InAs/GaSb strained layer superlattice detectors with nBn design

We have investigated the electrical and optical properties of an nBn based Type-II InAs/GaSb strained layer superlattice detector as a function of absorber region background carrier concentration. Temperature-dependent dark current, responsivity and detectivity were measured. At T = 77 K and V_b = 0.1 V, with two orders of magnitude change in doping concentration, the dark current density increased from ～0.3 mA/cm² to ～0.3 A/cm². We attribute this to a depletion region that exists at the AlGaSb barrier and the SLS absorber interface. The device with non-intentionally doped absorption region demonstrated the lowest dark current density (0.3 mA/cm² at 0.1 V) with a specific detectivity D¹ at zero bias equal to 1.2 × 10¹¹ Jones at 77 K. The D¹ value decreased to 6 × 10¹⁰ cm Hz^1/2/W at 150 K. This temperature dependence is significantly different from conventional PIN diodes, in which the D¹ decreases by over two orders of magnitude from 77 K to 150 K, making nBn devices a promising alternative for higher operating temperatures.     

Band engineered HOT midwave infrared detectors based on type-II InAs/GaSb strained layer superlattices

We report on heterostructure bandgap engineered midwave infrared photodetectors based on type-II InAs/GaSb strained layer superlattices with high operating temperatures. Bandgap and bandoffset tunability of antimonide based systems have been used to realize photodiodes and photoconductors. A unipolar barrier photodiode, pBiBn, and an interband cascade photovoltaic detector have been demonstrated with a 100% cutoff wavelength of 5 μm at 77 K. The pBiBn detector demonstrated operation up to room temperature and the cascade detector up to 420 K. A dark current density of 1.6 × 10⁻⁷ A/cm² and 3.6 × 10⁻⁷ A/cm⁻² was measured for the pBiBn and interband cascade detector, respectively, at 80 K. A responsivity of 1.3 A/W and 0.17 A/W was observed at −30 mV and −5 mV of applied bias for pBiBn and cascade detector, respectively, at 77 K. The experimental results have been explained by correlating them with the operation of the devices.     

Influence of the p-type doping on the radiometric performances of MWIR InAs/GaSb superlattice photodiodes

In this paper, quantum efficiency (QE) measurements performed on type-II InAs/GaSb superlattice (T2SL) photodiodes operating in the mid-wavelength infrared domain, are reported. Several comparisons were made in order to determine the SL structure showing optimum radiometric performances: same InAs-rich SL structure with different active zone thicknesses (from 0.5 μm to 4 μm) and different active zone doping (n-type versus p-type), same 1 μm thick p-type active zone doping with different SL designs (InAs-rich versus GaSb-rich and symmetric SL structures). Best result was obtained for the p-type doped InAs-rich SL photodiode, with a 4 μm active zone thickness, showing a QE that reaches 61% at λ = 2 μm and 0 V bias voltage.     

Analysis of the response time in high-temperature LWIR HgCdTe photodiodes operating in non-equilibrium mode

The paper presents the theoretical investigation of the active region parameters, especially the influence of thickness and doping, on the response time and current responsivity of high-temperature long wavelength infrared HgCdTe photodiodes operating at 230 K in non-equilibrium mode. Results of theoretical predictions of time constant were compared to the experimental data. The response time of the devices have been characterized using Nd:YAG laser, optical parametric generator with pulse width <25 ps and fast oscilloscope with suitable transimpedance amplifier as a function of detector design, temperature and bias. The reverse bias applied to the photodiode causes Auger-suppression and improve the performances of the devices. This way the response time decreases to the value below 1 ns at the good current responsivity increased to the value of about 6 A/W and what is a promising parameter in view of potential telecommunication applications. Due to the series resistance of electrical connections, the response time of the devices is mainly limited by RC constant while the calculations show that the time constant of the Auger suppressed structures should be limited by the drift time of carriers.     

Evolution of array format of longwave infrared Type-II SLS FPAs with high quantum efficiency

In the remarkably short span of 2 years, longwave infrared focal plane arrays (FPAs) of Type-II InAs/GaSb strained layer superlattice (SLS) photodiodes have advanced from 320 × 256 format to 1024 × 1024 format while simultaneously shrinking the pitch from 30 μm to 18 μm. Despite a dark current that is presently higher than state-of-the-art mercury cadmium telluride photodiodes with the same ∼10 μm cutoff wavelength, the high pixel operability and high (∼50%) quantum efficiency of SLS FPAs enable excellent imagery with temporal noise equivalent temperature difference better than 30 mK with F/4 optics, integration time less than 1 ms, and operating temperature of 77 K or colder. We present current FPA performance of this promising sensor technology.     

