Kamran Keramatnejad

Fast Growth of GaN Epilayers via Laser-Assisted Metal–Organic Chemical Vapor Deposition for Ultraviolet Photodetector Applications

In this study, we successfully developed a carbon dioxide (CO2)-laser-assisted metal-organic chemical vapor deposition (LMOCVD) approach to fast synthesis of high-quality gallium nitride (GaN) epilayers on Al2O3 [sapphire(0001)] substrates. By employing a two-step growth procedure, high crystallinity and smooth GaN epilayers with a fast growth rate of 25.8 μm/h were obtained. The high crystallinity was confirmed by a combination of techniques, including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and atomic force microscopy. By optimizing growth parameters, the ∼4.3-μm-thick GaN films grown at 990 °C for 10 min showed a smooth surface with a root-mean-square surface roughness of ∼1.9 nm and excellent thickness uniformity with sharp GaN/substrate interfaces. The full-width at half-maximum values of the GaN(0002) X-ray rocking curve of 313 arcsec and the GaN(101̅2) X-ray rocking curve of 390 arcsec further confirmed the high crystallinity of the GaN epilayers. We also fabricated ultraviolet (UV) photodetectors based on the as-grown GaN layers, which exhibited a high responsivity of 0.108 A W-1 at 367 nm and a fast response time of ∼125 ns, demonstrating its high optical quality with potential in optoelectronic applications. Our strategy thus provides a simple and cost-effective means toward fast and high-quality GaN heteroepitaxy growth suitable for fabricating high-performance GaN-based UV detectors.

Resonant and nonresonant vibrational excitation of ammonia molecules in the growth of gallium nitride using laser-assisted metal organic chemical vapour deposition

The influence of exciting ammonia (NH3) molecular vibration in the growth of gallium nitride (GaN) was investigated by using an infrared laser-assisted metal organic chemical vapor deposition method. A wavelength tunable CO2 laser was used to selectively excite the individual vibrational modes. Resonantly exciting the NH-wagging mode (v2) of NH3 molecules at 9.219 μm led to a GaN growth rate of 84 μm/h, which is much higher than the reported results. The difference between the resonantly excited and conventional thermally populated vibrational states was studied via resonant and nonresonant vibrational excitations of NH3 molecules. Resonant excitation of various vibrational modes was achieved at 9.219, 10.35, and 10.719 μm, respectively. Nonresonant excitation was conducted at 9.201 and 10.591 μm, similar to conventional thermal heating. Compared to nonresonant excitation, resonant excitation noticeably promotes the GaN growth rate and crystalline quality. The full width at half maximum value of the XRD r...

Skin effect mitigation in laser processed multi-walled carbon nanotube/copper conductors

In this study, laser-processed multi-walled carbon nanotube (MWCNT)/Cu conductors are introduced as potential passive components to mitigate the skin effect of Cu at high frequencies (0–10 MHz). Suppressed skin effect is observed in the MWCNT/Cu conductors compared to primitive Cu. At an AC frequency of 10 MHz, a maximum AC resistance reduction of 94% was observed in a MWCNT/Cu conductor after being irradiated at a laser power density of 189 W/cm2. The reduced skin effect in the MWCNT/Cu conductors is ascribed to the presence of MWCNT channels which are insensitive to AC frequencies. The laser irradiation process is observed to play a crucial role in reducing contact resistance at the MWCNT-Cu interfaces, removing impurities in MWCNTs, and densifying MWCNT films.

Ultraviolet laser photolysis of hydrocarbons for nondiamond carbon suppression in chemical vapor deposition of diamond films

In this work, we demonstrate that ultraviolet (UV) laser photolysis of hydrocarbon species alters the flame chemistry such that it promotes the diamond growth rate and film quality. Optical emission spectroscopy and laser-induced fluorescence demonstrate that direct UV laser irradiation of a diamond-forming combustion flame produces a large amount of reactive species that play critical roles in diamond growth, thereby leading to enhanced diamond growth. The diamond growth rate is more than doubled, and diamond quality is improved by 4.2%. Investigation of the diamond nucleation process suggests that the diamond nucleation time is significantly shortened and nondiamond carbon accumulation is greatly suppressed with UV laser irradiation of the combustion flame in a laser-parallel-to-substrate geometry. A narrow amorphous carbon transition zone, averaging 4 nm in thickness, is identified at the film–substrate interface area using transmission electron microscopy, confirming the suppression effect of UV laser irradiation on nondiamond carbon formation. The discovery of the advantages of UV photochemistry in diamond growth is of great significance for vastly improving the synthesis of a broad range of technically important materials.

