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Abstract
The effects of different p-GaN layer thickness on the photoelectric and thermal properties of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) were investigated. The results revealed that appropriate thinning of the p-GaN layer enhances the photoelectric performance and thermal stability of DUV-LEDs, reducing current crowding effects that affect the external quantum efficiency and chip heat dissipation. The ABC + f(n) model was used to analyse the EQE, which helped in identifying the different physical mechanisms for DUV-LEDs with different p-GaN layer thickness. Moreover, the finite difference time domain simulation results revealed that the light-extraction efficiency of the DUV-LEDs exhibits a trend similar to that of damped vibration as the thickness of the p-GaN layer increases. The AlGaN-based DUV-LED with a p-GaN layer thickness of 20 nm exhibited the best photoelectric characteristics and thermal stability.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapid development of the semiconductor industry, third-generation wide-bandgap semiconductor materials and devices are being widely used in solid-state lighting, optical storage, electronic power devices, and other fields [1]. Group III nitride semiconductor materials, including InN, GaN, AlN, their ternary alloys AlGaN, and InGaN, offer a broad range of light-emitting wavelengths, ranging from deep ultraviolet to infrared [2]. These materials exhibit excellent characteristics such as high-temperature and high-pressure resistance. AlGaN is a wide direct bandgap semiconductor material, that is typical of third-generation wide-bandgap semiconductor materials. By changing the components in AlGaN, the bandgap can be continuously adjusted from 3.4 eV to 6.1 eV, covering the UV band range from 210 nm to 360 nm, which makes it an ideal material for the preparation of deep ultraviolet light-emitting diodes (DUV-LEDs).
AlGaN-based DUV-LEDs have great application prospects in many fields, such as water and air purification, surface sterilization, 3D printing, UV lithography, non-visual communication, ink curing, and anti-counterfeit detection, owing to their compactness and portability, low energy consumption, easy integration, and long life [3]. Consequently, AlGaN-based DUV-LEDs have attracted significant attention and possess a vast potential for further development, making them ideal replacements for traditional UV light sources. Despite the considerable progress achieved in AlGaN-based DUV-LEDs, the luminous efficiency of these devices remains low compared to that of InGaN-based blue LEDs [4,5]. There are many obstacles to achieving high-efficiency DUV-LEDs, such as a large dislocation density in the AlGaN epitaxial layer with high Al content owing to lattice mismatch, strong polarisation effects, and serious energy band tilt, leading to a reduced electron-hole wave function overlap [6,7]. Researchers have made many efforts to enhance the optical power and efficiency of AlGaN-based DUV-LEDs, such as nanopatterning of the AlN buffer layer to reduce the dislocation density, and the utilisation of different UV reflective materials has been explored as a way to improve the light extraction efficiency (LEE) of AlGaN-based DUV-LEDs [8–10]. Moreover, designing an appropriate pattern for the p electrode has been shown to effectively decrease the current crowding effect [11].
Typically, p-AlGaN and p-GaN serve as hole injection layers in DUV-LEDs, the thickness of p-GaN layer generally exceeded 100 nm in prior studies [12,13]. However, because the bandgap energy of GaN is 3.4 eV, there is substantial absorption of DUV light with a wavelength of 280 nm, which results in a decreased LEE of DUV-LEDs. Furthermore, compared to the n-GaN layer, the thickness of the p-GaN layer is generally small, leading to a short distance between the multiple quantum wells (MQWs) and the p-side metal reflector; consequently, the thickness of the p-GaN layer is closely related to the interference field, light intensity distribution, radiation pattern, and LEE of DUV-LEDs [14]. Therefore, optimising the thickness of the p-GaN layer is imperative. To minimise the optical loss of DUV-LEDs on the p side, the p-GaN layer is commonly thinned to reduce light absorption [15]. However, there are few studies on the effects of the p-GaN layer thickness on the photoelectric characteristics, carrier competitive recombination mechanism, and thermal properties of AlGaN-based DUV-LEDs.
