Ultrafast carrier thermalization and trapping in silicon-germanium alloy probed by extreme ultraviolet transient absorption spectroscopy

Semiconductor alloys containing silicon and germanium are of growing importance for compact and highly efficient photonic devices due to their favorable properties for direct integration into silicon platforms and wide tunability of optical parameters. Here, we report the simultaneous direct and energy-resolved probing of ultrafast electron and hole dynamics in a silicon-germanium alloy with the stoichiometry Si0.25Ge0.75 by extreme ultraviolet transient absorption spectroscopy. Probing the photoinduced dynamics of charge carriers at the germanium M4,5-edge (∼30 eV) allows the germanium atoms to be used as reporter atoms for carrier dynamics in the alloy. The photoexcitation of electrons across the direct and indirect band gap into conduction band (CB) valleys and their subsequent hot carrier relaxation are observed and compared to pure germanium, where the Ge direct (ΔEgap,Ge,direct=0.8 eV) and Si0.25Ge0.75 indirect gaps (ΔEgap,Si0.25Ge0.75,indirect=0.95 eV) are comparable in energy. In the alloy, comparable carrier lifetimes are observed for the X, L, and Γ valleys in the conduction band. A midgap feature associated with electrons accumulating in trap states near the CB edge following intraband thermalization is observed in the Si0.25Ge0.75 alloy. The successful implementation of the reporter atom concept for capturing the dynamics of the electronic bands by site-specific probing in solids opens a route to study carrier dynamics in more complex materials with femtosecond and sub-femtosecond temporal resolution.

gas cell filled with 28 Torr of xenon. Polarization-assisted amplitude gating (PASSAGE) 22 allows for optimizing the XUV continuum [see pulse characterization in supplementary material, Sec. S3, the $4 fs pulse spectrum and phase that generate HHG is shown in Fig. S3(a) and the corresponding time domain in Fig. S3(c)]. A small fraction of the VIS-NIR pulse is split off as a pump pulse, time delayed, and focused collinearly with the XUV probe pulse onto the sample. The pump pulse duration is measured to 5.4 fs [spectrum with spectral phase and time domain characterization is shown in Figs. S3(b) and S3(c), respectively]. An aluminum filter removes the pump light, and the transmitted XUV light is spectrally analyzed by a flat-field spectrometer comprising a grating and X-ray CCD camera. A shutter periodically blocks the pump arm in order to acquire transient absorption spectra DA meas E; s ð Þ ¼ A p E; s ð ÞÀ A E ð Þ, which are defined as the difference between the absorbance of the excited (pump on) absorbance A p and the static absorbance A (pump off) for different time delays s. The experiment is performed at a repetition rate of 100 Hz in order to reduce heating of the sample. The integration time per spectrum was set to 1 s, i.e., one hundred laser pulses, in order to exploit the full dynamic range of the detector. The thin-film samples are raster-scanned on a grid spaced by 150 lm over an area of $2.5 Â 2.5 mm 2 . The positions within the grid are randomized over the time delay scan. Pump-on and pump-off spectra are separately measured on each sample at each time delay s. In addition to the Si-like silicon-germanium thin film and the nanocrystalline germanium thin film, both of which are 100 nm thick, a gas cell filled with argon is periodically moved into the beam. A full sequence, i.e., taking six spectra (pump on/off on three samples) and translating samples into the beam, takes about 8 s. Measurements of germanium and silicon-germanium alloy were performed at the same time for consistency in comparing the ultrafast photoresponse of the two materials. At each time delay, XUV absorption spectra of germanium and silicon-germanium alloy with and without 800 nm photoexcitation are taken in series and this procedure is subsequently performed for all time steps. The noise level on the raw experimental data for both samples is $DA ¼ 5 Â 10 À3 . Line shape changes in the argon 3s3p 6 4p autoionizing state 23 are used for in situ correction of time-delay drifts in the system and to calibrate time zero (see also supplementary material, Sec. S3). The VIS-NIR pump pulse has an energy of 2.9 lJ resulting in an intensity of $2 Â 10 11 W=cm 2 inside the semiconductor samples (note for the refractive index n Ge % n Si 0:25 Ge 0:75 % 4:560:2 around 800 nm, Refs. 24 and 25) generating carrier densities of N e;Ge ffi 8 Â 10 20 1 cm 3 and N e;SiGe ffi 5 Â 10 20 1 cm 3 in germanium and silicon-germanium, respectively. Transient absorption data of both germanium and silicon germanium thin films shown here consist of five averages and the time delay s was scanned with 0.6 fs time steps around time zero and with 3.3 fs time steps out to 1.5 ps. An upper bound for the instrumental response function was determined to be $6 fs by transient absorption measurements in argon. An in-depth description of the apparatus and more technical details are reported elsewhere. 13 A detailed characterization of the optical setup can be found in supplementary material, Sec. S3. A detailed report on XUV transient absorption measurements in germanium can be found elsewhere; 13 here, we refer to a specific set of measurements that were performed at the same time on both materials measuring both at each time delay step, i.e., in a non-sequential manner.
