Selective wet etching of lattice-matched AlInN–GaN heterostructures

Selective wet etching of lattice-matched AlInN–GaN heterostructures

ARTICLE IN PRESS Journal of Crystal Growth 300 (2007) 254–258 www.elsevier.com/locate/jcrysgro Selective wet etching of lattice-matched AlInN–GaN he...

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ARTICLE IN PRESS

Journal of Crystal Growth 300 (2007) 254–258 www.elsevier.com/locate/jcrysgro

Selective wet etching of lattice-matched AlInN–GaN heterostructures F. Rizzia,b, K. Bejtkaa,b, P.R. Edwardsb, R.W. Martinb, I.M. Watsona, a

Institute of Photonics, SUPA, University of Strathclyde, Glasgow G4 0NW, UK Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, UK

b

Available online 20 December 2006

Abstract Wet etching of AlInN–GaN epitaxial heterostructures, containing AlInN layers with InN mole fractions close to 0.17 has been studied. One molar aqueous solution of the chelating amine 1,2-diaminoethane (DAE) proved to selectively etch the AlInN layers, without the need for heating above room temperature, or photo-assistance. In experiments with a (0 0 0 1)-oriented AlInN-on-GaN bilayer, the mode of removal of the AlInN layer was predominantly lateral etching, initiated from the sidewalls of pit defects in the AlInN layer. The lateral etch rate was estimated at 60 nm/h. The GaN buffer layer surface was roughened concurrently with etching of the AlInN, although the DAE solution has no effect on as-grown GaN (0 0 0 1) surfaces. The roughening of the GaN surface is tentatively attributed to the charge accumulation layer expected at the AlInN–GaN heterointerface. The DAE etchant also proved effective at removing buried AlInN layers from trilayer and more complex multilayer structures, leading to the prospect of epitaxial lift-off processes, and the fabrication of threedimensional engineered microstructures. These capabilities were demonstrated by the production of suspended microdisk structures from a GaN–AlInN–GaN trilayer, using a combination of dry and wet etching. r 2006 Elsevier B.V. All rights reserved. PACS: 68.55.a; 81.05.Ea; 81.65.Cf; 82.45.Jn Keywords: A1. Etching; A3. Metalorganic vapor phase epitaxy; B1. Nitrides; B2. Semiconducting III–V materials

1. Introduction In many III–V semiconductor alloy systems, highly selective wet etch process have been developed, which, for example, allow the production of complex three-dimensional microstructures by removal of a sacrificial layer in an epitaxial multilayer [1]. In contrast, simple wet etches operating at moderate temperatures are uncommon in the case of III-nitride semiconductors, and high-density plasma etch techniques are dominant in device processing. The most successful wet etch technique applied to III-nitrides is photoelectrochemical (PEC) etching, in which photogenerated holes play a critical role, and experimental configurations of moderate complexity are needed. Such PEC processes have been applied to bandgap-selective etching of InGaN–GaN [2–4] and AlGaN–GaN structures

Corresponding author. Tel.: +44 141 548 4120; fax: +44 141 552 1575.

E-mail address: [email protected] (I.M. Watson). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.017

[5], and examples of three-dimensional microstructures fabricated in this way include microdisk lasers [4]. Interest has recently increased in various types of structures containing GaN and the ternary alloy AlInN, which can be lattice matched to GaN on the wurtzite-phase basal plane at an InN mole fraction of 17% [6–9]. The AlInN alloy system offers new opportunities for the application of selective etch processes to III-nitrides, and we have previously demonstrated an 5:1 vertical etch rate selectivity between GaN and AlInN in reactive ion etching [8]. The direct bandgap of Al0.83In0.17N is estimated as 4.3 eV, 0.9 eV higher than the bandgap of GaN [6,10]. Therefore, development of bandgap-selective PEC etch processes for AlInN–GaN heterostructures, in which GaN is preferentially dissolved, may be feasible. In contrast, this paper describes room-temperature wet etching of AlInN–GaN structures in the absence of above-bandgap illumination. A chelating amine etchant was used to selectively remove AlInN layers close to the lattice-matched composition from bilayer and multilayer heterostructures. Our

