We have investigated the properties of a commercial light-emitting diode (LED) structure containing an InGaN single quantum well (SQW) by scanning tunneling luminescence (STL). Data was acquired under ambient conditions, i.e., in air and at room temperature, using our unique STL microscope with a novel light collection geometry. Scanning tunneling microscopy (STM) images revealed the presence of hexagonal pits in the structure, with STL images showing strong luminescence from these pits. The variation of STL intensity with bias voltage shows the STL threshold at --2.1 V is numerically similar to the peak position of the SQW luminescence band. A slight shoulder at --2.8V corresponds to the plateau of the delocalised absorption profile, observed in macroscopic measurements. The peak observed at --3.2 V is close to the observed GaN band edge emission

A.A. BASKI

C.W. LITTON

D. HUANG

F.YUN

H. MORKOC

J.JASINSKI

M.A. RESHCHIKOV

P. VISCONTI

R. CINGOLANI

T.KING

Z. LILIENTAL-WEBER

Nguyen, Ngoc Duy

Germain, Marianne

Schmeits, Marcel

Schineller, Bernd

Heuken, Michael

Open Repository and Bibliography - Liège

Investigation of defect levels in Mg-doped GaN Schottky structures by thermal admittance spectroscopy

Kirilyuk, V.

Hageman, P.R.

Tichelaar, F.D.

Larsen, P.K.

NARCIS 

Photoluminiscence study of piezoelectric polarization in strained AlxGa1-xN/GaN single quantum wells

Nguyen, N. D.

Germain, M.

Schmeits, M.

Schineller, B.

Heuken, M.

Publikationsserver der RWTH Aachen University

The formation of two-dimensinal electron gas (2DEG) in AlGaN/GaN heterostructure field effect transistors was investigated using synchrotron radiation photoemission spectroscopy and high-resolution X-ray diffraction. The surface band bending, interfacial strain and 2DEG density were evaluated as a function of AlGaN barrier thickness and composition. The 2DEG density increased with the thickness, but the surface Fermi level was independent of the thickness of AlGaN layer. This suggested that the polarization-induced 2DEG originated from the AlGaN barrier, which is unintentionally doped due to the nonstoichiometry of nitrogen deficiency and donor-like impurities.X11sciescopu

Jang, HW

Jeon, CM

Kim, KH

Kim, JK

Bae, SB

Lee, JH

Choi, JW

Lee, JL

포항공과대학교

physica status solidi (b)

Investigation for the formation of polarization-induced two-dimensional electron gas in AlGaN/GaN heterostructure field effect transistors

