III-nitride Quantum Dots Research Articles

Highly Uniform Array of Hexagonally Symmetric Micro‐Pyramid Structures for Scalable and Single Quantum Dot Emitters (Adv. Mater. Interfaces 8/2023)

Uniform Single Quantum Dot Emitters Controlling the position, size and shape of group III-nitride quantum dots is critical for the development of scalable single quantum emitters operable at room temperature. In article number 2202085, Yong-Hoon Cho and colleagues present a methodology for highly uniform, site-controlled GaN micro-pyramidal structures with a high degree of hexagonal symmetry by adjusting growth conditions under the self-limited growth mechanism. By successively growing a thin InGaN layer on the uniform pyramidal arrays with growth rate anisotropy, high-performance single photon emission from the apex of each pyramid can be achieved with suppressed background emission from side quantum wells of the pyramids.

Impact of Quantum Dots on III-Nitride Lasers: A Theoretical Calculation on Linewidth Enhancement Factors

III-nitride quantum dot (QD) lasers have potential applications in visible light regime, yet one of its most important dynamic parameters, the linewidth enhancement factor (LEF), has been rarely studied. We studied the LEF of InGaN QD lasers with a comprehensive theory model and found that the LEF values of InGaN QD lasers can be smaller and more robust against inhomogeneous broadening than those of InGaAs QD lasers. In addition, the LEF improvement from quantum well to QD in III-nitride semiconductors is much larger than that in III-arsenide. It is believed that the large confinement energy in III-nitrides and atomic-like density of states of QDs contribute to these advantages of III-nitride QD lasers. Our results not only reveal the advantages of using III-nitride QD for high spectral purity, low-chirp laser applications, but also provide a general guideline for designing QD lasers with lower LEF values in various semiconductor materials.

Control of the 3-Fold Symmetric Shape of Group III-Nitride Quantum Dots: Suppression of Fine-Structure Splitting.

Controlling the in-plane symmetry of wide-bandgap semiconductor quantum dots (QDs) is essential for room temperature quantum photonic applications using polarization entangled photon pairs. Herein, we report the formation of 3-fold symmetric group III-nitride QDs at the apex of a triangular pyramid via a self-limited growth mechanism. We employed the in-plane rotational symmetry of the c-plane of a Wurtzite crystal and the large built-in piezoelectric field to reduce fine-structure splitting. The QDs exhibit emission that is distinguishable from that of sidewall quantum wells, and the biexciton-exciton cascade possesses a single-photon nature. We observed the relatively low optical polarization anisotropy and small fine structure splitting under the measurement limit (270 μeV) with the 3-fold symmetric QD. In contrast with current strategies that consider group III-nitride QDs as strongly polarized single-photon emitters, our approach for controlling the QD symmetry provides a new perspective on such QDs, as polarization-entangled photon pairs.

Non-Polar Wurtzite (1120) GaN/AlN Quantum Dots for Highly Efficient Opto-Electronic Devices

In III-nitride quantum dots (QDs), optical transition rate is very low because of the large built-in electrostatic field caused by the spontaneous polarization (SP) and piezoelectric (PZ) effects. In this work, we study the screening potential which is a solution of the self-consistent Hartree equation taking into account the built-in electrostatic field and its effect on light emission characteristics of non-polar wurtzite (WZ) (112¯0) GaN/AlN QD. It is found that the light emission intensity of the non-polar (112¯0) GaN/AlN QD structure is expected to be about four times larger than that of the c-plane (0001) GaN/AlN QD structure because the y-polarized matrix elements in the non-polar QD are larger than that in the c-plane QD. These predictions indicate that non-polar GaN/AlN QD structure have strong potential for highly efficient opto-electronic devices.

