Micro-photoluminescence of GaN quantum dots embedded in 100 nm wide cylindrical AlN pillars

Individual pillars were etched from a sample embedding a single plane of GaN/AlN quantum dots, deposited by molecular beam epitaxy on a sapphire substrate. Pillars with diameters ranging from 0.1 to 5 μm were fabricated by electron-beam lithography and SiCl₄ reactive ion etching. The PL from a single pillar could be measured by using a confocal microscope, with a spatial resolution of 600 nm. We report an intense PL signal from pillar diameters as small as 0.1 μm at room temperature. By increasing the power of the excitation laser from 0.05 to 200 μW, we induced a blue-shift of the PL energy peak from 2.38 to 2.86 eV, accompanied by a substantial broadening of the PL line. This is explained by the photo-induced screening of the internal electric field, which is close to 10 MV/cm in GaN/AlN heterostructures. Finally we report and tentatively explain a photodarkening effect, i.e., the progressive decrease of the PL intensity over two orders of magnitude, after one hour of continuous laser excitation. However, this effect does not seem to be correlated to the etching process.     

Enhancement of field emission of SnO2 nanowires film by exposure of hydrogen gas

The effect of hydrogen (H₂) gas exposure on the field emission properties of tin oxide (SnO₂) nanowires films synthesized by the carbon thermal reduction vapor transport method was investigated. The exposure of H₂ gas results in the reduction of the turn-on voltage for driving a current of 10 nA from 7.6 V/μm to 5.5 V/μm and the increase of the field current based on 10 V/μm from 0.47 μA to 2.1 μA. The Fowler–Nordheim plot obtained from the current–voltage data supports that the field emission enhancement of SNW film is attributed to the reduction of the work function by the H₂ exposure.     

Design and fabrication of infrared detectors based on lattice-matched InAs0.91Sb0.09 on GaSb

The InAs_0.91Sb_0.09 ternary compound grown on GaSb substrates is a promising alloy for light detection in the 3–5 μm window. Nevertheless, its development is still limited due to difficulties occurring during device processing. For example, the use of dry etching for the processing of InAs_0.91Sb_0.09 p–i–n photovoltaïc detectors induces a strong leakage current along the mesa edge. In this letter, we show an improvement of the R₀A characteristic by several orders of magnitude at low temperature by using an ion beam etching (IBE) followed by a wet chemical etching. This optimized and reliable device processing allows us to demonstrate that the detector performance is actually limited by the diffusion current of holes. Finally, we discuss the ability of an n-type barrier made of the InAs/AlSb super-lattice to prevent hole diffusion and to improve the R₀A characteristic of these detectors.     

Dry etching characteristics of GaN using Cl2/BCl3 inductively coupled plasmas

ICP power/RF power, operating pressure, and Cl₂/BCl₃ gas mixing ratio are altered to investigate the effect of input process parameters on the etch characteristics of GaN films. The etch selectivity of GaN over SiO₂ and photoresist is studied. Although higher ICP/RF power can obtain higher GaN/photoresist etch selectivity, it can result in faceting of sidewall and weird sidewall profile due to photoresist mask erosion. Etch rates of GaN and SiO₂ decrease with the increase of operating pressure, and etch selectivity of GaN over SiO₂ increases with the increasing operating pressure at fixed ICP/RF power and mixture component. The highest etch selectivity of GaN over SiO₂ is 7.92, and an almost vertical etch profile having an etch rate of GaN close to 845.3 nm/min can be achieved. The surface morphology and root-mean-square roughness of the etched GaN under different etching conditions are evaluated by atomic force microscopy. The plasma-induced damage of GaN is analyzed using photoluminescence (PL) measurements. The optimized etching process, used for mesa formation during the LED fabrication, is presented. The periodic pattern can be transferred into GaN using a combination of Cl₂/BCl₃ plasma chemistry and hard mask SiO₂. Patterning of the sapphire substrate for fabricating LED with improved extraction efficiency is also possible using the same plasma chemistry.     

