In situ sensors for monitoring and control in molecular beam epitaxial growth of Hg1−xCdxTe

The current status of the implementation and refinement of two wafer state sensors forin situ monitoring and control during molecular beam epitaxial (MBE) growth of Hg_1−xCd_xTe will be reported. First a rapid scan spectral ellipsometer has been developed and employed for precisely measuring compositions of Hg_1−xCd_xTe alloys during growth. MBE films in the composition range x = 0.20 to 0.30 have been grown andin situ spectra taken at the growth temperature (180°C) and at room temperature. The MBE films were treated as single layers without the need to invoke any surface film (due to surface roughness, oxide, or of any different composition) as required for exsitu data. The least squares fit over the whole spectral range was used as a measure of the precision. The film composition was also determinedex situ by wavelength dispersive analysis of x-rays and by Fourier transform infrared (FTIR) spectrometry after verifying that there was no lateral variation. A precision of better than ±0.0015 has so far been demonstrated usingin situ spectral ellipsometry for Cd composition or CdTe mole fraction, x, measurements. This compares with ±0.003 for single wavelength ellipsometry. The composition of Hg_1−xCd_xTe films were also monitored during growth. A spectral pyrometer based on a FTIR spectrometer has also been developed for substrate temperature measurements during growth. The spectral pyrometer measures both the emission and reflectance to give the emissivity of a growing sample over a range of wavelengths spanning the peak of the grey body emission. From the reflectivity measurements, the thickness (in excess of 1 μm) of the growing film is also determined from the interference fringes. The spectral ellipsometer is only capable of measuring thicknesses up to C.a 5000°A (i.e. optically thin). Excellent agreement is obtained between thein situ (at growth temperature) andex situ (at room temperature) thickness measurements. The small discrepancy can be explained by the refractive index of Hg_1−xCd_xTe being 5% higher at the growth temperature than at room temperature. The combination ofin situ sensors now provides a means of continuously monitoring the composition and thickness of the growing Hg_1−xCd_xTe film.     

HgCdZnTe quaternary materials for lattice-matched two-color detectors

As the number of bands and the complexity of HgCdTe multicolor structures increases, it is desirable to minimize the lattice mismatch at growth interfaces within the device structure in order to reduce or eliminate mismatch dislocations at these interfaces and potential threading dislocations that can degrade device performance. To achieve this we are investigating the use of Hg_1−x−yCd_xZn_yTe quaternary alloys which have an independently tunable lattice constant and bandgap. Lattice matching in Hg_1−x−yCd_xZn_yTe structures can be achieved using small additions of Zn (y<0.015) to HgCdTe ternary alloys. We have investigated some of the basic properties of Hg_1−x−yCd_xZn_yTe materials with x≈0.31 and 0≤y≤0.015. The quaternary layers were grown on (112)CdZnTe substrates using MBE and the amount of Zn in the layers was determined from calibrated SIMS measurements. As expected, the lattice constant decreased and the bandgap increased as Zn was added to HgCdTe to form Hg_1−x−yCd_xZn_yTe. Hall-effect results for both n-type (In) and p-type (As) Hg_1−x−yCd_xZn_yTe layers were very similar to HgCdTe control samples. We have also utilized x-ray rocking curve measurements with (246) asymmetric reflections as a novel sensitive technique to determine the correct amount of Zn needed to achieve lattice matching at an interface. MWIR/LWIR n-p-n two-color triple-layer heterojunction structures were grown to evaluate the effects of minimizing the lattice mismatch between the widest bandgap p-type collector layer, using Hg_1−x−yCd_xZn_yTe, and the HgCdTe MWIR and LWIR collector layers and compared to structures that did not incorporate the quaternary. Sequential mode two-color detectors were fabricated using a 256 × 256, 30 μm unit cell design. There were several interesting findings. Macro defects predominantly affected the LWIR band (Band 2) operability and had little effect on the MWIR band (Band 1). The incorporation of Hg_1−x−yCd_xZn_yTe p-type collector layers had little effect on MWIR detector performance, but overall the LWIR performance was generally better. These initial detector results indicate that the use of Hg_1−x−yCd_xZn_yTe alloys in multicolor detector structures are potentially promising and should be pursued further.     

