Piezoelectric field effects in InGaN quantum wells

Abstract

D e p m n e " t o / P h r s i c s a~A s t~"~~~, CordiffUniversily. P.O. Box 913. Cardiff CF24 3YB. Woles, U. K. Tel. +44 (0)2920 874458 Fax. +M (OJ2920 840567 emil: smowronpml?&& Weng W. Chow Semiconductor Material and Devices Depmnent, Sondin Nnrionnl Lnbomton'es, Albuquerque. NM 87185-OM))Abstract:. We have measured piezoelectric fields in p-i-n LED structures using the quantum confined 'Stark effect and photocurrent absorption. The results agree with calculations of the absorption where material parameters are interpolated from the binaries. >The piezoelectric field in InGaN quantum wells plays an important role in the operation of nitride bas@ LEDs and lasers, affecting the emission wavelength, the gain spectrum and spontaneous recombination current. Values of the piezoelectric field strength reported in the literature vary by more than a factor of two even for the same nominal indium content and it is therefore uncertain which value of piezoelectric constant should be used when modelling, or understanding the performance of, these structures. In this work we have measured the piezoelectric field strength for a number of p-i-n structures and hence determined the piezoelectric constant. We have measured the amplitude and energy of the peak of the photocurrent absorption spectrum as a function of the externally applied reverse bias.The reverse bias acts to.oppose the piezoelectric field reducing the effect of the piezoelectrically induced quantum confined Stark effect until at a critical value of bias the quantum well is square. At this bias the overlap of.elecmn g d hole wavefunctions (and hence the amplitude of the absorption) and the lowest electron to heavy hole transition energy are maximised (see figure 1). At higher bias the well is skewed in the opposite sense and the amplitude and energy of the absorption peak decrease.We.have measured absorption'specga as a function of the reverse bias and an example of the raw data is plotted in figure 2. The peak amplitude of the photocurrent absorption and the energy of the peak of the lowest transition energy are plotted in figure 3 as a function of the externally applied reverse bias. All the data shown are for a sample consisting of 5 Ino.lsG%.ssN quantum wells, each 3nm wide with 7nm wide Ina.03G%.p7N baniers as described in reference 1. Both the amplitude and transition energy have a maximum at the same value of external voltage of 8M.5 V. The piezoelectric field is derived from this voltage using the expression [2]:where V is the externally applied bias, V,, is the built in voltage, N is the number of wells, L, is the well width, 4, is the width of the undoped active region and dd is the depletion width. To correctly determine the depletion width of a p-i-n structure we make use of an analytic approximation we have derived that includes the effect of the field in the intrinsic region.-We confirm that this is a good approximation over the full range of reverse bias we employ using the software package SimwinTM. Using this approximation we determine the piezoel...