Peng Feng, Ce Xu, Jie Bai, Chenqi Zhu, Ian Farrer, Guillem Martinez de Arriba, and Tao Wang
The microdisplays for augmented reality and virtual reality require ultrasmall micro light-emitting-diodes (μLEDs) with a dimension of ≤5 μm. Furthermore, the microdisplays also need three kinds of such μLEDs each emitting red, green, and blue emission. Currently, in addition to a great challenge for achieving ultrasmall μLEDs mainly based on III-nitride semiconductors, another fundamental barrier is due to an extreme difficulty in growing III-nitride-based red LEDs. So far, there has not been any effective approach to obtain high indium content InGaN as an active region required for a red LED while maintaining high optical performance.
In this paper, we have demonstrated a selective epitaxy growth approach using a template featuring microhole arrays. This allows us to not only obtain the natural formation of ultrasmall μLEDs but also achieve InGaN with enhanced indium content at an elevated growth temperature, at which it is impossible to obtain InGaN-based red LEDs on a standard planar surface.
By means of this approach, we have demonstrated red μLEDs (at an emission wavelength of 642 nm) with a dimension of 2 μm, exhibiting a high luminance of 3.5 × 107 cd/m2 and a peak external quantum efficiency of 1.75% measured in a wafer form (i.e., without any packaging to enhance an extraction efficiency). In contrast, an LED grown under identical growth conditions but on a standard planar surface shows green emission at 538 nm. This highlights that our approach provides a simple solution that can address the two major challenges mentioned above.
Yuefei Cai, Chenqi Zhu, Wei Zhong, Peng Feng, Sheng Jiang, Tao Wang
There is a significantly increasing demand on developing a microLED (μLED) based microdisplay which may be the only display system that can meet the requirements for augmented reality/virtual reality systems, helmet-mounted displays, and head‐up displays. However, a number of fundamental challenges that cannot be met by any existing technologies need to be overcome before such a microdisplay with satisfied performance becomes possible.
In this paper, a different type of integration concept using an epitaxial approach is proposed, aiming to monolithically integrate μLEDs and high electron mobility transistors (HEMTs) on a single chip. This concept can be potentially realized by using a selective epitaxial overgrowth method on a predefined HEMT template featuring microhole masks.
Finally, the proposed epitaxial integration concept is translated into a prototype, demonstrating an 8 × 8 microLED microdisplay, where each μLED is electrically driven by an individual HEMT which surrounds its respective μLED via the gate bias of the HEMT.
Direct Epitaxial Approach to Achieve a Monolithic On-Chip Integration of a HEMT and a Single Micro-LED with a High- Modulation Bandwidth
Yuefei Cai, Jack I. H. Haggar, Chenqi Zhu, Peng Feng, Jie Bai, and Tao Wang
Visible light communications (VLC) require III-nitride visible micro-light-emitting diodes (μLEDs) with a high-modulation bandwidth. Such μLEDs need to be driven at a high injection current density on a kA/cm2 scale, which is about 2 orders of magnitude higher than those for normal visible LED operation.
μLEDs are traditionally fabricated by dry-etching techniques where dry-etching-induced damages are unavoidable, leading to both a substantial reduction in performance and a great challenge to viability at a high injection current density. Furthermore, conventional biasing (which is simply applied across a p−n junction) is good enough for normal LED operation but generates a great challenge for a single μLED, which needs to be modulated at a high injection current density and at a high frequency.
In this work, we have proposed a concept for an epitaxial integration and then demonstrated a completely different method that allows us to achieve an epitaxial integration of a single μLED with a diameter of 20 μm and an AlGaN/GaN high-electron-mobility transistor (HEMT), where the emission from a single μLED is modulated by tuning the gate voltage of its HEMT.
Furthermore, such a direct epitaxial approach has entirely eliminated any dry-etching-induced damages. As a result, we have demonstrated an epitaxial integration of monolithic on-chip μLED-HEMT with a record modulation bandwidth of 1.2 GHz on industry-compatible c-plane substrates.
