AlGaN-Based Deep Ultraviolet Light-Emitting Diodes with Thermally Oxidized Al x Ga2-x O3 Sidewalls.

AlGaN and GaN sidewalls were turned into Al x Ga2-x O3 and Ga2O3, respectively, by thermal oxidation to improve the optoelectrical characteristics of deep ultraviolet (DUV) light-emitting diodes (LEDs). The thermally oxidized Ga2O3 is a single crystal with nanosized voids homogenously distributed inside the layer. Two oxidized Al x Ga2-x O3 layers were observed on the sidewall of the AlGaN layer in transmission electron microscopy images. The first oxidized Al x Ga2-x O3 layer is a single crystal, while the second oxidized Al x Ga2-xO3 layer is a single crystal with numerous nanosized voids inside. The composition of Al in the first oxidized Al x Ga2-x O3 layer is higher than that in the second one. The thermal oxidation at high temperature degrades the quality of the p-GaN layer and increases the forward voltage from 8.18 to 11.36 V. The thermally oxidized Al x Ga2-x O3 sidewall greatly enhances the light extraction efficiency of the lateral light of the DUV LEDs by combined mechanisms of holey structure, graded refractive index, high transparency, and tensile stress. Consequently, the light output power of the DUV LEDs increases from 0.69 to 0.88 mW by introducing a 420 nm thick Al x Ga2-x O3 sidewall oxidized at 900 °C, in which the enhancement of light output power can reach 27.5%.

Introduction

Deep ultraviolet (DUV) light is a very important light source in many fields, such as UV photolithography, high-density optical data storage, water purification, and portable chemical/biological agent detection/analysis systems.[1−5] However, most DUV light sources are not efficient enough. Recently developed AlGaN-based DUV light-emitting diodes (LEDs) possess enhanced light output efficiency because the optimization of epitaxy techniques has improved the material quality of AlGaN.[6−11] Therefore, AlGaN-based DUV LEDs are replacing conventional DUV light sources, such as mercury and xenon lamps. Despite the great enhancement in DUV LED performance, DUV LEDs still have high defect density, poor carrier injection, and low carrier confinement, which decline the output power. Many methods have been proposed to reduce the defects of high-Al AlGaN on sapphire, such as epitaxial lateral overgrowth AlN/sapphire templates,[12] high-temperature-annealed and sputtered AlN/sapphire,[13] multilayer AlN buffers,[14,15] and low-defect density bulk AlN.[16] Many studies proposed Mg-doped Al(In)GaN/Al(In)GaN superlattice structure to replace the electron blocking layer (EBL) to deal with low carrier confinement and hole injection issues.[15,17−22] This method can effectively increase the hole concentration and band offset of EBL to improve hole injection and electron confinement. In addition to EBL with an Al(In)GaN/Al(In)GaN superlattice structure, the use of the wide band gap interlayer,[23] serrated p-AlGaN region,[24] and step[25] and linear[26,27] grading profiles of p-AlGaN was proposed to replace traditional p-AlGaN EBL. The p-AlGaN inserted layer with an aluminum percentage less than the last barrier[28] and a thick Mg-doped last barrier[29] increased hole injection efficiency, electron blocking efficiency, and hole concentration in the inserted layer. Besides, the spontaneous and piezoelectric polarization in AlGaN/AlGaN multiple quantum wells (MQWs), which reduce the overlap of the electron and hole wave functions in MQWs, degrade light output power.[30] Several methods have been reported to overcome the polarization effect in MQWs, such as the nonpolar/semipolar orientation of MQWs,[31−33] silicon dopant in the quantum barrier,[34] graded quantum well structure,[35] polarization matched AlInGaN quaternary barrier,[36] and introducing the large misoriented sapphire substrate.[37]

