Further Reading
Further Reading
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- Detached Melt and Vapor Growth of InI in SUBSA Hardware
Abstract
Abstract
Indium iodide (InI) is ideal for space experiments in the SUBSA Transparent Furnace (LINK) because it is non-toxic, and has a relatively low melting point of only 351 C (~160 C below the melting point of InSb used in the SUBSA experiments in 2002). InI melts and evaporates congruently, allowing directional solidification from the melt at a rate of 5 mm/hr and growth from the vapor at ~ 5 mm/week. InI is considered to be one of the very few semiconductor materials that that could compete with CdZnTe (CZT) and HgI2 as detectors of nuclear radiation at room temperature. CZT is expensive, toxic and has to be grown very slowly (~ 5 mm/day). HgI2 is soft, toxic and has to be grown from the vapor phase at ~ 1 cm/month.
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During the past 3 years, using distillation and zone refinement, we demonstrated reduction of metallic impurities in InI down to the ppb level. However, the concentration of defects in InI single crystals grown on Earth remains high.
Here, we propose to reduce the density of defects in InI single crystals, by growing InI crystals in the SUBSA hardware, under microgravity conditions. Our objective is to experimentally obtain material properties approaching those expected from theory, establishing InI as a room temperature detector material that can compete against the expensive and difficult to grow cadmium zinc telluride (CZT) and HgI2 based detectors.
The SUBSA transparent furnace will allow precise seeding and real-time visualization of the growth processes. We propose to grow:
Three single crystals from the melt. Since the crystalline perfection is much greater during detached solidification, the ampoules will be designed to promote dewetting. Thus, stress-induced defects will be minimized.
Three samples using physical vapor transport process. As a rule, vapor growth experiments conducted in microgravity were not disturbed by convection, and have given crystals with superior properties, e.g. higher mobility, etc.
The low melting point of InI will result in low temperature gradients, which are calculated to be ~20 K/cm. Reference quality crystals will be obtained and used for detector fabrication and testing. The detector testing will be done at Radiation Monitoring Devices Inc. (RMD).
We propose to prepare InI charges in a form of ultra high purity single crystals at the PI’s laboratory at IIT. We produced 17 flight ampoules for the SUBSA investigation in 2002, and still have the Sample Ampoule Assemblies (SAA) which were fabricated by Tec-Masters Inc. In the interest of cost and time, the SAA’s can be refurbished with new ampoules and thermocouples, and reused. Detached growth of InI in the SUBSA transparent furnace will allow the dewetting process to be visualized and studied. This will advance the development of the process and equipment, for detached growth on Earth, for the future fabrication of InI and other stress-sensitive semiconductor materials.
Fig. Control Center at Techshot Inc in April 2018
Fig. Astronaut David St Jacque at the ISS, in front of the MSG loading the SUBSA furnace
2. Diffusion Coefficients of Dopants in Si and Ge Melts
2. Diffusion Coefficients of Dopants in Si and Ge Melts
Abstract
Abstract
Electronic properties of single semiconductors crystals depend on the diffusion coefficients of impurities and dopants dissolved in the melt. The diffusion coefficients appear in the governing equation which control the transport of impurities in the melt from which single crystal are grown. Therefore, the diffusion coefficients are required input into the key equations and finite element (FE) models used by industry to optimize a crystal growth process or zone refining.
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Diffusion is a slow process, affected by minute levels of natural convection. The experiments and FE investigations conducted so far indicate that:
On Earth, the values of the diffusion coefficients measured in the capillaries and under strong magnetic fields, are inflated by the natural the unavoidable natural convection.
In orbital laboratories, i.e. in “microgravity”, the level of buoyancy forces driving natural convection is reduced by approximately 10^5 to 10^6 times. Yet, convective interference with diffusion was reported in several investigations.
Considering the above, the key goal of our SUBSA investigation (1995 to 2004) was to achieve diffusion controlled growth of Te and Zn doped InSb and measure the diffusion coefficients.
We propose to conduct, on earth and in microgravity, (a) diffusion experiments using bundles of capillary tubes and (b) directional solidification experiments. The key goal is to determine precisely the diffusion coefficients of Ga, B, Sb and Si in germanium. Furthermore, our goal is to develop two facilities on Earth, one at MSFC using the 5 Tesla axial magnet, and one at IIT using the transverse 1 Tesla magnets. Both should enable measuring diffusion coefficients of impurities/dopant in molten semiconductors. Again, the data obtained in microgravity, will be used to assess and calibrate the proposed facilities.
In preparation for the microgravity experiments, ground based experiments will be conducted under creeping-flow condition, using for single crystal growth using:
5 Tesla axial magnetic field for Bridgman growth in 1.5 cm diameter tubes (with and without the baffle).
5 Tesla axial magnetic fields for Bridgman growth in capillary tubes.
The pioneering BPS approach: 2 cm diameter crystals of Ge and Si crystals will be grown from 5 cm diameter melts. The small size ensures laminar forced convection in the melt since buoyancy-driven convection is minimized. Thus, the BPS model, other models and finite element (FE) modeling will be used to determine the diffusion coefficients.
Furthermore, the diffusion coefficients will be measured “directly” using the capillary- reservoir method (see Shashkov and Gurevich) which is simple and sufficiently accurate:
In a Bridgman furnace, under 5 Tesla axial magnetic field, using spiral-shaped capillary tube.
In Czochralski furnaces, as described in the BPS paper.
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