Liam Lewis

Carrier distribution in InGaN/GaN tricolor multiple quantum well light emitting diodes

Carrier transport in InGaN light emitting diodes has been studied by comparing the electroluminescence (EL) from a set of triple quantum well structures with different indium content in each well, leading to multicolor emission. Both the sequence and width of the quantum wells have been varied. Comparison of the EL spectra reveals the current dependent carrier transport between the quantum wells, with a net carrier flow toward the deepest quantum well.

Dual gratings for enhanced light trapping in thin-film solar cells by a layer-transfer technique

Thin film solar cells benefit significantly from the enhanced light trapping offered by photonic nanostructures. The thin film is typically patterned on one side only due to technological constraints. The ability to independently pattern both sides of the thin film increases the degrees of freedom available to the designer, as different functions can be combined, such as the reduction of surface reflection and the excitation of quasiguided modes for enhanced light absorption. Here, we demonstrate a technique based on simple layer transfer that allows us to independently pattern both sides of the thin film leading to enhanced light trapping. We used a 400 nm thin film of amorphous hydrogenated silicon and two simple 2D gratings for this proof-of-principle demonstration. Since the technique imposes no restrictions on the design parameters, any type of structure can be made.

On the specific contact resistance of metal contacts to p-type GaN

Extraction of the specific contact resistance, ρc, using the established circular transmission line measurement (c-TLM) and a series resistance measurement across mesa-isolated diodes is compared for a non-alloyed Pd/Ag contact to p-GaN. The limitations of the c-TLM technique are discussed and it is shown that for ρc values below 10−4 Ω cm2 both unintentional submicron errors in the actual radii and uncertainty in the resistance measurements can lead to order of magnitude changes in the extracted ρc. An additional current–voltage measurement across a mesa-isolated diode is proposed. The accuracy of the extracted ρc in this case requires consistency of the intrinsic diode characteristic, namely the ideality, at the bias voltages used for extraction, which is in turn related to the carrier transport and recombination properties. From a comparison with the c-TLM, we conclude that the resistance should be extracted at current densities <10 A cm−2 as junction heating changes the diode ideality.

Free-standing gallium nitride Schottky diode characteristics and stability in a high-temperature environment

Schottky diodes have been fabricated using low-resistivity n-type free-standing GaN substrates with a reduced defect density lowly doped n-type epi-layer and an Ni/Ti/Pt/Au Schottky contact metalization. A thermionic field emission current transport mechanism was identified with a Schottky barrier height of about 0.75 eV and a diode ideality of 1.1 measured at 25 ?C, both of which increase with measurement temperature up to 200 ?C. The diodes were subjected to long-term testing under forward current (1.3 A cm?2) or reverse voltage (?3.5 V) biased storage at 300 ?C in N2 for 466 h and were also monitored under non-biased storage conditions for up to 1000 h at 350 ?C and 400 ?C in N2 or at 300 ?C for 1500 h in air. Except for the non-biased storage test at 400 ?C, the diodes show <10% drift in ideality and barrier height during the long-term storage tests. For the 400 ?C test, there is a significant increase in both barrier height and ideality over a relatively short storage period (48 h). This to be the first reported study on the long-term stability of Schottky diodes on free-standing GaN and while no catastrophic (e.g. thermal runaway) degradation of any of the diodes was observed, it is proposed that optimized thermal annealing of the Ni-based Schottky contact metalization in the temperature range 350?400 ?C is necessary for stable long-term operation at high temperature.

Low-resistance Ni-based Schottky diodes on freestanding n-GaN

Schottky diodes formed on a low doped (5×1016cm−3) n-type GaN epilayer grown on a n+ freestanding GaN substrate were studied. The temperature dependent electrical characteristics of Ni contacts on the as-grown material are compared with an aqueous, potassium hydroxide (KOH) treated surface. In both cases the diodes are dominated by thermionic emission in forward bias, with low idealities (1.04 at room temperature) which decrease with increasing temperature, reaching 1.03 at 413K. The Schottky barrier height is 0.79±0.05eV for the as-grown surface compared with 0.85±0.05eV for the KOH treated surface at room temperature. This is consistent with an inhomogeneous barrier distribution. The specific on-state resistance of the diodes is 0.57mΩcm2 The KOH treatment reduces the room temperature reverse leakage current density at −30Vto1×10−5Acm−2 compared to 6×10−2Acm−2 for the as-grown samples.

