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Abstract
Three different configurations of GaN are analysed to show robust and tunable light emitters from localised point defects induced in GaN crystal by a femtosecond laser (fs-laser). Localised irradiations of GaN are achieved using a fs-laser. The laser-induced damage threshold is found at a fluence of 130 ± 10 mJ/cm2. Raman spectroscopy allows for the characterization of irradiated GaN crystal while quasi-resonant photoluminescence mapping reveals a defect-related visible emission corresponding to the fs-laser irradiated area. From three different configurations of GaN, emission peaks vary from 620 to 680 nm-wavelengths for thin film, MBE intrinsically doped and Mg-doped NWs of GaN, respectively. The red emission in GaN is localized thanks to the new laser-induced fabrication and the engineering of the defect emission paves the way to further lighting applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Gallium nitride (GaN), a wide bandgap semiconductor (Eg=3.4 eV), is a key material for LEDs, laser diodes and photo-detectors operating from visible to deep UV spectral range [1–3]. Epitaxial growth techniques allow formation of high quality material with low density of dislocations and low concentration of point defects. Some point defects in GaN crystal are known to be optically active, emitting red, yellow, green or blue luminescence [4–7]. These defects may act as 0D quantum emitters. Nitrogen vacancy (NV) center is also predicted [8] but the energy states of defects are not always identified [9,10]. The red luminescence is observed at 1.8 eV (689 nm) and the zero phonon line at 2.38 eV (at room temperature) is attributed to a transition from the conduction band to a deep acceptor of unknown origin [11].
In view of the attractive optical properties of these optically-active defects, it would be advantageous to use them in optical devices. However, integration of defects in optical device requires a good control over their position and density, which is very difficult due to the random nature. Of their formation Therefore, the device design relies today on other nanometer-sized light emitters obtained from molecules, metallic nanoparticles, semiconductor quantum dots or nanowires [12].
Here we propose using femtosecond laser-based process, already widely used for micro-fabrication [13–15] for controlled fabrication of efficient GaN-based micro- and nanometer-size light emitters from induced point defects. This is a low-cost and large scale technique in comparison to other nanotechnology approaches based on electron beam lithography and etching. Such technique has already been demonstrated in the fabrication of NV center in diamond for single photon emitters [16].
In this work, femtosecond laser (fs-laser) radiation is used to controllably induce optically active defects in GaN crystal and we show that these defects can act as robust light emitters emitters as it has been shown recently in Berhane et al. [17] although the origin of the emission may not be the same. Three different configurations of GaN have been studied: MOCVD-grown thin film, MBE intrinsically doped and Mg-doped nanowires (NWs). In each case, we show a localised ablation and melting of the GaN structure by fs-laser irradiation over an area of 1 to 2.5 μm2 depending on the laser fluence. Characterization by scanning electron microscope (SEM) evidences sharp holes, the laser-induced damage threshold is reached at a fluence of 130 ± 10 mJ/cm2 for all samples. Raman spectroscopy mapping is used to characterize GaN crystal while quasi-resonant photoluminescence (PL) mapping reveals a red emission coincident with fs-laser irradiated area. It is found that the emission from defects can be tuned. The emission peaks vary from 626 to 680 nm-wavelength while the emission lifetimes are 0.81, 1.37 and 1.19 ns for thin film, MBE intrinsically doped and Mg-doped NWs of GaN, respectively. Laser patterning of defects in wide bandgap materials with one-micrometer precision and 3D capability of the patterning is appealing for deterministic sources for photonics in GaN system. Such tunable engineering of the emission wavelength and lifetime also add more functionalities in GaN lighting applications.
2. Methodology
Three structures of high quality GaN crystal have been used in this study: GaN thin film, as-grown MBE intrinsic and Mg-doped NWs of GaN. The layer of GaN, commercially available, is 1 μm thin and grown by Metal-Organic Chemical Vapour Deposition (MOCVD) on a Sapphire substrate. Two variations of GaN NWs have been grown using a Molecular-Beam Epitaxy (MBE) system: intrinsically doped and Mg-doped NWs. Both samples are grown on Si(111) with a thin AlN buffer layer. In both cases, NWs show a diameter of 40–50 nm, grow along the c-axis axis and are N-polar [18,19].
