A III-V compound semiconductor is an alloy, containing elements from groups III and V in the periodic table.
Different material systems combining these elements have been produced, the most commonly known of which is GaAs.
Within the III-V semiconductors there are the nitride semiconductors subset. At Warwick, there is extensive research into nitride semiconductors in the Surface, Interface and Thin Film Group.
GaN and its alloys offer many advantages compared to a III-As system, particularly a much wider range of energy bandgaps. Optoelectronic devices based on nitride ternary alloys can operate at energies in the mid ultraviolet all the way to infrared. The AlGaN/GaN heterojunction has a large band discontinuity that can allow GaN devices to have improved output power density and improved thermal conductivity means they can operate effectively at higher temperature.
Recent interest in short wavelength light emitting diodes (LEDs) and laser diodes (LDs) has led to the development of nitride based blue LEDs and ultraviolet LDs, with a wide range of applications.
GaN normally crystallizes in the hexagonal/wurtzite structure, which is subject to strong internal spontaneous and piezoelectric fields. These fields can reduce the efficiency of LEDs and LDs by reducing the overlap of wavefunctions in the quantum well. For this reason, attempts have been made to grow non-polar bulk hexagonal crystals, but these produced poor results.
The GaN can instead be grown directly in the non-polar direction in a cubic structure, which eliminates the internal fields. In addition, the material can be cleaved in perpendicular planes and there is improved electron and hole mobility due to the symmetric structure. Growing purely cubic GaN structures is difficult due to their thermodynamic instability, which leads to hexagonal inclusions forming, especially in thicker layers. Research is currently being undertaken to improve cubic GaN growth techniques. Bulk cubic GaN would be useful as a substrate for devices as strain and dislocation effects would be minimised at the interface.
Once device that also makes use of III-N materials is the Resonant Tunnelling Diode (RTD) that has many useful properties, the most notable being its Negative Differential Resistance (NDR). Efforts are being undertaken to improve the quality of these devices, indicated by their peak to valley current ratio (PVR). An NDR made of cubic nitride material would benefit from more reproducible tunnelling without the internal fields of hexagonal structures.
Indium nitride is a direct narrow bandgap semiconductor (~0.7 eV at room temperature), with potential in high speed optoelectronics and solar cells. When combined with gallium to produce the ternary alloy InGaN, the bandgap is tunable over the range 0.7 - 3.4 eV, from the infra-red to the ultraviolet, encompassing the entire visible spectrum, as shown in the figure below comparing the lattice constant to the room temperature bandgap of many compounds.
Bandgap vs. lattice constant for many semiconductor compounds .
InN has extreme properties, particularly an extreme electron accumulation at all surfaces, in contrast to most other III-V compounds that exhibit an electron depletion layer. InN is a heavily unintentionally n-type doped system, due to defects within the lattice, and the property of the branch point energy being well above the conduction band minimum, making the defects donor-like. As a result p-type doping is proving a difficult task, and currently research into doping the system with Mg is ongoing in different research groups.
InN exists in both a wurtzite, and zinc blende structure, but more commonly as wurtzite. The zinc blende structure requires specific growth conditions, including the use of a zinc blende substrate. The atomic structure of wurtzite InN at the surface has been investigated through the use of ion scattering spectroscopy to determine the reconstruction at the surface. The results of the ion scattering has led to the proposal of a model involving three In adlayers at the surface of the the polar InN (0001).
In tri-layer model, as published in .
Dilute Magnetic Semiconductors
In an effort to incorporate magnetism into existing semiconductor structures, a magnetic dopant (Cr, Mn, Fe, Ni, Co, Gd) is included into the material during growth. The exchange coupling between these dopants leads to magnetic properties.
T. D. Veal, C. F. McConville, and W. J. Schaff (Eds), Indium Nitride and Related Alloys (CRC Press, 2009).
R. E. Jones, et al, Physical Review Letters, 96, 125505, (2006).
- T. D. Veal and P. D. C. King and M. Walker and C. F. McConville and Hai Lu and W. J. Schaff, Physica B, 401-402, 351, (2007)