p to n conversion in SWIR mercury cadmium telluride with ion milling

Exposure to specific damage introduced by either ion implantation or ion milling converts p-type short wavelength infrared (SWIR) HgCdTe to n-type in a manner similar to the conversions in medium wavelength infrared (MWIR) or long wavelength infrared (LWIR) mercury cadmium telluride. However, the depth of conversion for SWIR Hg_1−xCd_xTe, with x=0.48, is approximately 300% smaller when compared to the depth of conversion for MWIR HgCdTe for an identical degree of ion milling. The depth of conversion, or the n/p junction depth, tracks linearly the extent of surface removals by ion milling when the metal vacancy concentration is held invariant. These results can be correlated to the interaction between metal vacancies and a product of the lattice damage process resulting from ion milling. The observation of a linear dependence of this depth on the degree or time of ion milling rules out the existence of a diffusive barrier in the transfer of this product for both MWIR and SWIR HgCdTe.     

Critical thickness in the HgCdTe/CdZnTe system

We present an analysis of the critical thickness of Hg_1−xCd_xTe on Cd_1−yZn_yTe substrates as a function of x and y and show that a very tight control of the substrate composition is needed to produce dislocation-free epi-layers. Hg_1−xCd_xTe layers on relaxed underlayers of different compositions of Hg are also examined.     

Molecular beam epitaxial growth of HgCdTe on CdZnTe(311)B

A series of n-type, indium-doped Hg_1−xCd_xTe (x∼0.225) layers were grown on Cd_0.96Zn_0.04Te(311)B substrates by molecular beam epitaxy (MBE). The Cd_0.96Zn_0.04Te(311)B substrates (2 cm × 3 cm) were prepared in this laboratory by the horizontal Bridgman method using double-zone-refined 6N source materials. The Hg_1−xCd_xTe(311)B epitaxial films were examined by optical microscopy, defect etching, and Hall measurements. Preliminary results indicate that the n-type Hg_1−xCd_xTe(311)B and Hg_1−xCd_xTe(211)B films (x ∼ 0.225) grown by MBE have comparable morphological, structural, and electrical quality, with the best 77 K Hall mobility being 112,000 cm²/V·sec at carrier concentration of 1.9×10⁺¹⁵ cm⁻³.     

Application of urbach rule optical absorption to composition measurement of Cd1−yZnyTe

The optical absorption coefficient of Cd_1−yZn_y Te near the fundamental band edge was measured at room temperature using transmission spectroscopy. Like in other II–VI semiconductors, it was found that the absorption coefficient exhibits an exponential dependence on incident photon energy according to Urbach’s rule. It was also found that the exponential parameters depend on composition, y, of Cd_1−y Zn_yTe. A technique is described for determining the composition of Cd_1−y Zn_yTe from optical transmission spectroscopy. This technique has been implemented in the manufacturing of Cd_1−yZn_yTe substrates for lattice matched epitaxial growth of HgCdTe.     

Vapor phase equilibria in the Cd1−xZnxTe alloy system

Cd_1−xZn_xTe compounds of different compositions have been prepared at temperatures ranging from 400 to 1000°C by annealing elemental Te in sealed quartz ampoules, in an atmosphere comprising vapors of Cd and Zn whose partial pressures were varied by varying the composition of the binary Cd_1−yZn_y alloys which provided the Cd and Zn vapors in these annealing experiments. The chemical compositions of the resulting Cd_1−xZn_xTe compounds have been analyzed using electron probe microanalytical techniques. Results indicate that presence of a 0.5%Zn along with Cd in a closed or semi-closed system may prove to be beneficial in preventing decomposition and/or formation of a metal/non metal phase during annealing of Cd_0.96Zn_0.04 Te substrates. Using the thermodynamic data in the literature for the binary Cd_1−yZn_y alloys and with the assumption that the activities of the Cd and Zn components are weakly dependent on temperature, the partial pressures of Cd and Zn in equilibrium with the Cd_1−xZn_xTe compounds at various temperatures have been evaluated.     

