Self-assembled GaN Quantum Dots Research Articles

Coulomb blockade: Toward charge control of self-assembled GaN quantum dots at room temperature

We present capacitance–voltage [C(V)] measurements of self-assembled wurtzite-GaN quantum dots (QDs). The QDs are embedded in a charge-tunable diode structure and were grown by molecular beam epitaxy in the Stranski–Krastanov growth method. The internal electric fields present in GaN and its alloys together with its wide bandgap make this material system an ideal candidate for high-temperature quantum applications. Charges and the internal electric fields influence the energy spacing in the QDs. We correlate photoluminescence measurements with C(V) measurements and show single-electron charging of the QDs and a Coulomb blockade energy of around 60 meV at room temperature. This finding demonstrates the possibility of quantum applications at room temperature.

Formation of GaN quantum dots by molecular beam epitaxy using NH3 as nitrogen source

Self-assembled GaN quantum dots (QDs) in AlxGa1−xN (0.3 ≤ x ≤ 1) were grown on c-plane sapphire and Si (111) substrates by molecular beam epitaxy using ammonia as nitrogen source. The QD formation temperature was varied from 650 °C to 800 °C. Surprisingly, the density and size of QDs formed in this temperature range are very similar. This has been explained by considering together experimental results obtained from reflection high-energy electron diffraction, atomic force microscopy, and photoluminescence to discuss the interplay between thermodynamics and kinetics in the QD formation mechanisms. Finally, possible ways to better control the QD optical properties are proposed.

Strain- and surface-induced modification of photoluminescence from self-assembled GaN/Al0.5Ga0.5N quantum dots: strong effect of capping layer and atmospheric condition

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al0.5Ga0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al0.5Ga0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen.

AlGaN-Based Light Emitting Diodes Using Self-Assembled GaN Quantum Dots for Ultraviolet Emission

Self-assembled GaN quantum dots (QDs) grown on Al0.5Ga0.5N have been used as the active region of light emitting diodes (LEDs). The LED emission wavelength exhibits a strong shift towards higher energies with increasing current density, which allows obtaining an emission in the UV range (down to 375 nm) above 100 A/cm2. Together with this shift, a reduction of the electroluminescence (EL) peak full width at half maximum (FWHM) is observed. These features are a consequence of the quantum confined Stark effect caused by the built-in electric field in the heterostructure. At larger current densities, an opposite behavior (i.e., an increase of the FWHM) is observed concomitant with the appearance of an additional peak on the EL high energy side. This characteristic has been confronted with calculations and attributed to a transition between the lowest electron state and the first excited hole state in the QDs.

Effect of Eu-implantation and annealing on the GaN quantum dots excitonic recombination

Undoped self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer layers were submitted to thermal annealing treatments. Changes in the balance between the quantum confinement, strain state of the stacked heterostructures and quantum confined Stark effect lead to the observation of GaN QD excitonic recombination above and below the bulk GaN bandgap. In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence. For samples implanted with high fluence, a broad emission band at 2.7 eV was tentatively assigned to the emission of large blurred GaN QD present in the damage region of the implanted SL. This emission band is absent in the SL structures implanted with lower fluence and hence lower defect level. In both cases, high energy emission bands at approx. 3.9 eV suggest the presence of smaller dots for which the photoluminescence intensity was seen to be constant with increasing temperatures. Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent. Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.

Morphology and strain of self-assembled semipolar GaN quantum dots in (112¯2) AlN

GaN quantum dots (QDs) grown in semipolar (112¯2) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques. The embedded (112¯2)-grown QDs exhibited pyramidal or truncated-pyramidal morphology consistent with the symmetry of the nucleating plane, and were delimited by nonpolar and semipolar nanofacets. It was also found that, in addition to the (112¯2) surface, QDs nucleated at depressions comprising {101¯1} facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (112¯2). Based on quantitative high-resolution TEM strain measurements, the three-dimensional QD strain state was analyzed using finite-element simulations. The internal electrostatic field was then estimated, showing small potential drop along the growth direction, and limited localization at most QD interfaces.

