The spin relaxation time $T_{1}$ in zinc blende GaN quantum dot is investigated for different magnetic field, well width and quantum dot diameter. The spin relaxation caused by the two most important spin relaxation mechanisms in zinc blende semiconductor quantum dots, {i.e.} the electron-phonon scattering in conjunction with the Dresselhaus spin-orbit coupling and the second-order process of the hyperfine interaction combined with the electron-phonon scattering, are systematically studied. The relative importance of the two mechanisms are compared in detail under different conditions. It is found that due to the small spin orbit coupling in GaN, the spin relaxation caused by the second-order process of the hyperfine interaction combined with the electron-phonon scattering plays much more important role than it does in the quantum dot with narrower band gap and larger spin-orbit coupling, such as GaAs and InAs.
We report on the exciton spin dynamics of nanowire embedded GaN/AlN Quantum Dots (QDs) investigated by time-resolved photoluminescence spectroscopy. Under a linearly polarized quasiresonant excitation we evidence the quenching of the exciton spin relaxation and a temperature insensitive degree of the exciton linear polarization, demonstrating the robustness of the optical alignment of the exciton spin in these nanowire embedded QDs. A detailed examination of the luminescence polarization angular dependence shows orthogonal linear exciton eigenstates with no preferential crystallographic orient…
The optical emission of InGaN quantum dots embedded in GaN nanowires is dynamically controlled by a surface acoustic wave (SAW). The emission energy of both the exciton and biexciton lines is modulated over a 1.5 meV range at ~330 MHz. A small but systematic difference in the exciton and biexciton spectral modulation reveals a linear change of the biexciton binding energy with the SAW amplitude. The present results are relevant for the dynamic control of individual single photon emitters based on nitride semiconductors.
Grazing incidence anomalous x-ray scattering was used to monitor in situ the molecular beam epitaxy growth of GaN/AlN quantum dots (QDs). The strain state was studied by means of grazing incidence Multi-wavelength Anomalous Di raction (MAD) in both the QDs and the AlN during the progressive coverage of QDs by AlN monolayers. Vertical correlation in the position of the GaN QDs was also studied by both grazing incidence MAD and anomalous Grazing Incidence Small Angle Scattering (GISAXS) as a function of the number of GaN planes and of the AlN spacer thickness. In a regime where the GaN QDs and t…
We report our theoretically investigation on the spin-orbit coupling and $g$-factor of the $X$-valley in cubic GaN. We find that the spin-orbit coupling coefficient from $sp^3d^5s^\ast$ tight-binding model is 0.029\,eV$\cdot$Å, which is comparable with that in cubic GaAs. By employing the ${\bf k}\cdot{\bf p}$ theory, we find that the $g$-factor in this case is only slightly different from the free electron $g$-factor. These results are expected to be important for the on-going study on spin dynamics far away from equilibrium in cubic GaN.
In situ growth regime characterization of cubic GaN using reflection high energy electron diffraction
采用等离子体辅助分子束外延技术在3C-SiC（001）衬底上生长了立方GAN层。原位反射高能电子衍射被用来定量确定在生长过程中的GAN表面的Ga覆盖。以电子束强度为探针，找到了在表面形成1 ML Ga覆盖层时c-GaN的最佳生长条件。当在生长期间建立1 ML的Ga覆盖时，1微米厚的c-GaN层具有2.5 nm的最小表面粗糙度。这些样品还显示了（002）摇摆曲线的半峰全宽最小值。
Starting with empirical tight-binding band structures, the branch-point (BP) energies and resulting valence band offsets (VBOs) for the zincblende phase of InN, GaN and AlN are calculated from their k-averaged midgap energy. Furthermore, the directional dependence of the BPs of GaN and AlN is discussed using the Green's function method of Tersoff. We then show how to obtain the BPs for binary semiconductor alloys within a band-diagonal representation of the coherent potential approximation (CPA) and apply this method to cubic AlGaN alloys. The resulting band offsets show good agreement to avai…
A pencil-like morphology of homoepitaxially grown GaN nanowires is exploited for the fabrication of thin conformal intrawire InGaN nanoshells which host quantum dots in nonpolar, semipolar and polar crystal regions. All three quantum dot types exhibit single photon emission with narrow emission line widths and high degrees of linear optical polarization. The host crystal region strongly affects both single photon wavelength and emission lifetime, reaching subnanosecond time scales for the non- and semipolar quantum dots. Localization sites in the InGaN potential landscape, most likely induced …
Periodic structures of GdN particles encapsulated in a single crystalline GaN matrix were prepared by plasma assisted molecular beam epitaxy. High resolution X-ray diffractometery shows that GdN islands, with rock salt structure are epitaxially oriented to the wurtzite GaN matrix. Scanning transmission electron microscopy combined with in-situ reflection high energy electron diffraction allows for the study of island formation dynamics, which occurs after 1.2 monolayers of GdN coverage. Magnetometry reveals two ferromagnetic phases, one due to GdN particles with Curie temperature of 70K and a …
The structural and electronic properties of new structural cubic (GaN)$_1$/(ZnO)$_1$ superlattice have been investigated using two different theoretical techniques: the full potential-linearized augmented plane wave (FP-LAPW) method and the linear combination of localized pseudo atomic orbital (LCPAO). The new modified Becke-Johnson (mBJ) exchange potential is chosen to improve the bandgap of the superlattice and effective masses. The bandgap is found to be slightly indirect and reduced from those of pure GaN and ZnO. The origin of this reduction is attributed to the $p-d$ repulsion of the Zn-…
Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN quantum dots are investigated systematically and the exciton spin relaxation in GaN quantum dots is calculated by first setting up the effective exciton Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists of the long- and short-range parts, is derived within the effective-mass approximation by taking into account the conduction, heavy- and light-hole bands, and especially the split-off band. The scheme applied in this work allows the description of excitons in both the strong and weak confinemen…