Improved electrical parameters of vacuum annealed Ni/4H-SiC (0 0 0 1) Schottky barrier diode

The reported work has been focused on the improvement of electrical parameters of Schottky diode using vacuum annealing at mild temperature in Ar gas ambient. Nickel Schottky barrier diodes were fabricated on 50 μm epitaxial layer of n-type 4H-SiC (0 0 0 1) substrate. The values of leakage current, Schottky barrier height (?_B), ideality factor (η) and density of interface states (N_SS) were obtained from experimentally measured current–voltage (I–V) and capacitance–voltage (C–V) characteristics before and after vacuum annealing treatment. The data revealed that ?_B, η and reverse leakage current for the as-processed diodes are 1.25 eV, 1.6 and 1.2 nA (at ?100 V), respectively, while for vacuum annealed diodes these parameters are 1.36 eV, 1.3 and 900 pA (at same reverse voltage). Improved characteristics have been resulted under the influence of vacuum annealing because of lesser number of minority carrier generation due to incessant reduction of number of available discrete energy levels in the bandgap of 4H-SiC substrate and lesser number of interface states density at Ni/4H-SiC (0 0 0 1) interface.     

Development of n-type Te-doped GaSb substrates with low carrier concentration for FPA applications

Undoped GaSb is p-type with the residual acceptor concentration of about 1e17 cm⁻³ due to the gallium vacancies and gallium in antimony site. Counter-doping of GaSb with low level of Te can reduce the net carrier concentration resulting in higher optical transparency in a broad IR spectral range. In this work, the carrier concentration, mobility and sheet resistance of n-type and p-type Te-doped GaSb substrates were measured using Hall method at 300 K and 77 K. The Hall carrier concentration data at 300 K were correlated with the absorption coefficients of GaSb in the IR spectral range. An empirical relationship between these values was established. Based on this correlation, we discuss application of FTIR spectroscopy for non-destructive optical screening of the substrates that allows construction of the carrier concentration distribution map across GaSb wafers. Investigations of the electronic properties of the low-doped p-type and n-type GaSb substrates upon cooling down to 77 K indicate the reduction of the hole carrier concentration background for both GaSb types. This is evident from the decrease in the Hall-measured carrier concentration for p-type GaSb. For n-type GaSb, an increase in the carrier concentration is observed due to the reduction of the hole carrier concentration background.     

Performance improvement of long-wave infrared InAs/GaSb strained-layer superlattice detectors through sulfur-based passivation

We report on effective sulfur-based passivation treatments of type-II InAs/GaSb strained layer superlattice detectors (100% cut-off wavelength is 9.8 μm at 77 K). The electrical behavior of detectors passivated by electrochemical sulfur deposition (ECP) and thioacetamide (TAM) was evaluated for devices of various sizes. ECP passivated detectors with a perimeter-to-area ratio of 1600 cm^?1 exhibited superior performance with surface resistivity in excess of 10⁴ Ω cm, dark current density of 2.7 × 10^?3 A/cm², and specific detectivity improved by a factor of 5 compared to unpassivated devices (V_Bias =  ? 0.1 V, 77 K).     

Demonstration of large format mid-wavelength infrared focal plane arrays based on superlattice and BIRD detector structures

We have demonstrated the use of bulk antimonide based materials and type-II antimonide based superlattices in the development of large area mid-wavelength infrared (MWIR) focal plane arrays (FPAs). Barrier infrared photodetectors (BIRDs) and superlattice-based infrared photodetectors are expected to outperform traditional III–V MWIR and LWIR imaging technologies and are expected to offer significant advantages over II–VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow InAs/GaSb superlattice pin photodiodes and bulk InAsSb structures on GaSb substrates. The coupled quantum well superlattice device offers additional control in wavelength tuning via quantum well sizes and interface composition, while the BIRD structure allows for device fabrication without additional passivation. As a demonstration of the large area imaging capabilities of this technology, we have fabricated mid-wavelength 1024 × 1024 pixels superlattice imaging FPAs and 640 × 512 MWIR arrays based on the BIRD concept. These initial FPA have produced excellent infrared imagery.     

