Advanced Semiconductor Laboratory
Developing cutting-edge technologies based on III-nitride semiconductors

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Category Archives: Discussion

Double-heterojunction Nanorods and Light-sensitive LEDs

Mohsen Alowayed - July 26, 2017

One of the recent developments in optoelectronic devices is the so called double-heterojunction nanorod (DHNR).​

Optimization of nanowire array structure to enhance light extraction efficiency

Fatimah Alowa - July 26, 2017

Light extraction efficiency of conventional UV LED nanowires (NWs) remains very low​

Parity-time time symmetry in photonics: part 1

Ronghui Lin - July 14, 2017

One of the most fundamental axioms is that the Hamiltonian H of a system, which specifies the energy levels and time evolution of the system, is Hermitian.​

AlN:Be

Kuang-Hui Li - June 26, 2017

Could we replace Mg by Be as p-dopant for AlN?

CdTe solar cells

Carlos G Torres - June 26, 2017

My previous research back in Mexico focused on the development of low-cost Cadmium Telluride (CdTe) solar cells. They is considered as thin film technology because the active layers are just a few microns thick. I want to show you a quick overlook of this II-VI device technology.

Rectifying behavior realized without p-n junction

Wenzhe Guo - June 26, 2017

Rectifier is used to convert the alternating current (AC) to direct current (DC), due to rectifying behavior that current increases sharply in the condition of forward bias, whereas little current flows in the reverse bias, as shown in Figure 1[1]. At present, solid-state electronic devices with rectifying behavior are mostly achieved by the incorporation of conventional p-n junction that requires doping technology. However, without doping technology to form p-n diode, the rectifying behavior still could come out of electronic devices, like magnetic tunnel junction, molecule diode.

Doping of AlN and its alloy

Kuang-Hui Li - April 26, 2017

Doping of semiconductor is very crucial since pure semiconductor material couldn’t make into a device. Doping of nitride material is not trivial like other III-V material, and it is not fully understood yet. AlN is the hardest compared to GaN and InN.

Truncated cones or cylinders?

Yangrui Hu - April 26, 2017

​When I did simulations about deep ultraviolet LED, I found that people always use cylinders to simulate nanowires. However, nanowires are not usually cylinders but truncated cones in the experiment, which means the radius of the nanowire becomes larger as growing. So I want to study the case with truncated cones. As the following figure shown, the bottom radius and the height are fixed and theta becomes one of the key parameters to describe the nanowire.

Multiresonator circuits and RFID chips

Nasir Alfaraj - April 25, 2017

​A multiresonating circuit generates a unique spectral signature in response to an interrogation signal sent by an external tag reader (interrogator). A spectral signature is a collection of reflected and attenuated interrogation signals at varying frequencies that collectively define a spectral ID. Spiral resonators are designed to resonate at a particular frequency to create a stopband. When the signal is attenuated at a particular frequency, it signifies a logic “0”; otherwise, it is logic “1”. Furthermore, the stopbands introduce “phase ripples” to the interrogation signal at their resonant frequencies, which can be identified as a phase variation at the interrogator end.

Playing guitar with photons (part 2): Optomechanical method

Ronghui Lin - April 24, 2017

Another method to change the frequency of a photon is by optomechanical devices. The concept that wave will change its frequency when reflected by a moving object is widely known as the Doppler Effect. ​​​​

Hexagonal Boron Nitride, direct bandgap or indirect bandgap?

Feng Wu - February 27, 2017

Hexagonal Boron Nitride, direct bandgap or indirect bandgap?
Hexagonal boron nitride has attracted a lot of attentions because of its unique properties, such as a wide bandgap, low dielectric constant, high thermal conductivity, and chemical inertness. In spite of this rising interest in hBN and the large number of studies devoted to this material, with its seemingly simple crystal structure, the very basic question of the nature of its bandgap remains controversial, that is whether it is a direct bandgap or indirect bandgap.

III-Nitride Based High Electron Mobility Transistor

Wenzhe Guo - February 26, 2017

In 1956, three physicists were awarded the Nobel Prize for the discovery of transistor effect. Transistors have contributed to the development and improvement of integrated circuits and hence the extraordinary progress of information technology. Conventional transistors are realized based on the n-doped or p-doped Si material, which are limited in the high-frequency and high-power application. 

