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The effect of polarization-matched Al0.25In0.08Ga0.67N electron-blocking layer (EBL) on the optical performance of ultraviolet Al0.08In0.08Ga0.84N/ Al0.1In0.01Ga0.84N multi-quantum well (MQW) laser diodes (LDs) was investigated. The polarization-matched Al0.25In0.08Ga0.67N electron blocking layer (EBL) was employed in an attempt to reduce the polarization effect inside the active region of the diodes. The device performance which is affected by piezoelectric was studied via drift-diffusion model for carrier transport, optical gain and losses using the simulation program of Integrated System Engineering Technical Computer Aided design (ISE TCAD). The optical performance of the LD using quaternary Al0.25In0.08Ga0.67N EBL was compared with the LD using ternary Al0.3Ga0.7N EBL where both materials have the same energy band gap of Eg = 3.53 eV. The self-consistent ISE-TCAD simulation program results showed that the polarization-matched quaternary Al0.25In0.08Ga0.67N EBL is beneficial as it confines the electrons inside the quantum well region better than ternary Al0.3Ga0.7N EBL. The results indicated that the use of Al0.25In0.08Ga0.67N EBL has lower threshold current and higher optical intensity than those for Al0.3Ga0.7N EBL. The effect of Al0.25In0.08Ga0.67N EBL thickness on the performance of LDs has also been studied. Results at room temperature indicated that lower threshold current, high slope efficiency, high output power, and high differential quantum efficiency DQE occurred when the thickness of Al0.25In0.08Ga0.67N EBL was 0.25 µm.
©2011 Optical Society of America
1. Introduction
Ultraviolet AlInGaN LDs have been extensively developed for several important applications, such as high density optical storage systems [1]. Although these optoelectronic devices have already been commercialized, superior device performance and short emission wavelength are expected to be the next challenge. The quaternary AlInGaN alloy allows the independent control of the band gap and of the lattice parameter of III-nitride group; therefore, this alloy is indeed the most promising material due to its high band gap and superior lattice match over AlGaN [2].
Many studies used AlInGaN EBL instead of the conventional ternary AlGaN EBL. This is attributed to the built-in polarization which can be reduced by using quaternary AlInGaN as an EBL [3]. The built-in polarization causes a strong deformation to the quantum well accompanied by a strong electrostatic field. Under these circumstances, the electrons and holes wave-functions separate in the quantum well, leading to a reduction in the photon emission rate and internal quantum efficiency [4]. Furthermore, Piprek et al found that the polarization of the AlGaN EBL has a strong effect on the laser threshold current due to the large polarization charges [5].
However, the strong electrostatic field in the active region, which is one of the major challenges, still needs to be solved to achieve high power and high-efficiency laser diodes. Specifically, the existence of strong electrostatic field may lead to quantum confined Stark effect (QCSE) and poor overlap of the electron and hole wave-functions. Consequently, the radiative recombination rate, internal quantum efficiency (IQE), and optical performance of the laser diode can extremely be reduced. In order to reduce the impact caused by the piezoelectric effect, some useful methods have been proposed recently. For example, the ideas of employing a heavily doped barrier and non-polar growth orientation are of great interest. Nevertheless, the ultra-high carrier concentration (1019 cm−3) may lead to free carrier absorption and the non-polar growth orientation may result in high dislocation defects [6]. Many studies concentrated on using quaternary AlInGaN as an EBL with ternary alloy active region instead of using quaternary AlInGaN as an EBL with quaternary AlInGaN active region. The ratio between In mole fraction to Al mole fraction has been ≈1:5. Many researchers believe that AlInGaN has many advantages as an EBL and active region in the same structure.
In this paper the effect of type and thickness of EBL on the threshold current and performance of AlInGaN MQW LD was studied. In addition, the composition of Al and In for the quaternary active region has been investigated has been investigated.
2. Laser diode structure and parameters
Two dimensional (2-D) ISE-TCAD laser simulation program which has many advanced physical models [7,8] has been used to solve the Poisson equation which is
where NA is the acceptor doping density (cm−3); ND is the donor doping density (cm−3); q is the electron charge; and n and p are the number of the electrons and holes respectively; ε is the permittivity of medium; and φ is the potential energy.The current continuity equations for electron and hole which are,
where Jn and Jp are the current density of the electrons and holes; Gn and Gp are the electron generation rate and hole generation rate; and Rn and Rp are the electron recombination rate and hole recombination rate, respectively.The photon rate equation, the scalar wave equation, and the carrier drift-diffusion model which includes Fermi statistics and incomplete ionization, were including in our simulation models. 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.
