Zinc Oxide (ZnO) is considered an excellent material for UV light-emitting diodes (LEDs) and lasers. A major obstacle for successful applications has been the difficulty of p-type doping. Though important progress has been made, the observation of light induced type conversion [1] underlines the necessity to understand trapping effects in these material.
We have grown ZnO films of thicknesses about 500 nm on sapphire by means of a low temperature cyclic pulsed laser deposition process. The electronic properties are strongly dependent on the deposition conditions: in as-deposited samples, we observe a ninefold decrease of the dark conductivity from σd=103 S/cm at a substrate temperature of T = 25°C to σd=1e-6 S/cm at T = 800°C. The initial photoresponse of the samples is excellent, but becomes strongly reduced after prolonged light soaking.
In order to assess the minority carrier transport properties we used the steady-state photocarrier grating (SSPG) method. This method uses the spatial interference of two coinciding laser beams and allows therefore to measure diffusion length of the order of the laser wavelength. It can be considered complementary to Hall effect measurements, that are not very sensitive for low mobility materials and may in the case of type conversion lead to contradictory interpretations. The obtained hole diffusion lengths are typically in the range of several hundreds of nanometers, and show only small variations as a function of temperature.
Based on the criteria of ambipolar transport and the Einstein relation, we obtain for the hole mobility values that are in general agreement with values from Hall measurements done on p-type ZnO samples.
The experimental results in particular and the anomaly of type conversion in wide gap semiconductors in generalwill be analyzed in the context of a previously published minority carrier dominated thermalization-recombination process [2].
References
1. D. C. Look, B. Claflin, Ya. I. Alivov, and S. J. Park, phys. stat. sol. (a) 201, 2203 (2004);
2. M. Niehus and R. Schwarz, phys. stat. sol. (c) 3, 1637 (2006) |