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Electronic and structural characterization of InN heterostructures grown on β-LiGaO2 (001) substrates
Vacuum 119 (2015) 106e111
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Electronic and structural characterization of InN heterostructures grown on b-LiGaO2 (001) substrates Jianli Wang*, Long Pu, Gang Tang, Junting Zhang Department of Physics, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 March 2015 Received in revised form 6 May 2015 Accepted 7 May 2015 Available online 18 May 2015
The structural and electronic properties of a wurtzite InN (11e20) film on an orthorhombic LiGaO2 (001) substrate were systematically studied by first-principle calculations. The In adsorption atoms are more favorable than the N atoms, the NeN dimer will be formed with increasing N coverage on LiGaO2 (001) surface. The calculated surface grand potentials show the perfect O2-termination is more stable than those defective O2-terminated LiGaO2 (001). The InN/O2-terminated LiGaO2 (001) interface is energetically favorable interface among the atomic arrangements of the InN (11e20)kLiGaO2 (001) interfaces. The antisite defects InN may act as a potential source for p-type behavior of InN on the O2-terminated LiGaO2 (001) surface. © 2015 Elsevier Ltd. All rights reserved.
Keywords: LiGaO2 (001) InN Surface/interface Defects First-principles calculations
1. Introduction Indium nitride (InN) is a promising material for near-infrared optoelectronics, high-efficiency solar cells, and high-speed electronics owing to its considerably narrower direct band gap (0.7e0.8 eV) [1,2] and superior electron transport characteristics in nitride semiconductors [1,3]. The growth of high-quality InN single crystals is difficult due to the low dissociation temperature and high equilibrium vapor pressure of nitrogen molecules, this material is usually observed by the heteroepitaxial growth. Meanwhile, InN has been usually grown using highly reactive nitrogen sources such as NH3 or N2 plasma, which causes nitridation of the substrate surfaces just before the epitaxial growth. This limits the substrates for the epitaxial growth of InN to chemically stable materials such as Si [4], SiC [5] and Al2O3 [6]. The suitable substrate materials play an important role for the high-quality epitaxial growth. The gas phase chemistry involved the AlGaInN epitaxial growth by metalorganic chemical vapor deposition was investigated using first principle calculations [7]. Furthermore, the wurtzite InN has been mainly grown along the [0001] c-axis with strong spontaneous and strained-induced piezoelectric polarization [8,9]. The polarization causes the
* Corresponding author. E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.vacuum.2015.05.004 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
internal electric field, which leads to quantum confined Stark effect (QCSE) and hence a reduced radiative combination probability of the carriers [10]. The strained-induced piezoelectric polarization field can be reduced by using lattice-matched substrates in heteroepitaxial nitride device structures. The spontaneous polarization field caused by polar atomic arrangement along the c-axis with ionic chemical bonds, however, is an inherent physical property of the wurtzite semiconductors. No fixed interface or surface polarization charges exist in the cubic nitrides [001] growth direction, because there is no polar atomic arrangement. Unfortunately, the present quality of the cubic nitride materials is unsuitable for device production. Another approach to resolve the spontaneously induced charge is to use wurtzite materials with either no polarization field or a reduced one in the growth direction and respectively across the active device regions. The nonpolar (11e20) a-InN and (1e100) m-InN planes can eliminate QCSE from occurring. Actually, the application of nonpolar orientated III-nitride films [11] for device fabrication is a key issue with the aim to avoid the strong internal electric fields in active regions of optoelectronic devices and to improve their efficiency. LiGaO2 (LGO), a ternary mixed-metal oxide compound semiconductor with a direct band gap of 5.26 eV [12], has a distortedwurtzite-type orthorhombic structure (space group Pna21) at ambient pressure [13]. The LGO crystal structure consists of an alternate stacking of two-dimensional array of oxygen tetrahedral centered on Ga and Li atoms. The space group of LGO belongs to
J. Wang et al. / Vacuum 119 (2015) 106e111
Pna21 and this means that LGO has polarity along the c-axis. Thus LGO crystal shows piezoelectric properties [14]. The melting point of LGO is about 1585 C and this compound exhibits a congruent melting [14]. Large LGO single crystals can be grown by the Czochralski method [12], and high purity single crystals several inches long have been reported [12]. Surface properties of LGO and etching have been investigated in epitaxial growth [15,16]. LGO can be cleaved to the cleavage faces that are promising lattice-matched substrate for epitaxial growth of nonpolar InN. LGO has an orthorhombic unit cell with dimensions a ¼ 5.402 Å, b ¼ 6.372 Å, and c ¼ 5.007 Å [13]. Its lattice mismatch with a-plane InN [17] is as small as 5.50% in the a-direction and 3.91% in the b-direction in experiment. Li and Yang [18] found that a-plane InN epitaxially grows on LGO (001) with high phase purity and smooth surface. The in-plane epitaxial relationships are InN [0001] kLGO [100] and InN [1-100] kLGO [010]. A better characterization of the composition dependence of the physical properties of the material is critical for further development of optoelectronic materials. The structural and electronic properties of the typical defects were studied in a hexagonal AlN network such as vacancies, antisites, and impurities [19]. The interface morphology and microstructure play an important role in affecting the physical properties of layer structure materials. To investigate the influence of the interfacial structure on electronic properties and the possible relationships between the interface microstructure, a clear understanding of the interface microstructure is especially meaningful. Although InN/LGO heterojunction shows many promising properties, there are few reports on the epitaxial growth of InN films on LGO substrates to date. Hence we present a systematic theoretical study of the structural and electronic properties of InN on b-LGO (001) substrates. 2. Methodology The present self-consistent total-energy pseudopotential calculations are performed using the Vienna ab initio simulation package (VASP) [20]. The projector-augmented wave [21] method € chl is used for the treatment of the core electrons. The elecof Blo tron exchange and correlation are treated within the generalized gradient approximation (GGA) using the PerdeweBurkeeErnzerhof [22] functionals. The Ga 3d and In 4d [23] electrons are treated as valence electrons. The O 2s2 and 2p4 electrons are considered as valence electrons. A plane-wave cutoff energy of 400 eV is used throughout for the plane-wave set. The energy differs by less than 0.2% with increasing cutoff energy. The forces on each ion are relaxed to less than 0.02 eV/Å. If we decrease the atomic forces to 0.002 eV/Å, the energy differs by less than 0.01%. The Brillouin-zone integrations are performed using a Gaussian smearing [24] of sigma ¼ 0.2 eV. This would accelerate the convergence. Within the computational scheme presented above, the optimized lattice constant of b-LGO used for constructing surface configurations is a ¼ 5.383 Å, b ¼ 6.363 Å, and c ¼ 5.052 Å, which is very close to experimental value [13] as well as other theoretical results (a ¼ 5.361 Å, b ¼ 6.255 Å, and c ¼ 5.361 Å) [25]. The InN lattice constants of the optimized wurtzite structure (a ¼ 3.6137 Å, c ¼ 5.8582 Å) [26] within GGA are in good agreement with the theoretical lattice parameters (a ¼ 3.5848 Å, c ¼ 5.8002 Å) [27] and the experimental values (a ¼ 3.535 Å, c ¼ 5.699 Å) [17]. The lattice mismatch for InN is estimated to be 5.50% in the a-direction and 3.91% in the b-direction in theory, if we assume the in-plane alignment of InN [0001]kLGO [100] and InN [1-100]kLGO [010]. For the theoretical modeling, the LGO (001) surface is modeled using a three-dimensional periodic surface slab model. Both LiGaterminated and O2-terminated surfaces are considered. In both cases, the unit cell includes an atomic slab with ten atomic LGO
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(001)-(2 2) layers (see Fig. 1(a) and seven point five cb-LGO lattice constants width (about 38 Å) of vacuum regions in thickness. We perform some tests with a bigger atomic slab which includes fourteen atomic LiGaO2 (001)-(2 2) layers and six point five cb-LGO lattice constants width of vacuum regions in thickness. The adsorption energy differs by less than 0.12 eV. The z-axis is taken as normal to the surface. For the adsorption studies presented here, only those high symmetry adsorption sites are examined (see Fig. 1(b) and (c)). The calculations are done with a (4 4 1) MonkhorstePack [28] mesh. Increasing the mesh to (6 6 1), the energy differs by less than 0.01%. The coverage is defined as the number of the adatom divided by the number of surface atoms. The adsorption energy per adsorption atom is defined by the equation [29,30] of Eads ¼ (Etotal Eref nEX)/n, where Etotal and Eref are the total energy of the adsorbed model and the clean surface model, n and EX are respectively the number and the energy of the adsorption atom X.
