Modeling studies of an impinging jet reactor design for hybrid physical–chemical vapor deposition of superconducting MgB2 films

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 1501–1507

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Modeling studies of an impinging jet reactor design for hybrid physical–chemical vapor deposition of superconducting MgB2 films Daniel R. Lamborn a, Rudeger H.T. Wilke b, Qi Li b, X.X. Xi b,c,d, David W. Snyder e, Joan M. Redwing a,c,d, a

Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA d Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA e Applied Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b c

a r t i c l e in fo

abstract

Article history: Received 1 September 2008 Received in revised form 14 December 2008 Accepted 19 January 2009 Communicated by R. Bhat Available online 25 January 2009

An impinging jet reactor was developed for the deposition of superconducting MgB2 thin films by hybrid physical–chemical vapor deposition, a technique that combines Mg evaporation with the thermal decomposition of B2H6 gas. A transport and chemistry model for boron film deposition from B2H6 was initially used to investigate the effect of carrier gas, Mg crucible temperature and gas flow rates on boron film growth rate and uniformity. The modeling studies, which were validated experimentally, demonstrated a reduction in B2H6 gas-phase depletion and an increased boron film growth rate using an argon carrier gas compared to hydrogen. The results were used to identify a suitable set of process conditions for MgB2 deposition in the impinging jet reactor. The deposition of polycrystalline MgB2 thin films that exhibited a transition temperature of 39.5 K was demonstrated at growth rates up to 50 mm/h. & 2009 Elsevier B.V. All rights reserved.

PACS: 74.70.Ad 74.78. w 81.15.Gh Keywords: A1. Computer simulation A1. Growth models A3. Chemical vapor deposition processes B1. Borates B2. Superconducting materials

1. Introduction The discovery of superconductivity in MgB2 (Tc39 K) [1] has stimulated interest in developing in situ growth techniques to facilitate the fabrication of Josephson junctions for superconducting electronics as well as thick coatings on conductive wires and tapes for magnetic resonance imaging (MRI) magnets, RF cavities and undulators for advanced light sources. The main challenge associated with the development of a suitable process is that high overpressures of Mg vapor must be present for MgB2 phase stability at the elevated temperatures desired for epitaxially oriented films (720 1C) [2]. Hybrid physical–chemical vapor deposition (HPCVD) is a promising technique that has been shown to produce epitaxial MgB2 thin films on (0 0 0 1) SiC substrates with a superconducting transition temperature greater than that of the bulk material (Tc42 K) and low residual resistivity [3,4].

Corresponding author at: Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA. Tel.: +1 814 865 8665. E-mail address: [email protected] (J.M. Redwing).

0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.116

The HPCVD technique, described in detail previously [3,5], combines the thermal evaporation of high-purity Mg pellets and the thermal decomposition of diborane (B2H6) gas to deposit MgB2 thin films at elevated substrate temperatures (700 1C) and increased reactor pressures (100 Torr) compared to high vacuum deposition methods. In the original reactor geometry used for HPCVD [3,5], the Mg pellets were placed on the same heated susceptor as the substrate. This arrangement limited the operational flexibility of the process by prohibiting independent temperature control of the Mg source and substrate. It also restricted the amount of Mg that could be placed within the reactor during a growth run which limits the deposition time and layer thickness. Consequently, there is strong interest in further developing the HPCVD process to expand the current capabilities and to introduce a greater degree of process flexibility. Recent reports have successfully demonstrated alternative reactor geometries for the HPCVD growth of MgB2 that incorporates separate heaters for the Mg source and substrate and enables an expanded range of substrate temperatures and deposition times [6–10]. Our initial dual-heater HPCVD design [6] used a resistive heater for substrate temperature control and

