Hole Transport Manipulation To Improve the Hole Injection for Deep

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Hole transport manipulation to improve the hole injection for deep ultraviolet light-emitting diodes Zi-Hui Zhang, Sung-Wen Huang Chen, Yonghui Zhang, Luping Li, Sheng-Wen Wang, Kangkai Tian, Chunshuang Chu, Mengqian Fang, Hao-Chung Kuo, and Wengang (Wayne) Bi ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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Hole transport manipulation to improve the hole injection for deep ultraviolet light-emitting diodes Zi-Hui Zhang 1 , Sung-Wen Huang Chen 2 , Yonghui Zhang 1 , Luping Li 1 , ShengWen Wang2 , Kangkai Tian 1 , Chunshuang Chu 1 , Mengqian Fang 1 , Hao-Chung Kuo 2,*, and Wengang Bi1,* 1

Institute of Micro-Nano Photoelectron and Electromagnetic Technology Innovation,

School of Electronics and Information Engineering, Hebei University of Technology, Key Laboratory of Electronic Materials and Devices of Tianjin, 5340 Xiping Road, Beichen District, Tianjin, 300401, P. R. China 2

Department of Photonics and Institute of Electro-optical Engineering, National Chiao

Tung University, Hsinchu 30010, Taiwan Abstract In this report, we propose to enhance the hole injection efficiency by adjusting the barrier height of the p-type electron blocking layer (p-EBL) for ~273 nm deep ultraviolet light-emitting diodes (DUV LEDs). The barrier height for the p-EBL is modified by employing a p-Al0.60Ga0.40N/Al0.50Ga0.50N/p-Al0.60Ga0.40N structure, in which the very thin Al0.50Ga0.50N layer is able to achieve a high local hole concentration, which is very effective in reducing the effective barrier height of the pEBL for holes. More importantly, besides the thermionic emission, such a p-EBL structure can also favor a strong intraband tunneling process for holes. As a result, we can obtain a more efficient hole injection into the quantum wells, leading to a remarkably improved optical power for the DUV LED with the proposed p-EBL architecture. Keywords: deep ultraviolet LED, hole injection, p-EBL, carrier transport

Deep ultraviolet light-emitting diodes (DUV LEDs) have found extensive applications in plenty of scopes such as water sterilization, air purification, biological disinfections, medical therapy, polymer solidification, etc.1 Nevertheless, the external quantum efficiency (EQE) for DUV LEDs at the current stage is low partly because of the poor hole injection.2 The hole injection for DUV LEDs is even more problematic due to the fact that the doping efficiency for Mg dopants of the Al-rich p-AlGaN layer

*)

Electronic mails: [email protected], [email protected]


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is even lower than that of the p-GaN layer.3 Therefore, it shall be advisable to increase the hole injection efficiency by increasing the hole density for the p-type hole injection layer, which can be obtained by adopting specially designed architectures. It has been suggested that the polarization-induced three-dimensional hole gas (3DHG) that is obtained by grading the Al composition in the AlGaN layer can enable a higher hole concentration.4-6 The polarization-induced-doping design numerically proves to be effective in facilitating the hole injection capability for the DUV LED which has been reported by Chang et al.7 In addition, the hole concentration for the p-type hole injection layer can also be increased by doping the last quantum barrier with Mg dopants,8, 9 such that the Mg dopants in Mg-doped last quantum barrier can be more efficiently ionized by the polarization induced electric field while the built-in electric field can deplete the holes and the holes are finally stored in the p-type hole injection layer. It is reported that the activation energy of Mg dopants for the GaN film can be even lower than 100 meV by adopting the Mg-In co-doping technology, by means of which the hole concentration can be increased to the order of 1018 cm-3.10, 11 Besides increasing the hole concentration for the p-type hole injection layer, another approach to boost the hole injection into the quantum well region is to energize holes,12, which is very helpful to facilitate the thermionic emission for holes to cross over the p-type electron blocking layer (p-EBL). On the other hand, the optical power for DUV LEDs can be further enhanced if the hole blocking effect by the p-EBL is minimized. For that purpose, several alternative p-EBL structures have been proposed and demonstrated, such as superlattice-structured p-EBL,13, 14 AlGaN/AlGaN superlatticestructured last quantum barrier.15 The enhanced Mg doping efficiency by the superlattice structure can be partly ascribed to the generated resonant states in the wells according to the theoretical model proposed by Liu et al. 16 In addition, the hole transport across the conventional bulk AlGaN based p-EBL is mainly determined by the thermionic emission which is very sensitive to the Al composition in p-EBL. The thermionic emission can be more favored once the Al composition or the thickness for the p-EBL is decreased, and this nevertheless may result in an even more serious electron leakage.17 In this report, we experimentally and numerically propose to manipulate the hole transport mechanism by a p-Al0.60Ga0.40N/Al0.50Ga0.50N/pAl0.60Ga0.40N structure.


