Equipment for Low Temperature Steady-State Growth of Silicon from Metallic Solutions Thomas Teubner,* Robert Heimburger, Klaus Bo¨ttcher, Torsten Boeck, and Roberto Fornari
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2484–2488
Institute for Crystal Growth, Max-Born-Strasse 2, 12489 Berlin, Germany ReceiVed January 31, 2008; ReVised Manuscript ReceiVed March 5, 2008
ABSTRACT: Low temperature steady-state solution growth of silicon from indium on Si(001) substrates with a vertical stack of feeding source, solution, and substrate in top down position is demonstrated. The equipment especially designed for this task is presented. By applying different heating strategies the vertical temperature difference in the crucible wall is adjusted to meet the demands of the three stages of processing, that is, Si dissolution and homogenization, growth, and recycling of the solute. Heating from either the bottom or sides results in a temperature difference that should be suitable for growth. However, only for side heating growth has been observed. In order to understand this effect, numerical modeling of heat transfer and fluid flow has been performed. Heating from the side initiates a flow path, which allows both effective saturation of solvent at the feeding source and significant supersaturation at the substrate. In this case the epitaxial growth of pyramids with {111} facets on Si(001) substrates occurs. In contrast, heating from the bottom does not result in supersaturation strong enough for growth.
1. Introduction Low temperature deposition of crystalline Si layers on amorphous substrates such as glass is one of the challenging tasks in the field of photovoltaic research. As grain boundaries reduce carrier lifetime in the absorption layer there is a need for microcrystalline films with grains of at least 50 µm size in diameter in order to reduce the density of these boundaries. Low temperature chemical vapor deposition1 or aluminum induced crystallization2,3 result in grain sizes in the low nanometer or micrometer scale, respectively. An alternative concept consists of seeding crystalline Si inside liquid In droplets, which are randomly predeposited on glass surface,4 and subsequent enlargement of these seeds in a steady-state solution growth process giving rise to the polycrystalline silicon layer. Equipment and processing suitable for the second step will be the subject of this paper. Solution growth using a permanent source has been applied in two cases for Si: (i) isothermal yo-yo solute feeding method,5 driven only by solutal density differences and (ii) the temperature difference method,6 which operates with a stationary temperature gradient in a diffusion-limited mode. The approach presented here is characterized by a vertical stack of source, solution, and substrate on top and is in principle comparable to the vertical steady-state liquid phase epitaxy.7 In addition to that, the equipment presented here gives the opportunity of changing the temperature distribution in the growth zone within certain limits in the course of the experiment and allows processing without opening the furnace to ambient air. This is important since the growth of Si, especially at low temperatures, requires rigorous shielding from oxidizing substances.8 The stack order and the direction of temperature gradient applied here realize in principle a temperature difference growth characterized by superposition of buoyancy-driven solutal and thermal flow. This promotes the flow of dissolved Si from feeding source to substrate, leading to a steep concentration gradient which provides the precondition for low temperature growth. First experiments of Si deposition using indium as metallic solvent have been carried out on single * To whom correspondence should be addressed. E-mail: Teubner@ ikz-berlin.de. Phone: 49-30-63923052. Fax: 49-30-63923003.
crystalline Si(001) wafers in the 600-700 °C temperature range in order to check the mass transport and saturation conditions for crystal growth.
