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Optically stimulated luminescence characteristics of heteroepitaxial diamond with different nitrogen concentrations: effects of pre-irradiation and luminescence center activation
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Abstract
This study delves into the optically stimulated luminescence (OSL) characteristics of heteroepitaxial diamond (HED) with different nitrogen concentrations, focusing on the effects of pre-irradiation and luminescence center activation. The research compares two HED samples with nitrogen concentrations of 3 ppb and 1 ppm, examining OSL decay behavior, dose-response characteristics, emission spectra, and fading properties. Key findings include the significant influence of nitrogen concentration on OSL sensitivity, with the higher nitrogen sample exhibiting an order of magnitude greater OSL intensity. Pre-irradiation was found to enhance OSL sensitivity in the nitrogen-rich sample, attributed to changes in the electronic occupancy of existing defect levels. The study also reveals that nitrogen concentration affects the thermal stability of traps responsible for OSL. The OSL emission spectra, consistent with NV- centers, remained unchanged by pre-irradiation, indicating that the enhancement originates from modifications in the occupancy or activation efficiency of existing luminescence centers. These insights highlight the potential of nitrogen-rich HED for advanced dosimetry applications, offering enhanced sensitivity and reusable capabilities.
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Abstract
This study systematically investigated the optically stimulated luminescence (OSL) characteristics of heteroepitaxial diamond (HED) with two different nitrogen concentrations (3 ppb and 1 ppm). The nitrogen-rich sample exhibited substantially higher OSL intensity than the low-nitrogen sample, reflecting an increased density of trapping sites and luminescent centers associated with nitrogen-related defects. Both samples showed weak dose dependence below 10 Gy and saturation at 100–200 Gy, consistent with the classical center-depletion model. A pronounced enhancement in OSL sensitivity was observed only in the nitrogen-rich sample following pre-irradiation up to 400 Gy. This enhancement is attributed to increased trapped-electron population, activation of previously inactive luminescent centers, and improved radiative recombination efficiency. The enhancement was completely removed by annealing at 500 °C for 60 s, indicating that the effect originates from metastable changes in defect occupancy rather than permanent defect formation. The OSL emission spectra exhibited a single broad band between 500 and 800 nm, peaking at 620–650 nm, corresponding to the phonon sideband of NV⁻-related luminescence. No zero-phonon line at 637 nm was resolved, consistent with the broad-band, phonon-assisted nature of the measured OSL emission. The spectral shape remained unchanged after pre-irradiation. Fading measurements further revealed that the nitrogen-rich diamond possesses more thermally stable trap levels than the low-nitrogen sample. These findings demonstrate that nitrogen incorporation is an effective strategy for tuning the OSL performance of HED and the potential of nitrogen-rich HED for high-dose and reusable dosimetry applications.
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1 Introduction
Diamond has attracted significant attention as an advanced electronic and optoelectronic material owing to its exceptional physical properties, including a wide bandgap (5.47 eV), high thermal conductivity, chemical inertness, mechanical robustness, and remarkable radiation hardness. In addition, the effective atomic number of diamond (Zeff ≈ 6) is close to that of soft tissue, making it a highly tissue-equivalent material suitable for radiation dosimetry in a wide range of medical applications such as diagnostic X-ray imaging, external beam radiotherapy, particle therapy, and boron neutron capture therapy (BNCT) [1‐3]. These advantages have motivated extensive research into diamond-based detectors and luminescent dosimeters over several decades.