The schematic diagram of the UV LD structure is shown in Fig. 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 layer.
The active region consists of 3 nm of Al0.08In0.08Ga0.84N DQW wells that are sandwiched between 6 nm of Al0.1In0.01Ga0.89N barriers. On top of the active region, there are four other layers. They are 0.02 µm of p-Al0.25In0.08Ga0.67N blocking layer, 0.1µm of p-GaN wave-guiding layer and a cladding layer of p-Al0.08Ga0.92N/GaN MD-SLS which consists of 80 pairs of 2.5 nm each, and finally 0.1µm of p-GaN contact layer. The doping concentration is 5 × 1018 cm−3 for p-type and 1 × 1018 cm−3 for n-type relevant layers. The LD area is (1µm x 400 µm) and the reflectivity of the two end facets equals 50% for each one.
The strain tensor in the plane of the epitaxial growth (in x or y axis) can be calculated by the following formula [9]:
where a is the natural unstrained lattice constant of the quantum well and ao is the lattice constant of AlInGaN. The perpendicular strain tensor (in z axis) can be obtained by the following equation [10]:where C13 and C33 represent the elastic stiffness constant of AlInGaN alloys.The physical parameters Q of the AlInGaN quaternary material which are more complicated than those of the ternary and binary alloys were interpolated linearly by the following formula [9]
The formula above applies to all the parameters except for the energy band gap which can be calculated by the following equations [11]
where x, y and z = 1-x-y represent the compositions of aluminium, indium, and gallium in the AlInGaN material system, respectively. b(AlInN), b(InGaN), and b(AlGaN) are the band gap bowing parameters of AlInN, InGaN, and AlGaN, respectively. The formulas for spontaneous polarization of ternary nitride alloys can be expressed as below [12]:The spontaneous polarization of quaternary AlInGaN can be calculated by using Eq. (6). The piezoelectric polarization of quaternary AlInGaN can be expressed as shown [6, 12, 13].
where ε can be calculated by using Eq. (4). The total polarization can also be expressed as below.The laser parameters of binary materials which have been used in our simulation are listed in the Table 1 below.
Table 1. Semiconductor parameters of binary III-nitride group
3. Simulation results and discussion
Figure 2 shows the output power versus current (L-I) curves of quaternary Al0.25In0.08Ga0.67N and ternary Al0.25Ga0.7N EBL at 300 K. In order to compare between them, the value of the energy band gap energy has to be the same (Eg = 3.53 eV) for both LD structures. As shown in the figure, the laser diode structure with quaternary EBL provides the lowest threshold current. This is attributed matching of the lattice between the blocking layer and active region layers which leads to a reduction in the built-in polarization. Consequently, the number of carriers inside the active region and the radiative recombination rates increase, and the number of escaped electrons outside the active region decreases.
Fig. 2 Output power versus current curves of quaternary Al0.08In0.08Ga0.84N laser diodes with used quaternary and ternary electron blocking layer EBL
Figure 3 shows the enhancement in the value of the optical intensity as a function of the vertical position in the active region of Al0.08In0.08Ga0.84N/ Al0.1In0.01Ga0.89N LD structure using the quaternary Al0.25In0.08Ga0.67N EBL in comparison with that of ternary Al0.3Ga0.7N EBL. This is attributed to a better lattice matching between the blocking layer and active region layers which resulted in reducing the spontaneous and piezoelectric polarization. The quaternary Al0.25In0.08Ga0.67N EBL has a refractive index that is higher than that of the ternary Al0.3Ga0.7N EBL. Consequently, this leads to an increase in the optical confinement because it accumulates more carriers in the active region, thus increasing population inversion for stimulated recombination. This means that the use of quaternary Al0.25In0.08Ga0.67N as an EBL is better than using ternary Al0.25Ga0.7N EBL as shown in Fig. 2 and Fig. 3.
Figure 4 shows the electrostatic potential, conduction band, and valence band profiles for top-down of the quaternary Al0.08In0.08Ga0.84N/Al0.1In0.01Ga0.89N MQW LD structure. It can be seen that the Al0.08In0.08Ga0.84N active region has a lower bandgap energy compared to that in the Al0.25In0.08Ga0.67N blocking layer and GaN waveguide layers. This will allow more carriers to accumulate in the active region and enhance the carrier recombination rate. Consequently, higher radiative recombination is achieved with this carrier’s confinement profile.
