Development of a 1K × 1K, 8–12 μm QWIP array

In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a 1,024 × 1,024 (1K × 1K), 8–12  μm infrared focal plane array (FPA). This 1 megapixel detector array is a hybrid using an L3/Cincinnati Electronics silicon readout integrated circuit (ROIC) bump bonded to a GaAs QWIP array fabricated jointly by engineers at the Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL). We have integrated the 1K × 1K array into an SE-IR based imaging camera system and performed tests over the 50–80 K temperature range achieving BLIP performance at an operating temperature of 57 K. The GaAs array is relatively easy to fabricate once the superlattice structure of the quantum wells has been defined and grown. The overall arrays costs are currently dominated by the costs associated with the silicon readout since the GaAs array fabrication is based on high yield, well-established GaAs processing capabilities. One of the advantages of GaAs QWIP technology is the ability to fabricate arrays in a fashion similar to and compatible with silicon IC technology. The designer’s ability to easily select the spectral response of the material from 3 μm to beyond 15 μm is the result of the success of band-gap engineering and the Army Research Lab is a leader in this area. In this paper we will present the first results of our 1K × 1K QWIP array development including fabrication methodology, test data and imaging capabilities.     

Development of a 4–15 μm infrared GaAs hyperspectral QWIP imager

In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a four band, 640 × 512, 23 μm × 23 μm pixel array which we have subsequently integrated with a linear variable etalon (LVE) filter providing over 200 spectral bands across the 4–15.4 μm wavelength region. This effort was a collaboration between NASA’s Goddard Space Flight Center (GSFC), the Jet Propulsion Laboratory (JPL) and the Army Research Laboratory (ARL) sponsored by the Earth Science Technology Office of NASA. The QWIP array was fabricated by graded molecular beam epitaxial (MBE) growth that was specifically tailored to yield four distinct bands (FWHM): Band 1; 4.5–5.7 μm, Band 2; 8.5–10 μm, Band 3; 10–12 μm and Band 4; 13.3–14.8 μm. Each band occupies a swath that comprises 128 × 640 elements. The addition of the LVE (which is placed directly over the array) further divides the four “broad” bands into 209 separate spectral bands ranging in width from 0.02 μm at 5 μm to 0.05 μm at 15 μm. The detector is cooled by a mechanical cryocooler to 46 K. The camera system is a fully reflective, f/4.2, 3-mirror system with a 21° × 25° field of view. The project goals were: (1) develop the 4 band GaAs QWIP array; (2) develop the LVE and; (3) implement a mechanical cryocooler. This paper will describe the efforts and results of this undertaking with emphasis on the overall system characteristics.     

Crosstalk between two-photon and two-color (two-photon) excitation in optical beam induced current generation with two confocal excitation beams

We analyze the influence of residual two-photon excitation (2PE) in two-color (two-photon) optical beam induced current (2CE-OBIC) generation in wide band gap semiconductor samples. 2CE-OBIC generation is accomplished with two confocal excitation beams of separation angle θ and wavelengths λ₁ and λ₂ where , λ_e = hc/E_b, h is the Planck’s constant, c is speed of light in vacuum, and E_b is the energy band gap. Because the conduction band of the sample is a continuum, at least one excitation beam would also contribute an undesirable 2PE-OBIC signal that degrades the signal-to-noise ratio of the measured 2CE-OBIC response and broadens the effective OBIC distribution in the sample particularly when θ ≠ 0 or π. We show that the deleterious effects of crosstalk are reduced by a careful selection of λ₁ and λ₂ and the relative excitation beam intensities. λ₁ and λ₂ should be chosen to minimize the ratio of the two-photon absorption coefficients (β₁, β₂) to the 2CE absorption coefficient β₁₂ or at least satisfy the constraint: β₁ + β₂  β₁₂. Keeping the two excitation intensities equal is beneficial only when β₁ = β₂. Otherwise, it is advantageous to bias the intensity ratio towards the wavelength with a lower 2PE absorption coefficient.     

