Fermi level influence on the adsorption at semiconductor surfaces—ab initio simulations

Abstract

Chemical adsorption of the species at semiconductor surfaces is analyzed showing the existence of the two contributions to adsorption energy: bond creation and charge transfer. It is shown that the energy of quantum surface states is affected by the electric field at the surface, nevertheless, the potential contribution of electron and nuclei cancels out. The charge transfer contribution is Fermi level independent for pinned surfaces. Thus for Fermi level pinned at the surface, the adsorption energy is independent on the Fermi energy, i.e., the doping in the bulk. The DFT simulations of adsorption of hydrogen at clean GaN(0001) and silicon at SiC(0001) surfaces confirmed independence of adsorption energy on the doping in the bulk. For the Fermi level nonpinned surfaces, the charge contribution depends on the position of Fermi level in the bulk. Thus adsorption energy is sensitive to change of the Fermi energy in the bulk, i.e., the doping. The DFT simulations of adsorption of atomic hydrogen at 0.75 ML hydrogen covered GaN(0001) surface confirmed that the adsorption energy may be changed by about 2 eV by the doping change from n- to p-type.

Adsorption of ammonia at GaN(0001) surface in the mixed ammonia/hydrogen ambient - a summary of ab initio data

“…21 Recently, intensive investigations of ammonia adsorption were conducted by the same authors. 6 They found that ammonia adsorbed on relatively low H-occupied surface decomposes into a NH 2 radical and a H adatom, which is in accordance with the results of Pignedoli et al and Bermudez. 17-20 The adsorption energy is about 2.8 eV for the hydrogen coverage up to 0.75 ML, but for higher coverage the energy is drastically reduced. Therefore, the adsorption is molecular in this region, locating the molecule in the 'on-top' position as the molecular adsorption energy, independent of the coverage, is close to 2.0 eV.…”

“…In the recent study we have shown how the adsorption energy depends on the electric charge transfer between solid bulk or the existing surface states and the new states emerging due to the presence of the adsorbate. 5,6 It was shown that in the absence of the charge transfer the adsorption energy attains the value determined by the energy of the bonding. The novel recently introduced idea is the energy dependence on the electronic charge transfer.…”

“…The novel recently introduced idea is the energy dependence on the electronic charge transfer. 5,6 The new phenomenon originates from the fact that the exchange of the electron between the Fermi level and the emerging surface state of different energies generates an additional energy gain. The energy difference of these two states may reach several electronvolts, affecting the adsorption energy considerably.…”

“…These investigations led not only to the considerable understanding of the system but also to the development of new methods, which are now sufficiently mature to simulate the surface taking into consideration the influence of the charged surface states and the fields close to the surface. [1][2][3][4][5][6] Recently, the dependence on the position of the Fermi energy and doping was also accounted for. 6,7 These new methods led to the discovery of new features, fundamentally changing the basic notions related to the processes occurring at the surface of a semiconductor and, introducing the dependence of adsorption energies on a pinned Fermi level surface and doping in the bulk for an unpinned Fermi level surface.…”

“…[1][2][3][4][5][6] Recently, the dependence on the position of the Fermi energy and doping was also accounted for. 6,7 These new methods led to the discovery of new features, fundamentally changing the basic notions related to the processes occurring at the surface of a semiconductor and, introducing the dependence of adsorption energies on a pinned Fermi level surface and doping in the bulk for an unpinned Fermi level surface.…”