Compositionally graded interface for passivation of HgCdTe photodiodes

A compositionally graded CdTe-Hg_1−xCd_xTe interface was created by deposition of CdTe on p-HgCdTe and subsequent annealing. The compositionally graded layer between CdTe and HgCdTe was formed by an interdiffusion process and was used for passivation. The composition gradient (Δx) in the interfacial region and the width of the graded region were tailored by adopting a suitable annealing procedure. The effect of process conditions on the interfacial profile and photoelectric properties such as lifetime and surface recombination velocity was studied in detail. Surface recombination velocity of the p-HgCdTe could be reduced to the level of 3,000 cm/s at 77 K, which represents very good passivation characteristics. The passivation layer formed by this method can be used for the fabrication of high performance and stable modern infrared detectors. Thus, a passivation process is developed, which is simple, effective, reproducible, and compatible with the HgCdTe device fabrication and packaging processes.     

Metalorganic chemical vapor deposition CdTe passivation of HgCdTe

CdTe epilayers are grown by metalorganic chemical vapor deposition (MOCVD) on bulk HgCdTe crystals with x ~ 0.22 grown by the traveling heater method (THM). The THM HgCdTe substrates are (111) oriented and the CdTe is grown on the Te face. The metalorganic sources are DMCd and DETe, and the growth is performed at subatmospheric pressure. Ultraviolet (UV) photon-assisted hydrogen radicals pretreatment plays a dominant role in the electrical properties of the resulting heterostructures. The requirements of a good passivation for HgCdTe photodiodes vis-a-vis the passivation features of CdTe/HgCdTe heterostructures are discussed. The effect of valence band offset and interface charges on the band diagrams of p-isotype CdTe/HgCdTe heterostructures, for typical doping levels of the bulk HgCdTe substrates and the MOCVD grown CdTe, is presented. Electrical properties of the CdTe/HgCdTe passivation are determined by capacitance-voltage and current-voltage characteristics of metal-insulator-semiconductor test devices, where the MOCVD CdTe is the insulator. It is found that the HgCdTe surface is strongly inverted and the interface charge density is of the order of 10¹²cm² when the CdTe epilayer is grown without the UV pretreatment. With the in-situ UV photon-assisted hydrogen radicals pretreatment, the HgCdTe surface is accumulated and the interface charge density is -4. 10¹¹ cm^-2.     

HgCdTe-MISFET fabrication with multi-step surface passivation and quantum effects

We have used multi-step surface passivation process integrating electrochemical reduction and UV exposure with native sulfidization by H₂S gas to obtain high quality ZnS/p-HgCdTe interface. It shows very low parasitic interface charge density of the order of 10¹⁰cm⁻². The insulating ZnS layer also exhibits very high resistivity of ∼10¹² Θcm. The resulting fabricated HgCdTe-MISFETs show 2D quantum effects. Magnetoresistance measured at 1.5K displays oscillations which begin to appear above the gate voltage of 10V. They are identified as the Shubnikov-de Haas oscillations involving three electronic subbands. The magnetotransport data are quantitatively analyzed with the calculated Landau level-fan diagram and confirm the 2D subband quantization of the inversion layer at the ZnS/p-HgCdTe interface. This result demonstrates successful role of the multi-step surface passivation for realizing 2D ZnS/HgCdTe interface which will provide high quality 2DEG resevoir basis in future Hg-based narrow-gap nanostructure device applications.     

MBE HgCdTe on Alternative Substrates for FPA Applications

Results of first-principles calculations and experiments focusing on molecular beam epitaxy (MBE) growth of HgCdTe on the alternative substrates of GaAs and Si are described. The As passivation on (2 × 1) reconstructed (211) Si and its effects on the surface polarity of ZnTe or CdTe were clarified by examining the bonding configurations of As. The quality of HgCdTe grown on Si was confirmed to be similar to that grown on GaAs. Typical surface defects in HgCdTe and CdTe were classified. Good results for uniformities of full width at half maximum (FWHM) values of x-ray rocking curves, surface defects, and x values of Hg_1−x Cd _x Te were obtained by refining the demanding parameters and possible tradeoffs. The sticking coefficient of As₄ for MBE HgCdTe was determined. The effects of Hg-assisted annealing for As activation were investigated experimentally and theoretically by examining the difference of the formation energy of As_Hg and As_Te. Results of focal-plane arrays (FPAs) fabricated with HgCdTe grown on Si and on GaAs are discussed.     

