Circular Mesa Research Articles

Enhanced Light Extraction in GaN-Based Light Emitting Diodes Fabricated with Increased Mesa Sidewalls

We report on the enhanced light extraction of GaN-based light emitting diodes (LEDs) fabricated with increased mesa sidewalls. It is shown that light extraction and operating voltages in LEDs typically have an inverse relationship that depends on the perimeter of the mesa sidewalls and the mesa area. Based on these observations, we designed an advanced LED incorporating regularly arrayed circular mesa holes to simultaneously maximize the mesa sidewall perimeter and area. Compared to a reference LED, the output power of our mesa-hole LED was enhanced by 12%, and its operating voltage was decreased by 0.1 V, resulting in a power efficiency improvement of 8.8%.

Generation of π modes in semiconductor vertical-cavity surface-emitting lasers

We have theoretically and experimentally studied optical emission from the side faces of mesa structures for vertical-cavity surface-emitting lasers (VCSELs). The results of spatially resolved spectral measurements show that a new type of lasing modes is found in VCSELs with circular mesa structures, which corresponds to the π mode propagation along the mesa diameter with reflection from the sloped walls and the bottom Bragg mirror.

GaAsSb-based backward diodes for highly sensitive millimetre-wave detectors

Highly sensitive millimetre-wave detectors based on p-GaAs0.51Sb0.49/i-In0.52Al0.48As/n-InxGa1-xAs backward diodes under zero-bias operation were developed on an InP substrate. Voltage sensitivity of 12300 V/W at 94 GHz was achieved with the GaAsSb-based diode which has a circular mesa with a diameter of 0.9 µm. To improve the sensitivity, the indium composition of the n-InxGa1-xAs layer and thickness of the i-In0.52Al0.48As layer were optimised. Furthermore, a self-aligned process was performed to minimise parasitic resistance at an ohmic contact layer. These GaAsSb-based diodes that are lattice-matched to InP are promising for easy integration with high-performance InP low-noise amplifiers.

GaN-based microdisk light emitting diodes on (111)-oriented nanosilicon-on-insulator templates

InGaN/GaN microdisk light emitting diodes (LEDs) on (111)-oriented nanosilicon-on-insulator (nano-SOI) substrates are demonstrated. The (111)-oriented thin SOI substrates are prepared by separation by implantation of oxygen (SIMOX) method. The InGaN/GaN LED structures are grown on these SIMOX templates by metal-organic chemical vapor deposition. The circular mesa patterns are created by standard LED processing steps including photolithography, inductively coupled plasma etching, and contact metallization. Due to the reflectivity changes at the bottom Si/SiO2 interfaces beneath AlN buffer, an improved light extraction from these LEDs has been observed. The room-temperature electroluminescence (EL) spectrum of the microdisk LEDs on SOI shows multiple interference peaks due to the reflections at the Fabry-Pérot boundaries and such effect resulted in an increased integrated EL intensity. For a comparative analysis of light extraction with respect to similar LED structures grown on bulk Si(111), a detailed reflectivity analysis has been carried out to verify the LED structures and to model the EL lineshapes.

Harmonic Analysis of AlN Piezoelectric Mesa Structures

AbstractIn this work we report the displacement response of piezoelectric Aluminum Nitride (AlN) thin film MESA in an electrical circuit consisting of a circular MESA in series with a resistance subjected to time varying electrical loads. ANSYS was utilized for the simulation of 3D piezoelectric structures; using coupled field analysis to understand the electro-mechanical behavior of AlN thin film mesas. ANSYS applies finite element analysis (FEM) method to simulate the transient piezoelectric trends. Ringing and overshoot effects were observed in the thin simulation results on applying pulse voltages of varying frequencies to the circuit. The fast rise time of the voltage pulse could be exciting these effects. The effect of fast rising pulse voltages on the RC time constant of the circuit is still unclear at this point and needs to be further investigated.

