Unique optical properties of AlGaN alloys and related ultraviolet emitters

Abstract

Deep UV photoluminescence spectroscopy has been employed to study the optical properties of AlxGa1−xN alloys (0⩽x⩽1). The emission intensity with polarization of E⊥c and the degree of polarization were found to decrease with increasing x. This is a consequence of the fact that the dominant band edge emission in GaN (AlN) is with polarization of E⊥c(E∥c). Our experimental results suggest that the decreased emission efficiency in AlxGa1−xN alloys and related UV emitters could also be related with their unique polarization property, i.e., the intensity of light emission with polarization of E⊥c decreases with x. It is thus concluded that UV emitters with AlGaN alloys as active layers have very different properties from InGaN and other semiconductor emitters.

Photoluminescence properties of AlN homoepilayers with different orientations

“…Due to the unique band structure of AlN near the ⌫ point, the PL intensity in a-plane AlN is enhanced by the fact that AlN exhibits a maximum emission in the directions perpendicular to the c direction ͑or in the E / /c measurement geometry͒. 19 In our measurement, the c direction of a-plane AlN lies in the substrate plane so that the maximum emitted light propagation direction is normal to the surface. Thus, optoelectronic devices with a-plane homoepilayers in their active region can exploit optimum quantum efficiency.…”

Demonstration of transverse-magnetic deep-ultraviolet stimulated emission from AlGaN multiple-quantum-well lasers grown on a sapphire substrate

“…As indicated by the dashed line in Figure 5, the rapid variation between TE-and TMdominance with respect to the change in lasing wavelength from 243 to 249 nm is distinct from the previous studies, wherein the spontaneous emission from AlGaN structures made a similar extent of polarization switch at a considerably longer wavelength span. 9,20,21 This can be attributed to the dramatic change in the ratio of TE-to-TM gain coefficients for the DUV AlGaN MQW lasers in the vicinity of TE-TM switch. 17 In summary, TM-dominant DUV stimulated emission from photo-pumped AlGaN MQW lasers grown by MOCVD on sapphire substrates were demonstrated at RT.…”

“…With an increased Al composition and thus a shorter emission wavelength, the split-off hole (CH) band moves closer to the conduction band relative to the HH band, which triggers the switch from TE-to transversemagnetic (TM)-polarized (E TM jj c-axis) emission when the CH band crosses over the HH band and thus becomes the topmost band. 9,10 The optical polarization can have a considerable impact on the performance of DUV LEDs and LDs. For top-or bottom-emitting LEDs grown on the c-plane substrates, TM-polarization can be detrimental for light extraction efficiency because the TM-polarized light is unlikely to emit from the surface that is parallel to the c-plane because of E TM jj c-axis.…”

Overcoming the fundamental light-extraction efficiency limitations of deep ultraviolet light-emitting diodes by utilizing transverse-magnetic-dominant emission

“…For bulk Al x Ga 12x N, the sign of D CR changes from positive to negative when x becomes greater than 0.25. 13 For the AlGaN MQW DUV LEDs, this transition occurs at an Al content higher than 0.25 due to the quantum confinement effect and the strain in the QWs. For AlGaN DUV LEDs grown on sapphire, the transition generally occurs at the wavelength of approximately 295 nm corresponding to an Al content of approximately 35%.…”

“…1 The dominant reason for such a low EQE is the poor light-extraction efficiency (LEE) caused by the strongly anisotropic light emission that is mostly in a direction within the c-plane of Al x Ga 12x N. 1,8,9 The strongly anisotropic emission, called sidewall emission in this study, is due to the unique valence band (VB) structure of AlGaN at high Al contents (x.,0.25). [11][12][13] The topmost VB of high-Al-content AlGaN is the crystal field split-off hole (CH) band, which has a P Z -orbital-like property. Therefore, AlGaN dominantly emits transverse magnetic (TM)-polarized light propagating perpendicular to the c-axis with the light's electric field parallel to the c-axis (E//c).…”