AlN Monolayer Research Articles

Leak path passivation by in situ Al-N for InGaN solar cells operating at wavelengths up to 570 nm

A leak path passivation (LPP) technology for InGaN solar cells with photo-response up to 570 nm was developed by inserting in situ monolayers of Al-N into active layers. The InGaN layer in the passivated sample is partially relaxed and incorporates more than 23.5% In. By adopting in situ Al-N LPP, the open circuit voltage increases from 0.96 V to 1.35 V under one sun illumination (1.45–1.68 V under 72 suns), and the dark shunt resistance increases from 3.6 kΩ cm2 to 12.6 kΩ cm2, leading to an increase in power conversion efficiency by a factor of 2.0–2.26 (1–72 suns). This in situ Al-N LPP approach paves a way to exploit the full potential of InGaN for high efficiency solar cell application, accepting the reality of defective high-In-content thick and relaxed InGaN.

Interactions of h-AlN monolayer with platinum, oxygen, and their clusters

In this paper, we investigate the adsorption properties of single Pt and O atoms, and PtO, Pt2O, and PtO2 clusters on hexagonal AlN monolayer as well as several substitutions in AlN structure. We employ density functional theory to study electronic structure and charge transfers by considering nonmagnetic and ferromagnetic states. PtO and Pt2O adsorbed AlN system has ferromagnetic ground state with 2.00 μB magnetic moment, while PtO2, Pt, and O adsorption lead to nonmagnetic structures. Pt adsorbed AlN system has the lowest adsorption energy with −3.175eV indicating the most stable structure energetically. Oxygen atom largely disrupts the AlN layer due to strong N–O repulsion caused by high electronegativities of N and O atoms. The substitution of AlN monolayer with Pt and O atoms also presents interesting features. The various substitutions are able to yield ferromagnetic structures with semiconducting (AlO), metallic (NPt), or half-metallic (AlPt) ground states. These properties can lead to possible applications in spintronics and nanoelectronics.

2D AlN crystal phase formation on (0001) Al2O3 surface by ammonia MBE

AbstractKinetics of a two‐dimensional (2D) AlN layer formation on (0001) sapphire (Al2O3) surface during nitridation at different ammonia fluxes is investigated by reflection high energy electron diffraction (RHEED). The process on the surface is described in the framework of a chemical reactions kinetic model including interaction between partially reduced aluminum oxide species (AlO) and chemisorbed NH2 particles. The experimentally determined AlN formation rates as functions of both the temperature and the ammonia pressure are successfully described by a simple set of kinetic equations. Calculated maximum rate of the process well agrees with the experimental values. Precision measurement of 2D AlN lattice parameters during the nitridation process detects the value of 3.01 Å. This value coincides exactly with a characteristic structure parameter of the oxygen‐deficient Al2O3 surface. We assume this coincidence results from flexibility of the 2D AlN monolayer which provides minimization of elastic stresses at the AlN/(0001)Al2O3 interface. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic and magnetic properties of an AlN monolayer doped with first-row elements: a first-principles study

Two-dimensional (2D) magnetic materials are the focus of one of the most active areas of nano-materials research.

Optical excitations and quasiparticle energies in the AlN monolayer honeycomb structure

We calculated the electronic structure and optical properties of the AlN monolayer based on first principle many-body Green’s function and Bethe–Salpeter equation formalism. We computed that the indirect (direct) band gap of the AlN monolayer honeycomb using quasiparticle correction is 5.36eV (6.07eV), but it has a value of 2.93eV (3.7eV) within the density functional theory. The calculations reveal that the optical absorption is sensitive to excitonic effects such as electron–hole interaction with binding energy of the first exciton of over 2.05eV within the GW+Bethe–Salpeter equation calculation. The enhanced excitonic effects in the AlN monolayer can be used to describe the optical properties in nano-optoelectronic devices.