F. YUN

M. A. RESHCHIKOV

T. KING

A. A. BASKI

C. W. LITTON

J. JASINSKI

Z. LILIENTAL WEBER

H. MORKOÇ

VISCONTI, Paolo

Archivio Istituzionale della Ricerca- Università del Salento

A Comparative Study of MBE-Grown GaN Films HavingPredominantly Ga- or N-PolarityF. Yun1) (a), D. Huang (a), M. A. Reshchikov (a), T. King (a), A. A. Baski (a),C. W. Litton (b), J. Jasinski (c), Z. Liliental-Weber (c), P. Visconti (a),and H. Morkoç (a)(a) Department of Electrical Engineering and Physics Department,Virginia Commonwealth University, 601 W. Main St., Richmond, VA 23284, USA(b) Air Force Research Laboratory (AFRL/MLPS), Wright Patterson AFB,OH 45433, USA(c) Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA(Received June 21, 2001; accepted August 1, 2001)Subject classification: 61.72.Dd; 61.72.Ff; 68.37.Ps; S7.14Wurtzitic GaN epilayers having both Ga and N-polarity were grown by reactive molecular beamepitaxy (MBE) using a plasma-activated nitrogen source on c-plane sapphire. The polarities wereverified by convergent beam electron diffraction (CBED). High-resolution X-ray diffraction, atom-ic force microscopy (AFM) imaging and transmission electron microscopy (TEM) were employedto characterize the structural defects present in the films. The different topographic features ofGa and N-polarity samples and their appearance after wet etching were correlated to themeasured X-ray rocking curve peak widths for both symmetric [0002] and asymmetric [101̄4]diffraction. For Ga-polarity samples, the [0002] diffraction is narrower than the [101̄4] diffraction,while for N-polarity ones the [0002] peaks are broader than [101̄4]. The half width of [101̄4] peaksfor both polarity types were in the range of 5–7 arcmin indicative of, among possibly other defects,a high density of pure edge threading dislocations lying parallel to the c-axis. The 1–2 arcmin[0002] linewidths of Ga-polarity samples suggest a low density of screw dislocations, whichcorresponds with the TEM observations where the screw dislocation density is less than 107 cm– 2.In N-polarity samples, however, the [0002] diffraction peak was typically wider than 5 arcmin,suggesting either a higher density of edge dislocations and inversion domains in N-polaritysamples, or the columnar structural features in AFM images, where the effective coherence lengthfor X-ray diffraction is drastically reduced.Introduction The potential application of III-nitride materials for electronic deviceshas generated extensive research on GaN grown by MBE [1, 2]. GaN is polar along thec-direction, which is the most commonly used orientation for MBE growth. The qualityof GaN films and performance of related device structures depend critically on thepolarity of the epilayers [3]. Study of the freestanding GaN template grown by hydridevapor phase epitaxy (HVPE) has demonstrated distinct features for Ga-polarity and N-polarity GaN [4]. X-ray rocking curve linewidth of both the symmetric and asymetricreflections are narrower on the Ga-polarity side than on the N-polarity side. The over-all defect density on the Ga-polarity side is more than an order of magnitude lowerthan that of N-polarity side. Moreover, the dependence of X-ray peak width on thedislocation structure has been studied on GaN films grown by metalorganic vaporphase deposition (MOCVD) [5], together with quantitative defect analysis by transmis-sion electron microscopy (TEM) [6, 7].1) Corresponding author; Fax: (804)828-4269; e-mail: fyun@mail1.vcu.eduphys. stat. sol. (b) 228, No. 2, 543–547 (2001)# WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0370-1972/01/22811-0543 $ 17.50þ.50/0Recently, we have established the topological growth pattern for Ga-polarity andN-polarity GaN films by MBE through control of buffer layer parameters [8]. Thisallows us to assess the structural properties specific to GaN polar structure. In thiswork, we examine both Ga- and N-polarity GaN films using atomic force microscopy(AFM), X-ray rocking curves (w-scan), and high-resolution TEM.Experimental The GaN thin films were grown by MBE using a rf-plasma nitrogensource. The substrate used was c-plane sapphire. Both GaN and AlN buffer layers wereoptimized [9] and used to obtain Ga-polarity and N-polarity films through control ofgrowth temperature and growth rate [8]. In particular, the Ga-polarity sample for TEManalysis was grown with an AlN buffer at 915 C for 20 min and the GaN epilayer at740 C for 2 h. The N-polarity sample for TEM was grown with GaN buffer layer at500 C for 1 h, followed by GaN epilayer grown at 725 to 790 C for 4.5 h.The polarity of the GaN films was monitored by in-situ RHEED pattern and wasestablished through wet chemical etching experiments, details of which can be foundelsewhere [10]. It was also confirmed by convergent beam electron diffraction (CBED)on the two samples for TEM