With advances in chemical vapor deposition (CVD) technologies, synthetic diamond films with controlled impurity concentrations and high crystalline quality have become widely available. This progress has enabled systematic investigations of defect-related luminescence processes, particularly thermoluminescence (TL). Numerous studies have demonstrated that CVD diamond exhibits stable TL glow curves, relatively high sensitivity, and acceptable reproducibility under various irradiation conditions [4‐7]. The TL response is known to depend strongly on the defect and impurity structure of the diamond. In particular, nitrogen and boron are key impurities that act as trapping and recombination centers, significantly influencing trap distributions, glow-peak positions, and the dose–response linearity of CVD diamond [8‐12]. Nitrogen-containing CVD diamonds have been reported to exhibit shifts in glow-peak temperatures, variations in TL efficiency, and changes in saturation behavior depending on nitrogen concentration and growth conditions [8, 9]. Likewise, boron incorporation can introduce deep acceptor states and substantially modify TL and related luminescence characteristics [10, 11, 13].
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In contrast to TL, optically stimulated luminescence (OSL) from diamonds has been far less explored. Only a limited number of studies have reported OSL from polycrystalline or nanocrystalline CVD diamond films, and these works generally describe relatively weak OSL signals and strong dependence on shallow trapping states [11‐15]. The OSL response in such films is highly sensitive to crystal defects such as grain boundaries, dislocations, and non-diamond (sp2-bonded) carbon phases, which complicates reproducibility and the interpretation of trap dynamics. Moreover, to the best of current knowledge, OSL studies on heteroepitaxial diamond (HED) films are extremely limited, and no systematic comparison of high-quality HED samples with different impurity concentrations has been reported so far.
HED has gained significant attention as it enables grain-boundary-free, large-area single-crystal diamond through epitaxial growth on iridium buffer layers. High-quality HED grown on Ir/YSZ/Si substrates has been reported in several studies [16‐18]. In contrast, HED grown on sapphire substrates is a unique material developed by Kim et al., and only a limited number of reports on sapphire-based HED growth exist [19, 20]. Sapphire-based HED exhibits exceptionally high crystalline uniformity, low defect density, and highly reproducible large-area step-flow growth on Ir buffer layers, with nitrogen concentration precisely controlled from the ppb to ppm range. These characteristics allow direct evaluation of the influence of intrinsic defects and impurity states on optical and dosimetric behaviors without complications arising from grain boundaries or structural heterogeneity. However, to date, no OSL studies have been conducted using high-quality sapphire-based HED, and the role of nitrogen concentration in determining OSL efficiency and trapping dynamics remains unexplored.
In this study, we present the first systematic comparison of the OSL properties of two sapphire-based HED samples grown using the same process, with nitrogen concentrations of 3 ppb and 1 ppm. Using TSL/OSL/RPL Automated and Integrated Measurement System (TORAIMS) equipped with 850 nm stimulation [21], we examine OSL decay behavior, dose–response characteristics, OSL spectra, and fading properties. By analyzing two high-quality HED samples with nitrogen concentrations differing by three orders of magnitude, this work provides new insights into the role of nitrogen-related defects in governing OSL efficiency and trapping dynamics.
2 Materials and methods
2.1 Materials
Two HED samples grown on sapphire substrates using an iridium buffer layer were used in this study [19, 20]. The diamond layers were synthesized by microwave plasma CVD under step-flow growth conditions on the Ir buffer layer, resulting in high crystalline quality. Raman spectroscopy confirmed a sharp diamond peak at 1332 cm⁻1, consistent with single-crystal diamond. After growth, the sapphire substrate and iridium buffer layer were removed, and the resulting diamond wafers were cut into single-crystal chips with dimensions of 10 × 10 mm2 and a thickness of 0.5 mm. The weight of the HED chips was 169 mg for HED-N_3ppb and 172 mg for HED-N_1ppm. Nitrogen concentrations were controlled during the growth process, producing the following two samples:
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・HED-N_3ppb: nitrogen concentration of approximately 3 ppb.
・HED-N_1ppm: nitrogen concentration of approximately 1 ppm.
Both samples were polished on the growth surface. No metal electrodes were deposited in order to avoid additional trapping or recombination originating from metal–diamond interfaces. Photographs of the samples are shown in Fig. 1.