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The sketched energy band diagram and the hole injection mechanism for the pAl0.60Ga0.40N(L2)/Al0.50Ga0.50N/p-Al0.60Ga0.40N(L1) EBL is shown in Fig. 1. For explicit illustration, we define layer L1 and layer L2 to represent different pAl0.60Ga0.40N layers for the proposed EBL architecture. Here we keep layer L1 thin in order to facilitate the intraband tunneling process for holes (i.e, P0 in Fig. 1) to enter the Al0.50Ga0.50N layer insertion layer.18,

19

Here, we purposely adopt a thin

Al0.50Ga0.50N layer embedded between layers L1 and L2. According to the our previous report in Ref. [19], normally the effective valence band barrier height of layer L2 can be defined as Φ h = ∆EV − kT × ln ( p / NV ) , where k represents the Boltzmann constant, T means the carrier temperature, NV stands for the effective density states for holes, and p is the hole concentration in the thin Al0.50Ga0.50N insertion layer. We purposely make the Al0.50Ga0.50N layer very thin so that a higher local hole concentration (i.e., p) can be easily obtained. Consequently, the effective valence band barrier height for the layer L2 can be reduced and the thermionic emission (i.e., P2) for holes can be improved. Experimental Methods

In order to demonstrate the effectiveness of the p-Al0.60Ga0.40N/Al0.50Ga0.50N/pAl0.60Ga0.40N EBL in promoting the hole injection and improving the optical power for DUV LEDs, we designed and grew the DUV LED structures that use the p-EBLs as depicted in Fig. 1 by using MOCVD growth on AlN template technology. The 4 µm thick high-quality AlN buffer layer on [0001] oriented sapphire substrate as the template was grown by HVPE technology (Provided by Nanowin corporation), which is then followed by 20-period AlN/Al0.50Ga0.50N superlattice structures to release the strain for the rest of the epi-layers. The electron injection layer comprises a 2 µm thick n-Al0.60Ga0.40N layer with a Si doping concentration of 1 × 1018 cm-3. 5-period 3 nm–Al0.45Ga0.55N/12 nm–Al0.56Ga0.44N multiple quantum wells (MQWs) are then grown to generate photons. In order to suppress the electron escape from the MQW region, we cap the MQW with a 10 nm thick p-Al0.60Ga0.40N EBL for Device A. Nevertheless, different than that for Device A, the p-EBL for Device B comprises the p-Al0.60Ga0.40N(L2)/Al0.50Ga0.50N/p-Al0.60Ga0.40N(L1) structure [see Fig. 1(b)]. Here, we fix the total thickness of the p-Al0.60Ga0.40N/Al0.50Ga0.50N/p-Al0.60Ga0.40N structure to 10 nm, for which the Al0.50Ga0.50N insertion layer is 3 nm thick, the remaining thicknesses of layers L1 and L2 are set to 2 nm and 5 nm, respectively and we


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purposely make layer L1 thin. The hole injection layer for both DUV LED devices is composed of a 50 nm thick p-Al0.40Ga0.60N layer, a 40 nm thick p-GaN layer. More importantly, we conduct the in-situ annealing at the temperature of 800 oC for 15 min to activate Mg dopants. The effective hole concentration for the Mg doped Al-rich pAlGaN layer and the p-GaN layer is roughly calculated to be 1 × 1017 cm-3 and 3 × 1017 cm-3, respectively. Finally, we cover a 10 nm thick heavily Mg-doped GaN layer (p+-GaN) to enable the ohmic contact. We perform the device fabrication after the epitaxial growth. The mesa is firstly obtained by using Inductively Coupled Plasma (ICP) etching and the mesa size is 650 × 320 um2. We then adopt Ti/Al as the n-type ohmic contact. A thin Ni/Au current spreading layer is also deposited on the mesa surface and is annealed in the O2 ambient for 5 min at the temperature of 550 oC. Lastly we deposit Ni/Au on the n-type contact and the current spreading layer to form the alloyed bonding pads for the nelectrode (Ti/Al/Ni/Au) and the p-electrode (Ni/Au/Ni/Au) at the same time. The investigated DUV LEDs are flip-chip devices so that the light can travel through the sapphire substrate. In addition, both DUV LEDs are numerically investigated by using APSYS simulator,2,