The electronic band structure (BS) of Si 0.25 Ge 0.75 and the pump-probe scheme are depicted in Fig. 1(b) schematically [(detailed band structure in supplementary material, Fig. S2(a)]. The band structure is obtained from density functional theory (DFT) 26 pseudopotential calculations carried out within the Virtual Crystal Approximation (VCA) 27 using the QUANTUMESPRESSO code. 28 Accordingly, the Si 0.25 Ge 0.75 random alloy assuming diamond crystal structure is described by a single effective pseudopotential built from a linear combination of normconserving Ge and Si pseudopotentials 29 including only the outermost occupied s and p shells. A plane wave kinetic energy cutoff of 100 Ry is employed and Brillouin zone integration is carried out over an 8 Â 8 Â 8 k-point grid. Exchange-correlation effects are treated at the level of the local-density approximation (LDA). 30 Fig. S2(a), for details] are consistent with electroreflectance measurements. 32 For the pump VIS-NIR pulse, which is centered around 1.65 eV photon energy (see the spectrum in supplementary material, Fig. S4), direct transitions into the C valley and indirect transitions into the X, L, and K valleys are possible. In Sec. III, we will show that carriers below the direct band gap are observed in the XUV transient absorption at zero time delay, indicating that a significant portion of carriers are photoexcited into the X and L valleys through phonon-mediated indirect transitions. This is in contrast to the germanium measurement where the photon energies of the pump pulse are completely above the optical gap (0.8 eV) such that the carriers are excited predominantly through direct transitions in to the C valley. 13 Employing the Ge atoms as reporters for the transient state in the alloy, the broadband XUV pulse [violet arrows in Fig. 1(b)] probes the transient states in the VB and CB at the Ge M 4,5 -edge around 30 eV [ Fig. 1(c)], which corresponds to excitation from the spin-orbit split 3d 3/2 and 3d 5/2 core electronic states. Transitions are possible to VB and CB states that are of 4p orbital character. First principles calculations using density functional theory (DFT) reveal that the VB is almost entirely of 4p orbital character and the CB is approximately 50% 4p character in the density of states (DOS) (Fig. 2). The total and partial DOS (PDOS) of Si 0.25 Ge 0.75 alloy were calculated by a DFT simulation of an ordered 4 atom supercell containing 3 Ge and 1 Si atoms described by norm-conserving pseudopotentials. The supercell was constructed by doubling the fcc primitive cell of Ge along the first lattice vector direction and substituting one Ge atom with Si. In contrast to the VCA simulation employed earlier for the band-structure calculation, such supercell simulations allow for the Ge 4p and 4s partial DOS to be estimated independently of Si-derived PDOS contributions while neglecting the effects of random disorder. The latter nevertheless yields an overall density of states around the bandgap very similar to the VCA simulation that approximates random disorder [see supplementary material, Fig.  2(b)]. The QUANTUMESPRESSO code was used to simulate the PDOS employing the same numerical parameters as before. A C-centered 12 Â 24 Â 24 k-point grid was used for the DOS calculation shown in Fig. 2.