An improved method for Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te surface chemistry characterization

Hg_1−xCd_xTe surface chemistry has been studied extensively with well-known tools such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES) in order to advance detector array operability, performance, and yield. Raytheon Vision Systems has pioneered the first application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) as a Hg_1−xCd_xTe surface diagnostic tool to provide unprecedented analysis capability, including analyzing a 0.5-μm-diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. In this study, data are presented illustrating surface chemistry geometry effects and photoresist redeposition due to common Hg_1−xCd_xTe processing steps including photolithography, bromine etching, and photoresist stripping.     

Arrays of 128×128 photodetectors based on HgCdTe layers and multilayer heterostructures with GaAs/AlGaAs quantum wells

A technology has been elaborated and photodetector modules based on Hg_1−x Cd_xTe/GaAs heterostructures and GaAs/AlGaAs multiquantum-well structures grown by molecular-beam epitaxy were fabricated for the 3–5 and 8–12 μm spectral ranges. The photosensitive HgCdTe layers were grown on the GaAs substrates with the intermediate buffer layer of CdZnTe. To decrease the surface effect on the recombination processes, the graded-gap Hg_1−x Cd_xTe layers with x increasing towards the surface were grown. A silicon multiplexer was designed and fabricated by CMOS/CCD technology with a frame rate of 50 Hz. The hybrid microassembly of the photodetector array and the multiplexer was produced by group cold welding on indium columns while monitoring the connection process. The fabricated 128×128 modules based on HgCdTe layers with the cutoff wavelengths 6 and 8.7 μm had a temperature resolution of 0.02 K and 0.032 K, respectively, at a temperature of 78 K and a frame rate of 50 Hz. The photosensitive GaAs/AlGaAs multilayer quantum well structures were fabricated by MBE. It is shown that the technology developed allows 128×128 multielement photodetector arrays (λ_peak=8 μm) to be produced with a temperature resolution of 0.021 K and 0.06 K at operating temperatures of 54 K and 65 K, respectively. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 9, 2001, pp. 1159–1166. Original Russian Text Copyright ? 2001 by Ovsyuk, Sidorov, Vasil’ev, Shashkin.     

Percolation problem in boron—Implanted mercury cadmium telluride

We used high-resolution x-ray diffraction to measure precisely structural modifications in variously composed Hg_1−xCd_xTe layers which were fabricated by different growth techniques and subjected to boron implantation to form p-n junctions. Analysis of implantation-induced features in the diffraction profiles allowed us to deduce the interstitials concentration remaining in the sample interior and, thus, to obtain important information on post-implantation defect migration. As a result, a percolation problem in the migration of Cd interstitials was discovered in samples with x<x_c (x_c=0.265 is the percolation threshold). Due to the percolation problem, the implanted samples having Cd content below and above x_c exhibited very different surface recovery, which was visualized by high resolution scanning electron microscopy. It was found that additional annealing at 250–300°C stimulates diffusion of formerly locked Cd interstitials and leads to the change in the conductivity type (n-p) at the expense of remaining non-compensated vacancies. The percolation problem in samples with x < x_c seems to be responsible for limited mobility of implanted boron and difficulties in boron activation in Hg_1−xCd_xTe-based devices for 8–12 μm atmospheric transparency window.     

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.     

Status of MBE technology for the flexible manufacturing of HgCdTe focal plane arrays

A robust process has been developed for the reproducible growth of in-situ doped Hg_1−xCd_xTe:As alloys by molecular beam epitaxy. Net hole concentrations in excess of 5 x 10¹⁷ cm⁻³, with peak mobilities >200 cm2/Vs were measured in Hg_0.74Cd_0.26Te:As films. The p-type layers were twin-free and consistently exhibit narrow x-ray rocking curves (<40 arc sec). The reproducible growth of small lots of p-on-n LWIR detector structures has been established. For a typical lot consisting of 13 layers, the average x-value of the n-type base layer was 0.226 with a standard deviation of 0.003. The lateral compositional uniformity across a 2.5 cm × 2.5 cm wafer was × = +- 0.0006. High performance p-on-n LWIR diodes were fabricated that exhibited R_oA_o values (0-fov at 78K) as large as 350 Q cm2 at 10.4 μm.     

