Properties Of InGaN Quantum Dots Research Articles

Surface morphology and optical properties of InGaN quantum dots with varying growth interruption time

The effect of different growth interruption time on the surface morphology and optical properties of InGaN quantum dots (QDs) grown on 2-inch silicon substrates is investigated. The surface becomes rougher and the photoluminescence intensity has been enhanced significantly when employing the growth interruption method. Temperature-dependent photoluminescence and excitation power-dependent photoluminescence both present unchanged peak energy and line-width of QDs. The sharp increase of PL intensity in medium temperature regime is attributed to the fingerprint of the existence of InGaN QDs. The shape of the QDs are further confirmed by the transmission electron microscopy with a size of 3 nm by 4 nm. Among the samples, a growth interruption time of 30 s gives the best optical performance.

Formation and properties of InGaN QDs: Influence of substrates

We examine the formation and properties of InGaN quantum dots (QDs) on free-standing GaN and GaN/sapphire templates, with and without buried InGaN/GaN QD superlattices (SLs). We use scanning tunneling microscopy (STM) and scanning tunneling spectroscopy to image the QDs and measure their electronic states. As the number of layers preceding the QDs increases (i.e., increasing substrate complexity), the total QD density increases. For free-standing GaN, STM reveals a mono-modal QD-size-distribution, consistent with a limited density of substrate threading dislocations serving as heterogeneous nucleation sites. For GaN/sapphire templates, STM reveals a bimodal QD-size-distribution, presumably due to the nucleation of additional ultra-small InN-rich QDs near threading dislocations. For multi-period QD SLs on GaN/sapphire templates, an ultra-high density of QDs, with a mono-modal size distribution is apparent, suggesting that QD nucleation is enhanced by preferential nucleation at strain energy minima directly above buried QDs. We discuss the relative influences of strain fields associated with threading dislocations and buried QD SLs on the formation of InGaN QDs and their effective bandgaps.

Size-dependent optical properties of InGaN quantum dots in GaN nanowires grown by MBE

Quantum dots in nanowires (DINWs) are considered as important building blocks for novel nanoscale semiconductor optoelectronic devices. In this paper, pure axial heterojunction InGaN/GaN DINWs are grown by using plasma-assisted molecular beam epitaxy (PA-MBE) system. The InGaN quantum dots (QDs) are disk-like observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The diameter of QDs can be controlled by the growth conditions of nanowires (NWs), while the thickness of QDs can be controlled by the growth time of InGaN. Temperature-dependent photoluminescence (TDPL) measurements demonstrate that the PL peak of DINWs with small and uniform sizes shows a general red shift with increasing temperature. However, the PL peak of DINWs with non-uniform sizes shows an abnormal blue shift with increasing temperature, which is due to different internal quantum efficiencies of the DINWs with different sizes.

Improved luminescence and thermal stability of semipolar (11-22) InGaN quantum dots

Semipolar (11-22)-oriented InGaN/GaN quantum dots (QDs) emitting in the 380–620 nm spectral range were synthesized by plasma-assisted molecular-beam epitaxy. The influence of the growth temperature on the properties of InGaN QDs has been investigated by photoluminescence and transmission electron microscopy. Growth temperatures low enough to prevent indium desorption provide a favorable environment to semipolar plane (11-22) to enhance the internal quantum efficiency of InGaN/GaN nanostructures.

Influence of wetting layers on the electric potentials and optical properties of InGaN quantum dots

This study investigates the influence of wetting layers (WLs) on the electric potentials and electronic structures of InGaN quantum dots (QDs). The single particle electron and hole energies of QDs with different thicknesses and indium compositions of the WLs are calculated using the four-band k ⋅ p Hamiltonian. Numerical results show that there is an average of 60 mV increase in the maximum electric potential drop per every 0.2 nm increment in the thickness of the WL or per every 0.05 increase in indium composition α of the WL. Consequently, the transition energy decreases significantly with increasing thickness and/or indium composition of the WL. We find that the strained bandgap shifts down by 125 meV leading to a 128 meV decrease in the transition energy per every 0.4 nm increment in the thickness of the WL, which corresponds to 5.6% and 4.8% of the strained bandgap and the transition energy, respectively, for the cases without the WL. Furthermore, there is also a 32.5 meV transition energy decrease per every 0.025 increase in indium composition α of the WL. Hence, it is important to consider the thickness and indium composition of the WL in the analysis of InGaN QDs.

