This summer, I successfully enrolled in the UGVR program hosted by the Stanford School of
Engineering for top Chinese undergraduate students, conducting research at Stanford University for two
months. On the final day, when my poster presentation “Simulation of AlGaN/GaN High Electron
Mobility Transistors” was highly praised by professors and graduate students from various departments
at Stanford, I felt immense satisfaction and gratification. I suddenly recalled my rewarding journey into
Applied Physics, starting with my success in the classroom and culminating in my research projects in
physics, materials and devices. This incredible journey reinforced my determination to pursue a Ph.D. in
Applied Physics, especially in three areas: (1) advanced characterization techniques, e.g. scanning probe
microscopy (SPM); (2) physics, materials and devices based on ferromagnetism materials (like LaCoO3,
LaFeO3) as well as III-V materials (like GaN, AlN, GaAs, InAs); (3) nanoelectromechanical systems
(NEMS) and microelectromechanical system (MEMS).
My excellent academic performance has proved to be crucial for my research. My performance in the
National Physics Olympiad earned me admission to University of Science and Technology a year ahead
of my peers. After three years’ study in the Department of Applied Physics, my accumulated major GPA
ranks among the top 5% of 257 students. Moreover, last semester I received an A in almost all of my
major courses, including Solid State Physics and Statistical Physics.
“Independence made me mature in research.” My first step into the depths of material science resulted
from my independent work in Professor Qingyou Lu’s group, centered on building a novel scanning tunnel
microscope (STM). The bottleneck was on the piezoelectric motor – the core part of STM – therefore it
was difficult to achieve high compactness, simplicity, reliability, and large output forces simultaneously.
In my work, I proposed a new piezoelectric motor (PM) design that has all the advantages above and
successfully implemented a whole set of STM. Firstly, I summarized all the drawbacks of the former PMs.
Then, I designed a PM that could reduce the frictional force when the sliding part is moving and increase
the frictional force when the sliding part is held. Additionally, I built the simulation model for the PM and
simulated its stepping process in COMSOL. With the topology optimized through simulation, I fabricated
the PM after the process of diamond wire cutting, high-precision grinding, epoxy bonding, wire leading,
annealing, and high-voltage testing. During testing, we found that the step size of our new PM increased
by more than 400% and the threshold voltage decreased by more than 50 V compared to a traditional
inertial motor. These improvements are invaluable for STM application to harsh environments. I also
acquired clear atomic resolution images of highly oriented pyrolytic graphite (HOPG) with a tip directly
carried by this PM despite conditions of large vibrations and large noise. To summarize my work, I
submitted a first-author paper to the Review of Scientific Instruments, a top journal on instrument
innovation, and filed a patent in China.
“A broad vision will help me build innovation.” My second step into the world of applied physics
began with an interesting material – LaCoO3. The strain and oxygen defects have attracted a great deal of
attention recently because of their correlation to the ferromagnetism (FM) origins. To develop a
comprehensive understanding of this area, I read several well-known textbooks about FM material,
including Physics of Ferromagnetism by Chikazumi Soshin and Introduction to the Theory of
Ferromagnetism by Aharoni Amikam and A. Arrott. I found that there still exists an important but widely
ignored issue: why was the saturated magnetic moment (1 uB/Co) of LaCoO3 only half of the calculated
value (2 uB/Co) or one quarter of total potential values? I realized that the reasons are directly correlated
to the FM origins, and a main obstacle for this study is the lack of a nanoscale magnetic characterizing
method. To overcome this issue, I proposed the idea of applying a magnetic force microscope (MFM) to investigate the magnetic domains of LaCoO3 films. While scanning, I found that crosshatch-like magnetic
domain patterns were only related to the distorted crystal structure of thick films. On the thinner films,
however, a totally different pattern was obtained with none line (without crosshatch) features observed.
To understand the experimental results, I first analyzed the sample topography and measured the sample
images during warming and cooling process across its curie temperature TC = 85 K with a magnetic field
of 0.3 T and 2.3 T. Then, I unveiled the comprehensive influence of different thicknesses on the samples’
surface magnetic domain pattern and excluded many controversial ferromagnetic origins in LaCoO3. In
summation, I not only contributed to a paper submitted to Nano Research, but also learned how to explore
the fundamental physics behind the novel material systems.
“Be courageous enough to step out of the comfort zone.” In Professor Senesky’s group at Stanford
University, I departed from the world of physics and materials, and instead turned to microelectronic
devices. In the summer, I independently built a simulation model from scratch for the AlGaN/GaN High
Electron Mobility Transistors (HEMTs). The surface traps in AlGaN/GaN HEMTs, which is the origin of
two-dimensional-electron-gas (2DEG), have an unclarified energy level distribution. Therefore, physicsbased
simulation is a powerful tool to understand the origins of every charge and trap in this device system.
In my study, I created a powerful model with more than 20 adjustable parameters, including dimensions,
electron/hole features, substrate material type, as well as coefficients of physical fields and their couplings,
etc., to describe the behavior of the AlGaN/GaN HEMT. However, many of the parameters are uncertain
and some even vary from device to device. To overcome these challenges, I proposed a strategy to correct
the parameters, by solving the value for each parameter through a process combining multivariate linear
regression, gradient decent algorithm and pivoting algorithm referring to the experimental data. With this
strategy, I successfully achieved an agreement between my simulation and experimental results with less
than 5% difference. In addition, I decided to use my simulation model to optimize device designs. The
former device in our lab was found to have low on-off ratio due to structure defects. With simulation, I
found the origin of leakage current and optimized the device design with my simulation. I then participated
in the fabrication of the new devices in the cleanroom. The on-off ratio has now been increased by 3 orders
of magnitude in the newly fabricated devices. My simulation work and codes have been published on
nanohubs (https://nanohub.org/projects/alganganinxlab). Moreover, we are also applying this model to
magnetic field sensors and I am preparing a first-author conference paper for Hilton Head.
I envision the Ph.D. program in Engineering and Applied Science, Electrical Engineering subfield at
Yale University as an ideal platform to pursue my dream of becoming a leading scientist and engineer. As
an undergraduate, each aspect in the numerous and splendid EE ignites my desire for knowledge and my
passion for research. Nevertheless, the research on photonic and nanomechanical devices such as the
recent work of integrated optomechanical single-photon frequency shifter, and real-time sensing devices
on integrated microfluidic platform, directed by Professor Hong Tang greatly sparks my interest. Professor
Jung Han’s research on wide-bandgap semiconductor materials, optoelectronic and microelectronic
devices, as well as nanoscale materials and devices also attract me. Furthermore, the work of Professor
Tso-Ping Ma on MIS devices based on SiC, GaN, GaP, SiGe, and InAs, ferroelectric thin films for memory
technology, and the potential commercial realization appeal to me.
I believe my independent thinking and research capabilities have qualified me for the requirements of
Northwestern University. My constant reflection on life enables me to be a future excellent scientist in the
field of Applied Physics, whether in academic institutions or corporate research and development
departments. I hope you find my diverse experiences and solid foundations in Applied Physics will allow
me succeed in your program.