InAs CoreâShell

Article pubs.acs.org/JPCC

Engineering the Effective p‑Type Dopant in GaAs/InAs Core−Shell Nanowires with Surface Dangling Bonds Changsheng Song,† Jiqing Wang,*,† Zhixiang Zhang,† Huibing Mao,† Qiang Zhao,† Pingxiong Yang,† and Huaizhong Xing‡ †

Key Laboratory of Polarized Materials and Devices, East China Normal University, Shanghai 200241, China Department of Applied Physics, Donghua University, Shanghai 201620, China

‡

S Supporting Information *

ABSTRACT: Using first-principles calculation based on density-functional theory, we investigated the effect of surface dangling bond on ptype doping mechanism and the electronic structures in wurtzite (WZ) and zinc blende (ZB) GaAs/InAs core−shell nanowires (NWs) along the [0001] and [111] directions, respectively. The results of the formation energies show that the surface dangling bond of the In atom is a kind of stable defect. Both in WZ and ZB core−shell NWs, we found it is easier and more stable to realize dopant in the GaAs core. Moreover, the position of Cd impurity plays a key role in the formation of p-type nanowires. The farther the distance between the impurity and the surface dangling In atom, the easier it is to form the p-type characteristic of the nanowires. In particular, it shows an intrinsic behavior when doping the Cd impurity near the surface dangling bond. The surface dangling bonds have an ability to capture the holes from the neighbor doping impurity, resulting in the deactivation of dopants. Meanwhile, the transfer of hole moves the valence band down to the lower energy levels and even can lead to a band anticrossing phenomenon in the conduction band. Our results highlight a new physical coupling between the doped state and surface dangling bonds in GaAs/InAs core−shell NWs, and open a new opportunity for the development of tailoring nanoscale electronic properties.

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INTRODUCTION Recent progress in nanotechnology of III−V nanowires (NWs) has great promise to integrate conventional optoelectronic devices. Especially, the core−shell NWs heterostructures have enabled the development of their electronic1−3 and optical properties.4,5 Applications such as tubular conductors,6 nanoimprint,7 solar cells,2 photodetectors,8 light-emitting diodes (LEDs),5and ultrahigh density transistors9 have been proposed. For example, the GaAs/AlGaAs core−shell NWs show enhanced photoluminescence intensity compared to pure GaAs NWs.10 Similarly, the electron mobility is increased by covering InAs NWs with InP shells.11 Prosperous realization of this outstanding performance depends on the degree of controlling their structures and physical properties. These applications can be considerably tailored by their diameter, morphology, doping, and surface behaviors. Evidently, intensive studies of the core−shell NWs’ properties, electronic structures, and their treatment are important and show a positive direction on the improvement of the performance of nanoscale device. Tailoring electronic properties of confined nanoscale core− shell NWs presents a fundamental issue. In particular, the ptype doping of core−shell NWs is still a difficult problem and great importance in optoelectronic application.12−14 Moreover, in the quantum confinement regime, the surface behavior induced by the surface dangling bonds (SDBs) is essentially © XXXX American Chemical Society

critical and dramatically affects the structural and the electronic properties of nanostructure.15−25 For example, the p-channel characteristics have been observed when Cd-doped InAs nanowires are passivated by surface ligands.26 Shu et al. have demonstrated that the charged SDBs are a significant obstacle for realizing the effective p-type doping of InAs nanowires but have less influence on the n-type doping.23 In addition, the surface functionalization in InAs 15 and ZnO 18 NWs is considered as the most useful way to lower the effect of dielectric confinement. The model of pseudo hydrogen atoms is reported as an approach to suppress the effects of surface reconstruction on both structural and electronic properties of the GaN/AlN core−shell NWs.24 From the above, the high surface sensitivity often becomes a detriment to device properties and weakens other technological impact of nanoscale materials. Therefore, how to achieve a reliable doping strategies in core−shell nanostructures with SDBs has become a central interesting issue. GaAs and InAs have two kinds of crystal structures: wurtzite (WZ) and zinc blende (ZB). Previous experimental and theoretical work, including ab initio calculations, mostly Received: July 16, 2014 Revised: October 7, 2014

