Admittance spectroscopy of GaAs/InGaP MQW structures
Introduction
Owing to important advantages in respect to the GaAs/AlGaAs heterostructure, such as a much lower chemical reactivity to oxygen and negligible DX centre related effects, the lattice matched GaAs/InGaP heterojunction has received considerable attention for electronic and optoelectronic applications, such as heterojunction bipolar transistors [1], [2], solar cells device systems [3], [4] and high power lasers [5]. In spite of the important role of the conduction and valence band offsets on the physical properties of a heterojunction, the values reported in literature appear widely scattered in the present case [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], so that their experimental determination is still of interest in this heterojunction. Admittance spectroscopy of multi-quantum well (MQW) systems [17], [18], [19], which is the most accurate one amongst electrical methods, has not yet been employed at our knowledge in the GaAs/InGaP heterostructure. In this work an accurate determination of the valence band discontinuity ΔEV at the interface of lattice matched GaAs/InGaP heterostructures is performed by admittance spectroscopy measurements in p+/GaAs/InGaP-MQW/n+ structures.
Section snippets
Experimental
p+/MQW/n+ structures, where an undoped and lattice matched GaAs/InGaP MQW region is interposed between a n+-GaAs (0 0 1) substrate and a p+-GaAs cap layer, were grown by low pressure MOVPE using tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP) as V-group element sources. The undoped MQW region consisted of 25–40 periods of 8 nm GaAs/12 nm InGaP with the interposition of a 1.5 nm GaAsP interlayer at each direct GaAs-on-InGaP interface to enhance the interface abruptness [20]. The
Results and discussion
The equilibrium band bending diagram of the p+/MQW/n+ structure is qualitatively sketched in Fig. 1 together with the equivalent electrical circuit. The MQW region is expected to be only partially depleted near the n+ GaAs contact, its averaged electrical conductivity resulting of p-type with net acceptor density in the 1016 cm−3 range. In fact undoped InGaP and GaAs layers grown at the same growth conditions resulted both of p-type with hole density of a few 1016 and 1015 cm−3, respectively. In
Acknowledgements
The authors are grateful to Dr. Renato Magnanini for his assistance in the electrical measurements and to Dr. Roberto Jakomin and Mr. Salvatore Vantaggio for their help in the MOVPE growth of the MQW heterostructures here investigated.
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