In this study, the DUV-LED with p-GaN thickness of 100 nm was used as the reference sample, and combined with the epitaxial layer preparation conditions, the DUV-LED with p-GaN layer thickness of 8 nm was selected as the thinnest sample, and the p-GaN thicknesses of 20 nm and 80 nm were selected from the interval of 8-100 nm, so that the DUV-LEDs with four p-GaN thicknesses were fabricated for the comparison of photoelectronic performance and thermal stability. The forward operating voltage, electroluminescence (EL) intensity, external quantum efficiency (EQE), LEE, light tracing, candela maps, and thermal properties of the DUV-LEDs were studied. Moreover, the ABC + f(n) model was employed to analyse the EQE curves and reveal the carrier recombination mechanism of the AlGaN-based DUV-LEDs.
2. Experiment
DUV-LEDs were grown on a c-plane sapphire substrate using metal organic vapour-phase epitaxy (MOCVD). The aluminium (Al), gallium (Ga) and nitrogen (N) sources were trimethylaluminum (TMAl), trimethylgallium (TMGa), and ammonia (NH3), respectively. The DUV-LEDs structure consisted of a 25 nm AlN template, 1.5 µm-thick AlN layer, 2 µm Si-doped Al0.45Ga0.55N layer, Si-doped Al0.4Ga0.6N/Al0.6Ga0.4N multiple quantum wells (MQWs), 30 nm-thick p-Al0.75Ga0.25N layer (electron-blocking layer), Mg-doped p-GaN layer. Notably, the different thickness of p-GaN layer can be effectively controlled during MOCVD by changing the gas flow rate, temperature, and pressure. The epitaxial samples were cleaned using H2SO2: H2O2: H2O = 5: 1: 1 for 10 min to remove the organic residues. A mesa pattern was formed by photolithography, and then inductively coupled plasma (ICP) etching was performed to produce the n-electrode n-Al0.45Ga0.55N. Subsequently, an isolation pattern was formed for electric isolation of the LED device. Ti/Al/Ni/Au layers were deposited by e-beam evaporation and annealed at 950 °C for 30 s to serve as the n-pad. ITO, as the p-contact electrode with p-GaN, was deposited by sputtering and annealed at 600 °C for 300 s. The ITO thin film was coated with a metal stack layer (Ni/Au) to save as p-pad. To solve the reliability problem, a 1 µm-thick SiO2 layer was deposited on the entire device by plasma-enhanced chemical vapour deposition (PECVD) at 240 °C. Lithography and ICP dry-etching processes were adopted to expose the Ni/Au stacked layer for further contact with bonding pad. Cr/Pt/Au/Sn layers were deposited for the p-and n-contact bonding pads.
After conventional LED processes were performed, the DUV-LED wafer underwent laser scribing to separate it into individual chips. Each LED chip's emissive region measured 20 × 20 mil. The flip-chip structure is used for these DUV-LEDs, because the emission light can be reflected by the p-electrode and also be propagated through the sapphire substrate side, thereby enhancing the light efficiency. The AlN ceramic submount has excellent thermal conductivity and stability, which can improve the heat dissipation effect of the devices. The DUV-LED chips were mounted on an AlN ceramic submount in a flip chip structure by gold-tin eutectic process, soldering at 240 °C for 5 min, the gold-tin ally material melt and formed a strong bond with the chip and the AlN ceramic substrate. In this study, DUV-LEDs with p-GaN layer thicknesses from thin to thick are referred to as samples A, B, C, and D, respectively. A schematic of the DUV-LED structure is shown in Fig. 1(a). Figure 1(b) shows the transmission electron microscopy (TEM) images of the p-GaN layers with four thicknesses. The p-GaN layer thickness of sample A-D are 8.1, 20.3, 81.7, 100.5 nm. The EL spectra of the packaged DUV-LEDs were measured using a calibrated integrating sphere (IS), computer, and electrical current source (Keithley 2400, Keithley Inc.). The thermal resistance and thermal imaging maps were obtained using TERALED & T3Ster thermal transient tester and Research-N2 thermal imager.
Fig. 1. (a) Schematic of the DUV-LED chip with four different thicknesses for the p-GaN layer and (b) the TEM images.