3. Results and discussion According to the results of the calculations, the second layer oxygen atom relaxes outward for LiGa-terminated LGO (001) surface and the upper layer oxygen atom relaxes inward (i.e. towards the bulk) for O2-terminated LGO (001) surface. For LiGa-terminated LGO (001) surface the upper layer metal Li atom relaxes inwards by 2.45% of bulk lattice constant bb-LGO, but the upper layer metal Ga atom relaxes outwards by 3.44% of bb-LGO. Outward relaxation of the second layer oxygen atom is 1.28% and 1.86% of bb-LGO. The upper layer oxygen atom for O2-terminated LGO (001) surface relaxes inwards by 1.95% and 3.33% of bb-LGO, while the second layer Li (Ga) atom relaxes outwards by 2.83% (0.82%) of bb-LGO. To understand the initial growth and adsorption behavior of InN on LGO substrate, we firstly focus on the structures of In or N adsorbate on LGO (001). The adsorption energies for In or N adatom on the LiGa-terminated and the O2-terminated LGO (001) surfaces are calculated for different adsorption sites, and the results are summarized in Table 1. For the LiGa-terminated LGO (001) surface, the adsorption energies of 1/16 monolayer (ML) In adsorption atoms at the H sites are shown to be lowered by 0.18, 0.07 and 0.17 eV/atom than the structures at the To, TGa and TLi sites. The
Fig. 1. (a) Three dimensional views of the LiGa-terminated and the O2-terminated LGO (001) surfaces; the top view of the two surface layer of the LiGa-terminated (b) and (c) the O2-terminated LGO (001) surface. H is the hollow site, top is the site directly above the surface/sublayer (second layer) atoms (abbreviate as TLi, TO and TGa).
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distances of the In adatom at the H site relative to the substrate Ga atoms are found to be 2.91 Å for the LiGa-terminated surface. When further layers of In are added, all the In atoms will move to the H sites with 1/4 or 1/2 ML In adsorption. The adsorption energies decrease with decreasing In coverage, 1/16 ML In adsorption at the H arrangement is the most stable. As for N adsorption on the LiGaterminated LGO (001) surface, the N atoms at the TGa and TLi sites will move to the H sites with 1/16 ML N adsorption and all the N atoms will move to the H sites with 1/4 ML N adsorption. All the surface structures induced by 1/16 or 1/4 ML nitrogen adsorption are unstable. However, the adsorption energies of 1/2 ML N adsorption atoms at the H or To sites get lower than that of 1/16 or 1/4 ML N adsorption. The reason is that the former of the NeN dimer. The charge density in Fig. 2 shows the chemical bonding between the N atoms is bonded better. Thus the LiGa-terminated LGO (001) surface is not suitable candidate for the growth of nonpolar wurtzite InN planes. We also investigate the stability of In and N adsorption on the O2-terminated LGO (001) surface. The adsorption energies of 1/ 16 ML In adsorption atoms that occupy the H adsorption sites are lower by 2.36, 4.16 and 2.45 eV/atom than that of In adsorption atoms at To, TGa and TLi sites. The distances between In and the underlying nearest O atoms are found to be 2.00 Å for the O2terminated surface. For a higher coverage, the adsorption energies of 1/4 ML In adsorption atoms that occupy the H sites are lower by 0.02 eV/atom than that at TGa (TLi) sites. Meanwhile, 1/4 ML In atoms will shift from the TO site to the H site and the total energy and the calculated relaxed atomic positions are the same. All the In atoms will move to the H sites with 1/2 ML In adsorption. The adsorption energies decrease with decreasing In coverage for In adsorption on the H sites of the O2-terminated LGO (001) surface. The adsorption energies of 1/16 ML N atoms are 2.70 eV/atom on the O2-terminated LGO (001) surface, which is obvious higher than that for In atoms. Thus the In adsorption atoms are more favorable than the N atoms. The reason could be coulomb attraction between In and underlying O in the top layer and repulsion between N and underlying oxygen. The adsorbing 1/4 ML N at TLi sites shows the lowest energy and thus is favorable. As the adsorption structure of 1/4 ML N atoms at TGa sites is unstable, the grown InN crystal may present nitrogen vacancy near the Ga atoms of the O2-terminated LGO (001) surface. Moreover, only the 1/2 ML N adsorption at the H sites presents the stable structure because the former of the NeN dimer on the O2-terminated LGO (001) surface (see Fig. 2(c)). The In or N adsorption atoms are more favorable O2-termination surface than the LiGa-termination surface for the LGO (001) surface among the structures examined. We have made no effort to introduce the temperature and pressure dependence here, so the results must be
used with caution when addressing questions of the relative stability of surfaces under specific external conditions, i.e., the growth temperature, atmospheric pressure, etc. Unlike the above atomically perfect surface structures, real LGO (001) surfaces are defective with missing atoms (vacancies) and/or substitutional defects. These point defects are thought to have an effect on the electrical and chemical behavior of the junction. It is important to study the role of impurities in junction formation for the development and manufacturing of optoelectronic devices. In the following, we shall compare structures with surface point defects. These defects include (a) O vacancy, VO, where one O atom is removed from O2-termination surface every 2 2 cell, (b) In substitutes O, InO, (c) N substitutes O, NO. We examined the structures using (2 2) surface unit cell. These models are simple for the real surfaces, where the defects probably are not in periodic arrays. Meanwhile, the stability of different surface configurations is believed to be very sensitive to chemical environment. Therefore, the effect of environment should be carefully considered to obtain the well-controlled LGO surfaces. We compute the surface grand potential of LGO surface (Ref. [31] and references therein) to estimate the relative stability of different terminations. Since, only the energy of the stoichiometric unit is defined in a compound material such as LGO, one must use the thermodynamic considerations to estimate the surface grand potential. We introduce the chemical potentials mLi, mGa, and mO of the Li, Ga, and O atomic species, respectively. One of the standard control parameters in actual experiments is the oxygen partial pressure in the vacuum chamber. The chemical potential of oxygen atoms is deviated from the energy of an oxygen atom in a free isolated O2 molecular and describes the abundance of oxygen in the environment. The oxygen chemical potential has to be smaller than that of an oxygen atom of O2 molecular. Otherwise an oxygen condensate will form at the surface. The chemical potential of In (N) is calculated from the total energy of thermal equilibrium bulk In (N2). The surface grand potential per unit area U can be given as:
