Improved Electrical Transport and Electroluminescence Properties of p

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Improved Electrical Transport and Electroluminescence Properties of PZnO/n-Si Heterojunction via Introduction of Patterned SiO Intermediate Layer 2

Zhifeng Shi, Di Wu, Tingting Xu, Yuantao Zhang, Baolin Zhang, Yongtao Tian, Xinjian Li, and Guotong Du J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10689 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 14, 2016

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The Journal of Physical Chemistry

Improved Electrical Transport and Electroluminescence Properties of p-ZnO/n-Si Heterojunction via Introduction of Patterned SiO2 Intermediate Layer Zhifeng Shi1, Di Wu1, Tingting Xu1, Yuantao Zhang2, Baolin Zhang2, Yongtao Tian1, Xinjian Li1,*, and Guotong Du2,* 1. Department of Physics and Laboratory of Material Physics, Zhengzhou University, Daxue Road 75, Zhengzhou, 450052, China 2. State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China Supporting Information Placeholder ABSTRACT: The authors report on the fabrication, temperature-dependent current-voltage and electroluminescence properties of p-ZnO:As/n-Si heterojunction diodes. The As-doped p-ZnO material was prepared by out-diffusion of arsenic atoms from a sandwiched GaAs interlayer on patterned SiO2/Si substrates. The introduction of hollow-shaped SiO2 patterned layer promotes the efficiency of carrier injection into the active layer and considerably lowers the emission onset of the studied diode. The current-voltage characteristics of the heterojunction were detailedly studied in the temperature range of 21-120 ° C to determine the dominant carrier transport mechanisms in different bias regions. The reverse saturation current, barrier height, and ideality factor were estimated from the thermionic emission model and found to be highly temperature dependent. An improved electroluminescence performance of the studied diode featuring an ultralow emission onset and an acceptable operation stability shows the potential of our approach. Long term stability of the diode without encapsulation in air-exposure environment was also investigated by monitoring the electroluminescence evolution with storage time, and the oxygen-related surface adsorption was identified as the main cause for the undesirable emission decay.

KEYWORDS: temperature-dependent, electroluminescence, emission decay

p-ZnO/n-Si,

INTRODUCTION Due to ever increasing commercial desire for shortwavelength light-emitting diodes (LEDs) and lasing diodes, ZnO material has attracted a great deal of attention owing to its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV).1-3 As the reliable and reproducible p-type ZnO material is still a challenge as a result, the performance of ZnO homojunction devices is typically inferior to that of ZnO-based heterojunction LEDs. Over the past decade, many studies have been conducted across the globe with an intention to acquire p-ZnO using group I or V elements as the ptype dopants,4-11 and the number of studies on optoelectronic devices implementing p-ZnO material shows a steady increase. For example, conventional ntype semiconductors n-GaN and n-SiC were frequently employed to combine with p-ZnO to construct heterojunction LEDs because of their same crystalline structure and closely matched lattice constants.10,12,13 While very few studies on p-ZnO/n-Si junctions featuring an acceptable electroluminescence (EL)

SiO2

patterned

layer,

carrier

transport

mechanisms,

performance are available.14,15 It is not only because their relatively large lattice mismatch but also an unsuitable type-II staggered band alignment between them, resulting in a favorable carrier recombination process in Si side. However, from the application point of view, the development of ZnO/Si heterostructured LEDs opens enormous opportunities for applications due to the wellknown advantages of Si material and its potential applications in Si-based optoelectronic integrated circuits. In previous reports, researchers often prepared p-ZnO/n-Si heterojunctions to confirm the p-type conductivity of ZnO by acceptor doping,14,16 and less attention has been paid to their detailed electrical characteristics and carrier transport mechanisms. Moreover, a deeper insight into the electrical characteristics and transport mechanisms of p-ZnO/n-Si heterojunctions, prior to their adoption and demonstration in electroluminescent nanodevices, is undoubtedly required considering potential application. Since ZnO is an important high-temperature (HT) material owing to its high exciton binding energy that ensures exciton survival well above room temperature

