Catalyst-Free Vertical ZnO-Nanotube Array Grown on p-GaN

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Catalyst-Free Vertical ZnO-Nanotube Array Grown on p‑GaN for UVLight-Emitting Devices Norah Alwadai,†,∥ Idris A. Ajia,† Bilal Janjua,‡ Tahani H. Flemban,⊥ Somak Mitra,† Nimer Wehbe,§ Nini Wei,§ Sergei Lopatin,§ Boon S. Ooi,‡ and Iman S. Roqan*,† †

Physical Sciences and Engineering Division, ‡Division of Computer, Electrical and Mathematical Sciences and Engineering, and Imaging and Characterization Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ∥ Department of Physics, College of Sciences, Princess Nourah Bint Abdulrahman University (PNU), Riyadh 11671, Saudi Arabia ⊥ Department of Physics, College of Science, Imam Abdulrahman Bin Faisal University (IAU), Dammam 31441, Saudi Arabia

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S Supporting Information *

ABSTRACT: One-dimensional (1D) structures-based UV-light-emitting diode (LED) has immense potential for next-generation applications. However, several issues related to such devices must be resolved first, such as expensive material and growth methods, complicated fabrication process, efficiency droop, and unavoidable metal contamination due to metal catalyst that reduces device efficiency. To overcome these obstacles, we have developed a novel growth method for obtaining a high-quality hexagonal, well-defined, and vertical 1D Gd-doped n-ZnO nanotube (NT) array deposited on p-GaN films and other substrates by pulsed laser deposition. By adopting this approach, the desired high optical and structural quality is achieved without utilizing metal catalyst. Transmission electron microscopy measurements confirm that gadolinium dopants in the target form a transparent in situ interface layer to assist in vertical NT formation. Microphotoluminescence (PL) measurements of the NTs reveal an intense ZnO band edge emission without a defect band, indicating high quality. Carrier dynamic analysis via time-resolved PL confirms that the emission of n-ZnO NTs/p-GaN LED structure is dominated significantly by the radiative recombination process without efficiency droop when high carrier density is injected optically. We developed an electrically pumped UV Gd-doped ZnO NTs/GaN LED as a proof of concept, demonstrating its high internal quantum efficiency (>65%). The demonstrated performance of this cost-effective UV LED suggests its potential application in large-scale device production. KEYWORDS: gadolinium, ZnO, vertical nanotube, GaN, UV light emitting, exciton localization

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INTRODUCTION UV-light-emitting diodes (LEDs) have many highly valuable applications, including solid-state lighting, photoelectrochemical hydrogen generation, photopolymerization, sterilization, environmental sensing, and treatment for a variety of diseases, including skin disorders and cancer.1,2 However, current UV-A LEDs (operating in the 320−370 nm range) based on GaN/ AlGaN multiple quantum wells (MQWs) suffer from many issues: first, the extremely low external efficiency ( 17.2 kW/cm3), the NBE energy remains roughly constant and is accompanied by peak broadening, indicating that QCSE has been fully screened, and band state filling has taken effect, whereby the emission is dominated by radiative recombination46 (the PDPL spectra at RT are shown in Figure S7 in the Supporting Information). Consequently, these PDPL results confirm the contribution of bound exciton radiative recombination in the NT emission. To investigate the carrier dynamics of our NTs, the correlation between internal quantum efficiency (IQE) and PDPL was studied.7 Figure 3c shows the IQE of our LED structure as a function of excitation power density (the injected carrier density). We calculated the IQE by applying the ABC model to the PDPL measurement data8

