Formation of photoelectron spectra of alloys niobium-molybdenum-zirconium

Crystals of niobium and its alloys obtained by low-energy implantation of molybdenum and zirconium ions were studied in a multifunctional installation. The energy distribution curves of N (E) photoelectrons before and after heating niobium – molybdenum – zirconium alloys were studied. The contribution of surface zones formed by molybdenum and zirconium atoms to photoelectron emission in the photon energy region of 8–10 eV was analyzed.


Introduction
Recently, as it is known, one of the most urgent tasks of physical electronics and nanotechnology is the creation of surface alloys of arbitrary composition with the necessary concentration of alloying elements. Alloys of refractory metals based on niobium, molybdenum, tantalum, and others are widely used in nuclear power, mechanical engineering, vacuum technology, etc. Experimental and theoretical studies of several alloys have shown that controlling the state of s-d electrons by introducing small concentrations of alloying elements and creating surface alloys allows us to solve some fundamental problems of an applied and theoretical nature. Alloys obtained by traditional methods have their drawbacks. One of the disadvantages of such alloys is the difficulty of obtaining atomically pure systems with a low content of carbon, oxygen, and sulfur atoms. The production of alloys by implanting ions of various refractory metals under ultrahigh vacuum conditions can eliminate this specified disadvantage. In this paper, using the example of implantation of low-energy Mo and Zr ions in a single crystal Nb (110), we study the change in the spectral dependence of the quantum photoemission yield (QPhY). The value of the photoemission work of the output is found, and the features of the formation of photoelectron spectra of N(E) in the near and vacuum ultraviolet are studied [1][2][3][4][5][6][7][8].

Materials and methods
The design of a multi-functional ultra-high-vacuum experimental setup with a solid-state ion source is described in [9][10][11][12][13]. Crystals of Nb, Mo, and Zr, placed in a tantalum cup, were heated to a temperature of 2000-2200 K to clean the surface from impurity elements. The working vacuum in the device chamber was (3-5)×10 -10 Torr. During the implantation of Mo and Zr ions, the residual gas pressure in the unit was no higher than (2-3)×10 -8 Torr. The flow of Mo and Zr ions in this mode is equal to ∼10 11 ionsm -2 s -1 , and the ion energy E = 3-3. 5 keV.
Monocrystals Mo(111), Nb(110), and polycrystalline zirconium were mounted in the manipulator of the experimental setup. The crystals were preheated to T = 2200-2400 K for more than 50 hours. The cleanliness of the surface was monitored by auger spectra. The Auger spectra of the samples were measured in the energy range from 80 to 600 eV after high-temperature heating, as well as in heated crystals. The spectral dependences of the QPhY were removed, and the value of the work function was determined by the method of straight and isothermal Fowler's curves. In this case, graphs of the dependence in the photon energy range from 3.95 to 5.15 eV were plotted. The sample temperature was maintained at room temperature when removing the QPhY. The photoelectrons' energy distribution curves were obtained at photon energies hv = 8.4 and 10 eV, and xenon and krypton resonance lamps were used as radiation sources.

