`
`
`
`J J ’ r
`
`Japanese Journal of Applied Physics (ISSN 002l-4922)
`
`Published monthly with the cooperation of the Japan Society of Applied Physics and the
`. - F
`Physical Society of Japan.
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`
`
`PUBLICATION BOARD (April l989-March 1990)
`Chairman
`Hiroshi TAKUMA
`VIN chairman
`Takuo SUGANO
`Edlrom-m-t‘luel
`Kunio TADA, Tadao SHIMIZU
`Treasurers
`Seishi KIKUTA, Kiyoshi KAWAMURA
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`Supervisors
`Secretaries
`Publit‘alian
`”'“’"'""
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`Ryuzo Ants, Yoshiyuki TAKEISIII
`Tatsuya KIMURA. Hiroyoshi SUEMATSU
`Minoru TANAKA, Masatoshi 0N0
`
`EDITORIAL BOARD (April 1989—erch 1990)
`
`Editors-in-Chief Kunio TADA (The University of Tokyo. Department of Electronic Engineering)
`Tadao SHIMIZU (The University of Tokyo. Department of Physics)
`
`Editors Yoshiaki AKIMOTO (National Research Laboratory of Metrology, Osaka Measurement System Center)
`Kenji GAMO (Osaka University, Faculty of Engineering Science, Department ot Electrical Engineering)
`Hidcki HASEGAWA (Hokkaido University. Department of Electrical Engineering)
`Takasu HASiIIMOTO (Tokyo Institute of Technology. Department of Applied Physics)
`lchiro HATI'A (Nagoya University, Department of Applied Physics)
`Altio HIRAKI (Osaka University. Faculty of Engineering, Department of Electrical Engineering)
`Yoshiji HORIKOSHI (NTT Basic Research Laboratorim)
`Noboru ICHINOSE (Wascda University. Department of Materials Science & Engineering)
`Takashi ITO (Fujitsu Laboratories Ltd.)
`Yukio KANEDA (Nagoya University, Department of Applied Physics)
`Akita KAWAZU (The University of Tokyo. Department of Applied Physics)
`Ken KIKUCHI (National Laboratory for High Energy Physics)
`Hideomi KOINUMA (Tokyo Institute of Technology. Research Laboratory of Engineering Materials)
`Makoto KONAGAI (Tokyo Institute of Technology. Department of Electrical & Electronic Engineering)
`Shin KOSAKA (EIeetrotechnical Laboratory)
`Katsuhiko KURUMADA (NT'I' Opto-clcctronies Laboratories)
`Hajime MAEDA (Tokyo Engineering University, Department of Electronics)
`Shintaro MIYAZAWA (NTT LSI Laboratories)
`Taneo NISHINO (Kobe University. Department of Electrical Engineering)
`Osamu NITI'ONO (Tokyo Institute of Technology. Department of Metallurgical Engineering)
`Masatada OGASAWARA (Keio University. Department of Instrumentation Engineering)
`Keiichi OGAWA (National Research Institute for Metals. Tsukuba Laboratory)
`Tomoya OGAWA (Gakushuin University. Department of Physics)
`Yoichi OKABE (The University of Tokyo. Department of Electronic Engineering)
`Koji OKANO (l'he University of Tokyo, Department of Applied Physics)
`Yuichi OKUDA (Tokyo Institute of Technology. Depanmenl of Applied Physics)
`Chuhci OSHIMA (Waseda University. Department of Applied Physics)
`Tadaslti SAITOII (Tokyo University of Agriculture & Technology. School of Electronic & Information Engineering)
`Hiroyuki SASABE (The Institute of Physical and Chemical Research)
`Yusaburo SEGAWA (The Institute of Physiwl and Chemical Research)
`Yasuhiko SYONO (Toholtu University. Institute for Material Research)
`Hideo SUGAI (Nagoya University. Department of Electrical Engineering)
`Ryo SUZUKI (Hitachi Ltd., Central Research Laboratory)
`Mikio TAKANO (Kyoto University. Institute for Chemical Research)
`Humihiko TAKEI (The University of Tokyo. the Institute for Solid State Physics)
`Hiroshi TOYAMA (The University of Tokyo. Department of Physics)
`Kiyoji UhHAItA (Keio University, Department of Physics)
`Tokuo WAKIYAMA (Tohoku University. Department of Electronic Engineering)
`Yasuhiro YAMAMOTO (Hosei University. Research Center of Ion Beam Technology)
`Tsutomu YAMASHITA (Nagaoka University of Technology. Department of Electronics)
`Takafumi YAO (Electrotechnical Laboratory)
`Toyohiko YATAGAI (The University of Tsukuba. Institute of Applied Physics)
`Sadafumi YOSI-IIDA (EIcctroteehnical Laboratory)
`Tonao YUASA (NEC Corporation. Opto-Eleetronics Research Laboratories)
`Yasuhito ZOH'I‘A (Toshiba Corporation, Research 8: Development Center)
`
`Overseas Editors
`
`Leo ESAKI (IBM T. J. Watson Research Center)
`Hideya GAMO (University of California Irvine. Department of Electrical Engineering)
`Nobuzo TERAO (Université Catholique de Louvain. Dept. dcs Sciences des Malériaux)
`
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`Published by
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`Publication Oflice, Japanese Journal of Applied Physics
`Daini-Toyokaiji Building, 24-8, Shinbashi 4—chome. Minato-ku Tokyo 105. Japan
`
`Copyright 1‘ 1990 by the Publication Board, Japanese Journal of Applied Physics. All rights reserved.
