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Abstract
We report on III-nitride-based micro-light-emitting diodes (µLEDs) operating at 450 nm wavelength with diameters down to 2 µm. Devices with a standard LED structure followed by a tunnel junction were grown by plasma-assisted molecular beam epitaxy. The emission size of µLEDs was defined by shallow He+ implantation of the tunnel junction region. The ion implantation process allows to create flat devices, applicable to further epitaxial regrowth. The shift of current density for the maximum external quantum efficiency as a function of µLEDs diameter was observed. This effect may be a fingerprint of the change in the external efficiency related to the lateral carrier diffusion (limited by holes) in InGaN quantum wells.
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1. Introduction
III-nitride micro-light-emitting diodes (µLEDs) have drawn a lot of attention recently due to their low power demand, high luminous efficacy, high brightness and contrast, high shock resistance and long lifetime. Depending on the chip size µLEDs find applications in augmented reality, smartwatches or smartphone displays with high brightness [1,2]. Although the operating principles for conventional broad-area LEDs and µLEDs are similar, the processing of µLEDs involves additional steps, which brings new challenges. The simplest fabrication process of µLEDs consists of epitaxial growth, followed by mesa etching to separate single microdevices and deposition of the insulator and metallic contacts [3]. Etching through the quantum wells (QWs) region can lead to the generation of defects on the walls of µLEDs. These defects act as centers of non-radiative recombination, which leads to the decrease in internal quantum efficiency (IQE) [2,4]. Several groups have worked to decrease the surface recombination. Methods included the passivation of the sidewalls using atomic layer deposition, the growth of µLEDs by selective area growth (SAG) or the modification of the structure for better current spreading [5–9]. Another limitation for the fabrication of µLEDs is related to upper p-type contact, where the geometry of the device requires the application of transparent p-type contact or challenging extraction of light through the thick substrate [10]. Therefore, there are still ongoing efforts to apply new designs of the epitaxial LED structure, which will simplify the processing scheme.
Recently, increased attention has been dedicated to the tunnel junctions (TJs) in III-nitride devices [11–20]. TJs resistance for wide bandgap semiconductors can be effectively reduced by using the piezoelectric fields in the region of the junction [12,13]. The TJs in the GaN-based optoelectronic structures open new possibilities for novel devices such as vertically integrated multicolor light-emitting diodes [21] or laser diodes [22]. The application of the TJ on the top of the µLEDs for efficient conductivity conversion from p-type to n-type can be a solution to the above-mentioned issues related to the fabrication of the µLEDs. It eliminates the need for transparent upper p-type contacts and creates more freedom in device design. However, there is a technological issue with the epitaxial growth of TJs. The majority of light-emitting devices are grown by metalorganic chemical vapor deposition (MOCVD). MOCVD-grown µLEDs have been reported to reach maximum quantum efficiencies at lower current densities, which might be a fingerprint of smaller contribution of non-radiative recombination channels in such structures [23]. Although this technology demonstrates the state-of-the-art highest quantum efficiencies of nitride LEDs, annealing of the samples is necessary to remove hydrogen in order to activate p-type conductivity [24]. However, in the case of TJs, it is difficult to activate p-type because p-type layers are buried below n-type layers. It is well known that the diffusion of hydrogen is blocked through n-type layers, therefore making the p-type layer activation process (which requires physical removal of hydrogen) ineffective for MOCVD [25]. To overcome this problem µLEDs with TJ have been fabricated by a hybrid approach using MOCVD and subsequent growth by ammonia molecular beam epitaxy (NH3-MBE) or by MOCVD [15,26]. It is also worth mentioning that there is substantial effort to activate p-type conductivity in TJs grown entirely by MOCVD [26]. Recent reports show that the reduction of the operating voltage of TJ devices grown by MOCVD is possible due to careful planning of the device structure with defined access to the p-type region either laterally (by etching trenches [27]) or vertically (by SAG [28]). These efforts allow for demonstrating low voltage MOCVD based µLEDs and laser diodes [29]. Alternatively, for making TJ devices, plasma-assisted molecular beam epitaxy (PAMBE) can be used. This technology does not require post-growth annealing of Mg doped layers, and there are no limitations for making a p-type layer buried below n-type one. In that context, the PAMBE technology can be very useful for developing new types of optoelectronic devices.