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The samples studied are wurtzite-phase epitaxial multilayers grown by metalorganic chemical vapour deposition (MOCVD). Detailed growth conditions for the AlInN layers were reported previously [10]. The growth regime employed produces a Ga-polar (0 0 0 1) surface during standard growth of GaN on sapphire (0 0 0 1), and it is assumed throughout that AlInN layers maintain a corresponding polarity. The AlInN-on-GaN bilayer studied was grown on a sapphire substrate, and is the same sample as that with the nominal AlInN growth temperature of 820 1C described in Ref. [10]. It has an AlInN thickness of 130 nm, and an InN mole fraction of 0.153 (70.005) from Rutherford backscattering (RBS) measurements. X-ray diffraction confirmed that the AlInN grew pseudomorphically with the relaxed GaN buffer layer, and high crystal quality was confirmed by a minimum RBS channelling yield for indium of 4%. Lateral etch trials were made with a multilayer structure on a free-standing GaN (0 0 0 1) substrate, containing two 70-nm AlInN layers grown under the same conditions as the bilayer, and which is described in more detail in Ref. [8]. Further, lateral etch experiments were performed with a GaN–AlInN–GaN trilayer on sapphire, which was previously used to demonstrate high selectivity between vertical etch rates in plasma etching in Ref. [8], and a preliminary demonstration of selective wet etching in Ref. [9]. The AlInN layer thickness in this structure is 300 nm, and it was grown at a faster rate than the AlInN in the bilayer. Consequences of this change in growth rate for the surface morphology of the AlInN layer will be discussed later. Wet etch experiments used chemicals of at least reagentgrade purity, made into solutions in deionized water. Concentrations are quoted in moles per liter. Most experiments were conducted at room temperature (20 1C) under fluorescent lighting, without any agitation of the solutions. Sample inspection after etching used a Filmetrics F20 spectroscopic reflectometer, which interrogated areas of a few square millimeters, and an FEI Sirion fieldemission scanning electron microscope (SEM). Inductively coupled plasma (ICP) etching of mesas from the trilayer sample was performed in a Surface Technology Systems Multiplex tool, using a standard Cl2-Ar recipe optimized for feature etching in Ga-polar GaN layers. 2.1. Etching results with the bilayer sample Many wet etch experiments performed with the AlInN–GaN bilayer in various solutions involved either no detectable etching, or complete and selective removal of the AlInN layer. Reflectance spectra provided straightfor-

a Reflectance (a.u.)

2. Sample description and experimental details

ward confirmation of these extremes of behavior. AlInN lattice-matched to GaN on the (0 0 0 1) plane has a refractive index contrast of 0.2 with GaN over the visible spectrum. Consequently, reflectance spectra of the asgrown bilayer show a long-period modulation of the fringe envelope, as illustrated in Fig. 1(a). Complete removal of the AlInN layer from the underlying GaN buffer layer removed the bilayer modulation from the reflectance spectrum, as illustrated in Fig. 1(b). Surface morphology changes discussed below also affected the reflectance amplitudes. The fitting routine available on the reflectometer could not determine individual AlInN and GaN layer thickness reliably, although acceptable values for the total GaN plus AlInN thickness were obtained from fits to single-layer models. Previous work, originally reported in 1996, investigated different wet etchants for AlInN layers with InN mole covering the entire composition range, which were grown by metalorganic molecular beam epitaxy directly onto GaAs or Si substrates [11]. In our study, we confirmed that various aqueous acid solutions had no detectable effect on the bilayer sample over periods of up to 48 h. Solutions tested included the mineral acids 11 M HNO3, 10 M HCl, 20 M HF, and two organic acids with complex-forming

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results indicate that the etching of AlInN is highly anisotropic, and is also polarity dependent, consistent with the known behavior of other III-nitride compounds.

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Wavelength (nm) Fig. 1. Reflectance spectra of the as-grown AlInN–GaN bilayer, (a), and of the same sample etched for 10 h in 1 M DAE solution, (b).

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properties: 1 M citric and 1 M tartaric acid. All these findings are consistent with the older study [11]. 1 M aqueous KOH solution fully removed the AlInN from the bilayer over 24 h at room temperature, and over 1 h at 80 1C, findings again consistent with Ref. [11]. However, KOH and similar alkaline solutions are well known to etch ¯ face [12], which and roughen GaN on the N-polar ð0 0 0 1Þ would be problematic for lift-off applications and production of three-dimensional structures. This motivated a search for milder and more selective basic etchants. A challenge in finding a suitable alkaline etchant for AlInN is that fact that indium is only weakly amphoteric, in contrast to aluminium and gallium [13]. The weak amphoteric character of indium means that it is relatively difficult to solubilize indium oxides and related phases in basic solutions. An attractive route to stabilizing In3+ ions in such media is the introduction of a chelating agent, which forms a stable complex. We have focussed so far on using the chelating amine 1,2-diaminoethane (henceforth DAE). DAE is a bidentate ligand, capable of occupying two adjacent co-ordination sites round an In3+ ion, and we have obtained similar results with the analogous tridentate ligand bis(2-aminoethyl)amine. DAE is a slightly stronger base in aqueous solution than NH3, such that the solutions employed in the etch work have pH values of 12. Fig. 2(a)–(d) shows the effect of a 1 M aqueous DAE solution on the bilayer in etch experiments of three different durations. The surface of the as-grown AlInN shown in Fig. 2(a) is basically smooth, but shows pits at an area density of 3  109 cm2. These features are thought most likely to originate from threading dislocations in the