as shown in Figure S5. The RT PL spectrum shown in Figure 2a includes a weak defect band, indicating that the ZnO NTs/ GaN structure exhibits high crystal quality.35 For comparison, the RT PL spectrum of a similar ZnO NT structure grown on sapphire (to avoid the emission from p-GaN), deposited at the same time using identical growth conditions, is shown in Figure S6b (Supporting Information). This spectrum exhibits a sharp and strong ZnO emission at 378 nm (3.25 eV), suggesting that the defect band can be due to the ZnO layer underneath the NTs, as it is known that point defects (e.g., oxygen vacancies) in ZnO are the cause of such defect band emission.14,15,36−40 As the temperature increases from 6 K to RT, the NBE emission exhibits a slight red shift. To explain this phenomenon, the contributions of free and bound excitons should be correlated with the temperature.41 The TDPL of the NBE exciton peak deviates from Varshni’s law (a typical Varshni behavior of a free exciton is indicated by the black line in Figure 2b)42 at low temperatures. A similar bound exciton behavior was observed in ZnO QWs, indicating that the NBE of the NTs is dominated by localized bound excitons. The localization energy of the bound excitons is around 9 meV, suggesting the presence of carrier confinement similar to that observed in the QWs.41 To interpret the TDPL findings, PDPL of ZnO NTs grown p-GaN was performed at RT. The dependence of the integrated PL (IPL) intensity of the NBE on the excitation power (P) is defined by IPL ∼ Pk, where the power factor k is positive. If the NBE emission is due to the bound exciton radiative recombination, the k value should be in the (1 < k < 2) range.43,44 Figure 3a shows a log−log plot of IPL vs power density. Under low and high excitation power densities, a linear dependence of log(IPL) on log(P) is observed. The best fit is achieved at k ∼ 1.18, suggesting that the emission is due to acceptor-bound exciton recombination at RT for all considered excitation powers. The correlation shown in Figure 3b

G = An + Bn2 + Cn3

(1)

where An is the Shockley−Read−Hall (SRH) nonradiative recombination rate, Bn2 represents the radiative recombination rate, and Cn3 denotes the carrier Auger-like nonradiative recombination rate. To calculate the IQE using the data yielded by PDPL analysis, it can be expressed in terms of the carriers generated by optical pump (Gopt), using the ABC model in eq 147 Gopt = Plaser(1 − R )α /(A spot hν)

(2)

where Plaser is the laser power incident on the LED sample, whereas R represents the Fresnel reflection at the sample surface, Aspot denotes the area of incident laser on the sample surface, hν is the incident energy, and α represents the ZnO absorption coefficient48 at the emission wavelength. The integrated PL intensity can be defined as IPL = βBn2 (β is a constant determined by the total collection efficiency of the PL and the excited active region volume). Consequently, eq 1 can be revised in terms of IPL by eliminating n yielding Gopt = A

iI y IPL I + PL + C jjjj PL zzzz βB β k βB {

3/2

= Q 1 IPL + Q 2IPL + Q 3(IPL)3/2

(3)

where Q1, Q2, and Q3 are the fitting parameters defined in terms of A, B, C, and β. After plotting IPL versus Gopt, the fitting coefficients can be obtained by applying eq 3 to the experimental data. Accordingly, at steady state, the IQE can be calculated as ηIQE =

Q IPL Bn2 = 2 Gopt Gopt

(4)

The plot in Figure 3c shows that the IQE increases rapidly as the excitation energy density increases, whereby the excitation power reaches 2500 kW/cm3, indicating performance superior to that achieved by UV LEDs based on GaN/AlGaN that suffer from IQE droop when optically pumped by laser. However, it has been demonstrated that LEDs based on AlGaN/GaN exhibit droop-free external efficiency at high current injections when electrically pumped.49 Nonetheless,

Figure 3. LED optical characteristics. (a) Integrated RT PL intensity output as a function of power density. (b) Peak energy (black curve) and FWHM (blue curve) of Gd-doped ZnO NT NBE peak as functions of excitation power density at RT. (c) IQE calculated from the integrated intensity NBE peak at RT as a function of excitation power density. (d) TRPL of Gd-doped ZnO NTs at RT and 77 K with a double-exponential decay fit. D

DOI: 10.1021/acsami.9b06195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) Schematic diagram of the Gd-doped ZnO NTs/p-GaN film LED device. (b) Energy band diagram of the n-ZnO/p-GaN heterojunction without applied bias. (c) I−V curves of Gd-doped ZnO NTs/p-GaN heterostructures. (d) RT EL spectrum of the LED. (e) EL intensity (blue) and EL peak energy (black) of the LED device as a function of injected current.

injection rates. This finding supports the assertion that radiative recombination is dominant and is accompanied by negligible Auger recombination, confirming the high efficiency of the obtained LED structure. We examined TRPL lifetimes at RT and low temperature (77 K) to confirm the dominance of radiative recombination contribution of the UV emission. Figure 3d shows the TRPL spectra of Gd-doped ZnO NT sample at the NBE peak, at low temperature and RT. As can be seen, the TRPL decay lifetime of the sample follows a nonexponential trend at both low temperature and RT. The TRPL spectra were fitted, with excellent convergence, using the following biexponential decay model54