It has already been reported, that for TJ µLEDs, the pattern that defines the current path (and therefore the area where light is emitted) can be formed by etching the TJ and subsequent regrowth of the conductive n-type GaN [30]. The interesting feature of such an approach is that the area of the light emission from µLEDs is defined by etching TJ located above the p-type region, while optically active QWs remain untouched. It also allows for placing top metallization outside the emitting area [31]. However, the formation of the µLEDs by the etching procedure creates non-planar devices and leads to surface roughening. This could be a problem if a subsequent regrowth is needed e.g. for TJ µLEDs red-green-blue displays with a stack of 3 µLEDs, each emitting a different color.
In this paper, we present a different approach to fabricating µLEDs. We used the shallow ion implantation of TJs to define the current aperture. Ion implantation is a widely used technique for altering the conductivity of many families of semiconductors, including GaN. It can be used to achieve either p-type or n-type conductivity in GaN layers or to electrically isolate layers by the creation of compensating point defects. It is a method used in many GaN-based electronic devices like transistors and lasers [32–36]. Different ions can be used for these purposes – one of them is helium He+ [37]. Helium is a light ion that can produce high-resistivity material by the introduction of point defects or defect complexes, which produce hole and electron deep-level traps. It was shown that He+ implantation can increase the resistivity of GaN up to 11 orders of magnitude [38].
In this work, we demonstrate nitride-based TJs µLEDs and µLEDs arrays grown by PAMBE. The surface of the devices was implanted with He+ ions to increase the resistivity of TJ and prevent current flow. The area, in which the µLEDs were formed, was left un-implanted. The approach with ion implantation for forming µLEDs patterns has three main advantages, it enables (1) shallow ion implantation for electrical isolation between microdevices, without parasitic impact on the active LED region; (2) fabrication of n-type top contact, that consists of a transparent n-type GaN layer above the active µLEDs region and it enables the application of n-type metal contacts, on the side of µLEDs; and most importantly, (3) the use of ion implantation leaves the surface of the whole wafer flat, ready for the next epitaxial process, paving a way for multicolor addressable µLED displays (e.g. n-p-n transistor structure can be grown for making addressable matrix).
2. Preparation of µLEDs with ion implantation
The µLED samples were fabricated in three steps: TJ LED structure epitaxy, defining µLEDs by ion implantation and regrowth of n-type conductive layer on the top. The process flow is presented in Fig. 1(a). Firstly, the epitaxial structure was grown by PAMBE on (0001) HVPE bulk GaN crystals with a threading dislocation density (TDD) of 107 cm−2. The details of the PAMBE growth of high-quality InGaN and GaN layers can be found elsewhere [39]. The TJ LED structure, presented in Fig. 1(b), starts with 200 nm GaN:Si (NSi = 3 × 1018 cm−3). The active region consists of 2.6 nm In0.17Ga0.83N single quantum well (SQW) surrounded by 20 nm In0.02Ga0.98N barriers. Then, a 20 nm Al0.13Ga0.87N:Mg electron blocking layer (NMg = 2 × 1019 cm−3) and 200 nm GaN:Mg (NMg = 1 × 1018 cm−3) were grown. The TJ area is composed of 60 nm In0.02Ga0.98N:Mg (NMg = 3 × 1019 cm−3), 3 nm In0.21Ga0.79N:Mg (NMg = 1.6 × 1020 cm−3), 3 nm In0.21Ga0.79N:Si (NSi = 2 × 1020 cm−3) and 20 nm In0.02Ga0.98N:Si (NSi = 5 × 1019 cm−3). The structure is capped with 100 nm GaN:Si (NSi = 3 × 1019 cm−3).
