Low-temperature Microphotoluminescence Research Articles

(Invited) Ultrathin Gap Nanoantenna: Crystal Phase Transitions and Luminescent Properties

We report Te-doped GaP nanowires (NWs) with positive tapering and radii measuring as low as 5 nm grown by the self-assisted vapor-liquid-solid mechanism using selective-area molecular beam epitaxy. The occurrence of the ultrathin nanoantenna showed a dependence on pattern pitch (separation between NWs) with a predominance at 600 nm pitch, and exhibited radius oscillations that correlate with polytypic zincblende/wurtzite segments. A growth model explains the positive tapering of the NW leading to an ultrathin tip from the suppression of surface diffusion of Ga adatoms on the NW sidewalls by Te dopant flux. The model also provides a relationship between the radius modulations and the oscillations of the droplet contact angle, predicting the quasi-periodic radius oscillations and corresponding crystal phase transitions. The GaP NWs showed strong low temperature micro-photoluminescence exciton-related emission, indicating a bandgap difference between the zincblende/wurtzite segments, with phonon replicas due to the transverse optical phonon mode. These results establish a link between dopants and the ability to control NW morphology, crystal phase and luminescent properties with possible applications in thermoelectrics, quantum emitters and photodetectors.

Оптические и спиновые свойства вакансионных кремниевых центров, созданных облучением протонами в гетероструктуре карбида кремния 6H/15R

Optically active silicon vacancy defects (VSi) with an electron spin S = 3/2 in a 6H-SiC/15R-SiC silicon carbide heterostructure grown by high-temperature sublimation technique have been studied. By means of low-temperature micro-photoluminescence (µ-PL) and electron paramagnetic resonance (EPR) techniques, we demonstrate the potential to generate five disparate types of VSi centers with distinct spectral properties in the aforementioned heterostructure using proton irradiation with E = 15 MeV. Wherein each type of VSi center is defined by its zero-phonon line (ZPL) and a distinct value of spin sublevel splitting in a zero magnetic field. As a result, we have demonstrated the scalability of the number of optically active spin centers that can be enclosed within a single crystalline matrix.

Optical and spin properties of silicon vacancy centers created by proton irradiation in a 6H/15R silicon carbide heterostructure

Optically active silicon vacancy defects (VSi) with an electron spin S=3/2 in a 6H-SiC/15R-SiC silicon carbide heterostructure grown by high-temperature sublimation technique have been studied. By means of low-temperature micro-photoluminescence (μ-PL) and electron paramagnetic resonance (EPR) techniques, we demonstrate the potential to generate five disparate types of VSi centers with distinct spectral properties in the aforementioned heterostructure using proton irradiation with E=15 MeV. Wherein each type of VSi center is defined by its zero-phonon line (ZPL) and a distinct value of spin sublevel splitting in a zero magnetic field. As a result, we have demonstrated the scalability of the number of optically active spin centers that can be enclosed within a single crystalline matrix. Keywords: silicon carbide, heterostructures, photoluminescence, electron paramagnetic resonance, proton irradiation, spin centers.

Low temperature micro-photoluminescence spectroscopy of microstructures with InAsP/InP strained quantum wells

Ridge microstructures were prepared by reactive ion etching (RIE) of a series of stacked InAs x P quantum wells (QWs) with step graded compositions grown on InP by molecular beam epitaxy. These microstructures were characterized by low temperature micro-photoluminescence. The photoluminescence (PL) emission associated with each of the QWs was clearly identified and a model for their line shape was implemented. PL line-scans were measured across etched ridge stripes of various widths in an optical cryostat, with a spatial resolution of 1 µm. The model for the PL spectra allowed accurate extraction of the local PL integrated intensities, spectral positions and line widths. Two different RIE processes, using CH4/H2 and CH4/Cl2, were investigated. The PL line-scans showed strong variations of the integrated PL intensities across the etched stripes. The PL intensities for all QWs increased gradually from the edge to the center of the ridge microstructures, over a length scale of 10–20 µm. On the other hand, the spectral peak position of the PL lines remained constant (within an accuracy of 0.2–0.4 meV, depending on which QW was considered) across the microstructures. These observations are discussed in terms of the mechanical stress induced by the RIE processes, the relaxation of the biaxial built-in compressive stress in the InAsP QWs (induced by the free surfaces at the vertical etched sidewalls), and also by the non-radiative recombination at these sidewalls. Altogether, this study illustrates the contribution that specially designed test structures, coupled with advanced spectroscopic characterization, can provide to the development of semiconductor photonic devices (e.g. lasers or waveguides) involving RIE processing.

