1.
Introduction

Vast promising applications for deep ultraviolet laser diode (DUVLD) in many fields such as wireless communication, medical instruments and chemical devices are still waiting for the higher performance of laser diode (LD) to be achieved^{[1, 2]}. Thus, one of the core obstacles yet to be overcome is the p-type conductivity mainly due to the higher activation energy of Mg-dopant^[3] for higher Al composition, which probably causes an increase in resistivity.



Many studies have shown and demonstrated that the potential for exploiting h-BN in far-ultraviolet (FUV) emitters are due to its good characteristics^{[4, 5]}. Also, the ternary BGaN and quaternary BAlGaN are found to be suitable for growing over (0001) 6H-SiC by MOVPE^[6]. Although B_0.01Ga_0.99N/Al_0.12Ga_0.88N MQWs LED had been successfully grown over 6H-SiC with a PL peak at 360 nm^{[7, 8]}, Mg-doped h-BN used as a contact and electron blocking layer (EBL) layer instead of p-AlN is successfully studied in Al-rich AlGaN LED^[9–11]. However, there is no laser diode (LD) structure simulation that investigated B_xGa_1–xN/Al_xGa_1-xN in deep ultraviolet (DUV). Therefore, this paper seeks to demonstrate a new LD structure that exhibits the potential of using wurtzite trinity B_0.313GaN/B_0.40GaN and AlGaN assumed to be grown over AlN substrate without any conventional contact layer.

2.
Simulation model

The simulated laser diode structure was conducted via using LASTIP-Crosslight simulator software with a self-consistency model for varies quantity calculations. Were mirror reflectivity factors of (0.05) for both facets were assumed and a cavity length of 1500 μm. The spacer-1 and spacer-2 layers in the Table 1 were proposed for reducing the band-offset between AlGaN and BGaN layers.

Layer material	Layer function	Thickness (nm)	Real refractive index
p-Al0.87GaN	p-WG	500	2.20
p-Al0.71–0.77GaN	p-CLL	150	2.263–2.239
i-Al0.80–0.65GaN	EBL-IT	10	2.228–2.286
p-Al0.57GaN	Spacer-1	10	2.317
i-B0.40GaN × (3)	QB	5	2.343
i-B0.313GaN × (2)	QW	4	2.286
n-Al0.61–0.58GaN	Spacer-2	10	2.30–2.313
n-Al0.77–0.71GaN	n-CLL	100	2.24–2.63
n-Al0.77GaN	n-WG-1	685	2.24
n-Al0.77GaN	n-WG-2	1800	2.24
i-AlN	Substrate	2000	2.15

Table1.
The contrived LD structure at room temperature for B_0.313GaN/B_0.40GaN MQW (p, p-type Mg-dopant; n, n-type Si-dopant; i, intrinsic/no-doping; CLL, cladding layer; WG, wave guide; EBL-IT, electron blocking layer-inverse tapered).



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Layer material	Layer function	Thickness (nm)	Real refractive index
p-Al0.87GaN	p-WG	500	2.20
p-Al0.71–0.77GaN	p-CLL	150	2.263–2.239
i-Al0.80–0.65GaN	EBL-IT	10	2.228–2.286
p-Al0.57GaN	Spacer-1	10	2.317
i-B0.40GaN × (3)	QB	5	2.343
i-B0.313GaN × (2)	QW	4	2.286
n-Al0.61–0.58GaN	Spacer-2	10	2.30–2.313
n-Al0.77–0.71GaN	n-CLL	100	2.24–2.63
n-Al0.77GaN	n-WG-1	685	2.24
n-Al0.77GaN	n-WG-2	1800	2.24
i-AlN	Substrate	2000	2.15

3.
Results and discussions

According to simulation results, it has been found that the minimum doping concentration that exhibits lasing at 277 nm for the given structure is N_A = 5 × 10¹⁷ cm^–3, N_D = 9 × 10¹⁶ cm^–3 with an OCF of 40.4 % as shown in Fig. 1. Therefore, different EBL design has been studied, firstly the conventional EBL contrivance which have a constant composition along the layer thickness assumed to be 10 nm, as well as an aluminum content of 80% (Al_0.8GaN), were adjusted.






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Figure1.
(Color online) The optical intensity confined within the active region for optical confining factor (OCF) of 40.4%.




Therefore, barely the structure produces an emission for P_max = 115 μW given at a maximum current injection of I_m = 550 mA and I_th = 150 mA as shown in Fig. 2. Whereas the V–I characteristic curve shows a semi-linear relationship for an approximated (biasing voltage) V_th = 7 V, with a DC resistance of 0.77 ?.






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Figure2.
(Color online) Illustrate the L–I and V–I electrical characteristic curves for the conventional EBL, tapered EBL, without EBL and inverse tapered EBL designs.




Secondly, for the tapered (linearly graded) EBL contrivance a composition of Al_0.65–0.80GaN was adjusted. Thus, the following quantities had been calculated by the simulator, were P_max = 1.73 W at I_max = 1.65 A observed in Fig. 2 as well as I_th = 600 mA, S.E. = 1.468 W/A, V_th = 5 V, R = 0.239 ?.



Thirdly, when a removal of EBL layer was established it shows an enhances in S.E. and threshold current almost by 15.8% and 8.3% achieved respectively, comparing to tapered design. Hence P_max = 2.02 W at I_max = 1.65 A, I_th = 550 mA and S.E. = 1.70 W/A. On the other side, V–I curve shows a biasing voltage V_th = 5 V, R = 0.262 ?, the lowest resistance is obvious since there is no EBL layer. Lastly, for an inverse tapered EBL contrivance as shown in Table 1 and in Fig. 3. gives P_max = 2.4 W, I_max = 1.65 A, I_th = 500 mA and S.E. = 1.91 W/A a very good enhances were achieved in S.E. and I_th by 12.3% and 9% respectively, in addition to same tapered design threshold voltage was observed but different resistance of 0.336 ?. Therefore, these results in well agree with published results at Ref. [12] in term of a better performance could be obtained, if tapered EBL design is applied instead of conventional EBL. Therefore this paper demonstrated an alternative opportunity of having higher performance for LD by using inverse tapered EBL design over tapered and conventional EBL.






