Temperature-dependent Cathodoluminescence Research Articles

Detection of an unintentional Si doping gradient in site-controlled GaN nanowires grown using a Si3N4 mask by spatially resolved cathodoluminescence and Raman spectroscopy

Highly uniform arrays of site-controlled GaN nanowires are synthesized by selective area growth using a Si3N4 mask and molecular beam epitaxy. Systematic modulation of the emission along the nanowire axis is observed in spectrally resolved cathodoluminescence linescans. We show that this intensity change is an indicator of unintentional Si incorporation during growth resulting from the interaction between the impinging Ga atoms and the mask material. The gradual reduction of the cathodoluminescence intensity along the nanowire highlights the important role of the growth geometry within the synthesis reactor, with shadowing from the elongating nanowires inhibiting the reaction with the mask. This gradient in Si doping is confirmed by the quenching of the longitudinal optical phonon line measured in Raman spectra along the nanowire axis. The corresponding carrier density is derived from the frequency of the coupled phonon–plasmon mode. The spectroscopic identification of inversion domain boundaries in the majority of the nanowires is also attributed to the Si incorporation. From temperature dependent cathodoluminescence experiments, we derive the activation energy for excitons bound to these defects.

Steady-state and dynamic characteristics of deep UV luminescence in rock salt-structured MgxZn1−xO

Temperature-dependent cathodoluminescence spectra were measured for rock salt-structured MgxZn1−xO films with x = 0.95–0.61. The Mg0.95Zn0.05O film exhibited the shortest deep UV peak wavelength of 199 nm (6.24 eV) at 6 K. Relatively high equivalent internal quantum efficiencies of 0.9%–11% were obtained. The Tauc plots, which were obtained from temperature-dependent optical transmittance measurements, exhibited large Stokes-like shifts of 0.7–0.9 eV at 6–300 K. Time-resolved photoluminescence (PL) signals at 7 K exhibited fast and slow decay components. The fast decay component had PL lifetimes of 2.59–3.08 ns, and the slow decay component far exceeded the measurement time range of 12.5 ns. The fast decay constant reflected the transfer lifetime of the photoexcited carriers to certain trapping centers. These centers were tentatively ascribed to Zn-related isoelectronic trapped-hole centers and may be a cause of the large Stokes-like shifts. The signals at 300 K exhibited very short PL lifetimes of 120–180 ps. The PL lifetimes were mainly attributed to the nonradiative recombination lifetime. Simultaneous decreases in the Zn-related isoelectronic trapped-hole centers and the nonradiative recombination centers were found to be necessary to improve the DUV emission properties of RS-MgxZn1−xO films.

Optical properties of Ga-doped AlN nanowires

We show that intentional Ga doping of AlN nanowires in the 0.01%–0.5% range leads to the spontaneous formation of nanometric carrier localization centers. Accordingly, for single nanowires, we observed a collection of sharp cathodoluminescence lines in a wavelength range spanning from 220 to 300 nm. From temperature-dependent cathodoluminescence, a ratio between the intensity at room temperature and 5 K of 20–30% is measured. We found that an ensemble of Ga-doped AlN nanowires exhibits a wide-band cathodoluminescence emission, which opens the path to the realization of efficient UV-C light emitting diodes covering a wide part of DNA absorption band.

Investigation of sidewall damage induced by reactive ion etching on AlGaInP MESA for micro-LED application

Previous reports have studied the impact of sidewall defects on AlGaInP micro light emitting diode (μLED) only by Current-Voltage-Luminescence (I-V-L) measurements. In this work, we propose an alternative approach to investigate these defects directly after MESA formation, by coupling optical characterization techniques together with Time-of-flight secondary ion mass spectrometry (TOF-SIMS) on AlGaInP square shaped pixels of different sizes formed by BCl3-based Reactive Ion Etching (RIE). It is found that for a 6 × 6 μm2 pixel, the light emission homogeneity is largely impacted by the sidewall defects. From emission efficiency map deduced by temperature-dependent cathodoluminescence measurements, we estimate that 84.5% of the 6 × 6 μm2 pixel exhibit a lower efficiency than the center. The carriers lifetime extracted from time-resolved photoluminescence (TRPL) measurements on larger pixel begins to decrease gradually at 3 μm from the sidewall due to non-radiative recombinations. On the other hand, the TOF-SIMS analysis shows that residues of boron and chlorine remain on the surface and sidewalls of the pixel after BCl3 etching. These results show the importance to characterize the μLEDs at the MESA step and the necessity to optimize the etching process and the passivation.

