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Scripta Materialia 59 (2008) 1171–1173

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Characterization of threading dislocations in GaN usinglow-temperature aqueous KOH etching and atomic force

microscopy

I. Han,a,* R. Datta,a S. Mahajan,a R. Bertram,b E. Lindow,b C. Werkhovenb and C. Arenab

aSchool of Materials, Arizona State University, P.O. Box 878706, 501 E Tyler Mall, Tempe, AZ 85287-8706, USAbGaNotec Inc, Tempe, AZ 85284-1808, USA

Received 26 May 2008; revised 29 July 2008; accepted 30 July 2008Available online 20 August 2008

We report a reliable and quick method for detecting threading dislocations (TDs) in GaN epitaxial layers grown on (0001) sap-phire using aqueous KOH etching maintained at 80 �C for 10 min. Atomic force microscopy characterization of topological profilesassociated with etch-pits reveal asymmetric and symmetric surface topologies. It is argued that high and low asymmetric profiles areassociated with a and a + c type TDs, whereas symmetric profiles are caused by c type TDs.� 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: GaN; Dislocation; Etching; Atomic force microscopy (AFM)

GaN is a wide band gap material with a widerange of optoelectronic and electronic applications[1,2]. Due to the lack of large area homoepitaxial sub-strates, GaN epitaxial layers are mostly grown on(0001) sapphire substrates, where the mismatch is�16% at room temperature. These layers contain largedensities of threading dislocations (TDs), ranging from1 � 108 cm�2 to 5 � 109 cm�2, depending on the proto-col employed during their growth [3–7]. TDs are knownto have adverse effects on the performance of GaN-based optoelectronic devices [1,2,8,9]. Estimating thedensities of TDs, understanding the characteristics andidentifying their type are very important for quick feed-back to the growing layers. Many different etching andtransmission electron microscopy (TEM)-based tech-niques have been reported so far for the determinationof different types of TD density [10–14]. Dry plasmaetching [10] and photoenhanced electrochemical (PEC)wet etching [13] of Group III nitrides have been widelyused for studying surface defects, cleaning prior to de-vice fabrication and improving electrical contacts. Themost commonly used etchants are hot phosphoric andsulfuric acid solutions, and molten KOH and PEC etch-ing in aqueous KOH solution [10,11,13,14]. However,

1359-6462/$ - see front matter � 2008 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2008.07.046

* Corresponding author. Tel.: +1 4809654441; e-mail:ilsu.han@asu.edu

there are several difficulties with these etching methods.GaN is rather inert due to its wide band gap and highbonding enthalpy. Hence an additional driving force,such as lasers, high temperature or ultraviolet light, is re-quired for etching. Among various etching methods fordefect delineation, Hino et al. [10] reported HCl vapor-phase etching of GaN at 600 �C, which led to the forma-tion of three distinct etch-pits related to edge (c, polygonshape), screw (a, well-defined hexagon) and mixed type(b, unclear hexagon) dislocations; furthermore, all thepits are wider than 600 nm. Stocker et al. [13] used var-ious hot etching solutions, such as molten KOH in eth-ylene glycol solution and H3PO4, which can etch GaNanisotropically and form pits on the surface by usingPEC etching. Shiojima [14] studied molten KOH etchingat 360 �C and found larger hexagonal pits from mixed-type and smaller ones from edge-type dislocations. InRefs. [13,14] high temperature was used to accelerateetching for easy detection of pits associated with varioustypes of TDs. It is possible that the formation of largerpits may hide many small pits related to edge TDs andthe high etching rate results in the disappearance ofatomic steps on the GaN surface. This may preventestablishing any relation between TDs and atomicstep-terrace structure, which is important for the identi-fication of different TDs. There is another technique re-ported, i.e. in situ (silane + ammonia) etching [12] in ametal-organic chemical vapour deposition (MOCVD)

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1172 I. Han et al. / Scripta Materialia 59 (2008) 1171–1173

reactor where one can identify various types of TDs, butthis technique again involves higher temperature andin situ treatment.

We report here a low-temperature TD detectionmethod using an aqueous KOH etching (which doesnot destroy atomic structure) and atomic force micros-copy (AFM) depth and topological profiling of etch-pits. The depth and topological profiling can distinguishthree different types of TDs. It reveals various types ofTDs clearly for both MOCVD and hydride vapor-phaseepitaxy (HVPE) grown samples and thus provides analternative method that avoids higher temperaturesand an in situ etching process. In this method, GaN lay-ers are first etched in an aqueous KOH solution at 80 �Cand then dislocations are detected by AFM using anatomically sharp tip (tip radius �2 nm, Nanosensors).

The GaN samples used for etching were grown byMOCVD and HVPE. MOCVD samples were grownon epitaxial-ready (0001) substrates in a horizontalAixtron 200RF reactor using trimethyl gallium andammonia as precursors. Hydrogen was used as carriergas. Very thin silicon nitride (referred to as SiN becauseof its unknown stoichiometry) layers were grown bysimultaneous flow of silane and ammonia immediatelyafter GaN nucleation layer and before the high-temper-ature growth in order to achieve a TD density�3 � 108 cm�2 [9]. The thickness of the MOCVD filmwas �3.4 lm and the thickness of the HVPE films was�30 lm. All GaN samples reported here have the Ga-polarity surface.