1-mm GaN buffer layer below the AlInN, and their mode of formation may be similar to pits seen in InGaN epilayers [14]. If the assumed association with threading dislocations is correct, the pits will extend to the AlInN–GaN interface in many instances. However, an alternative explanation of the origin of the pits is that they could be associated with inversion domains, as previously observed in AlGaN alloys [15]. Fig. 2(b) and (c) indicate that the mode of attack of the DAE solution on the AlInN layer is essentially lateral etching, presumably initiated from the pit sidewalls. The etch rate of the AlInN (0 0 0 1) crystal face appears negligible under these conditions. Assuming that removal of AlInN was close to complete after 3 h, and that the lateral etching proceeded from features uniformly distributed at an area density of 3  109 cm3, the corresponding average lateral etch rate is 60 nm/h. Full removal of the AlInN layer after 10 h left a significantly roughened GaN surface, as shown in Fig. 2(d). This morphology did not evolve when samples were exposed to the DAE solution for longer periods. The lowest points on the GaN surface are areas from which AlInN was removed most quickly, and these regions may contain clusters of threading dislocations in the GaN buffer layer. In rationalizing the etching behavior of the GaN buffer layer in the AlInN–GaN bilayer, we first note that 1 M DAE solutions have no detectable effect on standard GaN-on-sapphire films from our MOCVD growth. These show smooth, featureless surfaces under SEM examination. Pre-existing roughening of the GaN buffer layer during growth and/or substantial diffusion of aluminium or indium into the GaN, are not expected to be significant. The AlInN setpoint growth

Fig. 2. Secondary electron images at 651 tilt of the AlInN–GaN bilayer after various exposures to 1 M DAE solution: (a) as-grown; (b) etched for 1.5 h; (c) etched for 3 h; and (d) etched for 10 h.

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temperature was over 300 1C lower than the GaN growth temperature, and we also have depth profiles from secondary ion mass spectrometry, which help rule out interface roughening or interdiffusion [16]. The formation of a charge accumulation layer at the AlInN–GaN interface [7], and its interactions with threading defects, provide a possible explanation of how the reactivity of the GaN buffer layer surface is modified in the AlInN–GaN heterostructure. Further experiments with AlInN layers grown on defect-free GaN templates, and using structures with delta-doped layers introduced during growth [3], could help to understand this behavior more fully. The effects of stirring or agitating the DAE etch solution have not yet been studied in detail. SEM images of a bilayer sample etched for 1 h under ultrasonic agitation are very similar to Fig. 2(b), indicating that the agitation did not cause gross acceleration of the etching. However, we recognize that stirring and agitation could have a major effect on diffusion of reactants through micro-scale channels, as occurs in the lateral etching experiments discussed next. 2.2. Lateral etching results with multilayer samples The results discussed so far indicate the possibility of selectively removing AlInN layers from multilayer heterostructures by lateral etching. A demonstration is provided by Fig. 3(a), which shows the effect of exposing a fracture section through a structure grown on free-standing GaN,

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and containing two 70-nm lattice-matched AlInN layers, to 1 M DAE solution for 72 h. The image contrast indicates that the AlInN layers have been laterally etched in a highly selective fashion, and the measured thickness of layers in the structure accurately match their nominal values. No etching is evident around the two groups of three InGaN quantum wells incorporated into the top GaN layer in this structure [8]. Further experiments were conducted with a GaN–AlInN–GaN trilayer grown on sapphire (0 0 0 1). The 300-nm AlInN layer in this structure was grown at a rate 2.5 times higher than the AlInN layers discussed so far, by increasing the molar flow rates of the indium and aluminium sources. This growth condition resulted in a quite different AlInN morphology from that shown in Fig. 2(a), as was clarified by examination of an AlInN–GaN bilayer with an AlInN thickness of 260 nm. SEM images show no distinct pits, but rather a uniformly rough surface. Atomic force microscopy indicated a root mean square roughness of 14 nm. Fig. 3(b) shows a fracture section through the trilayer structure after exposure to 1 M DAE solution for 24 h. Partial removal of the AlInN layer has occurred, although the gap between the upper and lower GaN layers is still partially bridged by conical features. In the absence of pits in the AlInN layer, which could have been filled with GaN in the final growth step, these features are believed to consist of residual AlInN. Their uniform orientation suggests that their formation mechanism is polarity

