Search

The fast development of mid-wave infrared photonics has increased the demand for high-performance photodetectors that operate in this spectral range. However, the signal-to-noise ratio, regarded as a primary figure of merit for mid-wave infrared detection, is strongly limited by the high dark current in narrow-bandgap materials. Therefore, conventional mid-wave infrared photodetectors such as HgCdTe require cryogenic temperatures to avoid excessively high dark current. To address this challenge, we report an avalanche photodiode design using photon-trapping structures to enhance the quantum efficiency and minimize the absorber thickness to suppress the dark current. The device exhibits high quantum efficiency and dark current density that is nearly three orders of magnitude lower than that of the state-of-the-art HgCdTe avalanche photodiodes and nearly two orders lower than that of previously reported AlInAsSb avalanche photodiodes that operate at 2 µm. Additionally, the bandwidth of these avalanche photodiodes reaches ~7 GHz, and the gain–bandwidth product is over 200 GHz; both are more than four times those of previously reported 2 µm avalanche photodiodes.

We report Al0.3InAsSb/Al0.7InAsSb digital alloy n-barrier-n (nBn) photodetectors that operate in the 2-μm wavelength window. The AlxIn1-xAsySb1-y digital alloy material system exhibits near-zero valence-band offset, which is beneficial for nBn photodetectors. At 300 K these Al0.3InAsSb/Al0.7InAsSb nBn photodetectors have achieved specific detectivities of 1.7 × 10 10 and 3.0 × 10 10 Jones under 2-μm and 1.8-μm illumination, respectively. The dark current density at -0.5 V bias decreases from 3.1 × 10 -3 A/cm 2 at 300 K to 1.6 × 10 -10 A/cm 2 at 120 K, where the dominant dark current component is tunneling. The area-dependence of key performance parameters show that for mesa diameter ≤ 250 μm, surface defects assume a dominant role in the total noise.

Mid-IR is a useful wavelength range for both science and military applications due to its low atmospheric attenuation and ability to be used for passive detection. However, many solutions for detecting light in this spectral region need to be operated at cryogenic temperatures as their required narrow bandgaps suffer from carrier recombination and band-to-band tunneling at room temperature leading to high dark currents. These problems can be alleviated by using a separate absorption, charge, and multiplication avalanche photodiode. We have recently demonstrated such a device with a 3-µm cutoff using Al

We report the frequency response of Al0.3InAsSb/Al0.7InAsSb nBn photodetectors. The 3-dB bandwidth of the devices varies from ∼ 150 MHz to ∼ 700 MHz with different device diameters and saturates with bias voltage immediately after the device turn on. A new equivalent circuit model is developed to explain the frequency behavior of nBn photodetectors. The simulated bandwidth based on the new equivalent circuit model agrees well with the bandwidth and the microwave scattering parameter measurements. The analysis reveals that the limiting factor of the bandwidth of the nBn photodetector is the large diffusion capacitance caused by the minority carrier lifetime and the device area. Additionally, the bandwidth of the nBn photodetector is barely affected by the photocurrent, which is found to be caused by the barrier structure in the nBn photodetector.

In 1982, Capasso and co-workers proposed the solid-state analogue of the photomultiplier tube, termed the staircase avalanche photodiode. Through a combination of compositional grading and small applied bias, the conduction band profile is arranged into a series of steps that function similar to the dynodes of a photomultiplier tube, with twofold gain arising at each step via impact ionization. A single-step staircase was previously reported but did not demonstrate gain scaling through cascading multiple steps or report noise properties. Here we demonstrate gain scaling of up to three steps; measurements show the expected 2N scaling with the number of staircase steps, N. Furthermore, measured noise increased more slowly with gain than for photomultiplier tubes, probably due to the lower stochasticity of impact ionization across well-designed heterojunctions as compared with the secondary electron emission from metals. Excellent agreement was found between the experiments and Monte Carlo simulations for both gain and noise. A three-step staircase avalanche diode was demonstrated and pre-cited gain scaling was confirmed. The technology may be considered as a solid-state analogue to the photomultiplier tube.

