Low-temperature Cathodoluminescence Research Articles

Effect of Strain and Surface Proximity on the Acceptor Grouping in ZnO.

According to the present knowledge, the level of zinc oxide conductivity is determined by donor and acceptor complexes involving native defects and hydrogen. In turn, recently published low-temperature cathodoluminescence images and scanning photoelectron microscopy results on ZnO and ZnO/N films indicate grouping of acceptor and donor complexes in different crystallites, but the origin of this phenomenon remains unclear. The density functional theory calculations on undoped ZnO presented here show that strain and surface proximity noticeably influence the formation energy of acceptor complexes, and therefore, these complexes can be more easily formed in crystallites providing appropriate strain. This effect may be responsible for the clustering of acceptor centers only in certain crystallites or near the surface. Low-temperature photoluminescence spectra confirm the strong dependence of acceptor luminescence on the structure of the ZnO film.

Excitonic and deep-level emission from N- and Al-polar homoepitaxial AlN grown by molecular beam epitaxy

Using low-temperature cathodoluminescence spectroscopy, we study the properties of N- and Al-polar AlN layers grown by molecular beam epitaxy on bulk AlN{0001}. Compared with the bulk AlN substrate, layers of both polarities feature a suppression of deep-level luminescence, a total absence of the prevalent donor with an exciton binding energy of 28 meV, and a much increased intensity of the emission from free excitons. The dominant donor in these layers is characterized by an associated exciton binding energy of 13 meV. The observation of excited exciton states up to the exciton continuum allows us to directly extract the Γ5 free exciton binding energy of 57 meV.

Luminescence Properties of the Hexagonal Boron Nitride Epilayer

AbstractAs an emerging 2D ultrawide bandgap semiconductor, hexagonal boron nitride (h‐BN) is gaining significant attention for its superior properties and wide applications. Although optical properties of h‐BN have been partially revealed by using h‐BN bulk single crystal, luminescence properties of the h‐BN few‐layer are rare due to its poor crystallinity. In this work, the h‐BN epilayers have been synthesized on sapphire substrates by the submicron‐spacing vapor deposition method. The crystalline quality of the h‐BN epilayer is improved with the increase of the growth temperature, and the full width at half‐maximum of the Raman peak is only ≈13 cm−1. The low temperature cathodoluminescence spectra of h‐BN epilayers exhibit two strong deep ultraviolet (DUV) luminescence peaks at 5.49 and 5.35 eV, as well as a weaker and broader defect‐related emission band at ≈4.0 eV. The DUV emission band is ascribed to the recombination of bound excitons and its phonon replica. Based on the theoretically predicted energy levels, the 4.0 eV emission band is tentatively attributed to the radiative transition from C impurities occupying the B sites to the B vacancies. Moreover, the h‐BN‐based DUV photodetectors exhibit an outstanding performance, making it a promising prospect for optoelectronic applications.

Point defect localization and cathodoluminescence emission in undoped ε-Ga2O3

In this study, experimental and theoretical investigations have been performed on nominally undoped ϵ-Ga2O3 films deposited on (0001)-Al2O3 substrates by metal-organic vapor phase epitaxy using different O and Ga precursor ratios. Hydrogen and helium were used as carrier gas. Low-temperature cathodoluminescence (CL) broad emissions extending over the range 1.5–3.4 eV were deconvoluted in five peaks, whose position, integrated intensity, and full width at half maximum were investigated in the temperatures range 80 K–300 K. A non-monotonic behavior of the extracted CL peaks is observed, which is attributed to localization phenomena connected with families of point defects. The behavior of two main luminescence emissions with temperature has been simulated using the localized state ensemble model. The derived parameters agree with the experimental observations and provide a new interpretation of micro-and macroscale disorder inside ϵ-Ga2O3 and related potential fluctuations.

Correlating yellow and blue luminescence with carbon doping in GaN

Blue and yellow emission bands in carbon-doped GaN grown by MBE were investigated in low-temperature cathodoluminescence measurements performed in a scanning transmission electron microscope (STEM-CL) with high spatial resolution. Blue luminescence at 2.85 eV and two contributions in the spectral range of the yellow emission band around 2.2 eV separated by 120 meV are observed in carbon-doped material, whereas only one distinctive yellow luminescence contribution was found in unintentionally-doped GaN.

