Cedric Thomas

Quantum size effects in GaAs nanodisks fabricated using a combination of the bio-template technique and neutral beam etching.

We successfully fabricated defect-free, distributed and sub-20-nm GaAs quantum dots (named GaAs nanodisks (NDs)) by using a novel top-down technique that combines a new bio-template (PEGylated ferritin) and defect-free neutral beam etching (NBE). Greater flexibility was achieved when engineering the quantum levels of ND structures resulted in greater flexibility than that for a conventional quantum dot structure because structures enabled independent control of thickness and diameter parameters. The ND height was controlled by adjusting the deposition thickness, while the ND diameter was controlled by adjusting the hydrogen-radical treatment conditions prior to NBE. Photoluminescence emission due to carrier recombination between the ground states of GaAs NDs was observed, which showed that the emission energy shift depended on the ND diameters. Quantum level engineering due to both diameter and thickness was verified from the good agreement between the PL emission energy and the calculated quantum confinement energy.

Light-emitting devices based on top-down fabricated GaAs quantum nanodisks.

Quantum dots photonic devices based on the III–V compound semiconductor technology offer low power consumption, temperature stability, and high-speed modulation. We fabricated GaAs nanodisks (NDs) of sub-20-nm diameters by a top-down process using a biotemplate and neutral beam etching (NBE). The GaAs NDs were embedded in an AlGaAs barrier regrown by metalorganic vapor phase epitaxy (MOVPE). The temperature dependence of photoluminescence emission energies and the transient behavior were strongly affected by the quantum confinement effects of the embedded NDs. Therefore, the quantum levels of the NDs may be tuned by controlling their dimensions. We combined NBE and MOVPE in a high-throughput process compatible with industrial production systems to produce GaAs NDs with tunable optical characteristics. ND light emitting diode exhibited a narrow spectral width of 38 nm of high-intensity emission as a result of small deviation of ND sizes and superior crystallographic quality of the etched GaAs/AlGaAs layer.

Oxidation states of GaAs surface and their effects on neutral beam etching during nanopillar fabrication

We have investigated a new process for fabricating GaAs sub-20 nm nanopillars that uses a top–down combination of a bio-template and damage-free neutral beam etching. A two-dimensional array of nanoparticles composed of a protein shell embedded with a metal oxide core was formed on the top of a GaAs surface treated by neutral beam oxidation. Because of device requirements, three low-temperature oxygen techniques were investigated for removing the protein shell prior to the etching process: oxygen radical, oxygen neutral beam, and low-temperature oxygen annealing in vacuum (LT-OAV). X-ray photoelectron spectroscopy was used to monitor the effects of the different treatments on the GaAs surface. While the three processes could efficiently remove a protein shell, subsequent oxidation of the GaAs surface showed some differences in the oxide layer composition. Therefore, LT-OAV was selected considering its lower gallium oxide formation. A hydrogen radical process was then performed at temperatures lower than 400 °C to remove the oxide layer prior to etching. This process completely removed arsenide oxide and only residual gallium oxide was found on the surface afterwards. Etching was performed using a pure chlorine neutral beam of GaAs samples with metal oxide core etching masks. We found that control of the Ga-oxide amount on the surface is the key parameter for controlling the diameter and the density of nanopillars. Finally, high-aspect ratio nanopillars using stacked layers of GaAs and AlGaAs were obtained and showed no damage layer.

Temperature-Dependent Operation of GaAs Quantum Nanodisk LEDs with Asymmetric AlGaAs Barriers

Quantum dot photonic devices such as light-emitting diodes (LEDs), laser diodes (LDs), high-speed modulators, and semiconductor optical amplifiers are attractive because of their small threshold voltages, low power consumption, and temperature stability. We have studied and developed a defect-less top-down dry fabrication process for GaAs quantum nanodisk (QND) LEDs with diameters of less than 20 nm and thicknesses of 8 nm by employing a bionanotemplate, neutral beam etching, and asymmetric AlGaAs/GaAs regrowth via metalorganic vapor phase epitaxy. AlxGa1-xAs barriers with high (Al0.3Ga0.7As) and low (Al0.17Ga0.83As) aluminum contents were used between QNDs in the in-plane and vertical directions, respectively. This permits the control of the deep band energy offset between GaAs QNDs and AlxGa1-xAs barriers, leading to stable room-temperature operation. The temperature dependence of the optical properties of the QND LED was measured by electroluminescence, and we found that the energies and their transient behaviors were strongly affected by the band offset energies between the QNDs and the aluminum-rich barriers. Therefore, we could enhance the optical performances of our symmetric QND LED with low-Al-content barriers by developing asymmetric AlxGa1-x As barriers with low-Al-content vertical barriers and high-Al-content in-plane barriers.

