Material quality characterization of CdZnTe substrates for HgCdTe epitaxy

Cd_1−xZn_xTe (CZT) substrates were studied to investigate their bulk and surface properties. Imperfections in CZT substrates affect the quality of Hg_1−xCd_xTe (MCT) epilayers deposited on them and play a role in limiting the performance of infrared (IR) focal plane arrays. CZT wafers were studied to investigate their bulk and surface properties. Transmission and surface x-ray diffraction techniques, utilizing both a conventional closed-tube x-ray source as well as a synchrotron radiation source, and IR transmission micro-spectroscopy, were used for bulk and surface investigation. Synchrotron radiation offers the capability to combine good spatial resolution and shorter exposure times than conventional x-ray sources, which allows for high-resolution mapping of relatively large areas in an acceptable amount of time. Information on the location of grain boundaries and precipitates was also obtained. The ultimate goal of this work is to understand the defects in CZT substrates and their effects on the performance and uniformity of MCT epilayers and then to apply this understanding to produce better infrared detectors.     

Cadmium telluride growth on patterned substrates for mercury cadmium telluride infrared detectors

Patterned silicon-on-insulator (SOI) substrates have been proposed to grow low defect density CdTe. The CdTe epilayers so obtained will enable the growth of good-quality mercury cadmium telluride (HgCdTe) layers subsequently. This would increase the scope for better performance of infrared detectors fabricated on the HgCdTe epilayers. A background of our previous work on metal organic chemical vapor deposition (MOCVD) of GaN on nanopatterned alternative silicon substrates has been presented. Residual stress measurements were made on the GaN epilayers by spatially resolved Raman spectroscopy, which shows reduced strain in the epilayer growth on the patterned substrates. A theoretical approach of strain in planar substrates, compliant substrates, and patterned compliant substrates is presented with detailed plots.     

MBE growth and device processing of MWIR HgCdTe on large area Si substrates

The traditional substrate of choice for HgCdTe material growth has been lattice matched bulk CdZnTe material. However, as larger array sizes are required for future devices, it is evident that current size limitations of bulk substrates will become an issue and therefore large area Si substrates will become a requirement for HgCdTe growth in order to maintain the cost-efficiency of future systems. As a result, traditional substrate mounting methods that use chemical compounds to adhere the substrate to the substrate holder may pose significant technical challenges to the growth and fabrication of HgCdTe on large area Si substrates. For these reasons, non-contact (indium-free) substrate mounting was used to grow mid-wave infrared (MWIR) HgCdTe material on 3″ CdTe/Si substrates. In order to maintain a constant tepilayer temperature during HgCdTe nucleation, reflection high-energy electron diffraction (RHEED) was implemented to develop a substrate temperature ramping profile for HgCdTe nucleation. The layers were characterized ex-situ using Fourier transform infrared (FTIR) and etch pit density measurements to determine structural characteristics. Dislocation densities typically measured in the 9 10⁶ cm⁻² to 1 10⁷ cm⁻² range and showed a strong correlation between ramping profile and Cd composition, indicating the uniqueness of the ramping profiles. Hall and photoconductive decay measurements were used to characterize the electrical properties of the layers. Additionally, both single element and 32 32 photovoltaic devices were fabricated from these layers. A RA value of 1.8 10⁶-cm² measured at −40 mV was obtained for MWIR material, which is comparable to HgCdTe grown on bulk CdZnTe substrates.     

Absorption of Narrow-Gap HgCdTe Near the Band Edge Including Nonparabolicity and the Urbach Tail

An analytical model describing the absorption behavior of Hg_1−x Cd _x Te is developed. It simultaneously considers the contributions from non-parabolic conduction/light hole bands and parabolic heavy hole bands obtained from 14-band k·p electronic structure calculations and the Urbach tail. This model smoothly fits experimental absorption coefficients over energies ranging from the Urbach tail region to the intrinsic absorption region up to at least 300 meV above the band gap.     

Microstructure of Heteroepitaxial ZnTe Grown by Molecular Beam Epitaxy on Si(211) Substrates

The interface of ZnTe/Si(211) grown by molecular beam epitaxy was investigated by high-resolution transmission electron microscopy. Several types of defects such as misfit dislocations, stacking faults, agglomerations of vacancies, and precipitates were observed and studied by electron microscopy at the ZnTe/Si interface. The distribution of misfit dislocations at the interface was revealed with the assistance of the fast Fourier transformation filtering technique. A stick-and-ball interface model including misfit dislocation geometry is proposed. The possible origins of the stacking faults, vacancies, and precipitates are discussed.     

The Effect of Wet Etching on Surface Properties of HgCdTe

The surface of HgCdTe, grown by molecular-beam epitaxy and liquid-phase epitaxy, was studied by atomic force microscopy and x-ray photoelectron microscopy after etching in different solutions such as Br:methanol and HBr:H₂O₂:H₂O. Minority-carrier lifetime and surface recombination velocity were measured by photoelectron decay spectroscopy. The same measurements were repeated after exposure to air for periods from 2 h to 2 days. Although these surfaces are rather complicated, the main feature is that Br-based etchants produce elemental Te at the surface, which oxidizes rapidly in air. Without elemental Te, there is less Te oxide, even after longer exposure to air. The existence of elemental Te is correlated with higher surface recombination velocity. This can be explained in terms of band bending and band offsets at Te/HgCdTe and TeO₂/HgCdTe interfaces.     

