In ternary InGaN/GaN or GaN/AlGaN quantum wells (QWs) grown along the (0001) direction, presence of large internal strain results in a piezoelectric polarization and thus in a built-in electric field in the QW layers. Moreover, spontaneous polarization characteristic for nitrides crystallizing in wurtzite structures causes an increase of the built-in electric field in both InGaN/GaN and GaN/AlGaN QWs. The large internal electric fields induce in turn the separation of the electron and hole wavefunctions in the QW and a reduction of the optical transition matrix element. This effect is known as the Quantum Confined Stark Effect (QCSE). On the contrary, for the case of quaternary InAlGaN based QWs one may consider cases where the QWs are strained (with compressive or tensile strain) or unstrained. Particularly, one may expect that, for properly chosen compositions of barriers and QWs, the internal electric field is negligible since a term coming from the spontaneous polarization cancels a term from the piezoelectric effect. In such a case no reduction of the optical transition probability due to QCSE occurs.
In this work we analyze the pressure behavior of light emission and the photoluminescence decay time,τ, for structures consisting of InAlGaN based QWs. Both effects are very sensitive to the presence of the internal electric field in the QW systems.τ and the pressure coefficient of the light emission, dE_E/dp show drastic variation with the QW width in the QW system with strong built-in electric field in wurtzite InGaN/GaN and GaN/AlGaN QWs. On the contrary, τ and dE_E/dp remain almost independent of the QW width when the internal electric field in QWs is negligible. It particularly applies to cubic nitride QWs.
We have found surprisingly low magnitude of the built-in electric field in the studied structures. This result is not confirmed by calculations based on available data concerning nonlinear piezoelectric properties of the used nitride alloys.

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