Demonstration of 640 × 512 pixels long-wavelength infrared (LWIR) quantum dot infrared photodetector (QDIP) imaging focal plane array

We have exploited the artificial atom-like properties of epitaxially grown self-assembled quantum dots (QDs) for the development of high operating temperature long wavelength infrared (LWIR) focal plane arrays (FPAs). QD infrared photodetectors (QDIPs) are expected to outperform quantum well infrared detectors (QWIPs) and are expected to offer significant advantages over II–VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow multi-layer LWIR dot-in-a-well (DWELL) structures based on the InAs/InGaAs/GaAs material system. This hybrid quantum dot/quantum well device offers additional control in wavelength tuning via control of dot-size and/or quantum well sizes. DWELL QDIPs were also experimentally shown to absorb both 45° and normally incident light. Thus we have employed a reflection grating structure to further enhance the quantum efficiency. The most recent devices exhibit peak responsivity out to 8.1 μm. Peak detectivity of the 8.1 μm devices has reached 1 × 10¹⁰ Jones at 77 K. Furthermore, we have fabricated the first long-wavelength 640 × 512 pixels QDIP imaging FPA. This QDIP FPA has produced excellent infrared imagery with noise equivalent temperature difference of 40 mK at 60 K operating temperature.     

Surface deformation phenomena induced by electron-beam irradiation in strained InGaAs/AlGaAs layers on GaAs (1 0 0) and (3 1 1)B substrates

The strained InGaAs/AlGaAs layer structures have been grown on GaAs ( 10 0) and (3 1 1)B substrates in a horizontal low-pressure metalorganic vapor-phase epitaxy system at a temperature of 800°C. In the surface observation using a high-resolution scanning electron microscope, we have found that surface deformation phenomena induced by electron-beam irradiation in strained In_0.36Ga_0.64As,/Al_0.3Ga_0.7As layers on GaAs (1 0 0) and (3 1 1)B substrates. The change of the surface morphology was observed in real time on the display of SEM with the accelerating voltage of 30 kV and the irradiated time of 60–120 s. The surface deformation through mass transport seems to be the cause of the residual strain relaxation due to electron-beam irradiation.     

Luminescence from γ-irradiated humic acid

This study was conducted to investigate the ultraweak delayed radiochemiluminescence (RCL) spectra, kinetics and spectroscopic properties of humic acids (HAs) after γ-radiation exposure (absorbed doses of 1−10 kGy, Co-60) in model systems.

The kinetics and spectral distribution of RCL (340–650 nm) were measured using the single photon counting (SPC) method and cut-off filters.

The intensity of fluorescence (λ_ex=390, 440, 490 and 540 nm) covering the spectral range 400–580 nm was heavily dependent on the λ_ex and slightly increased with the absorbed dose of γ-radiation.

Absorption spectra (the range 240−800 nm) and color coefficients E_2.6/4 and E_4/6 of irradiated solutions indicated that post-radiative degradation/polymerization processes take place in the HA, changing their macromolecule size or properties.

Comparison of FTIR spectra and elemental analysis proved an increased O and decreased C atoms in irradiated samples. The data indicate on the radiolysis-induced degradation of native HA into fulvic-like acids with higher hydrophilicity and lower molecular size.     

640 × 512 pixel narrow-band, four-band, and broad-band quantum well infrared photodetector focal plane arrays

A 9 μm cutoff 640 × 512 pixel hand-held quantum well infrared photodetector (QWIP) camera has been demonstrated with excellent imagery. A noise equivalent differential temperature (NEDT) of 10.6 mK is expected at a 65 K operating temperature with f/2 optics at a 300 K background. This focal plane array has shown background limited performance at a 72 K operating temperature with the same optics and background conditions. In this paper, we discuss the development of this very sensitive long-wavelength infrared camera based on a GaAs/AlGaAs QWIP focal plane array and its performance in quantum efficiency, NEDT, uniformity, and operability. In the second section of this paper, we discuss the first demonstration of a monolithic spatially separated four-band 640 × 512 pixel QWIP focal plane array and its performance. The four spectral bands cover 4–5.5, 8.5–10, 10–12, and 13.5–15 μm spectral regions with 640 × 128 pixels in each band. In the last section, we discuss the array performance of a 640 × 512 pixel broad-band (10–16 μm full-width at half-maximum) QWIP focal plane.     