Investigation of excitonic recombination in single-crystal diamond with cathodoluminescence spectroscopy

Temperature-dependent cathodoluminescence (CL) measurements were performed for homoepitaxial chemical vapor deposited (CVD) diamond from 10 K to room temperature. With the increment of temperature, the unusual phenomena that the integrated intensities of free-exciton emission initially rise and then decrease, have been observed. The model based on trapping and thermal release of excitons was applied on the explanation of the unusual phenomena, and the binding energy of shallow traps in diamond at low temperatures has been obtained from the fitting experimental data. Besides, the broadening of the full width at half maximums (FWHMs) has been discussed. The exciton-phonon interactions at higher temperatures lead to the broadening of the width of the free-exciton emission.

Excitation-current-density and temperature dependences of deep UV cathodoluminescence in rocksalt-structured MgxZn1-xO films

Excitation-current-density and temperature-dependent cathodoluminescence (CL) spectra were measured for rocksalt-structured MgxZn1-xO films with x = 0.92–0.61. A deep UV luminescence (DUVL) band was predominantly observed in an energy range of 4.91–5.86 eV for all the films at 6–300 K. The analysis based on a rate equation model confirms that the DUVL band is attributed to the near-band-edge emission. Temperature-dependent CL spectra were quantified in terms of peak energy shifts and full width at half maximum values. Relatively high equivalent internal quantum efficiencies of 2.5%–11% were obtained for the RS-MgxZn1-xO films.

Solution-processed mixed halide CH3NH3PbI3−xClx thin films prepared by repeated dip coating

The mixed halide CH3NH3PbI3−xClx crystalline thin film has been prepared by two-step solution-processed repeated dip coating method at an ambient atmosphere. X-ray diffraction study reveals the presence of tetragonal and cubic phases in deposited film. Raman study confirms the metal halide bond in the inorganic framework and organic CH3 stretching/bending of C–H bond in CH3NH3PbI3−xClx perovskite. Scanning electron microscopy shows that cuboid and polyhedral-like crystal grains of 100 nm to 2 μm may find applications in optoelectronics. The perovskite CH3NH3PbI3−xClx thin film shows high spectral absorption coefficient of the order of 106 m−1. In optical band gap study, we found the coexistence of cubic and tetragonal perovskite phases. The energy band gap is dominated by cubic phase having Eg = 2.50 eV over tetragonal phase with band gap Eg = 1.67 eV. The room-temperature photoluminescence study confirms band edge, shallow and deep-level emissions. The temperature-dependent cathodoluminescence study shows red, green and ultraviolet emissions. The dominating green luminescence evolved for cubic phase at 2.51 eV. The red and ultraviolet emissions are also found for mixed-phase CH3NH3PbI3−xClx thin film, suitable for preparation of light-emitting devices.

Nonlayered Two-Dimensional Defective Semiconductor γ-Ga2S3 toward Broadband Photodetection.

Two-dimensional (2D) materials exhibit high sensitivity to structural defects due to the nature of interface-type materials, and the corresponding structural defects can effectively modulate their inherent properties in turn, giving them a wide application range in high-performance and functional devices. 2D γ-Ga2S3 is a defective semiconductor with outstanding optoelectronic properties. However, its controllable preparation has not been implemented yet, which hinders exploring its potential applications. In this work, we introduce nonlayered γ-Ga2S3 into the 2D materials family, which was successfully synthesized via the space-confined chemical vapor deposition method. Its intriguing defective structure are revealed by high-resolution transmission electron microscopy and temperature-dependent cathodoluminescence spectra, which endow the γ-Ga2S3-based device with a broad photoresponse from the ultraviolet to near-infrared region and excellent photoelectric conversion capability. Simultaneously, the device also exhibits excellent ultraviolet detection ability ( Rλ = 61.3 A W-1, Ion /Ioff = 851, EQE = 2.17× 104 %, D* = 1.52× 1010 Jones @350 nm), and relatively fast response (15 ms). This work provides a feasible way to fabricate ultrathin nonlayered materials and explore the potential applications of a 2D defective semiconductor in high-performance broadband photodetection, which also suggests a promising future of defect creation in optimizing photoelectric properties.

