxial strain, a transition from indirect to direct band gap occurs for GaN and InN, while the character of band gap for BN and AlN is not changed. Under tensile strain, only BN monolayer behaves as direct band gap semiconductor. In addition, when the shear strain is applied, only InN undergoes an indirect to direct band gap transition. Furthermore, the variations of band gap versus strain for III-nitride monolayers have been calculated. When a homogeneous uniform strain, in the range of [−10%, +10%], is applied to the monolayers, the band gap can be tuned for from 3.92 eV to 4.58 eV for BN, from 1.67 eV to 3.46 eV for AlN, from 0.24 eV to 2.79 eV for GaN and from 0.60 eV to 0.90 eV for InN.

Investigation of Substitution of Cu and Se Atoms on the Electronic and Magnetic Properties of GaN Monolayer Nanosheet

In this paper, some of the electronic properties of GaN monolayer nanosheet are investigated in its pure form and after substitution of Cu and Se atoms. Electronic properties have been calculated based on the density functional theory approach with full-potential linearized augmented plane wave method. The results show that GaN nanosheet is a nonmagnetic semiconductor and its energy gap is equal to 2.202 eV. Our results show that GaN nanosheet properties vary with substitution of Cu or Se atoms: GaSeN and GaCuN show half-metallic and metallic behaviors, respectively. Calculated magnetic moments for GaSeN and GaCuN are 1.00 and 1.127 µB, respectively.