InGaN Quantum Dots Research Articles

Electron microscopy investigations of V defects in multiple InGaN/GaN quantum wells and InGaN quantum dots

The mechanism of high emission of InGaN-based multiple quantum wells, which exhibit exceptionally high light emission efficiency despite their high defect density, is still not fully understood. Here, we deal with this problem, showing the details of structure and formation of V defects in the multiple quantum wells and reviewing interpretations proposed so far. Then, we show a structural investigation of three-dimensional high-density quantum dots, fabricated instead of quantum wells in the active layer. The shape and size of the InGaN quantum dots and the SiN(x) masks for the growth of the dots have been revealed using high-angle annular dark field scanning transmission electron microscopy, energy dispersive X-ray spectroscopy nanoanalysis and high-resolution transmission electron microscopy.

Microstructural characterisation of a prototype layer structure for a GaN-based photonic crystal cavity

A semiconductor multilayer consisting of an n-type GaN/sapphire pseudo-substrate with a double sacrificial layer (20 nm of InGaN and 20 nm of InAlN) and a GaN cavity structure on top incorporating an InGaN quantum dot active layer was grown. Atomic force microscopy (AFM) was used to measure the morphology of the upper surface of each layer of similar test structures and to assess the morphological evolution of the full structure during its growth. Transmission electron microscopy (TEM) was also used to assess defect incorporation into the structure. AFM showed that the incorporation of a double sacrificial layer was observed to have no detrimental affect on either the formation of the quantum dots, or the morphology of the top GaN capping layer. TEM highlighted the occurrence of threading dislocations propagating through the sacrificial layers into the capping GaN layer, but did not resolve the occurrence of defect generation in the sacrificial layers.

Cavity Enhancement of Single Quantum Dot Emission in the Blue

Cavity-enhanced single-photon emission in the blue spectral region was measured from single InGaN/GaN quantum dots. The low-Q microcavities used were characterized using micro-reflectance spectroscopy where the source was the enhanced blue output from a photonic crystal fibre. Micro-photoluminescence was observed from several cavities and found to be ~10 times stronger than typical InGaN quantum dot emission without a cavity. The measurements were performed using non-linear excitation spectroscopy in order to suppress the background emission from the underlying wetting layer.

Electroluminescence from a single InGaN quantum dot in the green spectral region up to150 K

We present electrically driven luminescence from single InGaN quantum dots embeddedinto a light emitting diode structure grown by metal–organic vapor-phase epitaxy. Singlesharp emission lines in the green spectral region can be identified. Temperature dependentmeasurements demonstrate thermal stability of the emission of a single quantum dot up to150 K. These results are an important step towards applications like electrically drivensingle-photon emitters, which are a basis for applications incorporating plasticoptical fibers as well as for modern concepts of free space quantum cryptography.

Back Cover (Phys. Status Solidi A 11/2009)

The Feature Article by Jarjour et al. (p. 2510) reviews the progress made so far towards the development of optically excited and electrically driven single photon sources based on nitride-based single quantum dots. The back cover picture illustrates the emission of single photons in the blue region of the spectrum from an InGaN/GaN quantum dot under nonlinear optical excitation in the near infrared. The atomistic structure of an InGaN quantum dot embedded in a GaN barrier was used in the atomistic simulation of the electronic structure of these quantum dots. The correlation measurements shown in the graph demonstrate the single-photon nature of the emission.

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.

Electrical control of the exciton spin in nitride semiconductor quantum dots

We report on the experimental evidence of the manipulation of the exciton spin in InGaN quantum dots through the application of an external electric field up to room temperature. Furthermore, we have found the exciton spin relaxation to be independent of temperature. These findings are highly promising for the potential future use of nitride semiconductor quantum dots in practical spintronic devices.

Phase separation and nonradiative carrier recombination in active regions of light-emitting devices based on InGaN quantum dots in a GaN or AlGaN matrix

Structures with InGaN nanolayers within GaN and AlGaN matrices, which constitute active regions of light-emitting devices, have been studied. Spectra and relative intensities of photoluminescence (PL) in the temperature range 20–300 K and the dependence of the position of the PL peak on the energy of the excitation photons have been measured. It is shown that, to account for the temperature dependence of the PL intensity, it is necessary to take into consideration, in addition to the nonradiative recombination via defects in the matrix and in the residual quantum well (RQW), an additional recombination channel with low activation energy, which is presumably associated with defects located close to quantum dots. It is demonstrated that structures with the AlGaN matrix show a larger decrease in the PL intensity upon an increase in temperature from 50 to ∼200 K, compared with structures with the GaN matrix. Analysis of the temperature dependence of the PL intensity in terms of the model that considers these three channels of nonradiative recombination shows that this dependence is associated with a decrease in the carrier’s localization energy relative to the ground state in the RQW. Such a decrease is due to suppression of the phase separation in InGaN layers grown within the AlGaN matrix. This behavior is confirmed by PL measurements at different excitation photon’s energies and leads, in addition to the lower localization energy, a decrease in the concentration of recombination centers is observed for these samples.