Secondly, a photolithography mask, that contains microcircles with a diameter in the range of 2 µm to 40 µm for arrays and 3 µm to 40 µm for single devices, was placed on the top of the sample and the ion implantation was carried out to define the size of the µLEDs. We performed simulation of the implantation of helium ions into LED structure using the Stopping and Range of Ions in Matter (SRIM) software [40]. The calculated distribution of vacancies is presented in Fig. 2. The energy of the implanted ions was 30 keV and the dose was 3×1013 cm−2. The TJ is located around 125 nm below the surface (before regrowth) – where the maximum vacancy concentration is expected, while SQW is at a depth of 430 nm. We have chosen the low ion energy for shallow implantation to influence only the electrical properties of TJ (and n-type InGaN and GaN layers above) while keeping the active region of the LED untouched by ions. This approach in principle reduces the probability of creation of point defects in QW and subsequent increase of nonradiative recombination of electrons and holes in this active LED region. The He+ implantation was performed to introduce a high density of vacancies in the area outside the photolithography mask to limit the conductivity of the TJ. This procedure defined the current path through the µLED and therefore the size of the µLEDs.
Fig. 2. Simulation of vacancy profile after He+ implantation of TJ LED structure using SRIM software.
Finally, subsequent regrowth of 200 nm GaN:Si was performed to grow a conductive layer on the top of the devices to make lateral contacts. After regrowth, the devices with sizes of 250 × 350 µm (containing arrays of µLEDs) or 150 × 300 µm (containing single µLEDs) were separated by reactive ion etching (RIE). On the Ga-polar side, n-type Ti/Al/Ni/Au metal contacts were deposited on the sides of the µLEDs as presented in Fig. 1. The same metal contacts were used for the bottom contact on the nitrogen polar side.
3. Experimental results and discussion
3.1. PAMBE regrowth and surface quality after ion implantation
We found, that the He+ implantation procedure, described above, suppresses not only p-type but also n-type conductivity. As an example, we measured the resistance of implanted and un-implanted GaN:Si (NSi = 3 × 1019 cm−3) layers using the Circular Transmission Line Measurements (CTLM). The total resistance as a variation of gap spacing between Ti/Al/Ni/Au contacts for implanted and un-implanted layers is shown in Fig. 3. We observed that after the implantation step, the upper n-type doped layer was insulating. Therefore, after implantation, a regrowth of 200 nm thick highly Si-doped (NSi = 3 × 1019 cm−3) GaN was performed by PAMBE to create a conductive layer to allow current flow into µLED from side contacts (see Fig. 1).
Fig. 3. Circular Transmission Line Measurements. Total resistance as a variation of gap spacing for the (Ti/Al/Ni/Au) /implanted GaN:Si layer and (Ti/Al/Ni/Au) /un-implanted GaN:Si layer.
In order to get more detailed insight into the possible extended defect formation after ion implantation, we performed additional experiments on very low TDD bulk Ammono-GaN crystals (with TDD of the order of 5 × 104 cm−2). The value of He+ energy and ion dose was used the same as for TJ µLEDs. After ion implantation, the 200 GaN:Si current spreading layer was grown by PAMBE. Atomic force microscopy (AFM) measurements and the defect selective etching (DSE) were performed to investigate the presence of extended defects. We compared this with un-implanted sample. Figures 4(a) and 4(b) present the surface morphology of un-implanted and implanted samples, respectively. We found that the GaN surface remains flat with visible atomic steps in both cases. Moreover, the number of dislocations revealed by DSE was comparable for implanted and un-implanted areas - at the level of 5–6 × 104 cm2 (see Fig. 4(c) and Fig. 4(d)), which is typical for low dislocation density Ammono-GaN crystals [41]. From these data, we can conclude that He+ implantation with the energy and dose used for our µLEDs processing does not produce additional extended defects in LED structures. Additionally, the surface is atomically smooth across the wafer, making possible further device growth and stacking.
Fig. 4. The AFM images of 200 nm GaN: Si grown on (a) un-implanted and (b) implanted Ammono-GaN substrate, (c) and (d) - the images of the GaN:Si surface after DSE grown on un-implanted (TDD = 5 × 104 cm−2) and implanted (TDD = 6 × 104 cm−2) Ammono-GaN substrate with 200 nm GaN regrowth, respectively.
