A. Passaseo, E. Piscopiello, A. Pomarico and R. Cingolani. 1. Istituto Nazionale di Fisica della Materia, Unita' di Lecce and Dipartimento di Ingegneria.
Mat. Res. Soc. Symp. Proc. Vol. 680E © 2001 Materials Research Society
Structural, optical and electrical properties of GaN films grown by metalorganic chemical vapor deposition on sapphire. P. Visconti 1,2, M. A. Reshchikov, F. Yun, K. M. Jones and H. Morkoç Virginia Commonwealth University, Dept. of Electrical Engineering, Richmond, VA, 23284
A. Passaseo, E. Piscopiello, A. Pomarico and R. Cingolani 1
Istituto Nazionale di Fisica della Materia, Unita’ di Lecce and Dipartimento di Ingegneria dell’Innovazione, Universita’ di Lecce, 73100, Lecce, ITALY.
M. Lomascolo, M. Catalano 2
Istituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR, Via Arnesano, 73100, Lecce, ITALY.
ABSTRACT Properties of GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on cplane of sapphire have been investigated using atomic force microscopy (AFM), wet etching for defect investigation, transmission electron microscopy (TEM), high-resolution X-ray diffraction, Hall effect measurements and low-temperature photoluminescence (PL). Tapping-mode AFM images of the as-grown samples showed atomically smooth surfaces (rms roughness ≈ 0.2 nm) consisting of terraces separated by about 3Å bi-layer steps. Hot H3PO4 chemical etching was used to produce hexagonal-shaped etch pits at the surface defect sites as revealed by AFM imaging. The obtained etch pit densities (9x108 - 2 x109 cm-2) were in agreement with the dislocation density found by plan-view and cross-sectional TEM observations. The full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve was about 4.8 and 3.9 arcmin for the symmetric (002) and asymmetric (104) directions, respectively. PL spectrum at 15 K demonstrated sharp peaks (FWHM ≈ 4 meV) in the excitonic region, which were attributed to free and bound excitons. The spectrum contained also weak PL bands with maxima at about 2.2, 2.9 and 3.27 eV, which have been attributed to three different acceptors. INTRODUCTION Nitride semiconductors and their heterostructures are very promising materials for optical emitters and detectors, and high power/temperature electronic devices [1,2]. They have been deposited by hydride vapor phase epitaxy (HVPE) , metalorganic chemical vapor deposition (MOCVD) , and by molecular beam epitaxy (MBE) . The most commonly used substrates, such as sapphire and silicon carbide poorly match with GaN material resulting in a high density of threading dislocations. These defects are believed to affect both optical and electrical properties and therefore to be responsible, for instance, for the high threshold-current required to achieve lasing in diodes. For this reason, to get to a better understanding of the material quality, a thorough characterization should be made. We have carried out a comprehensive analysis of the structural, optical and electrical properties of GaN films grown by MOCVD on sapphire substrates. E3.8.1
EXPERIMENTAL DETAILS The epitaxial growth of the unintentionally doped GaN layers was performed in a horizontal low pressure (LP)-MOCVD system (AIXTRON 200 AIX RF), equipped with a rotating substrate holder, heated by radio-frequency (RF) induction. Palladium purified H2 with a flow rate of 5.4 slm was used as a carrier gas, TMGa and pure ammonia (NH3) as source materials. The growth was performed on (0001) c-plane Al2O3 substrates cleaned in solvents, and then annealed in situ at 1100 °C under H2 flow. After the deposition of a 50 nm thick low temperature (T = 560 °C) GaN nucleation layer, the GaN epilayers were grown at temperature of 1150 °C and pressure of 50 mbar with a growth rate of 2 µm/h. The thickness of these films varied between 1.5 and 4µm. Investigation of the surface morphology was carried out using a Digital Instruments atomic force microscope (AFM) in tapping mode. Hot H3PO4-based chemical etching was used to determine the surface defect density by producing etch pits at the defect sites. Roomtemperature Hall effect measurements were made on the MOCVD grown GaN films using a Van der Pauw geometry patterns. Ohmic contact was made possible by evaporating metal compositions of Ti(300Å)/Al(1000Å)/Ti(300Å)/Au(200Å) and annealing at 900°C under N2 ambient. The samples were n-type with a free carrier concentration in the range 1×1017 - 2×1018 cm-3 and a mobility of about 70 cm2/Vs at room temperature. High resolution X-ray diffraction was performed using a Philips X’Pert MRD system equipped with a four-crystal Ge (220) monochrometer. The instrument has a resolution better than 10 arcsec under our measurement configuration. The CuKα1 line of X-ray source is used with a divergence slit of 2mm. There is no slit in the detector side. A JEOL 4000 FX analytical microscope operating at 400 kV accelerating voltage, with an interpretable resolution of 2.3nm and equipped with a