Origin of a Wide and Asymmetric Blue Luminescence Band in AlN

Article pubs.acs.org/JPCC

Origin of a Wide and Asymmetric Blue Luminescence Band in AlN Nanowires: VN, VAl, ON, and 3ON−VAl Surface Defects Yi-min Ding,† Jun-jie Shi,*,† Min Zhang,‡ Xin-he Jiang,† Hong-xia Zhong,† Pu Huang,† Ying-ping He,† and Xiong Cao† †

State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P.R. China College of Physics and Electronic Information, Inner Mongolia Normal University, Hohhot 010022, P.R. China

Downloaded by FLORIDA INTL UNIV on September 4, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.jpcc.5b06092

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ABSTRACT: The physical origin of a wide and asymmetric blue luminescence (BL) band from 350 to 750 nm in AlN nanowires (NWs) is clarified by first-principles calculations based on density functional theory and GW approximation together with the Bethe−Salpeter equation. Our results show that the band gap of the AlN NW with 1 nm diameter is 8.40 eV due to the strong one-dimensional quantum confinement, which is larger than that of AlN bulk (6.2 eV). The exciton binding energy is 1.75 eV owing to incomplete dielectric screening in AlN NWs. The defects including N vacancy (VN), Al vacancy (VAl), O impurity (ON), and 3ON−VAl have low formation energy and prefer to stay at the surface layer of AlN NWs. It is the combination of the optical transitions from the exciton ground state to these defect levels that determines the wide and asymmetric BL band in AlN NWs. Our work is useful for understanding the defect-related luminescence mechanism and improving the performance of AlN NW-based optoelectronic nanodevices.

1. INTRODUCTION In the past few years, great efforts have been devoted to fabricating high-quality AlN nanowires (NWs) by employing different methods, such as vapor−solid (VS) growth,1 direct nitridation,2−4 vapor transport and condensation,5,6 arc discharge,7 chemical vapor deposition,8−10 catalyst-assisted vapor−liquid−solid (VLS) growth,11 halide vapor-phase epitaxy,12 and molecular beam epitaxy.13 Compared with AlN bulk, one-dimensional (1D) AlN NWs have many superior properties, for example, their large surface-to-volume ratio, low dislocation density,14,15 high light extraction efficiency, low electron affinity, and high thermal stability.16 Technological advancements in AlN NWs have made them promising candidates for applications in high electron mobility transistors,17 light-emitting diodes (LEDs) and laser diodes (LDs),18 and photodetectors.15 Although the electronic structure and optical properties of AlN NWs have been investigated in recent years, some unresolved challenges still exist. The most notable issue is the defect-related origin of the wide and asymmetric blue luminescence (BL) band (350−750 nm) in AlN NWs,3,5 which has been debated for many years but still remains unclear now. The typical photoluminescence (PL) spectrum of AlN NWs (see Figure 5 of ref 5) has two remarkable characteristics, i.e., the large spectrum widening (350−750 nm) and asymmetry, under the condition of the optical excitation energy below the AlN band gap. The physical origin has been attributed to some different defects involved in AlN NWs, such as Al vacancy (VAl), interstitial Al (AlI), N vacancy (VN), O-related defects, and antisite and surface defects.19 Lei et al. observed a wide © XXXX American Chemical Society

