SCIENCE CHINA Technological Sciences • RESEARCH PAPER •
January 2011 Vol.54 No.1: 47–51 doi: 10.1007/s11431-010-4183-1
A study of the operating parameters and barrier thickness of Al0.08In0.08Ga0.84N/AlxInyGa1-x-yN double quantum well laser diodes A. J. GHAZAI1*, S. M. THAHAB2, H. ABU HASSAN1 & Z. HASSAN1 1
Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia; 2 Material Engineering Department, College of Engineering, University of Kufa, Najaf 00964, Iraq Received July 28, 2010; accepted October 9, 2010
The operating parameters such as the internal quantum efficiency (ηi), internal loss (αi) and transparent threshold current density (J0) of double quantum well laser diodes were investigated and identified using the program, Integrated System Engineering-Technical Computer Aided Design (ISE-TCAD). Various thicknesses (6, 7, 8, 10, 12 nm) of AlxInyGa1-x-yN barriers with (3 nm) Al0.08In0.08Ga0.84N wells as an active region were studied. The lowest threshold current (Ith), and the highest output power (Pop) were 116 mA and 196 mW respectively, at barriers thickness of 6 nm, Al mole fraction of 10% and In mole fraction of 1%, at an emission wavelength of 359.6 nm. AlInGaN, quaternary, UV laser diode, quantum well, barrier thickness Citation:
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Ghazai A J, Thahab S M, Hassan A H, et al. A study of the operating parameters and barrier thickness of Al0.08In0.08Ga0.84N/AlxInyGa1-x-yN double quantum well laser diodes. Sci China Tech Sci, 2011, 54: 47−51, doi: 10.1007/s11431-010-4183-1
Introduction
There has been great interest in producing semiconductor ultraviolet (UV) light emitting diodes (LEDs) and laser diodes (LDs) because they are used in many applications in different fields as in bio-agent detection systems, medical applications, and UV treatment. Therefore, researchers are trying to produce highly efficient, long-lived and inexpensive LEDs and LDs in order to fulfill these applications [1]. Quaternary InAlGaN was a challenge due to the different bond length and desorption temperature of the binary compounds, and the different surface mobility and desorption temperature of the growing species. The feasibility of InAlGaN quaternary alloys and InAlGaN based UV-LDs and LEDs structures has been demonstrated by sophisticated growth procedures using metal-organic chemical vapor deposition (MOCVD) [2–7]. There are also some *Corresponding author (email:
[email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2011
reports on InAlGaN quaternary alloys grown by molecular beam epitaxy (MBE) and these alloys were also grown on GaN templates by MOCVD or thick GaN layer [8]. Several experimental and simulation studies indicated that double quantum well (DQW) blue-violet InGaN LDs have many advantages, such as higher output power and efficiency with lower threshold current, depending on the specific wavelength of the LDs. Nakamura et al. (1998) experimentally studied the performance of several LDs with emission wavelength of 390–420 nm as a function of the number of InGaN well. They found that the lowest threshold current density was obtained when the number of InGaN well was two [9]. Chang and Kuo (2003) studied laser performance with an emission wavelength of 462 nm. They found that the threshold current density increased with the number of well layers. They found that the hole distribution was non-uniform between DQW and that this non-uniform hole distribution played an important role in laser performance detech.scichina.com
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crease when the number of well layers increased also [10]. The red shift is a result of increasing the piezoelectric field which arises from the strain between the well and barrier interface in InGaN/GaN multiple quantum well [11]. Therefore, the use of quaternary AlInGaN barrier in GaN quantum well (QW) laser increases the optical gain and decreases the threshold current density. This is because the piezoelectric field is zero at the interface, except for large band gap energy [12]. A laser diode with high power around 30 mW was commercially produced in September 2000 [13]. In this work, we studied an ultraviolet (UV) quaternary InAlGaN multiquantum well (MQW) LD by using ISE-TCAD software. Advanced physical models of semiconductor properties were used in order to obtain an optimized structure. The device performance, which is affected by piezoelectric and thermal effects, was studied via drift-diffusion model for carrier transport and optical gain and loss.
