Surface passivation and aging of InGaAs/InP heterojunction phototransistors

We report the effect of different surface treatment and passivation techniques on the stability of InGaAs/InP heterojunction phototransistors (HPTs). An In_0.53Ga_0.47As surface passivated with aqueous ammonium sulfide ((NH₄)₂S), aluminum oxide (Al₂O₃) grown by atomic layer deposition (ALD), and their combination is evaluated by using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). All samples were kept in the air ambient, and their performances were periodically measured to investigate their long-term stability. Raman spectroscopy revealed that the peak intensity of the GaAs-like longitudinal optical phonon of all passivated samples is decreased compared with that of the control sample. This is attributable to the diminution of the carriers near the passivated surfaces, which was proven by extracted surface potential (V_s). The V_s of all passivated samples was decreased to less than half of that for the control sample. XPS evaluation of As3d spectra showed that arsenic oxides (As₂O₃ and As₂O₅) on the surfaces of the samples can be removed by passivation. However, both Raman and XPS spectra show that the (NH₄)₂S passivated sample reverts back over time and will resemble the untreated control sample. When capped with ALD-grown Al₂O₃, passivated samples irrespective of the pretreatment show no degradation over the measured time of 4 weeks. Similar conclusions are made from our experimental measurement of the performance of differently passivated HPTs. The ALD-grown Al₂O₃ passivated devices show an improved optical gain at low optical powers and long-term stability.