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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

Thin Solid Films 527 (213) 9 15 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.

Selvamanickam a, a Department of Mechanical Engineering and Texas Center for Superconductivity, University of Houston, Houston, TX 7724, USA b SuperPower Inc, Schenectady, NY 1234, USA article info

2 Thin Solid Films 527 (213) 9 15 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: High mobility single-crystalline-like germanium thin films on flexible, inexpensive substrates R. Wang a, S.N. Sambandam b, G. Majkic a, E. Galstyan a, V. Selvamanickam a, a Department of Mechanical Engineering and Texas Center for Superconductivity, University of Houston, Houston, TX 7724, USA b SuperPower Inc, Schenectady, NY 1234, USA article info abstract Article history: Received 13 June 212 Received in revised form 13 December 212 Accepted 18 December 212 Available online 23 December 212 Keywords: rmanium Thin films Mobility Epitaxy Ion beam assisted deposition Flexible substrates Sputtering Photovoltaics Single-crystalline-like epitaxial germanium thin films with hall mobility values as high as 833 cm 2 /Vs have been demonstrated on inexpensive polycrystalline metallic substrates. The dependence of mobility of p-type and n-type germanium films on the deposition temperature has been examined and correlated to microstructural changes. The importance of crystallographic orientation of to achieve these high mobility values has been verified. The mobilities of the epitaxial germanium films on the film thickness for p-type and n-type films with multi-layer architecture with different epitaxial intermediate layers have been investigated. Results show that the increased mobility with increasing germanium film thickness is not just due to crystallographic texture improvement, but could also be a result of decreasing defect density. 212 Elsevier B.V. All rights reserved. 1. Introduction Multi junction III V solar cells are of great interest due to their high efficiency which can be over 4% [1,2]. Such high efficiencies have been possible only with expensive substrates such as germanium single crystal wafers. We had previously demonstrated an approach to fabricate single-crystalline-like germanium films on inexpensive, flexible metal substrates using continuous reelto-reel processing [3]. Single crystalline-like germanium films were made by using biaxially-textured templates made by ion beam assisted deposition (IBAD). IBAD has been used to achieve biaxial crystallographic texture in thin films on polycrystalline or amorphous substrates [4 6] and has been utilized to fabricate single-crystalline-like oxide films in lengths of over a kilometer [7]. IBAD-based templates have also been used to fabricate silicon and germanium thin films on glass substrates [8,9]. Thestructural and optical properties of the germanium films on inexpensive, flexible metal substrates were found to resemble those of bulk, single crystal germanium [3]. These single-crystalline-like germanium films have been used to demonstrate epitaxial thin film GaAs on inexpensive substrates [1]. Abbreviations: IBAD, Ion Beam Assisted Deposition; FIB, Focused Ion Beam Milling. Corresponding author at: Department of Mechanical Engineering, 48 Calhoun Rd, University of Houston, Houston, TX , USA. Tel.: address: selva@uh.edu (V. Selvamanickam). In our earlier report, we showed the importance of the structural matching of the intermediate layer for germanium film growth using IBAD templates. We demonstrated that, whose fluorite structure matches with the diamond structure of germanium, is a suitable intermediate layer to achieve epitaxial growth of germanium using IBAD template [3]. Grain-to-grain in-plane misorientation in the germanium film was reduced to 1 full-width-half-maximum (FWHM) as measured by phi-scan X-ray Diffraction measurements, by increasing the thickness of the germanium film [11]. In this paper, we report the influence of germanium film thickness on its Hall mobility values. We also investigate whether texture improvement by itself is responsible for an increase in mobility using films on films of different thicknesses grown of IBAD templates. Further, we report on a set of optimum process conditions for p-type and n-type thin films on IBAD templates. 2. Experimental details A schematic of the typical multilayer thin film architecture used in this work is shown in Fig. 1. The substrates consisted of an approximately 3 nm thick homo-epitaxial MgO film on IBAD MgO template grown on a 12 mm wide and 5 μm thick, electropolished Hastelloy C-276 tape. A detailed description of the substrate and oxide buffer layers used in the multi-layer architecture is described elsewhere [3]. In the first set of experiments, different intermediate epitaxial 4-69/$ see front matter 212 Elsevier B.V. All rights reserved.

Schematic of the multilayer architecture developed in this work for the growth of single-crystalline-like films on IBAD templates on metal substrates.

