Enhanced Performance of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes with Chirped Superlattice Electron Deceleration Layer
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AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) suffer from electron overflow and insufficient hole injection. In this paper, novel DUV LED structures with superlattice electron deceleration layer (SEDL) is proposed to decelerate the electrons injected to the active region and improve radiative recombination. The effects of several chirped SEDLs on the performance of DUV LEDs have been studied experimentally and numerically. The DUV LEDs have been grown by metal-organic chemical vapor deposition (MOCVD) and fabricated into 762 × 762 μm2 chips, exhibiting single peak emission at 275 nm. The external quantum efficiency of 3.43% and operating voltage of 6.4 V are measured at a forward current of 40 mA, indicating that the wall-plug efficiency is 2.41% of the DUV LEDs with ascending Al-content chirped SEDL. The mechanism responsible for this improvement is investigated by theoretical simulations. The lifetime of the DUV LED with ascending Al-content chirped SEDL is measured to be over 10,000 h at L50, due to the carrier injection promotion.
KeywordsAlGaN DUV LED SEDL MOCVD APSYS
Advance Physical Model of Semiconductor Devices
Bright-field scanning transmission electron microscopy
Carrier injection efficiency
Electron blocking layer
External quantum efficiency
Full width at half maximum
High-resolution X-ray diffraction
Internal quantum efficiency
Light extraction efficiency
Light output power
Metal-organic chemical vapor deposition
Superlattice electron deceleration layer
Threading dislocation density
X-ray rocking curve
In recent years, AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs), whose spectra ascribed to UVB (320 nm–280 nm) and UVC (280 nm–100 nm), have attracted much attention because of their applications in plant lighting, phototherapy, water purification, and air and surface sterilization [1, 2, 3, 4, 5, 6]. However, the light output power (LOP) of the state-of-the-art AlGaN-based DUV LEDs drops significantly as the light emission wavelength gets shorter [7, 8]. Those DUV LEDs suffer from low internal quantum efficiency (IQE), light extraction efficiency (LEE), and carrier injection efficiency (CIE) [9, 10, 11, 12, 13]. Generally, deficient IQE is caused by large density of defects and threading dislocations, while insufficient LEE is due to the polarization of AlGaN materials and the absorption by the nontransparent p-GaN contact layer [14, 15, 16, 17, 18]. Furthermore, electron overflow is the main reason for the poor CIE, which is on account of the inadequate hole density and the significantly imbalanced mobility of electron and hole in AlGaN materials [19, 20].
Conventionally, high-Al-content p-type AlGaN electron blocking layer (EBL) is used to suppress the electron overflow. But only a few holes can be injected into the active region through the barrier in the valence band introduced by the EBL, and even less holes can cross the barriers of the active region and transport to the quantum wells near n-type layers because of low activation efficiency of the Mg dopant and small mobility of holes . Various attempts have been made to improve electron and hole injection, such as hole barrier layer, specifically designed last barrier, EBL, and multiple quantum well structures [22, 23, 24, 25, 26]. Nevertheless, the performance of DUV LEDs is not substantially improved.
In this work, we have proposed a novel DUV LED structure with superlattice electron deceleration layer (SEDL) to decelerate the electron injection and restrain the electron overflow without compromising the hole injection. We have studied the effects of several SEDLs on the performance of DUV LEDs experimentally and numerically. The DUV LEDs have been grown by metal-organic chemical vapor deposition (MOCVD) and fabricated into 762 × 762 μm2 chips, exhibiting single peak emission at 275 nm. The external quantum efficiency (EQE) of 3.43% and operating voltage of 6.4 V were measured at a forward current of 40 mA, indicating that the wall-plug efficiency is 2.41% of the DUV LEDs with ascending Al-content chirped SEDL. The lifetime of the DUV LED with ascending Al-content chirped SEDL is measured to be over 10,000 h at L50. Furthermore, the mechanism of performance enhancement is investigated by theoretical simulation. It is verified that chirped SEDLs are able to equilibrate electron and hole injection into the active region, which promotes the radiative recombination in the first few quantum wells near n-type layers.