Correlation between visual defects and increased dark current in large-area Hg1−xCdxTe photodiodes

The Cross-Track Infrared Sounder (CrIS) program [an instrument on the National Polar-Orbiting Operational Environmental Satellite System (NPOESS)] requires photodiodes with spectral cutoffs denoted by short-wavelength infrared [γ_c(98 K) ∼5.1 μm], midwavelength infrared [γ_c(98 K) ∼9.1 μm], and long-wavelength infrared (LWIR) [γ_c(81 K) ∼15.5 μm]. The CrIS instrument also requires large-area (850-μm-diameter) photodiodes with state-of-art performance. Molecular beam epitaxy (MBE) is used to grow n-type short-wavelength infrared, midwavelength infrared, or LWIR Hg_1−xCd_xTe on latticematched CdZnTe. Detectors with p-type implants 7 μm in diameter are used to constitute the 850-μm-diameter lateral collection diodes (LCDs). The photodiode architecture is the double-layer planar heterostructure architecture. Quantum efficiency, I-V, R_d-V, and 1/f noise in photovoltaic Hg_1−xCd_xTe detectors are critical parameters that limit the sensitivity of infrared sounders. These are some of the parameters used to select photodiodes that will be part of the CrIS focal plane module (FPM). During fabrication of the FPM, the photodiodes are subject to a significant amount of handling while transitioning from part of newly processed Hg_1–xCd_xTe wafers to individual photodiodes mounted in a CrIS FPM ready to be flown on NPOESS. Quantum efficiency, I-V, noise, and visual inspections are performed at several steps in the detector’s journey. Initial I-V and visual inspections are conducted at the wafer level followed by I-V, noise, and quantum efficiency after dicing and mounting the photodiodes in leadless chip carriers (LCCs). A visual inspection is performed following removal of the detectors from the LCCs. Finally, the individual photodiodes are precision mounted on an FPM base, and I-V, noise, quantum efficiency, and visual inspections are performed again. Each step in the FPM fabrication process requires handling and environmental conditioning that can result in detector dark current and noise increase. Some photodiodes on the first flightlike FPMs fabricated exhibited an increase in dark current and noise characteristics at the FPM level as compared to the measurements performed when the photodiodes were in LCCs prior to integration into the FPM. The degradation observed resulted in an investigation to discern the cause of the performance degradation (baking at elevated temperatures, mechanical handling, electrical stress, etc.). This paper outlines the results of the study and the corrective actions that led to the successful manufacture of LWIR large detectors from material growth to insertion into flight FPMs for the CrIS program.     

Characterization of physical-chemical drivers of Hg1−xGdxTe etch rate using time of flight—Secondary ion mass spectrometry and optical interferometry

Time of flight-secondary ion mass spectrometry (TOF-SIMS) is a Hg_1−xCd_xTe surface diagnostic tool with unprecedented analysis capabilities, including analyzing a 0.5-μm diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. The present investigation demonstrates the power of TOF-SIMS when coupled with optical interferometry in understanding process reproducibility and uniformity critical to the fabrication of Hg_1−xCd_xTe detector arrays at RVS. Previous published works and unpublished studies at RVS have shown that geometry and fluid dynamics influence the lateral uniformity of surface chemistry, topography, and etch rates. By combining a set of photolithographically delineated features having various relative areas of photoresist-coated and uncoated regions in varying proximity to each other with various wet etching chemistries, we have exploited TOF-SIMS interrogation along with optical interferometry to investigate physical-chemical drivers of etch rate variation with window geometry orientation with respect to vertical gravity etchant fluid draining direction and proximity to other structures. This study has given us the ability to deconvolve two important etch rate drivers (depletion of etchant species and cross-contamination of etched windows) and elucidate their roles in enhancing and diminishing etch rates for features having far and close proximities to neighboring structures, respectively. This information allows a more judicious optimization of processing technology.     