U¼
1 ðEtot NLi mLi NGa mGa NO mO NX EX Þ S
(1)
with NLi, NGa, NO, and NX are the number of Li, Ga, O, and X (In or N) atoms in the slab, respectively. Etot is the total energy of the surface under consideration. S represents the surface area. The chemical potential mLGO of a condensed and stoichiometric phase of LGO is written as a sum of three terms representing the chemical potential of each species within the crystal:
mLGO ¼ mLi þ mGa þ 2mO :
(2)
Table 1 The adsorption energies per adsorption atom for In or N adatoms on the LiGa-terminated and the O2-terminated LGO (001) surfaces in eV.
LiGa-terminated surface In
N
O2-terminated surface In
N
H
To
TGa
TLi
1/16 1/4 1/2 1/16 1/4 1/2
3.16 2.44 2.71 0.42 1.34 0.10
2.98 e e 0.61 e 0.10
3.09 e e e e 3.33
2.99 e e e e 3.80
1/16 1/4 1/2 1/16 1/4 1/2
9.72 6.39 5.04 2.70 0.01 0.15
7.36 e e 1.11 0.39 0.06
5.56 6.37 e 1.57 0.44 1.47
7.27 6.37 e 2.49 0.11 1.91
J. Wang et al. / Vacuum 119 (2015) 106e111
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Fig. 2. Isosurfaces of the charge densities for 1/2 ML N adsorption at (a) H and (b) To sites at LiGa-terminated LGO (001) surface and (c) H sites at O2-terminated LGO (001) surface.
Since the surface is in equilibrium with the bulk LGO, we have
mLGO ¼ Ebulk (the bulk energy per formula unit in the orthorhombic structure). If we replace Eq. (2) with Eq. (1), we can eliminate the mGa and mLGO variables in the surface grand potential and obtain
1 U ¼ ½Etot NGa Ebulk NX EX mO ðNO 2NGa Þ mLi ðNLi NGa Þ S (3) Relying upon Eq. (3), one can deduce the range of the accessible values of U if the minimum and maximum values of the O and Li chemical potentials are known [32,33]. If we introduce the variation in the chemical potentials with respect to those computed for bulk the reference phases (DmO ¼ mO Emol O2 =2 and DmLi ¼ mLi ELi , respectively) in Eq. (3), we obtain
1 U ¼ f ½DmO ðNO 2NGa Þ þ DmLi ðNLi NGa Þ S with
" # Emol 1 O2 bulk ðNO 2NGa ÞELi ðNLi NGa Þ : f¼ Etot NGa Ebulk NX EX S 2 (4) f expresses the stability of the surface with respect to bulk LGO, molecular oxygen, and metallic Li, while (NO-2NGa) represents the excess (if positive) or the deficiency (if negative) in the number of O atoms of the terminations. Simultaneously, we can also eliminate the mLi(mO) and mLGO variables in the surface grand potential. If we introduce the variation of the chemical potentials with respect to those computed for the reference phases (DmO and DmGa ¼ mGa Ebulk Ga , or DmLi and DmGa, respectively), we obtain the surface grand potential:
1 U ¼ f ½DmO ðNO 2NLi Þ þ DmGa ðNGa NLi Þ S With
f¼
" # Emol 1 O Etot NLi Ebulk NX EX 2 ðNO 2NLi ÞEbulk ðN N Þ; Ga Li Ga S 2 (5)
and
U¼ h
1 N N DmLi NLi O þ DmGa NGa O S 2 2
With
h¼
1 N NO NO bulk Etot O Ebulk NX EX Ebulk E : N N Ga Li Ga Li S 2 2 2 (6)
f and h express the stability of the surface with respect to bulk LGO, molecular oxygen (metallic Li) and metallic Ga. The total energy calculations for an O2 molecule in gas phase were carried out using a cubic cell of 10 Å, which was found to be large enough to avoid interaction with other molecules in the neighboring cells. The relative values of the energies of an O atom in the O2 molecule in the gas phase, of the Li atom in the bulk cubic structure, and of the Ga atom in the pure a-Ga metal (a face-centered orthorhombic lattice with eight atoms per primitive cell) [34] are 4.92 eV, 1.90 eV, and 3.03 eV, respectively. Our computed value of the surface grand potential of LGO with respect to the Ga and Li atoms in their bulk phases, and the O atom in the gas phase is 8.08 eV. The thermodynamic stability of the (001) LGO surfaces may be estimated individually as functions of the chemical potentials by the Eqs. (4)e(6). The first-principle results for f,f, and h are listed in Table 2. The derivation of the upper and lower bounds of DmO, DmLi, and DmGa is detailed in Refs. [32,33]. The defective O2termination is unstable in the stability diagram of the O2-terminated LGO (001) surface (not shown) within the allowed region for the chemical potential of oxygen, lithium, and gallium, the perfect (001) LGO surface would predominate over the cleavage or growth of the LGO crystal along the [001] orientation. Note the pressure dependence [31] of oxygen is not considered here. No complete information regarding which termination is stable under specific conditions, so our atomistic simulation techniques offer a source of knowledge of surfaces in advance of experiment. The understanding and control of the interfaces are critical for the integration and fabrication of reliable heterojunctions. A fundamental issue is the difference between the surface lattice symmetry and the bulk. The geometry of the stable heterojunctions is schematically shown in Fig. 3 for the interfaces in the present study. The InN/LGO heterojunctions are modeled by a slab thickness of ten atomic layers (ALs) of LGO and six ALs of InN. Adsorption was allowed on only one side of the exposed surfaces, with the dipole moment corrected accordingly in the z direction. We also examine InN/LGO heterojunctions with nitrogen vacancies; since the In adsorption atoms are more favorable than the N atoms and
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J. Wang et al. / Vacuum 119 (2015) 106e111
Table 2 The surface energy f,f, and h, as defined in Eqs. (4)e(6), are given for the O2terminated surface under consideration in J/m2.
