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Silanna UV 4-chip 235nm LEDSilanna UV deep UV and far UV LEDs utilise high performance short period superlattice (SPSL), also known as "digital alloy" technology.

 

This article discusses the SPSL digital alloy approach, how it differs from the conventional bulk GaAlN method of producing UVC LEDs, and highlights the advantages of the Silanna UV technology.

 

Currently, most UVC LED manufacturers use the AlGaN material system as the basis for UVC LEDs. AlGaN is a ternary alloy semiconductor, whereby tuning the composition (i.e. the ratio between Al and Ga in the material) of the alloy, allows the bandgap to be adjusted to produce emissions from 210nm to 340nm, covering most of the UV spectrum. However, the AlGaN system suffers from various problems:-

 

  • Similar to other wide bandgap materials, it is extremely hard to dope AlGaN, especially for the shortest wavelengths.
  • High Al content AlGaN, which is essential for the far UV-range below 240nm, also suffers from light extraction issues due to the polarisation of the emitted light originating from the crystal fields within the AlGaN crystal lattice, resulting in significant reduction in light output at these wavelengths.

 

To overcome these problems an SPSL device comprises up to several hundred periods of repeating layers of binary AlN barriers and GaN wells, with each barrier and well thickness in the order of monolayers (ML) – single layers of molecules or atoms in the 1 – 5 Angstroms range, rather than nanometres. This allows the formation of an SPSL, whose properties, including bandgap and conductivity, can be tuned by adjusting the thickness of the constituent layers, a simple function of the deposition time of each layer. 

 

Below is a representation of the structure of competitor's LEDs with control the wavelength by fine-tuning ratio of Al and Ga in their AlxGa1-xN growth with Silanna UV's AIN/GaN SPSL which is tuned by changing the layer thickness:- 

 

Crystal vs SPSL

 

Silanna UV's SPSL digital alloy can be considered a new material system that is much easier to tune, with properties that are far superior to traditional AlGaN, producing UVC LEDs offering significant advantages to their customers.

Ability to easily tune wavelengths:

Below are representations of the active regions of conventional bulk AlGaN UVC LEDs which are prone to segregation and whose wavelength control requires the tuning of Al to Ga species ratio which can be hard to control - compared with binary SPSL digital alloy which is less prone to segregation and wavelength tuning only requires control of the deposition time.

 

Crystal vs SPSL Active Layers

 

Maintain high power at lower wavelengths:

Traditional AlGaN technology suffers from vertical light extraction issues at short wavelengths due to the transition of emission from vertical (TE) to lateral (TM) in high Al content AlGaN. Only a small amount of the generated photons are emitted vertically in these traditional structures - resulting in low efficiency. However, since the emission from an SPSL is governed by thickness and period rather than composition, the emissions originating from the GaN part of the SPSL remain mostly vertical through the whole spectrum, allowing high power to be maintained even at ultra-short wavelengths. 

Crystal vs SPSL Light Extraction

Superior Electrical Characteristics:

Because the emission wavelength of SPSL based digital alloy LEDs is tuned via thickness rather than the actual bandgap of the material, a higher conductivity can be maintained. In n-doped bulk AlGaN, reducing the wavelength requires a wider bandgap which means increasingly deep donors, whereas in SPSL the donors remain shallow relative to the quantum wells, regardless of wavelength. This allows a low drive voltage and high efficiency to be maintained even at the lowest emission wavelengths. 

 

Crystal vs SPSL Donor TX 600px

Silanna UV use their SPSL digital alloy technology through the entire device stack, taking full advantage of the superior properties described above. Using this approach, they have successfully demonstrated high performance Deep UVC and Far UVC LEDs down to 227nm. Of these, LEDs covering the 235nm and 255nm range are already in production. Products in other wavelength ranges are expected to be available in the near future.

 

For more information on Silanna UV please take a look at the UVC product page or contact APT.