J. Woodhead

Tailoring of internal fields in InGaAs/GaAs multiwell structures grown on (111)B GaAs

We present a study of internal field distributions in strained InGaAs/GaAs multiple quantum wells in p‐i‐n structures grown on (111)B‐oriented GaAs. Room temperature photocurrent spectroscopy shows clear blueshifting of the e1‐hh1 transition as the well fields are reduced by external bias. The relative length of total well to total barrier material is shown to be an important factor in determining the well and barrier fields. We demonstrate a photocurrent contrast ratio of 4.5:1 for only 3 V applied bias across a 25 quantum well In0.13Ga0.87As p‐i‐n diode and discuss the implication of our results to the design of high performance electro‐optic modulators and self electro‐optic effect devices in this material system.

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.

Electrical polarization control of vertical-cavity surface-emitting lasers using polarized feedback and a liquid crystal

We report electrical control of the polarization state of a vertical-cavity surface-emitting laser (VCSEL), The VCSEL is subject to strong external optical feedback (up to 6% of emission), with polarization controlled by a liquid-crystal (LC) element, It is found that the contrast ratio of the complete system can be enhanced compared to the contrast ratio of the LC element alone.

Relaxation of strain within multilayer InGaAs/GaAs pseudomorphic structures

Strained‐layer superlattice structures (SLSs) have been grown in InGaAs/GaAs with various GaAs barrier layer thicknesses. Photoluminescence measurements indicate that, in structures with thin barriers, the strained layers interact leading to the relaxation of strain, even though each individual well does not exceed the critical thickness for a single quantum well. These results suggest that a fuller understanding of the mechanisms by which strain relaxes in SLSs is important in order that the constraints on the design of devices using such structures can be known.

Carrier lifetimes in strained InGaAs/(Al)GaAs multiple quantum wells

The effect of strain, strain relief, and barrier design on the carrier lifetime in InGaAs/(Al)GaAs multiple‐quantum‐well samples is investigated. Carriers lifetimes are measured in samples with varying amount of strain, due to increasing indium concentration in the wells, as well as in samples subject to strain relief, with thick barriers and GaAs barriers. Lifetimes of the order of 0.5 ns are measured. The lifetime is sensitive to the presence of indium in the wells but remarkably insensitive to the indium concentration, the strain in the samples, and the barrier composition.

Development of a red VCSEL-to-plastic fiber module for use in parallel optical data links

This paper presents results that have emerged from the European funded ESPRIT Project, Bright Red Surface Emitting Lasers (BREDSELS-23455). The projects main objective has been to develop arrays of Vertical Cavity Surface Emitting Lasers (VCSELs) emitting in the region of 650 nm. These VCSEL arrays, suitably coupled to plastic fiber ribbon, are potentially ideal sources for high-speed plastic optical fiber networks. Linear 1 X 8 VCSEL arrays have been fabricated from wafers grown in multi-wafer MOVPE reactors. Individual VCSELs are shown to generate a peak room temperature power of 2 mW at 674 nm and are capable of operating continuous wave to a temperature of 60 degrees Celsius. The use of selective oxidation in the fabrication process is found to be essential in terms of providing effective heat sinking to the active region, while free carrier absorption is found to be a significant loss mechanism. A detailed description of the device results including modal behavior is presented along with the initial results from the plastic fiber ribbon module.

Room temperature visible (683-713 nm) all-AlGaAs vertical-cavity surface-emitting lasers (VCSELs)

Visible emitting all-AlGaAs vertical-cavity surface-emitting lasers (VCSELs) have been produced by metal organic vapor phase epitaxy (MOVPE) using ultra-high purity source reagents. Lasing was obtained at wavelengths in the range 683-713 nm using four 45 /spl Aring/ Al/sub 0.18/Ga/sub 0.82/As quantum wells in the active region. At room temperature, a minimum threshold current density of 3.8 kA.cm/sup -2/ was measured for a wavelength of 692 nm; this is the lowest value for an all-AlGaAs vertical-cavity laser operating at this wavelength. Growth of the epitaxial mirrors at 5.2 /spl mu/m/h/sup -1/ results in a total growth time of only two and a half hours.

Growth of pseudomorphic InGaAs/GaAs quantum wells on [111]B GaAs for strained layer, piezoelectric, optoelectronic devices

Abstract Pseudomorphically strained InGaAs/GaAs quantum wells grown on [111]B oriented GaAs substrates incorporate strong piezoelectric fields. Quantum confined interband optical transitions exhibit built-in Stark shifts to the red and additional, ‘forbidden’ transitions are observed because of the reduced symmetry. The novel properties of these structures can be exploited in such devices as electro-optic modulators with increased sensitivity, blue-shifting self electro-optic effect devices (SEEDs) with enhanced contrast, all-optical symmetric SEEDs (SSEEDs) and pseudomorphic high electron mobility transistors (HEMTs) with increased two-dimensional electron gas (2DEG) densities. At all stages of research, development and production successful device work necessitates reliable material growth of good crystalline quality. High quality material is routinely obtained for wafers misoriented towards 〈211〉. For substrates misoriented towards the 〈211〉 direction good quality material can also be achieved, but only when great care is taken to ensure that growth is carried out within the narrower optimum growth window. With the help of this understanding we have been able to produce devices which have been used successfully to observe the predicted improvements.

Carrier screening effects in piezoelectric strained InGaAs/GaAs quantum wells grown on the [111]B axis

Excitation‐power‐dependent blue shifts in photoluminescence and electroluminescence are studied in piezoelectric strained layer multiple quantum wells (MQWs) incorporated in p‐i‐n diodes. By investigating MQWs with different geometric structures and controlling external bias it is demonstrated that, in contrast to previous studies, these blue shifts cannot always be attributed to long‐range screening across a MQW and that screening must take place due to charge redistribution within individual wells. The results provide design rules to ensure this latter screening mechanism, which is subject to fast recovery.

Enhancement of a liquid-crystal modulator using an external-cavity VCSEL

We describe a vertical-cavity surface-emitting laser (VCSEL) and liquid-crystal modulator (LCM) system. The LCM controls external optical feedback into the VCSEL to produce a single switching element that has an enhanced contrast ratio compared to an LCM alone.