This paper presents the progress in the cadmium zinc telluride (CdZnTe) single crystal growth and high quality CdZnTe substrates fabrication at Shanghai Institute of Technical Physics (SITP). The 60 mm diameter ingots with almost 90% single-crystals were grown by the vertical Bridgman technique. The average single-crystal yield of all ingots grown in the last two years was about 60%. The large-size substrates (> 40 × 60 mm2) were obtained. The processing and assessment of CdZnTe substrates were introduced. Especially, the sliced wafers were etched by the bromine-methanol to remove slicing damage rather than by lapping and polishing. An Olympus BX51-IR transmission microscope with mapping function was employed to take images of the inclusions/precipitates in the full wafer and the surface of the wafers etched by the Evenson etchant. The inclusions with different shapes were investigated. The inclusions and precipitates were distinguished. The inclusions occurred at the crystal growth stage and the precipitates were separated from the nonstoichiometric crystals at the cooling stage. The both inclusions and precipitates mainly originated from the nonstoichiometric CdZnTe source materials. The mosaic structure of the etch pits profiles in the substrates has been eliminated. The etch pit density (EPD) of CdZnTe substrates was counted by the software and the average value of EPDs was less than 5 × 104 cm-2. After the defects of the substrates were measured, the substrates were polished. The surface flatness and roughness of the substrates were also improved by optimizing the technology.
Deposition in thermal ambience can obtain better CdTe passivation layers compared with general evaporation process. HgCdTe infrared focal plane arrays are fabricated to confirm the new process works well. Contrast n+-on-p planar photodiodes are manufactured from the same HgCdTe epilayer. Some use new process while others use general process. The performance of devices using new process shows a significant improvement. The device with general passivation process has a dark current of 7.8×10-7 A at 50 mV negative bias voltage, and the differential resistance at zero bias is 2.6×105 Ω. Meanwhile, the device with new passivation process has a dark current of
1.7×10-8 A at 50 mV negative bias voltage, and the differential resistance at zero bias is 8.0×105 Ω. Moreover, this new heating process provides a better thermal stability. The performance of devices with general passivation process declines after a long time baking at 70 °C. But the performance of the devices with heating passivation process improves a little after a long time baking even at 80 °C. The results show that CdTe deposition by vacuum evaporation in a thermal ambience can make a good HgCdTe surface passivation protection.
The carrier lifetimes of different types of p-type doped HgCdTe(x~0.23) long wavelength infrared (LWIR) epilayers
were measured which were Hg-vacancy, Au and arsenic doped ones prepared by Te-rich Liquid-phase epitaxy (LPE). By
comparing the lifetimes of Hg-vacancy and extrinsic doped HgCdTe, we focus on three primary mechanisms limiting the
lifetimes in these different p-type HgCdTe samples: radiative recombination, Auger recombination and Schokley-Read-
Hall (SRH) Recombination. The recombination mechanism in p-type HgCdTe is the SRH recombination at low
temperatures and Auger and radiative recombination at high temperature. It is found that the lifetime of As-doped and
Au-doped HgCdTe is far longer than that of Hg-vacancy-doped sample which is caused by the deep energy level of the
Hg-vacancy acceptor that is considered as a recombination center in HgCdTe. Also we found lifetime in those p-type
doped HgCdTe LWIR epilayers is limited by SRH by comparing the experimental lifetimes with the calculated data.
Impurity doping was found to have a main effect on minority carrier lifetime.
KEYWORDS: Mercury cadmium telluride, Liquid phase epitaxy, Arsenic, Data modeling, Annealing, Skin, Temperature metrology, Magnetism, Software, Chemical species
The Hall Effect and resistivity of arsenic-doped HgCdTe epilayers grown by Te-rich liquid phase epitaxy (Te-rich LPE)
have been measured in the temperature range between 20 and 300 K at a magnetic field of 2 kG. Some arsenic-doped
HgCdTe layers show anomalous n-type characteristic after activation annealing. A simplified two-layer model is applied
to describe the anomalous Hall Effect of the arsenic-doped HgCdTe layers. The results show that the anomalous
characteristic of the epilayers is due to the n-type layer in the surface, which may be caused by the surface oxidation.