The 100-nm-thick silicon-germanium alloy sample was fabricated by LPCVD 33 at 410 C on a 30 nm thick silicon nitride membrane. For 20 min, a silicon nucleation layer (100 sccm Si 2 H 6 , 300 mTorr pressure) was deposited, followed by flowing 160 sccm of SiH 4 and 40 sccm of GeH 4 to deposit a silicon-germanium alloy. XRD analysis confirmed the nanocrystalline structure of the sample (see supplementary materials, Fig. S5). Measuring the Raman spectra of the silicon-germanium film [ Fig. 1(d), solid blue line] reveals three characteristic Raman peaks corresponding to the optical vibrations of the Si-Si, Si-Ge and Ge-Ge bonds in the alloy. The molar fraction of germanium can be determined by analyzing the relative wavenumbers. Using the equations for the three peak wavenumbers given in Ref. 21, the spectrum suggests x ¼ 0:75 þ0:04 À0:03 for the sample used here. Further, the Raman spectra confirm proper alloying in the LPCVD deposition, since there is no evidence for a Si or Ge transverse optical (TO) phonon mode at the expected values of 521 cm À1 and 302 cm À1 , respectively. As expected, only the strong 302 cm À1 Ge TO phonon mode can be observed in the pure germanium sample [gray dashed-dotted line in Fig. 1(d)]. The determined molar mass fraction of x ¼ 0:75 suggests a Silike silicon-germanium alloy, 19 which is also qualitatively confirmed by the observed reduced FIG. 2. Density of states in Si 0.25 Ge 0.75 calculated using density functional theory (DFT). Bands with 4p orbital character can be probed via transitions from the 3d core-levels. Similar to pure germanium (Ref. 13), the valence band is almost primarily of 4p orbital character, while the conduction band is a mix of 4s and 4p orbital character. Midgap features are not included in this calculation, since a crystalline super-cell containing 75% Ge atoms and 25% Si atoms was assumed and defects causing trap states were not considered. optical absorption below 1.7 eV, i.e., less efficient indirect phonon-assisted excitation, in contrast to direct-gap germanium (supplementary material, Fig. S4). In addition, silicon-germanium alloy fabricated by LPCVD is known to exhibit a large number of point defects due to lattice size mismatch between silicon and germanium. Point defects result in dangling bonds and possible hydrogen contamination due to the fabrication process. However, the absence of distinct vibrational peaks from Si-H and Ge-H bonds 34 in the Raman measurement [640 cm À1 and 565 cm À1 , cf. Fig. 1(d)] indicates that the sample used has negligible hydrogen content. The presence of point defects in the sample also implies the presence of midgap states from localized defects and increased probability for carrier trapping.
After capturing the transient absorption spectra DA meas E; s ð Þ, the data are processed as outlined in Ref. 13. Briefly, using a measured static absorbance [ Fig. 1(c)], the measured transient absorption spectra can be decomposed into three components: state blocking, band shifts, and broadening (see supplementary material, Fig. S1). State blocking occurs when photoexcited electrons block otherwise possible XUV transitions in the CB, thus decreasing the absorption. Likewise, holes that are created in the VB by photoexcitation open up XUV transitions and increase the absorption. Band shifts can, for instance, be caused by band gap renormalization due to carrier-carrier screening in the VB and CB, 35 by phonon renormalization, or by corelevel shifts due to altered screening of the valence potential. 36,37 These shifts of the excited state spectrum produce broad features in the transient absorption spectra, whose shape and magnitude mainly depend on the shape and steepness of the edge structure. Broadening of the excited state spectra 38 can be induced by lifetime changes of the valence and/or core-level states following photoexcitation. The individual contributions can be retrieved from the transient absorption spectra by an iterative procedure outlined in-depth in Ref. 13. Furthermore, since the germanium M 4,5 -edge has contributions by two 3d core-level states that exhibit a spin-orbit splitting (DE so ¼ 0:58 eV, Ref. 39) comparable to the band gap in the materials considered here (DE gap;Ge;direct ¼ 0:8 eV, DE gap;Si 0:25 Ge 0:75 indirect ¼ 0:95 eV), it is necessary to separate these contributions, which is done by a Fourier method. 13 In the following, we restrict ourselves to describing the energy and time-dependent state blocking of the contribution due to the 3d 5/2 core-level DA SB;3d 5=2 E; s ð Þ, obtaining a direct representation of the dynamics of electrons and holes versus energy and time delay. Here referred to as carrier dynamics, in Si-like silicon-germanium alloy and monatomic germanium for comparison.

