Characterization of ultra-thin Y₂O₃ films as insulator of MFISFET structure

 TANG Ming-hua(唐明华)^1,2, ZHOU Yi-chun(周益春)^{1, 2}, ZHENG Xue-jun(郑学军)^{1, 2}, YAN Zhi(言  智)³,

 CHENG Chuan-pin(成传品)^{1, 2}, YE Zhi(叶  志)^{1, 2}, HU Zeng-shun(胡增顺)^{1, 2}

1.Faculty of Materials and Optoelectric Physics, Xiangtan University, Xiangtan 411105, China;

2. Key Laboratory for Advanced Materials and Rheological Properties of Ministry of Education,

Xiangtan University, Xiangtan 411105, China;

3. Shanghai Institute of Microsystem and Information Technology, Shanghai 200050, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The possibility of ultra-thin Y₂O₃ (yttrium sesquioxide) films as insulator of metal ferroelectric insulator semiconductor (MFIS) structure was investigated. The ultra-thin Y₂O₃ films with thickness of 10-40 nm were fabricated on p-type Si (100) substrates by molecular beam epitaxy(MBE) in vacuum and subsequently submitted to rapid thermal processing (RTP) in air ambient at 700, 800 and 900 ℃ for 30 min, respectively. The films were characterized by X-ray diffractometry and Raman spectroscopy. High frequency capacitance—voltage (C—V) characteristics and current—voltage (I—V) characteristics of the Y₂O₃/Si structure were analyzed. A Raman peak of the Y₂O₃ thin films was observed at 378 cm^-1. From the C—V data, these films exhibit dielectric constants ranging from 13 to 17.28, the hysteresis width (ΔV_FB) ranging from 0.07 to 0.22 V and the density of trapped charges ranging from 1.65×10¹¹ to 4.01×10¹¹ cm^-2. A leakage current of 4.75×10^-8 -9.0×10^-6  A/cm² at 1.5 MV/cm was observed. The results show that the Y₂O₃ buffer layers are suitable for non-volatile MFIS structure field-effect-transistors (FETs) memory application.

Key words: Y₂O₃; MBE; capacitance—voltage characteristics; current—votage characteristics; dielectric constant; MFIS structure

1 Introduction

Ferroelectric random access memory (FeRAM) with a metal-ferroelectric-semiconductor (MFS) structure has attracted much attention, because of its nonvolatile operation, the high access speed, low electrical power and small memory cell size[1-3]. However, some issues such as an inter-diffusion between ferroelectric films and Si[4] and formation of a SiO₂ layer with low dielectric constant exist in this kind of device. So a metal–ferroelectric–insulator–semiconductor field effect transistor (MFIS-FET) structure where an insulating buffer layer such as Y₂O₃, CeO₂, ZrO₂ and MgO thin film between the ferroelectric film and Si substrate has been proposed to solve these problems[5-8]. Among the available insulators, the Y₂O₃ thin films have great potential for MFIS-FET memory applications due to its a high electrical breakdown field (>4.4 MV/cm)[9,10], low dissipation factor (<0.005)[11], high resistivity (>10¹⁴ ??cm)[12], low leakage current(3× 10^-8 A/cm² at 1.8 MV/cm)[13], large band gap (5.8 eV)[14], low bulk trap density of (4-11)×10¹¹ cm^-2[15], a high k value of 10-20[16,17], high thermal stability up to 2 325 ℃, a low lattice mismatch (a₀ (Si)×2=1.086 nm; a₀ (Y₂O₃) = 1.060 4 nm) and a good chemical compatibility with silicon. Thus, the growth, microstructure and electrical properties of this buffer layer should be carefully investigated because it will ultimately determine the capacitance of the entire stacked structure and the performance of FeRAM. In this work, 10-40 nm Y₂O₃ thin films were deposited by molecular beam epitaxy (MBE) in vacuum and subsequently submitted to rapid thermal processing (RTP) in air ambient at 700, 800 and 900 ℃ for 30 min, respectively. Both physical and electrical properties measurements of Y₂O₃ films were carried out as a function of the post annealing temperature.

2 Experimental

Ultra-thin films of Y₂O₃ were deposited on 2″ p-type Si(100) wafers with resistance 0.1 ??cm by molecular beam epitaxy (MBE) under the ultrahigh vacuum (UHV) condition using sintered Y₂O₃ ceramics tablets as targets. The temperature of the p-type Si (100) single crystal substrate was at 275 ℃. The evaporation rate of Y₂O₃ thin films was 0.1 nm/s and the thickness was in-situ monitored with Maxtek MDC-360. The Y₂O₃ tablets were prepared by firing Y₂O₃ power (99.99%) for sintering. Prior to deposition, silicon wafers were etched with following procedure to eliminate the native oxide. Firstly, the Si wafers were etched for 20 s using 2.5%(mole fraction) HF solution and then rinsed with deionized water for 5 min in an ultrasonic cleaner. After the rinse, the wafers were again etched for 5 s in a 6:1 HF solution buffered by C₂H₅OH. Finally, they were dried with nitrogen gas 99.999 9% purity. Post-thermal treatment for the as-deposited Y₂O₃ samples was carried out by RTP in air ambient at 700, 800 and 900℃ for 30 min, respectively.

