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Abstract
In this work, we report an AlGaN-based ∼275 nm deep ultraviolet light-emitting diode (DUV LED) that has AlGaN based quantum barriers with a properly large Al composition. It is known that the increased conduction band barrier height helps to enhance the electron concentration in the active region. However, we find that the promoted hole injection efficiency is also enabled for the proposed DUV LED when the Al composition increases. This is attributed to the reduced positive polarization charge density at the last quantum barrier (LQB) and p-type electron blocking layer (p-EBL) interface, which can suppress the hole depletion effect in the p-EBL. Thus, the hole concentration in the p-EBL gets promoted, which is very helpful to reduce the hole blocking effect caused by the p-EBL. Therefore, thanks to the improved carrier injection, the proposed DUV LED increases the optical power and reduces the forward voltage when compared with the conventional DUV LED.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, considerable efforts have been paid to develop AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) due to the potential applications in wastewater treatment, air purification, medical sterilization and space communication, etc. [1–3]. Hence, one target is to develop DUV LEDs with high external quantum efficiency (EQE), strong optical power and large wall-plug efficiency (WPE). However, the EQE for DUV LEDs is still low at the current stage [4]. On one hand, the very low Mg doping efficiency in the Al-rich p-type layer strongly limits the hole concentration level [5]. On the other hand, the energy band discontinuity between the p-type electron blocking layer (p-EBL) and the p-AlGaN/p-GaN hole injection layer further retards the hole injection capability [6]. Thus, one of the methods to increase the hole injection efficiency is enhancing the hole concentration in the p-type layers. The proposed approaches include p-type GaN and AlGaN layer with Mg-δ-doping, p-AlGaN layer with the graded Al composition and superlattice structures for realizing the polarization doping [7–11]. Other methods include designing various p-EBL structures to reduce the hole blocking effect [12–14]. Most recently, DUV LEDs without p-EBL are also proven to be a potential solution strategy for favoring the hole injection if the electron injection efficiency is not sacrificed [15,16]. Besides engineering the hole injection process, research attention shall also be paid to electrons, such that electrons are difficult to be captured by the multiple quantum wells (MQWs), which is due to the large electron mobility [17]. Therefore, AlGaN quantum barriers (QBs) with the Al composition spike-structured barriers [18] and the n-AlxG1-xN/n-AlyGa1-yN (x > y) electron injection layers [19] are designed to reduce the drift velocity of the free electrons for DUV LEDs. Furthermore, for increasing the electron injection efficiency, a more direct method is to improve the electron confinement in the MQWs by properly increasing the thickness of the QBs [20] or using the staggered QBs [21]. In this work, we propose an easier approach by properly increasing the Al composition in the QBs for DUV LEDs. By doing so, the polarization discontinuity between the last quantum barrier (LQB) and the p-EBL can be reduced, which helps to decrease the positive polarization charge density at the LQB/p-EBL interface. As a result, the hole depletion effect within the p-EBL can be reduced, i.e., the hole concentration in the p-EBL gets increased, which in turn decreases the valence band barrier height for the p-EBL [22]. Other supplementary advantage for this design is that the QBs with the properly increased Al composition can increase the conduction band barrier height for both the QBs and the p-EBL. Therefore, the electron leakage current can be suppressed and the carrier concentration can be effectively confined in the active region. Finally, the electroluminescent (EL) spectra, the EQE, the optical power, the current-voltage (I-V) characteristics and the WPE are all improved for the proposed DUV LED.