III. RESULTS AND DISCUSSION
Here, high harmonic XUV light is established as a probe of electron dynamics through core-level transitions in semiconductors as a function of time. In Fig. 3, the carrier dynamics of germanium 13 and silicon-germanium versus time delay s are depicted. A positive time delay represents the NIR-VIS pump pulse arriving first and the broadband XUV pulse arriving later to probe the transient states. According to Ref. 13, the CB and VB in germanium are mapped to energies greater than 29.6 eV and less than 28.9 eV in the XUV, respectively [ Fig. 3(a)]. The general similarities between the transients of germanium and silicon-germanium [ Fig. 3(b)] strongly corroborate that the reporter atom concept 20 is successfully applied here to solids and that germanium atoms can be employed to probe the carrier dynamics of electrons and holes in the Si-like indirect gap alloy. Other recent findings by Santomauro et al. 40 suggest that this is expected to be true for carriers that are either localized near the germanium atoms or delocalized in the alloy.
In nanocrystalline germanium [ Fig. 3(a)], the carriers appear to decay symmetrically, which was described by a fast trap-assisted recombination [41][42][43][44] in which the lifetimes of the carriers associated with the trap are short (time constant of $1.1 ps). 13 In contrast, there is an asymmetry in silicon-germanium [ Fig. 3(b)], i.e., the electrons decay faster than the holes, whereas the hot holes relax up towards the VB edge during the first $400 fs and only slowly decay thereafter. The second apparent difference is that a signal with negative sign (less absorption) starts growing in the midgap of the alloy after $400 fs, continuously increasing towards the largest measured time delay of 1.5 ps [indicated by the black arrow in Fig. 3(b)].
It is instructive to compare the initial carrier distributions in the two materials, i.e., before relaxation processes set in. In Fig. 4, an energy slice of the data in Fig. 3 averaged from þ8 to 12 fs is shown for both materials, i.e., immediately following photoexcitation. With the valence band maximum (C 25' critical point) at approximately 28.9 eV (Ref. 13), the conduction band minimum at the C point of the silicon-germanium alloy appears at 30.6 eV, indicating that carriers excited over the direct gap should appear at energies above 30.6 eV. However, the initial carrier distribution in the conduction band spans from 31.5 eV down to about 29.6 eV, and the largest absolute value of the transient absorption signal in the CB appears at approximately 30 eV (Fig. 4). This implies that a significant portion of carriers is directly excited into the X and L valley by an indirect phonon-assisted process. For energies between 27 eV to 29 eV,  4. Comparison of the state blocking following photoexcitation after $10 fs in germanium and silicon-germanium alloy. A time slice of the state blocking averaged from s ¼ 8,…,12 fs is shown, which precedes the onset of carrier relaxation. Although the valence band (VB) distribution features a similar shape, the conduction band (CB) onset in silicongermanium is shifted to higher energies indicating a larger band gap. At the zero-crossing, an increase of the band gap by 0.17 eV compared to germanium can be measured. The letters designate valley assignments for germanium (blue letters) and silicon-germanium (red letters, cf. Fig. S5). The letters L, C, X, and K designate the conduction band minima in the respective valleys calculated for x ¼ 0:75 in relation to the valence band maximum at C 25' . corresponding to the VB, both systems have comparable spectral distributions. On the CB side, the onset of electron features is blue-shifted by DE ¼ 0:17 eV in silicon-germanium compared to germanium (Fig. 4) due to the band energies of silicon and germanium, which increases the band gap compared to pure germanium. For an alloy with x ¼ 0:75, the indirect band gap is given by the energy difference between the X valley in the CB and the C valley in the VB 19 [cf. Fig. 1(b)]. The indirect band gap DE CÀX (excitation from C point VB valley to the CB X valley) for x ¼ 0:75 is 0:95 eV (Refs. 19 and 32). With the direct band gap in germanium being DE Gap;Ge ¼ 0:8 eV, the measured blue shift of the CB can be explained by the band alignment in the alloy. Note that the XUV transient absorption measurements show that the increased band gap is solely due to blue-shifting of the CB edge, whereas the energy of the valence band remains as in pure germanium in relation to the Ge 3d core-levels within the instrumental resolution accessible here (dE inst % 0:07 eV).