Optical properties and modal gain of InGaN quantum dot stacks

AbstractWe present investigations of the optical properties of stacked InGaN quantum dot layers and demonstrate their advantage over single quantum dot layer structures. Measurements were performed on structures containing a single layer with quantum dots or threefold stacked quantum dot layers, respectively. A superlinear increase of the quantum dot related photoluminescence is detected with increasing number of quantum dot layers while other relevant GaN related spectral features are much less intensive when compared to the photoluminescence of a single quantum dot layer. The quantum dot character of the active material is verified by microphotoluminescence experiments at different temperatures. For the possible integration within optical devices in the future the threshold power density was investigated as well as the modal gain by using the variable stripe length method. As the threshold is 670 kW/cm2 at 13 K, the modal gain maximum is at 50 cm–1. In contrast to these limited total values, the modal gain per quantum dot is as high as 10–9cm–1, being comparable to the IIVI and III‐As compounds. These results are a promising first step towards bright low threshold InGaN quantum dot based light emitting devices in the near future (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MOCVD growth and optical study of InGaN quantum dots and their emitters on a high quality GaN layer grown using a high temperature AlN as buffer

AbstractThe optical properties have been investigated on InGaN quantum dots (QDs) with density up to 9× 1010/cm2 and excellent uniformity grown on a high quality GaN surface grown using high temperature AlN as a buffer layer on sapphire. A stimulated emission from the multiple layers of InGaN QDs has been observed under an optical pumping with a low threshold at room temperature indicating a superior optical quality. Moreover, InGaN QD based light emitting diodes (LEDs) with good performances have been grown and fabricated, and then the influence of the thermal annealing for p‐type GaN activation on the optical properties of the InGaN QD based LEDs has been studied. It has been found that the optimized annealing conditions for p‐type GaN activation can lead to significantly improved performance of the InGaN QD based LEDs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Incorporation of QD ensembles in separate confinement heterostructures for long wavelength emission

AbstractThis paper deals with the incorporation of temperature‐sensitive InGaN quantum dot (QD) ensembles into separate confinement double heterostructures grown on free‐standing GaN substrates. The laser diode (LD) structures were processed as ridge waveguide devices. The emission wavelength in electroluminescence (EL) of the 3 fold QD layers is around 485 nm.The structural quality and state of strain of the layers has been determined by high resolution X‐ray diffraction (HRXRD) measurements. Current‐voltage characteristics, light output power measurements and EL‐spectra demonstrate the good stability of the device. A comparable small blue shift was observed with increasing excitation current (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved capping layer growth towards increased stability of InGaN quantum dots

AbstractImprovements in GaN capping layer growth towards increased stability of InGaN quantum dots (QDs) will be presented. Surface quality is enhanced with changing the carrier gas during GaN capping layer growth from nitrogen to hydrogen. Without a dissolution of QDs it is possible to increase the temperature to 1050 °C applying a temperature ramp during growth of GaN.Dot stacks with an uniform and up to room temperature bright photoluminescence emission spectra is reported qualifying the structure to be incorporated into LED and laser structures. Finally, the evolution of the nucleation layer into flat and broad island‐like structures will be explained by the spinodal decomposition model (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth behaviour of InGaN/GaN self‐assembled quantum dots with different growth conditions in horizontal MOCVD

AbstractInGaN/GaN self‐assembled quantum dots (QDs) were grown on (0001) sapphire substrates by low pressure metalorganic chemical vapor deposition. The growth behavior of the InGaN/GaN QDs was examined under different experimental conditions. The InGaN/GaN QDs were formed in Stranski‐Krastanow growth mode by the continuous supply of metalorganic (MO) sources, whereas they were formed in the Volmer‐Weber growth mode by the periodic interruption of the MO sources. In addition, a quantum ring (QRs) structure was confirmed during the formation of InGaN QDs. The In content in the InGaN QDs appeared to increase with decreasing temperature. The photoluminescence spectrum (PL) showed peaks for the GaN buffer layer and the InGaN QDs layer at 362.2 nm (3.41 eV) and between 420–460 nm (2.96–2.70 eV), respectively. The composition of InGaN QDs was estimated from the PL peak position to be In0.12‐0.15Ga0.88‐0.85N. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of piezoelectric fields on excitonic complexes in InGaN quantum dots

