Search

As size requirements and pixel viabilities for infrared focal plane arrays (IRFPAs) continue to increase, resolution and sensitivity requirements for high performance advanced imaging systems must meet or surpass stringent demands. Strain layer superlattice (SLS) grown by molecular beam epitaxy (MBE) on 100mm GaSb has necessitated changes in crystal processing and finishing parameters. Device layer growth typically requires a thin (2-5 nm) and highly desorbable surface oxide on very flat substrates for successful MBE. This study compares the ability for rapid pre-epi desoprtion of three different chemo-mechanical (CMP) finishes on 100mm n:GaSb: CMP-1 with sequential double side polished (DSP), CMP-2 with sequential DSP, and CMP-2 with simultaneous double side polished (S-DSP). X-ray photoelectron spectroscopy (XPS) reveals the improvement from a CMP-1 (Ga-oxide rich) to CMP-2 (Sb-oxide rich) surface. No difference in surface chemistry was found between the CMP-2 of the sequential vs. simultaneous DSP. Tropel flatness measurements of the 100mm n:GaSb substrates show that both DSP and SDSP substrate batches yield excellent (<5m) wafer warp. However, initial studies have shown a more consistent wafer flatness with use of the simultaneous-DSP process. MBE growth on the Sb-rich surface was examined by high resolution XRD and resulted in a 64.7A periodicity and excellent FWHM (~20 arcsec) which verified the GaSb surface finish effectiveness. The resultant surface finish and flatness may provide a benefit for larger diameter GaSb IRFPA applications.

Engineered substrates such as large diameter (100mm) GaSb wafers need to be ready years in advance of any major shift in DoD and commercial technology, and typically before much of the rest of the materials and equipment for fabricating next generation devices. Antimony based III-V semiconductors are of significant interest for advanced applications in optoelectronics, high speed transistors, microwave devices, and photovoltaics. GaSb demand is increasing due to its lattice parameter matching of various ternary and quaternary III-V compounds, as their bandgaps can be engineered to cover a wide spectral range. For these stealth and spaced based applications, larger format IRFPAs benefit clearly from next generation starting substrates. In this study, we have manufactured and tested 100mm GaSb substrates. This paper describes the characterization process that provides the best possible GaSb material for advanced IRFPA and SLS epi growth. The analysis of substrate by AFM surface roughness, particles, haze, GaSb oxide character and desorption using XPS, flatness measurements, and SLS based epitaxy quality are shown. By implementing subtle changes in our substrate processing, we show that a Sb-oxide rich surface is routinely provided for rapid desorption. Post-MBE CBIRD structures on the 100mm ULD GaSb were examined and reveals a high intensity, 6.6nm periodicity, low (15.48 arcsec) FWHM peak distribution that suggests low surface strain and excellent lattice matching. The Ra for GaSb is a consistent ~0.2-4nm, with average batch wafer warp of ~4 m to provide a clean, flat GaSb template critical for next generation epi growth.

A key component for third generation FPA development is the megapixel strain layer superlattice (SLS) structures on GaSb substrates for advanced infrared detectors. A significant aspect that inhibits widespread application of large format device growth on GaSb is the starting substrate size. Recently, the Czochralski method resulted in the world's first 100mm GaSb boules. The 100mm GaSb substrates can be ultra-low doped (n~4-9x1015/cm3) for extended IR wavelength transparency. A plethora of changes to the manufacturing process is required for consistent 100mm GaSb growth and substrate polishing. In this study, we examined the surface quality of the 100mm GaSb as a function of a standard and experimental Polish "A" which incorporated an additional CMP step as well as a longer final polish time. Atomic force microscopy (AFM) and power spectral density (PSD) as a function of polish process measured the surface morphology. Interferometry was used to analyze free standing wafer flatness. Electron spectroscopy for chemical analysis (ESCA) determined surface oxide thickness, and successful MBE growth of a 400 period Complimentary Barrier Infrared Detector (CBIRD) structure assessed SLS based device suitability. The epi structure was examined by x-ray diffraction (XRD). The low 0.3-0.4nm Ra starting 100mm GaSb roughness values, the wafer flatness ~2.3m per 16 wafer batch, the low FWHM SLo = 15.48 arsec of the successful CBIRD epi growth and related high intensity XRD ~6.6nm periodicity peaks suggest that the modified polish provides the 100mm GaSb with a desirable epi ready character and excellent surface crystallinity for advanced IRFPA applications.