In addition, another problem with achieving the high efficiency of DUV LEDs is the low light extraction efficiency (LEE). The low light extraction of DUV LEDs would be attributed to the absorption of the p-GaN contact layer and the unique anisotropic optical polarization property of AlGaN MQWs.[38−41] Light is emitted as transverse electric (TE) or transverse magnetic (TM) polarization in AlGaN-based DUV LEDs. Light emission by the transition between conduction band to heavy hole (HH) band is primarily TE-polarized, and light emission by the transition between conduction band to crystal-field split hole (CH) band mainly is TM-polarized. The top valence band of AlN is a CH band, and that of GaN is a HH band; therefore, the proportion of TM-polarized light emission increases as the Al composition of AlGaN increases.[42−44] Therefore, high-Al AlGaN MQWs would dominantly emit TM-polarized light that propagates perpendicular to the c-axis with the light’s electric field parallel to the c-axis (E∥c). The contribution of TM polarization increases with decreasing wavelength and would also cause poor LEE. The LEE can be enhanced by introducing a flip-chip design,[38,45] a patterned sapphire substrate,[8,46] a patterned p-type layer,[47] or a nanowire structure.[48,49] Besides, Guo et al.[50] and Yan et al.[51] proposed substrate sidewall roughening and low refractive index antireflection coating, respectively, to enhance the LEE.

In this study, we proposed AlGaN-based DUV LEDs with thermally oxidized AlGa2–O3 and Ga2O3 sidewalls. The AlGaN and GaN sidewalls of the DUV LEDs were thermally oxidized into AlGa2–O3 and Ga2O3, respectively. The refractive index of the thermally oxidized AlGa2–O3 sidewall is lower than that of AlGaN. The low refractive index of the AlGa2–O3 sidewall could enhance the LEE of DUV LEDs. Besides, the optoelectrical properties of the DUV LEDs could be modified by the thermally oxidized AlGa2–O3 sidewall because of their crystal properties. The impacts of the AlGa2–O3 sidewall on the electrical and optical properties of AlGaN-based DUV LEDs, as well as the fabrication process, are discussed in this study.

Experiments

The epitaxy of AlGaN-based DUV LEDs on a 2 in. (0001) sapphire substrate was performed by a Thomas Swan close-coupled showerhead (31 × 2 in.) metal–organic chemical vapor deposition system. The details of epitaxy were similar to those in our previous study,[52] except for the p-layers. The p-layers used in the present study were a 20 nm-thick Mg-doped graded Al composition AlGaN layer and a 400 nm-thick Mg-doped GaN layer. The detail of the epitaxy structure is presented in Figure S1. The epitaxy wafers were processed to fabricate 300 μm × 300 μm DUV LEDs by standard processing steps. The mesa area of the DUV LEDs was formed by an inductively coupled plasma (ICP) dry etching process. A SiO2 layer was deposited on the mesa and exposed n-AlGaN. The p-GaN and AlGaN sidewalls were revealed after removing the SiO2 from the mesa sidewall by lithography and wet etching. Subsequently, the exposed sidewalls of the DUV LEDs were thermally oxidized in a furnace in ambient air with an oxygen (O2) flow of 10 SLM at 850, 900, and 950 °C. Thermally oxidized AlGa2–O3 and Ga2O3 sidewalls were prepared at different oxidation temperatures. Besides, the effects of the thickness of AlGa2–O3 and Ga2O3 were investigated. AlGa2–O3 and Ga2O3 samples with different thicknesses were prepared by varying the reaction time in the furnace. Eventually, the AlGa2–O3 sidewalls with average thicknesses of 140, 280, and 420 nm were obtained. The corresponding average thicknesses of the Ga2O3 sidewalls were 70, 140, and 210 nm because of the different oxidation rates of GaN. After sidewall oxidation, a 100 nm thick SiO2 was deposited on the sidewall. The purpose of the SiO2 layer was primarily to prevent moisture in the air. Besides, the refractive index of SiO2 (n = 1.52)[53] is higher than the refractive index of air (n = 1), and it would also improve light extraction due to reduced total internal reflection. Ti/Al/Ni/Au was deposited on the exposed n-AlGaN layer to be an n-pad, and Ni/Au was deposited on the p-GaN contact layer to form a transparent contact layer (TCL). Finally, Cr/Au was deposited on the TCL to be a p-pad. The detailed schematics of the DUV LED chip are presented in Figure . LEDs without sidewall oxidation were also prepared as reference LEDs for comparison.

