Self-assembled InGaN Quantum Dots Research Articles

Temperature dependence of optical study of InGaN QDs

Self-assembled InGaN quantum dots (QDs) were fabricated by metal-organic chemical vapor deposition. Abnormal temperature dependence of photoluminescence (PL) was observed. The integrated PL intensity of QDs sample shows a dramatic increase in a temperature range from 160 K to 215 K and reaches the maximum value at 215 K instead of 10 K as usual. To interpret this phenomenon, a theoretic model of temperature induced carrier redistribution mechanism is designed using rate equation, which fits closely with the experimental result. It is concluded that carriers’ redistribution from shallow QDs or wetting layer to deep QDs gives rise to the unique behavior for InGaN QDs structure.

InGaN quantum dots with short exciton lifetimes grown on polar c-plane by metal-organic chemical vapor deposition

An investigation of self-assembled polar InGaN quantum dots (QDs) on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD) is reported. The radiative exciton lifetime is measured by time-resolved photoluminescence at a low temperature of 18 K, where the non-radiative recombination can be negligible. A mono-exponential exciton decay with a radiative exciton lifetime of 480 ps for uncapped QDs is revealed. With an optimized GaN capping layer grown by a two-step method, a radiative exciton lifetime of 707 ps for the capped QDs is preserved. The short radiative exciton lifetime is much shorter than that for previously studied polar QDs and is even comparable with those grown along non-polar QDs, which is strong evidence of the reduction of built-in fields in these polar InGaN QDs.

Phosphor-free single chip GaN-based white light emitting diodes with a moderate color rendering index and significantly enhanced communications bandwidth

To achieve high quality lighting and visible light communication (VLC) simultaneously, GaN based white light emitting diodes (WLEDs) oriented for lighting in VLC has attracted great interest. However, the overall bandwidth of conventional phosphor converted WLEDs is limited by the long lifetime of phosphor, the slow Stokes transfer process, the resistance-capacitance (RC) time delay, and the quantum-confined Stark effect (QCSE). Here by adopting a self-assembled InGaN quantum dots (QDs) structure, we have fabricated phosphor-free single chip WLEDs with tunable correlated color temperature (CCT, from 1600 K to 6000 K), a broadband spectrum, a moderate color rendering index (CRI) of 75, and a significantly improved modulation bandwidth (maximum of 150 MHz) at a low current density of 72 A / cm 2 . The broadband spectrum and high modulation bandwidth are ascribed to the capture of carriers by different localized states of InGaN QDs with alleviative QCSE as compared to the traditional InGaN/GaN quantum well (QW) structures. We believe the approach reported in this work will find its potential application in GaN WLEDs and advance the development of semiconductor lighting-communication integration.

Evolution mechanism of InGaN quantum dots and their optical properties

InGaN self-assembled quantum dots (QDs) have been grown by metal-organic chemical vapor deposition. The surface morphologies of InGaN QDs were investigated for different number of periods in the range of one to five. Interfacial formation mechanism of InGaN QDs was investigated by phase separation theory and different strain in active zone. In-rich localized states and interfacial atomic images were observed by spherical aberration corrected transmission electron microscopy. Furthermore, the formation mechanism of InGaN QDs was discussed according to growth kinetics of indium (In) atom migration. The optical properties of QDs-like were tested by temperature-dependent photoluminescence. These results show internal quantum efficiency increases with increasing periodicity owing to increasing total accumulated strain. QDs are formed spontaneously owing to indium composition fluctuations in InGaN layer. The variations of peak energy with increasing temperature are explained by carrier dynamics as a result of size inhomogeneity and carrier localization in InGaN/GaN active zone. These results provide experimental data and theoretical basis for fabricating high quality InGaN QDs.

Ultrafast, Polarized, Single-Photon Emission from m-Plane InGaN Quantum Dots on GaN Nanowires.

We demonstrate single-photon emission from self-assembled m-plane InGaN quantum dots (QDs) embedded on the side-walls of GaN nanowires. A combination of electron microscopy, cathodoluminescence, time-resolved microphotoluminescence (μPL), and photon autocorrelation experiments give a thorough evaluation of the QD structural and optical properties. The QD exhibits antibunched emission up to 100 K, with a measured autocorrelation function of g(2)(0) = 0.28(0.03) at 5 K. Studies on a statistically significant number of QDs show that these m-plane QDs exhibit very fast radiative lifetimes (260 ± 55 ps) suggesting smaller internal fields than any of the previously reported c-plane and a-plane QDs. Moreover, the observed single photons are almost completely linearly polarized aligned perpendicular to the crystallographic c-axis with a degree of linear polarization of 0.84 ± 0.12. Such InGaN QDs incorporated in a nanowire system meet many of the requirements for implementation into quantum information systems and could potentially open the door to wholly new device concepts.

