Grown on InP by Atomic Layer Deposition - American Chemical Society

Feb 22, 2015 - Institute of Physics and Applied Physics, Yonsei University, 134 Shinchon-dong, Seoul 120-749, Korea. ‡. Department of Physics, Yonse...
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Defect States below the Conduction Band Edge of HfO2 Grown on InP by Atomic Layer Deposition Yu-Seon Kang,† Dae-Kyoung Kim,† Hang-Kyu Kang,† Sangwan Cho,‡ Sungho Choi,§ Hyoungsub Kim,§ Jung-Hye Seo,∥ Jouhahn Lee,∥ and Mann-Ho Cho*,† †

Institute of Physics and Applied Physics, Yonsei University, 134 Shinchon-dong, Seoul 120-749, Korea Department of Physics, Yonsei University, Wonju 220-710, Korea § School Department of Material Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea ∥ Division of Materials Science, Korea Basic Science Institute, Daejeon 305-806, Korea ‡

ABSTRACT: The electronic structure and nature of the defect states below the conduction band edge of an HfO2 gate dielectric grown on InP substrate prepared by atomic layer deposition was examined using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT). When the HfO2 dielectric was deposited on an InP substrate with an abrupt interface, the resulting HfO2 develops a tetragonal (t) structure, which minimizes the interfacial lattice mismatch. The O K-edge absorption features and DFT calculations indicated that additional structural distortion occurred by a q = −2 charged O vacancy (VO−2) in the t-HfO2. The electronic structure and the charge-transition levels of t-HfO2 with VO−2 were assigned based on a second derivative analysis of O K-edge features; 12 distinct pre-edge defect states below the Hf 5d conduction band edge were evident when the degeneracies resulting from Jahn−Teller splitting and crystal field splitting were removed. No changes in the electronic structure near valence band edge by VO−2 were observed in XPS valence spectra. Moreover, O vacancies in t-HfO2 lead to substantial midgap states caused by interstitial elemental In or P in the t-HfO2 due to the enhanced out-diffusion of elemental In or P through O vacancies. We conclude that both the defect states near the CBE and the midgap states could be controlled by the incorporation of nitrogen into the HfO2 using a thermal NH3 treatment.

I. INTRODUCTION The demands of SiO2/Si-based complementary metal oxide semiconductor (MOS) devices for faster and smaller devices have led to the employment of III−V channel materials and downscaling SiO2 gate dielectrics, respectively. Among the III− V compound semiconductors, GaAs, InGaAs, and InP have been actively studied due to their high electron mobility compared with Si.1−6 The downscaling of the SiO2 has now reached its physical limits due to problems associated with poor reliability, high-frequency dispersion, and high leakage currents. HfO2 with a high dielectric constant (κ) is considered to be one of the most promising candidates for replacing SiO2, and mass production has already been implemented.7 Recent research on HfO2 gate oxide materials has focused on dielectric properties, thermal stability, and suitable band offsets with III−V channel materials.8−16 The dielectric properties of HfO2 for MOS devices are strongly affected by their crystallinity, uniformity, and the extent of defect formation in the gate oxide. The crystallinity of a HfO2 gate oxide can have an influence on capacitance accumulation; at the same gate oxide thickness, tetragonal (t) HfO2 has a higher accumulation capacitance than monoclinic (m) HfO2 due to the higher permittivity of t-HfO2 than that of m-HfO2.17 In the case of HfO2, the most stable © 2015 American Chemical Society

crystal structure is a monoclinic phase (heat of formation energy of m-HfO2 is −1239.3 kJ/mol).18 However, our previous studies indicated the HfO2 has a tetragonal structure when it is deposited on an InP substrate with an abrupt interface to minimize interfacial lattice mismatch.5 The defect states of high dielectric oxides grown on III−V compound semiconductors can have several main origins. The first one is that the SiO2/Si interface has low gap states due to the nature of tetrahedral covalent bonding of Si and SiO2, while high dielectric has ionic bonding without a fixed coordination number, resulting in poor interface bonding quality because of the intrinsic complexity of HfO2/InP.19 There are only Si−O bonding dominating at the SiO2/Si interface, while HfO2/InP have In−O, P−O, Hf−In, and Hf−P bonds. The condition of the initial InP substrate plays an important role in interfacial quality. Cabrera et al. carried out first-principle calculations of InP surfaces.20 They showed that an annealing temperature of ∼300 °C during the ALD process leads to the formation of InPderived surface hydrophosphate species, namely, PO and P− Received: November 21, 2014 Revised: February 20, 2015 Published: February 22, 2015 6001

