TECHNOLOGY LEDs
a longer wavelength, while the optical gain for the xʹ-polarization shifts to a shorter wavelength. This high degree of anisotropy is not ideal for making an LED, and the optical gain is not as high as it can be in other material systems.
Cuprous halide LEDs We believe that one material system that could take the performance of an LED to an entirely new level is that of the copper compounds CuBr and CuCl. One of the most attractive attributes of the I-VII cuprous halides is their incredibly high optical gain: It is more than an order of magnitude higher than that of AlInGaN, thanks to a combination of inherent strong excitonic effects and negligible piezoelectric internal fields. What’s more, the lattice spacing in CuBr/CuCl quantum wells is close to that found in silicon substrates. That means cheap, widely available silicon substrates could be used as the foundation for producing devices that are free from misfit dislocations.
The I-VII cuprous halides, which include CuBr, CuCl and CuI, are direct bandgap semiconductors with a zincblend crystalline structure. They have piqued the interest of the research community with their very high exciton binding energies: For CuCl and CuBr, binding energies are 190 meV and 108 meV, compared with just 20 meV for GaN and 63 meV for ZnO. A high exciton binding energy is indicative of a strong attractive electron- hole Coulomb interaction, and ultimately enhanced optical transitions, even at temperatures well above room temperature.
We are not the first group to study these I-VII materials from a theoretical perspective. Efforts in this direction date back to the 1970s, led by Manual Cardona’s group at the Max Plank Institute for Solid-State Research. However, this initial study and those that have followed have been restricted to bulk materials. Our breakthrough is to consider quantum well heterostructures, the region found in real LEDs.
Our calculations, which include many-body effects such as
Figure 3. Optical gain spectra for CuBr/CuCl QW (green), ZnO/ Mg0.3
Zn0.7 O QW (blue), and In0.2 Ga0.8 N/Al0.2 In0.005 photon energy for carrier density of 5 x 1019 G0.7995 cm-2
bandgap renormalization, enhancement of optical gain due to excitonic effects and plasma screening, have determined the optical gain spectra for a CuBr/CuCl quantum well. Gain is 30 times that produced by an In0.2
Ga0.8 Zn0.7 O quantum well. N/Al0.2 In0.005 G0.7995
well, and significantly higher than that produced by a ZnO/ Mg0.3
Although this modelling effort shows that cuprous halide LEDs have tremendous promise, there is obviously a great deal of work still to do before they can make any commercial impact. The first step towards this is to establish a growth technology for forming high quality epitaxial films. Fortunately, some groundwork has already been carried out for this – in 2005 a partnership between scientists in Ireland reported the growth of a thin film of polycrystalline CuCl on a silicon (111) surface (this material produced strong room-temperature photoluminescence related to excitonic recombination).
In addition, the doping techniques need to be established, which could involve zinc and magnesium as n-type dopants and oxygen, sulphur, and selenium as p-type dopants. And once this has been accomplished, devices will have to be designed, shown to be robust and manufactured in high volumes. Only if all of this happens will cuprous halides be in with a chance of displacing nitrides in solid-state lighting.
Figure 2. (a) xʹ- and yʹ-polarized optical gain spectra for several crystal orientations and (b) the in-plane optical anisotropy as a function of the crystal orientation of the wurtzite In0.2
Ga0.8 N quantum well with a width of
3 nm. The decrease of the optical peak is attributed to the reduction of the optical matrix element for the xʹ-polarization. For the case of the yʹ- polarization, the optical gain peak increases significantly with the crystal angle. Note that the optical gain of the (0001)-oriented quantum well is calculated self-consistently, taking into account the piezoelectric and spontaneous polarizations
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www.compoundsemiconductor.net October 2013
© 2013 Angel Business Communications. Permission required.
Further reading S. H. Park and D. Ahn, Proc. SPIE 8625 862511-1, 201 D. Ahn and S. L. Chuang, Appl. Phys. Lett. 102 121114 (2013)
N QW (red) versus
N quantum
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