Doping Effect on Quantum Dot Laser Dynamics

Doping Effect on Quantum Dot Laser Dynamics

Rawaa Abas Abd Ali

Department of Physics, College of Sciences, Thi Qar University, Nassiriya, IRAQ

Abstract: In this paper, we study the effect of the doping processes on the wetting in quantum dot laser that it appeared on the concentrations of carriers of electrons (e) and holes (h) which lead to effect on the output power of quantum dot laser also this study was included the variation of time behaviors for the carriers in quantum dot laser at different values of current densities at fixed value of n- or p- doping in the wetting layer. The switch-on time is affected by the laser output by doped semiconductor QD laser.

Keywords: Quantum dot laser, Doping, Laser dynamics.

1. INTRODUCTION

   During the past decades, the performance of semiconductor lasers has been dramatically improved from a laboratory curiosity to a broadly used light source [1]. Owing to their small size and low costs, they can be found in many commercial applications ranging from their use in DVD players to optical communication networks. The rapid progress in epitaxial growth techniques allows to design complex semiconductor laser devices with nanostructured active regions and, therefore, interesting dynamical properties [3,4]. Future high-speed data communication applications demand devices that are insensitive to temperature variations and optical feedback effects, and provide features such as high modulation bandwidth and low chirp, as well as error- free operation. Currently, self-organized semiconductor

   electronic and optical properties [2], no serious proofs of strong advantages for device applications were presented. Quantum Dot Lasers (QDL) are now approaching a time in their development when they challenge current quantum well devices in terms of performance [7-9]. Many of the predicted advantages associated with QDL's, such as temperature insensitivity, large direct modulation bandwidth, wavelength tunability, reduced chirp and lower threshold current have been realized [10,11].

   An area of interest where QDL's currently fall short of predicted performance is with regard to modulation bandwidth. The extension of this modulation bandwidth is of great interest to application of QDL's as optical transceivers [6]. The doping processes have a great important on the concentrations of carriers in quantum dot laser which lead to more effective on the output power of laser [8].

2. THEORY

   The analytic and numeric investigations of the laser turn- on dynamics presented here are based on the model given in reference [7,8]. In the QD laser system the electrons are first injected into the WL before they are captured by the QDs.

   The following nonlinear rate equations (1-3) for the charge carrier densities in the QDs nb with b=e/h, the carrier densities in the wetting layer WL wb , and the photon density

   quantum dot (QD) lasers are promising candidates for

   n ph

   determine the dynamics (e and h stands for electrons

   telecommunication applications [2].

   Quantum dot (QD) heterostructures with size quantization of charged carriers in all three dimensions suitable for advanced research and applications were developed signicantly later than layered quantum well (QW) heterostructures [5]. The latter represented essentially the mainstream double heterostructure (DHS) concept [3] complemented by the ultimate reduction of the thickness of a narrow bandgap layer. Nevertheless some trends in the evolution of both types of size-quantized structures are similar [6].

   The two-dimensional layer structures were initially fabricated in non-coherent heterogeneous systems, such as ultrathin layers of metals or semi-metals on glass substrates [5]. In non-coherent systems each layer structure constituting the solid-state phase or material has its own lattice parameter and/or crystal orientation. Thus the crystal planes of the constituting materials (or phases of the same material) do not

   and holes, respectively) [7,8].

   The induced processes of absorption and emission are modulated by a linear gain

   Rind (ne , nh , nph) WA( ne nh N QD )nph .

   The spontaneous emission in the QDs is approximated by

   ~

   Rsp (ne , nh ) (W / N QD ) ne nh . The spontaneous recombination

   rate inwhere isis given by Rsp (we , wh ) BS we wh where BS is the bandband recombination coefficient in the WL.

   match. Consequently a lot of defects originate at the interface

   which hinder the realization of the intrinsic electric, optical, vibrational, etc properties that could be expected for ideally lattice- matched (or coherent) heterojunctions. In spite of

   g N QD / N sum is the optical confinement factor. geometric confinement factor.