2. Experimental Procedures The equipment, called liquid phase epitaxy module (LPE module), has been designed in close collaboration with a manufacturer of vacuum-based research instruments.9 The LPE module consists of two nested vessels connected to a handling vacuum transport module by a gate valve. Both double-walled vessels are water-cooled. The outer high vacuum-tight stainless steel vessel protects the inner growth furnace from ambient atmosphere and accommodates the electrical leads and tubing for motor-driven sample transport, heater current, process gases, and thermocouples. Two viewports on top of the chamber serve for control of fill level in the crucible by laser triangulation. The inner stainless steel chamber contains a growth crucible, the heaters, and a thermal shielding made by an onion-like stack of three shells of fused quartz with molybdenum sheets in-between. A simplified drawing of the growth chamber can be seen in Figure 1. To ensure purity and mechanical stability exclusively graphite, synthetic fused quartz and molybdenum are used in the hot zone. The 40 × 40 mm2 Si substrate is clamped on a graphite carrier and rests top down in the opening of the paddle-like end of the quartz glass sample holder. Two sample holders which are bearing these carriers can be turned motor-driven from outside into the growth chamber and enable the movement of the substrate from a waiting position in the hot zone into the solution by continued tilting and rotation. The carrier, which has a weight of 45.4 g, floats on the surface with a maximum draft of 2.6 mm when completely released by the sample holder due to its lower density compared to liquid indium. The gaps necessary for laser triangulation measurement and dipping movement of sample holder do not allow a complete thermal sealing of growth chamber from the outer vessel. The circular crucible containing the solution has an outer diameter of 130 mm and consists of high purity graphite. The 4 mm thick Si(001) feeding source, covering completely the bottom of the crucible, is fixed by graphite clamps and is capped by a 17 mm thick layer of indium. A conventional 4 in. diameter two-element boron nitride coated pyrolytic graphite heater (Boralectric HTR1004) below the graphite crucible (bottom heater), two self-designed meander graphite heaters, a ring-shaped around the crucible (side heater) as well as a plate-shaped heater 80 mm over the crucible (top heater) allow for the intended variation of vertical temperature gradients in the solution. To avoid thermal interference both elements of the bottom heater are interconnected to a single control loop. Temperature control has been realized by Eurotherm 2704 high performance multiloop controllers. The heaters are driven with a
10.1021/cg800120q CCC: $40.75 2008 American Chemical Society Published on Web 06/06/2008
Steady-State Solution Growth of Silicon
Crystal Growth & Design, Vol. 8, No. 7, 2008 2485
Figure 1. Simplified drawing of growth chamber: a - carrier with clamped substrate in intermediate (1) and growth (2) position, b crucible with drill hole for TCbot, c - drill hole for TCtop, d - Si feeding source, e - top heater, f - thermal insulation, g - bottom heater, h - side heater. Legend:
stainless steel,
quartz,
rigid graphite felt,
heater,
silicon,
fused graphite high purity graphite,
indium solution. maximum electrical power of 910 (bottom), 3420 (side), and 6550 W (top), respectively. Two type K thermocouples with hydrogen resistant sheaths (Thermocoax specification for sheath: Ih) are vertically immersed into the crucible wall slightly apart from each other at heights differing of 12 mm. Both these thermocouples provide the process values for the heater control loops. The top heater is connected to TCtop, whereas the signal of TCbot can be switched between bottom and side heater depending on the process stage. ∆T can be varied in certain limits in both directions. Four additional TCs are fixed in the immediate vicinity of heating elements for monitoring the heater action. The samples can be locked-in into the LPE module under high vacuum (better than 1 × 10-3 Pa) through the gate valve between a handling module and outer LPE module vessel. As the chamber is permanently flushed by Pd diffused hydrogen during the process (or when in idle state) the module is purged by technical argon prior to evacuation to fulfill the requirements for safe exhausting by a combination of turbo-molecular and Roots pump. Processing then again takes place in highly purified H2. The purity of gas is measured by a HIDEN HPR-20 residual gas analyzer connected directly to the growth site by an alumina capillary inside and a heated quartz capillary outside of the vessel. The impurity content in hydrogen is calculated as the pressure ratio of residual gas to H2. Hydrogen whenever present in the LPE module runs also through the HPR-20 gas lines and the analyzer chamber for maintenance of purity. The entire process can be divided into three stages: Si dissolution and homogenization, growth, and solute recycling. The governing parameter in each case is the temperature difference ∆T ) Ttop - Tbot between both thermocouples (TC) in the crucible wall (TCtop and TCbot) as can be seen in Figure 2. The first stage, characterized by heating up to growth temperature at a rate (R) of 10 K/min leads to melting of In and uniform dissolution of feeding Si into metallic liquid up to the saturation limit. A uniform temperature (∆T ) 0 K) can be adjusted by setting the temperature set points of both control loops to the same temperature. Prior to contact of the growth specimen to the solution, a dummy substrate fixed on
Figure 2. The three stages of processing: Si dissolution and homogenization (top), growth (center), and solute recycling (bottom), each with the respective temperature (T) vs height (z) plot. Arrows indicate the direction of movement of dissolved Si inside the solution. Figures mean: 1 - substrate, 2 - In solution, and 3 - Si feeding source. the second sample holder is moved into solution in order to mechanically wipe off SiO2 flakes floating on the surface. Before uniform mixing of Si is reached, the Si substrate is brought into contact with the solution to etch away the oxidized surface layer which forms during the short period of loading into the equipment after the final HF bath of the RCA cleaning procedure. During the second stage, supersaturation on top of the solution is initiated by lowering the set point of the top heater control loop. Stationary thermal conditions can be reached in a few minutes. The growth specimen stays in contact with the solution under stationary thermal conditions and is withdrawn at the end of growth period. With the third stage the dissolved Si is brought back to the source. Because of the lower density of Si with respect to In, this step has to be carried out with maximum positive vertical temperature gradient (∆T > 0 K) at the previously selected temperature level. During slow cooling (R ) -1 K/min) the maximum gradient has to be preserved.