Fig. 4 The electrostatic potential profile for top-down of the quaternary Al0.08In0.08Ga0.84N/Al0.1In0.01Ga0.89N MQW LD structure.
The results of the matching of the spontaneous polarization, piezoelectric polarization, total polarization between layers AlInGaN MQW LDs using Eqs. (6-16) were summarized in Table 2 .
Table 2. The matching of the spontaneous polarization, piezoelectric polarization, and total polarization of the quaternary AlInGaN MQW LDs layers as wells, barriers, and blocking layer. Corresponding values for the ternary AlGaN blocking layer are also included. The relevant parameters used in the calculations are also listed.
Figure 5 shows the effect of Al composition inside the quaternary AlxIn0.08Ga1-x-0.08N EBL of quaternary Al0.08In0.08Ga0.84N/Al0.1In0.01Ga0.89N LD structure on the threshold current. Increasing the Al mole fraction (x) leads to increase the AlxIn0.08Ga0.67N band gap energy, in turn it leads to increase the carrier confinement. When the Al mole fraction is x=0.23, a dramatic reduction on the threshold current occurs. As shown in Fig. 5 this indicates that the carrier confinement is maximized. Above x=0.23 to x=0.26, the threshold current reduces.
Fig. 5 The threshold current of LD structure using quaternaryAlxIn0.08Ga1- x -0.08N EBL versus Al mole fraction with fixed In concentration y=0.08
This is due to the increase of the optical confinement factor (OCF) with high Al content. In our design, the optimal value of an Al mole fraction is x=0.25 with In: Al≈1:3.
A fixed Al composition of x=0.25 with variant In (y) composition in the Al0.25InyGa1-0.25- yN EBL is plotted in Fig. 6 . Relatively, the threshold current has almost a constant value when the In mole fractions is between y=0.08 and above y=0.11. As a result our paper suggests the best ratio between In mole fraction to Al mole fraction is ≈1:3
Fig. 6 The threshold current of LD using quaternary Al0.25InyGa1-0.25- yN EBL versus In mole fraction with fixed Al concentration x=0.25
Figure 7 shows output power and voltage versus current L-I-V characteristics of LD with quaternary Al0.25In0.08Ga0.67N EBL at room temperature. Based on this, the value of the threshold current and threshold voltage is 66 mA and 3.4 V, respectively.
Fig. 7 Output power and voltage versus current characteristics of LD with quaternary Al0.25In0.08Ga0.67N EBL at room temperature
The effect of various the thickness of quaternary Al0.25In0.08Ga0.67N EBL has been investigated. The highest optical confinement factor (OCF) obtained when the thickness of AlInGaN EBL was 0.25 µm which attributed to maybe the change in the band offset ratio. Additionally, the lowest threshold current and high slope efficiency, high output power, and high DQE of Al0.25In0.08Ga0.67N EBL of thickness 2.5 nm (with corresponding values of 31.77 mA, 1.91 W/cm2, 267 mW, and 55.9%, respectively at emission wavelength of 359.6 nm are shown in Fig. 8 . The results are summarized in Table 3 below.
Fig. 8 The threshold current, output power, DQE, and OCF as a function of Al0.25In0.08Ga0.67N EBL thickness
Table 3. Threshold current, slope efficiency, output power, and external DQE as a function of quaternary Al0.25In0.08Ga0.67N EBL thickness
4. Conclusions
Comparative studies have been performance of between the influence of using quaternary Al0.25In0.08Ga0.67N EBL and ternary Al0.3Ga0.7N EBL both of band gap energy Eg = 3.53 eV on the optical performance of LD. The results indicated that the use of Al0.25In0.08Ga0.67N EBL produces lower threshold current and higher optical intensity than those for Al0.3Ga0.7N EBL. The best Al0.25In0.08Ga0.67N EBL thickness on the performance of LDs has also been 0.25 µm, which has the highest optical confinement factor (OCF). Results at room temperature indicate lower threshold current, high slope efficiency, high output power, and high differential quantum efficiency DQE obtained when the thickness of Al0.25In0.08Ga0.67N EBL and the ratio between Al and In mole fraction were 0.25 µm and 1:3, respectively.
Acknowledgments
The Authors would like to thank University Science Malaysia USM for the financial support under 1001/PFIZIK/843088 grant to conduct this research.
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