Deep UV Laser Research Articles

Deep-UV laser source based on χ(2) optical frequency conversion and χ(3) stimulated Raman scattering

By integrating χ(2) optical frequency conversion and χ(3) stimulated Raman scattering (SRS) technology, we demonstrated a new, to the best of our knowledge, deep-UV laser generation scheme near 200 nm in a non-cryogenic KD2PO4 (DKDP) crystal. Based on an Nd:YAG laser (1064 nm, ω1) and cascaded LiB3O5 and DKDP crystals, a 266 nm radiation was obtained firstly by the second- and fourth-harmonic generation (SHG and FHG) (ω4). The energy conversion efficiency from ω1 to ω4 was 24.8%. Meanwhile, the Stokes lights (ω R ) were stimulated by the Nd:YAG laser in a KGd(WO4)2 crystal with two polarization-dependent Raman shifts of 768 cm−1 and 901 cm−1. Finally, 3.5 mJ, 216.3 nm, and 3.1 mJ, 217 nm deep-UV laser sources were obtained in a DKDP crystal by the sum-frequency generation (SFG) of ω R and ω4. The total conversion efficiency from 1064 nm infrared to ∼200 nm deep UV was ∼3%. This scheme, by systematically combining the χ(2) and χ(3) nonlinear effects, overcame the phase-matching limitation of traditional schemes to acquire high-energy 200 nm wave band deep UV via the fifth-harmonic generation (FiHG) in the DKDP crystal, which may provide a new way for the deep-UV laser generation with high energy and high-peak power.

Defect-Free SiGe/Si Multistacks for Next-Generation CFET Architectures

The complementary FET (CFET) is the leading architecture choice for upcoming logic applications. Leveraging a stacked pMOS and nMOS design, this architecture is expected to enhance device performance with a reduced footprint [1,2]. The fabrication of the channels in these devices requires the deposition of SiGe/Si multistacks. The difference in etching rate of Si and SiGe is exploited, allowing for the selective removal of the SiGe layers and the resulting formation of free-standing Si channels.In this contribution, we present fully strained pseudomorphic SiGe/Si epitaxial multistacks for the use as channel regions in CFET devices. All layers were grown in a 300 mm ASM reduced-pressure chemical vapor deposition (RPCVD) reactor on Si(001) substrates.A targeted multistack structure for 1x1 nanosheet channel is displayed in Fig. 1(a). Two types of SiGe layers are present in the stack, having Ge concentration of 22% and 38%, respectively. Fig. 1(b) shows the X-ray diffraction (XRD) Omega-2Theta scan acquired for the deposited stack at the Si(004) Bragg reflection. The experimental spectrum is accurately simulated using a model closely resembling the nominal structure, thereby confirming that the thicknesses and germanium concentrations of the stack’s constituent layers are well within the targeted specifications. The X-ray reflectivity (XRR) spectrum acquired on the same sample and shown in Fig. 1(c) exhibits a high signal-to-noise ratio even at high incidence angles, suggesting a low surface roughness of the sample. Fig. 1(d) displays the asymmetric reciprocal space map (RSM) acquired around the Si(113) reflection. The stack's diffraction peaks are very narrow and horizontally aligned, indicating the absence of strain-relaxation in the sample. The within-wafer non-uniformity (WWNU) of thickness and Ge concentration was first minimized for single SiGe/Si bilayers. The optimized growth parameters were then successfully applied to the growth of multi-layered structures. Fig.2 depicts the haze map measured via deep-UV (DUV) laser scattering for the same multistack considering a 3 mm wafer edge-exclusion. The measured intensity ranges between 0.59 and 0.69 ppm, indicating a highly smooth surface morphology.AFM inspections, as shown in Fig. 3(a), confirm that the stack is fully strained, as evidenced by the absence of cross-hatch pattern in the examined areas with dimension 20x20 μm2. The RMS roughness (Rq) of the sample surface is found to be as low as 0.1 nm, which reduces to 0.07 nm on a 2x2 μm2 inspection area (Fig. 3(b)).The cross-sectional TEM analysis of a 3x3 nanosheet channel stack, as shown in Fig. 4(a), indicates that the layer thicknesses are highly consistent and in good agreement with the targeted nominal values. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping of the same cross-section also reveals a close match between the measured Ge concentration and the expected value. Furthermore, the high-resolution HAADF-TEM image presented in Fig. 4(b) demonstrates the excellent crystalline quality of the epitaxial stack and indicates the presence of sharp compositional transitions between the layers.Finally, Fig. 5 displays the Omega-2Theta spectra measured for the first and last 1x1 nano-sheet channel stacks fabricated in a batch processing of 25 wafers. Notably, the two spectra exhibit perfect overlapping, with identical peak positions and intensities, showcasing ideal run-to-run repeatability of the process.In this work, we have demonstrated the fabrication of complex SiGe/Si multistacks that exhibit excellent agreement with the targeted specifications, are free of relaxation-defects and have extremely low surface haze values. These results highlight the effectiveness of our growth process in overcoming typical challenges associated with the development of advanced CFET devices.[1] P. Schuddinck et al., 2022 IEEE Symposium on VLSI, pp. 365-366.[2] E. Park et al., IEEE Transactions on VLSI Systems, 31(2), , 2022, pp.177-187. Figure 1

