Optics and Photonics Journal, 2013, 3, 243-247 doi:10.4236/opj.2013.32B057 Published Online June 2013 (http://www.scirp.org/journal/opj) A Theoretical Study of Light Absorption in Self Assembled Quantum Dots Tarek A. Ameen, Yasser M. El-Batawy, A. A. Abouelsaood Department of Engineering Physics and Mathematics, Faculty of Engineering, Cairo University, Giza 12613, Egypt Email: tarek.amin@aucegypt.edu Received 2013 ABSTRACT Self assembled quantum dots have shown a great promise as a leading candidate for infrared detection at room tem- perature. In this paper, a theoretical model of the absorption coefficient of quantum dot devices is presented. Both of bound to bound absorption and bound to continuum absorption are taken into consideration in this model. This model is based on the effective mass theory and the Non Equilibrium Greens Function (NEGF) formalism. NEGF formalism is used to calculate the bound to continuum absorption coefficient. The results of the model have been compared with a published experimental work and a good agreement is obtained. Based on the presented model, the bound to bound ab- sorption coefficient component is compared to the bound to continuum absorption coefficient component. In addition, the effects of the dot dimensions and electron filling on the bound to continuum absorption coefficient are also investi- gated. In general, increasing the dot filling increases the absorption and decreasing the dots dimensions will increase the absorption and move the absorption peak towards longer wavelengths. Keywords: Absorption Coefficients; Non Equilibrium Greens Function; Self Assembled Quantum Dots 1. Introduction Self assembled quantum dots have attracted the attention due to the promise to improve the performance of many applications, like quantum dot infrared photodetectors (QDIP) [1], [2], and intermediate band solar cells (IBSC) [3]. For the QDIPs, it is reported that due to 3-dimen- sional confinement of the electrons in the quantum dots, QDIPs should have lower dark current than the conven- tional photodetectors at the same operating temperature, thus QDIPs can operate at higher temperature with the same signal to noise ratio [1]. Also QDIPs are very sen- sitive to the normal incidence unlike the QWIP [1]. While for the IBSC it should have higher efficiency than the conventional solar cell [4]. The proposed model is based on the non equilibrium Green’s function formula- tion (NEGF). NEGF formalism provides an approach to study the transport in quantum systems in the presence of open boundary conditions via the concept of self energy [5]. The NEGF formalism was used before for modeling the quantum dot-in-a-well (DWELL) structure [6-8]. This model calculated the responsivity not the absorption coefficient of DWELL structure and in arbitrary units. For calculating the bound to bound absorption coefficient of a self assembled quantum dot, a model was presented for the InAS/GaAs QDIP [9]. This model has led to an important result: for QDIP the in-plane polarized absorp- tion is dominant as long as the dot height is not very small compared to its base radius. This result was con- firmed experimentally [10]. There are many models that calculate the bound to bound absorption [9,11], but very little work has been done for the bound to continuum absorption. The intraband absorption coefficient α is an important parameter for the design of different quantum dot applications. In our model, the effective mass theory is used to build a hermitian Hamiltonian matrix for an isolated self as- sembled quantum dot. Diagonalizing this matrix gives the bound states and energies. Then, this Hamiltonian matrix with a nonhermitian self energy matrix are used to get the NEGF. Using the NEGF, the continuum states of the system have been calculated. Then, the bound to bound and bound to continuum absorption coefficients are both calculated. A comparison of our model and ex- perimental data of [12] has been done, showing a good agreement. 2. Theoretical Model A schematic of self assembled quantum dot islands is shown in Figure 1. The following assumptions are made: ● The self assembled quantum dot islands are as- sumed to have ideal conical shape with uniform size. ● Both the effects of the wetting layer and the cou- pling between neighboring dots are neglected. Copyright © 2013 SciRes. OPJ