Electrically Pumped III-N Microcavity Light Emitters Incorporating an Oxide Confinement Aperture
© The Author(s). 2017
Received: 22 September 2016
Accepted: 19 December 2016
Published: 5 January 2017
In this work, we report on electrically pumped III-N microcavity (MC) light emitters incorporating oxide confinement apertures. The utilized SiO2 aperture can provide a planar ITO design with a higher index contrast (~1) over other previously reported approaches. The fabricated MC light emitter with a 15-μm-aperture shows a turn-on voltage of 3.3 V, which is comparable to conventional light emitting diodes (LEDs), showing a good electrical property of the proposed structure. A uniform light output profile within the emission aperture suggesting the good capability of current spreading and current confinement of ITO and SiO2 aperture, respectively. Although the quality factor (Q) of fabricated MC is not high enough to achieve lasing action (~500), a superlinear emission can still be reached under a high current injection density (2.83 kA/cm2) at 77 K through the exciton-exciton scattering, indicating the high potential of this structure for realizing excitonic vertical-cavity surface-emitting laser (VCSEL) action or even polariton laser after fabrication optimization.
III-Nitride based materials are highly attractive for making vertical microcavity (MC) light emitters such as strongly coupled polariton lasers and conventional vertical-cavity surface-emitting lasers (VCSELs) due to their large exciton binding energies and wide spectra tuning range in the ultraviolet-visible region [1–12]. So far, electrically pumped polariton emitters and conventional VCSELs have been demonstrated in III-nitride based MCs at room temperature (RT). However, the p-side designs including indium-tin-oxide (ITO) transparent conducting layer (TCL) and dielectric materials based current aperture in the reported devices induce a negative guiding effect and lateral optical loss [13–15]. To provide a sufficient positive guiding effect, Cheng et al. utilized an intra-cavity low-index AlN layer to serve as a lateral optical/current aperture and achieved a high-quality-factor (Q) emission; however, no lasing action was observed in their MC light emitter due to the lack of TCL . A similar work, in which an oxidized AlInN aperture has been implanted in to a GaN-based LED, has been demonstrated by Castiglia et al. . In 2014, a GaN-based MC emitter featuring a Si-diffusion-defined current aperture developed by Yeh et al. shows a good current confinement capability . Although the current flow can be well-confined in the non-Si-diffused area, the near zero index contrast between diffused and non-diffused area cannot provide a sufficient lateral optical confinement. Recently, Leonard et al. proposed an approach using ion-implantation to define the current aperture. In their result, the index contrast between the implanted and the non-implanted p++GaN can be enhanced to 0.05 . In this letter, we propose an alternative design using self-aligned SiO2 aperture featuring a low index and high resistance. Within this structure, the index contrast between the p-GaN and surrounded SiO2 aperture can be enlarged to about 1 and provide a better lateral optical confinement. Through this approach, a multi-transverse mode behavior resulted from the strong lateral optical confinement can be observed in the measured spectra. Moreover, a strong excitonic gain emission generated from the devices with a small aperture size reveals the possibility of such devices to be pushed into lasing action or even polaritonic operation [19, 20].
Results and Discussion
In summary, we have demonstrated an III-N MC emitter featuring an oxide confinement aperture which can be driven by electrical injection. Thanks to the large index contrast between oxide and III-N (~1), our design can provide a better optical confinement than other reported fabrication techniques such as ion-implantation and Si-diffusion which index contrasts are about or even smaller than 0.1. From the CCD images and EL measurement results, oxide confinement aperture can provide a uniform light emission within the aperture but also maintain good electrical properties which are comparable with conventional LED devices. Also, a multi-transverse mode characteristic was observed under low injection condition. Interestingly, under high injection at 77 K, a pronounce X-X scattering with superlinearly increased intensity can be achieved in the fabricated MC devices with a 2.83 kA/cm2 threshold current density. Our demonstration suggests a powerful design for the electrically pumped III-N MC with excitonic gain which could be applied to low threshold MC laser devices including VCSELs and polariton lasers.
The authors would like to gratefully acknowledge Prof. H. C. Kuo at National Chiao Tung University for the technical support and would like to acknowledge Lextar Corporation in Taiwan for the high-quality ITO deposition and LLO process. This work has been supported in part by the MOE ATU program and in part by the National Science Council of the Republic of China in Taiwan under Contract No. NSC 102-2221-E-009-156-MY3.
YYL and YCL conceived and designed the experimental strategy. YCL and TCC performed the experiments. YYL and TCL wrote and modified the manuscript. SCW and TCL supervised the whole work and revised the final version of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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