Carrier concentration dependence of band gap shift in n-type ZnO:Al films

Abstract

Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2∕3 for ne≤4.2×1019 cm−3, which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4−8.4×1019 cm−3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widening and the latter an offsetting narrowing. The band gap narrowing (ΔEBGN) derived from the band gap renormalization effect did not show a good ne1∕3 dependence predicated by a weakly interacting electron-gas model, but it was in excellent agreement with a perturbation theory considering different many-body effects. Based on this theory a simple expression, ΔEBGN=Ane1∕3+Bne1∕4+Cne1∕2, was deduced for n-type ZnO, as well as p-type ZnO, with detailed values of A, B, and C coefficients. An empirical relation once proposed for heavily doped Si could also be used to describe well this gap narrowing in AZO.

Effect of pressure and Al doping on structural and optical properties of ZnO nanowires synthesized by chemical vapor deposition

“…Since Al is trivalent and Zn is divalent, Al can bond to more O atoms than Zn, and the Al 3 þ ions that occupy Zn 2 þ sites are positively charged (Al Zn in the Kroger-Vink notation). The commensurate increase in Al:ZnO electron concentration has been reported previously [2,27,28].…”

Structural, optical, and electrical properties of indium-doped cadmium oxide films prepared by pulsed filtered cathodic arc deposition

“…In heavily doped n-type semiconductors, the shift is due to the competitive effects of the Burstein-Moss (BM) band filling and the fundamental bandgap narrowing (BGN) [14][15][16]. Narrowing of the fundamental gap is the result of many-body effects and is also known as bandgap renormalization [17,18]. Good agreement between theory and experiment for the bandgap shift of heavily doped group IV, III-V, and II-VI semiconductors has been demonstrated [14,18,19].…”

“…The three terms in equation (6) represent the exchange energy of the majority carriers, the correlation energy, and the impurity interaction energy, respectively. Equation (6) has been demonstrated to be valid for AZO, ITO and other IV, III-V, and II-VI semiconductors [14,18,19,42]. …”

“…Narrowing of the fundamental gap is the result of many-body effects and is also known as bandgap renormalization [17,18]. Good agreement between theory and experiment for the bandgap shift of heavily doped group IV, III-V, and II-VI semiconductors has been demonstrated [14,18,19]. For pure CdO films, the bandgap shift has been extensively studied using the Burstein-Moss (BM) theory while accounting for the many-body effects [20,21].…”

Conductivity in transparent oxide semiconductors

“…In addition to this Moss-Burstein shift, which widens the optical energy gap, it has been found that many-body interactions lead to a shrinkage of the fundamental band gap with increasing free-carrier concentration [183]. This has been employed in the analysis of optical absorption in many TCOs [184][185][186][187][188][189], and has recently been shown to have a pronounced depth-dependent effect on the band gap size within their surface electron accumulation layers [150], as mentioned above. However, the origins of such band gap narrowing have also recently been questioned [190], with hybridization between states from the dopant impurity atoms and the host conduction band states rather than many-body interactions between the free carriers attributed as the cause of the bandgap shrinkage.…”