Micro-PL analysis of high current density resonant tunneling diodes for THz applications

Low-temperature micro-photoluminescence (μPL) is used to evaluate wafer structural uniformity of current densities >5mA/μm2 InGaAs/AlAs/InP resonant tunneling diode (RTD) structures on different length scales. Thin, highly strained quantum wells (QWs) are subject to monolayer fluctuations, leading to a large statistical distribution in their electrical properties. This has an important impact on the RTD device performance and manufacturability. The PL spot size is reduced using a common photolithography mask to reach a typical high Jpeak for a given RTD mesa size (1 ∼ 100 μm2). We observe that for lower strain-budget samples, the PL line shape is essentially identical for all excitation/collection areas. For higher strain-budget samples, there is a variation in the PL line shape that is discussed in terms of a variation in long-range disorder brought about by strain relaxation processes. The RTD operating characteristics are discussed in light of these findings, and we conclude that strain model limits overestimate the strain budget that can be incorporated in these devices. We also highlight μPL as a powerful nondestructive characterization method for RTD structures.

Single-photon generation from self-assembled GaAs/InAlAs(111)A quantum dots with ultrasmall fine-structure splitting

We present a novel semiconductor single-photon source based on tensile-strained (111)-oriented GaAs/InAlAs quantum dots (QDs) exhibiting ultrasmall exciton fine-structure splitting (FSS) of ≤ 8 µeV. Using low-temperature micro-photoluminescence spectroscopy, we identify the biexciton-exciton radiative cascade from individual QDs, which, combined with small FSS, indicates these self-assembled GaAs(111) QDs are excellent candidates for polarization-entangled photon-pair generation.

Photoluminescence spectra of intrinsic interstitials in Ⅱa diamond after near threshold energy irradiation

In this work, the intrinsic optical defects are created by near threshold energy electron irradiation and then the low temperature micro-photoluminescence (PL) spectroscopy is employed to investigate the dependences of laser power and measurement temperature. The optical centers include 1.673 eV, 2.091 eV and 2.253 eV. Compared with the emissions of 2.091 eV and 2.253 eV, the 1.673 eV emission has an obviously higher growth rate in intensity with the increase of laser power, a larger lattice relaxation, the stronger electron-phonon coupling strength, indicating that they are associated with the different defects. These results show that the 2.091 eV and 2.253 eV emissions are attributed to the self-interstitial defects.

Photoluminescence studies of nitrogen-vacancy and silicon-vacancy centers transformation in CVD diamond

In this study, the low-temperature micro-photoluminescence (PL) technology was employed to investigate the transformation of nitrogen-vacancy (NV), silicon-vacancy (SiV) centers in diamond crystal. Results showed that the NV and SiV luminescence were controlled by electron irradiation followed by thermal annealing. Both centers vanished together with the emergence of neutral single vacancy (GR1 center) after 200 keV electron irradiation. Interstitial related defects and vacancies were activated to diffuse by annealing (above ∼400 and 700 °C, respectively). The vacancies migrated to be captured by N and Si atoms due to the strain fields around the atoms attracted vacancies, and the NV and SiV centers appeared again in the PL spectra. In addition, compared the annealing behavior with NV center, the new emission at 639.7 nm was attributed to the nitrogen combined with carbon interstitials.