Fig. 1
Photographs of the HED samples [19, 20]: HED-N_3ppb (left) and HED-N_1ppm (right)
All OSL measurements were performed using a TORAIMS system [21], whose photograph and schematic diagram are shown in Fig. 2. The system consists of an 850 nm near-infrared stimulation light source, a photon-counting detector, optical filters, and a computer-controlled data acquisition unit.
Fig. 2
Photograph (left) and schematic diagram (right) of the TORAIMS. [21]
For OSL intensity measurements, the emitted luminescence was detected using a photon-counting head (H11890-210, Hamamatsu Photonics K.K.), and the signal was collected through a band-pass filter centered at 650 nm with a bandwidth of ± 100 nm. The OSL signal was recorded with a sampling interval of 1 s, repeated for 300 cycles, resulting in a total acquisition time of 300 s.
For OSL emission spectra, the luminescence was guided through an optical fiber and analyzed using a QEPro spectrometer (Ocean Optics). A 780-nm long-cut (IR-cut) filter was placed in front of the spectrometer to suppress the stimulation light. Spectral data were integrated over a total duration of 300 s. All measurements were conducted at room temperature in a dark environment to prevent ambient light interference.
2.3 Experimental procedures
All OSL measurements were performed following a standardized sequence unless otherwise noted. All measurements were conducted under temperature-controlled conditions at 40 ℃. Each sample was first annealed at 500 °C for 60 s and subsequently cooled to 40 ℃. A blank OSL measurement was then recorded under 850 nm stimulation to confirm the absence of residual trapped charge. After the blank measurement, the sample was irradiated with X-rays at a dose determined by the specific experiment. Finally, the OSL signal was recorded again under the same stimulation conditions.
2.3.1 OSL decay curves at 850 nm after 100 Gy
To evaluate the decay behavior of OSL at 850 nm stimulation, both HED-N_3ppb and HED-N_1ppm samples were irradiated with 100 Gy, and the OSL decay curves were subsequently recorded at 40 ℃ following the general procedure described above.
2.3.2 Dose–response characteristics
Dose–response curves were obtained by irradiating the samples with X-ray doses of 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 200, 500, and 1000 Gy. For each dose, the standard measurement sequence (annealing → blank OSL measurement → irradiation → OSL measurement) was repeated at 40 ℃. The OSL intensity was integrated over the 300 s acquisition period and plotted as a function of dose.
2.3.3 Influence of pre-irradiation on OSL sensitivity
2.3.3.1 Pre-irradiation dose dependence
The effect of accumulated radiation history on OSL sensitivity was studied by applying pre-irradiation doses of 0, 100, 200, 400, 600, 800, and 1000 Gy. After pre-irradiation, an OSL readout was performed at 40 ℃ to reset trapped electrons while preserving the activated luminescent centers. The sample was then irradiated with an irradiation dose of 10 Gy (HED-N_1ppm) or 100 Gy (HED-N_3ppb), followed by OSL readout at 40 ℃. This sequence was repeated for each pre-irradiation dose to evaluate changes in OSL sensitivity.
2.3.3.2 Dose–response after 400 Gy pre-irradiation
To further examine the role of pre-irradiation, a fixed pre-irradiation dose of 400 Gy was applied. After resetting the traps by OSL readout at 40 ℃, the samples were irradiated with the same set of doses used in Sect. 2.3.2. The resulting dose–response curves were compared with those obtained without pre-irradiation.
2.3.4 OSL emission spectra
OSL emission spectra were recorded only for the HED-N_1ppm sample, both with and without pre-irradiation. Without pre-irradiation, the sample was annealed, subjected to a blank OSL measurement, irradiated with 100 Gy, and then measured at 40 ℃ using the spectroscopic configuration described in Sect. 2.2. For pre-irradiated measurements, the sample was first exposed to 400 Gy, followed by OSL readout at 40 ℃ to reset shallow traps. A subsequent irradiation of 100 Gy was applied, and the OSL emission spectrum was then collected at 40 ℃.