6, 8, 9, 12, 19

which can well manage Schrödinger equation, Poisson’s

equations, current continuity equations, drift-diffusion equations for the passive layers, and the mean-free-path model for the active region that has been discussed in a indepth level in our previous works.20, 21 More importantly, we also take the thermally assisted intraband tunneling process into consideration by regarding the p-EBL as a rectangular barrier.22 Our models also take Shockley-Read-Hall (SRH) and Auger recombination processes into account that cause the carrier loss by nonradiative recombination, for which the recombination coefficients are set to 1 × 108 s-1, 1 × 1030

cm6s-1, respectively.2 The offset ratio between the conduction band and the valence

band is set to 50:50 for AlGaN/AlGaN (e.g., the MQW region and the p-EBL) and AlGaN/GaN (e.g., the hole injection layer) interfaces.23 We also assume a 40% polarization level which is linked with the polarization induced electric field for the [0001]

oriented

polarization-mismatched

heterojucntions.24

Furthermore,

the

calculated light for DUV LEDs by APSYS possesses both TE and TM photons, and in order to better reproduce the experimentally measured optical power, we empirically


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set the light extraction efficiency to 9% when the light escape surface is not specially treated for DUV LEDs. 2, 25

Results and Discussions We firstly measure the respective electroluminescence (EL) spectra for Devices A and B that are selectively shown in Fig. 2. The peak emission wavelength for both devices is centered at ~273 nm. Clearly we see that the EL intensity for Device B is higher than that for Device A. Since Device A differs from Device B only in the pEBL design, thus the enhanced EL intensity for Device B is well ascribed to the proposed p-Al0.60Ga0.40N(L2)/Al0.50Ga0.50N/p-Al0.60Ga0.40N(L1) EBL structure [see Fig. 1(b)]. Based on the collected EL spectra in Fig. 2, we are then able to get the optical power and the EQE in terms of different injection current levels as shown in Figs. 3(a), respectively. Apparently, the optical power for Device B is stronger than that for Device A, such that the optical power is experimentally increased by ~19.38% at the injection current level of 250 mA according to Fig. 3(a). In the meanwhile, we also reproduce the experimentally measured optical power and EQE by the numerical approach in Fig. 3(b). We can see that the experimentally measured and the numerically calculated values follow the similar trending which reflects the validity for the models and parameters that we set during calculations. Note, the numerically calculated hole injection is very sensitive to the material properties of the p-EBL, such as the polarization level, energy band offset, Mg ionization efficiency, SRH recombination coefficient, which are however strongly affected by different growth technologies. Therefore, they cannot be accurately known and are difficult to be precisely modelled at the current stage.26 Consequently, the calculated optical power and EQE show slight deviations from the experimentally measured ones, e.g., the EQE and the optical power in Fig. 3(b) is stronger than that in Fig. 3(a) for Device B, but the discrepancies here do not affect the conclusions of this work. Figs. 4(a) and 4(b) show the calculated valence bands for Devices A and B at the injection current level of 100 mA, respectively. Here we define Фh and ϕh as the effective barrier heights at different positions of the p-EBL. The values for Фh and ϕh are summarized in TABLE I. In Fig. 4(a), the holes have to overcome ϕh before they reach the position where Фh is defined. According to our computations, the values for


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Фh and ϕh are ~335.18 meV and ~206.51 meV in the p-EBL for Device A, respectively. Interestingly the value of ϕh for the p-Al0.60Ga0.40N of layer L1 [see Fig. 1(b)] is increased to ~217.40 meV compared to that for the conventional p-EBL structure. Meanwhile, we also define ϕH to represent the total valance band barrier height for the layer L1 that is ~234.64 eV. If we recall φh = ∆EV − kT × ln ( p / NV ) ,19 we can infer that the hole concentration at the p-EBL/p-Al0.40Ga0.60N interface for Device B decreases (details will be discussed subsequently in Fig. 5). This also illustrates that, besides the thermionic emission, the holes are also featured with a very significant intraband tunneling process from the p-Al0.40Ga0.60N layer into the thin Al0.50Ga0.50N layer by traveling through the p-Al0.60Ga0.40N layer of L1, therefore leading to a less hole accumulation at the p-EBL/p-Al0.40Ga0.60N interface. Fortunately, the Al0.50Ga0.50N insertion layer for the proposed p-EBL is thin (i.e., 3 nm in our case), and we are able to obtain a higher local hole concentration, which is very helpful to reduce the value of Фh to ~303.41 meV as we have discussed earlier [see Fig. 4(b)]. TABLE I Summarized values of Фh , ϕh and ϕH of the p-EBLs for Devices A and B, respectively.

