Microstructure and optical properties of GaN films

Mat. Res. Soc. Symp. Proc. Vol. 719 © 2002 Materials Research Society

Microstructure and optical properties of GaN films grown on porous SiC substrate by MBE F. Yun, M. A. Reshchikov, L. He, T. King, D. Huang, H. Morkoç, C. K. Inoki1, and T. S. Kuan1 Virginia Commonwealth University, Dept. of Electrical Engineering, Richmond, VA23284 1 Univ at Albany, SUNY, Dept of Physics, Albany, NY 12222 ABSTRACT GaN thin films were grown on porous SiC substrates using reactive molecular beam epitaxy with ammonia as the nitrogen source. Microstructure analysis and optical characterization were performed to assess the quality of the effect of pores on the growth and the quality of the GaN films. Results indicate that the GaN films on porous SiC are slightly less defective and more strain-relaxed (some completely relaxed) when grown on porous SiC substrate, as compared to growth on standard 6H-SiC substrates. Rocking curve FWHMs of 3.3 arcmin for (0002) diffraction and 13.7 arcmin for (10 1 2) diffraction were obtained for sub-micron thick GaN films. Excitonic transition with FWHM as narrow as 9.5 meV was observed at 15K on the GaN layer grown on porous SiC without a skin layer. INTRODUCTION High thermal conductivity of SiC substrates has provided the impetus for exploring GaN growth on SiC.1,2,3,4 In addition to the much better thermal conductivity of SiC as compared to the most commonly used substrate sapphire, the lattice mismatch between SiC and AlN is very small, and that between SiC and GaN is much smaller compared to sapphire. As alluded to earlier, the high thermal conductivity of the SiC substrate allows the fabrication of high power/temperature electronics devices. It also exhibits robust mechanical and chemical properties.5 Beside heat removal, high density of threading dislocations and point defects, and to some extent strain, in GaN films represent a bottleneck for improving device performance. To relieve the strain, researchers have opted to introduce multiple buffer layers,6 or grow GaN on GaN nano-columns formed on sapphire substrate.7 As for threading dislocations, approaches such as growth of GaN epilayers on top of porous materials, such as porous SiC (PSC) substrates, are being explored. PSC is of practical interest because it can be readily prepared by electrochemical anodization of conducting SiC substrates and in particular 6H-SiC (0001) substrates which were exploited in this work. There have been reports of SiC growth on PSC substrates8,9 by CVD, and GaN growth on porous GaN layer on top of SiC substrates.10,11 In all of these cases, growth on PSC was accompanied with improvements in defect reduction and stress alleviation. It is therefore interesting to study the growth of GaN directly on PSC substrate. With the formation of nanometer-scale pores, it is expected that the molecular beam epitaxial (MBE) growth of GaN on the PSC surface using ammonia nitrogen source might lead to lateral epitaxial overgrowth (LEO) as in metalorganic chemical vapor deposition (MOCVD), which leads to extended defect reduction. Ammonia based growth exhibits lateral growth, albeit to a smaller extent than MOCVD, and LEO on nanoscale with the aid of nanosized pores is of great interest. In this paper, we report the growth and results of our investigation on microstructure and optical properties of GaN films grown by MBE on PSC substrates, and compare the results with those obtained from GaN layers grown on nominal 6H-SiC substrates. F1.3.1

EXPERIMENTAL Nanometer scale (10-100nm) pores were formed by anodization in a dilute solution of HF. The process was carried out in an electrochemical cell under UV illumination. Pt was used as the cathode, while the substrate served as the anode. The GaN films were all grown under Ga-rich conditions to a thickness of 0.1-1.4 µm, on different types of PSC substrates. They include, a) on-axis (0001) PSC with a skin layer (~60 nm); b) off axis (8° miscut towards (11 2 0)) PSC with skin layer; and c) on-axis (0001) PSC with skin layer etched away in H at high temperature. MBE growth temperature was kept at 650°C, and the nitrogen source was ammonia. For thicker GaN films (>0.5 µm), a thin AlN buffer layer (~50 nm) was inserted between the GaN epilayer and the substrate. For comparison, we also grew a GaN layer with ammonia source on a standard 6H-SiC substrate at T=585°C, which was optimized for GaN/6H-SiC growth. High-resolution X-ray diffraction (XRD) was measured using a Philips MRD system equipped with Ge [220] four-crystal monochrometer. Atomic force microscopy (AFM) images were taken with Digital Instrument microscope. Cross-sectional transmission electron microscopy (TEM) technique was used to study the microstructure and dislocation density of GaN grown on PSC and 6H-SiC. Photoluminescence (PL) measurements were taken at 15K using a He-Cd laser (λ=325 nm) and a photomultiplier with photon counting.