3.2. Optical and electrical properties of implanted µLEDs
Circular microdevices with diameters of 2–40 µm were successfully fabricated. Exemplary images collected under an optical microscope of µLED arrays and single µLEDs are presented in Fig. 5, while in Fig. 6(a), we show the electroluminescence (EL) spectrum for different sizes of single µLEDs. We observed a strong blue emission from all of the processed μLEDs with the peak around λ=450 nm at j = 1kA/cm2. In Fig. 6(b) the evolution of the EL spectrum for different current densities for 20 µm µLED is presented. A typical blue-shift of the EL spectra is observed with the increase in current caused by screening of the built-in electric field and reduction of quantum confined Stark effect [42].
Fig. 5. Images of operating µLED arrays. µLEDs size: (a) 40 µm, (b) 20 µm, (c) 10 µm, (d) 5 µm, (e) 3 µm and (f) 2 µm. Images of operating single µLEDs with diameters of the following: (g) 40 µm, (h) 20 µm, (i) 10 µm, (j) 5 µm, (k) 3 µm.
Fig. 6. (a) Electroluminescence spectra of single μLEDs (with diameter d = 3- 40 μm) for current density 1kA·cm−2 and (b) EL of 20 μm μLED for different current densities.
The current density-voltage (j-V), light–current (L-I) and optical power density-current density characteristics of implanted µLEDs are shown in Fig. 7(a), 7(b) and 7(d), respectively. By careful choice of TJ design and doping levels, we could achieve smooth device morphology (RMS = 0.23 nm) with fairly small resistance of the TJ. More details of TJ optimization can be found in Ref. [20].
Fig. 7. (a) The j-V and (b) L-I and characteristics at room temperature of single µLEDs with diameters of 3, 5, 10, 20, and 40 µm operated at a wavelength of 450 nm. (c) Dependence of external quantum efficiency and (d) optical power density on current density for several µLED sizes.
From L-I characteristics presented in Fig. 7(b), we calculated the external quantum efficiency (EQE) according to the following relation:
where L is the measured optical power, I - the applied current, h - the Planck’s constant, q - the elementary charge value and v is the photon frequency [43].However, the value of the EQE is a product of the internal quantum efficiency (IQE), light extraction efficiency and injection efficiency. The IQE is described by the ABC model, which is the ratio between radiative recombination (Rr) and the sum of radiative and nonradiative (Rnr) recombination:
where A, B, C, and n represent Shockley–Read–Hall (SRH) recombination, radiative recombination, Auger recombination coefficients and carrier concentration, respectively [43].We would like to stress here, that the L-I measurements were performed on-wafer, and none of the methods to enhance the light extraction from the device was applied, meaning that only 4% of the light emitted from the active region was extracted and collected by the measurement setup [44,45].
The EQE as a function of the current density is presented in Fig. 7(c). A commonly observed in InGaN LEDs efficiency droop can be seen at higher current densities. It is important to notice that the maximum EQE shifted to higher current densities for smaller diameter µLEDs. The current density for maximum EQE for the largest devices (40 μm) is around 50 A/cm2 and for the smallest (3 μm) around 600 A/cm2. This is a well-known phenomenon in µLED in which the size of the device is defined by etching of the whole epitaxial structure. It is suspected that there is an increase in the A parameter at the side walls of the etched mesa structure and there is an increase in the part of carriers, which recombine non-radiatively (SRH) [46]. However, in this study there was no etching steps affecting the SQW and some point defects could be created by implantation. To get a more detailed insight into the number of point defects created by He+ implantation in the quantum well we probed the samples by cathodoluminescence (CL) spectroscopy. To examine the potential damage, we compared light intensity from the implanted and non-implanted regions at the QW energy and with an acceleration voltage of 15 kV at room temperature. In Fig. 8(a), we present the results of CL from 40 μm μLED taken at the wavelength 468 nm equal to the maximum emission from QW (at low excitation condition). Regions unexposed to the ion implantation have higher light intensity (around four times). Therefore, we conclude that ions create some density of point defects inside the QW, which is inconsistent with the simulations of vacancy profile (see Fig. 2). However, the number of point defects was not high enough to completely suppress CL from QW.
Fig. 8. (a) CL image of the μLED (d= 40 μm) at the quantum well energy for 15 kV. (b) SEM image of the μLED.
