Electron Blocking Layer Architecture to enhance the hole injection in the active region for 215 nm Ultra-violet Light Emitting Diode (UVC-LED)

doi:10.21203/rs.3.rs-2432229/v1

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Electron Blocking Layer Architecture to enhance the hole injection in the active region for 215 nm Ultra-violet Light Emitting Diode (UVC-LED)

https://doi.org/10.21203/rs.3.rs-2432229/v1

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This work engineered the p-Electron Blocking Layer (p-EBL) of the AlGaN-based deep ultraviolet (UVC) light-emitting diode (LED) to achieve a high Radiative Recombination Rate (RRR). Carrier concentration can be efficiently injected into the active region by inserting an undoped thin AlGaN layer between the doped p-AlGaN layer as a superlattice. Compared with the UVC-LED with the bulk p-AlGaN EBL, the superlattice LED improves the optical power, holes and electrons concentration, and RRR.

Hole injection

Deep Ultraviolet (DUV)

Lighting Emitting Diode (LED)

superlattice

Electron Blocking Layer (EBL)

UVC-LED

DUV-LED

SILVACO

TCAD simulation

Deep Ultra-violet Light Emitting Diode (UVC LED) comparing with mercury lamp has tunable bandgap energy, eco-friendly, harmless, high-power efficiency, lower weight and flexible functionalities including AC operation, and due to that, it has a substantial interest and attention in various fields such as biological disinfection, medical therapy, air purification and water sterilization [1]. The External Quantum Efficiency (EQE) of UVC-LED is extremely low, less than 10%, because of Internal Quantum Efficiency (IQE), Injection efficiency (IE), and Light Extracting Efficiency (LEE). Subsequently, to improve the IQE, the crystalline quality, the carrier injection efficiency, the current spreading, the self-heating effect, thermal management, and the polarization-induced electric field must be improved [2]. The reasons behind low IQE and LEE are density of defects, dislocation density, polarization of AlGaN material, and losses amount of absorption due to the nontransparent p-GaN pf p-contact layer [3]–[5].

The Multi-Quantum Wells (MQW) and Multi-Quantum Barriers (MQB) (i.e., the active region layer) have too low hole injection. Due to several reasons, such as the Mg doping efficiency of the Al-rich p-AlGaN is lower than that of the p-GaN [6]–[11]. The Mg ionization in p-AlGaN layer in UVC-LED is low than 1% in the room temperature, then the hole concentration is 10¹⁷ cm^− 3 [12]. As well as the hole mobility itself, regardless of the type of the material, its mobility too low and heavy effective masses due to its thermal energy activation is extremely high. Because of that, to obtain high Hole Injection Efficiency (HIE) in the active region in UVC-LED, the hole concentration must be increased, and to do that there are several methods such as increase the HIE from the p-Type Ohmic Contact into the Hole Supplier [13], [14], enhance the Hole Transport within the Hole Supplier [15], reduce the Hole Blocking Effect by the p-EBL, also increase the Hole Concentration in the MQW Region [16].

The Mg doping efficiency and the holes movement across the p-EBL can be increased by several methods, such as the three-dimensional hole gas (3DHG) as the [17] reported which the Mg doing concentration increased to 2.6 \(\times\) 10¹⁸ cm^− 3. Moreover, Mg-delta doping [18], [19], indium-surfactant-assisted Mg-delta doping, this method will increase the Mg doping concentration to 4.75 \(\times\) 10¹⁸ cm^− 3. As well as, when the structure was as a superlattice, the Mg ionization coefficient increased by the polarization-induced electric field, this is an application for the Poole-Frenkel effect. Whereas the effect suppresses the barriers height of the valence band by adding AlN layers as a stair-cased or by grading AlN composition [20]. By special architectures design for p-EBL to help holes to cross over the EBL to the active region and/or by doping the last quantum barrier with Mg dopants [21]. Zi-Hui Zhang et al. [10] reported that thin insertion layer embedded between p-type doped AlGaN layers improved the thermionic emission for the holes transfer mechanism. Resulted by working the insertion layer as intra-band tunneling process then alert the effective valence barriers height. While the Chun Shuang Chu et al [15] studied the impact of various thicknesses for the p-Al_0.50Ga_0.50N insertion layer at p-EBL on the hole and electron injections in the MQW also compared with the bulk p- Al_0.60Ga_0.40N as the EBL. The p-EBL with the smallest barrier height of the valence band energy of the last p-AlGaN layer improves the light output power of the 280 nm UVC-LED.

