Ali K. Okyay

Ge-Interface Engineering With Ozone Oxidation for Low Interface-State Density

Passivation of Ge has been a critical issue for Ge MOS applications in future technology nodes. In this letter, we introduce ozone oxidation to engineer Ge/insulator interface. Density of interface states (D_it) across the bandgap and close to the conduction band edge was extracted using conductance technique at low temperatures. D_it dependence on growth conditions was studied. Minimum D_it of 3 times 10¹¹ cm^-2V^-1 was demonstrated. Physical quality of the interface was investigated through Ge 3d spectra measurements. We found that the interface and D_it are strongly affected by the distribution of oxidation states and the quality of the suboxide.

C-shaped nanoaperture-enhanced germanium photodetector

We present a C-shaped nanoaperture-enhanced Ge photodetector that shows 2-5 times the photocurrent enhancement over that from a square aperture of the same area at 1310 nm wavelength. We demonstrate the polarization dependence of the C-aperture photodetector over a wide wavelength range. Our experimental observation agrees well with finite-difference time-domain simulation results.

Plasmonic backcontact grating for P3HT:PCBM organic solar cells enabling strong optical absorption increased in all polarizations

In P3HT:PCBM based organic solar cells we propose and demonstrate numerically plasmonic backcontact grating architectures for strong optical absorption enhanced in both transverse-magnetic and transverse-electric polarizations. Even when the active material is partially replaced by the metallic grating (without increasing the active layer film thickness), we show computationally that the light absorption in thin-film P3HT:PCBM is increased by a maximum factor of ~21% considering both polarizations under AM1.5G solar radiation and over a half-maximum incidence angle of 45° (where the enhancement drops to its half) compared to the same cell without a grating. This backcontact grating outperforms the typical plasmonic grating placed in PEDOT:PSS layer.

Effective dark current suppression with asymmetric MSM photodetectors in Group IV semiconductors

We have demonstrated for the first time, with both simulations and experiments, the application of an asymmetric metal electrode scheme in Group IV metal-semiconductor-metal photodetectors (MSM-PDs) to effectively lower dark current (I/sub dark/) without sacrificing the photocurrent (I/sub photo/) substantially. A new metric was introduced by normalizing the photocurrent-to-dark current ratio to the input optical power (NPDR) to provide an objective assessment of the detector performance. Improvement of at least 1.4 times in NPDR was obtained with asymmetric MSM-PDs. Finally, the impact of MSM sizing on NPDR was also addressed.

Superhydrophobic and omnidirectional antireflective surfaces from nanostructured ormosil colloids.

A large-area superhydrophobic and omnidirectional antireflective nanostructured organically modified silica coating has been designed and prepared. The coating mimics the self-cleaning property of superhydrophobic lotus leaves and omnidirectional broad band antireflectivity of moth compound eyes, simultaneously. Water contact and sliding angles of the coating are around 160° and 10°, respectively. Coating improves the transmittance of the glass substrate around 4%, when coated on a single side of a glass, in visible and near-infrared region at normal incidence angles. At oblique incidence angles (up to 60°) improvement in transmission reaches to around 8%. In addition, coatings are mechanically stable against impact of water droplets from considerable heights. We believe that our inexpensive and durable multifunctional coatings are suitable for stepping out of the laboratory to practical outdoor applications.

Experimental characterization of single-walled carbon nanotube film-Si Schottky contacts using metal-semiconductor-metal structures

We demonstrate that single-walled carbon nanotube (CNT) films make a Schottky contact on silicon by experimentally characterizing metal-semiconductor-metal (MSM) structures. We find that at temperatures above 240K, thermionic emission is the dominant transport mechanism across CNT film-Si contacts, and at lower temperatures tunneling begins to dominate. At high bias voltages, the CNT film MSM devices exhibit a higher photocurrent-to-dark current ratio relative to that of metal control devices. Our results not only provide insight into the fundamental electronic properties of the CNT film-Si junction but also opens up the possibility of integrating CNT films as Schottky electrodes in conventional Si-based devices.

