Metalorganic chemical vapor deposition of InN quantum dots and nanostructures.

Using one material system from the near infrared into the ultraviolet is an attractive goal, and may be achieved with (In,Al,Ga)N. This III-N material system, famous for enabling blue and white solid-state lighting, has been pushing towards longer wavelengths in more recent years. With a bandgap of about 0.7 eV, InN can emit light in the near infrared, potentially overlapping with the part of the electromagnetic spectrum currently dominated by III-As and III-P technology. As has been the case in these other III-V material systems, nanostructures such as quantum dots and quantum dashes provide additional benefits towards optoelectronic devices. In the case of InN, these nanostructures have been in the development stage for some time, with more recent developments allowing for InN quantum dots and dashes to be incorporated into larger device structures. This review will detail the current state of metalorganic chemical vapor deposition of InN nanostructures, focusing on how precursor choices, crystallographic orientation, and other growth parameters affect the deposition. The optical properties of InN nanostructures will also be assessed, with an eye towards the fabrication of optoelectronic devices such as light-emitting diodes, laser diodes, and photodetectors.

Effects of surface oxidation on the pH-dependent surface charge of oxidized aluminum gallium nitride.

HYPOTHESIS: The properties of the oxidized surface for common materials, such as silicon and titanium, are known to be markedly different from the reduced surface. We hypothesize that surface-oxidized aluminum gallium nitride ((oxidized-AlGaN)/GaN) surface charge behavior is different to unoxidized AlGaN (with ultrathin native oxide only), which can be validated via surfactant adsorption. Understanding these differences will explain why (oxidized-AlGaN)/GaN-based sensors are better performing than AlGaN ones, which has been previously demonstrated but not understood. EXPERIMENTS: The surface of an AlGaN/GaN structure was oxidized with hot piranha solution and oxygen plasma. AFM force measurements and imaging were performed to probe the charge properties of the surface in aqueous solutions of varying pH containing only an acid or base, or with an added ionic surfactant: cationic cetyltrimethylammonium bromide (CTAB) or anionic sodium dodecylsulfate (SDS). FINDINGS: The (oxidized-AlGaN)/GaN surface is positively charged at pH 4 and pH 5.5, although pH 5.5 should be close to the isoelectric point of the surface. The surface is negatively charged at pH 10 and pH 12, and sufficiently charged to attract cooperative adsorption of CTAB aggregates at pH 12. At pH 2, the evidence is inconclusive, but the surface is most likely positively charged. Compared to unoxidized AlGaN, the (oxidized-AlGaN)/GaN surface shows a wider range of surface charge magnitude over pH values between 2 and 12. This suggests that the (oxidized-AlGaN)/GaN surface has a higher surface hydroxyl group density than unoxidized AlGaN, which explains the higher sensitivity for pH sensors based on (oxidized-AlGaN)/GaN structures.

pH-Dependent surface charge at the interfaces between aluminum gallium nitride (AlGaN) and aqueous solution revealed by surfactant adsorption.

HYPOTHESIS: The net surface charge of AlGaN/GaN structures, where AlGaN is in contact with the solution, is controlled by the pH-dependent protonation and deprotonation of the surface hydroxyl groups and possibly the electron-deficient surface electronic states. We hypothesize that atomic force microscopy (AFM) force measurements of ionic surfactant adsorption can reveal how the AlGaN surface properties vary with pH. EXPERIMENTS: AFM force curves and images were used to probe the AlGaN/solution interface in water as a function of pH, and with added cationic surfactant cetyltrimethylammonium bromide (CTAB) or anionic surfactant sodium dodecylsulfate (SDS). FINDINGS: The AlGaN/solution interface is negatively charged at pH 12, has an isoelectric point near pH 5.5, and is positively charged at pH values less than 5.5. Surfactant adsorption data suggests AlGaN surface is somewhat hydrophobic at acidic pH. Compared to gallium nitride (GaN), at pH 2, AlGaN has a lower charge density and hydrophobicity, but at other values of pH, the surface properties of AlGaN and GaN are similar.

