InAs/GaSb superlattice infrared detectors

Future heterojunction InAs/GaSb superlattice (SL) detector devices in the long-wavelength infrared regime (LWIR, 8–12 μm) require an accurate bandstructure model and a successful surface passivation. In this study, we have validated the superlattice empirical pseudopotential method developed by Dente and Tilton over a wide range of bandgap energies. Furthermore, dark current data for a novel dielectric surface passivation for LWIR devices is presented. Next, we present a technique for high-resolution, full-wafer mapping of etch pit densities on commercial (1 0 0) GaSb substrates, which allows to study the local correlation between threading dislocations in the substrate and the electro-optical pixel performance. Finally, recent performance data for 384 × 288 dual-color InAs/GaSb superlattice imagers for the mid-wavelength infrared (MWR, 3–5 μm) is given.     

Fabrication of type-II InAs/GaSb superlattice long-wavelength infrared focal plane arrays

In this paper, we present an InAs/GaSb type-II superlattice (SL) with the M-structure for the fabrication of a long-wavelength (10 μm range) infrared (LWIR) focal plane arrays (FPA), which are grown by molecular beam epitaxy (MBE). The M-structure is named for the shape of the band alignment while the AlSb layer is inserted into the GaSb layer of InAs/GaSb SL. A 320 × 256 LWIR FPA has been fabricated with low surface leakage and high R₀A product of FPA pixels by using anodic sulfide and SiO₂ physical passivation. Experiment results show that the devices passivated with anodic sulfide obviously have higher R₀A than the un-sulphurized one. The 50% cutoff wavelength of the LWIR FPA is 9.1 μm, and the R₀A is 224 Ω cm² with the average detectivity of 2.3 × 10¹⁰ cm Hz^1/2 W⁻¹.     

Excitonic influences on the anticrossing gap of Wannier–Stark levels in a wide well superlattice

In this paper we compare different theoretical calculations with experimental data of anticrossings of a wide well semiconductor superlattice in the Wannier–Stark regime. Calculations have been performed by using a variation method and a numerical method. For the variation method we used different approaches to minimize energy. With the numerical model, continuum states were calculated and anticrossing of 1s states with continuum states were found. Calculations have been performed for h1e1(0)–h1e2( − 1) and h1e1(1)–h1e2(0) anticrossings and are compared with data from photocurrent experiments.     

Chirped Bloch oscillations in strain-balanced InGaAs/InGaAs superlattices

Bloch oscillations excited in a strain-balanced In_xGa_1 − xAs/In_yGa_1 − yAs superlattice by fs optical pulses at 1.55 μ m are investigated in time-resolved transmission spectroscopy. The transition from the coherent oscillatory motion to an incoherent drift transport of the electrons is observed via a transient frequency shift of the Bloch oscillations due to the associated screening of the applied electric field. These electric field changes are analyzed quantitatively as a function of the initial field strengths and excitation densities. The incoherent transport can be described by a drift-diffusion model. As a result, the carrier mobility in the superlattice is obtained on a picosecond timescale.     

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.     

Manufacturability of type-II InAs/GaSb superlattice detectors for infrared imaging

Type-II InAs/GaSb superlattice detectors and focal plane arrays (FPAs) with cut-off wavelength at 5.1 μm have been studied. For single pixel devices, dark current densities of 1 × 10⁻⁶ A/cm² and quantum efficiencies of 53% were measured at 120 K. From statistics of manufactured FPAs, an average FPA operability of 99.87% was observed. Furthermore, average temporal and spatial noise equivalent temperature difference (NETD) values of 12 mK and 4 mK, respectively, were deduced. Excellent stability of FPAs after non-uniformity correction was observed with no deterioration of the ratio between spatial and temporal noise during a two hour long measurement. Also after several cooldowns the ratio between spatial and temporal NETD stayed below 0.6.     

