First-principles Investigation Of The Surface Structures And Electronic Properties Of AlN Related Semiconductors

Aluminum nitride (AlN) is a wide bandgap semiconductor, which has some excellent properties such as large bandgap, high electron mobility, high hardness, high thermal conductivity, resistant to high-temperature, high piezoelectric coefficient, high surface acoustic velocity and high chemical stability. It has potential applications in deep ultra-violet emitters and detectors, high-power high-frequency electronic devices, resonator in bad condition, pressure transducer, surface acoustic wave device as well as micro-electro- mechanical systems. Especially, the alloys composited by AlN, GaN and InN are important optoelectronic materials at present whose emission spectra range from infrared to ultra-violet.Surface investigation is an important aspect in the field of semiconcuctor research because it has great influence on semiconductor's electronic, optical, acoustic and thermal properties as well as chemical stabiltiy. In addition, the investigation of surface structure can give some guidance to the growth of semiconductor films. In this dissertation, the structural and electronic properties of AlN(0001), (000(1|-)), (10(1|-)0) and (11(2|-)0) surfaces are investigated by first-principles calculation methods. The surface relaxation and reconstruction, the stability of surface defects, the adsorption of oxygen and water molecules are concerned. Sometimes GaN, InN and ZnO are also included. Special attentions are paid for the physical machanisms of these surface phenomenes in the whole work.The stability in electrostatics and electronic structure of wurtzite (0001) and (000(1|-)) surfaces are discussed. Reconstructions are thought to be expected to stabilize the two surfaces. The (2×2) reconstructions with Al vacancy or N adatom at the (0001) surface, and Al adatom at the (000(1|-)) surface, which satisfy the Electron Counting Rule, are found to be more stable than the ideal surfaces. The surface energy of AlN (0001) and (000(1|-))-Al can be also greatly decreased by the adsorption of oxygen atoms, which indicates that the polar surfaces of AlN is easy to be oxidated.The interactions of water molecule with the (0001) surfaces of AlN, GaN and ZnO are investigated systemically. Some common features of them are revealed. The stable adsorption configuration is dissociative (OH+H or O+2H) at low coverage (<1/2ML) but molecular at high coverage; a (2×1) reconstruction under 1ML coverage is found to be stable in general. Because of the differences in the number of valance electrons, the valance band offset and the magnitude of ionicity, the water molecules at GaN and AlN surfaces are easier to dissociate than at ZnO surface, and have larger adsorption energies. Besides there are some detailed differences in the adsorption configurations. The adsorption mechanisms are explored by analysing their energies and electronic structures. The covalent bond between water molecule and surface and the hydrogen bond between water molecules are found to be comparable for molecular adsorption, and for dissociative adsorption the adsorbates have strong nonlocal electrostatic interaction with surface cations in addition to the local covalent bond.The ideal structures of wurtzite (10(1|-)0) and (11(2|-)0) surfaces are stable in general because they satisfy the Electron Counting Rule. A fully occupied band above the valance band maximum and a empty band below the the conduction band minamum are introduced by the dangling bond of surface N and Al, respectively. The surface relaxation is studied and it is found that the wurtzite (10(1|-)0) and the zinc-blende (110) surfaces of allâ…¢-â…¤compounds slope after relaxation, however, rather differences exist betweenâ…¢-nitrides and the others. This phenomenon is explained detailedly based on a valance shell electron pair repulsion model and the electronegativity of species. The defect at (10(1|-)0) surface is studied and a planar-defect structure is found to be more stable in energy than the ideal surface for AlN. The origin is explored from electronic structure. The most important feature of this defect is that it can reverse the polarization orientation at surface. It is also predicted to exist at the lateral surface of AlN nanowires.Finally, the oxygen related AlN nonpolar surfaces are investigated. The stable oxygen incorporated (10(1|-) 0) surface is found to be V_Al-(ON)₃ for AlN but 2(O_N) for GaN and the reason of this difference is discussed. The adsorption of O is more stable than O₂ at AlN nonpolar surfaces. Atomic adsorption is thus the main adosrption morphology. The stable adsorpiton sites at the (10(1|-)10) surface lie above the Al-N dimers, but lie between the Al-N chains at the (11(2|-)0) surface. Because the N-O and Al-O bonds are short and strong, the diffusions of O adatoms are very limited. After adsorpion, the spin polarizaiton of O and O₂ are released and the original surface states in the bandgap are replaced by oxygen related ones.