Figure 1

Schematics of the AlGaN-based DUV LED chip in (a) cross-sectional and (b) top view.

Surface morphology was analyzed by scanning electron microscopy (SEM), and SEM was performed by JEOL JSM-7000 operated at 10 keV. Transmission electron microscopy (TEM) was performed by a JEOL JEM-2800F transmission electron microscope/scanning transmission electron microscope equipped with two energy-dispersive X-ray spectroscopy (EDX) detectors and operated at 200 keV. The wavelength of the excitation laser for the Raman spectroscopy was 532 nm. The intensity of the Raman spectra was recorded by a wavenumber step of 0.67 cm–1 in the range of 200–1000 cm–1. The current–voltage (I–V) characteristics of the fabricated DUV LEDs were measured by a Keysight B1500 and a HP-4156B semiconductor parameter analyzers for high- and low-current measurements, respectively. The output power and emission spectra of the LEDs were acquired at room temperature using a calibrated integrating sphere and a spectrometer (Ocean Optics USB2000), respectively.

Results and Discussions

Figure shows the tilted SEM images of the reference DUV LED and the DUV LEDs with sidewall oxidized at 850, 900, and 950 °C. The sidewalls of the DUV LEDs with thermal oxidation were Ga2O3 and AlGa2–O3 layers, which were derived from the oxidation of GaN and AlGaN sidewalls of the DUV LED. In the insets of Figure , the morphology of the oxidized sidewall presented clear grains, and the surface of the oxidized sidewall became rougher with increasing oxidation temperature. Figure shows the TEM images of the DUV LEDs with sidewalls oxidized at 850, 900, and 950 °C. The oxidation time at the oxidation temperatures of 850, 900, and 950 °C were 60, 30, and 15 min, respectively. All these DUV LEDs possess Ga2O3 and AlGa2–O3 sidewalls with similar average thicknesses. The average thickness of Ga2O3 is 210 nm (TEM images in Figure S2), and that of AlGa2–O3 is 420 nm. Figure d shows the oxidation rates of GaN and AlGaN to form Ga2O3 and AlGa2–O3 at different oxidation temperatures. According to TEM images in Figure , the oxidation rates of AlGaN at 850, 900, and 950 °C were 7, 14, and 28 nm/min. The oxidation rates of GaN at 850, 900, and 950 °C were 3.5, 7, and 14 nm/min. The oxidation rates doubled every 50 °C increase in oxidation temperature, and the oxidation rate of AlGaN was almost twice as fast as that of GaN at the same temperature. Al atoms in AlGaN are considered to be more reactive with O2 than Ga atoms, and the high Al content of Al0.6Ga0.4N tends to accelerate the oxidation rate. All these effects result in a higher oxidation rate of AlGaN compared with that of GaN. All the Ga2O3 sidewalls oxidized at 850, 900, and 950 °C showed single crystals with numerous domains ranging from several to several tens of nanometers in size. However, two oxidized AlGa2–O3 layers were observed on all the DUV LEDs oxidized at temperatures of 850, 900, and 950 °C. The two AlGa2–O3 layers show quite different characteristic contrast in TEM images. The first oxidized AlGa2–O3 layer, denoted as AlGa2–O3(I) in Figure , shows a nearly homogeneous contrast throughout the whole phase. TEM images of this kind of characteristic indicate that AlGa2–O3(I) is a single crystal. The second oxidized AlGa2–O3 layer, denoted as AlGa2–O3 (II) in Figure , shows a high density of black/white contrast variation. The size of black and white domains is much smaller than that of Ga2O3. The domain size in the second oxidized AlGa2–O3 seems to shrink with increasing oxidation temperature, and those domains of bright contrast in TEM BF (bright-field) images are identified to be voids by comparing TEM BF images and TEM DF (dark-field) images during TEM observation. Both holey Ga2O3 and AlGa2–O3(II) phases are single crystals determined by their corresponding selected area electron diffraction (SAED), as shown in Figure S3.