2. Numerical calculations and experimental measurements
To investigate the effect of the proposed device on the charge transport, two DUV LEDs are grown on the 2-inch [0001]-orientated sapphire substrates with a misorientation angle of 0.2° by the metal organic chemical vapor deposition (MOCVD) technology. Firstly, a 20 nm thick AlN nucleation layer is deposited at a low temperature on the substrate. Afterwards, a 2 µm thick AlN template is grown for achieving better crystalline coalescence. The 4 µm thick n-Al0.60Ga0.40N layer (n-doping = 8 × 1018 cm−3) as the electron injection layer is then grown on the AlN template. Next, we grow the active region composed of 5-pair Al0.45Ga0.55N/AlxGa1-xN MQW stacks, which have 3 nm thick quantum wells and 10 nm thick quantum barriers. The Al compositions (x) for Devices A and B are 0.55 and 0.61, respectively. After that, a 10 nm thick p-Al0.65Ga0.35N EBL and a 50 nm thick p-Al0.45Ga0.55N layer are grown. Lastly, the two devices are capped with an 80 nm thick p-GaN layers for forming p-type ohmic contacts. During the epitaxial growth, the Mg doping concentration is set to 4 × 1019 cm−3 for the p-type layers. However, considering the very low Mg ionization efficiency of even lower than 1% [23], the hole concentration for the p-type layers is in the scale 4 × 1017 cm−3, which number is reasonable according to the report by Kang et al. [24]. The value of threading dislocation density (TDD) for Devices A and B is estimated to be ∼3 × 109 cm−2. The schematic diagrams for Devices A and B are shown in Fig. 1.
Fig. 1. Schematic diagrams for Device A with the Al0.55Ga0.45N quantum barriers and Device B with the Al0.61Ga0.39N quantum barriers.
After the epitaxial growth, we make device mesas with the size of 350 µm × 350 µm by using the photolithography and the inductively coupled plasma etch (ICP) technology. Then, the Ti/Al/Ti/Au (20 nm/60 nm/30 nm/100 nm) metals are deposited on the n-Al0.60Ga0.40N layer by utilizing e-beam. The n-type Ohmic contact can be achieved by using the rapid thermal annealing (RTA) in the N2 ambient for 1 minute at the temperature of 650 °C. Ni/Au (10 nm/10 nm) current spreading layer is deposited and annealed in the O2 ambient for 3 minutes at the temperature of 450 °C. Finally, the Al/Ti/Au (800 nm/20 nm/100 nm) metals are deposited as the reflective p-electrode.
To reveal the in-depth device physics, we calculate the energy band, the carrier transport and the radiative recombination rate by using APSYS [6,14,17,19]. In our calculation models, we have taken the polarization effect at each lattice-mismatched AlGaN/(Al)GaN heterojunction into consideration by assuming the 40% polarization level [14]. The Auger recombination coefficient of 1.0 × 10−30 cm6/s that is used for Devices A and B has been considered [6,25]. The Shockley-Read-Hall (SRH) recombination lifetime is set to 10 ns [14,19]. The AlGaN/AlGaN energy band offset ratio is assumed to 50:50 [14,19]. Note, we also set other important parameters (e.g., energy gap, resistivity, dielectric constant, electron and hole mobility, crystal-field splitting holes, etc.) on III-nitride semiconductors in our simulation model [26].
3. Results and discussion
The measured EL spectra with varying injection current levels from 10 mA to 50 mA for Devices A and B are shown in Figs. 2(a) and 2(b), respectively. The peak emission wavelengths for Devices A and B are both ∼275 nm. Meanwhile, the EL intensity for Device B has been enhanced by about 40% when compared with Device A. If we further compare Figs. 2(c) and 2(d), we can find that the optical power and the EQE can be effectively increased by 44.9% and 44.0% for Device B. The conclusions regarding EQE and optical power agree with our previous predictions except that we further reveal the forward voltage, the leakage current and the WPE in this work [27]. Note, in our fabrication process, both Devices A and B are not packaged. Therefore, the EL spectra are collected from the bottom of the DUV LED chips by utilizing a calibrated integrating sphere as shown in the inset of Fig. 2(d).
Fig. 2. Measured EL spectra for (a) Device A and (b) Device B in terms of the current varying from 10 mA to 50 mA with the step of 10 mA. Measured (c) optical power and (d) EQE in terms of different injection current levels for Devices A and B, respectively. The inset of figure (d) shows the schematic diagram of the testing equipment for the calibrated UV integrating sphere.