UV-induced two-photon absorption in BiB3O6 single crystals

We show that BiB₃O₆ (BiBO) crystals, well known for their excellent second harmonic generation (SHG) properties, may also be of interest for third-order optical phenomena, particularly for two-photon absorption (TPA). Photoinduced TPA measurements were performed under illumination of excimer Xe–F laser (λ = 217 nm) as a photoinducing (pumping) beam. It created a thin surface layer (about 85 nm) that was a source of the observed photoinduced TPA. Raman shifted Nd-YAG laser radiation (λ = 1.9 μm) as well as its second and fourth harmonics (λ = 950 and λ = 475 nm, respectively) were used as fundamental (probing) beams of the TPA. The highest values of the TPA β coefficient were achieved for a polarization of the pumping light directed along crystallographic axis b. Quantum chemical simulations indicate on substantial contribution of UV-induced electron–phonon anharmonicity to the observed TPA. The obtained values of TPA coefficients indicate a possibility of using BiBO crystals as UV-operated optical limiters in a wide spectral range.     

Study of superconducting state parameters of alkali–alkali binary alloys by a pseudopotential

A detailed study of the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ^*, transition temperature T_C, isotope effect exponent and effective interaction strength N_OV of ten alkali–alkali binary alloys i.e. Li_1−xNa_x, Li_1−xK_x, Li_1−xRb_x, Li_1−xCs_x, Na_1−xK_x, Na_1−xRb_x, Na_1−xCs_x, K_1−xRb_x, K_1−xCs_x and Rb_1−xCs_x are made within the framework of the model potential formalism and employing the pseudo-alloy-atom (PAA) model for the first time. We use the Ashcroft’s empty core (EMC) model potential for evaluating the superconducting properties of alkali alloys. Five different forms of local field correction functions viz. Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used to incorporate the exchange and correlation effects. A considerable influence of various exchange and correlation functions on λ and μ^* is found from the present study. Reasonable agreement with the theoretical values of the SSP of pure components is found (corresponding to the concentration x = 0 or 1). It is also concluded that nature of the SSP strongly depends on the value of the atomic volume Ω₀ of alkali–alkali binary alloys.     

Z-scan determination of the third-order optical nonlinearity of a triphenylmethane dye using 633 nm He–Ne laser

The third-order nonlinear optical response of a triphenylmethane dye (Acid blue 7) was studied using the Z-scan technique with a continuous-wave He–Ne laser radiation at 633 nm. The magnitude and sign of the third-order nonlinear refractive index n₂ of aqueous solution of Acid blue 7 dye were determined; the negative sign indicates a self-defocusing optical nonlinearity in the sample studied. The negative nonlinear refractive index n₂ and nonlinear absorption coefficient β were estimated to be −1.88 × 10⁻⁷ cm²/W and −3.08 × 10⁻³ cm/W, respectively, corresponding to Re(χ⁽³⁾) = −8.35 × 10⁻⁶ esu, and Im(χ⁽³⁾) = −6.88 × 10⁻⁷ esu. The experimental results show that Acid blue 7 dye have potential applications in nonlinear optics.     

Operating temperature and the responsivity of split-off band detectors

A GaAs/AlGaAs heterojunction is used as a spin-split-off band IR detector operating at or around room temperature. This detector structure followed a similar layer architecture to the quantum well IR photo detectors (QWIP) and Heterojunction Interfacial Work function Internal Photoemission (HEIWIP) detectors. Compared to QWIPs, the emitter layer thickness is increased to avoid confinement. Unlike either the QWIPs or HEIWIPs, these detectors will have two energy gaps (barriers) to obtain the wavelength threshold which could be used to design detectors either for optimum operating temperature or optimum responsivity. The free carrier energy gap is determined by the Al fraction and the spin-split-off transition energy provides another handle on controlling the effective threshold of the detector. Unlike QWIPs, these will also detect normal incidence radiation. A preliminary detector showed a peak responsivity of 0.29 mA/W at 2.5 μm at room temperature.     

The dependence of I–V and C–V characteristics on temperature in the H-terminated Pb/p-Si(1 0 0) Schottky barrier diodes

