GalnN Quantum Wells Piezoelectricity

The theoretical piezoelectric field in fully-strained GalnN layers grown on GaN with (0001) orientation was reported as shown in FIGURE 1 [5], This figure shows that large piezoelectric fields in the order of MV/cm can be induced. Based on the sign of piezoelectric constants, the expected direction of the piezoelectric field is 0001 . In this calculation the spontaneous polarisation could be neglected, because the measurable field depends only on the difference of the total polarisations across the 

FIGURE 1 Calculated piezoelectric field along the 0001 direction in strained GalnN layer on GaN as a function of InN mole fraction. In this calculation, the piezoelectric constants in [4] were used (from [5]). 

FIGURE 2 PL peak energy of a Gao 84lno.i6N/GaN QW p-i-n structure as a function of the applied voltage. Solid circles represent the measured PL peak energies. Solid lines represent the calculated PL peak energies in the cases of the piezoelectric fields of -1.3,0 and 1.3 MV/cm (from [7]). 

MQWs as a function of well width. Solid and open threshold excitation power density in GalnN/GaN 

blue and green InGaN quantum well structure LEDs 

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When piezoelectric fields are taken into consideration, the oscillator strength of GalnN/GaN quantum wells decreases strongly with increasing well width or increasing magnitude of the field [6], This is due to the spatial separation of the electron and hole wavefimctions in the case of wide wells and strong fields. 

Time-resolved photoluminescence was also used to show that the spatial separation of the electron and hole wavefunctions due to the piezoelectric fields in GalnN/GaN QWs leads to a dramatic reduction in oscillator strength, particularly for thick quantum wells [6]. Due to the reduced oscillator strength for the lowest energy state, the optical absorption spectrum of the quantum wells is expected to be dominated by highly excited states close to the strained bulk bandgap. 

The optical properties of GalnN/GaN quantum wells differ somewhat from the well-known behaviour of other III-V-based strained quantum well structures, partly due to the rather strong composition and well width fluctuations, possibly induced by a partial phase separation of InN and GaN. The even more dominant effect seems to be the piezoelectric field characteristic for strained wurtzite quantum wells, which strongly modifies the transition energies and the oscillator strengths. However, the relative influence of localisation and piezoelectric field effect is still subject to considerable controversy. 

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