Small internal electric fields in quaternary InAlGaN heterostructures S.P. Łepkowski 1, P. Lefebvre 2, S. Anceau 1,2, T. Suski 1, H. Teisseyre 1, H. Hirayama 3, and Y. Aoyagi 3 1 UNIPRESS, Polish Academy of Sciences, Sokołowska 29/37, Warszawa, Poland 2 GES, Universite Montpellier II, F Montpellier Cedex 5. France 3 RIKEN (Institute of Physical and Chemical Research) 2-1Hirosawa, Wako-shi, Saitama , Japan GES Universite Montpellier 2 Introduction In the case of InGaN/GaN or GaN/AlGaN QWs [grown along the (0001) direction], we always deal with internal strain in the system, which results in a piezoelectric polarization and thus in a built-in electric field in the QW layers. Spontaneous polarization increases in the built-in electric field in both InGaN/GaN and GaN/AlGaN QWs. The large internal electric fields induced a Quantum Confined Stark Effect (QCSE), which causes the separation of the electron and hole wavefunctions and reduces the optical transition matrix element. On the contrary, for the case of InAlGaN based QWs we 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. The purpose of the work is to determine the magnitude of built-in electric fields in quaternary InAlGaN QWs showing intense light emission. Series ASeries B Theoretical determination of the magnitude of the built-in electric field Details of the growth procedure are in H. Hirayama et al., Appl. Phys. Lett., 80, 1589, (2002) 3MQWs of In 0.05 Al 0.20 Ga 0.75 well width : 1.3~4 nm 5nm barriers of In 0.02 Al 0.30 Ga 0.68 N strain reducer : 2nm 20 nm 30 nm Al 0.2 G 0.8 N 400nm 6H-SiC (0001) Substrate 3MQWs of In 0.05 Al 0.20 Ga 0.75 well width : 0.9~4.7 nm 5nm barriers of In 0.02 Al 0.60 Ga 0.38 N strain reducer : 2nm 20 nm 30 nm Al 0.2 G 0.8 N 400nm 6H-SiC (0001) Substrate Electric field in In x Al y Ga 1-x-y N/In 0.02 Al 0.30 Ga 0.68 N QW (4nm width) polarizations taken from V.Fiorentini et al., Appl. Phys. Lett. 80, 1204, (2002) In the samples the internal electric field is about 1.0 MV/cm Theoretical determination of the magnitude of the built-in electric field Electric field in In x Al y Ga 1-x-y N/In 0.02 Al 0.60 Ga 0.38 N QW (4nm width) polarizations taken from V.Fiorentini et al., Appl. Phys. Lett. 80, 1204, (2002) In the samples the internal electric field is about 3.5 MV/cm InAlGaN quaternary compounds are promising material for UV light emitters InAlGaN based QWs contra wurtzite GaN/AlGaN QWs quaternaries QWsGaN/AlGaN QWs Lack of built-in electric fields possible Built-in electric fields are omnipresent Presence of In-fluctuations (small non-radiative recombination) Absence of In-fluctuations (high non-radiative recombination) Very efficient photoluminescence and electroluminescence from InAlGaN based QWs have been obtained by H. Hirayama et al., Appl. Phys. Lett., 80, 1589, (2002) 1) small internal electric fields 2) large band offsetts 3) presence of In-fluctuations in QWs Difficulties : 1) poor knowledge of parameters (polarizations, band parameters) 2) technological obstacles in achieving alloys of arbitrary compositions Present solution : selection from a variety of grown samples these ones, having the most intensive PL signal Factors important for design quaternary based QWs having efficient light emission Our goal is to determine the value of internal electric field in these samples Photoluminescence spectra Photoluminescence decay times Observation: PL decay times of In 0.05 Al 0.20 Ga 0.75 N/ In 0.02 Al 0.30 Ga 0.68 N QWs do not depend on QW well width. Conclusion: the magnitude of internal electric field in QWs must be small (about 0.1 MV/cm) The magnitude of internal electric field in In 0.05 Al 0.20 Ga 0.75 N/ In 0.02 Al 0.60 Ga 0.38 N QWs is also much smaller (0.6 MV/cm) than theoretical predictions. Hydrostatic pressure experiment Series A Evolution of PL peak position with pressure Comparison of pressure coefficients for different systems Observation: pressure coefficients of In 0.05 Al 0.20 Ga 0.75 N/ In 0.02 Al 0.30 Ga 0.68 N QWs do not depend on QW well width. Conclusion: the magnitude of internal electric field in QWs must be small what agrees with time-resolved PL results. Conclusions 1) The magnitude of internal electric field in the studied quaternaries InAlGaN structures are very small comparing to the theoretical predictions. 2) Excellent light emission observed in this structures (even at room temperature) is related to the small values of the built-in electric field. Speculations on the reasons for unexpected small values of internal electric field in the studied samples : 1) Screening by unintentional doping 2) Poor knowledge of the polarizations for quaternaries 3) Huge In and Al. fluctuations can hide the influence of built-in electric field on the light emission In the QW structures presence of built-in electric field results in : 1)increase of photoluminescence decay time (also with QW width) 2)decrease of the pressure coefficient of emitted light energy (also with QW width) Improvement of temperature characteristics due to higher barriers in the series B