A Modelocked Ti:Sapphire Chameleon Ultra II (fs-laser) with a repetition rate of 80 MHz, laser pulse of 140 ± 20 fs and output power of 4.13 W at 800 nm is used for irradiating GaN samples. To attenuate and tune this high laser power, a cross polarization set-up is used by inserting a half wave plate (HWP) between two linear polarizers. Depending on the angle θ of the HWP, the laser power can be adjusted following Malus law (Iout = Iin cos2(2θ)). Two lenses forming a telescope allow to broaden the beam width and optimise the coupling with a ×40 Olympus objective, with 0.65 NA and 0.6 mm WD. Computer-controlled piezoelectric stage and a beam shutter fulfil a single-shot irradiation.
The Raman and PL mappings were performed using a 532 nm CW laser at 37 μW. Coupled to a WITec focus innovations microscope, the beam is focused on the sample with a ×100 Olympus objective with 0.8 NA. The scanning is done using a computer-controlled piezoelectric stage with a resolution of 0.5 μm/step. The emission is filtered at 570 nm with a long-pass filter and the data is processed with the associated WITec Project software.
The time-resolved photoluminescence and another photoluminescence set-up consists of a Picosecond Pulsed laser VisUV based on master oscillator fiber amplifier system from PicoQuant. This excitation source emits at 355 nm. The decay is measured using a Hamamatsu H7422-40 photo-sensor associated with a single-grating emission monochromator of 1200 grooves/mm density. The signal is then analyzed using single photon counting card.
3. Results and discussion
On each sample, defects are induced by focusing the fs-laser beam at the surface, as previously reported [20]. The focusing on sample’s surface is determined as a position where the smallest energy is necessary for introducing the damage, fs-laser irradiation is in situ monitored on a CCD camera. Any deviations from the focusing point (to the air or substrate side) requires a higher energy for damage since the laser-induced damage threshold (LIDT) inside bulk is larger than that at surface. Following this protocol, nine areas are irradiated one by one on each sample, see Table 1 for the pulse energy, fluence, and irradiation intensity by femtosecond-laser. Starting from area labeled ‘1’ in Fig. 1, the HWP is set to 144°, equivalent to 460 ± 30 mW laser power, the shutter is then opened for 2 s exposure. A computer-controlled piezoelectric stage allows to move by ∼5 μm then a new area is irradiated at lower energy. One last spot (label ‘9’), similar to the first one, marks the end of irradiations so that we can approximate the irradiated spots without damaging the surface, see Fig. 2. It is worth noticing that the fs-laser focusing, added to slight misalignments of the beam, result in asymmetric damaged areas, as seen in Fig. 1(a) for thin film GaN.
Table 1. Pulse energy, fluence and intensity of irradiation for each fs-laser induced defects in various structures of GaN.
Fig. 1 Structures of laser-induced GaN films and nanowires. Scanning-electron-microscope (SEM) images of localised irradiations by femto-second pulsed laser at 800 nm of a) MOCVD-grown GaN thin film, b) MBE intrinsically doped GaN nanowires and c) MBE-grown Mg-doped GaN nanowires. and a magnification of irradiation ‘9’ is shown in d), e) and f), respectively.
Fig. 2 SEM images of irradiated area ‘7’ (magnified) for a) MOCVD-grown GaN thin film, b) MBE intrinsically doped GaN nanowires and c) MBE-grown Mg-doped GaN nanowires. d) and e) are respectively the cross-section SEM images of b) and c) showing no penetration beyond the surface.
LIDT is a physical characteristic of optical component which defines a critical power or peak fluence of laser irradiation causing irreversible changes in materials structure. For the three samples we observe LIDT as the minimal pulse energy measured corresponding to an appearance of surface damage, as observed on SEM images in Fig. 1. LIDT is achieved at fluence equal to 130 ± 10 mJ/cm2 with a pulse energy of 2.3 nJ, which is area ‘6’. LIDT is the same for thin film and 1D nanostructures of GaN, the threshold energy is only dependent on the nature of GaN crystal.
The laser irradiations in all three cases result in structural deterioration of the GaN crystal. The laser-induced damage morphology is a good indicator of the physical mechanisms responsible for the damage initiation. SEM images (Fig. 1) show ablation in the center of irradiated areas, the edges of crater become blurred with melted crystal and residual debris. This matches a thermally induced laser damage followed by localised melting, then boiling, evaporation and plasma formation. The high pressure of melted gas results in blurred patterns redeposited on the nearby surface, clearly observed in Fig. 1(a) and 1(e) on GaN thin film sample. These results are reported as irradiation above ablation threshold in the center of the crater and a dissociation of Ga-N bonds on the edges where laser intensity is below ablation threshold resulting in a formation of Ga-rich phase [21].
