Conference Digest of Joint 29 th International conference on Infrared and Millimeter Waves and 12 th International Conference on Terahertz Electronics, Karlsruhe, Germany, September 27-Oct. 1, 2004 237 IR Light SI GaAs substrate GaAs barrier GaAs barrier n+ GaAs contact layer V n+ GaAs contact layer InAs QDs IR Light SI GaAs substrate GaAs barrier GaAs barrier n+ GaAs contact layer V n+ GaAs contact layer InAs QDs IR Light SI GaAs substrate GaAs barrier GaAs barrier n+ GaAs contact layer V n+ GaAs contact layer InAs QDs IR Light SI GaAs substrate GaAs barrier GaAs barrier n+ GaAs contact layer V n+ GaAs contact layer InAs QDs Fig. 1 Schematic of QDIP Novel Infrared Detectors Based on Semiconductor Quantum Dots Zhonghui Chen 1,2 , Eui-Tae Kim 2 , Zhengmao Ye 3 , and Joe C. Campbell 3 , and Anupam Madhukar 2 1 Physics Department, Brooklyn College, The City University of New York, Brooklyn, New York 11210, USA 2 Department of Materials Science and Physics, University of Southern California, Los Angeles, CA 90089, USA. 3 Microelectronics Research Center, University of Texas at Austin, Austin, Texas 78712, USA; e-mail: zchen@brooklyn.cuny.edu Abstract This paper reviews recent progresses in developing self- assembled InAs/GaAs quantum dots based infrared photodetectors, especially, with n-i(QDs)-n configuration. Introduction It has been well known that three dimensionally quantum confined nanostructures, so called quantum dots (QDs), promise superior optoelectronic device performances [1]. The main challenge to fabricate practical QD optoelectronic devices is synthesizing defect-free and uniform QD ensembles with desired optical transition energies. Self-assembled QDs are 3D islands formed spontaneously via epitaxial deposition of few monolayers of semiconductors on lattice-mismatched substrates (e.g., InAs/GaAs) [2-3]. Among various QD synthesis approaches, such as colloidal chemistry, epitaxial self-assembly (Stranski-Krastanov mode), and lithography, epitaxial self-assembly is arguably the most promising approach to synthesize QDs for optoelectronic devices because of its unique combination of great advantages: i) defect free (coherent) crystalline, ii) convenient electrical accessibility, and iii) easy fabrication of QD ensembles. In the past decade, significant progresses have been made in both understanding formation of the InAs/GaAs QDs and controlling their size/shape and uniformity [1]. For the InAs/GaAs QDs, narrow photoluminescence line width of ~25meV can be routinely achieved [4]. The self-assembled InAs/GaAs QDs is an excellent candidate for mid infrared photodetector applications for the following reasons: i) QDs are intrinsically sensitive to normally incident infrared radiation due to its 3D quantum confinement, ii) the material uniformity of InAs/GaAs over large area is expected to be much better than HgCdTe (the leading infrared photodetetctor material), iii) the processing of III-V semiconductor is much more mature than that of HgCdTe, and iv) the photoexcited carriers in QDs have longer life time than that in quantum wells due to suppressed electron-phonon scattering [5-6], which will lead to high photoconductive gain and high operating temperature. Similar to commercially available GaAs/AlGaAs quantum well infrared photodetectors [7], the InAs/GaAs QD infrared photodetectors (QDIPs) are based on intersubband transitions. Operating mechanism of QDIPs A schematic of a representative infrared photodetector structure based on InAs/GaAs QDs is shown in Fig. 1. An active InAs QD region consisting of multiple layers of InAs/GaAs QDs is located between highly-doped top and bottom GaAs contact layers. Figure 2 shows a schematic of the greatly simplified band diagram for electrons in the QDIP structure. Panel a) shows the band diagram of the QDIP at zero bias, while panel b) shows the band diagram of the QDIP at bias under infrared radiation. The InAs/GaAs QDs have type I band offset, i.e., electrons and holes are confined in the same low bandgap semiconductor. The incident mid-infrared photons induce intersubband transitions of the InAs/GaAs QDs in the active region, eventually leading to a change in current. Such a change of current upon infrared radiation corresponds to photocurrent. QDIPs with n-i(QDs)-n configuration For n-type QDIPs with the vertical electrical contacts shown in the Fig. 1, there are two classes of photodetector configurations: n-i(QDs)-n and n- n(QDs)-n. In the n-n(QDs)-n configuration, the active QD region between top abd bottom contacts is intentionally doped. In the n-i(QDs)-n configuration, that is intentionally undoped, and the electrons in the ground states of the QDs in the active region are transferred and/or injected from contacts. So far, QDIPs with highest detectivity are based on n-i-n configuration. The detectivity (D*) is defined as where I s is photocurrent, I n is noise current, and P is infrared photon power. The device area ( A) and band width ( ∆f ) are for normalization. Namely, the detectivity is a normalized signal-to-noise ratio. In past five years, we have extensively studied the QD infrared photodetectors based on intersubband transitions of the InAs/GaAs QDs. Surprisingly, Chen and Baklenov et al found that QDIPs with n-i(QDs)-n configuration show desired higher detectivity and lower dark current than that with n-n(QDs)-n configuration [8][9]. In order to enhance detectivity and to reduce dark current, we also introduced the concept of current blocking layer in QDIP structures (see Fig. 3) [8][9]. The AlGaAs layers placed beside a) j total j capture emitter collector Under bias (V) n+ GaAs n+ GaAs V QD-region No bias n+ GaAs n+ GaAs QD-region j excited b) a) emitter collector Under bias (V) n+ GaAs n+ GaAs V QD-region No bias n+ GaAs n+ GaAs QD-region b) emitter collector Under bias (V) n+ GaAs n+ GaAs V QD-region No bias n+ GaAs n+ GaAs QD-region No bias n+ GaAs n+ GaAs QD-region b) a) j total j capture emitter collector Under bias (V) n+ GaAs n+ GaAs V QD-region No bias n+ GaAs n+ GaAs QD-region j excited b) a) emitter collector Under bias (V) n+ GaAs n+ GaAs V QD-region No bias n+ GaAs n+ GaAs QD-region b) emitter collector Under bias (V) n+ GaAs n+ GaAs V QD-region No bias n+ GaAs n+ GaAs QD-region No bias n+ GaAs n+ GaAs QD-region b) Fig.2 Simplified band diagram of InAs/GaAs QDIPs. P f A I I D n s 2 / 1 * ) ( ∆ ⋅ =