On October 18, 2022, Beijing time, Professor Liao’s team at the University of California, Santa Barbara (UCSB) and the team of Professor Ren Zhifeng of the University of Houston published an article entitled “Persistent hot carrier diffusion in boron arsenide single crystals imaged by” in the journal Matter Ultrafast Electron Microscopy”.
This paper reports the direct imaging of photogenerated carriers in cubic boron arsenide single crystals using ultrafast scanning electron microscopy. This study found a hot carrier transport time of more than 200 picoseconds in boron arsenide single crystal due to its unique phonon structure. This value far exceeds that of monocrystalline silicon, further indicating the promise of cubic boron arsenide in devices based on hot carrier transport, such as hot carrier photovoltaics and light detectors.
The corresponding authors of the paper are Liao Huilin and Ren Zhifeng; First authors are Usama Choudhry and Fengjiao Pan.
Cubic boron arsenide is a group III-V semiconductor material that was first synthesized in the late 1950s. Due to the difficulty of preparing high-quality single crystals, the physical properties of cubic boron arsenide have been studied very limited in the past few decades. In 2013, Lindsay and Broido et al. used first-principles calculations to predict that cubic boron arsenide might have ultra-high thermal conductivity (over 2000 W/mK at room temperature), a value close to or even exceeding the best known thermal conductivity material diamond at room temperature. This unexpected nature is due to the unique phonon structure of cubic boron arsenide: the significant mass difference between boron atoms and arsenic atoms leads to a large frequency gap between the acoustic branch of its phonon and the optical branch, resulting in the three-phonon scattering process in which some acoustic phonons and optical phonons participate cannot occur, so that the thermally conductive acoustic phonons have a longer relaxation time. Tianli Feng and Xiulin Ruan of Purdue University then introduced the calculation of quadruple phonon scattering, correcting the predicted thermal conductivity value to 1300 W/mK. This room temperature thermal conductivity is still about 10 times that of monocrystalline silicon, indicating that monocrystalline cubic boron arsenide is a promising base material for heat dissipation of electronic devices. In the following years, Professor Ren Zhifeng’s team at the University of Houston developed and perfected a method for preparing high-quality cubic boron arsenide single crystals, and in 2018, he and his collaborators jointly reported the measured room temperature thermal conductivity of cubic boron arsenide single crystals in the experiment of more than 1000 W/mK, which is consistent with the results of other experimental groups, making an important breakthrough in the development of semiconductor heat dissipation materials. At the same time, Dr. Liu Dehuan of Professor Chen Gang of MIT (now assistant professor at Huazhong University of Science and Technology) and others used first-principles calculations to predict that cubic boron arsenide has high electron and hole mobility at the same time. In June 2022, Professor Ren Zhifeng’s group, Professor Chen Gang’s group at the Massachusetts Institute of Technology, Professor Jiming Bao’s group at the University of Houston, Professor Xinfeng Liu group of the Chinese Academy of Sciences and other collaborators jointly published an article in the journal Science to report the ultra-high bipolar carrier mobility (1600 cm2/Vs) of cubic boron arsenide measured in the experiment, which is much higher than the electron (1000 cm2/Vs) and hole (450 cm2/Vs) mobility in monocrystalline silicon. These findings show that single crystal boron arsenide has both ultra-high thermal conductivity and carrier mobility, indicating that it can not only be used as a heat dissipation substrate, but may even replace single crystal silicon as a new semiconductor core material.
The unique phonon structure of cubic boron arsenide not only leads to its ultra-high thermal conductivity, but also may give it a very long heat carrier transport time. In general semiconductor materials, electrons and holes excited by high-energy photons may have very high temperatures (thousands to tens of thousands of K) just after being excited. This part of the carriers is called photogenerated thermal carriers. Due to the interaction of electrons and phonons, these photogenerated heat carriers are generally reduced to room temperature by emitting phonons in a very short time (picosecond scale), and the extra energy they have is lost as thermal energy in the process, which cannot be used by ordinary photovoltaic or photodetector devices. If this additional energy can be collected and utilized before the photogenerated thermal carriers are cooled, theoretical analysis shows that the theoretical efficiency limit of photovoltaic solar cells can be increased from about 30% to about 65%. In polarized semiconductors, general hot carriers will first transfer their energy to polarimetric optical phonons, and then these optical phonons will transfer energy to low-frequency acoustic phonons, gradually reaching thermal equilibrium with the environment. In cubic boron arsenide, this cooling process is significantly delayed due to the large frequency gap between optical phonons and acoustic phonons, resulting in the temperature of photogenerated heat carriers not dropping rapidly. This process is known as the “hot phonon bottleneck.” Sadasivam et al. theoretically predicted in 2017 the important effect of this effect on the hot carrier cooling process in cubic boron arsenide. If this effect can be observed in experiments, cubic boron arsenide will be confirmed to be a “magic” semiconductor with ultra-high thermal conductivity, ultra-high carrier mobility and ultra-long thermal carrier transport time, and has extremely high application prospects in the future microelectronics, optoelectronics and photovoltaics.