Figure 2

Tilted top-view SEM images of (a) reference DUV LED and the DUV LEDs with sidewall oxidized at (b) 850, (c) 900, and (d) 950 °C. The insets are magnified top-view SEM images of the sidewalls of the associated DUV LEDs.

Figure 3

(a–c) TEM BF images of the Ga2O3 and AlGa2–O3 sidewalls. The oxidation temperatures of the Ga2O3 and AlGa2–O3 sidewalls were (a) 850, (b) 900, and (c) 950 °C. (d) Oxidation rates of GaN and AlGaN in different temperatures.

EDX was performed on all samples in the TEM analysis to explore the compositions of the oxidized Ga2O3 and AlGa2–O3 layers. Figure shows the EDX line profiles of Ga2O3 and AlGa2–O3 oxidized at 850, 900, and 950 °C. Approximately 1–2 at. % nitrogen was detected in the Ga2O3 and AlGa2–O3 sidewalls oxidized at 850 °C. The concentration of nitrogen in Ga2O3 and AlGa2–O3 oxidized at 900 and 950 °C is lower than the detection limit of EDX. The AlGa2–O3 oxidized at 850 °C shows a quick drop in Al composition from 1.5 (at close to SiO2 side) to 1.33 (at close to AlGaN side) in the AlGa2–O3 (I) layer. The AlGa2–O3 sidewalls oxidized at 900 and 950 °C shows a slow drop in Al composition from 1.33 to 1.28 and from 1.3 to 1.17 in the AlGa2–O3(I) layer. Different from the Al composition in AlGa2–O3(I) layers, the Al composition of AlGa2–O3(II) layers almost keeps constant through whole layers for all oxidized conditions. Therefore, the average Al composition of the AlGa2–O3(I) layer is higher than that of the AlGa2–O3(II) layer. The detailed mechanism of different Al composition profiles in two AlGa2–O3 layers is under investigation. Based on the SEM and TEM results, the Ga2O3 and AlxGa2–xO3 sidewalls of the DUV LEDs are formed via the thermal oxidation of the GaN and AlGaN layers.

Figure 4

EDX line scan of the composition profiles of the oxidized (a–c) Ga2O3 and (d,e) AlGa2–O3 sidewalls. The oxidation temperatures of Ga2O3 and AlGa2–O3 sidewalls were (a,d) 850, (b,e) 900, and (c,f) 950 °C.

The DUV LEDs with oxidized sidewalls were fabricated into devices to study their optoelectrical properties. Figure shows the I–V characteristics of the reference DUV LED, the DUV LEDs with 420 nm thick AlGa2–O3 oxidized at different temperatures, and the DUV LEDs with different thicknesses of AlGa2–O3 oxidized at 900 °C. The results showed that oxidation temperature and AlGa2–O3 thickness can affect the forward voltage (Vf) at 20 mA. Both oxidation temperature and oxidation time can increase the 20 mA Vf based on the I–V characteristics, as shown in Figure . The reference DUV LED had a 20 mA Vf of 8.18 V, while the 20 mA Vf of the DUV LEDs with 420 nm thick AlGa2–O3 oxidized at 850, 900, and 950 °C increased to 10.20, 11.36, and 13.06 V, respectively. Besides, the 20 mA V of the DUV LED with AlGa2–O3 thicknesses of 140, 280, and 420 nm increased to 9.60, 11.00, and 11.36 V, respectively. The 20 mA Vf of the DUV LEDs increased rapidly at oxidation temperature over 900 °C and increased steadily with the increase in oxidation time. Therefore, oxidation temperature was a more dominant factor for Vf than oxidation time. The high 20 mA Vf can be attributed to the increase in the series resistance of the DUV LEDs. The dynamic resistances of the DUV LEDs are shown in Figure . The dynamic resistances of the DUV LEDs increased with either elevated oxidation temperature or increased oxidation time, and the DUV LEDs with 420 nm thick AlGa2–O3 oxidized at 950 °C had the highest dynamic resistance of 654 Ω at 20 mA. The increase in the Vf of the DUV LEDs with oxidized sidewalls could be caused by the degradation of the p-GaN and p-AlGaN of the DUV LEDs during high-temperature oxidation. Lu et al.[54] reported that the carrier concentration of p-GaN decreases at annealing temperatures higher than 750 °C. The decrease of the carrier concentration raised the resistance of the p-layers of the DUV LEDs.

