Device structure and X-ray source

The In_0.5Ga_0.5P structure was grown on a (100) n-GaAs substrate by metalorganic vapour phase epitaxy using trimethylgallium, trimethylindium, arsine and phosphine as precursors and hydrogen as a carrier gas. For n-type and p-type doping, disilane and dimethylzinc were used, respectively. The epitaxial surface of the substrate had a miscut angle of 10° towards <111> A to avoid the spontaneous long-range ordering in the group III sublattice and related effects on the bandgap^{31, 32}. First, an n-GaAs buffer layer was grown, and this was subsequently followed by n-In_0.5Ga_0.5P, nominally undoped i-In_0.5Ga_0.5P and p-In_0.5Ga_0.5P layers with thicknesses of 0.1 µm, 5 µm and 0.2 µm, respectively. The doped layers had a carrier concentration of 2 × 10¹⁸ cm⁻³. The structure was completed with a 0.01 µm thick highly doped (1 × 10¹⁹ cm⁻³) p-GaAs contact layer to ensure a good Ohmic contact. Chemical wet etching techniques were used to fabricate a 400 μm diameter In_0.5Ga_0.5P mesa photodiode; a 1:1:1 K₂Cr₂O₇:HBr:CH₃COOH solution was used followed by a 10 s finishing etch in 1:8:80 H₂SO₄:H₂O₂:H₂O solution. Sidewall passivation techniques on the processed mesa In_0.5Ga_0.5P device were not used. Ti/Au (20 nm/200 nm) and InGe/Au (20 nm/200 nm) contacts were deposited on top of the GaAs top layer and onto the rear of the GaAs substrate to form the Ohmic top and rear contacts, respectively. The top Ohmic contact had an annular shape that covered 33% of the photodiode surface. The layers’ sequence, material compositions, thicknesses and doping types and levels in the fully fabricated structure are summarised in Table 1.

Layer details of the In_0.5Ga_0.5P photodiode.

A 206 MBq ⁵⁵Fe radioisotope X-ray source (Mn Kα = 5.9 keV, Mn Kβ = 6.49 keV) was used to illuminate the 400 μm diameter In_0.5Ga_0.5P mesa photodiode. The X-ray emitter was placed 5 mm away from the top surface of the detector. Figure 1 shows the schematic geometry of the source and detector.

Schematic geometry of the ⁵⁵Fe X-ray photovoltaic In_0.5Ga_0.5P cell; the figure is not drawn to scale.

The In_0.5Ga_0.5P device’s X-ray quantum efficiency was computed using the Beer-Lambert law and assuming complete charge collection in the p and i layers. X-ray quantum efficiencies (QE _NC) of 53% at 5.9 keV and 44% at 6.49 keV were obtained for the areas of the photodiode not covered by the top contact; whilst quantum efficiencies (QE _C) of 44% at 5.9 keV and 38% at 6.49 keV were found for the areas covered by the top contact. In_0.5Ga_0.5P total quantum efficiencies (QE) were calculated from a weighted sum of QE _NC and QE _C: total quantum efficiencies of 50% (QE _Kα) and 42% (QE _Kβ) were obtained at X-ray energies of 5.9 keV and 6.49 keV, respectively. The linear attenuation coefficients used in the QE calculations were 0.145 μm⁻¹ and 0.112 μm⁻¹ at 5.9 keV and 6.49 keV, respectively^{33, 34}; these values are higher than for many other semiconductors such as Si, GaAs, and Al_0.52In_0.48P^{33, 35}.