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(RT),17 it is desirable to investigate the HT electrical properties of ZnO/Si systems, and the obtained information will naturally contribute to further applications of the related nanodevices. In addition, to be employed more widely in optoelectronic fields, the improvement of EL performance of p-ZnO/n-Si heterojunction diodes is also important pursuits and certainly worthwhile subjects. Of particular concern is their emission onset and operation stability in harsh environment. A relatively high emission onset is the common failure of ZnO/Si systems because of the preference of carrier accumulation and recombination at narrow band gap Si, inducing an undesirable heating effect and seriously hampering the future applications. Some feasible scientific and technical approaches are therefore needed to address the above problem by optimizing the device structure, such as carrier injection and confinement configuration. In this work, a latemodel p-ZnO/n-Si heterojunction diode was fabricated by introducing an elaborate hollow-shaped SiO2 cladding pattern, which effectively prevents the lateral diffusion of injection current and ultimately lowers the emission onset of the diode. Investigations on the temperaturedependent current-voltage (I–V) and EL characteristics have been carried out, and carrier transport and recombination mechanisms have been discussed in detail. Experimental section The fabrication procedure of the p-ZnO/n-Si heterojunction diode started with a commercially available n-Si (111) substrate with a resistivity of 0.005 Ω·cm. A SiO2 dielectric layer with a thickness of ~110 nm was obtained by thermo-oxidizing silicon in dry oxygen at 1050 ° C. For the preparation of selective circular openings, the SiO2 cladding layer was patterned by conventional photolithography and wet etching method. The etching solution is HF:H2O:NH4F (28 mL:170 mL:113 g) and the etching time is 85 s, creating a series of hollow-shaped openings with a diameter of 100 μm and a separation of 500 μm, as shown in the lower pane of Figure 1b. Followed that, the As-doped ZnO film was prepared on the patterned SiO2/Si substrates by metalorganic chemical vapor deposition (MOCVD) combined with sputtering system, and the details of such a doping method can be found elsewhere.10,13 The electrical properties of ZnO:As film on sapphire substrate were examined by Hall measurements, which revealed a hole concentration of 1.92×1017 cm-3 and mobility of 0.68 cm2/V·s. The epitaxial ZnO:As layer is well-aligned but shows some columnar morphology. The average diameter and length of these rods are of ~80 and ~750 nm, respectively (Figure S1a). Excellent optical properties of the ZnO:As film was verified by the RT photoluminescence (PL) measurement, which exhibits a single intense near-band-edge (NBE) emission at 378 nm and a rather weak deep-level emission (DLE) band (Figure S1b), making the material suitable as the emitter in LEDs. Subsequently, vertical conducting LEDs based on p-ZnO/n-Si heterojunction were fabricated by evaporation with Au and Al metals as the n- and p-type

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electrodes on p-ZnO and backside of n-Si substrate, respectively. Note that the shape of deposited Au electrode was controlled by an elaborate rectangular shaped mask using tungsten filament as the braced frame, and the diameter of tungsten filament is 70 μm. From the surface optical microscopy image of four square device units (upper pane in Figure 1b), one can clearly find that each square Au electrode corresponds to an underneath hollow-shaped opening. The schematic diagram of the p-ZnO/n-Si heterojunction is presented in Figure 1a. Figure 1c shows the current distribution model of a single vertical conducting geometric unit, and a restricted lateral diffusion current offered by the patterned SiO2 cladding layer can be expected, and thus the injection efficiency of carriers into the active layer is improved, making the reduction of emission onset possible. Details of the positive role of such a device model will be interpreted later.

Figure 1. (a) Schematic diagram of the p-ZnO/n-Si heterojunction diode. (b) Upper pane: surface optical microscopy image of the prepared device showing four square units. Lower pane: pattered Si substrate with a hollow-shaped SiO2 cladding layer, displaying a series of openings with a diameter of 100 μm. (c) Current distribution model of a single vertical conducting geometric unit.

