Modification Of Polarization Field In III-nitride Semiconductors

Ⅲ-nitride semiconductors are very promising materials for their applications in optoelectronic devices(both emitters and detectors) and high power/temperature electronic devices.Being non-centro-symmetric and high degree ionicity,wurtziteⅢ-nitrides exhibit strong spontaneous and piezoelectric polarization effects,which influence carrier distributions,electric fields,and consequently a wide range of optical and electronic properties ofⅢ-nitrides and devices.In order to study the polarization effect onⅢ-nitrides,and to exploit the polarization field to advantage in nitride semiconductors and device engineering,comprehensive theoretical designs and heteroepitaxial growth of structuralⅢ-nitrides have been performed by the first-principles simulation and MOVPE technique.The site-selective codoped configurations have been designed and simulated for the first time by the first-principles calculation method to modify the polarization field in GaN.As indicated by the calculated electronic structures,the site-selective codoped Mg and Si dopants induce a localized polarization field in GaN,which will enhance the Mg concentration and reduce the activation energy of Mg acceptor.Mg- and Si-δ-codoped AlGaN/GaN superlattices(SLs) have been proposed and designed for the first time to modify the polarization field.The simulated results of the SLs structures show that conspicuous band bending occurs in the Mg- and Si-δ-codoped SLs,which indicates that the polarization field in SL has been intensified.Accordingly, the Mg- and Si-δ-codoped p-type AlGaN/GaN SLs have been grown by MOVPE.The hole concentrations and mobilities in the Mg- and Si-δ-codoped SLs have been measured to be larger than that of modulation-doped SL by Hall effect.Furthermore,by applying the Mg- and Si-δ-codoped SLs as the p-type layers,we have successfully fabricated deep UV-LED with superior current-voltage characteristics and high electroluminescence(EL) intensities of the wavelength down to 213 nm.Mg doped InGaN/GaN quantum well(QW) has been designed to modify the polarization field and the quantized levels in QW.The decrease band bending and the enlargement of the effective band gap in Mg-doped QW indicate that the polarization field has been reduced by Mg dopant.Accordingly,the undoped and Mg doped InGaN/GaN MQWs have been grown by MOVPE.At high injection-current level,two distinct EL emission peaks associated with the quantized level transitions are investigated in both undoped and Mg doped MQWs.Moreover,the energies of both two peaks in Mg-doped MQWs are higher,while the energy separation between them is smaller than that in undoped MQWs.These facts indicate that the quantized levels are modified by Mg dopant due to the reduction of polarization field.