Albert Birner

Silicon-Based Photonic Crystals**

In semiconductors electrons propagate in a periodic poten-tial, which originates from the atomic lattice. This modifiesthe dispersion relation of free electrons and a band structurewith a bandgap occurs in the case of semiconductors. Theincorporation of electrically active defects allows the manipu-lation of the electronic properties, which gave birth to a largevariety of electronic devices. There are distinct electrical andelectro-optical properties of the different semiconductormaterials, the dominant and most studied semiconductorbeing silicon.For more than ten years, the optical analogues to electronicsemiconductors, the so-called photonic crystals, have been thesubject of intense international research efforts. Photoniccrystals are materials with a periodically varying index ofrefraction. This allows the control of the propagation of elec-tromagnetic waves, similar to electrons in a semiconductorcrystal. By analogy with semiconductors, the periodicity of theunderlying lattice structure is of the same order of magnitudeas the wavelength of the electromagnetic radiation.Despite the far-reaching analogies between electronicwaves in semiconductors and electromagnetic waves in pho-tonic crystals, there are pronounced differences between thetwo as is noticeable from the corresponding equations of mo-tion. Electrons are described by a scalar wavefield. In con-trast, the electromagnetic field is vectorial by nature. Further-more, the time-independent Schrodinger equation allowssolutions with negative energy eigenvalues, whereas the corre-sponding wave equation in electrodynamics contains only thesquare of the eigenfrequencies, hence negative eigenvaluesare excluded from the outset. It may be inferred from the fewphotonic crystals that appear in nature, in contrast to ubiqui-tous semiconductor materials, that these differences have adisadvantageous effect on the likelihood of the formation ofphotonic bandgaps. From the multitude of the optical phe-nomena only, for example, the colorful speckles of opals, somecrystallites on the wings of butterflies and the spine of the sea-mouse

Polycrystalline nanopore arrays with hexagonal ordering on aluminum

Nanopore arrays with 6×108–5×1010 cm−2 pore densities were fabricated by self-organized anodization on aluminum. A two-step anodization process was used to oxidize aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays were obtained within domains of a few micrometers, which are separated from neighboring domains with different orientation of the pore lattice by domain boundaries, i.e., the nanopore arrays show characteristics analogous to two-dimensional polycrystalline structure. The interpore distance can be controlled by changing the electrolyte and/or the applied voltage.

Near-field visualization of light confinement in a photonic crystal microresonator

By using scanning near-field optical microscopy, we directly map the subwavelength confinement of light around a point defect in a two-dimensional photonic crystal microresonator. Comparison of our results with the outcome of three-dimensional finite-difference time domain calculations allows us to identify small imperfections in the structure that result in the spatial modification of the intensity distribution.

A model system for two-dimensional and three-dimensional photonic crystals: macroporous silicon

A review of the optical properties of two-dimensional and three-dimensional photonic crystals based on macroporous silicon is given. As macroporous silicon provides structures with aspect ratios exceeding 100, it can be considered to be an ideal two-dimensional photonic crystal. Most of the features of the photonic dispersion relation have been experimentally determined and were compared to theoretical calculations. This includes transmission and reflection of finite and bulk photonic crystals and their variation with the pore radius to determine the gap map. All measurements have been carried out for both polarizations separately since they decouple in two-dimensional photonic crystals. Moreover, by inhibiting the growth of selected pores, point and line defects were realized and the corresponding high-Q microcavity resonances as well as waveguiding properties were studied via transmission. The tunability of the bandgap was demonstrated by changing the refractive index inside the pores caused by an infiltrated liquid crystal undergoing a temperature-induced phase transition. Finally different realizations of three-dimensional photonic crystals using macroporous silicon are discussed. In all cases an excellent agreement between experimental results and theory is observed.

