Heterostructures of Oxides and Semiconductors - Growth and Structural Studies

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1 Heterostructures of Oxides and Semiconductors - Growth and Structural Studies Beamline 17B1 W20 X-ray Scattering beamline Authors M. Hong and J. R. Kwo National Tsing Hua University, Hsinchu, Taiwan H. Y. Lee and C. H. Hsu National Synchrotron Radiation Research Center, Hsinchu, Taiwan Part I: Epitaxial single crystal growth of γ-al 2 O 3 and Sc 2 O 3 on Si (111) and structural studies using high-resolution X-ray diffraction Part II: Thermodynamic stability of Ga 2 O 3 (Gd 2 O 3 )/GaAs interface 1. Epitaxial single crystal growth of γ-al 2 O 3 and Sc 2 O 3 on Si (111) and their structural properties (a) γ-al 2 O 3 Hetero-epitaxial growth between insulators and semiconductors is always fascinating in science and important in technology. For example, growth of single crystal GaN on sapphire has been essential for producing blue lasers and LED s. Epitaxial growth of insulators on Si is another example, on which a subsequent single-crystalline growth of other semiconductors such as GaN 5 may integrate high-power microwave devices or lasers with the most advanced Si-based electronic devices. In this work, we report the attainment of very high-quality cubic γ- Al 2 O 3 single crystal films with thickness as thin as 3.8 nm. The oxide films have (111) as the normal in parallel with (111) of the Si substrate and the films have [4 4 _ 0] in-plane axis in parallel with [2 2 _ 0] of the Si substrate. The rocking scans at γ-al 2 O 3 (222) position of films 3.8 and 11 nm shows a low FWHM of 0.6 and 0.3 degree, respectively. Atomic force microscopy (AFM) and x-ray reflectivity (XRR) all show a very smooth surface about 0.1~0.2 nm. The oxide/si interface is also atomically smooth of 0.1~0.2 nm as studied using XRR and cross-sectional TEM. In contrast to the previous efforts using MBE with precursors or gases, a high-purity sapphire was employed in this work. E-beam evaporation was used due to the high melting point of sapphire, and the deposited species (incoming flux) consist of only Al 2 O 3 molecules or clusters. Si (111) wafers were put into a multi-chamber MBE/UHV system, after being cleaned with an RCA method and an HF dip. The experimental procedure was described earlier. Streaky oxide RHEED patterns along the in-plane axes of [1 1 _ 0] and [11 2 _ ] (shown in Fig. 1(b)) of Si were observed after growth of oxide 1nm thick, indicating that a smooth single crystal γ-al 2 O 3 film formed on the Si (111) and with in-plane alignment between the oxide film and Si substrate. Single crystal X-ray measurements were carried out on a four-circle triple-axes diffractometer, using a 12 kw rotating anode Cu K-alpha source. A pair of graphite crystals is used to monochromatize and analyze the X-ray beam with a resolution of 0.01 Å -1 along the longitudinal and Å -1 along the transverse directions respectively. 27

2 Materials Physics The interface was found to be atomically sharp and smooth, as shown in Fig. 3(a), whose micrograph was taken with the electron beam directing along [112]Si. In the inset of Fig. 3 (a), the orientation relation in cross-sectional direction _(Si_ [112]) was found to be Si (112) // Al2O3 (224), Si [ 1 11] // Al2O3 [ 2 22]. Electron diffraction patterns from the plan-view of the hetero-structure (Fig. 3 (b)) show Si (111) // Al2O3 (222), Si [220] // Al2O3 [440]. The cubic γ-al2o3 and Si have significantly different atomic structures and lattice constants. The lattice constant of γ-al2o3 is 7.91Å and that of Si is 5.42 Å. Matching the two lattices over a unit cell dimension will result in a >30% lattice mismatch. It is intriguing that a highly ordered epitaxial growth was obtained in an unusually large mismatch for a hetero-epitaxial