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Abstract
The effect of atomic-layer deposition (ALD) sidewall passivation on the enhancement of the electrical and optical efficiency of micro-light-emitting diode (µ-LED) is investigated. Various blue light µ-LED devices (from 5 × 5 µm2 to 100 × 100 µm2) with ALD-Al2O3 sidewall passivation were fabricated and exhibited lower leakage and better external quantum efficiency (EQE) comparing to samples without ALD-Al2O3 sidewall treatment. Furthermore, the EQE values of 5 × 5 and 10 × 10 µm2 devices yielded an enhancement of 73.47% and 66.72% after ALD-Al2O3 sidewall treatments process, and the output power also boosted up 69.3% and 69.9%. The Shockley-Read-Hall recombination coefficient can be extracted by EQE data fitting, and the recombination reduction in the ALD samples can be observed. The extracted surface recombination velocities are 551.3 and 1026 cm/s for ALD and no-ALD samples, respectively.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As the demand of information displays keeps rising these years, novel technologies such as liquid crystals (LCs), organic light emitting diodes (OLEDs) were developed to meet this trend [1,2]. These technologies have been evolving into multi-billion-dollar industries and benefit our daily life greatly. The micro display with outstanding characteristics, such as excellent luminescence, high efficiency, low power consumption, high lifetime, fast response time, etc is a highly sought-after technology for next generation of displays [3–5]. Meanwhile, the request for an all-semiconductor micro display is still ongoing, and micro/mini LEDs have been the fore-runner these years. Micro-LED have been developed by many groups for Augmented Reality (AR)/Virtual Reality (VR) applications, wearable devices and extra-large displays [4,6,7]. The near-eye situation of AR and VR goggles inevitably push the resolution of the display much higher to meet the system requirement. The high resolution leads to the small pixels on the display. While the reduced chip size helps to dissipate the junction heat, provide uniform current spreading and improve the efficiency of light extraction, the fabrication of small size µ-LEDs, however, faces some challenges. When the chip size approaches the micron-level, the external quantum efficiency (EQE) drops significantly due to surface recombination and the sidewall damage [8,9]. The chemical contaminations and structural damages during the etching process of the µ-LED chip leads to the increase of the Shockley-Read-Hall (SRH) non-radiative recombination. The etched surface is bound with crystallographic defects, impurities, and dangling bonds that builds trap states within the bandgap and thus also serve as non-radiative recombination centers [10,11]. In the past, several reports have been published to discuss the effect of surface recombination and the sidewall treatment for the improvement of the micro LED efficiencies. The inclusion of TMA/nitrogen plasma and ALD passivation can effectively boost up the quantum efficiency of the micro LEDs [12]. The size-dependent effect of the micro LEDs were also investigated numerically to demonstrate the impact of the sidewall recombination [13].
Currently, the passivation of the sidewall using dielectric materials has been shown as an effective solution to mitigate the impact of sidewall defects. A plasma enhanced chemical vapor deposition (PECVD) technique has been widely used for sidewall passivation due to its fast deposition rate, but it will lead to plasma-induced sidewall damage which generates leakage current [8,14]. On the other hand, the atomic layer deposition (ALD) technique can provide better sidewall passivation with its nanometer-scale dielectric deposition [15–18]. However, the ALD process has a low deposition rate and the final thickness of the thin film is small compared to its PECVD counterpart. Whether the addition of such a thin layer of dielectric material can be examined properly by a comparative study with no-ALD samples. In this work, the ALD-Al2O3 sidewall treatment effect on the electrical and optical properties of InGaN-based µ-LED is investigated. By means of comparing the samples with and without depositing aluminum oxide (Al2O3) passivation layers, the size dependent characteristics of GaN based micro LEDs ranging from 5 to 100 µm can be further analyzed.