To reveal the impact of the Al composition of QBs on the hole and electron transport, we show the energy band profiles in the partial MQWs and p-EBL region for Devices A and B at 35 mA. According to Figs. 3(a) and 3(b), we can see the valence bands for the heavy hole (HH) and the crystal-field split-off hole (CH), which is due to the increased Al composition for the AlGaN material [28]. According to the insets of Fig. 3, the hole concentration in the p-EBL is increased for Device B, i.e., the hole depletion effect becomes less significant at the p-EBL side of the LQB/p-EBL interface, which results from the reduced the positive polarization charge density at the LQB/p-EBL interface. Meanwhile, the smaller effective valence band barrier height of the p-EBL for Device B (ϕB) is obtained which is 359.5 meV when compared with the 531.5 meV of the p-EBL for Device A (ϕA). This is attributed to the increased hole concentration therein [22]. In addition, the effective conduction band barrier heights of the p-EBL for Devices A and B (ΦA and ΦB) are 385.2 meV and 457.2 meV, respectively. The higher ΦB for Device B is due to the absent electron accumulation [29], which can be inferred by reading the positions of the conduction band and the quasi-Fermi level for electrons in the LQB and p-EBL [see Fig. 3(b)]. Moreover, the increased Al composition of QBs can also improve the electron confinement in the active region because of the enhanced conduction band offset of AlGaN/AlGaN heterojunction.
Fig. 3. Calculated energy band profiles for (a) Device A and (b) Device B. The insets of figures (a) and (b) show the hole concentration profiles in the p-EBL for Devices A and B. ΦA and ΦB denote the effective conduction band barrier height for p-EBL. ϕA and ϕB represent the effective valence band barrier height for p-EBL. Data are calculated at 35 mA.
Being consistent with the energy band profiles, we also present that the hole and electron distributions in the MQWs at 35 mA. If we look into Fig. 4(a), it is obvious that the hole concentration in the active region for Device B is larger than that for Device A due to the higher hole concentration within the p-EBL as shown in the inset of Fig. 3(b). Meanwhile, the electron concentration profiles in the active region [see Fig. 4(b)] for Devices A and B agree very well with the demonstrated values of the effective conduction band barrier height of p-EBL in Fig. 3, such that Device B has the larger electron concentration level in the MQWs. Thanks to the increased carrier concentration levels in the MQWs, the enhanced radiative recombination rate can be obtained from Device B as shown in Fig. 4(c).
Fig. 4. (a) Hole concentration profiles, (b) electron concentration profiles and (c) radiative recombination rate profiles in the MQWs for Devices A and B, respectively. Data are calculated at 35 mA.
Lastly, we present the I-V characteristics which are measured by Keithley 2400 and the WPE for Devices A and B in Figs. 5(a) and 5(b), respectively. As illustrated in Fig. 5(a), Device B possesses the reduced forward voltage and the suppressed leakage current when the devices are reversely biased. The reduced forward voltage is well attributed to the much better injection capability for electrons and holes. The suppressed leakage current before the device is turned on can be caused by the increased energy band offset of the proposed quantum barriers for Device B. In addition, it can be directly seen from Fig. 5(b) that the WPE can be enhanced by 52.6% at 35 mA for Device B.
Fig. 5. (a) Measured I-V performance curves in semi-log scale for Devices A and B. The inset of figure (a) shows the measured I-V characteristics curves in linear scale for Devices A and B. (b) Measured WPE in terms of the injection current level for Devices A and B.
4. Conclusions
The positive polarization charges at the LQB/p-EBL interface for the fabricated DUV LED can be controlled by properly increasing the Al composition of the QBs, which can effectively enhance the carrier concentrations in the MQWs. The increased hole concentration arises from the reduced hole depletion at the LQB/p-EBL interface. Therefore, the hole concentration increases and the valence band barrier height of the p-EBL is simultaneously suppressed. In the meantime, the energy band offset of the AlGaN/AlGaN heterojunction in the MQWs can also be enhanced by properly increasing the Al composition of QBs, which can better confine the electrons in the active region. As a result, the EQE and the optical power for the optimized DUV LED have been improved by 44.0% and 44.9% at 35 mA, respectively. The even better carrier injection also manifests the reduced forward voltage and the enhanced WPE. The accompanying contribution for the proposed structure also shows the reduced leakage current. Therefore, we strongly believe that the proposed structure can be very useful for making high-efficiency DUV LEDs.
Funding
National Natural Science Foundation of China (62074050, 61975051); Natural Science Foundation of Hebei Province (F2018202080, F2020202030); research fund by State Key Laboratory of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology (EERI_PI2020008); joint research project for Tunghsu Group and Hebei University of Technology (HI1909); Graduate Innovation Foundation of Hebei Province (CXZZBS2020027).
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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