Due to the large electron affinity
of InN and In-rich InGaN and InAlN, p-type doping has proven to
be a significant challenge. All nominally undoped films
have n-type conductivity, and even p-doped films have an n-type
surface layer, due to the accumulation of electrons on the surface
of the films. Only recently did we demonstrate successful
p-type doping of InN with magnesium acceptors, using techniques
to account for the effect of the n-type surface layer and allow
for a study of the bulk material [2]. Using an electrolyte
to make a blocking (Schottky) contact, we applied a voltage to
deplete the surface charge and measure the ionized space charge
(via a measurement of capacitance) beneath the surface.
This showed a net acceptor concentration in the bulk of Mg-doped
InN films. This measurement was supported by a study of
the electronic properties of InN films subjected to energetic
particle irradiation, which showed that Mg-doped films had
very different behavior from undoped films. This difference
was explained by the presence of p-type material in the bulk of
Mg-doped films, beneath the n-type layer on the surface.
In1-xGaxN
alloys of x less than or equal to 0.65 are also expected to exhibit a
surface electron layer, making verification of p-type doping of the
bulk difficult over this composition range. However, using the
same electrolyte-based capacitance-voltage measurement as described
above, we have been able to confirm successful p-type activity in
Mg-doped In1-xGaxN
films of x = 0.05, 0.30, 0.33, and 0.81. This is the first
demonstration of p-type doping across the whole composition range of In1-xGaxN.
[3] Analysis of the capacitance-voltage curves generated in these
measurements reveals an n-type surface accumulation layer masking
p-type material beneath in the x = 0.05, 0.30, and 0.33 samples.
In the x = 0.81 sample, no surface accumulation layer is observed and
the capacitance-voltage measurements show the material to be roughly
uniformly p-type.
[2] R. E. Jones, K. M. Yu, S. X. Li, W. Walukiewicz, J.W. Ager III, E. E. Haller, H. Lu, and W. J. Schaff, Evidence for p-type doping of InN, Phys. Rev. Lett. 96 (2006) 125505.
[3] D. M. Yamaguchi, J. W. Ager III, R. E. Jones, K. M. Yu, W. Walukiewicz, E. E. Haller, H. Lu, and W. J. Schaff, P-type InGaN alloys, Appl. Phys. Lett., submitted for review.
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