`This publication is partially supported by a Grant-in-Aid for Publication of Scientific Research Result from the Ministry of
`Education, Science and Culture.
`
`
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`Printed in Japan by Komiyama Priniting Co.. Ltd.
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`JAPANESE JOURNAL OF APPLIED PHYSlCS
`VOL 29. No. 2, FEBRUARY. I990. pp. 229-235
`
`Amorphous Silicon Thin-Film Transistors with SiOxNy/SiNx
`Gate Insulators
`
`Kouichi HtRANAKA and Tadahisa YAMAGUCHI
`
`Fujitsu Laboratories Ltd. J‘O—J Morfnosato-Wakamiya. Arsagt' 243—01
`(Received October 3, 1989; accepted for publication December to. 1989)
`
`A double layer of plasma chemical vapor deposition SiOKN, and SiN. was applied to the gate insulator of an amor-
`phous silicon (a-Si} thin—film transistor [TFT]. When a thin Sil’tl,t layer is inserted between the a-Si of an a-Si TFT and the
`Si0,N_, gate insulator, the density of trapped charges is found to decrease less than for a gate insulator with only SiN“
`when the thickness ofSiN, is decreased. No deterioration was observed in the switching characteristics of an avSi TFT
`with a SiO‘Nyi SiNJ| gate insulator. compared with a—Si TFTs with SiN, gate insulators commonly used in liquid crystal
`displays. The density of trapped charges is related to the threshold voltage shift.
`KEYWORDS:
`a‘Si. thin-film transistor, gate insulator, nitride. oxinirride. threshold voltage, subthreshold slope
`
`Introduction
`
`§1.
`Amorphous silicon thin-film transistors (a-Si TFTs)
`have been developed as switching elements for high-
`quaiity flat-panel displays?” as drive circuits for image
`sen50rs,‘-" and as basic logic elements.” In these
`devices,
`light exposure or gate bias stress produces a
`decrease in the on conductance and a shift of the
`threshold voltage.” Using an ambipolar TFT, Powell
`reported
`that
`there
`are
`two
`distinct
`instability
`mechanisms: 3. slow increase in the density of metastable
`fast states in the a-Si at low fields, and charge trapping in
`the SiN, at higher fields.” We reported the effect of SiNx
`gate insulators on threshold voltage and field-effect
`mobility in a-Si TFTs using different SiN,r composi-
`tions.'" In our previous paper, we found that the threshold
`voltage shift is reduced at the SiN, composition of x=l
`because the electron trapping in the SiN, increases when
`x< l , and the electron trapping in the deep states near the
`a—SifSiN, interface increases when x) 1. We also found
`that
`the field-effect mobility decreases as x increases
`because of the increase in the number of deep states in
`the a-Si near the a-Si/ SiN, interfacem Characterization
`of the near interface of the a-Si/Sileaycred structure by
`photoluminescence measurements indicates that
`these
`deep states are the results of lattice strain and/or
`macroscopic strain. ' " To improve performance and relia—
`bility, it is important to decrease the density of charge
`trapping states in the gate insulator and deep states in the
`a-Si near the gate insulator/a-Si interface.
`Some attempts to decrease the density of deep states at
`the gate
`insulator
`and a-Si
`interface have been
`teported.'3’-* Kim et at. reported that the field-effect mobil—
`ity, subthreshold slope, and stability of an a-Si TFT are
`enhanced by inserting a thin silicon-rich nitride layer be-
`tween the a-Si and the gate insulator.”’ In our recent
`paper, we investigated the effect of the Si0,,Ny gate in-
`sulator on the threshold voltage for a-Si TFTs, because
`SiOKNy films have a lower mechanical stress and a higher
`
`‘K. Hiranaka and T. Yamaguchi: to be published in J. Appl. Phys.
`(I990).
