Yan-Ling Hu

Double embedded photonic crystals for extraction of guided light in light-emitting diodes

Light-emitting diodes (LEDs) were fabricated in gallium nitride with two embedded photonic crystals (PhCs), creating a waveguide with a highly confined, strongly excited, and well-extracted fundamental mode. This structure improves upon previous PhC LED designs by reducing the extraction length of the fundamental mode and establishing a path to designs with very low absorption losses. Optical output was measured with angular-resolved electroluminescence. The extraction length of the fundamental mode was measured to be 21–39 μm along the PhCs’ Γ-M directions, which is much shorter than values reported for single-PhC devices. This structure opens the way to more efficient LEDs and lasers.

Recent progress in metal-organic chemical vapor deposition of

Progress in metal-organic chemical vapor deposition of high quality N-polar (Al, Ga, In)N films on sapphire, silicon carbide and silicon substrates is reviewed with focus on key process components such as utilization of vicinal substrates, conditions ensuring a high surface mobility of species participating in the growth process, and low impurity incorporation. The high quality of the fabricated films enabled the demonstration of N-polar (Al, Ga, In)N based devices with excellent performance for transistor applications. Challenges related to the growth of high quality N-polar InGaN films are also presented.

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We demonstrate high performance InGaN/GaN multiple quantum well solar cells with thin quantum barriers and spectral response extending to 460 nm. Devices grown on bulk (0001) GaN substrates with up to 50 quantum wells (QWs) outperform those grown simultaneously on sapphire due to the lower threading dislocation density. Increasing the number of QWs eventually leads to performance degradation of devices grown on both substrates. Solar cells are demonstrated with peak external quantum efficiencies up to 60%, open circuit voltages up to 2.28 V, fill factors up to 80%, and conversion efficiencies up to 2.4% under 1 sun AM0 equivalent illumination.

N-polar group-III nitrides

Experimental gain spectra at continuous-wave (CW) operation of laser diodes (LDs) fabricated from c-plane, nonpolar and semipolar GaN-based materials emitting in violet, blue and green spectral regions are presented. Gain spectra were obtained using the Hakki–Paoli method at high resolution. The ability of the setup to resolve the sharp Fabry–Perot longitudinal peaks of the lasers at all injection currents allowed us to accurately measure the current density to reach transparency on the devices characterized in this work, and determine both total losses and differential modal gain curves up to threshold. We present a comparison of transparency current density and modal gain for nonpolar and c-plane LDs in violet and blue regions. The main parameters for the analysis are the internal electric fields (for c-plane lasers), hole effective masses and valence band splittings. In a preliminary analysis, we investigated the gain spectrum of semipolar green LDs and observed a significantly lower total linewidth (homogeneous plus inhomogeneous) than reported elsewhere, which seems to indicate that inhomogeneous broadening is not the main issue governing the evolution toward high performance green LDs.

High performance thin quantum barrier InGaN/GaN solar cells on sapphire and bulk (0001) GaN substrates

Compound semiconductors belong to the most important materials for optoelectronic applications. Many of them exhibit favorable optical properties, such as a direct energy band gap (in contrast to silicon) and high-absorption coefficients over a wide spectral range. Moreover, varying the composition of the compound or substituting some of its elements often allows for controlled band gap engineering and optimization for specific applications. Because many compound semiconductors enable efficient conversion of light into electricity and vice versa, they are commonly used materials for optoelectronic devices.

Gain comparison in polar and nonpolar\ssty{/} semipolar gallium-nitride-based laser diodes

In this paper, we investigated the Ga incorporation effect in InAl(Ga)N/Al(Ga)N/GaN heterojunctions grown by a close coupled showerhead metal–organic chemical vapor reactor and proposed a grading growth strategy, where the indium composition was graded from Al(Ga)N to InAl(Ga)N, to mitigate the deleterious effect of Ga carry-over on the transport properties of two dimensional electron gas (2DEG). In contrast to non-graded samples grown by conventional growth strategy without grading, hall measurements revealed significant charge and mobility enhancements for the graded samples, with an electron mobility of 1300 cm2 V-1 s-1, a sheet charge density of 2.35 ×1013 cm-2 and a resultant low sheet resistance of 205 Ω/ compared to the non-graded sample with an low sheet charge density of 1.4 ×1013 cm-2 and mobility of 1100 ±50 cm2 V-1 s-1. The reason of the enhancements were then analyzed by transmission electron microscopy (TEM) and atom probe techniques, which revealed that grading strategy led to a higher average Al composition in the barrier layer.

