Bandgap and Defect Engineering for Semiconductor Holographic Materials
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rameters. Likewise, defect engineering in semiconductors provides flexibility in the choice of defects, their concentrations, and degree of compensation. Bandgap and defect engineering combined make customdesigned PR materials possible. Exciton Electrooptics The strongest electrooptic effect in semiconductors is the resonant excitonic electroabsorption for photon energies
Exciton-Lifetime-Broadening ^——"
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SI-GaAs
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1.40
1.41
1.42
0 kV/cm 3.75kV/cm 6.25 kV/cm
1.43
1.44
1.45
Energy (eV)
Figure 1. Absorbance of room-temperature excitons in semi-insulating GaAs. Lifetime broadening of the exciton dominates the electroabsorption of the fundamental band edge.
tuned near the fundamental bandgap. The excitonic transition lineshape is highly sensitive to electric fields, and can be broadened and its energy shifted. The electroabsorption is accompanied by an electric-field-induced change in the refractive index, called electrorefraction, required by the Kramers-Kronig relations. The refractive index and the absorption coefficient depend quadratically on electric field
(1)
where the refractive index n is a weak function of wavelength, and the quadratic electrooptic coefficients Si and s2 are strong functions of wavelength, with appreciable values only for photon energies close to the excitonic transition. Electroabsorption for a free-exciton transition is caused by exciton lifetime broadening (ELB). Field ionization reduces the exciton lifetime and increases the exciton transition linewidth. Despite severe thermal broadening at room temperature, ELB dominates the electrooptic effect in bulk semiconductors, and is a relatively strong and sensitive effect. The absorption by room temperature excitons in a thin film of GaAs is shown in Figure 1 for different electric field strengths. The electroabsorption and electrorefraction form absorption gratings and refractive index gratings during PR mixing. Excitons confined within quantum wells have stronger transitions and sharper lineshapes because the electron-hole pair wavefunction is localized, increasing the exciton binding energy and reducing the thermal ionization rate. Sharper and stronger transition lines produce larger electroabsorption. Changing the composition of the wells or barriers and changing the well and barrier widths all strongly influence the electrooptic coefficients in Equation 1, as does the direction of applied electric field. When an electric field is applied in the plane of the quantum wells, exciton lifetime broadening remains the dominant electrooptic effect. When a large electric field is applied normal to the quantum well planes, the quantum-confined Stark effect (QCSE) provides a much stronger electrooptic effect.1 The exciton oscillator strength shifts to lower energy under applied field, shown in Figure 2, producing large electroabsorption and electrorefraction, which are shown in Figure 3. Self-electrooptic-effect devices
MRS BULLETIN/MARCH 1994
J
Bandgap and Defect Engineering for Semiconductor Holographic Materials
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