Single-Crystal Y2O3 Epitaxially on GaAs(001) and (111) Using Atomic Layer Deposition

Single-crystal atomic-layer-deposited (ALD) Y2O3 films 2 nm thick were epitaxially grown on molecular beam epitaxy (MBE) GaAs(001)-4 × 6 and GaAs(111)A-2 × 2 reconstructed surfaces. The in-plane epitaxy between the ALD-oxide films and GaAs was observed using in-situ reflection high-energy electron diffraction in our uniquely designed MBE/ALD multi-chamber system. More detailed studies on the crystallography of the hetero-structures were carried out using high-resolution synchrotron radiation X-ray diffraction. When deposited on GaAs(001), the Y2O3 films are of a cubic phase and have (110) as the film normal, with the orientation relationship being determined: Y2O3(110)[001][1¯10]//GaAs(001)[110][11¯0]. On GaAs(111)A, the Y2O3 films are also of a cubic phase with (111) as the film normal, having the orientation relationship of Y2O3(111)[21¯1¯][011¯]//GaAs(111)[2¯11][01¯1]. The relevant orientation for the present/future integrated circuit platform is (001). The ALD-Y2O3/GaAs(001)-4 × 6 has shown excellent electrical properties. These include small frequency dispersion in the capacitance-voltage (CV) curves at accumulation of ~7% and ~14% for the respective p- and n-type samples with the measured frequencies of 1 MHz to 100 Hz. The interfacial trap density (Dit) is low of ~1012 cm−2eV−1 as extracted from measured quasi-static CVs. The frequency dispersion at accumulation and the Dit are the lowest ever achieved among all the ALD-oxides on GaAs(001).


Introduction
Single crystal rare earth (RE) oxides have been epitaxially grown on GaAs [1][2][3], Si [4][5][6][7], and GaN [8,9] using ultra-high vacuum (UHV) e-beam evaporation in a growth mode of molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Among various high κ dielectrics in amorphous and single-crystal forms to passivate GaAs(001), MBE-grown Gd 2 O 3 -based RE-oxides have given low interfacial trap densities (D it ) [10], thermal stability at high temperatures [11], and the first demonstration of inversion-channel enhancement-mode GaAs metal-oxide-semiconductor field-effect-transistor (MOSFET) [12,13]. Figure 1a shows the GaAs(001)-4ˆ6 reconstructed RHEED patterns along [110] and [110] directions. The GaAs RHEED patterns and their in-plane symmetry changed to those of cubic Y 2 O 3 (110) after the deposition of the ALD-Y 2 O 3~1 nm thick. Figure 1b shows Figure 1a shows the GaAs(001)-4 × 6 reconstructed RHEED patterns along 110 and 110 directions. The GaAs RHEED patterns and their in-plane symmetry changed to those of cubic Y2O3(110) after the deposition of the ALD-Y2O3 ~1 nm thick. Figure 1b shows the pattern for the oxide film 2.3 nm thick along the Y2O3 in-plane 001 and 110 directions. Note that the bulk lattice constant of bixbyite Y2O3, aY2O3 = 1.060 nm, is approximately twice that of GaAs, aGaAs = 0.565 nm. Thus, there exists a large lattice mismatch, d 2 ⁄ d /d of 6.2% and 32.7% along the in-plane [001] and [110] directions of Y2O3(110), respectively, where d stands for the lattice spacing along the commonly aligned directions.  Kikuchi arcs indicate the excellent crystallinity of GaAs growth with a well-ordered structure. Different from the epitaxial growth on GaAs(001), the co-existence of the RHEED patterns from both GaAs and the oxide was observed for the very first few cycles (2-6 cycles) of ALD-Y2O3 on GaAs (111)A as shown in Figure 2b,c of 4-and 6-cycle deposition. The RHEED streaks were fattened with the deposition of thin ALD-Y2O3 films, indicating that the films have the same in-plane symmetry, but with less crystallographic order and different lattice spacing. The patterns of GaAs were not observable at the 10-cycle film deposition as shown in Figure 2d, which are streakier than the ALD-Y2O3 with similar thickness on GaAs(001). Now comparing the RHEED patterns of the 22-cycle (2 nm thick) ALD-Y2O3 on GaAs(111) (Figure 2e) with those of ALD-Y2O3 with similar thickness (2.3 nm thick) on GaAs(001) (Figure 1b), the latter shows a sausage-like pattern with a 2× reconstruction along the in-plane 110 (Figure 1b).  