Functionalizing self-assembled GaN quantum dot superlattices by Eu-implantation

Self-assembled GaN quantum dots (QDs) stacked in superlattices (SL) with AlN spacer layers were implanted with Europium ions to fluences of 1013, 1014, and 1015 cm−2. The damage level introduced in the QDs by the implantation stays well below that of thick GaN epilayers. For the lowest fluence, the structural properties remain unchanged after implantation and annealing while for higher fluences the implantation damage causes an expansion of the SL in the [0001] direction which increases with implantation fluence and is only partly reversed after thermal annealing at 1000 °C. Nevertheless, in all cases, the SL quality remains very good after implantation and annealing with Eu ions incorporated preferentially into near-substitutional cation sites. Eu3+ optical activation is achieved after annealing in all samples. In the sample implanted with the lowest fluence, the Eu3+ emission arises mainly from Eu incorporated inside the QDs while for the higher fluences only the emission from Eu inside the AlN-buffer, capping, and spacer layers is observed.

Phonon modes in self-assembled GaN quantum dots

Phonons in GaN quantum dots (QDs) fabricated by Stranski–Krastanov growth are analyzed using Raman scattering and continuum models of phonon confinement. The QD samples are evaluated by Raman spectroscopy with four different excitation energies. Each excitation energy has a different penetration depth, and this makes it possible to distinguish Raman signals associated with the QDs from any other signals. In addition, the phonon frequency shift in GaN QDs are analyzed using theoretical models of the confined geometry and the internal stress, the calculated results are compared to the measured results.

Coulomb blockade in a self-assembled GaN quantum dot

The authors report on transport phenomena in single self-assembled GaN quantum dots using metallic leads with a nanoscale gap. The nitride device works as a single electron transistor. Measurement of the stability diagram at 12K shows Coulomb blockade regions with a single electron charging energy of about 10meV.

Raman study and theoretical calculations of strain in GaN quantum dot multilayers

Changes in strain and phonon mode energy in stacks of self-assembled GaN quantum dots embedded in AlN have been studied by means of Raman spectroscopy as a function of the number of periods. The ${E}_{2H}$ phonon modes related to the quantum dots and AlN spacers are clearly resolved, and their energies allow monitoring the state of strain of the dots and AlN spacers simultaneously. The evolution of the measured phonon frequencies and the associated strains are discussed in comparison with theoretical calculations of the inhomogeneous strain distribution in a system of coherent misfitting inclusions.

Reduction of the internal electric field in wurtzite a-plane GaN self-assembled quantum dots

We present a study of the emission of GaN∕AlN self-assembled quantum dots grown on a-plane 6H-SiC showing evidence of the suppression of the internal electric field. The strain in dots and barriers is determined by means of Raman scattering and the induced piezoelectric polarizations are estimated. These reveal a compensation of the spontaneous polarization and justify the lack of a quantum confined Stark effect found in the photoluminescence spectra. Strain effects and strong confinement are responsible for the partial depolarization of the emission and its energy dependence.

Photoluminescence of GaN quantum dots in AlN matrix

In the present work steady-state and time-resolved photoluminescence (PL) from self-assembled GaN quantum dots (QDs) embedded in AlN matrix has been studied. The maximum of low-temperature PL is near 2.55 eV that is below the band gap of bulk GaN. The PL peak is not shifted with variation of excitation power. When temperature is increasing the PL peak exhibits a red shift in accordance with a temperature dependence of the band gap of bulk GaN. A huge red shift of the PL maximum from 3.0 down to 2.2 eV is observed as a function of the delay time after the excitation pulse. The PL decay is non-exponential and spectral dependent. The PL decay time varies from about 10 μs to more than 100 μs over the spectrum. The experimental results interpreted in framework of a model taking into account an internal electric field in QDs and transport of the non-equilibrium carriers between QDs.

GaN quantum dots: from basic understanding to unique applications

The GaN self-assembled quantum dots constitute a very special and intriguing type of semiconductor nanostructure, mainly because they carry in their structure a giant internal electric field that can reach a value up to 7 MV/cm. In this report, we review the most important structural and optical properties of GaN quantum dots, and we discuss their advantages and limitations for blue-UV optoelectronic applications.

Formation of high-density GaN self-assembled quantum dots by MOCVD

We have successfully obtained GaN self-assembled quantum dots (QDs) with high density (>2×10 11 cm −2) by the Stranski–Krastanow growth mode in metalorganic chemical vapor deposition. The density of the QDs can be controlled between 10 8 and 10 11 cm −2 by changing an amount of GaN nominally deposited. The photoluminescence peaks from both the QDs and the wetting layer are clearly observed even in uncapped QD structures at room temperature.