3. Numerical Simulation The development of temperature field and fluid flow in the crucible has been numerically investigated in order to analyze different heating arrangements during growth (bottom or side heating) and to provide a more detailed picture of the arising thermal convection in the melt. The term melt is used since dissolution of silicon atoms into the indium melt and solidification at the growth interface is not yet included in the simulation. The numerical computation consists of two steps: (1) temperature field computation throughout the whole furnace whereby gas and melt are at rest, and (2) temperature and velocity field in the inner part within the ring-shaped side heater. Velocity calculations were done only for the melt. The radiative heat flux boundary condition in step 2 has been expressed by using relevant temperature data from the outer furnace parts computed in step 1. Numerical software packages with the Finite Element Method have been applied in both steps (1: FIDAP, 2: ENTWIFE). An axisymmetric geometry and steady-state conditions were selected. The numerical convergence has been reached by stepwise enlarging the gravitational acceleration from small to full value. Heat input at both side
2486 Crystal Growth & Design, Vol. 8, No. 7, 2008
Figure 3. Variation of mass spectrometric signal of water (mass 18 dotted line) and nitrogen/carbon monoxide (mass 28 - thin solid line) dependent on the temperature of top heater (monitoring TC of top heater-thick solid line).
and bottom heating elements was scaled in such a way that the measured temperatures at the thermocouple positions were reproduced by the computation. According to the maximum power of the individual heaters, the temperature level in case of bottom heating was around 600 °C and in case of side heating around 700 °C. With the above-mentioned data supply in step 2, realistic boundary conditions have been defined especially for the fluid region to avoid temperature fixing at the inner crucible wall. Hence, the buoyancy convection in the indium melt does not only change the temperature field distribution per se, but the temperature values along the melt boundary can vary, too. In the case of heating from the side, for instance, the initial temperature gap throughout the melt (at rest) of 20 K reduces to 16 K due to thermal convection which transports heat energy additionally to heat conduction. However, the ratio of kinematic viscosity to thermal diffusivity, the Prandtl’s number, is about 0.0082 for liquid indium. This small value means that the temperature field isoline pattern is predominantly shaped by heat conduction in combination with the heat flux boundary conditions and not by the melt convection. Hence, the temperature field in the region of Si feeding source, indium melt and substrate needs to be intentionally shaped by the heat transfer design of the total furnace.
4. Results and Discussion The strong desorption of water corresponding to a ratio of the HPR-20 signals of mass 18 to mass 2 of 1.36 × 10-6 during the first heating of the LPE module after opening to ambient atmosphere vanishes during subsequent experiments. The permanent flow of hydrogen through the LPE module and mass spectrometer reduces the content of residual gases to a level of 10-9, closely comparable with conditions usually met in UHV equipment. Nevertheless, also at very low water levels, the heating starts with a slight increase of mass 18 signal. But after reaching heater temperatures above 700 °C the mass 18 signal reduces concurrently with a mass 28 signal increase as can be seen in Figure 3. This is interpreted as a desorption of water followed by reaction of residual water in H2 atmosphere with hot graphite forming carbon monoxide and hydrogen at temperatures above 700 °C. Over the time, minor pinhole-like erosion of the heating elements causing an increase of electrical resistance can be observed. Conventional LPE graphite boats also undergo such corrosion, but the effect remains undetected because (1) carbon monoxide is not monitored in simple LPE
Teubner et al.
