Total Photoelectron Yield Spectroscopy Research Articles

Electron emission from H-terminated diamond enhanced by polypyrrole grafting

Electron emission plays an important role in diverse applications, from cold cathodes to chemical processes (solvated electrons, water purification), energy generation (thermionic or dye-sensitized solar cells), and even cancer treatment. Here we show that by surface treatment using electrochemically grown polypyrrole the secondary-electron emission and photoelectron emission from boron-doped diamond is enhanced even above the intensity of electron emission from the hydrogen-terminated surface with negative electron affinity. This enhancement is stable in air for at least one month and it persists also in vacuum after thermal annealing. Scanning electron microscopy, Kelvin probe force microscopy, total photoelectron yield spectroscopy as well as surface mapping by Auger and secondary ion mass spectroscopies are used to characterize and correlate the surface electronic and chemical properties. A model of the electron emission enhancement is provided.

(Invited) Photoemission-Based Characterization of Gate Dielectrics and Stack Interfaces

We have demonstrated quantitative evaluation of dielectric function and optical constants for gate dielectrics such as remote plasma-assisted chemical-vapor-deposition (CVD) SiO2 and plasma-assisted atomic-layer-deposition (ALD) alumina-silicate in the energy range of 5 ~ 30 eV from the combination of the analysis of energy loss signals (ELS) of core-line photoelectrons and the results of Kramers-Kronig transformation of measured ELS. We have also show quantification of the energy distributions of occupied gap states in gate dielectrics and dielectric stack interfaces such as CVD SiO2, ALD HfO2 and ALD hafnia-silicates on GaN by means of total photoelectron yield spectroscopy (PYS) in the photo-excitation energy range from 3 ~ 10eV.

Total photoelectron yield spectroscopy of energy distribution of electronic states density at GaN surface and SiO2/GaN interface

The energy distribution of the electronic state density of wet-cleaned epitaxial GaN surfaces and SiO2/GaN structures has been studied by total photoelectron yield spectroscopy (PYS). By X-ray photoelectron spectroscopy (XPS) analysis, the energy band diagram for a wet-cleaned epitaxial GaN surface such as the energy level of the valence band top and electron affinity has been determined to obtain a better understanding of the measured PYS signals. The electronic state density of GaN surface with different carrier concentrations in the energy region corresponding to the GaN bandgap has been evaluated. Also, the interface defect state density of SiO2/GaN structures was also estimated by not only PYS analysis but also capacitance–voltage (C–V) characteristics. We have demonstrated that PYS analysis enables the evaluation of defect state density filled with electrons at the SiO2/GaN interface in the energy region corresponding to the GaN midgap, which is difficult to estimate by C–V measurement of MOS capacitors.

Low-temperature formation of Ga-oxide/GaN interface with remote oxygen plasma and its interface properties

The Ga-oxide/GaN structures formed by remote oxygen plasma (ROP) exposure at various temperatures (Ts) and times have been systematically investigated. X-ray photoelectron spectroscopy clarified the formation of Ga2O3 layers with close-to-stoichiometric composition and a slight N incorporation of ∼6 at. %. Also, we found that a high Ts increases the intensity of a signal related to the N–O bond, which is located near the Ga-oxide/GaN interfaces. Total photoelectron yield spectroscopy (PYS) also revealed that the ROP exposure at Ts of 300 °C produces fewer filled defect states in the bandgap of GaN than at 500 °C. This difference in the filled defect states could be attributable to the amount of N–O bonds at the interface.

Characterization of remote O2-plasma-enhanced CVD SiO2/GaN(0001) structure using photoemission measurements

The control of chemical composition and bonding features at a SiO2/GaN interface is a key to realizing high-performance GaN power devices. In this study, an ∼5.2-nm-thick SiO2 film has been deposited on an epitaxial GaN(0001) surface by remote O2-plasma-enhanced chemical vapor deposition (O2-RPCVD) using SiH4 and Ar/O2 mixture gases at a substrate temperature of 500 °C. The depth profile of chemical structures and electronic defects of the O2-RPCVD SiO2/GaN structures has been evaluated from a combination of SiO2 thinning examined by X-ray photoelectron spectroscopy (XPS) and the total photoelectron yield spectroscopy (PYS) measurements. As a highlight, we found that O2-RPCVD is effective for fabricating an abrupt SiO2/GaN interface.