3. Results and discussion
Figure 2(a) shows that the forward voltage versus the injection current increased from 1 to 400 mA for the AlGaN-based DUV-LEDs. The forward voltages for samples A–D were 7.16 V, 6.52 V, 6.68 V, and 6.83 V, respectively, when the injection current was 200 mA. Notably, for samples B, C, and D, the thinner p-GaN layer contributed to a decrease in the vertical resistivity of the DUV-LEDs, consequently resulting in a reduction in the forward voltage. The lateral resistance of the current-spreading layer (CSL) was considerably augmented owing to the substantial discrepancy in thickness between it and the n-AlGaN layer, making it easier for holes to accumulate under the p-electrode. Sample A exhibited the highest forward voltage under the same current conditions may be due to the presence of a too-thin p-GaN layer cannot effectively screen the polarization effect induced elelctric field at the p-AlGaN/p-GaN interface, which caused holes to shift and locally aggregate within the p-GaN layer, limiting the hole injection efficiency, and the high ionization energy of Mg dopants in p-type AlGaN materials with a high Al composition lead to a low hole concentration in the p-AlGaN region, which further reduced the conductivity of the p-type region of the device (increased device resistance), exacerbated the current crowding phenomenon, resulting in an uneven hole concentration distribution [16]. The normalised EL spectra of the four samples at an injection current of 200 mA are shown in Fig. 2(b). The peak intensity initially increased and then decreased as the thickness of the p-GaN layer increased. This is because appropriately increasing the thickness of the p-GaN layer can suppress the extension of the MQW dislocations and improve the crystal quality of the epitaxial wafer such that more holes can be injected into the MQWs. However, further increases of the thickness of the p-GaN layer will no longer improve the hole injection efficiency [17]. Sample B exhibited the lowest operating voltage and the highest EL intensity, which indicates that appropriate thinning of the p-GaN layer enhances the electrical performance of AlGaN-based DUV-LEDs. Hence, it is crucial to select an appropriate p-GaN layer thickness for DUV-LED growth.
Figure 3(a) plots the light output power (LOP) as a function of current density measured at room temperature for the four samples. It is found that the LOP was notably enhanced as the current density increased and exhibits a linear relationship. For the EL spectrum measured by the IS systems, the energy of emitted phonons (Eph) and total radiant energy could be obtained, thus the number of emitted phonons calculated. By calculating the number of electrons under the injected current of I, the value of EQE could be obtained. Figure 3(b) shows the EQE versus current density for the four samples and the curves fitted using the ABC + f(n) model. As depicted in the figure, the peak EQE values of samples A–D are 3.24%, 3.88%, 3.61%, and 3.03% respectively. The superior EQE of sample B can be ascribed to the better current injection efficiency and the low absorption loss of DUV light by the p-GaN layer, consequently resulting in a higher EQE. Notably, sample B exhibited a remarkable enhancement of 28% in the peak EQE compared to sample D.
Fig. 3. (a)The LOP versus current density and (b) EQE fitted by the ABC + f(n) model of the four samples.
The ABC model is a classical model for analysing carrier recombination dynamics in MQWs. The traditional model mainly considers the Shockley–Read-Hall (SRH) non-radiative recombination, band-edge radiative recombination, and Auger recombination and does not include the carrier leakage mechanism in the analysis [18]. To accurately describe the carrier recombination process of DUV-LEDs with different p-GaN layer thicknesses, the carrier leakage term f(n) was introduced in this study. The expression of the ABC + f(n) model is as follows:
where Ф is the radiant flux, Eph is the energy of the emitted phonons, VQW is the volume of the active layer, ηext is the light extraction efficiency, n is the carrier concentration in the active region, I is the injection current, q is the elementary charge, A is the SRH non-radiative recombination coefficient, B is the radiative recombination coefficient, C is the Auger recombination coefficient, and D (f(n) ≈ Dn4) is related to the carrier leakage. The fitting results for each parameter are listed in Table 1.
