M. Çakmak

Structural, morphological, and optical properties of AlGaN/GaN heterostructures with AlN buffer and interlayer

AlxGa1−xN∕GaN (x∼0.3) heterostructures with and without a high-temperature (HT) AlN interlayer (IL) have been grown on sapphire (Al2O3) substrates and AlN buffer/Al2O3 templates by metal organic chemical vapor deposition. The effects of an AlN buffer layer (BL) grown on an Al2O3 substrate and an AlN IL grown under the AlGaN ternary layer (TL) on structural, morphological, and optical properties of the heterostructures have been investigated by high-resolution x-ray diffraction, spectroscopic ellipsometry, atomic force microscopy, and photoluminescence measurements. The AlN BL improves the crystal quality of the AlGaN TL. Further improvement is achieved by inserting an AlN IL between GaN BL and AlGaN TL. However, experimental results also show that a HT AlN IL leads to relatively rough surfaces on AlGaN TLs, and an AlN IL changes the strain in the AlGaN TL from tensile to compressive type. In addition, an AlN BL improves the top surface quality of heterostructures.

Molecular design of photoswitchable surfaces with controllable wettability

In this work, we developed a photocontrollable substrate which was prepared using an azobenzene-containing self-assembled monolayer (SAM) on the silicon surface via the chemisorption of 3-glycidoxypropyltrimethoxysilane (GPTS) and 4-(4′-aminophenylazo) benzoic acid (APABA). The prepared surfaces were chemically characterized by X-ray photoelectron spectroscopy (XPS). The reversible photoswitching performance of APABA molecules were investigated by UV spectroscopy in dimethylsulfoxide (DMSO) solution. To understand and control this reversible photoswitchable mechanism and wettability properties, contact angle measurements were performed by using a variety of liquids after UV and visible light irradiation. These contact angle results are used to approximate the components of the APABA-modified surface energy under UV and visible light using the Lifshitz–van der Waals/acid–base approach. The total surface energy (γs) after visible light irradiation (for trans formation) was calculated to be 37.28 mJ m−2, whereas the value after UV light exposure (for cis formation) was also calculated to be 36.95 mJ m−2. All the results demonstrate the great potential to control molecular events within and on the surfaces of molecular constructs using light.

Photocontrollable DNA hybridization on reversibly photoresponsive surfaces

Herein we have developed a new DNA sensor which displays the possibility of photocontrollable DNA hybridization by changing the orientation of azobenzene layers on the silicon wafer surface. Basically, the trans-form layers present a coordinating surface, the “on” state that can be switched “off” in the cis-form. Water contact angles of the prepared substrates have been measured after UV and visible light irradiation to understand the switchable properties of each form of the surface. The photocontrollable DNA chip was prepared using Cy3 labeled amino terminated single stranded probe-DNA (Cy3–ssDNA–NH2) molecules which were immobilized onto a COOH-terminated photoswitchable surface. The optimum hybridization conditions were performed with Cy5 labeled complementary-DNA (Cy5–ssDNA) using fluorescence microscopy. Furthermore, the photocontrolling of DNA hybridization onto prepared surfaces was verified by confocal microscopy before and after light irradiation. The percentage hybridization ratios from confocal microscopy for on and off positions of the DNA sensor were calculated to be 61% and 5%, respectively. These results show that the prepared surfaces can be reversibly photoswitched between two states efficiently. As a result we believe that the results demonstrate the great potential to control DNA hybridization within and on the surfaces of molecular constructs using light.

Adsorption and desorption of S on and off Si(001) studied by ab initio density functional theory

We present detailed ab initio density functional calculations of equilibrium atomic geometry, electronic states, and chemical bonding for the adsorption of elemental S on Si(001). Following recently reported room temperature low-energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy, and work function measurements by Papageorgopoulos et al. [Phys. Rev. B 55, 4435 (1997)], three different adsorption models have been studied: hemisulfide (2×1) structure, monosulfide (1×1) structure, and disulfide (1×1) structure. For hemisulfide and monosulfide structures, the calculated location of S above the Si(001) surface is in excellent agreement with the experiment. An analysis of surface free energy suggests that, in the allowed range of S chemical potential, the monosulfide structure is more stable than the hemisulfide and disulfide structures. A signature of desorption of the SiS unit is obtained from the study of the disulfide structure.