2 Laser structure and parameters used in the numerical simulation The laser simulation program [14,15] solved the Poisson equation, the current continuity equations, the photon rate equation and the scalar wave equation using the twodimensional (2D) simulator. The carrier drift-diffusion model, which includes Fermi statistics and incomplete ionization, was included in our simulation models. The Shockley Read-Hall (SRH) recombination lifetime of the electrons and holes is assumed to be 1 ns; however, this is a rough estimate since the type and density of recombination centers are sensitive to the technological process. Now we solve Poisson equation:
∇ ⋅ (ε∇φ ) = q (n − p − N D+ + N A− ),
(1)
where NA is the acceptor doping density (cm-3) ; ND , donor doping density (cm−3); ε permittivity of medium ; , potential energy; q, electron charge; and n, p are the number of electrons and holes respectively. Electron continuity equation: Hole continuity equation:
∂n +∇ ⋅ J n = q(Gn − Rn ); (2) ∂t
∂p + ∇ ⋅ J p = q (G p − R p ), (3) ∂t
where Jn, Jp are the current density of electron and hole; Gn, Gp are electron generation rate and hole generation rate; and Rn, Rp are the electron recombination rate and hole recombination rate, respectively. The schematic diagram of the UV LD structure is shown in Figure 1 which includes 0.6 μm GaN contact layer, a cladding layer of n-Al0.08Ga0.92N/GaN modulation-doped strained layer superlattice (MD-SLS) which consists of 80 pairs of 2.5 nm each, and 0.1 μm n-GaN waveguiding layers.
Figure 1 diode.
The schematic structure of an InAlGaN quantum well laser
The active region consists 3 nm Al0.08In0.08Ga0.84N DQW wells sandwiched between 6 nm Al0.1In0.01Ga0.89N barriers. On the top of the active region there are four other layers. They are p-Al0.25In0.08Ga0.67N blocking layer of 0.02 μm, p-GaN waveguiding layer of 0.1 μm and a cladding layer of p-Al0.08Ga0.92N/GaN MD-SLS which consists of 80 pairs of 2.5 nm, and finally 0.1 μm of p-GaN contact layer. Doping concentration of p-type is 5×1018 cm−3 and of n-type is 1×1018 cm−3. The LD area is (1 μm×400 μm), and the reflectivity of mirrors of both sides (left and right) is equal to 50%. The physical parameters (Q) of the quaternary AlInGaN material are more complicated than those of the ternary and binary alloys and were interpolated linearly by the following formula [16, 17]: Q (Al x In y Ga1- x - y N) = Q(AlN) x + Q(InN) y + Q(GaN)(1 − x − y ).
(4)
The formula above applies to all the parameters except for the band gap energy which can be calculated by the following equations [16, 17]: Eg (Al x In y Ga z N) =
xyEgu (AlInN) + yzEgu (InGaN) + xzEgu (AlGaN) xy + yz + zx
,
(5)
Egu (Al1-u In u N) = u Eg (InN) + (1 − u ) Eg (AlN) − u (1 − u ) B (AlInN),
(6)
Egv (In1-v Ga v N) = vEg (GaN) + (1 − v) Eg (InN) − v (1 − v) B (InGaN),
(7)
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Egw (Al1- w Ga w N) = wEg (GaN) + (1 − w) Eg (AlN) − w(1 − w) B (AlGaN),
(8)
where, u=
1− x + y 1− y + z 1− x + z , v= , w= , 2 2 2
(9)
where x, y, and z = 1−x−y which represent the compositions of aluminium, indium, and gallium in the material system of AlInGaN, respectively. B(AlInN), B(InGaN), and B(AlGaN) are the bandgap bowing parameters of AlInN, InGaN, and AlGaN, respectively.