`
`band-gap energy than SiNI and are more waterproof
`than SiO: films. Though the density of trapped charges
`related to the threshold-voltage shift in a—Si TFTs with
`SiOXN, gate insulators can be smaller than that with the
`commonly used SiN, gate insulators,
`the switching
`characteristics of a-Si TFTs with SiOxN, degrade with
`increased x]y due to an increase in deep states. We used
`photoluminescence measurements to show that
`these
`deep states are related to oxygen.
`This paper reports a-Si TFTs with double gate in-
`sulators of SiO,N,./SiN,. A thin SiNI layer inserted be-
`tween the a-Si and the SiOKNy gate insulator decreases the
`density of trapped charges without degrading the switch-
`ing characteristics. The density of the trapped charges,
`which relates to the threshold voltage shift. can be
`minimized by optimizing the thickness and the composi-
`tion of the SiOxNy. The total thickness of Si0,,1'\l,.;lSiN,r is
`fixed at 300 nm. Stress and photoluminescence measure-
`ments reveal that the decrease in trapped charges is due
`to reduction of the macroscopic strain and/ or increase in
`the barrier height preventing an electron from injecting
`to a trapped state in the gate insulator. The switching
`characteristics of an a-Si TFT with a SiOxNJ SiN, gate in-
`sulator do not depend greatly on the composition of the
`SiOxNy. The switching characteristics might be dependent
`not on macroscopic stress but on lattice strain, which is
`related to the difierence in Si-Si and Si—N bond lengths.
`
`§2. Experimental
`We used a-Si TFTs with an inverted staggered elec-
`trode configuration and an SD-nm NiCr gate electrode.
`Figure l
`is a cross-sectional view of an a-Si TFT with a
`Si0xNy/ Sil'slJ gate insulator. A 3DO-nrn SiO,,N,/SiN,r and
`a IOO-nm a-Si
`layer were sequentially deposited by
`plasma chemical vapor deposition (PCVD). We used
`SiI-It, NH3, N10, and H; gases for PCVD of SiO,N,, and
`Sil-I. and H; for a-Si deposition. In our previous paper,
`we described the preparation of the PCVD of SiO,,Ny and
`investigated the effect of the SiOrN, gate insulator on the
`performance of a-Si TFTs in detail.‘ For SiOxNylayers, a
`gas flow rate of 20% SiHr and H; was fixed at 42 sccm,
`and gas flow rate ratio R=N20/ (1130+ NHs) was set at
`
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`Kouichi HIRANAKA and Tadahisa YAMAGLCNI
`
`I E 0"
`
`E
`
`9
`
`7
`
`6
`
`5‘
`m
`V
`U)
`W 5
`
`4
`
`7
`
`6
`
`5
`
`4
`
`e
`e
`h)
`
`1
`N20/(N2'C5) + NH3)
`Fig. 2. Optical properties and relative permittivity as a function of
`gas ratio R=N;0/(N:O+NH,),
`
`
`
`2.0
`
`2.5
`
`3.0
`
`3.5
`
`1.0
`
`E'E:i
`
`‘3 0.5
`
`EE
`
`o
`
`Photon energy (IV)
`
`Fig. 3. Photoluminescence spectra of SiOIN, as a function of gas
`ratio R=N,0/(N,0+NH,).
`
`broad bands, an Hband near 2.5 eV, and an L band near
`2 eV. The peak position of the photoluminescence band
`EpL is defined as
`
`EPL= {EPL(_ 1/2)+Ept(l /2)}/2.
`
`where En.(- l/2) is the energy at the half-maximum of
`the photoluminescence band for
`lower energy, and
`EPL(l/2) is that for higher energy. We found that the
`peak energy of the photoluminescence in sto.N, layers
`has a maximum value at R=0.25 (x/y=0.58), and that
`when R>0.25, the optical gap energy increases and the
`peak energy decreases with the gas ratio R.
`Mechanical stress in Si0.N, films changes monotonical-
`ly from tensile to compressive when R is increased from 0
`to 0.63 and tends to have a constant compressive stress of
`2.2 X 10’ dyn/cm‘ when R>0.63, as shown in Fig. 4. The
`mechanical stress of the a-Si film used in an a—Si TFT is a
`compressive stress of 3.0 X 109 dyn/cm‘. These results in-
`dicate that the difference in stress between the gate in-
`sulator and the a-Si
`films decreases as R increases.
`Therefore, it is expected that macroscopic strain could be
`reduced in the a-Si near the gate insulator/ a-Si interface
`if SiOXN, is used as a bottom gate insulator.
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`
`
`NlCr
`TI
`.
`_
`,
`g-Sal 8'
`SiNx
`SiOx N Y
`
`
`
`.
`NtCr __
`
`Glass Substrate
`
`Fig. l. Cross-sectional view of an a»Si TFT with a double gate in-
`sulator of Si0.N, and SiN,.
