GaAs Nanowire Surfaces Research Articles

Self-selective formation of ordered 1D and 2D GaBi structures on wurtzite GaAs nanowire surfaces

Scaling down material synthesis to crystalline structures only few atoms in size and precisely positioned in device configurations remains highly challenging, but is crucial for new applications e.g., in quantum computing. We propose to use the sidewall facets of larger III–V semiconductor nanowires (NWs), with controllable axial stacking of different crystal phases, as templates for site-selective growth of ordered few atoms 1D and 2D structures. We demonstrate this concept of self-selective growth by Bi deposition and incorporation into the surfaces of GaAs NWs to form GaBi structures. Using low temperature scanning tunneling microscopy (STM), we observe the crystal structure dependent self-selective growth process, where ordered 1D GaBi atomic chains and 2D islands are alloyed into surfaces of the wurtzite (Wz) {11{bar{2}}0} crystal facets. The formation and lateral extension of these surface structures are controlled by the crystal structure and surface morphology uniquely found in NWs. This allows versatile high precision design of structures with predicted novel topological nature, by using the ability of NW heterostructure variations over orders of magnitude in dimensions with atomic-scale precision as well as controllably positioning in larger device structures.

Impact of gas adsorption on the stability and electronic properties of negative electron affinity GaAs nanowire photocathodes

The influence of CO, CO2, H2O, H2 and CH4 adsorption on the stability and electronic properties of negative electron affinity (NEA) GaAs nanowire surfaces activated by Cs/O and Cs/NF3 are systematically investigated via first-principles. The calculations indicated that GaAs nanowires activated with 3Cs/O are more susceptible to the surface contamination. After residual gas molecule adsorption, 3Cs/O activated surfaces exhibit direct bandgap character, while 3Cs/NF3 activated surfaces are inversely indirect bandgap. In addition, residual gas adsorption results in a notable increase of band gap, work function and electron affinity of GaAs nanowire surfaces. The incoporation of residual gas molecules also induces a new electric dipole [Cs-gas] with a direction from Cs to gas molecule. From the perspective of theoretical calculation, it is predicted that GaAs nanowires activated through Cs/NF3 has a stronger stability compared with Cs/O in the aspect of gas exposure.

Intrinsic point defects in pristine and Zn-doped GaAs nanowire surfaces: A first-principles investigation

Utilizing first-principles calculations, we have systematically investigated on the impact of surface point defects on the stability, electronic and optical properties of pristine and Zn-doped GaAs nanowires. Different defect types (antisite, interstitial and vacancy defect) and defect sites (surface, subsurface and core layer of nanowires) are considered to build defect models. Results show that AsGa and GaAs are respectively the most stable surface defect under As-rich condition and under Ga-rich condition, in both pristine and Zn-doped GaAs nanowires. Compared with the pristine model, Zn-doped GaAs nanowire will suffer a more significant surface defects under the same growth conditions due to lower formation energy. In addition, the band structure and electron density difference calculations indicate that GaAs, Asin and VGa act as acceptors, while AsGa, Gain and VAs act as donors. Interestingly, the incorporation of AsGa defect will notably weaken the p-type character of Zn-doped GaAs nanowires. Moreover, the optical absorption capability of pristine and Zn-doped GaAs nanowires will be subject to varying degrees of variation after introducing different defects.

Adsorption mechanism of Pt, Ag, Al, Au on GaAs nanowire surfaces from first-principles

The electronic and optical properties of metal (M) atoms adsorbed GaAs nanowires are systemically investigated utilizing first-principles calculations based on density functional theory. Different materials (M = Pt, Ag, Al and Au) and different coverages (1M, 2M, 3M and 4M) are considered to construct surface adsorption models. The calculations show that all metal-adsorbed GaAs nanowire surfaces are stable, and the difficulty of metal atom adsorption on nanowire surfaces follows the rule of Ag > Au > Al > Pt. In addition, the layer distance variation of nanowire surfaces after metal atom adsorption mainly take place near the outmost layer region. In 1M coverage case, the work function is reduced by Pt, Ag, Al adsorption, while increased by Au adsorption. Specially, Pt- and Al-adsorbed GaAs nanowire surfaces are direct band gap semiconductors, but Ag- and Au-adsorbed surfaces are indirect band gap. The adsorption of metals on GaAs nanowire surfaces are via chemisorption. Moreover, metal atom adsorption can enlarger the absorption coefficient of GaAs nanowires, which are gradually enhanced with increasing the coverage of metal atoms.

First-principles study on structural stability and electronic properties of GaAs nanowire undergoing surface oxidization

In this study, the structural stability and electronic properties for oxidized GaAs nanowire surfaces are systematically investigated through first-principles calculations based on density functional theory. Various oxidized nanowire surface models with two interaction types (adsorption and substitution) and different coverages of O atoms (one O, two O, three O and four O atoms) are built. The formation energy, atomic structure, Mulliken charge distribution, work function and dipole moment for each oxidized model are calculated and analyzed. The calculations indicate that the structural stability of oxidized nanowire surfaces is gradually enhanced with increasing O coverage, where O adsorption models are more energetically favorable than O substitution cases. In addition, O atoms interacting with GaAs nanowire surfaces have an obvious influence on the atomic structures near surface layer region, especially the thickness of the first bilayer. Moreover, the incorporation of O atoms leads to the redistribution of surface charge, which is further aggravated with higher coverage of O atoms. Overall, the dipole moment induced by O adsorption or substitution will increase surface work function and thus weaken the surface emission ability.