A Study of Sidewall Effects in HgCdTe Photoconductors Passivated with MBE-Grown CdTe

In order to evaluate the effectiveness of CdTe surface passivating layers, HgCdTe photoconductors with and without CdTe sidewall passivation were fabricated. As expected, photoconductors with CdTe sidewall passivation demonstrated significantly higher responsivity in comparison with those without sidewall passivation, indicating the effectiveness of molecular-beam epitaxially (MBE)-grown CdTe as a passivation layer in reducing surface recombination velocity. Characterization of the responsivity differences between photoconductors with and without sidewall CdTe passivation offers a potential method for measuring the interface/surface recombination velocity. This has been demonstrated in this paper by extracting the value of the surface recombination velocity using the Synopsys Sentaurus commercial modeling package to fit experimental responsivity data for fully and partially passivated devices.     

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.     

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.     

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.     

Examination of the effects of high-density plasmas on the surface of HgCdTe

High-density argon-hydrogen plasmas have been demonstrated to be very effective as etchants of CdTe, CdZnTe, and HgCdTe materials for focal plane array applications. Understanding the physical, chemical, and electrical characteristics of these surfaces is critical in elucidating the mechanisms of processing Hg_1−xCd_xTe. The ways in which these plasmas interact with HgCdTe, such as etch rates and loading, have been studied.^1–11 However, little is known on how these plasmas affect the first few atomic layers of HgCdTe. In this study, the effects of high-density plasmas on the surface of HgCdTe were examined. The combination of argon and hydrogen plasma etch leaves a well-ordered, near-stoichiometric surface determined by both x-ray photoelectron spectroscopy and reflection high-energy electron diffraction (RHEED). Starting with Hg_0.78Cd_0.22Te, we were able to produce surfaces with x=0.4 and a RHEED pattern sharp enough to measure 2×1 reconstruction.     

Effect of dislocations on performance of LWIR HgCdTe photodiodes

The epitaxial growth of HgCdTe on alternative substrates has emerged as an enabling technology for the fabrication of large-area infrared (IR) focal plane arrays (FPAs). One key technical issue is high dislocation densities in HgCdTe epilayers grown on alternative substrates. This is particularly important with regards to the growth of HgCdTe on heteroepitaxial Si-based substrates, which have a higher dislocation density than the bulk CdZnTe substrates typically used for epitaxial HgCdTe material growth. In the paper a simple model of dislocations as cylindrical regions confined by surfaces with definite surface recombination is proposed. Both radius of dislocations and its surface recombination velocity are determined by comparison of theoretical predictions with carrier lifetime experimental data described by other authors. It is observed that the carrier lifetime depends strongly on recombination velocity; whereas the dependence of the carrier lifetime on dislocation core radius is weaker. The minority carrier lifetime is approximately inversely proportional to the dislocation density for densities higher than 10⁵ cm⁻². Below this value, the minority carrier lifetime does not change with dislocation density. The influence of dislocation density on the R₀A product of long wavelength infrared (LWIR) HgCdTe photodiodes is also discussed. It is also shown that parameters of dislocations have a strong effect on the R₀A product at temperature around 77 K in the range of dislocation density above 10⁶ cm⁻². The quantum efficiency is not a strong function of dislocation density.     

Dislocation profiles in HgCdTe(100) on GaAs(100) grown by metalorganic chemical vapor deposition

We studied dislocation etch pit density (EPD) profiles in HgCdTe(lOO) layers grown on GaAs(lOO) by metalorganic chemical vapor deposition. Dislocation profiles in HgCdTe(lll)B and HgCdTe(lOO) layers differ as follows: Misfit dislocations in HgCdTe(lll)B layers are concentrated near the HgCdTe/CdTe interfaces because of slip planes parallel to the interfaces. Away from the HgCdTe/CdTe interface, the HgCdTe(111)B dislocation density remains almost constant. In HgCdTe(lOO) layers, however, the dislocations propagate monotonically to the surface and the dislocation density decreases gradually as dislocations are incorporated with increasing HgCdTe(lOO) layer thicknesses. The dislocation reduction was small in HgCdTe(lOO) layers more than 10 μm from the HgCdTe/CdTe interface. The CdTe(lOO) buffer thickness and dislocation density were similarly related. Since dislocations glide to accommodate the lattice distortion and this movement increases the probability of dislocation incorporation, incorporation proceeds in limited regions from each interface where the lattice distortion and strain are sufficient. We obtained the minimum EPD in HgCdTe(100) of 1 to 3 x 10⁶ cm^-2 by growing both the epitaxial layers more than 8 μm thick.     