`
`0, 0.25, 0.63, or 0.75 with a total gas flow rate of N20
`and NH, fixed at 64 sccm. The thickness of the SiN, was
`300 nm, 100 nm, or 30 nm with the total thickness of the
`SiO.N,/SiN. fixed at 300 nm. The other films (a-Si and
`n*a-Si) were deposited under constant conditions. Self-
`alignment was used to form the source-drain electrode
`out of n‘a—Si/Ti/ NiCr."’ The channel length was 20 am
`and the width-to-length ratio was 15.
`The relative permittivity so" of Si0.N, was obtained by
`measuring the capacitance at 10 MHz of the MIS struc-
`ture of NiCr/ SiO,N,/ n*-c-Si/Al. Optical gaps were ob-
`tained at an absorption coefficient of ex 10‘ (cm") by
`Rand and Wonsidler’s method."’ Electronic properties,
`switching characteristics. and threshold voltage were
`measured for a-Si TFTs after annealing at 200°C for 30
`min. The threshold voltage is defined as the gate voltage
`where the extrapolation of the saturation region in the
`curve of the square root of ID versus VG intersects the V0
`axis, as well as the voltage at which drain current is 100
`pA. To investigate a-Si TF1" stability, we estimated the
`dependence of the threshold voltage Va. shift on the gate
`voltage stress V.. under a drain voltage of 0V. The
`amount of V... shift was measured as the difference in V...
`before and after the application of V... We reported in
`detail on photoluminescence in the SiO.N.. layer in our
`previous paper.‘ A 300-nm SiOxN, layer was deposited
`on a ground quartz substrate (#600). We used a He-Cd
`laser (325 nm) as an excitation light source at room tem-
`perature to obtain the cw photoluminescence spectra.
`Mechanical stress in SiOxN, and SiOxNy/SiNx/a-Si
`layers is determined at room temperature by measuring
`the curvature of each layer-coated Si (100) crystal plane
`with the use of the Newtonian interference technique.
`
`§3. Results and Discussion
`
`3.1 Properties of SiO.N, layers
`Figure 2 gives the optical gap and the relative permit-
`tivity of $0.14, layers. The optical gap E, increases from
`5.1 to 6.6 eV when R increases from 0 to 0.75. The x/y of
`the 0/ N ratio was measured by secondary ion mass spec-
`troscopy (SIMS) as 0. 0.58, [.75. and 2.35 for R of 0,
`0.25. 0.63. and 0.75. The relative permittivity decreases
`from 6.4 to 4.7 when the R of the SiOXN, layer increases
`from 0 to 0.75.
`To investigate the localized states of the SiOxN, film.
`we measured the SioxN, photoluminescence spectrum, as
`reported previously”) Figure 3 shows the cw photolumi-
`nescence spectra of SiO,N, as a function of gas ratio R.
`The photoluminescence band contains two different
`
`
`
`Amorphous Silicon Thin-Film Transistors wilh SiO‘Nv/SiN.
`
`23l
`
`10" 13
`
`10I-10
`
`0
`
`20
`
`30
`
`10
`Va (V)
`
`Fig. 5. Comparison of the transfer characteristics of TFl's with
`WIL=15 when the drain voltage Va: 5 V. (a) Double gate insulator
`(gas ratio R=N10/(N:0+NH,) of SiO,N, is 0.75) The thicknesses
`of the SiO,N, layer and the SN, layer are 270 and 30 nm, respec-
`tively. (b) SiN, single gate insulator (The thickness is 300 nm.) (C)
`$0.14, single gate insulator (gas ratio R=O.75. The thickness is 300
`nm.)
`
`[__l'—‘_'—_l’—fl
`1012 L F: > 0
`_l
`H = 0.75
`SIOINy
`
`
`
`‘1‘
`'e
`
`a
`
`300 nm
`
`a = 0
`SIN ,
`300 nm
`
`R = 0.75
`SIO,.NyIStN,I
`30 nm
`
`
`
`..a
`
`10 6
`1o 5
`Surface field F; (V cm")
`
`10 7
`
`Fig. 6. Density of trapped charges vs surface field strength under a
`gate voltage stress for an a-Si TFT with a SiO‘N, gate insulator at
`R =0.75. a SiN. gate insulator, and a SiO,NV/SiNr gate insulator at
`R =0.7S.
`
`sulator on an a-Si TFT, we estimated the threshold
`voltage shift under various stress conditions where the
`gate voltage stress is varied under a drain voltage of 0 V.
`Figure 6 gives the density of trapped charges as a func-
`tion of gate voltage stress. The gate voltage stress is con-
`verted to a surface field, as mentioned before. The
`threshold voltage shift A V". is directly proportional to
`the density of trapped charges, which is given by
`
`Q! = easvnslA Kh/q(€ndon + 60nd")
`
`(2):
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`Tensile
`
`Compressive
`
`l o
`
`l
`
`1
`05
`
`1
`
`3
`
`{a
`
`E2e
`
`.3
`01
`
`g 0
`g
`
`0zi
`
`n
`
`-3
`
`N20/(N20+ NHa)
`
`Fig. 4. Stress in the SiOKN, layer for R=0. 0.25. 0.63. and 0.75.
