Phase segregation in Mg_xZn_(1-) _xO probed by optical absorption and photoluminescence at high pressure

摘要

The appearance of segregated wurtzite Mg_xZn_(1-) _xO with low Mg content in thin films with x > 0.3 affected by phase separation cannot be reliably probed with crystallographic techniques owing to its embedded nanocrystalline configuration. Here, we show a high-pressure approach which exploits the distinctive behaviors under pressure of wurtzite Mg_xZn_(1-) _xO thin films with different Mg contents to unveil phase segregation for x > 0.3. By using ambient conditions, photoluminescence (PL), and with optical absorption and PL under high pressure for x = 0.3, we show that the appearance of a segregated wurtzite phase with a magnesium content of x ∼ 0.1 is inherent to the wurtzite and rock-salt phase separation. We also show that the presence of segregated wurtzite phase in oversaturated thin films phase is responsible for the low-energy absorption tail observed above x = 0.3 in our Mg_xZn_(1-) _xO thin films. Our study has also allowed us to extend the concentration dependence of the pressure coefficient of the band gap from the previous limit of x = 0.13 to x ≈ 0.3 obtaining dE_g/dP = 29 meV/GPa for wurtzite with x ≈ 0.3 and 25 meV/GPa for the segregated x ≈ 0.09 wurtzite phase.