Effects of high-temperature AIN buffer on the microstructure of AlGaN/GaN HEMTs

Effects on AlGaN/GaN high-electron-mobility transistor structure of a high-temperature AlN buffer on sapphire substrate have been studied by high-resolution x-ray diffraction and atomic force microscopy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge-type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root-mean-square of 0.25 nm and a step termination density in the range of 108 cm−2. Due to the high-temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high-temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.

An ab initio calculation of the adsorption of H2S onto InP(110)-(1 × 1) surface

Atomic geometry, electronic states and bonding of H 2 S-adsorption on the InP(110) surface are studied by using ab initio calculations, based on the pseudopotentials and density functional theory. The adsorption of the molecule removes the relaxation of the clean InP(110) surface. The calculated average distance between the topmost InP layer and the sulphur atom is 1.87 A, which is in agreement with an X-ray standing-wave study. We also find that the fundamental band gap is free of surface states, indicating that the adsorption of H 2 S passivates the InP(110) surface. Furthermore, the calculated surface states agree well with angle-resolved photoemission measurements.

Atomic and electronic structure of S-terminated GaAs(001) surface

We have reported, from ab initio calculations, on the changes in the electronic and structural properties due to S adsorption on the GaAs(001) surface. In our investigation, we have considered the experimentally observed (2×6) reconstruction for S coverages of n/12 monolayers (MLs), with n=2, 4, 5, 6, 8, and 10. Electronic energy levels and density of states for all the six coverages of S have been discussed. Using the chemical potential argument our calculations suggest that the reconstruction with S coverage of 10/12 ML (the Tsukamoto model) represents the most energetically preferable structure for S/GaAs(001). However, while this adsorption geometry is consistent with the electron counting model, it does not passivate the GaAs(001) surface electronically. The most effective reduction in the density of states in bulk band gap region is obtained for the coverage of 0.5 ML with five mixed As–S dimers, though this geometry is inconsistent with the electron counting model for chemical passivation of the su...

Atomic and electronic structure of Bi/GaAs(001)-α2(2 × 4)

We report an ab initio pseudopotential calculation for the atomic and electronic structure of Bi/GaAs(001)-α2(2 × 4). Three structural models with Bi coverages of Θ = 1/4 are considered, containing one Bi dimer or two Bi-As mixed dimers. According to our calculations for this coverage, the atomic model of the Bi/GaAs(001)-(2 × 4) reconstruction is similar to the α2 structure of the clean GaAs(001)-(2 × 4) surface in which the top As dimer is replaced by a Bi dimer. This result is in agreement with the most recent scanning tunneling microscopy work.

Effect of Bleaching on Roughness of Dental Composite Resins

This study investigated the effect of three bleaching agents (Whiteness Perfect, Whiteness Super, Whiteness HP) on roughness of three dental resin composites (Admira, Durafill VS, Gradia Direct). Twenty disk-shaped standard specimens (10 × 2 mm) of each composite material were prepared and divided into four subgroups (n = 5). In each resin composite group, the unbleached specimens served as control; the other specimens were bleached with one of the bleaching agents according to the manufacturers instruction. Roughness values were assessed using the atomic force microscope and metallographic microscope. Two-dimensional and 3D images were also taken for detecting surface alterations of each specimen group. Although the surfaces of all specimens did not seem to be smooth, the unbleached control specimens showed more irregular areas compared with those of the bleached ones. Roughness values were decreased in bleached groups to some extent depending on the bleaching agents used.

Adsorption of GeH2 on the Si(001) surface

We present results of ab initio calculations, based on pseudopotentials and the density functional theory, for the adsorption of germanium dihydride (GeH 2 ) on the Si(001) surface, which is thought to take place during the atomic-layer-epitaxy process. Three adsorption sites are considered : (i) on-dimer (or bridge) site, (ii) an intrarow position between two neighbouring Si dimers with hydrogenated surface (GeH 2 + H + H), and (iii) an intrarow position under GeH 2 rich condition (GeH 2 + GeH 2 ). The atomic structure and surface electronic states for these structures are presented and the results are discussed in relation to recent experimental works, as well as recent theoretical work on the adsorption of SiH 2 . In particular, we find that all the three models are characterised by much longer Si-Si dimer length than on the clean Si(001)-(2 x 1) surface. We also find that for all these models the fundamental band gap is free of surface states and the system remains fully passivated. We do not find any significant energy difference between the first two models, although the on-dimer structure provides unfavourable Ge-Si bond angle.