Based on the model, a computer program is applied to fit the experimental curves of Hall parameters. The results show
that the Hall parameters primarily depend on the charge density of the n-type surface layer. The theoretical curves based
on the model are consistent well with the experimental data.
Dislocation cell structures in CdZnTe substrates and its behavior of threading into HgCdTe LPE epilayers were studied.
A kind of dislocation cell structure of which dislocations linearly pile up to three <1 1 0> orientations on the (1 1 1) B
face of CdZnTe crystal was found. The formation of this cell structure can be demonstrated by the enrichment of the
dislocations through slip during the growth or cooling process. By comparing the dislocation densities and distributions
of HgCdTe LPE epilayers with it of CdZnTe substrates at the constant region using an optical microscopy system, the
behavior of cell structures of CdZnTe substrates threading into HgCdTe LPE epilayers were also discussed. The results
show that the stored dislocation densities at the dislocation walls can significantly affect the behavior of dislocation cell
structures threading into HgCdTe LPE epilayers. Dislocation cell structures of which dislocation walls have higher
stored dislocation densities can appear in constant region of HgCdTe epilayers. But the dislocations of HgCdTe epilayers
growth on CdZnTe substrate with lower stored dislocation densities at the dislocation walls are almost uniformly
distributed.
CdTe surface passivation layers were deposited by thermal evaporation or electron beam evaporation on (111) HgCdTe
epilayers. The processes of CdTe layer deposition were carried out at different temperatures from 100°C to 250°C.
Furthermore, prepared samples were annealed at a temperature range between 150°C and 300°C. Directly, scanning
electron microscope (SEM) was used to evaluate the CdTe passivation layers. The structures of CdTe layers and the
interface of CdTe/HgCdTe were studied by scanning the cross section of the samples. The results showed that the
thermal treatments could merge grain boundaries. Otherwise, the compositional properties of samples were surveyed by
secondary ion mass spectroscopy (SIMS). A compact CdTe structure near the HgCdTe surface caused by heating
deposition and compositional interdiffusion at CdTe/HgCdTe interface were observed. Moreover, the X-ray diffraction
(XRD) curves of the layers showed that the CdTe crystal quality was improved by thermal diffusion. The experimental
results showed that both heating during the deposition process and annealing after growth can effectively improve the
quality of CdTe passivation layers.
The dislocation densities in HgCdTe films grown on CdZnTe by Liquid Phase Epitaxy (LPE) are calculated based on their effects on the x-ray rocking curves. The dislocation densities derived from three kinds of methods, i.e. FWHM of X-ray double axis diffraction, Williamson-Hall plot and Pseudo-Voigt function, are approximately the same. It is found that the thickness of HgCdTe epilayers about 10 um is large enough so that effect of crystallize size on the rocking curves width can be ignored. Because the intrinsic FWHM of HgCdTe and the instrumental function of high resolution X-ray diffraction are neglected in Williamson-Hall plot and Pseudo-Voigt function, the dislocation densities obtained by these methods are a little larger than those derived from the first kind of method. Among three kinds of methods, Pseudo-Voigt function method is the easiest one to fit the rocking curves and calculate the dislocation densities.
This paper describes some recent results on surface defects, uniformity, dislocation density as well as device applications of MBE growth of HgCdTe at the research center of advanced materials and devices. The features of different surface defects and their origins were studied by using SEM/EDX observations on HgCdTe epilayers with different growth conditions. A variety of surface defects was observed and the formation mechanism was discussed. A good uniformity was observed over 3-in HgCdTe wafers, the Stddev/mean in x and thickness were 1.2%, and 2.7%, respectively. It was found that the dislocation density was sensitive to growth parameters and the composition. The ZnCdTe substrates with 4% mole fraction were found to be suitable for LW HgCdTe, however, for the HgCdTe of shorter wavelengths different Zn composition is required. An average value of EPD of 4.2×105cm−2 was obtained for LW samples. The MBE grown HgCdTe were incorporated into some preliminary FPA devices.