In applications where high sensitivity is required, the internal gain mechanism of avalanche photodiodes can provide a performance advantage relative to p-i-n photodiodes. However, this internal gain mechanism leads to an excess noise that scales with gain. This excess noise term can be minimized by using materials systems in which impact ionization is initiated primarily by one carrier type. Recently, two Sb-based materials systems, AlInAsSb and AlGaAsSb, have exhibited exceptionally low excess noise, particularly for III–V compound materials. There are four important considerations that can impact the excess noise measurements in such low-noise materials. These considerations deal with the excess noise factor calculation method, measurement RF frequency, measurement wavelength, and the gain calculation method. In this paper, each of these factors is discussed, and their implications on excess noise are considered.

Avalanche photodiodes fabricated from AlInAsSb grown as a digital alloy exhibit low excess noise. In this article, we investigate the band structure‐related mechanisms that influence impact ionization. Band‐structures calculated using an empirical tight‐binding method and Monte Carlo simulations reveal that the mini‐gaps in the conduction band do not inhibit electron impact ionization. Good agreement between the full band Monte Carlo simulations and measured noise characteristics is demonstrated.

We report Al0.8In0.2As0.23Sb0.77 avalanche photodiodes with high gain ( $M>1300$ ) and low dark current at room temperature. Impact ionization coefficients for this material system are also extracted, indicating electron-dominant impact ionization. Low avalanche breakdown temperature dependence is demonstrated.

In this work, we demonstrate a novel top-illuminated avalanche photodiode (APD) with high-speed and wide dynamic range performance for coherent lidar applications, which needs simultaneous processing of weak light reflections from the object and a strong optical local-oscillator (LO) signal at the receiving end. By taking advantage of a partially depleted p-type In0.53Ga0.47As absorbing layer and a thin In0.52Al0.48As multiplication layer (88 nm), the demonstrated APDs exhibit a wide optical-to-electrical bandwidth (16 GHz) and high responsivity (2.5 A/W at 0.9 Vbr) near the saturation output current which is as high as >8 mA. Furthermore, the measured low excess noise (k = 0.14) characteristics of this device ensure its wide dynamic range performance. In addition, it should be noted that the measured nonlinear behaviors of these APDs are quite different from those of typical fast p-i-n photodiodes (PDs), which always show serious degradation in their O-E bandwidths at their saturation output. On the other hand, the measured bias dependent 3-dB O-E bandwidth of the demonstrated APDs can be pinned at 16 GHz without any degradation at around the saturation current output. Such distinct nonlinear APD behaviors can be attributed to the space-charge-screening effect induced gain reduction under high-power operation.

We investigate the room-temperature bandwidth performance of AlInAsSb avalanche photodiodes under 2-μm illumination. Parameter characterization denotes RC-limited performance. While measurements indicate a maximum gain-bandwidth product of 44 GHz for a 60-μm-diameter device, we scale this performance to smaller device sizes based on the RC response. For a 15-μm-diameter device, we predict a maximum gain-bandwidth product of approximately 144 GHz based on the reported measurements.

We report Al0.7InAsSb avalanche photodiodes grow as ternary–binary and binary–binary digital alloys. Their characteristics of ideality factor, activation energy, temperature-dependent excess noise, temperature stability, and impact ionization coefficients are compared.

Mid-IR is a useful wavelength range for both science and military applications due to its low atmospheric attenuation and ability to be used for passive detection. However, many solutions for detecting light in this spectral region need to be operated at cryogenic temperatures as their required narrow bandgaps suffer from carrier recombination and band-to-band tunneling at room temperature leading to high dark currents. These problems can be alleviated by using a separate absorption, charge, and multiplication avalanche photodiode. We have recently demonstrated such a device with a 3-µm cutoff using Al0.15In0.85As0.77Sb0.23, as the absorber, grown on GaSb. Here we investigate Al0.15In0.85As0.77Sb0.23 as a simple PIN homojunction and provide metrics to aid in future designs using this material. PL spectrum measurements indicate a bandgap of 2.94 µm at 300 K. External quantum efficiencies of 39% and 33% are achieved at 1.55 µm and 2 µm respectively. Between 180 K and 280 K the activation energy is ∼0.22 eV, roughly half the bandgap of Al0.15In0.85As0.77Sb0.23, indicating thermal generation is dominant.

