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Quantification of unintentional doping in non-polar GaN using scanning capacitance microscopy

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Quantification of unintentional doping in non-polar GaN using scanning capacitance microscopy
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     © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim pss current topics in solidstatephysics c      s      t     a      t     u     s     s     o      l      i      d      i   www.pss-c.com      p      h     y     s      i     c     a Phys. Status Solidi C 7 , No. 7–8, 1875–1877 (2010) /   DOI   10.1002/pssc.200983523   Quantification of unintentional doping in non-polar GaN using scanning capacitance microscopy Tongtong Zhu * , Menno J. Kappers, Michelle A. Moram, and Rachel A. Oliver Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Received 30 September 2009, accepted 3 March 2010 Published online 28 April 2010 Keywords  GaN, MOCVD, doping, electrical properties, annealing   * Corresponding author: e-mail tz234@cam.ac.uk, Phone: +44 1223 334368, Fax: +44 1223 334437 Unintentional doping in non-polar a -plane (11-20) gal-lium nitride (GaN) grown on r  -plane (1-120) sapphire has been characterized using scanning capacitance mi-croscopy. A conductive interface layer has been observed adjacent to the GaN/sapphire interface, and its average thickness has been quantified and found to increase as the duration of an initial period of three-dimensional growth is increased. By using a Si doped calibration staircase structure, the conductive interface region is found to con-tain an average carrier concentration of (2.5 ± 0.3) x 10 18  cm -3 . We also observe unintentionally-doped features ex-tending at 60° from the GaN/sapphire interface. We sug-gest that these inclined features may be due to impurity in-corporation around prismatic stacking faults. The carrier concentration decreases along the inclined feature with dis-tance from the GaN/sapphire interface. In addition, the in-clined features are found more frequently in a sample sub- jected to an additional annealing process, which may pro-vide evidence that the unintentional doping arises from oxygen diffusion from the sapphire substrate.  © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction  Growth of a -plane (11-20) gallium ni-tride (GaN) is one of the promising paths for achieving higher efficiencies for GaN-based optoelectronic devices [1] and doping level dependence of GaN-based electronic device properties [2], since devices grown on this non- polar orientation exhibit reduced internal electric fields. GaN grown on c -plane (0001) sapphire is often found to be unintentionally n-doped [3], with samples with the lowest dislocation density exhibiting the largest unintentionally-doped region [4]. It has been demonstrated that reduced dislocation density and unintentional doping in GaN-based electronic devices will lead to improved device perform-ance due to lower leakage currents and higher electron mobility [5]. In order to improve the performance of elec-tronic devices based on non-polar GaN, it is essential to understand and control the unintentional doping in this ma-terial. Scanning capacitance microscopy (SCM) has been recognized as an ideal tool to assess unintentional doping in GaN [6], given that it can provide two-dimensional in-formation on carrier concentration variation. Additionally, SCM amplitude and phase data measure the carrier con-centration and carrier type separately, providing greater in-sight than other scanning probe microscopy based electri-cal characterization techniques. To gain insight into unintentional doping in non-polar GaN, we focus on the identification and quantification of the unintentional doping in non-polar (11-20) GaN grown on r  -plane (1-120) sapphire by a three-dimensional (3D) – two-dimensional (2D) method. We will investigate a link  between unintentional doping and extended crystal defects. To help clarify the mechanism involved in the uninten-tional doping, we will assess the effect of annealing on un-intentional doping. 2 Experimental details  Non-polar (11-20) GaN samples were grown using a 3D–2D method by metalor-ganic vapour phase epitaxy in a Thomas Swan close-coupled showerhead reactor using trimethyl gallium, silane (at 50 ppm in hydrogen), and ammonia (NH 3 ) as precur-sors with hydrogen (H 2 ) as a carrier gas. After a low temperature nucleation layer was deposited at 540 °C on r  -plane sapphire, 3D islands were grown with a V/III ratio of 1310 at 1020 °C. Then a V/III ratio of 200 was used to promote lateral growth and coalesce the 3D is-lands to form a film. Sample A was grown with a low V/III  1876   Tongtong Zhu et al.: Quantification of unintentional doping in non-polar GaN  © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com        p       h     y     s       i     c     a p s s      s      t     a      t     u     s     