Photoluminescence ͑PL͒, Raman spectroscopy, and x-ray diffraction are employed to demonstrate the coexistence of a biaxial and a hydrostatic strain that can be present in GaN thin films. The biaxial strain originates from growth on lattice-mismatched substrates and from post-growth cooling. An additional hydrostatic strain is shown to be introduced by the presence of point defects. A consistent description of the experimental results is derived within the limits of the linear and isotropic elastic theory using a Poisson ratio ϭ0.23Ϯ0.06 and a bulk modulus Bϭ200Ϯ20 GPa. These isotropic elastic constants help to judge the validity of published anisotropic elastic constants that vary greatly. Calibration constants for strain-induced shifts of the near-bandedge PL lines with respect to the E 2 Raman mode are given for strain-free, biaxially strained, and hydrostatically contracted or expanded thin films. They allow us to extract differences between hydrostatic and biaxial stress components if present. In particular, we determine that a biaxial stress of one GPa would shift the near-band-edge PL lines by 27Ϯ2 meV and the E 2 Raman mode by 4.2Ϯ0.3 cm Ϫ1 by use of the listed isotropic elastic constants. It is expected from the analyses that stoichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom. ͓S0163-1829͑96͒03148-7͔

Chance discoveries of weapons, horse bones and human skeletal remains along the banks of the River Tollense led to a campaign of research which has identified them as the debris from a Bronze Age battle. The resources of war included horses, arrowheads and wooden clubs, and the dead had suffered blows indicating face-to-face combat. This surprisingly modern and decidedly vicious struggle took place over the swampy braided streams of the river in an area of settled, possibly coveted, territory. Washed along by the current, the bodies and weapons came to rest on a single alluvial surface.

We have performed a detailed investigation of the photoluminescence pressure dependence of heteroepitaxial GaN thin films on sapphire substrates. A comparison between as grown GaN on sapphire and free-standing GaN membranes, created using a laser assisted substrate liftoff process, revealed that the presence of the sapphire substrate leads to an energy gap pressure coefficient reduction of approximately 5%. This result agrees with the numerical simulations presented in this article. We established that the linear pressure coefficient of free-standing GaN is 41.4±0.2 meV/GPa, and that the deformation potential of the energy gap is −9.36±0.04 eV. Our results also suggest a new, lower value of the pressure derivative for the bulk modulus of GaN (B′=3.5).

The energies of photo-and electroluminescence transitions in In x Ga 1Ϫx N quantum wells exhibit a characteristic ''blueshift'' with increasing pumping power. This effect has been attributed either to band-tail filling, or to screening of piezoelectric fields. We have studied the pressure and temperature behavior of radiative recombination in In x Ga 1Ϫx N/GaN quantum wells with xϭ0.06, 0.10, and 0.15. We find that, although the recombination has primarily a band-to-band character, the excitation-power induced blueshift can be attributed uniquely to piezoelectric screening. Calculations of the piezoelectric field in pseudomorphic In x Ga 1Ϫx N layers agree very well with the observed Stokes redshift of the photoluminescence. The observed pressure coefficients of the photoluminescence ͑25-37 meV/GPa͒ are surprisingly low, and, so far, their magnitude can only be partially explained.