June 18, 2013…Hong Kong University of Science and Technology (HKUST) has developed silicon substrate growth of high-performance green and yellow nitride semiconductor LEDs. Details of the development were published in the May 29th issue of IEEE Electronic Device Letters. The researchers claim their 565nm yellow LEDs are the first multi-quantum well (MQW) devices produced on silicon.
In theory, using silicon as a substrate would lower material cost and enable economies of scale in mass production from the larger wafer diameters. However, the quality of nitride semiconductors on silicon suffer from the larger lattice mismatch compared with conventional, more expensive substrates of free-standing GaN, sapphire or silicon carbide (SiC).
The researchers note that producing longer-wavelength nitride semiconductor LEDs is challenging due to the difficulty of producing good-quality indium gallium nitride (InGaN) with higher indium concentration. Although growth on silicon is well established in nitride semiconductor-based transistor development, the researchers point out that it is only fairly recently that similar growth methods have been applied to LED device material.
The researchers used metal-organic chemical vapor deposition (MOCVD) to grow the initial template, grown on 2-inch silicon . The template consisted of aluminium nitride (AlN) nucleation, 8 pairs of aluminium nitride/gallium nitride (AlN/GaN) layers to create a superlattice (SL) as stress-balancing interlayer, and a 2μm GaN buffer.
The researchers deposited layers of SiO2 and indium tin oxide (ITO) and etching the ITO with hydrochloric (HCL) acid solution to form a mask, and finally used plasma etching to form Silicon dioxide (SiO2) nanorods. The density of nanorods was 2×109/cm2, giving a surface coverage of 35%. The nanorods acted as a mask in GaN re-growth with reduced dislocation density and improved crystalline quality.
Then the researchers grew the LED structure through MOCVD with re-growth of 800nm of GaN around the nanorods, an AlN/GaN SL interlayer, 2μm of n-type GaN, a 5-period multiple quantum well (MQW), and 200nm of p-GaN. The re-grown GaN had a dislocation density of 8×108/cm2, which was described by the researchers as “one of the lowest values reported for GaN-on-Si substrates, as determined by TEM”.
Materials suitable for emitting yellow (565nm) and green (505 and 530nm) light were prepared and formed into 300μm x 300μm LED chips.
As was to be expected, the light output power (LOP) decreased as the wavelength increased (Figure 2). At 20mA, the output at 505nm was 1.18mW. The respective values for 530nm and 565nm were 0.30mW and 74μW, respectively. Saturation of the light output power was achieved at 7.60mW (200mA), 2.72mW (180mA) and 0.52mW (160mA), for the 505nm, 530nm and 565nm devices, respectively.
The researchers stated, “This is the first report of fabricated 565nm yellow InGaN/GaN MQW LEDs on a silicon substrate, and the LOP of the 505nm LEDs was much higher than that for the LEDs on Si reported in the past.”
Apart from the improved material quality, the researchers believe that the nanorods also provide a scattering enhancement of light extraction from the devices.