Growth and Characterization of High-Performance GaN and AlxGa1−xN Ultraviolet Avalanche Photodiodes Grown on GaN Substrates

AbstractWide-bandgap III-nitride-based avalanche photodiodes (APDs) are important for photodetectors operating in UV spectral region. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates such as sapphire and SiC, a high density of defects is introduced, thereby causing device failure by premature microplasma breakdown before the electric field reaches the level of the bulk avalanche breakdown field, which has hampered the development of III-nitride based APDs. In this study, we investigate the growth and characterization of GaN and AlGaN-based APDs on free-standing bulk GaN substrates. Epitaxial layers of GaN and AlxGa1−xN p-i-n ultraviolet avalanche photodiodes were grown by metalorganic chemical vapor deposition (MOCVD). Improved crystalline and structural quality of epitaxial layers was achieved by employing optimum growth parameters on low-dislocation-density bulk substrates in order to minimize the defect density in epitaxially grown materials. GaN and AlGaN APDs were fabricated into 30μm- and 50μm-diameter circular mesas and the electrical and optoelectronic characteristics were measured. APD epitaxial structure and device design, material growth optimization, material characterizations, device fabrication, and device performance characteristics are reported.

Optoelectronic switches based on diffusive conduction

We study the process of diffusive conduction that we use in our optoelectronic switches to achieve rapid optical switching (on a picosecond time scale). We present the characteristic Green’s function of the diffusive conduction derived for arbitrary initial conditions. We also report the series solutions of the diffusive conduction obtained for different boundary conditions (V=0 and ∇V=0 along the device contact lines) in different device geometries (rectangular and circular mesas). Using these analytical results, we investigate the effect of boundary conditions on the switching operation and the steady state behavior in optical links. We demonstrate the feasibility of using such diffusive conductive optoelectronic switches to establish optical links in return-to-zero and non-return-to-zero coding schemes. For multichannel optical switching, we discuss possible use of a single optoelectronic switch to accommodate multiple channels at once, with >100 optical channels (with a 2000mm−2 channel density and <10% cross-talk), predicted on a 300×300μm2 mesa with a device switching bandwidth of >50GHz, leading to a 5Tb∕s aggregate transmission in principle. This approach of using multiple parallel channels on a single switch is completely opposite to the traditional idea of arraying many switches. This proposed scheme eliminates the need for on-chip switch integration and the need for the alignment of the optical channels to the integrated individual switches.

Zero dimensional exciton‐polaritons

AbstractThe cover picture from the article [1] shows, in the left parts of the three diagrams, the measured photoluminescence intensity as a function of energy and emission angle for a 3 μm (left diagram), 9 μm (middle diagram) and 19 μm‐diameter quasi‐circular mesa (right diagram). The white lines are the energy dispersions of 2D polariton modes. For clarity, intensities above 1485 meV have been multiplied by a constant factor, as indicated. The right parts of the diagrams contain intensity plots of the simulated polariton spectral density for cylindrical mesas of the same diameters, yielding very good quantitative agreement of the 0D polariton states in the circular mesas. This confirms the coexistence of 0D and 2D microcavity polaritons in these semiconductor structures.This paper is an invited presentation from the 8th International Workshop on Nonlinear Optics and Excitation Kinetics. Further articles from NOEKS 8 are published in phys. stat. sol. (c) 3, No. 7 (2006).

Theory of trapped polaritons in patterned microcavities

AbstractWe consider the system of a quantum well embedded in a planar semiconductor microcavity with a shallow circular mesa patterned on top of the cavity spacer. For this system we develop the linear coupling theory of polaritons. We then compute polariton eigenstates and the corresponding optical spectrum. The theory predicts the existence of laterally confined polariton states with a discrete energy spectrum, as well as continuum states above the finite mesa potential barrier. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Bright semipolar GaInN∕GaN blue light emitting diode on side facets of selectively grown GaN stripes

The authors demonstrate the fabrication and evaluation of bright semipolar GaInN∕GaN blue light emitting diodes (LEDs). The structures are realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes with triangular shape running along the ⟨112¯0⟩ direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140μm. Continuous wave on-wafer optical output powers as high as 700μW and 3mW could be achieved under dc conditions for 20 and 110mA, respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5nm for currents between 1 and 50mA. This is less than half the value measured on c-plane LEDs and confirms the reduced piezoelectric field in our LED structures.