f
f h
Clean
VO
InO
NO
0.09 0.09 0.09
0.07 0.07 0.66
1.08 1.08 1.67
0.58 0.58 1.18
the adsorption structure of 1/4 ML N atoms at TGa sites are unstable. The total energy of the perfect InN/LGO heterojunction is higher (1.10 eV) than the sum of the total energy of the nitrogen atom and the InN/LGO heterojunction with nitrogen vacancies. Thus nitrogen vacancies may present in the InN/LGO heterointerface. Moreover, the total energy of the InN/LGO heterojunction with nitrogen vacancies is lower (6.00 eV) than that of the InN/LGO heterojunction with ON (the first InN surface oxygen atoms occupy the nitrogen vacancies), which indicates that the oxygen atom will not diffuse from the O2-terminated LGO (001) to nitrogen vacancy site in the InN/LGO heterointerface. The total energy of the InN/LGO heterojunction with the antisite defects InN (In atom substitutes N atom) is lower (1.65 eV) than the sum of the total energy of the In atom and the InN/LGO heterojunction with nitrogen vacancies. So the antisite defects InN are stable in the InN/LGO heterointerface. For the relaxed perfect InN/LGO interface, the interplanar spacing normal to the surface (0.958 Å) is less than 1.851 Å (aInN/ 4þ3cLGO/16). After nitrogen vacancies introduced in the InN surface,
there is much larger relaxation in comparison to that for the perfect InN/LGO heterointerface. The interplanar distance is found to be 0.857 Å for the relaxed InN/LGO interface with nitrogen vacancies. The change of the interplanar distance is unobvious for In adsorption on the nitrogen vacancies in the InN/LGO interface. We analyze the electronic properties of the perfect and defective InN/ LGO interface. Qualitative insights into the electronic properties can be obtained by contrasting closely related electronic structures. As the partial density of states (PDOS) provide a general identification of the nature of the orbital, the effects of the interface atoms on the electronic properties of the interface can be obtained by contrasting the PDOS. Fig. 4 (a) displays the total density of states (TDOS) for the perfect InN/LGO heterojunction. The upper valence band ranges are mainly composed of In 4d5p, N 2p, Ga 4s4p and O 2p states, and the inegap states are mainly derived from the In 5s5p, N 2p, and O 2p states. After nitrogen vacancies introduced at the InN/LGO interface, some N 2p inegap states disappear in comparison with TDOS for the perfect InN/LGO heterojunction (Fig. 4(b)), which might impact on the interfacial electrically conducting. Meanwhile, the valence and conduction bands are shifted up in energy. The substitution of In for N in the InN/LGO interface reduces the inegap states in comparison with TDOS of the perfect Ge/SrO heterojunction (Fig. 4(c)); the TDOS change unobviously after the substitution of In for N defects at the InN/LGO interface in comparison with that of the InN/LGO heterojunction with N vacancies. 4. Summary In summary, the structural and electronic properties of InN on the LGO (001) substrate were studied using the DFT. The upper layer oxygen atom for O2-terminated LGO (001) surface relaxes inwards by 1.95% and 3.33% of bb-LGO, while the second layer Li (Ga) atom relaxes outwards by 2.83% (0.82%) of bb-LGO. The In or N adsorption atoms are more favorable O2-termination surface than the LiGa-termination surface for the LGO (001) surface among the structures examined. The In adsorption atoms are more favorable than the N atoms for both the LiGa-terminated and the O2-terminated LGO (001) surface. The NeN dimer will be formed with increasing N coverage on LGO (001) surface. The surface grand potentials of the perfect and defective (001) LGO as functions of the chemical potentials indicate that the perfect O2-terminations are stable. The interplanar spacing normal to the surface is 0.958 Å for the relaxed perfect InN/LGO interface. The effects of the intrinsic
Fig. 3. Schematic of the InN/O2-terminated LGO interfaces showing the composition of each layer. For the reduced InN/LGO interface, the nitrogen vacancies are simulated by removing the nitrogen atom at the top of Ga atom from the first layer of the InN surface.
Fig. 4. The TDOS of the valence and conduction band states of the perfect and defective InN/LGO heterojunctions. The vertical dotted line gives the location of Fermi level.
J. Wang et al. / Vacuum 119 (2015) 106e111
point defects of InN on the electronic properties are investigated by comparing the DOS of the various interface configurations on the O2-terminated LGO (001) substrate. The antisite defects InN and nitrogen vacancies at the InN/LGO interface reduce the occupied states at the Fermi level in comparison with TDOS for the perfect InN/LGO heterojunction. Acknowledgment This work has been supported by NNSFC (11347016) and the Fundamental Research Funds for the Central Universities under Grant no. 2014QNA65. We are grateful to the High Performance Computing Center of China University of Mining and Technology for the award of CPU hours to accomplish this work. References [1] Bhuiyan AG, Hashimoto A, Yamamoto A. J Appl Phys 2003;94:2779. [2] Butcher KSA, Tansley TL. Superlattices Microstruct 2005;38:1. [3] Walukiewicz W, Ager III JW, Yu KM, Liliental-Weber Z, Wu J, Li SX, et al. J Phys D 2006;39:R83. [4] Kumar M, Roul B, Shetty A, Rajpalke MK, Bhat TN, Kalghatgi AT, et al. Appl Phys Lett 2011;99:153114. [5] Ive T, Brandt O, Ramsteiner M, Giehler M, Kostial H, Ploog KH. Appl Phys Lett 2004;84:1671. [6] Jasinski J, Liliental-Weber Z, Lu H, Schaff WJ. Appl Phys Lett 2004;85:233. €m S, Hultman L, Janze n E. [7] Kakanakova-Georgieva A, Gueorguiev GK, Stafstro Chem Phys Lett 2006;431:346. [8] Waltereit P, Brandt O, Trampert A, Grahn HT, Menniger J, Ramsteiner M, et al. Nature 2000;406:865.
