Monte Carlo simulations applied to AlxGayIn(1-x-y)X quaternary alloys (X=As,P,N): A comparative study

We develop a different Monte Carlo approach applied to the A(x)B(y)C(1-x-y)D quaternary alloys. Combined with first-principles total-energy calculations, the thermodynamic properties of the (AI,Ga,In)X (X=As, P, or N) systems are obtained and a comparative study is developed in order to understand the roles of As, P, and N atoms as the anion X in the system AlxGayIn1-x-yX. Also, we study the thermodynamics of specific compositions in which AlGaInN, AlGaInP, and AlGaInAs are lattice matched, respectively, to the GaN, GaAs, and InP substrates. We verify that the tendency for phase separation is always towards the formation of an In-rich phase. For arsenides and phosphides this occurs in general for lower temperatures than for their usual growth temperatures. This makes these alloys very stable against phase separation. However, for nitrides the In and/or Al concentrations have to be limited in order to avoid the formation of In-rich clusters and, even for low concentrations of In and/or Al, we observe a tendency of composition fluctuations towards the clustering of the ternary GaInN. We suggest that this latter behavior can explain the formation of the InGaN-like nanoclusters recently observed in the AlGaInN quaternary alloys.712

Microscopic description of the phase separation process in AlxGayIn1-x-yN quaternary alloys

Ab initio total energy electronic structure calculations are combined with Monte Carlo simulations to study the thermodynamic properties of AlxGayIn1-x-yN quaternary alloys. We provide a microscopic description of the phase separation process by analyzing the thermodynamic behavior of the different atoms with respect to the temperature and cation contents. We obtained, at growth temperatures, the range of compositions for the stable and unstable phases. The presence of Al in InGaN is proven to "catalyze" the phase separation process for the formation of the In-rich phase. Based on our results, we propose that the ultraviolet emission currently seen in samples containing AlInGaN quaternaries arises from the matrix of a random alloy, in which composition fluctuations toward InGaN- and AlGaN-like alloys formation may be present, and that a coexisting emission in the green-blue region results from the In-rich segregated clusters.70

Theoretical prediction of ferromagnetic MnN layers embedded in wurtzite GaN

We studied, using the spin density functional theory, the manganese mononitride (MnN) grown on GaN in the wurtzite phase, forming the GaN/MnN heterostructures. We obtained a ferromagnetic ground state with a higher magnetic moment than the hypothetical wurtzite bulk MnN. This behavior can be explained in terms of the high magnetization of the MnN interface monolayers that have longer first and second neighbors bond lengths due to structure relaxation. We suggest that this system can be applied to the new spintronics technology by being able to provide spin polarized carriers in the important wide-gap nitride systems.88

Strain-induced ordering in InxGa1-xN alloys

The energetics and thermodynamic properties of cubic (c-)InxGa1-xN alloys are investigated by combining first-principles total energy calculations, a concentration-dependent cluster-based model, and Monte Carlo simulations. The search for the ground-state energies leads to the conclusion that biaxial strain suppresses phase separation, and acts as a driving force for chemical ordering in c-InxGa1-xN alloys. Ordered superlattice structures, with composition xcongruent to0.5 and stable up to T=1000 K, arises as the relevant thermodynamic property of the strained alloy. We suggest that the In-rich phases recently observed by us in c-GaN/InxGa1-xN/GaN double heterostructures are ordered domains formed in the alloy layers due to biaxial strain. (C) 2003 American Institute of Physics.82244274427

Structural properties and Raman modes of zinc blende InN epitaxial layers

We report on x-ray diffraction and micro-Raman scattering studies on zinc blende InN epitaxial films. The samples were grown by molecular beam epitaxy on GaAs(001) substrates using a InAs layer as a buffer. The transverse-optical (TO) and longitudinal-optical phonon frequencies at Gamma of c-InN are determined and compared to the corresponding values for c-GaN. Ab initio self-consistent calculations are carried out for the c-InN c-GaN lattice parameters and TO phonon frequencies. A good agreement between theory and experiment is found. (C) 1999 American Institute of Physics. [S0005-6951(99)00503-3].74336236

Theoretical study of strain-induced ordering in cubic InxGa1-xN epitaxial layers

Chemical ordering in cubic epitaxial InxGa1-xN layers is investigated by combining first-principles pseudopotential plane-wave total-energy calculations, a local concentration-dependent cluster-based method, and Monte Carlo simulations. It is found that for the unstrained or fully relaxed layers there are no stable ordered structures, indicating the tendency of the alloy to undergo phase separation, in agreement with previous calculations and experiment. The energetics of the InxGa1-xN layers pseudomorphycally grown on fully relaxed GaN (001) buffers shows that biaxial strain acts as the driving force for chemical ordering in the alloys. It is found that strained InxGa1-xN alloy comprises stable ordered structures which are (210)-oriented superlattices with composition in the range [0.5,0.63], the [AABB] alternation of planes (configuration "chalcopyrite") being the most stable phase.692