Single photon emission from top-down etched III-nitride quantum dots

We report the single photon emission properties of III-nitride quantum dots (QDs) fabricated by electrochemical etching method from an epitaxial wafer. Through such top-down fabrication, QDs with diameters of sub-10 nm are obtained, embedded in GaN nanoneedles. Owing to the size induced quantum confinement effect, the photoluminescence of the QDs exhibits a 3.35 nm blueshift compared with that of the epitaxial wafer. At low temperature, a second order correlation value down to 0.123 is observed, indicating a high-purity single photon emission. Our QDs manifest single photon emission at a temperature up to 130 K with a high degree of polarization of 0.69, comparable to those QDs synthesized by epitaxial growth. Our work demonstrates single photon emission are viable in top-down QDs by electrochemical etching III-nitride wafers.

Impact of quantum dots on III-nitride lasers: a theoretical calculation of threshold current densities

We present a theoretical study on the gain and threshold current density of III-nitride quantum dot (QD) and quantum well (QW) lasers with a comprehensive theory model. It is found that at transparency condition the injection current density of QD lasers is about 120 times lower than QW lasers in III-nitrides, while in III-arsenide it is about 15 times. It means that using QDs in III-nitride lasers could be 8 times more efficient than in III-arsenide. This significant improvement in III-nitrides is due to their large effective-masses and the large asymmetry of effective-masses between valence bands and conduction bands. Our results reveal the advantages of using QD for low threshold laser applications in III-nitrides.

Red and Near-Infrared III-Nitride Quantum Dot Lasers

InGaN/GaN self-organized quantum dots and similar dots in GaN nanowires are formed by strain relaxation and the luminescence from these nanostructures extends to longer wavelengths than generally obtained with quantum wells. We have exploited this advantage by incorporating these nanostructures in the gain region of edge-emitting diode lasers. Here, we describe the characteristics of 650 nm self-organized quantum dot lasers epitaxially grown on GaN and 1.3 μm dot-in-nanowire array lasers grown on (001) Si by molecular beam epitaxy. The devices are characterized by relatively low threshold currents, excellent temperature stability (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> > 200 K), and differential gain ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The highest measured small-signal modulation bandwidth of the dot-in-nanowire laser is 3.3 GHz. The experiments have been complemented by theoretical modeling of the self-organized quantum dots and lasers made with them. These new classes of devices open up new opportunities in applications such as displays, optical data storage, heads-up displays in automobiles, plastic fiber communication, and silicon photonics.

Ultraclean Single Photon Emission from a GaN Quantum Dot.

Wide bandgap III-nitride quantum dots (QDs) are promising materials for the realization of solid-state single-photon sources, especially operating at room temperature. However, so far a large degree of inhomogeneous broadening induced by spectral diffusion has compromised their use. Here, we demonstrate the ultraclean emission from single GaN QDs formed at macrostep edges in a GaN/AlGaN quantum well. As a likely consequence of the high growth temperature and hence a reduced defect density, spectral diffusion is heavily suppressed to levels at least 1 order of magnitude lower than conventional GaN QDs. A record narrow line width of as small as 87 μeV is obtained, while the low inhomogeneous broadening enables us to assess an upper limit of homogeneous broadening in the QDs (27 μeV). Furthermore, the uncontaminated emission facilitates the generation of ultraviolet single-photons with unprecedented purity (g(2)(0) = 0.02). The realization of high-quality GaN QDs will enable exploration of optoelectronic properties of III-nitrides, opening up the possibility of realizing single-photon quantum information systems operating at room temperature.

Charge-tunable indium gallium nitride quantum dots

III-Nitride quantum dots have emerged as a new chip-scale system for quantum information science, which combines electrical and optical interfaces on a semiconductor chip that is compatible with non-cryogenic operating temperatures. Yet most work has been limited to optical excitations. To enable single-spin based quantum optical and quantum information research, we demonstrate here quantized charging in optically active, site-controlled III-Nitride quantum dots. Single-electron charging was confirmed by the voltage dependence of the energy, dipole moment, fine structures and polarization properties of the exciton states in the quantum dots. The fundamental energy structures of the quantum dots were identified, including neutral and charged excitons, fine structures of excitons, and A and B excitons. The results lay the ground for coherent control of single charges in III-Nitride QDs, opening a door to III-Nitride based spintronics and spin-qubit quantum information processing.