RESULTS AND DISCUSSIONS GaN epitaxial layers were grown on porous SiC substrates, with and without skin layers, with ammonia MBE. We observed an improved surface smoothness on PSC even for a very thin layer of GaN (0.11 µm), as compared to the surface of GaN on 6H-SiC with comparable thickness (0.15 µm). From the AFM images shown in Fig. 1, it is seen that the surface of GaN layer grown on H-polished 6H-SiC substrate (Fig. 1(a)) exhibits well-coalesced step flow growth pattern with curved bi-layer step ledges which are terminated mostly likely at defects. The root-mean-square (rms) roughness of the 2x2µm image is 2.21 nm. On the other hand, the surface morphology of GaN on PSC indicates a uniform growth without steps in the image (Fig. 1(b)), while the curved ledge edges remain similar. It should be noted here that the small dark pits in the image are likely the defect sites. The image shown in Fig. 1(b) was taken after the removal of excess Ga droplets formed as a result of Ga-rich conditions employed. The rms roughness improved to 0.67 nm for the GaN layer grown on PSC. XRD pole figure indicates the film to be a highly oriented GaN crystal structure with (0001) 2x2 µm

2x2 µm

(a)

(b)

Fig. 1. AFM images of GaN films grown on (a) 6H-SiC substrate, and (b) PSC substrates. Vertical scales are (a) 30 nm, (b) 10 nm.

F1.3.2

direction aligned with (0001) direction of the substrate, i.e. the GaN film is 8 degrees off-axis towards (11 2 0) for the miscut PSC substrate, and is on-axis for other on-axis substrates. The best FWHM of the (0002) rocking curve diffraction was 3.3 arcmin, and that of (10 1 2) was 13.7 arcmin. It is remarkable that such sharp linewidths were obtained in ~0.1µm GaN film, which reflects reduction of defect density if one assumes that screw dislocation contributes mainly to (0002) diffraction, and edge dislocation dominates the linewidth of (10 1 2) diffraction. This is a promising indication that device quality GaN film can be obtained in very thin layers grown on PSC substrate.

(a)

(c)

(b)

(d)

(e)

Fig. 2. TEM images of (a) GaN grown on 6H-SiC, (b) pores in PSC substrate, (d) GaN on PSC with skin layer, (e) GaN on PSC with skin layer removed. Electron diffraction, (c), is used to extract strain relaxation in each layer.

Cross-sectional TEM images were taken on all three types of samples, namely, GaN layer grown on standard 6H-SiC substrate, on PSC substrate with skin layer, and on PSC substrate with skin layer removed prior to growth. Figure 2 shows the TEM images as well as the electron diffraction patterns indicating the strain conditions for each growth. From the images (Fig. 2(b) F1.3.3

