Zhijue Quan

Roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well light-emitting diodes

The roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well (MQW) light-emitting diodes are investigated by numerical simulation. The simulation results show that V-shaped pits cannot only screen dislocations, but also play an important role on promoting hole injection into the MQWs. It is revealed that the injection of holes into the MQW via the sidewalls of the V-shaped pits is easier than via the flat region, due to the lower polarization charge densities in the sidewall structure with lower In concentration and {10–11}-oriented semi-polar facets.

Electroluminescence from the sidewall quantum wells in the V-shaped pits of InGaN light emitting diodes

InGaN/GaN multi-quantum well (MQW) light emitting diodes with heavily Mg doped and unintentionally doped (UID) low-temperature Al0.2Ga0.8N electron blocking layer (EBL) were investigated. Broad short-wavelength electroluminescence peak, which has strong relative intensity to the main emission, was found in the UID-EBL sample at cryogenic temperatures. Study suggests that the broad peak is emitted by the sidewall MQWs. This result indicates that the electroluminescence of sidewall MQWs, in which the carrier density is high enough, can be detected at cryogenic temperatures. The lineshape variation with current density reveals detailed information on the process of carrier injection into the sidewall MQWs.

The effect of silicon doping in the barrier on the electroluminescence of InGaN/GaN multiple quantum well light emitting diodes

InGaN/GaN multiple quantum well (MQW) light emitting diodes were grown on silicon substrate by metal organic chemical vapor deposition. A different barrier was heavily doped with silicon based on the same structure. Temperature dependent electroluminescence was performed on the devices. The results reveal that heavily doping the barrier distant from the n-type layer with silicon causes two emission peaks. As the doped barrier gets closer to n-type layer, the energy gap between the two peaks becomes narrower. Silicon doped in the barrier is believed to generate p-n junction built-in field from the doped barrier towards p-type layer. This field compensates the piezoelectric field in the well(s) between the doped barrier and p-type layer. It results in higher emission energy of this (these) well(s). When the doped barrier gets closer to the n-type layer, the compensation is less significant.

A new interpretation for performance improvement of high-efficiency vertical blue light-emitting diodes by InGaN/GaN superlattices

The effect of InGaN/GaN superlattices(SLs) on quantum efficiency and forward voltage of vertical blue InGaN/GaN multiple quantum well(MQW)light-emitting diodes(LED) grown on Si substrate has been experimentally and theoretically investigated. We have prepared two LED samples, in which the 30 and 45 periods of SLs are inserted between MQW active layers and n-GaN layer, respectively. Electroluminescence measurement shows that the LED with 45 periods of SLs has higher quantum efficiency but lower forward voltage. It is observed that V-shaped pits grow up in size with an increase in SLs period number by means of scan transmission electron microscope and secondary ion mass spectrometry. Further numerical simulations confirm that the performance improvement of LED by SLs is mainly ascribed to enhancing hole injection from the V-shaped pits.

Reduction of the resistivity of Ag/p-GaN contact by progressive breakdown of the interfacial contamination layer

Ag contact was prepared on p-GaN that had been previously activated by annealing and then removing an Ag/Ni contact. The non-annealed Ag contact showed higher resistivity than the annealed Ag/Ni contact. But, we found that the resistivity of the Ag contact decreases gradually under an electrical stress. Through secondary ion mass spectroscopy analysis, we excluded the effect of enhancement of the holes concentration by the electrical stress and attributed the decrease of resistivity to the progressive breakdown of the contamination layer at the Ag/GaN interface. Our findings provide a way to obtain low-resistivity non-annealed Ag contact to p-GaN.

Effects of thickness ratio of InGaN to GaN in superlattice strain relief layer on the optoelectrical properties of InGaN-based green LEDs grown on Si substrates

InGaN-based multiple quantum well (MQW) green light-emitting diodes with a InGaN/GaN superlattice as a strain relief layer (SSRL) were grown on Si(111) substrates by metal organic chemical vapor deposition. The influences of the thickness ratio of InGaN to GaN in SSRL on the optoelectrical properties have been investigated. Electrical measurements show that the sample with a higher thickness ratio has a lower series resistance. This is mainly ascribed to the improvement of carrier vertical transport due to the thinner GaN in SSRL. However, it is found that the leakage current increases with the thickness ratio from 1:1 to 2.5:1, which could be attributed to the larger density of small size V-pits forming at the first few QW pairs. Compared with the smaller thickness ratio, the sample with a higher thickness ratio of InGaN to GaN in SSRL is found to exhibit larger strain relaxation (about 33.7%), but the electroluminescence measurement exhibits inferior emission efficiency. Carrier leakage via the small V-...