Atom Probe Tomography of Compound Semiconductors for Photovoltaic and Light-Emitting Device Applications

We have developed a dry etch process for the fabrication of lithographically defined features close to light emitting layers in the III-nitride material system. The dry etch was tested for its effect on the internal quantum efficiency of c-plane InGaN quantum wells using the photoluminescence of a test structure with two active regions. No change was observed in the internal quantum efficiency of the test active region when the etched surface was greater than 71 nm away. To demonstrate the application of the developed dry etch process, surface-etched air gaps were fabricated 275 nm away from the active region of an m-plane InGaN/GaN laser diode and served as the waveguide upper cladding. Electrically injected lasing was observed without the need for regrowth or recovery anneals. This dry etch opens up a new design tool that can be utilized in the next generation of GaN light emitters.

Charge and Mobility Enhancements in In-Polar InAl(Ga)N/Al(Ga)N/GaN Heterojunctions Grown by Metal–Organic Chemical Vapor Deposition Using a Graded Growth Strategy

In this study, a-plane GaN was grown on r-plane sapphire by sidewall lateral epitaxial overgrowth. Prismatic stacking faults (PSFs) in the window region of the GaN layer were identified to have an atomic configuration similar to the model presented by Northrup [Appl. Phys. Lett. 72, 2316 (1998)], which was demonstrated by comparing high-angle annular dark-field scanning transmission electron microscopy images with the atomic configurations of the Drum model, the Amelinckx model, and simplified Northrup model, respectively. The Northrup PSF structure has been further confirmed by the scattering contrast analysis in transmission electron microscopy experiments.

Low damage dry etch for III-nitride light emitters

In2O3 is important because it has been widely used as a transparent contact material and an active gas sensor material. To understand and utilize its intrinsic physics as a semiconductor, it is necessary to have In2O3 with a high material quality. In this article, single-crystalline (001)-oriented In2O3 thin films were grown on yttria-stabilized zirconia (001) substrate, and a group theory analysis and transmission electron microscopy (TEM) experiments were conducted to investigate the defects within the In2O3 film. Owing to the reduced symmetry of the bixbyite structure (space group Ia{\overline 3}) in comparison with the fluorite template (space group Fm {\overline 3}m), the formation of antiphase domains and 90° rotation domains in the In2O3 thin films is anticipated. This prediction is confirmed experimentally by TEM and high-angle annular dark-field scanning transmission electron microscopy images. The size of the enclosed domains ranges from 50 to 300 nm, and the major domain boundaries are along the (110), (1{\overline 1}0), (010) and (100) planes. The rotation domains are related by a fourfold rotation operation along the 〈001〉 directions, which will cause the permutation of the axes of the bixbyite structure.

Atomic structure of prismatic stacking faults in nonpolar a-plane GaN epitaxial layers

Planar (In,Ga)N layers were grown on nanostripe arrays composed of InGaN/GaN multi quantum wells (MQWs) by metal–organic chemical vapor deposition. The MQW nanostripe arrays with height to width aspect ratios of about 0.5 and 1 were fabricated from planar coherently strained InGaN/GaN MQW samples. Independent of their aspect ratio, the nanostripes exhibited elastic relaxation perpendicular to the stripe direction after pattern fabrication, resulting in an a⊥ lattice constant perpendicular to the stripe direction larger than that of the GaN base layer. In a subsequent step, (In,Ga)N layers were grown on top of the nanostripe arrays, leading to the formation of planar films with a similar a⊥ lattice constant as the MQW stripes beneath. Bright luminescence was recorded from the planar, partially relaxed re-grown (In,Ga)N layers grown on the stripe arrays with an aspect ratio of 1. Plastic relaxation of the MQW stripes was observed after (In,Ga)N regrowth for samples with a stripe aspect ratio of 0.5, leading to luminescence quenching.