Band offsets and charge storage characteristics of atomic layer deposited high-k HfO2/TiO2 multilayers

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Band offsets and charge storage characteristics of atomic layer deposited

high-k HfO

₂

/ TiO

₂

multilayers

S. Maikapa兲

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 and C.-H. Lin

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

T. C. Tien

Material Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan 310, Republic of China

L. S. Lee

J.-R. Yang

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

M.-J. Tsai

共Received 26 March 2007; accepted 30 May 2007; published online 25 June 2007兲

The band offsets and charge storage characteristics of atomic layer deposited high-k HfO2/ TiO2

multilayers with ten periods in p-Si/ SiO2/共HfO2/ TiO2兲/Al2O3 structure have been investigated.

The thickness of high-k HfO2 or TiO2film is ⬃0.5 nm for each layer, before and after annealing

treatment of 900 ° C for 1 min in N2 ambient. High-resolution transmission electron microscopy,

x-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy measurements on high-k HfO2/ TiO2 multilayers confirm the layer-by-layer structure after annealing treatment,

suggesting the HfO2/ TiO2multilayer quantum wells. The valence band offsets of HfO2and TiO2

films are found to be⬃3.1 and ⬃1.5 eV, respectively. The conduction band offsets are found to be ⬃1.7 eV for HfO2 films and ⬃0.9 eV for TiO2 films. The high-k HfO2/ TiO2 multilayers in p-Si/ SiO2/共HfO2/ TiO2兲/Al2O3/aluminum memory capacitor show a large capacitance-voltage

hysteresis memory window of⬃5 V at gate voltage of ±5 V, due to the charge storage in multilayer quantum wells. The hysteresis memory window of ⬃1.3 V at small gate voltage of ±1 V is also observed. The high-k HfO2/ TiO2multilayer memory structure can be used in future nanoscale flash

Nonvolatile memory devices achieving high-speed writ-ing and eraswrit-ing of data with a low gate voltage共Vg⬍5 V兲, consuming less power and allowing higher integration have an important position in the semiconductor industry for fu-ture nanoscale flash memory device applications. Due to poor retention and scaling issues in poly-Si-oxide-silicon-nitride-oxide-silicon nonvolatile memory devices,1the

high-k charge trapping layers such as Al₂O₃, La₂O₃, ZrO₂, and HfO2 in a metal-oxide-high-k-oxide-silicon structure are

reported.2–7 To further improve the scaling and to increase the program/erase speed, the high-k dielectric with a large barrier height such as Al2O3 can also act alternatively as a

blocking oxide for high-speed flash memory device applications.8–10 Recently, the high-k HfO2 charge trapping

layers in metal-aluminum-oxide-high-k-oxide-silicon 共MAOHOS兲 structure with a large gate voltage operation 共Vg⬎5 V兲 are also reported.9,10

To obtain a low gate voltage operation, high-speed, excellent retention, and highly scal-able flash memory devices, the high-k HfO2/ TiO2multilayer

quantum wells with an Al2O3 blocking oxide in MAOHOS

structure have been proposed. In this letter, the band offsets and excellent charge storage characteristics of atomic layer deposited high-k HfO₂/ TiO₂ multilayer quantum wells have been reported. The pure HfO2and TiO2charge trapping

lay-ers in MAOHOS structure have also been studied for com-parison.

p-type Si 共100兲 substrates 共4 in.兲 with a resistivity of

15– 25⍀ cm were cleaned by dipping in dilute HF for 1 min using the RCA process to remove native oxide from the sur-face. After cleaning the p-Si substrate, a tunneling oxide 共SiO2兲 with a thickness of 3 nm was grown by rapid thermal

a兲_{Author to whom correspondence should be addressed; electronic mail:} [email protected]

APPLIED PHYSICS LETTERS 90, 262901共2007兲

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oxide system at a temperature of 1000 ° C for 20 s. Then, the high-k HfO2 film with a thickness of 0.5 nm for each layer

was grown by atomic layer deposition共ALD兲 using hafnium tetrachloride共HfCl₄兲 at a substrate temperature of 300 °C. The high-k TiO₂ film with a thickness of 0.5 nm for each layer was deposited by plasma-enhanced atomic layer depo-sition 共PEALD兲 using titanium 共IV兲 isopropoxide precursor at a substrate temperature of 350 ° C. The high-k HfO2/ TiO2

multilayers with ten periods were deposited on SiO2

共3 nm兲/p-Si substrate. Basically, the total thickness of charge storage layer was⬃10 nm. For comparison, the pure HfO2 charge trapping layer with a thickness of 10 nm or