Nano-fabrication and related optical properties of InGaN/GaN nanopillars

Nanopillars with diameters down to 20 nm were fabricated from InGaN/GaN multiple quantum wells (MQWs) by using a nanosphere–SiO2 double mask and inductively coupled plasma (ICP) etching. Clear photoluminescence (PL) signals of the nanopillars were observed at room temperature, and the PL peak energy at 20 K showed a large blueshift of 220 meV compared with that of the original MQWs. The exciton activation energy in the temperature range of 100 ∼ 300 K increased from 33 meV for MQWs to 83 meV for nanopillars. According to the measurements and numerical simulation results, the strain relaxation effect is believed to play a dominant role rather than the quantum confinement effect in determining the emission wavelength of nanopillars. This work also demonstrates a promising method for obtaining III-nitride quantum dots.

Room-Temperature Triggered Single Photon Emission from a III-Nitride Site-Controlled Nanowire Quantum Dot

We demonstrate triggered single photon emission at room temperature from a site-controlled III-nitride quantum dot embedded in a nanowire. Moreover, we reveal a remarkable temperature insensitivity of the single photon statistics, and a g((2))[0] value at 300 K of just 0.13. The combination of using high-quality, small, site-controlled quantum dots with a wide-bandgap material system is crucial for providing both sufficient exciton confinement and an emission spectrum with minimal contamination in order to enable room temperature operation. Arrays of such single photon emitters will be useful for room-temperature quantum information processing applications such as on-chip quantum communication.

III-nitride semiconductors for intersubband optoelectronics: a review

III-nitride nanostructures have recently emerged as promising materials for new intersubband (ISB) devices in a wide variety of applications. These ISB technologies rely on infrared optical transitions between quantum-confined electronic states in the conduction band of GaN/Al(Ga)N nanostructures, namely quantum wells or quantum dots. The large conduction band offset (about 1.8 eV for GaN/AlN) and sub-picosecond ISB relaxation of III-nitrides render them appealing materials for ultrafast photonic devices in near-infrared telecommunication networks. Furthermore, the large energy of GaN longitudinal-optical phonons (92 meV) opens prospects for high-temperature THz quantum cascade lasers and ISB devices covering the 5–10 THz band, inaccessible to As-based technologies due to phonon absorption. In this paper, we describe the basic features of ISB transitions in III-nitride quantum wells and quantum dots, in terms of theoretical calculations, material growth, spectroscopy, resonant transport phenomena, and device implementation. The latest results in the fabrication of control-by-design devices such as all-optical switches, electro-optical modulators, photodetectors, and lasers are also presented.

Current–voltage characteristics in III-nitride quantum dot heterostructure based high electron mobility transistors

This paper proposes an accurate model to investigate the current–voltage characteristics of AlGaN/GaN high electron mobility transistors in presence of InN quantum dots. This work has been done for different InN quantum dot size and different distance between quantum dots. The presence of InN quantum dots will approximately cause a Gaussian peak in the linear regime of the current–voltage characteristics. So, applying this model and using the I–V characteristics one can find the position or size of quantum dots in the structure.

Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities (∼1010 cm−3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength.

High performances III-Nitride Quantum Dot infrared photodetector operating at room temperature

In this paper we present a novel long wave length infrared quantum dot photodetector. A cubic shaped 6nm GaN quantum dot (QD) within a large 18 nm Al(0.2)Ga(0.8)N QD (capping layer) embedded in Al(0.8)Ga(0.2)N has been considered as the unit cell of the active layer of the device. Single band effective mass approximation has been applied in order to calculate the QD electronic structure. The temperature dependent behavior of the responsivity and dark current were presented and discussed for different applied electric fields. The capping layer has been proposed to improve upon the dark current of the detector. The proposed device has demonstrated exceptionally low dark current, therefore low noise, and high detectivity. Excellent specific detectivity (D*) up to approximately 3 x 10(8)CmHz(1/ 2)/W is achieved at room temperature.

