Photoemission characterization of interface dipoles and electronic defect states for gate dielectrics

In the development of Metal-Insulator-Semiconductor (MIS) devices, gate dielectric technology is of great importance and includes major scientific and technological issues to be solved for required device performance and reliability [1]. In particular, characterization of electronic defects in dielectrics and at their interfaces with semiconductor substrates as well as energy band profiles has been imperative to gain a better understanding of physics of dielectric/semiconductor heterostructures [2, 3]. In this work, our recent achievements on the characterization of gate dielectrics and their stacked interfaces by means of photoemission techniques have been reviewed. First, we have shown how valuable the cut-off energy of photoelectrons is to measure directly the potential change due to electrical dipoles at the interface in stacked dielectrics [4]. And then, we have demonstrated how powerful total photoelectron electron yield spectroscopy (PYS) [5, 6] is to quantify the energy distribution of electronic defect states in gate dielectrics and at dielectric/semiconductor interfaces. The inner potential changes in dielectric stacks reflect in changes in the cut-off energy of secondary photoelectrons (SEs) measured in high-resolution x-ray photoelectron spectroscopy (XPS) [4]. After calibration of the kinetic energy of core-line signals from the underlying layer, an abrupt potential change due to electrical dipoles at the interface between dielectrics, resultant abrupt potential change can be measured as a change in the cut-off energy of SEs. The observation of cut-off energy provides us an advantage in simple and precise evaluation of the potential change due to electrical dipoles as compared to a discussion on dipole formation based on the energy shift of core-line signals which reflects not only the potential change due to dipoles but also the chemical shift, that is, change in the chemical bonding features. From SE spectra near the lowest limit in kinetic energy for the samples before and after the formation of various high-k dielectrics on thermally-grown thick SiO2, we found that, with the formation of either ultrathin Al2O3 or HfO2 or TiO2 on SiO2, the cut-off energy of SEs was shifted toward the higher kinetic energy side. On the other hand, in the cases of Y- and Sr-oxides with silicate formation at the interfaces, a slight opposite energy shift was detected. The analyses of the core line signals confirm that there is a linear correlation between the observed potential changes and the ratios in the oxygen anion density at the interfaces between SiO2 and high-k dialectics as suggested in Ref. [8]. In the photoelectron yield measurements, the total number of photoelectrons emitting from the sample is counted considering the incident photon flux, the yield spectrum is related to an integral over the occupied density of states existing near the sample surface [5, 6]. From photoelectron yield spectra of 2nm-thick SiO2 formed 500ºC by remote plasma enhanced CVD on n-type GaN(0001) before and after N2 anneal at 800ºC, we found that, with the N2 anneal at 800ºC, the yield from the defects was reduced markedly. The 1st derivative of the measured yield spectrum with respect to incident photon energy leads us to the energy distribution of occupied defect state densities in consideration of density of states of the GaN valence band, measured photoelectron yield from the GaN VB and photoelectron escape depth. As a result, occupied states are reduced down to ~1x1011cm-2eV-1 at the energy corresponding to the midgap of GaN near the SiO2/GaN interface with the N2 anneal at 800ºC. The defect state density near the conduction band edge, which was crudely estimated in consideration of electron occupation probability based on the Fermi-Dirac distribution, is in good agreements with the result obtained from the capacitance-voltage (C-V) analysis using the Terman method [7] Acknowledgements The authors wish to thank Assoc. Prof. K. Makihara and Dr. M. Ikeda for their supports about the experiments and Drs. N. Fujimura and N. X. Truyen for their contributions to sample fabrication and characterizations. References [1] For example, ECS Trans. 80(1) (2017). [2] Z. Yatabe et al., J. Phys. D: Appl. Phys. 49 (2016) 393001. [3] J. Robertson and R. M. Wallace, Materials Science and Engineering: R: Reports, 88 (2014) 1. [4] N. Fujimura et al., Jpn. J. Appl. Phys., 57 (2018) 04FB07. [5] A.Ohta et al., Microelectro. Eng., 178 (2017) 85. [6] A.Ohta et al., Jpn. J. Appl. Phys., 57 (2018) 06KA08. [7] N. X. Truhen, Jpn. J. Appl. Phys, 57 (2018) 01AD02. [8] K. Kita and A. Toriumi, Appl. Phys. Lett. 94 (2009) 132902