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Electronic and structural characterization of InN heterostructures grown on b-LiGaO2 (001) substrates Jianli Wang*, Long Pu, Gang Tang, Junting Zhang Department of Physics, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 March 2015 Received in revised form 6 May 2015 Accepted 7 May 2015 Available online 18 May 2015
The structural and electronic properties of a wurtzite InN (11e20) film on an orthorhombic LiGaO2 (001) substrate were systematically studied by first-principle calculations. The In adsorption atoms are more favorable than the N atoms, the NeN dimer will be formed with increasing N coverage on LiGaO2 (001) surface. The calculated surface grand potentials show the perfect O2-termination is more stable than those defective O2-terminated LiGaO2 (001). The InN/O2-terminated LiGaO2 (001) interface is energetically favorable interface among the atomic arrangements of the InN (11e20)kLiGaO2 (001) interfaces. The antisite defects InN may act as a potential source for p-type behavior of InN on the O2-terminated LiGaO2 (001) surface. © 2015 Elsevier Ltd. All rights reserved.
Keywords: LiGaO2 (001) InN Surface/interface Defects First-principles calculations
1. Introduction Indium nitride (InN) is a promising material for near-infrared optoelectronics, high-efficiency solar cells, and high-speed electronics owing to its considerably narrower direct band gap (0.7e0.8 eV) [1,2] and superior electron transport characteristics in nitride semiconductors [1,3]. The growth of high-quality InN single crystals is difficult due to the low dissociation temperature and high equilibrium vapor pressure of nitrogen molecules, this material is usually observed by the heteroepitaxial growth. Meanwhile, InN has been usually grown using highly reactive nitrogen sources such as NH3 or N2 plasma, which causes nitridation of the substrate surfaces just before the epitaxial growth. This limits the substrates for the epitaxial growth of InN to chemically stable materials such as Si [4], SiC [5] and Al2O3 [6]. The suitable substrate materials play an important role for the high-quality epitaxial growth. The gas phase chemistry involved the AlGaInN epitaxial growth by metalorganic chemical vapor deposition was investigated using first principle calculations [7]. Furthermore, the wurtzite InN has been mainly grown along the [0001] c-axis with strong spontaneous and strained-induced piezoelectric polarization [8,9]. The polarization causes the
* Corresponding author. E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.vacuum.2015.05.004 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
internal electric field, which leads to quantum confined Stark effect (QCSE) and hence a reduced radiative combination probability of the carriers [10]. The strained-induced piezoelectric polarization field can be reduced by using lattice-matched substrates in heteroepitaxial nitride device structures. The spontaneous polarization field caused by polar atomic arrangement along the c-axis with ionic chemical bonds, however, is an inherent physical property of the wurtzite semiconductors. No fixed interface or surface polarization charges exist in the cubic nitrides [001] growth direction, because there is no polar atomic arrangement. Unfortunately, the present quality of the cubic nitride materials is unsuitable for device production. Another approach to resolve the spontaneously induced charge is to use wurtzite materials with either no polarization field or a reduced one in the growth direction and respectively across the active device regions. The nonpolar (11e20) a-InN and (1e100) m-InN planes can eliminate QCSE from occurring. Actually, the application of nonpolar orientated III-nitride films [11] for device fabrication is a key issue with the aim to avoid the strong internal electric fields in active regions of optoelectronic devices and to improve their efficiency. LiGaO2 (LGO), a ternary mixed-metal oxide compound semiconductor with a direct band gap of 5.26 eV [12], has a distortedwurtzite-type orthorhombic structure (space group Pna21) at ambient pressure [13]. The LGO crystal structure consists of an alternate stacking of two-dimensional array of oxygen tetrahedral centered on Ga and Li atoms. The space group of LGO belongs to
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Pna21 and this means that LGO has polarity along the c-axis. Thus LGO crystal shows piezoelectric properties [14]. The melting point of LGO is about 1585 C and this compound exhibits a congruent melting [14]. Large LGO single crystals can be grown by the Czochralski method [12], and high purity single crystals several inches long have been reported [12]. Surface properties of LGO and etching have been investigated in epitaxial growth [15,16]. LGO can be cleaved to the cleavage faces that are promising lattice-matched substrate for epitaxial growth of nonpolar InN. LGO has an orthorhombic unit cell with dimensions a ¼ 5.402 Å, b ¼ 6.372 Å, and c ¼ 5.007 Å [13]. Its lattice mismatch with a-plane InN [17] is as small as 5.50% in the a-direction and 3.91% in the b-direction in experiment. Li and Yang [18] found that a-plane InN epitaxially grows on LGO (001) with high phase purity and smooth surface. The in-plane epitaxial relationships are InN [0001] kLGO [100] and InN [1-100] kLGO [010]. A better characterization of the composition dependence of the physical properties of the material is critical for further development of optoelectronic materials. The structural and electronic properties of the typical defects were studied in a hexagonal AlN network such as vacancies, antisites, and impurities [19]. The interface morphology and microstructure play an important role in affecting the physical properties of layer structure materials. To investigate the influence of the interfacial structure on electronic properties and the possible relationships between the interface microstructure, a clear understanding of the interface microstructure is especially meaningful. Although InN/LGO heterojunction shows many promising properties, there are few reports on the epitaxial growth of InN films on LGO substrates to date. Hence we present a systematic theoretical study of the structural and electronic properties of InN on b-LGO (001) substrates. 2. Methodology The present self-consistent total-energy pseudopotential calculations are performed using the Vienna ab initio simulation package (VASP) [20]. The projector-augmented wave [21] method € chl is used for the treatment of the core electrons. The elecof Blo tron exchange and correlation are treated within the generalized gradient approximation (GGA) using the PerdeweBurkeeErnzerhof [22] functionals. The Ga 3d and In 4d [23] electrons are treated as valence electrons. The O 2s2 and 2p4 electrons are considered as valence electrons. A plane-wave cutoff energy of 400 eV is used throughout for the plane-wave set. The energy differs by less than 0.2% with increasing cutoff energy. The forces on each ion are relaxed to less than 0.02 eV/Å. If we decrease the atomic forces to 0.002 eV/Å, the energy differs by less than 0.01%. The Brillouin-zone integrations are performed using a Gaussian smearing [24] of sigma ¼ 0.2 eV. This would accelerate the convergence. Within the computational scheme presented above, the optimized lattice constant of b-LGO used for constructing surface configurations is a ¼ 5.383 Å, b ¼ 6.363 Å, and c ¼ 5.052 Å, which is very close to experimental value [13] as well as other theoretical results (a ¼ 5.361 Å, b ¼ 6.255 Å, and c ¼ 5.361 Å) [25]. The InN lattice constants of the optimized wurtzite structure (a ¼ 3.6137 Å, c ¼ 5.8582 Å) [26] within GGA are in good agreement with the theoretical lattice parameters (a ¼ 3.5848 Å, c ¼ 5.8002 Å) [27] and the experimental values (a ¼ 3.535 Å, c ¼ 5.699 Å) [17]. The lattice mismatch for InN is estimated to be 5.50% in the a-direction and 3.91% in the b-direction in theory, if we assume the in-plane alignment of InN [0001]kLGO [100] and InN [1-100]kLGO [010]. For the theoretical modeling, the LGO (001) surface is modeled using a three-dimensional periodic surface slab model. Both LiGaterminated and O2-terminated surfaces are considered. In both cases, the unit cell includes an atomic slab with ten atomic LGO
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(001)-(2 2) layers (see Fig. 1(a) and seven point five cb-LGO lattice constants width (about 38 Å) of vacuum regions in thickness. We perform some tests with a bigger atomic slab which includes fourteen atomic LiGaO2 (001)-(2 2) layers and six point five cb-LGO lattice constants width of vacuum regions in thickness. The adsorption energy differs by less than 0.12 eV. The z-axis is taken as normal to the surface. For the adsorption studies presented here, only those high symmetry adsorption sites are examined (see Fig. 1(b) and (c)). The calculations are done with a (4 4 1) MonkhorstePack [28] mesh. Increasing the mesh to (6 6 1), the energy differs by less than 0.01%. The coverage is defined as the number of the adatom divided by the number of surface atoms. The adsorption energy per adsorption atom is defined by the equation [29,30] of Eads ¼ (Etotal Eref nEX)/n, where Etotal and Eref are the total energy of the adsorbed model and the clean surface model, n and EX are respectively the number and the energy of the adsorption atom X.
