Letter pubs.acs.org/NanoLett
Observation of the Long Afterglow in AlN Helices Lei Jin,† Huayu Zhang,† Ruiqun Pan,‡ Ping Xu,§ Jiecai Han,∥ Xinghong Zhang,*,∥ Quan Yuan,∥ Zhihua Zhang,⊥ Xianjie Wang,# Yi Wang,*,∇ and Bo Song*,#,∇ †
Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China Changchun University of Science and Technology, Changchun 130022, China § Department of Chemistry and ∥Centre for Composite Materials, Harbin Institute of Technology, Harbin 150080, China ⊥ Liaoning Key Materials Laboratory for Railway, School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China # Department of Physics and ∇Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, China ‡
S Supporting Information *
ABSTRACT: The coupling effect between nitrogen-vacancies (VN) and aluminum-interstitial sites (Ali) is investigated theoretically and experimentally in AlN helices. First-principles calculations predict a photoluminescence emission peak at approximately 600 nm in AlN doped with complex-defect (VNAli). A typical long afterglow (persistent luminescence) was observed in unintentionally doped AlN helices by introducing the complex-defect of (VNAli). An analysis of the luminescent characteristics indicated that the mechanism behind this afterglow is the complex-defect level and complex-defect density. These findings may further enrich the thoughts of defects in the wide band gap semiconductor of AlN. KEYWORDS: AlN, helix, long afterglow, complex-defect a visible PL emission at ∼600 nm (2.07 eV), which is assigned to the contribution of (VNAli) complex-defect.14 Further, in combination with its intrinsic lattice-vibration characteristic, the long-afterglow phenomenon is expected to emerge in AlN under suitable irradiation conditions. Experimentally, the anticipated long afterglow effect was observed in AlN helices with the presence of (VNAli). To the best of our knowledge, such typical long afterglow behavior has not been found in unintentionally doped AlN materials until now. Our study therefore provides new insight into understanding the physical properties of AlN with the intrinsic defects. The band structure of intrinsic wurtzite AlN (2H), and wurtzite AlN containing a complex-defect of (VNAli) are examined by first-principles density functional theory calculations. All the calculations are done on 108-atom (3 × 3 × 3) supercells, which were N54Al54 for intrinsic AlN, and N53Al55 for AlN containing one complex-defect of (VNAli), respectively. Because the interstitial Al atom has a tendency to sit at the octahedral interstitial O site,15 we put the Al atom at the O site, and removed the N atom directly beneath the interstitial Al in order to decrease the total energy as much as possible. For wurtzite AlN, the lattice constants (a = b = 3.13 Å, c = 5.01 Å)
A
lN has recently been the subject of intense research efforts due to its superior physical properties such as ultrawide direct bandgap, high melting point, and high thermal conductivity. These properties make it suitable as the ultimate material for the fabrication of laser diodes (LDs), photodetectors, and the deep-ultraviolet light-emitting diodes (DUVLEDs) over the past several decades.1−3 However, growth of AlN with high optical and structural quality as well as a high purity is very essential for the high performance of AlN-based optoelectronic devices. Therefore, it is highly desirable to understand the intrinsic features of native point defects including vacancy, antisite, interstitial, and their complexes because these defects have significant influence on the charge carrier density and dynamics in AlN.4−6 It is very important to shed light on this, both for a fundamental understanding as well as for their practical applications. Today, the characteristics of single defects are relatively well understood.7−11 For example, the well-known intrinsic defect of nitrogen vacancy, VN, in III− V nitrides was reported to introduce deep donor levels in wurtzite AlN, which has been verified to be able to adjust the long-range magnetic order in AlN whiskers.7 Another native defect, Ali, has been demonstrated to contribute to photoconductivity.8 In contrast to these well-investigated single defects, little is known about the features of complex-defect, such as (VNAli).12,13 In this Letter, we report on the luminescent properties of AlN that are related to the complex-defect of (VNAli). First-principles calculations predict © XXXX American Chemical Society
Received: June 10, 2015 Revised: August 24, 2015
