Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Solid-State Efecrrmics Vol. 40, zyxwvutsrqponmlkjihgfedcbaZY NOS l-8. pp. 357-361, 1996 003&1101(!6)0032%2 Copyright 0 1996 Elxvicr Science Ltd Printed in Great Britain. All rights ms~rvcd 0038-t lOI/ SI5.00 + 0.00 BAND-FILLING IN InP DOTS: SINGLE DOT SPECTROSCOPY AND CARRIER DYNAMICS M.-E. PISTOL, P. CASTRILLOt, D. HESSMAN, S. ANAND, N. CARLSSON, W. SEIFERT and L. SAMUELSON Department of Solid State Physics, Lund University, Box 118, S-221 00 Lund, Sweden Abatraet-We have measured band-filling in InP dots inbetween GaInP barriers. We find that band-filling occurs at very low optical power densities. About 200 times less optical power density is required for the InP dots, compared with quantum wells, for the same amount of band-filling. We have measured photon emission from single dots and also here we find band-filling. The time-evolution of the emission has been followed and also been modelled using a simple model. Good agreement between theory and experiment is found. The capture time into the dots is around 3 ns and the decay time constant is about 1 ns. zyxwvutsrqponmlkjihg I. INTRODUCllON 2. EXPERIMENTAL Semiconductor dots have a reduced density of states compared with quantum wells. Indeed the energy levels are discrete in this case and there is a finite number of energy eigenstates in a given energy interval (below the continuum). The carrier density in semiconductor dots that have a smaller lateral extension than the carrier diffusion length should be higher than in quantum wells for the same excitation power density[l]. It is thus expected that band-filling phenomena should occur at lower excitation power densities in dots compared with quantum wells and quantum wires, making dots more suitable for non-linear optical devices[l]. We demonstrate in this paper that band-filling is indeed easily obtained in dots, having lateral extensions of 50 nm. By using spatially selective photolumines- cence (PL), realized by the use of a mask with micrometer sized holes and magnifying optics, we can obtain emission spectra from single dots as well as spectrally highly resolved emission spectra from so-called wetting-layer quantum wells, which are also present in the sample. By using time-resolved PL we have followed the time-evolution of the charge carriers. The carriers have a thermal&d dis- tribution and the recombination time is about 1 ns for the whole population. In our samples we do not find any signi6cant reduction in energy intraband relaxation related to a decrease in the elec- tron-phonon scattering rate which has been pre- dicted to occur in dots(2]. We have modelled the timeevolution of the carrier density and we find good agreement between the model and the exper- iment. tOn leave from: Department of Electricity and Electronics, University of VaIIadoIid, Prado de la Magdalena s/n, E-4701 I VaIladoIid. Spain. The growth of the samples was by low-pressure Metal-Organic Vapour Phase Epitaxy, using trimethyl-indium, trimethyl-gallium, at-sine and phosphine on GaAs(OO1) substrates. The coherent InP dots were produced between 300 nm GaInP barriers by deposition of nominally 2.4 monolayers (ML) InP, followed by a growth interrupt of 3 s. These conditions result in a bimodal dot size distri- bution with large dots and small dots. For a more detailed description see Carlsson et a1.[3]. The large dots are very uniform and have a lateral size of 40-50 nm and a height of 13 nm, obtained from transmission electron microscopy and scanning electron microscopy. They are shaped in the form of truncated pyramids. The separation between the large dots was about 1 pm. The size of the smaller dots is much less and we consider them as being part of a “roughening” of the wetting layer which has a thickness of 1 and 2 ML. The large dots have a PL emission energy of about 150 meV higher than the calculated energy for biaxially strained dots. This experimental energy agrees, however, with calculations which include finite element simu- lations of the strain[4]. The theoretical estimations of the energy splitting between the lowest lateral confined levels in the dots are about l-2 meV. Since the average separation between the large dots (which we hereafter will refer to as dots) is a com- fortable 1 pm, our sample is very well suited for single-dot spectroscopy. Charge carriers which are within 1 pm of a dot are efficiently collected into the dots, as seen in cathodoluminescence exper- iments[3]. The carrier collection area is about five hundred times larger than the dot area. The excitation power density required to obtain band- filling compared with a quantum well should scale with the ratio of collection area to recombi- nation area. 357