system. Fig. 1: 1 In-situ RHEED patterns of Si (111) substrate (a) and_of γ-al2o3 (111) film 1.0 nm thick _ (b) along [1 10] and [11 2] axes. Note that the surface normal (111) γ-al2o3 film _ is parallel with (111) Si and the in-plane axe [4 4 0] of γ-al2o3 is _ in parallel with [2 20] of Si substrate. A single crystal X-ray theta two-theta scan along substrate surface of Si (111) on the oxide film 3.8 nm thick is shown in Fig. 2, in which a broad peak near 40 degree coincides with the (222) reflection of the cubic gamma phase of Al2O3. The broadness of the peak is caused by the thin thickness of the oxide films. The relatively strong oscillation at small angle reflectivity on all of our oxide films grown on Si (111) indicates that the film thickness is highly uniform and smooth. (a) The single crystalline nature of the γ-al2o3 film is further studied by scans along the major zone axes in reciprocal lattice. We find that γ-al2o3 (004) peak lies on the same zone axis with the Si (004) reflection, proving that the film is single crystalline and is aligned with the Si (111). (b) Fig. 3: Cross-sectional TEM image and electron diffraction pattern of a 3.8 nm γ-al 2O 3 film, showing a sharp interface and smooth surface (a). The electron diffraction pattern indicates that the film is well aligned with Si substrate. The In-plane electron diffraction pattern of the 3.8 nm γ-al2o3 film is shown in (b). Fig. 2: X-ray theta two-theta scan along Si (111) surface of (111)γ-Al2O3 film 3.8 nm thick and the mosaic scan of the γ-al2o3 (222) peak. 28

3 Fig. 4: X-ray theta two-theta scan using synchrotron radiation on the same film shown in Fig. 1 Note that our recent study using synchrotron radiation source at NSRRC has given much more detailed information on the crystallographic structures of both the thin film and the interface. This then will allow us to study γ-al 2 O 3 films with thickness in the range of 2 nm. In fact, an X-ray diffraction normal scan with synchrotron radiation on the same sample of Fig. 1 is shown in Fig. 4. In Fig. 1, the (222) peak of γ-al 2 O 3 is indicated with a small broad peak. In comparison, all the peaks of γ-al 2 O 3 are well resolved with sharp peaks. The Pendellösung fringe oscillations surrounding the (222) are clearly exhibited, indicating a highly uniform film with an atomically smooth surface and sharp oxide/si interface. (b) Sc 2 O 3 High-quality single-crystal Sc 2 O 3 films a few nanometer thick have been grown epitaxially on Si (111) despite a huge lattice mismatch. The films were Fig. 5: Single crystal scans along the surface normal in reciprocal space finds an epitaxial growth of the Sc 2 O 3 bixbyite phase. Fig. 6: A cross-sectional HRTEM image of the Sc 2 O 3 /Si(111) heterostructure in [ 112 _ ] Si projection, showing the Sc 2 O 3 film thickness of ~4 nm. The edge-type misfit dislocation is indicated by T. The arrow exhibits the inclined defects in Sc 2 O 3. electron beam evaporated from a Sc 2 O 3 target. Structural and morphological studies were carried out by X-ray diffraction and reflectivity (Fig. 5, along with very small rocking FWHM of at Sc 2 O 3 (222)), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM) (Fig. 6), and medium energy ion scattering (MEIS) (Fig. 7), with the initial epitaxial growth monitored by in-situ reflection high energy electron diffraction (RHEED). The synchrotron X-ray experiments were performed at wiggler beamline BL17B1 at the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The films have the cubic bixbyite phase with a remarkably uniform thickness and high structural perfection. The film surfaces are very smooth and the oxide/si interfaces are atomically sharp with a low average roughness of 0.06 nm. The films are well aligned with the Si substrate with an orientation relationship of Si (111) // Sc 2 O 3 (111), and an in-plane expitaxy of Si [ 1 _ 1 0 ] // Sc 2 O 3 [ 1 _ 0 1 ]. The density of the inclined defects in our sample was estimated to be ~10 14 m -2. This value is considerably low compared to that of the line defect, misfit dislocations (>10 16 m -2 ), frequently observed in epitaxial heterostructures. Since neither considerable inclined defects nor extensive dislocation networks were observed in the current system, we would expect that the corresponding rocking scans are little affected by such crystallographic defects. 29

