MOVPE growth of n-GaN cap layer on GaInN/GaN multi-quantum shell LEDs
Introduction
Recently, three-dimensional structures composed of GaInN/GaN multi-quantum shells (MQSs) and GaN nanowires have become promising for use in high-performance light-emitting devices because of their advantages such as nonpolar surface orientation, dislocation-free and tolerance of misfit strain owing the small crystal size [1], [2], [3], [4], [5], [6]. A nanowire here is a hexagonal columnar crystal grown in the vertical direction from the substrate surface. Its diameter is less than 1 μm, and the height is from 100 nm to μm orders. In particular, the core-shell geometry of GaInN/GaN grown on the m-plane can suppress the quantum confined Stark effect [7], [8], [9], which usually causes a significant efficiency droop in conventional thin film light-emitting diodes (LEDs) [10], [11].
The green gap or degradation of emission efficiency in longer wavelength LEDs could also be reduced because of the lack of internal polarization on the m-plane. A high durability of compressive strain may be attained in the MQS/nanowire structure because of the tiny individual structures, and it also is helpful in improving the emission efficiency of long-wavelength LEDs with a high InN molar fraction in GaInN quantum wells [12], [13], [14], [15], [16], [17]. Furthermore, it is possible to enhance the optical confinement factor to achieve low threshold lasing and high output for the MQS/nanowire-based laser diodes (LDs) [18]. Therefore, a very low threshold current density and a high energy-conversion efficiency are expected in MQS-LDs. In addition, stable single-transverse-mode operation, a low aspect ratio of beam divergence, and high level of catastrophic optical damage may also be possible in this new device. It has been reported that indium tin oxide (ITO) is commonly used as a transparent conducting electrode in similar kinds of LEDs [19], [20]. However, the absorption coefficient of ITO is reported to be about 2000 cm−1 in the blue-violet wavelength region [21], so it is not suitable for blue-violet lasers. Furthermore, uniform current injection to the vertical MQS active region is difficult when a thin ITO film is used.
A tunnel junction (TJ) and an embedded n-GaN cap layer to cover the MQS/nanowire structure is thought to be a suitable configuration for uniform current injection to the MQSs. A GaN-based TJ is composed of thin p+ and n+ layers with very high impurity concentrations, and very low resistance TJs have already been reported [22], [23], [24], [25]. Although the p+-layer has a certain light absorption coefficient, light absorption loss is negligible small because of the very small thickness [21]. Furthermore, since the current is diffused to the n-type layer through the low resistance [26], [27] and low-light-absorption Si-doped n-type GaN, uniform current injection becomes possible [28], [29]. For uniform current injection through the TJ, an n-GaN cap layer to cover the whole MQS/nanowire structure is indispensable. However, this kind of embedded growth on the tiny MQS/nanowire arrangement has not been reported or established. The difficulty of the growth of the embedded n-GaN cap layer is thought to be dependent on the height of the MQS/nanowire structure.
In this work, the growth conditions of an n-GaN cap layer are systematically studied to suppress Mg diffusion and void formation. The influence of the height of the MQS/nanowire structure in the growth of the n-GaN cap layer is also investigated.
Section snippets
Experimental methods
Before the growth of GaN nanowires, a SiO2 film with a thickness of 30 nm was deposited by the RF sputtering. Nanoimprinting lithography was employed to form a triangular arrangement of openings with a diameter of 300 nm at a pitch of 1200 nm in the SiO2 mask. Fig. 1 shows a schematic diagram of the total structure composed of the MQS/nanowire, the TJ, and the embedded n-GaN cap layer. The Si-doped n-type GaN nanowire was grown by MOVPE at 1145 °C through the openings of SiO2, where the growth
Results and discussion
The first series of samples was grown while varying the V/III ratio from 20 to 2200 with the growth temperature and the MQS height fixed at 800 °C and 1100 nm, respectively. Cross-sectional SEM images of two samples grown with V/III ratios of 20 and 2200 are shown in Fig. 5. It is clear that the size of the voids is smaller when the V/III ratio is 20. Fig. 6 shows the void height and top-bottom difference as a function of V/III ratio. In the graph, circles indicate void height, and triangles
Conclusions
The embedded growth of an n-GaN cap layer on MQS/nanowires through a TJ was investigated to improve current injection to the m-plane of the MQS. Different growth conditions of n-GaN cap layers were systemically studied to suppress the Mg diffusion and void formation. A very low V/III ratio of 20, an intermediate growth temperature of 800 °C, and a height of the MQS/nanowire structure of 700 nm were found to be the optimal conditions for completely eliminating void formation during the embedded
CRediT authorship contribution statement
Nanami Goto: Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Naoki Sone: Investigation, Formal analysis. Kazuyoshi Iida: Investigation, Formal analysis. Weifang Lu: Investigation, Formal analysis. Atsushi Suzuki: Investigation. Hideki Murakami: Investigation. Mizuki Terazawa: Data curation. Masaki Ohya: Data curation. Satoshi Kamiyama: Conceptualization, Data curation, Funding acquisition, Project administration, Supervision, Writing - review & editing.
Declaration of Competing Interest
No inappropriate relationships or financial problems to others are existing with the authors.
Acknowledgments
This work was supported by MEXT Private University Research Branding Project, MEXT Program for Research and Development of Next-Generation Semiconductor to Realize Energy-Saving Society, JSPS KAKENHI for Scientific Research A [No. 15H02019], JSPS KAKENHI for Scientific Research A [No. 17H01055], JSPS KAKENHI for Innovative Areas [No. 16H06416], and Japan Science and Technology CREST [No. 16815710].
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