Al-Doped ZnO Monolayer as a Promising Transparent Electrode Material

materials Article

Al-Doped ZnO Monolayer as a Promising Transparent Electrode Material: A First-Principles Study Mingyang Wu 1, *, Dan Sun 2 , Changlong Tan 2, *, Xiaohua Tian 2 and Yuewu Huang 3 1 2 3

*

School of Mechanical Engineering, Harbin University of Science and Technology, Harbin 150080, China College of Applied Science, Harbin University of Science and Technology, Harbin 150080, China; [email protected] (D.S.); [email protected] (X.T.) School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] Correspondence: [email protected] (M.W.); [email protected] (C.T.); Tel.: +86-451-8639-1231 (M.W.); +86-451-8639-0728 (C.T.)

Academic Editor: Andrea Li Bassi Received: 4 February 2017; Accepted: 25 March 2017; Published: 29 March 2017

Abstract: Al-doped ZnO has attracted much attention as a transparent electrode. The graphene-like ZnO monolayer as a two-dimensional nanostructure material shows exceptional properties compared to bulk ZnO. Here, through first-principle calculations, we found that the transparency in the visible light region of Al-doped ZnO monolayer is significantly enhanced compared to the bulk counterpart. In particular, the 12.5 at% Al-doped ZnO monolayer exhibits the highest visible transmittance of above 99%. Further, the electrical conductivity of the ZnO monolayer is enhanced as a result of Al doping, which also occurred in the bulk system. Our results suggest that Al-doped ZnO monolayer is a promising transparent conducting electrode for nanoscale optoelectronic device applications. Keywords: ZnO monolayer; transparent electrode; electrical conductivity; optical property

1. Introduction Transparent conducting oxide (TCO) films have been studied for various photo-electronic devices such as displays, piezoelectric transducers, and solar cells, due to their distinctive combination of optical transparency in the visible spectrum and high electrical conductivity [1–4]. Currently, Tin doped indium oxide (ITO) is indeed the most widely used TCO in commercial application. However, indium has some disadvantage such as toxicity and high cost, so it is hardly suitable for large-area applications [5]. In recent years, Al-doped ZnO (AZO) has been considered to be one of the best candidates as transparent electrode in thin film solar cells and flat-panel displays due to the lower cost arising from the abundance of Zn and Al, and better optical transmission in the visible region [6]. ZnO material can be applied to light emitting devices, due to the wide band gap (3.37 eV), a high exciton binding energy of 60 meV, and transparency under visible light [7,8]. Unfortunately, ZnO is characterized by a low electrical conductivity, making its use in the photovoltaic field difficult. The doping of different impurity atoms can lead to an improvement in the electrical and optical properties. Saniz et al. demonstrated that the Al substituted Zn site of ZnO is favorable energetically [9]. ZnO doped with group-III elements, such as Al and Ga, is regarded as a promising alternative material to ITO, owing to high transparency, low cost, non-toxicity, and high chemical stability [10]. Experimentally, several works show an improvement in the electrical conductivity of ZnO on doping with Al without any significant deterioration in the optical transmittance [11–13]. Therefore, Al-doped ZnO as one of the most promising transparent electronics materials has been widely explored. With the development of the TCO, it has become necessary to achieve high transparency and conductivity in the visible region. The two-dimensional (2D) materials have attracted extensive Materials 2017, 10, 359; doi:10.3390/ma10040359