2. Experimental details
The µ-LED structures are square-shaped mesas sized from 5 х 5 µm2, 10 х 10 µm2, 20 х 20 µm2, 50 х 50 µm2 and 100 х 100 µm2, and the emission wavelength is about 445 nm because of the InGaN/GaN active region in the device. In this study, two different types of passivation layers are used: (a) ALD Al2O3 +PECVD SiO2 passivation and (b) PECVD SiO2 passivation. These oxides are good passivation material due to their capability to stop the oxidation of the cation atoms in III-V materials at the sidewall/air interface [19]. A study was performed previously based on the same passivation material (SiO2) but different deposition technique (ALD vs. PECVD). The result showed the ALD device can survive longer under high current aging process [16]. We believe that the existence of ALD layer can greatly help the device performance. All of the blue LED wafers were grown on c-plane sapphire substrate using a metal-organic chemical vapor deposition (MOCVD) system. The main epitaxial layers include Si-doped GaN (n-GaN), the multiple quantum wells (MQWs) active region consisting of twelve pairs of InGaN wells and GaN barrier and Mg-doped GaN (p-GaN) epilayer. The fabrication process are as follows: the thickness of 100 nm indium tin oxide (ITO) was deposited first by electron-beam evaporation as a transparent conducting oxide (TCO). Moreover, the rapid thermal annealing under 450°C for 2 min is followed to form an ohmic p-contact. TCO layer will provide an effective current spreading layer and a clear emission window for the device. Next, to etch the TCO film, a wet etch procedure was used. The ITO etchant is the commercial product from Taiwan Maxwaves Co. The product number of the etchant is ITO-ETCH-B1, and it contains hydrochloric acid (HCl) and nitric acid. The wet etch of ITO layer was performed under the condition of 50°C solution temperature and the photoresist was patterned as the etching mask. The inductively coupled plasma-reactive ion etching (ICP-RIE) were used to achieve 1 µm depth mesa etch. After forming a µ-LED mesa, a sidewall passivation layer of 7 nm ALD-Al2O3 and/or 300 nm PECVD-SiO2 was deposited. The ALD system is PICOSUN R-200 Advanced system made by Picosun Oy. Moreover, ALD-Al2O3 was deposited by Al(CH3)3 (trimethylaluminum, TMA) and H2O as precursors which was deposited at 300 °C. The sidewall treatment process is consisted of a given number of identical cycles. The pulses are separated by a flush of N2 to assure the two chemicals never meet in the gaseous state. Next, the via-hole process to the ALD layer is proceeded by ICP-RIE etching process to open up proper contact area and then the second ITO deposition process is followed as re-distribution layer (RDL) which is regarded as an extended conduction layer. Subsequently, the electrode metal consisted of 50/300 nm of Ti/Au was deposited by electron-beam evaporation as the p-type and n-type electrode to complete the µ-LED device. The schematic diagram of the device is shown in Fig. 1(a), and an optical microscopic picture of the single device is in Fig. 1(b). The Transmission Electron Microscope (TEM) image shows ALD-Al2O3 layer coverage under different sidewall positions as shown in Fig. 1(c)-(f). The TEM was performed by an external vendor (Integrated Service Technology Inc., Hsinchu, Taiwan). The general preparation procedures include: the deposition of the protective Pt metal layer, first rough milling by focus ion beam (FIB) etch, partial removal of the side material of the target sample, 2nd FIB thinning, final cutting and the liftoff of the lamellar piece of the target sample. The model number of TEM system is JEM-2800F and the FIB’s is Helio-660. The images revealed that thickness of Al2O3 layer on the mesa surface is about 7.3 nm and it becomes 6.8 nm when the layer extends to the bottom of the trench in Fig. 1(e)(f). According to the definition of step coverage (which is equal to the thickness ratio between the top surface and the trench bottom of the layer), our ALD deposition’s step coverage is about 93.1% for the bottom surface. The current step coverage of the ALD layer is still not optimized to 100% between the top of mesa and the bottom of the trench. In ALD process, chemistry, stoichiometry, and kinetics are three important factors that can be discussed in the step coverage issue [20,21]. Chemistry means to find the suitable complementary reactants for the ALD process, and supplying them in an alternating way to the wafer surface. Sufficiently large quantities of these reactants need to be provided in the system to achieve stoichiometric condition. Finally, the diffusion distances that these reactants can reach on the surface and the sticking coefficients of the reactants to the surface will determine whether the kinetic model will work for the complicated surface (like trenches) or not. Because Al2O3 is a mature ALD material, we believe the basics of these three factors shall be fulfilled to a flat surface in our system. However, the process details such as the substrate temperature uniformity, the uniform distribution of precursors, the purge vs. pulse time, the amount of reactants for our micro LED structures are not optimized currently. The complex behavior of vapor by-products which could change the reactant adsorption or etch back the film in the process are also untapped in our process. We believe further investigation should be necessary to achieve 100% step coverage in ALD for a micro LED structure.