Effect of successive implantation of Ag+(Cu+) and Xe+ ions on the recombination properties of CdxHg1−x Te crystals

The effect of successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions on the recombination properties of Cd_xHg_1−x Te (0.2<x<0.3) crystals has been investigated. It is shown that after implantation of ions of one chemical element, followed by diffusion thermal annealing at temperatures below 150–200 K, recombination through local levels lying 30±5 meV below the conduction band bottom dominates. Successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions followed by diffusion thermal annealing changes the course of the temperature dependence of the lifetime of the nonequilibrium charge carriers. It was determined that for Cd_xHg_1−x Te crystals with x⋍0.20–0.25 in the temperature interval 700–200 K the lifetime of the nonequilibrium charge carriers is low (τ<0.15 μs) and does not depend on the temperature. For Cd_xHg_1−x Te crystals with x⋍0.3 recombination of nonequilibrium charge carriers occurs through two types of levels: in the temperature range 140–200 K — deep levels E _t1⋍E _c −51 meV and at lower temperatures (77–140 K) — through shallower levels E _t2⋍E _c −(16±2) meV. Fiz. Tekh. Poluprovodn. 31, 786–789 (July 1997)     

Determination of individual layer composition and thickness in multilayer HgCdTe structures

The reproducible molecular-beam epitaxy (MBE) growth of dual-band Hg_1−xCd_xTe (MCT) heterostructures requires routine post-growth wafer analysis for constituent layer thickness and alloy composition, therefore, demanding nondestructive characterization techniques that offer quick data feedback. This paper reports a multilayer structure model, which can be least-square fit directly to either Fourier transform infrared (FTIR) transmission or reflection spectra to provide individual layer thickness, alloy composition, and grading information for various complex structures. The model, we developed, is based on an accurate representation of both the real and imaginary parts of the MCT dielectric function across and above E _g as a function of alloy composition. The parametric, MCT optical-dielectric function for compositions varying between x=0.17 to x=0.5 was developed in the range from 400 cm⁻¹ to 4,000 cm⁻¹, based on a semi-empirical model for the absorption coefficient and extrapolation of the refractive index across E _g . The model parameters were refined through simultaneous fits to multiple reflection and transmission data sets from as-grown, double-layer planar heterostructure (DLPH) structures of variable thickness. The multilayer model was tested on a variety of simple DLPH structures with thick absorber layers (>8 μm) and was compared against traditional FTIR analysis and cross-section optical microscopy and showed good agreement in both composition and thickness. Model fits to dual-color MCT data and subsequent analysis of the internal parameter correlation have demonstrated that error bars on absorber layer composition and thickness could be as low as ∼0.0005 and ∼0.02 μm, correspondingly.     

Electron beam induced current study of ion beam milling type conversion in molecular beam epitaxy vacancy-doped CdxHg1−xTe

Ion milling has been used to type convert molecular beam epitaxy vacancy-doped Cd_xHg_1−xTe, and electron beam induced current measurements have been performed to study the pn-junction depth dependence on milling time, milling current and vacancy concentration. The junction depth seems to initially increase linearly with time for depths up to ∼ 4 μm, then possibly as the square root of time at larger depth. For given x, the depth increases with decreasing vacancy concentration. For the same annealing temperature, high x samples have lower carrier concentration and greater junction depth than low x samples. Up to 4 μm, junction depth is proportional to milling current density as well as milling time.     

Molecular beam epitaxial growth and characterization of lead telluride for laser diodes

The characteristics of PbTe films grown by molecular beam epitaxy (MBE) have been investigated. These films were grown on (100) oriented Tl-doped PbTe substrates under UHV conditions (~5×l0⁻⁹ Torr during deposition). Substrate surface contamination levels were studied with Auger electron spectroscopy. Oxygen, the dominant impurity observed, is rapidly thermally desorbed from PbTe, but is stable on Pb_1−xSn_xTe up to at least 410°C. Carrier concentration and mobility were measured with the Van der Pauw technique. The electron mobility increased strongly with increasing film thickness, varying from 4,000 to 14,000 cm²/volt-sec as the thickness increased from 2.0 to 7.3 μn. The film surface also became smoother with increasing film thickness. These results suggest the need for a buffer layer in a laser structure. Lasers grown with 6 μm thick buffer layers have exhibited extremely low threshold current densities (40 A/cm² at 13 K) and very high junction resistancearea products at zero-bias (0.7 Ω−cm² at 77 K), indicative of very high junction quality.     