4 Fig. 7: Backscattering energy spectrum (130 kev H + ) of a Sc 2 O 3 film on Si(111) in a normal incidence and substrate blocking geometry. A simulation (solid line) shows a fit for a 3.9 nm thick crystalline film with no detectable interfacial layer formation, to be compared to a simulation for an amorphous film (dashed line). The inset shows angular distributions for the Si and Sc signals. Figure 7 shows a MEIS backscattering energy spectrum for an uncapped Sc 2 O 3 film exposed to air prior to analysis. Observation of distinct blocking minima in Sc signal, shown in the inset in Fig. 7, and comparison of the Sc signal just below the surface Sc peak to a calculated yield for the same thickness amorphous film (the yield, shown as a dashed line in Fig. 7) indicate very good film crystalline quality. Remarkably no interfacial Sc and O peaks are observed, indicating a highly ordered Sc 2 O 3 lattice structure at the interface with Si. Sc 2 O 3 and Si have significantly different crystal structures, different bonding, and lattice constants. The bulk lattice constants of Si (5.43 Å) and Sc 2 O 3 (9.86 Å) are mismatched by 9.2% (relative to the doubled Si unit cell dimension). It is intriguing that a highly ordered epitaxial growth was obtained with this unusually large mismatch. References.S. Nakamura, et al., Appl. Phys. Lett. 64, 1687 (1994)..R. A. Mckee, et al. and M. F. Chrisholm, Phys. Rev. Lett. 81, 3014 (1998)..J. Kwo, et al., Appl. Phys. Lett. 77, 130 (2000)..S. Guha, et al., J. Appl. Phys. 90, 512 (2001)..Wakahara, et al., J. Crystal Growth 236, (2002)..Young-Chul Jung, et al., Journal of Crystal Growth 196, (1999)..J. T. Zborowski, et al., J. Vac. Sci. Technol. B 16(3), 1451 (1998)..M. Hong, et al., J. Vac. Sci. Technol. B 14(3), 2297 (1996)..S. Y. Wu, et al., Appl. Phys. Lett. 87, (2005)..M. Hong, et al., Res. Soc. Symp. Proc. 811, D9.5 (2005)..M. Hong, et al., Appl. Phys. Lett. 87, (2005). 2. Thermodynamic stability of Ga 2 O 3 (Gd 2 O 3 )/GaAs interface GaAs MOSFET offers potential advantages over Si-based MOSFET because of the high electron mobility, high breakdown field, and semi-insulating substrate of GaAs. Searching and identifying electrically and thermodynamically stable insulators on GaAs with a low interfacial density of states (D it ) has been one of the key challenges in the compound semiconductor devices over the past four decades. In-situ deposition of Ga 2 O 3 (Gd 2 O 3 ) dielectric film on GaAs surfaces produced MOS diode structures with a low D it. Subsequent employment of the Ga 2 O 3 (Gd 2 O 3 ) as a gate dielectric along with an ion implantation process led to the demonstration of the first enhancement mode GaAs MOSFETs with inversion on semi-insulating GaAs substrates in both n- and p- channel configurations. Previously, the oxide-gaas interface was found to be roughened in a high temperature (> 750 C) annealing for fabricating the inversion-channel GaAs MOSFETs, in which the annealing was inevitably needed to activate the ion implantation for ohmic contacts at source and drain regions. For the device processing, Ga 2 O 3 (Gd 2 O 3 ) should be thermodynamically stable with GaAs at temperatures of ~ 750 C or above. The interfacial roughness has to be controlled and minimized to a few Å, as was witnessed in the case of the perfected SiO 2 -Si interface. However, it was found out later that when the samples are exposed to air, they absorb water and form hydro-oxides. 30