DOI: 10.1021/jp511666m J. Phys. Chem. C 2015, 119, 6001−6008

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The Journal of Physical Chemistry C

the ALD chamber. We performed 26, 77, and 141 cycles of ALD to deposit the HfO2 film, ultimately achieving film thicknesses of ∼2, ∼6, and ∼11 nm, respectively. The HfO2 film thickness was confirmed by high-resolution transmission electron microscopy (HRTEM). After the ALD process, films were annealed at 600 °C by a rapid thermal process (RTP) for 1 min in an environment of N2 (PDA) or NH3 (PDN). The local structure and defect states below the CBE were examined by O K-edge XAS at the 8A1 beamline at the Pohang Light Source (PLS). XAS spectra were collected using the total electron yield (TEY) mode by measuring sample drain current (Is). XAS spectra were normalized using mesh grid current (Io) from a reference Au-coated mesh in the incident photon beam. The slit widths were set to give a energy resolution of 190 meV for the O K-edge. The energy scale of the XAS spectra was calibrated using the first- and second-order diffraction Ti L3,2 edge absorption feature of rutile TiO2. The equivalent d2 model was used to examine the second derivative spectra of O Kedge.7,15 Chemical reactions of HfO2 films grown on InP were examined by high-resolution X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, (Kratos)) using a monochromatic Al Kα X-ray source (hν = 1486.7 eV) with a pass energy of 20 eV. The binding energy of the measured core-level spectra of C 1s and P 2p was calibrated by core-level spectra using the C 1s spectrum (surface carbon, 284.9 eV for the C−H bond). Fitting curves were determined by Gaussian and Lorentzian distributions, in which the Gaussian distribution ratio was higher than 80%. In addition, the intensity ratio of the doublet in the P 2p core-level caused by spin−orbit splitting was determined by the transition probability during photoionization. To investigate the elemental distribution throughout the HfO2 gate dielectric, we obtained time-of-flight secondary ion mass spectroscopy (ToFSIMS, ION-TOF TOF.SIMS-5) data. For the depth profiles of Hf, N, and In elements, O2+ sputtering and Bi+ analysis guns were used. The density of the interfacial defect states (Dit) was determined by measuring parallel conductance (Gp/ω)max (measured by means of an Agilent E4980A LCR meter and B1500A semiconductor device analyzer) using a MOS capacitor with a sputter-deposited TiN top electrode with an area of 6.4 × 10−5 cm2 area and thickness of 600 nm, fabricated by a lift-off technique. The energy levels of the defect state were determined from frequency measurements by Shockley− Read−Hall statistics.16 To measure the energy level of the defect states in the range from 0.25 to 1.13 eV from the VB edge of InP, we obtained capacitance (Cm) and conductance (Gm) data from electrical measurements were using S-doped ntype and Zn-doped p-type substrates under the following measurement conditions: frequency between 316 Hz and 1 MHz and a temperature of 25 or 200 °C. The Dit is proportional to parallel conductance (Gp) and Gp/ω were calculated from Cm and Gm measurements as follows

