5 Fig. 1 Schematic drawing of the apparatus.

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2 RHEED Hydrogen doser Transfer Rod Gate Valve Sample QMS Ag cell Screen Pd cell Gate Valve Loading Chamber Transfer Rod 5 Fig. Schematic drawing of the apparatus.

3 .5 = nm Pd Ag Normalized d - = nm d Si Pd/Ag/Si TEM nm (a) (b) 3 [nm] Ag() RHEED ( )Pd nm RHEED ()(), ()( ) (c) (d) height [nm] -5 - θpd= ML θpd=5 ML θpd= ML -5 distance [nm] RMS [nm] (e) 6 8 [ML] Ag (a), Pd 5 (b), (c)(d): (a)-(c) RMS

4 3 8 L 3 α-h absorption [ML] α-h absorption [ML] H exposure [ L] L 5 L 675 L 5 L 8 L 5 Figure 3: α-h TD spectra at Pd( ML)/Ag(5 ML)/Si() surfaces. The H exposure corresponds to 675 L (triangle), 5 L (square), and 8 L (circle). The H exposure [ L] H dependence of the integrated α-h desorption amount is shown in the inset. The line is for eyes. as shown in Figs.. The AFM images of the Pd() surface at θ Pd =, 5 and ML, are shown in Fig. (a) to (c), respectively. The images size is.5.5 µm. The typical cross sections at each image 5 (indicated as lines) are shown in Fig. (d). The Ag() growth on the Si() surface has been established as Stranski-Krastanov mode [, ], and the three TDS dimensionalpd Ag islands formed with the atomically flat () terrace (Fig. (a)). The Pd() film was epitaxially grown at each Ag island, and it grew as layer-by-layer fashion at < ML (not shown). The roughness at the island surface became worse some extent as θ Pd increased (ex. typical surface height difference at the each island corresponds to θ Pd 7 ML at = ML). The surface roughness root mean square (RMS) values at each are plotted in Fig. (e). Though the surface became rough as the increased, H uptake is known to proceed mainly from the Pd() terrace site []. At different orientation of (), no H uptake from the terrace has been reported []. Therefore, we presumed that the H uptake from the Pd() terrace is the dominant absorption path in our TD measurement. 3.. H-TD spectra at Pd()/Ag()/Si() surface H-TD spectra observed at = ML at various H exposures are shown in Fig. 3. A sharp desorption signal was observed at 8 K whose peak temperature was consistent with that of the previously reported α signal []. Small β signals originate from the topmost layer were also observed around 35 K (not shown). In our highly H exposure conditions, the sur- α Pd Pd % 3 β signal β signal Figure : α-h TD spectra at the various θ Pd. The exposed H was fixed as 5 L. The Inset shows the β-h signals observed around 35 K. In comparison with TDS Pd the α-h signal, no dependence was observed in β-h signal. L α signal 5 ML 56 ML ML ML 7 ML Therefore, the α signal at 8 K was the dominant desorption signal at T e K. The α-h absorption amount estimated from the integration of the α-td spectrum is plotted in the inset of the Fig. 3. Initially, the α-h absorption rapidly increases with the exposure. The exposure dependence curve exhibits a saturation trend around the α-h absorption of 8 ML. In the previous Figure : α-h report, TD spectra α signal at theshowed various θnon-saturating Pd. The exposed Hfeature was fixed against as 5 the L. The H Inset exposure shows the from β-h signals the bulk observed Pd() around []. 35The K. Inobserved comparisonsat- uration the α-h signal, trend Pd no inθ our Pd dependence experiment wasreflects observedthe β-h limitation signal. of the Pd with RHEED thickness. In an assumption that H was absorbed in the Pd film % uniformly, the averaged H concentration at the largest H exposure in the Pd film was about %. The observed α-h concentra- Pd Therefore, the α signal at 8 K was the dominant desorption tion signal of at% Tis e close tok. thethe previously α-h absorption reported α-h amount concentration estimated in from - the 3 ML integration 6-8 the deep of % region the α-td of.6 spectrum % by NRA is plotted measurement in the inset of the Fig. 3. Initially, the α-h absorption rapidly increases []. Pd with TD the spectra exposure. observed The at exposure various θ dependence curve exhibits a Pd with H exposure of 5 saturation trend around the α-h absorption of 8 ML. In the L are shown in Fig.. α signals continuously increased with θprevious report, α signal showed non-saturating feature against Pd. In contrast, the β signals at 35 K did not show any clear the H θ exposure from the bulk Pd() []. The observed saturation for trend theintopmost our experiment surface saturation reflects the (inset limitation in Fig. of ). the The Pd Pd dependence, since the H exposure of 5 L was high enough θthickness. In an assumption that HPd/Ag was absorbed the Pd film Pd independent β signal implies that the influence of the Pd surface uniformly, roughness the averaged is not Htheconcentration essential in our at the measurement, largest H exposureincrease the Pd offilm α-hwas is due about to and Pd the %.. The observed α-h concentration θ of % is close to the previously reported α-h concentration Pd dependences of the integrated α-h signals in Fig. is shown in - 3 in Fig. ML 5. theifdeep α-hregion is formed of.6 only% atbythenra nearmeasurement surface limited []. region, H absorption amount has to show independent feature TD spectra above aobserved certain at. various However, α-h withabsorption H exposure continued of 5 to L increase are shown upintofig. =. 5 α signals ML. The continuously result indicates increased thatwith the α-h. formation In contrast, extends the β signals to the deep at 35 bulk Kregion did not beyond show any 3 clear ML