Photoemission study of gate dielectrics on gallium nitride

In the development of gallium nitride (GaN) power devices, gate dielectric technology on GaN is of great importance and includes major scientific and technological issues to be solved for required device performance and reliability[1]. In particular, characterization of electronic defects in dielectrics and at their interfaces with GaN as well as energy band profiles has been imperative to gain a better understanding of physics of dielectrics/GaN heterostructures. In this work, we have demonstrated how useful the analysis of energy loss signals of photoelectrons [2] is to characterize dielectric functions of dielectrics on GaN and how powerful photoelectron electron yield spectroscopy [3] is to quantify the energy distribution of electronic defect states in dielectric/GaN heterostructures. After wet-chemical cleaning of a GaN epitaxial layer with an Si concentration of 1.5x1016cm-3 on sapphire (0001) and subsequent dipping in a 4.5% HF solution, as a representative dielectric, SiO2 layers in the thickness range of 5-50nm were deposited at 500 ºC by remote plasma CVD, in which the decomposition of SiH4 was induced by remote plasma of Ar/O2 gas mixture to suppress ion dimages. AFM images confirm uniform depositon of SiO2 on GaN. In XPS measurements using monochromitzed AlKa radiation, Ga3d and N1s spectra from ~5nm-thick SiO2/GaN show no detectable interfacial oxidation and no change in stoikiometry near the SiO2/GaN interface. Also, in Si2p, O1s and valence band spectra from deposited SiO2, no difference from thermally-grown SiO2 was detectable. From the analysis of valence band spectrum taken for the ~5nm-thick SiO2/GaN, the valence band offset was determined to be 2.2 eV, from which the conduction band offsetwas derived to be 3.35eV in consideration of energy bandgap values of GaN and SiO2. The energy bandgap of deposited SiO2 was directly measured to be 8.95eV within an accracy of 50meV, being identical to the value measured for thermally-grown SiO2, from the onset energy of energy loss signals of O1s photoelectrons. For further information about electronic structures, taking into acount the relationship among energy loss spectrum, dielectric function and optical constants (n and k values) and using Kramers-Kronig relations for the real and imaginary parts of complex functions such as dielectric function and refractive index, elaborately measured energy loss signals of O1s and Si2p3/2 photoeelectrons were convered into the dielectric finction below ~30eV, in which the contribution of surface plasmon into the measured energy loss signals was first eliminated by difference signals between the cases taken at photoelectron take-off angles of 15 and 30º. As a result, peaks in k values corresponding to characteristic transitions including excitonic transition reported in SiO2 glass were discerned. For quantification of electronic defects in SiO2/GaN heterostructures without additional process steps, photoelectron yield spectra in the incident photon energy region of 3 -10eV, in which Xe arc and high brightness D2 lamps were used as light sources, were measured at each step of SiO2 thinning in a dilute HF solution. Occupied states located in energy region deeper than the conduction band edge of GaN were clearly detected although photoemissions from the GaN valence band became significant in the photon energy region over ~7eV with progressive SiO2 thinning. When measured photoelectron yield spectra were normalized with the yield from mainly from the GaN valence band for photons over ~9eV, almost no change in the yield due to defects was detectable until the SiO2 thickness was reduced down to ~1.5nm. With further SiO2 thinning, a marked decrease in the yield for defects was observed. The results indicates occupied defect states are located within ~1.5nm from the SiO2/GaN interface. Since the energy derivatives of the measured yield spectra lead us to energy distribution of occupied defect state density in consideration of density of states of the GaN valence band, measured photoelectron yield from the GaN valence band and photoelectron escape depth. As a result, occupied states as many as ~3x1011 cm-2eV-1 were detected even at the energy corresponding to the midgap of GaN near the SiO2/GaN interface. Acknowledgements This work was supported in part by NEDO through a co-operative research with AIST. Authors wish to thank Assoc. Prof. K. Makihara and Dr. M. Ikeda for their supports about sample characterization and Prof. Amano’s Laboratory, Nagoya Univ. for the preparation of epitaxial GaN on sapphire substrate. References [1] Z. Yatabe et al., J. Phys. D: Appl. Phys. 49 (2016) 393001. [2] T. Yamamoto et al., ECS Trans. vol.75, No.8 (2016) 777. [3] S. Miyazaki et al., Microelectro. Eng., 48 (1999) 63

