InGaN - Wet Etching

InGaN - Wet Etching

Material Name: InGaN
Recipe No.: 10350
Primary Chemical Element in Material: In
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Wet etching
Etchant (Electrolyte) Composition: InGaN-based LED structures were grown using a metallorganic chemical vapor deposition system on a C-face (0001) 2 in. diameter sapphire substrate. The LED structures consisted of a 3 µm thick n-type GaN layer, ten pairs of InGaN/GaN MQW active layers, and a 0.2 µm thick p-type GaN:Mg layer with a micropits-roughened surface (as-grown surface) shown in Fig. 1b. The active layers consisted of a 30 Å thick InGaN well layer and a 70 Å thick GaN barrier layer in a InGaN/GaN MQW structure. We cleaved a 2 in. LED wafer into two half-wafers to use for the mesa definition process in the dry-etching and wet-etching processes. In a standard LED (ST-LED), the p- and n-GaN regions are defined by the photoresist and etched by an ICP etcher as a 1.2 µm depth mesa region. The mesa region of the LEDs was 260 x 260 x µm2 in size. In the WME-LED devices, an 800 W Hg lamp was used as the front-side illumination source during the PEC wet mesa etching process. In the PEC wet-etching process, the Ti metal layer was deposited and patterned by a photolithographic process on a p-type GaN surface as a mesa region with a size of 260 x 260 µm2. Indium metal was deposited on the LED wafer’s edge as the anode electrode. A dc bias was applied to the p-type GaN:Mg surface as a positive 10 V, and the total exposure time was 5 h under Hg lamp illumination in unstirred DI water. The sequential PEC oxidation and oxide-removing process consisted of a 30 min oxidation process in DI water, followed by a 1 min oxide-removing process in a diluted HCl solution, with these dual processes repeated ten times. In this sequence, the GaN layer was oxidized as a GaOx layer in DI water. After the oxidation process, the oxidized samples were immersed in a diluted HCl solution to dissolve the GaOx layer. These sequential processes on the GaN-based epitaxial layers are defined as the PEC wetetching process. An illustrated figure of the experimental procedures is shown in Fig. 1a to describe the PEC wet mesa etching process. After the PEC wet-etching process, the top p-type GaN:Mg mesa region is still measured at 260 x 260 µm2 in size. The mesa structure of WME-LED consisted of a 0.2 µm thick p-type GaN:Mg layer, a 0.1 µm thick InGaN/GaN MQW layer, and a 0.9 .m thick n-type GaN:Si layer. The etching depth of the ST-LED and WMELED was defined as the same value of 1.2 .m depth through the ICP dry etching and PEC etching processes when comparing device performance. The Ni/Au ~3/5 nm. layer was deposited on the mesa region as a transparence contact layer (TCL) without a 10 µm width region around the mesa edge, then the Ti/Al and Ni/Au metal layers were deposited on n- and p-type regions as metal bonding pads. The ST-LED and WME-LED devices were located at the 2 in. LED wafer center near the cleaved line in order to analyze their optical and electrical properties. We measured ten LED dies of ST-LED and WME-LED samples located at the 2 in. LED wafer center near the cleaved line in order to analyze their optical and electrical properties. In addition, the photoluminescence (PL) emission wavelength of the LED epitaxial wafer was almost the same as that measured by the PL mapping system. The etched mesa sidewall surfaces of both LEDs were not passivated in this experiment. The micrographs of both LEDs were observed using scanning electron microscopy (SEM). The optical and electrical properties of both LED samples were measured using the optical spectrum analyzer, Ando 6315A, and the precision semiconductor parameter analyzer, HP 4156C.
Procedure (Condition): No data
Note: A photoelectrochemical wet mesa etching (WME) process was used to fabricate InGaN-based light emitting diodes (LEDs) as a substitute for the conventional plasma mesa dry etching process. The p-type GaN:Mg layer, InGaN active layer, and n-type GaN:Si layer were etched through a sequential photoelectrochemical oxidation and oxide-removing process to define the mesa region. The higher lateral wet-etching rate ~3.4 µm/h. of the InGaN active layer was observed to form a wider undercut structure which has 42.7% light output power enhancement compared to a conventional LED fabricated with the plasma dry etching process. The reverse current of a WME-LED was suppressed by avoiding plasma damage during the dry mesa etching process.
Reference: Chung-Chieh Yang, et al., Wet Mesa Etching Process in InGaN-based Light Emitting Diodes, Electrochemical and Solid-State Letters, 11 (7) H169-H172, 2008.

Figure 1: (Color online) (a) An illustrated figure of the experimental procedures has been added to describe the PEC wet-etching process. The fabricated procedures are listed as the following steps: (1) The PEC wet etching process occurs on the p-type GaN:Mg without any exposed n-type GaN layers in the DI water, (2) the higher lateral wet-etching rate occurs on the InGaN/GaN active layer without a p-type GaN:Mg layer, and (3) a wetetching process on the n-type GaN:Si is used to define the n-type GaN region for metal contact. (b) The schematic of the WME-LED structure is shown with an undercut structure under the top p-type GaN:Mg layer.

Figure 2: The SEM micrographs of the PEC-treated WME-LED are described as follows. (a) The mesa edge region consists of a p-type GaN:Mg layer, an undercut structure, and a microroughened n-type GaN:Si surface. (b) The top 0.2 µm thick p-type GaN:Mg layer has a high-density V-shaped micropits surface. (c) The wider undercut structure of the InGaN/GaN active layer can be observed. (d) The microroughened n-type GaN:Si surface on the mesa sidewall can be observed when the p-type GaN layer has split from the mesa structure during the sample cutting.