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of H-terminated Pb/p-Si/Al contacts fabricated by us have been measured in the temperature range of 77–300 K. The experimental values of the barrier height (BH) Φ_bo and the ideality factor n for the device range from 0.674 and 1.072 eV (at 300 K) to 0.352 and 2.452 eV (at 77 K), respectively. The ideality factors become larger with lowering temperature while the barrier height decreases. The Φ_bo(n) plot shows a linear dependence in the temperature range of 77–300 K that can be explained by the barrier inhomogeneity at the metal/semiconductor interface. The extrapolation of the linear Φ_bo(n) plot to n = 1 has given a homogeneous barrier height of approximately 0.713 eV for the Pb/p-Si(1 0 0) contact. A Φ_bo versus 1/T plot was drawn to obtain evidence of a Gaussian distribution of the BHs, and values of and σ_s = 80.5 mV for the mean BH and zero-bias standard deviation have been obtained from this plot, respectively. Then, a modified versus 1/T plot gives and A^* as 0.828 eV and 54.89 A/cm² K², respectively. Furthermore, an average value of −0.687 meV/K for the temperature coefficient has been obtained, the value of −0.687 meV/K for hydrogen terminated p-type Si differs from those given for p-type Si without hydrogen termination in the literature.     

Formation of AlxGa1−xAs periodic array of micro-hexagonal pillars and air holes by selective area MOVPE

A two-dimensional (2D) periodic array having air/semiconductor interfaces can be applied to photonic crystals (PCs), which are expected to control spontaneous emission and optical transports in the next-generation devices. In this paper, we report on the selective area metal-organic vapor phase epitaxial (SA-MOVPE) growth of a Al_xGa_1−xAs 2D periodic array on a GaAs (1 1 1)B substrate for application to 2DPCs having GaAs/AlGaAs heterostructures. Al_xGa_1−xAs (x=0, 0.25 and 0.50) growth was carried out on triangular lattice array of hexagonal GaAs openings and hexagonal SiN_x masks. A uniform Al_0.50Ga_0.50As hexagonal pillar array and a GaAs hexagonal air-hole array with a 1 μm-period were successfully obtained. The important growth parameter for uniform 2DPC structure formation by SA-MOVPE was clarified. Furthermore, we describe the successful demonstration of a 400 nm-period pillar array and an air-hole array, which corresponds to the optical communication wavelength λ=1.3–1.55 μm. The results indicate that SA-MOVPE method is very promising for the formation of uniform semiconductor 2DPCs without the occurrence of process-induced damages.     

Determination and analysis of the dispersive optical constants of the 5,5′,6,6′-tetraphenyl-2,2′-bi([1,3]dithiolo[4,5-b][1,4]dithiinylidene)–DDQ complex thin film

The synthesis and optical properties of the 5,5′,6,6′-tetraphenyl-2,2′-bi([1,3]dithiolo [4,5-b] [1,4]dithiinylidene)–2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) complex thin film were investigated by the optical characterization. The optical constants such as refractive index, extinction coefficient and absorption coefficient were determined using the transmittance T(λ) and reflectance R(λ) spectra and the refractive index dispersion was analyzed using single oscillator of Wemple–Didomenico model. The single oscillator energy E₀ and the dispersion energy E_d were calculated. The effect of temperature on refractive dispersion and optical band gap E_g is also discussed. As a result, the annealing temperatures have an important effect on refractive index of thin film.     

Overcoming absorption saturation with doping in p-type quantum well infrared photodetectors: modeling and experiment

Bound-to-continuum normal-incidence absorption in p-type GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) is strongest when the second light-hole (LH2) level is resonant with the top of the valence band QW. However, we found that such absorption saturates as a function of doping in the well. Using the envelope-function model (EFA), this paper shows that moving the LH2 resonance slightly deeper into the continuum avoids absorption saturation and produces optimal p-QWIP response. A suitable set of mid-IR samples was grown to test this conjecture and their photoresponse measured. The results indicate that absorption can be more than doubled through the use of the new p-QWIP designs. This result is explained by showing that the line of resonances in the continuum as a function of the in-plane wave vector eventually becomes a bound LH2 band in the well at some critical wave vector. Therefore, it is possible to avoid absorption saturation by matching this critical wave vector (i.e., well width and/or well depth) with the Fermi wave vector (i.e., doping in the well) for the desired QWIP (i.e., cutoff wavelength).     

Time-resolved electron transport studies on InGaAs/GaAs-QWIPs

Due to the short internal response time, quantum-well infrared photodetectors (QWIPs) are interesting for high-speed applications such as heterodyne spectroscopy or laser pulse monitoring. We studied the photocurrent transients of InGaAs/GaAs-QWIPs after irradiation with infrared laser pulses of 250 fs duration. The excitation wavelength of about 9 μm matches the peak wavelength of the QWIP structure. The photocurrent transient consists of two different dynamical components, representing the fast photoionization in the quantum-wells and the slow injection current that compensates the remaining space charge. The investigations of the different components as a function of temperature and bias voltage were performed on a nanosecond time-scale. The experimental separation of the two photocurrent contributions allows us to determine the photoconductive gain. The Fourier transform of the photocurrent transient was compared with other experimental methods including heterodyne detection and microwave rectification. The quantitative agreement between these different measurement techniques is excellent.     

