The wide-bandgap semiconductor ZnO has gained major interest in research community for its unique properties and wide range of applications. In this review article, we present synthesis techniques and a few emerging applications for ZnO. Common techniques for growing ZnO films are discussed briefly, and a detailed discussion of MOCVD growth of ZnO is provided citing previous experimental reports on this technique by our group and others. A few important and distinctive uses of ZnO are discussed for various applications focusing on the current limitations of ZnO to realize its feasibility in these applications.
The high crystalline quality, large junction surface area, and insensitivity to c-axis oriented polarization fields make core-shell
doped GaN nanowire p-n junctions exciting prospects for use as LEDs. The LED external efficiency depends upon
the spatial distribution of optical recombination within the device, which may be controlled through the use of radial
heterojunctions, such as quantum wells and electron blocking layers. In this work, we explore the impact of an axially
varying doping profile on the spatial distribution of optical recombination in a GaN nanowire LED.
The numerical simulation of the nanowire LED is carried out using the TiberCAD simulation package. This package
provides a finite-element-based solution to the drift-diffusion model of a nanowire. Simulations of core-shell nanowire
LEDs are performed with various doping profiles to produce variations in the optical recombination distribution
throughout the device.
In a core-shell device with a uniformly doped n-type core, the current density tends to travel primarily along the core, as
the mobility of electrons is much greater than that of holes in GaN devices. The optical recombination is concentrated
beneath the p-contact, where most of the current crosses the p-n junction. By properly setting a tapered doping profile in
the n-type core, it is possible to increase the uniformity of the optical recombination along the junction. In certain
geometries this will increase the emission efficiency of the nanowire LED.
An efficient modular approach is used to develop components for a 3D simulator for complex semiconductor
LED and laser structures. In this approach, only drift transport is simulated in bulk regions, while the active
region is simulated with models of varying complexity. The approach is tested using a basic vertical-cavity
surface emitting laser (VCSEL) structure, and comparisons are made with experimental data. This approach
is advantageous for fast simulation of complex photonic crystal LEDs and VCSELs, for which 2D simulation is
GaN epilayers and AlGaN/GaN superlattice structures have been deposited on (0001) ZnO substrates by
metalorganic vapor phase epitaxy (MOVPE) using GaN and AlN buffer layers. The growth conditions were first
optimized on GaN templates using N2 as carrier gas at relatively low temperature (<800 °C), which is suitable for
GaN growth on a ZnO substrate. Experimental results show that high interfacial quality can be achieved in the
superlattice by using TMIn as a surfactant. The optimized growth conditions were subsequently transferred to ZnO
substrates. The influence of growth temperature on the material quality was studied. A proper growth temperature for
both GaN cover layer and AlGaN/GaN superlattice can improve the structural and optical properties of the structures
on ZnO. This improvement is verified using x-ray diffraction, atomic force microscopy and photoluminescence
characterizations. The growth temperature must be chosen with these two factors in mind, with too low a growth
temperature leading to poor quality material and too high a temperature causing reactions at the GaN/ZnO interface
that degrade quality. AlN buffer layers on ZnO were also studied to increase subsequent GaN epilayer quality.
Effects of buffer layer growth conditions on optical and structural quality were studied.
We compare the threshold gain and modal discrimination of a range of large aperture VCSELs intended for high-power
single-mode operation. The threshold gain is calculated using a gain eigenvalue solver that enforces the threshold
condition of the mode (gain = loss). In this way, gain guiding is included automatically. We find that confining the
gain to a smaller area than the mode results in excellent threshold gain and modal discrimination, due to the large
difference in modal overlap factors between the fundamental and the second order mode.
The dispersion relation of a cavity surrounded by multi-layered photonic crystals is obtained using a fast, accurate
and generalized round trip operator. This will assist in the optical design of photonic crystal patterned lasers.
A 2D quasi-bandgap was obtained for the lowest order mode of a 1D multi-layered photonic crystal. Although
the method is demonstrated for 1D photonic crystal layers, the method is general and can be extended to two
Coupling between InGaAs/GaAs quantum dots is investigated using differential transmission spectroscopy. Degenerate measurements show an initial carrier relaxation time that is relatively independent of carrier density. Two-color pump-probe techniques are used to spectrally resolve the carrier dynamics, revealing transfer between quantum dots and a homogeneous linewidth of 12 nm at room temperature. The time constant for carrier escape is shown to increase from 35 ps at room temperature to 130 ps at 230 K. We then employ a rate equation model to simulate the performance of a semiconductor optical amplifier with QDs as the active region.