Growth and electrical characterization of type II InAs/GaSb superlattices for midwave infrared detection

Herein, we report a type II InAs/GaSb superlattice structure (SLS) grown on GaSb(1 0 0) substrates by molecular beam epitaxy (MBE) and its electrical characterization for mid-wavelength infrared detection. A GaSb buffer layer was grown under optimized SLS growth conditions, which can decrease the occurrence of defects for similar pyramidal structures. The complications associated with these conditions include oxide desorption of the substrate, growth temperature of the SLS, the V/III ratio during superlattice growth and the shutter sequence. High-resolution X-ray diffraction (HRXRD) shows the sixth satellite peak, and the period of the SLS was 52.9 Å. The atomic force microscopy (AFM) images indicated that the roughness was less than 2.8 nm. High-resolution transmission electron microscopy (HRTEM) images indicated that the SLS contains few structural defects related to interface dislocations or strain relaxation during the growth of the superlattice layer. The photoresponse spectra indicated that the cutoff wavelength was 4.8 μm at 300 K. The SLS photodiode surface was passivated by a zinc sulfide (ZnS) coating after anodic sulfide.     

Growth and characterization of strain-compensated InGaAs/GaAsSb type II multiple quantum wells on InP substrate

Strain-compensated InGaAs/GaAsSb type II multiple quantum wells (MQWs) were grown by molecular beam epitaxy (MBE) and their optical and electrical properties were studied. High-quality strain-compensated type II MQWs were successfully grown, which have longer emission wavelength than that of lattice-matched type II MQWs. PL peak energy at 300 K of the strain-compensated type II MQWs, where the InGaAs layer has 0.6% tensile strain and GaAsSb layer has 0.6% compressive strain, shows a red-shift of 43 meV, which is 12 meV larger than the calculated energy shift of 31 meV. In addition, the PL intensity and the electron mobility of the strain-compensated MQWs are comparable to those of the lattice-matched MQWs, suggesting that the crystal quality of the strain-compensated MQWs is good and are very promising for low dark current photodiodes in the 2 μm wavelength region.     

Investigation of Q-switched InP-based 1550 nm semiconductor lasers

High energy picosecond pulse generation from a two contact tapered 5 quantum well (QW) InGaAlAs/InP diode laser (1550 nm) is investigated using a passive Q-switching technique. Single peak pulses with pulse energies as high as 500 pJ and durations of typically hundreds of picoseconds are obtained from the device by applying reverse bias voltages in the range of 0 V to ?18 V to the absorber section of the device. It is also demonstrated that more symmetrical Q-switched pulses are obtained by reducing the duration of electrical pulses applied to the gain section of the laser. Such an improvement is attributed to the reduced time of the population inversion in the gain section due to shorter electrical pulse. We also show comparatively the dependence of optical spectra on the reverse bias voltage for diode lasers emitting at 1550 nm and 1350 nm, and demonstrate that better spectral output is obtained from AlGaInAs lasers emitting at a wavelength of 1550 nm.     

Mid-infrared InAsSb-based nBn photodetectors with AlGaAsSb barrier layers – Grown on GaAs,using an interfacial misfit array,and on native GaSb

InAsSb-based nBn photodetectors were fabricated on GaAs, using the interfacial misfit (IMF) array growth mode, and on native GaSb. At −0.1 V operating bias, 200 K dark current densities of 1.4 × 10⁻⁵ A cm⁻² (on GaAs) and 4.8 × 10⁻⁶ A cm⁻² (on GaSb) were measured. At the same temperature, specific detectivity (D^*) figures of 1.2 × 10¹⁰ Jones (on GaAs) and 7.2 × 10¹⁰ Jones (on GaSb) were calculated. Arrhenius plots of the dark current densities yielded activation energies of 0.37 eV (on GaAs) and 0.42 eV (on GaSb). These values are close to the 4 K bandgap of the absorption layers (0.32–0.35 eV) indicating diffusion limited dark currents and small valence band offsets. Significantly, these devices could be used for mid-infrared focal plane arrays operating within the temperature range of cost-effective thermoelectric coolers.     

CMOS monolithic optoelectronic integrated circuit for on-chip optical interconnection

A monolithic silicon CMOS optoelectronic integrated circuit (OEIC) is designed and fabricated using standard 0.35-μm CMOS technology. This OEIC monolithically integrates light emitting diode (LED), silicon dioxide waveguide, photodetector and receiver circuit on a single silicon chip. The silicon LED operates in reverse breakdown mode and can emit light at 8.5 V. The output optical power is 31.2 nW under 9.8 V reverse bias. The measured spectrum of LED showed two peaks at 760 nm and 810 nm, respectively. The waveguide is composed of silicon dioxide/metal multiple layers. The responsivity of the n-well/p-substrate diode photodetector is 0.42 A/W and the dark current is 7.8 pA. The LED-emitted light transmits through the waveguide and can be detected by the photodetector. Experimental results show that on-chip optical interconnects are achieved by standard CMOS technology successfully.     