2.3.5 Fading characteristics
The temporal stability of the OSL signal was investigated by varying the delay time between irradiation and OSL readout. Delay times of 0, 15, 30, 45, and 60 min were examined. During the fading experiments, the samples were stored at a controlled temperature of 40 ℃. For measurements without pre-irradiation, both HED-N_3ppb and HED-N_1ppm samples were irradiated with 100 Gy, and the OSL signal was recorded after each specified delay. For measurements with pre-irradiation, only HED-N_1ppm was analyzed. The sample underwent a 400 Gy pre-irradiation followed by trap reset, then a 100 Gy irradiation dose, and the OSL readout was obtained after the defined delay times, all at 40 ℃.
3 Results
3.1 OSL decay curves at 850 nm
Figure 3 shows the OSL decay curves of HED-N_3ppb and HED-N_1ppm measured after 100 Gy irradiation. The HED-N_1ppm sample exhibited an OSL intensity approximately one order of magnitude higher than that of HED-N_3ppb throughout the measurement. Under the present experimental conditions, the OSL signal of both samples decayed to the background level within approximately 300 s, indicating that the stimulation duration was sufficient for complete readout.
Fig. 3
OSL decay curves of HED-N_3ppb and HED-N_1ppm under 850 nm stimulation after 100 Gy X-ray irradiation
Before the onset of stimulation (0–10 s region in Fig. 3), a noticeable luminescence signal was detected, and its magnitude differed between the two samples. This signal is attributed to phosphorescence, indicating the presence of long-lived trapped carriers whose populations differ depending on nitrogen concentration.
3.2 Dose–response characteristics
The OSL dose–response curves for both samples under 850 nm stimulation are shown in Fig. 4. In both samples, the OSL efficiency increased for doses above approximately 10 Gy, followed by saturation in the range of 100–200 Gy. In contrast, the low-dose region (0.2–10 Gy) exhibited only a weak dependence of OSL intensity on dose.
Fig. 4
Dose–response curves of HED-N_3ppb and HED-N_1ppm measured under 850 nm stimulation
Because OSL intensity generally reflects the product of the number of luminescence centers and trapped electrons, these results suggest that irradiation above a certain threshold may modify the availability or population of active luminescence centers, thereby affecting the dose dependence.
The repeatability of the OSL intensity measurements was evaluated by repeated measurements under identical conditions (n = 4). At an irradiation dose of 100 Gy, the coefficient of variation (CV) was 0.98% for HED-N_1ppm and 8.7% for HED-N_3ppb. Because the signal level decreases at lower doses, the relative uncertainty in the 0.2–10 Gy region is expected to be comparable to or larger than these values. Therefore, variations in this low-dose region should be interpreted with caution, and no statistically significant distinction is claimed within 0.2–10 Gy.
3.3 Effect of pre-irradiation on OSL sensitivity
3.3.1 Pre-irradiation dose dependence
Figure 5 presents the dependence of OSL intensity on the pre-irradiation dose. For HED-N_1ppm, the OSL sensitivity increased progressively with increasing pre-irradiation dose and approached saturation at approximately 400 Gy. Above this dose, no significant additional enhancement was observed. Therefore, 400 Gy was selected as an appropriate pre-irradiation dose that provides sufficient sensitivity enhancement while avoiding unnecessary over-irradiation. In contrast, HED-N_3ppb exhibited no noticeable enhancement, indicating that the pre-irradiation effect is strongly dependent on nitrogen concentration.
Fig. 5
Dependence of OSL intensity on pre-irradiation dose for HED-N_1ppm (irradiation dose: 10 Gy) and HED-N_3ppb (irradiation dose: 100 Gy)
3.3.2 Dose–response behavior after 400 Gy pre-irradiation
The dose–response characteristics of HED-N_1ppm after a 400 Gy pre-irradiation are shown in Fig. 6. Pre-irradiation resulted in a substantial increase in OSL sensitivity, enabling clear dose dependence even at low doses. However, the slope of the dose–response curve was smaller than that of the non-pre-irradiated case in the 10–100 Gy region. Regardless of pre-irradiation, saturation occurred above 100 Gy, and the saturation intensities were comparable. These results suggest that the upper limit of OSL intensity is governed by the maximum product of luminescent centers and trapped electrons achievable under the given conditions.