   N sum is twice the density of the total QD and N QD

   g is the

   denotes

   some progress in the demonstration of the medications of twice the QD density of the lasing subgroup (the factor 2

   account for the spin degeneracy) . W is the Einstein coefficient and A is the wetting layer normalized area. is

   which can be integrated giving

   , j (t)

   N sum (n n ) N QD (wh we ) N QD (w0 w0 )

   the spontaneous emission coefficient is the injection e h e h

   current density, eo is the electronic charge . 2 is the optical intensity loss.

   Nonradiative carriercarrier scattering rates (nonlinear

   The microscopic calculations of the scattering rates would be doping the WL, since space charges will lead to band bending and deform the energy scheme; the relations of the scattering rates are given by researchers [9]. However, doping of the

   scattering rates)

   Sein

   and

   Shin

   for electron and hole capture

   WL will also change the charge conservation condition [8].

   w

   w

   into the QD levels, the QD levels.

   Seout and

   Shout

   for carrier escape from

3. RESULTS AND DISCUSSIONS

   Figure (1) represents an energy diagram of the band structure.

   By increasing

   0 or

   0 and keeping the other at the small

   e

   h

   value of

   102 e KT we are able to model n- or p-doping ,

   respectively. Because the rate equation system treats 2D

   densities, also the doping concentrations n w0e

   w0h are given per area.

   and p

   Figure (1): Energy diagram of the band structure across a QD. h labels the ground state (GS) losing energy. Ee and Eh mark the distance (in energy) of the GS from the QW band edge for electrons and holes, respectively [7].

   Doped wetting layer (WL):

   A doped WL can be implemented by choosing different initial conditions for electron and hole densities in the WL. Without

   In figure (2): Simulation result of the temporal variation of photon density nph for (a) different n-doping in the WL and

   (b) for different p-doping in the WL . Parameters as in Table 1; pump current density is ( j= 2.6 jth ). The threshold injection current density is calculated by using the same parameters of the system which are given in Table.1(

   jth 2883 A cm2 ) [9].

   Table 1: parameters used in the simulation [7]

   e

   h

   doping, the following initial conditions n0 0 , n0 0 ,

   n0 0 , w0 102 KT ,

   ph e e

   h

   and w0 102 e KT

   have b the Boltzmann where K

   is Boltzmann constant and T is temperature. Note that charge conservation is contained in the five -variable rate equation system, thus leading to only four independent dynamic variables that are related by [8]:

   In figure (2) we find that the switch-on time is affected (decreased) by the doped semiconductor QD laser at constant value of injection current density.

   N sum

   QD

   (ne nh ) N

   (wh we ) 0

   (a)

   (b)

   Figure (2): Simulation result of the temporal variation of photon density nph for (a) different n-doping in the WL and (b) for different p-doping in the WL

   . Parameters as in Table I; pump current is ( j= 2.6 jth ).

   In figure (3): Simulation of the temporal variation of electrons and holes densities in the QD ( ( ne , nh ) ) for p-

   n-doping ( n 1.5x1011cm2 ) as shown (a,b)for different injection current density j=(1.6,2.6,2.9,3.2,3.9) jth

   doping

   p 10x1011cm2

   as shown (a,b) and in figure (4):

   (a)

   (b)

   Figure (3): Simulation of the temporal variation of electrons and holes densities in the QD ( ne , nh ) for p-doping p 10x1011cm2 as shown (a,b) for different injection current density j=(1.6,2.6,2.9,3.2,3.9) jth .

   (a)

   (b)

   Figure (4): Simulation of the temporal variation of electrons and holes densities in the QD ( ne , nh ) for n-doping( n 1.5x1011cm2 )as shown (a,b) for different injection current density j=(1.6,2.6,2.9,3.2,3.9) jth .

4. CONCLUSIONS

From the results we find that the carriers of the system are affected by doping the wetting layer regions which lead to change the output intensity (or number of photons per unit of area). This effect can be appeared and related with the current injection density because the doping processes is enhanced the carriers on the wetting layer that lead to change the carriers in the active layer of quantum dot laser. Doping in quantum dot laser is important in the many applications such as increasing the activity of carriers in the lasing processes,

i.e. improving the output power of laser.

Thus, QD Laser with Doped Carrier Reservoir is a way of changing the confinement energies that lead to these effects on the laser output and we note that the switch-on time is affected (decreased) in the laser output by doped semiconductor QD laser.

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