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 Hot-Cold Effect on Optical Properties for Nitride-Based Green LEDs by Ammonia Source Preflow

In this study, with the NH3 preflow, an AlN monolayer was initially formed on the surface of a sapphire substrate by the decomposition of the NH3 source and Al2O3 substrate. It was found that threading dislocation density significantly decreased. At an injection current of 350 and 1500 mA, the light output powers of light-emitting diodes (LEDs) with NH3 preflow was increased by 2.7% and 12.5%, respectively. As the ambient temperature increased, the light output intensity decreased compared with room temperature, which could be attributed to the thermal-induced band-gap energy lowering and more thermally excited injection carriers escaping from the quantum wells without recombination. It also can be seen obviously that the hot/cold factors of LEDs with NH3 preflow at different injection currents were better than those of the conventional LEDs. However, the behaviors of hot/cold factors at higher injection carriers were mainly influenced by the efficiency droop effect mechanism.

Counting Tm dopant atoms around GaN dots using high-angle annular dark field images

High resolution Z-contrast STEM imaging is used to study the Tm doping of GaN quantum dots grown in AlN by molecular beam epitaxy (MBE). High-angle annular dark field (HAADF) imaging allows us to visualize directly individual Tm atoms in the AlN matrix and even to count the number of Tm atoms in a given AlN atomic column. A new visibility coefficient to determine quantitatively the number of Tm atoms in a given atomic column is introduced. It is based on locally integrated intensities rather than on peak intensities of HAADF images. STEM image simulations shows that this new visibility is less sensitive to the defocus-induced blurring or to the position of the Tm atom within the thin lamella. Most of the Tm atoms diffuse out of GaN dots. Tm atoms are found at different positions in the AlN matrix, (i) Above the wetting layer, Tm atoms are spread within a thickness of 14 AlN monolayers (MLs). (ii) Above the quantum dots all the Tm are located in the same plane situated at 2-3 MLs above the apex of the GaN dot, i.e. at a distance of 14 MLs from the wetting layer, (iii) In addition, Tm can diffuse very far from the GaN dot by following threading dislocations lines.

Ab Initio-based Monte Carlo Simulation Study for the Structural Stability of AlN Grown on 4H-SiC(11-20)

The Monte Carlo simulation is performed on the basis of our ab initio-based approach for adsorption-desorption behavior as a function of growth conditions. The distinctive atomic arrangements appear in subsequent adsorption of Al and N atoms after N and Al pre-depositions, respectively. After the N deposition, the subsequent Al adatom occupies the 4H lattice site similarly to the results for single Al on the surface. On the other hand, the N adatom tends to form the dimer structure with the pre-deposited Al. On the basis of these results, the electron counting Monte Carlo method is applied to investigate the atomic arrangements of AlN monolayer on the 4H-SiC(11-20) surface as a function of the III/V ratio. The simulated results imply that Al adatoms can easily occupy the 4H lattice sites to form the 4H-AlN layer at the III/V ratio greater than 10. In the range of the III/V ratio less than 0.1, the N adatoms reside in the stable interstitial sites to form the Al-N dimer layer or intermittently migrate to the 2H lattice site to form the 2H-AlN layer. This is qualitatively consistent with experimental results for the dependence of the structural stability on the III/V ratio. [DOI: 10.1380/ejssnt.2010.52]

Ab initio-based approach to structural modulation on 4H-SiC(1 1 2¯ 0) during MBE growth

We systematically investigate adsorption behavior of Al and N atoms and resultant atomic arrangements of AlN monolayer on 4H-SiC(1 1 2¯ 0) surfaces using ab initio-based approach. The calculated results for the single Al or N adatom on the 4H-SiC(1 1 2¯ 0) reveal that the stable adsorption site for the Al is near the lattice site of the 4H structure, whereas the N stays at the interstitial site located above top C atom. The N adatom desorbs except the most stable site at the conventional growth temperature ~1200 K. Furthermore, the distinctive atomic arrangements appear when successive adsorption of Al and N atoms after N and Al pre-depositions, respectively. Even after the N deposition, the successive Al adatom occupies the 4H-like lattice site similarly to the results for the single Al on the surface. On the other hand, the N adatom tends to form the dimer structure with the pre-deposited Al. On the basis of these results, the structural modulation of AlN monolayer on the 4H-SiC(1 1 2¯ 0) is discussed performing the electron counting Monte Carlo simulations under Al- and N-rich limits.