Playing guitar with photons: part 1: nonlinear optics conversion

Ronghui Lin - February 25, 2017

The conversion of photon frequency is an important function in quantum communication and quantum computation. Yet it is nontrivial because the frequency of a photon is related to the energy, thus to change the frequency of a photon, additional energy input is required. ​ 

π = 22/7

Nasir Alfaraj - February 25, 2017

High school students have often been misled to believe that 22/7 is either the actual value of π or an acceptable approximation to π. Here, I show that 355/113 is a better approximation of π in terms of both absolute and relative errors.​​​

My first month in KAUST

Crystal Hu - February 25, 2017

​I arrived at KAUST on Jan 16th and have been here for more than one month. During this month, it’s the peaceful environment in KAUST that touches my heart. The Red Sea is near our academic buildings. The fresh air, blue sky, warm sunshine, and the sound of water flow make up the campus. The culture here is very different with the culture in my country, China. But it is an interesting thing to experience a different life. I like to go swimming and play bowling after work. I am trying to study swimming now with my friends’ help. I am also not good at bowling, but after practicing many times, I do better now.

MOCVD growth regions

Kuang-Hui Li - February 23, 2017

There are three growth regions for MOCVD. They are kinetic limit, mass transport limit, and thermodynamic limit, as shown in Fig. 1. The kinetic limit happens at low temperature; the mass transport limit happens at medium temperature; the thermodynamic limit happens at high temperature.

Knowledge you should know about LED

Renad AlJefri - December 29, 2016

Light emitting diodes are lighting our lives and are ten times more efficient than the century-old great light bulb. Both are made and optimized with the knowledge of electromagnetics. It is true that Edison did not need to solve Gauss’s law to build an incandescent light, but to understand the colors of light you need to understand Planck's law of black-body radiation which is derived from energy distribution of photons gas in thermal equilibrium. ​
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Brief of MOSCAP

Sarah Alsaggaf - December 27, 2016

There are four essential devices in integrated circuits (IC) which are resistors, diodes, capacitors, and transistor. In general, a capacitor is a device that has two closely spaced conductors, which are called plates, separated by an insulator or a dielectric material such as air, foil or any other insulation. The function of the capacitor is to store the energy in the electric field between these plates; so it works as energy-storage in any device.  

Current Status of DUV Laser

Ronghui Lin - December 26, 2016

To date, the development of DUV laser diode is still at its early stage. DUV laser diodes are still in the lab, far from mass production. Most of these attempts make use of the AlGaN/AlN material ​system, which is one of the promising material systems that can be used to produce DUV laser with high performance. There are generally three methods to pump the laser diode:​​​​

 

High Temperature MOCVD grown AlGaN\AlN epilayers

Hamad S. Alotaibi - December 26, 2016

AlGaN material has been demonstrated as a promising candidate for UV devices. High-quality AlGaN is preferably grown on AlN\sapphire buffer layer over GaN\sapphire. AlGaN\GaN will suffer from a biaxial strain after exceeding a critical thickness due to the lattice mismatch between the AlGaN and GaN, especially with high Al composition [1]. Also, GaN template is not recommended for UV devices due to its high absorption of UV light.

Showerhead design for MOCVD

Kuang-Hui Li - December 26, 2016

​For MOCVD deposition, precursors will thermally decompose, then diffuse onto the substrate. However, there is an unfavorable process called parasitic reaction if a chemical reaction happens within precursors.

Gallium Oxide: a new semiconductor candidates for high power electric device

Feng Wu - December 26, 2016

Gallium oxide has attracted a considerable interest as a functional material for various applications. In these applications, high power high voltage electric device is very appealing because of its high breakdown voltage. The breakdown voltage can reach above 1000 V as shown in the Fig. 1. Besides, it is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt–growth methods.

Microscale energy harvesting

Nasir Alfaraj - December 25, 2016

Recent advances in nanotechnology have enabled rapid integration of microscale electronic energy-harvesting devices such as solar cells, thermoelectric and piezoelectric generators, and fuel cells. Due to their low-cost material and manufacturing advantages, these nanostructured devices are typically fabricated using solution processing methods and have the potential for growth while achieving an efficient energy-harvesting rate.

Cathodoluminescence and electron source

Wenzhe Guo - December 24, 2016

Over the last few years, the III-nitride AlGaN materials have been used to achieve deep ultraviolet laser sources, which triggers many new developments in various domains. However, realizing high-performance deep UV laser based on current injection and optical pumping remains challenging since p-doping and ohmic contact formation are difficult to achieve and inefficient. An alternative approach is in the case of cathodoluminescence based on electron beam pumping. Cathodoluminescence is the emission of photons of characteristic wavelengths by high energy electron beam bombarding semiconductor material. But to realize a compact and portable deep UV laser pumped by e beam still remains unavailable.

Difficulty in predicting the accurate bandgaps of semiconductors

Muwei Zhang - December 24, 2016

Band structures always play a very important role in materials application or devices design. It reveals the basic electronic properties like bandgaps, effective masses, etc. For decades, as a computer method of predicting the materials properties, the first principle calculation can give us very satisfied results such as total energy, Fermi levels, showing good agreements with the experiments. However, the theoretical description of band structures by the first principle calculation remains a very challenging problem, mainly due to the inaccuracy of density function exchange correlations.

