Chih-Li Chuang

Non-equilibrium quantum well populations and active region inhomogeneity in polar and nonpolar III-nitride light emitters

Strong disparity of electron and hole transport in III-nitride materials is commonly accepted as a main reason for inhomogeneous carrier injection in multiple-quantum well (MQW) active regions of light emitters operating in visible spectral range. In this work, we show that two more factors, specifically (i) excessive depth of III-nitride QWs and (ii) strongly non-equilibrium character of electron and hole populations in optically active QW, are responsible for the active region inhomogeneity in GaN-based light emitters. Modeling shows that electron and hole populations of deep III-nitride QWs are highly imbalanced and substantially deviate from thermodynamic equilibrium with corresponding mobile carrier subsystems in the device active region. In turn, large residual QW charges provide strong impact on the active region electrical uniformity and QW injection conditions. We demonstrate that, as a result of non-equilibrium effects in QW population, even nonpolar III-nitride light emitters with deep QWs suff...

Growth of monolithic full-color GaN-based LED with intermediate carrier blocking layers

Specially designed intermediate carrier blocking layers (ICBLs) in multi-active regions of III-nitride LEDs were shown to be effective in controlling the carrier injection distribution across the active regions. In principle, the majority of carriers, both holes and electrons, can be guided into targeted quantum wells and recombine to generate light of specific wavelengths at controlled current-densities. Accordingly we proposed and demonstrated a novel monolithic InGaN-based LED to achieve three primary colors of light from one device at selected current densities. This LED structure, which has three different sets of quantum wells separated with ICBLs for three primary red-green-blue (RGB) colors, was grown by metal-organic chemical vapor deposition (MOCVD). Results show that this LED can emit light ranging from 460 to 650 nm to cover the entire visible spectrum. The emission wavelength starts at 650 nm and then decreases to 460 nm or lower as the injection current increases. In addition to three primary colors, many other colors can be obtained by color mixing techniques. To the best of our knowledge, this is the first demonstration of monolithic full-color LED grown by a simple growth technique without using re-growth process.

Effect of active QW population on optical characteristics of polar, semipolar and nonpolar III-nitride light emitters

Quantum well (QW) population effects are compared in III-nitride light emitters with different levels of polarity. We show that wider nonpolar active QWs are characterized by increased QW transparency current and a reduced differential optical gain which consequently increases the laser threshold. We also show that high intra-QW recombination rates in nonpolar/semipolar structures make the QW populations strongly non-equilibrium and vulnerable to inhomogeneous QW injection. In the LED regime, structures with a different polarity level reveal different mechanisms of the efficiency droop. In polar structures, the droop is dominated by the electron leakage and is notably affected by the active region ballistic overshoot. The efficiency droop in semipolar/nonpolar structures is dominated by the combined effect of radiative time saturation and non-radiative Auger recombination.

The effects of thin capping layers between quantum wells and barriers on the quantum efficiency enhancement in InGaN-based light emitting diodes

We discovered that adding H2 to the carrier gas in GaN barrier growth improved the light emitting diode (LED) peak quantum efficiency and shifted the efficiency maxima toward lower currents (∼20 mA). This implies that the Shockley–Read–Hall nonradiative process can be suppressed via the introduction of combination carrier gas (H2/N2) during barrier growth. Further, 1–2 nm thick Al-In-Ga-N alloys were adopted as capping layers to circumvent H2 etching effect during growth interruption. It was then revealed that quantum efficiency was effectively enhanced for LEDs employed with these thin large bandgap capping layers, particularly at low injection levels. Numerical simulation suggested that the improved quantum efficiency can be ascribed to the increased electron capture rate in the active region as well as enhanced electron and hole wavefunction overlap, which correlated well with experimental results.

III-nitride monolithic LED covering full RGB color gamut

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.

Effect of electron blocking layer on inter-QW transport in III-nitride multi-QW LEDs

Strong disparity in electron and hole transport characteristics and excessive depth of optically active quantum wells (QWs) in III-nitride materials are the main causes of inhomogeneous carrier distribution and uneven QW injection in multi-QW light emitters of visible range. Both polar and nonpolar LED structures suffer from inhomogeneous injection. Undoped wide-bandgap electron blocking layer (EBL) located on the P-side of the active region can only make the situation worse by further reducing already insufficient hole injection. On the other hand, P-doped EBL facilitates the hole injection, improves the overall active region injection uniformity, and reduces the carrier leakage. We show, however, that EBLs act very differently in polar and nonpolar III-nitride multi-QW structures. While in nonpolar LED the p-doped EBL ultimately promotes the inter-QW carrier exchange, the injection efficiency in polar structure remains limited by strong electron leakage from the marginal p-side QW.

Etching formation of GaN micro optoelectronic device array

GaN based micro emitter optoelectronic device array has been proved to be the core component for wide variety of applications such as microdisplay, biosensor, projection etc. Etching is one of the key steps to form the GaN micro emitter array device, including inductively coupled plasma (ICP) dry etch and alkaline solution wet etch. This paper reports the recent progress made by Ostendo Technologies Inc in fabricating the ultra-high density, large aspect-ratio etching formed monolithic GaN micro emitter optoelectronic device array. The unit density reaches 1M per cm2, with good uniformity across the whole wafer. Perpendicular etching sidewall was achieved, with smooth surface roughness which is significance feature used for laser diodes (LDs) device.

Effect of internal electric field in c-Si solar cells

Built-in electric field of the P-N junction is expected to support the separation and enhance the collection of the charge carriers generated in solar cell absorber. If this is a case, the distribution of the electric field across the solar cell absorber should affect the device performance. Comparative modeling of P-N junction and P-i-N junction solar cells with strongly different distributions of internal electric field was carried out to reveal the performance difference. Detailed simulations show that the photovoltaic characteristics of monocrystalline silicone (c-Si) solar cells with N-type and P-i-N-type absorbers are nearly identical which indicates that the field-assisted carrier transport in the absorber layer is irrelevant to c-Si solar cell operation. Insensitivity of the device performance to the field distribution is explained by exceptionally low level of recombination loss in c-Si.

Injection inhomogeneity and lasing thershold in III-nitride multi-QW deep-UV laser diodes

Lasing threshold conditions have been analyzed in deep-UV (DUV) multiple-QW (MQW) III-nitride laser diode (LD) structures with different QW confinement depth. Shallow QWs with smaller internal polarization fields reveal better QW emission characteristics including lower QW transparency population and higher differential gain. In MQW LD structures, however, the high-gain operation of shallow QWs is hindered by insufficient QW injection due to increased carrier leakage from shallow-QW active region. Deep active QWs can attain higher operational populations and provide for higher LD optical gain; however, LDs designed with deeper active QWs suffer from MQW population non-uniformity due to increased inhomogeneity of carrier injection. Underpumped deep QWs reduce the total modal gain of the LD structure thus deteriorating the lasing threshold and LD power conversion (wall-plug) efficiency.