Cs and Cs-O co-adsorption on Zn-doped GaAs nanowire surfaces: A first-principles calculations

The Cs-only and CsO co-adsorption process of Zn-doped GaAs nanowire surface with negative electron affinity are investigated utilizing the first-principles calculations based on density function theory. The 1Cs, 2Cs, 3Cs and 4Cs adsorption models are built to simulate the process of Cs-only adsorption, while 3Cs/1O, 3Cs/2O and 3Cs/3O models are established to study CsO co-adsorption process. The adsorption energy, work function, Mulliken charge distribution and dipole moment for each adsorption model are discussed, respectively. During the Cs-only adsorption process, as increasing Cs coverage, the adsorption energy increases accordingly, indicating that the stability of Cs-only adsorption model is gradually weakened. After O adsorption on Cs-covered GaAs nanowire surface, the reduction of adsorption energy means that O incorporation can make the surface adsorption structure more stable. For Cs-only adsorption models, the work function obviously decreases owing to first dipole moment induced by Cs. However, excessive Cs adsorption on the Zn-doped nanowire surface will result in the ‘Cs-kill’ phenomenon. After O atom incorporation, the work function of Cs-covered nanowire surface further decrease, owing to the dual-dipole caused by Cs, O adatoms and nanowire surface atoms. The lowest work function can be obtained for 2O co-adsorption model, indicating the optimized atomic ratio of Cs/O is 3:2. These calculations provide an attempt to explore the adsorption mechanism of CsO on nanowire surfaces, and the results are expected to be verified by the experimental results in the future.

Tuning the electronic and optical properties of GaAs nanowire (10-10) surfaces by Zn doping: First-principles study

To reveal the tuning effect of Zn doping on the electronic and optical properties of GaAs nanowire (10-10) surfaces, pristine and Zn-doped GaAs nanowire surface models with different doping positions and doping concentration are built. The formation energy, work function, atomic structure, charge distribution, band structure and optical properties of these surfaces are discussed via first-principles calculations. Results show that Zn atom has a strong tendency to be close to the surface position to form p-type doping surface. The surface models become more and more unstable as increasing Zn doping concentration. Besides, the work functions of Zn-doped nanowire surfaces are reduced compared with the pristine one. In addition, the p-type conductivity is observed on GaAs nanowire surfaces after the doping of Zn atom. Moreover, the absorption coefficients of doped surfaces are smaller than that of pristine surfaces.

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.

Study on electronic and optical properties of alkali metal atoms adsorbed GaAs nanowires by first-principles calculations

This study presents a systematic investigation on the electronic and optical properties of GaAs nanowire surfaces by adsorption of alkali metal atoms (Li, Na, K, Cs) using a first-principles approach. Calculated adsorption energies show that all alkali metal atoms prefer to adsorb on the GaAs bridge site of GaAs nanowire surface, and Na adsorbed GaAs nanowire structure is the most stable. Under the joint action of surface dipole moment and band bending region, alkali metal atoms adsorption can obviously reduce the work function of GaAs nanowire surface. All alkali metal atoms carry positive charge due to the electron transfer from adatoms to nanowire surface atoms. After alkali metal atoms adsorption, the band gap of GaAs nanowire surface is narrowed and the Fermi level enters into the conduction band, which makes nanowire surfaces exhibit n-type conductivity. The absorption coefficient curves occur red-shifted phenomenon and the highest absorption peaks are slightly decreased due to alkali metal adsorption. We believe that these calculations are suitable for experimental exploration and useful for preparing alkali metal adsorbed GaAs-based nano-optoelectronic devices.

Early stages of Cs adsorption mechanism for GaAs nanowire surface

In this study, the adsorption mechanism of Cs adatoms on the (100) surface of GaAs nanowire with [0001] growth direction is investigated utilizing first principles method based on density function theory. The adsorption energy, work function, atomic structure and electronic property of clean surface and Cs-covered surfaces with different coverage are discussed. Results show that when only one Cs is adsorbed on the surface, the most favorable adsorption site is BGa-As. With increasing Cs coverage, work function gradually decreases and gets its minimum at 0.75 ML, then rises slightly when Cs coverage comes to 1 ML, indicating the existence of ‘Cs-kill’ phenomenon. According to further analysis, Cs activation process can effectively reduce the work function due to the formation of a downward band bending region and surface dipole moment directing from Cs adatom to the surface. As Cs coverage increases, the conduction band minimum and valence band maximum both shift towards lower energy side, contributed by the orbital hybridization between Cs-5s, Cs-5p states and Ga-4p, As-4s, As-4p states near Fermi level. The theoretical calculations and analysis in this study can improve the Cs activation technology for negative electron affinity optoelectronic devices based on GaAs nanowires, and also provide a reference for the further Cs/O or Cs/NF3 activation process.

Site control of very low density InAs QDs on patterned GaAs nano-wire surfaces

Self-organized InAs quantum dots (QDs) were grown on patterned GaAs substrates. A 2.5-ML-thick InAs grown on self-organized GaAs nano-wire surfaces showed QD formation with a very low density of 1.5×10 8 cm −2. The self-organized GaAs nano-wires were grown under low V/III ratio, and nano-wires were parallel to the [1 1 0] direction. On the other hand, the 2.5-ML-thick InAs on smooth (0 0 1) GaAs surfaces formed QDs with a density of 4.4×10 9 cm −2. We grew these low density InAs QDs on patterned GaAs, which had small A-planes. The InAs QDs were formed only on the small A-plane, which was positioned at the boundary between the square-patterned and planar areas. Except for the A-planes, no InAs QD formation was observed in a relatively large area. These results indicate that the combination of GaAs nano-wires and pattern substrates with small A-planes can produce InAs QDs at desired areas.