Electrochemical etching of AlGaN for the realization of thin-film devices

Abstract

Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 μm×1 μm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.

A 310 nm Optically Pumped AlGaN Vertical-Cavity Surface-Emitting Laser

“… 29 − 31 An alternative way to remove the substrate is to use Al-content- and doping-selective electrochemical etching of AlGaN, which has recently been demonstrated by our groups. 32 , 33 This method allows for the fabrication of AlGaN membranes with high Al content, smooth surfaces, and well-defined thicknesses. Owing to these properties, the membranes can be used for a wide variety of thin-film devices.…”

“…On a larger scale, the electrochemically etched surface has an inverted topography of that of the as-grown surface, similar to what we have reported previously. 32 The smooth etching was achieved by using a multilayered sacrificial layer, which promotes etching at the sacrificial layer and cavity interface; see Figure 1 d. This design enables incorporation of layers with lower Al content without degrading the crystal quality of the device layers, and the built-in polarization field yields sheets of high carrier concentrations, both of which enhance the etch selectivity 32 and result in smoother surfaces.…”

Thin-film flip-chip UVB LEDs realized by electrochemical etching

“…Recently, we have demonstrated that electrochemical etching can be applied to etch AlGaN with an Al content up to 50%. 25 This opens up the possibility to incorporate a lattice matched sacrificial AlGaN layer that can be selectively removed to realize TFFC UV LEDs of high quality. In this work, we demonstrate a TFFC ultraviolet B (UVB) LED using this method.…”

“…An etching voltage of 25 V vs the Ag/AgCl reference electrode was chosen to achieve complete removal of the n-doped Al 0:37 Ga 0:63 N sacrificial layer, as shown in Ref. 25, and still to prevent etching of the etch stop and doped device layers. The lower Al composition and higher doping of the sacrificial layer relative to the etch block layers on both the sides confine the etching to only the sacrificial layer.…”

“…In addition, time-resolved photoluminescence (PL) measurements yielded similar PL decay times of 340630 ps at room temperature for the quantum wells in the as-grown LEDs and in the TFFC LEDs, indicating no degradation of the active region. 25 Figure 5 shows the electroluminescence spectrum at room temperature of the TFFC LED at a current density of 0:8 A=cm 2 with a single emission peak at 311 nm. This corresponds well to the electroluminescence peak wavelength of 312 nm of the as-grown LED structure that was contacted using an In-dot, for which the emission was collected through the sapphire substrate.…”