Mercury Cadmium Telluride (MCT) Material Growth
Metal Organic Vapour Phase Epitaxy (MOVPE)
MOVPE is SELEX Galileo's preferred MCT wafer-scale material growth technology, and the Company has been developing the capability for over 25 years.
MOVPE uses the interdiffused multilayer growth process (IMP) within a horizontal, atmospheric-pressure reactor. IMP gives improved composition control, with CdTe and HgTe grown separately under optimised conditions, and enables acceptor doping to be achieved using arsenic without any post-growth anneal schedules. Donor doping is achieved using iodine. The composition, layer thickness and doping is controlled using software. The process time is approximately 1 week to produce and assess a layer. Assessment consists of Fourier transform infrared spectrometry for wavelength and thickness data, and secondary ion mass spectrometry for doping and composition profiles through the layer thickness. The current layer size is 75 mm diameter, grown on GaAs substrates for all production and R&D samples. The Company's new MOVPE reactor has 150 mm layer capability built in.
The advantages of MOVPE over other growth methods include:
- Lower cost than current MBE processes used by competitors
- Excellent result achieved for Medium Waveband (MW), Long Waveband (LW) and Dual-Waveband MW/LW detectors using low-cost GaAs substrates
- The MOVPE LW diode quality is world beating
- Low defect levels on all arrays including dual-waveband arrays
- Facilitates the growth of large format arrays currently using 75-mm diameter substrates, scaleable to 150 mm
MOVPE Reactor.
MOVPE MESA Structure. Optimised structure for low dark current and isolated absorber for minimum crosstalk.
Loophole MCT Technology
SELEX Galileo's 25 year development of the practice of Liquid Phase Epitaxy (LPE) comprises a Te-rich horizontal sliding boat process to create highly uniform MCT layers. Mercury telluride (overpressure source) is compounded at 750 °C then granulated, and the Tellurium-rich MCT growth solution is compounded at 500 °C, then pelleted. The substrate used is CdZnTe (~4% Zn). The sliding boat is high-purity graphite. The growth kit uses Pd-diffused H2 and a growth temperature of ~500 °C.
Loophole Structure
The process time is approximately 1 month from start to finished layer ready for use, although the actual layer growth process is only approximately 1 day. Layer sizes are approximately 20 x 40mm. Assessment consists of Fourier transform infrared spectrometry for wavelength mapping and Hall effect measurements for electrical assessment. LPE is used for current smaller 2-dimensional short-wave focal plane arrays such as Swift and Swan.
Advantages
- Substantial experience in loophole array fabrication at SELEX Galileo
- Mechanically very robust
- High fill factor
- Small detector pitch
However, Loophole does not utilise wafer scale processing to reduce cost at the array fabrication step as MOVPE does, and Heterojunction technology is not possible.
Bulk MCT
SELEX Galileo has over 40 years experience in this growth method. The capability has expanded from producing 12mm diameter crystals in the late 1960s, up to 70mm diameter crystals recently, the largest grown anywhere in the world to date. SELEX Galileo has produced more than 500,000 slices in total, of all wavelengths from 1 to 25 microns.
A modified Bridgman (vertical) bulk MCT growth technique known as ACRT (accelerated crucible rotation technique) is used to improve wavelength uniformity, and it is supplied only to make photoconductive arrays. This method of growth is now standardised on 20-mm diameter crystals. Assessment consists of Fourier transform infrared spectrometry for wavelength mapping and Hall effect measurements for electrical assessment. The process time is approximately 2 months from start to finished slices ready for use. Compounded and purified Cd, Te and Hg elements are crystallised in a simple 2-zone furnace, with constant slow rotation (2 rpm) or ACRT. The growth kits are housed in sealed cells for safety reasons (70 atm at ~800 °C).
Research is continuing to grow larger-diameter (50mm is the current standard) crystals for special applications with higher composition values, i.e. shorter wavelength material. The Company has produced material with a cut-off wavelength of <1.0 micron, (~88% Cd) and, of course, the tail ends of all bulk crystals eventually become mercury telluride, i.e. (0% Cd).
MCT Crystal and Schematic of our ACRT Crystal Growth Kit
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