Khalid Moumanis

Fourier-transform infrared and photoluminescence spectroscopies of self-assembled monolayers of long-chain thiols on (001) GaAs

Self-assembled monolayers (SAMs) of various thiols have shown the potential to protect freshly fabricated or chemically cleaned GaAs surfaces from oxidization, adsorption of foreign atoms, and∕or surface defect formation. We have employed an attenuated total reflection Fourier-transform infrared spectroscopic technique to investigate the process of formation of long-chain thiols, comprising ten or more methylene chains, on the surface of (001) GaAs. A strong infrared (IR) signal was measured for all the investigated GaAs-thiol interfaces. Varying the type of terminal groups, from hydrophilic to hydrophobic, significantly changes the IR intensity of the methylene stretching vibration, indicating different methylene chain orientation in SAMs. Consequently, these SAMs exhibited different passivation performance to the (001) GaAs surface as judged by the intensity of the GaAs-related photoluminescence signal.

Experimental evidence for mobile luminescence center mobility on partial dislocations in 4H-SiC using hyperspectral electroluminescence imaging

We report on the formation, motion, and concentration of localized green emission centers along partial dislocations (PDs) bounding recombination-induced stacking faults (RISFs) in 4H-SiC pin diodes. Electroluminescence imaging depicted the motion of these green emitting point defects during forward bias operation along carbon-core PDs that bound the RISFs. Following high temperature annealing, these green emitting point defects did not contract with the PDs, but remained in the final location during the expansion. This implies that the motion of these green emitting point dislocations is enabled through a recombination-enhanced motion, similar to the process for RISF expansion and contraction within SiC.

Surface morphology of SiO2 coated InP/InGaAs/InGaAsP microstructures following irradiation with the ArF and KrF excimer lasers

Successful fabrication of devices from quantum well-intermixed material requires efficient control of its surface morphology. To address this problem, we have employed atomic force microscopy to study surface morphology of InP/InGaAs/InGaAsP QW microstructure coated with dSiO2 = 50, 150, 190, 243 and 263 nm thick SiO2 films. Both ArF (193 nm) and KrF (248 nm) excimer lasers have been used to irradiate series of samples with up to 400 pulses of fluence 76 to 156 mJ/cm2. The roughness (σRMS) of SiO2 layer after both lasers irradiation and RTA decreases as the pulse number increases. Following RTA, a smoother surface morphology was observed for all irradiated samples. The cap InP layer was found to have a relatively smaller roughness (~ 0.4 nm) due to the protection provided by the SiO2 layer during excimer laser irradiation and high temperature RTA. For samples coated with 50- or 150-nm-thick SiO2 and irradiated by the ArF laser, the blueshift is only obtained when the SiO2 layer was ablated. However, the sample coated with 243-nmthick SiO2 (dSiO2 ≈ λKrF), following the 75-pulse-irradiation with the KrF laser at 124mJ/cm2 and RTA, showed a smooth surface (σRMS = 1.8 nm) and maximum blueshift of 74 nm achieved without removal of the SiO2 layer.

A study of binding biotinylated nano-beads to the surface of (001) GaAs

We have investigated the deposition of biotinylated nano-beads on the surface of GaAs. The deposition procedure involved either direct coating of (001) GaAs with nano-beads, or binding the nano-beads with avidin immobilized on the surface of (001) GaAs through the interface of biotin and the NH2 terminal group of 11-amino-1-undecanethiol (HS(CH2)11NH2). The efficiency of binding was tested by washing the samples in a solution of a commercial detergent and by subjecting them to a deionized water ultrasonic bath. The results indicate that nano-beads deposited directly on the surface of (001) GaAs withstand the detergent washing test but they are easily removed by ultrasonic washing. In contrast, the nano-beads attached to (001) GaAs through the avidin-biotin-thiol interface survive the ultrasonic washing tests.

Formation Kinetics of Mixed Self-Assembled Monolayers of Alkanethiols on GaAs(100)

We report on the formation kinetics of mixed self-assembled monolayers (SAMs) comprising 16-mercaptohexadecanoic acid (MHDA) and 11-mercapto-1-undecanol (MUDO) thiols on GaAs(100) substrates. These compounds were selected for their potential in constructing highly selective and efficient architectures for biosensing applications. The molecular composition and quality of one-compound and mixed SAMs were determined by the Fourier transform infrared absorption spectroscopy measurements. The formation of enhanced-quality mixed SAMs was investigated as a function of the molecular composition of the thiol mixture and the proportion of ethanol/water solvent used during their arrangement. Furthermore, the formation of mixed SAMs has been carried out by successive immersion of MHDA SAMs in MUDO thiol solutions and MUDO SAMs in MHDA thiol solution through the process involving thiol-thiol substitution. Our results, in addition to confirming that water-ethanol-based solvents improve the packing density of single thiol monolayers, demonstrate the attractive role of water-ethanol solvents in forming superior quality mixed SAMs.