`
`3.2 TFT characteristics
`A comparison of the transfer characteristics of three
`TFl‘s—one with a double gate insulator of Si0,N,/ SiN,
`and two with single gate insulators of Sil’sl,r and SiOxNy, re-
`spectively—is shown in Fig. 5. The gas ratio R of the
`SiO‘N, layer is 0.75. The thicknesses of the SiO,N, layer
`and the SiNx layer in the double gate insulator are 270
`and 30 nm, respectively. The thickness of the single gate
`insulator is 300 nm. The results, as shown in Fig. 5, show
`that the TFT with a double gate insulator exhibits steeper
`switching characteristics than that of a single SiOxN, gate
`insulator, and that the threshold voltage shifts to the
`positive gate voltage as the thickness of SiOxN, increases.
`The threshold voltage shift and the switching character-
`istics are dependent on the permittivity and the thickness
`of the gate insulator. Therefore, we estimated the charac-
`teristics of TFTs using the surface field on the a-Si in the
`gate insulator/a-Si interface. For the SiO,.N,/SiN,r gate
`insulator, the surface field F5 on the a-Si was calculated
`from
`
`Fe: swan VG/£n(endon + Gouda)
`
`(I)!
`
`where so. is the relative permittivity of the SiO,N, layer,
`a. that of the SiN,r layer, a. that of the a-Si layer, d0. the
`thickness of the SiO,,Ny layer, d. that of the SiN, layer.
`and Va the gate voltage.
`The threshold voltage shift and the switching character-
`isties of the SiOrN,/ SiN, gate insulator TFTs are depen-
`dent on the thickness of the gate insulator film. We
`selected a SiOXN, layer with R=0.75 to be the first gate in-
`sulator adjacent
`to the gate-electrode side. and in-
`vestigated the effect of the thickness of the SiN, gate in-
`sulator on the TFT when the total thickness of the gate
`SiOXN,/ SiN.r insulator was fixed at 300 nm. A SiOXN,
`film with this value of R has a stress value close to that of
`the a—Si film as shown in Fig. 4. Thus. a gate insulator of
`SiO,N, with R=0.75 is expected to increase the stability
`of a TFT by reducing the macroscopic strain near the
`gate insulator/a-Si interface.
`To investigate the etfect of the Si0.,l’~l,/SiN.r gate in—
`
`
`
`232
`
`Kouichi HIRANAKA and Tadahisa YAMMJUCHI
`
`do" + drl
`
`.-. 300 nm
`
`Fl = 0.75
`
`(Wdecade)
`
`S
`
`where e, is the permittivity in a vacuum and q is the elec-
`tric charge. Kaneko et of. reported the following formula
`for the threshold voltage shift of in TFT:
`
`A Vn.=A Vistlog (1'))‘6 exp (—41:7le
`
`(3).
`
`where V5: is the stress gate bias, I the operation time, T
`the absolute temperature,
`It Boltzmann’s constant, AE
`the activation energy for the charge trapping process,
`and A, or, and ,8 are constants.” The density of trapped
`charges per square centimeter can be expressed by the sur-
`face field at the SiOINyfa-Si interface as follows:
`
`Qr=A'F§‘(103(0)3 exp (*AE/kT)
`where A’ is a constant.
`
`(4).
`
`Figure 6 shows that when F5<8 x 103 Vfcm, the density
`of trapped charges Q, decreases as the thickness of the
`SiN, layer decreases from 300 to 30 nm, and that Q. of a
`TFT with a SiOxNy/SiNx gate insulator decreases more
`than that with only a Si0,,l\ly gate insulator. Q, of a TFT
`with the SiOxN, gate insulator when F,>8>< 10’ V/cm is
`the smallest of the three TFTs. However, the switching
`characteristics of a TFT of the SiOXN, gate insulator
`degrade as the gas ratio increases, as shown in Fig. 11.