Nitrogen Plasma Doping of Single-Crystal ZnTe and CdZnTe on Si by MBE

We have investigated in situ p-type doping of ZnTe and CdZnTe on Si(211) by molecular beam epitaxy using a radiofrequency (RF)-nitrogen plasma source for application to multijunction II–VI-based solar cells. CdZnTe would be used as a wide-gap top cell in a monolithic multijunction device, and ZnTe or CdZnTe could be used for the p-side of tunnel junctions. Highly p-type material is required for producing the high-quality tunnel junctions crucial for maintaining current flow, and p-doping of order 10¹⁷ cm^?3 is required for the generation of a large built-in potential in the absorber region of solar cells. Our uniformly doped films exhibited good Hall characteristics, especially considering the large lattice mismatch between Si and either ZnTe or CdZnTe. Crystal quality was examined by x-ray diffraction. Nitrogen incorporation was examined as a function of the source-gas dilution with argon. A sample with layers of CdZnTe doped using 1% to 100% nitrogen was grown on nominally undoped CdZnTe and analyzed using secondary-ion mass spectrometry. The nitrogen incorporation differed by only a factor of 10, despite the factor of 100 difference in the nitrogen concentration in the plasma, indicating a saturation effect.     

Correlation of CdZnTe(211)B substrate surface morphology and HgCdTe(211)B epilayer defects

We present results on the surface morphology and recombination lifetimes of molecular-beam epitaxy (MBE)-grown HgCdTe (211)B epilayers and correlate them with the roughness of the CdZnTe substrate surfaces. The substrate surface quality was monitored by in-situ spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction (RHEED). The SE roughness of the substrate was measured after oxide desorption in the growth chamber. The RHEED patterns collected show a strong correlation with the SE roughness. This proves that SE is a valuable CdZnTe prescreening tool. We also found a correlation between the substrate roughness and the epilayer morphologies. They are characterized by a high density of thin elongated defects, “needle defects,” which appear on most samples regardless of growth conditions. The HgCdTe epilayers grown on these substrates were characterized by temperature-dependent, photoconductive decay-lifetime data. Fits to the data indicate the presence of mid-gap recombination centers, which were not removed by 250°C/24-h annealing under a Hg-rich atmosphere. These centers are believed to originate from bulk defects rather than Hg vacancies. We show that Te annealing and CdTe growth on the CdZnTe substrates smooth the surface and lower substantially the density of needle defects. Additionally, a variety of interfacial layers were also introduced to reduce the defect density and improve the overall quality of the epilayer, even in the presence of less than perfect substrates. Both the perfection of the substrate surface and that of its crystalline structure are essential for the growth of high-quality material. Thus, CdZnTe surface polishing procedures and growth techniques are crucial issues.     

Correlation of arsenic incorporation and its electrical activation in MBE HgCdTe

The behavior of arsenic for p-type doping of MBE HgCdTe layers has been studied for various annealing temperatures and arsenic doping concentrations. We have demonstrated that arsenic is in-situ incorporated into HgCdTe layers during MBE growth. The carrier concentration has been measured by the Van der Pauw technique, and the total arsenic concentration has been determined by secondary ion mass spectroscopy. After annealing at 250°C under an Hg over pressure, As-doped HgCdTe layers show highly compensated n-type properties and the carrier concentration is approximately constant (∼mid 10¹⁵ cm⁻³) until the total arsenic concentration in the HgCdTe layers approach mid 10¹⁷ cm⁻³. The source of n-type behavior does not appear to be associated with arsenic dopants, such as arsenic atoms occupying Hg vacancy sites, but rather unidentified structural defects acting as donors. When the total arsenic concentration is above mid 10¹⁷ cm⁻³, the carrier concentration shows a dependence on the arsenic concentration while remaining n-type. We conjecture that the increase in n-type behavior may be due to donor arsenic tetramers or donor tetramer clusters. Above a total arsenic concentration of 1∼2×10¹⁸ cm⁻³, after annealing at 300°C, the arsenic acceptor activation ratio rapidly decreases below 100% with increasing arsenic concentration and is smaller than that after annealing at 450°C. The electrically inactive arsenic is inferred to be in the form of neutral arsenic tetramer clusters incorporated during the MBE growth. Annealing at 450°C appears to supply enough thermal energy to break some of the bonds of neutral arsenic tetramer clusters so that the separated arsenic atoms could occupy Te sites and behave as acceptors. However, the number of arsenic atoms on Te sites is saturated at ∼2×10¹⁸ cm⁻³, possibly due to a limitation of its solid solubility in HgCdTe.     

Selective epitaxy of cadmium telluride on silicon by MBE

CdTe B was grown on As-terminated Si(111) by molecular beam epitaxy (MBE). Nucleation and interface properties were studied by photoelectron spectroscopy, scanning tunneling microscopy, electron diffraction, and energy-dispersive spectroscopy of x-rays. Selective growth on Si(111) was investigated either by using SiO₂ as a mask, or by growing on a patterned CdTe seed layer. The highest temperature where CdTe nucleates on As-terminated Si(111) surfaces is typically in the range of 220–250°C. On a SiO₂ mask, CdTe nucleates at the same temperatures, leading to polycrystalline growth. However, homoepitaxy of CdTe is possible around 300°C. Hence, CdTe can be grown selectively on a patterned CdTe seed layer on Si(111). This is confirmed by scanning electron microscopy and scanning Auger microscopy.