Effects of gallium doping on properties of a -plane ZnO films on r -plane sapphire substrates by plasma-assisted molecular beam epitaxy

• Published in: Journal of vacuum science & technology. A, Vacuum, surfaces, and films Vacuum, surfaces, and films, 2011-05, Vol.29 (3), p.03A111-03A111-7
• Main Authors: Han, Seok Kyu, Lee, Hyo Sung, Lim, Dong Seok, Hong, Soon-Ku, Yoon, Nara, Oh, Dong-Cheol, Ahn, Byung Jun, Song, Jung-Hoon, Yao, Takafumi
• Format: Article
• Language: English
• ISSN: 0734-2101; 1520-8559
• DOI: 10.1116/1.3562162

The authors report on the structural, optical, and electrical properties of Ga-doped a -plane ( 11 2 ¯ 0 ) ZnO films grown by plasma-assisted molecular beam epitaxy. Ga doping level was controlled by changing the Ga cell temperatures from 350 to 470   ° C with an interval of 30   ° C . With up to Ga cell temperatures of 440   ° C , single crystalline Ga-doped a -plane ZnO films were grown; however, the sample with a Ga cell temperature of 470   ° C showed polycrystalline features. The typical striated surface morphology normally observed from undoped ZnO films disappeared with Ga doping. ZnO films doped with Ga cell temperatures up to 440   ° C did not show a significant change in full width at half maximum (FWHM) values of ( 11 2 ¯ 0 ) x-ray rocking curves by doping. The smallest FWHM values were 0.433° ( ϕ = 90 ° ) and 0.522° ( ϕ = 0 ° ) for the sample with a Ga cell temperature of 350   ° C . The polycrystalline ZnO film with excessive Ga doping at the Ga cell temperature of 470   ° C showed significantly increased FWHM values. Hall measurements at room temperature (RT) revealed that electron concentration began to be saturated at the Ga cell temperature of 440   ° C and electron mobility was drastically reduced at the Ga cell temperature of 470   ° C . The carrier concentration of Ga-doped ZnO films were controlled from 7.2 × 10 18 to 3.6 × 10 20   cm − 3 . Anisotropic electrical properties (carrier concentration and Hall mobility) were observed in measurements by the van der Pauw method depending on the direction ( c - or m -direction) for the undoped sample but not observed for the doped samples. RT photoluminescence (PL) spectra from the Ga-doped single crystalline ZnO films showed dominant near band edge (NBE) emissions with negligibly deep level emission. The NBE intensity in PL spectra increases with Ga doping.