Mode properties of telecom wavelength InP-based high-(Q/V) L4/3 photonic crystal cavities

We present finite-difference time domain simulations and optical characterizations via micro-photoluminescence measurements of InP-based L4/3 photonic crystal cavities with embedded quantum dots (QDs) and designed for the M1 ground mode to be emitting at telecom C-band wavelengths. The simulated M1 Q-factor values exceed 106, while the M1 mode volume is found to be 0.33 × (λ/n)3, which is less than half the value of the M1 mode volume of a comparable L3 cavity. Low-temperature micro-photoluminescence measurements revealed experimental M1 Q-factor values on the order of 104 with emission wavelengths around 1.55 μm. Weak coupling behavior of the QD exciton line and the M1 ground mode was achieved via temperature-tuning experiments.

(Invited) Effect of the Plasma Etching on InAsP/InP Quantum Well Structures Measured through Low Temperature Micro-Photoluminescence and Cathodoluminescence

Plasma etching is widely used for the fabrication of photonic devices with InP, GaAs, and their ternary or quaternary compounds. Generally speaking, these processes are rather well controlled in terms of the geometry and morphology of the fabricated features. However, the microscopic interaction between the etching ions / radicals and the semiconductors have not been paid much attention [1]. In order to investigate this issue, we design and grow dedicated quantum well (QW) samples. The samples are grown on InP substrates and include a series of strained InAsP QWs with different As/P compositions [1]. The luminescence from these samples is very intense and the lines associated with each QW are very sharp at low temperature, as illustrated in figure 1. Due to the different As/P compositions, each line can be related to a specific QW. The QWs are grown at different depths below the surface (between 300 nm and 1 µm). Thus, by examining the changes in shape and position of each line after plasma etching, it is possible to probe the interactions at different depths. In addition, we can also record micro-photoluminescence (µPL) spectra at very low temperature using an optical cryostat [2]. By choosing the excitation wavelength in the near infra-red (1064 nm), we avoid the presence of the InP-related luminescence lines on the spectra. This allows us to also investigate the lateral effects that plasma etching can produce e.g. on narrow stripes. Our etching experiments are performed using an inductively coupled plasma reactor, and different kinds of etching chemistries. As an example, we illustrate in figure 2 the different interactions taking place when etching a stripe (width: 50 µm) with CH4/H2 and Cl2/CH4/Ar. Figure 2 shows that the CH4/H2 etching broadens the PL lines for all QWs, whereas the Cl2/CH4/Ar in contrast yields much sharper lines than on the reference un-etched sample (figure 1). We have also investigated the intensity variation for each line across the ridges, and observed quantitative differences for the different etching processes. In another set of experiments, similar samples were measured using low temperature cathodoluminescence, while simultaneously biasing the samples at different positive and negative voltages (bias applied between surface and bulk). Strong peak shifts and broadening were observed, related to a quantum confined Stark effect [3], indicating the presence of charged species after some of the etching processes. These charged species interact with the built-in electric field in our samples. A mechanism describing the interaction between the plasma and the semiconductor material will be discussed based on the different luminescence measurements and mappings performed.[1] J.P. Landesman, J. Jiménez, C. Levallois, F. Pommereau, C. Frigeri, A. Torres, Y. Léger, A. Beck and A. Rhallabi, J. Vac. Sci. Technol. A, 34, 041304-1 (2016).[2] C. M. Haapamaki, PhD dissertation, McMaster University (2012).[3] L. Vina, E. E. Mendez, W. I. Wang, L. L. Chang, and L. Esaki, J. Phys. C: Solid State 20, 2803 (1987). Figure 1

(Invited) Effect of the Plasma Etching on InAsP/InP Quantum Well Structures Measured through Low Temperature Micro-Photoluminescence and Cathodoluminescence

Photoluminescence and cathodoluminescence spectral imaging were performed across rectangular stripes etched in samples with InAsxP1-x quantum wells of constant thickness and variable composition grown on InP. In particular, the effects of different etching chemistries (CH4/H2/Ar and Cl2/CH4/Ar) were investigated. The results discussed deal with modifications of the luminescence line shapes (which differ with etching process) and with the intensity variation of the emissions associated with the quantum wells across the stripes. The possible origins of these effects are investigated in terms of carrier recombination on the vertical sidewalls of the stripes and lateral diffusion of species from the plasma during etching. Cathodoluminescence measurements on samples under DC-bias also show the quantum confined Stark effect which is correlated to the material modifications induced by the etching.

