Crystal Systems- Silicon

HEM Silicon

Large multicrystalline silicon ingots, up to 58 cm square cross section weighing 200 kg, have been produced using the Heat Exchanger Method (HEM).

These ingots are produced by adaptation of a single crystal growth process, the Heat Exchanger Method, so that controlled directional solidification can be carried out at low cost. Growth is therefore achieved from the bottom of the crucible to the top, with a planar solid-liquid interface such that control is exercised even up to the top corners of a square cross section silicon ingot. During solidification, the ambient is controlled to yield low oxygen and carbon concentrations. After solidification is completed, the ingot is in situ annealed to reduce residual stress and produce uniform properties. From loading to unloading the cycle time is about 50 hours.

HEM multicrystalline silicon ingots can be used for many high technology applications. For solar cell applications, HEM silicon is characterized by long diffusion length, cm-size grains, vertical orientation of columnar grain boundaries, low oxygen concentrations and narrow resistivity range. The structural and electrical properties are uniform from top to bottom and center to edges of the ingot.

In research during investigation for potential of HEM silicon for photovoltaic applications, world's highest efficiency multicrystalline silicon solar cells - 18.6% (1 cm2 area) - were fabricated by Georgia Institute of Technology. HEM silicon is compatible with various solar cell processing techniques. High efficiency solar cells have been fabricated world wide.

HEM Silicon Ingot
Production Furnace

HEM furnaces with complete automation have been used for production of large multicrystalline silicon ingots upto 58 cm square, 200 kg. A custom, software-based computer program controls the processing cycle and includes:

a safety checklist for operation of furnace,
a checklist of pre- startup procedures,
processing involving Heat-up, Melt-in, Growth, Anneal and Cooldown cycles,
a separate cycle for maintenance,

a bridge cycle for future use.

Control features involve power, temperature, gas flow, chamber pressure, temperature gradients, etc. within a narrow specified range. Emphasis is on automation and reproducibility in a production environment. Labor requirements have been limited to loading and unloading of the furnace.

Characteristics of silicon produced in the HEM Silicon Ingot Production Furnace are:

58 cm square cross section ingot, 200 kg with a cycle time of 50 hours,
complete automation processing,
large grain size with vertical orientation of grain boundaries,
no impinging interfaces and nop grit structure,
narrow resistivity range,
world's record multicrystalline solar cells on small area devices-18.6%, 1 cm square area,
compatibility with various solar cell processes,

upgrading of poor quality meltstock.

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