The interface of metalorganic chemical vapor deposition-CdTe/HgCdTe

The metalorganic chemical vapor deposition (MOCVD) growth of CdTe on bulk n-type HgCdTe is reported and the resulting interfaces are investigated. Metalinsulator-semiconductor test structures are processed and their electrical properties are measured by capacitance-voltage and current-voltage characteristics. The MOCVD CdTe which was developed in this study, exhibits excellent dielectric, insulating, and mechano-chemical properties as well as interface properties, as exhibited by MIS devices where the MOCVD CdTe is the single insulator. Interfaces characterized by slight accumulation and a small or negligible hysteresis, are demonstrated. The passivation properties of CdTe/ HgCdTe heterostructures are predicted by modeling the band diagram of abrupt and graded P-CdTe/n-HgCdTe heterostructures. The analysis includes the effect of valence band offset and interface charges on the surface potentials at abrupt hetero-interface, for typical doping levels of the n-type layers and the MOCVD grown CdTe. In the case of graded heterojunctions, the effect of grading on the band diagram for various doping levels is studied, while taking into consideration a generally accepted valence band offset. The MOCVD CdTe with additional pre and post treatments and anneal form the basis of a photodiode with a new design. The new device architecture is based on a combination of a p-on-n homojunction in a single layer of n-type HgCdTe and the CdTe/HgCdTe heterostructure for passivation.     

Photo-enhanced native oxidation process forHg0.8Cd0.2Te photoconductors

Proposes an easy and reproducible vapor-phase photo surface treatment method to improve the device performance of the Hg_0.8 Cd_0.2Te photoconductive detector. We explore the effect of surface passivation on the electrical and optical properties of the HgCdTe photoconductor. Experimental results, including surface mobility, surface carrier concentration, metal-insulator-semiconductor leakage current, 1/f noise voltage spectrum, the 1/f knee frequency, responsivity R_λ, and specific detectivity D* for stacked photo surface treatment and ZnS or CdTe passivation layers are presented. These data are all directly related to the quality of the interface between the passivation layer and the HgCdTe substrate. We found that, by inserting a photo native oxide layer, we can shift the 1/f knee frequency, reduce the noise power spectrum, and achieve a lower surface recombination velocity S. A higher D* can also be achieved. It was also found that HgCdTe photoconductors passivated with stacked layers show improved interface properties compared to the photoconductors passivated only with a single ZnS or CdTe layer     

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.     

Passivation effect on optical and electrical properties of molecular beam epitaxy-grown HgCdTe/CdTe/Si layers

The effects of passivation with two different passivants, ZnS and CdTe, and two different passivation techniques, physical vapor deposition (PVD) and molecular beam epitaxy (MBE), were quantified in terms of the minority carrier lifetime and extracted surface recombination velocity on both MBE-grown medium-wavelength ir (MWIR) and long-wavelength ir HgCdTe samples. A gradual increment of the minority carrier lifetime was reported as the passivation technique was changed from PVD ZnS to PVD CdTe, and finally to MBE CdTe, especially at low temperatures. A corresponding reduction in the extracted surface recombination velocity in the same order was also reported for the first time. Initial data on the 1/f noise values of as-grown MWIR samples showed a reduction of two orders of noise power after 1200-Å ZnS deposition.     

Interface profile optimization in novel surface passivation scheme for InGaAs nanostructures using Si interface control layer

This paper attempts to control and optimize the interface atomic profiles of a novel surface passivation scheme for InGaAs nanostructures, using a silicon interface control layer (ICL). An in-situ x-ray photoelectron spectroscopy characterization technique was used to establish a process sequence that satisfies the conditions of maintenance of pseudomorphic matching to InGaAs, prevention of direct oxidation of InGaAs, and formation of a good SiO₂/Si interface with minimal suboxide components. It is shown that the above conditions can be satisfied by a new process that is a formation of the thermal SiO₂ at the SiO₂-Si interface by repetition of deposition/oxidation/annealing cycle. A large reduction of interface state density (N_ss) was realized by the optimization of the new process, resulting in a minimum N_ss of 4 × 10¹¹ cm⁻² eV⁻¹. The silicon ICL technique was successfully applied to the passivation of InGaAs wire structures.     