`We found that when F.> 8 X 10‘ V/cm, the dependence
`of the surface-field intensity or on the density of trapped
`charges for a TFT with a SiN, gate insulator is almost
`constant, and has little dependence on the thickness of
`the SiNx. These results indicate that the charge trapping
`mechanism is
`based
`on two
`different
`instability
`mechanisms. One mechanism is the charge trapping
`which takes place in the gate insulator, and the other is
`charge trapping at deep states near the gate insulator/ a-
`Si interface, as mentioned in our previous paper.” We
`believe
`that when Rs”. 8 X 10’ Vicm,
`the
`second
`mechanism has
`a more dominant
`effect
`on the
`
`dependence of the density of the trapped charges on the
`thickness of the SiN, layer. The charge trapping near the
`gate insulator/a—Si
`interface is dependent on the
`thickness of the SiNx and on the macroscopic strain as
`shown in Fig. 4. Judging from the difference between Q,
`of a TFT with a SiChNy gate insulator and that with a
`SiOXNy/SiNx gate insulator, the top gate insulator of the
`SiN, acts as the block layer to prevent an electron from in-
`jecting to a trapped state in the SiOKNy layer, and the bot-
`tom gate insulator of the SiOXNy reduces macroscopic
`strain near the gate insuiator/a-Si interface. However,
`when F,>8>< 14')5 V/cm, the charge trapping mechanism
`becomes independent of the thickness of the SiNx. These
`results show that when F.>B X 105 V/ cm, the charge trap-
`ping mechanism takes place in the thinner region of the
`SiNx gate insulator, which is less than 30 nm thick.
`Figure 7 shows the subthreshold slope S of the TFT , de»
`fined as S=dVG/d(log ID). We found that
`the sub-
`threshold slope of the TF1" shows a sharp decrease when
`the SiN, is deposited on the SiOxNy, but reaches a nearly
`constant value of 0.56 V / decade as the thickness of the
`SiNx increases from 30 nm to 300 nm. These results show
`that the widths of the tail states :of the a-Si film decrease
`when the SiN,r is deposited on the SiOXNy, and that the
`widths reach a constant value when the thickness of the
`SiN, increases above 30 nm. Thus, the field-efi'ect mobili-
`
`o
`
`100
`
`200
`
`300
`
`SW, layer thickness (nm)
`
`Fig. 'i'. Subthreshold slope of an a-Si TFT as a function of the
`thickness of the SiN, layer.
`
`ty of a TFT with a SiOINy/ SiN.r gate insulator is better
`than that of 3 TFT with only a Si0,,Ny gate insulator, and
`is as good as that of one with only a SiN,r gate insulator.
`We found that though the density of trapped charges is
`sensitive to macroscopic stress. the subthreshold slope is
`less sensitive to macroscopic strain. These results reveal
`that S is dependent not on macroscopic strain, but on lat-
`tice strain. Lattice strain near the gate insulator/a-Si in-
`terface is the result of the difference in Si—Si and Si—N
`bond length, and depends on the composition of the SiN,
`layers.”’ In our SiOXNy/ SiN, gate insulators, we used the
`same composition of SiNx. Therefore, we believe that our
`subthreshold slope is also dependent on lattice strain.
`Judging from the threshold voltage shift and sub-
`threshold slope, we determined that a 270-nm SiOxNy/
`30-nm SiN, gate insulator is best for an a—Si TFT.
`Figure 8(a) shows the density of the trapped charges in
`an a-Si TFT for R:0, 0.25, 0.63, and 0.75 under a
`positive gate voltage stress for 600 s. The thicknesses of a
`Si0,,l"~ly layer and a SiN, layer in the double gate insulator
`are 270 and 30 nm, respectively. The gate voltage stress is
`converted to the surface field at the a-Si film. We found
`that the density of trapped charges in an a-Si TFT with a
`SiOxNnyiN, double gate insulator is lower than that of
`the commonly used a-Si TFT with just a SiN, gate in-
`sulator,
`and has
`a minimum for R=0.25-0.63
`(xfy=0.58 - 1.75) when the surface field F. at the a-Si in-
`terface is less than 8 X 10’ W cm. When R is greater than
`0.63, the density of trapped charges increases slightly.
`The charge trapping density is less for F,< 8 X 10’ V/cm
`because macroscopic strain near the gate insulator f a-Si
`interface is reduced. As shown in Fig. 8(a), the constant
`or over the whole range of the surface-field strength
`depends on the composition of the SiOiNy in the gate in-
`sulator and on the gate voltage stress. We found that
`when fig-<8 X 105 V/cm, o: varies from 4.3 to 6.9 as R in-
`creases from 0 to 0.63. When R increases over 0.63. or
`decreases, and reaches a value of 5.0 for R=0.75. When
`F,>8 x 105 Vlcm,
`or
`increases as F,
`increases, and
`becomes independent of the composition of the SiO.N,.
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`6/9
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`6/9
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`SAMSUNG EX. 1008
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`
`
`Amorphous Silicon Thin-Firm Transistors with SEO,N_./ SIN,
`
`
`233
`
`
`
`
`
`Fs<0
`
`600 s
`
`
`
`10 5
`106
`Surface field F3 (V cm")
`(a)
`
`107
`
`10‘i
`1n5
`Surface field F5 (V cm")
`03)
`
`107
`
`Density of trapped charges as a function of surface field strength under bias stress voltage for 600 s (bias stress voltage
`Fig. 8.
