Small two-dimensional arrays of mid-wavelength infrared HgCdTe diodes fabricated by reactive ion etching-induced p-to-n-type conversion

The reactive ion etching (RIE) technique has been shown to produce high-performance n-on-p junctions by localized-type conversion of p-type mid-wavelength infrared (MWIR) HgCdTe material. This paper presents variable area analysis of n-on-p HgCdTe test diodes and data on two-dimensional (2-D) arrays fabricated by RIE. All devices were fabricated on x = 0.30 to 0.31 liquid-phase epitaxy (LPE) grown p-type (p = ∼1 × 10¹⁶ cm⁻³) HgCdTe wafers obtained from Fermionics Corp. The diameter of the circular test diodes varied from 50 μm to 600 μm. The 8 × 8 arrays comprised of 50 μm × 50 μm devices on a 100-μm pitch, and all devices were passivated with 5000 ? of thermally deposited CdTe. At temperatures >145 K, all devices are diffusion limited; at lower temperatures, generation-recombination (G-R) current dominates. At the lowest measurement temperature (77 K), the onset of tunneling can be observed. At 77 K, the value of 1/R₀A for large devices shows quadratic dependence on the junction perimeter/area ratio (P/A), indicating the effect of surface leakage current at the junction perimeter, and gives an extracted bulk value for R₀A of 2.8 × 10⁷ Ω cm². The 1/R₀A versus P/A at 195 K exhibits the well-known linear dependence that extrapolates to a bulk value for R₀A of 17.5 Ω cm². Measurements at 77 K on the small 8 × 8 test arrays were found to demonstrate very good uniformity with an average R₀A = 1.9 × 10⁶ Ω cm² with 0° field of view and D* = 2.7 × 10¹¹cm Hz^1/2/W with 60° field of view looking at 300 K background.     

Tl3VS4 as an acousto-optic and surface wave material

Single crystals of thallium vanadium sulphide (Tl₃VS₄) up to 2 cm in length were grown from melts using the Stockbarger technique. This material has acoustic velocities as low as 8.73 × 10⁴ cm/sec for bulk shear waves and 8.7 × 10⁴ cm/sec for surface (Rayleigh) waves. In a sample oriented (001)Λ(110), k² was found to be 1.39%, and the temperature coefficient of delay −54 ppm for surface waves. The optical transmission is from 0.75 to 10.5 μm, and the indices of refraction range from 3.156 at 0.749 μm to 2.808 at 5.26 μm. These properties make it an attractive candidate for use in acousto-optic and acoustic surface-wave devices.     

Characterization of Mg-doped InP grown by MOCVD using a bis(methylcyclopentadienyl)magnesium dopant source

The use of bis(methylcyclopentadienyl)magnesium (MCp₂Mg) as ap-dopant source for MOCVD-grown InP has been investigated. The Mg incorporation was nonlinear. The relationship between the H₂ flow through the MCp₂Mg bubbler and the Mg concentra-tion in the epilayers suggested that when [Mg] <20 ppb in the reactor it was mostly depleted from the gas mixture, probably by means of reaction with O₂ or H₂O, but at higher concentrations a large fraction of the Mg diffusing to the epilayers was incor-porated. For concentrations >10¹⁹ cm^s-3 the layer morphology deteriorated and stacking faults were observed by TEM, at a density greater than 10⁹ cm^s−2. Significant diffusion of Mg into the substrates during the growth was observed, with diffusion depths up to 0.1 μm at a concentration of 10¹⁹ cm^s−3 in S-doped, and up to 32 μm at 10¹⁷ cm^s-3 in Fe-doped substrates. These concentrations correspond to the S and Fe doping level in those substrates, and the results are explained in terms of the formation of a complex between the S or Fe dopants and the diffusing Mg, which immobilizes the latter species. At [Mg] >10¹⁸ cm^s−3, the net hole concentration, measured by means of electrochemical C-V pro-filing, decreases with increasing [Mg], indicating significant self compensation. Com-pensation at high [Mg] was also suggested by the effect of excitation power density on the peak shift of the donor to acceptor transition observed during photoluminescence measurements at 7 K.     