Passivation of HgCdTe p-n diode junction by compositionally graded HgCdTe formed by annealing in a Cd/Hg atmosphere

Cadmium telluride (CdTe) is being widely used for passivating the HgCdTe p-n diode junction. Instead of CdTe, we tried a compositionally graded HgCdTe as a passivation layer that was formed by annealing an HgCdTe p-n junction in a Cd/Hg atmosphere. During annealing, Cd diffuses into HgCdTe from the Cd vapor, while Hg diffuses out from HgCdTe, forming compositionally graded HgCdTe at the surface. The Cd mole fraction at the surface was constant regardless of the annealing temperature in the range of 250–350°C. Capacitance versus voltage (C-V) curves for p-type HgCdTe that were passivated with compositionally graded HgCdTe formed by Cd/Hg annealing at 260°C showed a smaller flat-band voltage than the one passivated by thermally deposited CdTe, indicative of the better quality of the passivation. A long-wave infrared (LWIR) HgCdTe p-n junction diode passivated by compositionally graded HgCdTe showed about a one order of magnitude smaller R_dA value than the one passivated by thermally deposited CdTe, confirming the effectiveness of the compositionally graded HgCdTe as a passivant.     

Auger lifetimes in ideal InGaSb/InAs superlattices

Quantitative calculations are reported of both band-to band Auger and radiative recombination lifetimes in thin-layered type II In_xGa_1−x Sb/InAs superlattices with energy gaps in the 5–17 μm range, using accurate band structure and numerical techniques. Results for an 11 μm superlattice are compared with similar calculations for bulk HgCdTe and a HgTe/CdTe superlattice having the same energy gap. The results show the n-type Auger rates to be comparable and the p-type rates to be suppressed by three orders of magnitude in some experimentally realizable structures. Thus, well fabricated III–V superlattices appear to be excellent candidates as a new class of infrarer detectors.     

Growth of HgTe Quantum Wells for IR to THz Detectors

We zone-engineered HgCdTe/HgTe/HgCdTe quantum wells (QWs) using the molecular-beam epitaxy (MBE) method with in situ high-precision ellipsometric control of composition and thickness. The variations of ellipsometric parameters in the ψ–Δ plane were represented by smooth broken curves during HgTe QW growth with abrupt composition changes. The form of the spiral fragments and their extensions from fracture to fracture revealed the growing layer composition and its thickness. Single and multiple (up to 30) Cd _x Hg_1−x Te/HgTe/Cd _x Hg_1−x Te QWs with abrupt changes of composition were grown reproducibly on (013) GaAs substrates. HgTe thickness was in the range of 16 nm to 22 nm, with the central portion of Cd _x Hg_1−x Te spacers doped by In to a concentration of 10¹⁴ cm⁻³ to 10¹⁷ cm⁻³. Based on this research, high-quality (013)-grown HgTe QW structures can be used for all-electric detection of radiation ellipticity in a wide spectral range, from far-infrared (terahertz radiation) to mid-infrared wavelengths. Detection was demonstrated for various low-power continuous-wave (CW) lasers and high-power THz pulsed laser systems.     

Large area depositon of Cd1-xZnxTe on GaAs and Si substrates by metalorganic chemical vapor deposition

Results of large-area (up to 1000 cm²/run) Cd_1-xZn_xTe heteroepitaxy on both GaAs and GaAs/Si substrates by metalorganic chemical vapor deposition (MOCVD) are presented. Cd_1-xZn_xTe (x = 0-0.1) films exhibited specular surface morphology, 1% thickness uniformity (standard deviation), and compositional uniformity (Δx) of ±0.002 over 100 mm diam substrates. For selected substrate orientations and deposition conditions, the only planar defects exhibited by (lll)B Cd_1-xZn_xTe/GaAs/Si films were lamella twins parallel to the CdTe/GaAs interface; these do not propagate through either the Cd_1-xZn_xTe layer or subsequently deposited liquid phase epitaxy (LPE) HgCdTe layer(s). Background Ga and As-impurity levels for Cd_1-xZn_xTe on GaAs/Si substrates were below the secondary ion mass spectroscopy detection limit. Preliminary results of HgCdTe liquid phase epitaxy using a Te-rich melt on Si-based substrates resulted in x-ray rocking curve linewidths as narrow as 72 arc-sec and etch-pit densities in the range 1 to 3 x 10⁶ cm².