3. Results and discussion According to the results of the calculations, the second layer oxygen atom relaxes outward for LiGa-terminated LGO (001) surface and the upper layer oxygen atom relaxes inward (i.e. towards the bulk) for O2-terminated LGO (001) surface. For LiGa-terminated LGO (001) surface the upper layer metal Li atom relaxes inwards by 2.45% of bulk lattice constant bb-LGO, but the upper layer metal Ga atom relaxes outwards by 3.44% of bb-LGO. Outward relaxation of the second layer oxygen atom is 1.28% and 1.86% of bb-LGO. The upper layer oxygen atom for O2-terminated LGO (001) surface relaxes inwards by 1.95% and 3.33% of bb-LGO, while the second layer Li (Ga) atom relaxes outwards by 2.83% (0.82%) of bb-LGO. To understand the initial growth and adsorption behavior of InN on LGO substrate, we firstly focus on the structures of In or N adsorbate on LGO (001). The adsorption energies for In or N adatom on the LiGa-terminated and the O2-terminated LGO (001) surfaces are calculated for different adsorption sites, and the results are summarized in Table 1. For the LiGa-terminated LGO (001) surface, the adsorption energies of 1/16 monolayer (ML) In adsorption atoms at the H sites are shown to be lowered by 0.18, 0.07 and 0.17 eV/atom than the structures at the To, TGa and TLi sites. The
Fig. 1. (a) Three dimensional views of the LiGa-terminated and the O2-terminated LGO (001) surfaces; the top view of the two surface layer of the LiGa-terminated (b) and (c) the O2-terminated LGO (001) surface. H is the hollow site, top is the site directly above the surface/sublayer (second layer) atoms (abbreviate as TLi, TO and TGa).
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distances of the In adatom at the H site relative to the substrate Ga atoms are found to be 2.91 Å for the LiGa-terminated surface. When further layers of In are added, all the In atoms will move to the H sites with 1/4 or 1/2 ML In adsorption. The adsorption energies decrease with decreasing In coverage, 1/16 ML In adsorption at the H arrangement is the most stable. As for N adsorption on the LiGaterminated LGO (001) surface, the N atoms at the TGa and TLi sites will move to the H sites with 1/16 ML N adsorption and all the N atoms will move to the H sites with 1/4 ML N adsorption. All the surface structures induced by 1/16 or 1/4 ML nitrogen adsorption are unstable. However, the adsorption energies of 1/2 ML N adsorption atoms at the H or To sites get lower than that of 1/16 or 1/4 ML N adsorption. The reason is that the former of the NeN dimer. The charge density in Fig. 2 shows the chemical bonding between the N atoms is bonded better. Thus the LiGa-terminated LGO (001) surface is not suitable candidate for the growth of nonpolar wurtzite InN planes. We also investigate the stability of In and N adsorption on the O2-terminated LGO (001) surface. The adsorption energies of 1/ 16 ML In adsorption atoms that occupy the H adsorption sites are lower by 2.36, 4.16 and 2.45 eV/atom than that of In adsorption atoms at To, TGa and TLi sites. The distances between In and the underlying nearest O atoms are found to be 2.00 Å for the O2terminated surface. For a higher coverage, the adsorption energies of 1/4 ML In adsorption atoms that occupy the H sites are lower by 0.02 eV/atom than that at TGa (TLi) sites. Meanwhile, 1/4 ML In atoms will shift from the TO site to the H site and the total energy and the calculated relaxed atomic positions are the same. All the In atoms will move to the H sites with 1/2 ML In adsorption. The adsorption energies decrease with decreasing In coverage for In adsorption on the H sites of the O2-terminated LGO (001) surface. The adsorption energies of 1/16 ML N atoms are 2.70 eV/atom on the O2-terminated LGO (001) surface, which is obvious higher than that for In atoms. Thus the In adsorption atoms are more favorable than the N atoms. The reason could be coulomb attraction between In and underlying O in the top layer and repulsion between N and underlying oxygen. The adsorbing 1/4 ML N at TLi sites shows the lowest energy and thus is favorable. As the adsorption structure of 1/4 ML N atoms at TGa sites is unstable, the grown InN crystal may present nitrogen vacancy near the Ga atoms of the O2-terminated LGO (001) surface. Moreover, only the 1/2 ML N adsorption at the H sites presents the stable structure because the former of the NeN dimer on the O2-terminated LGO (001) surface (see Fig. 2(c)). The In or N adsorption atoms are more favorable O2-termination surface than the LiGa-termination surface for the LGO (001) surface among the structures examined. We have made no effort to introduce the temperature and pressure dependence here, so the results must be
used with caution when addressing questions of the relative stability of surfaces under specific external conditions, i.e., the growth temperature, atmospheric pressure, etc. Unlike the above atomically perfect surface structures, real LGO (001) surfaces are defective with missing atoms (vacancies) and/or substitutional defects. These point defects are thought to have an effect on the electrical and chemical behavior of the junction. It is important to study the role of impurities in junction formation for the development and manufacturing of optoelectronic devices. In the following, we shall compare structures with surface point defects. These defects include (a) O vacancy, VO, where one O atom is removed from O2-termination surface every 2 2 cell, (b) In substitutes O, InO, (c) N substitutes O, NO. We examined the structures using (2 2) surface unit cell. These models are simple for the real surfaces, where the defects probably are not in periodic arrays. Meanwhile, the stability of different surface configurations is believed to be very sensitive to chemical environment. Therefore, the effect of environment should be carefully considered to obtain the well-controlled LGO surfaces. We compute the surface grand potential of LGO surface (Ref. [31] and references therein) to estimate the relative stability of different terminations. Since, only the energy of the stoichiometric unit is defined in a compound material such as LGO, one must use the thermodynamic considerations to estimate the surface grand potential. We introduce the chemical potentials mLi, mGa, and mO of the Li, Ga, and O atomic species, respectively. One of the standard control parameters in actual experiments is the oxygen partial pressure in the vacuum chamber. The chemical potential of oxygen atoms is deviated from the energy of an oxygen atom in a free isolated O2 molecular and describes the abundance of oxygen in the environment. The oxygen chemical potential has to be smaller than that of an oxygen atom of O2 molecular. Otherwise an oxygen condensate will form at the surface. The chemical potential of In (N) is calculated from the total energy of thermal equilibrium bulk In (N2). The surface grand potential per unit area U can be given as:
