Imaging Localized Energy States in Silicon-Doped InGaN Nanowires Using 4D Electron Microscopy

Abstract

Introducing dopants into InGaN NWs is known to significantly improve their device performances through a variety of mechanisms. However, to further optimize device operation under the influence of large specific surfaces, a thorough knowledge of ultrafast dynamical processes at the surface and interface of these NWs is imperative. Here, we describe the development of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) as an extremely surface-sensitive method to directly visualize in space and time the enormous impact of silicon doping on the surface-carrier dynamics of InGaN NWs. Two time regime dynamics are identified for the first time in a 4D S-UEM experiment: an early time behavior (within 200 picoseconds) associated with the deferred evolution of secondary electrons due to the presence of localized trap states that decrease the electron escape rate and a longer timescale behavior (several ns) marked by accelerated charge carrier recombination. The results are further corroborated by conductivity studies carried out in dark and under illumination.The dynamics of charge carriers at material surfaces and interfaces not only plays a pivotal role in controlling the applicability of nanoscale materials to optoelectronic and microelectronic devices, but is also one of the major challenges in the development of these practical energy technologies. However, real-space and real-time mapping of charge carrier dynamics selectively at material surfaces is beyond the reach of conventional time-resolved spectroscopic or static microscopic methods and has only been enabled by the recent development of four-dimensional scanning ultrafast electron microscopy (4D S-UEM). 1-10 This technique relies on using photoelectron packets to probe the secondary electrons (SEs) generated from the first few nanometers of the top surface of the material as a result of the interaction of the electron beam with the surface of the photoactive material, which is directly correlated with the excited state charge carrier population and hence the temporal behavior of SEs reflects the charge carrier dynamics, thus opening up a new dimension in imaging techniques: observing the change in charge carrier dynamics explicitly at the surface of the sample in real space and time.Several approaches have been developed to manipulate charge carrier dynamics in order to enhance the device functionality of photoactive materials; doping is one of the most effective ways among them. [11][12][13][14][15][16][17][18][19][20][21][22][23][24] In general, insertion of impurity species leads to the formation of additional levels in the bandgap of the host material that not only provide additional relaxation routes for the charge carriers, but may also alter the density of states and the band structure, resulting in significant changes in both the optical and electronic properties of the host material.For example, group III nitride semiconductors, in particular ternary InGaN nanowires (NWs), have gained much attention because of their widespread applicat...

Access to Ultrafast Surface and Interface Carrier Dynamics Simultaneously in Space and Time

“…Meanwhile, a higher concentration of trapping states can also quicken the subsequent electron–hole recombination after the initial photoexcited charge carrier separation, revealing the reason why the decay rates for doped sample with Si at 1200 °C is larger than both doped sample at 1150 °C and undoped sample in the longer time regime (Figure e). Moreover, these results were corroborated by numerical simulations and conductivity studies with/without light illumination and deepened the understanding of how Si doping affects the charge carrier dynamics and performance of InGaN/GaN nanowire-based optoelectronic devices …”

“…Therefore, the enhanced PCE can be reasonably attributed to the lower density of the surface states after treatment, which suppresses nonradiative recombination channels . Moreover, S-UEM is also capable of exploring how Si doping affects the photophysics of InGaN/GaN nanowires (Figure e,f) . Interestingly, it was demonstrated that, with Si doping, the specimen exhibits a faster charge carrier recombination in a longer time regime and a slower charge separation at a shorter time scale, compared to that with the undoped specimen.…”

Transition from Positive to Negative Photoconductance in Doped Hybrid Perovskite Semiconductors

“…[12] Very recently, NPC was reported for Si-doped InGaN nanowires, which was attributed to carrier-carrier scattering owing to high carrier concentration. [42] Since the carrier generation and recombination are energetic processes, [43] one expects the results could have dependence on the illumination conditions and measurement temperature.…”

Visualization of Charge Carrier Trapping in Silicon at the Atomic Surface Level Using Four-Dimensional Electron Imaging

“…Four-dimensional scanning ultrafast electron microscopy (4D S-UEM) is the sole technique capable of the surfaceselective visualization of the light-triggered carrier dynamics at the angstrom-nanometer scale. [48][49][50][51][52][53][54][55] 4D S-UEM uses an electron probing system to scan the top surface of the photoactive materials after being pumped with a photon pulse. As a result, secondary electrons are ejected from the top few nanometers (< 5 nm) and can be detected in real space and time.…”

“…However, the large penetration depth of light, especially in Si (∼1330 nm at a light wavelength of 515 nm), hinders the selectivity of discerning the native oxide layer that has at most a 2 nm thickness without the interference of the bulk properties when using a photon pump in the photon probe configuration. Four-dimensional scanning ultrafast electron microscopy (4D S-UEM) is the sole technique capable of the surface-selective visualization of the light-triggered carrier dynamics at the angstrom–nanometer scale. − 4D S-UEM uses an electron probing system to scan the top surface of the photoactive materials after being pumped with a photon pulse. As a result, secondary electrons are ejected from the top few nanometers (<5 nm) and can be detected in real space and time.…”