Edward B. Stokes

Diffusion and tunneling currents in GaN/InGaN multiple quantum well light-emitting diodes

We have studied the electrical characteristics and optical properties of GaN/InGaN multiple quantum well (MQW) light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition. It appears that there is an essential link between material quality and the mechanism of current transport through the wide-bandgap p-n junction. Tunneling behavior dominates throughout all injection regimes in a device with a high density of defects in the space-charge region, which act as deep-level carrier traps. However, in a high-quality LED diode, temperature-dependent diffusion-recombination current has been identified with an ideality factor of 1.6 at moderate biases. Light output has been found to follow a power law, i.e., L /spl prop/ I/sup m/ in both devices. In the high-quality LED, nonradiative recombination centers are saturated at current densities as low as 1.4 /spl times/ 10/sup -2/ A/cm/sup 2/. This low saturation level indicates that the defects in GaN, especially the high density of edge dislocations, are generally optically inactive.

Investigation of radiative tunneling in GaN/InGaN single quantum well light-emitting diodes

Abstract The mechanisms of carrier injection and recombination in a GaN/InGaN single quantum well light-emitting diodes have been studied. Strong defect-assisted tunneling behavior has been observed in both forward and reverse current–voltage characteristics. In addition to band-edge emission at 400 nm, the electroluminescence has also been attributed to radiative tunneling from band-to-deep level states and band-to-band tail states. The approximately current-squared dependence of light intensity at 400 nm even at high currents indicates dominant nonradiative recombination through deep-lying states within the space-charge region. Inhomogeneous avalanche breakdown luminescence, which is primarily caused by deep-level recombination, suggests a nonuniform spatial distribution of reverse leakage in these diodes.

Influence of defects on electrical and optical characteristics of GaN/InGaN-based light-emitting diodes

The microstructural, electrical and optical properties of GaN/InGaN light emitting diodes (LEDs) with various material quality grown on sapphire have been studied. Burgers vector analyses showed that edge and mixed dislocations were the most common dislocations in these samples. In defective devices, a large number of surface pits and V-defects were present, which were found to be largely associated with mixed or screw dislocations. Tunneling behavior dominated throughout all injection regimes in these devices. The I-V characteristics at the moderate forward biases can be described by I = I0 exp (eV/E), where the energy parameter E has a temperature-independent value in the range of 70 -110 meV. Deep level states-associated emission has been observed, which is direct evidence of carrier tunneling to these states. Light output was found to be approximately current-squared dependent even at high currents, indicating nonradiative recombination through deep-lying states in the space-charge region. In contrast, dislocation bending was observed in a high quality device, which substantially reduced the density of the mixed and screw dislocations reaching the active layer. The defect-assisted tunneling was substantially suppressed in this LED device. Both forward and reverse I-V characteristics showed high temperature sensitivity, and current transport was diffusion-recombination limited. Light output of the LED became linear with the forward current at a current density as low as 1.4x10-2 A/cm2, where the nonradiative recombination centers in the InGaN active region were essentially saturated. This low saturation level suggests optical inactivity of the edge dislocations in this LED.

Evanescent wave immunoprobe with high bivalent antibody activity

Abstract We have determined the kinetic response of an evanescent wave optical fibre immunosensor and its absolute sensitivity. Using both kinetic methods and optical determinations of bound antigen, we infer that there are 2·4 × 10 11 active antibodies per cm 2 probe area. We estimate that 75% of the active antibodies are in the bivalent form, with both binding site capable of binding antigen. Using the kinetic data and optical measurements of the number of antigens bound to the fibre, we estimate that the unstirred dead layer thickness next to the fibre surface in the stirred solution, across which the antigens must diffuse, is 55 μm. Approximately 10 9 FITC labelled antigens are necessary to achieve a threshold signal, defined as 1% of the signal obtained when the fibre is saturated with antigen.