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an inductively heated crucible for Mg temperature control. Although this design addressed the lack of independent temperature control in the original HPCVD configuration, the MgB2 growth rate was significantly reduced in this configuration due to substantial upstream depletion of B2H6 from the gas phase resulting from pre-reaction on heated surfaces and gas flows near the substrate surface. Our prior studies [11] demonstrated that the growth rate of MgB2 is limited by the B2H6 concentration in the reactor; consequently, reactor designs that enhance the gasphase transport of B2H6 to the substrate surface are expected to yield increased growth rates. We recently demonstrated high growth rates for MgB2 (110 mm/h) using an impinging jet reactor configuration in which B2H6 was introduced into the reactor via a water-cooled tube with the gas flow perpendicular to the substrate surface [8]. The high gas velocities obtainable in an impinging jet configuration are expected to reduce the gas residence time and therefore the extent of parasitic gas-phase decomposition of B2H6 upstream of the susceptor. In this study, a transport model of the impinging jet reactor was developed and was used to predict the gas-phase temperature and velocity profiles in this reactor geometry. Given the lack of information on the gas phase and surface chemistry of Mg and B2H6 and the associated reaction kinetics, it was not possible to develop a chemistry model for MgB2 deposition. Instead, a chemistry model for boron film deposition from B2H6 was used in combination with the transport model of the impinging jet to carry out a detailed study of the effect of process conditions and carrier gas on the growth rate and uniformity of boron thin films. The model was used to identify process conditions that yield reduced B2H6 gas-phase depletion, high boron film growth rates and uniform film thickness, which were subsequently verified experimentally. The results of the boron film growth studies were used to identify a suitable range of conditions for the deposition of MgB2 films in the impinging jet configuration.

Thermocouple Outlet

Heater Housing Cartridge Heater Substrate Flow Directing Cap

Mg Source

Induction Coil Jet Nozzle Thermocouple Alumina Cap Cooling water jacket Purge Inlet Ar/H2

Coolant Inlet

Coolant Outlet

Jet Inlet 2. Experimental procedure A schematic of the impinging jet configuration for HPCVD growth of MgB2 is shown in Fig. 1. The substrate was mounted on a resistive heater, which was encased in a custom housing to protect the heater leads and thermocouple from Mg vapor. The Mg pellets were placed in a stainless steel crucible that was inductively heated. The crucible had a hole in the middle where the B2H6 inlet tube was located, which permitted the direct delivery of a mixture of B2H6 and carrier gas, either H2 or Ar, to the substrate. A cone-shaped cap was placed on top of the crucible to help direct the flow of Mg vapor to the substrate and reduce Mg deposition on the inner diameter of the reactor vessel. The B2H6 was introduced into the reactor via a water-cooled quartz tube to reduce pre-heating of the gas. An alumina sleeve (or cap) was placed between the crucible and the water-cooled quartz inlet tube to reduce thermal gradients at the top of the inlet tube. In addition to the B2H6 jet inlet, an additional purge gas, either H2 or Ar, was used to reduce the back-diffusion of gases in the region upstream of the Mg crucible. Boron thin films were deposited on (0 0 0 1) sapphire substrates using the impinging jet reactor configuration shown in Fig. 1 but without the addition of Mg pellets. Diborane (5% B2H6 in H2) was used as the boron precursor with ultra-high-purity H2 or Ar as the carrier gas. The reactor pressure was 25 Torr, the purge gas flow rate was 400 sccm and the substrate temperature was set at 675 1C in all the experiments. The parameters varied in the experiments included the Mg crucible temperature (80–900 1C) and the inlet jet flow rate (100–825 sccm). The inlet jet mole fraction of B2H6 was held constant at 0.005 as the total jet flow

Ar/H2 B2H6 Fig. 1. Schematic of the impinging jet HPCVD reactor.

rate was varied. The boron film thickness was measured using scanning electron microscopy of the film/substrate cross-sections. MgB2 films were also grown on (0 0 0 1) sapphire substrates in the impinging jet configuration using an Ar carrier gas and conditions similar to those described above for the boron films. In this case, however, the substrate and Mg crucible temperatures were both held constant at 765 1C and the total inlet jet flow rate was varied from 125 to 225 sccm. The MgB2 film thickness was determined by measuring the height of a step that was etched into the film using HCl. X-ray diffraction measurements were performed using a Scintag model X2 y 2y diffractometer with a Cu X-ray tube and a Si(Li) peltier solid state detector. Resistivity measurement of the MgB2 films was performed using a van der Pauw geometry in a custom cryostat system over a temperature range of 300–30 K.

