Tahir Colakoglu

Silicon nanowire-silver indium selenide heterojunction photodiodes

Structural and optoelectronic properties of silicon (Si) nanowire-silver indium selenide (AgInSe2) thin film heterojunctions were investigated. The metal-assisted etching method was employed to fabricate vertically aligned Si nanowire arrays. Stoichiometric AgInSe2 films were then deposited onto the nanowires using co-sputtering and sequential selenization techniques. It was demonstrated that the three-dimensional interface between the Si nanowire arrays and the AgInSe2 thin film significantly improved the photosensitivity of the heterojunction diode compared to the planar reference. The improvements in device performance are discussed in terms of interface state density, reflective losses and surface recombination of the photogenerated carriers, especially in the high-energy region of the spectrum.

In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon

Silicon is an excellent material for microelectronics and integrated photonics1–3, with untapped potential for mid-infrared optics4. Despite broad recognition of the importance of the third dimension5,6, current lithography methods do not allow the fabrication of photonic devices and functional microelements directly inside silicon chips. Even relatively simple curved geometries cannot be realized with techniques like reactive ion etching. Embedded optical elements7, electronic devices and better electronic–photonic integration are lacking8. Here, we demonstrate laser-based fabrication of complex 3D structures deep inside silicon using 1-µm-sized dots and rod-like structures of adjustable length as basic building blocks. The laser-modified Si has an optical index different to that in unmodified parts, enabling the creation of numerous photonic devices. Optionally, these parts can be chemically etched to produce desired 3D shapes. We exemplify a plethora of subsurface—that is, ‘in-chip’—microstructures for microfluidic cooling of chips, vias, micro-electro-mechanical systems, photovoltaic applications and photonic devices that match or surpass corresponding state-of-the-art device performances.By exploiting dynamics arising from nonlinear laser–material interactions, functional microelements and arbitrarily complex 3D architectures deep inside silicon are fabricated with 1 μm resolution, without damaging the silicon above or below.

Electrical and photoelectrical properties of Ag-In-Se thin films evaporated by e-beam technique

In this study, the electrical and photoelectrical properties of the Ag‐In‐Se thin films deposited by the e-beam technique have been investigated by carrying out temperature dependent conductivity, photoconductivity under different illumination intensities and photoresponse measurements between 380 and 1050nm, as a function of annealing temperature. The measured conductivity values of the films at room temperature depending on the annealing temperatures vary in the range 1.9 × 10 −6 ‐5.2 � −1 cm −1 . Annealing the films above 200 ◦ C results in degenerate thin films. Photoconductivity of the films increases with illumination intensity and its value reaches about 300 times dark conductivity values as the illumination intensity is increased from 17 to 113mWcm −2 for as-grown samples. The relation between photocurrent and excitation intensity is of Iph ∝ φ n -type. n values varying in the range of 0.96 and 1.55 imply the supralinear photoconductivity with the two-centre model. The spectral distribution of photoconductivity shows three maxima located at 1.57, 1.77 and 2.01eV, which was governed by the splitting of p-like levels in the valence band including the effects of a tetragonal crystalline field (� CF) and spin‐orbit (� SO) interaction. The band gap energies of as-grown and the 200 ◦ C annealed films are determined to be 1.57eV and 1.68eV, respectively.

Effect of boron implantation on the electrical and photoelectrical properties of e-beam deposited Ag-In-Se thin films

In this study, e-beam evaporated Ag‐In‐Se (AIS) thin films were doped by the implantation of boron (B) ions at 75keV with a dose of 1 ×10 15 ionscm −2 and a subsequent annealing process was applied to the doped AIS films at different temperatures under nitrogen atmosphere. The effects of implantation and annealing on the electrical and photoelectrical properties of AIS thin films were investigated through temperature dependent conductivity, spectral photoresponse and photoconductivity measurements under different illumination intensities. The electrical conductivity measurements showed that the room temperature conductivity values were determined as 2.4 × 10 −7 (� cm) −1 ,1 .7 × 10 −6 (� cm) −1 and 8.9 × 10 −5 (� cm) −1 for B-doped films (B0), B-doped and annealed films at 200 ◦ C (B2) and at 300 ◦ C (B3), respectively. It was observed that the electrical conductivity improved as the annealing temperature increased up to 400 ◦ C at which the AIS thin films showed degenerate semiconductor behaviour. The spectral distribution of the photoresponse curves indicated three local maxima located at 1.63, 1.79 and 2.01eV for B0 type films, 1.65, 1.87 and 2.07eV for B2 type films and 1.73, 2.02 and 2.32eV for B3 type films at room temperature. These three different energy values were ascribed to the splitting of the valence band due to spin‐orbit interaction and crystalline lattice field effects. The first energy values of each set were determined to be energy band gaps of the AIS thin films. The photoconductivity measurements as a function of temperature and illumination intensity were performed on the B-doped AIS thin films in order to determine the nature of recombination processes in the films. The photoconductivity values were found to be thermally quenched for all types of thin films and the variation of photocurrent as a function of illumination intensity showed that the dependence of photocurrent on the intensity was supralinear. The two-centre recombination model was applied successfully in order to explain the photoconductivity behaviours of the films.

Laser-slicing of silicon with 3D nonlinear laser lithography

Recently, we have showed a direct laser writing method that exploits nonlinear interactions to form subsurface modifications in silicon. Here, we use the technique to demonstrate laser-slicing of silicon and its applications.

Optical waveguides written deep inside silicon by femtosecond laser

Photonic devices that can guide, transfer or modulate light are highly desired in electronics and integrated silicon photonics. Through the nonlinear processes taking place during ultrafast laser-material interaction, laser light can impart permanent refractive index change in the bulk of materials, and thus enables the fabrication of different optical elements inside the material. However, due to strong multi-photon absorption of Si resulting delocalization of the light by free carriers induced plasma defocusing, the subsurface Si modification with femtosecond laser was not realized so far [1, 2]. Here, we demonstrate optical waveguides written deep inside silicon with a 1.5-μm high repetition rate femtosecond laser. Due to pulse-to-pulse heat accumulation for high repetition rate laser, additional thermal lensing prevents delocalization of the light around focal point, allowing the modification. The laser with 2-μJ pulse energy, 350-fs pulse width, operating at 250 kHz focused in Si produces permanent modifications. The position of the focal point inside of the sample is accurately controlled with pumpprobe imaging during processing. Optical waveguides of ∼20-μm diameter, and up to 5.5-mm elongation are fabricated by translating the beam focal position along the optical axis. The waveguides are characterized with a 1.5-μm continuous-wave laser, through optical shadow-graphy (Fig. 1 a-b, e) and direct light coupling (Fig.1 c-d, f). The measured refractive index change obtained by quantitative shadow-graphy is ∼6×10−4. The numerical aperture of the waveguide measured from decoupled light is 0.05.