The morphological characterizations of As-doped ZnO films were performed by field emission scanning electron microscopy (FE-SEM; Jeol-7500F). The PL spectra were recorded at RT using a Zolix Omni-λ 5007 monochromator/spectrograph with a continuous wave He-Cd laser (325 nm, 30 mW) as the excitation source. The I–V characteristic of the device was measured using an Agilent B2900A semiconductor characterization analyzer and EL spectra were measured by using a homemade acquisition equipment including photomultiplier tube and lock-in amplifier systems. The Infrared (IR) EL spectra were detected at RT with a liquid-nitrogen-cooled Ge detector. Results and discussion Commonly, I–V characteristics of a diode provide a valuable source of information about several junction parameters such as turn-on voltage, rectification ratio (IF/IR, IF and IR for forward and reverse currents, respectively), series resistance (RS), reverse saturation current (I0), diode ideality factor (n), barrier height (BH,

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Φb) and so on. Therefore, it is essential to make an indepth understanding on the electrical characteristics and transport mechanism of p-ZnO/n-Si heterojunction diodes prior to their adoption and demonstration in electroluminescent nanodevices. In this work, temperature-dependent I–V measurements were carried out at different temperatures varying from 21 to 120 ° C to determine the transport mechanism controlling the conduction through p-ZnO/n-Si heterojunction. As shown in Figure 2a, typical I–V characteristics of the studied diode at 21, 50, 70, 100, and 120 ° C are presented. The diode shows well-behaved diode-like rectification behaviors at all temperature points. The turn-on voltage was found to be ~1.35 V at RT (21 ° C), comparable to the value reported by Zhang et al. in phosphorus-doped pZnO nanonail array/n-Si heterojunction.18 As seen in the left inset of Figure 2a, a high IF/IR of approximately 170 was determined at a bias of 5.0 V. Besides, the RS of the diode at 21 ° C was calculated from the slope of I/(dI/dV) versus I (shown in the right inset of Figure 2a), a typical value of the fitted RS was 0.71 kΩ. With increasing the working temperature, the forward and reverse I–V curves display the same dependence on working temperature, as clearly demonstrated in their semilogarithmic plots in Figure 2b. The turn-on voltage of the device chip decreases from 1.35 to 0.82 V gradually, and the trend can be attributed to the more carriers generated from the two semiconductor sides at higher temperatures. Due to the Richardson effect, carriers can be driven over the energy barrier at the ZnO/Si heterointerface by relatively smaller bias voltage at higher temperatures.19 And meanwhile, temperature-induced band gap shrinkage is another deserved factor. But even so, the IF/IR at 120 ° C still stays at a high level of 86.8, much better than other reports based on p-ZnO/n-Si heterojucntion,14,15,18 suggesting a good potential of EL performance of the studied diode operated at HT. In addition, the slope of the I–V curves at high-voltage region increases with the working temperature, indicating a change in RS of the diode. As shown in the inset of Figure 2b, a regular temperature dependence of RS can be found, which eventually reduces to 0.39 kΩ at 120 ° C. Taking into account of the temperature effect, I–V curves of the diode at three representative temperature points (21, 70, and 120 ° C) were fitted by the standard diode equation:20 (1) I  I 0 [exp( qV )  1] nkT where k is the Boltzmann’s constant, I0 is the reverse saturation current, n is the so-called quality or junction ideality factor, and T is the temperature in Kelvin. As shown in Figure 3a, the n values for low forward voltages were obtained by calculating the slope of lnI–V plots, or given by the equation n  (q / kT)(dV / d (ln I )) . The values are found to be 8.45, 7.56, and 6.75, respectively, lowest being at the highest temperature. Such a high value (n > 2) indicates that the diode is far from ideal according to

the Sah-Noyce-Shockley theory.21 And the ideal model is therefore not suitable to explain the current transport mechanisms though the heterojunction. High ideality factor is a very common feature observed in wide band gap heterojunctions, and is usually attributed to the presence of surface or interface states. According to the work of Wang et al. about ZnO/diamond p-n junction diode,22 a nonlinear characteristics of metalsemiconductor contact in the interim bias voltage range is also presumably responsible for such a high n value. As for the decrease of n at higher temperature (illustrated in Figure 3b), the well-known inversely-proportional relationship of n with T ought to bear mostly liability. Of course, the role of thermionic field emission, enhanced tunneling through the barrier, in addition to the generation-recombination process occurring in the depletion region can not completely ruled out to explain the temperature dependence of n at this stage of investigation.

Figure 2. (a) Experimental I–V characteristics of p-ZnO/nSi heterojunction in the temperature range of 21–120 ° C. Left inset: the I–V data measured at 21 ° C on a semilogarithmic scale. Right inset: I/(dI/dV) versus I curve showing the RS of heterostructure. (b) I–V characteristics of p-ZnO/n-Si heterojunction at five representative temperature points on a semilogarithmic scale. The inset shows the temperature dependence of RS and the equivalent circular of a real LED.