Electrical and low frequency noise properties of 4H-SiC p+-n-n+junction diodes

The electrical and low frequency noise properties of 4H-SiC p+–n–n+ junctions have been investigated at different temperatures. The forward current–voltage characteristics are described as the sum of a recombination current originating from carrier recombination on the sidewall of the circular mesa of the diodes and a diffusion volume current. In the current region where the recombination mechanism is dominant, the main noise sources are attributed to carrier recombination at the perimeter surface of the diode and to a generation-recombination noise related to a local trap level. In the current region where the diffusion current becomes important, the noise originates from mobility and diffusivity fluctuations in the space charge region of the p+–n junction. Analysis of the current and noise data allowed us to determine the density of surface states and the Hooge factor, characterizing the surface and the bulk quality of the diode. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Torsional vibrations of circular elastic plates with thickness steps

This paper presents a theoretical study of torsional vibrations in isotropic elastic plates. The exact solutions for torsional vibrations in circular and annular plates are first reviewed. Then, an approximate method is developed to analyze torsional vibrations of circular plates with thickness steps. The method is based on an approximate plate theory for torsional vibrations derived from the variational principle following Mindlin's series expansion method. Approximate solutions for the zeroth- and first-order torsional modes in the circular plate with one thickness step are presented. It is found that, within a narrow frequency range, the first-order torsional modes can be trapped in the inner region where the thickness exceeds that of the outer region. The mode shapes clearly show that both the displacement and the stress amplitudes decay exponentially away from the thickness step. The existence and the number of the trapped first-order torsional modes in a circular mesa on an infinite plate are determined as functions of the normalized geometric parameters, which may serve as a guide for designing distributed torsional-mode resonators for sensing applications. Comparisons between the theoretical predictions and experimental measurements show close agreements in the resonance frequencies of trapped torsional modes.

Few-electron spin and charge configurations in circular and rectangular vertical quantum dot mesas in a magnetic field: Experiment and full three-dimensional self-consistent simulations

We explore the spin and charge properties as the shape of a single vertical quantum dot mesa is changed from being circular to rectangular. The magnetic field dependence of the electrochemical potentials of both mesa geometries is measured and compared with a full three-dimensional multi-scale self-consistent approach without any prior assumptions about the shape and strength of the dot confinement potential. The calculated zero field electron addition energies are in good agreement with experimental data for both mesa geometries. Few-electron charging diagrams in a magnetic field are also computed. Consistent with the experimental data, the charging curves for the rectangular mesa dot in a magnetic field are “flatter” and exhibit less features than for a circular mesa dot.

Trapped torsional vibrations in elastic plates

We report observation and analysis of trapped torsional vibrations in elastic plates. Each trapping element consists of a circular mesa machined in cast aluminum plate, with an electromagnetic acoustic transducer used to generate oscillatory surface traction. Suitably applied traction induced torsional vibrations trapped in the mesa. The resonant frequencies, relative displacements and Q-values were measured, and an approximate theory was developed to analyze the trapping effect with good agreement between measurements and theory. It was found that these trapped torsional modes have Q-values exceeding 100 000 with pure in-plane motion, which is of practical importance for acoustic sensor applications.

Resonant detection of microwave radiation in a circular two-dimensional electron system with quantum point contacts

We demonstrate that a two-dimensional electron system with a circular mesa and with quantum point contacts along its periphery is an attractive device geometry for the resonant detection of electromagnetic radiation in the millimeter and microwave region. Its operation relies on the resonant excitation of edge magnetoplasmons and the quantum point contacts serve as rectifying elements. The resonant response can be tuned easily across a broad frequency range, for instance, by altering the magnetic field, the electron density or the radius of the disk shaped mesa.