Cathodoluminescence mapping of electron concentration in MBE-grown GaAs:Te nanowires

Cathodoluminescence mapping is used as a contactless method to probe the electron concentration gradient of Te-doped GaAs nanowires. The room temperature and low temperature (10 K) cathodoluminescence analysis method previously developed for GaAs:Si is first validated on five GaAs:Te thin film samples, before extending it to the two GaAs:Te NW samples. We evidence an electron concentration gradient ranging from below 1 × 1018 cm−3 to 3.3 ×1018 cm−3 along the axis of a GaAs:Te nanowire grown at 640 °C, and a homogeneous electron concentration of around 6–8 × 1017 cm−3 along the axis of a GaAs:Te nanowire grown at 620 °C. The differences in the electron concentration levels and gradients between the two nanowires is attributed to different Te incorporation efficiencies by vapor–solid and vapor–liquid–solid processes.

Low-Temperature Cathodoluminescence of Nitrogen-Doped ZnO Films Deposited at Low-Temperature by Atomic Layer Deposition

Low-Temperature Cathodoluminescence of Nitrogen-Doped ZnO Films Deposited at Low-Temperature by Atomic Layer Deposition

Growth of hexagonal boron nitride crystals at atmospheric pressure from Cu[sbnd]Cr flux

We report here the growth of hexagonal boron nitride crystals from CuCr flux. The transparent crystals have a maximum size of 6 mm and a thickness of 20 µm. The growth atmosphere, source of raw materials, and crystallization cooling rate were controlled to demonstrate their effect on the crystal quality and size. It is found that diluted nitrogen atmosphere, the use of boron nitride crucible as source material, and relatively low cooling rate ~2.5 °C/h can produce the best quality of hexagonal boron nitride crystals. We show that the growth proceeds in a layered mode on the smooth surface (0001) of hexagonal boron nitride and the triangular domain boundaries are verified to come from cracks. The X-ray diffraction and Raman spectroscopy confirm the crystals are of good crystallinity. The low-temperature cathodoluminescence spectrum reveals the intense 215 nm band-edge luminescence of the obtained h-BN crystals.

Quantitative Assessment of Carrier Density by Cathodoluminescence. II. GaAs Nanowires

Precise control of doping in single nanowires (NWs) is essential for the development of NW-based devices. Here, we investigate a series of MBE-grown GaAs NWs with Be (p-type) and Si (n-type) doping using high-resolution cathodoluminescence (CL) mapping at low- and room-temperature. CL spectra are analyzed selectively in different regions of the NWs. Room-temperature luminescence is fitted with the generalized Planck law and an absorption model, and the bandgap and band tail width are extracted. For Be-doped GaAs NWs, the bandgap narrowing provides a quantitative determination of the hole concentration ranging from about $1\times 10^{18}$ to $2\times 10^{19}$~cm$^{-3}$, in good agreement with the targeted doping levels. High-resolution maps of the hole concentration demonstrate the homogeneous doping in the pure zinc-blende segment. For Si-doped GaAs NWs, the electron Fermi level and the full-width at half maximum of low-temperature CL spectra are used to assess the electron concentration to approximately $3\times 10^{17}$ to $6\times 10^{17}$~cm$^{-3}$. These findings confirm the difficulty to obtain highly-doped n-type GaAs NWs, maybe due to doping compensation. Notably, signatures of high concentration (5--9$\times 10^{18}$~cm$^{-3}$) at the very top of NWs are unveiled.