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Results and discussion
The diagram of the band gap energy and carrier density of the DQW LD structure as a function of the vertical position is shown in Figure 2. The carrier density of the holes (p side) and the electrons (n side) is the highest in the quantum well active region. As a result, the carrier density of the well close to the p side has carrier density around 2.121×1019 cm−3 which is higher than the one close to the n side which has carrier density around 1.844×1019 cm-3. This is ascribed to the presence of p-Al0.25In0.08Ga0.67N blocking layer and Al0.08Ga0.92N/GaN superlattice layers which are useful in increasing the holes concentration by ionizing deep acceptors in the valence band of the barriers neighboring the narrow band gap material [18] and the holes have high effective mass, low mobility, and high band offset at the valence band. Figure 3 shows the optical intensity together with the refractive index profile of the DQW LD. A maximum optical intensity around 1.59×1015 can be
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observed inside the AlInGaN active region. This is due to the higher optical confinement achieved by the refractive index profile provided by GaN waveguide and AlGaN/GaN MD-SLS cladding layers. The effect of barrier thickness on the operating parameters can be seen in Figure 4. The figure indicates that at barrier thickness of 6 nm, the external differential quantum efficiency (DQE) and the slope efficiency increase till barrier thickness of 7 nm. Over 7 nm, they decrease with an increase of the barrier thickness and the output power and threshold current have the same behavior. Since the barrier width influences the strength of the internal fields in the wells and barriers, it also influences the absorption and gain. If the width of the barriers increases from 7 nm to 12 nm, the internal field in the barriers reduces. This is because the electrons are no longer localized as close to the well as before. The electrons are not confined in the well, but their confinement is provided by the tilted barriers. Thus, its localization is strongly influenced by the internal field in the barrier. However, at barrier thickness of 10 nm, the threshold current slightly increases; this is perhaps related to the changes in the band offset ratio between the conduction and valence bands. Thus, the spontaneous emission increases and the stimulated emission decreases. The values of these results are summarized in Table 1. The high output power is obtained with the lower barrier thickness 6 nm, but the high DQE and the high slope efficiency are obtained with barrier thickness 7 nm. While the lowest threshold current is obtained with the highest barrier thickness 12 nm. The study does not include the barrier thickness less than 6 nm because the authors have not observed any optical gain and consequently laser at this barrier thickness was not
Figure 2 The band gap and carrier density of the DQW LD structure as function of the vertical position.
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Figure 3
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The refractive index and optical intensity of the DQW LD structure as a function of the vertical position.
Figure 4 The threshold current, output power, slope efficiency and DQE of DQWs LD structure as a function of the barrier thicknesses.
Table 1
Performance parameters of (Al0.08In0.08Ga0.84N/Al0.1In0.01Ga0.89N ) LDs as a function of the barrier thicknesses
Thickness of barrier (nm)
Threshold current (mA)
Slope efficiency
Power output (mW)
DQE
6
116
0.668
196.2
0.193
7
110.2
0.725
115.8
0.210
8
101.6
0.662
109
0.192
10
104.2
0.652
111.9
0.189
12
99.28
0.659
113.2
0.191
obtained. This is attributed to several reasons such as, firstly, it is because of the electron leakage current and also the higher density of positive polarization charge at the interface between the barrier layer and blocking layer which enhances the electron leakage current [19].
Secondly, when the barrier thickness is reduced, the wave function of the adjacent wells begins to overlap and discrete levels broaden to minibands as the wave function in the neighboring wells is coupled together with the thin barrier that separates the neighboring wells [20].
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Conclusions 8
The laser performance of quaternary AlxInyGa1-x-yN DQW UV LDs especially on the effects of the barrier thickness on the operating parameters was studied and investigated. The lowest threshold current is 116 mA with a high power of 196.2 mW at an emission wavelength 359.6 nm. The optimized values for the Al mole fraction of the barrier, reflectivity of the mirror facets and cavity length of the investigated quaternary AlxInyGa1-x-yN DQW LDs are 0.1, 0.5, and 400 μm respectively. This work was conducted under Science Fund, Cycle 2007, of The Ministry of Science, Technology and Innovation, Malaysia. The financial support from Universiti Sains Malaysia is gratefully acknowledged.
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