OH states. However, when the InP surface is treated with HF/ (NH4)2S, the resulting surface has a higher film density and contains lower levels of hydrophosphate species. The other origin is that the gap states result from the oxygen vacancies, oxygen interstitials, III−V dangling bonds, and III−V dimer pairs.19 These defect states in HfO2 reduce a band gap; Lucovsky et al. reported that negatively charged O-atom vacancies in nanocrystalline transition-metal (TM) oxides induce defect states near the conduction band edge (CBE) of t-HfO2, thus resulting in a decrease in band gap.9 In addition, our previous study using an HfO2/InP system showed that the formation of O-atom vacancies in HfO2 accelerates both the out-diffusion of InP substrate elements and an increase in midgap defect density on the order of ∼1013 cm−2.5 These defect states in an HfO2 gate dielectric cause electron or hole trapping and Fermi level pinning problems.19,21 Thus, it is important to understand systematically the origin of the defect states on dielectric properties of HfO2 films on III−V materials and to develop various defect control technique for further development because these gate dielectric properties directly affect device properties. The focus of this study was on the structural properties and CBE electronic structure of an HfO2 gate dielectric grown on an InP substrate. The effects of oxygen vacancies in HfO2 on band alignment and relevant electrical properties were studied in detail using X-ray absorption spectroscopy (XAS) and various physical measurements. Among the analytical methods used to investigate the electronic and structural properties of molecules, XAS is one of the most powerful measurement tools, which enable the density of states (DOS) of the partially occupied or unoccupied electronic states to be determined. In addition, it is very sensitive to the local bonding environment, such as the number of valence electrons, their spin configuration, the symmetry, and coordination number of the unit cell of TM compounds.22 XAS data for the HfO2 gate dielectric grown on InP indicated four distinct orbital states in the first two peaks near the CBE by crystal field (C-F) splitting and Jahn−Teller (J−T) distortions, corresponding to a tetragonal structure with a tetrahedral symmetry. In addition, our data indicate that O-atom vacancies in t-HfO2 induce (i) unoccupied defect states near the CBE of HfO2 and (ii) occupied midgap states within the HfO2 band gap resulting from the out-diffusion of In or P atoms. Finally, the findings also confirm that both the defect states near CBE and the midgap states could be controlled by chemical reaction using NH3 treatment. After thermal nitridation, N fills up the oxygen vacancy sites. The two electrons trapped at the oxygen vacancy are transferred to an N 2p orbital at the top of the valence band (VB) due to the fact that its electronegativity values of 3.0 are higher than that of Hf (1.3). Finally, the oxygen-vacancy-related gap states disappear.

Gp

II. EXPERIMENTAL SECTION Three different thick HfO2 films (approximately 2, 6, and 11 nm) were deposited on an InP (001) substrate using atomic layer deposition (ALD) process with tetrakis-ethyl-methylamide hafnium Hf[N(CH3)(C2H5)]4 (TEMAHf) as the precursor and H2O vapor as the oxygen source. Before deposition of the HfO2 films, the InP substrate was cleaned for 5 min in a dilute solution (∼1%) of buffed oxide etchant (BOE, NH4F/HF 6:1). The substrate was then rinsed in deionized H2O and dried by blowing N2 over the substrate. The chemically etched substrate was immediately transferred to

ω

=

ωCox 2Gm [Gm 2 + ω 2(Cox − Cm)2 ]

The angular velocity ω (= 2πf) can be obtained because the frequency (f) is measured from 316 Hz to 1 MHz. Cox is the capacitance of the oxide in the accumulation region. In addition, density functional theory (DFT) calculations were employed to understand the origin of the defect states using supercell models. Local structure and formation energy were calculated using VASP code with the exchange correlation function of the generalized gradient approximation (GGA) 6002

