Impurity distribution and microstructure of Ga-doped ZnO films grown by molecular beam epitaxy

Abstract

We report microstructural characterization of heavily Ga-doped ZnO (GZO) thin films on GaN and sapphire by aberration-corrected scanning transmission electron microscopy. Growth under oxygen-rich and metal-rich growth conditions leads to changes in the GZO polarity and different extended defects. For GZO layers on sapphire, the primary extended defects are voids, inversion domain boundaries, and low-angle grain boundaries. Ga doping of ZnO grown under metal-rich conditions causes a switch from pure oxygen polarity to mixed oxygen and zinc polarity in small domains. Electron energy loss spectroscopy and energy dispersive spectroscopy spectrum imaging show that Ga is homogeneous, but other residual impurities tend to accumulate at the GZO surface and at extended defects. GZO grown on GaN on c-plane sapphire has Zn polarity and no voids. There are misfit dislocations at the interfaces between GZO and an undoped ZnO buffer layer and at the buffer/GaN interface. Low-angle grain boundaries are the only threading microstructural defects. The potential effects of different extended defects and impurity distributions on free carrier scattering are discussed. V C 2012 American Institute of Physics. [http://dx

Free‐electron concentration and polarity inversion domains in plasmonic (Zn,Ga)O

“…b). V‐type patterns have been also observed for (Zn,Ga)O layers grown on GaN/sapphire templates and assigned to low‐angle grain boundaries (). The thickness of the defect‐free layer separating IF #1 and IF #2 corresponds to the width of the ZnO NL indicating that defects are introduced close to the ZnO/(Zn,Ga)O interface.…”

A Revised Growth Model for Transparent Conducting Ga Doped ZnO Films: Improving Crystallinity by Means of Buffer Layers

“…The poor crystallinity of 1%GZO (Figures –5), in other words, the poor crystallinity of the grains with O polarity, can be explained by fact, that (00‐1) surface attracts the incorporation of donors (Ga or/and Zn). Excessive values of Ga (Al) and Zn contents inside abnormal grains with respect to film areas free from the abnormal grains were observed by Ogino et al and Joo et al Further Ogino speculated that this spatial fluctuation in the impurity concentration between abnormal grains and small ones is mostly the reason why abnormal grains formed. We propose instead that the undoped and slightly doped ZnO films are characterized by similar nucleation probability of grains featuring opposite polarities.…”

“…In particular, Al doped ZnO (AZO) and Ga doped ZnO (GZO) transparent conducting thin films are the most important alternative materials to expensive and scarce Sn doped In 2 O 3 (ITO) for applications as transparent electrode in solar cell, flat screen displays, and other electro‐optical devices . Despite the fact that AZO is cheaper, GZO thin films have some merits over AZO, such as a higher resistance to oxidation, low stress‐strain constraints on doping, and Ga, unlike Al, does not have tendency to segregate to interfaces and extended defects …”

Doping Effect of Electron Transport Layer on Nanoscale Phase Separation and Charge Transport in Bulk Heterojunction Solar Cells

“…% Ga concentration due to the formation of structural defects in the G-ETL and their consequent scattering of carriers at this high doping level. 19 Furthermore, the valleys of the P-ETL and the grain boundaries of all P-and G-ETLs yielded relatively high current values and could act as recombination centers and leakage current paths. These AFM results represent the effect of the Ga-doping in the ZnO ETL on the morphology and conductivity of the ZnO ETL, and subsequently on the performance of electronics and optoelectronic devices.…”