GaSb and related quaternary semiconductors offer a great potential for application in mid-infrared photovoltaics (PV) and thermophotovoltaics (TPV). The implementation of these materials, however, has been hampered by an inability to reproducible control of their surface properties. The surface treatment in sulphur containing solutions has received much attention and was applied to the improvement of the GaSb-based photodiodes electro-optical parameters. Little was reported about long-term stability of GaSb-based TPV cells.

In this work we have used heterojunction photodiodes grown by liquid phase epitaxy (LPE) on (100) n‑GaSb:Te substrates. The mesa-type devices with mesa size of 500 µm × 500 µm were formed using photolithography and reactive ion etching in BCl₃ plasma. AgTe/Cr/Au and Au/Zn/Au metallizations were applied for backside n-type and front p-type ohmic contacts, respectively. The contacts were alloyed in H₂ at 250°C for 3 min. The processing of device structures was completed by surface passivation of mesa side-walls.

(NH₄)₂S, Na₂S, and (NH₂)₂CS have been chosen as sulphur sources in either aqueous or C₃H₇OH solutions. Electrochemical processing has been carried out at the current density of 0.4 – 4 mA/cm² for 15 - 30 min at room temperature. The parameters of electrochemical treatment – the current density and processing time - were optimised so as to fabricate the thickest possible overcoat with acceptable surface morphology (surface roughness below 10 nm). The passivating coatings obtained as a result of electrochemical sulphuration processes have been characterised by complementary use of Variable Angle Spectroscopic Ellipsometry (VASE), and X-ray Photoelectron Spectroscopy (XPS). The photodiode characterisation involved measurements of the current-voltage (I-V) characteristics, spectral response, and low frequency noise at 300 K. The I-V measurements were done using a computer controlled Keithley SourceMeter 2400 programmable meter. The relative photoresponse spectra were measured with a transimpedance amplifier coupled to a Fourier transform infrared (FTIR) spectrometer. Long-term performance under air exposure has been monitored within the time of 12 - 36 months, in 3-moths intervals.

The results of VASE analyses have proven insulating character of passivating coatings (ε₂ = 0.014 and ε₂ = 0.029 at 1.1 eV). The spectroscopic dependence of ellipsometric functions ψ(λ) and Δ(λ) of n-GaSb surface passivated in 21%(NH₄)₂S-H₂O measured immediately after sulphur treatment and 3 years later were compared. No change of ellipsometric characteristics ψ(λ) and Δ(λ), has been observed which indicates on good stability of the coating.

The key parameters of passivated GaSb/In(Al)GaAsSb photodiodes were monitored during 12 months. It has been found that after 12 months exposure of the photodiodes to ambient environment, the electrical and optical characteristics are unchanged. The above result confirms the chemical stability of applied passivating coatings.

Part of the research was supported by the grant no. 3T11B 00926 from the Ministry of Education and Science, Poland.

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