 Open Access
 Total Downloads : 107
 Authors : Rawaa Abas Abd Ali
 Paper ID : IJERTV4IS100494
 Volume & Issue : Volume 04, Issue 10 (October 2015)
 Published (First Online): 25112015
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
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 switchon time is affected by the laser output by doped semiconductor QD laser.
Keywords: Quantum dot laser, Doping, Laser dynamics.

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 highspeed 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, selforganized 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 [79]. 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].

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 (13) 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 sizequantized structures are similar [6].
The twodimensional layer structures were initially fabricated in noncoherent heterogeneous systems, such as ultrathin layers of metals or semimetals on glass substrates [5]. In noncoherent systems each layer structure constituting the solidstate 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

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 pdoping ,
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 ndoping in the WL and
(b) for different pdoping 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 switchon 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 ndoping in the WL and (b) for different pdoping 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
ndoping ( 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 pdoping 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 ndoping( n 1.5x1011cm2 )as shown (a,b) for different injection current density j=(1.6,2.6,2.9,3.2,3.9) jth .

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 switchon time is affected (decreased) in the laser output by doped semiconductor QD laser.
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