On the Bandstructure in GaInN/GaN Heterostructures - Strain, Band Gap and Piezoelectric Effect
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Internet Journal Nitride Semiconductor Research
On the Bandstructure in GaInN/GaN Heterostructures Strain, Band Gap and Piezoelectric Effect Christian Wetzel1, Shugo Nitta1, Tetsuya Takeuchi1, Shigeo Yamaguchi1, H. Amano1 and I. Akasaki1 1Department
of Electrical and Electronic Engineering, Meijo University,
(Received Friday, August 28, 1998; accepted Tuesday, October 6, 1998)
A study of the optoelectronic properties of strained 40 nm Ga1-xInxN layers on GaN films is presented. The fact of pseudomorphic strain leads to a new interpretation of the film composition when derived from x-ray scattering. In addition we directly confirm that strain induces huge piezoelectric fields in this uniaxial system by the observation of Franz-Keldysh oscillations in photoreflection. As a function of composition (0 < x < 0.2) and strain we derive the electronic band gap energy and the piezoelectric field strength. We interpret both in terms of effective bowing parameters and piezoelectric coefficients, respectively. From a spatially resolved micro photoluminescence at room temperature we find no evidence for spatial band gap or composition variations of more than 60 meV over the length scale from 1 to 50 µm (x=0.187) in our material. At the same time, an observed discrepancy between photoluminescence peak energy and photoreflection band gap energy increases with x to some 160 meV. We attribute this redshift to photon assisted tunneling in the huge piezoelectric fields (Franz-Keldysh effect).
1
Introduction
Significant progress in the hetero-epitaxy of group-III nitrides [1] using low temperature deposited buffer layers [2] [3] [4], multiple buffer layer techniques [5] and lateral epitaxial overgrowth [6] have led to record breaking devices such as light emitting diodes and laser diodes [7] [8] in the UV, blue and green region as well as high frequency [9] and high power [10] switching devices. As a function of composition the alloys of GaN and InN should cover the entire spectral region from the very near UV to the red while AlGaN covers a wide range in the near UV. In detail, however, significant controversy exists about the compositional dependence of the electronic band gap in Ga1-xInxN and its heterostructures with, i.e., GaN (see. e.g. Refs. [11] [12]). Comparing electro- and photoluminescence results with photo voltage and electroabsorption data Chichibu et al. [13] [14] [15] find significant discrepancies in the band gap energy between absorption and emission energies amounting to some 100 to 300 meV. It has been proposed that this effect should be correlated with the difficulty to homogeneously incorporate high InN fractions during layer growth [16] [17] [14] [15]. Indeed inhomogeneous luminescence and structural properties as
revealed by spatially resolved experiments [16] [17] [18] have been reported to find grain-like features that could define areas of variable band gap energy. In the extreme case large electric dipole moments in quantum dot-like features [19] could result in high optical gain of the material whi
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