Determination of absorption length of CO2 and high power diode laser radiation for ordinary Portland cement and its influence on the depth of melting

The laser beam absorption lengths of CO₂ and a high power diode laser (HPDL) radiation for concrete have been determined. By employing Beer–Lambert’s law the absorption lengths for concrete of CO₂ and a HPDL radiation were 470±22 μm and 177±15 μm, respectively. Indeed, this was borne out somewhat from a cross-sectional analysis of the melt region produced by both lasers which showed melting occurred to a greater depth when the CO₂ laser was used.     

Micromachining of copper using Nd:YAG laser radiation at 1064, 532, and 355 nm wavelengths

The interaction phenomena of nanosecond time period Q-switched diode-pumped Nd:YAG laser pulses using 1064, 532 and 355 nm with 0.25 mm thick pure-copper foil was investigated at an incident laser intensity range of 0.5–57.9 GW/cm². For each sample, etch rate and surface structure were determined. Analysis of the results of the tests included scanning electron microscopy (SEM). A maximum etch rate of 13.3 μm per pulse was obtained for the etch rate tests carried out at 532 nm. The maximum etch rate obtainable for 1064 nm was 2.21 μm per pulse, and for 355 nm, 6.68 μm per pulse. The dramatic decrease in etch rate observed when processing at 1064 nm is thought to occur due the highly reflective nature of copper as the interaction wavelength is increased, plus the nature of the plasma formed above the material during the high-intensity laser–material interaction. This plasma then imparts energy to the surface of the processed area leading to surface melting of the area surrounding the hole as can be seen by the SEM photographs.     

GaN/AlN quantum dot photodetectors at 1.3–1.5 μm

We have demonstrated GaN/AlN quantum dots (QD) photodetectors, relying on intraband absorption and in-plane carrier transport in the wetting layer. The devices operate at room temperature in the wavelength range 1.3–1.5 μm. Samples with 20 periods of Si-doped GaN QD layers, separated by 3 nm-thick AlN barriers, have been grown by plasma-assisted molecular-beam epitaxy on an AlN buffer on a c-sapphire substrate. Self-organized dots are formed by the deposition of 5 monolayers of GaN under nitrogen-rich conditions. The dot height is 1.2±0.6 to 1.3±0.6 nm and the dot density is in the range 10¹¹–10¹² cm⁻². Two ohmic contacts were deposited on the sample surface and annealed in order to contact the buried QD layers. The dots exhibit TM polarized absorption linked to the s–p_z transition. The photocurrent at 300 K is slightly blue-shifted with respect to the s–p_z intraband absorption. The responsivity increases exponentially with temperature and reaches a record value of 10 mA/W at 300 K for detectors with interdigitated contacts.     

Etching and oxidation of InAs in planar inductively coupled plasma

The surface of InAs (1 1 1)A was investigated under plasmachemical etching in the gas mixture CH₄/H₂/Ar. Etching was performed using the RF (13.56 MHz) and ICP plasma with the power 30–150 and 50–300 W, respectively; gas pressure in the reactor was 3–10 mTorr. It was demonstrated that the composition of the subsurface layer less than 5 nm thick changes during plasmachemical etching.A method of deep etching of InAs involving ICP plasma and hydrocarbon based chemistry providing the conservation of the surface relief is proposed. Optimal conditions and the composition of the gas phase for plasmachemical etching ensuring acceptable etch rates were selected.     

