At the vacuum-ultraviolet (VUV) beam line of a synchrotron, an end station for photoluminescence (PL) coupled to a system to detect absorption is used to investigate the luminescence and absorption of materials. We analyzed a CVD diamond window in wavelength range 160 - 250 nm at 300 and 14 K. The PL excited with VUV light enabled an identification of nitrogen defects in diamond samples. The VUV PL technique is applicable to explore advanced materials including materials with similar wide band gaps, such as boron nitride and aluminum nitride.
Photoluminescence (PL) arises when a sample is excited with radiation and responds by relaxation with emission of light. As PL, the material might emit light no matter what its form, shape, size or state. A great advantage of this analytical technique is the lack of necessity of further sample preparation for materials in various forms. Photoluminescence is hence well developed; numerous applications are employed for diverse materials—biomolecules, polymers, ice aggregates and polycyclic aromatic hydrocarbons.
Typical sources of excitation of PL, comprising mercury lamps, xenon lamps, tungsten filaments, tungstenhalogen lamps and various lasers, thus operate in the visible and UV ranges. For these conventional and laser sources, to tune the energy to the vacuum ultraviolet (VUV) region is difficult. For some advanced materials, especially phosphors applied in a plasma-display panel (PDP), their excitation requires light with photon energy into the VUV region. Synchrotron radiation (SR) provides an intense and continuous source of VUV light, which serves effectively as excitation to produce photoluminescence in the VUV region [
For this purpose, we have established an end station, coupled to a beam line of a synchrotron for photoluminescence, to investigate the luminescence of phosphors [
To prepare a CVD diamond, the most general methods involve deposition of a diamond film with plasma discharges, heated filaments and combustion. With improved CVD techniques yielding excellent and controlled rates of growth of diamond, CVD diamonds of great quality can be produced for some industrial and optical applications, but these diamond samples might contain impurities and crystal defects resulting from their growth through the gaseous phase [
For measurement of PL, a monochromator (JobinYvon HR320) equipped with a diffraction grating (1200 lines/mm) and a photomultiplier tube (PMT, Hamamatsu R943-02) serve to record the PL spectra. To record the excitation (PLE) spectra, the dispersed emission was monitored at a selected wavelength, for which purpose
the CGM beam was scanned with a grating (450 lines/ mm). All PLE spectra were normalized with the spectral response curve of the CGM beam line. To measure the spectra of samples at a temperature below 100 K, we attached the sample holder to the cold head of a helium closed-cycle cryostat (APD HC-DE204S); this holder was mounted on a flange such that the sample was rotatable to about 45˚ with respect to both the incident VUV source and the entrance slit of the monochromator. The temperature of the cold head in the cryostat was controlled to better than ±1 K during the period of data collection.
For the absorption measurements, the VUV light from the synchrotron was perpendicularly intersected to the CVD diamond samples; afterwards, the transmitted light irradiated on a glass window coated with sodium salicylate. Subsequently, the converted visible light was measured with a photomultiplier tube (Hamamatsu R943-02) in a photon-counting mode.
IR absorption spectra were recorded with a spectrometer (Bomem DA8 FTIR or Nicolet Magna 860 FTIR) attached to the infrared beamline; a detector, HgCdTe or DTGS, spanned the mid-IR range, 500 - 4000 cm−1 or 400 - 4000 cm−1, respectively. The IR spectra were typically measured with resolution 0.5 to 1 cm−1 and 512 to 1000 scans.
CVD diamond discs (type IIa optical grade, diameter 15 mm) were purchased with thickness 1.2 ± 0.05 mm (Diamond Materials Advanced Diamond Technology Company) or 1.0 mm (Drukker International Asia Pacific Company).
Diamonds are rarely pure and might be contaminated with hydrogen, boron, nitrogen and other elements. These impurities in diamonds are distinguishable according to their infrared spectra, which have hence served to characterize and to classify diamond samples [
Apart from these intrinsic signals, some one-phonon absorption lines due to lattice defects and impurities are discernible in impure diamonds. As examples, the presence of two adjacent substitutional nitrogen atoms (N2) in a diamond, called an A center, might produce an absorption at 1282 cm−1, whereas a line at 1177 cm−1 characterizes a B center due to four nitrogen atoms (N4) on substitutional sites symmetrically surrounding a vacancy [
Photoluminescence is a sensitive and powerful technique to characterize optically the impurities, defects and absorption edges of natural and synthetic diamonds. Both emission and excitation spectra of various diamond samples have been measured under diverse conditions in IR, visible, UV and X-ray regions. According to the UV absorption spectra, light suitable for use for PL excitation should have a wavelength less than 226 nm for this diamond window of optical grade.
Upon excitation at 223 nm of this CVD diamond window at 300 and 14 K, the emission spectra are displayed in Figures 4(a) and (b), respectively. Although the emission signal, as shown for 300 K in
For the sample at 14 K, the maximum intensity of the PL emission is 12 times that of the same sample at 300 K; prominent features appear along the continuous curve. The characteristic emission feature recorded at 416.5 nm in
The photoluminescence excitation (PLE) spectra of a CVD diamond disc monitored with emission about 428 nm at 300 K and 417 nm at 14 K are presented in
When the sample was cooled to 14 K, PLE features appeared between 224 (5.53 eV) and 180 nm (6.80 eV), as shown in
are labeled as lines A(0) - A(4), respectively, of progression A of the N2 center [
In summary of our results, to identify impurities and defects in a CVD diamond window of optical quality, we applied three spectrometric methods. Hence, we found that IR and UV absorption techniques are insufficiently sensitive to monitor minute impurities and defects in diamonds, whereas the VUV PL technique provides an analytical ability for nitrogen defects. We contend that the PL technique excitation with VUV light from a synchrotron can provide unique spectra and extend the capability to explore advanced materials including those with similar wide band gaps, such as boron nitride and aluminum nitride.
National Science Council of the Republic of China (grant NSC102-2113-M-213-005-MY3) and National Synchrotron Radiation Research Center provided support.