We present numerical simulation of III-nitride monolithic multi-color LED covering full red-green-blue (RGB) color gamut. The RGB LED structure was grown at Ostendo Technologies Inc. and has been used in Ostendo proprietary Quantum Photonic Imager (QPI) device. Active region of our RGB LED incorporates specially designed intermediate carrier blocking layers (ICBLs) controlling transport of each type of carriers and subsequent carrier injection redistribution among the optically active quantum wells (QWs) with different emission wavelengths. ICBLs are proved to be essential elements of multi-color LED active region design requiring optimization both in material composition and doping level. Strong interdependence between ICBL parameters and active QW characteristics presents additional challenge to multi-color LED design. Combination of several effects was crucial for adequate simulation of RGB LED color control features. Standard drift-diffusion transport model has been appended with rate equations for dynamic QW-confined carrier populations which appear severely off-balanced from corresponding mobile carrier subsystems. QW overshoot and Auger-assisted QW depopulation were also included into the carrier kinetic model thus enhancing the non-equilibrium character of QW confined populations and supporting the mobile carrier transport across the MQW active region. For device simulation we use COMSOL-based program suit developed at Ostendo Technologies Inc.
Simulation of III-nitride color-coded multiple quantum well (MQW) LED structures was performed using as an
experimental benchmark dichromatic semipolar LEDs grown in Ga-polar and N-polar crystallographic orientations
(Y. Kawaguchi et.al, APL 100, 231110, 2012). Different QW depths in the color-coded LEDs and opposite interface
polarization charges in Ga-polar and N-polar structures provide different conditions for carrier transport across the LED
active regions. Combination of several effects was crucial for adequate reproduction of the emission spectra
experimentally observed in color-coded structures with violet-aquamarine and aquamarine-violet active region layouts.
A standard drift-diffusion transport model wascompleted with rate equations for nonequilibrium QW populations and
several high-energy transport features, including the effects of QW carrier overshoot and Auger-assisted QW
depopulation. COMSOL-based Optoelectronic Device Modeling Software (ODMS) developed at Ostendo Technologies
Inc. was utilized for device simulation.
III-nitride visible light emitters employ deep QWs and feature strong disparity of electron and hole transport in diode
structures. As a result, multi-QW active regions of such devices suffer from inhomogeneous carrier injection, large
residual charges of active QWs, and overall active region electrical non-uniformity which unfavorably affects the emitter
efficiency. In this work, we show that electron and hole populations of deep optically active III-nitride QWs are highly
imbalanced and substantially deviate from thermodynamic equilibrium with corresponding mobile carrier subsystems.
Non-equilibrium QW populations are self-consistently determined by carrier injection and light generation processes in
active QWs. In turn, QW residual charges impose strong feedback on the active region electrical uniformity. Our selfconsistent
modeling of QW radiative characteristics and multi-QW carrier transport in diode structures relates the effects
of non-equilibrium QW populations, inhomogeneous QW injection and residual QW charges to the structure internal
efficiency. Comparative modeling of polar and nonpolar diodes shows that in both types of structures the nonequilibrium
effects tend to decrease the QW operational electron populations; this trend benefits the active region
electrical uniformity. For device simulation, we use COMSOL-based Optoelectronic Device Modeling Software
(ODMS) developed at Ostendo Technologies Inc.
InGaN light emitting diodes (LEDs), which have become key components of the lighting technology owing to
their improved power conversion efficiencies and brightness, still suffer from efficiency degradation at high
injection levels. Experiments showing sizeable impact of the barrier height provided by an electron blocking layer
(EBL) or the electron cooling layer prior to electron injection into the active region strongly suggest that the electron
overflow resulting from ballistic and quasi-ballistic transport is the major cause of efficiency loss with increasing
injection. Our previous report using a first order simple overflow model based on hot electrons and constant LO
phonon scattering rates describes well the experimental observations of electron spillover and the associated
efficiency degradation in both nonpolar m-plane and polar c-plane LEDs with different barrier height EBLs and
electron injection layers. LEDs without EBLs show three to five times lower efficiencies than those with
Al0.15Ga0.85N EBLs due to significant electron overflow to the p-type region in the former. For effective means of
thermalization in the active region within their residence time and possibly longitudinal optical phonon lifetime, the
electrons were cooled prior to their injection via a staircase electron injector, i.e. an InGaN staircase structure with
step-wise increased In composition. The investigated m-plane and c-plane LEDs with incorporation of staircase
electron injector show comparable electroluminescence performance regardless of the status of EBL. This paper
discusses hot electron effects on efficiency loss, means to cool the electrons prior to injection.
The paper describes the heterostructures and device output parameters of Type-I quantum-well (QW) laser diodes with
InGaAsSb active regions designed for room-temperature operation near 2.3 μm and 3.1 μm. For both designs decrease of
the threshold current density and increase of the room-temperature output power have been achieved with increase of the
QW depth for holes. For the 2.3 μm laser diodes, confinement of holes in the QW embedded into the AlGaAsSb
waveguide was improved with increase of the hole energy level with compressive strain. Arrays of 1-mm-long 100-μmwide
laser diode emitters with a fill-factor of 30 % have been fabricated. A quasi-CW (30 μs, 300 Hz) output power of
16.7 W from a 4-mm-wide array has been obtained with conductive cooling. For the laser diodes designed for roomtemperature
operation above 3 μm, the hole confinement was improved by lowering the valence band energy in the
waveguide. Two approached were implemented: one with increase of the Al composition, and another with utilization of
quinternary InAlGaAsSb waveguide with increased As composition compared to the conventional AlGaAsSb
waveguide. With the quinternary waveguide approach, a room-temperature CW output power in excess of 130 mW and
a threshold current as low as 0.6 A have been obtained at λ = 3 μm from 2-mm-long 100-μm-wide emitters.
We propose a novel design of an electrically tunable type II mid-IR light source based on a InAs/AlSb/GaInAsSb/GaInSb heterostructure. The design combines the advantages of strong wavelength tuning due to the linear Stark effect and the presence of separate charge accumulation layers, which enables laser wavelength tuning without a change of the optical loss. Experiment shows a blue shift of the electroluminescence (EL) line at increasing bias current, commensurate with that expected form the linear Stark effect. The laser generation was observed at higher currents. The EL wavelength shifts from 2.79um to 2.38um ( ~ 80meV) at T=80K as the bias current increases from 97mA to 418mA, which provides the record combination of the wide tuning range and low relative change of the bias current.
Laser sources operating in spectral region 2 - 4 μm are in demand for ultra-sensitive laser spectroscopy, medical diagnostics, home security, industrial process monitoring, infrared countermeasures, optical wireless communications, etc. Currently, solid state lasers and optical parametric oscillators and amplifiers are used as coherent light sources in this spectral region. Solid state and parametric sources are being optically pumped by near infrared diode lasers. This intermediate energy transfer step from near infrared pumping diode to mid infrared emitting device reduces power-conversion system efficiency. Development of the highly efficient semiconductor diode lasers operating in 2 - 4 μm spectral region will significantly improve the performance of the many existing systems and enable new applications. In this work we will describe major breakthrough in the development of the high power room temperature operated mid-IR semiconductor lasers. The performance limitations of the devices based on type-I and type-II quantum well (QW) active region design will be analyzed. Future directions in device performance optimization and enhancement of the wavelength for high power room temperature operation will be discussed.
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