Metalorganic chemical vapor deposition of II-VI semiconductors for surface passivation of HgCdTe IR detectors

Infrared (IR) detectors have acquired extensive popularity due to their vast range of applications. They have found wide use in the fields of medicine, military applications, scientific measurements, communications, as well as, agriculture. Mercury cadmium telluride (Hg1-xCdxTe) with its tunable bandgap, spanning the entire IR region, is the material of choice for IR detectors today. Surface passivation of HgCdTe IR detectors is a very significant step in the fabrication process as it improves the device performance remarkably. This research focuses on investigating two potential II-VI semiconductors, namely cadmium telluride (CdTe) and cadmium sulfide (CdS), for surface passivation of HgCdTe IR detectors. The deposition techniques that have been employed---metalorganic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD), are of utmost importance in today's IC fabrication industry. These have several advantages like, excellent uniformity and conformality, which makes them irreplaceable for deposition of thin films on complex device structures with high aspect ratios.;CdTe has been the preferred material for surface passivation of HgCdTe IR detectors. Deposition of CdTe using MOCVD ensures good conformal coverage on mesa-etched, high aspect-ratio focal plane array (FPA) structures for modern day IR detectors. MOCVD of CdTe generally requires high temperatures (∼350°C), but HgCdTe surfaces cannot be exposed to such high temperatures as Hg is volatile and may deplete from the surface. A major part of this research revolves around designing and developing a hot-wall reactor with two clam-shell heaters to facilitate cracking of the precursors at a high temperature (∼600°C), while maintaining the substrate at a lower temperature (135°C--170°C). Deposition rates of 40--70 nm/hr were recorded by varying the temperature from 135°C to 170°C. Favorable conformal coverage on high aspect ratio HgCdTe structures was obtained. Significant improvement in minority carrier lifetime was demonstrated in HgCdTe samples passivated with CdTe at 135°C.;Further modification in the reactor design led to the increase in deposition rates of CdTe greatly. A novel graphite cracker cell was designed to be placed in the reactor tube and heated to a high temperature (∼600°C), which facilitated the efficient cracking of precursors. CdTe deposition rates increased from 50 nm/hr to ∼420 nm/hr. CdTe deposited on mesa-etched HgCdTe samples showed adequate conformal coverage. Minority carrier lifetime of planar HgCdTe samples improved notably. An additional annealing step at 250°C for 20 mins in the presence of H2 improved the lifetimes further, suggesting the formation of a graded interface between CdTe and HgCdTe.;CdS is a potent alternative to CdTe as a surface passivant of HgCdTe. The precursors readily react even at room temperature to produce CdS. This research work also includes the deposition and comparison of CdS films using both MOCVD and ALD. Commercial GaAs (100) and Si (110) substrates were used for the deposition. A deposition rate of ∼300 nm/hr has been obtained using CVD at room temperature, whereas, ALD yielded a rate of ∼70 nm/hr at 85°C. Both methods of deposition provided very good conformal coverage on high aspect ratio trench structures fabricated on Si (110) substrates. MOCVD provides much higher deposition rates, but ALD is the preferred method of deposition for uniform depositions over a large area.