N-rich Growth Conditions Research Articles

Increasing the bifunctional OER/ORR activity of 3d transition metals doped g-C3N3 by controlling the charge states

Pursuing efficient bifunctional oxygen reduction reaction/ oxygen evolution reaction (OER/ORR) electrocatalysts is crucial for realizing sustainable and renewable clean energy, yet it remains a formidable challenge. In this study, we employ density functional theory and machine learning (ML) predictions to systematically investigate the bifunctional OER/ORR activity of 3d transition metal (TM) atom-doped g-C3N3. Our findings reveal that N-rich and C-poor growth conditions can facilitate 3d TM doping into g-C3N3. The Ni occupation N in 0 (Ni×N@C54N53, including 54 C and 53 N atoms), Ni occupation N in +1 (Ni•N@C54N53) and Ni interstitial sites in +1 charge state (Ni•int@C54N54, including 54 C and 54 N atoms) meet the requirements of bifunctional OER/ORR electrocatalysts, which because they (1) have lower formation energies, and (2) lower overpotentials (smaller than 0.47V), (3) can maintain stability under 300K and (4) their band gaps are decreased, which leads to facilitating electron transfer. ML predictions identify the 3d band center (εd) and the number of 3d electrons (Ne) as the most effective descriptors for overpotentials of OER and ORR, respectively. This work can pave the way for understanding the origin of bifunctional OER/ORR activity of 3d-TM doped g-C3N3 and benefit the rational design of novel electrocatalysts for other catalytic reactions by considering the charge states.

The unexpected magnetism in 2D group-IV-doped GaN for spintronic applications

In this study, we systematically investigated the structural and magnetic properties of group-IV-doped monolayer GaN by first-principles calculations. Among the group-IV element, only Ge and Sn atoms with large atomic radii can form a buckling substituted doping structure with an in-plane magnetic moment of 1 μB per dopant. The compressive strains enhance the in-plane magnetic anisotropy, while tensile strains tend to destroy the magnetic moment of dopants. Both Ge and Sn atoms can stay on the same side of monolayer GaN to form anti-ferromagnetic semiconductors due to the large diffusion barrier for crossing to the monolayer GaN. The intrinsic Ga or N vacancies will eliminate the magnetic moments of group-IV dopants due to the charge transferring from the dopants to intrinsic vacancies. The N-rich growth conditions and a plentiful supply of Ge or Sn dopants to fill the intrinsic vacancies help to maintain the magnetic properties of group-IV-doped monolayer GaN.

The Theoretical Study of Unexpected Magnetism in 2D Si-Doped AlN

In this study, the structural and magnetic properties of Si-doped bulk and 2D AlN were systematically investigated by first-principles calculations. Si atoms prefer to substitute Al atoms in both bulk and 2D AlN under N-rich growth conditions. In bulk AlN, Si dopants exhibit a non-magnetic state, uniform distribution, and a strong anisotropic diffusion energy barrier. In contrast to that, Si dopants prefer to form a buckling structure and exhibit a magnetic moment of 1 μB in 2D AlN. At a low Si concentration, Si atoms tend to get together with antiferromagnetic coupling between each other. However, the magnetic coupling among Si atoms changes to ferromagnetic coupling as Si concentration increases, due to the enhanced exchange splitting and delocalized impurity states. At the extreme doping limit, monolayer SiN, along with its analogs GeN and SnN, is a ferromagnetic semiconductor with a large band gap and high Curie temperature. These results indicate that 2D AlN doped by group IV atoms has potential applications in spintronic devices.

Effect of disorder on superconductivity of NbN thin films studied using x-ray absorption spectroscopy

The superconducting transition temperature (T C) of rock-salt type niobium nitride (δ − NbN) typically varies between 9 to 17 K and the theoretically predicted value of 18 K has not been achieved hitherto. The low T C in δ − NbN has been assigned to some structural disorder which is always present irrespective of the microstructure (polycrystalline or epitaxial), methods or conditions adopted during the growth of NbN thin films. In this work, we investigate the atomic origin of such suppression of the T C in δ − NbN thin films by employing combined methods of experiments and ab initio simulations. Sputtered δ − NbN thin films with different disorder were studied using the synchrotron-based N and Nb K-edge x-ray absorption spectroscopy techniques. A strong correlation between the superconductivity and the electronic structure reconstruction was observed. The theoretical analysis revealed that under N-rich growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and results into a smeared electronic structure around Fermi-level. The role of electronic smearing on the T C is thoroughly discussed.