3. Modeling procedure Numerical process modeling was used to simulate the reactive flow conditions and predict the boron film deposition rate and film uniformity in the impinging jet reactor configuration. The computational model is similar to that previously developed for boron film deposition in the original HPCVD reactor [12]. The commercial software package CFD-ACE+ (ESI-CFD, Inc., Huntsville,

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4. Results and discussion

nozzle through the center of the crucible (see Fig. 1) upstream of the substrate. A combination of modeling and experiments was performed to determine the effect of the carrier gas and crucible temperature on boron film growth rate and film uniformity. Traditionally, H2 is used as the carrier gas in HPCVD to suppress the formation of MgO; however, in the impinging jet configuration, a carrier gas with a lower thermal conductivity is desirable to reduce heat transfer from the crucible to the gas phase and thereby reduce the extent of pre-reaction of B2H6. Argon is a nonreactive gas with a thermal conductivity approximately one order of magnitude lower than that of H2 [13], which makes it an ideal candidate for use as a carrier gas for MgB2 growth. For the initial studies, the carrier gas (H2 or Ar) flow rate was held constant at 500 and 50 sccm of the B2H6/H2 mixture was used to maintain a constant total flow rate of gases through the inlet jet of 550 sccm. The substrate heater temperature was held constant at 675 1C and the crucible temperature was varied over the range from 80 1C to 790 1C for Ar and 190 1C to 900 1C for H2. The purge gas was also varied between Ar and H2 using a set flow rate of 400 sccm. The total pressure of the system was held constant at 25 Torr. Fig. 2(a) shows the predicted boron film growth rates as a function of the radial position (distance from the center of the substrate) for the case of H2 vs. Ar carrier gas and varying Mg crucible temperatures. The choice of carrier gas significantly affects the boron film growth rate and the film uniformity. The boron growth rate is predicted to decrease as the crucible temperature is increased when H2 is used as the carrier gas.

H2

Ar Boron Film GrowthRate(nm/s))jj

12

Direction of increasing Tcrucible

10 8

Tcrucible= 790°C Tcrucible= 190°C

6 Tcrucible= 500°C

4 Tcrucible= 425°C 2 Tcrucible= 80°C 0 12

8

Direction of T = 900°C increasing Tcrucible crucible 4 4 0 Radial Position (mm)

8

12

Center of Substrate

20 Boron Film Growth Rate(nm/s)

AL) was used to numerically solve the coupled partial differential equations describing the conservation of mass, momentum and energy. As shown in Fig. 1, the region surrounding the substrate and crucible is symmetric with respect to the z-axis of the reactor tube; consequently, a 2D-axisymmetric model was used to reduce the computational complexity of the simulations. The transport model assumed laminar flow, multi-component diffusion, thermal diffusion and the effects of buoyancy-driven convection. The thermal profile of the substrate heater housing was simulated by assuming isothermal boundary conditions (675 1C) at all surfaces where the heater was in direct contact with the housing. The temperature profiles of the Mg crucible and flow-directing cap were modeled by simulating the induction heating of the stainless steel crucible using the measured resonant frequency (187 kHz) and peak current of the coil (200–500 A) as input parameters. Heat transfer via conduction, convection and radiation was included in the model. The physical properties of the gases were calculated using the ideal gas law and kinetic theory estimates. The thermal conductivity, viscosity and gas-phase diffusion coefficients were calculated using the Chapman–Enskog relationships [13]. The solid materials used in the simulations included stainless steel, quartz and sapphire (substrate material) and their respective properties (thermal conductivity, specific heat, emissivity, density, resistivity) were obtained from the literature [14–16]. A chemistry model developed previously for boron film deposition from B2H6 in the original single-heater HPCVD reactor was used in the simulation [12]. The gas-phase chemistry model included the reversible decomposition reaction, B2H6(g)32BH3(g) [17]. Given the lack of information on the surface chemistry of boron film deposition, a simple surface reaction model was developed. The model included the dissociative adsorption of BH3(g) on the surface to form adsorbed BH2* and 12H2(g), which was described via a sticking coefficient (g), and the surface decomposition of BH2* species to boron and H2 gas. The rate of BH2* decomposition on the surface was assumed to be orders of magnitude larger than the rate of BH3(g) adsorption such that the adsorption reaction was the rate-limiting step in the process. The surface reactions were assumed to occur on all heated surfaces in the reactor chamber. The sticking coefficient (g) of BH3(g) was the only adjustable parameter in the model. A value of 3  10 3 was obtained for g by fitting the model to experimentally measured growth rate data obtained from boron film deposition in the original single-heater HPCVD reactor over a wide range of growth conditions [12]. The sticking coefficient value obtained from the fit was in the range of values previously published for BH3 adsorption on a silicon surface (1 10 2–1 10 4) [17,18]. The transport and chemistry model for boron film deposition described above was used to predict the effect of process conditions on the growth rate and uniformity of boron films in the impinging jet reactor geometry. The parameters investigated included the Mg crucible temperature, the carrier gas type (H2 vs. Ar) and the flow rate of the B2H6/H2 gas mixture through the inlet jet tube. The model predictions were compared to the experimentally measured boron film deposition rates and were used to identify a suitable set of conditions for MgB2 film deposition in this reactor geometry.

