E.A. Khoo

PARTIAL SCREENING OF INTERNAL ELECTRIC FIELDS IN STRAINED PIEZOELECTRIC QUANTUM WELL LASERS : IMPLICATIONS FOR OPTOELECTRONIC INTEGRATION

The spectral electroluminescence characteristics of broad‐area (Al)GaAs/In0.23Ga0.77As/(Al)GaAs single quantum well separate confinement heterostructure lasers grown on (111)B GaAs have been studied under forward biased current injection. A room‐temperature threshold current density of 750 A/cm2 is measured for a 1000 μm laser. The subthreshold electroluminescence spectrum blue shifts with increasing current up to the point of lasing threshold. Our measurements reveal that lasing is achieved while there is a strong residual or ‘‘unscreened’’ electric field across the quantum well. Based on these observations we outline how piezoelectric quantum wells can be used to monolithically integrate a quantum well laser with a blue‐shifting electroabsorption modulator.

InxGa1-xAs/InP quantum well structures grown on [111]B InP

Abstract We report growth and characterization details of lattice matched and coherently strained In x Ga 1− x As/InP quantum well structures grown on misoriented [111]B InP substrates. Photoluminescence from a range of such structures, grown on substrates with optimum misorientation, show linewidths as good or better than equivalent [100] layers. Multiquantum well (MQW) samples with relatively modest compressive strain show X-ray diffraction data characteristic of highly uniform pseudomorphic quantum wells. With increased strain ( x = 0.63), relaxation is evident through a degradation of the diffracted peak widths and through the observation of defects in the surface morphology. Fabricated strained p-i(MQW)-n diode structures exhibit low reverse leakage current densities (e.g. j = 6 μA/cm 2 ). Room temperature photocurrent measurements on these devices show a strong excitonic blue shift (15 nm) with applied bias, as a consequence of the built-in piezoelectric field. The rate of peak shift, up to 8 nm/V, demonstrates excellent potential for low voltage optical modulator applications at around 1.55 μm wavelength.

Photoluminescence linewidths of piezoelectric quantum wells

The photoluminescence linewidths of strained InxGa1−xAs/GaAs quantum wells grown on (111)B GaAs substrates are shown to differ radically from those grown on (001) substrates. In (111)B structures, the linewidth in wide wells is broadened significantly compared with (001) structures. This broadening cannot be explained by well width or alloy fluctuations alone, and suggests the presence of another mechanism which we believe is due to the piezoelectric field and its sensitivity to screening under illumination.

Memory effects on piezoelectric InGaAs/GaAs MQW PIN diodes

Abstract Charge accumulation erects in piezoelectric multiple quantum well (MQW) InGaAs/GaAs PIN diodes grown on (111)B GaAs substrates have been studied regarding memory applications. Strain-induced piezoelectric fields allow new PIN structures with configurations of negative average electric field (NAF) active region. These new devices can store an electric dipole with spatially separated electrons and holes that have low recombination probability and thus long lifetimes. This produces a longrange screening of the field in the active region and hence a strong blue shift of the absorption band edge (maximum light transmission for reading purposes). Both a light pulse and a forward voltage pulse are able to create the dipole (data writing or charged device). The stored dipole can be removed by a reverse electrical pulse (data erasing or device discharge), resulting in a minimum light transmission across the device. Capacitance voltage and time resolved capacitance measurements, after single optical or electrical charging pulse at low temperature (20 K) have been used to determine the stored dipole behaviour. Capacitance transients analysis allowed study of the kinetics of the discharge process, which shows a non-exponential behaviour with storage times up to 10 3 sec, suggesting very long time refresh cycles. Time resolved photocurrent has been used to check read and write capabilities giving on-off ratios up to 30.

Intracavity piezoelectric InGaAs/GaAs laser modulator

Integration of a laser and modulator is shown to be possible in the InGaAs/AlGaAs material system by growing on a B GaAs substrate and utilizing the piezoelectric effect. The absorption characteristics of the modulator section are initially red shifted due to the built-in piezoelectric field and can be easily blue shifted with applied reverse bias. Since even under lasing conditions there is found to be a significant residual piezoelectric field in the quantum well, the modulator can be biased to a shorter wavelength than the lasing emission. Utilizing these effects a simple two-section laser-modulator device in which the absorber section lies within the laser cavity has been fabricated. The result show that the threshold current of the laser- modulator structure is controlled by the reverse bias voltage and hence absorption in the modulator section.

In/sub x/Ga/sub 1-x/As/InP multiquantum well structures grown on [111]B InP substrates

We report the growth and characterisation of In/sub x/Ga/sub 1-x/As/InP quantum well (QW) structures grown on [111]B InP substrates. Photoluminescence (PL) from a range of such structures, grown on substrates with optimum misorientation, show PL linewidths and peak positions equivalent to similar [100] structures. Structural studies, using transmission electron microscopy (TEM) and x-ray diffraction (XRD) show that multiquantum well (MQW) specimens exhibit a high degree of uniformity and a low level of defects. With increasing compressive strain (x>0.53) MQW samples eventually show relaxation, observed through an increase of the XRD peak width and through an increase in linear surface morphological features. Room temperature photocurrent measurements on strained p-i(MQW)-n diode structures show a strong excitonic blue shift, demonstrating excellent potential for low voltage, long wavelength, optical modulators.