Optical Properties of Heavily Doped n-type CdSe Quantum Dots for Intersubband Device Applications
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Optical Properties of Heavily Doped n-type CdSe Quantum Dots for Intersubband Device Applications Shengkun Zhang1, Xuecong Zhou1, Aidong Shen2, Wubao Wang1, Robert Alfano1, Hong Lu2, William Charles2, I. Yokomizo2 , Maria Tamargo2 , K. Franz3, and C. Gmachl3 1 Physics Department, City College of New York, 160 Convent Avenue, New York, NY, 10031 2 Chemistry Department, City College of New York, 160 Convent Avenue, New York, NY, 10031 3 Department of Electrical Engineering, Princeton University, Princeton, NJ, 08544 ABSTRACT In this research, interband and intersubband optical properties of heavily doped n-type CdSe quantum dots were investigated by temperature dependent photoluminescence (PL) spectroscopy, picosecond time-resolved PL spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Two doped and one undoped CdSe quantum dot samples with multiple QD layers were grown over ZnCdMgSe barrier layers on InP (001) substrate by molecular beam epitaxy. Heavy doping leads to decreasing of activation energy of nonradiative recombination centers, however, does not affect the luminescence efficiency of doped quantum dots. Time resolved PL experiments show that the PL decay times of the doped samples have weak dependence on dot size and are much longer than that of the undoped sample. The two doped CdSe QD samples show strong Intersubband IR absorption that peaked at 2.54 µm, 2.69 µm and 3.51 µm. INTRODUCTION Semiconductor quantum dots (QDs) have been extensively investigated for optical and optoelectronic devices due to their unique advantages over bulk and quantum well materials. One of the important applications is quantum dot infrared photodetectors (QDIPs) where intersubband (ISB) transition is applied. The expected advantages of QDIPs are their inherent sensitivity to normal-incidence radiation, high operating temperature, and low dark current. QDIP becomes a promising candidate as an alternative technology aiming to replace quantum well and HgCdTe infrared detectors. In(Ga)As/GaAs quantum dots grown on GaAs substrate [1], being probably the most explored quantum dot material system, are typically used in QDIPs. In this study, a different material system, CdSe QDs on ZnCdMgSe barriers, is introduced and experimentally tested for intersubband infrared device applications. Both interband and intersubband optical properties of heavily doped n-type CdSe QDs have been investigated. Intersubband absorption of these CdSe quantum dots was successfully observed in mid-infrared range. EXPERIMENTAL DETAILS Three CdSe quantum dot samples, A, B and C, were grown by molecular beam epitaxy (MBE) on (001) semi-insulating InP substrates in a dual-chamber Riber 2300P MBE system. The CdSe QDs were self-assembled over ZnCdMgSe materials that are lattice-matched to InP substrates. The CdSe QD/ZnCdMgSe structure was repeatedly grown for 50 periods for each
sample. In all the three samples, the ZnCdMgSe barrier layer thickness is kept at 18 nm. The band gap of the ZnCdMgSe barrier is about 3.0 eV at 77 K as determine
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