Fig. 1. (a)The schematic diagram of µ-LED device; (b) Top-view optical microscopy of the 5 х 5 µm2 device; (c)-(f) The TEM image of µ-LED with ALD sidewall treatment under different position.
The elemental composition of the ALD-Al2O3 sidewall passivation layer can be shown in Fig. 2. From the energy-dispersive X-ray spectroscopy (EDX) map of the individual elements in Fig. 2(b), we can see clear indium signals in the mesa region, which is the sign of InGaN QW structure. Another sets of Al and O layers demonstrate a uniform Al2O3 deposition on the surface of mesa. The strong signal of silicon in the right-hand region of the EDX figure shows the pre-dominant PECVD SiO2 layer outside of the ALD-Al2O3 layer.
After the devices were finished with semiconductor processes, the electrical and photonic characterization were carried out. The detailed current density versus voltage curves can be measured by Agilent 4156C precision semiconductor parameter analyzer. An integrating sphere system (Isuzu Optical SLM-12 system) with BaSO4 surface coating was used for optical spectrum collection, and a power supply (Keithley 2400 source meter) was used to apply the electrical signal to the device. The detected photons are collected and fed into an Ocean Optics QE65000 spectrometer, whose spectral range is suitable for visible light detection (360nm-1000 nm).
3. Results and discussions
The effectiveness of ALD passivation can be observed from the variations in the electrical current density-voltage (J-V) characteristics of µ-LED. Fig. 3 shows the J-V characteristics for devices with size of 5 × 5 µm2, 10 × 10 µm2, 20 × 20 µm2, 50 × 50 µm2 and 100 × 100 µm2 with and without 7-nm ALD sidewall treatment. Under forward bias, we saw better diode J-V characteristics for small devices with the ALD passivation layer. This behavior can be attributed to a better surface quality and the elimination of parasitic current with ALD sidewall treatment [12,22,23]. Figs. 3(a)(b) show the devices’ J-V characteristics. Between the ALD-coated and pure PECVD devices, we saw the changes in series resistance (Rs) from 8.26k Ohm (PECVD) to 4.73k Ohm (ALD) in 10 µm devices. The reverse leakage current, which is evaluated at -6 V, also shows signs of improvement when ALD treatment is presented. The 5-micron device shows about an order of magnitude of reduction (from 6.79 х 10−6 to 2.71 х 10−7 A/cm2), and the 10-micron device shows a 60% reduction, respectively. Large devices also demonstrate different degrees of improvements as we summarize them in Table 1. The major source of the process-related leakage currents can rise from the dry-etch caused defects that are located in the mesa side wall [24,25]. Once these defects are properly passivated, the leakage becomes lower. Further assessment of the reverse bias leakage current showed sometimes the large devices (such as 100 µm ones) having higher currents even under the protection of ALD passivation layer. The possible cause behind this situation might be the multiple sources of leakage. In general, we can categorize the device leakage into two sources: bulk and peripheral leakage [26,10]. In small devices, such as 5 µm and 10 µm ones, the peripheral leakage from the sidewall becomes important, while bulk leakage due to defects or dislocations should play more roles in the larger devices. For the bulk leakage, there are also several effects that help carrier to transport: Poole-Frenkel emission, carrier hopping through traps, phonon-assisted tunneling, etc. [27–29] It is very possible that the local defects in the 100 µm ALD device in Table 1 were assisting carrier transportation and causing higher leakage under reverse bias.
Fig. 3. The semi-log scale J-V characteristics of micro LEDs (a) without ALD (b) with ALD passivation layers (c) The forward bias J-V for 5 and 10 micron devices with and without ALD.
