An ensemble Monte Carlo framework is used to compare the impact ionization behavior important to avalanche photodiode (APD) performance in a band-engineered InAlAs/InAsSb type-II superlattice with same-energy gap bulk InAs and HgCdTe at 250 K. Impact ionization rates are computed directly from the electronic band structures. The same stochastic transport kernel is used for each material for consistency. A realistic treatment of impact ionization initial and final carrier states is employed in the transport simulations that considers energy and crystal momentum conservation. The major effects of band features on carrier states, transit path lengths between impact ionization events, and impact ionization coefficients support the role of band engineering in materials selection for high-performance APDs.
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.
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 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.
Avalanche Photodiodes (APDs) that target a wavelength of 1550 nm, have several applications ranging from optical communications to imaging to single photon detection. The DE-JTO will use an array of APDs to image the wavefront of its 1550 nm laser. The distinctive feature of an APD is high sensitivity due to the gain achieved by impact ionization of carriers. Because impact ionization is a stochastic process, it introduces excess noise that limits the signal to noise ratio of an APD. However, the excess noise may be reduced by engineering the k (=β/α) value of the device, where β and α are the impact ionization coefficients of holes and electrons, respectively. k can be engineered by band diagram engineering [1], band structure engineering [2], or dead space effect [3,4]. Combinations of these are also used [5]. Band diagram engineering enables the implementation of Capasso’s channeling APD [1]. In this design, electrons and holes are spatially separated in different channels with distinct materials and bandgaps. These channel materials are designed to minimize the impact ionization of one carrier and promote the other, thereby optimizing the k and excess noise. The two limitations of the Capasso design are (1) the leakage current due to doping the channels and (2) excess noise due to dual carrier injection. Firstly, to spatially separate the carriers between narrow and wide bandgap materials, type I band alignment with doping is suggested by Capasso. However, type II band alignment, due to the valance band offset, may inherently provide the field required for the spatial separation of carriers. And type II alignment avoids the doping that could lead to leakage currents. Secondly, channeling APD is a planar configuration leading to dual carrier injection that increases the excess noise. Using a window injection layer defined by lithography, a channeling APD with single carrier injection is designed.
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.
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