Towards Exciton-Polaritons in an Individual MoS2 Nanotube.

We measure low-temperature micro-photoluminescence spectra along a MoS nanotube, which exhibit the peaks of the optical whispering gallery modes below the exciton resonance. The energy fluctuation and width of these peaks are determined by the changes of the nanotube wall thickness and propagation of the optical modes along the nanotube axis, respectively. We demonstrate the potential of the high-quality nanotubes for realization of the strong coupling between exciton and optical modes when the Rabi splitting can reach 400 meV. We show how the formation of exciton-polaritons in such structures will be manifested in the micro-photoluminescence spectra and analyze the conditions needed to realize that.

Mid-Infrared Lasing of Single Wurtzite InAs Nanowire.

Mid-infrared (MIR) photonics is a developing technology for sensing materials by their characteristic MIR absorptions. Since silicon (Si) is a low-loss material in most of the MIR region, Si photonic structures have been fabricated to guide and confine MIR light, and they allow us to achieve sensitive and integrated sensing devices. However, since the implementation of MIR light sources on Si is still challenging, we propose a thick indium arsenide (InAs) nanowire as an MIR laser that can couple to Si photonic structures with material manipulation. In this study, thick InAs nanowires are grown on an indium phosphide substrate with a self-catalyst vapor-liquid-solid method and transferred to gold-deposited SiO2/Si substrates. Low-temperature microphotoluminescence (PL) spectroscopy shows that InAs nanowires exhibit broad PL peaking at a wavelength of around 2.6 μm (3850 cm-1 in frequency), which corresponds to the bandgap energy of wurtzite InAs. At high optical pump fluences, single InAs nanowire exhibits sharp emission peaks, while their integrated intensity and polarization degree increase abruptly at the threshold pump fluence. These nonlinear behaviors indicate that the MIR lasing action takes place in the InAs nanowire in its cavity mode. Our demonstration of the MIR nanowire laser expands the wavelength coverage and potential application of semiconductor nanowires.

Luminescence spatial characteristics of ZnSe:Fe

Two types of thermal-diffusion-based technological routes have been realized to obtain ZnSe:Fe plates demanded for laser application in the mid-IR spectral range. For the technological routes used, the iron doping process is shown to lead to the formation of auxiliary luminescence bands including (1) luminescence of residual tetrahedral donors and acceptors; (2) unknown bound exciton line located at 459 nm; (3) broad bands with maxima at 490, 520, 670, and 820 nm; and (4) IR band at 960 nm previously assigned to centers related to transition metals. Using the measurements of photoluminescence under conditions of two-photon excitation and measurements of low-temperature microphotoluminescence, the intensity profiles of the bands have been shown to be uncorrelated with the concentration profile of the optically active (Fe2+) iron. This trend does not depend on the technological routes used. Thus, heavily Fe doped areas, as well as adjacent regions in ZnSe:Fe plates produced by thermal-diffusion-based techniques, contain auxiliary centers governing luminescence in visible and near IR spectral ranges. These centers are not directly related to the optically active (Fe2+) iron.

Probing of single acceptors in a wide ZnSe-based quantum well via optical spectroscopy

Using low-temperature (5 K) micro-photoluminescence measurements of a ZnSe/ZnMgSSe quantum well, isolated (quantum) emitters produced by interacting acceptor and hydrogen-like donor have been found and studied. These emitters enable optical probing of a single acceptor (donor) center if the nature of the donor (acceptor) constituting the distant donor-acceptor pair is known. Here we demonstrate the simplest realization of proposed optical probing: frequencies of optically active phonons are successfully determined for different types of single residual acceptors in a ZnSe-based quantum well.

