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Improving polar InN surfaces

Using a simple nitridation process, it is now possible to grow up to 25nm thick polar InN films with the surface Fermi level close to the valence band maximum. Substrates used include silicon (111) and GaN (0001)
A Finnish-Russian-Swedish collaboration is claiming that it has used ammonia nitridation to produce the first polar InN with the surface Fermi level near the valence-band maximum.

In the past, the growth of polar InN films has been hindered due to metallic indium clusters, formed readily during growth, and unintentional n-type conductivity of nominally undoped films. These include surface electron-accumulation layers via the Fermi level pinning into the conduction band.

These issues have hampered, for example, the realisation of p-type InN layers. This makes it difficult to grow p-n junctions in InN layers, which are needed in devices such as those used in solar, power electronic and LED applications.

A team from the Institute of Solid State Theory, Friedrich Schiller University and the European Theoretical Spectroscopy Facility, all based in Germany, using ab initio calculations, recently predicted the Fermi level position for InN [1]. Furthermore, researchers at Ritsumeikan University in Japan and Seoul National University, Korea have shown that the plasma nitridations during growth stops transform even large indium clusters into two-dimensional InN islands, improving InN quality [2].

Now researchers, led by a team at the University of Turku in Finland, have shown that high electron concentrations can be avoided, which is promising for making p-type InN films.

They initially grew InN films on silicon (111) and GaN (0001) substrates by the nitridation of the indium-covered semiconductor surfaces with ammonia (NH3) or cracked N2 gas.

Following this treatment, the scientists found it was possible to grow up to 25nm thick polar InN films with the surface Fermi level close to the valence band maximum. In other words, they avoided the presence of electron accumulation layers. The plot below shows the Fermi level obtained using Scanning Tunnelling Spectroscopy (STS).



Analysis of current-voltage curves, measured by STS from well-defined surface areas, reveal that the surface Fermi level locates close to the valence-band maximum

 

Synchrotron-radiation In 4d core-level photoelectron spectra show that the nitridation transform metallic indium to InN

The substrate temperature during the nitridation was found to be one of the most crucial parameters in the formation of InN (000-1) films with the Fermi level near the valence-band maximum.

Indium was evaporated from a heated tantalum envelope onto the substrates, which were kept at room temperature before nitridations. The NH3 pressure was about 5 x 10-5 mbar during the nitridation. Temperature of indium-covered silicon (111) substrates was 400 - 450 °C during the nitridation, and the temperature for indium-covered GaN (0001) pieces was 530 - 580 °C, just below the decomposition temperature of InN.

The silicon (111) substrates were flash heated at about 1200 °C. STM indicates a smooth, well-defined (7x7)-reconstructed surface.


STM image showing two-dimensional nature of InN (000)/Si (111) sample (~ 800 nm x 800 nm)

To clean HVPE-grown n-type GaN (0001) substrate pieces, they were heated to around 600 °C in the NH3 atmosphere to get a sharp (1 x 1) diffraction.

STM image showing two-dimensional nature of InN (000)/GaN (0001) sample (~450nm x 450nm)

Further details of this work have been published in the paper, "Formation of polar InN with surface  Fermi level near the valence band maximum by means of ammonia nitridation" by J. Dahl et al in Physical Review B, 86, 245304 (2012). DOI: 10.1103/PhysRevB.86.245304

References:

[1] A. Belabbes, J. Furthmüller, and F. Bechstedt, Phys. Rev. B 84, 205304 (2011)

[2] T. Yamaguchi and Y. Nanishi, Appl. Phys. Expr. 2, 051001 (2009).

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