Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy

Abstract

Charge carrier dynamics in amorphous semiconductors has been a topic of intense research that has been propelled by modern applications in thin-film solar cells, transistors and optical sensors. Charge transport in these materials differs fundamentally from that in crystalline semiconductors owing to the lack of long-range order and high defect density. Despite the existence of well-established experimental techniques such as photoconductivity time-of-flight and ultrafast optical measurements, many aspects of the dynamics of photo-excited charge carriers in amorphous semiconductors remain poorly understood. Here, we demonstrate direct imaging of carrier dynamics in space and time after photo-excitation in hydrogenated amorphous silicon (a-Si:H) by scanning ultrafast electron microscopy (SUEM). We observe an unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron-hole separation and charge trapping induced by the atomic disorder. Our findings demonstrate the rich dynamics of hot carrier transport in amorphous semiconductors that can be revealed by direct imaging based on SUEM.

Pulling apart photoexcited electrons by photoinducing an in-plane surface electric field

“…The spatial and temporal dynamics of charged particles at the interfaces of materials is of vital consequence to several modern technologies, such as light-harvesting and semiconductor devices. For example, the mobility of carriers ( 1 ) and the underlying nature of diffusion ( 2 ) raise important questions relevant to semiconductor device technology. In the case of photocatalysis, where light energy is converted to chemical energy at the surface of a semiconductor, spatiotemporal dynamics of the photocarriers directly affects surface chemical reactions ( 3 , 4 ).…”

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

“…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.…”

Fast Reversible Phase Change Silicon for Visible Active Photonics

“…Both thermal and carrier effects induced by optical excitation can modify the refractive index. [ 54 ] We observe the reflection decay in Figure 3C,D is much slower than the typical carrier decay (1 ns) [ 55 ] but follows well the predicted temperature transient. Studies indicated the inverted position of the isentropic band gaps from c‐Si and a‐Si lead to positive and negative temperature coefficients (d n /d T ) [ 56 ] in 633 nm.…”