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DOI: 10.1021/acs.nanolett.5b02300 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters and the bond lengths along the c-axis (1.91 Å) were introduced to the calculations. The atomic positions and the volumes of supercells are optimized completely using general gradient approximation (GGA) within the amended form of Perdew− Burke−Ernzerhof (PBE) functional16 as adopted in the Vienna Ab initio Simulation Package (VASP).17 The electron and core interactions were considered by the frozen-core all-electron projector-augmented wave (PAW) method.18 The kinetic energy cutoff for the plane-wave basis sets was chosen to be 500.0 eV. Brillouin-zone sampling was done in 3 × 3 × 2 Monkhorst−Pack k-point sets. The positions of the atoms were relaxed until the residual Hellmann−Feynman forces are smaller than 0.01 eV/Å. Here, we employed a modified Becke−Johnson (mBJ) exchange potential,19 PBE-GGA correlation, and general potential linearized augmented planewave (LAPW)20 method adopted by the WIEN2K21 code to calculate the band gap (Eg) and band structures. A plane wave cutoff determined by the value of 7.0 for min (Rα) max (kn) was used. The k-meshes in the Brillouin-zone are the same as that in the structure optimizations. As shown in Figure 1a, the calculated direct Eg at point Γ is 5.50 eV, which agrees well with 5.55 eV as provided by the mBJ
the AlN crystal lattice (process III). Finally, the recombination process (between electrons and holes) from level D to E (induced by Ali from (VNAli)) will generate an ∼595 nm (2.08 eV) PL emission (process IV), exhibiting a typical long afterglow feature at room temperature (RT). The validation of such interesting optical phenomena induced by complex-defect of (VNAli) is essential to modulate the properties of AlN-based materials and devices via the defects engineering in the future. To verify the aforementioned theoretical predictions, we synthesized unintentionally doped AlN helices by physical vapor transport (PVT) method with high-purity AlN powder (99.999%, Merck) as a starting material.22,23 X-ray diffraction (XRD) pattern of as-synthesized sample is displayed in Figure 2a, which could be well indexed to the wurtzite AlN (2H). The lattice parameters calculated from the XRD data based on the Rietveld technique using the FULLPROF program are a = 3.112 Å, c = 4.977 Å, which are in good agreement with the standard value for 2H-AlN (a = 3.111 Å, c = 4.978 Å, ICDD-PDF no. 76-0565, space group P63mc). No characteristic peak associated with impurities or other secondary phases was detected. Raman spectrum in Figure 2b shows the E2 (low), E2 (high), E1 (TO) and largewave vector A1 (LO) mode, located at ∼247.1, ∼656.3, ∼668.5, and ∼889.1 cm−1, respectively, providing further evidence to demonstrate the formation of single-phase hexagonal AlN.24 Figure 2c,d shows the formation of AlN helices with a broad diameter distribution ranging from ∼100 to 400 nm. Energydispersive X-ray spectroscopy (EDX) inset in Figure 2c for an individual AlN helix displays an element ratio of 46.95:53.05 for N to Al. Figure 2e,f shows bright-field transmission electron microscope (TEM) images and high-resolution TEM (HRTEM) images of an individual AlN helix, respectively. The corresponding fringe spacing of ∼0.27 nm can be assigned to d100 values of hexagonal AlN based on the known ICDD data as mentioned above. The selected area electron diffraction (SAED) pattern was shown in the inset of Figure 2f. Obviously, on the basis of the typical Debye−Scherrer diffraction dots, single-crystal nature was verified in as-obtained AlN helices. These results further confirm that the as-prepared AlN helices possess good crystallinity. RT PL spectroscopy measurements, as shown in Figure 3a, were performed to evaluate the types of defects in as-prepared AlN helices. Two strong PL peaks, located at ∼330 nm (3.75 eV) and ∼430 (2.88 eV) nm as well as a broad emission band ranging from ∼575 to 625 nm with the peak located at ∼600 nm (2.07 eV) were observed, revealing the presence of at least three kinds of defects in as-prepared AlN helices. In contrast, only a very weak PL emission was observed in the parent AlN polycrystalline specimen. Generally, the PL band located at ∼330 nm is assigned to the contribution from VN,25,26 while the emission peak located at ∼430 nm was assigned to the effect of deep-level Ali defect,8 and the complex-defect of (VNAli) are reported to be responsible for the ∼600 nm emission band.14 Figure 3a demonstrates the successful formation of the desired complex-defect of (VNAli) as expected. On another note, the antisites and N interstitials (Ni) are energetically less favorable owing to the large lattice mismatch in the covalent radii of Al and N,10 and thus their potential contribution to PL emission can be precluded. In addition, the glow discharge mass spectrometry (GDMS) analysis revealed that total impurity element concentration (Mn, Fe, Mg, Zn, Cr, Si, rare-earth, etc.) is less than approximately 400 ppm in as-prepared AlN helices, which is 12 times less than the magnitude observed in parent
Figure 1. Band structure of intrinsic wurtzite AlN (a), and wurtzite AlN containing the (VNAli) complex-defect (b). The Fermi energy level is defined as 0 eV and plotted as the dotted line.