We propose a novel treatment that enhances the accuracy of the Effective Index Method (EIM) when used for gain-guided oxide-confined VCSELs. If a thin oxide is placed at or near a z-field null position, the diffraction caused by the oxide becomes negligible. Gain-guiding subsequently dominates and causes the EIM to break down. To circumvent this problem, we propose to use an artificial index-guided diffraction effect to simulate the gain-guided diffraction effect. This is achieved by increasing the oxide thickness and making a correction to the oxide index by taking a weighted sum between the original oxide index and the center region index at the oxide layer position. The weight is specifically chosen to be the mean z-field (normalized to its local z-field variation) at the position of the oxide. We show that this simple correction to the EIM successfully simulates the gain-guided diffraction effect and produces the correct transverse phase variation for oxide-apertured VCSELs when gain-guiding becomes the dominant mechanism. Therefore, the improved EIM is able to produce resonant wavelengths which are in excellent agreement to those of the vector Green's function method for the COST-268 VCSEL model, both in the gain-guided and index-guided regimes. Comparisons with an experimental model have also been made and excellent agreement is shown.
This work focuses on the effects of spatial hole-burning (SHB) on the modulation response of oxide-confined vertical-cavity surface-emitting lasers. The comprehensive laser diode simulator, Minilase, as well as a simple 1-D rate equation models are used as simulation tools in the studies. We demonstrate that, due to the non-uniform transverse optical intensity, carriers at different locations of the quantum well (QW) have different stimulated recombination rates, and therefore exhibit different dynamic responses under direct modulation. This non-uniformity is revealed to be responsible for an over-damping of the relaxation oscillation and the reduction of the modulation bandwidth. Due to the limit of this nonlinear effect, VCSELs with small oxide apertures show lower intrinsic maximum bandwidth compared with that of large aperture structures. Further simulations demonstrate that this damping effect can be greatly reduced by making the electrical aperture smaller than the optical aperture, thereby significantly improving the modulation response.
We demonstrate InGaAs/GaAs quantum dot lasers with multimode lasing at room temperature immediately above threshold. The lasing modes are separated by about ten times the Fabry-Perot mode spacing, with several dark modes in between the lasing modes. Rate equation simulations indicate that this multimode behavior can be explained by a homogeneous broadening that is on the order of the mode spacing.
It has been established both theoretically and experimentally that semiconductor microcavities are capable of modifying the spontaneous emission rate of electron-hole pairs within the microcavity. In particular, the microcavity may be used to re- shape the spectral distribution of radiation from a dipole source in order to enhance emission at certain frequencies and suppress emission other frequencies. It is desirable to exploit this effect to increase the efficiency of LEDs and SLDs. Numerical calculation may be used to evaluate the magnitude of the modification of spontaneous emission in practical microcavity structures. The Green's function based VCSEL mode solver we have developed is uniquely well-suited for such calculations. The method we have employed for calculating spontaneous emission rates in microcavities is presented, along with calculated results and comparisons with experimental data.
The semiconductor laser simulator Minilase has been integrated with a novel optical mode solver in order to simulate vertical cavity surface emitting lasers. The electronic and optical solvers are reviewed, and the interaction between the two solvers is presented in detail. The combined simulator recalculates the optical modes in response to changing electronic conditions without requiring prohibitively large computational resources.
Active cavity modes are presented as a useful mode set for the analysis of the optical problem in oxide-confined VCSELs. We define the active cavity modes via a novel integral eigenvalue equation containing the VCSEL cavity electromagnetic Green's function and the gain distribution. Photon rate equation parameters, such as the net modal gain and the spontaneous emission into a mode, are calculated using this mode set as a basis. An efficient method for calculating the active cavity modes has been developed in order to allow for rapid iterations with the electronic solver in the laser simulator MINILASE. The details of the implementation of this method are presented, as are sample calculated mode parameters.