Internal field effects in InGaN quantum wells

InGaN/GaN based quantum well structures are strained and due to the lack of a centre of symmetry large internal fields are present which skew the potential of the quantum wells, this has a large effect on the properties of these structures. InGaN/GaN based quantum well structures are studied using a theoretical method based on the Pade model and comparison made with experimental results. The importance of using a correct description for the depletion widths of p-i-n structures for use in interpreting measurements of the internal field is established in this work. Interpreting the results from reverse bias photocurrent absorption measurements of an In0.1Gao.9N quantum well structure, a value of-1.9 MVcm-1 for the internal field has been determined, which is within 5 % of the field of -1.8 MVcm-1 calculated using piezoelectric constants interpolated from the binaries. Comparison of the results of the Pade model and experiment demonstrate that the experimental absorption magnitude is an unreliable indication of the internal field due to the carrier extraction efficiency at low bias, hence the absorption peak energy should be used. Using a theoretical model governed by pumping and recombination processes time-resolved photoluminescence experiments performed at Sheffield University have been modelled. It was found that the observed shift of the emission peak arises from a delicate balance between the contributions from bandgap renormalisation, screening of the internal field and the Coulomb interaction. Comparisons between the free carrier and Pade models, found that the energy shift and magnitude of the peak intensity are underestimated when using the free carrier model. Although the internal field is strongly screened at high carrier density, the increase in the dipole matrix element is small. It was found that at threshold the screened internal field is still of the order of -1 MVcm-1, hence inclusion of the screened internal field is essential in laser gain-current calculation