exchange potential.18 Figure 1b displays two states in the gap induced by the complex-defect (VNAli). The upper duplicate and lower single defect-induced states are adjacent to each other. The Fermi energy level is located in the upper defectinduced state. Clearly, the lowest conduction band (LCB) as shown in Figure 1b is down-shifted and removed from other conduction bands. To determine whether the two defectinduced states are related with the complex-defect of (VNAli) or single defect (VN or Ali), we investigate the charge density isosurfaces (Supporting Information, Figure S1) and the partial densities of states (PDOS) (Supporting Information, Figure S2) for the defect-induced states. Distinctly, the above defectinduced band structure allows the emergence of a new PL emission peak. For instance, a photoexcited carrier (electron) jumps from defect energy level A (induced by (VNAli)) to conduction band B (process I). Subsequently, the carrier will drop from conduction band B to the trap level C (originated from the host lattice) driven by the low system energy (process II). Next, the carrier is gradually released from level C to D (originated from the host lattice) by the thermal vibration of B
DOI: 10.1021/acs.nanolett.5b02300 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. (a) XRD pattern of the as-prepared AlN helices. The bottom is the standard data for AlN (ICDD-PDF no. 76-0565). (b) Raman spectrum of the as-prepared AlN helices. (c) SEM image of the AlN helices, and the inset shows the EDX pattern. (d) Magnified SEM image as denoted by red box in (c). (e) TEM image of a single AlN helix. (f) HRTEM image for the dot as denoted by the blue box in (e). The inset shows the SAED pattern.
where I is the intensity; I1 and I2 are the constants; t is the time; and τ1 and τ2 are known as the decay time corresponding to a rapid and a slow decay procedure, respectively. Table 1 summarizes the fitting results for the above decay curve. Apparently, there are two decay process involved. The afterglow effect is visually displayed in images shown in Figure 3e−i, which were taken by a low noise high sensitivity CCD camera. Obviously, the long afterglow features could last for more than 5 min with a brightness ≥0.32 mcd/m2, making visually seen by the naked eye possible. Owing to the slow decay component of τ2, the weak afterglow features can still be viewed for more than 10 min after the removal of the 254 nm UV irradiation source. To the best of our knowledge, such PL features in unintentionally doped AlN has not been reported previously.36−39 To further probe the origin of the long afterglow feature, a 365 nm irradiation source was used to irradiate the as-prepared AlN helices for 2 min in dark room. However, only a weak purple color was observed (Figure 3j),
bulk AlN powder. Thus, the possible contribution for PL emission from these potential impurity elements could be ruled out.27−32 Notably, the observed PL peak centered at approximately 600 nm agrees well with the recombination process from level D to E with the emission of approximately 595 nm as predicted by defect-induced band structures in Figure 1b. To further verify the theoretical predictions, the afterglow decay curve was recorded, as represented in Figure 3b. A ST-86LA brightness meter was used to record the decay curve in a dark room after the samples were irradiated by a 254 nm UV irradiation source for 2 min (Figure 3c). The decay curve was well fitted to the following biexponential equation33−35 ⎛ t ⎞ ⎛ t⎞ I = I1 exp⎜ − ⎟ + I2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠
(1) C
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Figure 3. (a) The photoluminescence emission spectra of the as-prepared AlN helices under excitation of 200 and 325 nm light, and bulk AlN powder under excitation of 325 nm light. (b) Schematic illustration of the measurement of the afterglow effect. (c) Afterglow decay curve of the asprepared AlN helices from 0 to 300 s after excited by 254 nm light for 2 min. Symbols (○), experiment; red solid line, fitted by eq 1. (d) Optical images of the as-prepared AlN helices before excitation. (e−i) Optical images taken at different decay times after removal of the excitation source. (j) Optical images of the AlN helices excited by 365 nm light. (k) Optical image of the AlN helices taken immediately after the removal from the 365 nm excitation source.
