Effects of p doping on GaAs/AlGaAs SAM-APDs for X-rays detection

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.

Synchrotron Radiation Study of Gain, Noise, and Collection Efficiency of GaAs SAM-APDs with Staircase Structure

“…In the past, in-depth studies have been carried out on the behaviours of these devices regarding the doping levels of the various layers, the number of multiplication steps, and the fundamental role of the δ p-doped layer [35][36][37][38]. Thanks to these studies, it was found that a δ p-doping layer of carbon atoms of 2.5 × 10 12 cm −2 is necessary to keep the absorption region unbiased over the whole range of reverse biases, up to the breakdown voltage.…”

“…A possible explanation for this behaviour could be ascribed to small uncontrolled fluctuations of the C atom density in the δ p-doped layer from batch to batch. In this regard, it must be pointed out that we are working in a region where δ p-doped C layers are highly compensated [36], and a small excess of deposited C atoms above the optimal dose might increase defect formation due to atomic pairing [60], consequently decreasing the efficiency of the device.…”

Experimental Characterization of Separate Absorption–Multiplication GaAs Staircase Avalanche Photodiodes under Continuous Laser Light Reveals Periodic Oscillations at High Gains

“…As a result, the multiplication process has a higher associated noise, which can significantly degrade the signal-to-noise ratio [5]. Over the years, these devices have seen some significant evolutions to reduce the multiplicative noise via the separation of the "absorption and photon-electron conversion" region from the "multiplication" region (separate absorption-multiplication APDs, SAM-APDs), and the band-gap engineering in the multiplication region [6][7][8].…”