AlN, AlxGa1-xN, and GaN

AlN, AlxGa1-xN, and GaN - Wet Etching

Material Name: AlN, AlxGa1-xN, and GaN
Recipe No.: 10344
Primary Chemical Element in Material: Al, Ga
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Wet etching
Etchant (Electrolyte) Composition: AlN and GaN bulk substrates were obtained from HexaTech, Inc. and Kyma, Inc., respectively. GaN bulk crystals were prepared by hydride vapor phase epitaxy (HVPE) on sapphire substrates followed by substrate removal. AlN single crystal wafers were obtained from free standing AlN single crystalline boules grown by physical vapor transport (PVT). Detailed information on the substrate properties and wafer preparation can be found elsewhere. Average dislocation densities of the AlN and GaN wafers were estimated to be less than 10 exp(3) cm(2) and 10 exp(6) cm(2), respectively, based on X-ray topography.

Selective etching of the III-polar and N-polar surface was obtained using KOH aqueous solution. Etching temperature was maintained at 70 C, and the concentration of KOH was approximately 0.18 mol/L. AlGaN bulk crystals are currently not available. Consequently, investigation of the polarity etching selectivity of AlN and AlxGa(1-x)N (0 <= x <= 1) was performed using epitaxial films with thickness around 800 nm grown by metalorganic chemical vapor deposition (MOCVD) on 2-in. sapphire substrates. Using X-ray diffraction (XRD), the dislocation density of these films was estimated to be around 3 x 10 exp(10) cm(2). The investigated films were fabricated as lateral-polarity-structures (LPS) with alternating 10 lm wide III-polar and N-polar stripes. An inertness of III-polar material was established in agreement with the previous work on GaN and AlN. Consequently, LPS are ideal structures for KOH etching experiments, since the change of the height difference between the III-polar and N-polar stripe is a simple but reliable measure of the etching rate.

The surface morphology of the etched AlN and GaN bulk crystals was characterized by an Olympus BH-2 differential interference contrast (DIC) optical microscope. Scanning electron microscope (SEM) images were recorded using a JEOL 6400F field emission SEM operating at 5 kV. TEM cross-section samples were prepared by mechanical wedge polishing followed by Arþ ion milling (Fischione Model 1010). TEM imaging was performed with a JEOL 2000FX operated at 200 kV.

Initially, the surface morphology and etch rates of bulk AlN and GaN crystals were established to allow for comparison to the AlGaN layers. N-polar surfaces of AlN and GaN etched under 0.18 mol/L KOH at 70 .C for 60 min are shown in Figures 1(a) and 1(b), respectively. Both surfaces were covered with oriented hexagonal etching hillocks with sizes ranging from 1 µm to 10 µm. In contrast to the N-polar surface, III-polar GaN and AlN surfaces were nearly inert to KOH wet etching. Ga-polar and Al-polar surfaces remained atomically smooth after wet etching under the above condition and no etching pits or hillocks were observed. The etch rate of both polarities was determined and it was found that the N-polar surface had an etch rate around 500 nm/min for AlN and 40 nm/min for GaN, while the etch rates for IIIpolar AlN and GaN are extremely low. These values yielded an N-polar AlN to N-polar GaN selectivity higher than 12 and demonstrated the potential of KOH etching for substrate thinning application or substrate removal from DUV LEDs grown on AlN single crystalline substrates.

To further investigate the etching selectivity between AlN and AlGaN, similar wet etching was performed on AlGaN LPS grown on sapphire substrates. An optical microscope image of an as-grown Al0.6Ga0.4N LPS (a) and of the structure after 5 min of KOH etching (b) is shown in Figure 2. The 10 lm wide stripes with alternating III-polar and N-polar orientation can clearly be distinguished. After the KOH etching procedure, the III-polar material remained unchanged. In contrast, the N-polar stripes were significantly darker, which indicated a significant change of the surface and thickness of the material.

