Duc V. Dinh

Surface diffusion and layer morphology of ((112¯2)) GaN grown by metal-organic vapor phase epitaxy

(112¯2) GaN layers were grown by metal-organic vapor phase epitaxy on (112¯2) bulk GaN substrates and (101¯0) sapphire substrates. The growth temperature was varied between 950 and 1050 °C and the total reactor pressure between 50 and 600 mbar. The growth conditions show a strong impact on the yellow band luminescence properties, while weak impact on the threading dislocation density was observed. The layer morphologies exhibit undulations with two periods along GaN [11¯00] and one period along [112¯3¯]. The different period lengths are connected to anisotropic adatom surface diffusion lengths. Arrow like features on the surface originate from the interference of the undulations along [112¯3¯] and [11¯00].

Growth and characterizations of semipolar (112¯2) InN

We report on metal-organic vapor phase epitaxial growth of (112¯2) InN on (112¯2) GaN templates on m-plane (101¯0) sapphire substrates. The in-plane relationship of the (112¯2) InN samples is [1¯1¯23]InN||[0001]sapphire and [11¯00]InN||[12¯10]sapphire, replicating the in-plane relationship of the (112¯2) GaN templates. The surface of the (112¯2) InN samples and the (112¯2) GaN templates shows an undulation along [11¯00]InN,GaN, which is attributed to anisotropic diffusion of indium/gallium atoms on the (112¯2) surfaces. The growth rate of the (112¯2) InN layers was 3-4 times lower compared to c-plane (0001) InN. High resolution transmission electron microscopy showed a relaxed interface between the (112¯2) InN layers and the (112¯2) GaN templates, consistent with x-ray diffraction results. Basal plane stacking faults were found in the (112¯2) GaN templates but they were terminated at the InN/(112¯2) GaN interface due to the presence of misfit dislocations along the entire InN/GaN interface. The misfit dis...

Polarity determination of polar and semipolar (112¯2) InN and GaN layers by valence band photoemission spectroscopy

We demonstrate that the polarity of polar (0001), (0001¯) and semipolar (112¯2) InN and GaN thin layers can be determined by valence band X-ray photoemission spectroscopy (XPS). The polarity of the layers has been confirmed by wet etching and convergent beam electron diffraction. Unlike these two techniques, XPS is a non-destructive method and unaffected by surface oxidation or roughness. Different intensities of the valence band states in spectra recorded by using AlKα X-ray radiation are observed for N-polar and group-III-polar layers. The highest intensity of the valence band state at ≈3.5 eV for InN and ≈5.2 eV for GaN correlates with the group-III polarity, while the highest intensity at ≈6.7 eV for InN and ≈9.5 eV for GaN correlates with the N-polarity. The difference between the peaks for the group-III- and N-polar orientations was found to be statistically significant at the 0.05 significance level. The polarity of semipolar (112¯2) InN and GaN layers can be determined by recording valence band ph...

Comparative study of polar and semipolar (112⁻2) InGaN layers grown by metalorganic vapour phase epitaxy

InGaN layers were grown simultaneously on (112¯2) GaN and (0001) GaN templates by metalorganic vapour phase epitaxy. At higher growth temperature (≥750 °C), the indium content (<15%) of the (112¯2) and (0001) InGaN layers was similar. However, for temperatures less than 750 °C, the indium content of the (112¯2) InGaN layers (15%–26%) were generally lower than those with (0001) orientation (15%–32%). The compositional deviation was attributed to the different strain relaxations between the (112¯2) and (0001) InGaN layers. Room temperature photoluminescence measurements of the (112¯2) InGaN layers showed an emission wavelength that shifts gradually from 380 nm to 580 nm with decreasing growth temperature (or increasing indium composition). The peak emission wavelength of the (112¯2) InGaN layers with an indium content of more than 10% blue-shifted a constant value of ≈(50–60) nm when using higher excitation power densities. This blue-shift was attributed to band filling effects in the layers.

High Bandwidth Freestanding Semipolar (11–22) InGaN/GaN Light-Emitting Diodes

Freestanding semipolar (11-22) indium gallium nitride (InGaN) multiple-quantum-well light-emitting diodes (LEDs) emitting at 445 nm have been realized by the use of laser lift-off (LLO) of the LEDs from a 50-μm-thick GaN layer grown on a patterned (10-12) r-plane sapphire substrate (PSS). The GaN grooves originating from the growth on PSS were removed by chemical mechanical polishing. The 300 μm × 300 μm LEDs showed a turn-on voltage of 3.6 V and an output power through the smooth substrate of 0.87 mW at 20 mA. The electroluminescence spectrum of LEDs before and after LLO showed a stronger emission intensity along the [11-23]InGaN/GaN direction. The polarization anisotropy is independent of the GaN grooves, with a measured value of 0.14. The bandwidth of the LEDs is in excess of 150 MHz at 20 mA, and back-to-back transmission of 300 Mbps is demonstrated, making these devices suitable for visible light communication (VLC) applications.