Figure 5

Semi-log forward I–V curves of the reference DUV LED and the DUV LEDs with oxidized sidewalls. (a) AlGa2–O3 sidewalls with 420 nm thickness oxidized at different temperatures. (b) AlGa2–O3 sidewalls with different thicknesses oxidized at 900 °C.

The curves of the current-dependent light output power (L–I curves) and external quantum efficiency (EQE, EQE–I curves) of all the DUV LEDs are shown in Figure . The reference DUV LED had the lowest light output power and EQE at all currents, and it had a light output power and an EQE of 0.69 mW and 0.76% at 20 mA, respectively. The light output power of the DUV LEDs was enhanced by introducing sidewall oxidation. Either oxidation temperature or the thickness of oxidized AlGa2–O3 affected the enhancement of the light output power and EQE of the DUV LEDs. The 20 mA light output powers of the DUV LEDs with 420 nm thick AlGa2–O3 oxidized at 850, 900, and 950 °C were 0.79, 0.88, and 0.78 mW, respectively. The corresponding enhancement values in light output power were 14.5, 27.5, and 13% for the oxidation temperatures of 850, 900, and 950 °C, respectively. As the oxidation temperature elevated from 850 to 950 °C, the enhancement of light output power raised from 14.5 to 27.5% (the highest enhancement in this study) and then dropped to 13%. The light output power of the DUV LEDs with 140, 280, and 420 nm-thick AlGa2–O3 oxidized at 900 °C continuously increased from 0.69 mW to 0.83, 0.84, and 0.88 mW at 20 mA, respectively. The corresponding light output power enhancements were 20.3, 21.7, and 27.5% for the oxidized AlGa2–O3 with thicknesses of 140, 280, and 420 nm, respectively. Devices with a thicker AlGa2–O3 sidewall had a better enhancement in light output power and EQE, and the optimized oxidation temperature for the light output power of DUV LEDs was found to be around 900 °C.

Figure 6

Current-dependent output power and EQE curves of the reference DUV LED and the DUV LEDs with oxidized sidewalls. (a) AlGa2–O3 sidewalls with 420 nm thickness oxidized at different temperatures. (b) AlGa2–O3 sidewalls with different thicknesses oxidized at 900 °C.

