Morphological study of InN films and nanorods grown by H-MOVPE

Transcription

1 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society 0892-FF Morphological study of films and nanorods grown by H-MOVPE H.J. Park, S.W. Kang, O. Kryliouk, and T. Anderson Department of Chemical Engineering, University of Florida, Gainesville, FL U.S.A. ABSTRACT Hydride-Metalorganic Vapor Phase Epitaxy (H-MOVPE) was used to grow a series of films on c-al 2 O 3 substrates. Depending on the growth temperature and HCl/TMIn molar ratio, deposited as a continuous film or a collection of micro or nanorods, or no growth was observed. A chemical equilibrium analysis of the In-N-H-Cl system predicts both growth and etching regimes with the nanorod growth observed near the growth-etching transition. All rod structures demonstrated well faceted hexagonal structure with a near random orientation of the rods, while the films were polycrystalline. INTRODUCTION Although is the least studied of the group III-nitrides, it exhibits some interesting optoelectronic and electronic characteristics that make it suitable for a variety of applications. Device structures based on that have already been fabricated and suggested include MISFETs [1], gas and liquid sensors [2, 3], heterojunction solar cells [4], and terahertz radiation microwave devices [5]. Interest in has intensified due to recent reports that the value of the bandgap energy (0.6 to 0.8 ev [6 11]) may be considerably lower than the previously accepted value (~1.9 ev [12 20]). The growth of device quality, however, is challenging. A low growth temperature (~600 to 700 C) is required due to its low thermal stability relative to the other group III-nitrides. At low growth temperature, however, the amount of reactive nitrogen available for growth from NH 3 is very low and In droplet formation can occur. Therefore a very high NH 3 partial pressure is required to avoid nucleation of In liquid. Excessively high NH 3 will in turn generate considerable H 2 as a by product of NH 3 decomposition. H 2 is also a product of the apparently reversible deposition reaction, which will reduce the growth rate. Furthermore, the low deposition temperature makes it difficult to grow material of high structural quality due to the reduced surface mobilities of adatoms. Add in the lack of a suitable latticematched substrate, it is not surprising that the structural quality of has been relatively poor. There have been several morphological studies of growth, but all have used either MBE or MOCVD [21 29]. In the study reported here, Hydride-Metal Organic Vapor Phase Epitaxy (H-MOVPE) was used to grow. H-MOVPE is a growth technique that generates volatile InCl by reacting an organometallic precursor (TMIn in this study) with HCl in the source zone of a HVPE hot-walled reactor, and then combines the InCl with the group V hydride (NH 3 ) as in conventional HVPE. The advantage of H-MOVPE or HVPE is that the added HCl can prevent formation of In droplets [30] and thus reduce the need for excess NH 3 to allow growth of at a reasonable rate. It was also hoped that the surface morphology would improve given the deposition reaction is reversible. Furthermore, a hot-walled reactor facilitates increased extent of NH 3 cracking. Previous work on the growth of nanorods and nanowires [31 34] has not involved conventional deposition methods such as CVD and MBE but rather closed spaced vapour