Ultrasmall, Ultracompact and Ultrahigh Efficient InGaN Micro Light Emitting Diodes (μLEDs) with Narrow Spectral Line Width
Jie Bai, Yuefei Cai, Peng Feng, Peter Fletcher, Chenqi Zhu, Ye Tian & Tao Wang
Augmented reality and visual reality (AR and VR) microdisplays require micro light emitting diodes (μLEDs) with an ultrasmall dimension (≤5 μm), high external quantum efficiency (EQE), and narrow spectral line width.
Unfortunately, dry etching which is the most crucial step for the fabrication of μLEDs in current approaches introduces severe damages, which seem to become an insurmountable challenge for achieving ultrasmall μLEDs with high EQE. Furthermore, it is well-known that μLEDs which require InGaN layers as an emitting region naturally exhibit significantly broad spectral line width, which becomes increasingly severe toward long wavelengths such as green.
In this paper, we have reported a combination of our selective overgrowth approach developed very recently and epitaxial lattice-matched distributed Bragg reflectors (DBRs) embedded in order to address all these fundamental issues. As a result, our μLEDs with a diameter of 3.6 μm and an interpitch of 2 μm exhibit an ultrahigh EQE of 9% at ∼500 nm.
More importantly, the spectral line width of our μLEDs has been significantly reduced down to 25 nm, the narrowest value reported so far for III-nitride green μLEDs.
A Direct Epitaxial Approach To Achieving Ultrasmall and Ultrabright InGaN Micro Light-Emitting Diodes (μLEDs)
J. Bai, Y. Cai P. Feng, P. Fletcher, X. Zhao, C. Zhu & T.Wang
A direct epitaxial approach to achieving ultrasmall and ultrabright InGaN micro light-emitting diodes (μLEDs) has been developed, leading to the demonstration of ultrasmall, ultraefficient, and ultracompact green μLEDs with a dimension of 3.6 μm and an interpitch of 2 μm.
The approach does not involve any dry-etching processes which are exclusively used by any current μLED fabrication approaches. As a result, our approach has entirely eliminated any damage induced during the dry-etching processes.
Our green μLED array chips exhibit a record external quantum efficiency (EQE) of 6% at ∼515 nm in the green spectral region, although our measurements have been performed on bare chips which do not have any coating, passivation, epoxy, or reflector, which are generally used for standard LED packaging in order to enhance extraction efficiency.
A high luminance of >107 cd/m2 has been obtained on the μLED array bare chips. Temperature-dependent measurements show that our μLED array structure exhibits an internal quantum efficiency (IQE) of 28%.
It is worth highlighting that our epitaxial approach is fully compatible with any existing microdisplay fabrication techniques.
Confocal photoluminescence investigation to identify basal stacking fault’s role in the optical properties of semi-polar InGaN/GaN lighting emitting diodes
Y. Zhang, R. M. Smith, L. Jiu, J. Bai & T. Wang
High spatial-resolution confocal photoluminescence (PL) measurements have been performed on a series of semi-polar (11–22) InGaN light emitting diodes (LEDs) with emission wavelengths up to yellow.
These LED samples have been grown on our high crystal quality semi-polar GaN templates which feature periodically distributed basal stacking faults (BSFs), which facilitates the study of the influence of BSFs on their optical performance.
Scanning confocal PL measurements have been performed across BSFs regions and BSF-free regions. For the blue LED, both the emission intensity and the emission wavelength exhibit a periodic behaviour, matching the periodic distribution of BSFs. Furthermore, the BSF regions show a longer emission wavelength and a reduced emission intensity compared with the BSF-free regions.
However, with increasing indium content, this periodic behavior in both emission intensity and emission wavelength becomes weaker and weaker. When the indium content (and correspondingly, wavelength) increases up to achieve yellow emission, only random fluctuations have been observed.
It is worth highlighting that the influence of BSFs on the optical properties of semi-polar InGaN LEDs is different from the role of dislocations which normally act as non-radiative recombination centers.
Optical and polarization properties of nonpolar InGaN-based light-emitting diodes grown on micro-rod templates
J. Bai , L. Jiu, N. poyiatzis, P. Fletcher, Y. Gong & T. Wang
We have demonstrated non-polar a-plane InGaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) on sapphire, achieved by overgrowing on a micro-rod template with substantially improved crystal quality.