ϕh (meV)

Device A

Device B

~206.51

~217.40

ϕH (meV) Фh (meV)

~234.64 ~335.18

~303.41

To further clarify the origin for the improved optical power given by the proposed EBL structure, we also compute the hole concentration levels in the p-EBL, the pAl0.40Ga0.60N region and the MQW regions for Devices A and B, respectively [see Figs. 5(a) and 5(b)]. According to Fig. 5(a), we can get a lower hole concentration at the p-EBL/p-Al0.40Ga0.60N interface for Device B when compared to that for Device A, which is consistent with our earlier analysis. The suppressed hole accumulation therein results from the stronger intraband tunneling for holes to travel through the pAl0.60Ga0.40N layer of L1 [see Figs. 1(b) and 4(b)]. In the meanwhile, we also observe a larger hole concentration in the thin Al0.50Ga0.50N insertion layer of the proposed p-


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EBL for Device B as illustrated in Fig. 5(a), which is very effective to decrease the valence band barrier height of Фh as has been mentioned previously[see Fig. 4(b)]. Fig. 5(b) demonstrates the hole concentration profiles within the MQW region for both devices. Because of the improve hole tunneling efficiency into thin Al0.50Ga0.50N insertion layer and the correspondingly reduced valence band barrier height of Фh, the hole concentration level within the MQW region for Device B has been substantially improved. As a result, Fig. 5(c) shows a stronger radiative recombination rate across the MQW region for Device B than that for Device A. The enhanced radiative recombination leads to the improved optical power for the DUV LED with the proposed p-EBL. It is worthy of noting here that the hole concentration level in the thin Al0.50Ga0.50N insertion layer of the proposed p-EBL is ~ 3.5 × 1018 cm-3, and this value is smaller than the hole concentration in the MQW region. However, we believe that the thickness of the Al0.50Ga0.50N insertion layer is of crucially importance. A thicker Al0.50Ga0.50N insertion layer can cause a significant hole blocking effect which is not helpful to improve the hole concentration level in the MQW region. The hole concentration and the DUV LED performance can be further increased if the Al0.50Ga0.50N insertion layer can be more precisely controlled by well managing the MOCVD-based epitaxial technology and is made even thinner. On the other hand, although the Al0.50Ga0.50N insertion layer is set to 3 nm thick in our case, we do not observe an obvious and strong radiative recombination in the Al0.50Ga0.50N insertion layer, which is due to the fact that the 3 nm thick p-Al0.60Ga0.40N layer of L1 is not able to confine sufficient electrons in the Al0.50Ga0.50N insertion layer. Therefore, the holes will not be significantly consumed by recombining with electrons in the Al0.50Ga0.50N insertion layer and can be effectively captured by the MQW region, which also suggests that the position of the Al0.50Ga0.50N insertion layer is important and we have to make the p-Al0.60Ga0.40N layer of L1properly thin. Conclusions In

conclusion,

we

have

reported

a

p-Al0.60Ga0.40N(L2)/Al0.50Ga0.50N/p-

Al0.60Ga0.40N(L1) EBL for DUV LEDs. By adopting such a proposed p-EBL architecture, we are able to obtain both thermionic emission and intraband tunneling processes simultaneously during the hole transport. The effectiveness of the proposed p-EBL is demonstrated by producing the higher hole concentration within the MQW


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region and the stronger optical power at different injection current levels. We have also pointed out that the Al0.50Ga0.50N insertion layer has to be properly thin to reduce the hole blocking effect, and in the meanwhile, the Al0.50Ga0.50N insertion layer has to be properly close to the hole supplier so that the electrons are able to tunnel through the p-Al0.60Ga0.40N region of L1, and then the holes have little possibility to recombine with the electrons in the Al0.50Ga0.50N insertion layer that further guarantees a smooth hole injection into the active region. We believe that the proposed p-EBL structure is very important for the III-nitride community to progress the DUV LED performance and the corresponding physical interpretations provide even more understanding for the current device physics for DUV LEDs. Acknowledgements This work is supported by National Natural Science Foundation of China (Project Nos. 51502074, 61604051), Natural Science Fund for Excellent Young Scholars of Hebei Province (Project No. F2017202052), Program for 100-Talent-Plan of Hebei Province (Project No. E2016100010), Natural Science Foundation of Tianjin City (Project No. 16JCYBJC16200), Program for Top 100 Innovative Talents in Colleges and Universities of Hebei Province (Project No. SLRC2017032). The authors are also grateful to Epistar Corporation for providing DUV LEDs and Nanowin to supply AlN templates. References 1

