Gain and noise in GaAs/AlGaAs avalanche photodiodes with thin multiplication regions

Abstract

Avalanche photodiodes based on GaAs/AlGaAs with separated absorption and multiplication regions (SAM-APDs) will be discussed in terms of capacitance, response to light (gain and noise) and time response. The structures have been fabricated by molecular beam epitaxy introducing a p layer doped with carbon to separate the multiplication and the absorption regions. The thickness of the latter layer defines the detection efficiency and the time resolution of the structure, which in turn allows tailoring the device for specific scientific applications. Within the multiplication region a periodic modulation of the bandgap is obtained by growing alternating nanometric layers of AlGaAs and GaAs with increasing Al content; this staircase structure enables the tuning of the bandgap and subsequently provides a well-defined charge multiplication. The use of such staircase hetero-junctions enhances electron multiplication and conversely reduces-at least in principle-the impact of the noise associated to hole multiplication, which should result in a decreased overall noise, when compared to p-in diodes composed by a single material. The first part of this paper focuses on the electrical characteristics of the grown structure and on the comparison with the simulated behaviour of such devices. In addition, gain and noise measurements, which have been carried out on these devices by utilizing photons from visible light to hard X-rays, will be discussed and will be compared to the results of a nonlocal history-dependent model specifically developed for staircase APDs.

Accurate Nonlocal Impact Ionization Models for Conventional and Staircase Avalanche Photodiodes Derived by Full Band Monte Carlo Transport Simulations

“…The simulated devices feature d = 100 nm and E C = 0, 0.3 eV, 0.5 eV (these values are consistent with the conduction band step at a Al x Ga 1−x As/GaAs heterojunction in a staircase APD [14], [24], where E C 0.355 eV for x = 0.45 [25], [26]. Moreover, we have always applied an electric field along the x direction, E x = 400, 500 and 600 kV/cm, to ensure that all the carriers exit from the simulation domain.…”

“…In this Section, we show how the results for the electron's and hole's impact ionization coefficients, α(x|x ) and β(x|x ), obtained with FBMC simulations have been used to improve the NL-HD impact ionization model described in [9]. Our final goal is to model the impact ionization coefficients in the presence of a conduction band discontinuity and for low applied electric fields, so as to evaluate the performance of a single step AlGaAs/GaAs-based staircase APDs [14], [24]. recall that, in the NL-HD model of [9], the position dependent impact ionization coefficients are expressed as…”

“…Moreover, we have always applied an electric field along the x direction, E x = 400, 500 and 600 kV/cm, to ensure that all the carriers exit from the simulation domain. For all the simulations, since in GaAs/AlGaAs staircase APDs as the ones in [14] and [24] impact ionization mostly takes place in GaAs layers (due to its lower band gap w.r.t. AlGaAs), parameters for GaAs have been used.…”

Experimental and simulation analysis of carrier lifetimes in GaAs/AlGaAs Avalanche Photo-Diodes

“…Next generation X-rays detectors require higher count rate and higher quantum efficiency for medium and high photon energy. Avalanche photo-diodes (APDs) in III-V compounds are very promising with respect to silicon ones thanks to the higher atomic number of the material that allows for thinner detectors and to the larger band-gap that limits the leakage current [1], [2]. However, the APD development and optimization requires an in-depth understanding of the physical mechanisms involved in the device operation [3].…”

Modeling Approaches for Gain, Noise and Time Response of Avalanche Photodiodes for X-Rays Detection

“…We have in fact verified that trials corresponding to photons absorbed in the same position produces waveforms that are very similar in shape and amplitude. Time responses for APDs as those described in Section 3 have been measured in [41]. A 1:1 comparison with the simulation in Figure 12 is not possible due to the effects of the read-out parasitics and the fact that the fabrication process was not as mature as for the results shown in Figure 11.…”