Defect reduction in Hg1−xCdxTe grown by molecular beam epitaxy on Cd0.96Zn0.04Te(211)B

A study on preparation of Cd_0.96Zn_0.04Te(211)B substrates for growth of Hg_1−xCd_xTe epitaxial layers by molecular beam epitaxy (MBE) was investigated. The objective was to investigate the impact of starting substrate surface quality on surface defects such as voids and hillocks commonly observed on MBE Hg_1−xCd_xTe layers. The results of this study indicate that, when the Cd_0.96Zn_0.04Te(211)B substrates are properly prepared, surface defects on the resulting MBE Hg_1−xCd_xTe films are reduced to minimum (size, ∼0.1 m and density ∼500/cm²) so that these MBE Hg_1−xCd_x Te films have surface quality as good as that of liquid phase epitaxial (LPE) Hg_1−xCd_xTe films currently in production in this laboratory.     

HgCdTe composition determination using spectroscopic ellipsometry during molecular beam epitaxy growth of near-infrared avalanche photodiode device structures

The application of spectroscopic ellipsometry (SE) for real-time composition determination during molecular beam epitaxy (MBE) growth of Hg_1−xCd_xTe alloys with x>0.5 is reported. Techniques previously developed for SE determination of composition in long-wavelength infrared (LWIR) HgCdTe have been successfully extended to near-infrared HgCdTe avalanche photodiode (APD) device structures with x values in the range of 0.6–0.8. Ellipsometric data collected over a spectral range of 1.7–5 eV were used to measure depth profiles of HgCdTe alloy composition through the use of an optical model of the growth surface. The optical model used a dielectric-function database collected through the growth of a set of HgCdTe calibration samples with x ranging from 0.6 to 0.8. The sensitivity of this SE method of composition determination is estimated to be Δx ∼0.0002 at x=0.6, which is sufficiently low to sense composition changes arising from flux variations of less than 0.1%. Errors in composition determination because of Hg-flux variations appear to be inconsequential, while substrate-temperature fluctuations have been observed to alter the derived composition at a rate of −0.0004/°C. By comparing the composition inferred from SE and postgrowth 300 K IR transmission measurements on a set of APD device structures, the run-to-run precision of the Se-derived composition (at x=0.6) is estimated to be ±0.0012, which is equivalent to the precision achieved with the same instrumentation during the growth of mid-wavelength infrared (MWIR) HgCdTe alloys in the same MBE system.     

Optical and Structural Properties of CdTe Grown by Molecular Beam Epitaxy at Low Temperature for Resonant-Cavity-Enhanced HgCdTe Detectors

Investigation into resonant-cavity-enhanced (RCE) HgCdTe detectors has revealed a discrepancy in the refractive index of the CdTe layers grown by molecular beam epitaxy (MBE) for the detectors, compared with the reported value for crystalline CdTe. The refractive index of the CdTe grown for RCE detectors was measured using ellipsometry and matches that of CdTe with an inclusion of approximately 10% voids. X-ray measurements confirm that the sample is crystalline and strained to match the lattice spacing of the underlying Hg_(1−x)Cd_(x)Te, while electron diffraction patterns observed during growth indicate that the CdTe layers exhibit some three-dimensional structure. Secondary ion mass spectroscopy results further indicate that there is enhanced interdiffusion at the interface between Hg_(1−x)Cd_(x)Te and CdTe when the Hg_(1−x)Cd_(x)Te is grown on CdTe, suggesting that the defects are nucleated within the CdTe layers.     

Molecular beam epitaxial growth of Cd<Subscript>1−y</Subscript>Zn<Subscript>y</Subscript>Se<Subscript>x</Subscript>Te<Subscript>1−x</Subscript> on Si(211)

We report on the first successful growth of the quaternary alloy Cd_1−yZn_ySe_xTe_1−x(211) on 3-in. Si(211) substrates using molecular beam epitaxy (MBE). The growth of CdZnSeTe was performed using a compound CdTe effusion source, a compound ZnTe source, and an elemental Se effusion source. The alloy compositions (x and y) of the Cd_1−yZn_ySe_xTe_1−x quaternary compound were controlled through the Se/CdTe and ZnTe/CdTe flux ratios, respectively. Our results indicated that the surface morphology of CdZnSeTe improves as the Zn concentration decreases, which fits well with our previous observation that the surface morphology of CdZnTe/Si is poorer than that of CdSeTe/Si. Although the x-ray full-width at half-maximums (FWHMs) of CdZnSeTe/Si with 4% of Zn + Se remain relatively constant regardless of the individual Zn and Se concentrations, etched-pit density (EPD) measurements exhibit a higher dislocation count on CdZnSeTe/Si layers with about 2% Zn and Se incorporated. The enhancement of threading dislocations in these alloys might be due to an alloy disorder effect between ZnSe and CdTe phases. Our results indicate that the CdZnSeTe/Si quaternary material with low Zn or low Se concentration (less than 1.5%) while maintaining 4% total Zn + Se concentration can be used as lattice-matching composite substrates for long-wavelength infrared (LWIR) HgCdTe as an alternative for CdZnTe/Si or CdSeTe/Si.     