Ge–SiGe Quantum-Well Waveguide Photodetectors on Silicon for the Near-Infrared

We demonstrate near-infrared waveguide photodetectors using Ge-SiGe quantum wells epitaxially grown on a silicon substrate. The diodes exhibit a low dark current of 17.9 mA/cm2 at 5-V reverse bias. The photodetectors are designed to work optimally at 1480 nm, where the external responsivity is 170 mA/W, which is mainly limited by the fiber-to-waveguide coupling loss. The 1480-nm wavelength matches the optimum wavelength for quantum-well electroabsorption modulators built on the same epitaxy, but these photodetectors also exhibit performance comparable to the demonstrated Ge-based detectors at longer wavelengths. At 1530 nm, we see open eye diagrams at 2.5-Gb/s operation and the external responsivity is as high as 66 mA/W. The technology is potentially integrable with the standard complementary metal-oxide-semiconductor process and offers an efficient solution for on-chip optical interconnects.

Hollow cathode plasma-assisted atomic layer deposition of crystalline AlN, GaN and AlxGa1−xN thin films at low temperatures

The authors report on the use of hollow cathode plasma for low-temperature plasma-assisted atomic layer deposition (PA-ALD) of crystalline AlN, GaN and AlxGa1−xN thin films with low impurity concentrations. Depositions were carried out at 200 °C using trimethylmetal precursors and NH3 or N2/H2 plasma. X-ray photoelectron spectroscopy showed the presence of 2.5–3 at.% O in AlN and 1.5–1.7 at.% O in GaN films deposited using NH3 and N2/H2 plasma, respectively. No C impurities were detected within the films. Secondary ion mass spectroscopy analyses performed on the films deposited using NH3 plasma revealed the presence of O, C (both <1 at.%), and H impurities. GIXRD patterns indicated polycrystalline thin films with wurtzite crystal structure. Hollow cathode PA-ALD parameters were optimized for AlN and GaN thin films using N2/H2 plasma. Trimethylmetal and N2/H2 saturation curves evidenced the self-limiting growth of AlN and GaN at 200 °C. AlN exhibited linear growth with a growth per cycle (GPC) of ∼1.0 A. For GaN, the GPC decreased with the increasing number of deposition cycles, indicating substrate-enhanced growth. The GPC calculated from a 900-cycle GaN deposition was 0.22 A. Ellipsometric spectra of the samples were modeled using the Cauchy dispersion function, from which the refractive indices of 59.2 nm thick AlN and 20.1 nm thick GaN thin films were determined to be 1.94 and 2.17 at 632 nm, respectively. Spectral transmission measurements of AlN, GaN and AlxGa1−xN thin films grown on double side polished sapphire substrates revealed near-ideal visible transparency with minimal absorption. Optical band edge values of the AlxGa1−xN films shifted to lower wavelengths with the increasing Al content, indicating the tunability of band edge values with the alloy composition.

Volumetric plasmonic resonator architecture for thin-film solar cells

We propose and demonstrate a design concept of volumetric plasmonic resonators that relies on the idea of incorporating coupled layers of plasmonic structures embedded into a solar cell in enhanced optical absorption for surface-normal and off-axis angle configurations, beyond the enhancement limit of individual plasmonic layers. For a proof-of-concept demonstration in a thin-film organic solar cell that uses absorbing materials of copper phthalocyanine/perylene tetracarboxylic bisbenzimidazole, we couple two silver grating layers such that the field localization is further extended within the volume of active layers. Our computational results show a maximum optical absorption enhancement level of ∼67% under air mass 1.5 global illumination considering both polarizations.

Plasmonically enhanced hot electron based photovoltaic device

Hot electron photovoltaics is emerging as a candidate for low cost and ultra thin solar cells. Plasmonic means can be utilized to significantly boost device efficiency. We separately form the tunneling metal-insulator-metal (MIM) junction for electron collection and the plasmon exciting MIM structure on top of each other, which provides high flexibility in plasmonic design and tunneling MIM design separately. We demonstrate close to one order of magnitude enhancement in the short circuit current at the resonance wavelengths.