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Based on the equation 1, one can obtain I0 of the diode derived from the straight line intercept of lnI at V=0. As plotted in Figure 3b, the I0 increased from 1.52×10-8 to 8.11×10-8 A with increasing the temperature from 21 to 120 ° C. A relatively small value of I0 even at 120 ° C compared with other reports might be an indication of a good potential of the proof-of-concept diode working at high temperatures. According to the thermionic emission theory, the zero bias Φb can be calculated from the saturation current variation with the temperature by following the relation:20 (2) I  AA*T 2 exp( q / kT ) 0

b

where the reverse saturation current I0 is the function of temperature, A is the junction area, A* is the Richardson constant (A*=4πqm*k2/h3), and m* is the effective mass of the charge carriers. For ZnO, A* is theoretically estimated to be 32 A/cm2·K2 (me*=0.27m0). As summarized in Figure 3b, the value of Φb shows a clear temperature dependence. Because of the barrier inhomogeneities and presence of defects at heterojunction, the current conduction will be dominated by patches with lower Φb at lower temperatures. This means electrons are able to surmount the lower barriers and current flowing through defects or intermediate states at ZnO/Si interface is favored. While at higher temperatures, the patches with higher Φb take effect due to the fact that more electrons have sufficient energy to surmount such barriers. As a result, the dominant Φb is proportional to working temperature. In addition, the variation of lnI0 with the reciprocal of the temperature is plotted in Figure 3c; a nearly linear relation suggests that thermionic emission is the dominant conduction mechanism at low voltage. The activation energy (Ea) for carriers can be derived from the slope of the linear fitted line by assuming an Arrhenius-type relation I  I 0 exp(  Ea / kT ) .23 The obtained value of Ea in the region (294–393 K) is 0.15 eV. Since the Arrhenius behavior is a characteristic feature of multi-step tunneling model, we therefore consider that some shallow defect levels localized below the bottom of conduction band of ZnO are likely to assist the multi-trapping tunneling mechanism through electrons capture and its reemission towards n-Si side. Figure 3d presents the logI–logV plots of the studied heterojunction at three temperature points, and three distinct regions can be found dependent on the applied voltage. For region I (V < 0.3 V), current obeys the Ohmic law, in which current follows a linear relation due to thermally generated carrier tunneling process. At a moderate junction voltage (0.5 V < V < 1.5 V), the current increases exponentially following the relation of I (V , T )  I 0 (T ) exp(V ) , where α is a constant. This current transport behavior is often observed in the wide band gap p-n heterojunction controlled by recombinationtunneling mechanism.15,22 The constant α is calculated to be 4.65 V-1 by fitting the experimental data and it is independent of temperature (Figure S2). For region III,

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current increases with voltage and satisfies power law I ~ V2.65, I ~ V2.41, and I ~ V2.16 at 21, 70, and 120 ° C, respectively. In this region, current follows space charge limited conduction (SCLC) mechanism in which current conduction through heterojunction is due to single charge carrier.15 This is reasonable because of the apparent asymmetric conduction and valence band offsets of ZnO/Si heterojunction as we discussed later. In our case here, the exponents larger than 2 can be interpreted as an indication of trap-limited SCLC model with exponentially distributed traps,24 and the term of trap-limited means that the current is controlled by the traps via thermally activated carriers. Herein, the traps can be regarded as the localized states arising from dangling bonds, impurities, etc, which capture freecharge carriers and play an important role in the conduction process of semiconductors. In theory, the trap-limited SCLC process become dominant as the density of injected free carriers is much larger than that of thermally generated free-charge carriers. As a result, the exponent decreases and gradually approaches 2 with increasing the temperature from 21 to 120 ° C. It is because that more localized states located within the band gap of two semiconductors would be filled by the thermally excited carriers at high temperatures, and the direct consequence is that current conduction follows the trap-free SCLC process with the exponent near 2.

Figure 3. (a) Plots of current versus bias at three representative temperature points on a semilogarithmic scale to calculate the ideality factor. (b) Variation in Φb, n, and I0 as a function of working temperature. (c) A plot of ln(I0) versus 1/kT revealing the activation energy. (d) Loglog plots of the I–V characteristics of p-ZnO/n-Si heterojunction at the temperatures of 21, 70, and 120 ° C, respectively.

