Amphoteric nature of copper impurities in Bi₂Se₃ crystals

Abstract

ReflectivityR(λ) in infrared spectral region was measured on bismuth selenide crystals Bi₂Se₃(Cu) doped with copper and on the crystals Bi₂Se₃(2Cu+3Se) doped with copper and selenium in atomic ratio 2:3. Bi₂Se₃(Cu) crystals exhibit a pronounced shift of the reflectivity minimum towards shorter wavelengths, as compared with “pure” Bi₂Se₃, whereas in the Bi₂Se₃(2Cu+3Se) crystals the reflectivity minimum is shifted towards longer wavelengths, as compared with “pure” Bi₂Se₃ crystals.

Hall constant and theN/m _χ ratio, calculated from the reflectivity curves in the plasma resonance frequency region, were determined for “pure” Bi₂Se₃. Assuming the “single valley” model to hold for the conduction band of Bi₂Se₃, we calculated the effective mass of free current carriers:m _χ=m _⊥c =0.13m ₀. On the assumption thatm _⊥c is constant in a moderately broad range of free-carrier concentrations we then calculate the free carrier concentrations in Bi₂Se₃(Cu) and Bi₂Se₃(2Cu+3Se) and correlate them with the concentrations of copper, as determined by a chemical analysis. Discussion of possible point defects introduced by building of Cu atoms into the Bi₂Se₃ lattice leads to the conclusion that in Bi₂Se₃(Cu) crystals copper exist in the form of singly ionized interstitial atoms Cu ⁺ _i which act as donors, whereas in Bi₂Se₃(2Cu+3Se) crystals there arise most probably substitutional defects of copper on bismuth sites, Cu_Bi″, which carry double negative charge and act as acceptors.