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Therefore, Al-doped ZnO as one of the most promising transparent electronics materials has been widely explored. With the development of the TCO, it has become necessary to achieve high transparency2 and Materials 2017, 10, 359 of 14 conductivity in the visible region. The two-dimensional (2D) materials have attracted extensive interest recently due to their unique structure and properties differing from the corresponding bulk interest recently duesuccessful to their unique structure and properties from the corresponding bulk materials. After the fabrication of graphene in 2004differing [14], creating a new research field in materials. After the successful fabrication of graphene in 2004 [14], creating a new research field in 2D materials, an increasing number of other 2D graphene-like materials were successfully realized 2D materials, an[15–21]. increasing number of other 2D graphene-like materials successfully realized experimentally Among these materials, ZnO monolayer has were attracted much research experimentally these materials, ZnO monolayer has attracted much research density interest interest because[15–21]. of its Among optoelectronic characteristic [22–29]. Freeman et al. performed because of its optoelectronic characteristic [22–29]. Freeman et al. performed density functional functional calculations to prediction the possibility of forming ultrathin films of wurtzite ZnO with a calculations to prediction the possibility of forming ultrathin films of wurtzite two-monolayer-thick ZnO with a graphitic graphitic structure [22]. Tushce et al. were the first to successfully synthesize structure [22]. Tushce et al. were theAg first to successfully synthesize ZnO (0001) ZnO (0001) films deposited on an (111) surface, where the Zn two-monolayer-thick and O atoms are arranged in a films deposited onthe an Ag (111) surface, where the[23]. Zn and atoms are arranged in a planar as in planar sheet as in hexagonal BN monolayer In aOprevious study, we explored thesheet structure the hexagonal BN monolayer In a previous study, the as structure and optical properties and optical properties of ZnO[23]. monolayer, which werewe allexplored confirmed demonstrating potential for of ZnO monolayer, which were all confirmed as demonstrating potential for future optoelectronic future optoelectronic devices [24]. So far, numerous studies have been conducted on Al-doped ZnO devices [24].thin Sofilms far, numerous studies havethere beenhave conducted onany Al-doped in bulk and in bulk and forms [8–13]. However, not been previousZnO investigations onthin the films formsZnO [8–13]. However, been any previous on the Al-doped ZnO Al-doped monolayer inthere spitehave of itsnot unique structure andinvestigations properties differing from its bulk monolayer in spite of its unique structure and properties differing from its bulk counterpart. counterpart. In this systematically investigated the electrical and optical of Al-doped In thiswork, work,wewe systematically investigated the electrical and characterization optical characterization of ZnO bulkZnO andbulk monolayer using first-principle calculations. We found thatthat the the transparency in Al-doped and monolayer using first-principle calculations. We found transparency thethe visible light region in visible light regionofofAl-doped Al-dopedZnO ZnOmonolayer monolayerisissignificantly significantly enhanced enhanced compared to the bulk counterpart. counterpart.Moreover, Moreover, electrical conductivity the monolayer ZnO monolayer is enhanced as bulk thethe electrical conductivity of theofZnO is enhanced as a result a result of Al doping, which also occurred in the bulk system. It was therefore concluded that of Al doping, which also occurred in the bulk system. It was therefore concluded that Al-doped ZnO Al-doped ZnO a promising transparent electrode for nanoscale optoelectronic monolayer is amonolayer promisingis transparent conductingconducting electrode for nanoscale optoelectronic device device applications. applications. 2. Calculation Calculation Models Models and and Methods Methods 2. All the the calculations calculations were were performed performed using using CASTEP CASTEP code code based based on on density density functional functional theory theory All (DFT) [30]. The interaction between ions and electron is described by ultrasoft pseudopotentials [31]. (DFT) [30]. The interaction between ions and electron is described by ultrasoft pseudopotentials [31]. The cutoff cutoff energy energy of of 400 400 eV eV is The Brillouin Brillouin zone zone integrations integrations were were The is used used for for plane-wave plane-wave expansion. expansion. The performed by using a Monkhorst-Pack grid of 4 × 4 × 2 and 6 × 6 × 1 for Al-doped ZnO performed by using a Monkhorst-Pack grid of 4 × 4 × 2 and 6 × 6 × 1 for Al-doped ZnO bulk bulk and and monolayer, respectively energy was convergedtotoless lessthan than 10 10−6−6eV. eV. Structural Structural monolayer, respectively [32].[32]. The The totaltotal energy was converged optimization was thethe remaining forceforce on each than 0.02 eV/Å. order optimization wasperformed performeduntil until remaining on atom each was atomless was less than 0.02IneV/Å. to investigate the different concentrations of the Al atom, the models of 2 × 2 × 2 supercell for bulk In order to investigate the different concentrations of the Al atom, the models of 2 × 2 × 2 supercell wurtzite ZnO and 4 × 4 supercell for ZnO monolayer with one to three Zn atoms replaced by Al atoms for bulk wurtzite ZnO and 4 × 4 supercell for ZnO monolayer with one to three Zn atoms replaced by were considered. These models correspond to 6.25 at%, to 12.5 at%at%, and12.5 18.75at% at%and of Al in ZnO Al atoms were considered. These models correspond 6.25 18.75 at% bulk of Aland in monolayer, respectively. For example, Figure 1 shows the structures of 6.25 at% Al in ZnO bulk ZnO bulk and monolayer, respectively. For example, Figure 1 shows the structures of 6.25 at% Aland in monolayer. Moreover, a large vacuuma spacing of 15 Åspacing was taken to Å prevent mirror for ZnO bulk and monolayer. Moreover, large vacuum of 15 was taken to interactions prevent mirror ZnO monolayer systems. interactions for ZnO monolayer systems.

(a)

(b)

Figure Figure 1. 1. The Thestructures structures of of 6.25 6.25 at% at% Al-doped Al-doped (a) (a) bulk bulk ZnO ZnO and and (b) (b) ZnO ZnO monolayer. monolayer.

The DFT + Ud + Up method was used to describe the electronic structure accurately. Ma et al. suggested that for oxide materials the Up,O value of 7 eV is suitable for first principles calculations [33]. Wu et al. showed that when both the Ud value for Zn 3d and the Up value for O 2p were employed

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with 10 eV and 7 eV respectively, the calculated band gap and lattice parameter of pristine ZnO bulk are in excellent agreement with experimental values [34]. Therefore, we adopted Ud,Zn = 10 eV and Up,O = 7 eV in this study. All optical properties can be determined from the dielectric function ε of each compound. It describes the optical properties as a function of the frequency or the wavelength. It consists of two parts ε1 and ε2 : ε(ω ) = ε 1 (ω ) + iε 2 (ω ) (1) where ε1 and ε2 are given by [35]: ε 2 (ω ) =

w 4 dSk | Pnn0 (k)|2 ∑ ∇ωnn0 (k) πω 2 nn0

(2)

∞

ε 1 (ω ) = 1 +

2 w ω 0 ε 2 (ω 0 ) 0 dω P π ω0 2 − ω2 0

(3)

Pnn0 (k ) is the dipole matrix elements. ωnn0 (k) is the energy difference between initial and final states. Sk denotes the surface energy and P is the principal part of the integral. The coefficient of absorption α is proportional to the imaginary part ε2 : α(ω ) =

ωε 2 (ω ) c

(4)

the refractive index n(ω ) is a function of ε1 and ε2 : 1/2 1 1 n(ω ) = √ (ε21 (ω ) + ε22 (ω )) 2 + ε 1 (ω ) 2

(5)

the reflectivity R(ω ) can be given by [36,37]: n e − 1 ( n − 1)2 + k 2 R(ω ) = = e+1 n ( n + 1)2 + k 2

(6)

To calculate the electrical properties of the pristine and Al-doped bulk and monolayer ZnO systems, we fitted the calculated band structure into a Boltzmann package, which is based on semi-classic Boltzmann theory and the rigid band approach [38,39]. The dependence of the conductivity on transport distribution can be given by: σαβ (ε) =

δ(ε − ε i,k ) 1 σαβ (i, k ) ∑ N i,k δ(ε)