measurements. High-resolution X-ray measurements wereperformed on a Philips X’Pert MRD system equipped with a four-crystal Ge(220)monochromator. The instrumental broadening was 10 arcsec. CBED, conventionaland high-resolution TEM studies were perforemed using an 002B TOPCON micro-scope operated at 200 keV. AFM images were taken on Ga-polarity and N-polarityGaN samples before and after wet chemical etching by hot H3PO4.Results and Discussions The surface morphology of Ga- and N-polarity GaN films wasimaged by tapping mode AFM and is shown in Fig. 12), both as-grown and after chemi-cal etching by H3PO4 at 160 C. Distinct features of surface morphology can be ob-served between Ga- and N-polarity samples. The surface of as-grown Ga-polarity GaN544 F. Yun et al.: A Comparative Study of MBE-Grown GaN Films2x2µ m 2x2µ m2x2µ m 2x2µ ma bc dFig. 1 (colour). AFM images ofa) as-grown Ga-polarity GaN,b) Ga-polarity after etching for5 min, c) as-grown N-polarity GaN,and d) N-polarity after etching for30 s. Vertical scales for a) andb) are 10 nm, and for c) andd) 70 nm2) Colour figure is published online (www.physica-status-solidi.com).(Fig. 1a) is smooth, terraced, and well-coalesced, with a root-mean-square (rms) rough-ness of 3.5 nm. After exposure to etchants for 5 min, the surface of GaN remainsrelatively smooth, except for the presence of many hexagonal etch pits (Fig. 1b). Theas-grown surface of the N-polarity film (Fig. 1c) shows isolated columns with feature sizeof 0.2 mm, and a roughness (rms) of  50 nm. The N-polarity film is etched quickly byhot H3PO4 leaving only sporadic spots (Fig. 1d) of GaN after etching for only 30 s.Symmetric [0002] and asymmetric [101̄4] w-scans are plotted in Fig. 2 for the Ga- andN-polarity samples used for TEM measurements, respectively. The Ga-polarity samplehas a full-width at half-maximum (FWHM) of 1.98 arcmin for the [0002] peak, and5.0 arcmin for the [101̄4] peak. The N-polarity sample shows a much larger FWHM of5.26 arcmin for the [0002] peak, and 5.24 arcmin for the [101̄4] peak. Through accumu-lative statistics of more than 20 samples, we found that the FWHM figures of [0002]direction were between 1 and 2 arcmin for Ga-polarity GaN, and 5 and 7 arcmin for N-polarity. The FWHMs for [101̄4] direction were 5– 7 arcmin for both Ga- and N-polar-ity films. This suggests that the nature and density of dislocations dominating Ga- andN-polarity films are different.Three types of dislocations can be identified in GaN by their Burgers vectors. Theedge dislocation (with Burgers vectors of [100], [110], or [010]) is parallel to thegrowth plane (c-plane) and will produce an in-plane strain which can only affect theasymmetric diffraction [101̄4]. The screw dislocation (with Burgers vectors of [001],parallel to c-axis) will produce a shear strain that can broaden the symmetric diffraction[0002]. There are also dislocations with mixed Burgers vectors ([101], [011], [111])which can contribute to both the symmetric and asymmetric diffractions. In order todifferentiate between different types of dislocations and estimate their densities weapplied conventional TEM methods since application of the diffraction contrast undertwo-beam condition allows for observation of a specific type of dislocations.Shown in Figs. 3a and b are cross-sectional multi-beam bright field TEM imagestaken from Ga- and N-polarity GaN, repectively showing all types of dislocations. Alsoshown are the bright field images with a g-vector being parallel (g ¼ 0002, Figs. 3c andd) or perpendicular (g ¼ 112̄0, Fig. 3e and g ¼ 011̄0, Fig. 3f) to the c-axis. For both Ga-and N-polarity films, very high dislocation density was observed (Figs. 3a and b) at thephys. stat. sol. (b) 228, No. 2 (2001) 545Fig. 2. Symmetric and asymmetric X-ray rocking curves for a) Ga-polarity and b) N-polarity GaNgrown by MBEinitial stage of growth (buffer layer). Some of these dislocations terminate within the bufferlayer, others thread through the epilayers. The density gradually decreases with continuousgrowth. The total density of dislocations for the Ga-polarity sample (Fig. 3a) decreasesfrom D ¼ 1  1011 cm– 2 at the buffer region to 5  109 cm– 2 near the growth front.The total density for the N-polarity sample (Fig. 3b) decreases from D ¼ 2  1010 cm– 2at the bottom to 5  108 cm– 2 near the top surface. It should be noted that the totaldislocation density of N-polarity GaN is an order of magnitude lower than that ofGa-polarity samples.With the g ¼ 0002 reflection, only pure screw dislocations and mixed dislocations arevisible. With g ¼ 112̄0, only pure edge dislocations and mixed dislocations can be ob-served. Therefore, it can be concluded (from Figs. 3, c, d, e and f) that for the Ga-polarity sample, edge disloactions are dominant (95%), while the density of screwdislocations is very low (Ds < 107 