We report the noise characteristics of an AlInAsSb avalanche photodiode (APD) on an InP substrate. We observe low excess noise corresponding to an impact ionization coefficient ratio (k) of 0.012, and a dark current density of 55 μA/cm2 at a gain of 10 at room temperature. The performance of commercial APDs on InP substrates is limited by the excess noise and the performance of state of the art (SOA) APDs on InP substrates is limited by the dark current. The combination of low excess noise and low dark current of AlInAsSb leads to a significant performance improvement compared to commercial APDs and provides a potential candidate for low noise, SOA, commercial APDs for near-infrared applications. When combined in a separate absorber, charge and multiplication layer (SACM) architecture with an InGaAs absorption layer, the low noise characteristics of AlInAsSb point towards a superior InP substrate-based APD targeting 1.55 μm for applications such as optical communications and light detection and ranging (LiDAR).

We present gain, dark current and excess noise characteristics of PIN Al0.85Ga0.15As0.56Sb0.44 (hereafter AlGaAsSb) avalanche photodiodes (APDs) on InP substrates with 1000 nm thick multiplier layers. The AlGaAsSb APDs were grown by molecular beam epitaxy using a digital alloy technique (DA) to avoid phase separation. Current-voltage measurements give a peak gain of ~ 42, a breakdown voltage of – 54.3 V, and a dark current density at a gain of 10 of ~ 145 μA/cm2. Excess noise measurements of multiple AlGaAsSb APDs show that k (the ratio of electron and hole impact ionization coefficients) is ~ 0.01. This k-value is comparable to Si, which is widely used for visible and near-infrared APDs. The low dark current density and low excess noise suggest that such thick AlGaAsSb layers are promising multipliers in separate absorption, charge and multiplication (SACM) structures for short-wavelength infrared applications such as optical communication and LIDAR, particularly on a commercial InP platform.

Avalanche photodiodes (APDs) can provide high receiver sensitivity owing to their internal gain, however, the impact ionization gain mechanism is also a source of noise. Recently, AlxIn1-xAsySb1-y grown as a digital alloy has been used to fabricate APDs with excess noise as low as Si (k ~ 0.01). Operation of these APDs at telecommunication wavelengths (1300 nm to 1550 nm) or 2 µm has been achieved. This paper also discusses staircase APDs, which utilize compositional grading to mimic the dynodes of a photomultiplier tube, with a gain of 2x arising at each step providing extremely low noise avalanche gain.

We report on engineering impact ionization characteristics of In0.53Ga0.47As/Al0.48In0.52As superlattice avalanche photodiodes (InGaAs/AlInAs SL APDs) on InP substrate to design and demonstrate an APD with low k-value. We design InGaAs/AlInAs SL APDs with three different SL periods (4 ML, 6 ML, and 8 ML) to achieve the same composition as Al0.4Ga0.07In0.53As quaternary random alloy (RA). The simulated results of an RA and the three SLs predict that the SLs have lower k-values than the RA because the electrons can readily reach their threshold energy for impact ionization while the holes experience the multiple valence minibands scattering. The shorter period of SL shows the lower k-value. To support the theoretical prediction, the designed 6 ML and 8 ML SLs are experimentally demonstrated. The 8 ML SL shows k-value of 0.22, which is lower than the k-value of the RA. The 6 ML SL exhibits even lower k-value than the 8 ML SL, indicating that the shorter period of the SL, the lower k-value as predicted. This work is a theoretical modeling and experimental demonstration of engineering avalanche characteristics in InGaAs/AlInAs SLs and would assist one to design the SLs with improved performance for various SWIR APD application.

Recently, advances in imaging and LIDAR applications have stimulated the development of high-sensitivity receivers that operate at wavelengths of ≥ 2 µm, which has driven research on avalanche photodiodes (APDs) that operate in that spectral region. High quantum efficiency is a key performance parameter for these photodetectors. Increasing the thickness of the absorption region is a straightforward approach to increase the quantum efficiency. However, the primary source of dark current is the narrow-bandgap material used for 2-µm detection. Increasing its thickness results in higher noise. In this paper, we describe two approaches to enhance the quantum efficiency, both of which are superior to a conventional anti-reflection (AR) coating. For normal incidence at 2 µm, finite-difference time-domain (FDTD) simulations show the absorption can be enhanced by more than 100% with a triangular-lattice photonic crystal, and nearly 400% by applying a metal grating. This is achieved by coupling normal incidence light into the laterally propagating modes in the device. Moreover, the significantly higher absorption of the metal grating compared to the photonic crystal is due to the high coupling efficiency provided by the metal grating. This work provides promising methods and physical understanding for enhancing the quantum efficiency for 2-µm detection without increasing absorber thickness, which also enables low dark current and high bandwidth.