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dx.doi.org/10.1021/jp5070825 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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passivated by the pseudo-hydrogen atoms. We focus the NWs with WZ [0001] and ZB [111] growth directions, with one SDB per unit cell. A dangling bond is formed by removing one hydrogen atom from a surface In or As atom. All NWs present six {110} planes, forming hexagonal cross sections. The supercell used in our study contains 192 and 122 (exclusive of pseudo-hydrogen atoms) atoms for WZ and ZB core−shell NWs, respectively. The kinetic energy cutoff is chosen to be 300.0 and 360.0 eV for the two different NWs, respectively. Periodic boundary conditions are employed in the xy plane with a supercell by a vacuum region of 16 Å to eliminate the interaction between neighboring wires. The Monkhorst−Pack k-point mesh of 1 × 1 × 6 is found to provide sufficient accuracy in the Brillouin zone integration for both WZ and ZB NWs. We include 11 k-points in the band structure calculation along the K vector direction Γ(0,0,0) to Z(0,0,0.5). The geometrical structures have been relaxed until all the convergence tolerance of energy is less than 10−4 eV. Meanwhile, all the forces acting on each atom are lower than 0.01 eV/Å. The core−shell NWs supply a 1D band offset to form a confinement for the hole carriers. Therefore, the hole can be confined by the core regions and benefit the p-type dopant in WZ or ZB structures. We study Cd atoms as common p-type impurity substitution in the core and shell of GaAs/InAs core−shell for WZ or ZB NWs with single SDB (see Figure 1). Because of the shell

focused on the strain relaxation, electronic properties, surface adsorption, and impurity doping of pure GaAs,10,27−29 InAs,23,30−32 and InAs/InP core−shell NWs.11,13,33 GaAs/ InAs core−shell NWs are a promising candidate to implement the highly efficient p-type doping because of their similar crystal structures and the larger band gap difference (∼1.06 eV). Recent theoretical study of first-principles calculations has been performed on the stability and electronic properties of unpassivated GaAs nanowires.34 They found that the band gaps of nanowires are ruled by surface states of edge atoms and do not follow the trend by the quantum confinement effect. Shu et al.23 have studied the role of the SDBs on the doping of WZ and ZB InAs NWs and shown the different surface adsorption mechanisms of NH3 and NO2.32 All these results suggest that the elimination of surface states by passivating surface atoms is an effective way to improve the transport properties and active p-type dopant in nanostructures. It is noteworthy that it has been proved that the SDBs have a negative effect for enhancing efficiency and obtaining p-type doping in InAs and GaAs NWs. However, the microscopic mechanism is far from clear. For example, what is the mechanism and its variation tendency of the different dopant positions effect on the nanowire with SDBs? What is the interaction between the impurities doping at different sites and the surface dangling atom? To the best of our knowledge, the theoretical studies on the role of SDBs in the electronic properties and doping of GaAs/InAs core−shell NWs have not yet been reported. Thus, insight into these questions is essential to realize reliable and modulate p-type doping in GaAs/InAs core−shell NWs. In this paper, motivated by the theoretical and experimental accomplishments, especially to overcome the difficulties of the less efficient on p-type doping in core−shell NWs with surface dangling bonds and to understand the role of the SDBs in the doping core−shell NWs, we have carried out first-principles calculations and systematically investigated the characteristics of SDBs effect on p-type doping in both WZ and ZB GaAs/InAs core−shell NWs. The calculated Cd doping formation energies for the different Cd substitution sites indicate that the impurities prefer to stay in the GaAs region. Moreover, the results show that the position of Cd impurity plays a key role in determining whether the characteristics of p-type semiconductor will generate or not. The combination of In-SDB and Cd dopant complex gradually yields an intrinsic semiconductor characteristic as the distance decreases between the Cd atom and the surface dangling In atom.

Figure 1. Top view of representative ball-and-stick models of hydrogen-passivated Cd doping in WZ (a) and ZB (b) GaAs/InAs core−shell NWs with single surface dangling bond. Blue, red, green, and pink spheres represent In, As, Ga, and Cd atoms, respectively. Dark spheres on the boundary are passivating hydrogen atoms. Red box denotes the sites of the removed hydrogen. The numbers (1, 2, 3, etc.) show the sites of Cd impurities doping at core or shell regions in WZ (ZB) GaAs/InAs core−shell NWs.