pure TiO2 charge trapping layer with a thickness of 10 nm

was deposited on SiO2 共3 nm兲/p-Si substrate. Then, the

high-k Al2O3 film as a blocking oxide was grown by ALD

using trimethylaluminium 关Al共CH₃兲₃兴 precursor at a sub-strate temperature of 300 ° C. The thickness of Al₂O₃ film was ⬃15 nm. All high-k films were in situ deposited by ALD/PEALD cluster tool. The postdeposition annealing 共PDA兲 treatment at a temperature of 900 °C for 1 min in N2

ambient was carried out for all memory structures. The post-metal annealing treatment at a temperature of 400 ° C for 5 min was also carried out using N2 共90%兲 and H2 共10%兲

gases. All memory capacitors were fabricated using an alu-minum 共Al兲 metal gate electrode 共gate area: 50⫻50␮m2_兲

with a lithography process. To probe the thickness and mi-crostructure of high-k HfO2/ TiO2 multilayers,

high-resolution transmission electron microscopy共HRTEM兲 was performed using a FEI Technai F30 field emission system with an operating voltage of 300 kV and a resolution of 0.17 nm. X-ray photoelectron spectroscopy 共XPS兲 was per-formed to investigate the interaction between the ultrathin HfO2 and TiO2 layers for as-deposited and after PDA

treat-ment. To obtain the valence band and conduction band off-sets of high-k HfO₂/ TiO₂ multilayers, the ultraviolet photo-electron spectroscopy 共UPS兲 was carried out. Electrical characteristics of memory capacitors were performed using HP 4284A LCR meter and HP 4156B semiconductor ana-lyzer systems.

Figure 1共a兲 shows a HRTEM image of high-k

HfO2/ TiO2multilayers after PDA treatment. The thicknesses

of tunneling oxide 共SiO2兲 and blocking oxide 共Al2O3兲 are

found to be⬃3 and ⬃15 nm, respectively, while the thick-ness of HfO2 or TiO2 film is observed ⬃0.5 nm for each

layer. The thickness of as-deposited HfO₂ or TiO₂ film is also observed⬃0.5 nm for each layer 共not shown here兲. The high-k Al2O3 film as a blocking oxide shows partial

crystal-line, while the high-k HfO₂/ TiO₂ multilayers show fully crystalline with maintaining the layer-by-layer structure. Fig-ure1共b兲shows a HRTEM image with an elemental line pro-file共red color兲 of high-k HfO2/ TiO2 multilayers plotted by

the Gatan “DIGITAL MICROGRAPH.” The high-k HfO₂/ TiO₂ multilayers show a layer-by-layer structure, indicating that there is no intermixing between the HfO2 and TiO2 films

after high temperature annealing treatment of 900 ° C for 1 min in N2ambient, and it has also been confirmed by XPS

measurement below.

The XPS spectra of pure HfO2, pure TiO2, and HfO2

共0.5 nm兲/TiO2共0.5 nm兲 multilayers for as-deposited and

af-ter PDA treatment are shown in Fig.2. The binding energy of Hf 4f7/2 electrons is the same 共⬃16.8 eV兲 for all memory

structures of as-deposited and after PDA treatment 关Fig.

2共a兲兴, which is close to the reported value of binding energy

共⬃16.7 eV兲 of HfO2 films on Si substrate.11 The binding

energy of Ti 2p3/2electron is also the same共⬃458.8 eV兲 for

as-deposited and after PDA treatment 关Fig. 2共b兲兴, which is similar to the reported value.12Note that the binding energies of Hf–O and Ti–O bonds for HfO2/ TiO2 multilayers are

similar to the pure HfO₂and TiO₂films for as-deposited and after PDA treatment. It indicates that there is no interdiffu-sion or intermixing between the Hf and Ti atoms in HfO2/ TiO2 multilayers after high temperature annealing

treatment, which is corroborated by the HRTEM image. The valence band as well as the conduction band offsets of HfO2/ TiO2multilayers have been measured by UPS.

Fig-ure3共a兲shows the UPS spectra of pure HfO2, pure TiO2, and

HfO2 共0.5 nm兲/TiO2 共0.5 nm兲 multilayers with ten periods

on SiO2共3 nm兲/p-Si substrates after PDA treatment. A −4 V

bias is applied to the substrate to overcome the analyzer work function during UPS measurement. The valence band maximum共VBM兲关共Ef兲Si−共Ev兲high k兴 is found to be ⬃3.4 eV

for pure HfO2, ⬃1.8 eV for pure TiO2, and ⬃2.6 eV for

HfO2/ TiO2 multilayers. Note that the VBM value of

HfO2/ TiO2multilayer structure has an average value of the

VBM of pure HfO₂and TiO₂films. Considering a doping of ⬃1⫻1015_cm−3 _{for p-Si substrate, the energy difference of}