Plane-wave implementation of the real-space [formula omitted] formalism and continuum elasticity theory

In this work we demonstrate how second-order continuum elasticity theory and an eight-band k ⋅ p model can be implemented in an existing density functional theory (DFT) plane-wave code. The plane-wave formulation of these two formalisms allows for an accurate and efficient description of elastic and electronic properties of semiconductor nanostructures such as quantum dots, wires, and films. Gradient operators that are computationally expensive in a real-space formulation can be calculated much more efficiently in reciprocal space. The accuracy can be directly controlled by the plane-wave cutoff. Furthermore, minimization schemes typically available in plane-wave DFT codes can be applied straightforwardly with only a few modifications to a plane-wave formulation of these continuum models. As an example, the elastic and electronic properties of a III-nitride quantum dot system are calculated.

Matter Coupling to Strong Electromagnetic Fields in Two-Level Quantum Systems with Broken Inversion Symmetry

We demonstrate theoretically the parametric oscillator behavior of a two-level quantum system with broken inversion symmetry exposed to a strong electromagnetic field. A multitude of resonance frequencies and additional harmonics in the scattered light spectrum as well as an altered Rabi frequency are predicted to be inherent to such systems. In particular, dipole radiation at the Rabi frequency appears to be possible. Since the Rabi frequency is controlled by the strength of the coupling electromagnetic field, the effect can serve for the frequency-tuned parametric amplification and generation of electromagnetic waves. Manifestation of the effect is discussed for III-nitride quantum dots with strong built-in electric field breaking the inversion symmetry. Terahertz emission from arrays of such quantum dots is shown to be experimentally observable.

Growth of III‐nitride quantum dots and their applications to blue‐green LEDs

AbstractIn this paper we discuss the growth of InN, GaN and InGaN QDs by MBE on either GaN or AlN templates. InN QDs on GaN templates were found to occur without an InN wetting layer, a result consistent with the large lattice mismatch of 11% between InN and GaN. Self‐assembled GaN QDs were grown on AlN templates, using the modified Stranski‐Krastanov mode of growth. The microstructure and the size distribution of such QDs in a single layer or a superlattice structure were investigated by electron microscopy and atomic force microscopy. Finally, the self assembly of InGaN QDs on GaN templates using the Stranski‐Krastanov mode and the applications of such QDs to blue‐green LEDs are addressed. The results indicate that InGaN / GaN multiple quantum dots (MQDs) are highly strained and their emission at low injection is red shifted with respect to that of a single layer of QDs due to quantum confined Stark effect. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Strong carrier confinement and negligible piezoelectric effect in InGaN/GaN quantum dots

We report photoluminescence and time-resolved photoluminescence experiments on In x Ga 1− x N/GaN quantum dots grown by plasma-assisted molecular beam epitaxy. Single dot spectroscopy was used to prove the quantum dot origin of the luminescence. In addition, we show that both the photoluminescence intensity and the carrier recombination time remain constant upto 200 K, reflecting the strong confinement of the carriers. We also found that this strong confinement prevents the carriers from being affected by the huge internal electric field, which are generally reported in wurtzite-structured III-nitride quantum dots.

Growth Structure, and Optical Properties of III-Nitride Quantum Dots

ABSTRACTQuasi-zero-dimensional (0D) semiconductors have been the subject of considerable interest which is stemmed from their unique physical properties which in turn are conducive to devices such as low threshold lasers and light polarization insensitive detectors, in addition to exciting basic physical phenomena. A laboratory analogue of 0D systems is semiconductor quantum dots (QDs) wherein the electronic states are spatially localized and the energy is fully quantized, loosely similar to an atomic system, making it more stable against thermal perturbations. In addition, the electronic density of states near the band gap is higher than in 3D and 2D systems, leading to a higher probability for optical transitions. Furthermore, the electron localization may dramatically reduce the scattering of electrons by bulk defects and reduce the rate of non-radiative recombination. Semiconductor based and metal based dots have been produced, the former via self-assembly and also by lithographic methods in many II-VI, III-V, and group IV semiconductor. The aim of this paper is focused on III-Nitride based quantum dots covering their production and optical properties, as well as reporting on the GaN quantum dots produced by molecular beam epitaxy utilizing standard, ripening, metal spray followed by nitridation methods.