Exciton localization in polar and semipolar (112̅2) In0.2Ga0.8N/GaN multiple quantum wells

The exciton localization (ELZ) in polar (0001) and semipolar (112) InGa multiple-quantum-well (MQW) structures has been studied by excitation power density and temperature dependent photoluminescence. The ELZ in the (112) MQW was found to be much stronger (ELZ degree σ E ~ 40 –70 meV) compared to the (0001) MQW (σ E ~ 5−11 meV) that was attributed to the anisotropic growth on the (112) surface. This strong ELZ was found to cause a blue-shift of the (112) MQW exciton emission with rising temperature from 200 to 340 K, irrespective of excitation source used. A lower luminescence efficiency of the (112) MQW was attributed to their anisotropic growth, and higher concentrations of unintentional impurities and point defects than the (0001) MQW.

GHz bandwidth semipolar (112¯2) InGaN/GaN light-emitting diodes

We report on the electrical-to-optical modulation bandwidths of non-mesa-etched semipolar (112¯2) InGaN/GaN light-emitting diodes (LEDs) operating at 430-450 nm grown on high-quality (112¯2) GaN templates, which were prepared on patterned (101¯2) r-plane sapphire substrates. The measured frequency response at -3 dB of the LEDs was up to 1 GHz. A high back-to-back data transmission rate of above 2.4 Gbps is demonstrated using a non-return-to-zero on-off keying modulation scheme. This indicates that (112¯2) LEDs are suitable gigabit per second data transmission for use in visible-light communication applications.

Role of substrate quality on the performance of semipolar (112¯2) InGaN light-emitting diodes

We compare the optical properties and device performance of unpackaged InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) emitting at ∼430 nm grown simultaneously on a high-cost small-size bulk semipolar ( 112¯2) GaN substrate (Bulk-GaN) and a low-cost large-size ( 112¯2) GaN template created on patterned ( 101¯2) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ∼105 cm−2–106 cm−2 and basal-plane stacking fault (BSF) density of 0 cm−1, while the PSS-GaN substrate has the TDD of ∼2 × 108 cm−2 and BSF density of ∼1 × 103 cm−1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density.

Semipolar (202̅3) nitrides grown on 3C–SiC/(001) Si substrates

Heteroepitaxial growth of GaN buffer layers on 3C–SiC/(001) Si templates (4°-offcut towards [110]) by metalorganic vapour phase epitaxy has been investigated. High-temperature grown Al0.5Ga0.5N/AlN interlayers were employed to produce a single (203) GaN surface orientation. Specular crack-free GaN layers showed undulations along [10] with a root mean square roughness of about 13.5 nm (50 × 50 μm2). The orientation relationship determined by x-ray diffraction (XRD) was found to be [20]GaN ∥[10] and [034]GaN ∥[110]3C − SiC/Si . Low-temperature photoluminescence (PL) and XRD measurements showed the presence of basal-plane stacking faults in the layers. PL measurements of (203) multiple-quantum-well and light-emitting diode structures showed uniform luminescence at about 500 nm emission wavelength. A small peak shift of about 3 nm was observed in the electroluminescence when the current was increased from 5 to 50 mA (25–250 A cm−2).

Strongly nonparabolic variation of the band gap in In x Al1−x N with low indium content

80–120 nm thick In x Al1−x N epitaxial layers with 0 < x < 0.224 were grown by metalorganic vapour phase epitaxy on AlN/Al2O3-templates. The composition was varied through control of the growth temperature. The composition dependence of the band gap was estimated from the photoluminescence excitation absorption edge for 0 < x < 0.11 as the material with higher In content showed no luminescence under low excitation. A very rapid decrease in band gap was observed in this range, dropping down below 5.2 eV at x = 0.05, confirming previous theoretical work that used a band-anticrossing model to describe the strongly x-dependent bowing parameter, which in this case exceeds 25 eV in the x → 0 limit. A double absorption edge observed for InAlN with x < 0.01 was attributed to crystal-field splitting of the highest valence band states. Our results indicate also that the ordering of the valence bands is changed at much lower In contents than one would expect from linear interpolation of the valence band parameters. These findings on band gap bowing and valence band ordering are of direct relevance for the design of InAlN-containing optoelectronic devices.