Eutectic Formation, V/III Ratio and Controlled Polarity Inversion in Nitrides on Silicon[1

The crystallographic polarity of AlN grown on Si(111) by plasma assisted molecular beam epitaxy is intentionally inverted from N-polar to Al-polar at a planar boundary. The position of the inversion boundary is controlled by a two-step growth process that abruptly changes from Al-rich to N-rich growth conditions. The polarity inversion is induced by the presence of Si, which is incorporated from an Al-Si eutectic layer that forms during the initial stages of AlN growth and floats on the AlN surface under Al-rich growth conditions. When the growth conditions change to N-rich the Al and Si in the eutectic react with the additional N-flux and are incorporated into the solid AlN film. Relatively low levels of Al-Si eutectic formation combined with lateral variations in the Si incorporation lead to nonuniformity in the polarity inversion and formation of surprisingly narrow, vertical inversion domains. The results suggest that intentional incorporation of uniform layers of Si may provide a method for producing polarity engineered nitride structures.

Graphene/h-BN In-Plane Heterostructures: Stability and Electronic and Transport Properties

We present a first-principles study of structural, electronic, and transport properties of in-plane Gr:BN heterostructures in the form of graphene stripes embedded in a BN matrix. In our calculations, we consider carbon, nitrogen, and boron chemical potentials that are consistent with growth conditions (gas sources and temperatures) at either nitrogen-rich or boron-rich environments. Interestingly, we find that structures with excess of B atoms can be energetically more stable than structures with excess of N atoms even in N-rich growth conditions. The general trend is that N-rich growth conditions favor B/N stoichiometric heterostructures, while B-rich growth conditions favor heterostructures with excess of B atoms at the graphene/BN junctions, such that only B–C bonds occur at both edges of a graphene stripe region embedded in BN. We also investigate the dependence of magnetic properties and the band gap magnitudes of graphene stripe regions embedded in BN with several structural characteristics. We fin...

Highly efficient p-type doping of GaN under nitrogen-rich and low-temperature conditions by plasma-assisted molecular beam epitaxy

Highly efficient and reproducible p-type doping of GaN under nitrogen-rich and low-growth-temperature conditions was demonstrated with the plasma-assisted molecular beam epitaxy technique. The low-temperature range is approximately below 650 °C and refers to growth temperatures at which the thermal desorption of any excess Ga is negligibly slow. The Mg and hole concentrations obtained with the N-rich condition were more than one order of magnitude higher than those obtained with the Ga-rich condition while keeping all other conditions identical. The Mg doping under such N-rich conditions was also found to show Mg-mediated suppression of background impurities, good epitaxy quality on GaN templates, and relatively low surface roughness. Over the investigated growth temperature range from 580 °C to 650 °C, the Mg incorporation efficiency under the N-rich condition was found to be close to unity (70%-80%) and independent of the growth temperature. High hole concentrations of up to 2×1019 cm-3 and activation efficiencies of up to 16.6% were obtained. The result rules out the Mg surface sticking probability as the limiting mechanism for Mg incorporation in this temperature range, as it would be temperature dependent. Instead, the Mg incorporation rate was more likely governed by the availability of substitutional sites for Mg on the surface, which should be abundant under the N-rich growth conditions. Excellent diode characteristics and electroluminescence results were observed when this p-type doping method was employed in the growth of full device structures.

Ab-initio study of boron incorporation and compositional limits at GaN and AlN (0001) surfaces

Density functional theory calculations are employed to investigate B incorporation at the GaN(0001) and AlN(0001) surfaces. It is found that under typical metal-organic chemical vapor deposition (MOCVD) and metal rich molecular beam epitaxy (MBE) conditions, the maximum B contents at the surfaces are in the order of 3% for GaN and 15% for AlN. Under MBE N-rich growth conditions the calculations reveal a rehybridization enhanced solubility mechanism that dominates at the surface. This mechanism offers a promising route to kinetically stabilize B contents above the bulk solubility limit and as high as 25%.

Epitaxial ScAlN grown by molecular beam epitaxy on GaN and SiC substrates

ScxAl1-xN is a promising ultra-wide bandgap material with a variety of potential applications in electronic, optoelectronic, and acoustoelectric devices related to its large piezoelectric and spontaneous polarization coefficients. We demonstrate growth of ScxAl1-xN on GaN and SiC substrates using plasma-assisted molecular beam epitaxy with x = 0.14–0.24. For metal-rich growth conditions, mixed cubic and wurtzite phases formed, while excellent film quality was demonstrated under N-rich growth conditions at temperatures between 520 and 730 °C. An rms roughness as low as 0.7 nm and 0002 rocking curve full-width at half maximum as low as 265 arc sec were measured for a Sc0.16Al0.84 N film on GaN. To further demonstrate the quality of the ScAlN material, a high-electron-mobility transistor heterostructure with a Sc0.14Al0.86 N barrier, GaN/AlN interlayers, and a GaN buffer was grown on SiC, which showed the presence of a two-dimensional electron gas with a sheet charge density of 3.4 × 1013 cm−2 and a Hall mobility of 910 cm2/V·s, resulting in a low sheet resistance of 213 Ω/◻.