One of the reasons for the enhancement of light output power and EQE is the improvement of the LEE of the DUV LEDs by the thermally oxidized AlGa2–O3 sidewall. Angle-dependent light-emitting intensity measurements were performed on all DUV LEDs to understand the effects of the thermally oxidized AlGa2–O3 sidewall on the radiation patterns. Figure shows the polar plot of the radiation pattern of all the DUV LEDs. The reference DUV LED had a Lambertian-like radiation pattern, as shown in Figure . The Lambertian-like radiation pattern of the reference DUV LED extended at angles around 45° to become a half heart-shaped radiation pattern by introducing the thermally oxidized AlGa2–O3 sidewall. It indicates that the enhancement of light output power could be primarily attributed to improved lateral light emission. In Figure b, the lateral light emission intensity of the DUV LEDs at angles around 45° raised with the increasing thickness of AlGa2–O3 oxidized at 900 °C, and the light emission intensity at 90° showed a similar trend. However, in Figure a, the lateral light emission intensity of the DUV LEDs with 420 nm thick oxidized AlGa2–O3 at angles around 45° reached the maximum intensity at the oxidation temperature of 900 °C and then decreased from the maximum intensity at the oxidation temperature of 950 °C. The same behavior was also observed in the light emission intensities of these DUV LEDs at 90°. The corresponding refractive indexes of Al2O3 and Ga2O3 are 1.8 and 2.12, respectively, at the wavelength of 280 nm.[55,56] The average Al compositions of AlGa2–O3(I) oxidized at 850, 900, and 950 °C were 1.42, 1.31, and 1.24, and the estimated refractive indexes of AlGa2–O3(I) were around 1.89, 1.91, and 1.92, respectively. The average Al composition of AlGa2–O3(II) oxidized at 850, 900, and 950 °C were 1.22, 1.2, and 1.06, respectively, and the estimated refractive indexes of AlGa2–O3(II) were around 1.92, 1.93, and 1.95, respectively. These values were estimated by Vegard’s law, , where n is the refractive index of the material. These values are between the refractive indexes of Al0.6Ga0.4N (n = 2.36)[57] and SiO2 (n = 1.52).[53] Therefore, the AlGa2–O3 layer on the sidewall would have a graded refraction index. Besides, the void structure in Ga2O3 and the second oxidized AlGa2–O3 layer would enhance light scattering in the DUV LEDs. As a result, it extended the total internal reflection angle at the sidewall of the DUV LEDs and the angle of the light escape cone as well. The band gap of AlGa2–O3 was estimated to be 6.1 eV (monoclinic phase) or 7.0 eV (corundum phase),[58] and the thermally oxidized AlGa2–O3 would be transparent at the wavelength of 280 nm. (The detailed estimation is provided in Supporting Information). These phenomena also improved the lateral emission intensity of the DUV LEDs because of the improvement in LEE. High oxidation temperature will degrade the p-layers of the DUV LEDs because their Vf increases with increasing oxidation temperature, which will cause a negative impact on the light output power and EQE of the DUV LEDs. This deduction was verified by an annealing process without revealing the sidewall of the DUV LEDs. The sidewalls were covered by SiO2, and these DUV LEDs were annealed in ambient N2 at 850, 900, and 950 °C. The annealing time was the same as the oxidation time corresponding to the condition of the 420 nm-thick AlGa2–O3 at each temperature. Figure S4 in Supporting Information presents the I–V curves of the DUV LEDs with sidewalls protected by SiO2. The dynamic resistances of these DUV LEDs increase with the elevation of annealing temperature, which indicates that high annealing temperature could degrade the p-layers of the DUV LEDs. In Figure S5, the L–I curves show that high annealing temperature indeed lowers the light output power of the DUV LEDs, and the light output power drops with increasing annealing temperature. It also implies that the degradation of p-layers could cause the decline of output power. Compared with the effect of the degradation of p-layers, the enhancement of LEE played a dominant role in the DUV LEDs with the sidewall oxidized at 850–900 °C. The degradation of p-layers overwhelmed the enhancement of LEE at higher oxidation temperature because of the decline of the output power of the DUV LED with sidewall oxidized at 950 °C. Therefore, a trade-off between the degradation of p-layers and the enhancement of LEE occurred. Notably, the thickness of AlGa2–O3 oxidized at 900 °C may not yet reach the optimized thickness in our study, and the enhancement of LEE could be further improved.

Figure 7

Far-field emission patterns of the reference DUV LED and the DUV LEDs with oxidized sidewalls at 90 mA current. (a) AlGa2–O3 sidewalls with 420 nm thickness oxidized at different temperatures. (b) AlGa2–O3 sidewalls with different thicknesses oxidized at 900 °C.