cm– 2) and the density of mixed dislocations (Dm ¼1  108 cm– 2) is also much lower than the edge dislocations. The N-polarity sample, onthe contrary, shows no dominance by any dislocation type. Instead, it exhibits anequivalent amount of all three types of dislocations. Also pronounced in N-polarity filmis the large density (DID ¼ 1  1011 cm– 2) of inversion domains (IDs) which occur inthe Ga-polarity regions (see Fig. 3d). The density of IDs in the Ga-polarity film, how-ever, is found to be very low (less than 107 cm– 2).The features of Ga and N-polarity films in X-ray rocking-curve measurements can bewell correlated with dislocation characteristics observed in TEM images. The narrow[0002] FWHM for Ga-polarity GaN is consistent with the very low density of screw andmixed dislocations. For N-polarity GaN, the much wider FWHM of [0002] peak agreeswith the existence of a high density of screw dislocations, together with even higherdensity of IDs which may contribute to the broadening in a way similar to mixed dis-546 F. Yun et al.: A Comparative Study of MBE-Grown GaN Filmscomparable densities0.5 µm(c)(e)ggD~5x109 cm-2D~2x1010cm-2D~1x1011cm-2Ds<1x107 cm-2Dm~1x108 cm-2edge dislocations~95%Ga-polarity0.25µm(b)(d)(f)ggall three types ofdislocations (edge,screw and mixed)present withD~5x108 cm-2D~5x109 cm-2D~2x1010 cm-2N-polarity(a)Fig. 3. TEM images showing dislocations in a) Ga-polarity and b) N-polarity GaN films. Multiple-beam dark-field images of Ga and N-polarity films with g ¼ 0002 are shown in c) and d), respec-tively, and with g ¼ 112̄0 in e) and g ¼ 011̄0 in f)locations. The asymmetric [101̄4] rocking curve peaks for both Ga- and N-polarity filmshave a comparable FWHM, reflecting the TEM observation of comparable edge-dislo-cation densities in Ga- and N-polarity films.The symmetric X-ray diffraction peak linewidth can also be influenced by the featuresize of grains or columns with respect to Ga- and N-polarity. The well-coalesced surfacefeatures of the Ga-polarity sample (in Fig. 1a) indicates less influence of broadeningeffects due to its large coherent length. For the N-polarity film in Fig. 1b, the columnarstructure with lateral feature size of 0.2 mm will centainly contribute to the broaden-ing of the symmetric [0002] peak due to the size effect predicted by the Scherrer formu-la [11].Summary Distinct structural features from X-ray rocking curve measurements werefound in Ga-polarity and N-polarity GaN films grown by MBE. Ga-polarity samplesshow much narrower [0002] linewidth than N-polarity films, while both types of filmsshow comparable linewidths of the asymmetric [101̄4] diffraction. TEM images revealthe different nature and density of dislocations in Ga and N-polarity films, and are ingood agreement with X-ray data. The surface morphology measured by AFM alsoshows different features for Ga and N-polarity films.Acknowledgements The authors would like to thank Prof. K. J. Wynne and Ms. J. Uilkfor the use of a large area AFM. The VCU portion of this work was funded by grantsfrom AFOSR (Dr. G. L. Witt), NSF (Dr. L. Hess and Dr. G. Pomrenke), and ONR(Dr. C. E. C. Wood and Dr. Y. S. Park). The TEM group (J. J. and Z. L.-W.) supportedby Air Force Office of Scientific Research, through the U.S. Department of Energyunder Order No. AFOSR-ISSA-00-0011 would like to thank W. Swider for her excel-lent TEM sample preparation and would like to acknowledge the use of the facilities atthe National Center for Electron Microscopy at Lawrence Berkeley National Labora-tory.References[1] H. Morkoç, Nitride Semiconductors and Devices, Springer-Verlag, Berlin 1999.[2] S. J. Pearton, J. C. Zolper, R. J. Shul, and F. Ren, J. Appl. Phys. 86, 1 (1999).[3] H. Morkoç, R. Cingolani, and B. Gil, Solid-State Electron. 43, 1909 (1999).[4] F. Yun, M. A. Reshchikov, K. Jones, P. Visconti, H. Morko, S. S. Park, and K. Y. Lee, Solid-State Electron. 44, 2225 (2000).[5] B. Heying, X. H. Wu, S. Keller, Y. Li, D. Kapolnek, B. P. Keller, S. P. DenBaars, andJ. S. Speck, Appl. Phys. Lett. 68, 643 (1996).[6] D. Kapolnek, X. H. Wu, B. Heying, S. Keller, U. K. Mishra, S. P. DenBaars, and J. S. Speck,Appl. Phys. Lett. 67, 1541 (1995).[7] W. Qian, M. Skowronski, M.. DeGraef, K. Doverspike, L. B. Rowland, and D. K. Gaskill,Appl. Phys. Lett. 66, 1252 (1995).[8] D. Huang, P. Visconti, K. M. Jones, M.. A. Reshchikov, F. Yun, A. A. Baski, T. King, andH. Morkoç, Appl. Phys. Lett. 78, 4145 (2001).[9] F. Yun, M. A. Reshchikov, P. Visconti, K. M. Jones, D. Wang, M. Redmond, J. Cui,C. W. Litton, and H. Morkoç, Mater. Res. Soc. Symp. Proc. 639, G3.17 (2001).[10] P. Visconti, K. M. Jones, M. A. Reshchikov, F. Yun, R. Cingolani, H. Morkoç, S. S. Park, andK. Y. Lee, Appl. Phys. Lett. 77, 3743 (2000).[11] B. E. Warren, X-Ray Diffraction, Dover Publ. Co., New York 1990.phys. stat. sol. (b) 228, No. 2 (2001) 547

