Polarization Properties of Deep-Ultraviolet Optical Gain in Al-Rich AlGaN Structures

Abstract

The polarization properties of the amplified spontaneous emission (ASE) in the deep-UV wavelengths from AlGaN/AlN structures with 70% Al content have been investigated. Samples are prepared through a novel liquid phase epitaxy mode leading to strong compositional fluctuations. Large net modal gain is quantified to 230 nm after femtosecond UV optical pumping by the variable-stripe length technique. A strong transverse electric (TE)-polarization mode of the amplified emission has been demonstrated, together with an unpolarized emission giving rise to absorption only. These measurements provide an important observation in order to understand the origin of the optical gain in AlGaN alloys with high Al content. # 2012 The Japan Society of Applied Physics I n recent years, the demand for compact deep-UV sources has increased due to potential applications in free-space communications, biochemical agent detection, disinfection, and medical diagnostics. Group-III nitrides, particularly AlGaN materials, are especially suited to cover the UV spectral range by varying the Al content. 1) Sub-300 nm AlGaN-based light emitting devices have been demonstrated with an internal quantum efficiency of 50% at 250 nm 2) and a power efficiency of 40% at 240 nm. 3) Currently, an intense investigation is targeting the deep-UV lasing from these materials. Recent theoretical and experimental works on AlGaN multi-quantum wells (MQW) suggest the possibility of achieving lasing at wavelengths as short as 220 nm. 9) One of the issues still under debate in the literature is the origin of the gain and its polarization properties. The transition between the conduction band and the top valence band is mainly transverse electric (TE) polarized for GaN, and transverse magnetic (TM) polarized for the c-planeoriented AlN heterostructure. 10) A turnover point is expected in the AlGaN material, depending on the Al content, material strain, and thickness of the well. Zhang et al. 11) calculated the band structure and material gain of 3 nm AlGaN QWs. Above 60% Al, they expect a large TM material gain, while, the TE spontaneous emission rate becomes negligible with increasing Al content. Yamaguchi found an abrupt polarization switch at an Al composition of 76% if the thickness of the well was 1.5 nm. 12) Moreover, considering a (0001)-oriented c-plane AlGaN/AlN QW structure (well thickness is 2.5 nm), Park 13) found a turnover point occurring above 80% Al content. Some experimental evidences confirm that for a wavelength as long as 267 nm, the laser emission at low temperature is still TE polarized. 8) Note that all the calculations consider the well as the active region. Compositional fluctuation introduces a different order, and the role of the fluctuations in the optical emission has received much less attention. 9) In this letter, we report on the polarization properties of the amplified spontaneous emission (ASE) of the AlGaN/ AlN MQW structure with strong compositional fluctuations under femtosecond optical pumping. We demonstrate that ASE is TE polarized, while the TM emission gives rise to absorption in the material. The investigated samples consist of a 0.5-m-thick AlN cladding layer, followed by 10 periods of Al 0:7 Ga 0:3 N (1.5 nm)/AlN (40 nm) MQWs and a 100 nm AlN cladding layer. A schematic picture of the sample is shown i