Comparative study of semipolar [formula omitted], nonpolar [formula omitted] and polar [formula omitted] InGaN multi-quantum well structures grown under N- and In-excess by plasma assisted molecular beam epitaxy

InGaN multi-quantum well (MQW) structures were grown by plasma-assisted molecular beam epitaxy (PAMBE) on semipolar (202¯1), nonpolar m-plane (101¯0) and polar c-plane (0001) GaN substrates. The impact of In- and N-rich growth conditions on surface morphology, luminescence properties and structural quality was studied. Semipolar MQWs grown under N-excess have a smooth surface and narrow photoluminescence (PL) at room temperature at 420nm. Semipolar MQWs grown under In-rich conditions emitted at 414nm with slightly broader emission and higher surface roughness. Nonpolar m-plane MQWs grown under N-excess emitted at 411nm and its surface was very rough. When grown under In-rich conditions, nonpolar MQWs also emitted at 411nm but significant peak broadening was observed due to hillock formation resulting in surface roughening and inhomogeneous In incorporation. The c-plane reference MQW structures grown in the same growth run did not show PL at room temperature due to poor structural quality related to three-dimensional growth mode and structure relaxation.

Hole-induced d0 ferromagnetism enhanced by Na-doping in GaN

The d0 ferromagnetism in wurtzite GaN is investigated by the first-principle calculations. It is found that spontaneous magnetization occurs if sufficient holes are injected in GaN. Both Ga vacancy and Na doping can introduce holes into GaN. However, Ga vacancy has a high formation energy, and is thus unlikely to occur in a significant concentration. In contrast, Na doping has relatively low formation energy. Under N-rich growth condition, Na doping with a sufficient concentration can be achieved, which can induce half-metallic ferromagnetism in GaN. Moreover, the estimated Curie temperature of Na-doped GaN is well above the room temperature.

Deep level defects in N-rich and In-rich InxGa1−XN: in composition dependence

This study focuses on the presence and electronic properties of deep levels in In- and N-rich InxGa1−xN materials with different growth temperature, causing the different In composition, grown by nitrogen plasma-assisted molecular beam epitaxy (PAMBE). Semi-transparent Ag/Au Schottky contacts were formed on the InxGa1−xN layers and the device performances has been investigated with varying In contents. Increase in the leakage current density with the increase of In content has been observed in both In- and N-rich samples. Deep level optical spectroscopy has shown three deeper levels in both In-rich and N-rich InxGa1−xN layers. 1.4–1.6 eV level below the conduction band has been observed in all the samples, while the energy values of the other deep levels varies by sample to sample. The deepest levels are found to be the most prominent with the concentration in the range of 1016 cm−3 for high In content InxGa1−xN for both In-rich and N-rich growth conditions. Almost 4–5 times higher total trap concentrations have been observed for higher In InxGa1−xN sample.

Spontaneous growth of GaN nanowire nuclei on N- and Al-polar AlN: A piezoresponse force microscopy study of crystallographic polarity

The polarity of gallium nitride (GaN) nanowire nuclei grown on AlN layers was studied by piezoresponse force microscopy (PFM). N- or Al-polar AlN layers were grown by molecular beam epitaxy (MBE) on Si (111) substrates by use of Al- or N-rich growth conditions, respectively. Short and low density GaN nanowires were then grown on each AlN polarity type. PFM measurements verified the expected AlN layer polarity and further indicated that predominantly N-polar nanowires are produced for growth on both AlN polarity types. Cross-section scanning transmission electron microscopy (STEM) images further reveal that the nanowires on Al-polar AlN films are nucleated on regions in the AlN layer that contain inversion domains, which propagate into the GaN nanowire nuclei. PFM measurements were found to be a convenient technique for mapping the polarity of a statistically significant number of individual GaN nanowires.

Position-controlled growth of GaN nanowires and nanotubes on diamond by molecular beam epitaxy.

In this work the position-controlled growth of GaN nanowires (NWs) on diamond by means of molecular beam epitaxy is investigated. In terms of growth, diamond can be seen as a model substrate, providing information of systematic relevance also for other substrates. Thin Ti masks are structured by electron beam lithography which allows the fabrication of perfectly homogeneous GaN NW arrays with different diameters and distances. While the wurtzite NWs are found to be Ga-polar, N-polar nucleation leads to the formation of tripod structures with a zinc-blende core which can be efficiently suppressed above a substrate temperature of 870 °C. A variation of the III/V flux ratio reveals that both axial and radial growth rates are N-limited despite the globally N-rich growth conditions, which is explained by the different diffusion behavior of Ga and N atoms. Furthermore, it is shown that the hole arrangement has no effect on the selectivity but can be used to force a transition from nanowire to nanotube growth by employing a highly competitive growth regime.