Bias-selectable mid-/long-wave dual band infrared focal plane array based on Type-II InAs/GaSb superlattice

In this paper, a mid-/long-wave dual-band detector which combined PπMN structure and unipolar barrier was developed based on type-II InAs/GaSb superlattice. A relevant 320 × 256 focal plane array (FPA) was fabricated. Unipolar barrier and PπMN structure in our dual band detector structure were used to suppress cross-talk and dark current, respectively. The two channels, with respective 50% cut-off wavelength at 4.5 μm and 10 μm were obtained. The peak quantum efficiency (QE) of mid wavelength infrared (MWIR) band and long wavelength infrared (LWIR) band are 53% at 3.2 μm under no bias voltage and 40% at 6.4 μm under bias voltage of −170 mV, respectively. And the dark current density under 0 and −170 mV of applied bias are 1.076 × 10⁻⁵ A/cm² and 2.16 × 10⁻⁴ A/cm². The specific detectivity of MWIR band and LWIR band are 2.15 × 10¹² cm·Hz^1/2/W at 3.2 μm and 2.31 × 10¹⁰ cm·Hz^1/2/W at 6.4 μm, respectively, at 77 K. The specific detectivity of LWIR band maintains above 10¹⁰ cm·Hz^1/2/W at the wavelength range from 4.3 μm to 10.2 μm under −170 mV. The cross-talk, selectivity parameter at 3.0 μm, about 0.14 was achieved under bias of −170 mV. Finally, the thermal images were taken by the fabricated FPA at 77 K.     

Photodiode properties of molecular beam epitaxial InSb on a heavily doped substrate

Photodiodes of InSb were fabricated on an epitaxial layer grown using molecular beam epitaxy (MBE). Thermal cleaning of the InSb (0 0 1) substrate surface, 2° towards the (1 1 1) B plane, was performed to remove the oxide. Photodiode properties of МВЕ-formed epitaxial InSb were demonstrated. Zero-bias resistance area product (R₀A) measurements were taken at 80 K under room temperature background for a pixel size of 100 μm × 100 μm. Values were as high as 4.36 × 10⁴ Ω/cm², and the average value of R₀A was 1.66 × 10⁴ Ω/cm². The peak response was 2.44 (A/W). The epitaxial InSb photodiodes were fabricated using the same process as bulk crystal InSb diodes with the exception of the junction formation method. These values are comparable to the properties of bulk crystal InSb photodiodes.     

Excess noise in InAs/GaSb type-II superlattice pin-photodiodes

We characterize the low-frequency white noise behavior of a large set of InAs/GaSb superlattice infrared pin-photodiodes for the mid-wavelength infrared regime at 3–5 μm. For diodes with an increased dark current in comparison to the dark current of generation–recombination limited bulk material, the standard shot-noise model fails to describe the noise experimentally observed in the white part of the spectrum. Instead, we find that McIntyre’s noise model for avalanche multiplication processes is compliant with our data. We suggest that within high electric field domains localized around macroscopic defects, avalanche multiplication processes leading to increased dark current and excess noise.     

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.     

Modeling of electrical and optical characteristics of near room-temperature CdS/ZnSe based NIR photodetectors

Results of modeled photodetector characteristics in (CdS/ZnSe)/BeTe multi-well diode with p–i–n polarity are reported. The dark current density (J–V) characteristics, the temperature dependence of zero-bias resistance area product (R₀A), the dynamic resistance as well as bias dependent dynamic resistance (R_d) and have been analyzed to investigate the mechanisms limiting the electrical performance of the modeled photodetectors. The quantum efficiency, the responsivity and the detectivity have been also studied as function of the operating wavelength. The suitability of the modeled photodetector is demonstrated by its feasibility of achieving good device performance near room temperature operating at 1.55 μm wavelength required for photodetection in optical communication. Quantum efficiency of ∼95%, responsivity ∼0.6 A/W and D^* ∼ 5.7 × 10¹⁰ cm Hz^1/2/W have been achieved at 300 K in X BeTe conduction band minimum.     

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.     

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).     

High detectivity dilute nitride strained layer superlattice detectors for LWIR and VLWIR applications

This paper will describe the first-of-a-kind development and demonstration of dilute nitride strained layer superlattice detectors with detectivity as high as 4 × 10¹⁰ cm Hz^1/2/W and cut-off wavelength of 11-μm for an LWIR design and a cut-off wavelength of 22-μm for a VLWIR design. The developed dilute nitride SLS detectors are based on ultra-low leakage dilute nitride epitaxial layers and/or strained layer superlattices (SLS) of InAs/InAsSbN and InAs/GaInSbN that could enable high VLWIR detectivities at elevated temperatures and at low cost.     

Improved light extraction efficiency of GaN-based LEDs with patterned sapphire substrate and patterned ITO

To improve the light extraction efficiency of GaN-based light-emitting diodes (LEDs), periodic semisphere patterns with 3.5 μm width, 1.2 μm height, and 0.8 μm spacing were formed on sapphire substrate by dry etching using BCl₃/Cl₂ gas chemistry. The indium tin oxide (ITO) transparent conductive layer was patterned by wet etching to reduce the total internal reflection existing along between p-GaN, ITO, and air. At 350 mA injection current, the high power LED by integrating patterned sapphire substrate with patterned ITO technology exhibited a 36.9% higher light output power than the conventional LEDs.     