Evaluation of energy distribution of filled defects of Si oxide thin films from total photoelectron yield spectroscopy

We have demonstrated the how powerful total photoelectron yield spectroscopy (PYS) is to evaluate the energy distribution of filled defect states for dielectrics such as Si oxide prepared by thermal oxidation and electron beam evaporation. PYS system with wide measurement energy range from 3 to 10 eV have been developed by the combination of the two light sources with Xe-arc and high brightness D2 lamps in addition to the control of ambience in optical path. The filled electronic states density in Si oxide were converted from the measured PYS spectra in the consideration of the measured density of states of valence electron from in-situ XPS measurements.

Effects of remote hydrogen plasma on chemical bonding features and electronic states of 4H-SiC(0001) surface

We have demonstrated a novel dry cleaning process for the 4H-SiC surface using remote hydrogen plasma (H2-RP). The effects of H2-RP exposure on the chemical structures and electronic states of the wet-cleaned 4H-SiC surface have been evaluated by X-ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS). XPS shows that H2-RP exposure is effective in removing contaminants from the 4H-SiC surface. PYS indicates that the defect states at the surface are generated after the H2-RP exposure and such generated defect states are drastically decreased by annealing at 850 °C. The combination of H2-RP exposure with the subsequent annealing is found to be effective in 4H-SiC surface cleaning.

Photoemission Study on Chemical Bonding Features and Electronic Defect States of Thermally-Grown SiO2/4H-SiC Structure

Depth analysis of thermally grown SiO2/4H-SiC structure has been conducted systematically by high resolution x-ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS) to gain a better understanding of the energy band diagram and the energy distributions of the electronic defects. A SiO2 layer was grown on wet-chemically cleaned 4H-SiC surface by wet-oxidation at 1080 ºC. The energy band alignments have been evaluated by combination of the oxide bandgap values determined from energy loss spectra of O core level photoelectron with the valence band offsets obtained from valence band spectra. Depth profile of the defect states has been investigated from changes in the PYS intensity at each step of SiO2 thinning by dipping in a dilute HF solution. The photoelectron yield from filled defect states in the energy region corresponding to the 4H-SiC bandgap was significantly increased near the SiO2/4H-SiC interface.

Photoemission Study on Chemical Bonding Features and Electronic Defect States of Thermally-Grown SiO2/4H-SiC Structure