Issues in ZnO homoepitaxy

Zinc oxide (ZnO) bulk single crystals, which are of high purity and transparency with a large size of 2 in., are successfully grown by the hydrothermal method. The sliced substrates are chemomechanically polished to form an epi-ready surface. The impurities existing on the as-polished substrate surface are characterized before and after annealing by SIMS (secondary-ion mass spectroscopy), and a damaged surface layer due to chemomechanical polishing is evaluated by an optical method. We attempt to remove the layer damaged due to chemomechanical polishing with two approaches, chemical etching and thermal annealing in N₂, O₂ or high vacuum. The improvement of the surface morphology and crystallinity is evaluated by means of high resolution X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM). In the PL measurements, the relative intensity of the first-order longitudinal optical phonon replica of the free exciton (FX-1LO) is compared against varying etching depth. The relative intensity becomes weak with increasing etch depth and finally saturates at the etch depth of 5 μm. After the annealing process, we grow ZnO thin films on these ZnO(0001) substrates by plasma-assisted molecular beam epitaxy. Films grown directly on the substrate show a 3D growth mode in the initial stage of growth with various surface treatments. To overcome this problem, we employ a low temperature grown ZnO buffer layer (LT-ZnO), and a two-dimensionally grown high quality ZnO film is attained.     

Single InAs/InP quantum dot spectroscopy in 1.3–1.55 μm telecommunication band

We present an optical spectroscopy and photon correlation measurement at telecommunication wavelengths performed on single InAs/InP quantum dots. Two main approaches brought high optical quality: an application of a ‘double-cap’ growth method to metalorganic chemical vapor deposition, and fabrication of a small mesa structure using low-damage wet chemical etching. Sharp and discrete exciton transition lines have been observed on the single quantum dots, which widely cover the spectral range of 1.3–1.55 μm. Using a pulsed excitation source and gated single-photon detection modules, we observed a photon antibunching behavior for an isolated exciton emission line, indicating nonclassical light emission near the wavelength of 1.3 μm.     

Characterization of wide-band-gap semiconductors (GaN, SiC) by defect-selective etching and complementary methods

Two defect-selective etching approaches used for revealing and analysis of defects in wide-band-gap semiconductors (GaN, SiC) are described in detail: (i) orthodox etching in molten salts (KOH, NaOH) and hot acids (H₂SO₄,H₃PO₄) and (ii) electroless photo-etching (photoelectrochemical or PEC) in aqueous solutions of KOH. Characteristic features of these two techniques, their reliability and limitation in revealing different types of defects (dislocations, stacking faults, micro-defects and electrically active chemical non-homogeneities) will be discussed. Examples of the use of both etching approaches to reveal defects in bulk and epitaxial layers of different crystallographic orientation are given. Numerous references to previous work on calibration of the etch features by means of TEM, X-ray diffraction, Raman and PL methods are cited.     

Optimized ICP etching process for fabrication of oblique GaN sidewall and its application in LED

Inductively coupled plasma (ICP) etching of GaN is systemically investigated by changing ICP power/RF bias power, operating pressure, and Cl₂/BCl₃ gas mixing ratio. The hexagonal etch pits related to screw dislocation existing along GaN epitaxial layer were observed on the etched GaN surface after ICP etching. The intensity of band-edge emission is significantly reduced from the etched n-GaN surface, which reveals that plasma-induced damage are generated after ICP etching. The oblique sidewall is transferred into GaN using a combination of Cl₂/BCl₃ plasma chemistry and hard mask SiO₂. By adjusting ICP etching process parameters, oblique sidewalls with various oblique angles can be formed, allowing for conformal metal lines coverage across the mesa structures, which can play an important role in the interconnection of multiple microchips for light emitting diodes (LEDs) fabrication.     

Low temperature transport properties of thin SOI MOSFETs

Fabrication and 4.2 K mobility measurements of silicon-on-insulator (SOI) metal–oxide-field-effect-transistors are reported. The three sets of samples fabricated in this work include devices for which the SOI film thicknesses (t_SOI) are in the ranges of 10–15, 16–19 and 56–61 nm. The peak mobility of the devices that have the SOI film thickness above 16.5 nm is 1.9 m²/V s. The set of devices with thinnest channel (t_SOI=10–15 nm) suggest that the peak mobility decreases with decreasing t_SOI.     