Whereas the polarization induced positive interface charges at the last quantum barrier (LB) interface reduced the holes concentration in the p-EBL. Minimizing the mismatch between LB and the p-EBL reduced the polarization induced positive interface charges [22]. Jiahui Hu et al [23], investigated experimentally and numerically the effect of architecture the n-layer on the 275 nm UVC-LED performance. The Superlattice Electrons Declaration Layer (SEDL) was added after the n-layer before Active Region (AR), to control the electron overflow and enhance the carrier injection of the AR. As a result, the radiative recombination rate increased, as well as the UVC-LED achieved 3.43% and 2.42% of EQE and wall-plug efficiency (WPE) respectively. On the other hand, Shahzeb Malik et al [24], worked in 2021 on p-EBL experimentally on 222 nm UVC-LED. The authors worked on the design of the Al-graded p-AlGaN hole source layer (HSL), and as a result, the correct choice of Al composition at the interface as well as engineering the band diagram of p-AlGaN of the p-EBL/p-AlGaN of HSL/p-GaN of a contact layer (CL) are perfectly promising a huge enhancement of HIE across MQW. As a result, the level of electron and hole concentrations in the MQW is enhanced by 26% and 53%, respectively, subsequently, enhancing the radiative recombination rate (RRR) in the MQW.

In this paper, the work focuses on engineering numerically the bandgap energy of the p-EBL to manipulating the holes movement mechanism in the p-EBL via adding a thin intrinsic layer (insertion layer) Al_90%Ga_10%N with thickness of 2 nm between two p-type layers of Al_95%Ga_5%N, the first layer (near to p-layer) has a thickness of 10 nm then second layer (near to the active region layer) has a thickness of 8 nm.

The studied devices structures are shown in Fig. 1 The structure designed based on 8 um AlN as a buffer layer: 2e18 cm^-3n-type as intrinsic impurities, followed by 2 um of Al_0.89Ga_0.11N (electron source) doped by 1e18 cm^-3 n-type, the active region (photon source) has 3 multi-quantum wells Al_0.86Ga_0.14N (4nm)/ 4 quantum barrier Al_0.87Ga_0.13N (10 nm), covered by 20 nm p- type Electron-Blocking layer (p-EBL) doped by 1e19 cm^-3 p-type, 40 nm Al_0.69Ga_0.31N (holes source) doped by 1e19 cm^-3 p-type. The proposed p-EBL of the second structure has superlattice p-EBL with configuration of p-Al_0.95Ga_0.05N (10 nm) (L₁)/ Al_0.90Ga_0.10N (2nm) (L₂)/ p-Al_0.95Ga_0.05N (8 nm) (L₃) see Fig.1, L₂ purposely insertion as thin layer to obtain preferable moving for the holes from p-layer to active region as illustrated in [7], [10], [11], the total thickness of superlattice p-EBL fixed to be as bulk p-EBL 20 nm. The bandgap energy calculated by Vegard’s law and listed in Tabled 1, as well as the refractive index and the lattice constant calculated depending on these equations [25]–[27]. Moreover, the thickness and the type and concentration of the doping of the device listed in Table 1.

Table 1 Device parameters of each layer. Bandgap energy, refractive index, lattice constant, layers thickness and the type and concentration of the doping of the device.
Layers	Materials	Thickness (nm)		Doping concentration (cm^-3)	Energy (eV)	Lattice constant at 295 T (Å)		Refractive index
Layers	Materials	Thickness (nm)		Doping concentration (cm^-3)	Energy (eV)	a	c	Refractive index
p-layer	Al_69%Ga_31%N	40		1e19	5.7592	3.12047	4.992752	2.11512
Electron Blocking Layer (EBL)	Al_95%Ga_5%N	10	20	1e19	6.08	3.1158	4.98528	2.10251
	Al_90%Ga_10%N	2		-	5.90	3.1197	4.99152	2.11284
	Al_95%Ga_5%N	8		1e19	6.08	3.1158	4.98528	2.10251
Active Region (AR)	Al_86%Ga_14%N	4		-	5.72	3.12278	4.996448	2.12241
Active Region (AR)	Al_87%Ga_13%N	10		-	5.79	3.12201	4.995216	2.11991
n-layer	Al_89%Ga_11%N	2000		1e18	5.87	3.12047	4.992752	2.11512

