TECHNOLOGY LEDs
The density of these defects increases at higher indium concentrations, due to a greater lattice mismatch between GaN and InN and an increase in the miscibility gap in the InGaN crystal phase. To produce an LED emitting at 540 nm or more, the indium content must be greater than 25 percent – compared to just 14 percent for a blue LED. This increase in indium richness for the green emitter makes it very tricky to grow an active region with eight or nine quantum wells, a standard structure for a blue emitter. The higher indium content can result in high levels of accumulated strain, which can cause catastrophic crystal degradation.
Figure 2. Toshiba’s yellow LED emits a peak wavelength of around 570 nm and produces an external quantum efficiency of almost 20 percent at 20 mA
can reduce them. Escaping from the impairments resulting from of the QCSE can enhance the radiative efficiency and assist the fabrication of high-efficiency, longer-wavelength LEDs. In fact, superior optical properties of green LEDs using non-polar (1120) or semi-polar (2021) GaN substrates have already been reported. Furthermore, weakened electric fields can facilitate thickening of the quantum wells for reduction of average carrier density, achieving low droop operation which is one of the greatest concerns for GaN based LEDs [2]. However, there are challenges associated with the crystal growth of relatively high indium contents layers needed to form yellow or longer wavelength emitters on semi-polar and non-polar substrates.
Also, both of these foundations, which are formed by slicing GaN crystals, are prohibitively expensive. This means that the only practical way forward for producing longer wavelength LEDs is to build them on conventional c-plane sapphire (0001) substrates.
In addition to the issues associated with the QCSE, crystal degradation hampers the development of longer-wavelength, GaN-based LEDs. As indium content in the InGaN alloy increases, this layer is plagued with an increasing number of imperfections: threading dislocations, stacking faults, V-shape defects and indium-rich surface clusters. All of them act as non-radiative centres, hindering LED performance.
To address the green gap, our team from Toshiba Corporation of Kawasaki, Japan, has developed optimised growth conditions for forming a new active region by MOCVD. This structure, which is formed on conventional c-plane sapphire substrates, features a 1 nm-thick AlGaN capping layer directly above each InGaN quantum well.
There are several benefits associated with the addition of this thin AlGaN capping layer [3]. Its primary purpose is to shift the wave-function of the electrons toward the inside of the well, thereby increasing electron-hole overlap and radiative recombination. However, introducing the capping layer also creates a barrier to electron overflow from each well, so we do not need to employ a thick, conventional AlGaN electron- blocking layer above the active region. What’s more, the thin layer of AlGaN recovers the smoothness of the surface after the high-indium-content InGaN well, which has a very rough surface. In order to produce longer wavelength LEDs with a high external quantum efficiency, it is essential to realise a high degree of surface flatness prior to the growth of each quantum well.
We have worked hard to optimise the crystal quality of our new active region. To fabricate green, yellow and amber LEDs, we need to have an indium content in the InGaN quantum wells of 24 percent to 28 percent, which is a composition that is tricky to realise by MOCVD.
Figure 3. Toshiba’s novel LED structure is formed by MOCVD 46
www.compoundsemiconductor.net March 2014 The common approach to increasing
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