Avalanche multiplication in submicron AlxGa1−xAs/GaAs multilayer structures

Abstract

A systematic study of the role of band edge discontinuities on ionization rates in periodic AlxGa1−xAs/GaAs structures has been performed by measuring the electron and hole multiplication characteristics in a series of submicron AlxGa1−xAs/GaAs multilayer p–i–n and n–i–p structures. These structures comprise alternating 500Å AlxGa1−xAs and GaAs layers in the intrinsic multiplication regions, with a total thickness of up to 0.5 μm. The results show little dependence on initiating carrier type for multiplication region widths above 0.3 μm, nor on whether they originate in GaAs or AlxGa1−xAs. Only alloy-like behavior is observed at all values of multiplication up to the breakdown voltage in contrast to earlier work on single heterojunction structures where a large difference was seen at low values of multiplication between carriers starting in GaAs and AlxGa1−xAs. The microscopic aspects of hot carrier transport in these devices were studied numerically using a simple Monte Carlo model. Simulations suggest that the energy gained from the conduction band edge discontinuity from AlxGa1−xAs to GaAs is offset by the increased energy loss via the higher phonon scattering rate in the preceding AlxGa1−xAs layer. We conclude that AlxGa1−xAs/GaAs multilayer structures offer no electron ionization enhancement.

Boundary effects on multiplication noise in thin heterostructure avalanche photodiodes: theory and experiment

“…4). Furthermore, as an approximation, we take the phonon scattering mean free path and the phonon energy from Chia et al [29] (for 45% Al concentration) as 4 nm and 32.5 meV, respectively. With these estimates at hand, we estimate the total phonon loss in the region in the vicinity of the GaAs i-layer as approximately 0.325 eV, which is approximately 17% of the initial energy when the mean gain is 20.…”

“…In particular, we do not assume any localized change in the ionization coefficients as a result of bandedge discontinuity at the layer boundary beyond what is dictated by the type of material and the electric field. In fact, a recent Monte-Carlo study on Al Ga As/GaAs multilayers showed that bandedge discontinuities in multilayer structures offer no ionization-coefficient enhancement due to carrier energy losses brought about by phonon scattering [29].…”

Numerical simulation of impact ionization in Ge/AlxGa1−xAs avalanche photodiode

“…The method relies on the generation of random numbers to determine the occurrence of random events such as the termination of a carrier's free flight, selection of a scattering event, the change in flight angle and in momentum. A simple MC simulation program similar to that reported in the literature [3][4][5] has been used to study the hot carriers transport across the Ge-Al x Ga 1−x As heterojunction. Essential fitting parameters in the program such as phonon scattering and impact ionization rates, are obtained by fitting to the experimental data for bulk Ge ͑Refs.…”

“…10,11 On the other hand, it was found that when the i-region thickness increases to above 300 nm, ␤ / ␣ reduces toward an average characteristic between the Ge and Al x Ga 1−x As because the effects of dead space becoming less significant when the device length becomes thicker. 22 Thin i-region is also expected to give lower noise due to a narrower ionization probability distribution, 23 the GaAs/ Al x Ga 1−x As APD with similar device dimension has been studied and reported in the literature 5,23,24 and has been shown to exhibit low excess noise. In the Ge/ Al x Ga 1−x As APD, the greater suppression of the ionization of opposite carrier as compared to the GaAs/ Al x Ga 1−x As structure could result in an even lower excess noise.…”