Cathodoluminescence of undoped and Si-doped ɛ-Ga2O3 films

Cathodoluminescence (CL) investigations are performed on nominally undoped and Si-doped ε-Ga2O3 samples grown by metal-organic vapor phase epitaxy on (0001)-Al2O3 substrates, using different carrier gases. All films exhibit a broad low-temperature CL emission extending over the photon energy range 2–3.4 eV. Emission deconvolution suggests that four narrower bands centered at about 2.4, 2.75, 3.0 and 3.15 eV may well account for the broad band. While the position of these peaks results independent of the growth conditions, significant intensity differences are observed. A general reduction of the broad emission is evidenced as the Si concentration increases. No band-to-band recombination is observed. Temperature dependence of the CL signal shows a trend consistent with radiative transitions from the CB to deep acceptor states, probably of intrinsic nature.

Cathodoluminescence in single and multiwall WS2 nanotubes: Evidence for quantum confinement and strain effect

For nanoparticles with sub-10 nm diameter, the electronic bandgap becomes size dependent due to quantum confinement; this, in turn, affects their electro-optical properties. Thereby, MoS2 and WS2 monolayers acquire luminescent capability, due to the confinement-induced indirect-to-direct bandgap transition. Rolling up of individual layers results in single wall inorganic nanotubes (SWINTs). Up to the present study, their luminescence properties were expected to be auspicious but were limited to theoretical investigations only, due to the scarcity of SWINTs and the difficulties in handling them. By optimizing the conditions in the plasma reactor, relatively high yields of WS2 SWINTs 3–7 nm in diameter were obtained in this work, compared to previous reports. A correlative approach, transmission electron microscopy coupled with a scanning electron microscope, was adapted to overcome handling obstacles and for testing individual nanotubes by low-temperature cathodoluminescence. Clear cathodoluminescence spectra were obtained from WS2-SWINTs and compared with those of WS2 multiwall nanotubes and the corresponding bulk material. Uniquely, the optical properties of INTs acquired from cathodoluminescence were governed by the opposite impact from quantum size effect and strain in the bent triple S-W-S layers. The experimental findings were confirmed by the Density Functional and Time-Dependent Density Functional theoretical modeling of monolayer and bilayer nanotubes of different chiralities and diameters. This study provides experimental evidence of the quantum confinement effect in WS2 SWINTs akin to WS2 monolayer. The ability to tune the electronic structure with morphology or number of layers may be exploited toward photoelectrochemical water splitting with WS2 catalysts, devising field effect transistors, photodetectors, and so on.

Thallium lead iodide (TlPbI3) single crystal inorganic perovskite: Electrical and optical characterization for gamma radiation detection

TlPbI3 has been suggested as a candidate semiconductor material for room temperature gamma ray and charged particle detection, motivated by its wide bandgap (2.17–2.3 eV), and high density (6.6 gm/cm3) required for low noise and high intrinsic detection efficiency. In this work, we studied the optical and electronic properties of Bridgeman-grown TlPbI3 single crystal using ultraviolet–visible–infrared absorption spectroscopy, low temperature cathodoluminescence, temperature dependent current-voltage measurements, and charge carriers’ mobility-lifetime measurements. The fabricated Ag/TlPbI3/Ag device showed Ohmic behavior in temperatures ranging between −7 °C to 22 °C and exhibited high resistivity (~2 × 1011 Ω.cm at RT). Fermi level pinning at 0.72 eV above valence band minimum or below conduction band maximum was calculated from the temperature dependent resistivity measurement, and the 2.17 eV bandgap was determined from ultraviolet–visible–infrared absorption spectroscopy. The cathodoluminescence spectrum of TlPbI3 was resolved to two peaks at 1.45 eV and 1.48 eV indicating deep level traps. The mobility-lifetime product was estimated to be 3.43 × 10−5 cm2/V and 2.29 × 10−6 cm2/V for electrons and holes, respectively. Possible interpretations are presented suggesting the presence of vacancy type defects. Our results provide new essential information for developing such a new class of room temperature radiation detector material for a wide range of applications.