U¼
1 ðEtot NLi mLi NGa mGa NO mO NX EX Þ S
(1)
with NLi, NGa, NO, and NX are the number of Li, Ga, O, and X (In or N) atoms in the slab, respectively. Etot is the total energy of the surface under consideration. S represents the surface area. The chemical potential mLGO of a condensed and stoichiometric phase of LGO is written as a sum of three terms representing the chemical potential of each species within the crystal:
mLGO ¼ mLi þ mGa þ 2mO :
(2)
Table 1 The adsorption energies per adsorption atom for In or N adatoms on the LiGa-terminated and the O2-terminated LGO (001) surfaces in eV.
LiGa-terminated surface In
N
O2-terminated surface In
N
H
To
TGa
TLi
1/16 1/4 1/2 1/16 1/4 1/2
3.16 2.44 2.71 0.42 1.34 0.10
2.98 e e 0.61 e 0.10
3.09 e e e e 3.33
2.99 e e e e 3.80
1/16 1/4 1/2 1/16 1/4 1/2
9.72 6.39 5.04 2.70 0.01 0.15
7.36 e e 1.11 0.39 0.06
5.56 6.37 e 1.57 0.44 1.47
7.27 6.37 e 2.49 0.11 1.91
J. Wang et al. / Vacuum 119 (2015) 106e111
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Fig. 2. Isosurfaces of the charge densities for 1/2 ML N adsorption at (a) H and (b) To sites at LiGa-terminated LGO (001) surface and (c) H sites at O2-terminated LGO (001) surface.
Since the surface is in equilibrium with the bulk LGO, we have
mLGO ¼ Ebulk (the bulk energy per formula unit in the orthorhombic structure). If we replace Eq. (2) with Eq. (1), we can eliminate the mGa and mLGO variables in the surface grand potential and obtain
1 U ¼ ½Etot NGa Ebulk NX EX mO ðNO 2NGa Þ mLi ðNLi NGa Þ S (3) Relying upon Eq. (3), one can deduce the range of the accessible values of U if the minimum and maximum values of the O and Li chemical potentials are known [32,33]. If we introduce the variation in the chemical potentials with respect to those computed for bulk the reference phases (DmO ¼ mO Emol O2 =2 and DmLi ¼ mLi ELi , respectively) in Eq. (3), we obtain
1 U ¼ f ½DmO ðNO 2NGa Þ þ DmLi ðNLi NGa Þ S with
" # Emol 1 O2 bulk ðNO 2NGa ÞELi ðNLi NGa Þ : f¼ Etot NGa Ebulk NX EX S 2 (4) f expresses the stability of the surface with respect to bulk LGO, molecular oxygen, and metallic Li, while (NO-2NGa) represents the excess (if positive) or the deficiency (if negative) in the number of O atoms of the terminations. Simultaneously, we can also eliminate the mLi(mO) and mLGO variables in the surface grand potential. If we introduce the variation of the chemical potentials with respect to those computed for the reference phases (DmO and DmGa ¼ mGa Ebulk Ga , or DmLi and DmGa, respectively), we obtain the surface grand potential:
1 U ¼ f ½DmO ðNO 2NLi Þ þ DmGa ðNGa NLi Þ S With
f¼
" # Emol 1 O Etot NLi Ebulk NX EX 2 ðNO 2NLi ÞEbulk ðN N Þ; Ga Li Ga S 2 (5)
and
U¼ h
1 N N DmLi NLi O þ DmGa NGa O S 2 2
With
h¼
1 N NO NO bulk Etot O Ebulk NX EX Ebulk E : N N Ga Li Ga Li S 2 2 2 (6)
f and h express the stability of the surface with respect to bulk LGO, molecular oxygen (metallic Li) and metallic Ga. The total energy calculations for an O2 molecule in gas phase were carried out using a cubic cell of 10 Å, which was found to be large enough to avoid interaction with other molecules in the neighboring cells. The relative values of the energies of an O atom in the O2 molecule in the gas phase, of the Li atom in the bulk cubic structure, and of the Ga atom in the pure a-Ga metal (a face-centered orthorhombic lattice with eight atoms per primitive cell) [34] are 4.92 eV, 1.90 eV, and 3.03 eV, respectively. Our computed value of the surface grand potential of LGO with respect to the Ga and Li atoms in their bulk phases, and the O atom in the gas phase is 8.08 eV. The thermodynamic stability of the (001) LGO surfaces may be estimated individually as functions of the chemical potentials by the Eqs. (4)e(6). The first-principle results for f,f, and h are listed in Table 2. The derivation of the upper and lower bounds of DmO, DmLi, and DmGa is detailed in Refs. [32,33]. The defective O2termination is unstable in the stability diagram of the O2-terminated LGO (001) surface (not shown) within the allowed region for the chemical potential of oxygen, lithium, and gallium, the perfect (001) LGO surface would predominate over the cleavage or growth of the LGO crystal along the [001] orientation. Note the pressure dependence [31] of oxygen is not considered here. No complete information regarding which termination is stable under specific conditions, so our atomistic simulation techniques offer a source of knowledge of surfaces in advance of experiment. The understanding and control of the interfaces are critical for the integration and fabrication of reliable heterojunctions. A fundamental issue is the difference between the surface lattice symmetry and the bulk. The geometry of the stable heterojunctions is schematically shown in Fig. 3 for the interfaces in the present study. The InN/LGO heterojunctions are modeled by a slab thickness of ten atomic layers (ALs) of LGO and six ALs of InN. Adsorption was allowed on only one side of the exposed surfaces, with the dipole moment corrected accordingly in the z direction. We also examine InN/LGO heterojunctions with nitrogen vacancies; since the In adsorption atoms are more favorable than the N atoms and
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Table 2 The surface energy f,f, and h, as defined in Eqs. (4)e(6), are given for the O2terminated surface under consideration in J/m2.