EL spectra were collected by electrically stimulating the studied diode with positive voltage connected with the Au electrode. Figure 4a shows the corresponding EL spectra under different driving currents measured at RT, and an ultraviolet (UV) emission peak at ~383 nm as well as a broad defect-related visible emission at around 500 nm can be observed. As the current is increased, the corresponding intensities of both UV and visible

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components increase gradually, and an additional observation is that the band-to-band transitions increasingly dominate over the defect-related transitions. It is reasonable that injected carriers may preferentially fill the shallow defect-produced energy level, corresponding to the long-wavelength light emitting. Once such DL traps are saturated, the band-to-band transitions of the electrons and holes come into playing a dominant role. The overall emission of the diode appears blue-white, and visible to the naked eyes in an appropriately dark environment (see inset of Figure 4a with an injection current of 7.0 mA). Unlike other reports,18 the studied p-ZnO/n-Si heterojunction diode in our case possesses a low emission onset and a dominant UV contributor, indication of an improved carrier injection and recombination efficiency benefited from the elaborate hollow-shaped SiO2 cladding pattern. For a vertical conduction diode with a top-bottom electrode geometry, the total electric current consists of two components: vertical injection current under the Au top electrode and lateral diffusion current across the p-ZnO layer. Due to the columnar morphology of p-ZnO epitaxial layer, a high density of undesirable grain boundary, where impurities, disorders, and dangling bands have a preferential accumulation according to the work of Grovenor et al.,25 is inevitable. These localized states often capture free-charge carriers, playing an important role in the lateral conducting process, and are also the cause of leakage current of operating diodes. In our case here, a patterned SiO2 layer could act as an electrical isolation layer to restrict the lateral current flow due to the distinct thickness difference of SiO2 in the plane parallel to the substrate surface according to the electrical current flowing along the shortest path with minimal resistance. In addition, additional advantages provided by the separated nanorods in our case also deserve comment. The morphological characteristics of ZnO means that the carrier transport along the vertical direction of nanorods might be enhanced in some sense. Thus the efficiency of carrier injection into the active layer can be improved, and further the emission onset of the diode is reduced, allowing the UV emission to dominate at a low-current injection level. A low emission onset combined with a well-behaved rectifying I–V relationship (shown above) of the studied diode show the potential of our approach, and may also an indication of the reliability of As-doping p-type ZnO. Because the key concept in our approach is the elaborate geometric profile of SiO2 patterned layer, it is anticipated that our strategy is applicable to other material and device systems.

Figure 4. (a) RT EL spectra of the studied diode at different injection currents. The inset shows the corresponding colored photos at 7.0 mA. (b) Temperature-dependent EL spectra of the diode at a fixed current of 7.0 mA. (c) Shift of the UV emission peak and the integrated EL intensity of the diode as a function of working temperature. (d) Schematic band diagrams of the p-ZnO/n-Si heterojunction at zero (blue solid line) and forward bias (pink dashed line) showing two corresponding recombination processes.

The above discussions might be a positive signal that we can drive such a prototype device substantially above RT and/or above the emission onset at present because a relatively low operating current implies an insufficient heating effect, which would allow for a higher output and a better application potential. From another perspective, the EL proceeds or current flow occurs only through a limited area where the number of defects is finite and thus, DL traps may be saturating at a low-current injection level, and effective carrier injection and recombination behaviors can be guaranteed. As shown in Figure 4b, temperature-dependent EL measurements were carried out to verify the sensitivity of EL performance to operating temperature, and a drastic and regular evolution of spectral characteristics can be observed. Note that the driving current was set to 7.0 mA for comparison over the course of measurement. With the successive increase in operating temperature, the integrated EL intensity is gradually suppressed, as shown in Figure 4c. At 70 ° C, the emission intensity decreases by 54.06% and at 120 ° C, only less than 17% remains. The decay of spectra intensity could be the result of increasing probability of nonradiative recombination, which is likely to increase in proportion with junction temperature. Theoretically, heating effect will inevitably give rise to a rapid proliferation of structural defects, producing a number of nonradiative recombination centers in the depletion layer and thus, the carrier