Table 1. Fitting results of parameters by the ABC + f(n) model
To investigate the carrier recombination mechanism and efficiency droop for the four samples in detail, the fraction of each recombination term at different current densities were calculated, and are shown in Fig. 4. Parameter A is primarily associated with the crystal quality of the epitaxial wafer, and because a large number of defects and impurities are generated in it, a thicker p-GaN layer leads to a higher proportion of term A. Under a lower current density, carriers are more susceptible to capture by defect centres, thereby resulting in a larger contribution of SRH non-radiative recombination. With an increase in the current density, the carrier concentration increases, leading to an increased chance of collision and recombination between electrons and holes, and increasing the probability of band-edge radiative and Auger recombination. A p-GaN layer that is either too thin or too thick leads to an increase in the number of surface defects and interfacial states, these surface scattering and defects have an influence on the motion and interaction of carriers, resulting in a non-uniform local energy distribution and subsequently increasing the probability of Auger recombination. When the p-GaN layer is too thin, some electrons may penetrate the MQWs without being effectively utilised, thus increasing the leakage probability and decreasing the number of carriers involved in radiative recombination in the active region; thus, Term D of sample A is larger than those of the other samples. The fractions of term A and C are relatively small in sample B, which indicates that sample B has fewer defects generated during growth process, and the proportion of carrier leakage (term D) is the lowest, more carriers are injected into MQWs, thus sample B exhibits a high radiative recombination rate owing to the better crystal quality and hole injection efficiency. The droop efficiencies (defined as [(EQEmax – EQEmin)/EQEmax] ${\times} \; $100%) were calculated as 24.42%, 14.00%, 14.02%, and 14.17% for the four samples. The current crowding effect becomes more severe because of the ultrathin p-GaN layer in sample A, which leads to a decrease in the hole injection efficiency and makes it easier for electrons to leak from the active region, thus increasing the droop efficiency [19].
The finite-difference time-domain (FDTD) method has been widely employed to study the optical characteristics and LEE of GaN-based LED structures in which micro- or nanoscale objects are included. In this study, the LEE of an AlGaN-based flip-chip LED was investigated using a 2D-FDTD method. In the simulation model, the epitaxial structure of the AlGaN-based DUV-LEDs and the refractive index of the material were set according to Table 2 [20]. An actual LED device has a thick sapphire substrate, but sapphire is a transparent material with low refractive index and absorption coefficient, variations in sapphire substrate thickness has minimal impact on the mechanisms of light propagation and extraction, and the sapphire substrate thickness in previous simulations model was relatively thin [21–23]. Consequently, due to resource constraints, the model’s substrate thickness was set to 3 µm. The model was surrounded by a perfectly matched layer (PML) absorption boundary to prevent the re-reflection of the output light. In the FDTD simulation, a dipole source was set in the middle of the MQW active region and the centre wavelength was assumed to be 275 nm. To evaluate the LEE, the total dipole radiation power was first calculated in a small box surrounding the dipole source [24], and the radiation power outside the LED chip was calculated in the detection plane near the PML. The LEE was determined as the ratio of the power on the detection planes to the total radiation power. Figure 5 shows the relationship between the LEE of the DUV-LED and the p-GaN thickness. As the thickness of the p-GaN increased, the LEE exhibited a damping-like motion trend instead of a simple linear variation, and the oscillation period is about 50 nm. The underlying reason for this phenomenon is the interference between the light emitted to the sapphire substrate and the light emitted to the p-type electrode and then reflected to the side of the substrate [21]. When the p-GaN layer becomes thicker, the absorption of DUV light increases, resulting in a decrease in the amount of light propagating towards the p-electrode. Consequently, the variation in the amplitude of the LEE associated with the thickness change of the p-GaN layer decreases and may even stabilise. The simulated LEE values for samples A–D are 7.33, 10.69, 9.67, and 7.07%, respectively. In order to verify the accuracy of the simulation results, the internal quantum efficiencies (IQE) of samples A-D are obtained from temperature dependent photoluminescence (TDPL) measurements as 44.58, 45.06, 44.97, and 43.72%, respectively. According to the formula LEE = EQE/IQE, the LEE values of samples A-D are calculated as 7.27, 8.61, 8.03, and 6.93%. However, the influence of environmental factors is introduced in the actual measurement, which makes the measured values slightly lower than the simulated values. The simulation results can provide a reference for improving the LEE of DUV-LEDs.
