X-ray Spectroscopic Studies of the Bulk Electronic Structure of InGaN Alloys
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X-ray Spectroscopic Studies of the Bulk Electronic Structure of InGaN Alloys Cormac McGuinness, James E. Downes, Philip Ryan* and Kevin E. Smith** Department of Physics, Boston University, Boston, MA 02215 Dharanipal Doppalapudi and Theodore D. Moustakas Electrical and Computer Engineering Department, Boston University, Boston, MA 02215 ABSTRACT Synchrotron radiation excited soft x-ray emission and soft x-ray absorption spectroscopies are applied to the study of the electronic structure of InxGa1-xN alloys with (0 # x #0.29). The elementally resolved partial density of states of the valence and conduction bands may be measured using these spectroscopies. The x-ray absorption spectra indicate that the conduction band broadens considerably with increasing indium incorporation. The band gap evolution as a function of indium content derives primarily from this broadening of the conduction band states. The emission spectra indicate that motion of the valence band makes a smaller contribution to the evolution of the band gap. This gap evolution differs from previous studies on the AlxGa1-xN alloy system, which observed a linear valence band shift through the series (0 # x # 1). Instead in the case of InxGa1-xN the valence band exhibits a significant shift between x = 0 and x = 0.1 with minimal movement thereafter. Furthermore, evidence of In 4d -N 2p and Ga 3d - N 2p hybridisation is reported. Finally, the thermal stability of an In0.11Ga0.89N film was investigated. Both emission and absorption spectra were found to have a temperature dependent shift in energy, but the overall definition of the spectra was unaltered even at annealing temperatures well beyond the growth temperature of the film. INTRODUCTION It is abundantly clear that nitride based wide band gap semiconductors have great technological potential due to their application in light emitting devices coupled with their extreme hardness and the ability to sustain high temperatures.1-5 Optoelectronic devices made from alloys of InN, GaN and AlN cover the visible to ultraviolet range of the electromagnetic spectrum as their band gap energies are 1.9 eV, 3.4 eV, and 6.2 eV respectively. However, experimental data on the detailed electronic structure of these alloys is scarce, particularly in the case of InxGa1-xN. Although photoemission spectroscopy is a standard probe of electronic structure in solids, it is difficult to apply to InxGa1-xN since it requires that atomically clean surfaces be prepared before bulk properties can be measured.6 Successful methods exists for cleaning surfaces of GaN thin films, but obtaining clean surfaces of InxGa1-xN alloys suitable for photoemission studies is very difficult.7 However soft x-ray emission (SXE) and soft x-ray absorption (SXA) spectroscopies are ideal probes of the valence and conduction band density of states as in these spectroscopies there is a sampling depth of approximately 1000 Å, and they can thus be considered insensitive to atomic contamination or disorder of sample surfaces.8 The electronic structure
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