New Type of Quantum Emitters Related to Dislocations in Crystalline CdZnTe Revealed by Laser-Based Spectroscopy

Based on the measurements of spectra and polarization of low-temperature (5K) microphotoluminescence excited by stabilized continuous-wave laser pumping, we find a new type of isolated emitters formed due to the dislocations in crystalline CdZnTe. For an emitter of this type, zero-phonon luminescence is located 30–50 meV above the usual dislocation luminescence and is subjected to giant fluctuations of spectral position (~10 meV) and intensity. A noticeable degree of linear polarization in the plane containing directions indicates the strongly anisotropic nature of the corresponding electronic states and confirms their connection with dislocations.

Kinetics of Low-Temperature Microphotoluminescence of Exciton-Impurity Complexes in CdZnTe Single Crystals

Low-temperature (6.5 K) microphotoluminescence near the intrinsic absorption edge of CdZnTe alloy single crystals is studied under conditions of non-resonant and resonant excitation by picosecond pulses. Characteristic relaxation times for free excitons and exciton-impurity complexes of various types are determined. A significant (by a factor of 4–8) decrease in the photoluminescence signal decay time of exciton-impurity complexes on neutral donors during the transition to the resonant excitation mode is detected. The detected sharp decrease in the photoluminescence signal decay time can indicate the manifestation of collective effects in this system of emitters.

Isolated (Quantum) Emitters Generated with the Participation of Donor–Acceptor Pairs in the ZnSe/ZnMgSSe Heterostructure

Measurements of low-temperature (5 K) microphotoluminescence reveal the existence of isolated (quantum) emitters in ZnSe/ZnMgSSe quantum well with a radiation line experiencing abrupt displacements of its spectral position by several meV within 1–10 min. The unusual properties of the detected emitters are explained on the basis of a system with a large dipole moment in its ground state, e.g., a single donor–acceptor pair.

Two types of isolated (quantum) emitters related to dislocations in crystalline CdZnTe

Based on the measurements of spectra and polarization of low-temperature (5 K) microphotoluminescence, two types of isolated quantum emitters formed due to the dislocations in crystalline CdZnTe have been identified. For an emitter of the first type, the spectral position of zero-phonon emission line (ZPL) corresponds to the well-known dislocation luminescence of CdZnTe. In this case, position and intensity of ZPL demonstrate high stability over time. For an emitter of the second type, ZPL is located 30–50 meV above the usual dislocation luminescence and is subjected to giant fluctuations of spectral position (~ 10 meV) and intensity. For each of the two types of emitters, coupling with lattice is confirmed by the presence of phonon replicas associated with longitudinal optical phonons in CdZnTe. A noticeable degree of linear polarization in the plane containing 〈110〉 directions indicates the strongly anisotropic nature of the corresponding electronic states and confirms their connection with dislocations.

Determination of Dopant Density Profiles of Heavily Boron-Doped Silicon From Low Temperature Microphotoluminescence Spectroscopy

Low temperature microphotoluminescence spectroscopy ( μ -PLS) is employed to determine the inhomogeneous doping profiles for heavily boron-doped regions on silicon wafers. Samples having various Gaussian function doping profiles, in terms of surface dopant density and depth factor, are prepared via two-step thermal boron diffusion on high resistivity n-type silicon wafers. Measured PL spectra are normalized to the Si band–band luminescence peak, and PL components of undiffused Si are subtracted to resolve the doping peak. We show that the wavelength of the doping peak has a reliable and simple linear relationship with the measured surface dopant density on a semilog plot, and so establish a calibration curve which can be applied to estimate the surface dopant density. A second calibration curve for estimating the depth factor is also established after correcting the measured doping peak intensity to account for incomplete dopant ionization. We show the effectiveness of this method by reconstructing independently measured doping profiles using the surface dopant density and depth factors estimated from the PL spectra. Furthermore, by performing two-dimensional mapping, with μ -PLS measurements at 2 μ m spatial resolution, we are able to map the surface dopant density and diffusion depth factor of micron-scale, locally diffused features.