Characterization of free standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy

Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS), and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with angstrom precision. We determined that: i) the exposure to air induced the formation of an InAsO$_4$ layer on top of the stoichiometric InAs(QM); ii) the top interface between the air-side InAsO$_4$ and the InAs(QM) is not sharp, indicating that interdiffusion occurs between these two layers; iii) the bottom interface between the InAs(QM) and the native oxide SiO$_2$ on top of the (Si/Mo) substrate is abrupt. In addition, the valence band offset (VBO) between the InAs(QM) and the SiO$_2$/(Si/Mo) substrate was determined by HXPS. The value of $VBO = 0.2 \pm 0.04$ eV is in good agreement with literature results obtained by electrical characterization, giving a clear indication of the formation of a well-defined and abrupt InAs/SiO$_2$ heterojunction. We have demonstrated that HXPS and SW-HXPS are non-destructive, powerful methods for characterizing interfaces and for providing chemical depth profiles of nanostructures, quantum membranes, and 2D layered materials.

III-V compound semiconductors possess superb carrier transport and excellent optoelectronic properties, which render them widely used in high performance electronic and optoelectronic devices, such as high electron mobility transistors, heterostructure lasers and solar cells. 1,2,3 These III-V "alternative" semiconductors, such as InAs and InGaSb, have much higher electron/hole mobility than Si and are good candidates to replace Si as future channel materials in metal-oxide-semiconductor field-effect transistors (MOSFETs). 4,5,6,7 However, the growth of these semiconductors and their integration with the low-cost processing of Si technology present challenges, since high defect densities and junction leakage currents may occur at the interface between these compounds and the Si substrate 8 due the their lattice mismatch. In order to overcome the problems related to the growth of these semiconductors on Si or SiO 2 substrates and to their integration with the more mature Si-based processes, Javey et al. 9 developed a method for transferring ultrathin free-standing crystalline III-Vs films on a user-defined substrate. This transfer step led to high-performance III-V complementary metal-oxide-semiconductor (CMOS) and radio frequency (RF) circuits on both Si and plastic substrates. 10 However, the non-destructive characterization of these transferred films is still a challenge. In particular, their depth-resolved chemical composition and the intermixing/oxidation occurring at the interfaces have not been fully determined.
Low-and high-resolution transmission electron microscopy (TEM) characterization on such membranes has been performed by Javey et al.. 11 However, it is well known that the sample preparation for TEM analysis can induce damages to the material and can create interdiffusion at the interfaces. 12 Therefore, TEM has limitations as a technique for a nondestructive, quantitative characterization of the interfaces of quantum membranes. On the other hand, the characterization of these interfaces is fundamental for high performance nanoscale transistors. It is known that oxide layers on the surface 13 , or at the interface, strongly influence the electronic transport properties of these materials, especially in the case of nanowires and quantum membranes. For instance, the native oxide layer could shift the position of the surface Fermi-level or even induce Fermi-level pinning, which significantly degrades device performances. 14,15,11 In this paper, we show that hard x-ray photoemission spectroscopy (HXPS) and Standing Wave HXPS (SW-HXPS) can be used to characterize bulks and buried layers/buried interfaces without altering the samples. Standard XPS analysis provides information on the chemical and electronic states of each element at the surface. Moving to hard x-ray photon energies (2-15 keV), and thus to larger electron inelastic mean-free paths (IMFPs), allows one to obtain information on chemical and electronic states of the bulk and of the buried interfaces. 16 Among the many III-V alternative semiconductors, we chose to study free-standing InAs quantum membranes (InAs(QM)) which, in addition to their interesting properties, represent a more general class of 2-D materials. For instance, it has been reported 9,11 that when the thickness of InAs is reduced, especially when it is below its exciton Bohr radius (34 nm), strong quantum confinement effects start to emerge and its band structure can be precisely tuned from bulk to 2D by changing the thickness. In  (dimensions not to scale). Each nanoribbon is 15 nm thick and 300 nm wide and the distance between one ribbon and its neighbors is 300 nm. The multilayer period is 3.3 nm.
In order to perform the SW-HXPS experiment, free-standing crystalline nanoribbons of InAs(QM) were transferred onto a (Si/Mo)x80 periodic multilayer mirror with the bilayer period of d ML = 3.3 nm (see Figure 1), which generates a standing wave by reflecting the xrays at the incidence angle defined by the first-order Bragg reflection 17 where λ x is the incident photon wavelength, d ML is the period of the multilayer mirror, and θ inc is the incidence angle. The wavelength of the standing wave so generated is and matches the period of the multilayer mirror. For a given photon energy, by varying the angle inc θ around the Bragg reflection, the phase of the standing wave varies over π. As the antinodes of the electromagnetic field shift vertically through the sample, they highlight different depths in the sample. This provides depth selectivity to the photoemission process.
