Abstract
The soft X-ray spectroscopic performance of a GaAs/Al0.8Ga0.2As Separate Absorption and Multiplication (SAM) APD was assessed at room temperature using a 55Fe source. An energy resolution of 1.08 keV (FWHM) was achieved for the 5.9 keV X-rays, at an avalanche gain of 3.5. The avalanche gain also improved the minimum detectable energy from 4.8 keV at unity gain to about 1.5 keV at a gain of 5. Through avalanche statistics analyses, we confirmed that (i) the APD's FWHM was degraded by X-ray photon absorption within the avalanche region, and (ii) photon absorption in/near the n-cladding layer contributed to an undesirable secondary peak in the spectrum.
Highlights
The soft X-ray spectroscopic performance of a GaAs/Al0.8Ga0.2As Separate Absorption and Multiplication (SAM) APD was assessed at room temperature using a 55Fe source
To maintain spectral resolution, X-ray photons should be predominantly absorbed in a non-avalanche region in the APD [7], which favours the configuration of SAM APDs
A GaAsbased SAM APD [9] was reported for X-ray detection, with energy resolutions of 1.95 keV and 0.9 keV achieved at gains of unity and 4.1 respectively, for 13.9 keV X-rays
Summary
Details of the GaAs/Al0.8Ga0.2As SAM APD wafer grown on an n+ GaAs substrate by Molecular Beam Epitaxy (MBE) is shown in table 1 It had a 0.43 μm thick GaAs absorption layer and a 0.22 μm thick Al0.8Ga0.2As avalanche layer. The Al0.8Ga0.2As avalanche region was kept thin in order to (i) minimize undesirable X-ray photon absorption in the multiplication region [7], and (ii) minimize presence of defects caused by the slight-mismatch between Al0.8Ga0.2As and GaAs. Circular mesa diodes of diameters ranging from 50 to 400 μm were fabricated from the wafer by using standard photolithography. For reverse bias < 10 V, the depletion region did not include the i-Al0.8Ga0.2As avalanche region, allowing us to estimate the i-GaAs absorption layer thickness accurately. Information deduced from the C-V analysis were later used in modelling the APD’s X-ray response
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