Planar n-on-p ion milled mid-wavelength and long-wavelength infrared diodes on molecular beam epitaxy vacancy-doped CdHgTe on CdZnTe

Planar mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) photodiodes were fabricated by ion milling molecular beam epitaxy (MBE) Cd_xHg_1−xTe (CMT) layers with and without compositional grading in the layer. Linear arrays with 32 and 64 diodes, as well as test diodes of varying size, were fabricated. Good quantum efficiencies were measured, and MWIR diodes, with cutoff wavelength λ_CO=4.5 μm, had zero-bias resistance-area values (R₀A) in excess of 1×10⁷ Ωcm², whereas LWIR diodes with λ_CO=8.9−9.3 μm had R₀A=3×10² Ωcm² at 77 K. Comparison between a limited number of layers indicates that in layers with a gradient the RA values are a factor of ∼10 larger, and possibly more uniform, than in layers without a gradient.     

Shear deformation and strain relaxation in HgCdTe on (211) CdZnTe

Shear strain is present in Hg_0.68Cd_0.32Te epitaxial layers grown by molecular beam epitaxy on (211)-oriented Cd_1−yZn_yTe substrates. Differences in the substrate zinc composition led to lattice mismatch between the epitaxial layer and the substrate. The shear strain induced by the mismatch was measured using reciprocal space maps in the symmetric (422) and asymmetric (511) and (333) reflections. In addition, strain relaxation through the formation of misfit dislocations was confirmed using double crystal x-ray topography. Both the shear strain and the misfit dislocation density increased with increasing mismatch between the epitaxial layer and the substrate. Lattice-matched layers were free of misfit dislocations and exhibited triple axis diffraction rocking curve widths of approximately 6 arcsec. The combination of a thick epitaxial layer, a low index substrate, and the potential for lattice mismatch indicates that both shear strains and misfit dislocations must be considered in the structural analysis of HgCdTe/CdZnTe heterostructures.     

Growth and properties of Hg1-x Cdx Te epitaxial layers

The growth of epitaxial layers of mercury-cadmium-telluride (Hg_1-xCd_xTe) with relatively low x (0.2-0.3) from Te-rich solutions in an open tube sliding system is studied. The development of a semiclosed slider system with unique features permits the growth of low x material at atmospheric pressure. The quality of the films is improved by the use of Cd_1-yZ_yTe and Hg_1-xCd_xTe substrates instead of CdTe. The substrate effects and the growth procedure are discussed and a solidus line at a relatively low temperature is reported. The asgrown epitaxial layers are p-type with hole concentration of the order of 1·10¹⁷ cm⁻³, hole mobility of about 300 cm²·V⁻¹ sec⁻¹ and excess minority carrier life-time of 3 nsec, at 77 K.     

Band anticrossing in highly mismatched group II-VI semiconductor alloys

We have successfully synthesized highly mismatched Cd_1−yMn_yO_xTe_1−x alloys by high-dose implantation of O ions into Cd_1−yMn_yTe crystals. In crystals with y>0.02, incorporation of O causes a large decrease in the bandgap. The bandgap reduction increases with y; the largest value observed is 190 meV in O⁺-implanted Cd_0.38Mn_0.62Te. The results are consistent with the band anticrossing (BAC) model, which predicts that a repulsive interaction between localized states of O located above the conduction-band edge and the extended states of the conduction band causes the bandgap reduction. A best fit of the measured bandgap energies of the O-ion-synthesized Cd_1−yMn_yO_xTe_1−x alloys using the BAC model for y<0.55 suggests an activation efficiency of only ∼5% for implanted O in Cd_1−yMn_yTe.     

