Elsevier

Journal of Crystal Growth

Volume 311, Issue 7, 15 March 2009, Pages 2183-2186
Journal of Crystal Growth

High κ dielectric single-crystal monoclinic Gd2O3 on GaN with excellent thermal, structural, and electrical properties

https://doi.org/10.1016/j.jcrysgro.2008.10.079Get rights and content

Abstract

MBE-grown single-crystal Gd2O3 epitaxially on GaN has exhibited excellent thermal stability, withstanding rapid thermal annealing (RTA) at 1100 °C. The high-κ dielectric Gd2O3 thin film 10 nm thick has a monoclinic phase with a high degree of crystallinity, characterized by X-ray diffraction using synchrotron radiation. The orientation relationship is (4¯02)Gd2O3(0001)GaN and 010Gd2O321¯1¯0GaN. Interface between Gd2O3 and GaN remains atomically sharp after the RTA, as demonstrated using X-ray reflectivity and high-resolution transmission electron microscopy. Gd2O3/GaN metal-oxide-semiconductor capacitors show well-behaved capacitance–voltage characteristics with small dispersion and hysterisis, a high dielectric constant of ∼17, and a low electrical leakage current density of 4.6×10−9 A/cm2 at 1 MV/cm.

Introduction

GaN is a strong alternative channel contender for technologies beyond the 22–16 nm-node complementary metal-oxide-semiconductor (CMOS), due to its wide energy band gap (3.4 eV) and a high saturation velocity at high electrical fields; the former alleviates the adverse affects like drain-induced barrier lowering (DIBL) and band-to-band tunneling (BTBT), and the latter helps achieving high speeds in small devices. The epi-growth of GaN on Si, leading to the integration of GaN onto Si, has added to its attraction [1], [2]. Amorphous (Al2O3, Ga2O3(Gd2O3) [GGO], and HfO2) [3], [4], [5], [6] and single-crystal (Gd2O3, Sc2O3) [7], [8] high κ dielectrics on GaN with low interfacial densities of states (Dit) and low electrical leakage currents, essential in fabricating inversion-channel devices, have received research interest recently. Annealing at very high temperature of at least 1100 °C is needed to activate source/drain implanted ions for ohmic contacts in fabricating inversion-channel GaN MOS field-effect-transistors (MOSFET's) [9], [10]. On that regard, single-crystal high κ dielectrics offer advantages over its amorphous counterparts, as the re-crystallization of the amorphous films may lead to poly-crystalline films, resulting in a high electrical leakage current.

The epitaxy of two dissimilar materials has always received a great deal of interest, due to its pathway to new physical phenomena [11] and/or practical applications. Cubic Gd2O3 was found to grow epitaxially on GaAs [12] and Si [13], and was also employed as a gate dielectric for inversion-channel devices [14]. Thin Gd2O3 1.8 nm thick, with a hexagonal close-packed (hcp) phase, was epitaxially deposited on GaN [7]. However, no electrical measurements were performed. In particular, it was found that Gd2O3 tends to absorb moisture with air exposure [15], [16].

Therefore, in this work, Gd2O3 10 nm thick was grown on GaN to minimize the effect of moisture absorption on the electrical measurements. It was found, unexpectedly, that the film is of monoclinic phase with excellent crystallinity. Metal/Gd2O3(10 nm)/GaN capacitors exhibit capacitance–voltage (CV) characteristics with a high dielectric constant of approximately 17 and show a low electrical leakage current density of 4.6×10−9 A/cm2 at 1 MV/cm. The Gd2O3/GaN hetero-structure withstands rapid thermal annealing (RTA) at 1100 °C with the oxide/GaN interface remaining atomically sharp and smooth, as characterized using X-ray reflectivity (XRR), high-resolution X-ray diffraction (XRD) with synchrotron radiation and high-resolution transmission electron microscopy (HR-TEM).

Section snippets

Experimental procedures

Prior to the oxide deposition, n-type GaN (grown on a c-plane (0 0 0 1) sapphire substrate) was heated up to 700 °C in ultra high vacuum for surface cleaning. Gd2O3 was deposited on GaN using electron beam evaporation from a powder-packed and sintered Gd2O3 at a substrate temperature of 700 °C. In-situ reflection high energy electron diffraction (RHEED) was used to monitor the film growth.

The samples were rapid thermal annealed (RTA) at 1100 °C and dwelled there for 5 min in helium ambient, as the

Results and discussion

A GaN (0 0 0 1) (1×1) RHEED pattern (Fig. 1(a)) turned to a sharp streaky reconstructed (2×2) (Fig. 1(b)) with the 700 °C annealing, indicating the attainment of a clean and atomically ordered surface [21]. After the growth of Gd2O3 10 nm thick, the RHEED pattern shows a clear streaky (3×3) reconstructed feature (Fig. 1(c)), showing that the oxide surface is atomically smooth and ordered. The (3×3) reconstructed pattern is different from the (4×4) displayed in cubic single-crystal Sc2O3 [22], Y2O3

Conclusion

Gd2O3 oxide thin films 10 nm thick epitaxially grown on GaN were found to be a high-temperature monoclinic phase, neither the low-temperature cubic phase nor the previously reported hcp phase. The cystallinity of monoclinic Gd2O3 and its interfacial properties with GaN remain intact, after RTA to high temperatures, the minimum requirement for GaN dopant activation. Furthermore, the single-crystal Gd2O3 on GaN in this work exhibits better electrical properties than its counterpart amorphous GGO.

Acknowledgments

The authors wish to thank Department of Natural Sciences at National Science Council under grants of NSC-97-2120-M-007-008 and NSC-96-2628-M-007-003-MY3, Taiwan, Republic of China and HUGA Optotech Inc. for supporting this work.

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