Fig. 6
Dose–response curve of HED-N_1ppm after a 400 Gy pre-irradiation. Pre-irradiation increases OSL sensitivity and improves low-dose detectability
Figure 7 shows the OSL emission spectra of the HED-N_1ppm sample with and without pre-irradiation. A single broad emission band was observed between approximately 500 and 800 nm, with a maximum intensity near 620–650 nm. This spectral shape is characteristic of the phonon sideband (PSB) of NV⁻ centers. Although the zero-phonon line (ZPL) of the NV⁻ center is located at 637 nm [22‐24], no sharp ZPL peak was resolved, likely due to the limited spectral resolution and moderate signal intensity of the present measurements. No emission was detected in the blue region around 435 nm, indicating that neutral-vacancy-related GR1 centers do not contribute to the OSL process under the present conditions. OSL emission spectra of the HED-N_3ppb sample could not be reliably obtained because its OSL sensitivity was more than one order of magnitude lower than that of the HED-N_1ppm sample, resulting in insufficient signal intensity for meaningful spectral analysis under the present experimental conditions.
Fig. 7
OSL emission spectra of HED-N_1ppm with and without 400 Gy pre-irradiation
Importantly, the spectral shape and peak position were unchanged by pre-irradiation, implying that the pre-irradiation enhancement arises not from the formation of new luminescent centers but from changes in the electronic occupancy or recombination efficiency of existing NV-related centers.
3.5 Fading characteristics
The fading behaviors of the samples are summarized in Fig. 8. For HED-N_1ppm, the fading curves with and without pre-irradiation were nearly identical, implying that the stability of the trapped charge population is unchanged and that pre-irradiation does not introduce additional trap types. In contrast, HED-N_3ppb exhibited a more rapid decay, indicating the presence of shallower or less stable trapping states compared with HED-N_1ppm. These observations demonstrate that nitrogen concentration has a strong influence on the thermal stability of the traps responsible for OSL.
Fig. 8
Fading characteristics of HED-N_3ppb and HED-N_1ppm with and without pre-irradiation
In this study, two HED samples with different nitrogen concentrations (3 ppb and 1 ppm) were examined, and the results showed that the OSL behavior varies significantly within this concentration range. In addition, pre-irradiation was found to strongly modify the OSL sensitivity. The following discussion interprets these observations based on the phenomenological relationship:
where \({N}_{trap}\) is the number of trapped electrons, \({N}_{center}\) is the effective number of luminescent centers, and \({\eta }_{rec}\) denotes the radiative recombination efficiency. This relationship represents a simplified kinetic description of the OSL process that assumes negligible re-trapping under quasi-equilibrium conditions. Such phenomenological formulations have been widely used in the analysis of thermally and optically stimulated luminescence and are discussed in detail by Chen and McKeever [22]. While more rigorous kinetic models explicitly account for re-trapping probabilities, the present formulation provides a practical framework for interpreting relative changes in OSL sensitivity and efficiency between samples with different nitrogen concentrations.
4.1 Influence of nitrogen concentration on OSL sensitivity
Within the nitrogen concentrations investigated, the HED-N_1ppm sample exhibited approximately an order of magnitude higher OSL intensity than HED-N_3ppb. Nitrogen is known to participate in both trapping and luminescent defect formation in CVD diamond, particularly through vacancy-related complexes such as NV and other N–V–related defects [1, 2]. Therefore, an increase in nitrogen concentration is expected to increase both the density of trapping sites (\({N}_{trap}\)) and the density of radiative recombination centers (\({N}_{center}\)), which is consistent with the observed sensitivity difference.
