Hussein S. El-Ghoroury

Optical characteristics of III-nitride quantum wells with different crystallographic orientations

This article presents a direct comparison of calculated optical characteristics of polar, nonpolar, and semipolar III-nitride quantum wells. We show that the advantage of using wider quantum wells offered by nonpolar/semipolar technology is severely limited by narrower valence subband separation, thermal hole redistribution, and resulting optical gain degradation in wider wells. However, we emphasize the importance of using wider quantum wells to prevent electron leakage. We also show that gain characteristics of laser structures grown in nonpolar/semipolar orientations are less vulnerable to detrimental effect of nonradiative recombination.

Frequency offset estimation and correction in the IEEE 802.11a WLAN

The paper shows how to utilize the short training sequence, long training sequence and pilot subcarriers of the IEEE 802.11a frame, to estimate and equalize the effect of both carrier and sampling frequency offset. To reduce cost, the equalization process is performed digitally. Using computer simulation, we show that the presented scheme nearly eliminates the degradation due to frequency offset for all IEEE 802.11a modulation schemes. The performance measures are the root-mean-squared error (RMSE) and BER in a fading channel.

Optimum quantum well width for III-nitride nonpolar and semipolar laser diodes

The advantage of using wider quantum wells in III-nitride lasers offered by nonpolar/semipolar technology is limited by narrower valence subband separation, thermal hole redistribution, and resulting optical gain degradation in wider wells. We show that corresponding increase in radiative carrier lifetime in wider quantum wells can lower the laser threshold, thus inferring the existence of an optimum quantum well width for laser design.

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...

Modeling of injection characteristics of polar and nonpolar III-nitride multiple quantum well structures

Carrier confinement and injection characteristics of polar and nonpolar III-nitride quantum well (QW) light-emitting diode or laser diode structures are compared. We demonstrate that strongly inhomogeneous QW injection in multiple-QW (MQW) active region is one of the possible reasons holding back the advance of nonpolar laser structures. In polar structures, strong interface polarization charges induce the nonuniform carrier distribution among the active QWs so that the extreme p-side QW always dominates the optical emission. On the contrary, in nonpolar MQW structures, the inhomogeneity of QW populations is supported mainly by QW residual charges and the prevailing QW is the one closest to the n-side of the diode. For both polar and nonpolar structures, the QW injection inhomogeneity is strongly affected by the QW carrier confinement and becomes more pronounced in longer wavelength emitters with deeper active QWs. We show that in nonpolar structures indium incorporation into optical waveguide layers impr...

Modeling of III-Nitride Multiple-Quantum-Well Light-Emitting Structures

Spatial inhomogeneity of carrier injection across the multiple-quantum-well (MQW) active region of a semiconductor light emitter can impose severe limitations on the device efficiency. In III-nitride-based devices, the large disparity of electron and hole transport and the excessive depth of active QWs trigger the process of inhomogeneous QW injection which is further aggravated by strong dependence of QW radiative characteristics on the QW injection conditions due to 1) intra-QW screening of polarization fields in polar and semipolar materials, 2) phase-space filling effect in lowest QW subbands at higher levels of carrier injection, and 3) exceedingly nonequilibrium character of the electron and hole populations in deep QWs. All these tendencies become more pronounced in longer wavelength emitters. The residual QW charges provide strong feedback to the QW injection conditions, thus requiring a high level of self-consistency between the active region transport simulation and the QW emission modeling.

Display gamut reshaping for color emulation and balancing

Emerging next generation digital light projectors are using multiple LED/laser sources instead of one white lamp. This results in a color gamut much larger than any existing display or capture device. Though advantageous in theory, when used to display contents captured/processed at a smaller gamut, a large gamut expansion results in hue-shift artifacts. We present a hardware-assisted 3D gamut reshaping method that handles the gamut expansion in LED based DLP displays by hierarchical temporal multiplexing of the multiple primaries. This, in turn, results in a color emulation technique by which projectors with such large gamuts can also achieve a standard color gamut and white point - the two most important color properties in terms of display quality, with an additional advantage of increased brightness and dynamic range. The same method can also be used for color balancing across multiple projectors that are often used to create large-scale high resolution displays.

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.

Compression for full-parallax light field displays

Full-parallax light field displays utilize a large volume of data and demand efficient real-time compression algorithms to be viable. Many compression techniques have been proposed. However, such solutions are impractical in bandwidth, processing or power requirements for a real-time implementation. Our method exploits the spatio angular redundancy in a full parallax light field to compress the light field image, while reducing the total computational load with minimal perceptual degradation. Objective analysis shows that depending on content, bandwidth reduction from two to four orders of magnitude is possible. Subjective analysis shows that the compression technique produces images with acceptable quality, and the system can successfully reproduce the 3D light field, providing natural binocular and full motion parallax.

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.