The fs-laser induced craters are optically active. Fig. 3 shows irradiation ‘9’ observed under a conventional optical microscope, a mapping of the GaN Raman mode and a PL emission intensity map. For thin film sample, we observe from the Raman mapping (Fig. 3(a)) that the irradiation at 325 ± 20 mJ/cm2 fluence removes the fundamental resonant mode of GaN Wurtzite structure. The associated SEM image (Fig. 1(e)) distinguishes a central crater surrounded by melted area. The Raman mode of GaN does not appear neither in the crater nor the surrounding melted area. An excitation with a 532 nm CW laser however reveals a photoluminescence from this surrounding melted area, the emission is centred at 626 nm. The central crater does not emit light while the top melted area is the strongest emitter.
Fig. 3 Comparative study on irradiation with 460 ± 30 mW laser power (label ‘9’) in films and nanowires. A mapping of the GaN Raman peak and a PL mapping of defect related emission of a) MOCVD-grown GaN thin film, b) MBE intrinsically doped GaN nanowires and c) MBE-grown Mg-doped GaN nanowires. The Raman and PL mapping experiments were performed with a continuous wave laser under 532 nm excitation at 300 K. The Raman and PL spectra are shown in the inset, the star symbol identifies the normal region, the circle identifies the defect region. The intensities in the mapping are the integration of the spectra in the region between two red lines.
The observations are quite similar for irradiation of NWs. For intrinsically doped GaN NWs, the SEM (Fig. 1(f)) shows a crater, void of GaN surrounded by melted NWs collapsing towards the center. The Raman mapping of the GaN Raman mode (Fig. 3(b)) confirms the ablation of NWs in the crater as no scattering is observed. The surrounding pristine NWs show a relative uniform peak centered at 567.4 cm−1. The associated PL mapping shows a dim center while the emission from surroundings is bright, uniform and superimposed to the melted area of NWs. The emission is Gaussian and centred at 653 nm. Non-irradiated NWs do not show any emission. Irradiation of Mg-doped NWs results in an area of ∼3 μm wide with melted nanowires (see Fig. 1(c)), similar to intrinsically doped NWs. Irradiation of the thin film results in smaller (∼1 μm wide) crater. The associated Raman mapping (Fig. 3(c)) does not show the GaN mode in the center of irradiation whereas PL mapping exhibits an emission well distributed along the melted area. The emission is centred at 680 nm.
Group theory predicts four pairs of Raman active modes for hexagonal structure of GaN: A1 and E1 (Raman and infrared active), B1 (silent mode) and E2 (only Raman active) [22]. One set of A1 and E1 modes are acoustic while all others are optical modes. The A1 and B1 modes result from atomic displacement along the c-axis while E1 and E2 inform on displacement perpendicular to that axis. Fig. 4(a) shows the Raman spectra of pristine GaN, as measured during the mapping. Table 2 identifies all peaks observed. The peak , most easily observed, is identified around 568 cm−1 characteristic of Wurtzite GaN for MBE-grown nanowires on silicon, Mg-doped and intrinsically doped [23]. This peak is shifted at 577.6 cm−1 for MOCVD-grown GaN thin film on sapphire. This shift is correlated to the stress within the c-plane [24]. The strain coefficients of phonon modes vary according to GaN/Si and GaN/sapphire systems, varies with substrate material [25]. After fs-laser irradiation, as shown in Raman mappings in Figs. 3 and 5, the peak disappears.
Fig. 4 a) Raman spectroscopy and PL emission characteristics of laser-induced GaN films and nanowires. b) Comparative PL spectra of defect-related emission and c) lifetime decay curves of defect-related emission. The instrument response function is 0.39 ns. The time constants are obtained after deconvolution with the instrument response. The exponential decay fitting is shown with full line. The Raman spectroscopy has the same excitation source as that in Fig. 3. The PL spectra and decay curves were measured with 10 MHz pulsed laser under 355 nm excitation and at 300 K.
Fig. 5 Raman and PL mapping of laser-induced GaN films and nanowires. Comparative study of femto-second laser irradiation showing an optical image, a mapping of the GaN Raman mode and a PL mapping of defect-related emission of a) MOCVD-grown GaN thin film, b) MBE intrinsically doped GaN nanowires and c) MBE-grown Mg-doped GaN nanowires. The experiments were performed with a continuous wave laser under 532 nm excitation at 300 K.