4.2 Non-linear dose response and saturation
Both samples exhibited weak dose dependence in the 0.2–10 Gy region, followed by a rapid increase in OSL efficiency beyond 10 Gy and eventual saturation at 100–200 Gy. This trend agrees well with the classical model of center depletion in thermoluminescence/OSL systems [3]. At low doses, both electron traps and luminescent centers are far from saturation, and the OSL intensity is governed by the gradual increase in the number of trapped electrons and the associated radiative recombination probability. At intermediate doses, the population of trapped electrons increases more rapidly, leading to a pronounced enhancement of OSL intensity. At higher doses, the number of available luminescent centers approaches its upper limit (\({N}_{center,max}\)), resulting in saturation of the OSL response. The larger saturation level of HED-N_1ppm suggests a larger maximum number of available luminescent centers in the nitrogen-rich sample.
4.3 Mechanisms of pre-irradiation-induced OSL enhancement
A remarkable increase in OSL sensitivity was observed only in HED-N_1ppm when pre-irradiation doses up to 400 Gy were applied, whereas no enhancement occurred in HED-N_3ppb. This indicates that nitrogen concentration plays a crucial role in enabling pre-irradiation effects. Several mechanisms may contribute simultaneously:
4.3.1 Increase in trapped electron population or electron capture efficiency
Deep traps may be progressively filled by pre-irradiation, allowing subsequent irradiation to populate shallower or more efficient traps. In addition, the charge-state modification of nitrogen-related defects could increase their electron capture cross section, thereby enhancing \({N}_{trap}\) [4].
4.3.2 Increase in the effective number of luminescent centers
Although the OSL emission spectra remained unchanged with and without pre-irradiation (Fig. 7), indicating that the type of luminescent centers does not change, the effective number of centers contributing to radiative recombination may increase. Pre-irradiation may activate previously inactive centers or alter their charge states, thereby increasing \({N}_{center}\).
4.3.3 Improvement in radiative recombination efficiency (\({\eta }_{rec}\))
Pre-irradiation could suppress non-radiative recombination pathways or modify defect charge states in a manner that favors radiative recombination, thereby increasing the probability that recombination events result in photon emission [5].
In addition, OSL-like signals were observed under 450 nm stimulation, suggesting the presence of deeper traps. However, under this stimulation condition, the OSL signal spectrally overlapped with photoluminescence (PL), making quantitative analysis difficult. Therefore, the present discussion focuses on results obtained under 850 nm stimulation, where reliable separation between stimulation light, PL, and OSL was achieved. The involvement of deep traps in the pre-irradiation-induced OSL enhancement is thus suggested but cannot be quantitatively evaluated under the present experimental conditions and remains an important subject for future investigation.
4.4 Reset of enhancement by annealing: evidence for a metastable state
The pre-irradiation enhancement completely disappeared after annealing at 500 °C for 60 s, indicating that the enhancement does not arise from the creation of new permanent defects. Instead, it results from metastable electronic occupancy of existing defect levels. Thermal energy released during annealing empties deep traps and resets defect charge states, thereby restoring \({N}_{trap}\), \({N}_{center}\) and \({\eta }_{rec}\) to their initial states.
4.5 Spectral invariance and trap stability
Figure 7 shows that the OSL emission spectra of the HED-N_1ppm sample consist of a single broad luminescence band extending from approximately 500 to 800 nm with a peak near 620–650 nm. This broad-band feature is consistent with phonon-assisted luminescence from NV⁻-related defect centers, which are known to exhibit a wide phonon sideband in this spectral region [23‐25]. Although NV⁻ centers possess a zero-phonon transition at 637 nm, no sharp ZPL peak was observed under the present measurement conditions. The absence of a resolvable ZPL indicates that the measured OSL emission arises predominantly from the phonon sideband rather than from isolated electronic transitions associated with NV⁻ centers. No detectable emission appeared in the blue region around 435 nm, confirming that GR1 (neutral vacancy) centers do not contribute to the OSL process.
