Mitch M.C. Chou

GaN homoepitaxy on freestanding (11̄00) oriented GaN substrates

We report homoepitaxial GaN growth on freestanding (1100) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition. Scanning electron microscopy, atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (1100) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (1100) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, our work forms the basis for developing high performance III-N optoelectronic devices.

Crystal growth and properties of LiAlO2 and nonpolar GaN on LiAlO2 substrate

In this study, the growth and properties of LiAlO2 material and a nonpolar GaN-based light-emitting-diode (LED) structure on LiAlO2 have been investigated. The LiAlO2 material is grown by the Czochralski pulling technique and is used as a substrate for nonpolar nitride growth. An improved surface roughness can be obtained by a four-step polishing process. With subsequent nitridation treatment, a pure M-plane (1010) GaN can be obtained. An electron microscope shows an abundance of cracks that are oriented parallel to the (001) and (100) planes of the LiAlO2 substrate on the rear surface of GaN. The absence of the polarization-induced electric field of a GaN-based LED structure on LiAlO2 was shown by using photoluminescence measurements. Therefore, this approach is promising to further increase the luminescence performance of GaN-based LEDs.

Defects and acoustic properties of LiAlO2

A potential piezoelectric crystal LiAlO2 with (100) orientation is grown by means of the Czochralski pulling method. The as-grown crystal is identified as a single phase with good uniformity by x-ray diffraction pattern. (001) Transmission electron microscope image showed a unique cross-hatched pattern which reveals a superlattice structure. Several cubic LiAlO2 specimen, 10.0mm×10.0mm×10.0mm were manufactured to characterize its elastic properties. The time-based pulse-echo transmission technique was employed to measure the acoustic velocities of longitudinal and transverse modes. The elastic constants of LiAlO2 were extracted from the acoustic velocity measurements at different propagation directions. It was found that the acoustic velocities of LiAlO2 are much higher than the current piezoelectric crystals, including quartz, LiNbO3, and Langasite family materials.A potential piezoelectric crystal LiAlO2 with (100) orientation is grown by means of the Czochralski pulling method. The as-grown crystal is identified as a single phase with good uniformity by x-ray diffraction pattern. (001) Transmission electron microscope image showed a unique cross-hatched pattern which reveals a superlattice structure. Several cubic LiAlO2 specimen, 10.0mm×10.0mm×10.0mm were manufactured to characterize its elastic properties. The time-based pulse-echo transmission technique was employed to measure the acoustic velocities of longitudinal and transverse modes. The elastic constants of LiAlO2 were extracted from the acoustic velocity measurements at different propagation directions. It was found that the acoustic velocities of LiAlO2 are much higher than the current piezoelectric crystals, including quartz, LiNbO3, and Langasite family materials.

Ultrasensitive nonlinear absorption response of large-size topological insulator and application in low-threshold bulk pulsed lasers

Dirac-like topological insulators have attracted strong interest in optoelectronic application because of their unusual and startling properties. Here we report for the first time that the pure topological insulator Bi2Te3 exhibited a naturally ultrasensitive nonlinear absorption response to photoexcitation. The Bi2Te3 sheets with lateral size up to a few micrometers showed extremely low saturation absorption intensities of only 1.1 W/cm2 at 1.0 and 1.3 μm, respectively. Benefiting from this sensitive response, a Q-switching pulsed laser was achieved in a 1.0 μm Nd:YVO4 laser where the threshold absorbed pump power was only 31 mW. This is the lowest threshold in Q-switched solid-state bulk lasers to the best of our knowledge. A pulse duration of 97 ns was observed with an average power of 26.1 mW. A Q-switched laser at 1.3 μm was also realized with a pulse duration as short as 93 ns. Moreover, the mode locking operation was demonstrated. These results strongly exhibit that Bi2Te3 is a promising optical device for constructing broadband, miniature and integrated high-energy pulsed laser systems with low power consumption. Our work clearly points out a significantly potential avenue for the development of two-dimensional-material-based broadband ultrasensitive photodetector and other optoelectronic devices.

Optical characteristics of nonpolar a-plane ZnO thin film on (010) LiGaO2 substrate

We investigate the optical properties of non-polar a-plane ZnO film grown on (010) LiGaO2 substrate by chemical vapor deposition. X-ray diffraction indicates the a-plane orientation and Raman spectroscopy reveals the phase purity. Four distinct features in the near band-edge range are identified as neutral-donor-bound-exciton (D°X), free-exciton (FX), free-to-bound, and donor-acceptor pair transitions. The thermal activation energy of D°X is 12 meV while an acceptor energy level of 124 meV is estimated. Temperature evolution of photoluminescence (PL) shows that the room-temperature luminescence is a mixture of free-to-bound and FX transitions. The polarization dependence of the near-band-edge transitions were investigated. The largest degree of polarization of 95.5% occurs in FX transition. It can be attributed to anisotropic in-plane strain and the nonpolar a-plane orientation.