this is the first work showing that droop-free IQE can be obtained when the carrier injection is achieved by optically pumping the UV LED by laser. This droop-free IQE performance might be due to the saturation of the nonradiative recombination centers, which peaks at 65% at 15.7 mW, followed by slight saturation. This high IQE can be ascribed to the dominant radiative recombination process as a result of high NT structural quality, as well as the absence of dislocations and the confinement effect due to NT nanodimensions. At high carrier injection densities, no behavior related to Cn3 given in eq 1 is observed, which may suggest that Auger recombination is insignificant under the conditions adopted in this study. The high binding exciton energy (60 meV), which considerably exceeds that of GaN (23 meV) as well as RT thermal energy (25 meV), suggests high stability and dominance of the bound excitons that cannot be dissociated at RT, leading to high efficiency. This can be the reason behind the absence of efficiency droop as the carrier injection density increases. Thus, as no efficiency droop has occurred, our findings confirm that the LED structure exhibits high performance, as the emission is significantly denominated by radiative recombination. Compared to the previous reports, this is the highest IQE value obtained for ZnO/p-GaN-based LEDs.50−53 To elucidate the high efficiency and dominant radiative recombination contribution, we investigated the intensity as a function of generated carrier density (Figure 3a). It should be noted that, according to eq 3, if An ≪ Bn2, the emission is dominated by radiative recombination, resulting in a slope of ∼1 in the log−log plot of power density (corresponding to Gopt) and IPL. On the other hand, when the emission is controlled by nonradiative recombination, i.e., when Bn2 ≪ An, the IPL is proportional to Gopt2 in eq 3, corresponding to a slope of ∼2.8 Hence, the slope of ∼1.18 in Figure 3a confirms that, at RT, radiative recombination dominates significantly over the entire carrier density range even at high carrier

It = A1e−t / τ1 + A 2 e−t / τ2

(5)

where A1 and A2 are the initial intensities of the fast and slow decay components, respectively, It is the time-dependent intensity of the sample emission, and τ1 and τ2 represent the fast and slow lifetimes, respectively. At 77 K, τ1 and τ2 were calculated at 3.9 and 4.2 ps, respectively, while 18.7 and 14.1 ps were obtained at RT. These results are in line with the values reported for ZnO nanostructures.55 Moreover, τ1 was attributed to pertain to the electron−hole scattering effects, inter-subband scattering (ΔE > hωLO), and surface recombination states of nanostructures, as the surface-to-volume ratio is high. On the other hand, τ2 was attributed to multiexcitation centers.55,56 There is a slight difference between the slow decay values, τ2, at low temperature and RT, indicating predominance of radiative recombination. The absence of droop and the dominance of radiative recombination may be due to a negligible dislocation density propagating into the NTs.20 While we can demonstrate that the interface between the two materials can be sufficient to create an ideal p−n junction diode with a relatively low turn-on voltage, to confirm that our structure can be potentially used for producing UV LEDs, we fabricated a UV LED as a proof of concept. The fabrication steps are shown in Figure S1. Gd dopants in ZnO found to E

DOI: 10.1021/acsami.9b06195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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be attributed to a negligible QCSE at high injected power density, which is consistent with the PDPL findings and the droop-free IQE behavior.8 This superior performance, as indicated by a high IQE, can be ascribed to different reasons: (i) the localized bound excitons due to the high binding energy of ZnO-bound excitons in NTs (60 > 25 meV at RT).69 Moreover, (ii) TEM images show that threading dislocation defects are absent in the wetting layer. Threading dislocations (which is a common problem in GaN UV LEDs) are among the key causes of nonradiative emission and low efficiency. It is also noteworthy that (iii) in contrast to GaN-based LEDs, the nanodimensions of dislocation-free NT structures assist in carrier confinement, which in turn leads to a negligible QCSE, resulting in droop-free and high-efficiency IQE.5,8,39

