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Series of green laser diodes (LDs) with different (In)GaN barrier layers are investigated. It is found that the optical confinement factor of multi-quantum well (MQW) always increases with increasing indium content of InGaN barrier layer, which results in a decrease of threshold current when indium content of InGaN barrier layer increases from 0 to 5%. However, when a high In content InGaN barrier is used (> 5%), both threshold current and slop efficiency of LDs deteriorate. It may be attributed to the waste of carriers in the potential well at the interface between the last barrier (LB) and the upper waveguide (UWG) layers, which is induced by the piezoelectric polarization effect in high In content InGaN LB layer. Therefore, a new LD structure using a thin thickness of the LB layer to reduce the effect of polarization shows a low threshold current and a high output power even when the In content of barrier layers is as large as 7%.
© 2017 Optical Society of America
1. Introduction
Science Nakamura [1,2] proposed the first InGaN-based multi-quantum well (MQW) laser diodes (LDs), they have attracted significant attentions due to numerous applications in high-density optical storage, small portable projector and laser display [3–7]. To data, InGaN based blue and violet LDs is commercialization. However, fabrication of InGaN-based green LDs is more challenging due to (1) a weak optical confinement of active region due to the decreased refractive index contrast between waveguide (WG) layer and cladding (CL) layer [8,9] and (2) a low radiative recombination rate resulting from high dislocation density and strong quantum confined stark effect (QCSE) [10,11]. During past years, increasing the thickness or In(Al) content of InGaN waveguide (WG) layer and AlGaN cladding (CL) layer are often used to realize a good optical field distribution in green LDs [12–14]. However, it often makes the peak of optical field deviate from active region, meanwhile, growth of thick and high In(Al) content InGaN(AlGaN) materials suffers great challenges. In order to improve the performance of InGaN based green LDs, InGaN barrier layer is used to instead of GaN barrier layer due to that they cannot only enhance the confinement factor but also help to increase the radiative recombination rate by decreasing the piezoelectric polarization electric field in InGaN well layers [15]. In this work, the variation of optical and electrical properties with indium content of InGaN barrier layers are investigated by the two-dimension simulator LASTIP and an optimized LD structure with low threshold current and high output power is obtained.
2. Experiment
The schematic diagrams of LD structure is shown in Fig. 1. It consists of a 1-μm thick Si-doped n-type GaN layer, a 1-μm thick Si-doped n-type Al0.08Ga0.92N cladding (CL) layer, a 80-nm thick Si-doped n-type GaN waveguide (WG) layer, another 115-nm thick In0.05Ga0.95N WG layer (lower WG (LWG) layer mentioned below consists of these two WG layer), an InGaN/(In)GaN MQW active region, a 110-nm thick In0.05Ga0.95N upper WG (UWG) layer, a 20-nm p-type Al0.15Ga0.85N electron blocking layer (EBL), a 450-nm thick Mg-doped p-type Al0.08Ga0.92N CL layer, and a 150-nm thick Mg-doped p-type GaN contact layer. The InGaN/(In)GaN MQW of all investigated LD structures consists of two 2.5 nm un-doped In0.37Ga0.63N well layers and three 17 nm un-doped (In)GaN barrier layers, but their indium content of (In)GaN barrier layers is 0%, 2%, 3%, 5%, 7% and 10%, respectively. They are named as LD0, LD2, LD3, LD5, LD7 and LD10. In addition, the properties of another LD structure, i. e., LD7-2, with an asymmetric InGaN/InGaN MQWs is also simulated. Its MQW structure is similar to that of LD7, but the thickness of its last barrier layer (LB) is only 1 nm, thinner than that of LD7. In this work, the optical and electrical characteristics of these LD structures are theoretically simulated by the Crosslight Device Simulation Software (LASTIP, Crosslight Software Inc.), which is a powerful device simulation program designed to simulate the operation of a semiconductor laser in two dimensions through self-consistently solving the Poisson’s equation and the current continuity equations using the finite element analysis [16,17]. During the simulation, both the p-type and n-type electrodes are set to be an ideal Ohmic contact. The cavity length and ridge width is set to be 600 and 3 μm, respectively. The screening factor of polarization is set to be 0.25. The absorption coefficients of n-type and p-type layers are set as 5 cm−1 and 50 cm−1, respectively, except for the heavily Mg-doped GaN contact layer whose absorption coefficient is taken as 100 cm−1. In addition, for the lasing wavelengths very close to 539 nm in this work, the refractive indexes of InGaN and AlGaN materials are obtained by using an approximate method as the following expressions:
where the refractive indexes of InN, GaN and AlN are 2.879, 2.435 and 2.247, respectively.3. Results and discussion
The calculated P-I-V diagrams of LD0, LD2, LD3, LD5, LD7 and LD10 are shown in Fig. 2(a). From P-I curves, the threshold current and the slop efficiency of these six LDs are obtained, and they are shown in Fig. 2 (b). It can be seen that the threshold current decreases abruptly when the indium content of InGaN barrier layers increases from 0 to 5%, then it increases slightly when the indium content increases to 7%. When the indium content further increases up to 10%, stimulated recombination is not observed anymore. In addition, it is found that the slope efficiency of LDs keeps nearly unchanged when the indium content of InGaN barrier layer increases from 0 to 3% and afterwards it decreases abruptly, i.e., the output power @ 120 mA decreases when the indium content of InGaN barrier layers increases to above 3%. It suggests that in these LD structure, In content of InGaN barrier layer should be equal to or lower than 3%, otherwise the LD performance will be deteriorated. For the I-V curves, it can be seen that they are nearly the same to each other for LD0, LD2 and LD3, while the series resistance decreases when the indium content of InGaN barrier layer increases to above 3% (for LD5 and LD7).
Fig. 2 (a). Output power and voltage versus current (P-I-V) curves of all studied LDs. Figure 2(b) shows the dependences of threshold current and slop efficiency of LDs on indium content of InGaN barrier layer.
In fact, the electric properties of LDs are closely related to their optical field distribution. To find the reason for the variation of threshold current and output power, optical field distribution of all LDs are simulated and shown in Fig. 3. It is noted that the optical field distribution becomes concentrated in and the peak of optical field moves toward MQW region when the indium content of InGaN barrier increases. Based on the optical field distribution of all LDs, the optical confinement factor and total optical loss of LDs are calculated and their dependences on indium content have been shown in the inset of Fig. 3. It is found that the optical confinement factor increases linearly with increasing indium content of InGaN barrier, and it increases by 35% when the indium content increases from 0 to 10%. It indicates that using a high In content InGaN barrier layer is beneficial for enhancing the optical confinement of active region due to the increase of its effective refractive index, while the total optical loss first increases slightly (only 5%) and afterwards becomes saturated when the indium content of InGaN barrier increases to 7%. The increase of optical loss is attributed to the fact that the optical filed distribution in the p-type region increases when the indium content of InGaN barrier increases. As a whole, the enhancement of the optical confinement with the increase of indium content in InGaN barrier layer should be responsible for the reduction of threshold current up to an In content as large as 5%.
Fig. 3 Simulated near optical field distribution for all LDs. The inset shows their optical confinement factor and total optical loss.
Meanwhile, it’s also noted that both threshold current and slop efficiency of LDs deteriorate seriously when a high In content InGaN barrier is used (>5%). It is known that an increase of the total optical loss may result in an increase of threshold current and a decrease of output power of LDs. However, the total optical loss increases slowly when the indium content is higher than 5%. Therefore, we are aware that such a small increase of total optical loss is not the main reason for the deterioration of electric properties for LDs with high In content InGaN barrier layers.
To further improve the performance of green LDs and find out why the electric properties of LD deteriorate when a high In content InGaN barrier is used, energy band diagrams of all LDs under zero bias voltage and under an injection current of 120 mA are drawn and the radiative recombination rate at injection current of 120 mA for each layer in LDs has been calculated. It can been seen that from Fig. 4, the radiative recombination rate in InGaN/(In)GaN MQWs is nearly the same except of QW2 in LD10, which is much lower than others. However, the radiative recombination rate in the InGaN last barrier (LB) layer and UWG layer increases obviously when the indium content of InGaN barrier layer increases, especially when it is higher than 5%. It indicates that, in LDs with high indium content InGaN barrier layers, many carriers escape from InGaN quantum well and recombine in LB layer and UWG layer at high injection current.