Comparing the critical points or valleys in the CB (Fig. 4, letters in blue for germanium and red for silicon-germanium), one finds that in the Si-like alloy the X critical point is redshifted, whereas the L and C critical points are blueshifted compared to germanium. The amount of blueshift of the C point is approximately 0.9 eV for x ¼ 0:75, which aligns the C critical point closely with the K critical point [cf. supplementary material, Fig. S2(a)]. Since for the same photon energy excitations into the K valley constitute an indirect transition, it is expected that the majority of the excitation that reaches the C/K critical points will excite into the C valley. The L and X critical points are energetically too close to be separated in the experiment, given that the core-hole life time s 3d of the Ge atoms limits the achievable energy resolution to dE Ge; s 3d % 0:24 eV (Ref. 39). In the silicon-germanium experiment (red shaded area in Fig. 4), the main excitation in the initial carrier distribution aligns with the X and L valleys, but a second weaker maximum can be observed at about $0.8 eV higher photon energy, signifying excitations of carriers excited to the C valley. Although XUV transient absorption on randomly oriented crystallites with linearly polarized pump and probe in the same polarization state does not resolve electron and crystal momenta, the X/L versus C/K valleys appear as discernible features due to the increased densities of states at the energies corresponding to these valleys.
In order to analyze the electron kinetics in the CB of silicon-germanium [ Fig. 5(a)], single exponentials are fit to each slice along the time delay axis with the initial amplitude at s ¼ 0 fs and a decay constant. Representing the fit result as a 2D map of energy versus time delay [Fig. 5(b)] shows good agreement with the data. The main feature around 29.9 eV has been assigned to the X and L valleys (cf. Fig. 4). The initial amplitudes [green line with shaded error bar in Fig. 5(c)] resemble the initial electron distribution, which compares to the CB signal in Fig. 4. It becomes apparent that the center of mass of the initial amplitude distribution lies above the CB valleys, indicating hot electrons with excess energy. At the same time, the short lifetimes of less than 400 fs (blue line with shaded error bars) at energies higher in the X/L bands between 30 and 30.2 eV indicate a fast relaxation of these hot electrons towards the X and L valley, where the lifetime continually increases, indicating that the carriers accumulate in the valleys. The process of hot electron relaxation towards a valley, i.e., intravalley scattering, is mainly mediated by carriers scattering with acoustic phonons, although optical phonons can be involved depending on the excited valley. Carriers in the L valley will ultimately scatter to the X valley and accumulate in the lower lying X valley. This relaxation pathway is more likely due to the electrons reducing their energy further by scattering into the X valley. For a recombination with holes from the VB, a symmetric decay of carrier signals would be expected [compare to germanium in Fig. 3(a)]. The decay, i.e., carrier removal, that takes place on a time scale of $900 fs in the X valley [see the CB edge region around 29.7 eV in Figs. 5(a)-5(c)], can be understood as either CB electrons scattering into trap states or electrons scattering into the VB by carrier recombination. Both processes can be mediated by carrier-phonon scattering, carrier-carrier scattering, or Auger processes. 45 At around 30.8 eV, a weaker feature with a less pronounced but similar shape is observed and attributed to electrons initially excited predominantly near the C point. Here, a comparable lifetime on the order of 0.9 ps is measured, which suggests that the phonon-assisted scattering from the C valley into the X or L valley, to further relax the electrons, is inefficient compared to what is observed in the germanium nanocrystal samples, where the lifetimes of higher lying CB valleys were significantly shorter than the lifetimes in the lower lying valleys. 13 These differences, however, may be due to the crystalline morphology and numbers of trap states in the two different samples. The relatively weak and sharp features at 30.6 and 30.5 eV [see Figs. 5(a) and 5(c)] could potentially relate to a weak excitation into the K valley, but the energetic proximity to the C critical point within the spectral resolution given by the natural lifetime of the Ge 3d core-level and the signal-to-noise ratio prohibit detailed analysis. Figure 5(d) shows energy slices for germanium and the Si 0.25 Ge 0.75 alloy at a longer time delay (s ¼ 1.4 ps). The weak midgap feature becomes clearly visible in the alloy at energies between 29.1 and 29.5 eV (black line). In germanium, the signal is zero within the error bars (dotted blue line), which can also be qualitatively assessed in Fig. 3(a). Comparing to the early signal (dashed red line) after excitation, one finds a shallow signal around zero absorbance in the respective midgap region (29.1-29.5 eV), as is expected for a band gap. This suggests that the midgap feature is due to trap states tentatively above the Fermi energy, which are not directly accessible by the VIS-NIR laser. These states can be populated after hot electrons relax to the CB edge from where the electrons subsequently scatter into these states. X-ray photoelectron spectroscopy studies on silicon-germanium alloys suggested the existence of these states mainly being localized on the germanium atoms and exhibiting p orbital character, 46 which together corroborates observability in the XUV transient absorption experiment.  