2 0892-FF transport on catalyzed or templated substrates. In these studies, pure indium metal or In 2 O 3 and NH 3 were used as the indium and the nitrogen sources. Catalytic effects were often utilized especially the use of sputtered thin (3 to 10 nm) Au layers. In this work, H-MOVPE was used to grow. We report the growth of nanorods at low NH 3 /TMIn ratio. The promise of improved structural quality in nanorods is that, once nucleated, the ratio of the contact area to free surface area is much less than in the film case, thus reducing substrate effects. This difference and the potential for dislocations to terminate at the sidewalls should significantly improve the structural quality of the material. Observations of the effects of the HCl/TMIn molar ratio and growth temperature on surface morphology are presented. EXPERIMENTAL PROCEDURE The details of H-MOVPE growth are presented elsewhere [35]. The indium and nitrogen sources were TMIn (TMIn solution, Epichem) and NH 3 (Anhydrous Grade 5, Matheson-Trigas). HCl gas (10 % HCl in 90 % N 2, Air Products) was added to the TMIn flow to yield volatile InCl according to overall reaction R1. The in-situ formed InCl then reacts with NH 3 downstream to deposit by the overall deposition reaction R2: In(CH 3 ) 3 (g) + HCl(g) InCl(g) + CH 4 (g) + C 2 H 6 (g) (R1) InCl(g) + NH 3 (g) (s) + HCl(g) + H 2 (g) (R2) In this study, c-al 2 O 3 was used as the substrate after cleaning in warm trichloroethylene, acetone, and methanol for 5 min. each followed by DI water rinse and nitrogen dry. After loading the substrate, the temperature of the reactor was increased at a rate of 15 C/min. HCl gas was first supplied for 5 sec to in-situ clean the surface before initiation of growth. The growth time was set at 1 hr and the carrier gas was nitrogen. The HCl/TMIn molar ratio was varied from 0 to 7 and the growth temperature was set to a value in the range 400 to 750 C. The NH 3 /TMIn ratio was typically set at 250, a value that is very low relative to conventional MOCVD, although this value was varied in the range 100 to for on set of runs. After the growth, the reactor was cooled to room temperature at a rate of ~15 C/min. The deposited material was then examined by SEM, AFM, and XRD to characterize the structural quality and surface morphology of the deposited material. RESULTS AND DISCUSSIONS Figure 1 shows SEM images of the grown at several growth temperatures at fixed HCl/TMIn ratio and NH 3 /TMIn values of 4 and 250, respectively. When the temperature is lower than 400 C or higher than 750 C, no growth occurred. Below 400 C the extent of NH 3 cracking is expected to be extremely low. It is also known that the decomposition temperature of is around 750 C and along with added etching by H 2 it is not surprising that no growth occurs above this temperature. At 550 C, sparse and short nanorods were observed. When the temperature was in the range 600 to 650 C, nanorods were grown. At 700 C the diameter of the rods increased to the micron range and their number density was low. For the reasons just stated, when the temperature is in the high or low regimes, the nucleation of is difficult, so low density rod growth is anticipated.

3 0892-FF Figure 1. SEM micrographs of film and nanorods. The temperature was varied from 400 to 750 C and the HCl/TMIn ratio and NH 3 /TMIn were set at 4 and 250, respectively. Deposition temperature: (a) 400 C, (b) 550 C, (c) 600 C, and (d) 700 C. A chemical equilibrium analysis was performed to estimate the growth conditions for to better understand In droplet formation and conditions where etching might be achieved. Since the deposition reactions in HVPE are reversible, chemical equilibrium near the growth surface can be closely approached in this deposition method, as has been shown for the group III arsenides and phosphides. All the thermochemical data related to, HCl, H 2, N 2, and InCl were obtained from the SUB94 database accompanying the ThermoCalc software [36]. It is noted from the results of this analysis that In droplets were not predicted to form in the temperature and HCl/In ranges typically used in HVPE. Also the growth-etch line shifted depending on the HCl/In ratio. This is perhaps best illustrated by comparing the standard Gibbs energy change of the etching reaction with the observed growth habit (see Figure 2) as a function of temperature. If the species were in their standard states, the growth-etch temperature is 623 C. This temperature coincides with about the middle of the nanorod growth regime. When the HCl/In ratio is greater than 1 and P H2 is less than 1 in a N 2 carrier, the growth-etch condition should be shifted to higher T. Apparently, a low thermodynamic driving force favors nanorod growth. To grow nanorods, the more stable atomic arrangements should prevail over the higher energy ones. This requires a harsh and selective growth environment. Therefore, nanorods should be grown only around this growth-etch transition temperature. Figure 2. Gibbs energy change of etching with HCl along with the observed deposition domains.