equipment and (2) the electrical resistance does play any role for such boats. The removal of residual water by hot carbon contributes to a clean indium solution surface. Nevertheless, a tendency exists for dissolved Si to react with residual water at the solution-gas interface leading to formation of thin SiO2 flakes floating on the surface. An indication of the presence of such flakes is the deterioration of in situ solution level measurement by laser triangulation due to diffuse light scattering. When heating predominantly from the top (the condition used both for substrate in situ etching and solute recycling) the maximum positive temperature difference which can be established in the crucible wall decreases only slightly with rising temperature from +10.8 K at Ttop ) 500 °C to +9.4 K at Ttop ) 700 °C. Positive ∆T creates the necessary vertical concentration difference between the surface of the solution and the interface of the feeding source in order to avoid the formation of silicon crystallites on the surface and to drive the dissolved silicon back to the feeding source during cooling down to room temperature. Moreover, under stationary conditions at 700 °C a considerable amount of silicon initially placed on the surface of the solution was completely dissolved and deposited to the source forming epitaxial structures as observed on inspection of feeding source after the experiment. However, this is a rather slow process. When heating from the side alone the maximum negative temperature difference increases significantly with rising temperature from -6.6 K at Tbot ) 500 °C to -13.6 K at Tbot ) 700 °C. Bringing a substrate into contact with the solution at constant heating power at a temperature of Tbot ) 700 °C and ∆T ) -10 K decreases the values of TCtop and TCbot permanently by about 6 and 7 K, respectively. This thermal behavior indicates an enhanced heat flow out of the bath through contact to the substrate carrier and gives the opportunity to start crystal growth with initial supersaturation comparable to the step cooling mode of conventional LPE. When applying positive ∆T the interaction of solution with the Si substrate leads always to etch pits on the substrate surface. That means the solution is permanently undersaturated near the surface under such thermal conditions. As an example a Si(001) substrate etched for 2 h at Ttop ) 700 °C and ∆T ) +8.2 K is shown in Figure 4. This procedure results in extended etch grooves. The coalescence of residual indium as spheres is promoted by SiO2 flakes adhering to the substrate surface. Starting at Ttop ) 600 °C with ∆T ) +7.5 K for in situ etching for 1 h and then switching to ∆T ) -9.2 K by heating exclusively from the side for 2 h resulted in pyramidal growth on Si(001) substrates (Figure 5). Heating from below leads to negative temperature difference in the crucible wall but does not establish growth conditions as anticipated (see Figure 6). In this condition only etching has been observed. Improved hydrogen purity in the LPE module leads to the complete coverage of etch pits with indium. An explanation for the different growth behavior when heating from the bottom or the side can be derived from numerical simulation. The two heating arrangements lead to very different temperature distributions both within the solution and in the whole region of the crucible. In both cases, the hydrodynamic computations lead to steady-state velocity distributions. Heating from the side puts the substrate at the temperature minimum within the solution region, and this provides the driving force for solution growth (Figure 7). This
Steady-State Solution Growth of Silicon
Figure 4. SEM micrograph of Si(001) substrate surface after contact with In for 2 h at Ttop ) 700 °C with ∆T ) +8.2 K. Indium is coalesced inside the etched grooves as spheres.
Crystal Growth & Design, Vol. 8, No. 7, 2008 2487
Figure 6. SEM micrograph of etch pits completely filled with indium on a silicon (001) surface with a density of 6.4 × 103 cm-2. After etching for 1 h at Ttop ) 600 °C and ∆T ) +7.3 K heating from below resulting in ∆T ) -4.4 K took place for 2 h.
Figure 5. SEM micrograph of Si pyramids grown on Si(001) substrate for 2 h at Tbot ) 600 °C with ∆T ) -9.2 K. The pyramid surface density is 2.1 × 103 cm-2. The preceding etching was performed for 1 h at Ttop ) 600 °C with ∆T ) +7.5 K.
heating mode leads to one dominant vortex in the free solution region, the velocity of which is directed upward very near to the inner crucible wall. Though the solute transport has not yet been included into the current simulation it can be expected that this kind of vortex supports the solute transport toward the substrate region because the vortex touches the area of the highest Si concentration next to the feeding source. Heating from the bottom provides the substrate region with a high temperature compared to the rest of the solution region, and hence a large concentration gradient between feeding source and substrate position (Figure 8) cannot arise. This heating mode generates a bundle of smaller vortices, with one being dominant, the velocity of which is directed downward very near to the inner crucible wall. Unfortunately, these multiple vortices act in the “low temperature” region (in the scale of the solution) where there is no obvious effect to support the solute transport.