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The second-derivative data shown in Figure 1a reveal that the first two peaks are further split into four peaks at 533.3, 534.3, 538.1, and 540.6 eV. In addition, the valley between eg and t2g became sharper with increasing film thickness. In general, the partial DOS of a metal d orbital is greatly affected by the local C-F by the surrounding oxygen orbital.22 Therefore, the O Kedge features indicate that the HfO2 film has a tetragonal structure, resulting from the C-F splitting and J−T d-state degeneracy removal; that is, the energy splitting of the Hf d orbital are due to the changes of local symmetry as a result of a tetragonal structure. Similarly, the t2g, which is split to two peaks, not three peaks, is due to the degeneration of dyz and dzx in the tetragonal structure of HfO2. This result is entirely consistent with our previous reports that the HfO2 has a tetragonal structure when the HfO2 deposited on an InP substrate with an abrupt interface to minimize interfacial lattice mismatch. Details of the mechanisms for this have already been discussed based on X-ray diffraction (XRD) and transmission electron microscopy (TEM) data.5 The pre-edge states were detected as indicated with a red arrow. These can result from (i) band overlapping of P 3sp or In 5sp states hybridized O p orbital by In2O3, In(PO3)3, and InPO4 or (ii) oxygen-atom vacancy states in HfO2 dielectric. To discuss the case of (i), we prepared thermally annealed InP substrate using a RTP, after which XPS and XAS measurements were carried out. The XPS data show multiple oxidation states (In2O3 at 444.7 eV, In(PO3)3 at 445.4 eV, and InPO4 at 445.7 eV) are generated during annealing process, as shown in the inset of Figure 1b. From O K-edge features of this sample, two main absorption peaks at 536.2 and 541.0 eV that resulted from P 3sp and In 5sp states were obviously observed. Therefore, the pre-edge states of the HfO2/InP were not affected by In- or P-related oxidation states, as indicated by a red arrow in Figure 1b. Concerning the case of (ii), Lucovsky et al. reported that negatively charged O-atom vacancies in nanocrystalline HfO2 induce defect states below the Hf 5d CBE of t-HfO2, resulting in a decrease in band gap.9 They interpreted the defect features using two-electron multiplet theory. We performed a secondderivative analysis of the O K-edge spectra based on the equivalent d2 model to confirm the pre-edge defect states in HfO2. Figure 2 shows (a) the second derivative spectra in pre-edge regime at a X-ray energy less than ∼532 eV and (b) the schematic energy band diagram for 11 nm thick film of t-HfO2.

PBESol. Geometry optimization for the unit cell of the P42/ nmc HfO2 structure was performed. The conventional cell for HfO2 was calculated using 7 × 7 × 7 k-points. To minimize interactions between charged defects, we used 3 3 2 (HfO2) supercells for the defect calculations. Gamma k-points for geometry optimization and 3 × 3 × 3 k-points were used to calculate the energy state and DOS. All calculations were carried out using a plane-wave cutoff energy of 500 eV.

III. RESULTS AND DISCUSSION Figure 1a presents the O K-edge XAS spectra of a HfO2 film grown on an InP substrate and its second derivative spectra as a

Figure 1. XAS O K-edge spectra of HfO2/InP in the upper figure (a) as a function of film thickness and (b) 11 nm thick HfO2. Second derivative O K-edge spectra are in the bottom of panel a. O K-edge spectra of annealed InP are in the bottom of panel b. The inset of panel b shows the In 3d XPS core-level spectra for annealed InP.

function of film thickness. The O K-edge spectrum is directly related to O p-projected unoccupied DOS by dipole selection rules. The O K-edge feature of the HfO2 film grown on InP includes the first two distinct peaks related to the hybridized orbitals of the O 2p + Hf 5d states, which are split into eg (having 2-fold degeneracy related to dx2−dy2 and dz2) and t2g (having 3-fold degeneracy related to dxy, dyz, and dzx) orbitals in tetrahedral symmetry.14 The broad peaks at higher energy are related to the hybridized orbitals of the O 2p + Hf 6sp states.

Figure 2. (a) Second-derivative O K-edge spectra of the 11 nm thick HfO2 in the pre-edge regime. (b) Schematic energy band diagram for t-HfO2 with O2− charged vacancy. 6003

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Figure 3. Atomic structures of t-HfO2 with (a) neutral O vacancy and (b) O2−-charged vacancy (red, oxygen; sky-blue, hafnium). (c) Formation energies versus Fermi energy for O vacancy in t-HfO2 with various charged states. Partial density of states for (d) intrinsic HfO2 and (e) HfO2 with O2− charged vacancy. (f) Energy band diagram of O vacancy defect states with various charged states for t-HfO2/InP. The filled circles and open circles correspond to occupied states and unoccupied states, respectively.

Figure 4. XAS O K-edge spectra of (a) 2, (b) 6, and (c) 11 nm thick HfO2 film on InP for as-grown and annealing environment of N2 (PDA) and NH3 (PDN).