The dislocations in HgCdTe films grown by LPE on CdZnTe (111)B substrates were studied by chemical etching. Among the etchants used for HgCdTe materials before, it was found that the Schaake etchant was an effective method to reveal the dislocations of HgCdTe (111)B films with a thickness over 5μm. Besides of the threading dislocations as reported before, another kind of dislocations induced by the stress was also observed by using Schaake etchant. Large quantities of such dislocations were observed in the areas nearby the melt droplet left on the LPE film and cutting line and on the surfaces of the HgCdTe films annealed by using a cap layer and unsuitable transportation of samples. The measurements of the depth profiles of the EPDs show that the most of the stress-induced dislocations are located in the surface layer and its density can be as high as 107cm-3. Whereas the EPD of the threading dislocation doesn't change a lot along the depth of the film in usual case. The Chen etchant can also be used to reveal the threading dislocations of the films thicker than about 10μm. The phenomena of two kinds of etch pits was also observed on the surface of HgCdTe film etched by the Chen etchant. The etch pit densities originated from threading dislocations are approximately the same for both etchants. But the stress-induced dislocations in the surface layer were not observed by using the Chen etchant except the area near the melt droplet.
The Cd-annealing effects on Cd1-xZnxTe wafers were studied by means of IR transmission and micro-Raman spectrum. The experiments and theoretical analysis demonstrated that the free carrier absorption related to the Cd vacancies resulted in the IR extinction as observed in the transmission spectra. The Raman spectra showed that Raman scattering is a more sensitive method to detect the fine Te precipitates in the Cd1-xZnxTe substrates. The Raman scattering peaks related to the Te precipitates could be found in both the as grown and the annealed samples. The relative intensity of the Te scattering peaks became weaker after Cd-annealing. This result also indicated that it was quite difficult to eliminate the fine Te precipitates entirely through annealing process.
The surface morphologies of both MBE and LPE HgCdTe films were observed by optical microscope and atom force microscope. Both HgCdTe films were grown on CdZnTe substrates. It was found that the surface morphology of LPE film is very sensitive to the orientation of CdZnTe substrate, while MBE film surface morphology is mainly dependent on the surface morphology of the substrate. The characteristic of the surface morphologies and the non- flatness of HgCdTe LPE and MBE films in different scales are studied.
Based on empirical rules for the intrinsic absorption coefficient and refractive index of Hg1-xCdxTe in Hougen's model, a novel calculation method determining the composition profile of epitaxy layer from room- temperature infrared transmittance spectroscopy is presented. The composition depth profile of Hg1-xCdxTe film samples grown by liquid-phase epitaxy and after annealing is determined using this method. The expression of the composition interdiffusion coefficient for Hg1-xCdxTe was deduced.
The recent progress in MBE growth of HgCdTe at the Research Center for Advanced Materials and Devices, and the National Laboratory for Infrared Physics is reported. It is found that the excellent compositional uniformity and reproducibility of HgCdTe can be archived by MBE technique. The results of surface morphology, dislocation density, electrical properties and focal plane array detectors are described in the paper.
The recent progress in MBE growth of HgCdTe at the Epitaxy Research Center for Advanced Materials, and the National Laboratory for Infrared Physics is described. It was found that the surface morphology is sensitive to the growth temperature and the flux ratio. The compositional reproducibility studied in a limited number of samples showed that a STDDEV for x-values of 0.0017 deviated from an average value of 0.229 was obtained. The epilayers showed excellent compositional uniformity across 2-inch wafers, the relative deviations for x-value and thickness were found to be 0.18 percent and 2.19 percent respectively. Ellipsometer was used for real-time monitoring the compositional variations during growth. The post growth annealing process was found to be effective in reducing the dislocation density, a reduction in dislocation density by approximately 50 percent could be obtained even by approximately 250 degrees C low temperature annealing. Electrical properties of epilayers are described, and a p-type in situ vacuum annealing process was demonstrated. MBE grown p-HgCdTe epilayers were successfully incorporated into 32 X 32 focal plane arrays detectors.
This is the first report on the mechanism of As incorporation into MOCVD-grown HgCdTe/CdTe/GaAs films. HgCdTe/CdTe/GaAs wafers have been grown by metalorganic chemical vapor deposition (MOCVD). The atomic absorption spectrometer was utilized to analyze As content in the As-grown HgCdTe epilayers. The integrated GC-MS analytical system was utilized to observe the decomposition products of DMCd and/or DETe at different temperatures in the presence of GaAs. The mechanism of As incorporation into epilayers is presented.
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