The advent of non-c-plane GaN substrates opened up new possibilities for the fabrication of nonpolar and semipolar nitride devices [1]. The majority of the light emitting diodes (LEDs) and laser diodes (LDs) is grown by metal-organic vapour phase epitaxy (MOVPE), but there are few exceptions that include optically pumped nonpolar laser [2] or recently reported semipolar LEDs [3] fabricated by plasma assisted molecular beam epitaxy (PAMBE).

In this work we report on the efficient LEDs and electrically driven LDs emitting in near-UV range grown by PAMBE on semipolar (2021) ammono-GaN substrates. We present smooth semipolar surface morphologies after growth under metal-rich conditions obtained by atomic force microscopy (AFM) and we confirm the high structural quality of the structures presenting detailed transmission electron microscopy (TEM) studies.

In Fig. 1 we show the high resolution TEM image of the multi-quantum well In_0.06Ga_0.94N/In_0.01Ga_0.99N (MQW) structure that has been implemented in the active region of the LED and LD. The QWs are very uniform with sharp interfaces that, along with the efficient n- and p-type doping, was essential to achieve both, electroluminescence and lasing from the semipolar devices. Using secondary ion mass spectroscopy (SIMS) we found similar Mg and Si concentrations for the (Al,In,Ga)N layers grown on semipolar (2021) GaN substrate in comparison to c-polar (0001) layers grown under the same growth conditions. The electroluminescence of the semipolar LED peaked at 387 nm while the optical output power obtained from nonprocessed wafer structure was 0.3 mW @ 100 mA. We will discuss the detailed electrical LED characteristics and its structural quality.

The semipolar LDs were processed with a ridge waveguide oriented along [1120] direction. The light-current-voltage (L-I-V) characteristics for room-temperature pulse mode operation are shown in Fig. 2(a). The threshold current density is 13.2 kA/cm^-2 and the threshold voltage is about 10.8 V. The high resolution laser emission spectra is shown in Fig. 2(b), while the inset shows the magnified part of the spectrum. Laser emission wavelength is 388.2 nm. We observe a very small wavelength difference between the stimulated and spontaneous emission that is attributed to (i) the small electric fields in the semipolar structure and (ii) homogenous QWs and high interface quality in the active region. Further improvements in the LD electrical characteristics is expected for the devices processed with a ridge waveguide along the [1014] direction [4].

Acknowledgements: Authors would like to thank A. Feduniewicz-Żmuda, B. Grzywacz, A. Nowakowska-Siwińska, A. Sarzyńska for help with sample preparation and device processing. This work has been partially supported by the National Science Centre Grant No. 02950, the National Centre for Research and Development Grant No. IT13426, INNOTECH 157829, the European Union within IAPP project SINOPLE grant No. 230765 and European Union funds by the European Social Fund.

References:

[1]  R.M. Farrell, E.C. Young, F. Wu, S.P. DenBaars and J.S Speck, Semicond. Sci. Technol. 27, 024001 (2012).

[2] H. Teisseyre, C. Skierbiszewski, A. Khachapuridze, A. Feduniewicz-Zmuda, M. Siekacz, B. Łucznik, G. Kamler, M. Krysko, T. Suski, P. Perlin, I. Grzegory, and S. Porowski, Appl. Phys. Lett. 90, 081104 (2007).

[3] M. Sawicka, C. Cheze, H. Turski, G. Muziol, S. Grzanka, C. Hauswald, O. Brandt, M. Siekacz, R. Kucharski, T. Remmele, M. Albrecht, M. Krysko, E. Grzanka, T. Sochacki, C. Skierbiszewski, Appl. Phys. Lett. 102, 111104 (2013).

[4] W. G. Scheibenzuber, GaN-Based Laser Diodes Towards Longer Wavelengths and Short Pulses, PhD Thesis, Berlin Heidelberg (2012).

Fig 1: High resolution transmission electron micrograph (HRTEM)  of the semipolar multi-quantum well structure grown by PAMBE that builds the LED and LD active region. Both the surface and interfaces are very smooth. The thickness of the respective layers is indicated in the image, dash lines mark the position of the interfaces.

Fig 2: (a) Light-Current-Voltage characteristics of the semipolar LD with laser stripe along the [1120] direction. (b) Room temperature high resolution spectra of the semipolar laser diode below (pink and violet curves) and above the lasing threshold. Inset shows the magnified spectrum around the lasing wavelength 388.2 nm..

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