Structural characterization of ZnO/ Er2O3 core/shell nanowires

Zinc oxide/erbium oxide core/shell nanowires are of great potential value to optoelectronics because of the possible demonstration of laser emission in the 1.5 μm range. In this paper we present a convenient technique to obtain structures of this composition. ZnO core nanowires were first obtained by a vapor–liquid–solid (VLS) method using gold as a catalyst. ZnO nanowires ranging from 50 to 100 nm in width were grown on the substrates. Erbium was incorporated into these nanowires by their exposure to Er(tmhd)₃ at elevated temperatures. After annealing at 700 C in air, the nanowires presented 1.54 μm emission when excited by any of the lines of an Ar⁺ laser. An investigation of nanowire structure by HRTEM indicates that indeed the cores consist of hexagonal ZnO grown in the 001 direction while the surface contains randomly oriented Er₂O₃ nanoparticles. EXAFS analysis reveals that the Er atoms possess a sixfold oxygen coordination environment, almost identical to that of Er₂O₃. Taken collectively, these data suggest that the overall architectures of these nanowires are discrete layered ZnO/ Er₂O₃ core/shell structures whereby erbium atoms are not incorporated into the ZnO core geometry.     

Dry etching of ITO by magnetic pole enhanced inductively coupled plasma for display and biosensing devices

The dry etching of indium tin oxide (ITO) layers deposited on glass substrates was investigated in a high density inductively coupled plasma (ICP) source. This innovative low pressure plasma source uses a magnetic core in order to concentrate the electromagnetic energy on the plasma and thus provides for higher plasma density and better uniformity. Different gas mixtures were tested containing mainly hydrogen, argon and methane. In Ar/H₂ mixtures and at constant bias voltage (−100 V), the etch rate shows a linear dependence with input power varying the same way as the ion density, which confirms the hypothesis that the etching process is mainly physical. In CH₄/H₂ mixtures, the etch rate goes through a maximum for 10% CH₄ indicating a participation of the radicals to the etching process. However, the etch rate remains quite low with this type of gas mixture (around 10 nm/min) because the etching mechanism appears to be competing with a deposition process. With CH₄/Ar mixtures, a similar feature appeared but the etch rate was much higher, reaching 130 nm/min at 10% of CH₄ in Ar. The increase in etch rate with the addition of a small quantity of methane indicates that the physical etching process is enhanced by a chemical mechanism. The etching process was monitored by optical emission spectroscopy that appeared to be a valuable tool for endpoint detection.     

Laser-induced etching of titanium by Br2 and CCl3Br at 248 nm

A quartz crystal microbalance (QCM) has been used to study the KrF* excimer laser-induced etching of titanium by bromine-containing compounds. The experiment consists of focusing the pulsed UV laser beam at normal incidence onto the surface of a quartz crystal coated with 1 m of polycrystalline titanium. The removal of titanium from the surface is monitored in real time by measuring the change in the frequency of the quartz crystal. The dependence of the etch rate on etchant pressure and laser fluence was measured and found to be consistent with a two-step etching mechanism. The initial step in the etching of titanium is reaction between the etchant and the surface to form the etch product between laser pulses. The etch product is subsequently removed from the surface during the laser pulse via a laser-induced thermal desorption process. The maximum etch rate obtained in this work was 6.2 Å-pulse^–1, indicating that between two and three atomic layers of Ti can be removed per laser pulse. The energy required for desorption of the etch product is calculated to be 172 kJ-mole^–1, which is consistent with the sublimation enthalpy of TiBr₂ (168 kJ-mole^–1). The proposed product in the etching of titanium by Br₂ and CCl₃Br is thus TiBr₂. In the etching of Ti by Br₂, formation of TiBr₂ proceeds predominantly through the dissociative chemisorption of Br₂. In the case of etching with CCl₃Br, TiBr₂ is formed via chemisorption of Br atoms produced in the gas-phase photodissociation of CCl₃Br.     