Methods and Experimental Section
Epitaxy by MOCVD
Following the MOCVD growth, DUV LEDs were fabricated with standard processing techniques. First, mesa structures with square and finger geometries were formed by dry-etching down to 150 nm below the top of Si-doped Al0.6Ga0.4N n-type contact layer, followed by a 900 °C annealing to repair the etching damage. Then, Ti/Al/Ni/Au n-contact metal stack was deposited and annealed at 850 °C in nitrogen atmosphere. Subsequently, an ITO film was evaporated and annealed at 250 °C for the use of p-contact, followed by thick electrode evaporation, passivation layer deposition, pad evaporation, and stealth dicing into 762 × 762 μm2 chips.
To illuminate the mechanism of performance enhancement of DUV LEDs, the band diagram, optical properties, and carrier transport characteristics of this structure were simulated by solving the Schrödinger equation, Poisson’s equation, the carrier transport equations, and the current continuity equation self-consistently by Crosslight APSYS (Advance Physical Model of Semiconductor Devices) programs . The Shockley-Read-Hall (SRH) recombination time is set to be 1.5 ns for all layers except the p-type inserted layer as 1 ns because the SRH lifetime is dependent upon the doping level . The internal loss is 2000 m−1 . The bowing parameter b is 1 eV, and the band-offset ratio is assumed to be 0.7/0.3 for AlGaN materials . The Auger recombination coefficient is set to be 1 × 10−30 cm6/s to fit the experiment . In this simulation, the built-in interface charges due to the spontaneous and piezoelectric polarization are calculated based on the method proposed by Fiorentini et al. . Furthermore, taking the screening by defects into consideration, the surface charge densities are assumed to be 40% of the calculated values .
Results and Discussion
Crystalline quality characterization of AlN and n-type AlGaN layers of samples A, B, C, and D by high-resolution X-ray diffraction along symmetric (002) plane and asymmetric (102) plane. Threading dislocation density (TDD) was calculated according to ref. 
8.87 × 108
1.33 × 109
8.83 × 108
1.30 × 109
8.76 × 108
1.35 × 109
8.56 × 108
1.27 × 109
The effects of the chirped superlattice electron deceleration layer on the DUV LEDs are investigated experimentally and numerically. The results indicate that chirped SEDLs are able to equilibrate electron and hole injection into the active region, which promotes the radiative recombination in the first few quantum wells near n-type layers. The increase of radiative recombination further leads to the enhancement of DUV LED device performance. The AlGaN-based DUV LEDs have been fabricated into 762 × 762 μm2 chips, exhibiting single peak emission at 275 nm. External quantum efficiency of 3.43% and operating voltage of 6.4 V are measured at a forward current of 40 mA, demonstrating that the wall-plug efficiency is 2.41% of the DUV LEDs with ascending Al-content chirped SEDL. The lifetime of the DUV LED with ascending Al-content chirped SEDL is measured to be over 10,000 h at L50, due to the carrier injection promotion. Further improvement can be expected by introducing laser lift-off, surface roughening, reflecting electrode, and encapsulation. In general, the designed DUV LED with chirped SEDL shows satisfactory electrical property, favorable optical performance, and desirable reliability, which is promising for high-efficiency water purification and surface sterilization.
The authors thank Haili Zhang engineer in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in the XRD test.
JH and JZ grew the AlGaN-based DUV LED with and without SEDL and wrote this manuscript. YZ, HZ, and HL carried out the chip process. QC simulated the structures by APSYS. MS and SD worked on the measurement. JD and CC managed the experiments and simulation. All authors read and approved the final manuscript.
This work is supported by the Key Project of Chinese National Development Programs (Grant No. 2016YFB0400901), the Key Laboratory of infrared imaging materials and detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences (Grant No. IIMDKFJJ-17-09), the National Natural Science Foundation of China (Grant No. 61774065), and the Director Fund of WNLO.
The authors declare that they have no competing interests.
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