Xingtong Liu

Effects of GaN/AlGaN/Sputtered AlN nucleation layers on performance of GaN-based ultraviolet light-emitting diodes

We report on the demonstration of GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at 375u2009nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN nucleation layers (NLs) and ex-situ sputtered AlN NL. The threading dislocation (TD) densities in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NLs were determined by high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), which revealed that the TD density in UV LED with AlGaN NL was the highest, whereas that in UV LED with sputtered AlN NL was the lowest. The light output power (LOP) of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL owing to a decrease in the absorption of 375u2009nm UV light in the AlGaN NL with a larger bandgap. Using a sputtered AlN NL instead of the AlGaN NL, the LOP of UV LED was further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. In the sputtered AlN thickness range of 10–25u2009nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest, revealing that optimum thickness of the sputtered AlN NL is around 15u2009nm.

Comparative study of GaN-based ultraviolet LEDs grown on different-sized patterned sapphire substrates with sputtered AlN nucleation layer

GaN-based ultraviolet-light-emitting diodes (UV LEDs) with 375 nm emission were grown on different-sized patterned sapphire substrates (PSSs) with ex situ 15-nm-thick sputtered AlN nucleation layers by metal–organic chemical vapor deposition (MOCVD). It was observed through in situ optical reflectance monitoring that the transition time from a three-dimensional (3D) island to a two-dimensional (2D) coalescence was prolonged when GaN was grown on a larger PSS, owing to a much longer lateral growth time of GaN. The full widths at half-maximum (FWHMs) of symmetric GaN(002) and asymmetric GaN(102) X-ray diffraction (XRD) rocking curves decreased as the PSS size increased. By cross-sectional transmission electron microscopy (TEM) analysis, it was found that the threading dislocation (TD) density in UV LEDs decreased with increasing pattern size and fill factor of the PSS, thereby resulting in a marked improvement in internal quantum efficiency (IQE). Finite-difference time-domain (FDTD) simulations quantitatively demonstrated a progressive decrease in light extraction efficiency (LEE) as the PSS size increased. However, owing to the significantly reduced TD density in InGaN/AlInGaN multiple quantum wells (MQWs) and thus improved IQE, the light output power of the UV LED grown on a large PSS with a fill factor of 0.71 was 131.8% higher than that of the UV LED grown on a small PSS with a fill factor of 0.4, albeit the UV LED grown on a large PSS exhibited a much lower LEE.

Numerical and experimental investigation of GaN-based flip-chip light-emitting diodes with highly reflective Ag/TiW and ITO/DBR Ohmic contacts

We demonstrate two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with highly reflective Ag/TiW and indium-tin oxide (ITO)/distributed Bragg reflector (DBR) p-type Ohmic contacts. We show that a direct Ohmic contact to p-GaN layer using pure Ag is obtained when annealed at 600°C in N2 ambient. A TiW diffusion barrier layer covered onto Ag is used to suppress the agglomeration of Ag and thus maintain high reflectance of Ag during high temperature annealing process. We develop a strip-shaped SiO2 current blocking layer beneath the ITO/DBR to alleviate current crowding occurring in FCLED with ITO/DBR. Owing to negligibly small spreading resistance of Ag, however, our combined numerical and experimental results show that the FCLED with Ag/TiW has a more favorable current spreading uniformity in comparison to the FCLED with ITO/DBR. As a result, the light output power of FCLED with Ag/TiW is 7.5% higher than that of FCLED with ITO/DBR at 350 mA. The maximum output power of the FCLED with Ag/TiW obtained at 305.6 A/cm2 is 29.3% larger than that of the FCLED with ITO/DBR obtained at 278.9 A/cm2. The improvement appears to be due to the enhanced current spreading and higher optical reflectance provided by the Ag/TiW.

Reflectance bandwidth and efficiency improvement of light-emitting diodes with double-distributed Bragg reflector

Distributed Bragg reflectors (DBR) with metal film on the bottom have been demonstrated to further improve the light output power of GaN-based light-emitting diodes (LEDs). Periods of TiO2/SiO2 stacks, thickness of metal film, and material of metallic reflector were designed and optimized in simulation software. The maximal bandwidth of double-DBR stacks have reached up to 272xa0nm, which was 102xa0nm higher than a single-DBR stack. The average reflectance of LEDs with wavelength from 380xa0nm to 780xa0nm in double-DBR stacks is 95.09% at normal incident, which was much higher than that of a single-DBR stack whose average reflectance was 91.38%. Meanwhile, maximal average reflectance of LEDs for double-DBR stacks with an incident angle from 0 to 90° was 97.41%, which was 3.2% higher than that of a single-DBR stack with maximal average reflectance of 94.21%. The light output power of an LED with double-DBR stacks is 3% higher than that of an LED with a single-DBR stack, which was attributed to high reflectance of double-DBR stacks.

