InSb infrared p–i–n photodetectors grown on GaAs coated Si substrates by molecular beam epitaxy
Introduction
InSb has been an important semiconductor for infrared detector applications in the 3–5 μm atmospheric window, and there has been significant interest in large area InSb focal plane arrays (FPAs). InSb offers important advantages over HgCdTe, the other widely used infrared material, such as relatively easy large area homogeneous growth and stronger covalent bonding. While InSb photodiodes on InSb substrates have extensively been researched, there has been very limited work on the growth of this material on alternative substrates such as Si1, 2, 3, 4, 5. However, for the development of large area FPAs and monolithic integration with the Si readout circuit, growth of this material on Si substrates is indispensable. Growth on a larger bandgap Si substrate eliminates the need for a yield lowering substrate thinning process necessary to decrease the InSb substrate attenuation. Growing the InSb detectors on a Si substrate also facilitates the integration of the detector array and the Si read out circuit. Obviously, high quality growth of InSb on Si is very important to increase the yield and decrease the cost of infrared detector systems. However, the large lattice mismatch between InSb and Si (19%) seems to be an important obstacle in growing device quality InSb on Si substrate.
While there have been only few reports1, 2, 3, 4, 5on the growth of InSb on Si, growth of this material on GaAs substrates has been studied by various groups6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18. In most of these studies, a high density of dislocations at the InSb/GaAs interface, and an improvement in the crystallinity and Hall mobility with increasing thickness were observed. It was also reported that the Hall mobility anomalously decreased with decreasing temperature. Similar behaviour was observed in InSb grown on Si[2]. While some authors suggested that this might be due to dislocation scattering in n-type material[10], others attributed this to the generation of a p-type layer at the interface due to acceptor-like defects which compensates the Hall coefficient of an otherwise n-type layer[18]. However, Hall measurements on low bandgap material can be quite deceptive, and due to electron accumulation at the surface, these measurements can reflect typical n-type behaviour for p-type material with anomalous dependence of mobility on temperature[19]. Due to the material related problems, there are only very few studies reported on the fabrication of InSb and InAsSb photodetectors on GaAs and Si substrates20, 21, 22.
Recently, high quality growth of InSb on GaAs and Si was reported1, 4. In this paper, we report the fabrication, characteristics and dark current analysis of InSb p–i–n photodetectors grown on GaAs coated Si substrates.
Section snippets
Growth and fabrication
The p–i–n detector structure used in this study is shown in Fig. 1. The detector epitaxial layers were grown on 2 μm GaAs coated, 3 inch Si substrates by molecular beam epitaxy at a substrate temperature of 395°C using uncracked elemental sources. Be and Te were used for p- and n-type dopants, respectively. The detailed growth optimization process is presented elsewhere[1]. In the latter, InSb epilayers grown on Si substrates showed excellent morphology with an X-ray full width at half maximum of
Characterization results, modelling and discussion
The photodetectors were mounted inside a cryostat and I–V measurements were taken at various temperatures between 77 K and 180 K. Fig. 2 gives the current–voltage (77 K) and differential resistance characteristics of a 400×80 μm2 detector. The zero bias differential resistance (R0) at 77 K was around 8.3 kΩ and the dark current was measured to be 13 μA under a reverse bias of 0.1 V. It was observed that at low temperatures (T≤100 K), the differential resistance was almost independent of the temperature
Conclusion
In conclusion, InSb p–i–n detectors with reasonably good 77 K performance have successfully been grown and fabricated on Si substrates, and the feasibility of monolithic integration of large area InSb detector arrays with the Si read out circuit has been shown. A detailed analysis of the dark current showed that R0A limiting mechanisms at 77 K are ohmic shunt leakage and tunnelling. The tunnelling current can be reduced if a lower unintentional doping density in the active layer is achieved.
Acknowledgements
This research is sponsored by NATO's Scientific Affairs Division in the framework of the Science for Stability Programme under the grant code: TU-MICROSYSTEMS. The authors wish to thank Prof. Manijeh Razeghi for supervising the growth of the detector epilayers and Eric Michel for his contribution to this work through material growth. The authors would also like to thank Prof. Tayfun Akin for useful discussions.
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