Improved quality of nonpolar m-plane GaN [101¯0] on LiAlO2 substrate using a modified chemical vapor deposition

Nonpolar GaN crystal on lattice-matched [100] γ-LiAlO2 substrate was grown by a newly designed chemical vapor deposition (CVD) reactor. Following the CVD growth, x-ray diffraction indicated that the GaN film was oriented in the nonpolar m-plane with [101¯0] orientation. Further structural characterizations and defect analysis of nonpolar GaN material was performed using transmission electron microscope. Low-temperature photoluminescence was dominated by neutral donor bound excitons and the yellow luminescence was negligible. Raman spectroscopy showed that the as-grown GaN {101¯0} epilayer on [100] γ-LiAlO2 substrate were indeed of good quality. Compared to the previous report, nonpolar GaN with an improved quality was demonstrated by modifying the inner structure of the CVD reactor.

Defects in m-face GaN films grown by halide vapor phase epitaxy on LiAlO2

Free-standing wafers (50 mm diameter) of GaN were grown by halide vapor phase epitaxy on lattice-matched γ-LiAlO2. We report a transmission electron microscopy study of defects and defect densities in these wafers. The growth direction is [101_0]. Stacking faults in the basal plane are seen when viewing the specimen in the [12_10] direction with an average spacing of less than 100 nm. Convergent beam electron diffraction measurements show no switch in the polarity and thus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations are found to have a correlated arrangement with a density of 3×108 cm−2 when viewing the [12_10] direction and widely varying (depending upon location) when viewing in the [0001] direction. These dislocations act as “seeds” for postgrowth surface features that directly exhibit the correlated nature of these threading dislocations.

InGaN/GaN single-quantum-well microdisks

We have grown In{sub x}Ga{sub 1-x}N/GaN quantum wells atop GaN microdisk with {gamma}-LiAlO{sub 2} substrate by using plasma-assisted molecular beam epitaxy. The structural and optical properties of the samples were analyzed by transmission electron microscopy, x-ray diffraction, cathodoluminescence, and photoluminescence measurements. Based on the measured results, we obtained the indium concentration of the In{sub x}Ga{sub 1-x}N/GaN single quantum well to be x = 0.25 with a band-gap energy of 2.31 eV, which is consistent with the bowing effect of bulk In{sub x}Ga{sub 1-x}N: E{sub g}(x) = [3.42 - x * 2.65 - x * (1 - x) * 2.4] eV.

m-plane (101̱0) InN heteroepitaxied on (100)-γ-LiAlO2 substrate: Growth orientation control and characterization of structural and optical anisotropy

Heteroepitaxial growth of m-plane (1010) InN film on (100)-γ-LiAlO2 (LAO) substrate has been realized by plasma-assisted molecular-beam epitaxy. Surface treatment of LAO substrate plays an important role in controlling the resultant phase and purity of m-plane InN. X-ray diffraction, reflection high-energy electron diffraction, electron back scatter diffraction, and transmission electron microscopy (TEM) studies revealed formation of pure m-plane InN film using substrate preannealed at 800 °C but without any nitridation. In contrast, using substrate with nitridation but otherwise identical pretreatment and growth conditions, c-plane (0001) InN columnar structure was grown, instead of m-plane InN film. Structural anisotropy of the m-plane InN epitaxied on LAO is attributed to the I1 type base-plane stacking faults according to the modified Williamson–Hall and TEM analyses. A rectangular-to-rectangular atomic stacking sequence and a commensurately lattice-matched condition in epitaxial direction of [1210]...

Extremely high-performance visible light photodetector in the Sb 2 SeTe 2 nanoflake

The photocurrent was performed in the Sb2SeTe2 topological insulator at a wavelength of 532 nm. It exhibits extremely high performance that the responsivity and the photoconductive gain reach 2293 AW−1 and 5344 at 1 V. This high photoresponse is orders of magnitude higher than most reported values in topological insulators and two-dimensional transitional metal dichalcogenides. This finding suggests that the Sb2SeTe2 nanoflake has great potential for future optoelectronic device applications.