EFFECT OF LASER-ASSISTED RESONANT EXCITATION ON THE GROWTH OF GaN FILMS

Gallium nitride (GaN) films were grown using laser-assisted metal organic chemical vapor deposition (LMOCVD). The vibrational mode (1084.63 cm−1) of ammonia (NH3) molecules was resonantly excited using a wavelength-tunable CO2 laser at a wavelength of 9.219 µm due to its high absorption cross-section. Through wavelength-matched resonant excitation of the NH3 molecules, highly c-axis oriented GaN films were successfully deposited on sapphire (α-Al2O3) substrates at low temperatures (250 to 600 °C). The strong (0001) GaN peak in X-ray diffraction spectra confirmed the good crystalline quality of GaN films. Additionally, the resonant vibrational excitation of NH3 in LMOCVD promoted the GaN growth rate considerably compared to that synthesized by MOCVD without resonant vibrational excitation of NH3 molecules.Gallium nitride (GaN) films were grown using laser-assisted metal organic chemical vapor deposition (LMOCVD). The vibrational mode (1084.63 cm−1) of ammonia (NH3) molecules was resonantly excited using a wavelength-tunable CO2 laser at a wavelength of 9.219 µm due to its high absorption cross-section. Through wavelength-matched resonant excitation of the NH3 molecules, highly c-axis oriented GaN films were successfully deposited on sapphire (α-Al2O3) substrates at low temperatures (250 to 600 °C). The strong (0001) GaN peak in X-ray diffraction spectra confirmed the good crystalline quality of GaN films. Additionally, the resonant vibrational excitation of NH3 in LMOCVD promoted the GaN growth rate considerably compared to that synthesized by MOCVD without resonant vibrational excitation of NH3 molecules.

Reducing graphene-metal contact resistance via laser nano-welding

Publisher’s Note: This paper, originally published on February 17, 2017, was withdrawn per author request.

Thermally Stable and Electrically Conductive, Vertically Aligned Carbon Nanotube/Silicon Infiltrated Composite Structures for High-Temperature Electrodes

Traditional ceramic-based, high-temperature electrode materials (e.g., lanthanum chromate) are severely limited due to their conditional electrical conductivity and poor stability under harsh circumstances. Advanced composite structures based on vertically aligned carbon nanotubes (VACNTs) and high-temperature ceramics are expected to address this grand challenge, in which ceramic serves as a shielding layer protecting the VACNTs from the oxidation and erosive environment, while the VACNTs work as a conductor. However, it is still a great challenge to fabricate VACNT/ceramic composite structures due to the limited diffusion of ceramics inside the VACNT arrays. In this work, we report on the controllable fabrication of infiltrated (and noninfiltrated) VACNT/silicon composite structures via thermal chemical vapor deposition (CVD) [and laser-assisted CVD]. In laser-assisted CVD, low-crystalline silicon (Si) was quickly deposited at the VACNT subsurfaces/surfaces followed by the formation of high-crystalline Si layers, thus resulting in noninfiltrated composite structures. Unlike laser-assisted CVD, thermal CVD activated the precursors inside and outside the VACNTs simultaneously, which realized uniform infiltrated VACNT/Si composite structures. The growth mechanisms for infiltrated and noninfiltrated VACNT/ceramic composites, which we attributed to the different temperature distributions and gas diffusion mechanism in VACNTs, were investigated. More importantly, the as-farbicated composite structures exhibited excellent multifunctional properties, such as excellent antioxidative ability (up to 1100 °C), high thermal stability (up to 1400 °C), good high velocity hot gas erosion resistance, and good electrical conductivity (∼8.95 Sm-1 at 823 K). The work presented here brings a simple, new approach to the fabrication of advanced composite structures for hot electrode applications.

