Thermal Stability in HgCdTe IR Photodiodes

Abstract

Packaging of HgCdTe photodiode detector arrays in a dewar involves degassing at elevated temperatures for several days so as to achieve vacuum integrity. This sustained exposure to relatively high temperatures can influence the HgCdTe bulk material properties, p-n junction integrity, and the passivant-HgCdTe interface. This work investigates the effects of bake-out treatment on the performance of HgCdTe based photodiodes formed using a new plasma induced type conversion process. Experimental results of a series of experiments in both long-wavelength infrared (LWIR) and mid-wavelength infrared (MWIR) devices are presented.Bulk lifetime was used as an indicator of the change in the bulk material resulting from baking in vacuum and was measured by photoconductive decay. These measurements did not show any appreciable changes as a result of baking. Modification of the doping profile in the n-p junction may also result from high temperature baking. Doping profiles of the photodiodes were studied by measuring the junction capacitance-voltage relation before and after baking. The results of these tests after baking showed no changes to C-V measurements from those before bake.The effect of baking on the passivant/HgCdTe interface was also examined by carrying out surface recombination velocity measurements by photoconductive decay on samples with different passivation layers.Variable area HgCdTe photodiodes have also been fabricated and studied to understand the effect of the surface condition of the performance of the devices. Initial bake tests on LWIR devices show that the technology is stable when employing a double layer passivation technique. Bake tests on the more advanced MWIR technology indicates that the plasma induced type conversion process produces stable photodiodes with state of the art performance.

Plasma induced type conversion in mercury cadmium telluride

“…After baking for 2 h at 80 • C, there is a significant drop of R 0 A and the dark current mechanism changes from g-r to tunnelling. This is attributed to the out-diffusion of mercury [61]. In the case of CdTe however, there is an improvement in the photodiode characteristics, as shown in figure 7.…”

“…Dell et al [61] have investigated the effect of surface passivation on the stability of junctions formed by reactive ion etching using CH 4 :H 2 plasma. They used ZnS, CdTe and CdTe+ZnS passivation for n-on-p junctions formed on Hg 1−x Cd x Te (x = 0.3).…”

“…In the case of CdTe however, there is an improvement in the photodiode characteristics, as shown in figure 7. The interface grading is thought to be responsible for this improvement [61]. In the third experiment, junction formation was performed using CdTe as a mask with ZnS patterning.…”

Compositionally graded interface for passivation of HgCdTe photodiodes

“…Efforts are still being made to tailor a desirable and thermally stable interface. 9 One needs to create and maintain an appropriate physical and chemical constitution of the CdTeHgCdTe heterostructure to tailor the resulting photoelectrical interface characteristics suitable for a given device design and architecture. To make this process really useful and reproducible for the detector array production, a correlation between the controllable physical/chemical constitution of the interface and the measurable photoelectrical properties has to be established.…”