Fig. 3. Secondary electron images showing lateral etching of buried AlInN layers in various multilayer heterostructures: (a) fracture section through a structure on free-standing GaN, containing two 70-nm AlInN layers, which correspond to the dark contrast features after etching; (b) fracture section ¯ through the GaN–AlInN–GaN trilayer structure, showing residual AlInN features between the two GaN layers, and the smooth N-polar GaN ð0 0 0 1Þ surface left by the etching; (c) oblique view of a cylindrical mesa produced by ICP etching; (d) pedestal structure produced by wet etching of a narrower cylindrical mesa, again produced by ICP etching. Wet etch conditions for the different structures are noted in the main text.

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specific, and they resemble in shape and orientation features seen in the early stages of selective lateral PEC etching of InGaN–GaN structures [2]. Faceting of the GaN fracture surface is also noticeable, and is thought to be caused by preferential etching of certain crystal planes by the DAE solution. However, the smoothness of the N¯ surface exposed by the wet etch is polar GaN ð0 0 0 1Þ noteworthy, and is especially relevant to the use of selective wet etching in the processing of microcavity structures on FS–GaN substrates, as proposed in Ref. [8]. Fig. 3(c) and (d) show the effect of 1 M DEA solution over 72 h on micron-scale cylindrical mesas, prepared from the trilayer structure by ICP etching. The ICP process left smooth sidewalls, and did not cause undercutting of the AlInN layer. Preferential wet lateral etching of the AlInN created undercut structures, as most clearly seen in Fig. 3(d), which shows a disk of GaN supported by a single tapered pillar of residual AlInN. The average lateral etch rate of the AlInN layer in this structure was estimated to be 6 nm/h, which is a factor of 10 lower than the lateral etch rate observed in the bilayer etching experiments. Faceting of the originally smooth GaN sidewalls is again evident. The lateral etch rate estimated for the AlInN layer is therefore, strictly a lower estimate, because of the possibility of a significant lateral etch rate for the GaN layers. Further work is necessary to determine whether or not the faceting of GaN surfaces is a self-limiting process, involving removal only of surface material damaged by dry etching or cleaving. 3. Conclusions Aqueous solutions of the chelating amine DAE proved effective in room-temperature etching of AlInN epitaxial layers designed to be approximately lattice-matched to GaN. Experiments with a AlInN–GaN bilayer of high crystallographic quality indicated that chemical attack on the (Al,In)-polar AlInN (0 0 0 1) surface was very slow, but the 130-nm AlInN layer was selectively removed from the underlying GaN buffer layer on a timescale of hours. The etching process was predominantly lateral in character, and proceeded from pit defects in the AlInN layer, at an estimated rate of 60 nm/h. In the light of these observations, it would be of interest to re-examine previously reported results on the vertical wet etching of AlInN layers [11], to assess whether the AlInN microstructure and/or polarity played a role in its susceptibility to etching. An unexpected finding in our bilayer etching experiments was the roughening of the GaN buffer layer accompanying the removal of the AlInN. This modification of the reactivity of the Ga-polar GaN (0 0 0 1) surface is tentatively associated with the two-dimensional electron gas expected to form at the GaN–AlInN interface. Demonstrations were also made of the effectiveness of DAE solutions in

selectively etching AlInN layers buried within more complex GaN–AlInN heterostructures. Such a process offers the potential for developing epitaxial lift-off, as well as routes to three-dimensional engineered microstructures. Importantly, the scope for varying lattice mismatch in the AlInN–GaN system allows structures containing sacrificial AlInN layers to be grown free from mismatch strain if required. The much greater etch rates of AlInN over GaN in DAE solutions contrasts with the slower vertical etching of AlInN we observed in reactive ion etching [8]. The wet etching processes discussed could also possibly be used in conjunction with the chemical conversion of buried AlInN layers into coherent oxide layers by anodic oxidation [6].

Acknowledgments The authors acknowledge funding from the EU projects CLERMONT2 (MRTN-CT-2003-503577) and STIMSCAT (STREP Contract 517769), and thank Dr. K. Lorenz (ITN, Sacave´m, Portugal) and Dr. C. Liu (Strathclyde University) for characterization of as-grown bilayers.

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