Importantly, the spectral shape and peak position were unchanged by pre-irradiation. This spectral invariance demonstrates that pre-irradiation does not introduce new luminescent centers or modify the radiative recombination pathways. Combined with the complete removal of the enhancement by annealing, these findings support the interpretation that the increased OSL sensitivity originates from metastable changes in the electronic occupancy or activation efficiency of existing NV-related centers, rather than from the formation of new defect species.
Fading measurements further revealed a strong dependence on nitrogen concentration. The nitrogen-rich HED-N_1ppm exhibited more thermally stable trap levels, while the HED-N_3ppb sample showed a more rapid loss of signal, indicating a higher population of shallower or less thermally stable traps. These results demonstrate that nitrogen incorporation not only enhances the density of luminescence centers but also influences the distribution and thermal stability of trap levels that govern the OSL response.
4.6 Implications for materials design and dosimetry applications
These results indicate that the OSL performance of HED can be tuned through nitrogen incorporation and control of the electronic occupancy of trap levels. Nitrogen-rich HED provides, higher densities of traps and luminescent centers, more stable trap structures, and pre-irradiation-induced sensitivity enhancement, making it a promising material for high-dose and potentially real-time dosimetry applications. The reversibility of the enhancement through annealing may also be advantageous for reusability in detector systems.
5 Conclusion
In this study, the OSL characteristics of HED with two different nitrogen concentrations (3 ppb and 1 ppm) were systematically investigated. Within the range of nitrogen concentrations examined, clear differences were observed in OSL sensitivity, dose response, and trap stability. The nitrogen-rich sample exhibited substantially higher OSL intensity, reflecting an increased density of trapping sites and luminescent centers associated with nitrogen-related defects.
Both samples showed weak dose dependence below 10 Gy and saturation at 100–200 Gy, consistent with the classical center-depletion model. A pronounced sensitivity enhancement was observed only in the nitrogen-rich sample following pre-irradiation up to 400 Gy, whereas no enhancement occurred in the low-nitrogen sample. This enhancement is attributed to a combined increase in the trapped-electron population, activation of previously inactive luminescent centers, and improved radiative recombination efficiency. The complete removal of the enhancement by annealing at 500 °C for 60 s demonstrates that the effect arises from reversible, metastable changes in the electronic occupancy of existing defect levels rather than from the formation of new defects.
The OSL emission spectra consisted of a single broad band between 500 and 800 nm, peaking at 620–650 nm, consistent with NV−-related luminescence. The absence of spectral changes after pre-irradiation confirms that the types of luminescent centers remain unchanged, supporting the conclusion that the enhancement originates from modifications in the occupancy or activation efficiency of existing NV−-related centers.
Fading measurements revealed that the nitrogen-rich diamond possesses more thermally stable trap levels, while the low-nitrogen sample is dominated by shallower traps. These findings demonstrate that controlled nitrogen incorporation is an effective means of tuning the OSL performance of HED. The enhanced sensitivity, trap stability, and reversible pre-irradiation response indicate that nitrogen-rich HED is a candidate for high-dose and reusable dosimetry applications.
Acknowledgements
This work was supported in part by research funding from Orbray Co., Ltd. (Project No. GG5-1170).
Declarations
Conflict of interest
The remaining authors declare that they have no conflicts of interest related to this work.
Ethical approval
No human participants/animals are involved in this study. Since the authors have nothing to declare and no human participants/animals are involved in this study, no informed consent is needed.
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Optically stimulated luminescence characteristics of heteroepitaxial diamond with different nitrogen concentrations: effects of pre-irradiation and luminescence center activation
Authors
Kiyomitsu Shinsho
Go Okada
Koji Koyama
Keitaro Hitomi
Seong Woo Kim
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