1503

Ar: 500°C 15

Ar: 700°C H2: 300°C Ar: 600°C

10

H2: 500°C 5

H2: 600°C

H2: 700°C 0 0

3

9 6 12 15 Distance Across Substrate (mm)

18

4.1. Effect of carrier gas and crucible temperature The effect of Mg crucible temperature was first investigated because the B2H6 gas comes into direct contact with the heated jet

Fig. 2. (a) Predicted and (b) experimentally measured boron film growth rate profiles as a function of the Mg crucible temperature for H2 and Ar as the carrier gas. Substrate temperature 675 1C, total inlet jet gas flow rate 550 sccm, purge gas flow rate 400 sccm, reactor pressure 25 Torr.

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This is due to pre-heating of the gas mixture in the inlet line, which results in premature gas-phase decomposition of B2H6 and deposition of boron on surfaces of the jet nozzle and flowdirecting cap upstream of the substrate. This is problematic for the growth of MgB2 thin films where crucible temperatures greater than 700 1C are required to obtain a high Mg partial pressure. The model predictions demonstrate that the lower thermal conductivity of Ar reduces the extent of heat transfer into the gas stream, enabling increased boron film growth rates to be obtained at high crucible temperatures (4700 1C). The growth rate uniformity, however, is significantly worse with Ar carrier gas. In the central region of the substrate, which is directly above the impinging gas jet, a decrease in boron film growth rate is observed at lower crucible temperatures (o600 1C). Boron film growth experiments were performed under identical conditions for comparison to the model predictions. As shown in Fig. 2(b), the experimentally measured boron growth

H2Carrier

ArCarrier

Tcrucible= 760°C

Tcrucible= 780°C

Temperature (°C) 960 800 600 400 200 25

Temperature (°C) 1030 1000 800 600 400 200

25

rates with H2 as the carrier gas are in the range of those predicted by the model and decrease with increasing crucible temperature. The boron thickness profile is not symmetric across the substrate, which was observed for all films in this study. This is believed to be due to slight misalignments of the B2H6 inlet jet tube and substrate heater with respect to the central axis of the reactor. In the case of Ar carrier gas, the experimentally measured boron growth rate was not strongly dependent on Tcrucible. Over the crucible temperature range of 500–700 1C, the maximum boron film growth rate remained relatively constant at 16.370.8 nm/s with a similar uniformity profile. These results are generally consistent with the model predictions (Fig. 2(a)) for growth in an Ar carrier gas in the area outside the central portion of the substrate. The experimental results, however, do not exhibit the drop in growth rate at the center of the substrate with decreasing crucible temperature that was predicted by the model. This is likely due to the misalignment of the inlet tube and substrate heater mentioned above, which is expected to produce asymmetric temperature and velocity profiles in the reactor compared to the model predictions. The gas-phase temperature and BH3 concentration profiles (Fig. 3) obtained from the model illustrate the effects of carrier gas on the boron film growth rate at high crucible temperatures. The use of H2 as the carrier gas (Fig. 3(a)) results in substantial heating of the B2H6/H2 gas mixture as it passes through the heated crucible region, which results in a pronounced decrease in the gas-phase partial pressure of BH3 near the substrate surface (Fig. 3(c)). In the case of Ar, the lower gas thermal conductivity produces a ‘‘cold-finger’’ effect, which reduces the gas-phase temperature at the center of the substrate (Fig. 3(b)). This reduces the extent of B2H6 pre-reaction and results in an increase in the BH3 partial pressure near the substrate surface (Fig. 3(d)) and thereby an increased boron film growth rate. However, the significant variation in gas temperature and BH3 partial pressure in the region near the substrate surface also produces a large variation in film thickness as a function of position as was observed experimentally with Ar (Fig. 2(b)). As the crucible temperature is further reduced, this cold-finger effect becomes more pronounced and leads to a reduction in the gas temperature and BH3 concentration near the substrate surface, which is responsible for the drop in the predicted boron film growth rate in the central region of the substrate shown in Fig. 2(a).