All 12 peaks were detected and are composed of six terms: three singlets and three triplets represented by group theory and multiplet theory. (The energy ordering in increasing energy is 3F, 3P, 1G, and 1S.)9,23,24 This result is in good agreement with previous reports and can be explained by C-F splitting and J−T distortions.9 During film deposition or post-deposition annealing (PDA), the HfO2 film is crystallized and, in this state, may still contain a significant concentration of oxygen vacancies. The removal of a neutral O atoms from t-HfO2 results in two electrons being confined within the vacancy site bounded by Hf atoms. At that moment, t-HfO2 breaks the tetrahedral symmetry with a tetragonal extension in the zdirection and distortions in the x−y plane by J-T distortion to

minimize repulsions between the two Hf orbitals by Pauli exclusion. In this process, degenerated energy bands of 3F and 3 P triplets were separated and 12 different energy levels were evident by degeneracy removal by C-F splitting and J−T distortion, as shown in Figure 2a. The negative ion states are caused by an atomic properties, not ligand-field-defined properties.9 Figure 2b displays the oxygen vacancy induced by the absorption states within the band gap of the t-HfO2 (∼5.8 eV) with the three singlet negative ion states near the CBE. The molecular orbital structure and the charge-transition levels strongly support the formation of two negatively charged oxygen vacancies (VO−2) in t-HfO2. 6004

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The Journal of Physical Chemistry C We performed DFT calculations to understand pre-edge absorption features in more detail. Figure 3 shows the atomic structure of t-HfO2 containing (a) neutral oxygen vacancy and (b) VO−2 after geometry optimization. Structural distortion of t-HfO2 was obviously observed around the O-atom vacancy site when the t-HfO2 contains the VO−2 charged states. We confirmed that the formation of VO−2 in t-HfO2 was most likely over the Fermi energy because it has the lowest formation energy, as shown in Figure 3c. The noteworthy fact is that there were no gap states in the intrinsic t-HfO2, whereas many gap states were generated in t-HfO2 containing the VO−2 charged states, as shown in Figure 3e. These defect states were composed of the band of the O 2p states and Hf 5d states, which could be caused by local symmetry distortions. From the partial density of states (PDOS) results, a schematic band diagram for various charged states of O vacancy was obtained by scissoring correction, as shown in Figure 3f. The scissoring operation was performed using the experimentally obtained band parameters including band gap and band offsets obtained by REELS and XPS.5,25 The O vacancy in t-HfO2 results in various defect states within the band gap of HfO2. Specially, the unoccupied defect states at ∼0.3 eV below the conduction band (CB) minimum were generated in t-HfO2 containing VO−2 charged states. This result is completely consistent with previous O K-edge feature at the pre-edge regime, which showed that the O vacancy in t-HfO2 results in defect states at ∼0.4 ± 0.2 eV below the CBE. From DFT and XAS results, we concluded that (i) there are pre-existing O vacancies in tetragonal HfO2 grown on InP, which may be combined with two negatively charged states, and (ii) these oxygen vacancies, which cause structural distortions, generate pre-edge defect states below the Hf 5d eg CBE states. Figure 4 shows O K-edge XAS spectra of HfO2 before and after PDA as a function of film thickness. Significant changes in the O K-edge features were observed after thermal annealing at 600 °C; that is, the intensity ratio of the first two peaks was altered and the full width at half-maximum (fwhm) of the absorption peaks was increased. The changes in the absorption feature are caused by the formation of the multiple oxidation states (In2O3, In(PO3)3, and InPO4) during the annealing process. The absorption features of multiple oxidation states are shown at the bottom of Figure 4, which are related to excitation to the P 3sp or In 5sp states hybridized with the O 2p orbital. Moreover, broad absorption peaks after PDA in both the 6 and 11 nm thick HfO2 indicate that the oxidation states formed at the surface of the HfO2 film, considering the probing depth of the electron yield in HfO2, are ∼5 nm. This result is related to the out-diffusion of the elemental InP substrate. Our previous study and other researchers showed that the out-diffusion of elemental In or P occurred during the film deposition and postannealing process, and the concentration of the out-diffused element was considerably film-thickness-dependent.5 The changes in the absorption feature after PDA are in good agreement with result showing that the formation of multiple oxidation states in HfO2 film decreased with increasing film thickness. To understand the changes in absorption features caused by chemical reactions in HfO2/InP, we collected spectra for In 3d, as shown in Figure 5. Peak deconvolution of In 3d spectra was carried out using Gaussian−Lorentzian fitting within the detection limit of XPS after Shirley-type background subtraction. From the equilibrium phase diagram of an In−P−O ternary system, the broad peaks related to multiple oxidation