Optical properties of InGaN quantum dots in monolithic pillar microcavities

The integration of InGaN quantum dots into GaN-based monolithic microcavities grown by metal-organic vapor-phase epitaxy is demonstrated. Microphotoluminescence spectra reveal distinct spectrally sharp emission lines around 2.73 eV, which can be attributed to the emission of single InGaN quantum dots. The samples are structured into airpost pillar microcavities. The longitudinal and transversal mode spectra of these cavities are in good agreement with theoretical calculations based on a vectorial transfer-matrix method. Quality factors up to Q=280 have been achieved.

Fully coupled and semi-coupled piezoelectric models on the optical properties of InGaN quantum dots

This paper investigates the differences between the fully coupled and the semi-coupled piezoelectric models for determining strain fields, piezoelectric potentials and optical properties of wurtzite InGaN quantum dots (QDs) in three different shapes. Through the calculations, we show that the relative difference of the x-component strain inside the dot remains almost unchanged regardless of the shapes and the sizes of the dot. On the other hand, a large relative difference for the z-component strain is obtained with the use of the semi-coupled model. We also find that the semi-coupled model clearly overestimates the piezoelectric potential, and the transition energy difference increased with increases in the dot size and indium composition. Consequently, the semi-coupled model causes a great amount of distortion in predicting the optical properties of InGaN QDs. It is thus evident that the fully coupled model for calculating the electromechanical fields and optical properties of InGaN QDs may be more appropriate according to our numerical examples.

The influence of a capping layer on optical properties of self-assembled InGaN quantum dots

Optical properties of InGaN quantum dots (QDs) with and without a GaN capping layer have been investigated, showing a major difference between each other. Compared with the InGaN QDs with a GaN capping layer, those grown under identical conditions but without the GaN capping layer showed much stronger photoluminescence (PL) emission and a ∼350 meV blueshift in emission energy. The excitation power-dependent PL measurements indicated that the emission energy of the QDs with the capping layer showed a large blueshift with increasing excitation power, while there was a negligible shift in the QDs without the capping layer. The major difference between them is attributed to existence of a strong quantum-confined Stark effect (QCSE) in the QDs with the capping layer, while there is no clear QCSE observed in the uncapped QDs. The transition energy has been calculated for both QDs within the framework of effective-mass approximation and variational approach, showing a good agreement with the experimental data. The results obtained should be highly taken into account in investigating the optical properties of InGaN QDs on a GaN surface.

Optical properties of InGaN quantum dots grown by SiNx nanomasks

InGaN quantum dots (QDs) deposited on SiNx nanomasks have been investigated by atomic force microscopy, photoluminescence (PL), and photoluminescence excitation (PLE) measurements. It was found that the size of QDs can be well controlled by SiNx nanomasks, enabling the manipulation of quantum confinement effect. The PL spectra of InGaN QDs contain several fine structures, and the main peaks can be attributed to families of QDs with different sizes. The emission arising from InGaN QDs and GaN buffer layer can be clearly distinguished based on PLE measurement, which can be used to improve the interpretation in the previous reports. Our study indicates that the quantum confined Stark effect due to piezoelectric field plays a very important role in the optical properties of InGaN QDs, which is very useful for the application of optoelectronic devices.

The Structural and Optical Properties of Self-assembled InGaN/GaN Quantum Dots Grown by Molecular Beam Epitaxy

ABSTRACTThe structural and optical properties of InGaN quantum dots grown by plasma-assisted molecular beam epitaxy (MBE) have been characterised using atomic force microscopy, high-resolution transmission electron microscopy (TEM), Z-contrast scanning TEM, micro-photoluminescence (PL), temperature dependent PL and time-resolved PL. The uncapped InGaN nano-islands have densities of ∼1.5 × 1011 cm−2, heights of (1.7 ± 1.0) nm and diameters of (10 ± 4) nm. These parameters are not substantially changed during overgrowth of a GaN cap and the resulting quantum dots have a composition of In0.15Ga0.85N. The observation of narrow luminescence peaks in micro-PL measurements proves light emission from discrete energy states and the optical properties indicate strong confinement of carriers in the quantum dots and an unusually weak impact of piezoelectric field effects.

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photoluminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system.