Oxygen pressure dependence of physical and electrical properties of LaAlO3 gate dielectric

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Abstract

High k LaAlO3 (LAO) films were deposited directly on silicon substrates in various oxygen pressures by laser molecular-beam epitaxy technique. The influence of oxygen pressures during film fabrication on the physical and electrical properties of LAO films was studied. High resolution transmission electron microscopy measurements indicate that the thermo stability of LAO films in contact with silicon substrates is greatly affected by oxygen pressures, and thicker interfacial layer would be expected for LAO films deposited in high oxygen pressure. Capacitance–voltage (CV) and leakage current measurements indicate that the effective oxide thickness, leakage current, flatband voltage and hysteresis loop characteristics are affected by the oxygen pressure during film fabrication. Larger EOT, lower leakage current and smaller hysteresis loop is expected to be obtained for LAO films deposited in higher oxygen pressure or lower vacuum. When oxygen pressure is below or equal to 0.1 Pa, the absolute value of VFB increases with the decrease of oxygen pressure. When oxygen pressure is above 0.1 Pa, the VFB value begins to decrease slowly.

Introduction

With the continued scaling of MOS technology, the thin SiO2 gate oxide must eventually be replaced by a high dielectric constant material [1]. Currently, there is much work being done in developing high-k dielectrics to replace silicon oxide as gate dielectric in the future MOSFETs [2], [3], [4], [5], [6]. Among the many potential high-k materials, LaAlO3 has recently attracted much attention due to its many advantages such as medium dielectric constant, high bandgap, and amorphous structure up to high temperature [7], [8], [9], [10]. It is known that oxide films deposited by pulsed laser deposition or laser molecular beam epitaxy technique (LMBE) often have oxygen vacancies, which plays important role in the electrical performance of the film [11], [12]. To reduce the oxygen vacancies in the films, incorporation of oxygen with high pressure during film formation is the method often used [13]. One of the characteristics desirable for the high-k dielectric is that it has good thermal stability with silicon substrate [1]. However, for most of the high-k materials, possible interfacial reactions between gate oxide and silicon substrate will occur when they are fabricated in oxygen containing ambient [1], [14], [15]. Good electrical performance and thermal stability with silicon are necessary for high-k dielectrics used as alternative gate dielectrics. Thus, it is necessary to understand the growth mechanism that controls these factors and influences both the electrical properties and thermal stability with silicon of the LAO thin films. In this paper, the effect of oxygen pressure on the structure and electrical properties of laser-fabricated LAO high-k gate dielectric was studied.

Section snippets

Experimental details

LAO films were deposited by a LMBE system, equipped with in situ reflective high-energy electron diffraction (RHEED). Details of the system have been reported elsewhere [16]. Substrates were 2-inch n-Si (1 0 0) wafers with resistivity of 4  6 Ω cm. After wet-chemical cleaning, the Si substrate was dipped into a buffered HF (10%) solution for 60 s to remove the amorphous SiO2 layer from the silicon surface, leaving a hydrogen-terminated surface. Then the Si substrate was immediately moved into the

Results and discussions

For most of the high k oxide films, the thermodynamic stability of them in contact with silicon is a critical issue for the application of alternative gate dielectrics in silicon-based devices [1]. Furthermore, the thermodynamic stability of high k oxide films is tightly correlated to the content of oxygen during film growth [15]. The impact of oxygen pressure on the thermodynamic stability of LAO films was studied by HRTEM first. Fig. 1(a) and (b) show the HRTEM images of LAO films deposited

Conclusion

In conclusion, LAO films were deposited by laser MBE in various oxygen pressures. The thermodynamic stability of LAO films in contact with silicon is greatly affected by the oxygen pressures during film fabrication, and higher oxygen pressure is favorable for the interfacial reactions. The EOT of LAO films increases with the increase of oxygen pressure. On the contrary, the corresponding leakage current decreases with the increase of oxygen pressure. The mechanism of determining the shift of VFB

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    Citation Excerpt :

    Therefore, the increase in the leakage current density of samples that are deposited at high oxygen pressure with oxygen pressure is reasonably attributed to an increase in the number of leakage current paths that are formed by more structural defects in the oxide films. No interfacial layer between LAO films and Si-substrate (even at a high substrate temperature of 700 °C) or ITO-substrate has been examined at an oxygen pressure of lower than 0.4 Pa [7,9]. Based on the experimental results of Edon et al. [5], the leakage current density decreased as the oxygen pressure increased for samples that were deposited in a circumstance with a low oxygen pressure (< 0.4 Pa) because more oxygen atoms compensated for the oxygen deficiencies of the vaporized species as the frequency of collision or reaction between the evaporated species and oxygen increased.

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