Troy J. Baker

Strain-induced polarization in wurtzite III-nitride semipolar layers

This paper presents growth orientation dependence of the piezoelectric polarization of InxGa1−xN and AlyGa1−yN layers lattice matched to GaN. This topic has become relevant with the advent of growing nitride based devices on semipolar planes [A. Chakraborty et al., Jpn. J. Appl. Phys., Part 2 44, L945 (2005)]. The calculations demonstrate that for strained InxGa1−xN and AlyGa1−yN layers lattice matched to GaN, the piezoelectric polarization becomes zero for nonpolar orientations and also at another point ≈45° tilted from the c plane. The zero crossover has only a very small dependence on the In or Al content of the ternary alloy layer. With the addition of spontaneous polarization, the angle at which the total polarization equals zero increases slightly for InxGa1−xN, but the exact value depends on the In content. For AlyGa1−yN mismatched layers the effect of spontaneous polarization becomes important by increasing the crossover point to ∼70° from c-axis oriented films. These calculations were performed u...

Demonstration of a semipolar (101¯3¯) InGaN∕GaN green light emitting diode

We demonstrate the growth and fabrication of a semipolar (101¯3¯) InGaN∕GaN green (∼525nm) light emitting diode (LED). The fabricated devices demonstrated a low turn-on voltage of 3.2V and a series resistance of 14.3Ω. Electroluminescence measurements on the semipolar LED yielded a reduced blueshifting of the peak emission wavelength with increasing drive current, compared to a reference commercial c-plane LED. On-wafer measurements yielded an approximately linear increase in output power with drive current, with measured values of 19.3 and 264μW at drive currents of 20 and 250mA, respectively. The external quantum efficiency did not decrease appreciably at high currents. Polarization anisotropy was also observed in the electroluminescence from the semipolar green LED, with the strongest emission intensity parallel to the [12¯10] direction. A polarization ratio of 0.32 was obtained at a drive current of 20mA.

Characterization of Planar Semipolar Gallium Nitride Films on Spinel Substrates

Specular, planar semipolar gallium nitride films were grown by hydride vapor phase epitaxy. Planar films of (1013) and (1122) GaN have been grown on (1010) m-plane sapphire. The in-plane epitaxial relationship for (1013) GaN was [3032]GaN ∥[1210]sapphire and [1210]GaN ∥[0001]sapphire. The in-plane epitaxial relationship for (1122) GaN was [1121]GaN ∥[0001]sapphire and [1100]GaN ∥[1210]sapphire. The (1013) films were determined to have N-face sense polarity and a threading dislocation density of 9×108 cm-2. The (1122) films have Ga-face sense polarity and have a threading dislocation density of 2×1010 cm-2. The basal plane stacking fault density was 2 ×105 cm-1 for both orientations. The RMS roughness of the films was under 10 nm for a 5 ×5 µm2 area.

Milliwatt Power Blue InGaN/GaN Light-Emitting Diodes on Semipolar GaN Templates

Growth of semipolar Group-III nitrides based devices offers a means of reducing the deleterious effects of the polarization-induced electric fields present in the polar quantum wells. We report on the fabrication of blue InGaN/GaN multiple-quantum well light-emitting diodes (LEDs) on semipolar (10-1-1) and (10-1-3) oriented GaN templates. A maximum on-wafer continuous wave output power of 190 µW was measured at 20 mA for 300×300 µm2 devices, and output power as high as 1.53 mW was measured at 250 mA. Drive-current independent electroluminescence peak at 439 nm was observed for the LEDs grown on both the planes. The current–voltage characteristics of these LEDs showed rectifying behavior with a forward voltage of 3–4 V at 20 mA.

Light-polarization characteristics of electroluminescence from InGaN∕GaN light-emitting diodes prepared on (112¯2)-plane GaN

Light polarization and emission spectra from InGaN∕GaN quantum-well light-emitting diodes (LEDs) were investigated. The LEDs were prepared on the (112¯2) plane of wurtzite GaN. Polarization and spectrum measurement was performed at different observation angles with respect to the LED surface. Partially polarized electroluminescence was confirmed at any angle of observation, where the emission intensity tended to be greater when a polarizer was aligned along the c axis of the InGaN∕GaN LED structure. The results clearly indicated the inclination of the c axis relative to the LED surface. As a result, two light polarizations were identified and they were assigned to two different electronic transitions in relation to emission peak energies. Possible alteration of the valence-band structure was suggested due to the induced strain.

First-moment analysis of polarized light emission from InGaN/GaN light-emitting diodes prepared on semipolar planes

A first-moment analysis has been applied to electroluminescent spectra of polarized light from InGaN/GaN light-emitting diodes prepared on (1011) and (1013) planes. The normalized first moment (the first moment divided by the zeroth moment) of emission spectra clearly fluctuated due to polarizer rotational angle, even though the spectral peak shift was not obvious. A δ-function model of emission spectra, where two δ-functions represented two polarization components of the electroluminescence, was proposed to explain the observed fluctuation. This was reproduced very well by the model, and fitting parameters appeared to be reasonable numerical values.

Defect-mediated surface morphology of nonpolar m-plane GaN

The role of extended defects in determining the atomic scale surface morphology of nonpolar {11¯00} m-plane gallium nitride has been elucidated. The heteroepitaxially grown m-GaN films are commonly reported to yield striated surface morphologies (slate morphology) correlated with their high densities of basal plane stacking faults. Here, the growth window was explored to allow nonslate morphologies for hydride vapor phase epitaxy. Lateral epitaxial overgrowth was then utilized to produce m-GaN films with three regimes of different extended defect contents. Elimination of stacking faults from the m-GaN yielded step-flow features with an average step height of 4–7 ML even for slate morphology growth conditions.

Photoelectrochemical Properties of Nonpolar and Semipolar GaN

The photoelectrochemical and electrical properties of nonpolar (1120)-oriented and semipolar (1122)-oriented GaN were compared with those of (0001)-oriented GaN. Flatband potentials were obtained in the order of (1120)<(0001)<(1122). The highest photocurrent at a zero bias had been expected for the (1120) sample considering the flatband potential, but the photocurrent of the (1120) sample was the lowest among the three. This could have been due to the electric properties of the (1120) sample used. The surface morphology changes indeed by the photoelectrochemical reactions are also discussed.

Plane Dependent Growth of GaN in Supercritical Basic Ammonia

The plane dependence of GaN grown in supercritical basic ammonia was investigated. Seed crystals with various surface crystallographic orientations were prepared and loaded for four separate growth runs. The growth thickness and crystal quality of GaN grown on each seed was evaluated by caliper and X-ray diffraction (XRD), respectively. These parameters were highly dependent on the orientation of the seed crystal. We achieved high crystalline quality with high growth rates utilizing semi-polar seed crystals. Also, the non-polar m-plane showed the narrowest XRD full width at half maximum (FWHM).

Lateral epitaxial overgrowth of (0001) AlN on patterned sapphire using hydride vapor phase epitaxy

Coalesced, crack-free (0001) AlN films were grown on stripe patterned sapphire substrates without AlN seed layers using hydride vapor phase epitaxy. Using templates with stripes oriented in the ⟨112¯0⟩sapphire direction, lateral epitaxial overgrowth AlN films were coalesced over trench regions as wide as 10 μm despite parasitic sidewall and trench growth. Using transmission electron microscopy, a reduction in the dislocation density from 1.6×109 cm−2 in the seed region to less than 1.0×108 cm−2 in the wing region was demonstrated. Atomic force microscopy and cathodoluminescence measurements were also performed to assess the material quality.