Photoluminescence measurements show one main emission peak at 418 nm along with another weak peak at 448 nm. Wavelength mapping measurements carried out by using a high spatial-resolution confocal PL system indicate that the two emissions origin from different areas associated with the underlying micro-rod patterns. Electroluminescence measurements exhibit a negligible blue-shift of 1.6 nm in the peak wavelength of the main emission when the driving current increases from 10 to 100 mA, indicating that the quantum confined Stark effect is effectively suppressed in in the nonpolar LED.
A polarization ratio of 0.49 is obtained for the low-energy emission (~448 nm), while the main emission (~418 nm) shows a polarization ratio of 0.34. Furthermore, the polarization ratios are independent of injection current, while the energy separation between m-polarized and c-polarized lights increases with the injection current for both emissions.
Y. Gong, L. Jiu, J. Bruckbauer, J. Bai, R. W. Martin & T. Wang
A novel overgrowth approach has been developed in order to create a multiple-facet structure consisting of only non-polar and semi-polar GaN facets without involving any c-plane facets, allowing the major drawbacks of utilising c-plane GaN for the growth of III-nitride optoelectronics to be eliminated.
Such a multiple-facet structure can be achieved by means of overgrowth on non-polar GaN micro-rod arrays on r-plane sapphire. InGaN multiple quantum wells (MQWs) are then grown on the multiple-facet templates.
Due to the different efficiencies of indium incorporation on non-polar and semi-polar GaN facets, multiple-colour InGaN/GaN MQWs have been obtained. Photoluminescence (PL) measurements have demonstrated that the multiple-colour emissions with a tunable intensity ratio of different wavelength emissions can be achieved simply through controlling the overgrowth conditions.
Detailed cathodoluminescence measurements and excitation-power dependent PL measurements have been performed, further validating the approach of employing the multiple facet templates for the growth of multiple colour InGaN/GaN MQWs.
It is worth highlighting that the approach potentially paves the way for the growth of monolithic phosphor-free white emitters in the future.
N. Poyiatzis, M. Athanasiou, J. Bai, Y. Gong & T. Wang
Carrier transport issues in a (11–22) semi-polar GaN based white light emitting diode (consisting of yellow and blue emissions) have been investigated by detailed simulations, demonstrating that the growth order of yellow and blue InGaN quantum wells plays a critically important role in achieving white emission.
The growth order needs to be yellow InGaN quantum wells first and then a blue InGaN quantum well after the growth of n-type GaN. The fundamental reason is due to the poor hole concentration distribution across the whole InGaN quantum well region. In order to effectively capture holes in both the yellow InGaN quantum wells and the blue InGaN quantum well, a thin GaN spacer has been introduced prior to the blue InGaN quantum well.
The detailed simulations of the band diagram and the hole concentration distribution across the yellow and the blue quantum wells have been conducted, showing that the thin GaN spacer can effectively balance the hole concentration between the yellow and the blue InGaN quantum wells, eventually determining their relative intensity between the yellow and the blue emissions.
Based on this simulation, we have demonstrated a monolithically multi-colour LED grown on our high quality semi-polar (11–22) GaN templates.
L. Jiu, Y. Gong & T. Wang
Non-polar (11–20) GaN with significantly improved crystal quality has been achieved by means of overgrowth on regularly arrayed micro-rod templates on sapphire in comparison with standard non-polar GaN grown without any patterning processes on sapphire.
Our overgrown GaN shows massively reduced linewidth of X-ray rocking curves with typical values of 270 arcsec along the  direction and 380 arcsec along the [1–100] direction, which are among the best reports. Detailed X-ray measurements have been performed in order to investigate strain relaxation and in-plane strain distribution.
The study has been compared with the standard non-polar GaN grown without any patterning processes and an extra non-polar GaN sample overgrown on a standard stripe-patterned template. The standard non-polar GaN grown without involving any patterning processes typically exhibits highly anisotropic in-plane strain distribution, while the overgrown GaN on our regularly arrayed micro-rod templates shows a highly isotropic in-plane strain distribution. Between them is the overgrown non-polar GaN on the stripe-patterned template.