Estimation of activation energy and surface reaction mechanism of chlorine neutral beam etching of GaAs for nanostructure fabrication

To understand the etching mechanisms of GaAs materials through the neutral beam etching (NBE) process developed in our laboratory, we investigated the effect of substrate temperature on etching conditions. The etch rate as a function of wafer temperature was found to increase with temperature. The apparent activation energy was calculated to be an average of about 8.6 ± 1.4 kJ mol−1. However, as the vapour pressure of the etch product (GaClx) was above the chamber pressure, the etching mechanism was assumed to be temperature independent. It has been suggested that residual Ga2O3 oxide on the GaAs surface is responsible for the temperature dependence of the etch rate. A comparison with reactive ion etching (RIE) was done and a lower activation energy was calculated (4.0 ± 0.3 kJ mol−1). We argue that etching of residual oxide on the GaAs surface is more efficient with RIE because of the energetic ions and ultraviolet photons. It should be noted that when substrate temperature was increased, the etching rate ratio between NBE and RIE decreased, suggesting a stronger effect of chemical etching on the etching mechanism.

Impact of artificial lateral quantum confinement on exciton-spin relaxation in a two-dimensional GaAs electronic system

We demonstrate the effect of artificial lateral quantum confinement on exciton-spin relaxation in a GaAs electronic system. GaAs nanodisks (NDs) were fabricated from a quantum well (QW) by top-down nanotechnology using neutral-beam etching aided by protein-engineered bio-nano-templates. The exciton-spin relaxation time was 1.4 ns due to ND formation, significantly extended compared to 0.44 ns for the original QW, which is attributed to weakening of the hole-state mixing in addition to freezing of the carrier momentum. The temperature dependence of the spin-relaxation time depends on the ND thickness, reflecting the degree of quantum confinement.

Narrow line-width photoluminescence spectrum of GaAs nanodisks fabricated using bio-template ultimate top-down processes

Quantum dot optoelectronic devices are very attractive for their low power consumption, temperature stability, and high-speed modulation. We developed an ultimate defect-free, top-down fabrication process for sub-20-nm diameter GaAs quantum nanodisks (NDs) by using a combination of a bio-template and neutral beam etching. Metal-organic vapor phase epitaxy was used to make stacked layers of GaAs/AlGaAs multiple quantum wells for etching and for regrowth of AlGaAs barrier layer after nanopillar fabrication (embedding GaAs NDs). To fabricate high-uniformity GaAs NDs array, surface condition such as oxide layer is very critical to etch GaAs/AlGaAs stacked layers with neutral beam. To make high quality GaAs NDs a small amount of oxide is better. To decrease the surface oxide ratio, we investigated oxygen processes such as oxygen radical treatment or low-temperature oxygen annealing under vacuum to remove ferritin protein shell. As a result, we could mitigate the surface oxide formation and achieved a high-uniformity and high-density GaAs NDs array. Very narrow line-width photo emission full-width at half maximum of less than 30 meV) was observed from NDs at 7 K confirming the high quality of GaAs NDs.

Optical properties of quantum energies in GaAs quantum nanodisks produced using a bio-nanotemplate and a neutral beam etching technique

We demonstrated that the lattice-matched GaAs quantum nanodisks (QNDs) embedded in an AlGaAs matrix were fabricated by our original top-down nanoprocess. Lattice-matched GaAs QNDs are very attractive in quantum cryptography because the spin relaxation time of QNDs might be longer than that of strained quantum dots. Quantum levels of QNDs were investigated by the photoluminescence (PL) technique. The minimum diameter and thickness of QNDs were 7 and 8 nm, respectively. PL peaks of QNDs at 1.64 and 1.66 eV were observed to be higher than that of multiple quantum wells (MQWs) observed at 1.57 eV. It is suggested that these peaks are due to the diameter distribution of QNDs. The calculated quantum levels were in good agreement with the present experimental results. The observation of the PL peaks from QNDs demonstrates that the quantum level is strongly confined not only in the perpendicular direction but also in the lateral direction.

Fabrication of InGaAs quantum nanodisks array by using bio-template and top-down etching processes

III-V compound semiconductor quantum dot (QD) optical devices, such as high-power lasers and high-speed modulators, have great potential for the future of telecommunications and quantum cryptographic communication. We developed a top-down method for fabricating InGaAs quantum nanodisks (NDs) arrays by using bio-template and neutral beam etching (NBE) processes. Damage-free InGaAs/GaAs nano-pillar structures were successfully fabricated for the first time. After NBE, InGaAs NDs were embedded in the GaAs barrier layer by metalorganic vapor phase epitaxy. Subsequently, the photoluminescence was measured and the emission originating from the NDs could be directly detected.

Photoluminescence of high-density and sub-20-nm GaAs nanodisks fabricated with a neutral beam etching process and MOVPE regrowth for high performance QDs devices

III-V compound semiconductor quantum dots photonic devices are very attractive because of their low power consumption, temperature stability, and high-speed modulation. We studied and developed a defect-free top-down fabrication process for sub-20-nm GaAs nanodisks (NDs) that uses bio-template and neutral beam etching. We successfully fabricated 100-nm-high nanopillars embedding 4- and 8-nm-thick GaAs quantum well and 30-nm-thick Al0.3Ga0.7As barrier-stacked structures. The nanopillars were mounted by metalorganic vapor phase epitaxy. We measured visible light photoluminescence at a low temperature originating from the GaAs NDs. Nanodisks fabricated by the top-down process have a great potential for use in high-performance III-V photonic devices.