introduce donor band underneath the conduction band minimum increase the n-type characteristics of ZnO,21 which can lead to the formation of an ideal p−n junction, which is required for efficient LED devices. Figure 4a shows the schematic diagram of the fabricated Gd-doped n-ZnO NTs/pGaN UV LED. As it is known that the efficient LED devices should be formed through p−n heterojunction, light is emitted through radiative recombination of electron and hole carries formed in the n-type and p-type layers, respectively. The p−n heterojunction mechanism can be elucidated from the energy band diagram shown in Figure 4b, based on the carrier diffusion process and Anderson’s model.57 At RT, the n-type ZnO and p-type GaN band gaps are 3.37 and 3.4 eV, respectively, and the corresponding electron affinities are 4.35 and 4.20 eV.58 When n-ZnO and p-GaN are attached, a small conduction band offset (ΔEc = 0.15 eV) and valence band offset (ΔEv = 0.13 eV) are produced. This slight band offset leads to the same barrier heights for electrons and holes, which is preferred for light-emitting devices, as it allows a high rate of electron−hole radiative recombination. Consequently, under forward-bias, the holes drift from GaN to ZnO NTs, due to which electrons in the ZnO NTs are recombined with these drifted holes, yielding efficient UV LED emission.59 We measured the current−voltage (I−V) characteristics of the Gd-doped ZnO NTs/GaN LED device, as shown in Figure 4c. To extract the emission from the NTs only, the free space between the individual NTs was filled with a PMMA thin layer, obtaining emission from the p−n junction between n-ZnO NTs and p-GaN. The forward-bias current is nonlinear, indicating reasonable p−n junction quality. The device junction exhibits diode properties with a turn-on forwardbias voltage well above 3 V. Under reverse bias above 10 V, a low leakage current (5.6 × 10−9 A) was measured at the junction, indicating a well-fabricated junction between Gddoped ZnO NTs and the metal electrode.60 The electroluminescence (EL) spectrum produced by the LED device based on Gd-doped ZnO NTs grown on a p-GaN shown in Figure 4d was measured under 120 mA forward-bias current. The RT EL spectrum is characterized by strong UV emission (at λ = 371 nm) without a defect band observed in the EL spectra under different forward-bias currents (2−120 mA), as shown in Figure S8 (Supporting Information). These results indicate superior device quality compared to UV LEDs based on ZnO films and nanostructures grown on GaN that are reported in pertinent literature.61 In these previous studies, low UV emission was obtained, and a dominant defect band was noted in the EL spectra of ZnO/GaN devices. Thus, our results demonstrate that highly efficient LED devices based on NT structures can be obtained. The broad blue peak in the EL spectra is due to donor− acceptor pair (DAP) emission,21 whereas the oscillations in the spectrum can be ascribed to the interference fringes between the reflections from the sapphire/GaN film/ZnO wetting layer interface.62,63 Similar UV emission with DAP characteristics from LEDs based on GaN/AlGaN quantum well was reported in pertinent literature.64−68 The p-GaN films contain high density of acceptors (see Figure S6a in the Supporting Information), whereas donors are introduced in ZnO NTs through Gd dopants, as demonstrated previously.15,21 Figure 4e shows the EL peak intensity and peak energy as a function of injected current, indicating that the former increases with the applied current, whereas the latter remains relatively constant (∼3.33 eV). Such peak energy behavior can

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CONCLUSIONS In this work, we reported for the first time on the successful growth of high-quality, catalyst-free well-ordered, and vertically aligned Gd-doped ZnO NT array of well-defined hexagonal shape grown on a p-GaN template by PLD using a one-step method. Our LED structure is characterized by high light efficiency as well as absence of efficiency droop at high carrier injection densities through nanosized junctions. A detailed assessment of optical properties indicated that recombination was dominated by bound excitons and radiative recombination, with a negligible Auger recombination contribution. The NT-based LEDs exhibit adequate UV EL performance, confirming high optical and structural quality. We demonstrated that the characteristics of the high exciton binding energy of ZnO, the nanostructure confinement, and superior NT quality can pave the way toward designing cost-effective high-efficiency nanoscale UV LEDs characterized by high IQE without any efficiency droop.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06195. Surface energy and kinetic energy effect mechanism; fabrication steps of the LED structure; SEM images of Gd-doped ZnO sample grown on different substrates; DSIMS positive depth profiling; SEM images of pure ZnO sample grown on p-GaN; PL spectra obtained at 6 K and RT; full PL spectra of both GaN and ZnO NTs on sapphire; PDPL spectra; and full EL spectra under different injection current densities (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Idris A. Ajia: 0000-0003-3156-4426 Sergei Lopatin: 0000-0003-3916-3803 Iman S. Roqan: 0000-0001-7442-4330 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acsami.9b06195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS I.S.R. and all members of the research team involved in this project acknowledge the financial support from General Directorate of Research Grants (405-34-AT), from King Abdul-Aziz City of Science and Technology (KACST), Kingdom of Saudi Arabia.

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DOI: 10.1021/acsami.9b06195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.9b06195 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX