Charge separation in coupled InAs quantum dots and strain-induced quantum dots W. V. Schoenfeld, a) T. Lundstrom, and P. M. Petroff Materials Department, University of California, Santa Barbara, California 93106 D. Gershoni Physics Department and Solid State Institute, Technion, Haifa, Israel 32000 Received 14 December 1998; accepted for publication 17 February 1999 We present an InAs self-assembled quantum dot structure designed to spatially separate and store photo-generated electron-hole pairs. The structure consists of pairs of strain-coupled quantum dots. Separation of electron-hole pairs into the quantum dots and strain-induced quantum dots has been observed using power-dependant photoluminescence and bias-dependent photoluminescence. © 1999 American Institute of Physics. S0003-69519904015-2 A greater knowledge of the growth of self-assembled quantum dots QDs and of their electrical and optical prop- erties is starting to yield quantum dot based devices, e.g., QD infrared detectors, 1–3 QD lasers, 4–7 and QD memory devices. 8–13 The ability to trap, localize, and store carriers within a QD makes it an attractive medium for memory ap- plications. Such devices also offer the potential for multiple storage bits per device by utilizing the size distribution of the self-organized QDs. In this letter we introduce a QDs device designed to spatially separate and store photo-generated electron-hole pairs. The structure consists of two GaAs quantum wells QW of different thicknesses, separated by a thin AlAs bar- rier. An InAs QDs layer is inserted in the thick QW to allow carrier localization within the device. The InAs QDs layer also creates strain-induced quantum dots SIQDs within the thin GaAs QW that are coupled to the InAs QDs. 14 In the structure photo-generated electrons and holes are spatially separated into the InAs QDs and SIQDs, respectively see Fig. 1. An incident photon pulse creates electron-hole pairs within the thin QW. The thicknesses of the GaAs QWs are selected such that their first electron state is either above for the thin QW or below for the thick QW surrounding the InAs QDs layer the X-band minimum of the AlAs barrier. Electrons are able to tunnel into the InAs QDs from the thin GaAs QW through the X-band minimum, while the respec- tive holes remain in the thin QW since there is no interme- diate tunneling route available for holes. Photoluminescence PL samples were grown by mo- lecular beam epitaxy and a growth rate calibration was per- formed through reflection high-energy electron diffraction oscillations. After deposition of an AlAs/GaAs 40 (20 Å/20 Å) short-period supperlattice SPS for smooth- ing, a 500 Å Al 0.5 Ga 0.5 As barrier was grown, followed by a 31 Å GaAs QW in which an InAs QDs layer QDs samples or wetting layer reference samples was inserted. Next, the 100 Å AlAs barrier was deposited, along with the thin 25 Å GaAs QW, a 500 Å Al 0.5 Ga 0.5 As barrier, and a 50 Å GaAs capping layer. Samples used for biased photoluminescence spectroscopy BPL were identical to the PL samples with the addition of a n + -GaAs back gate and a thin SPS layer above the structure to decrease the leakage current through the device. A semitransparent Cr/Au Schottky contact was thermally evaporated on the surface to provide the top con- tact. A typical PL spectrum of a QDs sample is shown in Fig. 2. A QDs peak is present at about 1.32 eV in the PL spectra of the QDs sample transition A of inset in Fig. 2. As ex- pected, PL spectra from the reference sample did not contain such a peak. The strong PL lines around 1.51 eV originate from the GaAs buffer layer in the sample. The broad peak full width at half maximum  FWHM=72 meV observed at 1.8 eV comes from the SIQDs transition B. This peak is associated with an indirect type-II recombination between electrons in the AlAs X minimum and heavy holes in the 25 Å QW. Previous PL studies on AlAs/GaAs superlattices have demonstrated strong luminescence from such a spatially indirect recombination due to the ultrafast transfer 0.4 psec of electrons from the GaAs QW to the X-band mini- mum in the AlAs. 15,16 The SIQDs arise from strain fields above the InAs QDs. The piezopotential band gap modulation 17 resulting from the strain field and the strain induced deformation potential induce carrier localization in the 25 Å QW. Results obtained from theoretical calculations of the piezoelectric and deformation potentials for a 25 Å GaAs QW located 100 Å above an InAs QD indicate a pi- ezoelectric potential of 30 meV for holes and electrons. a Electronic mail: 6500wvs0@ucsbuxa.ucsb.edu FIG. 1. Band diagram schematic of the charge separation process. APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 15 12 APRIL 1999 2194 0003-6951/99/74(15)/2194/3/$15.00 © 1999 American Institute of Physics Downloaded 31 Dec 2000 to 132.68.1.29. Redistribution subject to AIP copyright, see http://ojps.aip.org/aplo/aplcpyrts.html.