In this latest Matter paper, Professor Liao Chunlin’s team at UCSB directly photographed the photogenerated heat carrier transport process in cubic boron arsenide single crystal prepared by Professor Ren Zhifeng’s team using ultrafast scanning electron microscopy (SUEM). SUEM was invented in 2010 by the late Professor Ahmed Zewail, a Nobel Prize winner in chemistry at Caltech, an imaging technique that can combine the nanoscale spatial resolution of general scanning electron microscopy with the femtosecond temporal resolution of ultrafast lasers. In layman’s terms, SUEM can shoot “movies” that occur during microscopic transport at the nanoscale using extremely fast “shutters”. Liao’s team has further developed this technology at UCSB, and UCSB’s SUEM device is currently the only one in operation at an American university (the group of Professor Fu Xuewen of Nankai University in China has also developed this technology). Figure 1 shows a schematic of the basic principle of SUEM and the appearance of boron arsenide single crystals tested in the experiment and other crystal quality test results.
Figure 1: SUEM device schematic and sample topography, XRD, EDS, and Raman test data.
Figure 2A shows a time-resolved SEM image taken by SUEM, allowing direct observation of the photogenerated carrier distribution over time and space over time and space. The white brightness contrast in the image reflects the distribution of photogenerated carriers at a given delay time point after light excitation. It can be seen from the image that within a few hundred picoseconds after light excitation, photogenerated carriers spread rapidly across the sample surface, and the diffusion range is much larger than the spot size used for excitation. The rate of diffusion then gradually decreases and stabilizes. This rapid diffusion process is the hot carrier transport process caused by the initial high temperature of photogenerated carriers. These SUEM images visually show that hot carrier transport in cubic boron arsenide lasts hundreds of picoseconds. Further quantitative analysis uses SUEM images taken by a two-dimensional Gaussian distribution fit (Figure 2B) to assess the transport time of hot carriers.
Figure 2: A: Time-resolved image of the hot carrier distribution taken by SUEM; B: Quantitative fitting of experimental images using a two-dimensional Gaussian distribution.
Figure 3 shows the quantitative analysis of SUEM images of two different samples: the spatial distribution radius of photogenerated carriers varies with delay time. Quantitative results clearly show the rapid diffusion and gradual stabilization of photogenerated heat carriers in the initial stage. Through theoretical model fitting, SUEM data show that the average heat carrier transport time on the surface of cubic boron arsenide exceeds 200 picoseconds, which is about 3 times that of crystalline silicon. Results measured at different positions on different sample surfaces show that the hot carrier transport time of cubic boron arsenide may be longer under ideal conditions (without impurities).
Figure 3: Photogenerated carrier distribution radius over time.
Figure 4 shows SUEM data at higher light excitation intensities. Higher photoexcitation intensity results in a higher concentration of electron-hole pairs being excited, while more mobile holes are the first to diffuse from the excited region, resulting in a SUEM image of a unique luminous contrast around the center of the peripheral dark contrast. From the changes in the spatial and temporal distribution of dark contrast and bright contrast, the hot carrier transport time of electrons and holes can be extracted respectively, which are on the order of hundreds of picoseconds.
Figure 4: SUEM image at higher light excitation intensity.
This study experimentally verifies the ultra-long heat carrier transport time caused by the thermophonon bottleneck effect in cubic boron arsenide, and proves the superior thermal and photoelectric transport properties of cubic boron arsenide due to its unique phonon structure, indicating its important application prospects as a new generation of semiconductor materials in the fields of microelectricity, photovoltaics and photovoltaics. The study also demonstrates SUEM’s ability as a newly developed measurement technique to study microscopic transport processes in new materials. (Source: Web of Science)
Related Paper Information:https://doi.org/10.1016/j.matt.2022.09.029