Table 1. Decay Time of Two Exponential Components for As-Prepared AlN Helices value error
I1 (mcd/m2)
τ1 (s)
I2 (mcd/m2)
τ2 (s)
210.2 3.1
4.8 0.1
39.3 0.9
84.6 2.4
and no fingerprint of afterglow feature was found after the removal of the 365 nm excitation source (Figure 3k), which excludes the possible contribution of 430 nm PL emission for the observed afterglow effect. Thus, the observed long-lasting luminescence can be assigned undoubtedly to the 600 nm emission band, corresponding to complex-defect of (VNAli). It is thought that the long afterglow features are mainly governed by the traps and then induced by a gradual release of the charge carriers from the trapping centers.40,41 Therefore, two factors can determine the performance of the long afterglow: (i) trap depth, which decides the minimum energy required to release the charge carriers; and (ii) trap density. In general, a common thermal disturbance could release these charge carriers stored in shallow traps and then a short afterglow will generate. In contrast, it is difficult to release the charge carriers stored in deep traps to the excited state. Thus, both appropriate trap density and trap depth are highly required to produce an excellent long afterglow. To gain further information about the trap level, thermoluminescence (TL) measurements were performed. As shown in Figure 4, asprepared AlN helices show an asymmetric broad TL band in the temperature range from 20 to 210 °C. The TL curve can be divided into two peaks, the first centered at approximately 68 °C and the second located at approximately 125 °C. For parent bulk AlN, a relatively weak band centered at approximately 80
Figure 4. Thermoluminescence (TL) curve of AlN helices in the region of 20 to 210 °C. 254 nm UV light was used to irradiate the AlN helices for 2 min. Symbols (•,○), experiment; symbols (▲), fitted by eq 2 for total simulated curve.
°C was observed. Here, it is well accepted that the TL band located at low and high temperature corresponds to the shallow and deep traps, respectively.42 Compared with AlN powder, the first peak in the TL curve of AlN helices may be assigned to the contribution from single defect in AlN host, associated with the shallow traps. The second peak is attributed to the complexdefect of (VNAli) corresponding to the deep traps. Herein, it is easy to understand that AlN powder processes a lower concentration of the complex-defect than that of AlN helices, as confirmed by PL measurements in Figure 3a, leading to poor or even absence of the long afterglow features. It thus can be D
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where cα = 0.976 + 7.3 (μg−0.42), and bα is 0. Using eq 3, the trap depth was calculated to be ∼0.30 eV, which is in good agreement with the theoretical prediction of ∼0.35 eV, as shown in Figure 1b. Note that large value of b suggests a very complicated physical process,43 which needs further investigation. On the basis of the above discussions, we propose a mechanism to explain the long afterglow phenomena of the asprepared AlN helices with complex-defect of (VNAli) in Figure 5. Under 254 nm irradiation, numerous electrons and holes are created (process I). Next, the energy is transferred to the luminescence center D by the carriers (process II) and then emits as an immediate luminescence (process III). At the same time, some of the carriers are trapped by the defect-induced energy level C instead of being moved to the ground state (process IV). After removal of the irradiation source, with the assistance of the thermal disturbances, electrons and holes trapped by the defect energy level will be gradually released, transferred to the luminescence center (process V), and then emit as a persistent luminescence (process VI). Our theoretical and experimental studies convincingly suggest that the unintentionally doped AlN helices with a complex-defect induced luminescence emission may be a new family of materials that shows long-lasting phosphorescence. So far, we have investigated the (VNAli)-induced afterglow features in AlN using first-principles calculations and asprepared unintentionally doped AlN helices. This study provides a new chance to probe the optical features of AlN with intrinsic defects and will benefit the design and fabrication of novel AlN-based optoelectronic devices via the defectsengineering in future. In addition, the observed afterglow feature demonstrates its potential applications in the field of outdoor and emergency lighting, and so forth, which could save the electrical energy significantly than the conventional LED and pave the further way to meet the severe environment and energy problems. However, it should be noted that there still exists some unclear issues about this feature, such as the