Some studies reported that DUV LEDs emit a high proportion of TM-polarized light which propagates to the lateral direction (parallel to the c-axis) inside the DUV LEDs.[47,48] The thermally oxidized AlGa2–O3 sidewall could effectively extract TM-polarized light out of the DUV LED chip. The impacts of the thermally oxidized AlGa2–O3 sidewall on TE- and TM-polarized emissions of the DUV LEDs are also investigated. The electroluminescence (EL) signal was collected from the sample at an angle of 15° to the bottom surface of the sample by a spectrometer. The EL emissions with TE polarization (E⊥c) and TM polarization (E∥c) could be distinguished by a polarizer (a Glan–Taylor prism) inserted between the lenses. The polarization-resolved EL spectra are provided in Figure S6 in Supporting Information. The degree of polarization (DOP) is defined as , where ITE and ITM are the integrated EL intensities of TE and TM polarization, respectively. Figure shows that the DOP of the DUV LEDs varied with the oxidation temperature and the thickness of the AlGa2–O3 sidewall. The DOP decreased with either the elevation of oxidation temperature or the increase in the thickness of the AlGa2–O3 sidewall. This result means that the thermally oxidized AlGa2–O3 sidewall enhanced the TM-polarized emission in the DUV LEDs. The TM-polarized emission could escape out of the DUV LEDs through the thermally oxidized AlGa2–O3 sidewall and contribute to the lateral light emission. Therefore, the improvement of LEE would predominate the enhancement of the total light output power of DUV LEDs. Besides, Zhang et al.[59] reported that the compressive strain in AlGaN MQWs would induce the enhancement of TE-polarized light emission. By contrast, the relative tensile stress would enhance the TM-polarized light emission of the DUV LEDs. The decrease in DOP with the increase in oxidation temperature or the increase in thickness of the AlGa2–O3 sidewall might be attributed to changes in the stress of the DUV LEDs and hinted that the thermally oxidized AlGa2–O3 sidewall might cause a relative tensile stress on the DUV LEDs compared with the reference DUV LED.

Figure 8

Dependence of DOP at 45 mA current on the oxidation temperature and thickness of AlGa2–O3 sidewalls. (a) AlGa2–O3 sidewalls with 420 nm thickness oxidized at different temperatures. (b) AlGa2–O3 sidewalls with different thicknesses oxidized at 900 °C.

Raman spectroscopy was performed to study the stress status of the reference DUV LED and the DUV LEDs with the thermally oxidized AlGa2–O3 sidewall. Figure shows that the Raman spectra at the chip centers of the DUV LEDs varied with the oxidation temperature and thickness of the AlGa2–O3 sidewall. All samples show two main peaks in Raman shift from 450 to 800 cm–1. The peak near 572 cm–1 was E2high (GaN), and the peak around 659 cm–1 was E2high (AlN). Two broad peaks with low intensity were found between the E2high (GaN) and E2high (AlN) peaks, and these two peaks are attributed to the AlGaN layer. Davydov et al.[60] reported that the Raman spectrum of AlGaN has an E2high (GaN-like) peak and an E2high (AlN-like) peak. The broad peak around 608 cm–1 is the E2high (GaN-like) peak, and the broad peak around 640 cm–1 could be the E2high (AlN-like) peak. The peaks of the E2high (GaN), E2high (GaN-like), and E2high (AlN-like) of the DUV LEDs with thermally oxidized AlGa2–O3 shifted to lower wavenumbers compared with those in the reference DUV LED. The dependences of the E2high (GaN-like) Raman peak of AlGaN varied with the oxidation temperature and thickness of the AlGa2–O3 sidewall are presented in Figure . The E2high vibration mode is sensitive to the biaxial stress (σa), and the relative biaxial stress of the layer could be determined by σa = −Δω/kRa,a, where Δω and kRa,a are the shift in Raman peak position and absolute calibration constant, respectively.[61] A positive E2high Raman peak shift (moving to a higher wavenumber) indicates compressive stress, and a negative E2high Raman peak shift (moving to a lower wavenumber) indicates tensile stress. As presented in Figure a, in the case of a 420 nm thick oxidized AlGa2–O3 sidewall, the Raman peak of AlGaN obviously moved to a lower wavenumber with the increase in oxidation temperature, which means that the oxidized AlGa2–O3 sidewalls could alleviate the compressive stress of AlGaN layers. In Figure b, the Raman peak of AlGaN shifted to a lower wavenumber with the increase in thickness of AlGa2–O3, which suggests that a thick oxidized AlGa2–O3 sidewall could also mitigate the compressive stress of AlGaN layers. If we took the AlGaN Raman peak of the reference DUV LED as the standard, the E2high Raman peak shift of the DUV LEDs with thermally oxidized AlGa2–O3 sidewall was negative. The negative Raman peak shift indicated that the DUV LEDs with thermally oxidized sidewall would be under tensile stress compared with the reference DUV LED. In other words, the thermally oxidized AlGa2–O3 sidewall could change the stress of the DUV LEDs toward tensile stress. The absolute calibration constants of GaN and AlN are 4.2 and 4.04 GPa–1 cm–1, respectively.[61,62] The absolute calibration constant of AlGaN was estimated by Vegard’s law, and the maximum relative tensile stress was 1.97 GPa for the DUV LEDs with 420 nm thick AlGa2–O3 oxidized at 950 °C. The results of Raman spectroscopy measurements also matched the results of DOP, as shown in Figure . Internal quantum efficiency (IQE) is suppressed by the quantum-confined Stark effect (QCSE) because of the spontaneous and piezoelectric polarization of AlGaN MQWs. The relative tensile stress would be able to reduce the band bending in the AlGaN MQWs of the DUV LEDs and improve the spatial overlap of the electron wave function in the conduction band and the hole wave function in the valence band. The IQE of AlGaN MQWs would be enhanced by the relative tensile stress. Although the IQE of AlGaN MQWs could be improved, the proportion of TM-polarized emission would increase. The TM-polarized emission could be effectively extracted by the oxidized AlGa2–O3 sidewall, and the enhancement of light output power contributed to the synergy of improved LEE and IQE.