(7)

where, N denotes the number of k-points that are sampled in the Brillouin zone and εi,k presents the band structure. The k-dependent transport tensor is read as: σαβ (i, k) = e2 τi,k vα (i, k)v β (i, k)

(8)

with, i and k stand for the band index and wave vector, respectively, and τ denotes the relaxation time, να (i,k) is the α component of the group velocities, while e is the electron charge. By fitting the transport distribution over energy, the electrical conductivity can be obtained as a function of the temperature, T, and the chemical potential, µ, via the following equations: ∂ f µ ( T, ε) 1w σαβ ( T, µ) = σαβ − dε Ω ∂ε

(9)

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where α and β stand for the tensor indices, Ω, µ, and f denote the volume of the unit cell, the Fermi level of carriers, and the carrier Fermi-Dirac distribution function, respectively. Owing tothe complexity of the electron scattering mechanisms in the material, the exact solution of the Boltzmann equation cannot be achieved. To overcome this issue, the relaxation time is treated as an energy-independent constant, which is known as the relaxation time approximation. This approach has been demonstrated to be a reasonable good approximation for evaluating the electrical transport properties of several materials [40–42]. 3. Results and Discussion 3.1. Structural Properties Firstly, we explored the structural properties of Al-doped ZnO bulk and monolayer. Table 1 shows the lattice parameters for the optimized structure of the studied systems at various Al concentrations. The lattice parameters of pristine bulk ZnO are a = 3.313 Å and c = 5.329 Å, which are close to the experimental values of a = 3.249 Å and c = 5.206 Å [43] and in good agreement with other theoretical calculations [44], indicating that our calculations are reliable. Starting from a structure cut from the ZnO wurtzite crystal and terminating with the (0001) polar surface, atomic relaxations from the DFT calculations indicate that ZnO monolayer stabilizes in a hexagonal BN structure. The calculated Zn-O bond length in the ZnO monolayer is 1.91 Å, which is lower than the corresponding bond length in bulk ZnO (2.01 Å). This is due to the fact that the sp2 hybridization in the 2D honeycomb structure is stronger than the sp3 hybridization in wurtzite crystal. Moreover, it is found that the lattice constant a of Al-doped ZnO monolayer decreases with increasing Al concentration. This phenomenon is also observed in bulk ZnO. The reason is attributed to the fact that the radius of Al atom is smaller than that of Zn. Table 1. The lattice parameters and formation energies of Al-doped ZnO bulk and monolayer. Compounds

Concentration

a (Å)

c (Å)

Formation Energy (eV)

bulk

0% 6.25% 12.5% 18.75%

3.313 3.312 3.307 3.306

5.329 5.315 5.320 5.324

−0.096 −0.162 −0.231

monolayer

0% 6.25% 12.5% 18.75%

4.437 4.426 4.421 4.408

-

−0.059 −0.135 −0.199

The formation energy is an important parameter to describe material stability. It is well known that material systems with lower formation energies are more stable than those with higher formation energies. The negative formation energy means the formation of the structure is thermodynamically favorable. To evaluate the plausibility of doping Al atoms in ZnO bulk and monolayer, we calculated their formation energies, as shown in Table 1. The definition of formation energy was given in our previous work [24]. From Table 1, it can be seen that formation energies of both Al-doped ZnO bulk and monolayer are found to be negative, signifying that Al atoms are suitable for doping into ZnO systems. More interestingly, we found that the value of the formation energy for the Al-doped ZnO monolayer is lower than that of the bulk counterpart at the same Al concentration. We can conclude that the preparation of Al-doped ZnO monolayer could be rather easily achieved in experiments, because Al-doped ZnO bulk has been successfully prepared. Our results of formation energies would be useful for designing the growth process and application of Al-doped ZnO monolayer.

18.75%

4.408

-

−0.199

3.2. Electronic Properties

8

8

6

6

4

4

2

Energy (eV)

Energy (eV)

Next, we turn to the effect of Al doping on the electronic properties of ZnO bulk and monolayer, Materials 2017, where 10, 359 we calculated the band structure of Al-doped ZnO bulk and monolayer as shown 5 of 14 in Figures 2 and 3, respectively. From Figure 2a, the calculated band structure shows that pristine ZnO bulk is a direct band gap semiconductor with the conduction band minimum (CBM) and the 3.2. Electronic valence band Properties maximum (VBM) located at the same G point of the Brillouin zone. The VBM orbitals are mainly dueturn to Zn the CBM orbitals are mainly from O 2p Theand corresponding Next, we to 3d thestates effectand of Al doping on the electronic properties of states. ZnO bulk monolayer, band gap 3.37 eV,the which in good of agreement experiment [45]. When the Al where we is calculated bandisstructure Al-dopedwith ZnOthe bulk and monolayer as shown in concentration in ZnO bulk is large than 12.5 at%, the Fermi level shifts upward into the conduction Figures 2 and 3, respectively. From Figure 2a, the calculated band structure shows that pristine band as seen Figure 2c,d, which indicates that this is an n-type semiconductor. means ZnO bulk is aindirect band gap semiconductor with thesample conduction band minimum (CBM)Itand the that shallow states are created the Fermi level in Brillouin the bottom of the band, valence band donor maximum (VBM) locatedaround at the same G point of the zone. Theconduction VBM orbitals are leading to the increase of carrier concentration. mainly due to Zn 3d states and the CBM orbitals are mainly from O 2p states. The corresponding band From 3a, itisisinseen that the directwith band gap of pristine[45]. ZnOWhen monolayer eV) is much gap is 3.37 Figure eV, which good agreement the experiment the Al(4.03 concentration in larger thanisthat of bulk the quantum confinement effect. from Figure 3b–d, ZnO bulk large than material 12.5 at%,due theto Fermi level shifts upward into theMoreover, conduction band as seen in it can be seenwhich that the band structure of Al-doped ZnO monolayer is similar to thatthat of Al-doped ZnO Figure 2c,d, indicates that this sample is an n-type semiconductor. It means shallow donor bulk. Fermi level alsothe moves the band the Alband, concentration large than states The are created around Fermiinto level in conduction the bottom of the when conduction leading toisthe increase 12.5 at%. of carrier concentration.