Silicon carbide (SiC) has received increasing attention in the field of high temperature and high power electronics. One of major concerns in the fabrication of SiC MOSFETs is a large density of electrically active defects in SiC and near the SiO2/SiC interface which deteriorate device performance and its reliability. So far, it has been reported that thermal oxidation of SiC causes residual C incorporation in SiO2 network near the SiO2/SiC interface as a result of incomplete out-diffusion of COx molecular units [1]. Such excess C including carbon clusters and Si/C dangling bonds in the region near the interface have often been discussed as a possible cause of the interface traps [2, 3]. In this work, depth analysis of thermally grown SiO2/4H-SiC structure has been conducted systematically by high resolution x-ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS) to gain a better understanding of the energy band diagram and the energy distributions of the electronic defects.After wet-chemical cleaning of n-type Si-face 4H-SiC(0001) surface, SiO2 layers were grown by wet oxidation at 1080 ºC. To investigate the energy band alignment, XPS measurements were performed utilizing monochromatized AlKα radiation. PYS analysis was also carried out in the photon energy range from 3.4 to 5.6 eV to evaluate the energy distribution of the filled defect states [4]. Figure 1 shows energy loss spectra of O1s photoelectrons taken for 3.4 and 21.8 nm-thick SiO2/4H-SiC. From the onset of the energy loss signals [5], the Eg value of SiO2 in the thickness range from 3.4 to 21.8 nm on 4H-SiC was determined to be 8.9 ± 0.1 eV, which is quite consistent with the Eg value of SiO2 on Si. On the other hands, O 1s energy loss signals for SiO2/4H-SiC show an increase in small loss signals at about 7 eV from the zero-loss energy position by x-ray irradiation over 8 hours (data not shown), which imply the defect generation. Valence band (VB) offset (ΔEV) between SiO2 and 4H-SiC was evaluated from VB spectra as shown in Fig. 2. Measured VB spectrum of 3.4 nm-thick SiO2/4H-SiC was deconvoluted into two components due to the SiO2 and 4H-SiC substrate by using the reference spectrum of 21.8 nm-thick SiO2/4H-SiC. ΔEV at SiO2/4H-SiC interface was evaluated to be ~2.75 eV. Based on the measured values of Eg of SiO2 and ΔEV, the energy band diagram of SiO2/4H-SiC is drawn as shown in Fig. 3 in consideration of the known values of Eg and electron affinity of 4H-SiC [5, 6]. Thus, the conduction band offset (ΔEC) between SiO2 and 4H-SiC was found to be 2.92 eV. In the PYS measurements, the photoelectron yields from the samples were measured as a function of photon energy in the range from 3.4 to 5.6 eV. In Fig. 4, the EC denotes the 4H-SiC conduction band bottom measured from the vacuum level. From the energy band diagram of SiO2/4H-SiC (Fig. 3), the valence electrons in SiO2 and 4H-SiC cannot be emitted out by the irradiation of photons in this energy region. Therefore, the observed yields from the samples were attributable to the emission from the filled defect states distributed in the SiO2 and at the SiO2/4H-SiC interface. Notice that the yield due to the defect states in the photon energy region around 4 eV from the vacuum level was increased with a decrease in the SiO2 thickness, which indicates the formation of the filled defects in the region near the SiO2/4H-SiC interface. To get a clear insight into the depth profiles of the defects, PYS measurements were conducted repeatedly at each step of SiO2 thinning by dipping in a dilute HF solution. PYS analysis of 21.8 nm-thick SiO2/4H-SiC structure also shows that filled defects located in the energy region near the conduction band bottom of 4H-SiC becomes its maximum as the remained SiO2 thickness is around 2-3 nm. Acknowledgements: This work was supported in part by Grant-in Aids for Young Scientists (A) No. 15H05520 from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by JSPS Core-to-Core Program of International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration. References 1) V.V. Afanas’ev, et. al, Appl. Phys. Lett., 68 (1996) 2141. 2) J. L. Cantin, et. al., Phys. Rev. Lett., 92 (2004) 015502. 3) S. Wang, et. al., Phys. Rev. Lett., 98 (2007) 026101. 4) S. Miyazaki, et. al., Microelec. Eng., 48 (1999) 63. 5) L. Patrick, et. al., Phys. Rev., 137 (1965) A1515. 6) J. Robertson, J. Vac. Sci. Technol. A, 31 (2013) 050821. Figure 1

Electronic defect states in thermally-grown SiO2/4H-SiC structure measured by total photoelectron yield spectroscopy

Total photoelectron yield spectroscopy has been applied to evaluate the energy distribution of electronic defect states for thermally grown SiO2/4H-SiC structure. A SiO2 layer with the thickness of 3.4nm, 7.6nm, and 21.6nm was grown on wet-chemically cleaned 4H-SiC surface by wet-oxidation at 1080°C. X-ray photoelectron spectroscopy analysis shows no significant structural inhomogenities in the SiO2 layer in the thickness range over ∼2nm. Depth profile of the defect states has been investigated from PYS measurements in combination with SiO2 thinning by dipping in a dilute HF solution. The photoelectron yield from filled defect states in the energy region corresponding to the 4H-SiC bandgap is significantly increased near the SiO2/4H-SiC interface.

Electron emission suppression from hydrogen-terminated n-type diamond

By total photoelectron yield spectroscopy (TPYS), we have found a thermal instability of a negative electron affinity in hydrogen-terminated n-type diamond films, which has never been observed for intrinsic and p-type diamond films. Experimentally, we have succeeded in detecting surface defect states and surface valence band extended states on the same n-type sample after soft annealing from 100 to 300 °C, which allowed us to evaluate the surface Fermi level and surface band bending using phosphorus doping parameters. Our results show that “quasi-positive” electron affinity prevents electron emission from an n-type diamond film even though its surface has a negative electron affinity.

界面エレクトロニクス 金属／高誘電率絶縁膜ゲートスタックの光電子分光分析

The method to determine the energy band alignments of high-k dielectrics to Si(100) is described, in which the energy band gap values and the valence band lineups are measured from the analyses of energy loss signals of core line and valence band spectra measured by X-ray photoelectron spectroscopy (XPS), respectively. Also, it is demonstrated how useful the measurement of the cut-of energy for photoemission is to evaluate the effective work function in metal/high-k dielectric. Total photoelectron yield spectroscopy (PYS) is highlighted as a powerful way to quantify energy distribution of electronic defect states in a dielectric or at the dielectric/Si interface in the energy region corresponding to the Si band gap.