Table 1. The reversed bias leakage current density (A/cm2) for micro LEDs with different sizes
Figure 4(a)(b) present the spectra of 5 µm devices with and without ALD sidewall treatment process from 2 A/cm2 to 200 A/cm2. The shoulder in the long-wavelength side of the peak in the ALD device could be the result of local variation in the indium composition, which was reported previously [30,31]. In the same process run, the similar situation was also observed in other no-ALD devices. The peak wavelength of the emission spectrum is around 445 nm, and the emission peak shifts towards shorter wavelength when the current increases. As shown in the inset of Fig. 4, the Δλpeak between 2 and 200 A/cm2 changes to -7.76 nm for the no-ALD device and -9.36 nm for the ALD one. The linewidth of the emission spectrum (FWHM) also changes from 16 nm to 24 nm for the no-ALD device while the ALD one has its FWHM between 15.7 and 27 nm. On the other hand, the characteristics of the light output power of the 5 × 5 and 10 × 10 µm2 devices are demonstrated in the Fig. 4(c). The power densities of 5 × 5 and 10 × 10 µm2 devices increased from 63.2 W/cm2 to 107.0 W/cm2 and from 70.2 W/cm2 to 119.3 W/cm2 after ALD sidewall treatment. In comparison to those without ALD sidewall passivation, the 5 × 5 and 10 × 10 µm2 devices with sidewall treatments yielded an increase of 69.3% and 69.9% respectively in light output power. Although both types of samples have similar dielectric coating on the mesa sidewall, the ALD devices have better current injection than that of the PECVD devices as shown in Figs. 3(a) to (c). Since the injected carriers are directly recombined for spontaneous emission, higher currents could mean more photonic output. [12,22] The light output power of the 5 × 5 µm2 device is still lower than that of the 10 × 10 µm2 device, as smaller devices are more likely to suffer from greater impact caused by sidewall damage from surface recombination and non-radiative recombination [32].
Fig. 4. A series of spectra for a 5 µm device (a) without the ALD layer and (b) with the ALD layer. The insets are the FWHM and the peak wavelength at various current levels. The current levels are from 2 A/cm2 to 200 A/cm2. (c)The output power density of 5×5 µm2 and 10×10 µm2 with ALD sidewall treatment and without ALD sidewall treatment.
The EQE measurements of the samples with and without ALD sidewall treatment process are shown in Fig. 5(a)(b). From the plots, the µ-LED devices for 5 х 5 µm2, 10 х 10 µm2 exhibited lower droop efficiency than other chip size LEDs at current density from 10 A/cm2 to 100 A/cm2, which can be attributed to uniform current spreading. The droop efficiency is defined as (EQEpeak – EQEcurrent density) / EQEpeak х 100%. For 5 × 5 µm2, 10 × 10 µm2, 20 × 20 µm2, 50 × 50 µm2 and 100 × 100 µm2 without ALD devices, the droop efficiencies are 28.3%, 23.2%, 63.3%, 34.5% and 28.3%. After ALD sidewall treatment passivation, it can be seen that the efficiency droops about 18.4%, 25.0%, 28.8%, 29.8% and 24.3% in Fig. 5. In Fig. 5, different shape of EQE profile versus current density can be observed. The main reason is due to the different setting of x-axis (current density). In the linear scale, due to the rapid change at low current density, one will see different peaked profiles (or shape). Once you transform the linear scale to the log scale in x-axis, the EQE profile become bell shape as we will see later. Meanwhile, some higher-than-expected droop in the high current range was found among these devices, which might rise from the lack of acceptor in the region close to quantum well [33,34].
Fig. 5. (a) EQE as function of current density for different LED sizes without ALD-Al2O3 sidewall treatment process; (b) EQE as function of current density for different LED sizes with ALD-Al2O3 sidewall treatment process.
On the other hand, the comparison of the size-dependent peak EQE of various µ-LED with and without ALD sidewall treatment process are shown in Fig. 6(a). The gap of peak EQE of the ALD/no-ALD devices of the same size grows narrower as the mesa size increases. For the 5-micron devices, the difference(ΔEQE) between the ALD and no-ALD devices is large: 16.6% vs. 28.8%, which corresponds to a 73.47% enhancement. But for the 100-micron case, the incremental gap becomes 6.538%. The detailed numbers are summarized in Table 2. This narrowing gap indicates that the ALD sidewall treatment impacts more on the performances of 5 х 5 µm2 and 10 х 10 µm2 devices. The current density where the EQE peak happens can also be evaluated for both ALD-coated and no-ALD samples. The performances of the individual devices are varied, but the good devices for ALD-coated 5-micron size can demonstrate the peak current density around 9 A/cm2, while the best devices for no-ALD samples are higher than 10 A/cm2, as shown in Fig. 6(b).