Mid-wavelength infrared p-on-n Hg1−xCdxTe heterostructure detectors: 30–120 kelvin state-of-the-Art performance

We report on Hg_1−xCd_xTe mid-wavelength infrared (MWIR) detectors grown by molecular-beam epitaxy (MBE) on CdZnTe substrates. Current-voltage (I-V) characteristics of HgCdTe-MWIR devices and temperature dependence of focal-plane array (FPA) dark current have been investigated and compared with the most recent InSb published data. These MWIR p-on-n Hg_1−xCd_xTe/CdZnTe heterostructure detectors give outstanding performance, and at 68 K, they are limited by diffusion currents. For temperatures lower than 68 K, in the near small-bias region, another current is dominant. This current has lower sensitivity to temperature and most likely is of tunneling origin. High-performance MWIR devices and arrays were fabricated with median R_oA values of 3.96 × 10¹⁰ Ω-cm² at 78 K and 1.27 × 10¹² Ω-cm² at 60 K; the quantum efficiency (QE) without an antireflection (AR) coating was 73% for a cutoff wavelength of 5.3 μm at 78 K. The QE measurement was performed with a narrow pass filter centered at 3.5 μm. Many large-format MWIR 1024 × 1024 FPAs were fabricated and tested as a function of temperature to confirm the ultra-low dark currents observed in individual devices. For these MWIR FPAs, dark current as low as 0.01 e⁻/pixel/sec at 58 K for 18 × 18 μm pixels was measured. The 1024 × 1024 array operability and AR-coated QE at 78 K were 99.48% and 88.3%, respectively. A comparison of these results with the state-of-the-art InSb-detector data suggests MWIR-HgCdTe devices have significantly higher performance in the 30–120 K temperature range. The InSb detectors are dominated by generation-recombination (G-R) currents in the 60–120 K temperature range because of a defect center in the energy gap, whereas MWIR-HgCdTe detectors do not exhibit G-R-type currents in this temperature range and are limited by diffusion currents.     

Characterization of HgCdTe Diodes on Si Substrates Using Temperature-Dependent Current-Voltage Measurements and Deep Level Transient Spectroscopy

Reverse current in diodes can be dominated by generation processes, depending exponentially on temperature according to the rate-limiting step in the generation process. In this report, the current-voltage-temperature (IVT) relationship is analyzed for several midwave infrared and long-wave infrared (MWIR x = 0.295, LWIR x = 0.233) Hg_1−x Cd _x Te (MCT) diodes. The energy varied from diode to diode. At high reverse biases, the energy tends toward the band gap energy. Close to zero bias, the energy ranged from 0.06 to 0.1 eV. Deep level transient spectroscopy (DLTS) showed a broad peak centered at 55–80 K for the MWIR MCT. Comparison of the DLTS spectrum to a simulation based on the energy and capture cross section from a rate window analysis shows that the peak is a band of traps. The capacitance transient amplitude increased as the filling pulse increased from 1 μs to 0.1 s, consistent with capture at a dislocation. A shift to lower temperatures for the peak was also observed when the diodes are cooled under forward bias. The shift is reversible, indicating that the traps consist at least partially of a bistable defect.     

Heteroepitaxy of HgCdTe(112) infrared detector structures on Si(112) substrates by molecular-beam epitaxy

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 10⁶ cm⁻² for 8 μm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 10⁶ cm⁻². These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R₀A = 1.64 × 10⁴ Ωm₂ (0 FOV) for devices with 7.8 μm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.     

Optical absorption by transitions between subbands of light and heavy holes in <Emphasis Type="Italic">p</Emphasis>-Mn<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te

Absorption spectra of p-Mn_xHg_1−x Te epitaxial films with x=0.12–0.19 in the wavelength range of 2–24 μm at 300 and 80 K were measured. The absorption caused by transitions of charge carriers between subbands of light and heavy holes is calculated according to Kane’s theory taking into account the effect of remote bands. The values of the effective masses of heavy and light holes are obtained as functions of solid-solution composition and temperature. The analysis of low-temperature absorption spectra was carried out; the ionization energy of the acceptor level is evaluated. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 1, 2002, pp. 49–51. Original Russian Text Copyright ? 2002 by Nesmelova, Baryshev, Andreev. Deceased.     

