Faiz Rahman

Silicon nitride micromesh bolometer array for submillimeter astrophysics

We present the design and performance of a feedhorn-coupled bolometer array intended for a sensitive 350-mum photometer camera. Silicon nitride micromesh absorbers minimize the suspended mass and heat capacity of the bolometers. The temperature transducers, neutron-transmutation-doped Ge thermistors, are attached to the absorber with In bump bonds. Vapor-deposited electrical leads address the thermistors and determine the thermal conductance of the bolometers. The bolometer array demonstrates a dark noise-equivalent power of 2.9 x 10(-17) W/ radicalHz and a mean heat capacity of 1.3 pJ/K at 390 mK. We measure the optical efficiency of the bolometer and feedhorn to be 0.45-0.65 by comparing the response to blackbody calibration sources. The bolometer array demonstrates theoretical noise performance arising from the photon and the phonon and Johnson noise, with photon noise dominant under the design background conditions. We measure the ratio of total noise to photon noise to be 1.21 under an absorbed optical power of 2.4 pW. Excess noise is negligible for audio frequencies as low as 30 mHz. We summarize the trade-offs between bare and feedhorn-coupled detectors and discuss the estimated performance limits of micromesh bolometers. The bolometer array demonstrates the sensitivity required for photon noise-limited performance from a spaceborne, passively cooled telescope.

Nanotexturing of GaN light-emitting diode material through mask-less dry etching

We describe a new technique for random surface texturing of a gallium nitride (GaN) light-emitting diode wafer through a mask-less dry etch process. This involves depositing a sub-monolayer film of silica nanospheres (typical diameter of 200 nm) and then subjecting the coated wafer to a dry etch process with enhanced physical bombardment. The silica spheres acting as nanotargets get sputtered and silica fragments are randomly deposited on the GaN epi-layer. Subsequently, the reactive component of the dry etch plasma etches through the exposed GaN surface. Silica fragments act as nanoparticles, locally masking the underlying GaN. The etch rate is much reduced at these sites and consequently a rough topography develops. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) inspections show that random topographic features at the scale of a few tens of nanometres are formed. Optical measurements using angle-resolved photoluminescence show that GaN light-emitting diode material thus roughened has the capability to extract more light from within the epilayers.

Emission characteristics of photonic crystal light-emitting diodes

Experimentally measured optical properties of photonic crystal LEDs are reported here. Photonic crystal and photonic quasi-crystal structures were fabricated on GaN epilayer LED wafer material using both direct-write electron beam lithography and nanoimprint lithography. Some of these structures were processed to make finished LEDs. Both electroluminescence and photoluminescence measurements were performed on these structures. Devices were characterized for their current-voltage characteristics, emission spectra, far-field emission pattern, and angular emission pattern. These results are useful for fabricating photonic crystal LEDs and assessing their operational properties.

Polythiophene-based charge dissipation layer for electron beam lithography of zinc oxide and gallium nitride

The ability of thin polythiophene layers to dissipate accumulated charge in the electron beam lithography (EBL) of wide bandgap semiconductors, such as zinc oxide and gallium nitride, is demonstrated. A quick and inexpensive processing method is demonstrated for EBL exposure of dense and high-resolution patterns in a hydrogen silsesquioxane (HSQ) negative-tone resist deposited on bulk ZnO samples and with GaN/AlN on sapphire substrates. For the former, experimental results are given for three different cases: where no charge dissipation layer was used as well as cases where 40-nm-thick Al and 100-nm-thick conductive polymer layers were used on the top of the HSQ resist. For the latter material, EBL exposure was investigated for pure HSQ and for HSQ with a thin conductive polymer layer on top. Based on the scanning electron microscope observations of the resulting photonic crystal (PhC) pattern, conventional Al and the proposed polymer approach were compared. Good agreement between these results is reported, while the new method considerably simplifies sample processing. Spin-coatable conducting polymer may be easily removed due to its solubility in water, which makes it a perfect solution for the processing of amphoteric oxide samples, i.e., zinc oxide. Gallium nitride processing also benefits from polymer dissipation layer usage due to extended exposure range and the avoidance of dense pattern overexposure in HSQ.