`is converted to surface field strength at an a-Si interface} for gas ratios R=N30/{N,O+NH3). The thicknesses of the
`SiO‘N, layer and the Silt},r are 210 and 30 nm, respectively. (.1) Density of trapped charges when F90. (b) Density of trap-
`ped charges when F,< U.
`
`SiOXNy/BO-nrn SiN. gate insulator for various SiOxN,
`gate insulators under constant voltage stress. The stress
`voltage was chosen to equal the surface field strength, F;
`at the a-Si interface, where IFSI =6.8 X 105 V/cm for 600
`s. We found that the density of trapped charges for an a-
`Si TFT is at least 8.8 3-(10lcl cn't'2 at R=0.25. This is only
`28% of the value for an a-Si TFT with the commonly
`used SiN, gate insulator. These results correlate well
`with the finding that the total mechanical stress of the
`SiOxNy,’ SiNx/ a-Si
`layered structure is minimum at
`R=0.25, as shown in Fig. 10. For a TFT with a $0.14.;
`SiNx gate insulator at R=0.7S, the slight increase in the
`density of trapped charges suggests that charge trapping
`is due to trapping of electrons that have come through
`
`
`
`Stress(1O9dynicm2)
`
`O
`
`Tensile
`
`SiOxNyISilea-Si
`
`_|
`
`
`
`o
`
`0.5
`
`1
`
`N201(N20 + NH3)
`
`Stress in the SiQ‘NJlSiNx/a-Si layered structures for R=0.
`Fig [0.
`0.25. 0.63. and 0.?5.
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`Kaneko er a}. reported the dependence of the constant a
`on the gate stress. They also found that the constant a: is
`1.9 for convened surface field values at the a-Si interface
`from F,=l X 105 Vfcm to 8 X105 V/cm.“” Figure 8(b)
`shows the density of trapped charges in an a-Si TFT as a
`function of negative gate voltage stress for 600 s. We
`found that when R<0, or is almost independent of the
`composition of the SiOxNy, and has a value of 2.9.
`Kaneko et all. reported that or is a constant value of 3.9
`for a negative gate voltage stressm Figure 9 shows the
`density of trapped charges in an a—Si TFT with a 270-nm
`
` i
`
`T'fi
`
`i
`
`lFsl = 6.8):105 V cm"
`5L sous
`
`Fs"
`
`
`
`o
`
`0.5
`
`1
`
`N20/(N20+NH3)
`
`Fig. 9. Density of trapped charges in an a-Si TFT as a function of the
`gas ratio R=N=OI(N20 + NH,) for a surface field strength converted
`from bias stress voltage |F.i =63 X it)5 Vlcm for 6003.
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`Kouichi HIMNAM and Tadahisa YAMAGUCHI
`
`SleNylSiNx/a-Sl
`
`
`
`R=0
`
`0.25
`
`0.75
`
`Fig. 12. Band models of the SiO,N,/SiN,/a-Si layered structure for
`R=0. 0.25. and 0.75.
`
`R=0, 0.25, 0.75, as shown in Fig. 12. We assume that
`EpL indicates the trapped states in each gate insulator and
`that the energy band structures of the SiO.N,. SiNn and
`a-Si are symmetrical. For R=0. the band model is the
`same as that used in the active matrix switches for liquid
`crystal displays. The band model of the SiO.N,/SiN,/a-
`Si structure predicts a minimum density of trapped
`charges in an a-Si TFT with R=0.25. For R>0.25, the
`density of trapped charges increases as R increases. We
`presented the band model of the Si0.N,/ SiN,/ a—Si struc-
`ture for R=0.75. The threshold voltage shift is due to
`charge trapping in the gate insulator via direct tunnelling
`from the a-Si layer to the gate insulator. and to charge
`trapping in deep states near the gate insulator/a-Si inter-
`face.” We believe that the density of trapped charges
`decreases at R=0.25 because of the decrease in the
`number of deep states near the gate insulator/a-Si inter-
`face. The decrease in the number of deep states results
`from reduced macroscopic strain and decreased charge
`trapping in the gate insulator. The total stress of the
`Si0,N,/ SiNJ a-Si layered structure at R = 0.25 has a com-
`pressive minimum of 8.8)(10‘dyn/cm2 in our TFl‘s.
`Therefore the deep states related to macroscopic strain
`are expected to be minimized. in fact, the influence of the
`stress on the threshold-voltage shift for an a-Si TFT with
`a SiN, gate insulator is reported in several papers.""" For
`a TFT with a SiO,N,/SiN,, gate insulator at R=0.7S, the
`total stress approaches the stress of the a-Si
`film.