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Figure3.
(Color online) Schematic illustrations for the right half of simulated structure, real cavity length are 1500 μm.




Since the conventional EBL structure exhibits a very low emission power compared to other designs, probably due to low doping concentrations. Hence, we applied an increase in the doping concentration magnitude order, thus it had been found that an increase in N_A and N_D by two orders of magnitude up to N_A = 5 × 10²⁰ cm^–3, N_D = 9 × 10¹⁹ cm^–3 lead to a clear stimulated emission in the conventional structure. Therefore, the simulated results for those concentrations shows same L–I and V–I characteristic curves for tapered, inverse tapered and without EBL designs, were P_max = 103.8 W at I_max = 51.5 A, I_th = 500 mA, S.E. = 2.0 W/A, V_th = 4.6 V and R = 0.144 ? as shown in Figs. 4(a) and 4(b). While the conventional structure shows enhance in the S.E. (3.14 W/A) by 57% compared to the other three designs in this high accepter and donor concentrations. Were P_max = 50 W at I_max = 15.9 A, I_th = 130 mA V_th = 4.8 V, although a very good enhancement in S.E. is shown by the conventional design a higher resistance of 0.563 ? compared to previously mentioned resistance for the other structures. In addition to a limitation in the maximum emitted power by almost 51% is observed, these limitations probably due to: (1) The electron leakage into the p-cladding layers, as shown in Fig. 5. (2) The band-offset height that forms as a result of EBL composition difference between the QB/Spacer-2, directly coupled to the resistance.






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Figure5.
(Color online) Showing the relationship between the averages calculated electron concentrations within the P-Cladding layer for different accepter and donor concentrations for the four EBL designs.






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Figure4.
(Color online) Showing the (a) L–I and (b) V–I curve for the two LD structure for the conventional EBL, tapered EBL, without EBL and inverse tapered EBL designs with the N_A = 5 × 10²⁰ cm^–3, N_D = 9 × 10¹⁹ cm^–3.




It has been found that the electron concentrations within the quantum well for the three designs (tapered, inverse tapered and without EBL) almost 10¹⁹–10²⁰ cm^–3. While the conventional EBL contrivance found to be 10¹⁸–10¹⁹ cm^–3 which means lower concentration by one order of magnitude from the sufficient concentrations, therefore it is the probable reason for why there is no an apparent stimulated emission for the conventional structure, shown in Fig. 2 especially for lower accepter and donor concentrations.



For more elaboration of the first reason regarding the leakage of current/electrons, an analysis for the electron concentration in the p-cladding layer is summarized for the four different doping concentrations as shown in Fig. 5. Thus, it found that for a concentration of N_A = 5 × 10¹⁷ cm^–3, N_D = 9 × 10¹⁶ cm^–3 the three designed structures show a low concentration of 10¹⁰ cm^–3. While a larger amount of leakage current found for the conventional design ~ 10¹⁹ cm^–3 almost similar quantity of electrons found in QW, that may mean there is no blocking happened for the electrons as could be observed from Fig. 6. This larger amount of leakage electron it has been evident to be due to insufficient EBL barrier/offset. While an attempt to increase the EBL composition for the given N_A and N_D concentrations seems to increase the V_th.






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Figure6.
(Color online) Vertical cut view for the LD structure that illustrates the band diagram for the active region and EBL layer, with the N_A = 5 × 10¹⁷ cm^–3, N_D = 9 × 10¹⁶ cm^–3 for the four different EBL layers.




In addition to that, it will lead to a reverse increase of EBL height in the valence band, thus forming an undesired hole blocking layer that may directly degrade the p-type conductivity. On the other side, a tendency to increase the doping concentrations around 10¹⁹ cm^–3 gives p-cladding layer concentration approximately to 10¹⁷ cm^–3 for the three structures and 10¹¹ cm^–3 for a conventional design. These higher theoretical N_A and N_D i.e. > 10¹⁸ cm^–3 it might be difficult to implements practically, while N_A and N_D < 10¹⁷ cm^–3 have a higher possibility to be implemented for the demonstrated aluminum content.

4.
Conclusion

The contrivances of a new combination between BGaN and AlGaN alloys were proposed, for B_0.313GaN/B_0.4GaN quantum well/quantum barrier assumed to be grown over AlN substrate. Hence a lasing in a deep ultraviolet range in 277 nm were observed, with a maximum emitted power of 2.4 W at an injection current of 1.65 A, threshold current of 500 mA and slope efficiency of 1.91 W/A that represents 9 %, 12.3% enhancement compared to tapered design respectively. With the lowest reported accepter and donor concentration for DUV devices of N_A = 5 × 10¹⁷ cm^–3, N_D = 9 × 10¹⁶ cm^–3.



In addition to an investigation regarding different EBL designs were shown. Therefore, we found it agreed well with the published results, moreover this paper proposes a probable better EBL design, especially for low doping concentrations, called inverse tapered EBL.

Acknowledgement

This work is provisioned by National Key Research and Development Program (Nos. NKRDP 2016YFE0118400), the Key project of Science and Technology of Henan Province (No. 172102410062), National Natural Science Foundation of China (No. 61176008), and National Natural Science Foundation of China Henan Provincial Joint Fund Key Project (No. U1604263).

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Appendix A