The semiconductor-passivating layer interfaces, as well as the dielectric properties of the passivating layers, play important and very often dominant roles in determining HgCdTe device performance. With a narrow bandgap, HgCdTe infrared detectors are strongly influenced by the quality of the passivation layer(s). The surface band bending is often of the order of the bandgap energy for HgCdTe materials, even those used for short-wave and mid-wave infrared detection, and can easily accumulate, deplete, or invert the surface, drastically affecting device performance. The situation is worse for long and very long wave infrared detectors. Surface recombination processes can be enhanced in narrow bandgap materials like HgCdTe, and become the dominant loss mechanism for photo-generated excess carriers. High-quality photodiode detectors are limited by generation-recombination within the depletion region, tunnelling through the depletion region and surface/interface effects. Surface leakage is another surface-related current mechanism. The 1/f noise is surface related, and is associated with surface charge tunnelling into and out of the passivation interface. For ‘n-type/Barrier/n-type’ (nBn) heterostructure HgCdTe detectors, the absorber is covered with the barrier which consists the passivation layer itself, yet surface related phenomena impact greatly on the performance of nBn detectors. Surface passivation technology can greatly improve the HgCdTe/insulator interface, leading to a reduction of 1/f noise and generation-recombination noise, and an increase of responsivity and detectivity of HgCdTe IR detectors. Understanding the fundamental properties of interface states in narrow bandgap semiconductors is essential to the systematic development of techniques to ameliorate their effects and improve device performance.
The surface and interface chemistry of II-VI compounds has not been as extensively studied in the open literature as that of III-V compounds, and there is a lack of consensus on key questions related to II-VI surfaces and interfaces. Passivation techniques for HgCdTe have been developed using empirical approaches over a long time, with detailed information about surface conditioning, passivation material properties, deposition conditions and annealing processes often retained as proprietary knowledge. The absence of published work on a rigorous physical understanding of interfaces in narrow bandgap materials is the motivation for this work, and also the challenge.
In this thesis the interface effects in HgCdTe materials and devices have been investigated, concentrating on two passivant materials: CdTe and silicon nitride. The surface and interface effects in molecular beam epitaxy (MBE) low-temperature grown CdTe passivated HgCdTe structures have been studied, employing photoconductive devices and gated photodiode devices. In order to determine the effectiveness of this low-temperature deposited CdTe passivating film, photoconductors were utilised to investigate the effectiveness of the passivation by comparing photoresponsivity between devices with and without sidewall CdTe passivation. Surface recombination simulations of the photodetectors were performed to understand the behaviour of the passivation and estimate the surface recombination velocity at the interfaces of CdTe passivated surfaces. This is a new and effective way of estimating surface recombination velocities. The gated HgCdTe photodiode, passivated by MBE low-temperature grown CdTe was used as a tool to investigate passivation properties and performance, allowing the band bending at the surface to be controlled by varying bias through the gate. This allowed the magnitudes of dark current and dynamic resistance to be manipulated by changing the conditions at the passivant/semiconductor interface in the photodiode, and therefore change the dominant surface recombination mechanism.
The capabilities of low-temperature processing, good surface insulation and hydrogenated films make SiNx a suitable choice for passivating HgCdTe. In this thesis studies have been carried out to investigate SiNx thin films for surface passivation of HgCdTe epitaxial layers without the need for a CdTe intermediate capping layer. Conventionally, high-quality SiNx films for surface passivation layers are deposited at temperatures in the range 200 °C to 750 °C. These temperatures are much higher than the maximum allowed for HgCdTe processing temperature (typically < 120 °C) that can be used without a Hg overpressure to prevent dissociation of the HgCdTe. Inductively-coupled plasma-enhanced chemical vapour deposition (ICPECVD) systems with a high-density plasma source offer the ability to deposit relatively high quality SiNx films using a minimal thermal budget. SiNx films in this thesis were deposited at low temperatures (80 °C - 130 °C) employing a Sentech SI500D ICPECVD system with a high-density and low ion energy plasma source [1, 2]. The low ion energy of the plasma source enables the SiNx film to be deposited on the HgCdTe without significant surface damage. Prior to SiNx films being deposited on HgCdTe, a series of SiNx films were firstly deposited on CdTe/GaAs and Si substrates under different deposition conditions to examine the influence of ICP power, deposition temperature, and NH3/SiH4 flow ratio on the properties of SiNx films themselves.
The SiNx/HgCdTe metal-insulator-semiconductor structures were utilised as a tool in studying the interface between SiNx and HgCdTe. Interface trap density, Dit, was considered as the measure in evaluating surface passivation performance and in correlating passivation quality with other film properties. The SiNx/n-Hg0.68Cd0.32Te interface characteristics were investigated employing capacitance-voltage and conductance-frequency measurements, and the corresponding Dit were extracted from the high-frequency and low-frequency capacitance-voltage characteristics, and also by the conductance method. Analysis of the SiNx/n-Hg0.68Cd0.32Te MIS structures indicated that Si-rich SiNx film deposited at 100 °C by ICPECVD exhibit electrical characteristics suitable for surface passivation of HgCdTe-based devices. That is, interface trap densities in the range of mid-1010 cm-2eV-1, and fixed negative interface charge densities of ~ 1011 cm-2 [1, 2]. In addition, the relationship between different bond concentrations in the SiNx and surface passivation performance has been explored using infrared absorbance spectra. The Si-H and N-H bond concentrations were found to be directly correlated with passivation performance, such that SiNx films with a combination of high [Si-H] and low [N-H] bond concentrations were found to be suitable as electrical passivation layers on HgCdTe. This could be a useful criterion for optimising the passivation quality of SiNx films for HgCdTe-based devices.
|Qualification||Doctor of Philosophy|
|Award date||19 May 2016|
|Publication status||Unpublished - 2015|