3 1 R. Wang et al. / Thin Solid Films 527 (213) 9 15 CeO2 Intermediate epi layer Biaxially-textured IBAD + epimgo Amorphous alumina + yttria Inexpensive metal substrate Fig. 1. Schematic of the multilayer architecture developed in this work for the growth of single-crystalline-like films on IBAD templates on metal substrates LMO (1) LMO (1) STO (1) (a) / /LMO, P type target, 75 C (b) / /LMO, N type target, 59 C (c) / /STO, N type target, 59 C MgO MgO MgO LMO LMO STO (4) (4) theta layers between MgO and were deposited. Samples were made with and without SrTiO 3 and LaMnO 3 intermediate epitaxial layers. R.F. magnetron sputtering was used to grow thin films of MgO, LaMnO 3, SrTiO 3, and, in a reel-to-reel continuous system. The deposition chamber was evacuated to a base pressure of Pa and the substrates were heated using a lamp heater up to a maximum of 82 C for subsequent thin film growth. The optimum temperatures for the epitaxial growth of on SrTiO 3, LaMnO 3 and MgO were found to be 78 C, 82 C and 8 C respectively controlled with a typical linear tape travel speed of 1.4 cm/min. The thickness of the film was controlled by changing the tape speed. Epitaxial films were deposited on these different films using n-type and p-type doped sputter targets, in an Ar-4%H 2 atmosphere. The n-type target used was % pure with arsenic as the dopant. The p-type target was % pure and boron doped. (b) (c) (4) (4) (4) (4) Fig. 2. XRD out-plane theta-2theta scan of (a) p- on /LaMnO 3 /IBAD MgO template, (b) n- on /LaMnO 3 /IBAD MgO template and (c) n- on /SrTiO 3 /IBAD MgO template. (a) The substrate temperature was varied from 53 to 75 C during deposition. The out-of-plane texture of all deposited films was measured using X-ray diffraction (XRD). Detailed pole figure analysis was conducted using a Bruker two dimensional neral Area Detector Diffraction System XRD and phi scans extracted from the pole figures were used to determine in-plane texture values. Copper kα source was used in all XRD measurements. The microstructure of the samples was analyzed using scanning electron microscope (SEM) model LEO 1525 and transmission electron microscope (TEM) model JEOL 2FX at operating voltages of 15 kv and 2 kv respectively. Samples for cross sectional microstructure analysis were prepared by FEI 235 focused ion beam (FIB) milling system at an acceleration voltage of 3 kv and an ion emission current of 2 μa. Mobility measurements at room temperature were performed on germanium films using an Ecopia HMS-5 hall measurement system. 3. Results and discussion Fig. 2 shows theta-2theta XRD measurements on thin films for three different architectures. In all cases, the (111) and other non (1) orientations are found to be absent in the XRD data. It is well-known that an intermediate layer of LaMnO 3 or SrTiO 3 can reduce the lattice mismatch between and MgO on the IBAD template [12]. has the fluorite structure with atoms in the eight tetrahedral locations, and its basal plane projection matches well with the atomic locations of the diamond structure of. Such a structural match with is not possible with rock-salt MgO, perovskites LaMnO 3 and SrTiO 3 because of absence of atomic registry in their basal planes. We had shown previously that does not grow epitaxially on MgO and LaMnO 3 [3]. Hence was chosen to be the film for epitaxial growth of even though it has a significant lattice mismatch of 4.6%. Texture measurements using XRD (11) pole figures reveal a spread in the in-plane texture of LaMnO 3 and SrTiO 3 on IBAD MgO templates to be 5.7 and 5.8 respectively. Results from the pole figure measurements of the subsequently deposited 25 nm thick films on the three different intermediate epitaixal layers are shown in Fig. 3. The XRD (111) pole figure shows a strong four-fold symmetry without Fig. 3. XRD (111) pole figure analysis of (a) /IBAD MgO template, (b) /SrTiO 3 /IBAD MgO template, and (c) /LaMnO 3 /IBAD MgO template.