Temperature-dependent cathodoluminescence mapping of InGaN epitaxial layers with different In compositions

We performed temperature-dependent cathodoluminescence (CL) mapping of InGaN epitaxial layers, for which the presence of potential barriers around threading dislocations was previously confirmed by photoluminescence (PL) mapping using scanning near-field optical microscopy. The dark spot contrast in the panchromatic CL intensity images decreased with increasing temperature owing to the activation of nonradiative recombination processes. The change in the distribution of monochromatic CL (MCL) intensity with increasing temperature was ascribed to a carrier diffusion process in which thermally activated carriers diffuse to the potential minima. The change in the MCL intensity images of the InGaN epitaxial layer with higher In composition at higher temperatures was remarkable, indicating that the carriers cannot readily move to the potential minima owing to the higher potential barrier. This tendency is consistent with the temperature dependence of the intensity ratio of the two emission components previously observed in macroscopic PL measurements.

The fluorescence self-healing mechanism and temperature-sensitive properties of a multifunctional phosphosilicate phosphor

The temperature-dependent fluorescence characteristic is a key index of rare-earth ion-doped functional materials. In this paper, the structure, photoluminescence property, trap distribution and self-healing mechanism are studied in detail by XRD, photoluminescence spectra, decay times, the temperature-dependent fluorescence characteristic and cathodoluminescence spectrum. We developed a multicationic site phosphosilicate phosphor Ca8Al2(PO4)6(SiO4): Ce3+, Mn2+ to obtain the luminous self-healing property. In this work, we tried to change the energy and density distributions of traps by designing and adjusting synthesis scheme of target material and finally realized self-suppression of emission declined by energy compensation from traps or energy transfer between Ce3+ and Mn2+. As we expected, photoluminescence intensity of Ce3+ and Mn2+ at 250 °C is 40% and 300%, respectively, of their initial intensity at ambient temperature for co-doped representative sample, and it indicates that the emission degeneration of Mn2+ is obviously suppressed with the increase in temperature. A highly thermally sensitive fluorescence intensity ratio is obtained in a broad temperature range, and it implies that this material could be applied to a temperature sensing sensor. The fitting and calculated results show a good signal discriminability with the maximum absolute sensitivity 0.0097 K−1 and maximum relative sensitivity 2.1% K−1, respectively.

Optical and electrical properties of all-inorganic Cs2AgBiBr6 double perovskite single crystals.

In this work, we explored the possibility of using Cs2AgBiBr6, a double perovskite crystal, for radiation detection. Cs2AgBiBr6 crystals were grown using the solution growth technique. The resistivity of the as-grown Cs2AgBiBr6 crystal is larger than 1010 Ω cm, which is high enough to ensure low leakage current for fabrication of semiconductor radiation detectors. Using the temperature-dependent resistivity measurements, we estimated that the Fermi level is at 0.788 eV above the valence band and the material is a p-type semiconductor. From the low-temperature cathodoluminescence measurements, two near band gap energies at 1.917 eV and 2.054 eV were revealed.

