Istituto dei materiali per l'elettronica ed il magnetismo     
Rimada Herrera J. C., Prezioso M., Nasi L., Gombia E., Mosca R., Trevisi G., Seravalli L., Frigeri P., Bocchi C., Franchi S. Electrical and structural characterization of InAs/InGaAs quantum dot structures. In: EXMATEC 2008 - 9TH International Workshop on Expert Evaluation & Control of Compound Semiconductor Materials & Technologies (Lodz (Poland), 1st-4th June 2008). Abstract, pp. 131 - 132. UniversitÓ di Lodz, 2008.
Self assembled InAs quantum dots (QD) embedded in metamorphic InGaAs confining layers are attracting considerable attention for the possibility to obtain QD emission in the spectral window for optical communication at 1.3 Ám and eventually at 1.55 Ám. One of the approaches to redshift the emission of InAs/InGaAs quantum dots nanostructures is the increase of InAs coverage to enlarge the QD size. However the increase of InAs coverage beyond a critical value is expected to result in QD ripening, i.e. in an uncontrolled enlargement of a number of islands which coexist with the stable smaller ones during the Stransky-Krastanov growth mode. It has been recently reported that ripened islands in InAs/GaAs QD structures relax through edge-type dislocations which propagate towards the surface in the form of V-shaped defects, introducing strong depletions of free carriers and deep levels near the QD plane and giving rise to a reduction of the PL intensity [P. Frigeri et al., J. Appl. Phys. 102, 083506 (2007)]. Nevertheless detailed studies on the ripening effects in InAs/InGaAs QD structures have not yet been reported. In this work we present the results of an electrical and structural characterization of InAs/ In0.15Ga0.85As QD structures having InAs coverages that are, respectively, below and above the expected critical value for QD ripening. The structures, deposited by MBE on a semi-insulating GaAs substrate, consist of InAs QDs with coverage of 2.0 and 3.0 ML, respectively, embedded in n-In0.15Ga0.85As layers with a doping level of 3x1016 cm-3. Planar Schottky barriers have been prepared by conventional photolitographic techniques. The samples have been investigated by capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), transmission electron microscope (TEM) and atomic force microscopy (AFM) techniques. C-V profile measurements show the appearance of a clear accumulation peak at temperatures lower than 150 K on the sample with 2.0 ML coverage. On the contrary a depletion of free carriers centered near the QD plane is observed on the sample with 3.0 ML coverage in the investigated temperature range (40-300 K). Deep levels, that we relate to extended defects, have been detected in the upper confining layer and near the QD region on both samples, the higher concentrations being observed in the sample with higher coverage. TEM investigations revealed a network of misfit dislocations at the InGaAs/GaAs interface accompanied by threading dislocations propagating through the whole thickness of the structures. In addition to these defects, V-shaped dislocations nucleating at the QD plane are revealed in the 3.0 ML coverage sample, whose origin could be related to the large-sized islands which form by increasing the InAs coverage, as observed by AFM. The results obtained by the above techniques will be compared and discussed. The work has been partially supported by the "SANDiE" Network of Excellence of EU, contract no NMP4-CT-2004-500101. The AFM characterization has been carried out at CIM, University of Parma.
Subject Quantum Dots
Deep Levels
Schottky barriers

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