Results and discussion
After thermal cleaning of the samples under study, the Auger spectra of the surface of Nb, Mo, and Zr single crystals were taken. Figure 1 shows the Auger spectra after implantation of Mo and Zr ions in Nb (110), where the main peaks of niobium, the peaks of molybdenum and zirconium, and a small peak of carbon are visible. Figure 1 shows the Auger spectra after implantation of Mo and Zr ions in Nb (110), where the main peaks of niobium, the peaks of molybdenum and zirconium, and a small peak of carbon are visible. The maximum surface concentration of the implanted molybdenum and zirconium atoms is reached after heating the sample to 1350-1400 K. Further heating of the crystal (over 1400 K) leads to a decrease in the surface concentration of molybdenum and zirconium. To find the distribution of Mo and Zr atoms in-depth in Nb(110), the crystal surface was etched layer-by-layer with argon ions. At the current density and ion energy used, Ep = 1 keV, one layer of Mo and Zr atoms was removed in ~ 6-7 min. After removing ~8-10 atomic layers from the crystal surface, the concentration of Mo and Zr atoms decreased to ~1 at.%. Quantitative calculations were carried out by the method of the element sensitivity factor. Under the same surface treatment conditions, the spectral dependences of the QPhY of crystals studied before and after implantation of the Nb(110) surface with zirconium ions were removed, and the values of the photoemission work of the output were calculated. If before implantation for Nb (110) e= 4.52 ± 0.02 eV, then after implantation, this value was equal to e= 4.38-4.40 eV, i.e., there is a decrease in the output operation by ∼0.15 eV. To facilitate the interpretation of the obtained N(E) photoelectron spectra for the Nb-Mo-Zr alloy, the energy distribution curves of the photoelectrons were taken from the Mo(110) surface and the atomically pure surface of zirconium. Figure   In contrast to pure niobium, the N (E) curve had a broad maximum in the low-energy region, and after heating at T = 1400 K, a decrease in the intensity of this maximum was observed. The same figure shows the photoelectron spectrum for a pure Mo (111) single crystal (curve 1). Comparing these three curves, we can say the following. The C peak at 1.2-1.3 eV is most likely associated with the density of electronic states of hafnium since the position of this maximum did not change after the photoelectron spectra were taken at hν = 10 eV. We associate peaks A and C with indirect optical transitions [13,14] for a pure Mo (111) single crystal. Thus, the photoelectron spectra for pure Mo (111) exhibit maxima due to the band structure of molybdenum, and in the spectrum obtained after doping with zirconium ions, an additional maximum appears at 1.2-1.3 eV below the Fermi level [15][16][17][18][19][20].
It is shown in [10] that when low-energy ions of refractory metals are implanted, the alloying atoms are mainly distributed in the near-surface region of the metal matrix with a depth of up to ten atomic layers. Suppose before implantation of the Mo(111) KVF sample there is a significant increase in the quantum yield of the photoemission as a function of the energy of the incident photons, then after implantation. In that case, there is a weak dependence Y =f(hν). This is probably due to a violation of the order of the structure of the surface atomic layer and a change in the surface reflection coefficient after the implantation of hafnium atoms. Secondly, the introduced ions form a solid substitution solution with the matrix, which leads to a change in the lattice structure. Photoelectrons involved in the emission from the surface layer lose part of their energy due to the formation of defects and radiation damage during ion insertion. It should also be noted that these processes are affected by the heating of the crystal and the presence of residual gases in the installation. The features of the electronic states of the surface layer of the alloy are clearly shown on the energy distribution curves taken at hv = 8.4 eV after heating at T = 1400 K. The origin of the low-energy maximum in pure metals is mainly due to photoelectron's loss of energy during electron-electron inelastic scattering [11]. During ion doping, a cascade of collisions develops in a solid. In this case, many defects such as vacancies and interstitial atoms are formed in the matrix metal. Thus, a significant increase in the amplitude of the low-energy maximum is due to the energy losses of photoelectrons on structural and point defects. The decrease in the amplitude is associated with some ordering of the structure after heating the crystal. Directed photoelectron emission from the surface of niobium (110) was investigated. Our data for the pure Nb(110) surface are in good agreement with the results [21][22][23][24][25]. The position of peaks A and B with energies 0.5 and 1. eV below the Fermi level is independent of the photon energy. Such features for the N(E) curves in the indirect transition model [10] correspond to the maxima of the density of the electronic states of the valence band. It should be noted that the peak located directly near the Fermi level (∼0.5 eV) is very sensitive to surface contamination by adsorbed gases.

Conclusion
Apparently, this peak is associated with the surface resonance level. Analysis of the N(E) curves taken at photon energies of 10 and 8.4 eV from the surface of ion-doped niobium shows that additional maxima at energies of 1.25-1.3 eV below the Fermi level are due to implanted molybdenum and zirconium atoms [14]. Implanted niobium atoms create their own energy sub-bands associated with zirconium d-electrons [15]. Apparently, these electronic subbands are located in the forbidden zone in the direction of the symmetry ГN band structure of niobium [26,27]. Analyzing the results obtained, we note that the model calculations of the electron output from the bulk and surface states carried out by several authors [9,[25][26][27][28][29] showed that the contribution of the surface states is carried out from ~ 2.5 atomic layers.