Spatial Distribution of Defect Luminescence in ZnO Nanorods: An Investigation by Spectral Cathodoluminescence Imaging

The spatial distribution of ubiquitous green luminescence (GL) in ZnO nanorods is investigated using cathodoluminescence (CL) spectral imaging. The vertically aligned, single‐crystal nanorods exhibit a strong GL emission at 2.42 eV at 80 K, attributable to oxygen vacancies. The spectral imaging reveals the GL emission is predominantly located in the surface layer of nanorods; the thickness and intensity of this layer decreases rapidly at elevated temperatures. On the other hand, the near‐band‐edge emission is weakest near the nanorod edges. The temperature‐dependent CL maps are consistent with the properties of a model in which singly ionized oxygen vacancies are stabilized by the surface band bending, which leads to the GL enhancement at the expense of near‐band‐edge emission. These results demonstrate the utility of spectral CL imaging to map the spatial distribution of defect luminescence in nanostructured materials.

Modeling and interpretation of UV and blue luminescence intensity in β-Ga2O3 by silicon and nitrogen doping

Temperature-dependent cathodoluminescence spectra of (2¯01) Si-doped β-Ga2O3 single crystals and (010) N-doped epitaxial films were comprehensively shown to investigate their electronic structure and defect states. The decrease in the self-trapped exciton (STE) emission at low temperatures in heavily Si-doped crystals implied the Debye-Hückel screening of STEs with the critical charge density larger than 2 × 1018 cm−3. The analysis based on the rate equation model suggested a significant influence of the donor-acceptor-pair recombination involving a SiGa donor on the UV luminescence band in the Si-doped crystals. The blue luminescence band was suppressed for the heavily N-doped epitaxial films with a N concentration of 1 × 1018 cm−3, implying the decrease in the oxygen vacancy (VO) concentration by N doping. The increase in the NO acceptor concentration as well as the decrease in the VO concentration were found to contribute to the compensation of residual Si donors resulting in high resistivity of N-doped epitaxial films.

High Internal Quantum Efficiency Ultraviolet Emission from Phase-Transition Cubic GaN Integrated on Nanopatterned Si(100).

Ultraviolet emission characteristics of cubic (c-) GaN enabled through hexagonal-to-cubic phase transition are reported. Substrate patterning and material growth are shown to affect phase purity and emission characteristics of c-GaN as studied by electron backscatter diffraction, and photo- and cathodoluminescence, respectively. Raman study shows a tensile strain in the c-GaN. Time-resolved photoluminescence reveals c-GaN band edge emission decay time of 11 ps. The ultraviolet emissions from both phases of GaN are linearly polarized in the same direction, which is along the ⟨112̅0⟩ and ⟨110⟩ directions of hexagonal GaN and c-GaN, respectively. Temperature-dependent (5.7 to 280 K) cathodoluminescence studies reveal an internal quantum efficiency of ~29% at room temperature along with intrinsic and extrinsic defect energy levels of ~124 and ~344 meV, respectively, of the phase-transition c-GaN. Using the IQE value and carrier decay lifetime, a radiative lifetime of 38 ps is extracted. Overall, photonic properties of phase-transition c-GaN and their dependence on substrate patterning and material growth are reported.

An energy self-compensating phosphosilicate material applied to temperature sensors.

For years, researchers have been exploring effective methods of sustaining the emission intensity of phosphors with increasing temperature by suppressing emission loss. In this work, we developed a multi-cationic site and lattice-distorted phosphosilicate phosphor, Ca8Al2P6SiO28:Ce, Eu. To obtain luminous-self-healing properties, we attempted to change the energy depths and density distributions of the traps to achieve self-suppression of emission loss by energy compensation from the traps or energy transfer between Ce3+ and Eu2+/Eu3+. The temperature-dependent emission spectra indicate that the luminescence of Ce3+ presents similar change trends in both single and co-doped samples. Meanwhile, the change trends of the Eu2+/Eu3+ emission intensities show obvious differences. Combined with the thermoluminescence curves, decay times, temperature-dependent fluorescence characteristics and cathodoluminescence spectra, we speculate that the traps play an important role in the luminescence of Ce3+ due to the smaller energy difference of the Ce3+ excited states and the conduction band. The abnormal luminescence of Eu2+/Eu3+ mainly results from the energy transfer of Ce3+ to Eu2+/Eu3+. For this phenomenon, a high thermal sensitive fluorescence intensity ratio is obtained in a broad temperature range, which implies that this material can be applied in temperature sensors.

