Charge storage characteristics of atomic layer deposited RuOx nanocrystals

Charge storage characteristics of atomic layer deposited RuO

nanocrystals

Department of Electronic Engineering, Chang Gung University, Tao-Yuan, Taiwan 333, Republic of China T. Y. Wang

Department of Material Science Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China

P. J. Tzeng, C. H. Lin, and L. S. Lee

Electronic and Opto-electronic Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan 310, Republic of China

J. R. Yang

Department of Material Science Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China

M. J. Tsai

Electronic and Optoelectronic Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan 310, Republic of China

共Received 2 April 2007; accepted 24 May 2007; published online 19 June 2007兲

The charge storage characteristics of atomic layer deposited RuOxnanocrystals embedded in

high-k HfO₂/ Al₂O₃films in a metal/Al₂O₃/ RuOx/ HfO2/ SiO2/ n-Si structure have been investigated. The size and density of RuOxnanocrystals have been measured using transmission electron microscopy.

The RuOx nanocrystals show a density of ⬃1⫻1012/ cm2 and a diameter of 5 – 8 nm. A large

hysteresis memory window of ⬃13.3 V at a gate voltage of 9 V has been observed for RuOx

nanocrystal memory capacitors. A hysteresis memory window of 0.7 V has also been observed under a small sweeping gate voltage of 1 V. A promising memory window of RuOxnanocrystals has

been observed as compared with those of pure HfO2and Al2O3charge trapping layers, due to charge storage in the RuOx metal nanocrystals. The RuOx nanocrystal memory capacitor has similar

leakage current with the pure HfO2and Al2O3charge trapping layers. The RuOxmemory capacitor

has a large breakdown voltage of ⬃13.8 V. © 2007 American Institute of Physics. 关DOI:10.1063/1.2749857兴

Nonvolatile memory devices with a low gate voltage op-eration, consuming less power and allowing higher integra-tion with high-speed writing and erasing of data have an important role in semiconductor industry for future nanos-cale flash memory device applications. Silicon nitride 共Si3N4兲 charge trapping layers in a polycrystalline-silicon– oxide–silicon-nitride–oxide–silicon共SONOS兲 structure with poor retention and scaling problem have been reported.1The nonvolatile memory devices with high-k charge trapping lay-ers in SONOS structure have been reported by several researchers.2–5To improve the device performance, memory device structures with nanocrystals 共or quantum dots兲 have been reported for the possible solution of next generation of nonvolatile memory device applications.6–21 However, for the integration of nanocrystals into the memory device struc-ture, it is a challenging task to control the highly reproduc-ible memory device with a high spatial density, small size, and narrow size distribution of the nanocrystals. Recently, the memory structure with ruthenium共Ru兲 nanocrystals has also been reported.17 To get high density, small size, and narrow size distribution of nanocrystals, the memory devices with metal nanocrystals formed by atomic layer deposition 共ALD兲 have not yet been reported. In this letter, the memory device structure with ruthenium oxide 共RuOx兲 nanocrystals

formed by atomic layer deposition has been investigated. The RuOx is an attractive candidate for metal nanocrystal

memories because it has a large work function of⬃4.8 eV to bring about deep quantum well. Furthermore, high-k materi-als with a large barrier height, such as Al2O3film, are inter-esting alternatives as a blocking oxide to improve the device performance and scaling. A large memory window with a low gate voltage共Vg⬍5 V兲, small size 共5–8 nm兲, high

den-sity 共⬃1⫻1012_{/ cm}2_{兲, and good uniformity have been} ob-served for atomic layer deposited RuOx nanocrystals in a

platinum/Al₂O₃/ RuOx/ HfO2/ SiO2/ n-Si structure for nanos-cale high-performance flash memory device applications. The pure HfO2 and Al2O3charge trapping memory devices have also been fabricated for comparison.

N-type Si 共100兲 substrate with a resistivity of ⬃1 Ohm-cm was cleaned by the RCA process to remove native oxide from the surface. After cleaning the n-type Si substrate, a tunneling oxide共SiO2兲 with a thickness of 3 nm was grown by rapid thermal oxidation system at 1000 ° C for 15 s. The high-k HfO2film as a wetting layer was grown by ALD using hafnium tetrachloride共HfCl4兲 precursor at a sub-strate temperature of 300 ° C. The thickness of HfO2 film was⬃2 nm. Then, the ruthenium oxide 共RuOx兲 layer with a

thickness of ⬃2 nm was grown by ALD using di-ethylcyclopentadienyl ruthenium关Ru共EtCp兲2兴 precursor at a substrate temperature of 350 ° C. Then, the high-k Al2O3film as a blocking oxide was grown by ALD using trimethylalu-a兲_{Author to whom correspondence should be addressed; electronic mail:}

[email protected]

APPLIED PHYSICS LETTERS 90, 253108共2007兲

minum 关Al共CH3兲3兴 precursor at a substrate temperature of 300 ° C. The H2O precursor was used for oxygen. The thick-ness of Al2O3film was⬃20 nm. The precursor temperatures were 185 ° C for HfCl4, 100 ° C for Ru共EtCp兲2and 23 ° C for Al共CH3兲3. Due to an unoptimized process, the oxygen can be included into the Ru film, resulting in a RuOxlayer, by ALD.