Time-dependent femtosecond pump-probe spectroscopic ellipsometry studies on zincblende gallium-nitride (zb-GaN) are performed and analyzed between 2.9-3.7eV. An ultra-fast change of the absorption onset (3.23eV for zb-GaN) is observed by investigating the imaginary part of the dielectric function. The 266nm (4.66eV) pump pulses induce a large free-carrier concentration up to $4\times 10^{20}$cm$^{-3}$, influencing the transition energy between conduction and valence bands due to many-body effects, like band filling and band gap renormalization, up to $\approx$500meV. Additionally, the absorpti…
We compare the quality factor values of the whispery gallery modes of microdisks incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor (Q) resonant modes on the whole spectrum which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor which reflect the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs based microdisk…
DFT modelling of stacking faults in hexagonal and cubic GaN
University College London（伦敦大学学院）；London Centre for Nanotechnology, University College London（伦敦大学学院纳米技术伦敦中心）；Research Centre for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS)（国立材料研究所（NIMS）材料纳米结构研究中心（WPI-MANA)）
We report a correlative microscopy study of a sample containing three stacks of InGaN/GaN quantum dots (QDs) grown at different substrate temperature, each stack consisting of 3 layers of QDs. Decreasing the substrate temperature along the growth axis leads to the proliferation of structural defects. However, the luminescence intensity increases towards the surface, in spite of the higher density of threading dislocations, revealing that the QD layers closer to the substrate behave as traps for non-radiative point defects. During atom probe tomography experiments combined with in-situ micro-ph…
High resolution coherent nonlinear optical spectroscopy of an ensemble of red-emitting InGaN quantum dots in GaN nanowires is reported. The data show a pronounced atom-like interaction between resonant laser fields and quantum dot excitons at low temperature that is difficult to observe in the linear absorption spectrum due to inhomogeneous broadening from indium fluctuation effects. We find that the nonlinear signal persists strongly at room temperature. The robust atom-like room temperature response indicates the possibility that this material could serve as the platform for proposed exciton…
RF plasma assisted MBE growth of Scandium Nitride (ScN) thin films on GaN (0001)/SiC, AlN (0001)/Al2O3 and on 6H-SiC (0001) hexagonal substrates is found to lead to a face centered cubic (rock-salt) crystal structure with (111) out-of-plane orientation instead of hexagonal orientation. For the first time, cubic (111) twinned patterns in ScN are observed by in-situ electron diffraction during epitaxy, and the twin domains in ScN are detected by electron backscattered diffraction, and further corroborated with X-ray diffraction. The epitaxial ScN films display very smooth, sub nanometer surface …
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 micro-PL and photon autocorrelation experiments give a thorough evaluation of the QDs structural and optical properties. The QD exhibits anti-bunched 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 …
We have examined the formation mechanisms of GaN quantum dots (QDs) via annealing of Ga droplets in a nitrogen flux. We consider the temperature and substrate dependence of the size distributions of droplets and QDs, as well as the relative roles of Ga/N diffusivity and GaN nucleation rates on QD formation. We report on two competing mechanisms mediated by Ga surface diffusion, namely QD formation at or away from pre-existing Ga droplets. We discuss the relative roles of nucleation and coarsening dominant growth, as well as the polytype selection, on various substrates. The new insights provid…
Quantum well (QW) heterostructures have been extensively used for the realization of a wide range of optical and electronic devices. Exploiting their potential for further improvement and development requires a fundamental understanding of their electronic structure. So far, the most commonly used experimental techniques for this purpose have been all-optical spectroscopy methods that, however, are generally averaged in momentum space. Additional information can be gained by angle-resolved photoelectron spectroscopy (ARPES), which measures the electronic structure with momentum resolution. Her…
In this paper, we used the 8-band k$\cdot$p model with valence force field considerations to investigate the effect of size and shape on electronic and optical properties of cadmium selenide quantum dots. Major factors related to their properties including band mixing probabilities, spatial charge distributions, transition matrix elements and Fermi factors were studied. Volumetrically larger CdSe dots were found to have smaller band-gaps but higher transition matrix elements and Fermi factors. The maximum optical gain for dots was observed to have an initially positive and then negative correl…
In this paper we systematically analyze the electronic structures of polar and nonpolar wurtzite-InN/GaN quantum dots and their modification due to the quantum-confined Stark effect caused by intrinsic fields. This is achieved by combining continuum elasticity theory with an empirical tight binding model to describe the elastic and single-particle electronic properties in these nitride systems. Based on these results, a many-body treatment is used to determine optical absorption spectra. The efficiency of optical transitions depends on the interplay between the Coulomb interaction and the quan…
We report the control of spontaneous emission from CdSe/ZnS core-shell quantum dots coupled to novel open-access optical microcavities. The cavities are fabricated by focused ion beam milling, and provide mode volumes less than a cubic micrometre. The quantum dot emission spectrum, spatial modes, and lifetime are all modified substantially by the presence of the cavity, and can be tuned by actively varying the cavity length. An increase in emission rate of 75% is achieved at room temperature, attributed to the Purcell effect in the 'bad emitter' regime. We demonstrate a high degree of control …
Single photon emitters play a central role in many photonic quantum technologies. A promising class of single photon emitters consists of atomic color centers in wide-bandgap crystals, such as diamond silicon carbide and hexagonal boron nitride. However, it is currently not possible to grow these materials as sub-micron thick films on low-refractive index substrates, which is necessary for mature photonic integrated circuit technologies. Hence, there is great interest in identifying quantum emitters in technologically mature semiconductors that are compatible with suitable heteroepitaxies. Her…