Advances in composition control for 16 µm LPE P-on-n HgCdTe heterojunction photodiodes for remote sensing applications at 60K

With good composition control in both p-type cap and n-type base LPE layers, it is possible to make barrier-free two-layer P-on-n HgCdTe heterojunction photodiodes with very long cutoff wavelengths. Diode arrays with good R_oA operability, good quantum efficiency, and low 1/f noise at 60K have been demonstrated at cutoff wavelengths to 16.3μm. The diode performance continues to improve at lower temperatures, following a diffusion-current trend to at least 35K. Measured R_oA values of 2×10⁵ ohm-cm² for an 18 μm cutoff at 35K are the highest reported at this very long wavelength. A simple defect model applied to the area dependence of R_oA at 40K implied a defect areal density of 3×10⁴ cm⁻² and a defect impedance of 3×10⁶ ohm.     

Effect of copper and chlorine on the properties of SiO2 encapsulated polycrystalline CdSe films

The effects of different copper doping concentrations on the properties of SiO₂ encapsulated CdSe films have been investigated. Two methods were used to dope the films with copper: ion implantation and diffusion from a surface layer. The room temperature dark resistivity of films annealed in oxygen at 450°C was found to increase as the copper concentration was increased until a maximum resistivity of 10⁸ ohm cm occurred at a copper concentration of 10²⁰ atoms cm⁻³. The room temperature resistivity in the light was found to be independent of the copper concentration and whether the films were annealed in argon or oxygen. During annealing the grains grew from 0.03 μm to 0.3 μm and this growth was independent of the doping or the annealing ambient. The energy levels, carrier mobilities, and microstructure of the annealed films were dependent on the method of doping. The ion implanted films had an additional energy level at 0.33 eV and their mobility was a factor of 4 smaller than films doped by the surface diffusion method, whose mobilities were 20 to 35 cm²V⁻¹ s⁻¹. The addition of chlorine to copper doped films had no effect on either the resistivity or photosensitivity but slowed the response times of the photocurrent by a factor of 10. No energy levels were observed which could be associated with the copper nor was the copper found to affect the density of the observed intrinsic levels at 0.65 and 1.1 eV.     

Imaging one-dimensional and two-dimensional planar photodiode detectors fabricated by ion milling molecular beam epitaxy CdHgTe

Imaging one-dimensional (1-D) and two-dimensional (2-D) arrays of mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) planar photodiodes were fabricated by ion milling of vacancy-doped molecular beam epitaxy Cd_xHg_1−xTe layers. Sixty-four-element 1-D arrays of 26×26 μm² or 26×56 μm² diodes were processed. Zero-bias resistance-area values (R₀A) at 77 K of 4×10⁶ Θcm² at cutoff wavelength λ_CO=4.5 μm were measured, as well as high quantum efficiencies. To avoid creating a leakage current during ball bonding to the 1-D array diodes, a ZnS layer was deposited on top of the CdTe passivation layer, as well as extra electroplated Au on the bonding pads. The best measured noise equivalent temperature difference (NETD) on a LWIR array was 8 mK, with a median of 14 mK for the 42 operable diodes. The best measured NETD on a MWIR array was 18 mK. Two-D arrays showed reasonably good uniformity of R₀A and zero-bias current (I₀) values. The first 64×64 element 2-D array of 16×16 μm² MWIR diodes has been hybridized to read-out electronics and gave median NETD of 60 mK.     

Molecular-beam epitaxial growth of HgCdTe infrared focal-plane arrays on silicon substrates for midwave infrared applications

Molecular beam epitaxy has been employed to deposit HgCdTe infrared detector structures on Si(112) substrates with performance at 125K that is equivalent to detectors grown on conventional CdZnTe substrates. The detector structures are grown on Si via CdTe(112)B buffer layers, whose structural properties include x-ray rocking curve full width at half maximum of 63 arc-sec and near-surface etch pit density of 3–5 × 10⁵ cm⁻² for 9 μm thick CdTe films. HgCdTe p⁺-on-n device structures were grown by molecular beam epitaxy (MBE) on both bulk CdZnTe and Si with 125K cutoff wavelengths ranging from 3.5 to 5 μm. External quantum efficiencies of 70%, limited only by reflection loss at the uncoated Si-vacuum interface, were achieved for detectors on Si. The current-voltage (I-V) characteristics of MBE-grown detectors on CdZnTe and Si were found to be equivalent, with reverse breakdown voltages well in excess of 700 mV. The temperature dependences of the I-V characteristics of MBE-grown diodes on CdZnTe and Si were found to be essentially identical and in agreement with a diffusion-limited current model for temperatures down to 110K. The performance of MBE-grown diodes on Si is also equivalent to that of typical liquid phase epitaxy-grown devices on CdZnTe with R₀A products in the 10⁶–10⁷ Θ-cm² range for 3.6 μm cutoff at 125K and R₀A products in the 10⁴–10⁵ Θ-cm² range for 4.7 μm cutoff at 125K.     