To form the RuOxnanocrystals, a postdeposition annealing

共PDA兲 treatment at a temperature of 900 °C for 1 min was performed in N2 共90%兲 and O2 共10%兲 gases. The platinum 共Pt兲 metal gate electrode 共gate area: 1.12⫻10−4_cm2_{兲 was} used for all memory capacitors. The postmetal annealing was performed with a temperature of 400 ° C for 5 min in N2 共90%兲 and H2共10%兲 gases. To investigate the charge storage characteristics, the memory capacitor structures were de-signed such as S1: n-Si/ SiO2共3 nm兲/Al2O3共20 nm兲/Pt, S2:

n-Si/ SiO2共3 nm兲/HfO2共2 nm兲/Al2O3共20 nm兲/Pt, and S3: n-Si/ SiO₂共3 nm兲/HfO₂共2 nm兲/RuOx共2 nm兲/Al2O3共20 nm兲/Pt. The memory capacitors 共S1 and S2兲 were fabricated for comparison. To probe the size and microstructure of RuOxnanocrystals, high-resolution transmission electron

mi-croscopy was carried out using a FEI Tecnai F30 field emis-sion system with an operating voltage of 300 kV and a reso-lution of 0.17 nm. Electrical characteristics of all memory capacitors were performed using a HP 4284A LCR meter and HP4156B semiconductor analyzer systems.

Figures1共a兲and1共c兲show the cross-sectional TEM im-ages of the n-Si/ SiO2/ HfO2/ RuOx/ Al2O3 共sample: S3兲 structure for as deposited and after PDA treatment, respec-tively. The thicknesses of SiO2, HfO2, RuOx, and Al2O3 films are found to be 3.0, 2.0, 2.0, and 20 nm, respectively, for the as-deposited sample. The HfO2and RuOxfilms show

partial crystallinity while the Al2O3 film shows amorphous nature. After the annealing treatment, clear RuOx

nanocrys-tals embedded in HfO2and Al2O3films have been observed.

The average diameter of RuOxnanocrystals is 5 – 8 nm and

the thickness is about 3 nm. The thicknesses of SiO2, HfO2, and Al2O3films are found to be 3.0, 1.0, and 17 nm, respec-tively. The lattice constants of Ru film 共hexagonal兲 are cal-culated: a = 0.275 nm, b = 0.275 nm, c = 0.443 nm. The lattice constants of monoclinic HfO2 films are found to be

a = 0.511 nm, b = 0.517 nm, and c = 0.529 nm, while those

values are found to be a = 0.448 nm, b = 0.443 nm, and

c = 0.309 nm for the orthorhombic RuO₂films. Note that the HfO₂film as a wetting layer has been used because the RuOx

film cannot be directly deposited by ALD on SiO2 without HfO2 film. The reason for RuOx film deposition on HfO2 layers is unclear. It is also beneficial that the high-HfO2layer can be used as a part of tunneling oxide.

The elemental compositions of

n-Si/ SiO2/ HfO2/ RuOx/ Al2O3 共sample: S3兲 structure were in-vestigated by energy dispersive x-ray spectroscopy 共EDS兲 analysis with a spot size of⬃0.5 nm and a spatial resolution of⬃1 nm. Figures1共b兲and1共d兲show the elemental concen-trations of O, Hf, Si, Ru, and Al measured by EDS for the as deposited and after annealing treatment. The numbers indi-cated on the curve in Figs.1共b兲 and1共d兲 correspond to the numbers shown in the TEM image. It is estimated that the SiO₂, HfO₂, and Al₂O₃ films show close stoichiometric for the as-deposited and annealing treated samples. Average con-centrations of Hf, Ru, and O atoms are found to be 20, 14, and 54 at. %, respectively, for the as-deposited sample, while those values are found to be 20, 18, and 47 at. %, respec-tively, for the annealed sample. After annealing treatment, it is shown that the Ru-rich RuOxnanocrystal is formed in our

memory structure. The high density of ⬃1⫻1012_{/ cm}2 _for RuOxnanocrystals measured by plan-view TEM is observed

共Fig.2兲. The diameter of nanocrystals is 5–8 nm. The

thick-ness of nanocrystal is about 3 nm. It indicates that the shape of nanocrystal is likely a thick disk. The density and size of RuOxnanocrystals can be controlled by changing the

thick-ness of the RuOxlayer.