Figure 7. Heating from the side - temperature field (top) and velocity vector field (bottom). The left-hand boundary is the axis of rotational symmetry. The temperature difference between successive isotherms represents 0.8 K.
5. Conclusions It has been shown that the home-designed LPE module allows safe and controllable Si growth from metallic solutions under hydrogen atmosphere at low temperatures with a low level of contaminants. By establishing different heating conditions, growth and etching of silicon can be controlled. In particular the temperature distribution in the furnace can be varied within certain limits by adjusting the power of the three heaters surrounding the crucible. The vertical temperature difference necessary for stationary growth has been found to be of the order of magnitude of -10 K and can be established by combined heating from the sides and the top. Growth on
2488 Crystal Growth & Design, Vol. 8, No. 7, 2008
Teubner et al.
higher when the crucible is heated from the side. Growth then takes place on the surface of the substrate. Determination of Si growth rates at 600 °C especially on {111} facets will be the subject of further investigation. Acknowledgment. The initial work was supported by the German Federal Ministry of Economics and Technology under contract AiF KF0116401. More recent activities have been sponsored by BP Solar. Many thanks to Ralf Steudten and Doreen Schneider of Roth & Rau AG both for the engaged realization of our ideas leading to equipment with high functionality and for their continuous support. The authors thank K. Andrew Cliffe (Nottingham University, UK) for helpful advice when using the ENTWIFE package. Thanks also to Marlen Schulze for the SEM micrograph in Figure 4.
References
Figure 8. Heating from the bottom - temperature field (top) and velocity vector field (bottom). The left-hand boundary is the axis of rotational symmetry. The temperature difference between successive isotherms represents 1.3 K.
Si(001) substrates was demonstrated using the side heater for adjusting the temperature Tbot. In contrast, growth failed when heating from the bottom. This behavior is explained by results of numerical calculations of the temperature and velocity field. While in both cases (heating from the side and from the bottom) a negative temperature difference was calculated, the velocity field of solution between feeding-substrate and sample holder exhibits opposite algebraic signs. However, the path available for both saturation and supercooling is longer and the temperature difference between feeding source and substrate is much
(1) Edelman, F.; Chack, A.; Weil, R.; Beserman, R.; Khait, Yu L.; Werner, P.; Rech, B.; Roschek, T.; Carius, R.; Wagner, H.; Beyer, W. Structure of PECVD Si:H films for solar cell applications. Sol. Energy Mater. Sol. Cells 2003, 77, 125–143. (2) Nast, O.; Puzzer, T.; Koschier, L. M.; Sproul, A. B.; Wenham, S. R. Aluminium induced crystallization of amorphous silicon on glass substrates above and below the eutectic temperature. Appl. Phys. Lett. 1998, 73, 3214–3216. (3) Aberle, A. G. Progress with polycrystalline silicon thin-film solar cells on glass at UNSW. J. Cryst. Growth 2006, 287, 386–390. (4) Boeck, T.; Teubner, Th.; Schmidt, K.; Wilde, P.-M. A method to grow silicon crystallites on glass. J. Cryst. Growth 1999, 198/199, 420–424. (5) Sukegawa, T.; Kimura, M.; Tanaka, A. Gravity effect on dissolution and growth of silicon in the In-Si system. J. Cryst. Growth 1988, 92, 46–52. (6) Thomas, B.; Mu¨ller, G.; Wilde, P.-M.; Wawra, H. Growth of polycrystalline silicon thin films using the temperature difference method. In Proceedings of the 14th European PhotoVoltaic Solar Energy Conference; Ossenbrink, H. A.; Helm, P.; Ehman, H.; Eds.; H.S. Stephens & Associates: Bedford, UK, 1997; Vol. 2, pp 1483-1486. (7) Long, S. I.; Ballantyne, J. M.; Eastman, L. F. Steady-state LPE growth of GaAs. J. Cryst. Growth 1974, 26, 13–20. (8) Bergmann, R.; Kurianski, J. The role of hydrogen in silicon liquid phase epitaxy. Mater. Lett. 1993, 17, 137–140. (9) Roth & Rau AG. http://www.roth-rau.de (accessed January 31, 2007).
CG800120Q