Figure 9

Raman spectra of the reference DUV LED and the DUV LEDs with oxidized sidewalls. (a) AlGa2–O3 sidewalls with 420 nm thickness oxidized at different temperatures. (b) AlGa2–O3 sidewalls with different thicknesses oxidized at 900 °C.

Figure 10

Dependence of the E2high (GaN-like) Raman peak of the reference DUV LED and the DUV LEDs on oxidation conditions. (a) AlGa2–O3 sidewalls with 420 nm thickness oxidized at different temperatures. (b) AlGa2–O3 sidewalls with different thicknesses oxidized at 900 °C.

Conclusions

AlGaN and GaN sidewalls were turned into AlGa2–O3 and Ga2O3 sidewalls, respectively, by thermal oxidation to improve the optoelectrical characteristics of DUV LEDs. Many nanosized voids were found in the thermally oxidized Ga2O3. Two oxidized AlGa2–O3 layers were found on the AlGaN sidewalls of the DUV LEDs and showed different crystalline structures in TEM images. The first oxidized AlGa2–O3 layer is a typical single crystal, and the second oxidized AlGa2–O3 layer is a holey single crystal. The first oxidized AlGa2–O3 layer has a higher Al content than the second one. The high oxidation temperature degraded the quality of the p-layers and increased the Vf from 8.18 to 11.36 V. Nevertheless, the thermally oxidized AlGa2–O3 with the holey structure is transparent at the wavelength of 280 nm and has a graded refractive index between those of AlGaN and SiO2. These properties will enhance the LEE of the DUV LEDs. Therefore, the thermally oxidized AlGa2–O3 sidewall greatly enhances the LEE of lateral light in the DUV LEDs, which will predominate the enhancement of the light output power of DUV LEDs. In addition, the thermally oxidized AlGa2–O3 sidewall can enhance TM-polarized light emission because it induces a relative tensile stress in the DUV LEDs. The relative tensile stress can reduce the band bending in AlGaN MQWs caused by QCSE to increase the IQE of DUV LEDs. As a result, the light output power of the DUV LEDs increased from 0.69 to 0.88 mW after the introduction of a 420 nm thick AlGa2–O3 oxidized at 900 °C, in which the enhancement of light output power is able to reach 27.5%.