`However, the density of trapped charges increases more
`than that at R=0.25. This can be explained by our band
`model. In our previous paper, we found that the lower-
`energy shift of the peak in the photoluminescence spec-
`trum correlates well with the increase in the density of
`trapped charges in an a-Si TFT with a SiOXN, gate in-
`sulator.‘ This tendency also correlates well with the in-
`crease in the density of trapped charges in an a-Si TFT
`with a double SiO,.Ny gate insulator. The trapped states
`distribute in the region more than 30 nm from the
`Silea-Si interface, as shown in Fig. 6. The decrease in
`trapped charges can be explained by the increase in the
`difference between the peak energy of photoluminescence
`for an SiOKN, film and the energy of the conduction band
`in an a-Si layer. This energy difi‘erence might reflect the
`
`layer. The SiOxN, at
`the SiN,r layer into the SioxN,
`R=0.75 has more deep states than the SioxN, at
`R<0.75, as shown in Fig. 3.
`Figure 11 shows the dependence of the subthreshold
`slope S for an a-Si TFT with a SiO,N,/SiN, gate in-
`sulator on the composition of the SiOxN, film, compared
`with that of an a-Si TF’I‘ with only a SiO,N,. gate in-
`sulator. As reported in our previous paper,
`the sub-
`threshold slope for an a-Si TF1" with a SiO,.Ny gate
`insulator increases as the content of the oxygen in the
`SiOXN, film increases.‘ Though the density of trapped
`charges in an a-Si TF1“ with a SiOxNy gate insulator is
`reduced at R =O.25, degradation of the switching charac-
`teristics (the increase in the subthreshold slope) is a
`serious problem in a-Si TFTs. The degradation of the sub-
`threshold slope and the field-effect mobility might be
`caused by the increase in the widths of the tail states in
`the a-Si film, which is caused by an increase in the
`number of deep states near the SiO,N,/a-Si interface.
`The possibility of an increased number of deep states is
`supported by the enhancement of the lower-energy side
`of the photoluminescence band of the SiOxNy when the
`oxygen content
`is
`increased, as
`shown in Fig. 3.
`Therefore, the deep states in the SiOXN, film might cause
`the deep traps that occur near the SioxNyla-Si interface
`and which degrade the switching characteristics,
`in-
`cluding the field-effect mobility and the subthreshold
`slope. We found that the subthreshold slope for an a-Si
`TFT with a SiOXNy/ SiN, gate insulator is independent of
`the composition of the SiO,N,, and is the same as that
`with a SiN, gate insulator. These results indicate that the
`thin SiN, layer prevents oxygen in the SiOXN, from diffus—
`ing into the a-Si and/or makes the structure of the
`sequentially grown a-Si better than that of the SiO,N,
`layer. This results in improvment of the field-effect
`mobility and the subthreshold slope. The subthreshold
`slope is the same if the SiN, gate insulator adjacent to the
`a~Si
`is the same. which is mainly dominated by the
`nearest-neighbor interatomic distance.
`The decrease in the density of trapped charges in an a-
`Si TFT with a SiO,N,,/SiN,, double gate insulator could
`be explained by the band model of SiOxNyl SiNyla-Si for
`
` o
`
`1
`
`0.5
`mommomm)
`
`Fig. ll. Subthreshold slope of an a-Si TFl' as a function of the gas
`ratio R. The white circle denotes the subthreshold slope of an a-Si
`TFI with a Si0,N, gate insulator and the black circle denotes that
`with a Si0,N,/SiN. double gate insulator.
`
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`
`
`Amorphous Siiicon Thin-Film Transistors with SiOXNyISiN.
`
`235
`
`barrier height for electrons to tunnel directly from the a-
`Si into the gate insulator. The density of trapped charges
`in the SioxN, at R=0.25 is at its minimum since the
`energy difference between the photoluminescence peak
`energy of the SiO,N, and the energy of the conduction
`band in a—Si is the largest in our samples. In an a-Si TFT
`with a $0.14.} SiN. at R=0.75, the density of trapped
`charges is slightly more than that at R2025. This result
`indicates that a small number of charges could be trap-
`ped in the SioxNy layer above the SiNX gate insulator. We
`believe that the density of trapped charges at R=O.75 is
`smaller than that at R=0 since the macroscopic strain
`near
`the gate insulator/a-Si
`interface at R=0.75 is
`smaller than that at R=0.
`
`the charge trapping in the
`When F.>8x105V/cm.
`thin 30-nm SiN, layer becomes dominant in all of our
`samples. When F, < 0. the charge trapping in the thin 30-
`nm SiN. is also dominant.
`A layer of SiN.r between the gate insulator and the a-Si
`layer might prevent oxygen in the SiO..Ny film from diffus-
`ing into the a-Si. It also might improve the structure of
`the sequentially gro