Room-temperature Cathodoluminescence Spectra Research Articles

Fabrication of wurtzite ZnS nanobelts via simple thermal evaporation

Mass production of uniform wurtzite-ZnS nanobelts is achieved by a simple thermal evaporation method using Au as the catalyst. The as-synthesized ZnS nanobelts are single crystalline, usually several tens of microns in length and several hundreds of nanometers in width. Most of the nanobelts grow along [01 1̄ 0] direction. Stacking faults are commonly observed in these nanobelts. The room-temperature cathodoluminescence spectrum of such nanobelts reveals three peaks, which may be ascribed to surface states, defects, and impurity-induced emissions, and is consistent with the nanobelt microstructure. The growth mechanism of the nanobelts is discussed.

Effect of Remote Hydrogen Plasma Treatment on ZnO Single Crystal Surfaces

ABSTRACTWe have studied the effects of hydrogen plasma treatment on the defect characteristics in single crystal ZnO grown at Eagle-Picher by chemical vapor transport. Depth-dependent cathodoluminescence (CL) spectra, temperature-dependent (9–300 K) and excitation intensity-dependent photoluminescence (PL) spectra reveal significant changes resulting from unannealed exposure of n-type ZnO to a remote hydrogen plasma. Low temperature PL spectra show that this hydrogen exposure effectively suppresses the free-exciton transition and redistributes intensities in the bound-exciton line set and two-electron satellites with their phonon replicas. The resultant spectra after hydrogenation exhibit a new peak feature at 3.366 eV possibly related to a neutral donor bound exciton. A simple thermal analysis of the activation energy for the 3.366 eV line yields 5–10 meV. Hydrogenation also produces a violet 100 meV-wide peak centered at 3.16 eV. Remote plasma hydrogenation produces similar changes in room-temperature CL spectra: near-band edge emission intensity increases with hydrogenation. Furthermore, this new emission increases with proximity to the free ZnO surfaces, i.e., with decreasing the energy of the incident electron beam from 3.0 down to 0.5 keV. Subsequent annealing at 450 °C completely restores both the PL and CL spectra in the sub-band gap range. The appearance of a new bound-exciton feature at 3.366 eV with H plasma exposure, the near-surface nature of the spectral changes, and the reversibility of spectral features with annealing indicate a direct link between H indiffusion and appearance of a shallow donor.

Alloying effects in organometallic vapor phase epitaxial grown Zn xCd 1− xSe epilayers on (0 0 1) InP

We report on the growth of Zn x Cd 1− x Se pseudo-binary alloys on (0 0 1) InP substrate, at temperature of 360°C, by organometallic vapor phase epitaxy. The compositions x in the alloy were estimated from the peak energies in the room temperature cathodoluminescence (CL) spectrum and were confirmed by X-ray diffraction (XRD). We found that the composition in the alloy was difficult to control by adjusting the precursor flow rates and the flow ratios. Moreover, the growth is unstable by phase separation of binary compounds and formation of novel structures with compositions x close to 0, 0.25, 0.5, 0.75 and 1. The formation of these structures is complicated and does not depend on the precursor flow rates and flow ratios during growth. We also found that Se treatment of InP substrate before the growth facilitates the formation of these structures.

Recovery treatments for ion-induced defects in high-quality homoepitaxial CVD diamond

Effects of vacuum annealing and hydrogen plasma exposure on ion-implantation-induced defects have been investigated in case of high-quality chemical-vapor deposited (CVD) diamond mainly using cathodoluminescence (CL) measurements. The well-focused 30-keV Ga ions were implanted into regions with different ion doses from 1×10 12 to 1×10 15 ions/cm 2. The free-exciton emission and the N– V center were observed at 235 and 575 nm, respectively, in room temperature CL spectra for as-grown homoepitaxial CVD diamond. The former vanished completely after all the implantation processes examined while the latter was destroyed more strongly with increasing ion doses. On one hand, the band edge emissions at 235 nm were hardly recovered even after any treatments examined. On the other hand, the CL peak at 575 nm reappeared either after a 30-min vacuum annealing at 900°C for the Ga dose of 1×10 12 ions/cm 2 or after a suitable hydrogen plasma treatment for all the Ga ion dosages examined. Thus, it is found that the band edge emission signal is required to investigate the beam damages to ‘high’ crystalline quality diamond. A removal of the damaged surface layer by the plasma etching is also discussed in relation to the recovery process mainly for the 575-nm peak at heavier ion doses.