Spin configurations in circular and rectangular vertical quantum dots in a magnetic field: Three-dimensional self-consistent simulations

The magnetic field dependence of the electronic properties of real single vertical quantum dots in circular and rectangular mesas is investigated within a full three-dimensional multiscale self-consistent approach without any \'a priori assumptions about the shape and strength of the confinement potential. The calculated zero field electron addition energies are in good agreement with available experimental data for both mesa geometries. Charging diagrams in a magnetic field for number of electrons up to five are also computed. Consistent with the experimental data, we found that the charging curves for the rectangular mesa dot in a magnetic field are flatter and exhibit less features than for a circular mesa dot. Evolution of the singlet-triplet energy separation in the two electron system for both dot geometries in magnetic field was also investigated. In the limit of large field, beyond the singlet-triplet transition, the singlet-triplet energy difference continues to become more negative in a circular mesa dot without any saturation within the range of considered magnetic fields while it is predicted to asymptotically approach zero for the rectangular mesa dot. This different behavior is attributed to the symmetry ``breaking'' that occurs in the singlet wave-functions in the rectangular mesa dot but not in the circular one.

Regrowth dynamics of InAs quantum dots on the GaAs circular mesa

We report MBE regrowth of InAs quantum dots on GaAs circular mesas, prepared by optical lithography. Because of better strain relaxation, the possibility of quantum dots growth near the lithographic edge is high. Under controlled growth conditions, quantum dots appear only close to the edge. Under these conditions, we discuss the possible influence of crystal orientation to the quantum dots formation, as well as geometrical factors, such as the lateral size of the mesa, and the depth and steepness of the lithographic step. With the full control of the quantum dots formation, we measured the photoluminescence spectrum of the buried dots, as well as the real space image from a CCD camera. The results indicate that quantum dots only form at the edge. Besides the physical location, all the other parameters are quite similar to self-assembled quantum dots formed on planar surfaces.

Lateral oxidation kinetics of AlxGa1−xAs layer by capacitance technique

Capacitance voltage (C–V) technique is used to determine the kinetics of lateral oxidation of AlxGa1−xAs layer in a circular mesa structure. The oxide layer width determined by the C–V technique agrees with optical measurements. Additionally, the C–V technique has the advantage of determining different types of defects in the oxide, which are important for projecting the reliability of vertical cavity surface emitting laser devices. The wet oxidation between 400 and 435 °C shows negative charges in the oxide.

Effects of Coulomb interactions on spin states in vertical semiconductor quantum dots

The effects of direct Coulomb and exchange interactions on spin states are studied for quantum dots contained in circular and rectangular mesas. For a circular mesa a spin-triplet favored by these interactions is observed at zero and nonzero magnetic fields. We tune and measure the relative strengths of these interactions as a function of the number of confined electrons. We find that electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle state degeneracy, and find that the spin-configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states. For a rectangular mesa we observe no signatures of the spin-triplet at zero magnetic field. Due to the anisotropy in the lateral confinement single-particle state degeneracy present in the circular mesa is lifted, and Coulomb interactions become weak. We evaluate the degree of the anisotropy by measuring the magnetic field dependence of the energy spectrum for the ground and excited states, and find that at zero magnetic field the spin-singlet is more significantly favored by the lifting of level degeneracy than by the reduction in the Coulomb interaction. We also find that the spin-triplet is recovered by adjusting the level degeneracy with magnetic field.

Anisotropic radiation pattern from InGaAlP quantum well mesa-like microdisks

To overcome the isotropic directional emission of an ideal circular microdisk, two kinds of cylindrical mesa-like InGaAlP single quantum well (SQW) microdisks emitting at a visible red wavelength of 0.66 μm have been fabricated. An anisotropic luminescence pattern was revealed by the microscopic fluorescence (FL) image. FL intensity, preferentially enhanced with twofold symmetry, appeared at the circumference of the InGaAlP SQW microdisks. Our results demonstrated that anisotropic radiation can be achieved by geometry shaping of the disks on the top view two-dimensional boundary slightly deformed from circular shape and/or on the side-view cross-section of the circular mesa by wet etching anisotropic undercut.