The conduction band diagrams under zero bias voltage and under the fixed injection current of 120 mA are shown in Fig. 5. It can be seen that, under zero bias voltage, the tilt (slope) of conduction band for LB layer decreases when indium content of InGaN barrier increases and the tilt turns to an opposite direction when the indium content is higher than 5%. In fact, the tilt of energy band is related to the total electric field and it is a vector sum of the built-in electric field, the spontaneous polarization-induced electric field and the piezoelectric polarization-induced electric field in (In)GaN barrier layer. For Ga-polar (In)GaN material, the direction of spontaneous polarization-induced electric field is the same as the built-in electric field and the piezoelectric polarization-induced electric field has a direction opposite to the built-in electric field [18]. When the indium content of (In)GaN barrier layer increases from 0 to 5%, the piezoelectric polarization-induced electric field in the barrier layers increases. It results in a decrease of total electric field in LB layer. The tilt of energy band thus decreases. When the indium content of InGaN barrier increases to above 5%, the piezoelectric polarization-induced electric field is larger than built-in electric field. The orientation of both electric field and the tilt of energy band for LB layer changes to an opposite direction. Therefore, a potential well forms at the interface between LB and UWG layers, as marked in Fig. 5(a). In this case, many injected electrons will enter into this potential well and accumulate here. As for LD7 and LD10, the quasi-Fermi levels of electrons in part of LB layer and UWG layer have enter into conduction band at the injection current of 120 mA, as shown in Fig. 5(b). It indicates that there-is a very high density of carriers. It will result in an increased radiative recombination rate in LB layer and UWG layer at high injection current, in a well agreement with the calculated radiative recombination rate in Fig. 4. It is known that this recombination does not contribute to lasing. Therefore, the waste of carriers induced by large piezoelectric polarization electric field in InGaN LB layer should be responsible for the decrease of output power of LDs when the indium content of InGaN barrier layers increases too high. In addition, just because a large amount of carriers are accumulated in LB and UWG layers, the series resistances of LD5, LD7 and LD10 are lower than other LDs. Therefore, a lower voltage is obtained at high injection current when a high In content of InGaN barrier layer is used, as has been shown in Fig. 2.
Fig. 5 Conduction band diagrams under zero bias voltage (a) and under an injection current of 120 mA (b), where the dashed lines in Fig. 5(b) mark the quasi-Fermi levels.
Based on the discussion mentioned above, to decrease the leakage of carriers by eliminating the potential well at interface between LB and UWG layers should be a better method to improve the performance of InGaN/InGaN MQW green LDs. Therefore, another LD structure (LD7-2) is also simulated. The indium content of its InGaN barrier layers is taken as 7% (the same as for LD7) to ensure a good optical confinement, but its LB thickness is only 1 nm, much thinner than that in LD7. Its conduction band diagram together with that of LD7 are shown in the inset of Fig. 6. It can be seen that the potential well disappears when the thickness of LB layer reduces to 1 nm. Therefore, the leakage of electron will decrease in LD7-2. The P-I and V-I curves of LD0, LD3, LD7 and LD7-2 are calculated and their P-I curves are shown in Fig. 6. As we would expect, the slop efficiency of LD7-2 is significantly increased and the threshold current is nearly unchanged compared with LD7. Meanwhile, it is found that the threshold current of LD7-2 is also lower than that of LD0 and LD3, and the output power@120mA is higher, respectively. In addition, the voltage of LD7-2 at an injection current of 120 mA is larger than that of LD7, and is nearly the same as those of LD0 and LD3. It indicates that an asymmetric MQW structure with high indium content InGaN barrier and thin LB layer is suitable for fabricating high performance green LDs in comparison with symmetric InGaN/(In)GaN MQW structure.
Fig. 6 P-I curves for LD0, LD3, LD7 and LD7-2, the inset is the conduction band diagrams of LD7 and LD7-2 under the injection current of 120 mA.
4. Conclusion
Seven InGaN/(In)GaN MQW green laser diodes (LDs) with different MQW structures are investigated. It is found that due to the increase of the optical confinement factor with increasing indium content of InGaN barrier layers, the threshold current decreases when the indium content of InGaN barrier layer is equal to or lower than 3%. However, when it is higher than 5%, the electric properties of LDs become worse. It is attributed to the increase of leakage carriers induced by piezoelectric polarization effect in high In content InGaN LB layer. Therefore, to increase the optical confinement and to decrease the leakage carriers by using an asymmetric MQW structure is beneficial for improving the performance of green LDs.
Funding
National Key Research and Development Program of China (Grant No. 2016YFB0401801), National Natural Science Foundation of China (Grant Nos. 61674138, 61674139, 61604145, 61574135, 61574134, 61474142, 61474110, 61377020, and 61376089), Science Challenge Project (JCKY2016212A503) and One Hundred Person Project of the Chinese Academy of Sciences.
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