The valence band barrier height of the superlattice p-EBL was reduced compared to the bulk p-EBL, see Fig. 2(c). The superlattice produces intra-band tunneling for holes. This intra-band tunneling reduces the effective valence band barrier height whereas the valence band energy of superlattice p-EBL is altered therefore causing low thermionic emission efficiency for holes, as shown in Fig. 2 (b-c). According to [7], [10], [11] if the last layer in the p-EBL which interacted with the interface of the last quantum barrier in the MQW turns thin, the barrier height of the valence band energy decreased. Therefore, as has been illustrated in Fig. 3 the superlattice p-EBL has the smallest barrier height of the valence band energy. As a result of the intra-band tunneling effect, the holes move easier to the active region layer and overcome the valence band barrier height, which agrees with previous studies [9] and [7]. Subsequently, the hole concentration increased and the hole mobility of Al-rich in p-AlGaN in the superlattice p-EBL moves to overcome the p-EBL in a faster and easier way than in bulk p-EBL from the p-layer into the active region [10].

The optical power versus the injection current for the studied UVC-LEDs comparing with experiment result of reference [10] is shown in Fig. 3 (a). When compared to the experiment reference, it is markedly lower than that of the studied UVC-LED devices. It is clearly the superlattice p-EBL produced the strongest power density, whereas the superlattice p-EBL produces higher optical power than p-EBL by ~ 15%. The difference in the optical power for the investigated UVC-LEDs is well attributed to the different hole injection efficiency in the active region due to the different architectures [11], [28]. Whereas the Fig. 3 (b) is shown the power spectral density of the studied UVC-LED devices. The peak emissions of the studied samples are all around 215 nm. This also indicates that superlattice p-EBL is available for the enhancement of UVC-LED device performance by improving the hole injection efficiency in the active region [10].

Figure 4 (a) shows the carrier concentration for both devices of bulk and superlattice p-EBL in the active region. It is obviously that the superlattice p-EBL has more carrier concentration than bulk p-EBL. The high effective valence band hole barrier makes the holes accumulate in the bulk p-EBL then increase the hole depletion. Where the superlattice p-EBL can relaxed the hole depletion by improving the holes thermionic emission energy to across the EBL [11]. As results, the holes concentration increases in the active region as shown in Fig. 4(b). Surprisingly, the electron concentration profiles in the active region increased as well. Therefore, the superlattice p-EBL enhanced the electron concentration in the MQW by suppressing the electron leakage to the p-EBL. We attribute that due to the high polarization induced positive charge at the last quantum barrier, and that attracts the electrons, then increasing the electrons concentration. The superlattice p-EBL shows the highest potential electron barrier and that indicated effective blocking of electrons. The enhanced concentration for electrons and holes in the active region improved the radiative recombination rate, as shown in Fig. 4(c). More importantly, Fig. 4(c), proves that the superlattice p-EBL can in fact increase the electron and hole (carrier) concentrations in the active region, thus improving the radiative recombination rate of the UVC-LED devices.

In conclusion, we have reported a p-Al_0.95Ga_0.05N/Al_0.90Ga_0.10N/p-Al_0.95Ga_0.10N EBL for UVC-LEDs. By architecture the p-EBL, the holes movement easier and faster from p-EBL into the active region. The thermionic emission energy which is responsible about the holes moving into the active region and intra-band tunneling processes that allows the holes transport into the active region. The well architecture of the p-EBL attributed to the high holes’ injection efficiency in the active region as well as the electrons concertation. High carrier concentration leads to increase the radiative recombination rate in the active region, enhance the power spectral density, and in overall in the UVC-LED performance.

The barrier height is adjusted via insert an undoped thin layer as a superlattice layer as follow p-Al_0.95Ga_0.05N/Al_0.90Ga_0.10N/p-Al_0.95Ga_0.05N to make the holes’ movement easier to the MQW layer subsequently increasing the hole concentration in the active layer. Therefore, the carrier concentration increased including the electron concentration. As a results, the radiative recombination rate in the active region layer improved as well as the optical power for the UVC-LED with the superlattice EBL design.

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Availability of data and material

The data of this study are available from the corresponding author upon reasonable request

Competing interests

The authors declare that they have no competing interests.

Funding

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Authors' contributions

Abdullah Alharbi and Hadba Hussain conceived and designed the experiments
Hadba Hussain and Renad Ali: writing the code, studying the devices, analyzed the results.

Abdullah Almogbel: contribute to the data analysis and revised

Abdullah Alharbi: supervision, contribute to the data analysis and revised

The manuscript was written with input from all authors

Acknowledgements

Not applicable

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No competing interests reported.

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Electron Blocking Layer Architecture to enhance the hole injection in the active region for 215 nm Ultra-violet Light Emitting Diode (UVC-LED)

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