English

A comparative Study of MBE-Grown GaN Films Having Predominantly Ga- or N-Polarity

Radboud Repository

Christianen, P.C.M.

Photoluminescence study of piezoelectric polarization in strained AlxGa1-xN/GaN single quantum wells

Availability of reliable and quick methods to investigate defects and polarity in GaN films is of great interest.W e have used photo-electrochemical (PEC) and hot wet etching to determine the
defect density.W e found the density of whiskers formed by the PEC process to be similar to the density of hexagonal pits formed by wet etching and to the dislocation density obtained by transmission electron microscopy (TEM).Hot wet etching was used also to investigate the polarity of MBE-grown GaN films together with convergent beam electron diffraction (CBED) and atomic
force microscopy (AFM).W e have found that hot H3PO4 etches N-polarity GaN films very quickly resulting in the complete removal or a drastic change of surface morphology.On the contrary, the
acid attacks only the defect sites in Ga-polar films leaving the defect-free GaN intact and the morphology unchanged.T he polarity assignments, confirmed by CBED experiments, were related to the as-grown surface morphology and to the growth conditions

Investigation of Defects and Polarityin GaN Using Hot Wet Etching, Atomic Forceand Transmission Electron Microscopyand Convergent Beam Electron DiffractionP. Visconti (a, b, c), D. Huang (a), M. A. Reshchikov (a), F. Yun (a),T. King (a), A. A. Baski (a), R. Cingolani (b), C. W. Litton (d),J. Jasinski (e), Z. Liliental-Weber (e), and H. Morkoç (a)(a) Department of Electrical Engineering and Physics Department,Virginia Commonwealth University, Richmond, VA 23284, USA(b) NNL – National Nanotechnology Laboratory of INFM,Unit of Lecce and Department of Innovation Engineering, University of Lecce,Via per Arnesano, I-73100, Lecce, Italy(c) Istituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR-IME,Via per Arnesano, I-73100, Lecce, Italy(d) Air Force Research Laboratory, Wright Patterson AFB, OH 45433, USA(e) Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA(Received July 2, 2001; accepted July 18, 2001)Subject classification: 61.72.Ff; 68.37.Lp; 68.37.Ps; 68.55.Ln; S7.14Availability of reliable and quick methods to investigate defects and polarity in GaN films is ofgreat interest. We have used photo-electrochemical (PEC) and hot wet etching to determine thedefect density. We found the density of whiskers formed by the PEC process to be similar to thedensity of hexagonal pits formed by wet etching and to the dislocation density obtained by trans-mission electron microscopy (TEM). Hot wet etching was used also to investigate the polarity ofMBE-grown GaN films together with convergent beam electron diffraction (CBED) and atomicforce microscopy (AFM). We have found that hot H3PO4 etches N-polarity GaN films very quicklyresulting in the complete removal or a drastic change of surface morphology. On the contrary, theacid attacks only the defect sites in Ga-polar films leaving the defect-free GaN intact and the mor-phology unchanged. The polarity assignments, confirmed by CBED experiments, were related tothe as-grown surface morphology and to the growth conditions.Introduction Successful fabrication of GaN-based devices depends on the ability togrow epitaxial films on sapphire with low defect density. The poor match in both latticeparameter and thermal expansion coefficient results in a high dislocation density (DD).Wurtzite GaN is a polar material that has two different planes along its c-axis. The(0001) plane is the Ga-terminated face while the (0001) plane is the N-face [1]. It isknown that the surface and bulk properties of GaN layers depend greatly on the polar-ity [2]. Therefore the availability of reliable and quick methods to determine the defectdensity and the polarity of GaN films is of great importance.We have investigated defects in GaN films grown by MBE using PEC method [3]and hot wet etching [4]. We found the whisker density to be similar to the etch pit1) Corresponding author; Tel.: +39-0832-320231; Fax: +39-0832-326351;e-mail: paolo.visconti@unile.itphys. stat. sol. (b) 228, No. 2, 513–517 (2001)# WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0370-1972/01/22811-0513 $ 17.50þ.50/0densities (EPD) for samples etched under precise conditions. TEM studies confirmedthe DDs obtained by etching which increased our confidence in the validity of themethods used. Additionally, we have demonstrated that hot H3PO4 can be used to de-termine the polarity of GaN films. Already, NaOH and KOH based aqueous solutionsat different temperatures have been demonstrated to attack N-polar surface whereasthe morphology of Ga-polar GaN films remains unchanged [5–7]. Similarly, we