The Magnitude of the Piezoelectric Effect in InGaN Quantum Wells
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ABSTRACT A photoluminescence from a multiquantum GaN/InGa 1 .xN/GaN well structure ( x varies between 0.1 to 0.4) was investigated at various temperatures, pumping powers. While the temperature dependence of the peak position indicates normal band to band character of radiative recombination, the large pumping power induced "blue"shift of the peak position (up to 200 meV) can be observed. This kind of shift cannot be easily explained by the band tailing effect but is most likely the result of the screening of the strain-induced piezoelectric field. By evaluating the theoretical values of the piezoelectric filelds in the quantum well, we can show that in order to account for the experimental results we have to assume the partial relaxation of the strain.
INTRODUCTION In.Gal.,N is presently the only material used commercially as an active layer in high brightness optoelectronic devices emitting in the green-to-UV spectral region"'2. However, there are many basic technological problems related to the application of this material in the devices (mostly the quantum well structures). First, the InN/GaN sytem is not lattice matched (11% difference between the lattice constant of these two compounds). Secondly, the small missibility in the InN/GaN system leads to large fluctuation of the indium content and, frequently, also to phase separation 3,4. The combination of disorder effects and of strain makes the understanding of the optical processes in InGaN quantum wells complicated and challenging. In this paper, we focus on the piezoelectric field generated by strain and its consequences on the optical processes in InGaN/GaN multiple quantum wells.
SAMPLES AND EXPERIMENTS The MOCVD grown, QW structure consists of 1.8 gm thick GaN layer grown on sapphire, followed by six quantum wells. Each quantum well consists of a 50 A thick In.Gal-xN layer sandwiched between two 200 A thick GaN barriers. The quantum wells are divided into three pairs inwhich the x value is 0.1 for the first pair of wells, 0.2 for the second pair and 0.4 for the third pair. Photoluminescence was excited by a 25 mW He-Cd 325 nm laser line with a spot size diameter of 10gtm. The photoluminescence light was dispersed by a double 1404 Spex spectrometer and detected by a GaAs photomultiplier. ' On leave from High Pressure Research Center, Warsaw, Poland, e-mail: [email protected] 187 Mat. Res. Soc. Symp. Proc. Vol. 512 ©1998 Materials Research Society
RESULTS Spectra obtained for different excitation powers are shown in Fig. 1. 100 % of power equals a power density of 10 kW/cm 2 . We can distinguish the luminescence from all three pairs of quantum wells. Their energetic positions lie below that of bulk InGaN material 5. We will refer to this fact as a "red" shift later in the text. All three lines exhibit remarkable small linewidths. A value of 110 meV for a nominally 40 % In concentration in the well is to our knowledge the smallest value reported (see Reference 6 for the analysis of the linewidth in InGaN material). The line originating from the Ino.lo
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