3.37 eV

2

0

0

-2

-2

-4

-4

G

F

Q

Z

Spin up

G

(a)

F

Z

Q

G

G

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Q

Z

Spin up

Spin down

G

F

Z

Q

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Spin down

(b)

4

4

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Energy (eV)

0

-2

-4

0

-2

-4 -6

-6

-8

-10

G

F

Q

Spin up

Z

G

(c)

F

Q

Z

Spin down

G

-8

G

F

Q

Spin up

Z

G

(d)

F

Q

Spin down

Z

G

Figure Figure 2. 2. The The band band structures structures of of (a) (a) pristine pristine ZnO ZnO bulk; bulk; (b) (b) 6.25 6.25 at%; at%; (c) (c) 12.5 12.5 at%; at%; (d) (d) 18.75 18.75 at% at% Al-doped Al-doped ZnO bulk. ZnO bulk.

From Figure 3a, it is seen that the direct band gap of pristine ZnO monolayer (4.03 eV) is much larger than that of bulk material due to the quantum confinement effect. Moreover, from Figure 3b–d, it can be seen that the band structure of Al-doped ZnO monolayer is similar to that of Al-doped ZnO bulk. The Fermi level also moves into the conduction band when the Al concentration is large than 12.5 at%. Furthermore, the band gaps of Al-doped ZnO bulk and monolayer as a function of Al concentration are shown in Figure 4. For the bulk ZnO structure, the band gap decreases with increasing Al concentration at the range of less than 12.5 at%. The reason lies in the fact that the conduction band moves to the low-energy region as Al atoms are incorporated into bulk ZnO, leading to the decrease of band gap. However, when the Al concentration is higher than 12.5 at%, the band gap increases slightly with increasing Al concentration. In terms of Al-doped ZnO monolayer, the band gap decreases first when the Al concentration is less than 6.25 at%, and then increases with further increase in Al concentration. Particularly, it is observed that the band gap of Al-doped ZnO

6

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Energy (eV)

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4.03 eV

2

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0

-2

-2

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84

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40

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Energy (eV) (eV) Energy

Energy(eV) (eV) Energy

monolayer is obviously larger than that of ZnO bulk counterpart. Consequently, it is to be expected -4 -4 G that the band-edge absorption of Al-doped could be with Al K K M G M ZnOGmonolayer M blueG shifted G K G M increasing K Materials 2017, 10, 359 6 of 13 Spin down Spin up Spin up Spin down concentration compared to the(a) Al-doped ZnO bulk counterpart. (b)

4.03 eV

2 -2

0 -4

0-4

-2 -6

-4 -8

2-2

-2-6

G G

KK

M M

G G

Spin up Spin up

(a) (c)

K

M

K M Spin down Spin down

-4-8

GG

GG

KK

M M

Spin Spinup up

G G

(b) (h) d

KK

MM

Spin Spindown down

G G

4

4

Figure 3. The band structures of (a) pristine ZnO monolayer; (b) 6.25 at%; (c) 12.5 at%; (d) 18.75 at% 2 Al-doped2 ZnO monolayer. 0

Energy (eV)

Energy (eV)

0 Furthermore, the band gaps of Al-doped ZnO bulk and monolayer as a function of Al concentration are shown in Figure 4. For the bulk ZnO structure,-2the band gap decreases with increasing Al -2 concentration at the range of less than 12.5 at%. The reason lies in the fact that the conduction band -4 moves to the-4 low-energy region as Al atoms are incorporated into bulk ZnO, leading to the decrease of band gap.-6 However, when the Al concentration is -6higher than 12.5 at%, the band gap increases slightly with increasing Al concentration. In terms of Al-doped ZnO monolayer, the band gap -8 -8 decreases first is less than 6.25 at%, increases with further G whenK the K Mand then G M Al concentration K M G G G G K M Spin up Spin the up band gap Spinof down Spin down it is observed that increase in Al concentration. Particularly, Al-doped ZnO (c) (h) d monolayer is obviously larger than that of ZnO bulk counterpart. Consequently, it is to be expected Figure The band band structuresofof ofAl-doped (a) pristine pristineZnO ZnOmonolayer monolayer;could (b) 6.25 6.25 at%; (c)shifted 12.5 at%; at%; (d) 18.75 18.75 at% at% Al that the band-edge absorption be blue with increasing Figure 3. The structures (a) ZnO monolayer; (b) at%; (c) 12.5 (d) Al-doped ZnO monolayer. concentration Al-doped compared to the Al-doped ZnO bulk counterpart.