Chlorine termination of selenium dangling bonds decreases tellurium-diffusion-induced acceptor states in hexagonal selenium

In this study, we examined the effects of doping chlorine (Cl) into crystalline selenium (c-Se) on diminishing the tellurium-diffusion-induced shallow acceptor states associated with the dangling bonds that appear at the ends of Se chains by theoretical analysis with the first-principles calculations. In this model, the negatively charged dangling bonds can be neutralized by inserting monovalent Cl atoms. Furthermore, the defect termination by doping Cl into fabricated Se thin films was experimentally demonstrated. Low-temperature cathodoluminescence measurements were implemented to show that acceptor-related emissions from the c-Se films were significantly decreased by Cl termination of the dangling bonds, experimentally confirming the results of the simulations.

(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.

Luminescence behavior of semipolar (101¯1) InGaN/GaN “bow-tie” structures on patterned Si substrates

In this work, we report on the innovative growth of semipolar “bow-tie”-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs) and their structural and luminescence characterization. We investigate the impact of growth on patterned (113) Si substrates, which results in the bow-tie cross section with upper surfaces having the (101¯1) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channeling contrast imaging (ECCI) identifies the black spots as threading dislocations propagating to the inclined (101¯1) surfaces. Line defects in ECCI, propagating in the [12¯10] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the center of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region.

Strong Coupling of Excitons in Hexagonal GaN Microcavities

The GaN planar hexagonal microcavities are grown by the selective vapor-phase epitaxy technique. The spectra are measured by the low-temperature cathodoluminescence method using a scanning electron microscope. The obtained spectra show a huge Rabi splitting (~100 meV). Numerical simulation of the spatial distribution of the intensities of modes in a hexagonal cavity is carried out. Certain modes can have a high spatial localization leading to strong coupling with the exciton and huge Rabi splitting. The fraction of excitons in polariton modes, which correlates with the intensity of exciton radiation associated with these modes, is theoretically calculated for hexagonal-shaped microcavities. Thus, the form of the dependence of the radiation probability on the eigenfrequencies of the structure is obtained.

Anomalous Edge Emission from Zinc Selenide Heavily Doped with Oxygen

The specific features of the emission spectra of pure chemical-vapor deposited (CVD) ZnSe condensates grown with an excess of selenium and with doping with oxygen are studied. The anomalous 477(490)-nm edge emission prevailing in the low-temperature cathodoluminescence spectra of ZnSe(O) is studied in order to elucidate the nature of this emission. The relation of this emission to the stoichiometry composition of ZnSe and to the concentration of dissolved oxygen is established. The data suggesting the role of the copper impurity are obtained. On the basis of the band-anticrossing theory, a band model is developed to interpret the nature of the basic luminescence bands of ZnSe(O) and ZnSe(O,Cu) in the near-edge spectral region. It is shown that the model for ZnSe(O) is similar to the model previously proposed for ZnS(O) and CdS(O).

Аномальное краевое свечение ZnSe, сильно легированного кислородом

The features of the emission spectra of high-purity CVD ZnSe condensates grown with excess selenium and strong oxygen doping were studied. Investigated prevalent in the spectra of low-temperature cathodoluminescence of ZnSe∙O abnormal edge emission 477-490 nm to ascertain its nature. The connection of this luminescence with the stoichiometric composition of ZnSe and the concentration of dissolved oxygen is established.Obtained data indicating the role admixture of copper. Based on the theory of anti-crossing zones, a band model is presented, explaining the nature of the main luminescence bands of ZnSe∙O in the near edge region of the spectrum. It is shown that the model is identical for ZnSe, ZnS and CdS.

Сильная связь экситонов в микрорезонаторах GaN гексагональной формы

The GaN planar hexagonal microcavities were grown by a selective vapor phase epitaxy. The spectra were measured by the low-temperature cathodoluminescence method in-situ scanning electron microscope. The obtained spectra show a huge Rabi splitting (in order of 100 meV). Numerical simulation of the spatial distribution of the modes’ intensities of a hexagonal resonator is shown. Some modes can have strong spatial localization, leading to strong coupling with the exciton and huge Rabi splitting. Theoretically, we calculated the fraction of excitons in polariton modes, which correlates with the intensity of exciton radiation associated with these modes, for microcavities of a hexagonal shape. Thus, we have obtained the form of the dependence of the radiation probability on the eigenfrequencies of the structure.