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.

Microscopic origin ofn-type behavior in Si-doped AlN

In contrast to a long held belief, it has been shown that $n$-type AlN can be achieved through Si-doping. This is unexplainable from the current theoretical understanding, a situation that hinders further progress in AlN-based ultraviolet (UV) technologies. From first-principles calculations, we find that $n$-type behavior arises under N-rich growth conditions due to high Si solubility and to the formation of $V$${}_{\mathrm{Al}}$-bound Si clusters. We show that metal-rich growth may lead to weak $n$-type behavior due to oxygen impurities binding and deactivating cation vacancies. We provide clues for designing production processes for $n$-type AlN as a base material for potential new UV sources.

In-rich InGaN thin films: Progress on growth, compositional uniformity, and doping for device applications

A number of In-rich InGaN films with In contents in the 20–40% range have been grown at moderately low temperatures on sapphire and silicon substrates at high growth rates using a versatile molecular beam epitaxy-type technology that utilizes an energetic beam of N atoms called energetic neutral atom beam lithography and epitaxy to overcome reaction barriers in the group III-nitride system. Extensive characterization results on the crystalline, optical, and electrical properties of the In-rich InGaN materials are reported. It was found that N-rich growth conditions are required to produce materials that have excellent crystallinity, uniform compositions, and bright band edge photoluminescence. For In-rich InGaN growth on sapphire, electrical transport measurements show reasonably low carrier concentrations and high mobilities. Successful p-type doping of In-rich InGaN with ∼20% and ∼40% In contents is demonstrated, and preliminary results on the formation of a p–n junction are reported. For In-rich InGaN growth on Si, the film structural properties are somewhat degraded and carrier concentrations are considerably higher.

Effect of Mg Doping on the Photoluminescence of GaN:Mg Films by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

We investigate undoped GaN and Mg-doped GaN grown by rf plasma-assisted molecular beam epitaxy (MBE) with different Mg concentrations by photoluminescence (PL) at low temperature, Hall-effect and XRD measurements. In the PL spectra of lightly Mg-doped GaN films, a low intensity near band edge (NBE) emission and strong donor-acceptor pair (DAP) emission with its phonon replicas are observed. As the Mg concentration is increased, the DAP and NBE bands become weaker and a red shift of these bands is observed in the PL spectra. Yellow luminescence (YL) is observed in heavily Mg-doped GaN. The x-ray diffraction is employed to study the structure of the films. Hall measurement shows that there is a maximum value (3.9 × 1018 cm−3) of hole concentration with increasing Mg source temperature for compensation effect. PL spectra of undoped GaN are also studied under N-rich and Ga-rich growth conditions. Yellow luminescences of undoped Ga-rich GaN and heavily Mg-doped GaN are compared, indicating the different origins of the YL bands.

High In content InxGa1−xN grown by energetic neutral atom beam lithography and epitaxy under slightly N-rich conditions

Two series of In-rich InGaN films with compositions of ∼25% and ∼35% In, grown over a substrate temperature range from 490 to 620 °C, show how the film properties improve as the growth temperature is lowered below the InN decomposition temperature of ∼550 °C in vacuum. These InGaN films have been grown using a novel growth technique utilizing energetic N atoms as the active growth species. Under N-rich growth conditions, these InGaN films show how compositional uniformity, crystallinity, band edge photoluminescence, and surface morphology are improved as growth temperatures are reduced. The results emphasize the importance of energetic N atoms and lower substrate temperatures for overcoming difficulties associated with growing high-quality In-rich InxGa1−xN thin film materials. Utilizing energetic N atoms allows for the growth of high-quality, thick (>500 nm) InxGa1−xN films at temperatures below 500 °C.

Electronic structure of 1/6〈202¯3〉 partial dislocations in wurtzite GaN

The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {112¯0} (a-plane) and {11¯00} (m-plane) grown GaN. Their importance is established by recent experimental results, which correlate the partial dislocations (PDs) bounding I1 BSFs to the luminescence characteristics of GaN. PDs are also found to play a critical role in the alleviation of misfit strain in hetero-epitaxially grown nonpolar and semipolar films. In the present study, the energetics and the electronic structure of twelve edge and mixed 1/6〈202¯3〉 PD configurations are investigated by first principles calculations. The specific PD cores of the dislocation loop bounding the I1 BSF are identified for III-rich and N-rich growth conditions. The core structures of PDs induce multiple shallow and deep states, attributed to the low coordinated core atoms, indicating that the cores are electrically active. In contrast to edge type threading dislocations no strain induced states are found.