RF-MBE GROWTH OF GaN ON SAPPHIRE FOR GAS SENSING APPLICATION

Besides SiC , a group III-V nitrides are also suitable large-band gap semiconductor materials for high-temperature gas sensor devices. In this paper, we present the study of the H2 sensitive device fabricated based on n-type GaN wafer. The GaN thin film with AlN buffer layer was grown on sapphire by RF plasma-assisted molecular beam epitaxy (RF-MBE). A few monolayers of AlN were deposited using high flux Al before the growth of the AlN buffer layer. This step is speculated to be able to form a better relaxed layer for the subsequent growth of the AlN buffer layer. Pt contacts with thickness of about 150 nm were then deposited on the GaN/AlN/Al2O3 using the sputtering system. Gas detection was carried out at room temperature. Prior to the current–voltage (I–V) measurements, the samples were annealed at temperatures ranging from 200°C to 600°C in N2 ambience. A significant change of current in the Pt/ GaN gas sensor was observed for the 600°C annealed sample when exposed to 0.5% H2 in N2 gas.

Vertical electron transport study in GaN/AlN/GaN heterostructures

In this work, we investigate the electronic structure and vertical electron transport through GaN/AlN/GaN single-barrier structures with different AlN thickness, grown by plasma-assisted molecular beam epitaxy . Conductive and capacitive characterization has been performed, and the experimental results are interpreted by comparison with 1D self-consistent simulations. Capacitive measurements reveal a complete depletion of the top GaN layer, and the formation of a two-dimensional electron gas at the bottom interface of the AlN barrier, even for barrier thicknesses of 0.5 nm (2 monolayers of AlN). Conductive atomic force microscopy reveals discrete leakage current locations with a density of ∼10 7 cm − 2 , more than one order of magnitude lower than the dislocation density in these samples. These results are promising for the fabrication of resonant tunnelling diodes using the GaN/AlN material system.

In situ resonant x-ray study of vertical correlation and capping effects during GaN∕AlN quantum dot growth

Grazing incidence anomalous x-ray scattering was used to monitor in situ the molecular beam epitaxy growth of GaN∕AlN quantum dots (QDs). The strain state was studied by means of grazing incidence multiwavelength anomalous diffraction (MAD) in both the QDs and the AlN during the progressive coverage of QDs by AlN monolayers. Vertical correlation in the position of the GaN QDs was also studied by both grazing incidence MAD and anomalous grazing incidence small angle scattering as a function of the number of GaN planes and of the AlN spacer thickness. In a regime where the GaN QDs and the AlN capping are mutually strain influenced, a vertical correlation in the position of QDs is found with as a side effect an average increase in the QD width.

Ordering in undoped hexagonal AlxGa1–xN grownon sapphire (0001) with 0.09 < x < 0.247

AbstractIn this issue's Editor's Choice [1], multiple ordering in AlxGa1–xN grown by low pressure metal‐organic vapor phase epitaxy on sapphire is observed by comprehensive high resolution X‐ray diffraction as well as transmission electron microscopy investigations.The cover figure shows a detail of the reciprocal space map taken on a AlxGa1–xN (x = 0.167) sample in the vicinity of the &1macr; 0 1 30 spot. The sapphire asymmetric reflection &1macr; &1 macr; 2 12 is also visible. It is concluded that besides the disordered alloy, an ordered phase with two consecutive AlN monolayers in a 3.1 nm large unit cell is present.The first author, Marguerite Laügt, is a researcher at the Centre de Recherche sur l'Hétéro‐Epitaxie et ses Applications of the CNRS at Parc de Sophia Antipolis.

Ordering in undoped hexagonal AlxGa1–xN grown on sapphire (0001) with 0.09 < x < 0.247

AbstractMultiple ordering has been observed both by high resolution X‐ray diffraction (HRXRD) and transmission electron microscopy (TEM) in AlxGa1–xN grown by low pressure metal‐organic vapor phase epitaxy (LP‐MOVPE) on sapphire (0001) with x varying from 9 to 25 percent. This is shown to take place even simultaneously in the same sample; beside the disordered alloy, AlGaN2 (1:1), and Al0.25Ga0.75N (3:1), there is an additional type which a quite large unit cell. In the latter, the c parameter is six times larger than that of the disordered alloy. Through the analysis of X‐ray diffraction data and comparison with TEM observations, a structure model is proposed. Taking into account the super‐lattice reflections intensity and symmetry, we conclude that this ordered phase contains two consecutive AlN monolayers in the 3.1 nm unit cell that corresponds to Al0.16Ga0.84N (10:2).

