Optical studies of carrier dynamics and non-equilibrium optical phonons in nitride- based semiconductors

Gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and their alloys have long been considered very promising materials for device applications.^1,2 Semiconductor alloys such as In x Ga_1-x N have been successfully used in the fabrication of blue-green light emitting diodes (LEDs) and laser diodes (LDs).^2-7 Recently, growth of high-quality InN as well as In_x Ga_1-x N have been demonstrated.^8-10 In particular, progress in the manufacture of very high-quality, singlecrystal InN thin films has opened up a new challenging research avenue in the IIInitride semiconductors. 11 InN, together with its alloys of GaN and AlN, enable the operation of LEDs and LDs ranging in spectral wavelength from infrared all the way down to deep ultraviolet. It has also been predicted that InN has the lowest electron effective mass among all the III-nitride semiconductors. 12 As a result, very high electron mobility and very large saturation velocity are expected. It was found by ensemble Monte Carlo (EMC) simulations that InN possesses extremely high transient electron drift velocity.^13-15 InN had been reported to have a bandgap of ≅.1.89 eV¹⁶ In contrast to this general belief, recent experimental results on highquality InN samples have suggested that the bandgap of InN should be around 0.8 eV.^17-24 Some researchers argued that the bandgap of InN was about 1.89 eV as reported in the literature, and the narrow bandgap reported recently might be due to radiative emission from a defect level, like the well-known yellow luminescence of GaN. Others believed that the previous studies were carried out in poor-quality InN films and the films might be contaminated by impurities such as oxygen, which can result in a deep level state well above the conduction band of InN, and as a result they showed a higher bandgap energy value than the real one.