Broadband full-color monolithic InGaN light-emitting diodes by self-assembled InGaN quantum dots

We have presented broadband full-color monolithic InGaN light-emitting diodes (LEDs) by self-assembled InGaN quantum dots (QDs) using metal organic chemical vapor deposition (MOCVD). The electroluminescence spectra of the InGaN QDs LEDs are extremely broad span from 410 nm to 720 nm with a line-width of 164 nm, covering entire visible wavelength range. A color temperature of 3370 K and a color rendering index of 69.3 have been achieved. Temperature-dependent photoluminescence measurements reveal a strong carriers localization effect of the InGaN QDs layer by obvious blue-shift of emission peak from 50 K to 300 K. The broadband luminescence spectrum is believed to be attributed to the injected carriers captured by the different localized states of InGaN QDs with various sizes, shapes and indium compositions, leading to a full visible color emission. The successful realization of our broadband InGaN QDs LEDs provide a convenient and practical method for the fabrication of GaN-based monolithic full-color LEDs in wafer scale.

Strong localization effect and carrier relaxation dynamics in self-assembled InGaN quantum dots emitting in the green.

Strong localization effect in self-assembled InGaN quantum dots (QDs) grown by metalorganic chemical vapor deposition has been evidenced by temperature-dependent photoluminescence (PL) at different excitation power. The integrated emission intensity increases gradually in the range from 30 to 160 K and then decreases with a further increase in temperature at high excitation intensity, while this phenomenon disappeared at low excitation intensity. Under high excitation, about 40% emission enhancement at 160 K compared to that at low temperature, as well as a higher internal quantum efficiency (IQE) of 41.1%, was observed. A strong localization model is proposed to describe the possible processes of carrier transport, relaxation, and recombination. Using this model, the evolution of excitation-power-dependent emission intensity, shift of peak energy, and linewidth variation with elevating temperature is well explained. Finally, two-component decays of time-resolved PL (TRPL) with various excitation intensities are observed and analyzed with the biexponential model, which enables us to further understand the carrier relaxation dynamics in the InGaN QDs.

Temperature Dependence of Emission Properties of Self-Assembled InGaN Quantum Dots

Emission properties of self-assembled green-emitting InGaN quantum dots (QDs) grown on sapphire substrates by using metal organic chemical vapor deposition are studied by temperature-dependent photoluminescence (PL) measurements. As temperature increases (15–300 K), the PL peak energy shows an anomalous V-shaped (redshift—blueshift) variation instead of an S-shaped (redshift—blueshift—redshift) variation, as observed typically in green-emitting InGaN/GaN multi-quantum wells (MQWs). The PL full width at half maximum (FWHM) also shows a V-shaped (decrease—increase) variation. The temperature dependence of the PL peak energy and FWHM of QDs are well explained by a model similar to MQWs, in which carriers transferring in localized states play an important role, while the confinement energy of localized states in the QDs is significantly larger than that in MQWs. By analyzing the integrated PL intensity, the larger confinement energy of localized states in the QDs is estimated to be 105.9 meV, which is well explained by taking into account the band-gap shrinkage and carrier thermalization with temperature. It is also found that the nonradiative combination centers in QD samples are much less than those in QW samples with the same In content.

Recent progresses on InGaN quantum dot light-emitting diodes

InGaN quantum dots (QDs) have attracted many research interests in recent years for their potentials to realize long wavelength visible emission from green to red, which can pave a way to fabricate the phosphor-free white light emitting diodes (LEDs). In this paper, we reported our recent progresses on InGaN QD LEDs, the discussions were dedicated to the basic physics model of the strain relaxation in self-assembled InGaN QDs, the growth of InGaN QDs with a growth interruption method by metal organic vapor phase epitaxy, the optimization of GaN barrier growth in multilayer InGaN QDs, the demonstration of green, yellow-green and red InGaN QD LEDs, and future challenges.

Growth of self-assembled InGaN quantum dots on Si (111) at reduced temperature by molecular beam epitaxy

Self-assembled InGaN quantum dots (QDs) were grown by RF plasma-assisted molecular beam epitaxy on Si (111). The growth of InGaN QDs has been investigated in situ by reflection high energy electron diffraction. It is found that a low-temperature growth of InGaN instantaneously leads to the formation of three-dimensional islands. Field emission scanning electron microscopy and atomic force microscopy revealed that diameters were small enough to expect a zero-dimensional quantum effect which is around 20nm and the height of the dots was around 6.0nm. X-ray diffraction (XRD) measurement indicated that the InGaN was epitaxially grown on silicon substrate. Photoluminescence measurement exhibits a sharp and intense band edge emission of InGaN at 1.7 eV. There was an excellent agreement with the result of mole fraction measured from XRD by employing Vegard's law which is In molar fraction is 0.5.

Origins of Spectral Diffusion in the Micro-Photoluminescence of Single InGaN Quantum Dots

We report on optical characterization of self-assembled InGaN quantum dots (QDs) grown on three GaN pseudo-substrates with differing threading dislocation densities. QD density is estimated via microphotoluminscence on a masked sample patterned with circular apertures, and appears to increase with dislocation density. A non-linear excitation technique is used to observe the sharp spectral lines characteristic of QD emission. Temporal variations of the wavelength of emission from single QDs are observed and attributed to spectral diffusion. The magnitude of these temporal variations is seen to increase with dislocation density, suggesting locally fluctuating electric fields due to charges captured by dislocations are responsible for the spectral diffusion in this system.