In order to determine the thickness and etch rate, crosssectional SEM images of AlxGa(1-x)N LPS before and after etching were recorded. It was found that the etch rate of the III-polar AlGaN was below 2 nm/min while the etch rate of the N-polar surface was significantly higher. This was consistent with the earlier findings on GaN and AlN. Figure 3 shows SEM images of LPS with two different Al compositions. The red dashed line represents the inversion domain boundaries. In the as-grown AlN LPS, the N-polar and III-polar stripes equal in thickness even though the surface morphology differed (Figure 3(a)). After the sample was etched in 0.09 mol/L KOH aqueous solution for 6 min, the N-polar region (~800 nm thick) was completely removed, exposing the sapphire substrate, see Figure 3(b). For the sample with 60% Al content, as revealed by Figures 3(c) and 3(d), the height difference between the N-polar and III-polar Al0.6Ga0.4N changed from 140 nm to 450 nm, which indicated a decrease in the etch rate with respect to AlN.

The selectivity between the N-polar AlN over N-polar AlxGa(1-x)N as a function of Al composition is shown in Figure 4. An etching selectivity to AlN greater than 1 was observed over the whole compositional range of AlGaN. It was further observed that the selectivity increased as Al composition decreased. The maximum etching selectivity of around 12 was achieved between AlN and GaN LPS. The etch selectivity obtained under similar conditions for low dislocation density bulk AlN and GaN crystals was approximately 13. This conforms to the data obtained for the LPS, although the dislocation density in these structures was several orders of magnitudes higher than that of the bulk AlN and GaN materials. This indicated that under these mild etching conditions, the dislocations did not play a role in the etching behavior, and this etching scheme was demonstrated to be a very universal substrate removal method for materials grown by different methods with different crystalline quality. The formation and development of etching hillocks could only be attributed to different bonding conditions of crystallographic planes, which was an intrinsic property of IIInitride crystal structure.

In order to demonstrate the relationship between etching hillocks and underlying threading dislocations, thus further generalize the etching behaviour for various materials’ system, cross-sectional TEM was performed on N-polar AlN and GaN after KOH etching at 70 C for 30 min, respectively. As shown in Figure 5, no dislocations were observed beneath the hillocks for either of the samples (GaN and AlN) and either of the diffraction conditions (screw and edge dislocation). Thus, the etch hillocks could not be associated with dislocations.
Procedure (Condition): No data
Note: A controllable and smooth potassium hydroxide-based wet etching technique was developed for the AlGaN system. High selectivity between AlN and AlxGa(1-x)N (up to 12x) was found to be critical in achieving effective substrate thinning or removal for AlGaN-based deep ultraviolet light emitting diodes, thus increasing light extraction efficiency. The mechanism of high selectivity of AlGaN as a function of Al composition can be explained as related to the formation and dissolution of oxide/hydroxide on top of N-polar surface. Cross-sectional transmission electron microscopic analysis served as ultimate proof that these hillocks were not related to underlying threading dislocations.
Reference: W. Guo, R. Kirste, I. Bryan, Z. Bryan, L. Hussey, P. Reddy, J. Tweedie, R. Collazo, and Z. Sitar, KOH based selective wet chemical etching of AlN, AlxGa1-xN, and GaN crystals: A way towards substrate removal in deep ultraviolet-light emitting diode, Applied Physics Letters 106, 082110 (2015); doi: 10.1063/1.4913705.

Figure 1: 1. SEM micrographs of N-polar AlN (a) and GaN (b) etched in 0.18 mol/L KOH solutions at 70 C for 60 min.

Figure 2: Optical microscope image of an Al0.6Ga0.4N LPS with 10 µm stripes before (a) and after KOH etching (b).

Figure 4: Etch selectivity (AlN/AlxGa1.xN) as a function of Al composition under KOH (0.09 mol/L, 50 C) etching condition.

Figure 3: Cross-sectional SEM images of AlN LPS before (a) and after (b) etching under 0.09 mol/L KOH solution at 50 C for 6 min; Al0.6Ga0.4N LPS before (c) and after (d) etching under 0.09 mol/L KOH solution at 50 C for 6 min.

Figure 5: Cross-sectional TEM images of AlN ((a) and (b)) and GaN ((c) and (d)) for two diffraction conditions after KOH etching for 30 min. No correlation between the hillocks and dislocations is observed.