4 R. Wang et al. / Thin Solid Films 527 (213) Hall mobility (cm 2 /V sec) N type p type of a biaxially-textured film to achieve high mobility was evaluated by testing randomly oriented polycrystalline film on IBAD template. A randomly-oriented film was fabricated by deposition on a randomly-oriented films grown without active heating on IBAD MgO templates with SrTiO 3 and LaMnO 3 intermediate layers. Due to the plasma bombardment during deposition, substrate temperature reached 122 C. Fig. 7 shows the out of plane XRD scans of and layers deposited without active heating. grown on on both intermediate layers is found to have both (1) and (111) orientations. The other peaks in the figure belong to the substrate along with high order peaks of and. (311) and Temperature ( o C) Fig. 4. Hall mobilities of p- and n- thin films deposited at different substrate temperatures on /LaMnO 3 /IBAD MgO templates. secondary orientations which is evidence of biaxial texture. Fig. 3 shows that films grow with a preferred (1) orientation with 45 rotated epitaxy on SrTiO 3 and LaMnO 3 and with cube-on-cube epitaxy on MgO. develops an orientation similar to the texture and also shows a similar strong four-fold symmetry [11]. 1.4 μm thick p-type and n-type films were sputtered on to /LaMnO 3 /homo-epi MgO/IBAD MgO templates using borondoped p- and arsenic-doped n- targets at different substrate temperatures ranging from 53 C to 75 C. Hall mobility measurements were made on these films and the results are shown in Fig. 4. The hole mobility of the p-type was found to be low at deposition temperatures below 63 C and then increase at higher deposition temperatures. This increase could be due to the activation of boron dopants in the films at the higher deposition temperatures. The increase in mobility may also be attributed to the increase in the grain size at higher deposition temperatures. Fig. 5 shows scanning electron micrographs (SEM) of the surface of p-type films deposited at 63 C and 75 C. It is shown that the grain size of the p-type film increases from 2 nm to 1 μm as the deposition temperature is increased from 63 C to 75 C. Fig. 6 shows theta-2theta XRD scans around the (4) peak of films deposited at different substrate temperatures. The (4) peak intensity is found to increase with higher substrate temperatures, which is likely due to increased ad-atom mobility on the film growth surface. Fig. 4 showed that the highest mobility, 833 cm 2 /Vs, was achieved in p- films deposited at 75 C. Previous work on thin films deposited by physical vapor deposition on glass and silicon wafer substrates have reported mobility values between 12 and 4 cm 2 /Vs [13,14]. In this work, films made with an n-type target exhibit electron mobility at deposition temperatures below 65 C, as shown in Fig. 4. A maximum value of 343 cm 2 /Vs was achieved at 58 C. Surprisingly, at 65 C and above, the films exhibited a reduction in mobility and hall measurements with positive polarity of charge carriers. The transition from negative charge carriers to positive charge carriers at 65 C could be due to the sublimation of As dopants from the growing film above this temperature. As a consequence, there could be redistribution of dopants and creation of vacancies resulting in increase in defect density. It has been reported that defect states due to dislocations and point defects in an n-type film act as trap states and exhibit positive hall mobility [14]. It is also possible that another mechanism could be activated at higher deposition temperatures that resulted in transition to positive charge carriers. From our results, the layer is necessary for achieving a preferred (1) orientation in subsequently-grown. The importance Fig. 5. Surface microstructure of (a) p- deposited at 75 C, (b) p- deposited at 63 C and (c) n- deposited at 59 C on /LaMnO 3 /IBAD MgO template.

12 R. Wang et al. / Thin Solid Films 527 (213) 9 15 13 12 11 1 9 8 7 6 5 4 3 2 1 (4) A 2theta=65.9 o (4) B 2theta=66.4 o 65.8 66. 66.2 2 theta ( ) 59 o C 63 o C 67 o C 71 o C 75 o C Fig. 6. XRD scan of epitaxial films deposited at different substrate temperatures on IBAD templates.