UV photoluminescence from GaN self‐assembled quantum dots on Al x Ga 1– x N surfaces grown by metalorganic chemical vapor deposition

We have successfully grown GaN self-assembled quantum dots (QDs) with high density on an AlxGa1–xN surface, by the Stranski-Krastanow growth mode in a metalorganic chemical vapour deposition. An abrupt transition from two-dimensional to three-dimensional growth mode is observed by depositing over the critical thickness of GaN. The density of the QDs can be changed between 109 and 1010 cm–2. Furthermore, intense photoluminescence (PL) peaks from both the QDs and the wetting layer are clearly observed even in the uncapped QD structures at room temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation and optical properties of stacked GaN self-assembled quantum dots grown by metalorganic chemical vapor deposition

Multiple-layer stacked GaN quantum dots (QDs) with intense photoluminescence (PL) have been successfully grown by the Stranski-Krastanow growth mode in metalorganic chemical vapor deposition. Transmission electron microscopy (TEM) and scanning TEM analyses showed vertically aligned QDs resulting from a strain field induced by the buried islands. A redshift and linewidth narrowing of the PL spectra for the multilayered structures indicate that the size of the QDs becomes larger and more uniform throughout the layering process. The PL intensity increased with increasing the number of stacked layers, indicating that the QDs can be stacked without introducing new nonradiative recombination centers.

Direct comparison of recombination dynamics in cubic and hexagonal GaN/AlN quantum dots

We report on time-integrated and -resolved photoluminescence (PL) data on self-assembled GaN quantum dots (QD's) embedded in AlN, in both cubic [zinc-blende (ZB)] and hexagonal [wurtzite (Wz)] crystallographic phases. The comparison of the optical properties of ZB and Wz nitride QD's allows us to distinguish pure dimensionality effects from the influence of the large polarization-induced electric fields present in the Wz nanostructures. Specifically, the PL energy of the ZB QD's is always higher than the bulk cubic GaN band-gap energy, in contrast to the Wz QD's where a 7-MV/cm polarization field gives rise to below-gap PL emission for sufficiently large QD's. As a further consequence of the internal field, the low-temperature PL decay times of the Wz QD's are significantly longer than the ZB ones, and increase strongly with the QD height in contrast to the ZB ones, which exhibit only a small size dependence. For both types of QD's, the PL intensity is found to be weakly dependent on temperature, underscoring the strong zero-dimensional exciton localization in the GaN/AlN system. In spite of the strong localization, however, we show that the nonradiative channels cannot be neglected and have a significant contribution in the PL decay time for both ZB and Wz QD's.

Optical properties of hexagonal and cubic GaN self‐assembled quantum dots

Optical characteristics of hexagonal GaN (h-GaN) and cubic GaN (c-GaN) self-assembled quantum dots (QDs) were investigated by means of photoluminescence (PL), PL excitation, cathodoluminescence (CL), and time-resolved PL experiments. We observed a Stokes-like shift between PL excitation absorption edge and PL emission from the h-GaN QDs as well as Al(Ga)N base layer. With increasing temperature, the PL intensity of Al(Ga)N base layer or GaN quantum wells was dramatically decreased while that of GaN QDs was not changed much. We also observed wavelength-resolved CL images reflecting strong carrier localization in the QD confinement. From the time-resolved PL experiments, we found that the measured lifetime of the h-GaN QDs emission increased with decreasing emission energy (i.e., increasing QD size), while that of the c-GaN QDs kept almost constant. Therefore, we conclude that the h-GaN QD emissions are strongly influenced by built-in interal electric field as well as carrier localization in the QDs.

Effect of thermal treatment on structure of GaN self‐assembled quantum dots grown by MOCVD

We report the effect of thermal annealing on the formation of GaN self-assembled quantum dots (QDs) grown by low-pressure metalorganic chemical vapor deposition (MOCVD). Atomic force microscopy (AFM) analysis shows that the density of the QDs rapidly increases and then decreases with increasing the growth interruption time. This is due to the combined effect of the QD formation process and the ripening process. The flow rate of NH3 during the growth interruption also strongly influences the formation of the QDs. These results show that the growth interruption at growth temperature plays an important role in the formation of the QDs.

Enhanced radiative efficiency in gan quantum dots grown by molecular beam epitaxy

Self-assembled GaN quantum dots (QDs), grown on AlN by molecular beam epitaxy, were investigated by time-resolved photoluminescence spectroscopy. We investigate the emission mechanism in GaN QDs by comparing the carrier recombination dynamics in single and multiple period QDs. At 100 K, the PL decay time in single period QD structures is considerably shorter than in stacked QDs. Compared to single period QDs, the room temperature PL efficiency is considerably enhanced in 20 period QDs due to the reduction in nonradiative recombination processes.