Modeling and circuit simulation of GaN-based light emitting diodes for optimum efficiency through uniform current spreading

Uniform current spreading is desirable for both light emitting diodes (LEDs) performance and reliability. It enhances optical efficiency because the joule losses due to current crowding in some parts of the die would be eliminated. The LED design for optimal light extraction and uniform current spreading is therefore a necessity. In this paper we report on preliminary current spreading results obtained from circuit simulation, using Pspice and Aimspice, for LED designs with and without an n-metal ring as well as the epi-up and flip chip LEDs. For the epi-up, both the lateral and vertical resistances of the transparent metals were taken into account. Whereas in the flip chip, the lateral resistance was negligibly small thus only the vertical component contributed to the total p-lump resistance. The n-lateral resistance in the active mesa was critical to uniform current spreading. It was found that the lower the n-lateral resistance, the more uniform the current spreads and flows through the active region. In both the epi-up and flip-chip structures, the contact resistance of the p-metal (including the thin Ni/Au transparent metal) dominated the total p-lump resistance. The larger this value, with fixed n-layer lateral resistance, the more uniform the current spreads in the device. However, high p-contact resistance is not desirable as it reduces the overall efficiency of the device due to excessive heating and increased leakage current. Therefore, for uniform current spreading, the n-lateral resistance should be made small while maintaining an optimum p-lump resistance to achieve a high efficiency.

Thermal Stability Studies of CdSSe/ZnS Quantum Dots in GaN/Quantum Dots/GaN Wafer Bonded System

Wafer bonding technology is applied to the GaN/quantum dots/GaN system, where CdSSe/ZnS core/shell Quantum Dots serve as both the active layer and the binding layer. Photoluminescence is observed from quantum dots following wafer bonding at various temperatures under ultra high vacuum conditions. Temperature dependences of bond strength and photoluminescence properties are characterized. Annealing at elevated temperature to form wafer bonds degrades photoluminescence intensity and slightly blue shifts the emission wavelength. Transmission Electron Microscopy is used to study the physical properties of vacuum annealed QDs, in which enlargement of QD particle size is observed.

Photocurrent spectroscopy investigation of deep level defects in Mg-doped GaN and Mg-doped AlxGa1−xN(0.20<x<0.52)

Pulsed infrared photocurrent spectroscopy is used to investigate deep levels in highly resistive metal organic chemical vapor deposition-grown, magnesium-doped aluminum gallium nitride metal-semiconductor-metal test structures in the range of aluminum fraction from x=0.0 to x=0.52. Some background level of photocurrent is observed at all infrared pump wavelengths between 1.35 and 4.0 μm. The photocurrent decay time is a decreasing function of aluminum fraction. A peak photocurrent energy is observed for each aluminum fraction. With increasing aluminum fraction, the peak blueshifts and narrows.

Wafer bonding technique based GaN/Quantum Dots/GaN system

Integration of colloidal semiconductor quantum dots with epitaxial semiconductor materials offers the possibility of a wide variety of new device structures. In this work, wafer bonding processes for layered GaN/Quantum Dots/GaN structures are described. Bonding is achieved by thermal reorganization of bidentate ligand thiophenethiol which initially caps the CdSe/ZnS core/shell quantum dots. Annealing temperature dependent bond strength feature is characterized. Maximum bond strength is achieved after annealing at 350°C. Effect of annealing on quantum dot photoluminescence is also investigated.

CdSe-ZnS quantum dot Langmuir films employing thiophene thiol as a capping ligand

Core-shell CdSe-ZnS quantum dots have been one of the most actively researched nanomaterials in the past decade. In this paper we introduce thiophene thiol (TPT) as a capping ligand on the CdSe-ZnS heterostructure. These conjugated ligands allow for improved carrier transport in comparison to their saturated, long chain hydrocarbon counterparts. By demonstrating a robust and inexpensive method to deposit close packed monolayers of these nanostructures, we hope to open the door for a new family of low cost inorganic devices employing them.

Luminescent Properties of CdSe Quantum Dots Subjected to MBE GaN Growth Temperatures

Semiconductor quantum dots have received an enormous amount of attention due to their luminescent properties over a broad range of optical wavelengths that makes them attractive for integration into the next generation of lighting and display devices. Also, their ability to absorb a broad range of wavelengths makes them useful for solar cell applications. Cadmium selenide (CdSe) quantum dots are of interest due to their intense luminescence in the middle of the visible spectrum where it has been difficult to achieve efficient semiconductor light sources.