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DETAILS OF CALCULATIONS All the first-principles calculations are carried out on the basis of the density functional theory (DFT) as implemented by the Vienna ab initio simulation package (VASP).35,36 The exchange−correlation energy is treated in the generalized gradient approximation (GGA) using the PW91 functional.37,38 Although the well-known fact that DFT/GGA underestimates band gap and results in relatively shallow transition energies and higher formation energies of semiconductors, the trends elucidated by the DFT results are proved to still be valid, as shown in the previous numerous studies on the band gap of semiconductor nanostructures.17,19,23,25 We simulate only thin NWs; the diameters of the considered GaAscore/InAsshell hydrogenated NWs are approximately 3.0 nm (core, ∼1.3 nm) and 2.0 nm (core, ∼1.2 nm) for WZ and ZB NWs, respectively. The surface dangling bonds on the surface are

region of our core−shell NWs is a binary InAs material, we can obtain three types of SDBs: In-SDB, As-SDB, and pair-SDB. The In-SDB and As-SDB are obtained by removing a hydrogen atom from the In and As atoms of the nanowire surface, respectively. Pair-SDB is formed by In and As-SDB pair. In our work, we focus on the In-SDB and As-SDB for WZ and ZB NWs only including single surface dangling bond. The effect of the In−As-pair-SDB for the Cd doping in GaAs/InAs NWs lies between the In-SDB and As-SDB. The comparison results for these three types SDBs are shown in the Figure S1 (Supporting Information). The Cd impurity is used to substitute a Ga atom in the core or an In atom in the shell of the core−shell NWs. We choose several different substitution sites (as shown by the numbers in Figure 1). The numbers 1, 2 and 6, 7 denote the doping positions of Cd substitutional impurities in the shell B



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the nearer the Cd dopant is to SDB, the lower the values of formation energies will be. On the basis of the similar results of DFT calculations for WZ and ZB GaAs/InAs core−shell nanostructures, in our following discussion, we only show the results of Cd doping in various positions for WZ GaAs/InAs NWs with In-SDB. The corresponding results of ZB NWs are summarized in Figures S2 and S3 (Supporting Information). Figure 3a shows the band structures of the hydrogenpassivated WZ GaAs/InAs core−shell NWs with a Cd

regions, and the numbers 3, 4, 5 show the sites of core region in WZ NWs (see Figure 1a). With increasing index numbers, the distance to the dangling bond increases. Meanwhile, the numbers 1, 5, 6 and 2, 3, 4 (as shown in Figure 1b) denote the positions of Cd substitutional impurities in the shell and core regions for ZB core−shell NWs, respectively. To describe the effect of surface dangling bonds of GaAs/InAs core−shell and the stability of the distribution of Cd impurities in the NWs, we calculate the Cd formation energies by the following expression: Ef = ESDB(doped) − ESDB(undoped) +

∑ Δniμi i

(1)

where ESDB(doped) and ESDB(undoped) are the total energies of GaAs/InAs core−shell NWs containing SDB with Cd impurity and without Cd dopant, respectively. μi is the chemical potential of the atomic species i in the As-rich regime, and Δni is the difference between the number of atomic species i in the Cd-doped and undoped NWs.

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RESULTS AND DISCUSSION The resulting formation energies of each SDB for Cd atoms in different positions in WZ and ZB GaAs/InAs core−shell NWs are presented in Figure 2a and Figure 2b, respectively. On the Figure 3. Band structures of the wurtzite GaAs/InAs core−shell NWs with (a) Cd dopant without SDB and (b) In-SDB and Cd dopant complex at different doping positions. Site 1 and site 5 (a) represent the Cd doping positions corresponding to the shell and core regions, respectively. In-SDB-number (b) shows the different Cd substitutional positions corresponding to the Figure1a. The red, blue, and pink lines demonstrate the impurities, surface defects, and intrinsic bands, respectively. The horizontal dashed lines denote the position of the Fermi level.

substitutional dopant at the shell (site 1) and core (site 5) regions, respectively. Both of them show a typical p-type characteristic of nanostructure. In contrast, the band structures of core−shell NWs with different Cd dopant positions undergo a significant change when the In-SDB is introduced. The combination of In-SDB and Cd dopant complex gradually yields an intrinsic semiconductor characteristic as the distance decreases between the Cd atom and the surface dangling In atom, as shown in Figure 3b. For example, it shows a p-type behavior at site 6 (see In-SDB-6 in Figure 3b). However, when the Cd impurity is doped close to the surface dangling bond (see In-SDB-1), it unexpectedly represents an intrinsic characteristic of semiconductor. Moreover, the results show that the energy difference between impurity state (red line) and surface state (blue line) increases gradually as the dopant position gets closer to the dangling bond, which originates from the capture of hole of Cd dopant by In SDB. The holes from Cd dopant are captured by the SDBs when they are set very closely to each other, so the nanowire loses its acceptor characteristic and results in the energy reduction of Cd impurity state (bonding state) and the energy increase of surface state (antibonding state). The material gradually transforms into an intrinsic semiconductor from the p-type nanostructure. More importantly, when Cd is doped at sites 1 and 2, because of the intense hybridization interaction between the intrinsic band of conduction and surface state, we can find that the surface state (blue lines) can transit from band gap into the CB and interacts with the lowest CB (pink lines). It is indicated in In-SDB-1 and -2 of band structures in Figure 3b.