Anomalous temperature dependence of photoluminescence in self-assembled InGaN quantum dots

Self-assembled InGaN quantum dots (QDs) were fabricated by metal-organic chemical vapor deposition. Abnormal temperature dependence of photoluminescence (PL) was observed. The integrated PL intensity of QDs sample shows a dramatic increase in a temperature range from 160 K to 215 K and reaches the maximum value at 215 K instead of 10 K as usual. To interpret this phenomenon, a theoretic model of temperature induced carrier redistribution mechanism is designed using rate equation, which fits closely with the experimental result. It is concluded that carriers’ redistribution from shallow QDs or wetting layer to deep QDs gives rise to the unique behavior for InGaN QDs structure.

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.

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 the GaN barrier and growth temperature for the GaN barrier on InGaN QDs has been investigated, in combination with optical pumping measurements. Our results indicate that both the thickness of the GaN barrier and growth temperature for the GaN barrier should be carefully chosen. Otherwise, either the formation of the InGaN QDs can be prevented or the InGaN QDs that have been formed can be destroyed.

Fabrication and Characterization of InxGa1-xN Quantum Dots Using Nitridation of Nano-alloyed Droplet Growth Technique

We have studied the InGaN quantum dots (QDs) grown on a GaN pseudo template on a sapphire substrate (0001) using metal organic chemical vapor deposition (MOCVD). Self assembled InGaN QDs was successfully fabricated using nitridation of nano alloyed droplet (NNAD) method. The density, average diameter, and height of the InGaN QDs were estimated to be 1.0×108 cm-2, 50 nm, and 13 nm, respectively. The structural and optical properties of InGaN QDs were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), and photoluminescence (PL). From the SEM image, it is clearly seen that In+Ga alloy droplets are uniformly distributed on GaN template. Further, InGaN QDs uniform distribution on the GaN/sapphire surface was observed by AFM. Broad emission from the QDs was observed around 580 nm which indicate the grown QDs have relatively high In composition due to low growth temperature. The broad and separated PL peak of QDs induced by the fluctuation in size and composition. Our result suggests that self-assembled InGaN QDs are fabricated well by using NNAD technique.

Fabrication and characterization of self-assembled InGaN quantum dots by periodic interruption growth

A trial was carried out to fabricate self-assembled InGaN QD structures through periodic interruption growth, which is new method for supplying the source materials in a metal organic chemical vapor deposition (MOCVD) reactor. The growth of InGaN QDs was interrupted periodically by stopping the ammonia gas for selected time periods during periodic interruption growth. The change in surface morphology was investigated during the progress of InGaN QD growth. In addi- tion, the morphological and optical characteristics were compared with respect to the period time of 3 or 5 seconds for growth and interruption. InGaN QDs of 25–35 nm in lateral size were grown on GaN surfaces with a density of approximately 4–15 × 1010 cm–2. The composition of QDs was estimated to be In0.14Ga0.86N. Periodic interruption growth enables the fabrication of self-assembled InGaN QDs with high density and uniform-size. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-assembled InGaN quantum dots on GaN emitting at 520nm grown by metalorganic vapor-phase epitaxy

Abstract Self-assembled InGaN quantum dots (QDs) have been grown using metalorganic vapor-phase epitaxy (MOVPE), without using anti-surfactant. Using 120 s annealing, InGaN QDs have been successfully formed with a circular base diameter of 40 nm and an average height of 4 nm, with QDs density of 4×10 9 cm −2 . The InGaN QDs have peak photoluminescence (PL) wavelengths of 519 and 509 nm for samples without and with a GaN upper barrier, respectively. The full-width half-maximum (FWHM) of the PL spectra ranges from 56.6 up to 69.6 nm. These results demonstrates that high In-content InGaN QDs can be grown by MOVPE, and can potentially be utilized as the active media for light-emitting diodes (LEDs) and semiconductor laser diodes for green emission.

Green light-emitting diodes with self-assembled In-rich InGaN quantum dots

A green light-emitting diode (LED) was fabricated using self-assembled In-rich InGaN quantum dots (QDs). The photoluminescence studies showed that the QDs provide thermally stable deeply localized recombination sites for carriers with negligibly small piezoelectric field. The electroluminescence spectra of the LED showed a peak in the green spectral range and the dominant peak was blueshifted with increasing injection current due to the distribution of depth of the potential wells of QDs. The output power of the LED increased with increasing injection current, indicating that the potential wells are thermally stable and deeply localized in the QDs.

Characteristics of III-nitride MSM photodiodes with SAQDs

A novel metal-semiconductor-metal photodiode with InGaN self-assembled quantum dots was fabricated and compared with a conventional InGaN metal-semiconductor-metal photodiode. It was found that the InGaN QD photodiode with lower dark current can operate in the normal incidence mode. We achieved a much larger photocurrent to dark current contrast ratio from the fabricated photodiodes with nanoscale InGaN self-assembled quantum dots. The scanning near-field optical microscopy results revealed that the devices made by InGaN nanostructures could have better absorption for the near-field light with the wavelength of 457–514 nm.

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.