RESEARCH REVIEW
Flaws exposed in ZnO Hall measurements Hall measurements on annealed samples can indicate the wrong carrier type.
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 and 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.
This discovery is important because ZnO is a very promising material for the manufacture of light sources and sensors in the portion of the spectrum between blue and near UV. Today, 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.
Figure 2. (a) C-2
allows calculating a donor concentration of about 1016 cm-3
versus UE plot recorded at f = 10 kHz in 0.2 M Na2 cm-3
. (b) C-2
HPO4
solution. The linear fitting 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
These arise in the samples following high temperature annealing, which may falsify the Hall measurements, giving a different carrier type to the real one.
According to the team, over the years different ways for realising p-type ZnO films have been undertaken, often with non-reproducible and questionable results. Some of these results are even less convincing, considering the high hole concentration and mobility that 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. The team’s 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.
) recorded at three different irradiating light wavelengths, solution: 0.1 M ABE and potential scan rate 10 mV s-1
Figure 1. Photocurrent versus applied potential (UE
. 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 verses Ag/AgCl is readable. All curves are related to a sample annealed in air at 600°C for 1 hour
Hall effect measurements – resistivity, mobility, and carrier concentration – were carried out 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 to 28.5 cm2
V-1 60
www.compoundsemiconductor.net July 2013
cm-3 s-1
. and hole mobilities up
The mechanism responsible for 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.
R. Macaluso et. al. J. Appl. Physics 113 164508 (2013)
What’s more, the resistivity after annealing decreased by about an order of magnitude, indicating an apparent profound change in the electrical properties of the films. In contrast, photocurrent and CV measurements performed on the same samples revealed n-type conductivity. The photocurrent was, in fact, anodic, decreasing with the applied potential, UE
, which is the voltage
applied to the electrode (the ZnO/ InP sample) during the photoelectrical measurements performed in aqueous solution (see Figure 1).
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, see Figure 2(a)). The n-type conductivity of these samples was further confirmed by CV measurements employing a mercury probe (Fig. 2(b)).
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