Figure 2. Formation energies (eV) of Cd substitutional doping in the different core and shell sites of GaAscore/InAsshell with In-SDB and AsSDB for (a) WZ NW with diameter d = 3.0 nm and (b)ZB NW with diameter d = 2.0 nm. The dashed line distinguishes between the core and shell regions in core−shell nanostructures.

whole, by adjustment of the different doping positions, the energy values range from 0.6 to 2.2 (2.4) eV for WZ (ZB) structures. It is worth noting that the lower values appear in the core regions of Cd doping and show a nearly homogeneous tendency. The In-SDB and As-SDB have little effect on the Cd formation energies in the core because the GaAs core is surrounded by InAs shell in our core−shell NWs. However, there is a big increased energy for Cd doping in the shell regions. It means that the Cd dopant prefers to stay at the GaAs core. More importantly, the low formation energies of Cd impurities in the GaAs region thus contribute to the higher hole carrier concentration than the dopant in the InAs region. For both Cd doping WZ and ZB NWs with SDB, we find that the formation energies of In-SDB are much lower than those of AsSDB. This suggests that it is easier and more stable to realize the p-type dopant in NWs with In-SDB. Particularly, In the case of In-SDB, the positions of Cd atom doping in the shell play a key role in the stabilities of our core−shell NWs. It shows that C



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(0.1−0.6). The inset shows that the strength of coupling interaction between the Cd impurity state and surface state decreases with the increase of the distance between the Cd atom and the In-SDB and saturates after site 5. Thus, when Cd is doped near the surface dangling In atom, more hole carriers of Cd dopant are captured by In-SDB and there is stronger hybridization interaction between the intrinsic band of CB and surface state, which moves the valence band down, and the material shows an intrinsic behavior of semiconductor (see Figure. 3b). In order to verify the hybridization coupling between the surface state and CB state, we select the Cd doping at site 1 in the InAs shell region. The band structure as shown in Figure 5a demonstrates an intrinsic semiconductor characteristic. Here, we label the lowest conduction band and the second lowest conduction band as 888 and 889, respectively. Because of the existence of the orbital hybridization coupling, bands 888 and 889 generate band anticrossing phenomenon when k z approaches the anticrossing zone. In fact, band 888 (889) is composed of intrinsic (surface) state and surface (intrinsic) state, which are distinguished with pink lines and blue lines, respectively, as shown in the band structure of Figure 5 a. In this case, we deliberately choose a special kz (0.0 0.0 0.2) near the anticrossing point to prove the existence of the two states in one band. Figure 5b shows the two different charge density distribution at (0.0 0.0 0.0) Γ point. Band 888 at (0.0 0.0 0.0) (see Figure 5b, top panel) is the intrinsic state, which corresponds to the band 889 at (0.0 0.0 0.2) (see Figure 5c, bottom panel). Moreover, the surface states consist of band 889 at (0.0 0.0 0.0) (see Figure 5b, bottom panel) and band 888 at (0.0 0.0 0.2) (see Figure 5c, top panel), respectively. Thus, the hybridization of surface state with CB can be clearly observed. The engineering of the effective p-type dopant with SDB can also be observed by Bader charge analysis for WZ GaAs/InAs core−shell NWs. We compare the charge transfer for both the impurity Cd and the In atom with and without SDB,

Because of localization of surface state and a small range of dispersion, a small gap at a special k point can be found in CB when strong coupling occurs. However, the interaction decreases, as the wave vector is far away from the particular k point. Hence, the energy anticrossing between surface and the lowest CB happens at sites 1 and 2. Next, we further discuss in detail the energy variation of impurity and surface states for the Cd doping at different positions in WZ NWs with In-SDB. As shown in Figure 4, the

Figure 4. WZ GaAs/InAs core−shell NWs with Cd dopant of In-SDB. The energy variation of impurity and surface bands as a function of different Cd atoms positions is shown. The inset shows the strength of interaction between these two states.

energies of the impurity state gradually increase with the changing positions of the Cd dopant in turn and the range of energy varies from −0.2 to 0.0 eV. On the other hand, for surface band, the energy gradually decreases with the variational sites, which is opposite to impurity band. Additionally, the energy of surface state is more sensitive to doping position than that of impurity. For instance, the energy variation of the farthest two Cd positions (sites 1 and 7) even reaches 0.5 eV