Photoluminescence and Raman spectroscopy of wide bandgap semiconductors damaged by deep-UV laser irradiation

The effects of a pulsed, focused, deep-UV (4.66 eV) laser on wide and ultra-wide bandgap semiconductors were investigated with photoluminescence (PL) and Raman spectroscopy. Three semiconductor single crystals were studied: silicon carbide (6H-SiC), gallium nitride (GaN), and gallium oxide (β-Ga2O3). Atomic emission lines from neutral Ga or Si were observed during the laser-damage process. For all three semiconductors, PL mapping (3.49 eV laser excitation) of the damaged material revealed visible emission bands in the 2.6–2.8 eV range, attributed to point defects. Raman spectra (2.33 eV excitation) showed a reduction in the Raman peak intensities in the damaged region, along with weak PL bands around 1.9–2.1 eV.

High-power picosecond UV and deep-UV laser sources delivering powers of 30 W at 355 nm, 10 W at 266 nm, and 5 W at 213 nm.

Utilizing LiB3O5, β-BaB2O4 crystals, and an Nd:YVO4 laser with an average power of 70 W and a repetition rate of 100 kHz, we systematically demonstrated and operated high-repetition-rate, high-power, all-solid-state, UV, and deep-UV picosecond laser sources via high-efficiency third-, fourth-, and fifth-harmonic generation (THG, FHG, and FiHG). The maximum output powers of the radiation at 355, 266, and 213 nm reached 31.2, 10.6, and 4.86 W, respectively, and the highest conversion efficiencies from the 1064 nm infrared laser beam to its third, fourth, and fifth harmonics were up to 44.6, 15.3, and 7.16%, respectively. The intensity autocorrelation traces of the generated 355, 266, and 213 nm radiation were measured based on a two-photon absorption (TPA), and the extracted pulse durations were 7.7, 6.1, and 5.9 ps, respectively. This work validates the performance of the β-BaB2O4 crystal in obtaining deep-UV radiation, laying the foundation for compact high-power deep-UV devices. Especially the power of 213 nm radiation may be the highest power, to our knowledge, for the picosecond deep-UV radiation near the wave band of ∼200 nm.

Effect of Li concentration on second harmonic generation from widegap ZnMgO thin films

Widegap ZnMgO thin films hold great potential for applications to the high-efficiency generation of deep UV laser via frequency conversion, because of low light propagation loss and reversible polarization. In this work, we studied the effect of Li concentration on the second harmonic generation (SHG) from a-axis oriented ZnMgO thin films with an optical bandgap of about 3.96 eV. The SHG intensity per unit thickness from the ZnMgO thin film with a small Li concentration is approximately 10.7 times larger than the one without Li incorporation. The Li incorporation obviously enlarges interplanar spacing of ( 112̅0 ) plane and reduces the leakage current by three orders of magnitude. X-ray photoelectron spectroscopy measurements suggest the Li incorporation enhances SHG via enhancing electronic polarization. These findings uncover defect complexes based on Li interstitials in oxygen octahedron play a very important role in enhancing the nonlinear polarization of widegap ZnMgO thin films under optical frequency electric field.

Photochemistry of Photoinduced-Reaction Generated Bubbles.