Figure 5. (a−c) XPS In 3d core-level spectra as a function of film thickness and annealing conditions. In the In 3d spectra, the InP, In2O3, In(PO3)3, and InPO4 bonding states correspond to binding energies of 444.4, 444.7, 445.4, and 445.7 eV, respectively.

states are likely composed of In2O3 at 444.7 eV (ΔG ≈ −198.6 kcal/mol), In(PO3)3 at 445.4 eV (ΔG ≈ −610 kcal/mol), and InPO4 at 445.7 eV (ΔG ≈ −287 kcal/mol).18 The XPS spectra after PDA show that multiple oxidation states were generated and this resulted from interfacial reactions between out-diffused InP substrate elements and interdiffused oxygen impurities by the following thermodynamic process.5,26 (i) Formation of In2O3 and In(PO3)3: 3InP + 6O2 → In2O3 + In(PO3)3 (ΔG ≈ −753.4 kcal/mol) (ii) Formation of InPO4 from In2O3 and In(PO3)3: 4In2O3 + 8P → 5InP + 3InPO4 (ΔG ≈ −158.6 kcal/mol) and 8In + 4In(PO3)3 → 3InP + 9 InPO4 (ΔG ≈ −158.6 kcal/mol) These data also show that the formation of oxidation states in HfO2 after PDA is gradually suppressed when the film thickness is increased. These results support the changes in absorption featured after PDA as a function of film thickness in Figure 4; that is, the absorption peak became broad as a result of the overlapping (P 3sp or In 5sp + O 2p) states with (Hf 5d + O 2p) states. The notable observation is the multiple oxidation states were controlled during the PDA in a NH3 ambient (PDN), as shown in Figure 5. From the above chemical equation, the In or P elements are needed for the oxidation process, and they take part in the chemical reaction by the outdiffusion of InP substrate elements. The multiple oxidation states were suppressed by controlling the out-diffusion of In or P elements through the incorporation of N in the HfO2 film. Figure 6a shows O K-edge XAS spectra of the 11 nm thick HfO2 before and after the annealing process. Changes in the intensity ratio of absorption peaks after PDA were observed, whereas no changes in the absorption peaks after PDN were observed. This is consistent with the XPS result, showing that interfacial reactions were suppressed in nitride HfO2/InP by controlling the out-diffusion of In and/or P elements. Moreover, the absorption peaks below the CBE were suppressed after PDN, as shown in Figure 6b. From the previous XAS and DFT, the pre-edge states in the as-deposited film were induced by VO−2 in HfO2, and these defect states are associated with Hf 5d states. The reduced band-edge defect states can be attributed to N incorporation into O-vacancy sites of the HfO2 during PDN. Nitrogen bonded to Hf leads to Hf 5d orbital features on different pairs of Hf atoms with O and N neighbors, eliminating coherent Hf 5d orbital features and 6005

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leading to decreased defect intensity and soft pre-edge absorption features.10 Nitrogen incorporation into an Ovacancy site of the HfO2 may have occurred in all regions of the film, as shown in the TOF-SIMS data of Figure 6c. Figure 6d shows the VB spectra of an 11 nm thick HfO2 film before and after PDN. In contrast with the change in the CB features, the VB features were barely changed with the PDN treatment. The VB electronic structure is composed of dominantly O 2p orbital, whereas the CB is the localized Hf 5d orbital. Therefore, the VB features are rather insensitive to local coordination. Under the detection limit in XPS, the valence spectra of as-grown HfO2 indicated that there are no changes in the VB defect states by VO−2 in t-HfO2. To investigate the effect of an O vacancy in HfO2 on electrical properties, we fabricated an MOS capacitor using TiN as a gate electrode. Figure 7a−c provides information on the multifrequency C−V characteristics of MOS capacitors comprising an 11 nm HfO2 film on InP for an as-grown and annealing environment of N2 (PDA) and NH3 (PDN). The data were obtained at room temperature with frequency ranging from 316 Hz to 1 MHz. The forming gas process was not carried out in the case of the defect analysis. Large frequency dispersions in accumulation and depletion were observed in all samples. This large frequency dispersion resulted from the interfacial defect states.27 The conductance method was used to evaluate the interface density of states (Dit) between the HfO2 film and the InP substrate. Figure 7d shows the density of the interfacial defect states (Dit) of HfO2/InP as a function of annealing conditions. In HfO2/InP after PDA, the Dit at ∼3 eV from the VB edge of the HfO2 increased