共Ef兲Si−共Ev兲Si is ⬃0.3 eV. So, the valence band offsets

关⌬Ev⬇共Ev兲Si−共Ev兲high k兴 for pure HfO2 and TiO2 films are

found to be⬃3.1 and ⬃1.5 eV, respectively, which are simi-lar to the reported values.12,13The band gap energies共Eg兲 of pure HfO2 and TiO2 films measured by XPS method14 are

⬃5.9 and ⬃3.5 eV, respectively 共not shown here兲. Assuming a band gap energy of Si substrate共Eg⬇1.1 eV兲, the conduc-tion band offsets 关⌬Ec⬇共Eg兲high k−共Eg兲Si−⌬Ev兴 are

ob-FIG. 1. 共Color online兲共a兲 HRTEM image of high-k HfO₂共0.5 nm兲/TiO₂ 共0.5 nm兲 multilayers with ten periods in p-Si/SiO2/共HfO2/ TiO2兲/Al2O3 structure after annealing treatment of 900 ° C for 1 min in N2ambient.共b兲 HRTEM image with elemental line profile byDIGITAL MICROGRAPHsoftware shows the clear HfO2/ TiO2layer-by-layer structure共red color兲.

FIG. 2. 共Color online兲 XPs spectra of 共a兲 Hf 4f and 共b兲 Ti 2p signals for pure HfO2, pure TiO2, and HfO2共0.5 nm兲/TiO2共0.5 nm兲 multilayers with ten periods of as-deposited and after annealing treatment.

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served⬃1.7 eV for pure HfO2 and ⬃0.9 eV for pure TiO2

films. Considering all energies in pure HfO₂and TiO₂films, a schematic energy band diagram of the HfO2/ TiO2

multilayer memory structure with ten periods共or nine quan-tum wells兲 is shown in Fig. 3共b兲. In this memory structure, the electrons can be stored in quantum wells under a small positive gate voltage and the stored electrons can be erased easily under a small negative gate voltage, due to small con-duction band offset共⌬Ec⬇0.9 eV兲 of TiO2films. So, a large

memory window with a small operation voltage can be ex-pected and it has been confirmed by C-V measurement be-low.

Figure4共a兲shows a counterclockwise C-V共1 MHz兲 hys-teresis of high-k HfO2共0.5 nm兲/TiO2共0.5 nm兲 memory

ca-pacitor with different sweeping gate voltages共Vg兲. A holding time was 1 s during C-V measurement. A large hysteresis memory window of⌬V⬇5 V at Vg= 5 V is observed. A hys-teresis memory window of⌬V⬇1.3 V at an extremely low

gate voltage operation of ±1 V is also observed. The

memory window is ⬃1.6 V for single quantum well

device.15 The hysteresis memory window of high-k HfO2/ TiO2 multilayers is increased with increasing the

sweeping gate voltage up to 5 V, while there is no hysteresis memory window of pure HfO2 trapping layer up to Vg

= 5 V关Fig.4共b兲兴. The pure TiO2 charge trapping layer does

not show any hysteresis memory window up to Vg= 10 V. It indicates that the charge can be stored in HfO2/ TiO2

multilayer quantum wells. The charge loss of multilayer quantum wells is less than 10% after 10 years of retention for high-k HfO₂/ TiO₂ multilayer memory transistors 共not shown here兲. Large memory window 共⌬V⬎5 V兲 with a low gate voltage operation共Vg⬍5 V兲 can be used in future mul-tilevel charge storage high-density flash memory device ap-plications.

In conclusion, the excellent charge storage characteris-tics of high-k HfO2/ TiO2 multilayer quantum wells have

been observed. The high-k HfO2/ TiO2 multilayer quantum

well structure has been confirmed by physical and electrical measurements. A large memory window with a small gate voltage operation of 5 V in HfO2/ TiO2 multilayer quantum

wells has been observed as compared with those of pure HfO2 and TiO2 charge trapping layers, due to the charge

storage in quantum wells. The high-k HfO2/ TiO2multilayer

quantum wells pave the way in future nanoscale flash memory device applications.

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FIG. 3.共Color online兲共a兲 UPS spectra of pure HfO2, pure TiO2, and HfO2 共0.5 nm兲/TiO2共0.5 nm兲 multilayers with ten periods after annealing treat-ment. 共b兲 Schematic energy band diagram of the HfO2共0.5 nm兲/TiO2 共0.5 nm兲 multilayers with nine quantum wells.

FIG. 4.共Color online兲共a兲 Capacitance vs gate voltage characteristics of the HfO2/ TiO2 multilayer quantum well memory capacitor. 共b兲 Hysteresis memory window of HfO2/ TiO2multilayers, pure HfO2, and TiO2films.

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