UV light can create and grow bubbles (herein referred to as PIRGBs for photoinduced-reaction generated bubbles) at liquid/solid interfaces through photoinduced reactions that produce gases. Unlike the simple experience of blowing water bubbles through a straw, in which the bubbles quickly move away from their nucleation sites, not only can a deep UV laser beam create PIRGBs in liquid acetone, but also can hold and grow them. Free bubbles could be attracted to the excitation region from millimeters away, indicating that the reactions cause radial inward flow on the liquid surface. The radial flow can be due to imbalanced surface tensions at the interfaces. Raman measurements reveal that the gases in the PIRGBs include C2H6, CO, and H2, and in liquid acetone, sp2-carbon species are detected upon the UV excitation. Time series Raman measurement discloses a photocarbonization process in which small acyclic carbon species gradually form small clusters with carbon rings and eventually produce a large piece of amorphous carbon at the top of a PIRGB in pure liquid acetone. The photocarbonization may open new avenues for development of carbonaceous materials. Using PIRGB, miniature or microscale gas production reactors can be developed for producing gases.

Optical transmission enhancement of ionic crystals via superionic fluoride transfer: Growing VUV-transparent radioactive crystals

The 8−eV first nuclear excited state in Th229 is a candidate for implementing a nuclear clock. Doping Th229 into ionic crystals such as CaF2 is expected to suppress nonradiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of Th229 prohibits the growth of high-quality single crystals with high Th229 concentration; radiolysis causes fluoride loss, increasing absorption at 8eV. These radioactively doped crystals are thus a unique material for which a deeper analysis of the physical effects of radioactivity on growth, crystal structure, and electronic properties is presented. Following the analysis, we overcome the increase in absorption at 8eV by annealing Th229-doped CaF2 at 1250∘C in CF4. This technique allows to adjust the fluoride content without crystal melting, preserving its single-crystal structure. Superionic state annealing ensures rapid fluoride distribution, creating fully transparent and radiation-hard crystals. This approach enables control over the charge state of dopants, which can be used in deep-UV optics, laser crystals, scintillators, and nuclear clocks. Published by the American Physical Society 2024

Deep-UV Raman spectroscopy: A novel heuristic method to characterize volcanologically relevant glasses on Mars

Deep ultraviolet Raman spectroscopy is an essential component of the Perseverance rover operating on Mars. Here we propose a proof of concept of deep-UV Raman structural characterization of volcanologically relevant silicate glasses to provide a suitable analytical method of UV Raman spectra collected on Mars. The results show few but substantial spectral differences concerning those obtained by conventional Raman scattering using visible light laser sources. The evolution of the UV Raman spectra between 825 and 1300 cm−1 is confirmed to be more sensitive to the silica network's short-range structure than that below 700 cm−1 when compared to their visible counterpart. We adopted a Gaussian fitting model to parametrize the number of bridging oxygens resulting from the tetrahedral‑oxygens stretching vibrations along a sub-alkaline join of volcanic glasses. We used these parameters to empirically provide a relation with the glass silica content. The model was externally validated with glasses having different and known compositions (in particular different amounts of iron, alkali and alumina) extending the validation set also to other magmatic series (i.e. alkaline and shoshonitic). Our approach enables a fast screening of deep-UV Raman spectra collected on Mars to disentangle the glass structure and retrieve its silica content within a given magmatic series. This method is relevant for planetary exploration and applies both to microanalysis of dry or hydrate volcanologically relevant glasses and general technical glasses since deep-UV Raman spectroscopy is particularly effective in reducing the unwished photoluminescence and increasing the signal-to-noise ratio.

Continuously wavelength tunable, continuous wave laser ideal for UV Raman spectroscopy

AbstractWe utilize a novel, high‐power, tunable, continuous wave (CW) deep UV laser to measure resonance Raman spectra of phenolate solutions with high signal‐to‐noise ratios (SNR). In UV resonance Raman (UVRR), increased coupling of the excitation light with a chromophore can transfer molecules into excited states that cause increased heating and photochemistry. Deep UV lasers have traditionally utilized high peak powers to enable efficient single‐pass nonlinear conversion from visible into near infrared light. Nonlinear phenomena such as the formation of transient radical species, Raman saturation, thermal heating, and dielectric breakdown can introduce extraneous light sources that can complicate the interpretation of the Raman spectrum. Dielectric breakdown can increase the baseline, increase noise, and sometimes saturate the detector, preventing Raman detection. Spontaneous Raman scattering intensities should scale linearly with the excitation light intensity. However, this linear behavior does not always occur with pulsed laser excitation. This occurs because stimulated Raman scattering can cause a superlinear intensity response, or transient absorption can cause sublinear intensity responses. CW laser excitation excites samples with electric fields that are much lower than typical pulsed laser excitation. This eliminates the nonlinear responses. The geometry of our new CW laser enables high gain in the harmonic generation cavities that achieve high harmonic generation efficiencies. Average power in the deep UV is >30 mW for wavelengths as short as 206 nm. In the work here, we demonstrate that CW excitation is ideal for resonance Raman measurements in general to reduce spectral complexity.