Etching experiments were carried out using a 45%KOH:H2O (1:3) aqueous solution. The solution washeated to 80 �C; an etching time of 10 min was foundto be suitable to delineate dislocations clearly underAFM. Figure 1a and b show typical AFM images ofKOH-etched surfaces of as-grown GaN by MOCVDand HVPE, respectively. Both Figure 1a and b show awell-defined step-terrace structure (representing an

Figure 1. (a) AFM image after aqueous KOH etching for 10 min froma MOCVD-grown sample (5 lm � 5 lm scan). (b) AFM image afteraqueous KOH etching for 10 min from HVPE-grown sample(5 lm � 5 lm scan). (c) AFM images of a type pit, c type and a + c

type, respectively, shown in (a) and (b).

excellent quality surface for device structure growth)where three kinds of etch-pit related to three differentTDs are clearly observed on the etched surface and aremarked by their usual notation in Figure 1c [12]. Theyhave been identified as follows. Screw-type TDs appearon steps or at the end of the step; mixed-type TDs ap-pear at the end of step; and edge-type TDs appear onthe terraces [15]. The etching rate is higher surroundingthe dislocation region compared to dislocation-freeareas due to the strain associated with them and pitsare formed at TDs after etching. TDs appear to havethe same appearance in etched MOCVD and HVPEgrown samples. Figure 2 shows the GaN surface afterhigh-temperature KOH etching: the step-terrace struc-ture has been lost, preventing any direct correlation be-tween types of TDs and step terrace structures. Plan-view TEM was carried out (Fig. 3) to confirm the TDdensity obtained by AFM (Fig. 1b) and a TD densityof 2–4 � 108 cm�2 was obtained from both techniques.

Beam et al. [16] used a computer simulation and sug-gested that a protrusion and a depression are formed,respectively, at the compressive and tensile side of anedge TD due to strain relaxation at the free surface.We have obtained surface topological line profile along

Figure 2. AFM images after molten KOH etching at 280 �C as afunction of time (3 lm � 3 lm scan): (a) for 1 min, (b) for 5 min fromthe MOCVD-grown sample. Images reveal that the atomic step-terracestructure of GaN is lost after etching.

Figure 3. Plan-view TEM image with (a) g = <0002> and (b) alongZA = <11–20>, calculated TD density is 2–4 � 108 cm�2. One canidentify different TDs from (a). Pair of dots corresponding to screwTDs marked with black circle. The same TD density number isobtained from AFM [11].

Figure 4. Dislocation line profile for each dislocation: (a) a type, (b) c

type, (c) a + c type.

I. Han et al. / Scripta Materialia 59 (2008) 1171–1173 1173

various directions from edge, screw and mixed TDs onthe (000 1) plane of GaN surface.

Figure 4 shows the topological profiles from three dif-ferent TDs. The interesting observation is the asymme-try in profile across edge-type TDs as shown in Figure4a. The topological profile from screw TDs is symmet-ric, and for the mixed type the profile is asymmetricagain as shown in Figure 4b and c, respectively. Theidentification of different TDs from the (0001) surfaceis explained below. The identification of different TDsfrom the (000 1) surface is explained below. The identi-fication of TDs by AFM is based on the idea of the ori-entation of Burgers vector with respect to the (00 01)plane of GaN [17]. For screw-type (b = <0 001>) andmixed-type dislocations (b = 1/3<11–23>), the Burgersvector is perpendicular (90�) and at an angle (�58�) withrespect to the (0001) plane of GaN, respectively. Hence,these dislocations will give rise to steps equal to the mag-nitude of their Burgers vector and within easy detectionlimit of the AFM tips routinely used in the laboratory(1–2 A depth resolution). The Burgers vector of anedge-type TD is 1/3 <11–20> and parallel to the(0001) plane. Therefore, a protrusion (hillock) and adepression (indentation) are produced due to Poisson’scontraction and expansion at the compressive and ten-sile side of the edge TDs, respectively, or a pit formationdue to higher internal energy than the surface tension[15,16]. However, after etching, all types of dislocationsare clearly observed on the surface due to pit formation.The pit size and depth are different for the three differentTDs due to the differing strains associated with them.Previous researchers, using TEM, have noted only thatthe etch-pits are hexagonal and polygonal, with sizesranging from 100 to 600 nm in diameter and from 50to 200 nm in depth. However, most etch-pits terminatedin atomic steps in these studies. In addition, we have notseen any experimental evidence relating the surfacetopology to the various types of dislocations.

In summary, KOH wet etching was carried out onMOCVD and HVPE GaN samples to determine TDdensity. We have found that there is no termination ofatomic steps. This method can delineate a, a + c and c

type TDs in GaN under AFM using an atomically sharptip. In particular, a type threading dislocations can eas-ily be identified by this technique. The TD densityobtained through AFM is in excellent agreement withthat obtained from TEM. Most importantly, the AFMtopographic profiles associated with etch-pits revealasymmetric and symmetric surface topologies, whichhave been discussed. Our result suggests that the asym-metric line profile is related to the protrusion anddepression from the compressive and tensile side of anedge TD. The protrusion and depression height was cal-culated to be �1.2 A. The topological profile from screwTDs is symmetric, and for the mixed type the profile isasymmetric again due to the edge component and atom-ic step.

The authors are grateful to GaNotec, Inc. for finan-cial support.

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