Magnetic properties of GaN/MnxGa1-xN digital heterostructures: First-principles and Monte Carlo calculations

The energetic and magnetic properties of wurtzite GaN/MnxGa1-xN digital heterostructures are investigated by first-principles total energy calculations, within the spin density-functional theory, and Monte Carlo simulations. In a wurtzite GaN model sample, periodic in the c axis, we replace a GaN monolayer (a plane) by a plane with composition MnxGa1-xN, and study its properties for varying the GaN spacer layer thickness and Mn concentration x. The 100% MnN monolayer possesses an antiferromagnetic (AFM) ground state when, in the periodic sample, it is isolated from the other MnN monolayers by more than four GaN spacer layers. The case of submonolayers (x < 1) is studied by Monte Carlo simulations based on an Ising Hamiltonian, whose parameters are obtained from ab initio calculations on five configurations. At 700 degrees C, up to the concentration of 8% Mn, the two-dimensional (2D) alloy is stable. However, above this concentration, there is a strong tendency to the formation of MnN clusters with an AFM ground state defined by ferromagnetic Mn rows coupled antiferromagnetically with other Mn rows. The behavior of the magnetization with the temperature is completely different in these two concentration regimes, with the 2D MnN cluster being very stable, whereas the 2D alloy presents low magnetic transition temperatures.732

Statistical model applied to A(x)B(y)C(1-x-y)D quaternary alloys: Bond lengths and energy gaps of AlxGayIn1-x-yX (X=As, P, or N) systems

We extend the generalized quasichemical approach (GQCA) to describe the A(x)B(y)C(1-x-y)D quaternary alloys in the zinc-blende structure. Combining this model with ab initio ultrasoft pseudopotential calculations within density functional theory, the structural and electronic properties of AlxGayIn1-x-yX (X=As, P, or N) quaternary alloys are obtained, taking into account the disorder and composition effects. Results for the bond lengths show that the variation with the compositions is approximately linear and also does not deviate very much from the value of the corresponding binary compounds. The maximum variation observed amounts to 3.6% for the In-N bond length. For the variation of band gap, we obtain a bowing parameter b=0.26 eV for the (Ga0.47In0.53As)(z)(Al0.48In0.52As)(1-z) quaternary alloy lattice matched to InP, in very good agreement with experimental data. In the case of AlGaInN, we compare our results for the band gap to data for the wurtzite phase. We also obtained a good agreement despite all evidences for cluster formation in this alloy. Finally, a bowing parameter of 0.22 eV is obtained for zinc-blende AlGaInN lattice matched with GaN.732

Magnetic properties of MnN: Influence of strain and crystal structure

For manganese mononitride (MnN), the total energy versus lattice constant is obtained using the spin density functional theory. Instead of the tetragonally distorted NaCl structure, we study the zinc blende and wurtzite structures in which AlN, GaN, and InN crystallize. The ground state with nonmagnetic, antiferromagnetic (AFM), or ferromagnetic (FM) arrangement of spins depends on the polymorph of MnN and on the lattice constant. At equilibrium lattice constants, in zinc blende it is AFM in [100] direction, and in wurtzite it is FM. The zinc blende polytype of MnN under hydrostatic pressure at the InN lattice constant presents FM ground state. For the wurtzite polytype at the GaN and AIN lattice constants, the AFM is the ground state, but goes back to a FM ground state for the InN lattice constants. For both, structures, the system presents a half-metallic state at InN lattice constants (with a total magnetic moment of 4 mu(B) per Mn atom) instead of the metallic state obtained for smaller lattice constants. Results indicate that the FM or the AFM state of Ga1-xMnxN and In1-xMnxN may be related to, relaxed, or strained, MnN incorporations or Mn-rich composition fluctuations. (c) 2005 American Institute of Physics.861

Phase separation and gap bowing in zinc-blende InGaN, InAlN, BGaN, and BAlN alloy layers

We present first-principles calculations of the thermodynamic and electronic properties of the zinc-blende ternary InxGa1-xN. InxAl1-xN, BxGa1-xN, and BxAl1-xN alloys. They are based on a generalized quasi-chemical approximation and a pseudopotential-plane-wave method. T-x phase diagrams for the alloys are obtained, We show that due to the large difference in interatomic distances between the binary compounds a significant phase miscibility gap for the alloys is found. In particular for the InxGa1-xN alloy, we show also experimental results obtained from X-ray and resonant Raman scattering measurements, which indicate the presence of an In-rich phase with x approximate to 0.8. For the boron-containing alloy layers we found a very high value for the critical temperature for miscibility. similar to9000 K. providing an explanation for the difficulties encountered to grow these materials with higher boron content. The influence of a biaxial strain on phase diagrams, energy gaps and gap bowing of these alloys is also discussed. (C) 2002 Elsevier B.V. B.V. All rights reserved