High-pressure investigation of InGan quantum wells
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ABSTRACT We have studied the pressure dependence of photoluminescence for various InGaN quantum wells of different Indium compositions. In contrast to classical quantum well systems such as GaAs/AlGaAs, the pressure coefficients of the photoluminescence peak position of wells of similar parameters can vary greatly, depending on conditions of growth. Generally, the pressure coefficient of the emission peak is much lower than the one predicted for a GaN-InN system. We show that this behavior can be related in most cases to the influence of the thick GaN layer compressibility on the strain in the InGaN quantum wells. However, in some cases (Nichia devices), the pressure coefficient of the emission peak seems to be unrelated to the shift of the energy gap of the material, suggesting the involvement of strongly localized electronic states.
INTRODUCTION InGa-.,N is at the present the most important material in short-wavelength optoelectronics'. However, the mechanism of the radiative emission in InGa 1 .xN quantum wells (QW) is still far from being understood. Usually, the photoluminescence peaks observed in such structures do not follow the changes in the energy gap which result from temperature or external pressure changes. These observation were previously explained through the use of strong carrier localization on potential fluctuation of the alloy 2,3. These fluctuations are not necessarily the only reason for the anomalous optical behavior of these structures. Since the In.Ga-,_N quantum wells are not lattice matched to GaN (a thick GaN layer is usually grown first on the typical sapphire substrate), they should be highly strained (if the growth has a pseudomorphic character). The presence of the piezoelectric effect due to strain can and does influence the optical properties of these structures. Very recent evidence, suggests the possibility of the existence of very highly strained QWs of In 1Gal.,N for x as high as 0.2 4,5 The use of high pressure, as it is commonly used in the study of semiconductors, can help us in perceiving the difference between cases where the radiative transition involves localized states and the cases where it involves normal band to band recombination. However, as a first step, we have to say what kind of behavior we expect from strained pseudomorphic layers. PSEUDOMORPHIC In.Ga-.,N ON GaN.
SOn leave
from High Pressure Research Center, Warsaw, Poland, e-mail: [email protected]
399 Mat. Res. Soc. Symp. Proc. Vol. 512 © 1998 Materials Research Society
InGalxN QWs, we have to know the pressure coefficient of the energy gap of this material. Unfortunately, there is a lack of experimental data on the pressure induced changes in the energy gap of InN. This compound is virtualy impossible to grow as a single crystal with decent quality, or as a single crystalline epilayer. Through the rest of this publication, we will assume that dEg/dp = 33 meV/GPa for InN. This value was calculated from first principles by Christensen et a16 . It is surprising that this far softer material has a lower
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