Figure3共a兲shows a good clockwise hysteresis of RuOx

nanocrystal memory capacitors with different sweeping gate voltages 共Vg兲. A small capacitance equivalent thickness is

found to be ⬃9.3 nm. The high-frequency 共1 MHz兲 capacitance-voltage 共C-V兲 has been measured with a hold time of 100 ms. A large hysteresis memory window of 13.3 V at sweeping gate voltage of Vg= 9 V is observed, due

to the high density of RuOxmetal nanocrystals. Yim et al.17

reported the hysteresis memory window of⬃6.7 V at a large sweeping gate voltage of 10 V for Ru nanocrystal memory device. A hysteresis memory window of⬃0.7 V is also ob-served under an extremely low gate voltage of ±1 V, due to deep quantum well 共high work function of ⬃4.8 eV兲 of RuOx nanocrystals and small conduction band offset

FIG. 1. 共Color online兲 High-resolution transmission electron microscopy 共TEM兲 images of Al2O3/ RuOx/ HfO2/ SiO2/ n-Si structure for 共a兲 as-deposited and共c兲 900 °C, 1 min samples. Average elemental concentrations of oxygen共O兲, silicon 共Si兲, hafnium 共Hf兲, ruthenium 共Ru兲, and aluminum 共Al兲 for the 共b兲 as-deposited and 共d兲 annealed samples have been shown. Clear RuOxmetal nanocrystals have been observed for annealed sample.

FIG. 2. 共a兲 Plan-view transmission electron microscopy image of RuOx nanocrystals in Al₂O₃/ RuOx/ HfO2/ SiO2/ n-Si structure and 共b兲 high-resolution TEM image of a single RuO2nanocrystal.

共⌬Ec⬃1.7 eV兲 of HfO2films. It indicates that the charge can be stored in the RuOxnanocrystals under small positive gate

voltage and the stored charges can be erased easily under small negative gate voltage applications. The RuOx metal

nanocrystal memory devices formed by ALD show the best hysteresis memory characteristics as compared with reported nanocrystal memory devices in the literatures.17,18 The amount of stored charges in RuOxnanocrystals can be

esti-mated using the relation Q = Coxx共+VFB兲, where Cox 共⬇3.7⫻10−7_{F / cm}2_{兲 is the capacitance density at} accumula-tion region and +V_FB 共⬇4.4 V兲 is the flatband voltage shift under a positive gate voltage of Vg⬇6 V. Thus, the electron

density stored in RuOx nanocrystals is estimated to be

⬃1⫻1013_{/ cm}2_{. It indicates that one RuO}

x nanocrystal can

store about ten electrons, which is similar to the reported results on HfO2nanocrystals.11The hysteresis memory win-dow of RuOx nanocrystal capacitor increases with an

in-crease of the sweeping gate voltage up to 9 V 关Fig. 3共b兲兴. The nanocrystal capacitor has a large hysteresis memory window as compared with those of the pure HfO2and Al2O3 charge trapping layers. Large memory windows with a low gate voltage operation of RuOxnanocrystal memory

capaci-tor can be used in future nanoscale flash memory device applications.

The leakage current density of RuOxnanocrystal

capaci-tor is similar to those of the pure HfO₂ and Al₂O₃ charge trapping layers up to a gate voltage of 9 V 共Fig. 4兲. The

RuOxnanocrystal capacitor shows the breakdown voltage of

−13.8 V and leakage current density of 5⫻10−10_{A / cm}2_{at a}

gate voltage of −5 V. A breakdown voltage 共−13.8 V兲 of RuOxnanocrystal is lower共slightly兲 as compared with that of

the breakdown voltage of 17 V for pure HfO2 charge trap-ping layer, and it may be due to the contamination共slight兲 of RuOx metals in the Al2O3 blocking oxide after annealing treatment.

In conclusion, the excellent charge storage characteris-tics of atomic layer deposited RuOx nanocrystal capacitors

have been observed. A large hysteresis memory window of 13.3 V at a sweeping gate voltage of 9 V, low leakage cur-rent density of 5⫻10−10_{/ cm}2_{at a gate voltage of −5 V, and} high breakdown voltage of −13.8 V have been investigated. The atomic layer deposited RuOx nanocrystal memory

ca-pacitor can be used in future nanoscale high-speed flash memory device applications.

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FIG. 3.共Color online兲 共a兲 Capacitance vs sweeping gate voltage character-istics of RuOxnanocrystal memory capacitors.共b兲 The hysteresis memory window increases with increasing the sweeping gate voltage.

FIG. 4.共Color online兲 Leakage current densities of RuOxnanocrystal, pure HfO₂, and Al₂O₃charge trapping layers.