Kikuchi arcs indicate the excellent crystallinity of GaAs growth with a well-ordered structure. Different from the epitaxial growth on GaAs(001), the co-existence of the RHEED patterns from both GaAs and the oxide was observed for the very first few cycles (2-6 cycles) of ALD-Y 2 O 3 on GaAs (111)A as shown in Figure 2b,c of 4-and 6-cycle deposition. The RHEED streaks were fattened with the deposition of thin ALD-Y 2 O 3 films, indicating that the films have the same in-plane symmetry, but with less crystallographic order and different lattice spacing. The patterns of GaAs were not observable at the 10-cycle film deposition as shown in Figure 2d    The large lattice mismatch and different bonding of the deposited oxide and semiconductor substrate, namely ionic versus more covalent bonds between Y 2 O 3 and GaAs, has not prevented the epitaxial growth of ALD-Y 2 O 3 on both GaAs(001) and (111). One also observed that the strained pseudomorphic growth did not occur even for very thin thicknesses such as 1 nm, which already exhibited Y 2 O 3 (110) as surface normal to the underlying GaAs(001).

Results and Discussion
The observation of the ALD epitaxial growth was made possible using our unique setup of connecting MBE chamber whose vacuum is maintained below 10´1 0 Torr, while the pressure in the ALD reactor is in the order of a few Torr during deposition. After the ALD, the reactor was pumped down to 10´9 Torr prior to the sample being transferred to the MBE chamber equipped with RHEED via the UHV modules.
Synchrotron radiation source gives a high sensitivity to X-ray diffraction in studying very thin films with thickness in the range of a few nano-meters. We will start with the crystallographic structures of ALD-Y 2 O 3 film 2.3 nm thick on GaAs(001)-4ˆ6, and then move to the same oxide of a similar thickness on a different orientation of (111). The former structure was studied earlier [35]. The XRD radial scan along the surface normal of the former hetero-structure is shown in Figure 3a, in which the location of the broad peak appearing at the scattering vector along the surface normal q 001~3 .006 r.l.u. GaAsr001s , reciprocal lattice unit of GaAs along the [001] direction with a value of 2π/a GaAs Å´1, was very close to that of the (440) reflection of cubic Y 2 O 3 . Furthermore, no other peak except the GaAs reflections was observed in the radial scan. These observations indicated that the Y 2 O 3 film had a cubic structure and was (110) oriented. From the periodicity, ∆~0.247 r.l.u. GaAsr001s shown in Figure 3a, of the interference fringe near the Y 2 O 3 (440) diffraction peak, the film thickness was estimated to be~2.3 nm. In the region near the GaAs(002) reflection, the satellites and oscillation fringes were caused by the interference of the underlying AlGaAs/GaAs superlattice designed for blocking the diffusion of structural defects. The large lattice mismatch and different bonding of the deposited oxide and semiconductor substrate, namely ionic versus more covalent bonds between Y2O3 and GaAs, has not prevented the epitaxial growth of ALD-Y2O3 on both GaAs(001) and (111). One also observed that the strained pseudomorphic growth did not occur even for very thin thicknesses such as 1 nm, which already exhibited Y2O3(110) as surface normal to the underlying GaAs(001).
The observation of the ALD epitaxial growth was made possible using our unique setup of connecting MBE chamber whose vacuum is maintained below 10 −10 Torr, while the pressure in the ALD reactor is in the order of a few Torr during deposition. After the ALD, the reactor was pumped down to 10 −9 Torr prior to the sample being transferred to the MBE chamber equipped with RHEED via the UHV modules.