Compliant Micron-Sized Patterned InGaN Pseudo-Substrates Utilizing Porous GaN.

The compliant behavior of densely packed 10 × 10 µm2 square patterned InGaN layers on top of porous GaN is demonstrated. The elastic relaxation of the InGaN layers is enabled by the low stiffness of the porous GaN under layer. High resolution X-ray diffraction measurements show that upon InGaN re-growths on these InGaN-on-porous GaN pseudo-substrates, not only was the regrown layer partially relaxed, but the degree of relaxation of the InGaN pseudo-substrate layer on top of the porous GaN also showed an increase in the a-lattice constant. Furthermore, methods to improve the surface morphology of the InGaN layers grown by metal-organic chemical vapor deposition (MOCVD) were explored in order to fabricate InGaN pseudo-substrates for future optoelectronic and electronic devices. The largest a-lattice constant demonstrated in this study using this improved method was 3.209 Å, corresponding to a fully relaxed InGaN film with an indium composition of 0.056.

pH-dependent surface properties of the gallium nitride - Solution interface mapped by surfactant adsorption.

HYPOTHESIS: The surface charge of gallium nitride (GaN) in contact with solution is controlled by pH via surface protonation and deprotonation, similar to silica. Ionic surfactants adsorb on surfaces via electrostatic and hydrophobic interactions and can be utilized to reflect the surface charge of GaN. EXPERIMENTS: The surface charge properties of Ga-polar GaN in solution were probed as a function of pH using atomic force microscopy (AFM). AFM soft-contact images and force curves were used to study the pH-dependent adsorption of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium dodecylsulfate (SDS) on GaN surfaces. To further confirm the AFM results, GaN/AlGaN/GaN heterostructure-based ion sensing devices were used to measure the surfactant adsorption over the same pH range. FINDINGS: SDS aggregates adsorb on GaN below pH 2.75 while CTAB aggregates adsorb above pH 10. This shows that the GaN surface carries substantial net positive charge at low pH, and negative charge at high pH. There is no clear SDS or CTAB adsorption on the GaN surface between pH 3 and 9.75, which indicates the surface is weakly charged. GaN/AlGaN/GaN heterostructure-based devices confirm these results, and demonstrate the utility of these devices for measuring surfactant adsorption.

Origin of defect-insensitive emission probability in In-containing (Al,In,Ga)N alloy semiconductors.

Group-III-nitride semiconductors have shown enormous potential as light sources for full-colour displays, optical storage and solid-state lighting. Remarkably, InGaN blue- and green-light-emitting diodes (LEDs) emit brilliant light although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional LEDs. Here we explain why In-containing (Al,In,Ga)N bulk films exhibit a defect-insensitive emission probability. From the extremely short positron diffusion lengths (<4 nm) and short radiative lifetimes of excitonic emissions, we conclude that localizing valence states associated with atomic condensates of In-N preferentially capture holes, which have a positive charge similar to positrons. The holes form localized excitons to emit the light, although some of the excitons recombine at non-radiative centres. The enterprising use of atomically inhomogeneous crystals is proposed for future innovation in light emitters even when using defective crystals.

Simultaneous four-photon luminescence, third-harmonic generation, and second-harmonic generation microscopy of GaN.

We demonstrate what is to our knowledge the first example of four-photon luminescence microscopy in GaN and apply it to quality mapping of bulk GaN. The simultaneously acquired second- and third-harmonic generation can be used to map the distribution of the piezoelectric field and the band-tail state density, respectively. Through spectrum- and power-dependent studies, the fourth power dependence of the band edge luminescence is confirmed. The superb spatial resolution of the four-photon luminescence modality is also demonstrated. This technique provides a high-resolution, noninvasive monitoring and tool for examining the physical properties of semiconductors.