Band gap energy (eV)

Furthermore, the band gaps 5.0 of Al-doped ZnO bulk and monolayer as a function of Al concentration bulk ZnO are shown in Figure 4. For the bulk ZnO structure, the band gapZnO decreases with increasing Al monolayer 4.5 concentration at the range of less than 12.5 at%. The reason lies in the fact that the conduction band moves to the low-energy region4.0as Al atoms are incorporated into bulk ZnO, leading to the decrease of band gap. However, when the Al concentration is higher than 12.5 at%, the band gap increases 3.5 slightly with increasing Al concentration. In terms of Al-doped ZnO monolayer, the band gap decreases first when the Al concentration is less than 6.25 at%, and then increases with further 3.0 increase in Al concentration. Particularly, it is observed that the band gap of Al-doped ZnO monolayer is obviously larger than that of ZnO bulk counterpart. Consequently, it is to be expected 2.5 that the band-edge absorption of Al-doped ZnO monolayer could be blue shifted with increasing Al concentration compared to the Al-doped ZnO bulk counterpart. 2.0 0 at.%

6.25 at.%

12.5 at.%

18.75 at.%

5.0

bulkand ZnO monolayer. Figure Figure 4. 4. The The band band gap of Al-doped ZnO bulk monolayer ZnO

Band gap energy (eV)

4.5

4.0

3.5

3.0

2.5

2.0

0 at.%

6.25 at.%

12.5 at.%

18.75 at.%

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To further investigate investigate the theeffect effectofofAl Aldoping dopingonon the conductance in detail, Figure 5 presents the conductance in detail, Figure 5 presents the the total density of states (DOS) of ZnO bulk and monolayer doped with different Al concentrations. total density of states (DOS) of ZnO bulk and monolayer doped with different Al concentrations. The results obviously indicate that Al doping makes the Fermi level of ZnO monolayer enter the conduction band when the Al concentration is large than 12.5 at%. This phenomenon occurred in the Al-doped ZnO bulk system as well. well. Upon Al doping, electrons are introduced into the conduction conduction band ofofZnO monolayer, which increases the concentration of free electrons, resulting in the decrease in ZnO monolayer, which increases the concentration of free electrons, resulting in the the electronic resistance of Al-doped ZnO bulk and monolayer. The occupied states of electrons in the decrease in the electronic resistance of Al-doped ZnO bulk and monolayer. The occupied states of conduction the Fermiband level are generally related to the concentration. 5, electrons inband the near conduction near the Fermi level aredonor generally related From to theFigure donor it can be seen that theFigure Al 3s orbital to the the Al occupied states around the Fermi level. These concentration. From 5, it cancontributes be seen that 3s orbital contributes to the occupied states donor around the Fermi could be around considered the origin of the be conductivity in the aroundstates the Fermi level. Theselevel donor states the as Fermi level could consideredincrease as the origin Al doped ZnO sample. Furthermore, we findZnO thatsample. the donor states extend andthe deeply into the of the conductivity increase in the Al doped Furthermore, wegently find that donor states conduction band with increasing Al concentration, thus with the conductance Al-doped ZnO extend gently and deeply into the conduction band increasing of Alboth concentration, thusbulk the and monolayer is significantly enhanced. Therefore, less energy is required to liberate electrons from conductance of both Al-doped ZnO bulk and monolayer is significantly enhanced. Therefore, less the material, resulting in the increase in conductance. The enhanced of ZnO monolayer energy is required to liberate electrons from the material, resulting conductance in the increase in conductance. due to Al doping, which is similar to the doping effect in ZnO bulk, provides a basis for its potential The enhanced conductance of ZnO monolayer due to Al doping, which is similar to the doping effect application a transparent electrode. in ZnO bulk,asprovides a basis for its potential application as a transparent electrode. 60

50

TDOS

0 0.28

Zn-4s Zn-4p Zn-3d

0.21 0.14 0.07 0.00

O-2s O-2p

0.8

0.4

Density of states (electrons/eV)

30


TDOS

25

0 0.28

Zn-4s Zn-4p Zn-3d

0.21 0.14 0.07 0.00 0.8

O-2s O-2p

0.4 0.0

Al-3s Al-3p

0.15 0.10 0.05

0.0 -10

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0

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0.00

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(b)

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TDOS

TDOS

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0 0.28

Zn-4s Zn-4p Zn-3d

0.21 0.14 0.07 0.00

O-2s O-2p

0.5

0.0 0.12

Al-3s Al-3p


25


0

Energy (eV)

(a)

50

-5

0 0.28

Zn-4s Zn-4p Zn-3d

0.21 0.14 0.07 0.00 0.8

O-2s O-2p

0.4 0.0 0.12

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0.06

0.06

0.00

0.00 -15

-10

-5

0

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(c)

(d) Figure 5. Cont.

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60

60

TDOS

TDOS

40

40

0 0.49 0.42 0.35 0.28 0.21 0.14 0.07 0.00

Zn-4s Zn-4p Zn-3d

O-2s O-2p

1.2

0.6



20 20

Zn-4s Zn-4p Zn-3d O-2s O-2p

0.6 0.0 0.6

Al-3s Al-3p

0.3 0.0

0.0 -10

-5

0

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-10

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-5

0

5

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(e)

60

(f)

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TDOS

TDOS

40

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20

0 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1.2

Zn-4s Zn-4p Zn-3d

O-2s O-2p

0.6 0.0

Al-3s Al-3p

0.4


20


0 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1.2

0 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1.2

Zn-4s Zn-4p Zn-3d

O-2s O-2p

0.6 0.0 0.4

Al-3s Al-3p

0.2

0.2 0.0 -15

-10

-5

0

Energy (eV)

(g)

5

10

0.0 -15

-10

-5

0

Energy (eV)

5

10

(h)

Figure 5. The total and partial total density of states (DOS) of (a) pristine bulk ZnO; (b) 6.25 at%; Figure 5. The total and partial total density of states (DOS) of (a) pristine bulk ZnO; (b) 6.25 at%; (c) 12.5 at%; (d) 18.75 at% Al-doped bulk ZnO; (e) pristine ZnO monolayer; (f) 6.25 at%; (g) 12.5 at%; (c) 12.5 at%; (d) 18.75 at% Al-doped bulk ZnO; (e) pristine ZnO monolayer; (f) 6.25 at%; (g) 12.5 at%; (h) 18.75 at% Al-doped ZnO monolayer. (h) 18.75 at% Al-doped ZnO monolayer.