Investigation of Highly c-axis Oriented AlN Thin Film Re-growth

ABSTRACTThe properties of AlN films grown on a first set of differently treated AlN films have been studied by XRD, XPS, AFM, in-plane stress and interferometry. All films were deposited by dc pulsed sputtering at 300°C. The first set of films consisted of smooth, pure c-axis oriented AlN monolayers with narrow rocking curve width and excellent piezoelectric coefficient grown on platinized substrate in a large thickness range of 35–2000 nm. Subsequently, a 1000 nm AlN layer has been added. The use of a strongly alkaline developer during intermediate lithography steps and the time elapsed between the two steps of AlN re-growth were found to degrade the overall quality of the films. It's been shown that the alkaline developer etches down, increases the roughness and contaminates the surface of the AlN monolayer with mainly oxides and hydroxides while the exposure of the films to air develops a native oxide layer. By reducing the air exposure time and by the use of RF plasma cleaning prior to the re-growth process, a good piezoelectric coefficient can be recovered.

AlN/GaN short-period superlattices with monolayer AlN for optical-device applications

A series of AlN/GaN short-period superlattices with monolayer AlN were prepared by hot-wall epitaxy. The superlattice structure was confirmed by X-ray diffraction through a comparison between the measured diffraction curves and simulated ones, which indicates good controllability of the HWE technique. Interband photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured. Relatively strong PL emissions compared with conventional AlGaN/GaN superlattices were observed, which can be attributed to the small piezoelectric effect. A small Stokes shift should be noted, which may suggest small interface fluctuation in consistence with the XRD observation. The energy shifts of the band-edge emission with the GaN well thickness agree well with the calculated results within the envelope wave-function framework at least for the well thickness above 2 nm . Optical device applications such as light emitting devices in the ultraviolet and mid-infrared regions via interband and intersubband electron transitions are discussed.

High Quality AlN and GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

We describe the growth of high quality AlN and GaN on Si(111) by gas source molecular beam epitaxy (GSMBE) with ammonia (NH3). The initial nucleation (at 1130−1190K) of an AlN monolayer with full substrate coverage resulted in a very rapid transition to two-dimensional (2D) growth mode of AlN. The rapid transition to the 2D growth mode of AlN is essential for the subsequent growth of high quality GaN, and complete elimination of cracking in thick ( > 2 μm) GaN layers. We show, using Raman scattering (RS) and photoluminescence (PL) measurements, that the tensile stress in the GaN is due to thermal expansion mismatch, is below the ultimate strength of breaking of GaN, and produces a sizable shift in the bandgap. We show that the GSMBE AlN and GaN layers grown on Si can be used as a substrate for subsequent deposition of thick AlN and GaN layers by hydride vapor phase epitaxy (HVPE).

High Quality AlN and GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

AbstractWe describe the growth of high quality AlN and GaN on Si(111) by gas source molecular beam epitaxy (GSMBE) with ammonia (NH3). The initial nucleation (at 1130-1190K) of an AlN monolayer with full substrate coverage resulted in a very rapid transition to two-dimensional (2D) growth mode of AlN. The rapid transition to the 2D growth mode of AlN is essential for the subsequent growth of high quality GaN, and complete elimination of cracking in thick ( > 2 μm) GaN layers. We show, using Raman scattering (RS) and photoluminescence (PL) measurements, that the tensile stress in the GaN is due to thermal expansion mismatch, is below the ultimate strength of breaking of GaN, and produces a sizable shift in the bandgap. We show that the GSMBE AlN and GaN layers grown on Si can be used as a substrate for subsequent deposition of thick AlN and GaN layers by hydride vapor phase epitaxy (HVPE).