Gatan Imaging Filter (GIF) was used for TEM work. TEM data were obtained by acquiring energy-selected image series over the electron energy-loss range of interest, by using an energy-step and energy-selecting slit width of 20 eV. PL was excited with He-Cd laser (325 nm) and was analyzed with grating spectrometer and photomultiplier tube. Unfocused laser beam gave an excitation density of about 0.1 W/cm2. The samples were attached to the cold finger of the closed cycle cryostat to obtain the sample temperature of about 15 K. RESULTS AND DISCUSSION The surface morphology of the MOCVD-grown layers has been investigated by tapping mode AFM. The AFM images of figure 1a and 1b (respectively for GaN bulk layers with thickness of 4 and 1.5 µm) reveal atomically smooth as-grown surfaces consisting of terraces separated by ~3Å high bi-layer steps. The rms roughness is ~0.2 nm (vertical scale = 3nm). The surface is atomically smooth and no pits are revealed at the surface step terminations. We have used H3PO4 based hot wet etching to determine the surface defect density in the MOCVD GaN samples. By varying the time and temperature, we were able to optimize the etching process to reveal clearly, by AFM imaging, the size and density of the pits formed at the defect sites. During the etching process, a careful balance must be struck to ensure that every defect is delineated, but not over-etched to avoid underestimating the defect density [6,7,8]. Shown in figure 2a and 2b (5x5 and 2x2 µm2 respectively) is the surface morphology after etching in H3PO4 for 5 minutes at 160 oC for two GaN films with thickness of 1.5 and 4µm, respectively. The etch pits, with density of about 8x108 - 1x109 cm-2, are of hexagonal shape and their size ranges from 30 to 200 nm in diameter and from 10 to 50 nm in depth. E3.8.2
5µm x 5 µm
2µm x 2µm
FIG. 1 (a) AFM image (5x5 µm2) of the as-MOCVD-grown GaN sample (thickness =4µm). The smooth surface consists of terraces separated by 3Å high bi-layer steps. The rms roughness is ~0.2 nm. (b) AFM image of the 1.5µm thick MOCVD -grown GaN film. The vertical scale ranges from 0 to 3 nm for both AFM images.
5µm x 5 µm
2µm x 2µm
FIG.2 (a) AFM image (5x5 µm2) of the surface morphology (sample 1.5µm thick) after etching in H3PO4 for 5 min. at 160 C. The EPD is ~1x109 cm-2. The vertical scale ranges from 0 to 20 nm. (b) AFM image (2x2 µm2) of the surface morphology (sample 4µm thick) after etching in H3PO4 for 5 minutes at 160 oC. The EPD is ~8x108 cm-2. The vertical scale ranges from 0 to 10 nm. o
X-ray diffraction rocking curves (omega scan) demonstrate the high crystalline quality of the GaN films, with the full width at half maximum (FWHM) of the (0002) symmetric peak in the range of 4.6-5.0 arcmin, and the FWHM of (10-14) asymmetric peak in the range of 3.6-4.0 arcmin. The well-defined Gaussian shape of the diffraction curves also indicates the absence of influence of relative tilting or mosaic structure inside the layer, in contrast to the very broad and multi-peak feature  of the reported freestanding GaN template. Plan-view and cross-sectional TEM analysis was carried out to investigate the structural defects embedded in the MOCVD GaN films and to estimate their density (Fig. 3 and 4). Figure 3a is a bright field (BF) cross-sectional image of the sample, showing the defects present. All the defects in the GaN epilayer originate from those in the GaN buffer layer, showing typical contrast features of some defects already reported in literature [10,11,12,13,14]. Some of these, start at the GaN buffer layer and stop at the GaN epilayer, others start at the buffer layer and propagate through the entire GaN epitaxial layer (threading defects). Defects belonging to the former class present a typical dislocation-like contrast. Conventional diffraction contrast analysis E3.8.3
showed that dislocations with Burgers vectors parallel to c, c + a and a were present, as already reported by Ponce et al. for GaN homoepitaxial films grown by MOCVD . Concerning the latter class of defects, the threading defects, some of those, show a novel and complex contrast, not reported in literature, which is the combination of some contrast features already observed for other defects, like inversion domains, nanotubes and dislocations . A more accurate characterisation of these threading defects, to confirm our hypothesis, is still in progress. Multiple dark field imaging along the noncentrosymmetric [11-20] zone axis was used to reveal inversion domains (for more details see ref. ). In the multiple dark field images, obtained with the primary beam oriented parallel to the [11-20] zone axis, some of the threading defects appear bright when imaged with g = +(0002) and dark with g = -(0002) in comparison to the surrounding matrix. No contrast difference between the domain and the matrix was observed in bright field imaging. This is the typical behaviour of inversion domains.