Fig. 5(e). The signal associated with the electrons at the CB edge (X/L valleys) decays with a time constant of $0.9 ps in silicon-germanium and follows a single exponential decay [the green dashed line in Fig. 5(e)] as discussed in detail in the previous paragraphs. At the same time, although the CB signal decays, a signal grows in the midgap [black line in Fig. 5(e)]. The midgap feature is found to have a negative sign [cf. Fig. 5(d)], which is indicative of electrons contributing to the state blocking. Qualitatively the sign and relative energy of the midgap feature corroborates the assignment of the midgap states to trap states near the CB edge 47 into which the electrons relax and accumulate. This observation suggests that electrons scatter from the bottom of the CB into nearby trap states where a significant population of electrons builds up, so that population can be observed in the XUV. This buildup further suggests that carrier recombination with holes takes place on a time scale longer than measured here. This is supported by the hot holes in the VB relaxing towards the VB edge during the first $400 fs following photoexcitation [see the blueshift of the VB feature in Fig. 2(b)], but in general only exhibiting a slower decaying signal [red line in Fig. 5(b)] compared to the electrons at the CB edge. Combined, this supports a picture where after $1.5 ps, a large number of electrons is trapped in states near the CB edge at the X point, whereas the holes accumulate at the VB edge near the C point. The carrier recombination across the indirect band gap subsequently requires carrier-phonon scattering to overcome the momentum difference, which in principle would render the recombination process, which is similar to the phonon-assisted excitation process, less efficient compared to materials that have a direct band gap.

IV. CONCLUSION
In this work, XUV transient absorption measurements allow for probing of electron dynamics in a silicon-germanium alloy. The ability to resolve lifetimes of different valleys in the CB after excitation across the direct and indirect band gap simultaneously with hole dynamics adds to capabilities that optical pump-XUV probe techniques offer. In the present experiments, the germanium atoms serve as reporter atoms for the alloy. This method, employing XUV absorption from core-levels, allows direct access to the dynamics of both carrier species that are relevant for the electronic properties of a semiconductor with ultrafast temporal resolution. Silicongermanium with a germanium content of 75% as used in this work is an indirect band gap material and the CB has silicon-like properties. It is found that the CB in the alloy exhibits a static blueshift compared to monatomic germanium thus effectively increasing the band gap, while the alignment of the VB with respect to the 3d core-level is the same as in pure germanium. The large spacing of the X/L versus C CB valleys in the alloy enables valley-sensitive lifetime measurements between carriers excited across the direct versus indirect gap. The comparable lifetime of carriers in the X/L versus C valleys suggests a small scattering cross section for the higher lying C valley towards the X valley, in contrast to germanium where previously it was observed that decreasing lifetimes occur for increased energy above the lowest band edge. A specific feature observed in the alloy is a midgap feature that is assigned to trap states near the CB minimum into which electrons accumulate following intravalley relaxation. The indirect gap in the silicon-like alloy appears to reduce the cross section for recombination with the holes that accumulate at the top of the VB, causing an asymmetry in the carrier decay between VB and CB. A more quantitative analysis would require knowledge of the k-dependent transition dipoles for the XUV and knowledge about density and localization of these trap states.
A next step for employing XUV transient absorption spectroscopy for site-specific study of silicon-germanium alloys can involve measuring the dynamics at the silicon L 2,3 -edge which allows access to other CB and possibly midgap features in addition to the germanium M 4,5 -edge features observed here. This could reveal further insight into the localization and kinetics of the trap states. The direct and valley-resolved access into the VB and CB with, in principle, subfemtosecond temporal resolution renders germanium an ideal component in alloys for studying carrier dynamics using XUV transient absorption spectroscopy. Further, the measurements can be employed along with first principles calculations to improve the understanding of photoexcitation in indirect band gap materials. 48 The findings presented here hold great promise for studying carrier dynamics in ternary and quaternary semiconductor alloys that include fractions of germanium, which are becoming increasingly important for highly integrated and highly efficient photonics devices. 16,49,50 SUPPLEMENTARY MATERIAL See supplementary material for the result of decomposing the transient absorption data into components of electronic state blocking, XUV absorption edge shifts and broadening; a detailed electronic band structure; a characterization for both 800 nm pulses for high harmonic generation and photoexcitation, and a characterization of the samples.