Impact of carrier-density screening on Urbach-tail energies and optical polarization in ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>Al</mml:mi> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/Ma

Aluminum gallium nitride [( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>Al</a:mi> </a:math> , <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>Ga</c:mi> </c:math> ) <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:mrow> <e:mi mathvariant="normal">N</e:mi> </e:mrow> </e:mrow> </e:math> ] presents an ideal platform for designing ultraviolet (UV) light emitters across the entire UV spectral range. However, in the deep-UV spectral range ( <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mo>&lt;</h:mo> <h:mn>280</h:mn> </h:math> nm) these emitters exhibit very low quantum efficiencies, which in part is linked to the light-polarization characteristics of ( <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:mi>Al</j:mi> </j:math> , <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mi>Ga</l:mi> </l:math> ) <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mrow> <n:mrow> <n:mi mathvariant="normal">N</n:mi> </n:mrow> </n:mrow> </n:math> quantum wells (QWs). In this study, we provide insight into the degree of optical polarization of ( <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:mi>Al</q:mi> </q:math> , <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mi>Ga</s:mi> </s:math> ) <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:mrow> <u:mrow> <u:mi mathvariant="normal">N</u:mi> </u:mrow> </u:mrow> </u:math> QW systems operating across the UV-C spectral range by means of an atomistic multiband electronic structure model. Our model not only captures the difference in valence-band ordering in <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"> <x:mrow> <x:mi>Al</x:mi> <x:mi mathvariant="normal">N</x:mi> </x:mrow> </x:math> and <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"> <ab:mrow> <ab:mi>Ga</ab:mi> <ab:mi mathvariant="normal">N</ab:mi> </ab:mrow> </ab:math> but it also accounts for alloy-disorder-induced band-mixing effects originating from random alloy fluctuations in ( <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"> <db:mi>Al</db:mi> </db:math> , <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"> <fb:mi>Ga</fb:mi> </fb:math> ) <hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline"> <hb:mrow> <hb:mrow> <hb:mi mathvariant="normal">N</hb:mi> </hb:mrow> </hb:mrow> </hb:math> QWs. The latter aspect is often not captured in widely employed continuum-based models. The impact of alloy disorder on the electronic structure is studied in terms of Urbach-tail energies, which reflect the broadening of the valence-band density of states due to carrier-localization effects. We find that especially in wider wells, Urbach-tail energies are reduced with increasing carrier densities in the well, highlighting that alloy-disorder-induced carrier-localization effects in ( <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"> <kb:mi>Al</kb:mi> </kb:math> , <mb:math xmlns:mb="http://www.w3.org/1998/Math/MathML" display="inline"> <mb:mi>Ga</mb:mi> </mb:math> ) <ob:math xmlns:ob="http://www.w3.org/1998/Math/MathML" display="inline"> <ob:mrow> <ob:mrow> <ob:mi mathvariant="normal">N</ob:mi> </ob:mrow> </ob:mrow> </ob:math> QWs are also tightly linked to electrostatic built-in fields. Our calculations show that for QWs designed to emit at the longer-wavelength end of the UV-C spectrum, carrier density and well width are of secondary importance for their light-emission properties, meaning that one observes mainly transverse electrical polarization. However, for ( <rb:math xmlns:rb="http://www.w3.org/1998/Math/MathML" display="inline"> <rb:mi>Al</rb:mi> </rb:math> , <tb:math xmlns:tb="http://www.w3.org/1998/Math/MathML" display="inline"> <tb:mi>Ga</tb:mi> </tb:math> ) <vb:math xmlns:vb="http://www.w3.org/1998/Math/MathML" display="inline"> <vb:mrow> <vb:mrow> <vb:mi mathvariant="normal">N</vb:mi> </vb:mrow> </vb:mrow> </vb:math> QWs with high <yb:math xmlns:yb="http://www.w3.org/1998/Math/MathML" display="inline"> <yb:mi>Al</yb:mi> </yb:math> contents, we find that both well width and carrier density will impact the degree of optical polarization. Our calculations suggest that wider wells will increase the degree of optical polarization and may therefore be an option to improve the light-extraction efficiency in deep-UV light emitters.