emission band centered at 426 nm and regarded oxygen impurity as a key factor.2 Chen et al. divided the broadening and asymmetric emission band (350−700 nm) into three major optical transitions at 416, 480, and 564 nm and attributed them to VN, 3ON−VAl, defect complex and Al-rich surface defect states.5 Shen et al. reported an emission band ranging from 380 to 750 nm and attributed it to VN.7 Byeun et al. assigned an emission band centered at 500 nm to oxygen impurity.12 Recently, Zuo et al. found a strong ultraviolet emission at 345 nm and attributed it to VN.6 Li further found that the luminescence intensity of AlN NWs, synthesized via the reaction of aluminum powder and nitrogen gas at around 800 °C without catalyst, is more than 5 times stronger than that of the commercial AlN powder, and the emission spectrum has been attributed to VN and O-related defects.20 All of the aforementioned analyses are derived from the electronic structures of AlN bulk rather than the ones of AlN NWs.21−24 To the best of our knowledge, the theoretical investigation is still absent at present for the PL characteristics of AlN NWs. Our purpose here is to investigate the electron structures of defect-involved AlN NWs and clarify the defect-related origin of the wide BL bands by using the advanced first-principles calculations. We find that the band gap of AlN NW (∼1 nm) is 8.40 eV, and the exciton binding energy is about 1.75 eV because of the strong 1D quantum confinement and Received: June 25, 2015 Revised: August 31, 2015

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DOI: 10.1021/acs.jpcc.5b06092 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Top (a) and side (b) view of AlN NW. Electronic band structure calculated with the GW method (c) and the corresponding density of states (DOS) (d) for the AlN NW. The optimized Al−N bond length (in Å) is indicated in (a). The energy band gap (Eg) and top valence bandwidth (VBW) are 8.40 and 5.71 eV, respectively.

incomplete dielectric screening. The VN, VAl, ON, and 3ON−VAl complex defects, assembled at the surface layer of AlN NWs, dominate the observed wide and asymmetric BL band.

the ith constituent, which has been added to (ni > 0) or removed from (ni < 0) the host material; and μi is its chemical potential. The Fermi level EF is referenced to the valence band maximum (VBM) EV of the host. The range of EF is from VBM to conduction band minimum (CBM). A correction term ΔV[D] is introduced in order to align the electrostatic potential in the charged defect supercell with that in the host. The chemical potentials closely depend on the growth conditions and can be determined by μAl + μ N = μAlN (2)

2. CALCULATION METHODS Our first-principles calculations are based on the density functional and many-body perturbation theory. The equilibrium geometry and defect formation energy are calculated by using Vienna ab initio simulation package (VASP) code.25−27 The projector-augmented wave (PAW) potentials are used to describe the interaction of core and valence electrons, and the generalized gradient approximation (GGA) with Perdew, Burke, and Ernzerhof (PBE) functional is selected in our electron structure calculations.28 The energy cutoff is set to 450 eV, and structural optimization is carried out until the residual forces converge to less than 0.01 eV/Å. The 15 Å vacuum region and the Monkhorst−Pack k-sampling 1 × 1 × 8 are adopted. Moreover, the GW correction and Bethe−Salpeter equation calculations are carried out with the BerkeleyGW software package.29 Before the GW calculations, the groundstate Kohn−Sham wave functions and eigenvalues are obtained using a local density approximation (LDA) with Troullier− Martins norm-conserving pseudopotentials, as implemented in QUANTUM ESPRESSO code.30 A kinetic energy cutoff of 50 Ry is used for the wave function. Convergence with respect to the cutoff energy and k-point sampling has been carefully checked. In order to know the stability of defects, we first calculate their formation energy Ef in AlN NWs. In general, Ef of a defect D in charge state q can be calculated as follows31 E f [D(q)] = Etot[D(q)] − Etot[host] −

(3)

μ N ≤ μ N[N ]

(4)

μAl ≤ μAl[bulk]

(5)

2 3

2

The AlN NWs are usually grown under N-rich or N-poor conditions, which directly determine the kind and concentration of defects involved in AlN NWs. We thus consider these two limited cases to calculate the defect formation energy. For the N-rich condition, μN = μN[N2] (the energy of a N atom in N2 molecule). For the N-poor condition, μAl = μAl[bulk] (the energy of an Al atom in metal Al). For the O impurity included in AlN NWs, its chemical potential μO can be calculated from eq 3.

