Electrical, Optical, and Analytical Characterization of Bulk Hydrothermal ZnO Crystals Doped with Indium

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Bulk In-doped ZnO crystals have been grown hydrothermally by adding In2O3 to the normal nutrient. The major growth occurs along the [10−10] direction rather than the usual [0001] and [000−1] directions; thus, a c-plane crystal has a plate-like shape. Secondary-ion mass spectroscopy (SIMS) measurements reveal an In concentration of about 1.6×1019cm−3 in the bulk, with lesser amounts of Al (2.5×1018), Ga (1.8×1017), Fe (6.5×1018), and Li (1.1×1018). Low-temperature photoluminescence (PL) from the Zn face (0001) shows strong, relatively sharp donor-bound exciton (D°X) lines at 3.3609 and 3.3624eV, attributed to Al and H, respectively, along with much weaker lines at 3.29, 3.22eV, and 2.35eV, 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 (000−1) 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 15–320K 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 50K (n=7.8×1018cm−3, μ=48cm2/Vs, and ρ=0.017Ωcm) and (2) from 250 to 320K (n=7.3×1018cm−3,μ=83cm2/Vs, and ρ=0.010Ωcm). A mobility analysis based on degenerate electrons gives donor ND and acceptor NA concentrations of about 1.5×1019 and 7.5×1018cm−3, respectively. Remarkably, within error, ND≈[In]+[Al]+[Ga] and NA≈[Fe]+[Li].