Table 2. Material parameters of the DUV-LED epitaxial layers
Moreover, Fig. 6 shows the light tracing and candela maps of the four samples simulated by TracePro software, which is utilized for simulating and optimizing optical system based on light tracing techniques. Given that the p-GaN thickness of the four samples are different, while all other structural aspects remain consistent and have the same effect on the light propagation, so the difference in the light tracing results among these samples is predominantly attributed to the variations in p-GaN thickness. The light tracing image of sample B have a large number of lights that can be extracted from the p-GaN layer because the total internal reflections is reduced, and the possibility of improvement of photons escaping from the semiconductor to the air. A comparison of the total flux and the number of light rays for the four samples can also be seen in the candela diagram. In conclusion, the photons escaping from the semiconductor to the air increased as follows: Sample D < Sample A < Sample C < Sample B.
The thermal performance is an important factor affecting the stability and lifetime of AlGaN-based DUV-LEDs [25]. The heat generated in the chip mainly originates from three sources: the heat generated by non-radiative recombination occurring in the active region, Joule heat generated by device resistance, and local heat caused by contact resistance. Figure 7(a) and 7(b) show the infrared thermal image and thermal resistance of the four samples at an injection current of 200 mA and thermal deposition temperature of 25 °C. For sample A, the number of carriers participating in radiative recombination is lower, while the non-radiative recombination generates more heat, and the chip resistance is higher because of the current crowding effect, which in turn leads to an increase in the Joule heat generated during operation. Moreover, in sample A, the p-GaN layer is too thin, which may lead to instability in the metal-semiconductor contact and lead to an increase in contact resistance. Taking the above factors together, the thermal resistance and chip surface temperature of sample A are higher than those of the other samples, which adversely affects the heat dissipation effect. For samples B, C, and D, based on the carrier recombination mechanism, sample B exhibits the highest proportion of radiative recombination, so the heat generated from non-radiative recombination is the lowest compared with sample C and D. The V-I curves reveals that sample B has the lowest device resistance, followed by sample C and D. Consequently, under the same operating current, the heat generated by non-radiative recombination and Joule heating for sample B-D follows: sample B < sample C < sample D. Additionally, an increase in the p-GaN layer thickness will increase the heat conduction path length, rendering heat dissipation more difficult and increasing the chip temperature [26]. Figure 7(c) shows the variation in the maximum temperature of the chip surface with the operating current measured by the infrared thermographic camera, using the temperature at an injection current of 400 mA as a normalised reference. It is clear that the temperature drift of sample A is the most severe as the injection current increases. In contrast, sample B exhibits the best thermal stability and heat dissipation performance owing to its lower non-radiative recombination generated heat, smaller device resistance, and shorter thermal conduction path.
Fig. 7. (a) Surface temperature distribution at injection current of 200 mA, (b) thermal resistance of the four samples, and (c) normalized temperature drift versus current.
4. Conclusions
In this study, the effects of p-GaN layer thickness on the operating voltage, EL intensity, EQE, carrier recombination mechanism, LEE, and thermal properties of AlGaN-based DUV-LEDs were investigated. The results revealed that the AlGaN-based DUV-LED with a p-GaN layer thickness of 20 nm exhibited the best performance in all the above aspects. This result suggests that appropriately reducing the thickness of the p-GaN layer can enhance its photoelectric characteristics and thermal stability. However, if the p-GaN thickness is too small, it will cause a current crowding effect and degrade the performance of AlGaN-based DUV-LEDs. Therefore, it is important to select an appropriate p-GaN layer thickness to improve the performance of AlGaN-based DUV-LEDs.
Funding
Compound semiconductor technology Collaborative Innovation Platform project of FuXiaQuan National Independent Innovation Demonstration Zone (3502ZCQXT2022005); Fujian Province Central Guidance Local Science and Technology Development Fund Project In 2022 (2022L3058); Science and Technology Plan Project in Fujian Province of China (2021H0011); Natural Science Foundation of Fujian Province (2023J06012); National Natural Science Foundation of China (11904302, 62274138).
Acknowledgments
The authors would like to thank Prof. Ray-Hua Horng and Devices Lab (http://rhhlab.wixsite.com/astdl) of the National Yang Ming Chiao Tung University for their helpful support experimental studies.
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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