Development of Molecular Beam Epitaxially Grown Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te for High-Density Vertically-Integrated Photodiode-Based Focal Plane Arrays

Hg_1−x Cd _x Te samples of x ~ 0.3 (in the midwave infrared, or MWIR, spectral band) were prepared by molecular beam epitaxy (MBE) for fabrication into 30-μm-pitch, 256 × 256, front-side-illuminated, high-density vertically-integrated photodiode (HDVIP) focal plane arrays (FPAs). These MBE Hg_1−x Cd _x Te samples were grown on CdZnTe(211) substrates prepared in this laboratory; they were ~10-μm thick and were doped with indium to ~5 × 10¹⁴ cm⁻³. Standard HDVIP process flow was employed for array fabrication. Excellent array performance data were obtained from these MWIR arrays with MBE HgCdTe material. The noise-equivalent differential flux (NEΔΦ) operability of the best array is 99.76%, comparable to the best array obtained from liquid-phase epitaxy (LPE) material prepared in this laboratory.     

Lpe growth of Hgl−xCdxTe using conventional slider boat and effects of annealing on properties of the epilayers

The epitaxial layers of Hg_1−xCd_xTe (0.17≦×≦0.3) were grown by liquid phase epitaxy on CdTe (111)A substrates using a conventional slider boat in the open tube H₂ flow system. The as-grown layers have hole concentrations in the 10¹⁷− 10¹⁸ cm⁻³ range and Hall mobilities in the 100−500 cm²/Vs range for the x=0.2 layers. The surfaces of the layers are mirror-like and EMPA data of the layers show sharp compositional transition at the interface between the epitaxial layer and the substrate. The effects of annealing in Hg over-pressure on the properties of the as-grown layers were also investigated in the temperature range of 250−400 °C. By annealing at the temperature of 400 °C, a compositional change near the interface is observed. Contrary to this, without apparent compositional change, well-behaved n-type layers are obtained by annealing in the 250−300 °C temperature range. Sequential growth of double heterostructure, Hg_l−xCd_xTe/Hg_l−yCd_yTe on a CdTe (111)A substrate was also demonstrated.     

Ten-inch molecular beam epitaxy production system for HgCdTe growth

Growth of Hg_1−xCd_xTe by molecular beam epitaxy (MBE) has been under development since the early 1980s at Rockwell Scientific Company (RSC), formerly the Rockwell Science Center; and we have shown that high-performance and highly reproducible MBE HgCdTe double heterostructure planar p-on-n devices can be produced with high throughput for various single- and multiplecolor infrared applications. In this paper, we present data on Hg_1−xCd_xTe epitaxial layers grown in a ten-inch production MBE system. For growth of HgCdTe, standard effusion cells containing CdTe and Te were used, in addition to a Hg source. The system is equipped with reflection high energy electron diffraction (RHEED) and spectral ellipsometry in addition to other fully automated electrical and optical monitoring systems. The HgCdTe heterostructures grown in our large ten-inch Riber 49 MBE system have outstanding structural characteristics with etch-pit densities (EPDs) in the low 10⁴ cm⁻² range, Hall carrier concentration in low 10¹⁴ cm⁻³, and void density <1000 cm². The epilayers were grown on near lattice-matched (211)B Cd_0.96Zn_0.04Te substrates. High-performance mid wavelength infrared (MWIR) devices were fabricated with R₀A values of 7.2×10⁶ Ω-cm² at 110 K, and the quantum efficiency without an antireflection coating was 71.5% for cutoff wavelength of 5.21 μm at 37 K. For short wavelength infrared (SWIR) devices, an R₀A value of 9.4×10⁵ Ω-cm² at 200 K was obtained and quantum efficiency without an antireflection coating was 64% for cutoff wavelength of 2.61 μm at 37 K. These R₀A values are comparable to our trend line values in this temperature range.     

Liquidus isotherms,solidus lines and LPE growth in the Te-rich corner of the Hg-Cd-Te system

Liquidus isotherms for the Hg_1−xCd_xTe primary phase field in the Te-rich corner of the Hg-Cd-Te ternary system have been determined for temperatures from 425 to 600‡C by a modified direct observational technique. These isotherms were used to help establish conditions for the open-tube liquid phase epitaxial growth of Hg_1−xCd_xTe layers on CdTe_1−ySe_y substrates. Layers with x ranging from 0.1 to 0.8 have been grown from Te-rich HgCdTe solutions under flowing H₂ by means of a horizontal slider technique that prevents loss of Hg from the solutions by evaporation. Growth temperatures and times of 450–550‡C and 0.25–10 min, respectively, have been used. The growth solution equilibration time is typically 1 h at 550‡C. Source wafers, supercooled solutions, and (111)-oriented substrates were employed in growing the highest quality layers, which were between 3 and 15 Μm thick. Electron microprobe analysis was used to determine x for the epitaxial layers, and the resulting data, along with the liquidus isotherms, were used to obtain solidus lines. In addition to EMP data, optical transmission results are given. This work was sponsored by the Department of the Air Force and the U. S. Army Research Office.     

