Ultrafast spectroscopy of impact ionization and avalanche multiplication in GaAs

Abstract

Femtosecond carrier dynamics in biased AlxGa1−xAs heterostructure diodes is investigated tracing transient modifications of the Franz-Keldysh absorption spectrum. The nonlinear optical response is sensitive to the number of electron-hole pairs in the high-field region of the sample. As a result, the dynamical buildup of a nonequilibrium carrier avalanche due to impact ionization for electric fields F⩾350kV∕cm is directly analyzed in the time domain. The time scale of the carrier multiplication is found to be in the order of 10ps depending on the number of photoinjected carriers. Monte Carlo simulations in a simplified band structure agree well with the experiment.

Impact ionization dynamics in silicon by MV/cm THz fields

“…So far, experimental studies on IMI have mainly been based on transport measurements made on doped p-n junction Si devices with a quasistatic electric field bias [13][14][15]. Ultrafast optical pump-probe studies of IMI in a different material system (GaAs p-i-n heterostructure) has been performed [16], with spatiotemporal dynamics of the IMI process studied at low (10 K) temperature, using external fields below the static breakdown limit of 450 kV cm −1 , and with the initial carrier concentration defined by optical excitation. Thus, previous investigations have been limited to using electric field strengths in the sub-MV/cm range OPEN ACCESS RECEIVED…”

Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

“…Combining our new experimental and theoretical efforts with the findings of ref. 20, we can quantitatively account for carrier multiplication in both bulk and quantum dots with impact ionization, without invoking mechanisms such as the coherent superposition of single-and multi-exciton wavefunctions 4 or the occurrence of virtual single-exciton states 5 . Furthermore, it is evident that carrier multiplication is more efficient in bulk than in quantum dots.…”

Competition between isotropic and strongly anisotropic terms in the impact ionization rate of narrow- and middle-gap cubic semiconductors

“…Since in practice the output characteristics arising from impact ionization depend on many details, including specific band structure of a given material and characteristics of the relaxation processes giving a particular form of the non-equilibrium distribution function, the problem of calculation, say, of the I-V characteristics, starting directly from a band structure model is in no sense easy, from both conceptual and technical points of view. Therefore, the most popular method for modelling is Monte Carlo [3][4][5][6][7][8][9][10][11][12][13] , but it is obviously dependent on a particular relation between the impact ionization rate and the a) Electronic mail: afanasiev.an@mail.ru energy of a hot electron initiating the process, W(E). Phenomenologically, the rate must grow like a power of the excess energy above a threshold,…”

“…Analytic expression for the cubic term can be found in the only paper by Gelmont et al 21 , but without any derivation. Thus, a proper analytic answer for W(E) has been inaccessible to the specialists in Monte Carlo modelling, so they prefer using some arbitrary values of the power n (and prefactor C) such as n = 2.5 and n = 4.3 22 , n = 5.2 3 , n = 3 4,6,23 , n = 3.9 24 , n = 1.85 25 . Some theoretical studies were focused on giving efficient recipes for the proper choice and numerical solution of the band models suitable for the realistic modelling 26,27 , but incorporation of the band calculations into Monte Carlo modelling seems too complicated to be practical.…”