3.3. Optical Properties 3.3. Optical Properties In this section, we present and compare the calculated optical properties of pristine and In this section, we present and compare the calculated optical properties of pristine and different different concentrations of Al-doped ZnO bulk and monolayer. It is well known that the good TCOs concentrations of Al-doped ZnO bulk and monolayer. It is well known that the good TCOs should should have low absorption and reflectivity coefficient with large transmittance in the large have low absorption and reflectivity coefficient with large transmittance in the large wavelength region. wavelength region. In order to describe the optical properties of these structures, we calculated the In order to describe the optical properties of these structures, we calculated the imaginary part of imaginary part of the dielectric function through the formulas mentioned in the above section of the dielectric function through the formulas mentioned in the above section of calculation methods. calculation methods. The reflectivity, the absorption coefficient, and the transmittance can be The reflectivity, the absorption coefficient, and the transmittance can be calculated from the dielectric calculated from the dielectric function. The imaginary parts of the dielectric functions of ZnO bulk function. The imaginary parts of the dielectric functions of ZnO bulk and monolayer doped with and monolayer doped with different Al concentrations were calculated and are compared in Figure 6. different Al concentrations were calculated and are compared in Figure 6. From this figure, we can From this figure, we can see the imaginary part of the dielectric function of Al-doped ZnO bulk is see the imaginary part of the dielectric function of Al-doped ZnO bulk is smaller than that of pristine smaller than that of pristine ZnO bulk, and has a blue shift to the higher energy side. This result is ZnO bulk, and has a blue shift to the higher energy side. This result is consistent with a previous consistent with a previous study [46]. Moreover, the main peak width of the imaginary part of the study [46]. Moreover, the main peak width of the imaginary part of the dielectric function narrows dielectric function narrows following increasing Al concentration, indicating that the range of following increasing Al concentration, indicating that the range of absorption frequency narrows and absorption frequency narrows and the average optical transmittance increases. On the other band, the average optical transmittance increases. On the other band, for the Al-doped ZnO monolayer, the for the Al-doped ZnO monolayer, the imaginary part of the dielectric function is much smaller than imaginary part of the dielectric function is much smaller than that of Al-doped ZnO bulk. The main that of Al-doped ZnO bulk. The main peak at about 4.69 eV of pristine ZnO monolayer moves to the peak at about 4.69 eV of pristine ZnO monolayer moves to the higher energy side after Al doping. higher energy side after Al doping. Moreover, the energy value of this main peak decreases with Moreover, the energy value of this main peak decreases with increasing Al concentration. In addition, increasing Al concentration. In addition, after the 6.25 at% Al is doped into the ZnO monolayer, a after the 6.25 at% Al is doped into the ZnO monolayer, a new peak is formed at low energy, due to new peak is formed at low energy, due to the transition between the Al-3s donor occupied states the transition between the Al-3s donor occupied states around the Fermi level and the unoccupied around the Fermi level and the unoccupied Zn-4s and Zn-4p states in the conduction band. The new Zn-4s and Zn-4p states in the conduction band. The new peak is enhanced and shifts to lower energy peak is enhanced and shifts to lower energy on increasing the Al concentration to 12.5 at%.

increasing absorption in the visible region for 18.75 at% Al-doped bulk ZnO. For the pristine ZnO monolayer, the optical absorption coefficient calculated between 200 nm and 1200 nm is lower than that of ZnO bulk in the UV and visible regions, and presents a decline at about 255 nm. When the Al is doped, the optical absorption edge of ZnO monolayer has a clear blue-shift to a shorter wavelength region with increasing Al concentration. Meanwhile, following the increase of Al Materials 2017, 10, 359 9 of 14 concentration, the absorption coefficient decreases and becomes small in the visible region. This is due to the fact that Al doping decreases the concentration of Zn, resulting in a decrease of on increasing the Al concentration 12.5 at%. of Furthermore, thismonolayer peak becomes intense the absorption. However, the absorptiontocoefficient Al-doped ZnO increases in when the range Al-doped concentration increases to 18.75 at%. The reason may be that doping of high Al concentration from 600 nm to 1200 nm. Particularly, it is worth noting that the absorption coefficient obviously results in in thethe occupied states widening. increases range of 800 nm to 1200 nm when the Al concentration is 12.5 at%. 2.0

bulk ZnO bulk 6.25 at.% Al bulk 12.5 at.% Al bulk 18.75 at.% Al monolayer ZnO monolayer 6.25 at.% Al monolayer 12.5 at.% Al monolayer 18.75 at.% Al



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Figure 6. The Theimaginary imaginary part of the dielectric function of bulk ZnOand bulk and monolayer doped with Figure 6. part of the dielectric function of ZnO monolayer doped with different different Al concentrations. Al concentrations. 30000