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Efficiency in AlGaN Deep-Ultraviolet Light-Emitting Diodes. Appl. Phys. Express 2013, 6, 062101. 26

Piprek J.; Li Z. M. S. Sensitivity analysis of electron leakage in III-nitride light-

emitting diodes. Appl. Phys. Lett. 2013, 102, 131103. Figures captions Fig. 1 Schematic energy band diagrams and the hole injection mechanisms for (a) DUV LED with the conventional p-Al0.60Ga0.40N EBL and (b) DUV LED with the pAl0.60Ga0.40N(L2)/Al0.50Ga0.50N/p-Al0.60Ga0.40N(L1) EBL. P0 represents the hole transport by intraband tunneling through layer L1. P1 denotes the thermionic emission for holes to cross over the p-Al0.60Ga0.40N EBL in Fig. (a) and layer L1 in Fig. (b), respectively. P2 means the thermionic emission for holes to cross over layer L2 in Fig. (b). EC and EV represent the conduction band and the valence band, respectively. Fig. 2 Experimentally tested EL spectra for Devices A and B at the current levels of 20 mA an 60 mA, respectively. Fig. 3 (a) Experimentally tested and (b) numerically computed EQE and optical power at different injection current levels for Devices A and B, respectively. Fig. 4 Calculated valence band alignment in the nearby region of (a) the pAl0.60Ga0.40N EBL for Device A and (b) the p-Al0.60Ga0.40N(L2)/Al0.50Ga0.50N/pAl0.60Ga0.40N(L1) EBL for Device B. Data are collected at the current of 100 mA. Фh, ϕH and ϕh denote the effective valence band barrier height at different positions for both p-EBLs. Ev and Efh represent the valence band edge and the quasi-Fermi level for holes, respectively. Fig. 5 Calculated hole concentrations (a) in the p-EBL and the p-Al0.40Ga0.60N region, (b) in the MQW region, and (c) calculated radiative recombination rate in the MQW region for Devices A and B, respectively. Data are collected at the current of 100 mA.


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Fig. 1 Schematic energy band diagrams and the hole injection mechanisms for (a) DUV LED with the conventional p-Al0.60Ga0.40N EBL and (b) DUV LED with the p-Al0.60Ga0.40N(L2)/Al0.50Ga0.50N/pAl0.60Ga0.40N(L1) EBL. P0 represents the hole transport by intraband tunneling through layer L1. P1 denotes the thermionic emission for holes to cross over the p-Al0.60Ga0.40N EBL in Fig. (a) and layer L1 in Fig. (b), respectively. P2 means the thermionic emission for holes to cross over layer L2 in Fig. (b). EC and EV represent the conduction band and the valence band, respectively. 1510x1669mm (96 x 96 DPI)


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Fig. 2 Experimentally tested EL spectra for Devices A and B at the current levels of 20 mA an 60 mA, respectively. 748x531mm (96 x 96 DPI)


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Fig. 3 (a) Experimentally tested and (b) numerically computed EQE and optical power at different injection current levels for Devices A and B, respectively. 1055x748mm (96 x 96 DPI)


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Fig. 4 Calculated valence band alignment in the nearby region of (a) the p-Al0.60Ga0.40N EBL for Device A and (b) the p-Al0.60Ga0.40N(L2)/Al0.50Ga0.50N/p-Al0.60Ga0.40N(L1) EBL for Device B. Data are collected at the current of 100 mA. Фh, ϕH and ϕh denote the effective valence band barrier height at different positions for both p-EBLs. Ev and Efh represent the valence band edge and the quasi-Fermi level for holes, respectively. 1006x748mm (96 x 96 DPI)


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ACS Photonics

Fig. 5 Calculated hole concentrations (a) in the p-EBL and the p-Al0.40Ga0.60N region, (b) in the MQW region, and (c) calculated radiative recombination rate in the MQW region for Devices A and B, respectively. Data are collected at the current of 100 mA. 1485x1031mm (96 x 96 DPI)


Hole Transport Manipulation To Improve the Hole Injection for Deep

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