3. RESULTS AND DISCUSSION 3.1. Pristine AlN NW. First, we optimized the lattice structure of wurtzite AlN bulk on the basis of the minimization of the total energy. The optimized lattice parameters are a = b = 3.115 Å and c = 4.977 Å, which are very close to the experimental values a = b = 3.112 Å and c = 4.982 Å.32 Our calculated GW band gap is 6.13 eV, which is in good agreement with experiments (6.2 eV).33 Then, we further optimized the lattice parameters and calculated the electron band structures of hydrogen-passivated pristine [0001] AlN NWs. The optimized AlN NW structure is shown in Figure 1a and 1b. We can find from Figure 1a that the Al−N bond length (1.92 Å) in the

∑ niμi i

+ q(E V + E F + ΔV [D])

2μAl + 3μO = μ Al O

(1)

where Etot[D(q)] is the total energy of the supercell containing the defect D in charge state q; Etot[host] is the total energy in the same supercell without the defect; ni is the atom number of B


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antisite, and surface defects are possible to contribute the broad BL band in AlN NWs. It has been known that these defects prefer to stay at the NW surface.34,38,39 This is because, if defects exist in the interior of the NW, they will need extra energy to break down the inner perfect crystal structure, and the whole system requires a large lattice relaxation. The free surface of NWs permits an efficient elastic relaxation of atoms. Hence the defects have the smallest formation energy at the NW surface layer. Here we will pay our attention to these surface defects in order to clarify the origin of the observed wide and asymmetric BL band in AlN NWs. Our calculations show that the formation energies of AlI, ON−VAl, and antisite defects are high and will not be discussed any more. 3.2.1. VN at the AlN NW Surface. We can see from Figure 3a that VN is easier to form under N-poor conditions than under


surface layer is slightly larger than the one in the core (1.91 Å), which is in excellent agreement with the previous theoretical result 1.91 Å.34 This is because of the hydrogen passivation in the NW surface layer. The above statements clearly show that our calculations are accurate and reliable. Furthermore, we can see from Figure 1c that our accurate GW band gap of hexagonal AlN NW (∼1 nm) is 8.40 eV, which is larger than the ordinary DFT-GGA band gap of 4.934 and 5.309 eV.35 Compared with AlN bulk, the band gap is dramatically increased by more than 2 eV in the AlN NW due to its strong 1D quantum confinement.36 Figure 1d further shows that the VBM (CBM) of the AlN NW is mainly determined by the N-p (Al-s)state. The calculated imaginary part of the dielectric functions, closely related to the optical absorption, is shown in Figure 2

Figure 3. (a) Formation energy versus Fermi level for the N vancancy. (b) Configuration-coordinate diagram illustrating the optical transition process related to N vancancy in the AlN NW with 1 nm diameter.

N-rich conditions. The formation energy of VN in the case of EF approaching the VBM is larger than that when EF approaches the CBM. The VN has four transition levels within the band gap at 1.49 (+1/0), 2.43 (0/−1), 3.66 (−1/−2), and 4.68 eV (−2/ −3) above the VBM. The calculated optical transitions are presented in Figure 3b. Considering the lowest formation energy of V3− N , we thus chose the −2/−3 transition level at 4.68 eV as the initial state. The electron at this level can be excited to the exciton ground state via a photon absorption at 1.99 eV, leaving VN in the −2 charge state with the V3− N atomic configuration. Losing the excess energy through the fast lattice relaxation, V N2− returns back to its initial states V N3− , accompanied by a photon emission at 1.96 eV. Our calculated emission peak of VN is consistent with experiments.1,7 We further investigate the optical transition process related to the VN defect in AlN NW with diameter of 1.5 nm. Similar to Figure 3, we find that the electron at the −2/−3 transition level at 4.61 eV can be excited to the exciton ground state by means of a photon absorption at 1.92 eV, and then a photon at 1.87 eV can be emitted after the fast lattice relaxation. The difference of the emitting photon energies between the 1 and 1.5 nm AlN NWs is small and within 0.1 eV. This clearly shows that the defect-involved optical transition depends insensitively on the diameter of AlN NWs. Therefore, for simplicity, we will focus our attention on the 1 nm AlN NW in our following calculations. It is worthwhile to note that the negative defect formation energy is a very common result in many previous calculations.40−43 The negative formation energy just indicates that the defect is easy to form in the system, which does not mean an unlimited production of the defects. With increasing defect density, the defect formation energy will obviously increase.44 This is because the system is different from the previous one if more defects are created.