To investigate the crystalline nature of the Y₂O₃ films, X-ray diffraction (XRD, D/Max 2500PC) and Raman spectroscopy (Japan Spectroscopic CO., LTD) were employed. The electrical measurements were conducted using MIS configurations with aluminium (Al) as top electrodes, with diameter of 0.2 mm sputtered through a metal shadow mask. The backside of the silicon substrates was etched in hydrofluoric acid metallized by sputtering a 100 nm thick Al layer. Thus, the electrical measurements were carried out on Al/Y₂O₃/

p-Si (100) /Al capacitor structures. Capacitance-voltage (C—V) curves were measured using Agilent 4284 LCR meter at 1 MHz with bias sweep rate of 0.1 V/s. The current—voltage(I—V) curves were measured with HP 4156 semiconductor parameter analyzer. All measureme-

nts were carried out with thickness of 40 nm Y₂O₃ thin films at room temperature.

3 Results and discussion

Fig.1 shows the XRD patterns of the 40 nm-thick Y₂O₃ thin films in their as-deposited state as well as after different annealing temperatures. It is noted that the as-deposited amorphous Y₂O₃ films show the effect of amorphous-to-crystalline transformation, the broad peaks at 2θ=29.14? and 48.5? appear, which corresponds to the 222, 440 reflections, respectively, in agreement with the results of KANG et al[18] and WILK et al[19]. The Raman spectra of the 40 nm thick Y₂O₃ thin films at different annealing temperatures are shown in Fig.2. A

Fig.1 XRD spectra of Y₂O₃ films annealed at different temperatures

Fig.2 Raman spectra of Y₂O₃ films annealed at different temperatures

well resolved Raman peak of Y₂O₃ thin films is observed at 378 cm^-1, which is in good agreement with that reported by WANG et al[20].

Fig.3 displays the well-behaved high frequency capacitance—voltage(C—V) curves of Y₂O₃ thin films annealed at different temperatures. A relatively little flat band voltage and small hysteresis width (ΔV_FB) are observed in all samples. The accumulation capacitance (C_acc) of the as-deposited amorphous Y₂O₃ film is smaller than the crystallized Y₂O₃ films, however, the C_acc of the crystallized Y₂O₃ films decreases as the annealing temperature increases. This is due to the growth of the interfacial amorphous SiO₂ layer upon annealing process[21], which is confirmed on SEM observation. It is also observed that the flat band voltage shifted negatively upon annealing temperature, which indicates that negative fixed charge is compensated by positive charge generated during post-annealing process [22]. The hysteresis width (ΔV_FB) decreases from 0.22 V to 0.07 V as the annealing temperature increased from 700 ℃ to 900 ℃, which is consistent with that reported by WILK et al[19]. The dielectric constant of the Y₂O₃

Fig.3 High frequency C—V characteristics upon different annealing temperatures at 1 MHz

Fig.4 Dependencies of relative dielectric constant (ε_r) and hysteresis width (ΔV_FB) on annealing temperature

thin films can therefore be determined from the measured value of accumulation capacitance (C_acc), using the relation by

                                (1)

where  A is the area of diode, and d is the thickness of dielectric layer. Fig. 4 displays the obtained ε_r value and the hysteresis width as a function of the RTP temperature. A dielectric constant at 1 MHz ranging from 13 to 17.28 is observed (κ decreases with increasing annealing temperature), which is in good agreement with the results obtained by MANCHANDA et al[23,24]. The density of oxide trapped charges can be quantitatively calculated from the C—V curves using following formula as

                           (2)

where  C_acc is the accumulation capacitance, ΔV_FB is the hysteresis width, q is the electron charge and A is the electrode area. The density of the trapped charges decreased from 4.01×10¹¹ to 1.65×10¹¹ cm^-2 as the annealing temperature increases.

Fig.5 Current—voltage (I—V) curves of Y₂O₃ thin films annealed at different temperatures

Fig.5 illustrates that the leakage current densities of Y₂O₃ thin films decrease with increasing annealing tmperature. The observed leakage current density at an applied electric field of 1.5 MV/cm is about 4.75×10^-8 -9.0×10^-6 A/cm², depending on morphology, deposition and postannealing conditions [24]. Breakdown fields are measured as E_BD≈3.5 MV/cm. The decrease of the leakage current level is due to the densification of the Y₂O₃ thin films as well as the growth of the interfacial oxide layer.

4        Conclusions

High-quality ultra-thin Y₂O₃ films (10- 40 nm) on p-type Si(100) substrates were obtained by MBE in vacuum. The as-deposited amorphous Y₂O₃ films and crystalline films after RTP treatment were found. A well resolved Raman peak of Y₂O₃ thin films is seen at 378 cm^-1. The dielectric constant, hysteresis width and the density of the trapped charges decrease as the annealing temperature increases and the leakage current density can be reduced to 4.75×10^-8 A/cm² at 1.5 MV/cm electric field. Breakdown fields are measured as E_BD≈3.5 MV/cm. These results suggest that the Y₂O₃ buffer layers are suitable for non-volatile MFIS structure field-effect-

transistors memory application.

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(Edited by LONG Huai-zhong)

Foundation item: Project(05FJ2005) supported by the Key Project of Scientific and Technological Department of Hunan Province, China; Project(05C095) supported by the Research Funds of Educational Department of Hunan Province, China; Project(05JJ30208, 05JJ30126) supported by the Natural Science Foundation of Hunan Provincial , China

Corresponding author: ZHOU Yi-chun; Tel: +86-732-8293586; fax: +86-732-8292468. E-mail: zhouyc@xtu.edu.cn