Optically immersed HgCdTe photovoltaic detectors in the 2.5 to 3.2 μm wavelength region operating at near room temperatures have been developed based on HgCdTe graded structure materials grown by opened tube isothermal vapor phase epitaxy (ISOVPE) method on lattice matched CdZnTe substrate. Fourier transformation infrared spectroscopy (FTIR) measurement combined with continuous step wet etching was applied to adjust the cutoff wavelength. The devices were designed and fabricated by traditional n-on-p planar junction process. Optical immersion of micro-lenses by CdZnTe substrate was used to improve the performance of the devices and the hyper-hemispherical micro-lens with a diameter of 1.5mm was made by single point diamond turning method. The optical response area was tested by laser beam induced current (LBIC) scanning measurement, and the result showed that the devices with hyper-hemispherical immersion micro-lens could get a 1mm×1mm response area as designed. The current-voltage characteristic of the devices were measured, and all the devices showed a little increase in the values of zero biased resistance, which was due to a decreased background radiation acceptance angle caused by a hyper-hemispherical structure. The photo response signal and dark noise were also measured before and after the micro-lens fabrication. The signal showed an increase by 20-30 times due to the enlarged photo response area, and the dark noise showed a little decrease which was also due to a limited background radiation acceptance angle. As a result, a multiple factor of four in detectivity enhancement could be achieved by the adoption of hyper-hemispherical immersion micro-lens structures.
In this paper, photoresist reflow method was used to fabricate microlens array on the sapphire substrate which possesses high mechanical strength and transmittance in broad spectrum. High etch selectivity of sapphire over photoresist was obtained through adjusting ICP etching parameters. To test the fabrication process, a geometric model of square aperture microlens was built by finite element method. The validation of this model was done by comparing the surface profiles of three samples reflowed under different condition with the geometric model. In all three cases the simulation results were close to the experiment results. So the model was justified. On the other hand, the fabrication process was found to be repeatable because the surface profile of fabricated microlens was close to the theoretical surface profile of reflowed photoresist. The geometric model can be used to check the repeatability of photoresist reflow process and to predict the surface profile of microlens with irregular aperture.
CdZnTe is the most suitable epitaxial substrate material of HgCdTe infrared detectors, because its lattice constant is able to achieve full match with HgCdTe’s lattice constant. It is always needed to etch CdZnTe substrate during the process of device separation or when we want to fabricate micro optical device on CdZnTe substrate. This paper adopts the more advanced method, Inductive Coupled Plasma-Reactive Ion Etching(ICP-RIE). The etching conditions of ICP-RIE on CdZnTe substrate are explored and researched. First of all, a set of comparative experiments is designed. All of CdZnTe samples with the same component are polished by chemical mechanical polishing before etching. Then all samples are etched by different types of etching gases(CH4/H2/N2/Ar) and different ratios of gases as we designed. The etching time is all set to 30 minutes. After that, the surface roughness, etching rate, etching damage and the profile of etched mesas are tested and characterized by optical microscope, step profiler and confocal laser scanning microscope (CLSM), respectively. It is found that, Ar gas plays the role of physical etching, but the etching rate will decline when the concentration of Ar gas is too high. The results also show that, the introduction of N2 causes more etching damage. Finally, combination of CH4/H2/Ar is used to etch CdZnTe substrate. The ratio of these gases is 2sccm/2sccm/10sccm. The testing results of optimized etching show that, the maximum etching rate reaches up to 20μm/h and the etched CdZnTe surface is smooth with very low etching damage. At last, aimed at the shortcoming of photoresist’s degeneration after long-time etching, the ICP etching process of CdZnTe deep mesa is studied. Double-layer or triple-layer photoresist are spin-coated on CdZnTe substrate during the process of lithography. Then ICP etching is carried out with the optimized condition. It is seen that there is no more phenomena of degeneration.
In this paper, we study the magneto-transport properties of ion-etching-induced p-to-n type converted layers in Hg1-
xCdxTe (x=0.24) single crystal with the help of mobility spectrum analysis (MSA) technique. Hall measurement shows
that the residual p-HgCdTe completely converted to n-type after ion etching. By step-by-step chemical etching, MSA
reveals that ion-etching-induced conversion results in a damaged surface layer with low electron mobility while a bulk n-type
region exhibits higher electron mobility. It can be observed that the mobility of the surface electrons is
independence of temperature in the measured temperature range. In contrast, the bulk electrons exhibit classical behavior
of n-HgCdTe with characteristics that are strongly dependence on temperature. The Hall data from different thickness
shows the bulk n-layer is uniform with high mobility and lower concentration.