Figure 2. Calculated imaginary part of the dielectric function ε2 for hexagonal AlN NW with and without the electron−hole (e−h) interaction.

for the AlN NWs. The optical absorption occurs at 6.65 eV within the deep-ultraviolet (UV) region. The exciton binding energy, given by the difference between the electronic and optical band gaps, is found to be 1.75 eV in the AlN NW, which is much larger than that in AlN bulk (44 meV).37 This is because of the strong 1D quantum confinement and incomplete dielectric screening in small diameter AlN NWs. In order to know the difference of the electronic structures of AlN NWs with different sizes, we further calculate both the electronic structure and the imaginary part of the dielectric functions of AlN NWs with diameter of 1.5 nm. Similar to Figures 1 and 2, we find that the GW band gap is 7.8 eV, and the optical absorption occurs at 6.5 eV. Moreover, the exciton binding energy is reduced to 1.3 eV due to the large NW diameter. Above all, we know that the electronic and optical band gaps and the exciton binding energy decrease monotonically with increasing NW diameter and will approach the corresponding values of AlN bulk if the NW diameter becomes large enough. The exciton effect increases with decreasing NW diameter. 3.2. Defect-Involved AlN NW. According to Figure 2, the pristine AlN NWs should emit the light in the deep-UV region. However, the typical room-temperature PL spectra (see Figure 5 of ref 5) exhibit a wide and asymmetric BL band within the region of 350−750 nm2,3,5−7,12 with the excitation energy below the band gap of AlN NWs. We can thus confirm that the defects must play an important role in the wide BL band. According to ref 19, VAl, AlI, VN, ON, ON−VAl, 3ON−VAl, C


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The Journal of Physical Chemistry C 3.2.2. VAl at the AlN NW Surface. We can see from Figure 4a that VAl is easier to form under N-rich conditions, and its

2.68 (0/−1), and 3.89 eV (−1/−2) above the VBM. Similar to VN, our calculated optical transitions are presented in Figure 5b. The electron at the −1/−2 level can be excited to the exciton ground state via a photon absorption at 2.80 eV, and then a photon at 2.74 eV can be emitted after the fast lattice relaxation. The calculated emission peak of ON is in good agreement with experiments.5 We further calculate the formation energy of ON−VAl, 2ON− VAl, and 3ON−VAl and find that the 3ON−VAl defect complex has the lowest formation energy. We can see from Figure 5c that the formation energy of 3ON−VAl under N-poor is the same as that under N-rich conditions. The formation energy of 3ON−VAl decreases monotonically if EF increases. Two transition levels are found in the band gap at 2.63 (0/−1) and 3.67 eV (−1/−2) above the VBM. Similar to VN, our calculated optical transitions are shown in Figure 5d. The electron at the −1/−2 level, absorbed a photon at 3.07 eV, can be excited to the exciton ground state, and then a photon at 2.94 eV can be emitted after the fast lattice relaxation. The calculated emission peak of 3ON−VAl is completely consistent with experiments.3,5 It is interesting to note that, similar to the aforementioned PL spectra (350−750 nm) of AlN NWs (see Figure 5 of ref 5), the AlN films and nanobelts have also a distinguished wide PL band from 350 to 550 nm (please refer to Figure 2 of ref 46), approximately. Obviously, these PL spectra should not be due to the band gap emission of AlN and are referred to as defectrelated emission.46−48 According to Sun et al.,46 the wide emission band from 350 to 550 nm in AlN thin films is probably caused by the oxygen impurities. The emission band from 300 to 500 nm in AlN thick layers observed by Eriguchi et al. has been tentatively attributed to the defect complexes between Al vacancy and O impurities or DX-like shallow donor.47 Li et al.48 suggested that the emission band with width of 100 nm centered at 2.54 eV in AlN nanobelts is due to the nitrogen vacancy. Generally, the defects are easy to gather at the surface layer of samples, as we have analyzed in the above. Furthermore, considering that the AlN NWs will turn into AlN bulk if the NW diameter becomes large enough and the defectinvolved optical transition processes depend insensitively on the NW diameter, we thus know that our above analyses can also be applied to the other dimensional AlN samples.