Figure 5. Band structure of WZ GaAs/InAs core−shell NWs with In-SDB for Cd doping in the InAs shell at site 1 (a). The dotted line represents the Fermi level. Labeled arrows indicate the surface state and lowest intrinsic electronic states whose charge density distributions along the K vector directions (0.0 0.0 0.0) and (0.0 0.0 0.2) respectively are shown in panels b and c. The dashed line shows the core region in core−shell nanostructures. D



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particular, the position of Cd impurity plays a key role in determining whether the behaviors of p-type semiconductor will be generated or not. The farther the distance between the impurity and the surface dangling In atom is, the easier to form the p-type characteristic of the nanowires. Inversely, it shows an intrinsic behavior when doping the Cd impurity near the In atom. The hybridization between the intrinsic and surface states can even lead to a band anticrossing phenomenon in the conduction band. Moreover, according to the Bader charge analysis, when the Cd doping position is near In-SDB, the hole supplied by Cd can accept the unpaired electron of In-SDB and thus hinder the formation of p-type nanowires. To achieve the p-type doping limitation from the SDBs and increase the doping efficiency of GaAs/InAs core−shell NWs, it is required to adopt remote doping or avoid the formation of SDBs. Surface passivation and molecule adsorption of nanowires are effective ways to achieve it. According to our calculations, our proposed p-type doping in NWs can realize high efficiency and controlled impurity dopant engineering in GaAs/InAs core− shell NWs.

respectively (see Figure 6). Here Bader charge transfer is defined as ΔQi = Qatom − Qsc, where Qatom and Qsc are the

Figure 6. Charge transfer ΔQi = Qatom − Qsc of Cd and In (with dangling bond) atoms in WZ GaAs/InAs core−shell NWs (a) without and (b) with In-SDB, respectively. Qatom and Qsc are demonstrated as the Bader charge non-self-consistent and self-consistent, respectively. The position of the dashed line denotes the Bader charge transfer equaling zero.

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ASSOCIATED CONTENT

S Supporting Information *

Bader charge non-self-consistent and self-consistent, respectively. In the case of GaAs/InAs core−shell NWs without InSDB, Figure 6a demonstrates that ΔQi of Cd traces an oscillating curve as the doping sites are away from surface dangling site. However, because of the saturated bond of the surface In atom, the charge variation basically keeps a constant value of around zero. On the other hand, after the hydrogen of surface In atom is removed, an In-SDB is formed, and the ΔQi of Cd has the same tendency as that without SDB, as shown in Figure 6b. In contrast, we note that the ΔQi of In affected by the different positions is more obvious. The variation of Bader charge reveals a nearly linear decreasing trend occurring from site 1 to site 6. It suggests that the nearer the impurity Cd doped is to the surface dangling atom, the more is the Bader charge transfer. The hole supplied by Cd can accept the unpaired electron of In-SDB and decrease the number of charge of In atom. Here, the positive ΔQi of In atom denotes losing electrons, while a negative one means gaining electrons. Therefore, we can conclude that the dangling bond can cause ptype doping failure when the Cd atom is doped closely enough to the SDB, which is not due to the transfer of electrons but results from the capturing of the hole by the defect energy level induced by the surface dangling bond. Particularly, for remote doping Cd impurity (like site 5 of Figure 3) in the GaAs-core, due to the weak interaction between the Cd and In atoms, Cd impurity state obtains more electrons, and 1D hole gas is generated in InAs-shell region as consequence of the existence of a type I band-offset at the GaAs/InAs interface.

Comparison results of three types of surface dangling bonds (In-SDB, As-SDB, and In−As-pair-SDB) for the Cd doping in wurtzite GaAs/InAs NWs, shown in Figure S1; band structures and energy variation of impurity and surface bands as a function of different Cd impurity positions in zinc blende crystal GaAs/ InAs core−shell nanostructure with In-SDB, shown in Figures S2 and S3, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities and National Science Foundation of China (Grants 60990312, 61274100, and 11174048).

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REFERENCES

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CONCLUSIONS In summary, we investigate the characteristics of surface dangling bonds on variational positions of p-type doping in WZ and ZB GaAs/InAs core−shell NWs along the [0001] and [111] directions, respectively. On the basis of the firstprinciples calculations, the calculated Cd doping for the different substitution sites shows that the impurities prefer to stay in the GaAs region. Moreover, comparing the doping in WZ and ZB core−shell NWs of In-SDB and As-SDB, we found that it is easier and more stable to realize it for In-SDB. In E



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