Measurement of minority carrier lifetime in n-type MBE HgCdTe and its dependence on annealing

Results are presented for minority carrier lifetime in n-type molecular beam epitaxy Hg_1−xCd_xTe with x ranging from 0.2 to 0.6. It was found that the lifetime was unintentionally degraded by post-growth annealing under Hg saturated conditions in a H₂ atmosphere that was both time and temperature dependent. This effect was minimal or non-existent for x∼0.2 material, but very strong for x ≥ 0.3. Hydrogen was identified as responsible for this degradation. Identical annealing in a He atmosphere avoids this degradation and results in neartheoretical lifetime values for carrier concentrations as low 1 × 10¹⁵ cm⁻³ in ≥0.3 material. Modeling was carried out for x∼0.2 and x∼0.4 material that shows the extent to which lifetime is reduced by Shockley-Real-Hall recombination for carrier concentrations below 1 × 10¹⁵ cm⁻³, as well as for layers annealed in H₂. It appears that annealing in H₂ results in a deep recombination center in wider bandgap HgCdTe that lowers the lifetime without affecting the majority carrier concentration and mobility.     

Characterization of a new planar process for implementation of p-on-n HgCdTe heterostructure infrared photodiodes

We report on a new, simple process to fabricate planar Hg_1−yCd_yTe/Hg_1−xCd_xTe (x<y) heterostructure photodiodes with p-on-n configuration. The material used for this demonstration was a double-layer p-on-n heterostructure that was grown by a liquid-phase-epitaxy technique. The p-on-n planar devices consisted of an arsenic-doped p-type epilayer (y=0.28) on top of a long-wavelength infrared n-type epilayer (x=0.225, =10 m). The ion-beam-milling p-type to n-type conversion effect was used to delineate the active device element, and to isolate the planar device. Detailed analysis of the current-voltage characteristics of these diodes as a function of temperature show that they have high performance, and that their dark current is diffusion-limited down to 60 K. The results show that over a wide range of cut-off wavelengths, the R₀A product values are close to the theoretical limit. Electro-optic properties of a 2-D array of small diodes with a 60- m pitch are presented, and demonstrate the potential of the new process for implementation of 2-D arrays. The electrical properties of the photodiodes are stable following long-term annealing at 80°C for 48 hours.     

MBE HgCdTe heterostructure p-on-n planar infrared photodiodes

We recently succeeded in fabricating planar Hg_1−yCd_yTe/Hg_1−xCd_xTe (x<y) heterostructure photodiodes with the p-on-n configuration. Here we discuss early results in detail and present new results on an expanded range of infrared operation. The material used for this demonstration was grown by molecular beam epitaxy on lattice-matched CdZnTe substrates. The p-on-n planar devices consist of an arsenic-doped p-type epilayer (y∼0.28) atop a long wavelength infrared n-type epilayer (x=0.22–0.23). The planar junctions were formed by selective pocket diffusion of arsenic deposited on the surface by ion implantation. Detailed analysis of the current-voltage characteristics of these diodes as a function of temperature shows that they have high performance and that their dark currents are diffusion-limited down to 52K. Low frequency noise measurements at a reverse bias voltage of 50 mV resulted in noise current values (at 1 Hz) as low as 1×10⁻¹⁴ amps/Hz^0.5 at 77K. Average R_oA values greater than 10⁶ Ω-cm² at 40K were obtained for these devices with cut-off wavelength values in the 10.6 to 12 μm range. Seventy percent of these devices have R_oA values greater than 10⁵ Ω-cm² at 40K; further studies are needed to improve device uniformity. These results represent the first demonstration that high performance long wavelength infrared devices operating at 40K can be made using HgCdTe material grown by a vapor phase epitaxy growth technique.