Mise en page 1 - Laboratoire National des Champs Magnétiques ...

SEMICONDUCTORS AND NANOSTRUCTURES 2009InP/GaP self-assembled quantum dots under extreme conditionsSemiconductor quantum dots are promising for the fabricationof novel optoelectronic devices and a number of investigationshave been carried out to understand the electronicand optical properties of quantum dot systems. One powerfulmethod toward understanding the electronic states andshell structure is to investigate photoluminescence (PL) inmagnetic fields.5 nm is inferred, which is smaller than the radius of thequantum dots of about 10 nm. Thus, the measurement providesfurther evidence that the electrons belong to the InPvalleys and suggests a type-I band alignment for InP/GaPquantum dots [Dewitz et al., Appl. Phys. Lett. 95, 151105(2009)].The quantum dot material systems that have been the mostextensively investigated using magnetoluminescence areInGaAs/GaAs and InP/InGaP quantum dots. The InP/GaPquantum dot material system has received significantly lessattention. It is very interesting, however, for both a fundamentalunderstanding of quantum dots based on directindirectband gap semiconductors, as well as its potentialapplications for visible light emitters.Figure 73: Pressure dependence of the magneto-photoluminescencededuced values of the confinement length l 0 (squares) andthe exciton effective Bohr radius a ∗ (triangles) within the quantumdots. This reveals an increase of the quantum confinement in theplane of the dots.Figure 72: Pressure dependence of the energy of the two photoluminescencelines up to 1.2 GPa. These two lines correspond tothe direct radiative recombination between the ground and the firstexcited states of the dots. An unusually low pressure coefficientis measured (indicated by the slopes of the solid lines, which arelinear fits to the data).We have first investigated the radiative recombination inInP/GaP self-assembled quantum dots measured in highmagnetic fields. Using magneto-photoluminescence spectroscopythe reduced effective mass is determined to havea value of 0.094 m 0 expected for electron states associatedwith the InP valley. Using the determined effective massand the diamagnetic shift in the photoluminescence peakat low magnetic fields, an average exciton radius of aboutIn addition, we have also performed cryogenichigh-pressure photoluminescence and magnetophotoluminescenceexperiments in the 1 GPa range, inseveral cycles. As previously reported [Goñi et al., Phys.Rev. B 67, 075306 (2003)], the luminescence is quenchedat 1.2 GPa, and the intensity decreases continuously. However,our experiments do not show any signature of theΓ → X crossover reported to occur between ambient pressureand 0.3 GPa. We observe in fact a linear and continuousincrease of both the ground state and the first excitedexcitonic state photoluminescence features up to 1.2 GPawhere the emission is suppressed. A very low pressure coefficientis measured which may originate from a decreaseof the quantum confinement along z driven by a strong increaseof the effective mass. On the other hand, in the planeof the dots, an increase of the quantum confinement is evidenced[Millot et al., accepted in High Pressure Research].M. Millot, S. George, J. Leotin and J.M. BrotoC.v. Dewitz, F. Hatami and W.T. Masselink (Humboldt-Universitt, Berlin, Germany) and J. Gonzalez (DCITIMAC,Santander, Spain)52