Multi-spectral Photodetectors on GaAs Substrates using Metamorphic Epitaxy and Hybrid III-V Heterostructures

Realization of a high-performance monolithically integrated multi-spectral photodetector technology that is capable of simultaneous and independent detection in multiple bands of the electromagnetic spectrum, from the infrared (IR) to the ultra-violet, with inherent optical registry would have great impact in numerous important applications including military surveillance, environmental sensing, failure analysis, and medical imaging. However, the current technology either uses bulky optics or is mostly limited to the mid-wave and long-wave infrared (3-15 um) wavelengths. The primary obstacles to achieving such a technology has been the need to integrate lattice-mismatched materials (to access different bandgaps) on to a single substrate and the need for novel device design methodologies.

In this regard, monolithic, epitaxially-integrated, vertically-aligned, multi-band photodetector architecture has been demonstrated in this dissertation, via the successful growth and fabrication of metamorphic back-to-back n-i-p/p-i-n In_zGa_1-zP/In_xGa_1-xAs visible/near-IR dual-detector devices on GaAs substrates. In_zGa_1-zP and In_xGa_1-xAs alloys with lattice constants between GaAs and InP were chosen for this work due to the richness of direct bandgaps (corresponding to visible and near-IR detector cut-off wavelengths) available. Bandgap tunability for the detector materials was achieved using lattice grading by utilizing metamorphic In_xGa_1-xAs step-graded buffers grown using solid-source molecular beam epitaxy. In addition to its importance in multi-spectral detection, bandgap tunability also facilitates the maximization of detector performance at target wavelengths, which was demonstrated by the high-performance In_zGa_1-zP and In_xGa_1-xAs p-i-n photodetectors achieved with different cut-off wavelengths.

The excellent photodetector performances achieved in this work, is a direct result of the high metamorphic material quality obtained using the In_xGa_1-xAs step-graded buffers. The structural and electronic quality of the different metamorphic materials were studied using high-resolution triple-axis x-ray diffractometry (TA-XRD), transmission electron microscopy (TEM) and deep-level transient spectroscopy (DLTS) measurements, to characterize the impact of metamorphic buffer design and growth conditions on material properties. For buffer design optimization, a comprehensive study of the impact of In_xGa_1-xAs metamorphic buffer steps design on metamorphic material quality, including TDD and phase segregation, and its corresponding impact on photodetector performances were performed. It was found that the variation in TDD (calculated using plan-view TEM and electron-beam induced current (EBIC) experiments), within the range of values investigated in this work (low-10⁵ to low-10⁷ cm-2), has only a minimal impact on the In_xGa_1-xAs p-i-n photodetector properties such as dark current. This is attributed to the high intrinsic carrier concentration value of the low bandgap In_xGa_1-xAs resulting in higher diffusion current, effectively masking the effect of SRH generation-recombination effects. In addition, it was observed that the effect of residual strain in the final buffer layer has no measurable impact on In_xGa_1-xAs p-i-n photodetectors for In_xGa_1-xAs with x < 0.20.

However, in comparison, In_0.68Ga_0.32P p-i-n photodetectors were found to be extremely sensitive to the presence of residual or interface strain resulting in phase segregation of InGaP into In-rich and Ga-rich areas, which in turn resulting in a significant increase in the TDD values. As a result of the increased TDD and phase segregation, In_0.68Ga_0.32P p-i-n photodetectors with even slight tensile interface strain, displayed orders of magnitude higher dark current densities compared to devices with no strain. To address this, all the different photodetectors utilized a compositional overshoot layer as the final buffer step followed by a perfectly lattice-matched step-back layer to eliminate any residual strain in the photodetector layers. In terms of the dual-detector performance, the required simultaneous and independent operation of sub-detectors was achieved using a back-to-back n-i-p/p-i-n design combined with optimized photomask design and fabrication processes. This allowed the realization of desired results such as passage of light through the top sub-detector before being absorbed by the bottom sub-detector. The prototype In_0.61Ga_0.39P/ In_0.14Ga_0.86As demonstrated excellent material and photodetector properties including complete electrical isolation of the sub-detector elements (no measureable electrical cross-talk or charge leakage) and relatively low optical cross-talk of < -35 dB for most of each band, with a minimal cross-talk between 690 nm and 720 nm, which could be further minimized using thicker top In_0.61Ga_0.39P sub-detector device layers. The sub-detectors closely matched the individual photodetector properties of low, room temperature reverse bias (-2 V) dark current densities, high responsivities and high specific detectivity values of 4x10^-8 A•cm^-2, 0.41 A/W and 8.6x10¹¹ cm•Hz^1/2/W for the In_0.14Ga_0.86As sub-detectors (at 980 nm) and 7x10^-12 A•cm^-2, 0.30 A/W and 2.0x10¹⁴ cm•Hz^1/2/W for the In_0.61Ga_0.39P sub-detectors (at 680 nm), respectively, confirming the high-level of success achieved with the integration process. The demonstrated multi-detector technology could be easily extended to other wavelength bands by using optimized metamorphic epitaxy. In addition, there is great potential in integrating these visible/near-IR dual-photodetectors with quantum dot infrared multi-band photodetectors that operate in the MWIR and LWIR wavelengths, allowing for vertically integrated multi-spectral detectors capable of simultaneously and independently detecting individual bands of wavelengths from short-visible to long-wave infrared wavelengths.