320 × 256 Short-/Mid-Wavelength dual-color infrared focal plane arrays based on Type-II InAs/GaSb superlattice

Short-/Mid-Wavelength dual-color infrared focal plane arrays based on Type-II InAs/GaSb superlattice are demonstrated on GaSb substrate. The material is grown with 50% cut-off wavelength of 2.9 μm and 5.1 μm for the blue channel and red channel, separately at 77 K. 320 × 256 focal plane arrays fabricated in this wafer is characterized. The peak quantum efficiency without antireflective coating is 37% at 1.7 μm under no bias voltage and 28% at 3.2 μm under bias voltage of 130 mV. The peak specific detectivity are 1.51 × 10¹² cm·Hz^1/2/W at 2.5 μm and 6.11x10¹¹ cm·Hz^1/2/W at 3.2 μm. At 77 K, the noise equivalent difference temperature presents average values of 107 mK and 487 mK for the blue channel and red channel separately.     

InGaAsP/InP photodetectors targeting on 1.06 μm wavelength detection

InP-based InGaAsP photodetectors targeting on 1.06 μm wavelength detection have been grown by gas source molecular beam epitaxy and demonstrated. For the detector with 200 μm mesa diameter, the dark current at 10 mV reverse bias and R₀A are 8.89 pA (2.2 × 10⁻⁸ A/cm²) and 3.9 × 10⁵ Ω cm² at room temperature. The responsivity and detectivity of the InGaAsP detector are 0.30 A/W and 1.45 × 10¹² cm Hz^1/2 W⁻¹ at 1.06 μm wavelength. Comparing to the reference In_0.53Ga_0.47As detector, the dark current of this InGaAsP detector is about 570 times lower and the detectivity is more than ten times higher, which agrees well with the theoretical estimation.     

Distinction investigation of InGaAs photodetectors cutoff at 2.9 μm

Using double heterojunction structure with linearly graded In_xAl_1–xAs as buffer layer and In_0.9Al_0.1As as cap layer, wavelength extended In_0.9Ga_0.1As detectors with cutoff wavelength of 2.88 μm at room temperature have been grown by using gas source molecular beam epitaxy, their characteristics have been investigated in detail and compared with the detectors cutoff at 2.4 μm with similar structure as well as commercial InAs detectors. Typical resistance area product R₀A of the detectors reaches 3.2 Ω cm² at 290 K. Measured peak detectivity reaches 6.6E9 cm Hz^1/2/W at room temperature.     

Mid-wavelength type II InAs/GaSb superlattice infrared focal plane arrays

We have demonstrated 384 × 288 pixels mid-wavelength infrared focal plane arrays (FPA) using type II InAs/GaSb superlattice (T2SL) photodetectors with pitch of 25 μm. Two p-i-n T2SL samples were grown by molecular beam epitaxy with both GaAs-like and InSb-like interface. The diode chips were realized by pixel isolation with both dry etching and wet etching method, and passivation with SiN_x layer. The device one with 50% cutoff wavelength of 4.1 μm shows NETD ∼ 18 mK from 77 K to 100 K. The NETD of the other device with 50% cutoff wavelength at 5.6 μm is 10 mK at 77 K. Finally, the T2SL FPA shows high quality imaging capability at the temperature ranging from 80 K to 100 K which demonstrates the devices’ good temperature performance.     

Aδ-doped In0.24Ga0.76As/GaAs pseudomorphic high electron mobility transistor using a graded superlattice spacer

A new δ -doped In_0.24Ga_0.76As/GaAs pseudomorphic high electron mobility transistor (HEMT) using a graded superlattice spacer grown by molecular beam epitaxy (MBE) has been successfully fabricated and investigated. The present device structure demonstrated a more than 40% enhancement of electron mobility and 20% higher product value of electron mobility and two-dimensional electron gas (2DEG) concentration than those of the conventional HEMT with single undoped spacer under the same growth specifications. Superior device characteristics were achieved by employing the thickness-graded superlattice spacer to accommodate the lattice-mismatch-induced strain and to improve the interfacial quality. For a gate length of 1 μ m, the maximum drain-to-source saturation current density and extrinsic transconductance of the present HEMT design are 165 mA mm ^− 1and 107 mS mm ^− 1, respectively, at room temperature.