All-solid-state CW Pr3+:YLF green laser at 522 nm end-pumped by a high-power fiber-coupled 444 nm blue LD module

Continuous wave (CW) green lasers have a lot of important applications in many fields, including holography, interferometry, atom cooling and trapping, and quantum optics, and they are usually achieved by frequency-doubling 1 µm lasers based on the Nd3+ gain media. In this paper, we present an all-solid-state CW green laser with an output wavelength of 522 nm, which was directly attained by employing a Pr3+:YLF crystal pumped with a high-power fiber-coupled blue laser diode (LD) module as the gain medium. Due to the negative thermal lens effect of the Pr3+:YLF crystal, the designed laser resonator had to be lengthened with the increase in the incident pump power. As a result, when a 0.5% doped Pr3+:YLF crystal was employed as the gain medium and the incident pump power was 12 W, the length of the resonator was optimized to 311.3 mm and the maximum output power of 522 nm green laser was up to 886 mW. The obtained conversion efficiency and beam quality M2 were 11.25% and 1.15, respectively. The long-term power stability within 4.5 h was better than ±1.5% at an output power of 700 mW. The obtained watt-level green laser can also be used to generate high power CW deep UV laser for laser processing of silicon and organic materials, inspection, etc.

Towards Efficient Electrically-Driven Deep UVC Lasing: Challenges and Opportunities

The major issues confronting the performance of deep-UV (DUV) laser diodes (LDs) are reviewed along with the different approaches aimed at performance improvement. The impact of threading dislocations on the laser threshold current, limitations on heavy n- and p-doping in Al-rich AlGaN alloys, unavoidable electron leakage into the p-layers of (0001) LD structures, implementation of tunnel junctions, and non-uniform hole injection into multiple quantum wells in the active region are discussed. Special attention is paid to the current status of n- and p-type doping and threading dislocation density reduction, both being the factors largely determining the performance of DUV-LDs. It is shown that most of the above problems originate from intrinsic properties of the wide-bandgap AlGaN semiconductors, which emphasizes their fundamental role in the limitation of deep-UV LD performance. Among various remedies, novel promising technological and design approaches, such as high-temperature face-to-face annealing and distributed polarization doping, are discussed. Whenever possible, we provided a comparison between the growth capabilities of MOVPE and MBE techniques to fabricate DUV-LD structures.

Enhanced performance of an Ag(100) photocathode by an ultra-thin MgO film

Metal photocathodes are widely utilized as electron sources for particle accelerators for their ease of use, high durability, and fast response time. However, the high work function (WF) and low quantum efficiency (QE) typically observed in metals necessitate the use of high power deep UV lasers. Metal oxide ultra-thin films on metals offer a route to photocathodes with a lower WF and improved QE while maintaining photocathode durability and response time. We show how the photocathode performance of an Ag(100) single crystal is enhanced by the addition of an ultra-thin MgO film. The film growth and WF reduction of 1 eV are characterized, and the QE and mean transverse energy (MTE) are measured as a function of illumination wavelength. An eightfold increase of QE is achieved at 266 nm without adding to MTE through additional surface roughness, and the resistance of the photocathode to O2 gas is greatly improved.

High-peak-power picosecond deep-UV laser sources.