Transmission of a microcavity structure in a two-dimensional photonic crystal based on macroporous silicon

Photonic crystals consist of regularly arranged dielectric scatterers of dimensions on a wavelength scale, exhibiting band gaps for photons, analogous to the case of electrons in semiconductors. Using electrochemical pore formation in n-type silicon, we fabricated photonic crystals consisting of air cylinders in silicon. The starting positions of the pores were photolithographically pre-defined to form a hexagonal lattice of a=1.58mm. The photonic crystal was microstructured to make the photonic lattice accessible for optical characterization. Samples with dierent filling factors were fabricated to verify the gap map of electric and magnetic modes using Fourier-transform infrared (IR) spectroscopy. The complete band gap could be tuned from 3.3 to 4.3mm wavelength. We were able to embed defects such as waveguide structures or microcavities by omitting certain pores. We carried out transmission measurements using a tunable mid-IR optical parametric oscillator. The resonance is compared with theoretical expectations. # 2001 Elsevier Science Ltd. All rights reserved.

Data retention analysis on individual cells of 256Mb DRAM i n 110nm technology

In DRAM, every memory cell experiences an individual mixture of leakage currents which consume part of the stored charge and lead to a wide distribution of data retention time (t/sub Ret/). This distribution consists of an intrinsic (main) and an extrinsic (tail) branch. The formalism of activation energies (Ea) provides information about the mechanisms involved. Activation energies of single cells in a 256M DDR memory chip and their dependence on negative gate bias (VNWLL) as well as body bias (VBB) have been investigated intensively for the first time. The worst tail cells - all within a small retention time interval - show a twofold and wide distribution of activation energies. The lower Ea distribution can be altered with VNWLL, whereas the higher Ea distribution only alters with VBB. Going from tail towards main distribution, the percentage of cells belonging to the low Ea part continuously decreases and finally disappears. We therefore conclude that a gate induced mechanism (GIDL) is the main component responsible for DRAM retention tail.

All-optical ultrafast tuning of two-dimensional silicon photonic crystals via free-carrier injection

Summary form only given. Photonic crystals are emerging as a potential disruptive technology for the coming decade. The functionality of photonic crystals could be dramatically enhanced by providing a means to tune their spectral properties. In the past, tunable photonic crystals have been demonstrated using liquid crystals to control the refractive index, for which the optimal response time is milliseconds. Recently, it was suggested that thermally injected free-carriers could be used to tune photonic crystals. Here we demonstrate an ultrafast, all-optical-tunable, two-dimensional photonic crystal, where control of band edge wavelength and absorption is achieved through optical free-carrier injection.

Highly birefringent silicon two-dimensional photonic crystals

Summary form only given. Photonic crystals, even those without a complete, photonic band gap, possess numerous novel optical properties associated with the polarization, dispersion and anisotropy characteristics of the photonic bands. One such property is birefringence, which is always present for wavelengths comparable to the lattice constant. Although recent experiments have shown moderately large birefringence in this regime, many theoretical studies have shown that photonic crystals may also possess large birefringence in the long-wavelength limit. Here we report measurements of very large birefringence in two-dimensional silicon photonic crystals in the long-wavelength regime, and find excellent agreement with our calculations for these structures.

Capacitance Enhancement by Mesopore Formation for sub 100nm Deep Trench DRAM Technology

We have successfully applied the electro-chemical method of mesopore formation in deep trenches (DTs) to increase the surface of deep-trench capacitors for DRAMs. The length of the mesopores is controlled by the etching time and was up to 60 nm. Subsequently, the diameter of the mesopores, was increased to above 20 nm by an isotropic wet etch. By sufficient tuning of the arsenic doping concentration of the trench side-walls, the density of the mesopores was adjusted to 400/μm2. Thus, a surface area increase of the DTs of up to 150% was achieved.

Retention Tail Improvement for Gbit DRAMs through Trap Passivation confirmed by Activation Energy Analysis

A very efficient method to reduce gate induced drain leakage (GIDL) as the dominant leakage path in the tail part of DRAM data retention time distribution is presented. Different to other reports, GIDL is addressed by trap passivation instead of lowering of electric fields. Stable passivation of traps is achieved by implantation of fluorine into S/D regions of 512Mbit and 1Gbit DRAMs in 110 nm technology. It was found that the position of the F-implant within the process flow plays a key role to enable trap reduction and retention tail improvement. Systematic implant experiments were carried out resulting in a fail count reduction of up to 40 %. Detailed activation energy analysis on individual memory cells confirms the validity of the retention tail model and the selective reduction of GIDL traps by fluorine implantation