Excitonic localization at macrostep edges in AlGaN/AlGaN multiple quantum wells

Double peaks at wavelength of 276 and 290 nm are observed for AlGaN/AlGaN multiple quantum wells (MQWs). Cathodoluminescence (CL) mappings identify that the emission at 290 nm originates from the macrostep edges. Potential minima induced by local variation of QW thickness and Ga incorporation are found along the step edges, where quantum wires (QWRs) are formed. The lateral advance rate of macrostep (∼310 nm/h) is obtained by investigating the distribution of QWRs. Temperature-dependent CL spectrum suggest that thermal quenching for 290 nm emission is dramatically suppressed compared with that for conventional QWs emission, which shows excitonic localization characteristics of QWRs.

Cathodoluminescence properties of Pr, Tm co-implanted GaN thin films

In this work, Pr and Tm co-doped GaN thin films were prepared by ion implantation. After a thermal annealing treatment for the lattice recovery and ions activation, temperature-dependent cathodoluminescence (CL) spectroscopy was applied to investigate the luminescent properties. At room temperature, the intensity ratio of blue emission to infrared emission of Tm ions was decreased as the implantation dose of Pr ions was increased, which is due to the energy transfer between Pr and Tm ions. In addition, the 467 nm blue emission of Tm and the 528 nm green emission of Pr were observed only at low temperature. A model was proposed to illuminate the light emission mechanism of Pr and Tm co-implanted GaN thin films.

Optical and electron beam studies of gamma-irradiated AlGaN/GaN high-electron-mobility transistors

ABSTRACTThe impact of 60Co gamma-irradiation on n-channel AlGaN/GaN high-electron-mobility transistors was studied by means of temperature-dependent electron beam-induced current (EBIC) and cathodoluminescence (CL) techniques. For the doses up to ∼250 Gy, an enhancement of minority carrier transport was observed as evident from the EBIC measurements. This enhancement is associated with internal electron irradiation induced by the primary gamma photons. For the doses above ∼250 Gy, deterioration in minority carrier transport was explained by carrier scattering on radiation-induced defects. It is shown that calculated activation energy from the EBIC and CL measurements follows exactly the same trend, which implies that the same underlying phenomenon is responsible for observed findings.

Cathodoluminescence Studies of Gamma-Irradiation Effects on AlGaN/GaN High Electron Mobility Transistors (HEMTs)

The effects of 60Co gamma-irradiation on AlGaN/GaN High Electron Mobility Transistors (HEMTs) were studied by means of temperature dependent Cathodoluminescence (CL). The CL spectra were examined for several devices with radiation exposures of gamma-ray doses up to 1000 Gy, and for temperature ranging from 25°C to 125°C. Gamma-irradiation causes the CL intensity of HEMTs to decrease by creating defects that act as non-radiative recombination centers. The activation energy was observed to decrease as dosage was incremented to 200 Gy. The decrease in activation energy is related to Compton electron induced increase in carrier lifetime. However for doses above 200 Gy, the activation energy increases as the dose increases. This behavior could be explained by the formation of large electrically active defect complexes through the addition of new radiation defects to the defects formed at low doses.

Shallow carrier traps in hydrothermal ZnO crystals

Native and hydrogen-plasma induced shallow traps in hydrothermally grown ZnO crystals have been investigated by charge-based deep level transient spectroscopy, photoluminescence and cathodoluminescence microanalysis. The as-grown ZnO exhibits a trap state at 23 meV, while H-doped ZnO produced by plasma doping shows two levels at 22 meV and 11 meV below the conduction band. As-grown ZnO displays the expected thermal decay of bound excitons with increasing temperature from 7 K, while we observed an anomalous behaviour of the excitonic emission in H-doped ZnO, in which its intensity increases with increasing temperature in the range 140–300 K. Based on a multitude of optical results, a qualitative model is developed which explains the Y line structural defects, which act as an electron trap with an activation energy of 11 meV, being responsible for the anomalous temperature-dependent cathodoluminescence of H-doped ZnO.