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.     

Stacking-fault formation and propagation in 4H-SiC PiN diodes

Stacking-fault growth in SiC PiN diodes has been examined using light-emission imaging and stressing at 80 A/cm² and 160 A/cm². Dark areas in the emission develop because of stacking faults and the current capability of the diode drops. More detailed images are produced by reducing the current by a factor of 1000. The low-current images are bright lines at dislocations bounding the stacking faults and at or near the stacking-fault intersection with the surface. Stacking faults nucleate 1–2 μm below the surface. Most, but not all, continue growing until they span the diode. Growth dynamics and their dependence on the current density are discussed. An erratum to this article is available at .     

A 220 GHz dynamic frequency divider in 0.5 μm InP DHBT technology

A high performance 3 inch 0.5 μ m InP DHBT technology with three interconnecting layers has been developed. The epitaxial layer structure and geometry parameters of the device were carefully studied to get the required performances. The 0.5×5 μm² InP DHBTs demonstrated f_t=350 GHz, f_max=532 GHz and BV_CEO=4.8 V, which were modeled using Agilent-HBT large signal model. As a benchmark circuit, a dynamic frequency divider operating from 110 to 220 GHz has been designed, fabricated and measured with this technology. The ultra-high-speed 0.5 μm InP DHBT technology offers a combination of ultra-high-speed and high breakdown voltage, which makes it an ideal candidate for next generation 100 GHz+mixed signal integrated circuits.     

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.     

Mechanism of oxide thickness and temperature dependent current conduction in n+-polySi/SiO2/p-Si structures — a new analysis

The conduction mechanism of gate leakage current through thermally grown silicon dioxide (SiO₂) films on (100) p-type silicon has been investigated in detail under negative bias on the degenerately doped n-type polysilicon (n⁺-polySi) gate. The analysis utilizes the measured gate current density J_G at high oxide fields E_ox in 5.4 to 12 nm thick SiO₂ films between 25 and 300℃. The leakage current measured up to 300℃ was due to Fowler–Nordheim (FN) tunneling of electrons from the accumulated n⁺-polySi gate in conjunction with Poole Frenkel (PF) emission of trapped-electrons from the electron traps located at energy levels ranging from 0.6 to 1.12 eV (depending on the oxide thickness) below the SiO₂ conduction band (CB). It was observed that PF emission current I_PF dominates FN electron tunneling current I_FN at oxide electric fields E_ox between 6 and 10 MV/cm and throughout the temperature range studied here. Understanding of the mechanism of leakage current conduction through SiO₂ films plays a crucial role in simulation of time-dependent dielectric breakdown (TDDB) of metaloxide–semiconductor (MOS) devices and to precisely predict the normal operating field or applied gate voltage for lifetime projection of the MOS integrated circuits.     

Electrical and optical properties of oxygen doped GaN grown by MOCVD using N2O

Oxygen doped GaN has been grown by metalorganic chemical vapor deposition using N₂O as oxygen dopant source. The layers were deposited on 2″ sapphire substrates from trimethylgallium and especially dried ammonia using nitrogen (N₂) as carrier gas. Prior to the growth of the films, an AIN nucleation layer with a thickness of about 300? was grown using trimethylaluminum. The films were deposited at 1085°C at a growth rate of 1.0 μm/h and showed a specular, mirrorlike surface. Not intentionally doped layers have high resistivity (>20 kW/square). The gas phase concentration of the N₂O was varied between 25 and 400 ppm with respect to the total gas volume. The doped layers were n-type with carrier concentrations in the range of 4×10¹⁶ cm⁻³ to 4×10¹⁸ cm⁻³ as measured by Hall effect. The observed carrier concentration increased with increasing N₂O concentration. Low temperature photoluminescence experiments performed on the doped layers revealed besides free A and B exciton emission an exciton bound to a shallow donor. With increasing N₂O concentration in the gas phase, the intensity of the donor bound exciton increased relative to that of the free excitons. These observations indicate that oxygen behaves as a shallow donor in GaN. This interpretation is supported by covalent radius and electronegativity arguments.     

