Photo-electrical and transport properties of hydrothermal ZnO

Abstract

We performed the studies of optical, photoelectric, and transport properties of a hydrothermal bulk n-type ZnO crystal by using the contactless optical techniques: photoluminescence, light-induced transient grating, and differential reflectivity. Optical studies revealed bound exciton and defect-related transitions between the donor states (at ∼60 meV and ∼240 meV below the conduction band) and the deep acceptor states (at 0.52 eV above the valence band). The acceptor state was ascribed to VZn, and its thermal activation energy of 0.43 eV was determined. A low value of carrier diffusion coefficient (∼0.1 cm2/s) at low excitations and temperatures up to 800 K was attributed to impact the recharged deep acceptors. Electron and hole mobilities of 140 and ∼80 cm2/Vs, correspondently, were determined at room temperature. The decrease of carrier lifetime with excitation was ascribed to increasing rate of radiative recombination at low temperatures and nonradiative recombination above the room temperature.

Long-lived excitons in thermally annealed hydrothermal ZnO

“…6 is approximated as 1/τ(ΔN fc ) = 1/τ bulk + 1/τ S + B rad (ΔN fc )ΔN fc , where B 0 = (0.7 ± 0.1)x10 −11 cm 3 /s is the nondegenerate bimolecular recombination coefficient, while the factor N b = 1.3 × 10 19 cm −3 appears due to carrier degeneracy (N b limits the high injection free carrier radiative lifetime to 11 ns). A similar B 0 value of 1.6 × 10 −11 cm 3 /s was determined in ZnO [ 21 ]. For comparison, in GaN value of B 0 = 2 × 10 −11 cm 3 /s was observed [ 43 ].…”

“…The comparison with the experimental data collected at higher excitation in Ref. [ 21 ] shows a sharp D increase due to the degeneracy of free carriers. The diffusion length L D = (D × τ bulk ) is calculated to be 1.4 μm in the A2 sample.…”

“…5 a) the acceptor traps provide holes by thermal activation, leading to the formation of excitons at very large delays, which can be called delayed luminescence. Excitons are formed when activated holes bind the electrons available from n-type doping of ∼10 15 cm −3 (60 meV donors [ 21 ]), which can be related to Al impurities [ 33 ]. The slow decay activates with temperature with 390 meV energy.…”

“…Particularly, thermal annealing in oxygen and vacuum were figured out to enhance the picosecond lifetimes up to the values of 2–6 ns [ 17 ]. In our previous research, we studied in detail a standard ZnO substrate, grown by hydrothermal method [ 21 ]. The exciton lifetimes measured were rather short - ∼220 ps, which are not application-relevant.…”