The results presented demonstrate the major advantages of using our regularly arrayed micro-rod templates for the overgrowth of non-polar GaN, leading to both high crystal quality and isotropic in-plane strain distribution, which is important for the further growth of any device structures.
Spatially-resolved optical and structural properties of semi-polar (112¯2) Al x Ga1−x N with x up to 0.56
Jochen Bruckbauer, Zhi Li, G. Naresh-Kumar, Monika Warzecha, Paul R. Edwards, Ling Jiu, Yipin Gong, Jie Bai, Tao Wang, Carol Trager-Cowan & Robert W. Martin
Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields.
Crack-free semi-polar (112¯2) Al x Ga1−x N epilayers with AlN contents up to x = 0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs).
Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these regions is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging.
Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.
M. Athanasiou, R. M. Smith, J. Pugh, Y. Gong, M. J. Cryan & T. Wang
An optically pumped multi-color laser has been achieved using an InGaN/GaN based micro-disk with an undercut structure on a silicon substrate. The micro-disk laser has been fabricated by means of a combination of a cost-effective microsphere lithography technique and subsequent dry/wet etching processes.
The microdisk laser is approximately 1 μm in diameter. The structure was designed in such a way that the vertical components of the whispering gallery (WG) modes formed can be effectively suppressed. Consequently, three clean lasing peaks at 442 nm, 493 nm and 522 nm have been achieved at room temperature by simply using a continuous-wave diode laser as an optical pumping source.
Time–resolved micro photoluminescence (PL) measurements have been performed in order to further confirm the lasing by investigating the excitonic recombination dynamics of these lasing peaks. A three dimensional finite-difference-time-domain (FDTD) simulation has been used for the structure design.
M. Athanasiou, R. M. Smith, S. Ghataora & T. Wang
Highly polarised white light emission from a hybrid organic/inorganic device has been achieved.
The hybrid devices are fabricated by means of combining blue InGaN-based multiple quantum wells (MQWs) with a one-dimensional (1D) grating structure and down-conversion F8BT yellow light emitting polymer. The 1D grating structure converts the blue emission from unpolarised to highly polarised; Highly polarised yellow emission has been achieved from the F8BT polymer filled and aligned along the periodic nano-channels of the grating structure as a result of enhanced nano-confinement.
Optical polarisation measurements show that our device demonstrates a polarisation degree of up to 43% for the smallest nano-channel width. Furthermore, the hybrid device with such a grating structure allows us to achieve an optimum relative orientation between the dipoles in the donor (i.e., InGaN/GaN MQWs) and the diploes in the acceptor (i.e., the F8BT), maximising the efficiency of non-radiative energy transfer (NRET) between the donor and the acceptor. Time–resolved micro photoluminescence measurements show a 2.5 times enhancement in the NRET efficiency, giving a maximal NRET efficiency of 90%.
It is worth highlighting that the approach developed paves the way for the fabrication of highly polarised white light emitters.
The most successful example of large lattice-mismatched epitaxial growth of semiconductors is the growth of III-nitrides on sapphire, leading to the award of the Nobel Prize in 2014 and great success in developing InGaN-based blue emitters.
However, the majority of achievements in the field of III-nitride optoelectronics are mainly limited to polar GaN grown on c-plane (0001) sapphire. This polar orientation poses a number of fundamental issues, such as reduced quantum efficiency, efficiency droop, green and yellow gap in wavelength coverage, etc. To date, it is still a great challenge to develop longer wavelength devices such as green and yellow emitters.
One clear way forward would be to grow III-nitride device structures along a semi-/non-polar direction, in particular, a semi-polar orientation, which potentially leads to both enhanced indium incorporation into GaN and reduced quantum confined Stark effects. This review presents recent progress on developing semi-polar GaN overgrowth technologies on sapphire or Si substrates, the two kinds of major substrates which are cost-effective and thus industry-compatible, and also demonstrates the latest achievements on electrically injected InGaN emitters with long emission wavelengths up to and including amber on overgrown semi-polar GaN.
Finally, this review presents a summary and outlook on further developments for semi-polar GaN based optoelectronics.
J. Bai, B. Xu, F. G. Guzman, K. Xing, Y. Gong, Y. Hou and T. Wang
We demonstrate semipolar InGaN single-quantum-well light emitting diodes(LEDs) in the green, yellow-green, yellow and amber spectral region.