In our case, the standing wave, which travels perpendicularly to the multilayer and to the sample surfaces deposited on it, allows us to obtain a quantitative chemical depth profile of the InAs(QM) and of its interfaces with vacuum and with the SiO 2 /mirror substrate. The vertical resolution is approximately 1/10 of the SW period, which is ≈ 0.3 nm for the mirror used in this study. 18,19 The InAs(QM) in the shape of nanoribbons was epitaxially grown on the [111] plane. Each nanoribbon is 15 nm thick and 300 nm wide and the distance between one ribbon and its neighbors is 300 nm ( Figure 1). The (Si/Mo) multilayer onto which these InAs(QM) nanoribbons were transferred, was prepared at the Center for X-ray Optics of the Lawrence Berkeley National Laboratory, and consists of 80 (Si/Mo) bilayers, each bilayer having a thickness of 3.3 nm. The termination layer of this (Si/Mo) mirror was chosen to be Si, which, exposed to air, gives rise to a thin layer of native silicon oxide (SiO 2 ). In this way, the bottom interface between the InAs(QM) and the SiO 2 /(Si/Mo) substrate is a good approximation to the interface between the InAs(QM) and a typical silicon-based substrate. Before transferring the InAs(QM) on it, the mirror was cleaned with acetone, isopropyl alcohol, and de-ionized water. 9 At the photon energy of 4.0 keV used in our measurements, the Bragg angle is θ Bragg ≈ 2.7°. In order to scan over the first order Bragg reflection, the incidence angle of the incoming x-ray beam was varied between 2.2° and 3.8°, in steps 0.02°. The HXPS spectra were obtained at the Advanced Light Source (ALS) (Beamline 9.3.1) and at SOLEIL Synchrotron (GALAXIES Beamline). 20 The p-polarized x-ray photon energy was set to hν = 4.0 keV and the spectral total energy resolution was ≈ 500 meV for the data acquired at ALS and ≈ 250 meV for the data acquired at SOLEIL. At hν = 4.0 keV, the IMFP, as estimated from the TPP-2M formula, 21 is ~8 nm for InAs. As a consequence, not only the top surface, but also the bottom interface between the 15 nm thick InAs(QM) and the SiO 2 (Si/Mo) substrate could be characterized by HXPS. The binding energies of the HXPS spectra were calibrated using Au 4f and Au E F before and after each data acquisition. Figure 2 shows the relevant core level (CL) spectra of the InAs(QM) and of the SiO 2 /(Si/Mo) mirror. Various chemically-shifted components are also indicated. Since the InAs(QM) sample was exposed to air before the HXPS analysis, the presence of C 1s is probably due to surface contamination. Although C 1s was fit with two components, their rocking curves (RCs) are identical, thus, there is only one effective depth for whatever C contaminant species are present. In addition to this qualitative information, an analysis of the experimental RCs using an x-ray optical program, the Yang X-Ray Optics (YXRO) code 17,25 , can provide more quantitative information on the depth, chemical state, composition and interdiffusion of the four layers described above. In order to accurately predict the RCs, the YXRO code, in addition to the sample optical parameters, takes into account the differential photoelectric cross sections and the photoelectron IMFPs. The calculated RCs reported in this paper were obtained by combining the YXRO code with a global black box optimizer 26   The agreement between the experimental RCs and the simulations from our best-fit is remarkable (Figure 3a). The results from the optimized depth profile are reported in Figure   3b. The top layer of adventitious carbon is ≈ 1.5 nm thick, in good agreement with the C 1s core level relative intensity as simulated by the surface analysis program SESSA. 27 The C 1s RC is special, in that there are two contributors, on top of the InAs(QM) and on top of the  29,30 The VBO is the key parameter for designing high performance nanoscale transistors and electronic/photonic devices etc., therefore the determination of this parameter is critically important. So far, only a limited amount of experimental and theoretical information regarding band alignment at the interfaces of nanoparticles or quantum membranes is available. 31,32 Experimentally, the most direct and reliable way for determining the band alignment is by using core levels and valence band maximum binding energies from XPS, as originally demonstrated by Kraut et al.. 29,30 This method has been widely used in semiconductor applications, even though it does not consider differences in screening of core and valence holes near an interface compared to the reference bulk materials.
Chambers et al. 33 and references therein, explored the influence on the measurement of VBO by XPS of the core-level peak broadening due to chemical effects and electronic charge redistributions at the surface and interface for thin-film heterojunctions. In their studies of semiconductor and transition-metal heterostructures, this group has noticed that core-level binding energies of epitaxial interfaces of metals on oxides, and oxides on metals do not change in a significant way as a function of film thickness, implying that differences in screening of core and valence holes near an interface compared to pure material do not significantly impact the determination of VBO by using the Kraut method, provided that chemical effects and electronic charge redistributions are not involved. We therefore believe that it is appropriate to determine the VBO between the InAs(QM) nanoribbons transferred on the SiO 2 /(Si/Mo) substrate using this method, which says that where ∆E VBO is the VBO of layer A (InAs) relative to layer B (mirror substrate) , ' ' In conclusion, we have demonstrated that HXPS and SW-HXPS are powerful nondestructive methods that can be used to characterize the interfaces in novel materials and nanostructures such as quantum membranes. For instance, HXSP and SW-HXPS provided the stoichiometry, the depth and the thickness of the oxide overlayer on these InAs(QM) nanoribbons, which acts as passivation layer, useful in order to prevent dangling bonds. In addition, SW-HXPS showed that the interface between the InAs(QM) and its oxidation layer is not sharp, indicating that some interdiffusion occurred and that the oxidation is not entirely homogenous. On the contrary, the bottom interface between the InAs(QM) and the substrate is atomically abrupt, which is a crucial prerequisite for successful applications of high performance nanoscale transistors. In addition, the VBO between these InAs(QM) nanoribbons and the SiO2/(Si/Mo) substrate was determined. The obtained value of 0.2±0.04eV is in good agreement with literature results giving a clear indication of the formation of a well-defined and abrupt SiO 2 -InAs heterojunction.