GaN buffer 100 nm
FIG. 3 (a) Bright field cross-sectional TEM image of the sample with (0002) reflection excited. All the defects observed in the GaN epilayer originate from those in the GaN buffer layer. (b) High magnification cross-sectional TEM image taken near the top Ga-terminated surface of the GaN film.
FIG. 4(a) and (b) Two bright field low-magnification planar TEM images taken in the  zone axis from the GaN sample. From plan-view study, we estimated the total density of defects to be about 3 109 cm-2.
The same kind of defect (i.e. with contrast features like inversion domains), when imaged in the [0-110] zone axis, shows a peculiar contrast when imaged respectively over and underfocus. The defects are bright when imaged over focus and dark when imaged under focus. The relative strong phase contrast can be attributed to a different density which in turn can be related E3.8.4
to the different stoichiometric composition of the defect with respect to the surrounding matrix. In order to investigate the nature of these defects, a sample in planar geometry was prepared. TEM observations in the plan-view geometry revealed the presence of a symmetric strain field associated with the threading defects, showing the classical non contrast line normal to the operating reflection, as can be seen from the picture in fig.4b (see arrows). All these features confirm the complex structure of the defect, whose nature requires further investigation to be elucidated. On the basis of the available results we can move the working hypothesis that this kind of defect is constituted by cylindrical inversion domain, with a deficient stoichiometry giving rise to a lower density and, consequently, to a cylindrically symmetric strain field. In conclusion, by TEM analysis in planar geometry (Fig. 4a) we estimated the total surface defect density to be about 3x109 cm-2, very close to the etch pit density (EPD) calculated by defect revealing wet chemical etching. Low-temperature PL spectrum of one of the MOCVD-grown GaN samples is shown in figure 5. The peaks at 3.486 eV and 3.493 eV (the latter is seen as a shoulder) have been attributed to the free A and B excitons, respectively. A peak at 3.479 eV with the FWHM of about 4 meV was related to the neutral donor-bound exciton. A shoulder at about 3.45 – 3.46 eV can be attributed to acceptor-bound exciton . Two peaks at about 3.400 and 3.305 eV, more pronounced at high excitation power, are phonon replicas (with one and two LO phonons, respectively), unresolved from several excitonic transitions. The characteristic emission with few peaks in the range 2.9 – 3.3 eV is associated with the shallow donor-acceptor pair (DAP) transitions. In the main peak of the DAP band we can distinguish two small peaks at 3.275 and 3.29 eV. The latter is probably due to transitions from the conduction band to the shallow acceptor. The DAP band, as well as the yellow band (a broad band with a maximum at about 2.25 eV) saturate with increasing excitation power (figure 5) so that the DAP band disappears in the high-power PL spectrum. Instead, a new broad band with a maximum at about 2.88 eV emerges in the blue region of the spectrum. The blue band has been previously observed in undoped GaN grown by MOCVD and related to a native defect involving a gallium vacancy . From the optical characterization of the studied samples we conclude that the samples have good optical quality. This is evident from relatively small widths of the exciton lines and from the saturation of intensity of defect-related PL with increasing excitation power.
PL Intensity (a.u.)
TT = 15 3 0KK E xc ita tion D e nsity
3 .4 5
1 0 0 W /c m
0.1 W /c m
2 .5 3 P h o to n E n e rgy (e V )
FIG. 5. Low temperature photoluminescence spectrum of the MOCVD-grown GaN sample. The inset shows an excitonic part of the spectrum at excitation density of 0.1 W/cm2.