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bulk ZnO bulk ZnO Furthermore, In Figure 7a, we wavelength for bulkshow 6.25 at.% Althe absorption coefficient as a function ofbulk 6.25 at.% Al visible light region bulk 12.5 at.% Al 12.5 at.% Al visible pristine light region ZnObulk pristine and Al-doped ZnO bulkbulk and monolayer. It is clear that the bulk has a low 4 18.75 at.% Al bulk 18.75 at.% Al monolayer ZnO monolayer ZnO absorption coefficient in the visible and infrared (IR) regions. Moreover, the absorption coefficient monolayer 6.25 at.% Al 20000 monolayer 6.25 at.% Al monolayer 12.5 at.% Al monolayer 12.5 at.% Al increases from the visible to ultraviolet (UV) regions. When the absorption 3 the Al is doped in ZnO bulk, monolayer 18.75 at.% Al monolayer 18.75 at.% Al coefficient decreased and shifted to the lower wavelength region in the visible and UV regions. When 2 slowly increased in the range of 276 nm to the Al concentration is 12.5 at%, the absorption coefficient 10000 1070 nm. Meanwhile, it can be observed that the shallow donor states induce increasing absorption in the visible region for 18.75 at% Al-doped bulk ZnO. 1For the pristine ZnO monolayer, the optical absorption coefficient calculated between 200 nm and 1200 nm is lower than that of ZnO bulk in the UV and0200visible 400 regions,600and presents a1000decline at about0200 255 nm.400When 600 the Al is800doped,1000the optical 800 1200 1200 absorption edge of ZnOWavelength monolayer region (eV) has a clear blue-shift to a shorter wavelength Wavelength (nm) with increasing (a) Al concentration. Meanwhile, following the increase of Al concentration,(b) the absorption coefficient decreases and becomes small in the visible region. This is due to the fact that Al doping decreases the concentration of Zn, resulting in a decrease of absorption. However, the absorption coefficient of Al-doped ZnO monolayer increases in the range from 600 nm to 1200 nm. Particularly, it is worth noting that the absorption coefficient obviously increases in the range of 800 nm to 1200 nm when the Al concentration is 12.5 at%. The reflectivity and transmittance are important parameters to describe the optical property of TCOs. Figure 7b,c show the reflectivity and transmittance of Al-doped ZnO bulk and monolayer, respectively. The reflectivity of pristine ZnO bulk is low in the visible region and the average transmittance of pristine ZnO bulk is around 94%. For Al-doped ZnO bulk, the average transmittance increases with Al concentration in the UV and visible regions. This case is also reported in many experimental studies concerning Al-doped ZnO thin films [47]. The conventional ITO electrode has a transmittance of about 80% [48]. Therefore, Al-doped ZnO bulk is a high transparency material and has promise for transparent electronics. Nevertheless, when the Al concentration reaches 18.75 at%, the reflectivity rises rapidly and the transmittance declines rapidly at 300 nm due to the high carrier concentration.

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Figure Materials 2017,6. 10,The 359 imaginary part of the dielectric function of ZnO bulk and monolayer doped with 10 of 14 different Al concentrations. 30000

5 bulk ZnO bulk 6.25 at.% Al bulk 12.5 at.% Al bulk 18.75 at.% Al monolayer ZnO monolayer 6.25 at.% Al monolayer 12.5 at.% Al monolayer 18.75 at.% Al

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bulk ZnO bulk 6.25 at.% Al bulk 12.5 at.% Al bulk 18.75 at.% Al monolayer ZnO monolayer 6.25 at.% Al monolayer 12.5 at.% Al monolayer 18.75 at.% Al 400

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(c) Figure 7. (a) Absorption coefficient; (b) reflectivity; (c) transmittance of Al-doped ZnO bulk and Figure 7. (a) Absorption coefficient; (b) reflectivity; (c) transmittance of Al-doped ZnO bulk monolayer. and monolayer.

The reflectivity and transmittance are important parameters to describe the optical property of For the pristine ZnOthe monolayer, theand reflectivity is obviously lower and transmittance is far TCOs. Figure 7b,c show reflectivity transmittance of Al-doped ZnOthe bulk and monolayer, larger than that of bulk ZnO, as shown in Figure 7b,c. When Al is doped, the reflectivity of ZnO respectively. The reflectivity of pristine ZnO bulk is low in the visible region and the average monolayer decreases with increasing in the and UV regions. the transmittance of pristine ZnO bulkAlisconcentration around 94%. Forvisible Al-doped ZnO bulk, However, the average reflectivity slowly increases at 560 nm for Al-doped ZnO monolayer at 12.5 at% Al concentration. transmittance increases with Al concentration in the UV and visible regions. This case is also When thein Almany concentration is at 18.75 at%,concerning the reflectivity speed increases the UV The average reported experimental studies Al-doped ZnO thininfilms [47].region. The conventional transmittance 6.25 Al-doped ZnO monolayer reaches 98% in Al-doped the visibleZnO rangebulk (390–760 nm). ITO electrode of has a at% transmittance of about 80% [48]. Therefore, is a high When the Al concentration is 12.5 at%, the highest visible transmittance of about 99% is achieved. transparency material and has promise for transparent electronics. Nevertheless, when the Al However, when the Al18.75 concentration is 18.75 at%, the rapidly transmittance the Al-doped declines ZnO monolayer concentration reaches at%, the reflectivity rises and theoftransmittance rapidly rapidly decreases in the UV region. Overall, the transparency in the visible light region of the Al-doped at 300 nm due to the high carrier concentration. ZnO For monolayer is significantly enhanced of bulk lower counterpart. particular, 12.5isat% the pristine ZnO monolayer, thecompared reflectivitytoisthat obviously and theIntransmittance far Al-doped transmits moreinlight. larger thanZnO thatmonolayer of bulk ZnO, as shown Figure 7b,c. When Al is doped, the reflectivity of ZnO monolayer decreases with increasing Al concentration in the visible and UV regions. However, the 3.4. Transport Properties reflectivity slowly increases at 560 nm for Al-doped ZnO monolayer at 12.5 at% Al concentration. we concentration investigated the electrical transport propertiesspeed in the increases pristine and Al doped ZnO bulk WhenHere, the Al is at 18.75 at%, the reflectivity in the UV region. The and monolayer using the transport ZnO equations. We calculated conductivity average transmittance of Boltzmann 6.25 at% Al-doped monolayer reaches the 98%electrical in the visible range σ/τ of pristine and Al-doped ZnO bulk and monolayer ashighest a function of the relaxation time τ at room (390–760 nm). When the Al concentration is 12.5 at%, the visible transmittance of about 99% temperature. However,when one ofthe theAl major shortcoming of calculating σ value is the of is achieved. However, concentration is 18.75 at%, the the transmittance of knowledge the Al-doped the relaxation time relation. To overcome this shortcoming, we used the relaxation time relationship ZnO monolayer rapidly decreases in the UV region. Overall, the transparency in the visible light region of the Al-doped ZnO monolayer is significantly enhanced compared to that of bulk counterpart. In particular, 12.5 at% Al-doped ZnO monolayer transmits more light. 3.4. Transport Properties Here, we investigated the electrical transport properties in the pristine and Al doped ZnO bulk and monolayer using the Boltzmann transport equations. We calculated the electrical conductivity σ/τ of pristine and Al-doped ZnO bulk and monolayer as a function of the relaxation time τ at room

