Flitsyian, E.; Chernyak, L.; Malinauskas, T.; Aleksiejunas, R.; Jarasiunas, K.; Lim, W.; Pearton, S. J.; and Gartsman, K., "Optical and electron beam studies of carrier transport in quasibulk GaN" (2009).Cite as: Appl. Phys. Lett. 95, 092101 (2009); https://doi.

It has been recently discovered that electron injection into Phosphorus-, Lithium-, Antimony-or Nitrogen-doped ZnO semiconductor, using electron beam from a Scanning Electron Microscope, as well as a forward bias application to the p-n junction or Schottky barrier, leads to a multiple-fold increase of minority carrier diffusion length and lifetime (1-4). It has also been demonstrated that forward biasing a ZnO-based photovoltaic detector results in a several-fold responsivity enhancement due to a longer minority carrier diffusion length in the detector's p-region as a result of electron injection (5, 6). The observed electron injection effects were attributed to the charging of the metastable centers associated with the above-referenced impurities.With p-type doping of ZnO becoming possible, it is very likely that minority carrier (bipolar) devices such as LEDs, laser diodes, and transparent p-n junctions can be achieved in the near future (7,8,9). Besides effective p-type doping and robust ohmic contact fabrication, attaining the optimum performance of bipolar devices hinges upon overcoming an additional challenge -a short minority carrier diffusion length (usually ≤ 1µm), which is common to direct band gap semiconductors. Given the fact that the diffusion length is a critical parameter defining performance of p-n junction devices, it is imperative to find ways for its improvement. Our recent findings indicate that the latter parameter in ZnO can be noticeably enhanced by electron injection. The observed novel effect was attributed to electron trapping on impurity-related levels (10, 11). Electron injection in bulk ZnO substratesThe experiments were carried out on commercially available (Tokyo Denpa (TD)) bulk ZnO substrates grown by hydrothermal technique. Secondary Ion Mass Spectroscopy (SIMS) measurements performed on these substrates revealed the presence of lithium (Li) in the crystal on the level of ~ 4x10 16 cm -3 (Li is often added to ZnO to increase the resistivity of initially n-type samples) (12). Room temperature Hall measurements showed the samples to be a weak n-type with an electron concentration of ~ 10 14 cm -3 and mobility of ~ 150 cm 2 /Vs. The samples under investigation were cleaved perpendicular to c-plane thus exposing the non-polar a-plane of ZnO. This was motivated by the observations that the latter crystallographic plane results in a better quality of Schottky contacts, as opposed to those deposited on the c-plane. Schottky barriers were, therefore, fabricated by electron beam evaporation of 100 nm-thick Au layer on ZnO a-plane and subsequent lift-off.The experiments were carried out in-situ in a Philips XL30 Scanning Electron Microscope, which is integrated with a Gatan MonoCL3 cathodoluminescence system allowing wavelength-dependent and temperature-dependent optical measurements. A sample temperature in the cathodoluminescence measurements varied from 25 to 125 o C. For each temperature, periodic CL measurements were carried out under an SEM magnification of 4,000 at different locat...

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