Synchrotron radiation source gives a high sensitivity to X-ray diffraction in studying very thin films with thickness in the range of a few nano-meters. We will start with the crystallographic structures of ALD-Y2O3 film 2.3 nm thick on GaAs(001)-4 × 6, and then move to the same oxide of a similar thickness on a different orientation of (111). The former structure was studied earlier [35]. The XRD radial scan along the surface normal of the former hetero-structure is shown in Figure 3a, in which the location of the broad peak appearing at the scattering vector along the surface normal q001 ~ 3.006 r.l.u.GaAs[001], reciprocal lattice unit of GaAs along the [001] direction with a value of 2π/aGaAs Å −1 , was very close to that of the (440) reflection of cubic Y2O3. Furthermore, no other peak except the GaAs reflections was observed in the radial scan. These observations indicated that the Y2O3 film had a cubic structure and was (110) oriented. From the periodicity, Δ~0.247 r.l.u.GaAs[001] shown in Figure 3a, of the interference fringe near the Y2O3(440) diffraction peak, the film thickness was estimated to be ~2.3 nm. In the region near the GaAs(002) reflection, the satellites and oscillation fringes was caused by the interference of the underlying AlGaAs/GaAs superlattice designed for blocking the diffusion of structural defects. As to the growth of ALD-Y2O3(111) on GaAs(111)A, the Y2O3 layer also has a cubic structure but with its (111) planes parallel with the GaAs(111) surface. XRD radial scan (theta versus two-theta scan) along surface normal of the 2.0 nm thick ALD-Y2O3 is displayed in Figure 4a  As to the growth of ALD-Y 2 O 3 (111) on GaAs(111)A, the Y 2 O 3 layer also has a cubic structure but with its (111) planes parallel with the GaAs(111) surface. XRD radial scan (theta versus two-theta scan) along surface normal of the 2.0 nm thick ALD-Y 2 O 3 is displayed in Figure 4a  Previous study on MBE-Y2O3 grown on GaN(0001) showed that Y2O3 can exist in hexagonal phase as the film thickness 3 nm and its crystalline structure resembles that of the cubic phase in many aspects [36]. It is risky to identify the phase by the specular reflections alone and thus essential to examine the positions of the off-normal reflections.
For the Y2O3 film grown on GaAs(001), the azimuthal φ scan across cubic Y2O3(622) reflection shows four sharp peaks (Figure 3b). The larger angular separation ~110° agreed well with the calculated 109.5° between the (622) and (262) pair and between the (622) and (262) pair. On the other hand, the smaller separation 70° matches the angular spacing between the (622) and (622) pair and between the (262) and (262) pair. The four evenly spaced broad peaks were the tails of the GaAs{113} reflections. The epitaxial relationship between the Y2O3 film and GaAs substrate deduced from the relative position of the reflections is Y2O3(110) 110 //GaAs(001) 110 and only one rotational domain exists. With the determined orientation, we estimated the lattice constant of Y2O3 had a small, ~0.3%, dilation along the surface normal. More data is required to evaluate the bi-axial lateral strains.
Azimuthal φ scans across the (2 22) reflection of cubic Y2O3 on GaAs(111) is depicted in Figure  4b. Three evenly spaced sharp peaks yield the characteristic 3-fold symmetry along cubic [111] axis. The weak broad peaks between the intense peaks originate from the tail of the nearby GaAs{1 11} reflections. The 60° offset between the two sets of reflections elucidates the B-type cube-on-cube growth, i.e., Y2O3[21 1 ]//GaAs [2 11], consistent with the observed RHEED patterns, and there exists only one rotational variant. From the diffraction peak positions, we derived that the Y2O3(111) film is compressively strained by 0.6% along the growth direction and tensile strain by 0.9% laterally. The observed strain is much less than the calculated lattice mismatch, indicating a significant lattice relaxation, most probably through the generation of misfit dislocation at the interface.
The intensities were displayed in an arbitrary unit to illustrate the signals from both Y2O3 and GaAs. The intensity of the GaAs{111} reflections of the (111)-oriented GaAs in Figure 4b should not be compared directly with that of the {113} reflections of the (001)-oriented GaAs in Figure 3b. The much more intense GaAs(113) tail was resulted from the relatively weak Y2O3{622} reflections.