4.2. Effect of the inlet jet gas flow rate

PBH3 (Torr)

PBH3 (Torr) 0.0490.04 0.03 0.02 0.01

0

0.036 0.03

0.02

0.01

0

Fig. 3. Simulated gas-phase temperature profiles for (a) H2 and (b) Ar as the carrier gas and corresponding BH3 partial pressure profiles for (c) H2 and (d) Ar in the impinging jet reactor. The Mg crucible temperatures were 760 1C (H2) and 780 1C (Ar). Substrate temperature 675 1C, total inlet jet gas flow rate 550 sccm, purge gas flow rate 400 sccm, reactor pressure 25 Torr. The vertical lines are the inner and outer quartz walls of the double-walled reactor used for water cooling.

In the impinging jet configuration, the gas-phase temperature and concentration profiles are expected to be strongly dependent on the flow rate of gases through the inlet jet tube. The transport and chemistry model was therefore used to investigate the effect of inlet jet flow rate on boron film growth rate and uniformity for the two different carrier gases considered in this study (H2 and Ar). In the calculations, the substrate and Mg crucible temperatures were held constant at 675 and 700 1C, respectively, and the total gas flow rate through the inlet jet tube was varied over the range from 110 to 825 sccm holding the B2H6 inlet partial pressure (i.e. inlet concentration) constant at 0.125 Torr. Fig. 4(a) shows the predicted boron film growth rates as a function of the radial position (distance from the center of the substrate) for the case of H2 vs. Ar carrier gas and varying inlet jet gas flow rates. When H2 is used as the carrier case, the growth rate is predicted to increase with increasing inlet jet gas flow rate due to reduced pre-heating of B2H6 and depletion of BH3 from the gas phase. A similar trend is observed with Ar carrier gas; however, at the highest inlet jet flow rate (825 sccm), a drop in growth rate at

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Ar

4.3. MgB2 film growth

H2

825 sccm 550 sccm 275 sccm 165 sccm 110 sccm

10 8 6

Center of Substrate 18

4 2 0 12

8

4

0

4

8

12

Radial Position (mm)

25 Boron Film Growth Rate (nm/sec)

Although the chemical mechanism for MgB2 is not fully understood, the results of boron film modeling and validation experiments have important implications on the choice of process conditions for the growth of MgB2 films using the impinging jet

Increasing Jet Flow Rate

Center of Substrate 110 sccm 165 sccm

20

275 sccm 550 sccm

15

MgB2 Film Growth Rate (nm/sec)

Boron Film Growth Rate (nm/sec)

12

1505

125 sccm

16

225 sccm

14 12 10 8 6 4 2 0 0

3

6 9 12 Distance Across Substrate (mm)

15

18

825 sccm

10

5

0 0

3

6 9 12 15 Distance Across Substrate (mm)

18

Fig. 4. (a) Predicted boron film growth rate profiles as a function of the total inlet jet gas flow rate for an Ar and H2 carrier gas and (b) experimentally measured boron film growth rate using only Ar as the carrier gas. The substrate temperature was constant at 675 1C and the Mg crucible temperature was constant at 700 1C. The purge gas flow rate was 400 sccm and the reactor pressure was 25 Torr.

the center of the substrate is again predicted to occur due to the ‘‘cold-finger’’ effect described previously. In both cases, the film uniformity is predicted to degrade with increasing inlet jet gas flow rate. The effect of inlet jet gas flow rate on boron film growth rate and uniformity was experimentally investigated using process conditions identical to those used in the model. In this case, only Ar carrier gas was used given the similar trends predicted for Ar and H2 in Fig. 4(a). As shown in Fig. 4(b), boron growth rate and uniformity follow the trends predicted by the model, although the experimentally measured growth rates are higher than the model predictions by a factor of approximately two. At the highest inlet jet gas flow rate (825 sccm), a small decrease in growth rate was measured in the central region of the substrate slightly offset from the central axis, although it is not as pronounced an effect as that predicted by the model (Fig. 4(a)). The results demonstrate that the growth rate of the boron films can be increased by the use of an Ar carrier gas and increased inlet jet gas velocities; however, the film uniformity is significantly degraded at high inlet jet gas flow rates due to nonuniformities in the gas temperature and BH3 partial pressure across the substrate surface.

Fig. 5. (a) MgB2 film growth rate profiles and corresponding SEM images of the film surface for inlet jet gas flow rates of (b) 125 sccm and (c) 225 sccm in an Ar carrier gas. The substrate and Mg crucible temperatures were 765 1C, the reactor pressure was 25 Torr and the Ar purge gas flow rate was 100 sccm.