Electrical and optical characteristics of a Si-doped (Al)GaInAs digital alloy/AlInAs-distributed Bragg mirrors on InP

We report electrical and optical characteristics of a Si-doped (Al)GaInAs digital alloy/AlInAs Bragg mirror lattice matched to InP grown by molecular beam epitaxy. A 98.2% reflectivity with a 107 nm stop band width centred at 1.54 μ m is obtained. An average voltage drop of 16 mV per period at a current density of 1 KA cm ^− 2is observed for a mean electron concentration of about 5.5  ×  10¹⁸cm ^− 3. The influence of structural and intrinsic properties of the heterostructure on the electrical resistivity and optical reflectivity is analysed.     

A proposed organic Schottky barrier photodetector for application in the visible region

The fabrication and characterization of an organic photodetector (OPD) in the form of ITO coated glass/polycarbazole (PCz)/Al Schottky contact is reported. The device has been fabricated in our laboratory for the first time using the polymer synthesized by us. The device has been subsequently characterized in respect of electrical and optical properties in order to explore its potential for possible use as a detector in the visible region at 650 nm. It is observed that the detector exhibits a reasonably high value of peak detectivity (∼6 × 10⁶ cm Hz^1/2 W⁻¹) near zero bias voltage at V = 0.2 V.     

Photoelectrical characteristics of the InAsSbP based uncooled photodiodes for the spectral range 1.6–3.5 μm

The electrical characteristics of photodiode structures on the base of InAS/InAsSbP heterojunctions, that have a high room temperature differential resistance and operate in the mid-infrared region over the wavelength range 1.6–3.5 μm are reported. At the difference frequency, C–V measurements are showed that at small biases and temperatures which are higher than 160 K, the measured capacity is increase with decreasing frequency. It is possible to explain by presence the deep recombination centers in space charge region of the investigated structures. Have been studied the avalanche multiplication of the photocurrent and the temperature dependence of the monochromatic power–voltage sensitivity in the temperature range 77–300 K.     

Performance of mid-wave T2SL detectors with heterojunction barriers

A heterojunction T2SL barrier detector which effectively blocks majority carrier leakage over the pn-junction was designed and fabricated for the mid-wave infrared (MWIR) atmospheric transmission window. The layers in the barrier region comprised AlSb, GaSb and InAs, and the thicknesses were selected by using k · P-based energy band modeling to achieve maximum valence band offset, while maintaining close to zero conduction band discontinuity in a way similar to the work of Abdollahi Pour et al. [1] The barrier-structure has a 50% cutoff at 4.75 μm and 40% quantum efficiency and shows a dark current density of 6 × 10⁻⁶ A/cm² at −0.05 V bias and 120 K. This is one order of magnitude lower than for comparable T2SL-structures without the barrier. Further improvement of the (non-surface related) bulk dark current can be expected with optimized doping of the absorber and barrier, and by fine tuning of the barrier layer design. We discuss the effect of barrier doping on dark current based on simulations. A T2SL focal plane array with 320 × 256 pixels, 30 μm pitch and 90% fill factor was processed in house using a conventional homojunction p–i–n photodiode architecture and the ISC9705 readout circuit. High-quality imaging up to 110 K was demonstrated with the substrate fully removed.     

Thermal annealing of GaSb quantum dots in GaAs formed by droplet epitaxy

We investigate effects of annealing on GaSb quantum dots (QDs) formed by droplet epitaxy. Ga droplets grown on GaAs are exposed to Sb molecular beam and then annealed at T_a=340–450 °C for 1 min to form GaSb QDs. An atomic force microscope study shows that with the increase of T_a, the average diameter of dots increases by about 60%, while their density decreases to about 1/3. The photoluminescence (PL) of GaSb QDs is observed at around 1 eV only for those samples annealed above T_a=380 °C, which indicates that the annealing process plays an important role in forming high quality GaSb QDs.     

The effect of compositional disorder on electronic band structure in GaxIn1 − xAsySb1 − yalloys lattice matched to GaSb

A pseudopotential formalism coupled with the virtual crystal approximation are applied to study the effect of compositional disorder upon electronic band structure of cubic Ga_xIn_1 − xAs_ySb_1 − yquarternary alloys lattice matched to GaSb. The effects of compositional variations are properly included in the calculations. Our theoretical results show that the compositional disorder plays an important role in the determination of the energy band structure of Ga_xIn_1 − xAs_ySb_1 − y/GaSb and that the bowing parameter is dominated by the group V-anion-based sublattice. Moreover, the absorption at the fundamental optical gaps is found to be direct within a whole range of the x composition.