The Problem of Electron Concentration in Heavily Doped n-Type ZnO Structures

This article analyzes the effect of donor doping on the electron concentration in zinc oxide (ZnO). The electron concentration strongly influences the photoluminescence intensity, so its correct determination is very important. It is shown that when the donor concentration exceeds 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>17</roman></sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-<roman>3</roman></sup>, an excited donor band (EDB) is formed with the average energy located near the bottom of the conduction band (CB). The Mott concentration is about 5 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>19</roman></sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-<roman>3</roman></sup> and this is the threshold value for the formation of a ground donor band (GDB) with an average value located about 55 meV below the CB. The densities of states in the EDB and GDB are orders of magnitude higher than in the CB, so optical transitions mainly occur between the EDB or GDB and the valence band, since conduction electrons mainly fill these bands. In this article, the density of states in the GDB, EDB, and CB was determined and the electron concentrations in them calculated. Experimental data from other authors on doping with aluminum atoms were used to verify the calculation results. Strong doping with Al atoms leads to a decrease in lattice constants, which translates into an increase in the energy gap. When doping exceeds 5 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>19</roman></sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-<roman>3</roman></sup>, a decrease in the value of the optical energy gap occurs, which is associated with the formation of GBD. By taking GBD and EDB into account, this phenomenon could be explained theoretically.