Avalanche photodiodes (APDs) are used in short- and mid-wave infrared applications such as optical communication, LIDAR and 3D imaging [1] due to their internal gain, which improves the signal to noise ratio (SNR). However, the multiplication gain ( M ) gives rise to excess noise, caused by the stochastic nature of impact ionization, which can significantly degrade the SNR of APDs. The excess noise is quantitatively measured by excess noise factor, F(M) that is expressed by McIntyre’s local field theory [1] , F(M) = kM + (1-k)[2-(1/M)] where k is the ratio of the impact ionization coefficients for electrons and holes. According to the equation above, the low excess noise factor in APDs can be attained by a low k value.

An avalanche photodetector (APD) based on the AlxIn1-xAsySb1-y digital alloy materials system has recently attracted extensive attention due to its extremely low excess noise. Device defects are a critical factor limiting the performance of APDs. In this work, we use low frequency noise spectroscopy (LFNS) to characterize the property of the defects in AlxIn1-xAsySb1-y APDs grown by molecular beam epitaxy (MBE) using the digital alloy technique. Based on low frequency noise spectroscopy results carried out before and after device oxidation, two surface defects and one bulk defect have been identified, which could provide useful information for the future optimization the material growth and device fabrication processes.

We present a method of determining the background doping type in semiconductors using capacitance-voltage measurements on overetched double mesa p-i-n or n-i-p structures. Unlike Hall measurements, this method is not limited by the conductivity of the substrate. By measuring the capacitance of devices with varying top and bottom mesa sizes, we were able to conclusively determine which mesa contained the p-n junction, revealing the polarity of the intrinsic layer. This method, when demonstrated on GaSb p-i-n and n-i-p structures, determined that the material is residually doped p-type, which is well established by other sources. The method was then applied on a 10 monolayer InAs/10 monolayer AlSb superlattice, for which the doping polarity was unknown, and indicated that this material is also p-type.

We report InGaAs/InP based p-i-n photodiodes with an external quantum efficiency (EQE) above 98% from 1510 nm to 1575 nm. For surface normal photodiodes with a diameter of 80 µm, the measured 3-dB bandwidth is 3 GHz. The saturation current is 30.5 mA, with an RF output power of 9.3 dBm at a bias of −17 V at 3 GHz.

We report avalanche photodiodes (APDs) fabricated from the digital alloy Al0.8In0.2As0.23Sb0.77 (lattice-matched to GaSb). The APDs exhibit high avalanche multiplication and low excess noise.

Digital alloy Al0.85Ga0.15As0.56Sb0.44 and random alloy Al0.79In0.21As0.74Sb0.26 avalanche photodiodes with a 1-μm multiplication layer exhibit low excess noise under 543-nm laser illumination comparable to Si avalanche photodiodes. This has motivated a study of the optical characteristics of these materials. The absorption coefficients and complex refractive indices were extracted via variable-angle spectroscopic ellipsometry. Features of three semiconductor layer fitting approaches are compared, and a Kramers–Kronig-consistent basis spline function was chosen due to its flexibility and accuracy of approximating optical constants of quaternary materials. The external quantum efficiency has been calculated based on the extracted absorption coefficients and is shown to agree well with the measured external quantum efficiency.

Background doping polarity is a key parameter in the design of numerous electrical and optoelectronic devices. It is especially critical for avalanche photodiodes (APDs). Recently, high-performance APDs have been demonstrated based on the AlInAsSb digital alloy materials system. A critical element of this work was the determination of the background doping polarity of the molecular beam epitaxial grown wafers. In this work, we determine the unintentional background doping polarity of Al0.7InAsSb using the double mesa capacitance-voltage technique. We fabricated two p-i-n Al0.7InAsSb structures: one with p-type background polarity and the other with n-type. The measurements indicate that devices with different background doping polarities show different capacitance relations to the mesa diameters; moreover, the relationship reverses at high voltage in a p-type background device. Subsequent simulations reveal that this reversal is caused by electrical field confinement after the depletion reaches the smaller top mesa. These findings are consistent with reports of reduced surface leakage current in double and triple mesa structures.