Ultrafast deep-UV laser sources have extensive applications across a wide number of fields, whether biomedicine, photolithography, industrial processing, or state-of-the-art scientific research. However, it has been challenging to obtain deep-UV laser sources with high conversion efficiency and output peak power. Here, we simultaneously demonstrated high-peak-power picosecond deep-UV laser sources at two typical wavebands of 263.2 and 210.5 nm via the efficient fourth- and fifth-harmonic generation. The highest peak power of 263.2 and 210.5 nm laser radiations were up to 2.13 GW (6.72 ps) and 1.38 GW (5.08 ps). The overall conversion efficiencies from the fundamental wave to the fourth and fifth harmonic were up to 42.9% and 28.8%, respectively. The demonstrated results represent the highest conversion efficiencies and output peak powers of picosecond deep-UV laser sources at present to our knowledge. Additionally, we also systematically characterized the deep-UV optical properties of typical birefringent and nonlinear borate crystals, including α-BaB2O4, β-BaB2O4, LiB3O5, and CsLiB6O10 crystals. The experiments and obtained numerous new optical data in this work will contribute to the generation of ultrahigh-peak-power deep-UV and vacuum-UV laser sources and crucial applications in both science and industry, such as high-energy-density physics, material science, and laser machining.

AlGaN-Based Deep UV Laser Diodes without an Electron Blocking Layer and with a Reduced Aluminum Composition of Quantum Barriers

AlGaN-Based Deep UV Laser Diodes without an Electron Blocking Layer and with a Reduced Aluminum Composition of Quantum Barriers

220 nm deep-UV coherent source based on fifth-harmonic generation in ADP and DKDP crystals

Based on the 1.1 μm laser radiation obtained by parametrically amplifying Yb:YAG laser using frequency doubling of Nd:YAG laser, a 220 nm deep-UV coherent source was systematically presented via the efficient fifth-harmonic generation (FiHG) in NH4H2PO4 (ADP) and KD2PO4 (DKDP) crystals. In ADP crystals, noncritical phase-matching (NCPM) fourth-harmonic generation (FHG) and FiHG were realized at 109.4 and 31.6 °C, respectively. For DKDP crystals, we demonstrated the critical phase-matching FHG at 57.5 °C and verified that NCPM FiHG can be achieved at 38.7 °C, which was the first demonstration of the FiHG using the DKDP crystal to our knowledge. The energy-dependent conversion efficiencies, angular acceptances, and temperature acceptances of these nonlinear processes were systematically measured. The highest total conversion efficiencies from 1.1 μm to the fifth harmonic in ADP and DKDP crystals were 17.5% and 23.6%, respectively. Owing to the large-aperture availability of KDP-family crystals, this work paves the way for the generation and application of high-energy and high-peak-power deep-UV laser radiation.

Deep-UV laser direct writing of photoluminescent ZnO submicron patterns: an example of nanoarchitectonics concept

ABSTRACT Micro- and nanopatterning of metal oxide materials is an important process to develop electronic or optoelectronic devices. ZnO is a material of choice for its semiconducting and photoluminescence properties. In the frame of the nanoarchitectonics concept, we have developed and investigated a new process that relies on direct writing laser patterning in the Deep-UV (DUV) range to prepare photoluminescent microstructures of ZnO at room temperature, under air. This process is based on a synthesis of colloidal ZnO nanocrystals (NCs) with a careful choice of the ligands on the surface to obtain an optimal (i) stability of the colloids, (ii) redissolution of the non-insolated parts and (iii) cross-linking of the DUV-insolated parts. The mechanisms of photocrosslinking are studied by different spectroscopic methods. This room temperature process preserves the photoluminescence properties of the NCs and the wavelength used in DUV allows to reach a sub-micrometer resolution, which opens new perspectives for the integration of microstructures on flexible substrates for optoelectronic applications.

Spin-Polarized Photoemission from Chiral CuO Catalyst Thin Films

The chirality-induced spin selectivity (CISS) effect facilitates a paradigm shift for controlling the outcome and efficiency of spin-dependent chemical reactions, for example, photoinduced water splitting. While the phenomenon is established in organic chiral molecules, its emergence in chiral but inorganic, nonmolecular materials is not yet understood. Nevertheless, inorganic spin-filtering materials offer favorable characteristics, such as thermal and chemical stability, over organic, molecular spin filters. Chiral cupric oxide (CuO) thin films can spin polarize (photo)electron currents, and this capability is linked to the occurrence of the CISS effect. In the present work, chiral CuO films, electrochemically deposited on partially UV-transparent polycrystalline gold substrates, were subjected to deep-UV laser pulses, and the average spin polarization of photoelectrons was measured in a Mott scattering apparatus. By energy resolving the photoelectrons and changing the photoexcitation geometry, the energy distribution and spin polarization of the photoelectrons originating from the Au substrate could be distinguished from those arising from the CuO film. The findings reveal that the spin polarization is energy dependent and, furthermore, indicate that the measured polarization values can be rationalized as a sum of an intrinsic spin polarization in the chiral oxide layer and a contribution via CISS-related spin filtering of electrons from the Au substrate. The results support efforts toward a rational design of further spin-selective catalytic oxide materials.