The PL mappings in Fig. 3(a), 3(b) and 3(c) show a spatial distribution of the emission intensity from defects induced by fs-laser with the excitation of 532 nm continuous wave laser, and Fig. 4(b) shows the normalised PL spectra from respective samples from 355 nm 10 MHz pulsed laser excitation. PL spectra excited at 532 nm in the inset of Fig. 3 are slightly broader than the PL spectra excited at 355 nm due to the different point of excitation at defects, which may give a small broadening fluctuations with the emission. As an example, in GaN thin film sample, the emission full width at half maximum (FWHM) of spectrum excited at 532 nm is 60 ± 1 nm while that at 355 nm is 40 ± 1 nm nm. The difference in the width is also an indicator that defects are actually created in small nanoparticles after laser radiation. Such broadband emission was observed in laser-radiated samples of hBN bulk crystals [26] but not observed for atomic-thick layer hBN [20]. In different GaN systems, we notice a distinct shift in the emission. For MOCVD-grown GaN thin layer, MBE intrinsically doped and Mg-doped GaN NWs, the defect emission peaks at 626, 653 and 680 nm, respectively. The emission is spatially matches the irradiated area for each sample as shown in Fig. 3. The PL emission is well distributed along the melted area in each sample. While the thin film and intrinsically doped NWs show a crater void of GaN in the centre, the irradiation of Mg-doped NWs results in a crater with melted GaN wires all along (probably due to thin wires and randomly orientation), as seen in Figs. 1.e, 1.f and 1.g.
Time-resolved PL measurements shows a fast lifetime decay for thin film GaN and a slower recombination for NW samples. The samples were excited with a 355 nm pulsed laser associated to a microscope to probe the irradiated area. In all three cases, the luminescence decay is exponential (mono-exponential for the GaN layer and bi-exponential for NWs). Lifetime increases with distance between the defect-related states [9]. The fast decay of GaN thin film emission (τ1 = 0.80 ± 0.01 ns, single-exponential decay) compared to intrinsically doped NWs (τ1 = 1.10 ± 0.03 ns) and Mg-doped GaN NWs (τ1 = 0.95 ± 0.01 ns) suggests defects of different nature. Furthermore, the decay of undoped and Mg-doped GaN NWs is bi-exponential, indicating an additional recombination state with characteristic times of τ2 = 14.43 ± 1.85 ns and τ=13.09 ± 0.64 ns, respectively.
The effect of fs-laser irradiation on a larger scale is analysed by scanning all irradiated areas, as shown in Fig. 5. It is clear from the PL mappings that the centers of emissions are localised precisely at the fs-laser irradiated areas. Only the areas ‘7’ and ‘8’ do not show neither a structural defect nor any effect on the Raman mode nor any PL defect-related emission. Defect-related emission only appears above the LIDT for fluence above 130 ± 10 mJ/cm2 with structural deterioration.
Red luminescence bands have been reported at various energy positions and several defects may contribute to emission in this range. Red band in GaN grown by Hydride vapour phase epitaxy (HVPE) has been observed at 1.85 eV (670 nm) and Optically Detected Magnetic Resonance (ODMR) characterization reveal a transition from shallow donors to deep level defects (at low temperature) [29]. Undoped GaN sample grown by MBE show red emission ranging from 1.8 to 2.0 eV (620 to 690 nm) while the band transition and related defect are unclear [30]. From literature, in Mg-doped GaN grown by MBE, the red band is located near 1.7–1.8 eV, and ODMR characterization indicated a transition from a deep donor to a shallow acceptor [33]. While in other reports, this transition is attributed to deep donor-deep acceptor transitions [31,32]. Red luminescence band is more specifically observed in Ga-rich MBE-grown GaN [9].
4. Conclusion
We show that a femtosecond laser-assisted ablation of GaN creates nanoemitters that exhibit a gaussian photoluminescence localised at the ablation area. The emission peaks at 626, 653 and 680 mn for GaN thin film, intrinsically doped and Mg-doped nanowires, respectively. The ablation fluence threshold for a 140 fs pulse is about 131 mJ/cm2. From Raman observations, GaN is removed on the thin film sample for fluence above the threshold. For samples of GaN nanowires, fluence above threshold tends to melt the targeted area, incorporating defect in the structure. These defects are sources of the visible luminescence, located at 626, 653 and 680 nm-wavelength for thin film, intrinsically doped and Mg-doped NWs of GaN, respectively. Such tunable emissions from defects in GaN are suitable nanosized light sources. For further applications, engineering nanometer-sized craters can be achieved using the second harmonic of fs-laser (at 387 nm) [21]. To enhance this emission, light-matter interaction using photonic crystals [27] and plasmonic antenna can be explored [28].
Funding
Ministry of Education (MOE2016-T2-1-052 and MOE-RG-170/15); National Research Foundation of Singapore (NRF-CRP12-2013-04).
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