A Systematic Study of Plasma Activation of Silicon Surfaces for Self Assembly

We study the plasma activation systematically in an attempt to simplify and optimize the formation of hydrophilic silicon (Si) surface critical for self-assembly of nanostructures that typically uses piranha solution, a high molarity cocktail of sulfuric acid and hydrogen peroxide at elevated temperatures. In the proposed safer and simpler approach, O2 plasma is used under optimized process conditions in a capacitively coupled parallel-plate chamber to induce strong hydrophilic behavior on silicon surfaces associated with the formation of suboxide groups. Surface activation is validated and studied via contact angle measurements as well as XPS spectra and consequently optimized using a novel atomic force spectroscopy approach, which can streamline characterization. It is found that plasma power around 100 W and exposure duration of ∼65 s are the most effective parameters to enhance surface activation for the reactive ion etcher system used. Other optimum plasma process conditions for pressure and flow-rate are also reported along with temporal development of activation, which peaks within 1 h and wears off in 24 h scale in air. The applicability of the plasma approach to nanoassembly process was demonstrated using simple drop coating and spinning of polystyrene (d < 500 nm, 2.5-4.5% w/v) and inkjet printing on polydimethylsiloxane.

Photonic Crystal LEDs

LEDs that have photonic crystals etched on them can direct the emission of light at preferred angles and enhance brightness without consuming more power. These devices have the potential to revolutionize the lighting industry-and spur widespread adoption of environmentally friendly lighting solutions.

Photolithography with polymethyl methacrylate (PMMA)

Polymethyl methacrylate (PMMA) is widely used as an electron beam resist but is not used as a photoresist because of its insensitivity to electromagnetic radiation with wavelengths longer than about 300 nm. In this paper we describe a technique for performing conventional photolithography with high molecular weight PMMA at the widely used 365 nm i-line wavelength. The technique involves photosensitizing PMMA with Irgacure 651—a commercially available photo-initiator that can cause PMMA strands to cross-link. Optimum amount of Irgacure can produce a negative tone photoresist with adequate photosensitivity and plasma etch resistance. We describe this technique in detail with complete processing conditions and discuss the effects of varying Irgacure 651 concentration in PMMA as well as changes in UV exposure dose. We also show example structures patterned with commonly available materials and equipment. Finally, we show that it is possible to carry out gradient lithography with this approach, in order to produce structures in relief in photosensitive PMMA.

High-Color Definition Lighting with Broadband LEDs

With crisper color and more nuanced shading, broadband color and full-spectrum white LEDs are helping to extend the applications of solid-state lighting-into photography, material inspection, art and shop displays, and even Hollywood movies.

Gated Lateral p-i-n Junction Device for Light Sensing

We describe a silicon-based lateral p-i-n junction device for light sensing applications. This device is based on metal-oxide-semiconductor (MOS) architecture and, therefore, has a gate for controlling its electrical operating point. Device fabrication is described in brief, followed by a description of the devices electrical and optoelectronic properties including current-voltage characteristics and optical transfer characteristics. It shows good linearity and high optical responsivity of 20 and 16 A/W for red and blue light, respectively. The associated gate can be used to control the quiescent operating point thus making it easy to interface the detector with ordinary metal-oxide-semiconductor field-effect transistors (MOSFETs).

Fluid phase passivation and polymer encapsulation of InP/InGaAs heterojunction bipolar transistors

This paper reports on the development of effective passivation techniques for improving and stabilizing the characteristics of InP/InGaAs heterojunction bipolar transistors. Two different methods for carrying out sulfur-based surface passivations are compared. These include exposure to gaseous hydrogen sulfide and immersion treatment in an ammonium sulfide solution. The temporal behaviour of effects resulting from such passivation treatments is reported. It is shown that liquid phase passivation has a larger beneficial effect on device performance than gas phase passivation. This is explained in terms of the polarity of passivating species and the exposed semiconductor surface. Finally, device encapsulation in a novel chalcogenide polymer is shown to be effective in preserving the benefits of surface passivation treatments. The relevant properties of this encapsulation material are also discussed.