news review Bandgaps: Think they’re constant? Think again!
The shrinkage of the fundamental bandgap near the surface of InAs and other compound semiconductors could offer a new route in bandgap engineering. A team of European researchers has found that the semiconductor bandgap is not necessarily constant and varies with surface distance.
The partnership between researchers from the UK, France, Spain and Denmark has looked at the surfaces of some compound semiconductors that can support a quasi two-dimensional electron gas. (Q2DEG) Here, electrons can move freely parallel to the surface but are confined to this region. Traditional semiconductors like silicon and GaAs have a depletion of carriers close to the surface. For a long time, this has been thought of as the norm.
However, new materials are being found which exhibit electron accumulation. For example, Philip King, one of the researchers in this study pointed out that he and others have observed the presence of a Q2DEG zone at the surface of In-rich InGaN and InAlN alloys. In this latest investigation, the localized surfaces of InAs and CdO were investigated for quantum-well states supposedly intrinsic to these materials. Both materials exhibited similar phenomena.
The scientists used the ASTRID synchrotron to obtain Angle-Resolved Photo-Emission Spectroscopy (ARPES) data. This powerful technique directly images the electronic structure of the Q2DEG at the surface, and supplies information regarding the interactions between particles. The InAs(111)B sample was grown by MBE and was silicon-doped to 6x1017cm3. An amorphous arsenic cap was grown and removed in-situ by annealing at 3500C. The CdO was grown by metal-organic vapor phase epitaxy (MOVPE). The figure below shows the quantum well states at the CdO surface. Similar trends were exhibited InAs.
The researchers were surprised to find that interactions between the particles at the surface caused the bandgap to become smaller close to the surface of the material compared to in the bulk. Phil King commented that it was not the fact that the particle interactions caused a reduction of
the bandgap that was unexpected, but the fact that the changes occurred over macroscopic distances within the sample (i.e. the bandgap changes approaching the surface of the materials). These results showed the presence of a complicated interplay between several degrees of freedom within the materials.
He added “The magnitudes of the changes in bandgap are also larger than might be expected considering traditional ICs, which is testament to the greater flexibility and variety of properties that can be achieved in some of these emerging compound semiconductor materials.”
Regarding the results, he also explained, “The conventional one-electron picture of 7surface space-charge in semiconductors was different to the electronic structure observed from ARPES, indicating that many-body interactions play an unexpectedly large role in the Q2DEG in these materials.”
The relevance of this discovery should serve several purposes. The fact that the bandgap of these semiconductors becomes smaller when approaching the surface is essentially due to interactions between the electrons within the surface electron accumulation layer. Essentially these results could provide a stepping-stone in the advancement of bandgap engineering; the authors suggest
even an entirely new route to spatially- inhomogeneous bandgap engineering, eventually leading to tuning the functionality of electronic devices.
Furthermore, as King said, “The surface electronic properties of materials are crucial in any device application, as an electrical contact must always be made to the surface of a material (indeed, in his 2000 Nobel prize lecture, Herbert Kroemer remarked that “the interface is the device”)”.
“This so-called surface electron accumulation may, for example, make obtaining Schottky (rather than Ohmic) contacts difficult, but could potentially be beneficial for certain device applications such as terahertz generation or chemical sensors.”
Traditionally, the way to tune the bandgap of materials is to alloy two or more semiconductors together that have different bandgaps to start with. The findings in this paper show that the bandgap of a single material may be spatially modulated by controlling how strong the interactions are. This could be controlled by changing the doping levels in the material. King says he hopes these results could add an additional tool to band structure engineering.
Further details on this work will be available in the journal ‘Physical Review Letters’.
[1] Colakerol, L, et al, Phys. Rev. Lett. 97, 237601 (2006) [2] King, P.D.C. et al., “Surface Band-Gap Narrowing in Quantized Electron Accumulation Layers”, Phys. Rev. Lett. in press”
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www.compoundsemiconductor.net July 2010
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