f
f h
Clean
VO
InO
NO
0.09 0.09 0.09
0.07 0.07 0.66
1.08 1.08 1.67
0.58 0.58 1.18
the adsorption structure of 1/4 ML N atoms at TGa sites are unstable. The total energy of the perfect InN/LGO heterojunction is higher (1.10 eV) than the sum of the total energy of the nitrogen atom and the InN/LGO heterojunction with nitrogen vacancies. Thus nitrogen vacancies may present in the InN/LGO heterointerface. Moreover, the total energy of the InN/LGO heterojunction with nitrogen vacancies is lower (6.00 eV) than that of the InN/LGO heterojunction with ON (the first InN surface oxygen atoms occupy the nitrogen vacancies), which indicates that the oxygen atom will not diffuse from the O2-terminated LGO (001) to nitrogen vacancy site in the InN/LGO heterointerface. The total energy of the InN/LGO heterojunction with the antisite defects InN (In atom substitutes N atom) is lower (1.65 eV) than the sum of the total energy of the In atom and the InN/LGO heterojunction with nitrogen vacancies. So the antisite defects InN are stable in the InN/LGO heterointerface. For the relaxed perfect InN/LGO interface, the interplanar spacing normal to the surface (0.958 Å) is less than 1.851 Å (aInN/ 4þ3cLGO/16). After nitrogen vacancies introduced in the InN surface,
there is much larger relaxation in comparison to that for the perfect InN/LGO heterointerface. The interplanar distance is found to be 0.857 Å for the relaxed InN/LGO interface with nitrogen vacancies. The change of the interplanar distance is unobvious for In adsorption on the nitrogen vacancies in the InN/LGO interface. We analyze the electronic properties of the perfect and defective InN/ LGO interface. Qualitative insights into the electronic properties can be obtained by contrasting closely related electronic structures. As the partial density of states (PDOS) provide a general identification of the nature of the orbital, the effects of the interface atoms on the electronic properties of the interface can be obtained by contrasting the PDOS. Fig. 4 (a) displays the total density of states (TDOS) for the perfect InN/LGO heterojunction. The upper valence band ranges are mainly composed of In 4d5p, N 2p, Ga 4s4p and O 2p states, and the inegap states are mainly derived from the In 5s5p, N 2p, and O 2p states. After nitrogen vacancies introduced at the InN/LGO interface, some N 2p inegap states disappear in comparison with TDOS for the perfect InN/LGO heterojunction (Fig. 4(b)), which might impact on the interfacial electrically conducting. Meanwhile, the valence and conduction bands are shifted up in energy. The substitution of In for N in the InN/LGO interface reduces the inegap states in comparison with TDOS of the perfect Ge/SrO heterojunction (Fig. 4(c)); the TDOS change unobviously after the substitution of In for N defects at the InN/LGO interface in comparison with that of the InN/LGO heterojunction with N vacancies. 4. Summary In summary, the structural and electronic properties of InN on the LGO (001) substrate were studied using the DFT. The upper layer oxygen atom for O2-terminated LGO (001) surface relaxes inwards by 1.95% and 3.33% of bb-LGO, while the second layer Li (Ga) atom relaxes outwards by 2.83% (0.82%) of bb-LGO. The In or N adsorption atoms are more favorable O2-termination surface than the LiGa-termination surface for the LGO (001) surface among the structures examined. The In adsorption atoms are more favorable than the N atoms for both the LiGa-terminated and the O2-terminated LGO (001) surface. The NeN dimer will be formed with increasing N coverage on LGO (001) surface. The surface grand potentials of the perfect and defective (001) LGO as functions of the chemical potentials indicate that the perfect O2-terminations are stable. The interplanar spacing normal to the surface is 0.958 Å for the relaxed perfect InN/LGO interface. The effects of the intrinsic
Fig. 3. Schematic of the InN/O2-terminated LGO interfaces showing the composition of each layer. For the reduced InN/LGO interface, the nitrogen vacancies are simulated by removing the nitrogen atom at the top of Ga atom from the first layer of the InN surface.
Fig. 4. The TDOS of the valence and conduction band states of the perfect and defective InN/LGO heterojunctions. The vertical dotted line gives the location of Fermi level.
J. Wang et al. / Vacuum 119 (2015) 106e111
point defects of InN on the electronic properties are investigated by comparing the DOS of the various interface configurations on the O2-terminated LGO (001) substrate. The antisite defects InN and nitrogen vacancies at the InN/LGO interface reduce the occupied states at the Fermi level in comparison with TDOS for the perfect InN/LGO heterojunction. Acknowledgment This work has been supported by NNSFC (11347016) and the Fundamental Research Funds for the Central Universities under Grant no. 2014QNA65. We are grateful to the High Performance Computing Center of China University of Mining and Technology for the award of CPU hours to accomplish this work. References [1] Bhuiyan AG, Hashimoto A, Yamamoto A. J Appl Phys 2003;94:2779. [2] Butcher KSA, Tansley TL. Superlattices Microstruct 2005;38:1. [3] Walukiewicz W, Ager III JW, Yu KM, Liliental-Weber Z, Wu J, Li SX, et al. J Phys D 2006;39:R83. [4] Kumar M, Roul B, Shetty A, Rajpalke MK, Bhat TN, Kalghatgi AT, et al. Appl Phys Lett 2011;99:153114. [5] Ive T, Brandt O, Ramsteiner M, Giehler M, Kostial H, Ploog KH. Appl Phys Lett 2004;84:1671. [6] Jasinski J, Liliental-Weber Z, Lu H, Schaff WJ. Appl Phys Lett 2004;85:233. €m S, Hultman L, Janze n E. [7] Kakanakova-Georgieva A, Gueorguiev GK, Stafstro Chem Phys Lett 2006;431:346. [8] Waltereit P, Brandt O, Trampert A, Grahn HT, Menniger J, Ramsteiner M, et al. Nature 2000;406:865.
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