X-ray Photoemission for Probing Charging/Discharging Dynamics

Abstract

A novel technique is introduced for probing charging/discharging dynamics of dielectric materials in which X-ray photoemission data is recorded while the sample rod is subjected to (10.0 V square-wave pulses with varying frequencies in the range of 10 -3 to 10 3 Hz. For a clean silicon sample, the Si2p(Si 0 ) peak appears at correspondingly -10.0 eV and +10.0 eV binding energy positions (20.0 eV difference) with no frequency dependence. However, the corresponding peak of the oxide (Si 4+ ) appears with less than 20.0 eV difference and exhibits a strong frequency dependence due to charging of the oxide layer, which is faithfully reproduced by a theoretical model. In the simplest application of this technique, we show that the two O1s components can be assigned to SiO x and TiO y moeties by correlating their dynamical shifts to those of the Si2p and Ti2p peaks in a composite sample. Our pulsing technique turns the powerful X-ray photoemission into an even more powerful impedance spectrometer with an added advantage of chemical resolution and specificity.Charge accumulation and dissipation (charging/discharging) in dielectric materials are vital processes for design and function of various devices and sensors. 1 This is especially important for SiO 2 , as the thickness of the dielectric layer is expected to shrink down to few atomic layers for the next generation of metal oxide semiconductor (MOS) devices. 2 Charge accumulation in the oxide layer occurs via various trapping mechanisms and is normally probed by electrical current-voltage and/or current-capacitance measurements. [3][4][5][6] Photoemission utilizing UV, X-rays, and lasers has also been employed for probing very fast (<10 -9 s) charging dynamics. 7-9 Core-level X-ray photoemission, XPS, is especially attractive, since additional chemical information can also be derived from the line positions of the corresponding peaks. However, the measured line positions are severely altered by local potentials developed due to the uncompensated charges resulting from photoelectron emission, especially for poorly conducting samples or regions (layers or domains) within such samples. [10][11][12][13][14][15][16] Various methods of charge compensation have been developed using low-energy electrons, ions, and/or photons. 17,18 On the other hand, one can also utilize XPS for understanding the mechanisms leading to and/or controlling of the charging/ discharging processes in materials which offer great possibilities for researchers in all fields. 19 Several applications have been reported, which utilize the charging, called controlled surface charging, for extracting chemical, physical, structural, and electrical parameters of various surface species. 20-26 Using a slightly different strategy and by applying voltage stress to the sample rod while recording XPS spectra, we have shown that the extent of charging can be controlled and various analytical and electrical information can be extracted. [27][28][29][30][31][32] In addition to static information derived from applic...

Impedance-type measurements using XPS

“…2, we display C1s, Si2p and Rb3d peaks recorded at 0.01 Hz, where each peak exhibits a slightly different shift due the different extent of charging they experience. This is similar to our previous observations with other composite samples [33][34][35]. Whereas, the Si2p peak separation is exactly the same as that of O1s (18.2 eV), the Rb3d and C1s peaks are separated by 18.1 and 18.0 eV, respectively, revealing that to some extent, the Rb and C moieties are electrically isolated (and chemically distinct) from each other.…”

“…Using a slightly different strategy and by applying voltage stress to the sample while recording XPS spectra, we have shown that the extent of charging can be controlled, as a result of which a range of analytical and electrical information can be extracted [26][27][28][29][30][31]. In addition to static information derived from application of d.c. voltage stress, dynamical information can also be extracted by application of the voltage stimuli in the form of square wave pulses, as we have recently reported [29,[32][33][34][35]. In our previous measurements we used an unmonochromatized X-ray source where, in addition to characteristic Mg Ka radiation, it also emits wide bremstrahlung radiation and low energy electrons, which are known to affect charging of the samples under investigation.…”

“…At high frequencies the sample does not have enough time to charge nor discharge resulting in twinned peaks of 20.0 eV separation, but as the frequency is lowered the positive peak starts to exhibit larger charging shifts approaching that of the negative d.c. bias. These frequency dependent XPS shifts are measurements that probe the impedance of the sample under consideration, and can yield additional information which can be related to the resistancecapacitance of the composite sample [32][33][34][35].…”

XPS for probing the dynamics of surface voltage and photovoltage in GaN

“…In conventional data gathering mode the sample is grounded, and the position as well as the intensity of the peaks are recorded in a static fashion [20]. Although several reports have appeared in the literature related to dynamical XPS measurements in the two extreme ends, very fast (nanoseconds to attoseconds) [21][22][23][24], and very slow (minutes to hours) [25][26][27] through various publications, the basic principles and techniques for implementing such measurements employing very simple modification to conventional instruments [28][29][30][31][32][33][34][35][36][37][38][39].…”

Probing hot-electron effects in wide area plasmonic surfaces using X-ray photoelectron spectroscopy

“…X-Ray Photoelectron Spectroscopy (XPS) was used to investigate surface photovoltaic and photoconductivity effects in nanocomposite surfaces without using femtosecond or picoseconds light or X-ray pulses. [14][15][16][17][18] Here, we extend such use of XPS to investigate HEE in plasmonic MIM structures. Laser excitation is used to illuminate the plasmonic surfaces (Figure 1(a)).…”