AbstractWe present an analysis of the optical properties of single InGaN quantum dots (QDs) grown by MOVPE. The samples were structured into mesas by focused‐ion‐beam etching and investigated by micro‐photoluminescence measurements. The QDs are characterized by the high temperature stability of their emission up to 150 K. Furthermore, the polarization of individual QD emission lines was analyzed giving an insight into their geometrical shape. Time‐resolved microphotoluminescence measurements on the excitonic and biexcitonic transition of a single quantum dot yields a radiative recombination lifetime of 2.06 ns for the exciton. The data can be fitted by a simple model for cascaded emission confirming the expected refilling of the excitonic state by biexcitonic recombination. In addition, the influence of piezoelectric fields on the exciton and biexciton emission and on their binding energy in single QDs was investigated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Non‐linear excitation and correlation studies of single InGaN quantum dots

AbstractThe exact mechanism for non‐linear sub‐bandgap excitation of single InGaN quantum dots (QDs) has been investigated using interferometric autocorrelation techniques based around a near‐infrared (NIR) excitation laser. We identify unambiguously the heavily favoured two‐photon absorption (TPA) as being crucial in the observed background reduction for such Nitride structures and compare systematically the relative merits of both the linear (blue) and non‐linear (NIR) excitation regimes on several apertures of a masked sample. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stranski–Krastanov growth of InGaN quantum dots emitting in green spectra

Self-assembled InGaN quantum dots (QDs) were grown on GaN templates by metalorganic chemical vapor deposition. 2D–3D growth mode transition through Stranski–Krastanov mode was observed via atomic force microscopy. The critical thickness for In0.67Ga0.33N QDs was determined to be four monolayers. The effects of growth temperature, deposition thickness, and V/III ratio on QD formation were examined. The capping of InGaN QDs with GaN was analyzed. Optimized InGaN quantum dots emitted in green spectra at room temperature.

Practical enhancement of photoluminescence by metal nanoparticles

We develop a simple yet rigorous theory of the photoluminescence (PL) enhancement in the vicinity of metal nanoparticles. The enhancement takes place during both optical excitation and emission. The strong dependence on the nanoparticle size enables optimization for maximum PL efficiency. Using the example of InGaN quantum dots (QDs) positioned near Ag nanospheres embedded in GaN, we show that strong enhancement can be obtained only for those QDs, atoms, or molecules that are originally inefficient in absorbing as well as in emitting optical energy. We then discuss practical implications for sensor technology.

Optical and microstructural study of a single layer of InGaN quantum dots

Two typical kinds of InGaN quantum dots (QDs) have been grown on sapphires under different conditions through modifying the NH3 flow rate using metal-organic chemical vapor deposition: small spherical dots with a high dot density and large truncated pyramidal dots with a low dot density. The small dots have been found typically coherent and defect-free, while a strain relaxation has often been observed in the large dots. Consequently, this leads to a massive difference in optical properties between them. The optical properties have been investigated by means of temperature-dependent and excitation power-dependent microphotoluminescence measurements. It has been found that the small spherical QDs show higher optical quantum efficiency and much weaker piezoelectric field induced quantum-confined Stark effect than the large truncated QDs. Based on the energy balance between the strain and surface energy, the influence of V/III ratio on the transition from two-dimensional to three-dimensional growth mode during the QD growth has been discussed.

Electronic and optical properties of InGaN quantum dot based light emitters for solid state lighting

In this paper, we have made a systematic study of the electronic and optical properties of InGaN based quantum dot light emitters. The valence force field model and 6×6k⋅p method have been applied to study the band structures in InGaN or InN quantum dot devices. Piezoelectric and spontaneous polarization effects are included. A comparison with InGaN quantum wells shows that InGaN quantum dots can provide better electron-hole overlap and reduce radiative lifetime. We also find that variation in dot sizes can lead to emission spectrum that can cover the whole visible light range. For high carrier density injection conditions, a self-consistent method for solving quantum dot devices is applied for better estimation of device performance. Consequences of variations in dot sizes, shapes, and composition have been studied in this paper. The results suggest that InGaN quantum dots would have superior performance in white light emitters.

Study of the formation of InGaN quantum dots on GaN surface

The effect of growth temperature on the surface morphology of InGaN layers with quantum dots (QDs) on GaN surface has been studied. The optimal conditions for formation of a dense uniform InGaN/GaN QD array with lateral sizes of 20–30 nm, which is characterized by long-wavelength photoluminescence in the range of 420–470 nm, are found.