and (e)), the porous structure of the PSC substrate is clearly observed, with porous template thickness of ~2.8 µm, and two types of discernable pore sizing around 10 and 100 nm each. Most of these pores do not run along the c-direction, instead, they start from the substrate surface and penetrate into the substrate like cone shape chains, which extend in lateral directions of the substrate. In Fig. 2(d), a very thin skin layer covering the pores at the substrate interface can be identified underneath the AlN buffer layer. In Fig. 2(e), it is clear that this skin layer has been removed (by the high temperature H-etching employed following the process to render the substrate porous), leaving the AlN buffer layer directly grown on the porous network of SiC. The dislocation distribution in the GaN layers grown on top of these three substrates is quite different. For GaN layer grown on standard 6H-SiC substrate (Fig. 2(a)), a dislocation density of about 1x1010cm-2 is observed, together with twins forming on the c-plane of GaN layer. For the GaN layer grown on PSC substrate with skin layer, we observe from Fig. 2(d) that most of the structural features are threading dislocations, some of them emanate from the substrate pores, others from the AlN buffer layer. Along the growth direction, some of the dislocations loop, bend, merge, and annihilate one another, during the early stage of growth, while others thread through the epilayer. The total density of dislocations in the 1.4 µm thick GaN film is estimated to be ~5x109 cm-2. When the skin layer (non-porous layer) is removed from the PSC surface, the growth quality of GaN epilayer has significantly improved over that with skin layer. As shown in Fig. 2(e), the 0.77 µm GaN film looks less defective, with a total dislocation density of about ~1x109 cm-2, though the interface is much roughened due to the presence of large quantity of porous structure exposed on the surface after etching. It can be also observed that some of the exposed pores are backfilled with Al and/or Ga droplets during the MBE growth.

Fig. 3. TEM image of GaN grown on tilted PSC substrate 8° toward (11 2 0). Non-growth open tubes (arrow) and misfit lines can be identified. Ga/Al backfilling is shown in the image.

From the TEM image, a tentative growth mechanism can be suggested, though speculative at this juncture, for GaN growth on PSC. At the beginning of growth, AlN buffer grows on the nonporours regions of SiC, including possibly the sidewalls of SiC pores, leading to columns of AlN. This is then followed by GaN growth on top of the AlN columns. In an enlarged TEM image in Fig. 3, one can see the formation of open tubes (marked by arrows) in the initial stages of GaN growth, on top of the exposed pores. As the growth progresses, lateral epitaxy is enhanced, so we see the closing of some of the open tubes at the upper part of the GaN layer. These tubes may serve as relief mechanism of strain caused by the lattice and thermal mismatch during growth.

F1.3.4

The degree of strain relaxation in each type of the films was examined by electron diffraction pattern, as exemplified in Fig 2(d). Keeping in mind that the in-plane lattice mismatch between GaN and SiC crystals is ∆a / a = 3.42%, for GaN grown on standard 6H-SiC, the 0.15µm film shows a compressive in-plane lattice mismatch of ∆a / a = 3.1%, while a 0.11µm GaN film grown on PSC substrate shows a less compressive in-plain mismatch of ∆a / a = 3.2%. These numbers translate into 90% and 92% strain relaxation respectively. The thicker GaN films grown on PSC, both with and without skin layer, have 100% relaxation, i.e. ∆a / a = 3.5%, as determined by their electron diffraction patterns. The above results indicate that GaN layer is less strained when grown on PSC substrate than on 6H-SiC substrate. The effects on the GaN layer of miscut PSC substrate are worth discussing. First, we note that, for the same growth parameters used in MBE, the GaN growth rate on miscut PSC substrate was only about ½ of that on c-plane (on-axis) PSC substrate (0.38µm/hr vs. 0.70µm/hr). If this effect is only caused by the misorientation of the substrate, it may be useful in studying the anisotropic growth of GaN on PSC. We need to verify that the substrate temperatures were identical as the ammonia cracking efficiency is an activated process. 12 Secondly, the presence of misfit dislocations was observed at the tilted GaN/PSC interface and the horizontal misfit lines (Fig. 3), due to the 8-degree miscut towards (11 2 0) orientation. These additional dislocation sites could also have adverse effect on the optical properties. Fig. 4 shows the PL spectra measured at 15K for samples corresponding to those depicted in Fig. 2(b) and (c), i.e. GaN grown on PSC with or without skin layer. The spectra show the best FWHM of GaN excitonic peak to be ~9.5 meV for GaN grown on PSC with the skin layer removed, while GaN films with the skin layer exhibit a FWHM of 14-16 meV. The quantum efficiency of PL from thick layers (both with and without skin layer) is quite high (about 1.6%), whereas that from the thin layer is low (about 0.1%), indicating a reduction of nonradiative defects (presumably dislocations) in thicker layers. The features of the PL spectra for all three 10