Figure 4. Same as in Figure 3 but for the Al vancancy.

formation energy decreases monotonically if EF increases. Three transition levels are found within the band gap at 1.48 (0/−1), 3.52 (−1/−2), and 4.47 eV (−2/−3) above the VBM. Similar to VN, our calculated optical transitions are presented in Figure 4b. The electron at the −2/−3 level can be excited to the exciton ground state by means of a photon absorption at 2.21 eV, and then a photon at 2.16 eV can be emitted after the fast lattice relaxation. The calculated emission peak of VAl is in excellent agreement with experiments.5 3.2.3. O-Related Defects at the AlN NW Surface. It has been known that the oxygen atom and its related defects have a significant influence on the electron structures and optical properties of AlN NWs.5 Generally, the oxygen atoms in AlN NWs have three possible sources. The first one is that the oxygen atoms can be released from the quartz tube used in the high-temperature growth process; the second one is that the oxygen atoms are absorbed on the NW surface after the sample is removed from the preparation vacuum chamber;45 and the third one is that the oxygen atoms can be introduced in the synthesis of AlN NWs via the reaction of Al2O + 2NH3 → 2AlN + H2O + 2H2.5 Our calculated formation energy of ON is shown in Figure 5a. We can see from Figure 5a that ON is easier to form under N-poor condition than under N-rich condition. The formation energy of ON in the case of EF approaching the VBM is larger than that when EF approaches the CBM. There are three transition levels within the band gap at 1.73 (+1/0),

4. CONCLUSIONS In conclusion, we have calculated the defect formation energy and investigated the defect-induced optical transitions in AlN NWs by state-of-the-art first-principles calculations. The physical origin of the wide and asymmetric BL band has been clarified. We find that the band gap of small diameter AlN NWs (∼1 nm) is 8.40 eV, which is larger than that in AlN bulk (6.2 eV), because of the strong 1D quantum confinement. The exciton binding energy is 1.75 eV, which is much larger than that in AlN (44 meV), due to incomplete dielectric screening in AlN NWs. The VN, VAl, ON, and 3ON−VAl defects are gathered at the surface layer of AlN NWs. The emission peaks from the exciton ground state to these defect levels are at 1.96, 2.16, 2.74, and 2.94 eV, respectively. Their combination dominates the observed wide and asymmetric BL band in AlN NWs. We believe that our understanding for the defect-related luminescence mechanism in AlN NWs has significant meaning to improve the performance of NW-based optoelectronic nanodevices, such as nano-LEDs and nano-LDs.

Figure 5. Same as in Figure 3 but for the ON (a,b) and 3ON−VAl (c,d). The 3ON−VAl defect complex has almost the same formation energy under both N-poor and N-rich conditions. D


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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-10-62757594. Fax: +86-10-62751615. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2012CB619304) and the National Natural Science Foundation of China (11474012, 11364030, 11404013, 61204013). We used computational resource of the “Explorer 100” cluster system of Tsinghua National Laboratory for Information Science and Technology.


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Origin of a Wide and Asymmetric Blue Luminescence Band in AlN

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