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We report on the investigation of effective SiO2 passivated layer deposited at low temperatures. Comparison with the unpassivated photodiodes, at 77 K, the dark current density is reduced by one order of magnitude is achieved by introducing SiO2-passivated layer deposition technology at a lower temperature of 75 °C. The temperature-dependence and bias-dependence of the dark current are studied experimentally and correlated to the theory, and then the contribution of each dark current mechanism is also identified. Introduction InAs/GaSb Type-II superlattices (T2SL) have been a promising alternative to replace the leading Mercury-Cadmium-Telluride (MCT) system for infrared detection and imaging. Due to its great advantage for band structure engineering, which results in greater flexibility in controlling the band gap [1], suppression of Auger recombination [2], and design of novel device architectures [3-5], this material system has demonstrated rapid progress. However, the performance of T2SL photodetectors, especially in long-wavelength infrared region, is still limited by the high dark current. The identification of each individual dark current mechanism, such as surface leakage, diffusion, generation-recombination (GR) and trap-assisted tunneling (TAT) current, is very important for the understanding of the pin photodiode and then to improve SL device electrical and optical performances. Among these current mechanisms, the minimal of surface leakage is crucial to be resolved for reducing the total dark current and obtaining the high resolution imaging system. Generally, surface leakage is believed to originate from the abrupt termination of the periodic crystalline structure, contamination from processing, and fixed charges within the passivation layer, which can generate band bending on mesa-sidewalls, this band bending causes electron accumulation or type inversion at sidewall surfaces, resulting in a conduction channel along sidewalls [6, 7]. Several surface passivation attempts have been proposed to minimize the dark currents, thereinto, SiO2 passivation, which can be compatible with standard semiconductor fabrication procedures, is still the dominant and most suitable passivation technique for T2SL application. However, SiO2 film deposition usually utilizes a plasma enhanced chemical vapor deposition process conducted at high temperatures and exposure of SLs materials to high temperatures could degrade the overall device performance [6]. So special care needs to be taken to develop a low-temperature process of SiO2 deposition technology to prevent SLs period intermixing and improve the electrical performance. In this paper, we introduce a low-temperature inductively coupled plasma chemical vapor deposition technology (ICPCVD) and the SiO2-passivated layer was performed at a temperature of 75 °C to reduce the surface leakage current. The temperature-dependence and bias-dependence of the passivated and unpassivated photodiodes dark current is studied experimentally and correlated to the theory. 5th International Conference on Advanced Materials and Computer Science (ICAMCS 2016) © 2016. The authors Published by Atlantis Press 377 Materials and methods Samples presented in this paper have been grown by Riber C21T solid source molecular beam epitaxy system (MBE) equipped with As/Sb valved cracker cells and Ga/In SUMO cells. On the top of a 253 nm thick GaSb: Be p-type buffer, the device structure consisted of a 675 nm thick p-type InAs/GaSb: Be SLs contact layer, a 2.144 μm InAs/GaSb SLs unintentionally doped active region, a 675 nm thick n-type InAs: Si/GaSb SLs contact layer and capped with a 10 nm thick InAs: Si n-type capping layer. The superlattice design for the absorbing region used 13 monolayers (MLs) of InAs and 7 MLs of GaSb in one period. The single-pixel photodiodes with diameters ranging from 50-200 μm were fabricated. The samples were processed using standard photolithography and inductively coupled plasma etching techniques with Cl2 gas [8]. Then, the diodes were dipped in HCl: H2O2: H2O =1: 1: 10 solutions for 30 S to remove the native oxide film on the etched mesa sidewalls. Some diodes were passivated with 300 nm thick SiO2 dielectric layer, using the ICPCVD system at 75 °C, 160 °C and 350 °C and the process was carried out under the pressure of 90 mTorr, ICP power of 2400 W and RF power of 150 W. Flow rates were 17 sccm for SiH4 and 40 sccm for N2O. Subsequently, an ohmic contact was annealed at 375 °C for 45 S by depositing Ti (50 nm) /Pt (50 nm) /Au (300 nm) on the bottom and top contact layers of the samples. And then all the unpassivated and passivated samples were wire bonded to a leadless chip carrier for further electrical characterization. The dark current density was measured as a function of applied bias. For the measurement, the diodes were cooled from 300 K to 77 K with a closed cycle cryostat. All results presented in this paper were obtained for devices without antireflective (AR) coating. Results and discussion The electrical performance of the unpassivaed and SiO2-passivation diodes at different deposition temperatures was shown in Figure1. Comparison with unpassivated samples, the surface leakage current of passivated samples was effectively reduced. And at reverse bias region, it is clearly appeared that the current-density-voltage curve of the SiO2 dielectric layer performing at low temperature of 75 °C has the lower dark current and its performance was better than performed in typical temperature 160 °C and 350 °C. In addition, under forward bias, the diode of the low temperature deposition SiO2 dielectric layer has a low serial resistance, which is obtained from the current-density-voltage curve and makes this work relevant and useful to the T2SL detector community. The dark current in a given device is the sum of the diffusion current and the current originated from the GR and from TAT current. While at high bias, the band-to-band tunneling current (BTB) became larger, but photodiodes are normally operated in the low bias regime, and hence, the BTB tunneling current was reasonably neglected. Fig.1 the electrical performance of the SiO2-passivation diodes at different deposition temperatures.

Investigation of the lower temperature SiO2 surface passivation technology on InAs/GaSb superlattice detectors