Emission and material gain spectra of polar compressive strained AlGaN quantum wells grown on virtual AlGaN substrates: Tuning emission wavelength and mixing TE and TM mode of light polarization

Abstract

It is shown that compressively strained polar Al x Ga 1−x N/Al y Ga 1−y N quantum wells (QWs) of various contents grown on virtual Al Y Ga 1−Y N substrates (Y=20, 40, 60, 80, and 100%) are able to cover the whole UV-A, -B, and -C spectral range but their contents and widths have to be carefully optimized if they are to be used as the active region of light emitting diodes and laser diodes. The emission wavelength from AlGaN multi QWs can be tuned by both the QW width and barrier thickness, but the range of QW width for which an efficient luminescence is expected is very small (2-4 nm) due to a very weak electron-hole overlap for wider QWs. The most effective method for wavelength tuning in this QW system is content engineering, i.e., lowering Al concentration in the QW region. The decrease of Al concentration in the QW shifts the emission peak to red, broadens this peak, weakens its intensity, and changes its polarization from transverse magnetic (TM) to TM mixed with transverse electric (TE). For laser diodes the optimal QW design is more rigorous concerning the QW width since this width should be below 3 nm. Moreover it is shown that the TE and TM mode of materials gain overlap and are strongly blueshifted in comparison to emission spectrum.

Material Gain Engineering in Staggered Polar AlGaN/AlN Quantum Wells Dedicated for Deep UV Lasers

“…The electronic band structure of AlGaN QWs is calculated using 8-band kp Hamiltonian which is given in details in Ref. [37]. Because of the large band gap of AlGaN the interaction between the CB and the VB is neglected.…”

“…The valence band offset (VBO) between the Al x Ga 1-x N and AlN is defined for unstrained materials, see Ref. [37]. A linear approximation is used to obtain the valence band position in Al-GaN layers of different Al concentrations.…”

“…The influence of the compressive strain on the electronic band structure is calculated according to the Bir-Pikus model [48], details can be found in Ref. [37].…”

“…The switching between the TE and the TM polarized fundamental transition is present for lower Al concentrations if AlGaN QWs are grown on virtual AlGaN substrates, see Ref. [37]. Therefore AlN substrate is the most promising choice from the viewpoint of achieving the TE polarization for light emitters in the UV-C spectral region.…”

“…Theoretical studies of the material gain are very important for the development of semiconductor lasers and therefore they are often performed [25], [34]- [36]. Such studies were also performed for AlGaN QWs dedicated for UV emitters [37]- [40] but the issue of staggered AlGaN QWs is still open.…”