292487) RESEARCH ARTICLE Copyright © 2015 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoelectronics and Optoelectronics Vol. 10, 671–674, 2015 Effect of InAs/GaAs Quantum Dot Size on Infrared Photoresponse Characteristics Tien Dai Nguyen, Dong-Bum Seo, and Eui-Tae Kim ∗ Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea Two types of InAs/GaAs quantum dots (QDs), namely, relatively large/steep QDs and small/shallow QDs, are comprehensively studied for QD infrared photodetector (QDIP) applications. The QDIP with small QDs shows a broad photoresponse over a relatively wide range (∼5 m to 7 m), whereas its QDIP counterpart with large QDs yields a sharp photoresponse peak at ∼7.2 m with a full width at half maximum of 32 meV. The systematic interband and intraband studies suggest that the excited states involved in intraband transitions in QDIPs with small and large QDs have continuum and QD-bound states, respectively. The QDIP with large QDs shows higher photocurrent- to-dark current ratio than the QDIP with small QDs. Keywords: Quantum Dot, InAs, Infrared, Detectors, QDIP. 1. INTRODUCTION InAs/GaAs self-assembled quantum dots (QDs) have been extensively studied for various applications, such as pho- todetectors and lasers. 1–7 QD infrared (IR) photodetec- tors (QDIPs) have several advantages over quantum-well (QW) IR photodetectors (QWIPs). Three-dimensionally confined QDs make QDIPs intrinsically sensitive to nor- mally incident radiation, 1–7 unlike QWIPs. 8 Given the phonon bottleneck effects, 9 10 QDs can also have longer carrier relaxation time than QWs. Thus, QDIPs have potentially lower dark current and relatively higher photo- electric gain. 11 The performance of QDIPs is affected by various fac- tors such as QDIP structure, carrier doping, and growth conditions. 1–3 Among them, QD size and shape are the most important factors determining the nature charac- teristics of QD inter and intraband transitions for IR detection. 12–15 To some extent the QD size can be var- ied depending on desired detection bands of QDIPs. Cal- culation and experimental results show that the ground state interband transition energy and the energy level sep- arations increase (blueshift) with decreasing QD size. 12–15 Moreover, the binding energies, with respect to the GaAs conduction band edge, of carriers sitting in the ground and final electronic states of intraband transitions are sig- nificantly different depending on QD size and shape. In this study, QDIPs having two types of QDs (relatively large/steep QDs and small/shallow QDs) are compared to ∗ Author to whom correspondence should be addressed. each other. The two QDIPs show the apparent difference of QDIP performance such as IR photoresponse and dark cur- rent. Moreover, these QDIP studies reveal QD electronic structure and the nature of interband and intraband transi- tions and QD electronic states involved such transitions. 2. EXPERIMENTAL DETAILS QDIP samples were grown on semi-insulating GaAs (001) substrates through the use of solid-source molecular beam epitaxy (MBE) under As pressure of 7 × 10 6 Torr. A schematic of the InAs/GaAs QDIP structures is shown in Figure 1. The QDIP samples consisted of an undoped, active InAs QD region sandwiched between highly Si- doped GaAs contact layers. A Si-doped GaAs contact layer [1 × 10 18 cm 3 (0.7 m)/1 × 10 17 cm 3 (0.3 m)] was grown at a growth rate of 0.5 monolayer (ML)/sec at 600  C. An undoped 220 ML-thick GaAs layer was then grown at 0.25 ML/s at 600  C. The small and large QD lay- ers were formed with the use of 2.0 and 3.0 ML InAs deposition, respectively, at a growth rate of 0.22 ML/s at 500  C. The 3.0 ML InAs deposition was performed through punctuated island growth (PIG) approach with the deposition sequence 2.0 ML InAs + 60 s punctuation + 1.0 ML InAs. 16 The InAs QDs were followed by GaAs capping layer deposited under migration-enhanced epitaxy mode up to 40 ML at 0.25 ML/s at 350  C, and then by conventional MBE growth at 500  C. The total thick- nesses of capping layers were 100 and 150 ML for 2.0 and 3.0 ML InAs QDs, respectively. The QD growth and cap- ping procedure was repeated five times. Finally, a 0.5 m J. Nanoelectron. Optoelectron. 2015, Vol. 10, No. 5 1555-130X/2015/10/671/004 doi:10.1166/jno.2015.1823 671