The LEDs are grown on our overgrown semipolar (11-22) GaN on micro-rod array templates, which are fabricated on (11-22) GaN grown on m-plane sapphire. Electroluminescence measurements on the (11-22) green LED show a reduced blue-shift in the emission wavelength with increasing driving current, compared to a reference commercial c-plane LED. The blue-shifts for the yellow-green and yellow LEDs are also significantly reduced. All these suggest an effective suppression in quantum confined Stark effect in our (11-22) LEDs.
On-wafer measurements yield a linear increase in the light output with the current, and external quantum efficiency demonstrates a significant improvement in the efficiency-droop compared to a commercial c-plane LED. Electro-luminescence polarization measurements show a polarisation ratio of about 25% in our semipolar LEDs.
Hybrid III-Nitride/Organic Semiconductor Nanostructure with High Efficiency Nonradiative Energy Transfer for White Light Emitters
R. Smith, B. Liu, J. Bai, and T. Wang
A novel hybrid inorganic/organic semiconductor nanostructure has been developed, leading to very efficient nonradiative resonant-energy-transfer (RET) between blue emitting InGaN/GaN multiple quantum wells (MQWs) and a yellow light emitting polymer.
The utilisation of InGaN/GaN nanorod arrays allows for both higher optical performance of InGaN blue emission and a minimised separation between the InGaN/GaN MQWs and the emitting polymer as a color conversion medium. A significant reduction in decay lifetime of the excitons in the InGaN/GaN MQWs of the hybrid structure has been observed as a result of the nonradiative RET from the nitride emitter to the yellow polymer.
A detailed calculation has demonstrated that the efficiency of the nonradiative RET is as high as 73%. The hybrid structure exhibits an extremely fast nonradiative RET with a rate of 0.76 ns–1, approximately three times higher than the InGaN/GaN MQW nonradiative decay rate of 0.26 ns–1. It means that the RET dominates the nonradiative processes in the nitride quantum well structure, which can further enhance the overall device performance.
Room temperature plasmonic lasing in a continuous wave operation mode from an InGaN/GaN single nanorod with a low threshold
Hou Y, Renwick P, Liu B, Bai J, Wang T.
It is crucial to fabricate nano photonic devices such as nanolasers in order to meet the requirements for the integration of photonic and electronic circuits on the nanometre scale. The great difficulty is to break down a bottleneck as a result of the diffraction limit of light.
Nanolasers on a subwavelength scale could potentially be fabricated based on the principle of surface plasmon amplification by stimulated emission of radiation (SPASER). However, a number of technological challenges will have to be overcome in order to achieve a SPASER with a low threshold, allowing for a continuous wave (cw) operation at room temperature.
We report a nano-SPASER with a record low threshold at room temperature, optically pumped by using a cw diode laser. Our nano-SPASER consists of a single InGaN/GaN nanorod on a thin SiO2 spacer layer on a silver film. The nanorod containing InGaN/GaN multi-quantum-wells is fabricated by means of a cost-effective post-growth fabrication approach. The geometry of the nanorod/dielectric spacer/plasmonic metal composite allows us to have accurate control of the surface plasmon coupling, offering an opportunity to determine the optimal thickness of the dielectric spacer.
This approach will open up a route for further fabrication of electrically injected plasmonic lasers.
M. Athanasiou, R. Smith, , B. Liu & T. Wang
Optically pumped green lasing with an ultra low threshold has been achieved using an InGaN/GaN based micro-disk with an undercut structure on silicon substrates.
The micro-disks with a diameter of around 1 μm were fabricated by means of a combination of a cost-effective silica micro-sphere approach, dry-etching and subsequent chemical etching. The combination of these techniques both minimises the roughness of the sidewalls of the micro-disks and also produces excellent circular geometry. Utilising this fabrication process, lasing has been achieved at room temperature under optical pumping from a continuous-wave laser diode. The threshold for lasing is as low as 1 kW/cm2.
Time–resolved micro photoluminescence (PL) and confocal PL measurements have been performed in order to further confirm the lasing action in whispering gallery modes and also investigate the excitonic recombination dynamics of the lasing.