Vibrationally resolved band profiles for autoionisation of NO (cΠ) nℓλ (vR=0) Rydberg states into NO AΠ (vi=0, 1, 2) vibronic levels

Excitation spectra for dispersed VUV-fluorescence of NO (λ_fl=134–152 nm) were measured in the exciting-photon energy range between 16.9 and 24.8 eV using monochromatised synchrotron radiation at medium bandwidth of 29 meV. Fluorescence from A¹Π (v_i=0, 1, 2) vibronic NO⁺-levels and fluorescence from excited dissociation fragments NI (3s ²P_J) was observed simultaneously by recording the dispersed fluorescence with a monochromator–position-sensitive detector combination. The autoionisation of NO (c³Π) nℓλ (v_R=0) Rydberg levels into the NO⁺ A¹Π (v_i=0, 1, 2) vibronic levels was observed vibrationally resolved. Different Beutler–Fano profiles for autoionisation of one Rydberg level NO (c³Π) nℓλ (v_R=0) into the different vibronic NO⁺ A¹Π (v_i=0, 1, 2) levels are clearly visible. The dependence of the Beutler–Fano profiles on the quantum numbers n, ℓ, and λ of the Rydberg electron is discussed. For the direct photoionisation into the NO⁺ A¹Π (v_i=0, 1, 2) vibronic levels a non-Franck–Condon behaviour was observed.     

The fabrication and characterization of ZnO UV detector

ZnO films were deposited on GaAs substrates by radio frequency (rf) magnetron sputtering followed by an ambient-controlled heat treatment process for arsenic doping. In Hall measurements, the As-doped ZnO films showed the characteristics of p-type semiconductor. The ZnO thin film p–n homojuctions were then fabricated to investigate the electrical properties of the films. The p–n homojunctions exhibited the distinct rectifying current–voltage (I–V) characteristics. The turn-on voltage was measured to be 3.0 V under the forward bias. When ultraviolet (UV) light (λ = 325 nm) was irradiated on the p–n homojunction, photocurrent of 2 mA was detected. Based on these results, it is proposed that the p–n homojunction herein is a potential candidate for UV photodetector and optical devices.     

Relationship between electronic and crystal structure in Cu–Ni–Co–Mn–O spinels: Part B: Binding energy anomaly in Cu photoemission spectrum

In very rare circumstances, X-ray photoemission spectra of copper in spinel oxides exhibit a “negative binding energy shift”. The origin of such an anomalous XPS chemical shift was investigated. A metastable Ni_0.48Co_0.24Cu_0.6+xMn_1.68−xO₄ (0 < x < 0.6) spinel was fabricated at 600 °C using a low-temperature solution technique. The binding energy of the 2p_3/2 level of copper (930.8 eV) is found 1.9 eV lower than that of Cu⁰ (932.7 eV). XPS and EXAFS studies revealed that the post-thermal annealing between 600 and 800 °C undergoes an irreversible cubic-to-tetragonal phase transformation through oxidation–reduction reaction Cu¹⁺ + Mn⁴⁺  Cu²⁺ + Mn³⁺, and only tetrahedral Cu¹⁺ species in the cubic spinel shows this anomalous chemical shift. The negative shift of the core levels was correlated to an equal shift of the Cu 3d valence band levels. XPS valence bands from the samples annealed at different temperatures were compared to DOS calculations. The DOS computations were performed with FEFF-8.1 code using experimental crystal parameters established by the EXAFS analysis. It was found that the tetrahedral Cu¹⁺ in the 600 °C annealed sample exhibits localization of the 3d orbitals showing behavior characteristic to zinc. The completely filled and isolated 3d electron shell appears as a false valence band edge in the XPS spectrum. The position of the Cu 3d, and other core levels, is established by oxygen pinning the Cu valence band levels and by the fixed value of the p–d gap characteristic to the tetrahedral copper environment in this spinel.