We report an nBn photodetector based on the AlInAsSb digital alloy materials system, which has the advantage of a near-zero valence band offset. These photodetectors have achieved 28% external quantum efficiency, dark current densities of 2.6 × 10−3 A/cm2 at 300 K and 1.8 × 10−9 A/cm2 at 100 K with −0.5 V bias, and detectivity of 1.7 × 1010 Jones at room temperature under 2 μm wavelength illumination.

A high-sensitivity and a high-power photo-receiver is preferred in coherent lidar system. In this work, we demonstrate top-illuminated APDs, which can simultaneously exhibit low excess noise (k

We report separate absorption, charge, and multiplication (SACM) avalanche photodiodes (APDs) in the Al x In 1-x As y Sb 1-y material system for 2-μm detection. Gain, dark current, and low excess noise are demonstrated.

We demonstrate a III-V avalanche photodiode (APD) grown by heteroepitaxy on silicon. This InGaAs/InAlAs APD exhibits low dark current, gain >20, external quantum efficiency >40%, and similar low excess noise, k∼0.2, as InAlAs APDs on InP.

We report the gain, noise, and dark current characteristics of random alloy Al0.79In0.21As0.74Sb0.26 (hereafter AlInAsSb)-based avalanche photodiodes (APDs) on InP substrates. We observe, at room temperature, a low excess noise corresponding to a k value (ratio of impact ionization coefficients) of 0.018 and a dark current density of 82 μA/cm2 with a gain of 15. These performance metrics represent an order of magnitude improvement of the k-value over commercially available APDs with InAlAs and InP multiplication layers grown on InP substrates. This material is also competitive with a recently reported low noise AlAsSb on InP [Yi et al., Nat. Photonics 13, 683 (2019)], with a comparable excess noise and a room temperature dark current density almost three orders of magnitude lower at the same gain. The low excess noise and dark current of AlInAsSb make it a candidate multiplication layer for integration into a separate absorption, charge, and multiplication layer avalanche photodiode for visible to short-wavelength infrared applications.

We report on the demonstration of Al0.85Ga0.15As0.56Sb0.44 (hereafter, AlGaAsSb) avalanche photodiodes (APDs) with a 1000 nm-thick multiplication layer. Such a thick AlGaAsSb device was grown by a digital alloy technique to avoid phase separation. The current-voltage measurements under dark and illumination conditions were performed to determine gain for the AlGaAsSb APDs. The highest gain was ∼ 42, and the avalanche initiation occurred at 21.6 V. The breakdown voltage was found to be around −53 V. The measured dark current densities of bulk and surface components were 6.0 μA/cm2 and 0.23 μA/cm, respectively. These values are about two orders of magnitude lower than those for previously reported 1550 nm-thick AlAs0.56Sb0.44 APDs [Yi et al., Nat. Photonics 13, 683 (2019)]. Excess noise measurements showed that the AlGaAsSb APD has a low k of 0.01 (the ratio of electron and hole impact ionization coefficients) compared to Si APDs. The k of the 1000-nm AlGaAsSb APD is similar to that of the thick AlAsSb APDs (k ∼ 0.005) and 5–8 times lower than that of 170 nm-thick AlGaAsSb APDs (k ∼ 0.5–0.8). Increasing the thickness of the multiplication layer over 1000 nm can also reduce k further since the difference between electron and hole impact ionization coefficients becomes significant in this material system as the thickness of the multiplication layer increases. Therefore, this thick AlGaAsSb-based APD on an InP substrate shows the potential to be a high-performance multiplier that can be used with available short-wavelength infrared (SWIR) absorption layers for a SWIR APD.

We present a method of determining the background doping type in semiconductors using capacitance–voltage measurements on overetched double mesa p–i–n or n–i–p structures. Unlike Hall measurements, this method is not limited by the conductivity of the substrate. By measuring the capacitance of devices with varying top and bottom mesa sizes, we were able to conclusively determine which mesa contained the p–n junction, revealing the polarity of the intrinsic layer. This method, when demonstrated on GaSb p–i–n and n–i–p structures, concluded that the material is residually doped p-type, which is well established by other sources. The method was then applied to a 10 monolayer InAs/10 monolayer AlSb superlattice, for which the doping polarity was unknown, and indicated that this material is also p-type.