A crystal-processing machine using a deep-ultraviolet laser: application to long-wavelength native SAD experiments.

While native SAD phasing is a promising method for next-generation macromolecular crystallography, it requires the collection of high-quality diffraction data using long-wavelength X-rays. The crystal itself and the noncrystalline medium around the crystal can cause background noise during long-wavelength X-ray data collection, hampering native SAD phasing. Optimizing the crystal size and shape or removing noncrystalline sample portions have thus been considered to be effective means of improving the data quality. A crystal-processing machine that uses a deep-UV laser has been developed. The machine utilizes the pulsed UV laser soft ablation (PULSA) technique, which generates less heat than methods using infrared or visible lasers. Since protein crystals are sensitive to heat damage, PULSA is an appropriate method to process them. Integration of a high-speed Galvano scanner and a high-precision goniometer enables protein crystals to be shaped precisely and efficiently. Application of this crystal-processing machine to a long-wavelength X-ray diffraction experiment significantly improved the diffraction data quality and thereby increased the success rate in experimental phasing using anomalous diffraction from atoms.

High-power, high-efficiency, all-fiberized-laser-pumped, 260-nm, deep-UV laser for bacterial deactivation

We report a 5.8-W deep-ultraviolet (DUV) laser obtained from frequency-quadrupling of an all-fiberized ytterbium-doped fiber (YDF) master oscillator power amplifier (MOPA). The MOPA system delivers 585 ps pulses at 1040 nm with a maximum available output power of 23.5 W for nonlinear frequency conversion. A lithium triborate (LBO) crystal and a beta barium borate (BBO) crystal are employed for second- and fourth-harmonic generation (FHG), respectively. At a repetition rate of 1.6 MHz, a maximum DUV output power of 5.8 W is obtained at 260 nm with a corresponding pulse energy of 3.6 μJ and maximum peak power of at least 6.9 kW. A 1μm-to-260nm conversion efficiency of 26.4% is achieved at a DUV output power of 5.8 W. To the best of our knowledge these results represent the highest-average-power fiberized-laser-pumped DUV laser, as well as the most efficient DUV generation based on BBO crystals to date. We further demonstrate application of the pulsed DUV laser in bacterial disinfection achieving an inactivation efficiency of 99.999% for E-coli bacteria at a DUV exposure of 7 mJ/cm2.

Ultra-wide bandgap β-Ga2O3 films: Optical, phonon, and temperature response properties

Optical and phonon interactions of Ga2O3 thin films with nanocrystalline morphology were studied at extreme temperatures. The films were grown using a sputtering technique and analyzed via temperature response transmission, Raman scattering, and high-resolution deep-UV photoluminescence (PL). Raman modes indicated that the structure corresponds to the β-phase. The optical-gap at the range of 77–620 K exhibited a redshift of ∼200 meV, with a temperature coefficient of ∼0.4 meV/K. The optical-gap at room-temperature is 4.85 eV. The electron–phonon interaction model at that temperature range pointed to a low energy phonon, ∼31 meV, that is involved in the thermal properties of the optical-gap. Detailed Urbach energy analysis indicated that defects are the dominant mechanism controlling the band-edge characteristics even at an elevated temperature regime where phonon dominance is usually expected. Defects are attributed to the disordered forms of graphite that were detected via Raman scattering and to the granular morphology of the film. A deep-UV laser with an above-bandgap exaction line of 5.1 eV was employed to map the PL of the films. The highly resolved spectra, even at room-temperature, show a strong emission of ∼3.56 eV attributed to self-trapped holes (STHs). The STH is discussed and modeled in terms of the self-trapped exciton. Moreover, a very distinct but low-intensity emission was found at 4.85 eV that agrees with the value of the optical-gap and is attributed to bandgap recombination. The intensity ratio between the STH and that of the bandgap was found to be 6:1.