4 0892-FF (a) (b) (c) (d) Figure 3. SEM micrographs of films and nanorods. The HCl/TMIn ratio was varied from 0 to 7, while the growth temperature and NH 3 /TMIn ratio were set at 600 C and 250, respectively. Inlet ratio: (a) HCl/TMIn = 0, (b) HCl/TMIn = 0.3, (c) HCl/TMIn = 2, and (d) HCl/TMIn = 4. Figure 3 shows SEM images of films and nanorods grown at various HCl/TMIn molar ratios at constant growth temperature and NH 3 /TMIn ratio were set at 600 C and 250, respectively. In droplets were formed on films when no HCl was provided. When HCl/TMIn was between 0.3 and 1, a continuous film was grown that did not show any metallic In on the surface. At HCl/TMIn = 2, columnar polycrystalline films were grown. For intermediate values of HCl/TMIn ratios (3 to 5) the growth of nanorods was observed. To grow nanorods of the identical orientation, assuming the c-axis direction bond is stronger than the other bonds, the growth conditions should be selective to completely etch the more weakly bound adsorbed atoms. This should leave only those stronger bonds that lead to c- oriented rods. This is consistent with the growth of nanorods at relatively high temperature and high HCl/TMIn ratio. The diameters of the rods ranged from 50 to 260 nm and their length was ~ 1 µm depending on the HCl/TMIn ratio. The rod diameter showed a complex dependency on deposition temperature. The diameter decreased with increasing HCl/TMIn ratio in the low range, while at higher HCl/TMIn ratio the rod diameter increased to the µm range. No growth occurred when the HCl/TMIn ratio was greater than 7 presumably due to etching. The surface morphology of the polycrystalline films was characterized by AFM with an example shown in Figure 4 (1 µm x 1 µm scanned area). The growth conditions for this film were NH 3 /TMIn = 1000, HCl/TMIn = 0.3 and T = 560 ºC for 1 hr. The observed grain size was in the range 100 to 250 nm, suggesting an island coalescence growth mode (Volmer-Weber). The RMS roughness measured by AFM was 34 nm. To determine the film orientation, XRD θ-2θ scans were taken. The patterns contain only reflections attributed to hexagonal (Figure 5). Although individual nanorods were single crystalline (confirmed by TEM-Diffraction), the pattern matched the powder one, suggesting the nanorods were randomly oriented. Furthermore, the results show that within the detection limits neither crystalline indium oxide (In 2 O 3 ) nor metallic indium was present. An XRD pattern of an film grown at HCl/TMIn = 0.3 is also shown. The film is not single crystalline, although it shows a strong texturing along the c-axis. z = 10 nm RMS roughness = 34 nm Figure 4. AFM images of the films. NH 3 /TMIn = 1000, HCl/TMIn = 0.3, T = 560 ºC.

5 0892-FF (002) Al 2 O 3 (006) Intensity (log. a.u.) film (100) nanorods (002) (101) (101) Al 2 O 3 (006) Al 2 O 3 (006) (102) (102) PDF# theta Figure 5. Typical XRD patterns of nanorods and films grown on c-al 2 O 3. CONCLUSIONS films and nanorods, free of In inclusions, were successfully grown by H-MOVPE at low NH 3 /TMIn ratio. It was also observed that by adjusting the growth temperature and HCl/TMIn inlet molar ratio that the material morphology varied from continuous polycrystalline film to nanorod growth, with no material growth occuring at low and high deposition temperature. A thermodynamic analysis evaluated etching, film growth, and possible nanorods growth regimes. The experimental observations no In codeposition matched well with the thermodynamic predictions. The occurrence of nanorod growth was under conditions that were thermodynamically close to etching conditions. ACKNOWLEDGEMENTS The authors would like to thank the Major Analytical Instrumentation Center, Department of Materials Science and Engineering, University of Florida.. REFERENCES [1] H. Lu, W.J. Schaff, L.F. Eastman, Mater. Res. Soc. Symp. Proc. 693 (2002) 9. [2] O. Kryliouk, H.J. Park, H.T. Wang, B.S. Kang, T.J. Anderson, F. Ren and S.J. Pearton, J. Vac. Sci. Technol. B, 23(5), 1891(2005). [3] H. Lu, W.J. Schaff, L.F. Eastman, J. Appl. Phys. 96 (2004) [4] A. Yamamoto, M. Tsujino, M. Ohkubo, A. Hashimoto, Sol. Energy Mater. Sol. Cells 35 (1994) 53. [5] E. Starikov, P. Shiktorov, V. Gruzinskis, L. Reggiani, L. Varani, J.C. Vaissiere, Jian H. Zhao, Physica B 314 (2002) 171. [6] T. Inushima, V.V. Mamutin, V.A. Vekshin, S.V. Ivanov, T. Sakon, M. Motokawa, S. Ohoya, J. Cryst. Growth (2001) 481. [7] V.Yu Davydov, A.A. Klochikhin, V.V. Emtsev, S.V. Ivanov, V.V. Vekshin, F. Bechstedt, J. Furthmuller, H. Harima, A.V. Mudryi, A. Hashimoto, A. Yamamoto, A.J. Aderhold, J. Graul, E.E. Haller, Phys. Status Solidi b 230 (2002) R4.

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