The effect of nanometre-scale V-pits on electronic and optical properties and efficiency droop of GaN-based green light-emitting diodes

The development of efficient green light-emitting diodes (LEDs) is of paramount importance for the realization of colour-mixing white LEDs with a high luminous efficiency. While the insertion of an InGaN/GaN superlattice (SL) with a lower In content before the growth of InGaN/GaN multiple quantum wells (MQWs) is known to increase the efficiency of LEDs, the actual mechanism is still debated. We therefore conduct a systematic study and investigate the different mechanisms for this system. Through cathodoluminescence and Raman measurements, we clearly demonstrate that the potential barrier formed by the V-pit during the low-temperature growth of an InGaN/GaN SL dramatically increases the internal quantum efficiency (IQE) of InGaN quantum wells (QWs) by suppressing non-radiative recombination at threading dislocations (TDs). We find that the V-pit potential barrier height depends on the V-pit diameter, which plays an important role in determining the quantum efficiency, forward voltage and efficiency droop of green LEDs. Furthermore, our study reveals that the low-temperature GaN can act as an alternative to an InGaN/GaN SL structure for promoting the formation of V-pits. Our findings suggest the potential of implementing optimized V-pits embedded in an InGaN/GaN SL or low-temperature GaN structure as a beneficial underlying layer for the realization of highly efficient green LEDs.

Numerical simulation and experimental investigation of GaN-based flip-chip LEDs and top-emitting LEDs

We demonstrate a GaN-based flip-chip LED (FC-LED) with a highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) ohmic contact. A transparent ITO current spreading layer combined with Ta2O5/SiO2 double DBR stacks is used as a reflective p-type ohmic contact in the FC-LED. We develop a strip-shaped SiO2 current blocking layer, which is well aligned with a p-electrode, to prevent the current from crowding around the p-electrode. Our combined numerical simulation and experimental results revealed that the FC-LED with ITO/DBR has advantages of better current spreading and superior heat dissipation performance compared to top-emitting LEDs (TE-LEDs). As a result, the light output power (LOP) of the FC-LED with ITO/DBR was 7.6% higher than that of the TE-LED at 150xa0mA, and the light output saturation current was shifted from 130.9u2009u2009A/cm2 for the TE-LED to 273.8u2009u2009A/cm2 for the FC-LED with ITO/DBR. Owing to the high reflectance of the ITO/DBR ohmic contact, the LOP of the FC-LED with ITO/DBR was 13.0% higher than that of a conventional FC-LED with Ni/Ag at 150xa0mA. However, because of the better heat dissipation of the Ni/Ag ohmic contact, the conventional FC-LED with Ni/Ag exhibited higher light output saturation current compared to the FC-LED with ITO/DBR.

An InGaN/GaN Superlattice to Enhance the Performance of Green LEDs: Exploring the Role of V-Pits

Despite the fact that an InGaN/GaN superlattice (SL) is useful for enhancing the performance of a GaN-based light-emitting diode (LED), its role in improving the efficiency of green LEDs remains an open question. Here, we investigate the influence of a V-pits-embedded InGaN/GaN SL on optical and electrical properties of GaN-based green LEDs. We recorded a sequence of light emission properties of InGaN/GaN multiple quantum wells (MQWs) grown on a 0- and 24-pair InGaN/GaN SL by using scanning electron microscopy (SEM) in combination with a room temperature cathodoluminescence (CL) measurement, which demonstrated the presence of a potential barrier formed by the V-pits around threading dislocations (TDs). We find that an increase in V-pit diameter would lead to the increase of V-pit potential barrier height. Our experimental data suggest that a V-pits-embedded, 24-pair InGaN/GaN SL can effectively suppress the lateral diffusion of carriers into non-recombination centers. As a result, the external quantum efficiency (EQE) of green LEDs is improved by 29.6% at an injection current of 20 mA after implementing the V-pits-embedded InGaN/GaN SL layer. In addition, a lower reverse leakage current was achieved with larger V-pits.