Ultracompact display pixels: Tunnel junction nanowire photonic crystal laser
Graphical Abstract
Introduction
Visual reality (VR) and augmented reality (AR) displays require ultra-high definition pixels with smaller sizes, narrower pitches and vertical beam angles [1], [2]. There is a significant increase in interest in the development of III nitride micro-LEDs, a key component of micro-displays essential for smart watches, smart phones, visual reality and augmented reality devices [3]. Since this kind of device is typically used in small spaces or close to the eyes, especially for AR/VR micro-displays that require ultra-small LEDs, the micro-LEDs must exhibit high-brightness, high-resolution, and refresh rates. However, the manufacturing approach inevitably leads to serious damages during the top-down etching process, greatly enhancing non-radiative recombination, thus severely reducing the luminescence efficiency [4], [5]. These issues become incredibly severe as the size of micro-LEDs decreases. More importantly, it is necessary to significantly reduce the spectral linewidth of micro LEDs to achieve high-resolution micro-displays, especially in AR and VR. InGaN-based LEDs are well known to exhibit broad spectral linewidths, primarily as a result of alloy fluctuations and indium separation, which gets worse with increasing the wavelengths [6]. Unfortunately, the micro-LED devices have an insurmountable challenge in reducing the pixel pitch due to the wide divergence angle of light source. This limitation makes it impossible to realize real 3D projection displays that require ultra-high resolution, ultra-high definition, and high integration.
These critical issues can be addressed by using a laser system [7], [8], [9], [10], [11], [12], [13]. III-nitride based vertical cavity surface emitting lasers (VCSELs) are attractive for display light sources due to their superior characteristics such as single longitudinal mode, low-threshold current, circular output beam with limited divergence, and ideal integration with two-dimensional arrays [14], [15], [16], [17]. An essential component of the VCSEL is a distributed Bragg reflector (DBR), which consist of multiple alternative layers of materials with relatively large refractive index differences to provide very high reflectivity [18], [19], [20], [21], [22]. However, when the GaN-based DBRs are fabricated in an LD structure, they show high resistance. On the other hand, when they are formed outside as dual-dielectric DBRs, the volume ratio increases. In addition, GaN-based VCSELs still suffer from high threshold voltage due to high resistances by inefficient current conduction, as well as low crystal quality structure due to polarization fields and dislocations [23], [24], [25], [26], [27].
Compared to conventional planar structures, III-nitride based nanowires exhibit significantly reduced dislocations and defects owing to the effective lateral stress relaxation [28], [29]. Because of reduced dopant formation energy in the GaN nanowire structures, significantly improved dopant ionization and internal quantum efficiency can be realized [30], [31], [32]. Most importantly, the nanowire structure has some tremendous advantages including fine controllability of the direction of light extraction in the nanowire itself through structural innovation [33], [34], [35], [36]. It is worthwhile mentioning that such constructive nanowire structure can eliminate the chip fabrications and the DBR fabrications in the LD system. Furthermore, vertically aligned semiconducting nanowires are advantageous in terms of mode confinement, wave guiding cavity, structure stability from transferring process, and emission wavelength tunability. Given these advantages, III-nitride nanowires are considered promising in VCSELs [37], [38], [39], [40]. Since a photonic crystal structure can also change the optical density of states surrounding the InGaN active region, the internal quantum efficiency of LD will be affected by the modified radiative recombination rate under Purcell effect [41]. By using a Purcell effect in the optical micro-cavity, radiative lifetime can be reduced, thereby leading to the enhanced internal quantum efficiency [42]. Through the use of nano-structures in photonic crystal, it is possible not only to be flexible and transparent substrates, but also to change the direction and wavelength of the light emission by adjusting the optical mode. Furthermore, while epitaxial or dielectric DBR lasers are accompanied by high driving-voltages as well as high manufacturing costs, the photonic crystal laser can significantly reduce power consumption due to their smaller resistance, making the technology ideal for battery-powered products. Such photonic crystal-based pixel technology will provide an ideal solution for AR and VR displays.
On the other hand, these critical challenges can be also potentially addressed by employing the scheme of InGaN tunnel junction and the integration with photonic crystal nanowire LD structures. Because of high p-GaN resistance in conventional LEDs or LDs, ITO or flip-chip is essential for the current spreading, however, when the tunnel junction is introduced, not only p-GaN can be completely removed, but also the use of the current spreading layer can be terminated. In this context, it is expected that the integration of the tunnel junction structure and photonic crystal structure in the nanowire system, the photonic crystal based DBR-free VCSEL can dramatically improve the contact current spreading, internal quantum efficiency, and light extraction efficiency [43], [44].
In this paper, we have shown the world's first display implementation using an electrically injected DBR-free surface-emitting photonic crystal laser. We have also introduced, for the first time, a tunnel junction nanowire structure in the surface emitting photonic crystal laser system. With use of the tunnel junction, highly resistive p-GaN/metal contact was completely removed. The photonic crystal effect by photonic band edge mode eliminated the use of DBRs, which is essential for laser devices. We have further demonstrated that the spectral linewidth can be readily scalable by controlling the spacing of nanowires. It was confirmed that such scalable spectral linewidths remain virtually invariant under the different temperature range of 12–375 K, which are critical to color accuracy. An ultra-compact micro-display with a pitch of 10 µm and a pixel size of 4 µm2 was realized by electrically injected DBR-free surface-emitting photonic crystal laser devices. Such ultra-small laser device will be the most promising candidate for future high-definition display applications.
Section snippets
Epitaxial growth of tunnel junction nanowire heterostructures
GaN-based VCSELs need high-power density with high-current density to generate the high- efficient lasing [45], [46], [47], [48]. However, due to highly resistive p-GaN and ITO current spreading layers to metal contacts, it still suffers from high threshold current, low emission efficiency, and efficiency drop. Tunnel junctions have the ability to remove p-GaN to metal contacts through an interband connection [49]. It could enable high current density lasing with very low resistance contacts.
Conclusions
We have successfully developed the smallest display pixels using an electrically injected DBR-free surface-emitting photonic crystal laser. We have also introduced a tunnel junction nanowire structure in the surface emitting photonic crystal laser system. With use of the tunnel junction, highly resistive p-GaN/metal contact was completely removed. Furthermore, the photonic crystal effect by photonic band edge mode eliminated the use of DBRs, which is essential for laser devices. We have
Nanowire growth by Molecular Beam Epitaxy (MBE)
The tunnel junction nanowire structures were grown on 4 µm n-type GaN template on Al2O3 (0001) substrate using by radio-frequency (RF) plasma-assisted MBE system. The growth conditions for n-GaN:Si nanowire arrays included a substrate growth temperature of 920 °C, a nitrogen flow rate of 0.45 standard cubic centimeter per minute (sccm), forward plasma power of 350 W, and Ga beam equivalent pressure (BEP) of ~3.15 × 10−7 Torr. The high-doping region for n+-GaN was grown with Ga BEP of ~2.9 × 10−7
CRediT authorship contribution statement
Yong-Ho Ra: Conceptualization, Methodology, Investigation, Validation, Formal analysis, Writing - original draft & editing, Supervision. Cheul-Ro Lee: Conceptualization, Investigation, Formal analysis, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2018018638, JG), and (No. 2020M3F3A2A03082762).
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