Carrier localization and in-situ annealing effect on quaternary Ga1-xInxAsySb1-y/GaAs quantum wells grown by Sb pre-deposition

Using temperature-dependent photoluminescence spectroscopy, we have investigated and compared intrinsic InGaAs, intrinsic GaInAsSb, and p-i-n junction GaInAsSb quantum wells (QWs) embedded in GaAs barriers. Strong carrier localization inside the intrinsic GaInAsSb/GaAs QW has been observed together with its decrease inside the p-i-n sample. This is attributed to the effect of an in-situ annealing during the top p-doped AlGaAs layer growth at an elevated temperature of 580 °C, leading to Sb-atom diffusion and even atomic redistribution. High-resolution X-ray diffraction measurements and the decrease of both maximum localization energy and full delocalization temperature in the p-i-n QW sample further corroborated this conclusion.

Electroless nickel/gold Ohmic contacts to p-type GaN

A solution based approach to forming Ohmic contacts to p-type GaN is described. Electroless plated Ni∕Au contacts are shown to compare favorably with traditional evaporated contacts, with contact resistivities ρc in the region of 10−2Ωcm2. These values are readily achieved after a rapid thermal annealing in an O2 atmosphere. The tunneling nature of the contact is confirmed via temperature dependant measurements. X-ray diffraction measurements confirm the similarity between evaporated and plated contacts. Current-photocurrent (I-L) and current-voltage (I-V) measurements from light emitting diodes formed using an electroless p-type contact are shown. Electroless deposition of the contact metals allows for a reduction in processing time and cost.

Electro-optical and lasing properties of hybrid quantum dot/quantum well material system for reconfigurable photonic devices

We characterize the electro-optical and lasing properties of a hybrid material consisting of multiple InAs quantum dot (QD) layers together with an InGaAs quantum well (QW) grown on a GaAs substrate. Over 40 nm Stark shift of the InGaAs QW leading to 9 dB extinction ratio was demonstrated. Lasing operation at the QD first excited state transition of 1070 nm was achieved and together with < 10 ps absorption recovery the system shows promise for high-speed mode-locked lasers and electro-modulated lasers.

Characterization of planar microlenses made of high contrast gratings

We report on the focusing performance of reflective 2D high contrast grating lenses based on silicon. The combination of their subwavelength nature and their high refractive index contrast make it possible to create highly tolerant and planar microlenses. We used a rigorous mathematical code to design the lenses and verified their performance with finite element simulations. We also investigated the effects of grating thickness, angle and wavelength of incidence in these simulations. Experimentally, we show the evolution of the beam profile along the optical axis for a lens with a high (0.37) numerical aperture. We have explored a wide range of numerical apertures (0.1 – 0.93) and focal lengths (5 μm – 140 μm) and show that the lenses behave as expected across the full range. Our analyses demonstrate the large design flexibility with which these lenses can be made along with ease of fabrication and potential for a number of applications in micro-optics.

Focusing with planar microlenses made of two-dimensionally varying high contrast gratings

Abstract. We report on the focusing performance of reflective two-dimensionally varying high contrast grating lenses based on silicon. The combination of their subwavelength nature and their high refractive index contrast makes it possible to create highly tolerant and planar microlenses. We used a rigorous mathematical code to design the lenses and verified their performance with finite element simulations. We also investigated the effects of grating thickness, angle, and wavelength of incidence in these simulations. Experimentally, we show the evolution of the beam profile along the optical axis for a lens with a high (0.37) numerical aperture. We have explored a wide range of numerical apertures (0.1–0.93) and show that the lenses behave as expected across the full range. Our analyses demonstrate the large design flexibility with which these lenses can be made along with ease of fabrication and potential for a number of applications in micro-optics.