Study of the spatial response of reduced pitch Hg1−xCdxTe dual-band detector arrays

The third generation of infrared (IR) focal plane arrays (FPA) will have to detect simultaneously at least two different spectral bands. Small pixel pitches (less than 25–30 μm) will also be needed to achieve the resolution required for such devices without degrading performances, introducing difficult technological challenges. Among those is optical filling: the third generation will have to retain the high filling factor obtained with a planar single-color FPA. It thus appears necessary to estimate the sensible areas of small pitched dual-band pixels. Scanning a controlled IR spot on the FPA surface allows the obervation of the spatial response of a pixel. Such spotscan measurements have been carried out on Hg_1−xCd_xTe dual-band arrays. Different device structures have been tested: planar structures and NPN mesa structures, produced at LETI LIR. The spotscan bench test is first presented, in order to clarify characterization conditions and understanding. Measurement results are then discussed and compared with modeling, in order to identify the main physical parameters determining the optical area of such dual-band IR detectors. Emphasis on modulation transfer function (MTF) estimation and X talk is finally carried out.     

Heteroepitaxy of HgCdTe (211)B on Ge substrates by molecular beam epitaxy for infrared detectors

Epitaxial growth of (211)B CdTe/HgCdTe has been achieved on two inch germanium (Ge) by molecular beam epitaxy (MBE). Germanium was chosen as an alternative substrate to circumvent the weaknesses of CdZnTe wafers. The ease of surface preparation makes Ge an attractive candidate among many other alternative substrates. Best MBE CdTe growth results were obtained on (211) Ge surfaces which were exposed to arsenic and zinc fluxes prior to the MBE growth. This surface preparation enabled CdTe growth with B-face crystallographic polarity necessary for the HgCdTe growth. This process was reproducible, and produced a smooth and mirror-like surface morphology. The best value of the {422} x-ray double diffraction full width at half maximum measured from the HgCdTe layer was 68 arc-s. We present the 486 point maps of FWHM statistical values obtained from CdTe/Ge and HgCdTe/CdTe/Ge. High resolution microscopy electron transmission and secondary ion mass spectroscopy characterization results are also presented in this paper. High-performance middle wavelength infrared HgCdTe 32-element photodiode linear arrays, using the standard LETI/LIR planar n-on-p ion implanted technology, were fabricated on CdTe/Ge substrates. At 78K, photodiodes exhibited very high R₀A figure of merit higher than 10⁶ Ωcm⁻² for a cutoff wavelength of 4.8 μm. Excess low frequency noise was not observed below 150K.     

Analysis of reactor geometry and diluent gas flow effects on the metalorganic vapor phase epitaxy of AIN and GaN thin films on α(6H)-SiC substrates

The influence of diluent gas on the metalorganic vapor phase epitaxy of AlN and GaN thin films has been investigated. A computational fluid dynamics model using the finite element method was employed to improve film uniformity and to analyze transport phenomena. The properties of AlN and GaN thin films grown on α(6H)-SiC(0001) substrates in H₂ and N₂ diluent gas environments were evaluated. Thin films of AlN grown in H₂ and N₂ had root mean square (rms) roughness values of 1.5 and 1.8 nm, respectively. The surface and defect microstructures of the GaN thin films, observed by scanning and transmission electron microscopy, respectively, were very similar for both diluents. Low temperature (12K) photoluminescence measurements of GaN films grown in N₂ had peak intensities and full widths at half maximum equal to or better than those films grown in H₂. A room temperature Hall mobility of 275 cm²/V·s was measured on 1 μm thick, Si-doped, n-type (1×10¹⁷ cm⁻³) GaN films grown in N₂. Acceptor-type behavior of Mg-doped GaN films deposited in N₂ was repeatably obtained without post-growth annealing, in contrast to similar films grown in H₂. The GaN growth rates were ∼30% higher when H₂ was used as the diluent. The measured differences in the growth rates of AlN and GaN films in H₂ and N₂ was attributed to the different transport properties of these mixtures, and agreed well with the computer model predictions. Nitrogen is shown to be a feasible alternative diluent to hydrogen for the growth of AlN and GaN thin films.