Method for enhancing the favored transverse-electric-polarized emission of an AlGaN deep-ultraviolet quantum well

Abstract

An AlGaN quantum well (QW) structure of a deep-ultraviolet (UV) light-emitting diode (LED) needs to be well designed for controlling its band structure such that the heavy-hole (HH) band edge becomes lower than the split-off (SO) band edge and hence the transverse-electric (TE) polarization dominates the emission for achieving a higher light extraction efficiency. Here, we report the discovery of un-intentionally formed high-Al AlGaN nano-layers right above and below such a QW and their effects on the QW for changing the relative energy levels of the HH and SO bands. The comparison between the results of simulation study and polarization-resolved photoluminescence measurement confirms that the high-Al layers (HALs) represent the key to the observation of the dominating TE-polarized emission. By applying a stress onto a sample along its c-axis to produce a tensile strain in the c-plane for counteracting the HAL effects in changing the band structure, we can further understand the effectiveness of the HALs. The formation of the HALs is attributed to the hydrogen back-etching of Ga atoms during the temperature transition from quantum barrier growth into QW growth and vice versa. The Al filling in the etched vacancies results in the formation of an HAL. This discovery brings us with a simple method for enhancing the favored TE-polarized emission in an AlGaN deep-UV QW LED.

Enhanced TE-polarized emission of AlGaN-based deep-ultraviolet light emitting diodes by using an InAlN insertion layer

“…the transition from the conduction band and the split-off band. 4) The peak intensity of TE polarization spontaneous emission spectrum for the In x Al 1-x N insertion layer structure with x = 0.15 is increased by 93.24% as compared to that of the conventional structure. Furthermore, the TE-polarized R sp as a function of current density is also shown in Fig.…”

“…3) A main limiting factor is the poor LEE due to the absorption of p-type contact layer 2) and the strong TM-polarized emission. 4) For overcoming the absorption of p-type contact layer, the transparent p-AlGaN contact layer or thin-film flip-chip LED can be employed. 5,6) With increasing the Al-content, the TE-dominated transition can be switched as the TM-dominated transition for AlGaN-based LEDs.…”

“…In the literature, several methods have been suggested to overcome this problem. One is to adjust the valence band arrangement through the design of novel quantum well (QW) structure, such as the use of ultra-thin GaN layers [7][8][9] or high-Al AlGaN nano-layers 4) in the multi-QW (MQW) layers. The second is to control the optical polarization by the strain management of QW layers.…”

Improvement of TE-polarized emission in type-II InAlN–AlGaN/AlGaN quantum well*

“…[2] For the AlGaNbased LEDs, it is well known that the transverse-magnetic (TM) polarized emission will become dominant with increasing the Al-content due to the different valence band arrangement between AlN and GaN, which will lead to the lower surface light extraction efficiency for the c-axis grown LED structure. [5,6] Thus, the improvement of TE polarized emission by QW structure designs is important to enhance the light extraction efficiency.…”

AlGaN nano-shell structure on a GaN nanorod formed with the pulsed MOCVD growth

“…However, so far, the emission efficiencies of UV LEDs based on AlGaN quantum well (QW) growth on the c-plane are generally low, particularly those in the deep-UV range (<300 nm in wavelength). The low efficiencies are caused by a few factors, including low crystal quality, low light extraction of the transverse-magnetic (TM) emission component [8], and low p-type AlGaN conductivity. The dominating TM-polarized emission in a c-plane LED when Al content is high originates from the electron transition between the conduction and split-off valence bands, which emits light with c-axis-oriented polarization.…”