s     o       l       i       d       i c ratio only. Sample B was grown with 600 s at high V/III ratio growth followed by 4500 s at low V/III ratio. Sample C was grown with 2400 s at high V/III ratio growth fol-lowed by 5400 s at low V/III ratio. For the other two sam- ples, D and E, 3D islands were grown with 600 s at a high V/III ratio of 1800 followed by a coalescence step at a low V/III ratio of 57 for 2800 s. Sample E was subsequently annealed for 90 mins at 1070 °C in NH 3 , H 2  and enough Ga precursor to avoid GaN decomposition. In order to quantify the carrier concentration, another four layers of silicon-doped GaN with varying   doping con-centration separated by 200 nm thick undoped GaN spac-ers were grown on top of samples A–C. Dopant concentra-tions in the calibration staircase structure were profiled by secondary ion mass spectrometry (SIMS) and varied be-tween 6 x 10 17  cm -3  to 1 x 10 19  cm -3 . SCM studies on non-polar GaN were performed in cross-section with a Veeco Dimension 3100 atomic force microscopy with a commercial SCM application module. Sample preparation was performed by cleaving along the GaN [1-100] direction [7]. Processing of the SCM data ac-quired for this study was done using WSxM freeware [8]. SIMS analysis was carried out at Loughborough Surface Analysis Ltd. Quantification of the width of the   uninten-tionally-doped region has been achieved using an auto-mated routine which extracted the width from each line of ten 256 x 256 pixel SCM phase images of each sample. The automated routine is described elsewhere [4]. SCM amplitude images showing both the conductive interface region and the calibration staircase structure were used to quantify the carrier concentration in the unintentionally-doped region, following the method detailed elsewhere [9]. 3 Results  SCM phase images of the unintentionally-doped region for samples A –  C are shown in Fig. 1(a-c). For each image, there is a dark layer with varying thick-ness close to the GaN/sapphire interface, which corre-sponds to a layer of n-type material, given the imaging conditions used here [7]. The sapphire substrate is to the left of each dark layer and appears grey and noisy, since it is an insulator, which does not deplete, and thus shows no fixed phase response. The remaining GaN (to the right of each dark layer) also appears grey and noisy, indicating the carrier concentration is too low to be measured by the sys-tem, implying that the carrier concentration is less than 1.8 x 10 17  cm -3  [9]. In addition, an unintentionally-doped fea-ture extending at 60° from the GaN/sapphire interface is shown in Fig. 1(c). These features are frequently observed in samples B and C. The variation of the conductive interface layer width and the root mean square (rms) roughness of the top sur-face of the conductive interface layer with 3D growth time are shown in Fig. 2. It is shown that the average width of the conductive interface layer increases with increasing 3D growth time. The width measurement was based on at least ten images for each sample, resulting in a very small statis-tical error, represented by the positive error bars. The negative error bars correspond to an over-estimation due to the finite tip size and carrier diffusion out of the doped re-gion. Assuming the lower surface of the conductive inter-face layer is flat, the rms roughness of the top surface can  be determined by calculating the standard deviation from the mean of the interface thickness. It shows a similar trend to the conductive interface layer thickness.   Figure 3 illustrates the quantification of the carrier concentration in the unintentionally-doped region for sam- ple B using a ‘letter-box’ shaped SCM amplitude image. Firstly, it is worth noting that an unintentionally-doped feature inclined at 60° from the GaN/sapphire interface can also be seen in this image. Figure 3(a) shows that SCM amplitude signal increases along this inclined feature away from the GaN/sapphire interface. The high pixel density (4096 x 1024) image was then averaged in y-direction to form a line profile [Fig. 3(c)]. It shows that SCM ampli-tude decreases as the doping density increases, as heavily doped material is harder to deplete, thus giving a smaller capacitance change. Comparing the SCM line profile [Fig. 3(c)] with the known dopant concentrations from SIMS data, a calibration curve can be made [Fig. 3(d)]. Using this calibration curve, the unintentionally-doped region has Figure 2   Variation in conductive interface layer thicknessand rms roughness of the top surface of the conductive inter-face layer with 3D growth time.   Figure 1   SCM phase im-ages of samples with differ-ent growth conditions: a)sample A with 0 s 3Dgrowth; b) sample B with600 s 3D growth then coa-lescence; c) sample C with2400 s 3D growth then coa-lescence.  