Fig. 6. (a) Comparison of peak EQE values before/after ALD sidewall treatment, and the dashed line is to guide the eyes; (b) the normalized quantum efficiency of 5 µm devices with and without the ALD coating. The difference in peak current density can be observed.
Table 2. The maximal values of measured EQE for devices of different sizes in Fig. 5
Another important attribute we could extract from EQE data is the internal recombination mechanism. This mechanism is widely known as the ABC model which describes the Shockley-Read-Hall, the bimolecular radiative and Auger recombination processes in a LED device [16,27]. These radiative and non-radiative recombinations consume all the injected carriers and the total current density of a LED device can be shown as [16]:
, where Jtotal is the total current, q is the elementary charge, t is the thickness of the active layer, and n is the excess carrier density. The three parameters, A, B, and C, represent the SRH, bimolecular, and Auger coefficients, respectively. The EQE can be calculated as [14,16,10]: where ηLEE is the light extraction efficiency of the micro LED. By fitting the model to the measured results, we can extract the A, B, and C numerically. Fig. 7 shows the calculation and the measured EQE results.Fig. 7. The measured and calculated EQE profiles vs. current density of a ALD coated 100 µm micro LED and a 5 µm one. Solid curves are the fitted calculation based on the ABC model.
Among these A, B, and C numbers, the SRH coefficient A is particularly important because it is strongly size-dependent [26], and can be formulated by [14,26,10]:
, and A0, vs, Smesa, and Lmesa are the bulk SRH coefficient, the surface recombination velocity, the area of the mesa, and the side length of the mesa, respectively. Fig. 8 shows the extracted SRH coefficients of the devices with and without ALD layers plotted against the inverse of the side length of the device. As we could see in the figure, the error bars in no-ALD samples are wider, and this situation could indicate a more scattered distribution of SRH coefficients among the devices of the same size. The extracted surface recombination velocities for ALD and no-ALD devices are 551.3 and 1026 cm/s, respectively. According to previous studies, our results are within the reported values (300 to 10000 cm/s) [35]. The bulk SRH (A0) values are also different (ALD samples: 8.440 × 105 sec-1; no ALD samples: 1.485 × 106 sec-1), which might be caused by the local difference in crystalline quality, but both of them fall in line with what was reported previously (mid-105 to mid-106 sec-1) [36,37].Fig. 8. the extracted SRH coefficients of the devices with and without ALD layers plotted against the inverse of the side length of the device.
4. Conclusion
In this study, we successfully fabricated InGaN-based blue LED chip with various sizes (5 × 5 µm2, 10 × 10 µm2, 20 × 20 µm2, 50 × 50 µm2 and 100 × 100 µm2). The effect of sidewall damage is expected when chip sizes reduced to the microscale level. The ALD sidewall treatment process is crucial technique to reduce the leakage current for all chips as well as enhance the external quantum efficiency. The 5 × 5 µm2 devices can have maximal EQE at around 9 A/cm2 when ALD treatment is presented, and the no-ALD devices generally find their peak above 10A/cm2. The EQE enhancement of 5 × 5 µm2, 10 × 10 µm2 are 73.4% and 66.4% after ALD-Al2O3 sidewall treatments process, and significant output power increase is also observed. Further analysis by ABC model reveals the surface recombination velocity reduction and the decreasing SRH coefficients in the ALD devices. We hope our study on ALD treatment for micro LEDs can lead to an efficient and small-sized micro display panel which shall be the thrust force of the next generation micro display.
Funding
Ministry of Economic Affairs (111-EC-17-A-24-1579); Ministry of Science and Technology, Taiwan (108-2221-E-009-113-MY3, 110-2124-M-A49-003, 110-2218-E-A49-012-MBK, 110-2221-E-002-186-MY3).
Acknowledgments
The authors would like to thank Electronic and Optoelectronic System Research Laboratories, Industrial Technology Research Institute (ITRI) for the helpful discussion.
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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