Piezoelectric field

FIGURE 1 Calculated electron and hole quantisation energies in nitride quantum wells with and without piezoelectric field... 

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. 

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]). 

Disadvantages Low electron velocity, low current Extra gate bias required to compensate piezoelectric field due to inverted structure Low electron velocity, low current, difficulty in forming a short gate Complex layer in growth... 

The piezoelectric field induces charge at the heterojunction. Even mi unintentionally doped (with low background doping density) AlGaN/GaN heterostructure has a piezoelectric-induced sheet charge in the GaN channel, which is found by electrostatic analysis to be... 

Piezoelectric Field Piezoelectric Field Piezoelectric Stress Piezoelectric Strain... 

Since then, much effort has been expended on proving the advantage of nonpolar/semipolar LEDs over c-plane LEDs. The use of nonpolar nitrides potentially solves the problem of the large piezoelectric field, because no piezoelectric field is expected in the nonpolar heterostructures. 

Figure 5.1 Polar angle dependence of the piezoelectric field in coherently grown Ga0.93ln0.07N/GaN quantum well on GaN template. Zero degree means that the plane-normal axis is the c axis, while 90° means that the plane-normal axis is the a or...

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