Structural Dynamics of Retroviral Genome and the Packaging

Retroviruses can cause diseases such as AIDS, leukemia, and tumors, but are also used as vectors for human gene therapy. All retroviruses, except foamy viruses, package two copies of unspliced genomic RNA into their progeny viruses. Understanding the molecular mechanisms of retroviral genome packaging will aid the design of new anti-retroviral drugs targeting the packaging process and improve the efficacy of retroviral vectors. Retroviral genomes have to be specifically recognized by the cognate nucleocapsid domain of the Gag polyprotein from among an excess of cellular and spliced viral mRNA. Extensive virological and structural studies have revealed how retroviral genomic RNA is selectively packaged into the viral particles. The genomic area responsible for the packaging is generally located in the 5′ untranslated region (5′ UTR), and contains dimerization site(s). Recent studies have shown that retroviral genome packaging is modulated by structural changes of RNA at the 5′ UTR accompanied by the dimerization. In this review, we focus on three representative retroviruses, Moloney murine leukemia virus, human immunodeficiency virus type 1 and 2, and describe the molecular mechanism of retroviral genome packaging

Phylogenetic Insights into RT and Vpx/Vpr

The efficiency of reverse transcription to synthesize viral DNA in infected cells greatly influences replication kinetics of retroviruses. However, viral replication in non-dividing cells such as resting T cells and terminally differentiated macrophages is potently and kinetically restricted by a host antiviral factor designated SAMHD1 (sterile alpha motif and HD-domain containing protein 1). SAMHD1 reduces cellular deoxynucleoside triphosphate (dNTP) pools and affects viral reverse transcription step. Human immunodeficiency virus type 2 (HIV-2) and some simian immunodeficiency viruses (SIVs) have Vpx or Vpr to efficiently degrade SAMHD1. Interestingly, the reverse transcriptase (RT) derived from HIV-1 that encodes no anti-SAMHD1 proteins has been previously demonstrated to uniquely exhibit a high enzymatic activity. It is thus not irrational to assume that some viruses may have acquired or lost the specific RT property to better adapt themselves to the low dNTP environments confronted in non-dividing cells. This adaptation process may probably be correlated with the SAMHD1-antagonizing ability by viruses. In this report, we asked whether such adaptive events can be inferable from Vpx/Vpr and RT phylogenetic trees overlaid with SAMHD1-degrading capacity of Vpx/Vpr and with kinetic characteristics of RT. Resultant two trees showed substantially similar clustering patterns, and therefore suggested that the properties of RT and Vpx/Vpr can be linked. In other words, HIV/SIVs may possess their own RT proteins to adequately react to various dNTP circumstances in target cells

Search for Molecules against HIV-CA

Varieties of in vitro systems have been used to study biochemical properties of human immunodeficiency virus Gag-capsid protein (HIV Gag-CA). Recently, we have comparatively characterized HIV-1 and HIV-2 Gag-CA proteins using such technology, and have demonstrated that the NaCl-initiated CA-polymerization in vitro and the stability of CA N-terminal domain as judged by differential scanning fluorimetry (DSF) are inversely correlated. In this study, we found that ZnCl2 works as a competent initiator of the in vitro HIV-1 CA-polymerization at much lower concentrations than those of NaCl frequently used for the polymerization initiation. We also showed by DSF assays that ZnCl2 highly destabilize HIV-1 CA. Furthermore, PF74, a well-known inducer of premature HIV-1 uncoating in infected cells, was demonstrated to unusually promote the HIV-1 CA-disassembly in the presence of ZnCl2 as revealed by DSF assays. Taken together, we conclude that the DSF method may be useful as an efficient monitoring system to screen anti-HIV-1 CA molecules