foundthat a few seconds of etching in hot H3PO4 leads to the complete removal or a drasticchange in the morphology of N-polar GaN films. On the contrary, a defect free Ga-polar film is hardly affected by the hot acid. Only hexagonal pits associated with thesurface defects are formed after several minutes of etching. The polarity assignments,confirmed by CBED experiments, were related to the as-grown morphology and to thegrowth conditions. We found that the GaN films grown by MBE on high temperature(HT) AlN (> 890 C) and GaN (770–900 C) buffer layers show Ga- and N-polarity,respectively [8].Experimental Details The GaN samples consisted of unintentionally n-doped GaNlayers grown by MBE on c-plane of sapphire using radio-frequency activated N. Somesamples utilized GaN buffer layers grown at 800 C (HT). Others utilized AlN bufferlayers grown at 890–930 C (HT). Following the buffer layers, 1 mm thick GaN layerswere grown at a temperature between 720 and 850 C with growth rates in the range of0.3–1 mm/h under N-limited (Ga-rich) conditions. PEC etching was carried out in astandard electrochemical cell at room temperature using a 0.02M KOH solution and aHe–Cd laser. Further details of the PEC experimental set-up can be found elsewhere[9, 10]. The morphology of samples etched by PEC and wet etching was investigatedusing AFM and scanning electron microscopy (SEM). Additionally, some samples wereobserved by TEM to estimate the DD. The polarity was determined by hot H3PO4etching, CBED experiments and AFM investigation of their as-grown morphology. Ad-ditionally, in-situ RHEED patterns during the MBE growth supported the polarity as-signments.Results and Discussion PEC etching has been demonstrated to be suitable for DDestimation in n-type GaN films. Freestanding whiskers were obtained by selectivelyetching GaN between dislocation sites under precise etching conditions. With XTEManalysis, the presence of the dislocations in the whiskers was demonstrated and thewhisker density was found close to the DD [3]. We used slightly carrier-limited condi-tions to etch Ga-polar GaN selectively, leaving threading vertical wires on the surface.514 P. Visconti et al.: Investigation of Defects and Polarity in GaN1µmFig. 1. Plan-view SEM image of the PEC etchedsample. The density of the whisker-like features(white dots in the image) is 1–2  109 cm– 2The density of features (white dots in the SEM image of Fig. 1) formed by the PECprocess is  2  109 cm– 2, the same value obtained from the AFM investigation.Wet etching is a suitable technique to determine in Ga-polar GaN the density of defectspropagating to the surface. We have used hot H3PO4 as a defect etchant, which produceshexagonal pits at the defect sites. During etching, a careful balance must be struck toensure that every defect is delineated, but not over-etched to cause merging which wouldlead to an underestimation of the defect density. H3PO4 for 5 min at 160 C was used todetermine the defect density in Ga-polar GaN films grown by MBE. The AFM image ofFig. 2 shows the etched surface with EPD of 1  109 cm– 2. TEM analyses were carriedout in order to determine the effective DD. From XTEM observation (Fig. 3), the thread-ing dislocations were observed starting from the buffer/GaN interface and propagatingwithin the GaN layer. The total DD, near the surface, including screw (<1  107 cm– 2),mixed ( 1  108 cm– 2) and edge dislocations (about 95% of the total) is  4  109 cm– 2close to the values obtained by defect revealing etches.It is known that the electrical and optical properties of GaN layers depend greatly onthe polarity. We have investigated the polarity of GaN films grown by MBE on HT(T > 770 C) GaN buffer layers or HT (T  900 C) AlN buffer layers in order toobtain N- and Ga-polar GaN films, respectively. The polarity was determined by etch-ing in hot H3PO4 and AFM imaging of as-grown surface morphologies. The polarityassignments were supported by in-situ RHEED patterns and confirmed by CBED ex-periments. As reported in the literature, the surface of a Ga-polar film is either very flator shows stepped terraces. The surface of a N-polar film grown by MBE shows tallcolumns or terraces separated by deep troughs. Similar to NaOH- and KOH-basedetchings, we have found that hot H3PO4 etches N-polar GaN films very quickly result-ing in the complete removal or a drastic change of the morphology as revealed byAFM or optical microscopy. The etching rate is from 0.3 to 0.7 mm/min. On the con-trary, a defect free Ga-polar film is hardly affected by the hot acid. As shown in Fig. 2,only hexagonal pits associated with the surface defects are formed after several minutesof etching of Ga-polar films.phys. stat. sol. (b) 228, No. 2 (2001) 5155 µm0.5 µmFig. 