5 12 R. Wang et al. / Thin Solid Films 527 (213) (4) A 2theta=65.9 o (4) B 2theta=66.4 o theta ( ) 59 o C 63 o C 67 o C 71 o C 75 o C Fig. 6. XRD scan of epitaxial films deposited at different substrate temperatures on IBAD templates. (4) appear at 53.4 and 69.3 respectively. The competing growth of (111) and (1) could be a result of the faceted morphology of the underlying as seen in the surface microstructure in Fig. 8. This morphology is different from the smoother film surfaces from the (1) textured germanium films shown in Fig. 5. Mobility values of germanium films without preferred orientations were too low (b5 cm 2 /Vs) to be measured accurately. Previously, we demonstrated improvement in the in-plane texture of germanium films on IBAD templates with increasing germanium film thickness [11]. In this work, we investigated the influence of film thickness on carrier mobility for both p-type and n-type films deposited on IBAD templates with different intermediate epitaxial layers..7 to 5.6 μm thick p-type and n-type films were deposited on bi-axially textured /IBAD MgO templates. The previously optimized values of temperature for deposition of p-type and n-type were used in thick film fabrication. Fig. 9 displays the hall mobility values of both the p-type and n-type films of increasing thickness. The hole mobilities of p-type films are found to increase with film thickness at a similar rate with both LaMnO 3 and SrTiO 3 intermediate epitaxial layers. The mobility values of n-type films also increase with thickness but are found to depend significantly on the type of intermediate epitaxial layer. The n-type films with a LaMnO 3 intermediate buffer layer were found to exhibit the highest mobility followed by those using SrTiO 3 and then with (111) (a) /RT /LaMnO 3 (b) /RT /SrTiO 3 (4) theta ( ) Fig. 7. XRD scan of thin films on randomly-oriented layer on SrTiO 3 and LaMnO 3 buffer layers on IBAD templates. (b) (a) Fig. 8. SEM image of a randomly-oriented surface showing a faceted microstructure due to competing growth of (111) and (1) orientations. those without any intermediate epitaxial layer, for the film thickness range studied in this work. Next, we investigated whether the increase in mobility with increasing film thickness was a result of improved crystallographic texture. One method to separate the influence of texture is to test films of the same thickness deposited on epitaxial films with different texture values. For this purpose, we fabricated epitaxial films of different thickness on IBAD MgO templates followed by epitaxial deposition. The thickness of the films was varied by changing the deposition time: from 18, 225, 321, 563, 765, 1125 to 225 s. Subsequently, a 1.46 μm thick n-type film was deposited on these samples. Fig. 1 shows the theta-2theta XRD scan of the peak of made with different deposition times. The preferred texture of is seen to improve with increasing deposition time. A shift towards the equilibrium lattice constant of with increasing thickness is evident in the shift of the peak seen in Fig. 1. films deposited on the samples with different thickness were found to exhibit a strong (4) preferred orientation as shown in Fig. 11. However, the (4) peak intensity is found to increase with increasing thickness of only up to 321 s of deposition time and decrease with further increase in thickness. As seen in Fig. 11, the (4) peak intensity on deposited for 225 s decreased substantially indicating the deterioration of quality of the film deposited on thick films. Hall mobilities of the films deposited on the samples with different thickness were measured and the results are presented in Fig. 12. It is shown that the mobility increases sharply at low thicknesses and then decreases as the thickness increases. This trend is consistent with the change in the intensity of (4) with increasing thickness. The decreased mobility in germanium film with thicker could be due to the changes in the film morphology with increasing thickness as shown in Fig. 13. It is observed that the film deposited for 321 s has a grain size of about 2 nm whereas the films deposited for 1125 and 225 s have grains that are plate-like, 5 7 nm in diameter and a rougher surface. We suggest that the improved mobility with increased germanium film thickness is not due purely from the improved crystallographic texture. The cross sectional microstructure of a 5.6 μm thick epitaxial film on an IBAD MgO template with a SrTiO 3 intermediate layer was examined by transmission electron microscopy and the results are shown in Fig. 14a and b. As shown in Fig. 14a,

6 R. Wang et al. / Thin Solid Films 527 (213) p- Carrier Mobility (cm 2 V -1 s -1 ) / /SrTiO 3 / /LaMnO n- / /MgO 5 / /SrTiO 3 4 / /LaMnO Thickness of layer (µm) Fig. 9. Influence of epitaxial film thickness on the hall mobilities of n- and p- films prepared on different intermediate epitaxial layers on IBAD templates. abundant dislocations are seen close to the interface. Near the top surface of the thick film however, the dislocation density decreased substantially (Fig. 14b). Therefore, we believe that the decreasing defect density with increasing thickness of germanium on IBAD MgO templates is responsible for the improved mobility presented in Fig Summary Single-crystalline-like epitaxial thin films of p- and n- with high charge carrier mobilities have been demonstrated on flexible, inexpensive substrates. p- and n- films with hole and electron mobility values of 833 and 343 cm 2 /Vs respectively have been demonstrated on these substrates. The hole mobility of p-type was found to increase rapidly with deposition temperatures above 63 C which could be due to the increase in grain size from 2 nm to 1 μm and an increase in the (4) peak intensity. The s 225s 321s 563s 1125s 225s theta ( ) Fig. 1. XRD theta-2theta scan of peak of films made with different deposition times on IBAD MgO templates (4) 18 s 225 s 321 s 563 s 1125 s 225 s theta ( ) Fig. 11. XRD theta-2theta scan of (4) peak of on films made with different deposition times on IBAD MgO templates with LaMnO 3 intermediate layer. Electron mobility (cm 2 /Vs) Epi /CeO2/LaMnO 3 Epi /CeO2/SrTiO CeO2 deposition time (second) Fig. 12. Influence of film thickness of films on the hall mobility of.

14 R. Wang et al. / Thin Solid Films 527 (213) 9 15 a 5nm SrTiO 3 Near buffer interface b 5nm surface Near top surface Fig. 14. Transmission electron micrographs a 5.

SEM images of surface morphology of films deposited at (a) 321 s, (b) 1125 s and (c) 225 s.