The full width at half maximum (FWHM) of the Y2O3(444) rocking curve was 0.026°. The narrow FWHM of the rocking curve indicates the excellent crystallinity of ALD-Y2O3 film on GaAs(111)A, which is better than the ALD-Y2O3 grown on GaAs(001), of which the (440) rocking curve width is 0.033°.
A small lattice mismatch between deposited films and the substrates underneath is usually preferred for the epitaxial growth [3,37]. Numerous examples on the hetero-epitaxial growth with large lattice mismatches have nevertheless been demonstrated. These have led to excellent crystallographic characteristics, interesting scientific discoveries, and very useful important Previous study on MBE-Y 2 O 3 grown on GaN(0001) showed that Y 2 O 3 can exist in hexagonal phase as the film thickness ď3 nm and its crystalline structure resembles that of the cubic phase in many aspects [36]. It is risky to identify the phase by the specular reflections alone and thus essential to examine the positions of the off-normal reflections.
For the Y 2 O 3 film grown on GaAs(001), the azimuthal φ scan across cubic Y 2 O 3 (622) reflection shows four sharp peaks (Figure 3b). The larger angular separation~110˝agreed well with the calculated 109.5˝between the (622) Figure 4b. Three evenly spaced sharp peaks yield the characteristic 3-fold symmetry along cubic [111] axis. The weak broad peaks between the intense peaks originate from the tail of the nearby GaAs{111} reflections. The 60˝offset between the two sets of reflections elucidates the B-type cube-on-cube growth, i.e., Y 2 O 3 [211]//GaAs[211], consistent with the observed RHEED patterns, and there exists only one rotational variant. From the diffraction peak positions, we derived that the Y 2 O 3 (111) film is compressively strained by 0.6% along the growth direction and tensile strain by 0.9% laterally. The observed strain is much less than the calculated lattice mismatch, indicating a significant lattice relaxation, most probably through the generation of misfit dislocation at the interface.
The intensities were displayed in an arbitrary unit to illustrate the signals from both Y 2 O 3 and GaAs. The intensity of the GaAs{111} reflections of the (111)-oriented GaAs in Figure 4b should not be compared directly with that of the {113} reflections of the (001)-oriented GaAs in Figure 3b. The much more intense GaAs(113) tail was resulted from the relatively weak Y 2 O 3 {622} reflections.
The full width at half maximum (FWHM) of the Y 2 O 3 (444) rocking curve was 0.026˝. The narrow FWHM of the rocking curve indicates the excellent crystallinity of ALD-Y 2 O 3 film on GaAs(111)A, which is better than the ALD-Y 2 O 3 grown on GaAs(001), of which the (440) rocking curve width is 0.033˝.
A small lattice mismatch between deposited films and the substrates underneath is usually preferred for the epitaxial growth [3,37]. Numerous examples on the hetero-epitaxial growth with large lattice mismatches have nevertheless been demonstrated. These have led to excellent crystallographic characteristics, interesting scientific discoveries, and very useful important technologies [1,2,[38][39][40]. Here we demonstrate that, even with a large lattice mismatch, high-quality epitaxial Y 2 O 3 films have been grown on GaAs(001) and (111)A by ALD and the hetero-structures exhibit impressive electrical characteristics.
The systematic studies on the electrical performances of the ALD-Y 2 O 3 /p-and n-GaAs(001) are given in a separate publication [41]. Low frequency dispersion from 1 MHz to 100 Hz at accumulation in the CVs has been attained with~7% and~14% for p-and n-type GaAs(001)-4ˆ6, respectively, as shown in Figure 5a,b. These are the record low values among all the ALD-Al 2 O 3 and -HfO 2 on GaAs(001), of which the frequency dispersion at accumulation region of CVs on n-type GaAs(001) is high. For example, the values were reported to be~60%,~40%, and~23% in refs. [17,21,42], respectively. Note that the 23% was attained with frequency measured from 100 kHz to 100 Hz.
