How to obtain metal-polar untwinned high-quality (1 0 −1 3) GaN on m-plane sapphire
Introduction
GaN-based light emitting diodes (LEDs) grown on the (0 0 0 1) plane have very high efficiencies in the blue wavelength region [1]. However, their efficiency drops quickly towards the green and longer wavelengths. Commonly, strong piezo- and spontaneous polarization-induced fields are blamed for the loss of efficiency [2]. Different semi-polar GaN planes have been proposed for long wavelength LEDs, because lower polarization fields should increase radiative recombination. However, also a high Indium incorporation is needed, which was reported e.g. for (1 1 −2 2), (1 0 −1 1) and (2 0 −2 1) [3]. So far, semi-polar and non-polar GaN LEDs have not reached as high efficiency as (0 0 0 1) GaN LEDs [4]. Looking at the band structure, almost all investigated semi-polar planes have a negative polarization field across quantum-well (QW) in the active regions of which can decrease the confinement of hole states and thus greatly increase the carrier loss from the QW. Therefore, we want to realize the metal-polar (1 0 −1 3) surface orientation which has a weaker, but still positive polarization field.
There are very few reports on the preparation of metal-polar (1 0 −1 3) GaN [5], [6], [7], [8]. Single phase N-polar (1 0 −1 −3) GaN has been grown on (1 1 0) spinel [9]. And a few more reports exist on N-polar (1 0 −1 −3) GaN since it can be almost lattice matched to m-plane sapphire [10], [11]. However, the (1 0 −1 0) sapphire surface has the same bond configuration along [1 −2 1 0]sapphire and [−1 2 −1 0]sapphire directions. Hence, there are two possible orientations for the [0 0 0 1] direction of (1 0 −1 ±3) GaN [8]. This leads to an inherent twinning of (1 0 −1 ±3) GaN on m-plane sapphire, which is hard to overcome [12]. Furthermore, in metal-organic vapor phase epitaxy (MOVPE) N-polar (1 0 −1 −3) GaN layers are usually obtained due to sapphire nitridation, with one irreproducible exception [6]. Recently, untwinned (1 0 −1 3) GaN layers were reported using directional sputtering on (0 0 1) Si [7]. The full width at half maximum (FWHM) of the X-ray rocking curve of these layers is relatively broad, i.e., more than 3000 arcsec. Even after epitaxial lateral overgrowth the FWHM was still 1000 arcsec [8]. Furthermore, cracking of the layers is a challenge due to the strong mismatch in thermal expansion coefficient between Si and GaN and the anisotropic strain relaxation mechanism of semi-polar GaN.
This study reports on superior untwinned metal-polar (1 0 −1 3) GaN on m-plane sapphire by directional sputtering followed by MOVPE growth.
Section snippets
Experiment
The directional sputtering was performed using a direct current power source magnetron sputtering system (ULVAC ACS-4000). Sputtering target was a two inch 5 N aluminum target while 5 N argon or purified N2 were used as sputtering gas. The m-plane sapphire substrates were used as received.
The substrates were loaded into the sputtering chamber so that the [1 −2 1 0]sapphire direction pointed towards the Al target, which put the incident angle of Al at 35° ± 7° off the normal axis (Fig. 1). This
Results and discussion
The sputtering has been found crucial to determine the orientation and crystal quality. Fig. 2 shows the topography of an AlN layer after sputtering. The AlN layer is very thin (less than 10 nm), and the surface is very smooth in SEM, with a few, large crystallites. The root-mean-square (RMS) roughness value is typically below 0.3 nm for 2 µm × 2 µm AFM images.
The topography of a GaN layer after MOVPE growth is shown in Fig. 3. The typical chevrons of semi-polar GaN surfaces are pointing
Conclusion
In summary, we introduced directional sputtering of Al and AlN on m-plane sapphire using a two-step method. By subsequent MOVPE overgrowth, we obtained untwinned Ga-polar (1 0 −1 3) GaN with a narrow FHWM below 550 arcsec and low tilt of less than 1°. Further work will focus to smoothen the GaN surface by adjusting the growth conditions in MOVPE.
Acknowledgement
This research was supported by JST, Strategic International Collaborative Research Program, SICORP.
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