Theoretical calculations of a square multilayer Bragg–Fresnel lens by quantum theory

A theoretical method based on the quantum scattering theory is presented to evaluate the performances of a two-dimensional (2-D) focusing square multilayer Bragg–Fresnel lens. The numerical application results of the square multilayer Bragg–Fresnel lens working at 0.7 nm wavelength (W/Si 25 periods with a double layer thickness of 5.38 nm, the size of the diffraction pattern is about 291×291 μm, the size of the center square in the diffraction pattern is 21.4×21.4 μm, and the size of the smallest square in the diffraction pattern is 0.39×0.39 μm) are given. Our theoretical results are compared with the experimental results of the linear Bragg–Fresnel lens reported by other researchers; an analysis and a discussion are carried out regarding the advantages of an optical system based on the 2-D focusing square multilayer Bragg–Fresnel lens, in contrast to a Kirkpatrick–Baez optical system on the basis of a two-linear Bragg–Fresnel lens.     

Self assembled micro masking effect in the fabrication of SiC nanopillars by ICP-RIE dry etching

This report presents the results of the novel fabrication of 4H-SiC pillars with nanopores using ICP-RIE dry etching. Cl₂/Ar gas plasma with various mass flow rates was used in this etching process to produce SiC nanopillars without using patterned etch mask. Cylindrical pillars of 300 nm diameter and 500 nm height with smooth side walls were etched on SiC wafer. The etching condition for the optimized fabrication of SiC nanopillars is presented in this report. Each nanopillar has been produced with a nanosize pore at the center along its length and up to the middle of the cylindrical nanopillar; it is a unique feature has not ever been reported in case of SiC. Inclusion of oxygen was found influence the formation of nanopillars by the effect of SiO₂ micro masking. The formation of self assembled SiO₂ layer and its micro masking effect in the fabrication of this unique nanostructure has been investigated using TEM, STEM and EDAX measurements.     

InGaAsP/InP 1.55-μm lasers with chemically assisted ion beam-etched facets

Chemically assisted ion beam etching (CAIBE) involving an Ar ion beam and a halogen ambient gas (Cl₂, IBr₃) has been used to etch high-quality laser facets for InGaAsP/InP bulk lasers (1.55 m). We achieved eich rates of 40.0–75.0 nm min^–1 at substrate temperatures between-5 and +10°C. These low temperatures have allowed us to utilize UV-baked photoresists as well as PMMA as etch masks, facilitating very simple process development. Higher substrate temperatures (50 to 120°C) yield still higher etch rates, but at the expense of severely degraded surface morphologies. Angle resolved x-ray photoelectron spectroscopy (XPS) was investigated for observing etched InP surfaces. A disproportioned surface has been detected after etching in the higher temperature range; low temperatures yield stoichiometric surfaces.     

Lithium ion conductive glass–ceramics with Li3Fe2(PO4)3 and YAG laser-induced local crystallization in lithium iron phosphate glasses

The glasses with the composition of 37.5Li₂O–(25 − x)Fe₂O₃–xNb₂O₅–37.5P₂O₅ (mol%) (x = 5,10,15) are prepared, and it is found that the addition of Nb₂O₅ is effective for the glass formation in the lithium iron phosphate system. The glass–ceramics consisting of Nasicon-type Li₃Fe₂(PO₄)₃ crystals with an orthorhombic structure are developed through conventional crystallization in an electric furnace, showing electrical conductivities of 3 × 10^− 6 Scm^− 1 at room temperature and the activation energies of 0.48 eV (x = 5) and 0.51 eV (x = 10) for Li⁺ ion conduction in the temperature range of 30–200 °C. A continuous wave Nd:YAG laser (wavelength: 1064 nm) with powers of 0.14–0.30 W and a scanning speed of 10 μm/s is irradiated onto the surface of the glasses, and the formation of Li₃Fe₂(PO₄)₃ crystals is confirmed from XRD analyses and micro-Raman scattering spectra. The crystallization of the precursor glasses is considered as new route for the fabrication of Li₃Fe₂(PO₄)₃ crystals being candidates for use as electrolyte materials in lithium ion secondary batteries.