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reported by Ong et al. [49] using the same method as that for ZnO material. The relationship of relaxation time is given as following: τ = 2.53 × 10−5 T −1 n−1/3

(10)

where T is the temperature and n is the electron concentration. Materials 2017, 10, 359 11 of 13 First, we fixed the temperature at 300 K (room temperature), taking the electron concentration from our calculation for pristine andofAl-doped and Then monolayer ZnO. We obtained, relationship, the estimated values relaxationbulk times. we used these relaxationfrom timesthis to relationship, the estimated values of relaxation times. Then we used these relaxation times to calculate calculate the electrical conductivity σ as (σ/τ) × τ. The obtained values of the electrical conductivity the σ asthis (σ/τ) × τ. The obtained values of the electrical conductivity shown are electrical shown inconductivity Figure 8. From figure, we find that the electrical conductivity obviouslyare increases in Figure 8. FromAl this figure, we find electrical conductivity obviously with increasing with increasing concentration inthat boththe bulk and monolayer ZnO when theincreases Al concentration is less Al concentration in both bulk and monolayer ZnO when the Al concentration is less than 12.5 at%. than 12.5 at%. However, for the higher concentration of Al, the electrical conductivity decreased. By However, for the higher concentration of Al, the electrical conductivity decreased. By numerical numerical comparison, after Al doping, the electrical conductivity of ZnO monolayer is slightly comparison, the electrical conductivity ZnO monolayer is slightly loweristhan that lower than after that Al of doping, bulk material. However, for the ofmonolayer material, this value already of bulk material. However, for the monolayer material, this value is already relatively high. Therefore, relatively high. Therefore, combining the results of optical and transport properties, we suggest that combining of optical and transport properties, we suggest that Al-doped Al-doped the ZnOresults monolayer would be promising transparent electrode materialZnO formonolayer nanoscale would be promising transparent electrode material for nanoscale optoelectronic device applications. optoelectronic device applications. 8

bulk ZnO monolayer ZnO

3-1cm-1)

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Al doped concentration (at.%) Figure 8. The electrical conductivity of ZnO bulk and monolayer doped with different Al concentrations. Figure 8. The electrical conductivity of ZnO bulk and monolayer doped with different Al concentrations.

4. Conclusions

In summary, we studied the electronic structure and optical properties of Al-doped ZnO bulk 4. Conclusions and monolayer. We found that the band gap of the pristine and Al-doped ZnO monolayer is larger In summary, we studied the electronic structure and optical properties of Al-doped ZnO bulk and than that of the bulk system due to the quantum confinement effect. In addition, after Al is doped in monolayer. We found that the band gap of the pristine and Al-doped ZnO monolayer is larger than that the ZnO monolayer, the Fermi level shifts into the conduction band and there is a shallow donor of the bulk system due to the quantum confinement effect. In addition, after Al is doped in the ZnO state around the Fermi level. Moreover, the transparency in the visible light region of Al-doped ZnO monolayer, the Fermi level shifts into the conduction band and there is a shallow donor state around monolayer is significantly enhanced compared to bulk counterpart. In particular, the 12.5 at% the Fermi level. Moreover, the transparency in the visible light region of Al-doped ZnO monolayer Al-doped ZnO monolayer exhibits the highest visible transmittance of above 99%. Furthermore, our is significantly enhanced compared to bulk counterpart. In particular, the 12.5 at% Al-doped ZnO calculations show that the electrical conductivity of ZnO monolayer is enhanced by Al doping and monolayer exhibits the highest visible transmittance of above 99%. Furthermore, our calculations show exhibits a relatively high value. These results show that Al-doped ZnO monolayer is a promising that the electrical conductivity of ZnO monolayer is enhanced by Al doping and exhibits a relatively transparent conducting electrode for nanoscale optoelectronic device applications. high value. These results show that Al-doped ZnO monolayer is a promising transparent conducting electrode for nanoscale optoelectronic device applications. Acknowledgments: The authors acknowledge the supports of National Natural Science Foundation of China (Grant Nos. 51471064 and 51301054); the Program for New Century Excellent Talents (Grant No. 1253-NCET-009); and Program for Youth Academic Backbone in Heilongjiang Provincial University (Grant No. 1251G022). Author Contributions: Changlong Tan conceived and designed the experiments; Mingyang Wu, Dan Sun performed the experiments; Mingyang Wu, Dan Sun, and Xiaohua Tian analyzed the data; Dan Sun, Mingyang Wu, Xiaohua Tian, and Yuewu Huang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest

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Acknowledgments: The authors acknowledge the supports of National Natural Science Foundation of China (Grant Nos. 51471064 and 51301054); the Program for New Century Excellent Talents (Grant No. 1253-NCET-009); and Program for Youth Academic Backbone in Heilongjiang Provincial University (Grant No. 1251G022). Author Contributions: Changlong Tan conceived and designed the experiments; Mingyang Wu, Dan Sun performed the experiments; Mingyang Wu, Dan Sun, and Xiaohua Tian analyzed the data; Dan Sun, Mingyang Wu, Xiaohua Tian, and Yuewu Huang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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