Electrical, optical, and analytical characterization of bulk hydrothermal ZnO crystals doped with indium

Bulk In-doped ZnO crystals have been grown hydrothermally by adding In 2O 3 to the normal nutrient. The major growth occurs along the [1 0 −1 0] direction rather than the usual [0 0 0 1] and [0 0 0 −1] directions; thus, a c-plane crystal has a plate-like shape. Secondary-ion mass spectroscopy (SIMS) measurements reveal an In concentration of about 1.6×10 19 cm −3 in the bulk, with lesser amounts of Al (2.5×10 18), Ga (1.8×10 17), Fe (6.5×10 18), and Li (1.1×10 18). Low-temperature photoluminescence (PL) from the Zn face (0 0 0 1) shows strong, relatively sharp donor-bound exciton (D°X) lines at 3.3609 and 3.3624 eV, attributed to Al and H, respectively, along with much weaker lines at 3.29, 3.22 eV, and 2.35 eV, tentatively assigned, respectively, to a 1-LO-phonon replica of the D°X transitions, a donor–acceptor-pair transition, and the usual green band of unknown origin. The D°X-related PL from the O face (0 0 0 −1) is much broader because of significantly higher In, Li, and Fe concentrations; however, it also includes a weak, sharp In°X line evidently emanating from the bulk region. The Hall-effect measurements in the range 15–320 K reveal an unusual temperature variation of carrier concentration n, mobility μ, and resistivity ρ, namely the existence of two flat regions: (1) from 15 to 50 K ( n=7.8×10 18 cm −3, μ=48 cm 2/V s, and ρ=0.017 Ω cm) and (2) from 250 to 320 K ( n=7.3×10 18 cm −3, μ=83 cm 2/V s, and ρ=0.010 Ω cm). A mobility analysis based on degenerate electrons gives donor N D and acceptor N A concentrations of about 1.5×10 19 and 7.5×10 18 cm −3, respectively. Remarkably, within error, N D≈[In]+[Al]+[Ga] and N A≈[Fe]+[Li].