5 Pd Pd 5. S. Murakawa, A. Yamaguchi, M. Wasai, Y. Aoki, H. Ishimoto, R. Nomura, Y. Okuda, Y. Nagato, S. Higashitani, and K. Nagai, Spin-Dependent Acoustic Response in the Nonunitary A and A Phases of Superfluid 3 He under High Magnetic Fields, Phys. Rev. Lett.,, (5).. J. Sone, T. Ymagami, Y. Aoki, K. Nakatsuji, H. Hirayama, "Epitaxial growth of silicene on ultra-thin Ag() films", New Journal of Physics, 6, 95 (). 3. Y. Aoki, I. Iwasa, T. Miura, D. Takahashi, A. Yamaguchi, S. Murakawa, and Y. Okuda, Resonant Frequency Change of Torsional Oscillator Induced by Solid He in Torsion Rod, J. Phys. Soc. Jpn. 83, ().. Y. Yoshiike, H. Fukumoto, I. Kokubo, Y. Aoki, K. Nakatsuji, H. Hirayama, Regular ripples at the surfaces of heteroepitaxially grown Ag() ultra-thin films on Si() 3x 3-B substrates, Appl. Phys. Lett.,, 965 (). 5. Y. Aoki, Hydrogen absorption on thickness and lattice spacing controlled palladium film by thermal desorption spectrum, J. Jpn. Soc. Colour Mater. 86, 6-66 (3). Y. Aoki, I. Iwasa, T. Miura, A. Yamaguchi, S. Murawaka, and Y. Okuda, Frequency change of Torsional oscillator induced by solid He in torsion rod QFS3, Matsue, Japan, Aug. -6 (3). Y. Aoki, S. Nakajima, K. Nakatsuji, and H. Hirayama, H absorption depth profiling measurement at ultra-thin Pd() film by thermal desorption spectroscopy, AVS 59 th International Symposium & Exhibition, Tampa, Florida, USA, Oct. 8-Nov. (). TEMRHEEDPd/Ag() ( ) Ag/Si()Pd 3 ( ) () AOKI, Yuki Y. Aoki, K. Nakatsuji, H. Hirayama, H adsorption and absorption at epitaxially grown Pd film on Ag/Si(), ACSIN- & ICSPM, Tsukuba, Japan Nov. -8 (3).