Characterization of chemical bonding features and defect state density in HfSiO x N y/SiO 2 gate stack

We have studied depth profiling of the chemical composition, bonding features and defect states in 2 nm-thick HfSiO xN y (Hf/(Hf+Si) = ∼ 43%) films with N content of ∼13 at.% before and after post-deposition annealing (PDA) at 1050 °C in O 2 and N 2 ambience by using high-resolution x-ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS) in combination with oxide thinning by a dilute HF solution. NO x and Hf-N x ( x = 2, 3) bonding units existing in the sample before PDA were effectively eliminated or reduced by PDA and Si-N and Hf-N bonds were the main N bonding features after PDA. We found the formation of a region with a low Si content and a residue of Hf-N 2 bonding units in the HfSiO xN y film by O 2-PDA in comparison to N 2-PDA. The PYS measurements revealed a high density of filled defect states in such a Si-deficient region with a large number of Hf-N 2 bonds.

Depth Profiling of Chemical and Electronic Structures and Defects of Ultrathin HfSiON on Si(100)

We present in-depth profiling of chemical bonding features and defect state density in ultrathin HfSiOxNy (Hf/(Hf+Si)=~43%) films with average nitrogen contents up to ~18at.% by using x- ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS) in combination oxide thinning in a dilute HF solution. It is found that the oxygen content becomes its minimum around ~1.2nm from the surface while the nitrogen content becomes its maximum within ~1.5nm from the surface. The result suggests that the surface re-oxidation is promoted coincidentally with the diffusion of N atoms generated by thermal decomposition of the Hf-Nx units during the N2-annealing. The depth profile of the defect states, which was derived from the change in the yield, shows that the defect state density becomes its maximum in the near-surface region where oxygen deficiency becomes significant. It is likely that the imbalance in chemical coordination between anions and cations is responsible for the defect generation.

Field emission process of O-terminated heavily P-doped homoepitaxial diamond

Field emission characteristics of heavily phosphorus (P)-doped homoepitaxial diamond is measured. Temperature dependent resistivity measurements indicate that hopping dominant transport in heavily P-doped homoepitaxial diamond. Field emission current versus anode voltage characteristics divide into three distinct regions. The effective electron affinity of O-terminated diamond surface is calculated to be 1.5 eV from F–N plot. This value is in reasonable agreement with data evaluated from total photo-electron yield spectroscopy measurements. Using these data, we introduce field emission process of O-terminated heavily P-doped diamond: i) emission of electrons from the conduction band minimum, ii) depletion of electrons in conduction band and iii) field emission from the phosphorus doping level.

Photoemission Study of Ultrathin GeO2/Ge Heterostructures Formed by UV-O3 Oxidation

The chemical bonding features and electronic states of ultrathin Ge oxide layers prepared on Ge(100) by UV-O3 oxidation at room temperature or thermal oxidation at 550°C and 600°C were characterized by x-ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS), in comparison with those of ultrathin SiO2/Si(100) cases. We have found that the oxidation of Ge(100) by UV-O3 at room temperature proceeds in a layer by layer manner as well as the oxidation of Si(100) and that UV-O3 oxidation rates of Ge(100) and Si(100) are almost the same at room temperature in the oxide thickness region below 1nm. From the analysis of the onset of energy loss spectra of O1s and Ge2p3/2 photoelectrons, the energy bandgap of the GeO2 films was measured to 5.70±0.05 eV in the thickness range from 0.9∼1.9 nm, and the valence band offset at the interfaces between GeO2 and Ge(100) was determined to be 4.00±0.05 eV. It is also confirmed from PYS measurements that ultrathin GeO2/Ge(100) prepared by UV-O3 oxidation contains defect states being about one order of magnitude larger than ultrathin SiO2/Si(100) systems. [DOI: 10.1380/ejssnt.2006.174]

Impact of Rapid Thermal O2 Anneal on Dielectric Stack Structures of Hafnium Aluminate and Silicon Dioxide Formed on Si(100)