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injection efficiency and radiative recombination probability are reduced.2 Another observation concerns the change of UV/DLE ratio, which decreases with increasing temperature although both two contributors diminish simultaneously. Generally, the ratio of UV/DLE can be recognized as an evaluation criterion of ZnO material quality. Because of the presence of heat-induced structural defects incorporated in ZnO, the optical quality of ZnO:As films is bound to be affected, causing its p-type conductivity to deteriorate. Therefore, one possibility of added DL defects caused by the degradation of optical property of ZnO:As films ought to bear mostly liability for the above-mentioned observation. Although the broad-band (UV) and sub-bandgap (DLE) transitions could be understood as resulting from different radiative recombination processes, the recombination paths of UV component could be impeded or the excitation energy could be consumed with the undesirable DLE because the carriers may preferentially fill the lower energy level, as we stated above. Additionally, presumably due to the heating-induced band gap shrinkage, the NBE UV emission is observed to red-shift, as shown in Figure 4c.

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affinity (χ) and band gap (Eg) of two semiconductors are known from literatures. In the present case, values of χ (χZnO=4.35 eV, χSi=4.05 eV) and Eg (Eg,ZnO=3.37 eV, Eg,Si=1.12 eV) were taken,26,27 and a type-II staggered configuration is formed. Theoretically, the combination of electrons and holes proceeding at the Si side is more favorable than that in ZnO side by inspection of the band diagrams. In reality, we indeed detected the IR emission at ~1159 nm using a liquid nitrogen-cooled Ge detector (Figure 5a), and the spectra intensity in IR region is much stronger than that in UV-visible region. Herein, we consider that the IR emission at ~1159 nm comes from the recombination in Si material in spite of its indirect band gap feature. In order to confirm the origin of the IR emission, Al/SiO2/p-Si/Al metal-oxide-semiconductor (MOS) structure were also fabricated in a previous report by Li et al..28 The IR emission performance of the MOS structure was similar to that of ZnO/Si heterostructure, indicating that the detected IR emission indeed came from the recombination in Si side. However, the potential profile at the ZnO/Si hetero-interface depends strongly on the magnitude of the applied bias. The externally forward bias can effectively compensate the energy barrier of electrons, and thus electron transport or conduction from n-Si to p-ZnO takes effect, making the recombination of electrons injected from n-Si side with holes at the p-ZnO side possible. Moreover, we also calculate the whole width and distribution of space charge region (X0) based on the experimental results and other known parameters following the equation X 0  X n  X p  2VD ( n N A   p N D ) / qN A N D

Figure 5. (a) IR EL spectra of the studied diode at different injection currents. (b) Integrated EL intensity of the diode obtained after different storage periods with a fixed current of 7.0 mA, and the EL spectra measured before and after storage for 20 days. (c, d) Comparison on the PL spectra of ZnO:As films and I–V curves of the studied diode before and after storage for 20 days.

We now use the band structural model of p-ZnO/n-Si heterojunction to understand the carrier transport and recombination mechanisms. As shown in Figure 4d, the expected band alignments at zero (blue solid line) and forward bias (pink dashed line) were drawn according to the Anderson model assuming the continuity of vacuum levels and there are no imperfections at the heterointerface, and the electrical parameters such as electron

,20 where Xn and Xp are the width of positive and negative space charge regions, respectively; VD is the built-in potential at zero bias; NA and ND are the acceptor and donor concentrations, respectively; εn and εp are the relative dielectric constants of semiconductor (n-Si and p-ZnO). The calculated X0 is ~156 nm, distributing 14.56 nm and 141.44 nm at n-Si and p-ZnO sides, respectively. Combined with the factor of a lower mobility for holes (0.68 cm2V-1s-1) in our case than that of electrons (480 cm2V-1s-1) in Si, the conduction current contributed from electron transport will become more and more evident with increasing forward bias. To evaluate the stability of the studeid diode without any encapsulation, the EL performance was also monitored after different storage periods in air (RT, 4050% humidity). As shown in Figure 5b, the integrated EL intensity of the diode in UV-visible region was plotted against the storage time, and an obvious decreasing trend can be observed over the whole testing process. After 20 days storage, the device demonstrates a ~80% decay in the emission intensity although the typical UV and DLE from ZnO were still detected and their central positions remained unchanged (illustrated by the red and green curves in Figure 5b ). