F. Ranalli

Growth and optical investigation of self-assembled InGaN quantum dots on a GaN surface using a high temperature AlN buffer

InGaN quantum dots (QDs) with a high density up to 9×1010/cm2 have been obtained on a surface of high quality GaN grown using high temperature AlN as a buffer layer on sapphire substrates. X-ray diffraction measurements indicate that the full width at half maximum of rocking curve of the GaN in (0002) direction has been reduced to as narrow as 61 arc sec. The growth of the InGaN QDs has been found to be different from the formation of other III–V semiconductor QDs in the conventional Stranski–Krastanov mode. Too high NH3 flow rate leads to the InGaN QDs in a large diameter up to 50 nm with a density of ∼1010/cm2, while too low NH3 flow rate results in disappearance of the InGaN QDs. The growth mechanism for the InGaN QDs due to the change in NH3 flow rate has been discussed. The optical properties of the InGaN QDs have been investigated. A stimulated emission from the InGaN multiple QD layers has been observed under an optical pumping with a low threshold at room temperature. The influence of thickness of...

Optical investigation of exciton localization in AlxGa1−xN

The optical properties of AlxGa1−xN epilayers with x ranging from 0.08 to 0.52 have been studied by photoluminescence (PL). The temperature dependent PL of the AlxGa1−xN epilayers shows a classical “S-shape” behavior. This behavior is attributed to exciton localization due to compositional fluctuations in the AlxGa1−xN layers. The localization parameter σ extracted from temperature dependent PL, which gives an estimate of degree of localization, is found to increase with Al composition, up to a value of 52meV at the highest Al composition studied. Several phonon replicas are observed at the lower energy side of the main excitonic emission peak in these epilayers at low temperature. In all cases, the Huang-Rhys parameter has been estimated. The Huang-Rhys parameter is found to increase with x indicating that the degree of localization again increases with x. In addition, the Huang-Rhys parameter is found to increase with higher order phonon replicas.

Greatly improved performance of 340nm light emitting diodes using a very thin GaN interlayer on a high temperature AlN buffer layer

This letter reports a simple approach to significantly improve the performance of 340nm ultraviolet light emitting diodes (UV-LEDs) on an AlN buffer layer. Greatly improved optical and electrical properties of the 340nm UV-LED have been achieved by using a very thin GaN interlayer (10–20nm), deposited on AlN as a buffer layer directly on sapphire prior to growth of the UV-LED structure. Compared with the UV-LED without the thin GaN interlayer, the output power of the LED with it is increased by a factor of ∼2.2, and the applied bias voltage at 20mA drops from 6.5to∼5V. High resolution transmission electron observation indicates that the thin GaN interlayer can effectively stop the penetration of the dislocations in the AlN buffer layer into the overlaying AlGaN layer, while most of the dislocations in the AlN buffer layer in the UV-LED without the thin GaN interlayer can propagate into the overlying AlGaN layer. Therefore, the enhanced performance of the 340nm UV-LEDs results from a massive reduction in d...

Fabrication and optical investigation of a high-density GaN nanowire array

A high-density GaN nanowire array has been successfully fabricated through self-organized nanometer-sized holes as mask appearing in InGaN layer. The self-organized nanometer-sized holes are naturally formed during InGaN epitaxial growth using metalorganic chemical vapor deposition technology by modifying growth parameters. Scanning electron microcopy and atomic force microcopy have been used to characterize them. Optical investigation was carried out by room-temperature photoluminescence, which indicated that strong emission from an n-GaN nanowire array was observed at 367 nm, the near-band edge emission wavelength for n-type GaN. The results show that excellent optical properties of the GaN nanowire array can be obtained by this technique. It is important to point out that GaN-based nanolaser or nano-light-emitting diodes with different emission wavelengths can be potentially achieved using this technology.

Influence of annealing temperature on optical properties of InGaN quantum dot based light emitting diodes

Electron-luminescence (EL) and high-resolution transmission electron microscopy (TEM) measurements have been carried out on the InGaN quantum dot (QD) based light emitting diodes (LEDs) annealed at different temperatures for p-type GaN activation. The annealing temperatures are chosen based on the growth temperature for our InGaN QDs as a reference point. A significant improvement with a factor of up to ∼3.5 in EL intensity has been achieved when the annealing temperature is increased from 720to800°C. However, the EL intensity dramatically decreases if the annealing temperature further increases to 830°C. In addition, a clear blueshift in EL emission energy has been observed as a result of increasing annealing temperature. In combination with our TEM study, the change in optical properties of the QD based LEDs due to the thermal annealing can be attributed to the shrinkage of the QDs and then eventual mergence into the wetting layer if the annealing temperature is further increased. The data based on deta...

GaN hybrid microcavities in the strong coupling regime grown by metal-organic chemical vapor deposition on sapphire substrates

We observe exciton-photon strong coupling at low and room temperature in the ultraviolet spectral region in a GaN-based one wavelength bulk microcavity. The hybrid cavity is composed of 25 pairs of epitaxially grown Al0.2Ga0.8N∕Al0.6Ga0.4N distributed Bragg reflectors (DBRs) on the lower side of the cavity and 9 pairs of Si3N4∕SiO2 as the upper mirror, to obtain cavity Q values up to 160. Anticrossing is observed between the cavity mode and the bulk GaN exciton, showing the formation of polariton modes with normal mode splitting of 43 meV. The lower polariton dispersion is observed in both reflectivity and photoluminescence, with good agreement between the two obtained over a large tuning range of 40 meV. Good fits are obtained to the spectra of the Bragg mirrors. From simulations it is found that the reflectivity spectra of the Bragg mirrors are not significantly limited by absorption in the Al0.2Ga0.8N layers.

Influence of the GaN barrier thickness on the optical properties of InGaN/GaN multilayer quantum dot heterostructures

A study of InGaN/GaN multiple layer quantum dot (QD) structures with varying barrier thicknesses is reported. With increasing barrier thickness both a redshift in the photoluminescence (PL) peak energy and increase in the PL decay lifetime is observed. This is attributed to an increase in the size of the internal electric field and the influence on the electronic structure via the quantum confined Stark effect. Theoretical surface integral potential calculations support this interpretation. A minimum barrier thickness of 4 nm appears to be required for the formation of separate homogeneous QD layers.

Stimulated emission at 340 nm from AlGaN multiple quantum well grown using high temperature AlN buffer technologies on sapphire

It is necessary to further improve crystal quality of AlGaN multiple quantum well (MQW) structures on sapphire in order to achieve ultraviolet (UV) laser diodes. Two buffer technologies have been introduced based on our high temperature AlN buffer technology: modified “GaN interlayer” and “multiple porous AlN buffer.” The Al0.16Ga0.84N/Al0.05Ga0.95N MQWs have been grown on top of the two kinds of buffers on sapphire. High resolution x-ray diffraction measurements have confirmed that the crystal quality has been massively improved. As a result, an UV stimulated emission at 340 nm has been observed via optical pumping with a low threshold power of ∼6.6 kW/cm2 at room temperature. The developed approaches potentially provide a simple way for achieving electrical injection UV (including deep UV) laser.

Optical and microstructural studies of InGaN/GaN quantum dot ensembles

An optical and structural study of InGaN/GaN quantum dots (QDs) is reported. With increasing InGaN deposition time, the dominant emission changes from wetting layer (WL) to QDs, and a strong redshift of the emission occurs. Emission from localized WL states is observed, with a density and nature very different to that due to the QDs. Structural measurements reveal a disordered WL, consistent with the form of the WL photoluminescence excitation spectra.

Influence of crystal quality of underlying GaN buffer on the formation and optical properties of InGaN/GaN quantum dots

A study of InGaN quantum dots (QDs) grown on two different GaN templates—GaN growth using a conventional two-step approach and growth using our recently developed high temperature (HT) AlN as a buffer—is reported. The HT AlN buffer leads to a significant reduction in the dislocation density, particularly screw dislocations, in subsequently deposited GaN. This reduction is confirmed by a significant decrease in the (0002) x-ray diffraction rocking curve width. The GaN on the HT AlN buffer leads to a high density (1010/cm2) of InGaN QDs, whereas in contrast InGaN QDs on the conventional GaN layer grown using the two-step approach have a much smaller density (∼108/cm2). Furthermore, the carrier lifetimes for the QDs on the GaN/HT AlN have been found to be up to nine times longer than those for the QDs on the conventional GaN.