Structure and properties of dilute nitride gallium arsenic nitride alloy films
Dilute nitride semiconductor alloys are useful for a wide range of applications. A fundamental understanding of how various growth regimes affect the structural, optical and electronic properties is needed for further optimization of device performance. This thesis explores these issues in GaAsN.
We investigated the temperature-dependent mechanisms of growth for GaAsN films. At low temperatures, limited adatom surface mobility leads to layer-by-layer growth. As the temperature increases, the interplay between adatom surface diffusivity and the step-edge diffusion barrier leads to the formation of "mounds". For sufficiently high temperatures, adatoms overcome the step-edge diffusion barrier, resulting in layer-by-layer growth once again.
Using a combination of nuclear reaction analysis and Rutherford backscattering spectrometry, we observe significant composition-dependent incorporation of N into non-substitutional sites, presumably as either N-N or N-As split interstitials. The (2x1) reconstruction is identified as the surface structure which leads to the highest substitutional N incorporation, presumably due to the high number of group V sites per unit area available for N-As surface exchange.
For coherently strained films, a comparison of stresses measured via in-situ wafer curvature measurements, with those determined from x-ray rocking curves is used to quantify composition-dependent elastic constant bowing parameters. For films with x>2.5%, we observe that stress relaxation occurs by a combination of elastic relaxation via island formation and plastic relaxation associated with the formation of stacking faults.
Optical absorption measurements reveal a substitutional nitrogen composition-dependent band gap energy reduction, which is less significant than typical literature reports. However, when the data are corrected to account for the typical 20% incorporation of non-substitutional nitrogen, all measurements reveal a band gap reduction of ∼125 meV per 1% N. Thus, GaAsN band gap bowing is most significantly influenced by substitutional nitrogen and smaller than previously reported.
For bulk-like films, the electron mobility is observed to decrease with increasing N, independent of the arsenic species employed during growth. For GaAsN/GaAs:Si superlattices, the interface quality and electron mobilities are improved by controlling the N plasma flux using a pneumatic gate valve. In the AlGaAs/GaAsN channel layers, N-induced neutral scattering sources are identified as the dominant source of carrier scattering.