1The University of Tokyo, Japan; 2Japan Science and Technology Agency (JST) - ACCEL, Japan
We have developed a new epitaxial growth technique named PSD (pulsed sputtering deposition) which allows us to obtain high quality group III nitride films even at low substrate temperatures with high productivity. In this presentation, will demonstrate successful operation of various nitride devices such as LEDs and HEMTs on various new substrates with the use of PSD low temperature process. Low temperature photoluminescence of unintentionally doped PSD-GaN was dominated by free exciton with is indicative of high quality materials with low impurity concentrations. Maximum room temperature mobility of electrons for lightly Si doped PSD-GaN was as high as 1008 cm2/Vs at a carrier concentration of 2.1E16 /cm3 and that of holes for lightly Mg doped PSD-GaN with [Mg] of 7.9E17 /cm3 is 34 cm2/Vs. We have successfully fabricated LEDs and HEMTs on Si substrates by the use of the low temperature PSD technique. We will also discuss feasibility of preparation of large area nitride devices on low cost substrates such as metal foils, glass, polymer films. With the use of PSD low temperature process, we have successfully fabricated full-color RGB LEDs on metal foils and glass substrates. We have also successfully fabricated thin film nitride transistors on polymer films.
This work was partially supported by JSPS KAKENHI Grant Number JP16H06414.
4:30pm - 5:00pm Invited
RF-MBE Growth and Characterization of GaN on Graphene/Si(100) Substrates
Tsutomu ARAKI, Yasushi NANISHI
Ritsumeikan University, Japan
III-nitride semiconductors (GaN, AlN, InN and their alloys) have attracted considerable attention because of their outstanding materials properties. Si is one of the most attractive substrate for the growth of III-nitride semiconductors. Thus far, c-plane (0001) GaN-based devices such as LEDs and FETs grown on Si (111) substrates have already been commercialized. On the other hand, the c-plane (0001) GaN growth on Si (100) substrates has a great potential to realize the integration between the Si-based electronic devices on Si (100) and GaN-based optoelectronic devices. However, it is still difficult to grow high quality c-plane GaN on Si (100) substrates, owing to different crystal structure and lattice mismatch. Recently, GaN growth on graphene has attracted much attention. This technique enables us to grow GaN on various substrate materials which have essential difficulties in GaN growth.
In this talk, we report on GaN growth on graphene/Si (100) by RF-MBE. Graphene transferred onto Si (100) substrate was used as a substrate. GaN was grown directly (without buffer layer) on the graphene/Si (100) substrates at 770ºC. The characterization using SEM and XRD confirms that GaN grown on graphene/Si (100) shows c-axis well oriented hexagonal grain structure. The FWHM value of (0002) XRD rocking curve of GaN grown on graphene/Si (100) was 11.3 arc min. The CL spectrum had a peak near the band edge emission energy, around 3.4 eV. However, another peak was also observed at around 3.2 eV which originated from the cubic phase inclusion of GaN.
Microstructure of GaN columnar grains on graphene/Si (100) was also investigated by cross-sectional TEM observation. In single crystal GaN grains, few threading dislocations were observed even with very large mismatch between (0001) GaN and (100) Si. The effect of buffer layers on the GaN growth on graphene/Si (100) will also be discussed.
5:00pm - 5:15pm Oral
An Ultra-thin Compliant Sapphire Membrane for the Growth of Less Strained, Less Defective GaN
An ultra-thin (26 nm) sapphire (Al2O3) membrane was used as a compliant substrate for the growth of high quality GaN. The density of misfit dislocations per unit length at the interface between the GaN layer and the sapphire membrane was reduced by 28% compared to GaN on the conventional sapphire substrate. Threading dislocation density in GaN on the sapphire membrane was measured to be 2.4×108/cm2, which is lower than that for GaN on the conventional sapphire substrate (3.2×108/cm2). XRD and micro-Raman results verified that the residual stress in GaN on the sapphire membrane was as low as 0.02 GPa due to stress absorption by the ultra-thin compliant sapphire membrane.