5 During the annealing process, the hydro-oxides or other contaminations in the oxides, not the pure Ga 2 O 3 (Gd 2 O 3 ), react with GaAs, resulting in rough interfaces. In this work, Ga 2 O 3 (Gd 2 O 3 )/GaAs heterostructures have been annealed up to ~780 C. Studies using X-ray reflectivity (Fig. 1) and high resolution transmission electron microscopy (Fig. 2) have shown that the samples annealed under UHV have maintained smooth and abrupt interfaces with the interfacial roughness being less than 0.2 nm. The oxide remains as amorphous, an important parameter for device consideration. Current-voltage and capacitance-voltage measurements have shown low leakage currents (10-8 to 10-9 A/cm 2 ), a high dielectric constant of 15, and a low interfacial density of states (D it ) between gate dielectrics and GaAs. The attainment of a smooth interface between the gate dielectric and GaAs, even after high temperature annealing for activating implanted dopant, is a must to ensure the low D it and to maintain a high carrier mobility in the channel of the MOSFET. Table I lists the fitting results for the samples with the two different thermal processes. Dwelling at 300 C for 30 min in UHV allowed the hydro-oxides to be removed out of the air-exposed sample B, as evidenced from a rapid increase of the amount of H 2 O containing in the chamber detected using RGA (residual gas analyzer). The consequent annealing to high temperatures let GaAs be adjacent to hydrooxide free Ga 2 O 3 (Gd 2 O 3 ). Figure 3(a) of the J-E curves for the two samples shows a leakage current density of Ga 2 O 3 (Gd 2 O 3 ) on GaAs of about 10-8 to 10-9 A/cm 2 at low gate voltages. CV curves were obtained with frequencies varying from 1KHz to 1MHz and the dispersion in the CV curves of different frequencies was due to the equivalent circuit of complex impedance. The dielectric constant of Ga 2 O 3 (Gd 2 O 3 ) is calculated to be about The D it was estimated to be less than ~10 12 cm -2 ev -1 using the Terman method. Fig. 1: 1 Low angle X-ray reflectivity of Ga 2 O 3 (Gd 2 O 3 ) on GaAs annealing in UHV, with experimental data (dots), and a theoretical fit (line) Fig. 2: High-resolution cross sectional TEM picture of Ga 2 O 3 (Gd 2 O 3 ) on GaAs after air exposure and annealing in UHV (Sample B). Table 1: X-ray reflectivity studies on Ga 2 O 3 (Gd 2 O 3 )/GaAs sample No. oxide thickness (nm) thermal process air-oxide surface roughness (nm) interfacial roughness (nm) A 26.1 annealed to 780 C in UHV directly B 25 Exposed to air, then put back to UHV system, dwelled at 300 C for 30min before annealing to 780 C

6 Experimental Station X-ray scattering end station References.M. Hong, C. T. Liu, H. Reese, and J. Kwo, "Encyclopedia of Electrical and Electronics Engineering", Published by John Wiley & Sons, New York, 19, 87 (1999)..C. W. Wilmsen, "Phys. & Chem. of III-V Compound Semiconductor Interfaces", Plenum, New York, (1985)..M. Hong, et al., J. Vac. Sci. Technol. B, 14, 2297 (1996)..M. Passlack, et al., IEEE Transaction of Electron Devices, 44 (2), (1997)..F. Ren, et al., IEEE Int'l Electron Devices Meeting (IEDM) Technical Digest, 943 (1996), and Solid State Electronics, 41 (11), 1751 (1997)..Y. C. Wang, et al., Mat. Res. Soc. Symp. Proc. 573, 219 (1999)..M. Hong, et al., Appl. Phys. Lett. 76 (3), 312 (2000)..Y. L. Huang, et al., Appl. Phys. Lett. 86, (2005). Fig. 3: (a) Leakage current density J(A/cm 2 ) vs E (MV/cm) for Ga 2 O 3 (Gd 2 O 3 )/GaAs samples in different thermal processes and (b) C-V curves of a MOS diode made of Au/Ga 2 O 3 (Gd 2 O 3 ) (25nm)/GaAs after two-frequency corrections (Sample B) Contact mhong@mx.nthu.edu.tw 32