Wafer engineered long wave infrared photodiodes
Long-wavelength infrared (LWIR) detectors operating in 8 – 16 μm are crucial for military and commercial applications, including satellite-based surveillance, atmospheric pollution detection, and astrophysical imaging. HgCdTe (MCT) alloys are the well-established materials for LWIR detection with high performance and wavelength tunability. However, MCT alloys suffer from short carrier lifetimes due to high Auger recombination rate, and low electron and hole effective masses, resulting in high dark current due to tunnelling. Besides, the weak Hg-Te bond causes bulk, surface, and interface instabilities. Moreover, the MCT-based LWIR detectors require cryogenic cooling equipment with large size . Thus, there is an urgent need to develop an alternative to MCT materials.
Type-II superlattices (T2SLs) on InAs/GaSb was first proposed by G. A. Sai-Halasz , which has attracted considerable attention because of their Type-II broken-gap alignment with conduction band minimum of InAs lying below the valence band maximum of GaSb , as shown in Fig.1. The electrons are mainly confined in InAs layers, while holes are in GaSb layers. The overlap of electron (hole) wavefunctions between adjacent InAs layers (GaSb layers) forms conduction miniband (valence miniband). In comparison to conventional MCT-alloys, their lower tunnelling current due to higher effective mass of electrons and holes , lower Auger recombination rate because of split heavy hole and light hole bands by strain  and the spatial separation of electron and hole, stronger bonds, and higher structural stability  make the InAs/GaSb T2SL a potential alternative for LWIR detector application.
Since both InAs (6.0583 Å) and GaSb (6.0959 Å) are members of 6.1 Å family, the InAs/GaSb T2SLs provide the flexibility for the design and growth of novel device structures. LWIR InAs/GaSb T2SLs have been successfully applied in innovative device architecture for LWIR detection to realize high operating temperature (HOT) infrared detectors with a low dark current, such as nBn , pBp , pBn , p-π-M-n , PbIbN , and CBIRD . However, these structures were usually grown on GaSb substrates, which suffer from high cost, small wafer size, and a strong absorption coefficient of ~ 100 cm-1 for IR radiation above 5 μm . The size of GaSb substrates is limited, only up to 6 inches with a price of 500 – 600 USD per unit for a 2-inch wafer. Compared to GaSb substrates, Si wafers with large size up to 12 inches (300 mm) are available, and the price is usually around 100 – 200 USD per unit for a 12-inch wafer. Implementation of Si wafers as the substrate for the epitaxial growth of InAs/GaSb T2SLs is extremely attractive for the future development of LWIR detectors and focal plane arrays (FPAs). Traditional photodetector arrays are connected to a Si readout integrated circuit (ROIC) by using hybrid flip-chip bonding, which is expensive and involves several steps including wafer bonding and substrate thinning, limiting the yield and detector array size and restricting their potential for scale-up production . Direct heteroepitaxial growth of LWIR InAs/GaSb T2SLs on a Si substrate provides significant advantages due to the availability of low cost, large area, and mechanically robust Si wafers, which are compatible to ROIC, allowing for the fabrication of FPAs with reliable long-term thermal cycling. Therefore, heterogeneous integration of LWIR detectors on Si is a crucial step to the mass production of LWIR FPAs with high throughput and low cost. However, direct heteroepitaxial growth of T2SLs on Si faces challenges due to material dissimilarities, including the polar/non-polar III-V compound semiconductor/Si interface, large lattice mismatch (~ 14%) and different thermal expansion coefficients, which cause defects such as antiphase domains (APDs) and a high density of threading dislocations (TDs). These defects exert strong negative effects on the performance of optical devices. Thus, LWIR InAs/GaSb T2SLs on Si wafers with high quality are required. Despite the difficulty of the growth of Sb-based T2SLs on Si, some researchers have made a remarkable progress in the Sb-based MWIR detector on Si –. However, the epitaxial growth of LWIR T2SLs on Si is still not available and further study is required.
For the growth of LWIR InAs/GaSb T2SLs on Si wafers, a proper buffer design is essential. Two possible buffer structures have been proposed as indicated in Fig.2 and Fig.3. In the first design, a GaAs is deposited at first by using strained layers or a Ge/SiGe buffer layer. Then, a GaSb layer is grown on top of the GaAs with an interfacial misfit array (IMF) technique. To improve the deposition efficiency, the Ge/SiGe buffer layer on Si can be deposited by CVD. In the second design, a GaSb buffer layer can be deposited on Si by applying a thin AlSb nucleation layer and strained AlSb/GaSb superlattices.
Besides InAs/GaSb T2SLs, InAs/InAsSb T2SL has been proposed as a potential candidate for LWIR detector. Shorter Shockley-Read-Hall (SRH) minority carrier lifetimes of InAs/GaSb superlattices due to Ga-related defects were the main motivation to move the research to InAs/InAsSb superlattices . Compared to InAs/GaSb superlattice, infrared detectors based on Ga-free InAs/InAsSb type-II superlattices show longer minority carrier lifetimes , , which can be ascribed to the position of localized defects above the conduction band edge in a superlattice , providing a high defect-tolerance structure. Moreover, there is only one changing element (Sb) between InAs and InAsSb layers, which demands no strict interface control. In addition, the superlattice can be grown in a strain-balanced manner on a GaSb buffer layer, as the InAsSb layer is compressively strained with a thick InAs layer under tensile strain. Thus, there is a possibility of integration of LWIR InAs/InAsSb T2SLs on a Si substrate.
MBE was widely used for the deposition of Sb-based T2SLs and multi-wafer production of LWIR photodetector structures on 150 mm GaSb substrates has been demonstrated by S. A. Nelson et al. at IQE on industrial MBE equipment . However, to further improve the production yield and reduce the fabrication cost of LWIR detectors, the deposition of LWIR InAs/GaSb T2SLs by metalorganic chemical vapor deposition (MOCVD) plays an important role in mass production. The growth of InAs/GaSb T2SLs on GaSb , GaAs , and InAs substrates  have been reported, however, that on Si substrates grown by MOCVD is still not available. Since the GaAs-on-Si – and GaSb-on-Si ,  templates grown by MOCVD have been demonstrated, further study for integration of LWIR InAs/GaSb and InAs/InAsSb T2SLs on Si substrates is required, which paves the way for the scale-up production of LWIR detectors, as well as other devices based on antimonides.
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