Chemotaxis for enhanced immobilization of Escherichia coli and Legionella pneumophila on biofunctionalized surfaces of GaAs

The authors have investigated the effect of chemotaxis on immobilization of bacteria on the surface of biofunctionalized GaAs (001) samples. Escherichia coli K12 bacteria were employed to provide a proof-of-concept of chemotaxis-enhanced bacterial immobilization, and then, these results were confirmed using Legionella pneumophila. The recognition layer was based on a self-assembled monolayer of thiol functionalized with specific antibodies directed toward E. coli or L. pneumophila, together with the enzyme beta-galactosidase (β-gal). The authors hypothesized that this enzyme together with its substrate lactose would produce a gradient of glucose which would attract bacteria toward the biochip surface. The chemotaxis effect was monitored by comparing the number of bacteria bound to the biochip surface with and without attractant. The authors have observed that β-gal plus lactose enhanced the immobilization of bacteria on our biochips with a higher effect at low bacterial concentrations. At 100 and 10 bacteria/ml, respectively, for E. coli and L. pneumophila, the authors observed up to 11 and 8 times more bacteria bound to biochip surfaces assisted with the chemotaxis effect in comparison to biochips without chemotaxis. At 10(4) bacteria/ml, the immobilization enhancement rate did not exceed two times.

XPS study of InP/InGaAs/InGaAsP microstructures irradiated with ArF laser in air and deionized water

Excimer lasers, due to their compatibility with a large-scale industrial production, are attractive tools for precise processing of photonic and microelectronic materials. In this article, we discuss the effect of ArF excimer laser defect formation on the surface of InP/InGaAs/InGaAsP quantum well (QW) microstructures irradiated in air and deionized (DI) water environments. Structural defects on surfaces of such QW materials have been known to induce vacancy diffusion towards the QW region and lead to the so called quantum well intermixing (QWI) effect during the rapid thermal annealing step. Excimer lasers have been used to create surface defects on InP/InGaAs/InGaAsP microstructure and induce QWI during high temperature annealing. Chemical composition of the QW microstructures irradiated with ArF laser in air and DI water is studied with X-ray photoelectron spectroscopy to investigate both the formation and role of the surface defects in the laser-induced QWI process.

ArF excimer laser-induced quantum well intermixing in dielectric layer coated InGaAs/InGaAsP microstructures

It has been known that excimer laser irradiation of surfaces of III-V quantum well (QW) semiconductor microstructures can be used for selective area bandgap engineering of such materials [1]. Depending on the laser used, investigated microstructure and the irradiation environment both enhanced quantum well intermixing (QWI) leading to blue shifting of the bandgap energy [2], as well as suppressed QWI [3] processes have been reported. Excimer lasers are attractive for bandgap engineering as they can be used to pattern large size wafers, often in a single step, and without the need of using photolithography masks that normally are required to achieve selective area processing.To investigate the role of the solid environment and the influence of laser-induced surface defects on the amplitude of the QWI process, we have investigated InP/InGaAs/InGaAsP QW microstructure coated with 2 sets of SiO2 layers having distinctively different optical properties. An ArF (193 nm) laser has been used to irradiate a series of samples with up to 400 pulses of fluence 76 mJ/cm2. The bandgap shift of irradiated sites, following two 120 sec rapid thermal annealing steps at 650°C and 675°C, varies with the pulse fluence and number. A maximum of 120 nm blueshift of the bandgap was observed for the samples irradiated with 150 pulses of the laser. We have also investigated the ArF laser QWI on the QW microstructure coated with layers of Si3N4. We discuss the advantages of this approach for post growth fabrication of multibandgap QW material suitable for designing and manufacturing of monolithically integrated photonic devices.It has been known that excimer laser irradiation of surfaces of III-V quantum well (QW) semiconductor microstructures can be used for selective area bandgap engineering of such materials [1]. Depending on the laser used, investigated microstructure and the irradiation environment both enhanced quantum well intermixing (QWI) leading to blue shifting of the bandgap energy [2], as well as suppressed QWI [3] processes have been reported. Excimer lasers are attractive for bandgap engineering as they can be used to pattern large size wafers, often in a single step, and without the need of using photolithography masks that normally are required to achieve selective area processing.To investigate the role of the solid environment and the influence of laser-induced surface defects on the amplitude of the QWI process, we have investigated InP/InGaAs/InGaAsP QW microstructure coated with 2 sets of SiO2 layers having distinctively different optical properties. An ArF (193 nm) laser has been used to irradiate a series...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75°. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm(2), respectively, has decreased the CA to near 15°, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafers surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm(2) and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103°. The XPS results indicate ArF laser induced formation of Si-(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

Multi section bandgap tuned superluminescent diodes fabricated by UV laser induced quantum well intermixing

Superluminescent diodes are used in numerous sensing and testing applications. To achieve the wide emission spectrum and high power required from such devices, we studied an innovative design constituting in several different bandgap energy sections independently electrically pumped. Bandgap modification was obtained by the UV laser quantum well intermixing process.