2 Fig. 3Fig. 2. AFM image of the Ga-polar GaN surface morphology after etching in H3PO4 for 5 min at160 C. The EPD is 1  109 cm– 2. The vertical scale ranges from 0 to 10 nmFig. 3. Cross-sectional TEM image of a Ga-polar GaN layer. The total DD, near the top surface, isabout 4  109 cm– 2 (edge dislocations about 95% of the total)The surface morphology of an as-grown Ga-polar GaN film is shown in Fig. 4. AHT AlN buffer layer leads to Ga-polar film with a smooth, but pitted surface mor-phology with a rms roughness of 1.3 nm. According to the literature, the Ga-polarfilm shows a 2  2 RHEED pattern upon cool-down at temperatures between 280and 650 C. Shown in Fig. 5 is the typical surface morphology of a N-polar filmobtained using a HT GaN buffer layer. The rough morphology presents 50–100 nmhigh non-coalesced columns with a rms roughness as high as 20 nm. The RHEEDpattern upon cooling shows only the bulk 1  1 structure. Dipping of the N-polarGaN layer in hot (160 C) H3PO4 for only 20 s results in a drastic change of themorphology. Longer dipping in hot H3PO4 for just few minutes instead results in thecomplete removal of the film, obtaining the clean surface of sapphire substrate byAFM imaging. In the AFM image of Fig. 6, we show the surface morphology of a N-polar film after etching in hot H3PO4 for 20 s. Etching of the c-plane with an etchrate of  0.5 mm/min produces 300–400 nm high features on the sapphire substrate(rms roughness  150 nm).In order to confirm the polarity assignments, we carried out CBED experiments. Fora GaN layer grown on HT AlN buffer layer, the CBED study confirmed the Ga-polar-ity, whereas from XTEM study the density of inver-sion domains (ID) was estimated 1  107 cm– 2.For a GaN layer grown on HT GaN buffer layer,the CBED study again confirmed the assigned N-po-larity, whereas from TEM study the density of IDswas estimated 1  1011 cm 2.516 P. Visconti et al.: Investigation of Defects and Polarity in GaNGa-polarity5 µmN-Polarity2 µmFig. 4 Fig. 5Fig. 4. AFM image of Ga-polar GaN sample. The rms roughness is 1.3 nm. The vertical scaleranges from 0 to 20 nmFig. 5. AFM image of N-polar GaN sample. The rms roughness is 20 nm. The vertical scaleranges from 0 to 80 nm10 µmFig. 6. AFM image of N-polar GaN morphology after etch-ing in H3PO4 for 20 s at 160 C. The rms roughness is150 nm. The vertical scale ranges from 0 to 1200 nmConclusion PEC and hot wet etching experiments were carried out to determine thedefect density in GaN films. The density of whiskers formed by the PEC process wassimilar to the density of pits formed by wet etching and close to the DD obtained byTEM. Hot wet etching has been used also to determine the polarity of MBE-grownGaN layers. We found that few seconds of etching in H3PO4 produce a drastic changeof the morphology whereas few minutes result in the complete removal of N-polarGaN film. On the contrary, the hot acid attacks only the defect sites in Ga-polar filmsproducing pits, but leaving the defect-free GaN intact and the morphology unchangedafter several minutes of etching.Acknowledgements The authors would like to thank Prof. K.J. Wynne and Ms. J. Uilkfor the use of large area AFM. The VCU portion was funded by grants from AFOSR(Dr. G.L. Witt), NSF (Drs. L. Hess and G. Pomrenke), and ONR (Drs. C.E. C. Wood andY.S. Park). LBNL portion of this work was supported in part by Air Force Office ofScientific Research, through the U.S. Department of Energy under Order No.AFOSR-ISSA-00-0011. J.J. and Z.L.W. thank NCEM at Berkeley for the use of TEM facility.References[1] H. Morkoç, Nitride Semiconductors and Devices, Springer Verlag, Berlin/Heidelberg/NewYork 1999.[2] E. S. Hellman, MRS Internet J. Nitride Semicond. Res. 3, 11 (1998).[3] C. Youtsey, L. T. Romano, R. J. Molnar, and I. Adesida, Appl. Phys. Lett. 74, 3537 (1999).[4] P. Visconti, K. M. Jones, M. A. Reshchikov, R. Cingolani, H. Morkoç, and R.J. Molnar, Appl.Phys. Lett. 77, 3532 (2000).[5] M. Seelmann-Eggebert, J. L. Weyher, H. Obloh, H. Zimmermann, A. Rar, and S. Porowski,Appl. Phys. Lett. 71, 2635 (1997).[6] A. R. Smith, R. M. Feenstra, D. W. Greve, M.-S. Shin, M. Skowronski, J. Neugebauer, andJ. E. Northrup, Appl. Phys. Lett. 72, 2114 (1998).[7] J.L. Rouviere, J.L. Weyher, M. Seelmann-Eggebert, and S. Porowski, Appl. Phys. Lett. 73, 668(1998).[8] D. Huang, P. Visconti, K. M. Jones, M. A. Reshchikov, F. Yun, A. A. Baski, T. King, andH. Morkoç, Appl. Phys. Lett. 78, 4145 (2001).[9] P. Visconti, M. A. Reshchikov, K. M. Jones, F. Yun, D. F. Wang, R. Cingolani, and H. Mor-koç, Proc. MRS Fall Meeting, Boston, Nov. 2000, Vol. 639, no. G3.14.[10] P. Visconti, M. A. Reshchikov, K. M. Jones, D. F. Wang, R. Cingolani, H. Morkoç, R. J. Mol-nar, and D. J. Smith, J. Vac. Sci. Technol. B 19, 1328 (2001).phys. stat. sol. (b) 228, No. 2 (2001) 517