Silicon carbide Schottky diodes with thick i-regions are reported. Compared with previously reported p-i-n photodiodes, a shift of the absorption peak from 270 nm to 350 nm was observed. The responsivity curves of the Schottky diode are modeled and compared with the experimental data.Silicon carbide Schottky diodes with thick i-regions are reported. Compared with previously reported p-i-n photodiodes, a shift of the absorption peak from 270 nm to 350 nm was observed. The responsivity curves of the Schottky diode are modeled and compared with the experimental data.

We describe the molecular beam epitaxial growth, characterization, and device performance of conventional, staircase, and photoconductive avalanche photodetectors grown with AlInAsSb digital alloys. In particular, this is the first low-noise III-V avalanche photodiode alloy family and offers the band engineering flexibility necessary to achieve staircase avalanche photodiode operation.

Publisher’s Note: This paper, originally published on 5/4/2018, was replaced with a corrected/revised version on 3/7/2019. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance. Avalanche photodiodes (APDs) are a promising detector technology for light detection and ranging (LIDAR) systems needed for a variety of DoD and commercial applications. However, a new material that is sensitive to 1.55 μm and has low “excess noise” is needed to achieve the required signal-to-noise. The main issue for improving APD signal-to-noise is to reduce excess noise. Excess noise is inevitable in APDs because impact ionization must occur to obtain a high multiplication gain. One solution to reduce the excess noise is to develop a new material system with favorable impact ionization coefficients. The ratio of electron (α) and hole (β) impact ionization coefficients, defined as k value, is intrinsically defined by the material and is a dominant factor for the APD’s excess noise. In this work, we investigate InAs/AlSb type-II superlattice (T2SL) APD. The superlattices provide us with additional degrees of freedom to engineer the electronic band structure. Our work is building on previous, promising results with the quaternary system AlInAsSb. We have theoretically modeled an InAs/AlSb type II superlattice (T2SL) system that can provide flexibility to engineer the electronic band structure to achieve single carrier impact ionization and reduce the excess noise. The simulation of this T2SL predicts that InAs/AlSb has higher absorption and would work as an electron- APD with low k. We will discuss design, growth, fabrication and IV characterization of this photodiodes.

We report Al

We report temperature-dependence and temporal stability studies of Al x In 1−x As y Sb 1−y -based avalanche photodiodes (APDs). Multiplication gain and breakdown voltage of Al x In 1−x As y Sb 1−y APDs have shown low gain-temperature coefficients for a wide range of temperature.

We investigate carrier localization in the InAs/AlSb type-II superlattice (T2SL) material system using temperature- and excitation power (Iex)-dependent photoluminescence (PL). Evidence of carrier localization in T2SLs was observed by an S-shaped temperature dependence of the PL peak position. Analysis of the Iex-dependent PL at various temperatures also shows the existence of carrier localization in the T2SLs. The thermal activation energies in T2SLs were extracted to identify the nonradiative recombination mechanisms and the possible origins of localized states. We found that there are two thermal activation energies, E1 = 8.2–1.2 meV and E2 = ∼60 meV at various Iex. We interpret E1 as a thermal activation energy that comes from Anderson localization, associated with roughness due to As2 diffusion into the interfaces. This is because the extracted E1 values are comparable to the exciton binding energy of localization in various quantum structures. Carrier trapping at a state in the InSb interfacial layer (Tamm state) may account for the origin of E2. Based on previous reports, we believe that the 60 meV state might be a Tamm state if we consider thickness variations in the InSb interfacial layer for the T2SLs.

The unprecedented wide bandgap tunability (∼1 eV) of AlxIn1-xAsySb1-y lattice-matched to GaSb enables the fabrication of photodetectors over a wide range from near-infrared to mid-infrared. In this paper, the valence band-offsets in AlxIn1-xAsySb1-y with different Al compositions are analyzed by tight binding calculations and X-ray photoelectron spectroscopy measurements. The observed weak variation in valence band offsets is consistent with the lack of any minigaps in the valence band, compared to the conduction band.