Silanna Core Technology

Short Period Superlattice Technology (SPSL)

Currently, most UV-C LED manufacturers use the AlGaN material system as the basis for UV-C 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, can in theory allow the bandgap to be adjusted to produce emissions from 340 nm to 210 nm, 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, both n and p-type, especially in the shortest wavelength ranges. 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 in Deep UV-C LED technologies, Silanna uses a different approach to the generation of UV light. Instead of the common AlGaN approach, Silanna utilizes a short period superlattice (SPSL) technology.

Competitors: AlxGa1-xN
Tune wavelength by changing x

Silanna UV: AIN/GaN SPSL
Tune wavelength by changing thickness

Competitors: AlxGa1-xN
Tune wavelength by changing x

Silanna UV: AIN/GaN SPSL
Tune wavelength by changing thickness

In this approach, up to several hundred periods of repeating layers of binary AlN barriers and GaN wells are formed, 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 what is called an SPSL, also known as a digital alloy, whose properties, including bandgap and conductivity, can be tuned by simply adjusting the thickness of the constituent layers. This can essentially be considered a new material system that is much easier to tune, with properties that are far superior to traditional AlGaN. In particular, the use of SPSL digital alloy technology allows Silanna to standout from its competitors, providing the following advantages:

a. Ability to easily tune wavelengths

Using an SPSL approach, the emission wavelength of the active layer can be easily tuned via the thicknesses and period of constituent layers. This eliminates many of the problems faced by traditional AlGaN technologies such as composition fluctuation and segregation that makes process controllability extremely challenging at high compositions.

AlGaN Active Region

Bulk AlGaN is prone to segregation and wavelength tuning requires the tuning of Al to Ga species ratio which can be hard to control

SPSL Active Region

Binary SPSL is less prone to segregation and wavelength tuning only requires of easy change in deposition time

b. Maintains 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 light is emitted vertically in these traditional structures. 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.

AlGaN Quantum Well

Most emission is lateral leading to low power at low wavelength

AIN/GaN SPSL

Emission mostly remains vertical through the whole UV Spectrum 

c. Superior Electrical Characteristics

Because of the nature of SPSL based digital alloys where the emission wavelength is tuned via thickness rather than the actual bandgap of the material, a higher conductivity can be maintained by an SPSL layer compared to an optically equivalent or composition equivalent bulk AlGaN n-type layer. This allows a low drive voltage and high efficiency to be maintained even at the lowest emission wavelengths.

N-doped AlGaN

Short Wavelength implies wide bandgap resulting in deep donors

N-doped SPSL

Donor remain shallow to the GaN wells regardless of emission wavelength

At Silanna we use the SPSL technology through the entire device stack, taking full advantage of the superior properties of SPSL as described above. Using this approach, we have successfully demonstrated high performance Deep UV-C and Far UV-C 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.