CONCLUSIONS Structural, electrical and optical properties of MOCVD grown GaN layers were reported. Tapping-mode AFM images showed atomically smooth surfaces (rms roughness ≈ 0.2 nm) consisting of terraces separated by about 3Å bi-layer steps. Hot H3PO4 chemical etching revealed hexagonal-shaped etch pits with densities (9x108 - 2 x109 cm-2), in good agreement with planview and cross-sectional TEM observations. The FWHM of the X-ray diffraction rocking curve was about 4.8 and 3.9 arcmin for the symmetric (002) and asymmetric (104) directions, respectively. PL spectrum at 15 K demonstrated sharp free and bound excitonic peaks (FWHM ≈ 4 meV). The spectrum contained also weak PL bands with maxima at about 2.2, 2.9 and 3.27 eV, which have been attributed to three different acceptors. ACKNOWLEDGMENTS The authors would like to thank Prof. A. Baski for collaboration, Dr. D. F. Wang for sample preparation, and D. Cannoletta, I. Tarantini and T. King for his tireless technical assistance. The VCU portion of this work was funded by grants from AFOSR (Dr. G. L. Witt), NSF (Drs. L. Hess and G. Pomrenke), and ONR (Drs. C. E. C. Wood and Y. S. Park). REFERENCES  H. Morkoç, Nitride Semiconductors and Devices (Springer, Heidelberg, 1999).  H. Morkoç, A. Di Carlo and R. Cingolani, Condensed Matter News (in press).  R. J. Molnar, W. Goetz, L. T. Romano, N. M. Johnson, J. Cryst. Growth 178, 147 (1997).  H.M. Manasevit, F.M. Erdmann and W.I. Simpson, J. Electrochem. Soc. 118, 1864 (1971).  S. Yoshida, S. Misawa and A. Itoh, Appl. Phys. Lett. 26, 461 (1975).  P. Visconti, K. M. Jones, M. A. Reshchikov, R. Cingolani, H. Morkoç and R. J. Molnar, Appl. Phys. Lett. 77, 3532, (2000).  P. Visconti, K. M. Jones, M. A. Reshchikov, F. Yun, R. Cingolani, H. Morkoç, S. S. Park and K. Y. Lee, Appl. Phys. Lett. 77, 3743, (2000).  J. Jasinski, W. Swider, Z. Liliental-Weber, P. Visconti, K. M. Jones, M. A. Reshchikov, F. Yun, H. Morkoç, S. S. Park and K. Y. Lee, Appl. Phys. Lett. 78, 2297, (2001).  F. Yun, M. A. Reshchikov, K. Jones, P. Visconti, H. Morkoç, S. S. Park, and K. Y. Lee, Solid-State Electronics 44, 2225 (2000).33  F. A. Ponce, J. S. Major, W. E. Plano, and D. F. Welch, Appl. Phys. Lett. 65, 2302 (1994).  P. Ruterana, G, Nouet, W. Van der Stricht, I. Moerman, and L. Considine, Appl. Phys. Lett. 72, 1742 (1998) ; D. Doppalapudi, S. N. Basu, and T. D. Moustakas J. Appl. Phys. 84, 1389 (1998).  L. T. Romano, B. S. Krusor, M. D. McCluskey, D. P. Bour, and K. Nauka, Appl. Phys. Lett. 73, 1757 (1998).  Z. Liliental-Weber, Y. Chen, S. Ruvimov, and J. Washburn, Phys. Rev. Lett. 79, 2835 (1997).  X. H. Wu, C. R. Elsass, A. Abare, M. Mack, S. Keller, P. M. Petroff, S. P. DenBaars, J. S. Speck, and S. J. Rosner, Appl. Phys. Lett. 72, 692 (1998).  F. A. Ponce, D. Cherns, W. T. Young, and J. W. Steeds, Appl. Phys. Lett. 69 (6), 770 (1996).  E. Piscopiello, M.Catalano, M. Vittori Antisari, A. Passaseo, R. Cingolani, M. Berti, A. V. Drigo, Proc. Royal Microscopy Society (2001), to be published.  G. Pozina, J. P. Bergman, T. Paskova, and B. Monemar, Appl. Phys. Lett. 75, 4124 (1999).  M. A. Reshchikov, F. Shahedipour, R. Y. Korotkov, B. W. Wessels, and M. P. Ulmer, J. Appl. Phys. 87, 3351 (2000).