The current density-field (JE) characteristics (insets of Figure 5a,b) showed low leakage current densities <10´8 A/cm 2 at˘1 MV/cm for the MOSCAPs of ALD-Al 2 O 3 (4 nm)/Y 2 O 3 /(2.3 nm)/p-and n-GaAs(001); the low leakage allows the reliable quasi-static CV measurements. Low D it values of (1-3)ˆ10 12 cm´2eV´1, extracted from the quasi-static CVs [43,44], are shown in Figure 5c. Moreover, while the GaAs(001) MOSCAPs using other ALD-oxides always showed large D it peak values at the mid-gap [17,20,22], the D it spectrum here showed a flat distribution across whole bandgap. Thus, Fermi level at the Y 2 O 3 /GaAs interface can be moved effectively across the bandgap of GaAs with applying gate voltage, the key to high performance device.  [1,2,[38][39][40]. Here we demonstrate that, even with a large lattice mismatch, high-quality epitaxial Y2O3 films have been grown on GaAs(001) and (111)A by ALD and the hetero-structures exhibit impressive electrical characteristics.
The systematic studies on the electrical performances of the ALD-Y2O3/p-and n-GaAs(001) are given in a separate publication [41]. Low frequency dispersion from 1 MHz to 100 Hz at accumulation in the CVs has been attained with ~7% and ~14% for p-and n-type GaAs(001)-4 × 6, respectively, as shown in Figure 5a,b. These are the record low values among all the ALD-Al2O3 and -HfO2 on GaAs(001), of which the frequency dispersion at accumulation region of CVs on n-type GaAs(001) is high. For example, the values were reported to be ~60%, ~40%, and ~23% in refs. [17,21,42], respectively. Note that the 23% was attained with frequency measured from 100 kHz to 100 Hz.
The current density-field (JE) characteristics (insets of Figure 5a,,b) showed low leakage current densities <10 −8 A/cm 2 at ± 1 MV/cm for the MOSCAPs of ALD-Al2O3 (4 nm)/Y2O3/(2.3 nm)/p-and n-GaAs(001); the low leakage allows the reliable quasi-static CV measurements. Low Dit values of (1-3) × 10 12 cm −2 eV −1 , extracted from the quasi-static CVs [43,44], are shown in Figure 5c. Moreover, while the GaAs(001) MOSCAPs using other ALD-oxides always showed large Dit peak values at the mid-gap [17,20,22], the Dit spectrum here showed a flat distribution across whole bandgap. Thus, Fermi level at the Y2O3/GaAs interface can be moved effectively across the bandgap of GaAs with applying gate voltage, the key to high performance device.

Experimental Section
MBE was employed for the epi-layer growth of GaAs(001) and (111), and ALD for the high κ Y2O3 films. Both the MBE chamber and the ALD reactor are in a multi-chamber ultrahigh vacuum (UHV) growth/analysis system, which also includes an arsenic-free metal/oxide MBE chamber, and two analysis chambers of scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) [42]. The aforementioned chambers are connected via transfer modules, which maintained 10 −10 Torr to ensure maintenance of the pristine sample surfaces free of contamination. After the MBE growth of the GaAs epi-layers 30-50 nm thick with Si and Be as the n-and p-dopants, respectively, the samples were transferred in-situ under UHV to an As-free MBE chamber to attain GaAs(001)-4 × 6 and (111)A-2 × 2 surface reconstructions by annealing the samples to 550 °C; these were monitored by in-situ RHEED and were confirmed by low-energy electron diffraction (LEED) in the photoemission chamber at the nearby National Synchrotron Radiation Research Center (NSRRC)

Experimental Section
MBE was employed for the epi-layer growth of GaAs(001) and (111), and ALD for the high κ Y 2 O 3 films. Both the MBE chamber and the ALD reactor are in a multi-chamber ultrahigh vacuum (UHV) growth/analysis system, which also includes an arsenic-free metal/oxide MBE chamber, and two analysis chambers of scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) [42]. The aforementioned chambers are connected via transfer modules, which maintained 10´1 0 Torr to ensure maintenance of the pristine sample surfaces free of contamination.