LaNiO3 and Cu3Ge contacts to YBa2Cu3O7-x films

Metallization of high-T_c superconductors using low resistivity metal oxides and Cu-Ge alloys has been investigated on high quality pulsed laser deposited epitaxial YBa₂Cu₃O_7-x (YBCO) films. Epitaxial LaNiO₃ (LNO) thin films have been grown on YBCO films at 700°C using pulsed laser deposition. The specific resistivity of LNO was measured to be 50 μΩ-cm at 300K which decreases to 19 μΩ-cm at 100K indicating good metallicity of the LNO films. The contact resistance of LNO-YBCO thin film interface was found to be reasonably low (of the order of 10^-4Ω-cm² at 77K) which suggests that the interface formed between the two films is quite clean and LNO can emerge as a promising metal electrode-material to YBCO films. A preliminary investigation related to the compatibility of Cu₃Ge alloy as a contact metallization material to YBCO films is discussed. The usage of other oxide based low resistivity materials such as SrRuO₃ (SRO) and SrVO₃ (SVO) for metallization of high-T_c YBCO superconductor films is also discussed.     

Uniformity in HgCdTe diode arrays fabricated by reactive ion etching

Two-dimensional, midwavelength infrared (MWIR) HgCdTe detector arrays have been fabricated using reactive ion etching (RIE). Detector-to-detector uniformity has been studied in the devices fabricated with CdTe- and ZnS-passivation layers. Mapping of the doping profile, passivant/HgCdTe interface electrical properties, and diode impedance-area product (R₀A_j) in a two-dimensional array of diodes has been carried out. Temperature and perimeter/area dependence of the dark current are studied to identify the bulk and surface current components. Maximum R₀A_j=2×10⁷ Θcm² was achieved in CdTe-passivated, 200×200 μm² diode arrays. It demonstrates that CdTe-passivated, RIE-processed HgCdTe is a feasible technology.     

Minority carrier diffusion lengths in liquid phase epitaxial InGaAsP and InGaAs

Minority carrier diffusion lengths were determined for InGaAsP and InGaAs layers grown by liquid phase epitaxy on (100)-InP substrates by measuring the variation of the short circuit photocurrent as a focussed laser beam was scanned along a beveled (~1°) p-n junction. The effect of lattice-mismatch on the hole diffusion length (λ_p) for n-type unintentionally doped InGaAsP layers (λg=1.15 μm) was investigated for mismatch values from -0.25% to +0.31%, with the longest diffusion length (L_p = 1.5 μm) occurring when the epitaxial layer was lattice-matched to the substrate. As the amount of mismatch increased, L_p decreased. Electron diffusion lengths, L_n, were determined for lattice-matched quaternary and ternary layers grown from Zn doped melts over a wide range of hole concentrations. At the lowest hole concentrations, p = 3 × l0¹⁵ and 1.4 × 10¹⁶ cm⁻³, the electron diffusion lengths were 3.5 and 2.5 μm for the quaternary and ternary, respectively. As the hole concentration increased, L_n decreased and at the highest concentration (p = 5 × 10su18,cn⁻³) L_n was 0.13 μm for InGaAsP and 0.83 un for InGaAs.     

Modeling and simulation of emissivity of silicon-related materials and structures

A brief review of the models that have been proposed in the literature to simulate the emissivity of silicon-related materials and structures is presented. The models discussed in this paper include ray tracing, numerical, phenomenological, and semi-quantitative approaches. A semi-empirical model, known as Multi-Rad, based on the matrix method of multilayers is used to evaluate the reflectance, transmittance, and emittance for Si, SiO₂/Si, Si₃N₄/SiO₂/Si/SiO₂/Si₃N₄ (Hotliner), and separation by implantation of oxygen (SIMOX) wafers. The influence of doping concentration and dopant type as well as the effect of the angle of incidence on the radiative properties of silicon is examined. The results of these simulations lead to the following conclusions: (1) at least within the limitations of the Multi-Rad model, near the absorption edge, the radiative properties of Si are not affected significantly by the angle of incidence unless the angle is very steep; (2) at low temperatures, the emissivity of silicon shows complex structure as a function of wavelength; (3) for SiO₂/Si, changes in emissivity are dominated by substrate effects; (4) Hotliner has peak transmittance at 1.25 μm, and its emissivity is almost temperature independent; and (5) SIMOX exhibits significant changes in emissivity in the wavelength range of 1–20 μm.     

Elimination of low frequency gain in InAlAs/InGaAs metal-semiconductor-metal photodetectors by silicon nitride passivation

We report the reduction of low frequency gain and surface recombination in InAlAs/InGaAs metal-semiconductor-metal (MSM) photodetectors, by surface passivation with plasma-enhanced chemical vapor deposition (PECVD) of silicon nitride. A corresponding improvement in device speed, measured in the frequency-domain, is demonstrated. Large (100 μm)² devices exhibit neartransit-time-limited bandwidth (>10 GHz) following passivation, and device characteristics have been stable over a period of several months. We propose a physical model for electron trapping at the free surface, to explain the low frequency gain in nonpassivated devices and its subsequent elimination.     

Mesoporous tin oxide nanospheres for a NOx in air sensor

Mesoporous tin oxide(SnO₂/with a high surface area of 147.5 m²/g has been successfully synthesized via self-assembly process, combining the driven forces of water-evaporation and molecular interactions. Scanning electron microscope, X-ray diffraction, transmission electron micrograph, Fourier transform infrared and BrunauerEmmett-Teller were employed to analyze the morphology and crystal structure of the as-synthesized mesoporous materials. As a gas sensor, mesoporous SnO₂ shows impressive performances towards NOx gas with high selectivity and stability as well as ultra high sensitivity about 94.3 to 10 ppm NO_x gas at 300℃. The best response time of the sample S-500 is about 3.4s to 10 ppm NO_x at 450℃.     

Molecular-beam epitaxial growth and high-temperature performance of HgCdTe midwave infrared detectors

Results are reported on the molecular-beam epitaxial (MBE) growth and electrical performance of HgCdTe midwave-infrared (MWIR) detector structures. These devices are designed for operation in the 140–160 K temperature range with cutoff wavelengths ranging from 3.4–3.8 μm at 140 K. Epitaxial structures, grown at 185°C on (211)B-oriented CdZnTe substrates, consisting of either conventional two-layer P-n configurations or three-layer P-n-N configurations, were designed to examine the impact of device performance on variation of the n-type base layer (absorber) thickness and the inclusion or omission of an underlying wide-bandgap buffer layer. Devices were grown with absorber thicknesses of 3 μm, 5 μm, and 7 μm to examine the tradeoff between the spectral response characteristic and the reverse-bias electrical performance. In addition, 5-μm-thick, wide-bandgap HgCdTe buffer layers, whose CdTe mole fraction was approximately 0.1 larger than the absorber layer, were introduced into several device structures to study the effect of isolating the device absorbing layer from the substrate/growth initiation interface. The MBE-grown epitaxial wafers were processed into passivated, mesa-type, discrete device structures and diode mini arrays, which were tested for temperature-dependent R₀A product, quantum efficiency, spectral response, and the I-V characteristic at temperatures close to 140 K. External quantum efficiencies of 75–79% were obtained with lateral optical-collection lengths of 7 μm. Analysis of the temperature dependence of the diode R₀A product indicates that the device impedance is limited by the diffusion current at temperatures above 140 K with typical R₀A values of 2×10⁶ Ω cm² for a detector cutoff of 3.8 μm at 140 K. An alloy composition anomaly at the absorbing-layer/buffer-layer interface is believed to limit the observed R₀A products to values approximately one order of magnitude below the theoretical limit projected for radiatively limited carrier lifetime. Device electrical performance was observed to be improved through incorporation of a wide-bandgap buffer layer and through reduction of the absorbing layer thickness. An optimum spectral response characteristic was observed for device structures with 5-μm-thick absorbing layers.