7 14 R. Wang et al. / Thin Solid Films 527 (213) 9 15 a 5nm SrTiO 3 Near buffer interface b 5nm surface Near top surface Fig. 14. Transmission electron micrographs a 5.6 μm thick epitaxial thin film on /SrTiO 3 /IBAD MgO template near (a) buffer interface and (b) top surface. Fig. 13. SEM images of surface morphology of films deposited at (a) 321 s, (b) 1125 s and (c) 225 s. mobility of n-type is found to undergo a transition at 65 C and exhibits a positive polarity of charge carriers. Randomly-oriented films on similar substrates showed poor mobility values confirming the importance of crystallographic texture to achieve the high mobility values. The surface morphology of randomly-oriented films was found by SEM to be faceted and completely different from the smoother (1) textured films. The influence of film thickness and intermediate epitaxial layers on mobility was also investigated in this work. The hole mobility values of p-type films were found to increase with film thickness for both LaMnO 3 and SrTiO 3 intermediate epitaxial layers. The mobility values of n-type films increased with thickness as well, with the highest values achieved with an LaMnO 3 intermediate epitaxial layer. In order to determine if improved crystallographic texture with increased film thickness is the primary reason for improved mobility, films on different thickness values were investigated. The intensity of the (4) peak as well as the mobility of the films were found to decrease beyond a certain thickness and was likely due to changes in microstructures for the thicker films. Cross sectional TEM analysis of thick films showed a clear and substantial reduction in defect density with increasing film thickness which is believed to be the primary reason for the increased mobility. References [1] J.F. isz, D.J. Friedman, J.S. Ward, A. Duda, W.J. Olavarria, T.E. Moriarty, Appl. Phys. Lett. 93 (28) [2] R.R. King, D.C. Law, K.M. Edmondson, C.M. Fetzer, G.S. Kinsey, H. Yoon, R.A. Sherif, N.H. Karam, Appl. Phys. Lett. 9 (27)

8 R. Wang et al. / Thin Solid Films 527 (213) [3] V. Selvamanickam, S. Sambandam, A. Sundaram, S. Lee, A. Rar, X. Xiong, A. Alemu, C. Boney, A. Freundlich, J. Cryst. Growth 311 (29) [4] J.M.E. Harper, J.J. Cuomo, R.J. Gambino, H.R. Kaufman, in: R.J. Kelly, O. Auciello (Eds.), Surface Modification by Ion Bombardment: Fundamentals and Applications, Elsevier, Amsterdam, The Netherlands, [5] C.P. Wang, K.B. Do, M.R. Beasley, T.H. balle, R.H. Hammond, Appl. Phys. Lett. 71 (1997) [6] A.T. Findikoglu, W. Choi, V. Matias, T.G. Holesinger, Q.X. Jia, D.E. Peterson, Adv. Mater. 17 (25) [7] X. Xiong, S. Kim, K. Zdun, S. Sambandam, A. Rar, K.P. Lenseth, V. Selvamanickam, IEEE Trans. Appl. Supercond. 19 (29) [8] J.R. Groves, J.B. Li, B.M. Clemens, V. LaSalvia, F. Hasoon, H.M. Branz, C.W. Teplin, Energy Environ. Sci. 5 (212) 695. [9] V. Selvamanickam, C. Jian, X. Xiong, G. Majkic, E. Galtsyan, in: Proc. 38th IEEE Photovoltaic Specialists Conference (PVSC), Austin, TX, 212, p [1] A. Freundlich, C. Rajapaksha, A. Alemu, A. Mehrotra, M.C. Wu, S. Sambandam, V. Selvamanickam, in: Proc. 35th IEEE Photovoltaic Specialists Conference (PVSC), Honolulu, HI, 21, p [11] V. Selvamanickam, S. Sambandam, A. Sundaram, S. Lee, G. Majkic, A. Rar, A. Freundlich, in: Proc. 35th IEEE Photovoltaic Specialists Conference (PVSC), Honolulu, HI, 21, p [12] P.N. Arendt, S. Foltyn, MRS Bull. (24) 543. [13] C.Y. Tsao, J. Wong, J. Huang, P. Campbell, D. Song, M.A. Green, Appl. Phys. A 12 (21) 989. [14] H. Kobayashi, N. Inoue, T. Uchida, Y. Yasuoka, Thin Solid Films 3 (1997) 138.

Next Generation High-Efficiency Low-cost Thin Film Photovoltaics

Next Generation High-Efficiency Low-cost Thin Film Photovoltaics Investigators Bruce Clemens, Professor, Materials Science and Engineering, Stanford University, Alberto Salleo, Assistant Professor, Materials