5:15pm - 5:30pm Oral
Low Density of Deep Level Defects in GaN Light-Emitting Diodes Grown by MOCVD on an 8 inch Si (111) Substrate
Xuan Sang NGUYEN1, Tong Hua LEE2, Li ZHANG1, Kadir ABDUL1, Chew Beng SOH2, Eugene FITZGERALD1,3, Aaron AREHART4, Steve RINGEL4, Soo Jin CHUA1,5
1Singapore MIT Alliance for Research and Technology, Singapore; 2Engineering Cluster, Singapore Institute of Technology, Singapore; 3Department of Materials Science and Engineering, Massachusetts Institute of Technology, United States; 4Department of Electrical and Computer Engineering, The Ohio State University, United States; 5Department of Electrical and Computer Engineering, National University of Singapore, Singapore
In this paper, deep level defect density observed in the p-GaN layer of GaN LED grown on silicon substrate with a new buffer layer and device structure was characterized using deep level transient spectroscopy (DLTS). The LED was grown by MOCVD on 8 inch Si (111) wafer. Reduction of 5 times in the deep level traps density is observed in the two LED grown using buffer layers with different threading dislocation density, 3x109 cm-2 and 5x108 cm-2. Further reduction of 1 order of magnitude in trap density was observed in GaN LED with 13QWs compared with the LED structure of 5 QWs. The GaN LED devices with 13QWs has only 1 electron trap level revealed in the p-layer with the activation energy of 0.65 eV and capture cross section of 2 x 1015 cm-2. The total defect density of the 13 QWs is 1.05 x 1014 cm-3 which is comparable with GaN epi-layer grown on sapphire substrate. These LEDs are among the best reported grown on 8 inch silicon giving a power output of 90 mW at 30mA.
5:30pm - 5:45pm Oral
Laser Assisted Chemical Etching of GaN and its Application
Hong ZHU, Guangyu ZHANG, Ngaiyeen CHONG, Lingling SUN
Temasek Polytechnic, Singapore
High brightness, blue Light Emitting Diodes (LEDs) are now finding use in a rapidly growing range of applications. To support further market growth, many efforts are put in to improve device output efficiency and to lower production costs. One of the key and challenging processes in the LED chip fabrication is the etching of GaN material.
A new laser assisted wet chemical etching of GaN, a new technique which the chemical etching is greatly enhanced with the help of laser as compared to normal plasma etching of GaN and the application of the developed technique in the fabrication of vertical LED process.
We have done extensive experiments to have a good understanding of the relationship between the experiment parameters (such as Laser power, focus offset, laser pulse frequency, laser scan speed, number of scan passes, chemical concentration) and the etching. The laser assisted chemical etching of GaN experiments achieved good result. In K2S2O8 chemical solution, 355nm withwavelength laser can etch through all the GaN epitaxial layers of 4.6 um thickness down to the sapphire substrate with good etching edge. The mechanism of the etching was that K2S2O8 had reaction with the Ga decomposed from GaN resulted from laser processing.
Laser assisted chemical etching of GaN has been applied in the fabrication process of Vertical LED to form dicing pattern on the P side of GaN wafer.
5:45pm - 6:00pm Oral
Effect of Laser Frequency on Growth of GaN Epitaxial Layer on Si (111) Substrate by Laser Molecular Beam Epitaxy
Group III-nitrides have become most important semiconductor after silicon due to their wide, direct band gap and are the backbone of commercially available light-emitting diodes (LEDs), especially general lighting. Currently, there is an increasing interest to develop III-nitrides on Si substrates because they are low cost, highly thermal conductive and has a matured device technology. However, growth of GaN on Si is a challenging task due to the large lattice and thermal mismatch, which affects the quality of grown material. Laser molecular beam epitaxy (LMBE) is a relatively new technique for GaN epitaxial growth in which laser is used to ablate the target material. The laser power assists the kinetic energy of growth precursors for a better surface mobility so that the growth temperature can be significantly lowered. Here, we have grown GaN epitaxial layers on silicon (111) at various laser frequencies in the range of 10-30 Hz at a growth temperature of 700 °C using LMBE. The growth was carried out using laser ablation of a polycrystalline GaN solid target under r.f. nitrogen plasma ambient. The influence of laser repetition rate on the crystalline and optical properties of GaN epilayers has been studied. The entire growth sequence was monitored in-situ using reflection high energy electron diffraction (RHEED) technique. RHEED observation confirmed the epitaxial growth of hexagonal GaN along c-axis on silicon (111) substrate. It is observed that the GaN growth at low flux rate (10 Hz) considerably improves the crystalline property. Atomic force microscopy exhibits an island growth of GaN and the island size increased with increasing frequency from 10 to 30 Hz. From room temperature photoluminescence, a strong near band edge emission is obtained at about 367 nm increase with a negligible deep band emission. The results will be presented in detail.