concluded that growth of AlN helices via the PVT process increases the complex-defect concentration to a suitable level and then allows the emergence of the afterglow. By using the general order kinetics equation36,38,43 (eq 2), the key parameters including trap intensity and depth could be approached (Table 2). ⎛ E ⎞ I(T ) = s″n0 exp⎜ − A ⎟ ⎝ kT ⎠ ⎛ s″(b − 1) ⎜1 + ⎝ r
∫T
T
0
b /1 − b ⎛ E ⎞ ⎞ exp⎜ − A ⎟dθ ⎟ ⎝ kθ ⎠ ⎠
(2)
Table 2. Parameters of TL Curve for AlN Helices parameter
value
glow-peak position Tm, ±5 °C glow-peak Half width ωT, ±2 °C form factor μg, ±0.01 activation energy EA, ± 0.03 eV effective frequency factor s″, ± 0.1 × 104 s−1 initial electron concentration n0, ± 0.1 × 104 cm−3 kinetic order b, ± 0.10 determination coefficient R2
69 90 0.66 0.30 2.7 × 104 2.6 × 106 3.84 0.994
Here, I (T) represents the TL intensity; n0 is the concentration of trapped charges at the initial time; kB is the Boltzmann’s constant; r is the speed of the heating (K/s); and E is the activation energy (trap depth). The kinetics order, b, is obtained on the basis of the peak asymmetry. Here, the activation energy for the second deconvoluted peak can be estimated according to the following equation43 ⎛ T2 ⎞ Eα = cα⎜k m ⎟ − bα(2kTm) ⎝ α ⎠
(3)
Figure 5. Schematic illustration of the long afterglow mechanism in AlN with the complex-defect of (VNAli). E
DOI: 10.1021/acs.nanolett.5b02300 Nano Lett. XXXX, XXX, XXX−XXX
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Performance Computing Center of Harbin Institute of Technology for calculation resource.
dependence of decay life and brightness on the complex-defect concentration. In conclusion, we report on the luminescence properties of AlN helices with complex-defect of (VNAli). The detailed optical measurement exhibited a typical afterglow feature after UV irradiation. These results support the assumption based on the first-principles calculations that the complex-defect of (VNAli) will create a typical afterglow characteristic in AlN. Such unintentionally doped AlN helices with (VNAli)-induced afterglow are of great interest in the growing family of fluorescent materials. Methods. Approximately 10 g of AlN powder (99.999%, Merck) was put in the traditional tantalum carbide crucible. Then, the tantalum carbide crucible was put in the center of a graphite crucible. Both of them were put in center of a radio frequency-heated furnace. Prior to heating, the chamber of the furnace was evacuated to a pressure of 10−3 Pa before being charged with high purity N2 (99.9999%) to a pressure of 105 Pa. This process was carried out three times. The N2 pressure was then maintained at approximately 3−8 × 104 Pa. The temperature was then raised to 1800−1900 °C from RT at a rate of 20−25 °C/min and kept at the target temperature for 10−50 min in order to control the defect types and concentrations. The PL spectra of the as-prepared samples were obtained by the Hitachi F-4500 fluorescence spectrophotometer. The afterglow measurement was performed on a ST86LA brightness meter after being excited by a 254 nm UV light source. The TL curves of the as-prepared samples were collected on a thermoluminescent dosimeter (FJ427A1, Beijing Nuclear Instrument Factory) from RT to 200 °C with a heating rate of ∼0.5 °C/s.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b02300. Charge density isosurfaces (Figure S1) and partial densities of states (Figure S2) for the defect-induced states. (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Tel: +86-0451-86403753. Fax: +86-0451-86403753. *E-mail:
[email protected]. Tel: +86-0451-86403753. Fax: +860451-86403753. *E-mail:
[email protected]. Tel: +86-0451-86403753. Fax: +86-0451-86403753. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (Grants 50902037, 51172055, 51372056, 51472064, 21471039, 10904024), Fundamental Research Funds for the Central University (Grants HIT.BRETIII.201220, PIRS of HIT A201502, HIT.NSRIF.2012045, HIT.ICRST.2010008, HIT.NSRIF.2012040), International Science & Technology Cooperation Program of China (2012DFR50020), and the Program for New Century Excellent Talents in University (NCET-13-0174). Y.W. thanks the High F
DOI: 10.1021/acs.nanolett.5b02300 Nano Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.nanolett.5b02300 Nano Lett. XXXX, XXX, XXX−XXX