3–7-nm-thick HfAlOx (Hf/Hf+Al=29%) films were deposited on thermally grown ∼0.9-nm-thick SiO2/Si(100) by atomic layer chemical vapor deposition (ALCVD) at 350°C. After postdeposition anneal (PDA) in 0.2% O2 ambience at 1050°C for 1 s, we evaluated the compositional profile of HfAlOx/SiO2 dielectric stack structures by X-ray photoelectron spectroscopy (XPS) in combination with oxide thinning in a dilute HF solution and observed Si diffusion and incorporation into the HfAlOx network by a few at.% through the pregrown SiO2 layer. The energy band alignment in the dielectric stack structure on Si(100) after the PDA was determined by the energy band gap of the ALCVD HfAlOx films and the valence band offset at the interfaces between HfAlOx and Si(100), which were measured by analyzing the energy loss spectra of O1s photoelectrons and the valence band spectra of the dielectric stack structures, respectively. Also, by the total photoelectron yield spectroscopy (PYS) of the dielectric stack structures, filled defect states as many as 1–3×1017 cm-3 eV-1 were estimated in the HfAlOx layer in the energy region of 0.4 to 0.8 eV from the energy position corresponding to the Si valence band top.

Band diagrams of intrinsic and p-type diamond with hydrogenated surfaces

We investigated the correlation between surface conductivity and surface band profiles of hydrogenated single crystal diamond (type IIa and IIb) by a combination of total photoelectron yield spectroscopy and I–V measurements. Electron and exciton contributions in the photoyield spectra are clearly separated and their response to thermal annealing and subsequent air exposure of the samples is found to be strictly correlated with the occurrence of surface conductivity. From this correlation we prove a long-ranging upward band bending in samples that show surface conductivity irrespective of bulk doping. This upward band bending drops to zero or even turns to downward bending when samples are annealed to temperaturesabove 200 °C in vacuum and loose their surface conductivity. Both changes are fully reversible by air exposure. The results are analyzed in terms of the two currently discussed models for the surface-near hole layer that gives rise to the p-type surface conductivity, namely the electrochemical surface transfer model and the hydrogen induced sub-surface acceptor model. Whereas the former offers a straightforward interpretation, the latter requires a number of conditions to be met in order to be consistent with our results. One is that donor-like surface defects such as dangling bonds with densities exceeding the typical carrier sheet density of the conductive layer have to be assumed. Furthermore, a critical upper limit exists for the product of the carrier sheet density and the extension of the sub-surface acceptor layer into the bulk. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure and energy band offsets for ultrathin silicon nitride on Si(1 0 0)

For the interfaces of CVD Si 3N 4 on Si(1 0 0) and directly-nitrided Si(1 0 0), chemical bonding features, energy band offsets and defect state density distributions have been studied using high-resolution X-ray photoelectron spectroscopy and total photoelectron yield spectroscopy. At nitride–Si(1 0 0) interfaces, SiN bonding states in which each Si atom is bonded with one or three N atoms are formed predominantly, being presumably related to the structural strain induced by SiN bonding at the interface. For nearly stoichiometric SiN x ( x≥1.3) in the thickness range of 1.0–17 nm which was prepared by 750 °C CVD or 700 °C direct-nitridation, the energy band gap was determined to be 5.4±0.1 eV from the energy loss spectra of N 1s photoelectrons. By analyzing the valence band spectra of thin SiN x /Si(1 0 0) heterostructures, the valence band offset between such SiN x and Si(1 0 0) was obtained to be 1.9±0.1 eV. For the direct-nitridation of Si(1 0 0) at 600 or 700 °C, an interface state density as low as ∼10 10 eV −1 cm −2 near Si midgap was confirmed by total photoelectron yield measurements.

Characterization of high-k gate dielectric/silicon interfaces

We have shown that, for thermally evaporated Ta 2O 5 or ZrO 2 thin films on Si(1 0 0), O 2 annealing at 300–500 °C causes the formation of an interfacial silicon oxide layer as thin as 1–2 nm which can be interpreted in terms of their high permeability to oxygen. And we have demonstrated how useful the energy loss spectra of photoexcited electrons from core levels such as O 1s are to measure the energy bandgaps of very thin insulators. With the combination of measured bandgaps and valence band lineups determined for X-ray photoelectron spectroscopy valence band spectra, we have determined the energy band alignments of Ta 2O 5 and ZrO 2 with Si(1 0 0) before and after the O 2 annealing at 500 °C. In addition, we have demonstrated that total photoelectron yield spectroscopy provides us direct information to quantify the energy distributions of both the defect states in the high-k dielectrics and the dielectric/Si(1 0 0) interface states over nearly entire Si bandgap.