ZnO-based LEDs Begin To Show Full-color Potential
The attractiveness of ZnO LEDs stems from the potential for phosphor-free spectral coverage from the deep ultraviolet to the red, coupled with a quantum efficiency that could approach 90% and a compatibility with high-yield low-cost volume production. These LEDs could even one day outperform their GaN-based cousins, which offer a narrower spectral range, thanks to three key characteristics – superior material quality, an effective dopant and the availability of better alloys.
The superior material quality is seen in the low defect densities of ZnO layers. At MOXtronics, our development of a viable p-type dopant has provided hole-conducting layers for ZnO-based devices. And our growth of BeZnO layers has shown that it is possible to fabricate ZnO-based high-quality heterostructures (see "The advantages of ZnO over GaN").
ZnO also promises very high quantum efficiencies, and ultraviolet detectors based on this material have produced external quantum efficiencies (EQE) of 90%, three times that of equivalent GaN-based detectors. The physical processes associated with detection suggest that similarly high efficiency values should be possible for the conversion of electrical carriers to photons. So it is plausible that ZnO LEDs will have an EQE upper limit that is three times higher than that of GaN-based devices.
Finding the right dopant
However, ZnO is yet to fulfill all of its promise because of the delay in developing p-doped material. Early progress throughout the community was hampered by focusing efforts on using nitrogen as a p-type dopant. Nitrogen was the first choice because it was an effective dopant in ZnSe, and also because it was deemed, erroneously, to be of a suitable size to sit on an oxygen lattice site. Although we also tried to obtain p-type doping using nitrogen, a switch to arsenic enabled us to report the first successful p-type doping of ZnO in 1997. By 2000 we could produce hole concentrations into the 1017 cm–3 range with this approach.
Later in 2000 we reported our hybrid beam deposition (HBD) process that offers a viable approach to growing doped and undoped ZnO films, alloys and devices. The HBD process is comparable to MBE. However, it uses a zinc oxide plasma source, which is produced by illuminating a polycrystalline ZnO target with either a pulsed laser or an electron beam, and a high-pressure oxygen plasma created by a radio-frequency oxygen generator. Additional sources for either doping or ZnO-based alloy growth can be added to the growth chamber by conventional evaporation or injection methods.
We used the HBD process to fabricate the first ZnO-based ultraviolet detectors (see "Highly efficient detectors"), ultraviolet LEDs, FETs (Ryu et al. 2006), and red, green, blue and white phosphor-coated LEDs. Our LEDs incorporate BeZnO (see figure 2), an alloy that allows bandgap engineering into the ultraviolet and the formation of multiple quantum wells and other heterostructures.
Why BeZnO beats MgZnO
BeZnO alloys of varying composition have provided a significant boost towards the development of the deep ultraviolet high-power LEDs. These alloys do not phase segregate, because BeO and ZnO have the same hexagonal crystal structures, and the extremely high-energy bandgap of BeO could potentially lead to devices emitting at just 117 nm. Ultraviolet LEDs containing BeZnO alloys produce a narrow spectral profile, with very little emission in the visible, suggesting that the alloy is of high crystal quality.
Until we had produced BeZnO films, the primary choice for a compatible higher bandgap alloy was ZnMgO, a material developed by a group at Tohoku University, Toyo University, Tokyo Institute of Technology, and Japan s Institute of Physical and Chemical Research. In 1997 this team reported that crystal phase separation occurs between MgO and ZnO when the atomic fraction of magnesium exceeds 0.33, which corresponds to a bandgap of 3.99 eV. The separation is driven by different crystalline structures; MgO is a cubic structure with a lattice spacing of 0.422 nm, while ZnO is a hexagonal wurtzite structure with a lattice spacing of 0.325 nm.
We recently produced and characterized the first ultraviolet LEDs made from ZnO and BeZnO (figure 3). The device s emission can be tuned from the deep ultraviolet to around 380 nm, the wavelength associated with ZnO. Our devices have been built with several different active layer structures, including double heterostructures and single or multiple quantum wells, to try to improve efficiencies and optical output powers.
Our latest ultraviolet LEDs have a typical wall-plug efficiency of 0.1%, which would equate to an efficacy of 0.6 lm/W if the emission were in the visible spectrum. Although the efficiency is far lower than that of GaN LEDs, we are making rapid progress by addressing the various phenomena that degrade device performance. If progress continues at the same rate we will produce LEDs with a 1% wall-plug efficiency within one year, 1–5% within two years, and about 10% or more within three years (see figure 4).
Our ZnO LED development program has used various substrates manufactured by several vendors, and has shown that the LED s performance is directly dependent on the substrate s material type and crystalline quality. Single-crystal ZnO produces the best devices. This material has been available for many years, and interest is rapidly increasing for the growth of high-quality single-crystal ZnO with a diameter of 50 mm or more that could be used for ZnO-based LEDs and other optoelectronic devices.
Major improvements in the efficiency and power output of ZnO ultraviolet and visible LEDs are still needed to enable these devices to compete in the market place. Advances will depend on the availability of higher quality single-crystal substrates and improved processes for producing reliable and highly-ohmic electrical contacts to various different layers. Additional bandgap engineering development is needed for the ultraviolet C-band (100–280 nm) and visible region, along with optimization of the multiple quantum well and related structures in the device s active region.
With the output power of our ZnO LEDs increasing rapidly, these devices appear to have a promising future. We expect them to first be deployed in white-light lamps and replace incandescent sources in applications such as liquid-crystal display backlights. The promise of emission from the ultraviolet through the visible will then allow ZnO LEDs to target applications where no other single semiconductor material can operate today. At this time, for example, red– green–blue sources that are fabricated on a single wafer will offer unique advantages for the development of bright, compact displays and projectors. Laser diodes built from ZnO-based materials could also be produced that emit in the visible and ultraviolet, and offer compact alternatives for larger tube-type laser sources, ushering in a new era for color printing.
A Ohtomo et al. 1998 Applied Physics Letters 72 2466.
Y R Ryu et al. 2006 Applied Physics Letters 88 241108 (and references therein).