Figure 6. (a) XAS O K-edge spectra of 11 nm thick HfO2 film on InP as a function of annealing condition. (b) O K-edge spectra in pre-edge regime. (c) TOF-SIMS depth profiles of Hf, In, and N elements for 11 nm thick HfO2/InP. (d) XPS valence band spectra of 11 nm thick HfO2 film for as-grown and PDN annealing.

Figure 7. Multifrequency C−V characteristics of (a) as-grown HfO2, (b) HfO2 with PDA annealing, and (c) HfO2 with PDN annealing. (d) Density of interfacial defect states of 11 nm thick HfO2 measured by conductance as a function of annealing conditions. 6006

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The Journal of Physical Chemistry C substantially, from 1.0 × 1013 to 1.1 × 1014 eV−1 cm−2. From the DFT calculations, ∼3 eV defect states from the HfO2 VB edge are caused by the presence of interstitial In or P in t-HfO2. As previously discussed in the case of XAS and XPS, O vacancies in t-HfO2 lead to substantial midgap states caused by interstitial elemental In or P in the t-HfO2 due to the enhanced out-diffusion of In or P elements through O vacancy. However, Dit increased slightly from 1.0 × 1013 to 3.5 × 1013 after PDN, which indicates that the incorporation of N into HfO2 controls the out-diffusion of In and P. Figure 8 shows the molecular orbital diagram of HfO2 grown on InP substrate. The diagram can be completed including the

field splitting (L-FS) and Jahn−Teller splitting (J−TS). (ii) In addition, O vacancies in t-HfO2 induce the midgap states within the band gap of HfO2 by the out-diffusion of the elemental InP substrate. (iii) Both the defect states near CBE and the midgap states could be controlled by PDN. N bonded to O vacancy site in t-HfO2 effectively suppressed the interfacial reactions by controlling out-diffusion of In or P atoms from the substrate.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-2-2123-5610;. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by an Industry-Academy joint research program between Samsung Electronics-Yonsei University.

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REFERENCES

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Figure 8. Molecular orbital energy diagram above valence band of HfO2 grown on InP. The diagram evaluated by O K-edge XAS.

VO−2-induced defect states in t-HfO2, as evaluated by XAS and conductance measurements. There are two contributions to the molecular orbital feature of HfO2 grown on InP: (i) Lattice mismatch values between the HfO2 film and the InP substrate. This determines the main feature of an electronic structure based on local symmetry. As shown in Figure 8, the degenerated Hf 5d orbital distinctly splits four orbital states, resulting from tetrahedral symmetry as a result of C-F splitting and J−T distortion. (ii) Pre-existing O vacancies in HfO2 during the ALD process. This induced unoccupied or partially occupied pre-edge defect states below the Hf 5d CBE. In addition, O vacancies in t-HfO2 enhance the out-diffusion of elemental In or the P substrate, resulting in the formation of occupied midgap states within the band gap of HfO2. The proposed diagram indicates that O vacancies in t-HfO2 gate dielectric induce substantial defect states within the band gap of the HfO2, and these can cause charge trapping and Fermi level pinning problems. The combined results suggest that gate dielectric properties of HfO2 for use in MOS devices can be improved by incorporating nitrogen into the HfO2.

IV. CONCLUSIONS We investigated the molecular orbital structure and defect states below the CBE of HfO2 grown on InP by ALD. The results for the absorption intensity ratio and peak position in O K-edge features show that the molecular orbital structure of HfO2 exhibits a characteristic of 8-fold O coordination with a tetragonal structure. In addition, structural distortion of t-HfO2 was observed around the O-atom vacancy site when the t-HfO2 contains VO−2 charged states. (i) An O vacancy in t-HfO2 results in the pre-edge defect states below the CB by ligand6007

DOI: 10.1021/jp511666m J. Phys. Chem. C 2015, 119, 6001−6008

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DOI: 10.1021/jp511666m J. Phys. Chem. C 2015, 119, 6001−6008