Effects of p-AlGaN EBL thickness on the performance of InGaN green LEDs with large V-pits

The effects of the p-AlGaN electron blocking layer (EBL) thickness on the performance of InGaN/GaN multiple quantum wells (MQWs) green light emitting diodes (LEDs) was investigated. It was observed that increasing the thickness of the p-AlGaN EBL could reduce the leakage current and improve the efficiency of green LEDs with large V-pits. It is proposed that increasing the EBL thickness leads to a thicker p-AlGaN on the sidewalls of V-pits, which provides a thicker energy barrier and consequently screens dislocations more effectively. The leakage current (at −5 V) of LEDs with a 40 nm EBL is about an order of magnitude lower than that of LEDs with a 20 nm EBL. With the increase in EBL thickness, at low current densities, the external quantum efficiency (EQE) firstly decreases and then increases afterwards, which could be attributed to the competition between the enhancement of the radiative recombination rate and the reduction of the hole injection efficiency. At operating current density, there is a positive correlation between EQE and the thickness of the EBL. This is attributed to the improved electron confinement in the active region by preventing electrons overflowing to the p-type layer. Meanwhile, the efficiency droop is obviously suppressed when the thickness of the EBL increases from 20 nm to 40 nm. However, further increasing the thickness of the EBL may deteriorate the EQE and efficiency droop. Packaged green LED chips with an optimized EBL emit 260 mW (dominant wavelength: 520 nm) at 350 mA (35 A cm−2), and the EQE reaches 31.2%.

Performance enhancement of yellow InGaN-based multiple-quantum-well light-emitting diodes grown on Si substrates by optimizing the InGaN/GaN superlattice interlayer

A specially designed InGaN/GaN superlattice (SL) interlayer was inserted between n-GaN and a multiple quantum well to enhance the performance of yellow light-emitting diodes (LEDs) grown on Si (111). The number of SL periods was determined to be the key to enhancing the external quantum efficiency and reducing forward voltage. Our results show that more SLs could suppress nonradiative recombination by eliminating micron-scale indium-rich clusters and could promote hole injection with increased V-pit size. However, too many SLs reduce the effective luminescence area and lead to many voids formed in the p-type layer. We demonstrate that 32 is the optimum number of SLs for yellow InGaN/GaN LEDs, obtaining a high light output power of 63 mW with a dominant wavelength of 568 nm, and a low forward voltage of 2.38 V at 200 mA (20 A/cm2).

Carrier transport via V-shaped pits in InGaN/GaN MQW solar cells*

Carrier transport via the V-shaped pits (V-pits) in InGaN/GaN multiple-quantum-well (MQW) solar cells is numerically investigated. By simulations, it is found that the V-pits can act as effective escape paths for the photo-generated carriers. Due to the thin barrier thickness and low indium composition of the MQW on V-pit sidewall, the carriers entered the sidewall QWs can easily escape and contribute to the photocurrent. This forms a parallel escape route for the carries generated in the flat quantum wells. As the barrier thickness of the flat MQW increases, more carriers would transport via the V-pits. Furthermore, it is found that the V-pits may reduce the recombination losses of carriers due to their screening effect to the dislocations. These discoveries are not only helpful for understanding the carrier transport mechanism in the InGaN/GaN MQW, but also important in design of the structure of solar cells.

Study on the band alignment of GaN/CH3NH3PbBr3 heterojunction by x-ray photoelectron spectroscopy

A GaN/CH3NH3PbBr3 heterojunction was fabricated by depositing a GaN thin layer on a CH3NH3PbBr3 single crystal by plasma enhanced atomic layer deposition. The band alignment of the GaN/CH3NH3PbBr3 heterojunction was studied by x-ray photoelectron spectroscopy. The valance band offset (VBO) is directly determined to be 0.13 ± 0.08 eV. The conduction band offset is deduced from the VBO and the band gaps, which turned out to be 1.39 ± 0.12 eV. Thus, the band alignment of the GaN/CH3NH3PbBr3 heterojunction is determined to be type-I. These results show that GaN is a promising material for carrier confinement in halide perovskite based light emitting devices.