Sihao Xia

Doping process of p-type GaN nanowires: A first principle study

The process of p-type doping for GaN nanowires is investigated using calculations starting from first principles. The influence of different doping elements, sites, types, and concentrations is discussed. Results suggest that Mg is an optimal dopant when compared to Be and Zn due to its stronger stability, whereas Be atoms are more inclined to exist in the interspace of a nanowire. Interstitially-doped GaN nanowires show notable n-type conductivity, and thus, Be is not a suitable dopant, which is to be expected since systems with inner substitutional dopants are more favorable than those with surface substitutions. Both interstitial and substitutional doping affect the atomic structure near dopants and induce charge transfer between the dopants and adjacent atoms. By altering doping sites and concentrations, nanowire atomic structures remain nearly constant. Substitutional doping models show p-type conductivity, and Mg-doped nanowires with doping concentrations of 4% showing the strongest p-type conductiv...

Theoretical study for heterojunction surface of NEA GaN photocathode dispensed with Cs activation

For the disadvantages of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, new-type NEA GaN photocathodes with heterojunction surface dispensed with Cs activation are investigated based on first-principle study with density functional theory. Through the growth of an ultrathin n-type GaN cap layer on p-type GaN emission layer, a p–n heterojunction is formed on the surface. According to the calculation results, it is found that Si atoms tend to replace Ga atoms to result in an n-type doped cap layer which contributes to the decreasing of work function. After the growth of n-type GaN cap layer, the atom structure near the p-type emission layer is changed while that away from the surface has no obvious variations. By analyzing the E-Mulliken charge distribution of emission surface with and without cap layer, it is found that the positive charge of Ga and Mg atoms in the emission layer decrease caused by the cap layer, while the negative charge of N atom increases. The conduction band moves downwards after the growth of cap layer. Si atom produces donor levels around the valence band maximum. The absorption coefficient of GaN emission layer decreases and the reflectivity increases caused by n-type GaN cap layer.

Differences in optoelectronic properties between H-saturated and unsaturated GaN nanowires with DFT method

To investigate the influences of dangling bonds on GaN nanowires surface, the differences in optoelectronic properties between H-saturated and unsaturated GaN nanowires are researched through first-principles study. The GaN nanowires along the [0001] growth direction with diameters of 3.7, 7.5 and 9.5 A are considered. According to the results, H-saturated GaN nanowires are more stable than the unsaturated ones. With increasing nanowire diameter, unsaturated GaN nanowires become more stable, while the stability of H-saturated GaN nanowires has little change. After geometry optimization, the atomic displacements of unsaturated and H-saturated models are almost reversed. In (0001) crystal plane, Ga atoms tend to move inwards and N atoms tend to move outwards slightly for the unsaturated nanowires, while Ga atoms tend to move outwards and N atoms tend to move inwards slightly for the H-saturated nanowires. Besides, with increasing nanowire diameter, the conduction band minimum of H-saturated nanowire moves to the lower energy side, while that of the unsaturated nanowire changes slightly. The bandgaps of H-saturated nanowires are approaching to bulk GaN as the diameter increases. Absorption curves and reflectivity curves of the unsaturated and H-saturated nanowires exhibit the same trend with the change of energy except the H-saturated models which show larger variations. Through all the calculated results above, we can better understand the effects of dangling bonds on the optoelectronic properties of GaN nanowires and select more proper calculation models and methods for other calculations.

Theoretical research on bandgap of H-saturated Ga1−xAlxN nanowires

Based on first-principles plane-wave ultra-soft pseudopotential method, bandgaps of Ga1−xAlxN nanowires with different diameters and different Al constituents are calculated. After the optimization of the model, the bandgaps are achieved. According to the results, the bandgap of Ga1−xAlxN decreases with increasing diameter and finally, closed to that of the bulk. In addition, with increasing Al constituent, the bandgaps of Ga1−xAlxN nanowires increase. However, the amount of the increase is lower than that of the bulk Ga1−xAlxN with the increase of Al constituent.

A first-principles study of the effects of different Al constituents on Ga1−xAlxN nanowires

In order to investigate the influences of different Al constituents on Ga1−xAlxN nanowires, the formation energy, stability, band structure, densities of states and optical properties of Ga1−xAlxN nanowires with different Al constituents are calculated using first-principles plane-wave ultrasoft pseudopotential method. Results show that Ga1−xAlxN nanowires become more stable with increasing Al constituent. Bandgap of Ga1−xAlxN nanowires increases as the Al constituent increases but with a lower amplification compared with bulk Ga1−xAlxN. The peaks of static dielectric constants show a decreasing trend and move towards high-energy side as Al constituent increases. The absorption of Ga1−xAlxN nanowires shows an interesting phenomenon that it firstly increases and then decreases slightly as the Al constituent increases. Reflectivity of Ga1−xAlxN nanowires is much smaller than that of the bulk. The optical properties of Ga1−xAlxN nanowires show a blueshift effect as Al composition increases. According to these calculations, it is found that Ga1−xAlxN nanowires are appropriate to be applied into photoelectric detecting materials by adjusting the Al constituent of Ga1−xAlxN nanowires.

Adsorption of residual gas molecules on (10–10) surfaces of pristine and Zn-doped GaAs nanowires

Using first-principles calculations, the effects of residual gas molecules (H2O, CO, CO2, H2 and N2) adsorption on the photoelectric properties of pristine and Zn-doped GaAs nanowire surfaces are investigated. Total energy calculations show that p-type doping surface is beneficial to reduce the damage of residual gases to cathodes and improve the stability of GaAs nanowire photocathodes. After adsorption of gas molecules, the electrons are transferred from surface to adsorbates, leading to a dipole moment pointing from surface to residual gas molecules, which obstructs the escape of electrons and increases the work function of photocathodes. Through Zn doping, the charge transfer between gas molecules and nanowire surface is reduced and the force of dipole moment induced by gas molecules is weakened. Besides, the conduction energy bands shift toward higher energy region and the band gap increased after adsorption of residual gas molecules. Moreover, residual gas adsorption will weaken the absorption characteristic of GaAs nanowire photocathodes.

First-principle study on electronic structure and optical properties of GaN nanowires with different cross-sections

This paper explores the properties of intrinsic gallium nitride (GaN) nanowires (NWs) in terms of formation energy, band structure, density of state (DOS) and optical properties with plane-wave ultrasoft pseudopotential method based on first-principles. Results show that after relaxation, N atoms of the outer layers move outwards, while Ga atoms move inwards, and the relaxation of surface atomic structure appears less obvious with the increasing cross-sectional area. Comparing different cross-sections of GaN NWs, it is found that the formation energy decreases and the stability goes stronger with the increasing size. With the increasing cross-section, the bandgap is decreased. Moreover, through comparative investigation in optical properties between GaN NWs and bulk GaN, a valuable phenomenom is found that the static dielectric constants of GaN NWs are notably lower, which contributes remarkably to the excellent absorbing performance of GaN NWs.