Photo- and Cathodoluminescence of the Combustion-synthesized Al2O3–TiB2 Composites

Al2O3–TiB2 composites were synthesized by the combustion reaction 3TiO2 + 3B2O3 – (10 + x)Al, with x ranging from 0 to 4.71. The products consisted of corundum, TiB2, and a trace of Ti2O3 for the reactions with x = 0 and x = 1.4. However, the products of the reaction with x = 2.96 contained corundum and TiB2 only. For products of samples with x = 0, 1.4, and 2.96, similar room-temperature absorption spectra were observed in the ultraviolet and visible range at wavelengths of 265, 706, and 844 nm. Room-temperature photoluminescence spectra of the composites showed one visible band at about 720 nm. Room-temperature cathodoluminescence (CL) spectra of the composites synthesized by reactions with x = 0 and 1.4 exhibited two visible bands centered at 400 and 710 nm, respectively. From samples synthesized with x = 2.96, another visible band centered at 570 nm was observed in the CL spectrum. Comparing secondary electron images to panchromatic and monochromatic CL images of the composites, it was found that the light-emitting phase is TiB2-containing corundum. The 710-nm band is attributed to the transition between 1T1 (et) and 3T2 (et) excited states, and the 570- and 400-nm bands to the transitions between 3T1 (et) or 3A2 (e2) and 1T1 (et) excited states and 3T1 (t2) ground state of Ti2+ in corundum, respectively.

Cathodoluminescence study of europium-doped zirconia and cassiterite powders

The cathodoluminescence spectra of ZrO 2:Eu 3+ powders of average crystallite size 15–35 nm and SnO 2:Eu 3+ powders of crystallite size 40–50 nm, obtained after different heat treatments, were studied. The change in the room-temperature cathodoluminescence spectra of Eu 3+, measured between 400 and 750 nm by 4–15 kV excitation, showed that prolonged heat treatment (24 h/950 °C) leads to the formation of non-centrosymmetric Eu 3+ sites in both systems. In the ZrO 2:Eu 3+ systems two spectrum types were detected, for samples heated at relatively low and high temperatures, respectively. These observations are explained qualitatively by the formation of different configurations of oxygen vacancies in the process of crystal growth under heat treatment. Quantitative cathodoluminescence measurements show an increase of the luminous efficiency of the powders which is connected with particle agglomeration during heat treatment.

Cathodoluminescence and photoluminescence of highly luminescent CdSe/ZnS quantum dot composites

We report room-temperature cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals, 33 and 42 Å in diameter, embedded in a ZnS matrix. The thin-film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. Cathodoluminescence and photoluminescence are dominated by the sharp band-edge emission characteristic of the initial nanocrystals. The emission wavelength can be tuned in a broad window (470–650 nm) by varying the size of the dots. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and the voltage and current density applied.

Characterisation of MOCVD grown Ga1-xAlxAs for double heterostructure lasers

The n-type doping of Ga1-xAlxAs (x=0.25 to 0.35) with hydrogen selenide and silane has been studied. The room-temperature cathodoluminescence spectra and electron mobilities of epilayers with various doping levels have been measured and compared. Most n-Ga1-xAlxAs layers exhibit a luminescence emission at approximately=1.2 eV. It is shown that the ratio of the band-edge peak intensity to the approximately=1.2 eV emission peak intensity is a measure of the quality of the Ga1-xAlxAs. Double heterostructure lasers incorporating material that exhibits this 1.2 eV emission show increased threshold current density when the n-type doping level of the Ga1-xAlxAs is greater than 5*1017 cm-3.