Skin Effect Suppression in Infrared-laser Irradiated PlanarMulti-walled Carbon Nanotube/ Cu Conductors

Skin effect suppression in planar multi-walled carbon nanotube (MWCNT)/Copper (Cu) conductors was realized at the 0-10 MHz frequency range through infrared laser irradiation of MWCNTs, which were coated on the surface of the Cu substrate via the electrophoretic deposition (EPD) method. The effect of laser irradiation and its power density on electrical and structural properties of the MWCNT/Cu conductors was investigated using a wavelength-tunable CO2 laser and then comparing the performance of the samples prepared at different conditions with that of pristine Cu. The irradiation at λ=9.219 µm proved to be effective in selective delivery of energy towards depths close to the interface, compared to the conventional rapid thermal processing (RTP) annealing method. At f=10 MHz, the ac resistance of the laser irradiated MWCNT/Cu conductors was reduced by more than one order of magnitude compared to its original value for the pristine Cu. Impedance measurements and structural characterizations indicate that this technique results in successful implementation of the nanotubes on the surface of the metallic substrate to operate as current channels with saturated skin depths and reduced contact resistance at the interface. Therefore, the limited performance of Cu conductors at high frequencies can be modified. Additionally, it was further demonstrated that the impedance reduction and the suppression of the skin effect in MWCNT/Cu conductors are in direct relation with the irradiated power density and can thus be easily controlled by this parameter.Skin effect suppression in planar multi-walled carbon nanotube (MWCNT)/Copper (Cu) conductors was realized at the 0-10 MHz frequency range through infrared laser irradiation of MWCNTs, which were coated on the surface of the Cu substrate via the electrophoretic deposition (EPD) method. The effect of laser irradiation and its power density on electrical and structural properties of the MWCNT/Cu conductors was investigated using a wavelength-tunable CO2 laser and then comparing the performance of the samples prepared at different conditions with that of pristine Cu. The irradiation at λ=9.219 µm proved to be effective in selective delivery of energy towards depths close to the interface, compared to the conventional rapid thermal processing (RTP) annealing method. At f=10 MHz, the ac resistance of the laser irradiated MWCNT/Cu conductors was reduced by more than one order of magnitude compared to its original value for the pristine Cu. Impedance measurements and structural characterizations indicate that th...

Influence of resonant and non-resonant vibrational excitation of ammonia molecules in gallium nitride synthesis

Attempts on the selective promotion of gallium nitride (GaN) growth were investigated by deploying laser-assisted vibrational excitation of reactant molecules, which deposits energy selectively into specific molecules and activate the molecules towards the selected reaction pathways. Laser-assisted metal organic chemical vapor deposition (LMOCVD) of GaN was studied using a wavelength-tunable CO2 laser. The NH-wagging modes (υ2) of ammonia (NH3) precursor molecules are strongly infrared active and perfectly match the emission line of the CO2 laser at 9.219, 10.350, and 10.719 µm. On-and off-resonance excitations of molecules were performed via tuning the incident laser wavelengths at on-resonant wavelength 9.219 µm and off-resonant wavelength of 9.201 µm. The on-resonant vibrational excitation allowed a largest fraction of the absorbed laser energy coupled directly into NH3 molecules whereas energy coupling under off-resonant excitations is less efficient in energy coupling and influencing the GaN growth process. The GaN deposition rate was enhanced by a factor of 2.6 accompanied with an improvement of crystalline quality under the on-resonant excitation. Optical emission spectroscopic (OES) studies confirmed that the on-resonant vibrational excitation effectively promotes the dissociation of NH3 molecules and creates N-containing species favoring the GaN growth. This study indicates that the resonant vibrational excitation is an efficient route coupling energy into the reactant molecules to surmount the chemical reaction barrier and steering the growth process.Attempts on the selective promotion of gallium nitride (GaN) growth were investigated by deploying laser-assisted vibrational excitation of reactant molecules, which deposits energy selectively into specific molecules and activate the molecules towards the selected reaction pathways. Laser-assisted metal organic chemical vapor deposition (LMOCVD) of GaN was studied using a wavelength-tunable CO2 laser. The NH-wagging modes (υ2) of ammonia (NH3) precursor molecules are strongly infrared active and perfectly match the emission line of the CO2 laser at 9.219, 10.350, and 10.719 µm. On-and off-resonance excitations of molecules were performed via tuning the incident laser wavelengths at on-resonant wavelength 9.219 µm and off-resonant wavelength of 9.201 µm. The on-resonant vibrational excitation allowed a largest fraction of the absorbed laser energy coupled directly into NH3 molecules whereas energy coupling under off-resonant excitations is less efficient in energy coupling and influencing the GaN growth p...