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5. Summary and conclusions An impinging jet reactor configuration was developed in order to gain more process flexibility and increase the growth rate of superconducting MgB2 thin films deposited by HPCVD. A transport and chemistry model of boron film deposition from B2H6 gas in the impinging jet reactor was initially used to evaluate the effect of process conditions and carrier gas type on boron film growth rate and uniformity in order to identify conditions that minimized B2H6 pre-reaction and depletion upstream of the substrate. The modeling results, which were validated with experimental data, indicated that the use of an Ar carrier gas, which has a lower thermal conductivity than H2, reduces heat transfer from the Mg crucible and thereby suppresses depletion of B2H6 in the impinging jet reactor. The effect of inlet jet gas flow rate on boron growth rate and uniformity was also evaluated to identify conditions that yielded increased boron film growth rates with good thickness uniformity. The boron film growth rate studies were used to identify an initial set of process conditions for the growth of MgB2 thin films in an Ar carrier gas in the impinging jet reactor. The MgB2 growth rate and structural properties were found to be strongly dependent on the inlet jet flow rate. At lower inlet flow rates, epitaxial MgB2 films were

(101) 3000 2500 (110) Intensity(counts)

2000

(102) (201)

(100)

1500

(112)

(111) 1000

(002)

(001)

(200)

500 0 20

30

40

50 θ-2θ

60

70

80

90

35 8 30

ρ (μΩ-cm)

HPCVD reactor configuration. While the use of a lower thermal conductivity Ar carrier gas reduces pre-heating of the B2H6 gas via heat transfer from the Mg crucible, it also introduces a significant non-uniformity in the gas-phase temperature profile, particularly at high inlet jet gas flow rates, which degrades the boron film thickness uniformity and is expected to impact MgB2 film deposition in a similar way. As a result, for the MgB2 film growth experiments, reduced inlet jet gas velocities were used to obtain an increased deposition rate while maintaining good film uniformity across a 1 1 cm2 central region of the substrate. The MgB2 films were deposited using a substrate temperature of 765 1C and a crucible temperature of 765 1C. The total inlet gas jet flow rate was varied between 125 and 225 sccm total using Ar as the carrier gas. The Ar purge gas flow rate was 100 sccm, the reactor pressure was 25 Torr, the B2H6 partial pressure was 0.125 Torr and the growth time was 3 min. The MgB2 film growth rate as a function of distance across the substrate is shown in Fig. 5(a) for two films grown using a total inlet jet gas flow rate of 125 and 225 sccm. The average growth rate of the MgB2 film grown with a total inlet jet gas flow rate of 125 sccm was 1.270.4 nm/s across the substrate surface with good thickness uniformity. As shown in Fig. 5(b), the film exhibited a textured surface similar to that previously obtained for epitaxially oriented MgB2 films deposited in the single-heater and dual-heater HPCVD reactor configurations [3,6]. An increase in the inlet jet gas velocity to 225 sccm resulted in an increase in the maximum growth rate of the MgB2 films to 15 nm/s (54 mm/h), which is suitable for the growth of thick MgB2 layers. However, the film thickness varies significantly across the substrate surface as observed for boron film deposition under similar conditions (Fig. 4(b)). The thickness profile is asymmetric again due to misalignment of the substrate heater and inlet jet tube. The thick MgB2 deposited at the higher inlet jet gas flow rate exhibits a rough surface morphology (Fig. 5(b)) consisting of misaligned faceted crystals. X-ray diffraction measurements of this film (Fig. 6(a)) indicate that it is polycrystalline consistent with prior reports [19]. The temperature-dependent resistivity profile obtained on this sample is shown in Fig. 5(b). A transition temperature (Tc) of 39.5 K was measured with a sharp transition (DTc0.1 K). The Tc value is slightly lower than previous epitaxially oriented films grown by HPCVD [3].

25 ρ (μΩ-cm)

1506

6 4 2 0

20

38

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39 40 T (K)

41

10 5 0 0

50

100

150

200

250

300

Temperature (K) Fig. 6. (a) X-ray diffraction spectrum and (b) plot of resistivity vs. temperature for MgB2 film deposited in an Ar carrier gas with an inlet jet gas flow rate of 225 sccm.

obtained on the sapphire substrates with good thickness uniformity. At higher inlet flow rates, polycrystalline MgB2 films were obtained at growth rates up to 50 mm/h.

Acknowledgements This work was supported in part by the NSF under Grant nos. DMR-03060746 (X.X.X and J.M.R.) and DMR-0405502 (Q.L.) and by ONR under Grant nos. N00014-06-1-1019 (J.M.R.) and N00014-071-0079 (X.X.X).

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