3.3. Comparison of ion implanted and etched µLEDs
We prepared additional µLEDs from the same wafer, which were left un-implanted and instead reactive ion etching (RIE) was used to define the device size. After etching of 220 nm by RIE, during which TJ was removed on the entire device surface apart from the µLED area, the GaN:Si regrowth was performed to place the metal contacts on the sides of the µLED. A comparison of the µLEDs structures prepared by ion implantation and RIE is shown in Fig. 9(a) and Fig. 9(b), respectively. The results of EQE measurements performed on µLEDs prepared by RIE show similar behavior as for the implanted ones. We observe an increase in current density at which the maximum EQE occurs for smaller μLEDs diameters. The lower values of current density for EQEmax for RIE samples result from the fact, that this sample was taken from the edge of the wafer where the quantum efficiency was much smaller. Figure 9(c) shows the dependence of the current density j, at which the maximum EQE is observed, on µLEDs diameter for implanted and etched structures. As can be seen, there is a substantial increase in current density j for EQEmax for µLEDs size below 20 µm.
Fig. 9. (a) The schematics of implanted μLEDs and (b) etched μLEDs. (c) Dependence of current density at which the maximum of EQE is observed on the diameter of etched and implanted μLEDs.
In the RIE process ions, which have insignificant energies (∼10 eV), interact only with the surface of the sample and do not destroy QWs. Nonetheless, results of EQE measurements are similar to those obtained for implanted structures. Therefore, we conclude that to explain the performance of the TJ µLEDs not only change of the material parameters A, B, and C plays a role, but also other physical process should be involved. We suspect a change in the carrier density n in the SQW, which can be caused by lateral carrier diffusion in InGaN. It was recently demonstrated, that for high-quality InGaN SQW the lateral carrier diffusion depends on the current density and can range from a few up to a few tens of microns [47]. The origin of this effect is following - diffusion length for carriers ${L_D}$ depends strongly on the value of relaxation time τ (${L_D} = \sqrt {D\tau } $). Here, D is the diffusion coefficient limited by the hole mobility. For high IQE structures and low current densities, where τ is high (limited by SRH, τ =1/A), lateral carrier diffusion could dramatically influence the lateral carrier distribution in SQW, especially for the devices with diameters smaller than 10 µm [47,48]. Having this in mind, we postulate that a possible explanation of the current density behavior for EQEmax could be related to the lateral carrier diffusion, which effectively increases the size of µLEDs. In other words, the active area of the µLEDs could be larger than the one taken from the geometrical size of the TJ surface. Another argument supporting this hypothesis is the non-symmetrical “leakage” current evident for low bias level seen in Fig. 7(a). A more detailed investigation of this effect will be the subject of our future study.
4. Conclusions
We have successfully fabricated TJ nitride µLEDs and arrays of µLEDs with diameter down to 2 μm. The LED wafers were grown entirely by PAMBE. The key point enabling the efficient fabrication was the implementation of the TJ inside the LED structure. We have grown TJs with atomically smooth morphology and fairy small resistance. The size of µLEDs was defined by He+ implantation. The surface of the implanted µLED, examined with AFM, has remained atomically smooth. Neither the increase in the dislocation density appeared. Such properties ensure excellent prospects for further applications that require epitaxial regrowth. This approach paves a way for a novel device architecture with addressable RGB µLEDs arrays. However, for further application, the parameters of ion implantation need to be improved to reduce the number of point defects in the quantum well. We observe the increase in the current density for the maximum of the external quantum efficiency as the dimensions of µLED are reduced. This may be a fingerprint of the change in the external efficiency related to the lateral carrier diffusion (limited by holes) in InGaN quantum wells. This phenomenon will be a subject of future studies.
Funding
Fundacja na rzecz Nauki Polskiej (TEAM TECH POIR.04.04.00-00-210C/16-00); Narodowe Centrum Badań i Rozwoju (LIDER/35/0127/L9/17/NCBR/2018); Narodowe Centrum Nauki (2019/35/D/ST3/03008, 2019/35/N/ST7/02968).
Acknowledgments
We would like to thank the Institute of Electron Technology in Warsaw for their assistance and advice on ion implantation. In particular, special thanks to Maciej Kozubal for your help in ion implantation optimization and simulations.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time, but may be obtained from the authors upon reasonable request.
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