9

T = 15 K

PL Intensity (rel. units)

Sam ple

10

8

10

7

10

6

10

5

10

4

1 2 3

2

2.5

3

3.5

Photon Energy (eV)

Fig. 4. Low-temperature PL spectra of GaN grown on porous SiC. Sample 1— 0.11 µm-thick GaN layer; sample 2 — 1.4 µm-thick GaN layer; sample 3 — 0.77 µm-thick GaN layer with etched skin layer of SiC.

samples are similar in the sense that they include excitonic emission, shallow donor-acceptor band with the main peak at about 3.26 eV, and a very weak yellow luminescence band at about F1.3.5

2.2 – 2.3 eV. The overall quality of the samples is good, especially for the GaN layer grown on top of the porous SiC with skin layer removed (high radiative efficiency, narrow excitonic peak, and smaller contribution from the defect-related PL bands). SUMMARY We have characterized GaN epitaxial films grown on different PSC substrates, both with and without the non-porous skin layer, for their structural and optical properties. Discernable reduction in dislocation density was demonstrated in GaN grown on PSC with the skin layer removed prior to the growth of GaN. Specifically, the dislocation density is reduced by an order of magnitude (to ~1x109cm-2) when compared with GaN grown on standard 6H-SiC substrates. The reasonably smooth surface morphology, narrow x-ray linewidth, and sharp and intense PL, suggest that GaN films grown on PSC substrates may be beneficial to high power, high temperature devices based on GaN. ACKNOWLEDGEMENTS The authors would like to thank Dr. C. D. Lee and Prof. R. M. Feestra for cutting and Hetching of SiC substrates and reading of the manuscript. The work at both institutions was funded under a DoD DURINT program and monitored by C. E. C. Wood of ONR. The work at VCU also benefited from grants by AFOSR, NSF and ONR. REFERENCES 1 2

3

4 5 6

7 8 9

10

11

12

S.T. Strite and H. Morkoç, J. Vac. Sci & Technol., B10, 1237(1992). M.E. Lin, S. Strite, A. Agarwal, A. Salvador, G.L. Zhou, N. Teraguchi, A. Rockett and H. Morkoç, Appl. Phys. Letts. 62, 702(1993). H. Morkoç, “Nitride Semiconductors and Devices”, Springer Verlag 1999. ISSN 0933-033x, ISBN 3-54064038. H. Morkoç, in "Wide Energy Bandgap Electronics" Eds. S. Pearton and F. Ren, World Scientific, in press. V. E. Chelnokov, A. L. Syrkin, and V. A. Dmitriev, Diamond Relat. Mater. 6, 1480(1997). Y. Ishihara, J. Yamamoto, M. Kurimoto, T. Takano, T. Honda, H. Kawanishi, Jpn. J. Appl. Phys. 38, L1296(1999). K. Kusakabe, A. Kikuchi, and K. Kishino, J. Cryst. Growth, in press, 2002. J. E. Spanier, G. T. Dunne, L. B. Rowland, and I. P. Herman, Appl. Phys. Lett. 76, 3879(2000). M. Mynbaeva, S. E. Saddow, G. Melnychuk, I. Nikitina, M. Scheglov, A. Sitnikova, N. Kuznetsov, K. Mynbaev, and V. Dmitriev, Appl. Phys. Lett. 78, 117(2001). A. Nikolaev, Y. Melnik, M. Blashenkv, N. Kuznetsov, I. Nikitina, A. Zubrilov, D. Tsvetkov, V. Nikolaev, V. Dmitriev, and V. Soloviev, MRS Internet J. Nitride Semicond. Res. 1, 45(1996). M. Mynbaeva, A. Titkov, A. Kryganovskii, V. Ratnikov, K. Mynbaev, H. Huhtinen, R. Laiho, and V. Dmitriev, Appl. Phys. Lett. 76, 1113(2000). Currently, we do not exclude the possibility of some fluctuations in growth temperatures which could strongly affect the available reactive N for growth due to the fact that NH3 cracking rate is exponentially dependent on temperature.

F1.3.6