New laser applications

Altynay Kaidarova - December 19, 2016

​Since 1960 lasers have driven scientific innovation into a facet of modern life. Many scientific, military, medical and commercial laser applications were developed since then. The question is whether the laser is going to continue to drive such advances in the future.

Quantum Structures

Sarah Alsaggaf - November 2, 2016

There are two well-known essential ways in order to manufacture materials in a nanoscale which are top-down approach and bottom-up approach. The top-down approach essentially means synthesizing nanomaterials by breaking down the bulk amount of materials into nanoscales. Chemical Vapour Deposition (CVD) is one example of bottom-up approach where nanomaterials are synthesized by assembling the particles together to make a larger structure.​

Pi-Pi* Semiconducting Bandgaps and Narrow Absorption Bandwidths

Renad AlJefri - October 26, 2016

Pi-Pi* Semiconducting Bandgaps
Organic semiconductors are made from conjugated organic materials, which have an alternating single bond (sigma bond) and double bonds (sigma and pi bond). The semiconductors’ nature arises from the delocalized and weakly held pi electrons. The pi to pi* energy transition controls the electronic and optical properties of the materials.​
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Alternative current in conductor

Kuang-Hui Li - October 26, 2016

When one applies alternative current on a conductor, most of the current will flow through a thin layer near the surface instead of through the whole conductor. This phenomenon is called skin effect, and that is exactly the reason why power cable contains copper fiber bundle​​​​

Planar MOSFETs vs. FinFETs

Nasir Alfaraj - October 26, 2016

The metal-oxide-semiconductor field-effect transistor (MOSFET) is the basic element used in designing and fabricating modern high-performance integrated circuits (ICs) for switching or amplifying electronic signals. MOSFETs enjoy considerable advantages in terms of integration feasibility:

Vision Art by Electrons––The Imaging Technology

Muwei Zhang - October 25, 2016

​Thousands of years, people always hold great interests in recording the marvelous scenes in their lives. However, our eyes are just like a high-resolution cache video cameras without a sharing button of Twitter or Facebook. ​

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High temperature thermal annealing

Feng Wu - October 25, 2016

​For the group-III nitrides grown by MOCVD, it was well established that a buffer layer, either GaN or AlN, grown at low temperature, played a key role in the crystal quality of the epitaxial layer grown on this buffer layer. However, the dislocation density of III-nitride layers grown on sapphire is still very high.  ​

Random laser ABC

Ronghui Lin - October 25, 2016

When light interacts with random structure, it will undergo multiple scattering, such interacts are discovered everywhere in nature, such as clouds, white paint, powders and even human tissue[1] . While for scientist, their role is to create order and find the order, this random interaction can also provide us with new paradigm for device design, such as random laser.

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Competitive Advantage of Plasma Etching in PV devices

Altynay Kaidarova - September 25, 2016

In the semiconductor industry, there are two major techniques of etching: wet and dry (plasma).

Seven reasons why group-III-nitride semiconductors have emerged as the materials of choice for optoelectronic as well as for high-temperature and high-power electronic devices

Nasir Alfaraj - August 30, 2016

​Seven reasons why group-III-nitride semiconductors have emerged as the materials of choice for optoelectronic as well as for high-temperature and high-power electronic devices​​​​​​

Organic Semiconductors and Organic Photovoltaics

Renad A AlJefri - August 30, 2016

Organic solar cells and panels have the potential to make solar to electricity conversion widely available and accessible to humankind.​

UV water purification

Hamad Al-Otaibi - August 29, 2016

​Water supplies in Saudi Arabia are very scarce due to the lack of rains and rivers in the kingdom. 50% of the drinking water comes from seawater desalination and around 40% comes from the non-recovered underground water.​​

(III+V)/2=IV

Ronghui Lin - August 26, 2016

​Graphene is a peculiar material that continuously attracts the attention of the science community. It has a unique two dimension structure in which each carbon atoms are connected with 3 neighboring carbon atoms with σ bonds, forming a series of hexagonal structures in the plane. ​​​

Polarity of group-III nitrides

Feng Wu - August 25, 2016

For the group-III nitrides grown by MOCVD, it is well known that nitridation before the deposition is beneficial to the improvement of structural and optical properties of nitride layers. While the chemical processes of the nitridation step are studied in detail, very little and contradictory work is presented on structural aspects that mediate polarity. ​

Time to restart efficiency droop debate again?

Xiaohang Li - July 25, 2016

Prof Van de Walle's group published an APL paper in 2009 stating that the Auger coefficient of blue and green InGaN (~2.5eV) is very large (~1e-30 cm6s-1). This becomes one of the strongest evidences about why Auger recombination is THE dominant reason for efficiency droop​.
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