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Can you rely on Hall to assess p doping of ZnO films?

Sometimes not. In samples which have been annealed at a high temperature, Hall measurements may indicate the wrong carrier type. In this case, other characterisation techniques, such as CV and photocurrent-based measurements, are more reliable
Hall effect measurements performed on ZnO films annealed after growth on InP substrates can be misleading with respect to the real nature of the analysed material.

That’s the key finding of a study by scientists at the Università di Palermo, Italy and Thales, Research & Technology in France. This team found, from Hall measurements, that a change in the electrical properties of the films, from n-type to p-type, was not confirmed by both capacitance voltage (CV) and photocurrent-based measurements. Instead, the ZnO films remained n-type after post-growth annealing.

The team’s findings are important, because ZnO is a very interesting material, which could be used for the manufacture of light sources and sensors in the portion of spectrum between blue and near UV. However, this material suffers from poor p-type doping, in terms of reliability, stability, and reproducibility, making it difficult to fabricate a high performing ZnO LED.

Investigations by the researchers from Italy and France involved measurements made on samples grown at 400°C and annealed afterwards in air at 600°C.

These results indicate that carrier type identification in ZnO films thermally treated after growth should be approached with caution, because of artefacts such as profound structural and electrical changes at the ZnO/Substrate interface. These arise in the samples following the high temperature annealing, which may falsify the Hall measurements, giving a different carrier type to the real one. In this case, other characterisation techniques, such as CV and photocurrent-based measurements are more reliable and could be used instead.

According to the team, over the years, different ways for realising p-type ZnO films have been undertaken, often with non reproducible and arguable results. Some of these results are even less convincing considering the high hole concentration and mobility which have been reported.

This is not in line with both the standard electron transport theory of ZnO and the majority of experimental research works that have been published. It is possible that many of the most controversial results may be ascribed to an incorrect assignment of the p-type doping after Hall effect measurements.

ZnO films were grown on undoped InP substrates by pulsed laser deposition (PLD) at 400°C and 10-2 mbar oxygen pressure, and subsequently annealed in air at different temperatures for 1 hour.

Hall effect measurements (resistivity, mobility, and carrier concentration) were performed before and after annealing together with a detailed photoelectrical investigation performed in aqueous solution and CV measurements.

The Hall effect measurements suggested that ZnO films annealed at 600°C for 1 hour exhibited an anomalously high p-type conductivity with hole concentrations up to 9.2 × 1019 cm−3 and hole mobilities up to 28.5 cm2/V s.

What's more, the resistivity after annealing decreased by about an order of magnitude (from 1.7 × 10-2 to 4.2 × 10-3 Ω cm), indicating an apparent profound change in the electrical properties of the films.

Photocurrent and CV measurements performed on the same samples revealed, instead, n-type conductivity. The photocurrent was, in fact, anodic, decreasing with the applied potential UE (Fig. 1), and the differential capacitance C of the film increased as the electrode potential moved toward the cathodic direction, as expected for a n-type semiconductor (Mott-Schottky representation, Fig. 2(a)).

UE is the electrode potential. It is the voltage applied to the electrode (the ZnO/InP sample) during the photoelectrical measurements performed in aqueous solution.


Figure 1. Photocurrent versus applied potential (UE) recorded at three different irradiating light wavelengths, solution: 0.1 M ABE and potential scan rate 10 mV s-1. The inset shows a zoom of the plot in the region where the photocurrent becomes zero. A flat band potential UFB of about -0.6 V vs. Ag/AgCl is readable. All curves are related to a sample annealed in air at 600°C for 1 hour

The n-type conductivity of these samples was further confirmed by CV measurements employing a mercury probe (Fig. 2(b)).

The mechanism responsible of the p-type doping measured by Hall measurements can be ascribed to the formation of a very thin, high conducting layer at the ZnO/InP interface due to zinc ion migration into the InP substrate. This high conductive layer dominates the Hall effect measurements and instead is invisible to both CV and photocurrent-based methods.

Figure 2 (a) C-2 versus UE plot recorded at f = 10 kHz in 0.2 M Na2HPO4 solution. The linear fitting allows calculating a donor concentration of about 1016 cm-3.

Figure 2 (b) C-2 versus applied voltage for the Hg n-ZnO junction realised with a Hg contact area of 0.432 mm². The linear fitting allows calculating a donor concentration of 3.7 × 1016 cm-3

All plots are related to a sample annealed in air at 600°C for 1 hour.

This work is described in detail in the paper, “Erroneous p-type assignment by Hall effect measurements in annealed ZnO films grown on InP substrate”, by R. Macaluso et al in the Journal of Applied Physics, 113, 164508, published online on 30th April 2013. http://dx.doi.org/10.1063/1.4803080

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