Improved carrier collection in intermixed InGaAs/GaAs quantum wells

Abstract

We have used photoluminescence up conversion to study the carrier capture times into intermixed InGaAs/GaAs quantum wells. We have found that the capture into the intermixed wells is markedly faster than capture into the reference ͑unintermixed͒ quantum wells. The reasons for the significant reduction in the capture time is related to the shape of the intermixed quantum well. Such a reduction in the capture time is beneficial both in terms of the quantum efficiency and the frequency response of intermixed optoelectronic devices. © 1998 American Institute of Physics. ͓S0003-6951͑98͒01849-X͔Quantum well intermixing ͑QWI͒ is a method of considerable recent interest due to its wide applicability in optoelectronics. 1 QWI is based on the modification of the shape of the quantum well, and allows the post growth adjustment of several key parameters, such as the effective band gap, the optical absorption coefficient, and the refractive index. This flexibility enables the monolithic integration of optoelectronic devices, such as lasers, modulators, waveguides, amplifiers, etc. onto a single chip. 2 QWI has also been used to fabricate blueshifted 3 and multiplewavelength laser diodes, low threshold lasers, and lasers with saturable absorbers, in addition to improving the performance of high power lasers. 1 In this letter, we shall show that in addition to modifying the quantum well parameters mentioned above, QWI can also have a significant impact on the carrier capture into ͑intermixed͒ quantum wells. We shall show that in intermixed InGaAs/GaAs quantum wells, the carrier capture is faster than in similar but nonintermixed quantum wells. This efficient carrier capture can explain the relatively high quantum efficiency of intermixed light emitting devices, and may also result in improved frequency response of optoelectronic devices in general. The carrier capture times were determined from time resolved photoluminescence measurements using the photoluminescence up-conversion technique.The photoluminescence ͑PL͒ up-conversion experiments 4 were performed using a femtosecond self-mode locked Ti:sapphire laser, using a LiIO 3 nonlinear crystal. The laser was tunable between 750 and 900 nm, the pulse width was 80 fs, the repetition rate 85 MHZ, and the output power was 200 mW at ϭ780 nm. The time resolution of our system was approximately 200 fs. The optically excited carrier concentration was approximately 4ϫ10 10 carriers/cm 2 . The samples were mounted in a variable temperature, closed cycle, He cryostat, the temperature of which could be varied between 8 and 300 K. In this study we shall discuss results obtained on a number of In x Ga 1Ϫx As/GaAs quantum well ͑QW͒ samples grown by metalorganic chemical vapor deposition. Each sample contained two QWs: a 5-nm-wide In 0.15 Ga 0.85 As well and a 5-nm-wide In 0.3 Ga 0.7 As well embedded between 50-nm-thick GaAs barriers. To achieve intermixing, we used room temperature proton implantation in combination with rapid thermal annealing ͑RTA͒. Details of the implantation conditions...

Spin‐polarized surface‐emitting lasers

“…4. The sign of Π C is consistent with majority spin injection as is typically observed for spin-LED containing Fe/AlGaAs or Fe/Al 2 O 3 tunnel injectors [5,9]. Since the Fe layer is incorporated outside of the resonant cavity, a parasitic polarization arising from magnetic circular dichroism (MCD) is absent in our device as longitudinal and transverse optical confinement are ensured by the DBR mirrors and airpost geometry, respectively, such that the emission never passes through the magnetic layer.…”

“…Above threshold the carrier lifetime is on the order of a few tens of picoseconds and is limited primarily by the QW carrier capture time, which includes both diffusion in the barrier and capture by the well. Techniques such as QW intermixing [9] and impurity-mediated resonant tunnelling [10] have been shown experimentally to reduce the carrier capture time in InGaAs/GaAs QWs from 45 ps to 20 ps at 12 K and from 80-150 ps to 600 fs over the temperature range 10-150 K, respectively. Application of such techniques to the design of spin-VCSELs could result in larger values of Π C for a given, yet small, spin polarization.…”

Picosecond photoluminescence dynamics in an InGaAs/GaAs quantum-well heterostructure

“…The trapping time is commonly determined from the rise time of the PL signal for the basic transition in the QW. In the simplest case, e.g., in [5], the trapping time of charge carriers in a QW is determined in terms of the rise time of the PL signal for the basic transition in the QW; thus, this is done without separation of the trap ping time proper and the cooling time of charge carri ers in the QW. To determine the trapping time of charge carriers, Blom et al [6] measured the lumines cence rise times for the basic transition in a QW at excitation photon energies higher and lower than the band gap of the QW's barriers.…”

“…Femto second spectroscopy has become one of the basic methods for studying the ultrafast processes associated with various mechanisms of scattering and the subse quent recombination of charge carriers. This method provides a means for comprehensively examining the physical processes that control charge carrier dynam ics at all stages, from photoexcitation, followed by time evolution of charge carrier states, to the transition to equilibrium distribution [4][5][6].…”

Possibility of improved frequency response from intermixed quantum-well devices

“…QWI has already been shown to be a useful and efficient method in several devices, including laser diodes, quantum well infrared photodetectors (QWIPs) and optical waveguides [1][2][3]. We have recently shown that carrier capture into intermixed InGaAs/GaAs quantum wells is markedly faster than capture into similar but nonintermixed quantum wells [4]. We argued that this improved carrier collection is due to the fact that intermixed quantum wells are wider at the top of the well and contain extensive concentration gradients along the growth direction.…”