Phys. Status Solidi C 7 , No. 7–8   (2010) 1877 www.pss-c.com    © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim   ContributedArticle Figure 3   Quantification of carrier concentration in theconductive interface region: a) SCM amplitude image of the conductive interface region and the calibrationdopant staircase structure; b) SCM dC/dV amplitudeincreases along the inclined feature away from theGaN/sapphire interface; c) SCM amplitude data of theSi-doped calibration staircase structure is averaged inslow-scan (vertical) direction to form a line profile; d) Acalibration curve is constructed between log (dopingconcentration from SIMS) and SCM amplitude data,showing a roughly linear relationship.  been found to have an average carrier concentration of (2.5 ± 0.3) x 10 18  cm -3 .  No SIMS measurements have been carried out on the interface region in this sample. However, previous SIMS studies have shown similar level of oxygen concentration in the conductive interface region in GaN grown on c - plane sapphire [6], thus suggesting that oxygen could be the source for the unintentional doping in non-polar GaN. In addition, the SCM amplitude increases along the 60° in-clined feature with distance from the GaN/sapphire inter-face corresponding to a decrease in carrier concentration [Fig. 3(b)]. This observation is consistent with uninten-tional doping occurring due to oxygen diffusion from the sapphire substrate. 4 Discussions  We will briefly consider the srcin of the 60° inclined features and the cause of the unintentional doping. Transmission electron microscopy studies on the samples studied here have found that dislocation density decreases with increasing 3D growth time, and the pris-matic stacking faults (PSFs) extend through the films at 60° from the GaN/sapphire interface [10]. Hence, the unin-tentionally-doped features inclined at 60° to the GaN/sapphire interface observed in SCM could be due to enhanced impurity incorporation around PSFs. SCM studies of samples D and E offer further insight into the mechanism involved in forming the inclined fea-tures. Figure 4 shows montages of SCM phase images for the unannealed (D) and annealed (E) samples (Below the dark layer is the sapphire substrate). By comparing 30 SCM phase images from each sample over a total distance of 90 μ m, we observe that the inclined features occur in sample E much more often than in sample D. A Student’s t-test suggests a confidence interval greater than 98% that the average density of the inclined features is higher in the annealed sample than the unannealed sample. This further support the suggestion that the doping in the inclined fea-tures arises from diffusion of oxygen from the sapphire substrate, since annealing would allow further diffusion to occur. The alternative dopant incorporation mechanism -contamination from precursor gases during growth is highly unlikely in an annealing experiment. 5 Conclusions  Unintentional doping in non-polar a - plane (11-20) GaN has been identified and quantified using scanning capacitance microscopy. Dislocation reduction has been achieved by increasing the 3D growth time, while the average width of the unintentionally-doped region close to the GaN/sapphire interface increases. It has been found that the unintentionally-doped region has an average carrier concentration of (2.5 ± 0.3) x10 18  cm -3 . We suggest that the inclined features extending at 60° from the GaN/sapphire interface are related to impurity incorpora-tion around PSFs. Quantification of our data leads us to suggest that doping in the inclined features arises as a con-sequence of oxygen diffusion from the sapphire substrate. Acknowledgements This work has been funded by the EPSRC (EP/E035167/1). RAO would like to acknowledge fund-ing from the Royal Society. References [1] B. Imer, M. Schmidt, B. Haskell, S. Rajan, B. Zhong, K. Kim, F. Wu, T. Mates, S. Keller, U. K. Mishra, S. Naka-mura, J. S. Speck, and S. P. DenBaars, Phys. Status Solidi A 205 , 7, 1705 (2008). [2]   M. Kuroda, H. Ishida, T. Ueda, and T. Tanaka, J. Appl. Phys. 102 , 093703 (2007). [3] C. Mavroidis, J. J. Harris, M. J. Kappers, N. Sharma, C. J. Humphreys, and E. J. Thrush, Appl. Phys. Lett. 79 , 1121 (2001). [4] S. Das Bakshi, J. Sumner, M. J. Kappers, and R. A. Oliver, J. Cryst. Growth 311 , 232 (2009). [5] H. Yu, M. K, Ozturk, S. Ozcelik, and E. Ozbay, J. Cryst. Growth 293 , 273 (2006). [6] J. Sumner, S. Das Bakshi, R. A. Oliver, M. J. Kappers, and C. J. Humphreys, Phys. Status Solidi B 245 , 896 (2008). [7] J. Sumner, R. A. Oliver, M. J. Kappers, and C. J. Hum- phreys, Phys. Status Solidi C 4 , 2576 (2007). [8] I. Horcas, R. Fernández, J. M. Gómez-Rodríguez, and J. M. Gómez-Rodríguez, Rev. Sci. Instrum. 78 , 013705 (2007). [9] J. Sumner, R. A. Oliver, M. J. Kappers, and C. J. Hum- phreys, J. Vac. Sci. Technol. B 26 , 611 (2008). [10] C. F. Johnston, M. J. Kappers, and C. J. Humphreys, J. Appl. Phys. 105 , 073102 (2009). Figure 4   Montages of SCM phase images for a) sample D (unan-nealed) and b) sample E (annealed).  
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