Four-dimensional imaging of carrier interface dynamics in p-n junctions

Abstract

Traveling a long way past the junction Diodes are central components of modern electronic circuits. They essentially consist of two semiconductors sandwiched together, with one deficient in electrons (p), the other enriched (n). Najafi et al. used ultrafast electron microscopy to study the dynamics in a silicon diode on a time scale of trillionths of a second. They discovered that when light excites the diode's charge carriers, those carriers migrate much farther past the p-n junction than standard models would imply. The authors explain the results using a ballistic transport model. Science , this issue p. 164

Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

“…Najafi et al applied SUEM to directly visualize the photo-excitation, electron-hole separation and recombination in space and time at a silicon p-n junction [34]. As shown in Fig.…”

“…SUEM experiences the same charging issue as the normal SEM, so in principle is not suitable to study electrically insulating materials, although the environmentalmode SUEM [29] can be a potential solution. SUEM has been utilized to image ultrafast photocarrier dynamics on the surface of a wide range of materials, including crystalline semiconductors [28,30], semiconducting nanowires [31] and nanocrystals [32], amorphous semiconductors [33], semiconductor p-n junctions [34] and two-dimensional materials [35], and these applications have resulted in intriguing observations such as ballistic transport of photocarriers across a p-n junction [34], superdiffusion of photocarriers in heavily-doped semiconductors [30] and spontaneous spatial separation of electrons and holes in amorphous semiconductors [33]. Whereas there has been an abundance of recent reviews of ultrafast electron microscopy [8,9,[36][37][38], we dedicate this article specifically to SUEM, with an emphasis on the current understanding of various physical processes that contribute to the contrast images observed in SUEM from a users' perspective.…”

Super-diffusion of excited carriers in semiconductors

“…For example, pump-probe microscopy with visible light is inherently limited by diffraction, and it remains a non-routine approach in spite of recent progress3. Studies of the spatiotemporal evolution of excited carriers have recently become possible using scanning ultrafast electron microscopy (SUEM)4567, a technique that combines the spatial resolution of electron microscopy and the time resolution of ultrafast lasers.…”

“…For example, exciting a silicon p - n junction with a laser5 induces a so-called super-diffusive transport regime in which the distance covered by excited carriers over a time τ is significantly longer than the diffusion length L D =, where D 0 is the room temperature carrier diffusivity. Similarly, ultrafast laser-induced demagnetization of ferromagnetic films has been explained using super-diffusion of charge carriers and spin, but this process is still the subject of considerable research to unravel its mechanism89.…”

Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy

“…A new era of spatiotemporal imaging started with the invention of four dimensional (4D) ultrafast electron microscopy, which merged unprecedented spatial resolution with the time-resolved technique. [43][44][45][46][47][48][49][50][51][52][53][54][55] In particular, the invention of scanning ultrafast electron microscopy (S-UEM) provided the unique opportunity to selectively map charge carrier dynamics on the surface of a material with nm spatial and subpicosecond temporal resolutions. [56][57][58][59][60][61][62] In S-UEM, the optical pulse generated from a femtosecond (fs) laser system is used to generate electron packets from the tip of the scanning electron microscope, instead of the continuous electron beam used in the conventional setup.…”