Investigation of Defects and Polarity in GaN using hot wet Etching, Atomic Force and Transmission Electron Microscopy and Convergent Beam Electron Diffraction

Manson-Smith, S.K.

Trager-Cowan, C.

O'Donnell, K.P.

University of Strathclyde Institutional Repository

Scanning tunnelling luminescence studies of nitride semiconductor thin films under ambient conditions

The polarization dependence of biexcitonic signals and quantum beats between A-excitons (X-A) and A-biexcitons (XAXA) in a high-quality GaN epilayer is measured by spectrally-resolved and time-integrated four-wave mixing. With cross-linear polarised light, mixed beats with two periods are observed: the first beating period corresponds to the energy splitting between X-A and XAXA, and agrees well with the calculated XAXA binding energy, while the second beating period corresponds to that between X-A and donor bound excitons ((DX)-X-0). The temperature-dependent homogeneous linewidth shows that the (DX)-X-0 has a larger acoustic phonon coupling coefficient than the XAXA. We also measured the polarization dependent B-biexciton (XBXB) signal. The effective masses for the A- and B-hole were deduced from the binding energy

Kyhm, K

Taylor, RA

Ryan, JF

Aoki, T

Kuwata-Gonokami, M

Beaumont, B

Gibart, P

Oxford University Research Archive (ORA)

Comparison of exciton-biexciton with bound exciton-biexciton dynamics in GaN: Quantum beats and temperature dependence of the acoustic-phonon interaction

Scanning tunnelling luminescence studies of nitride semiconductor thin films under ambient conditions

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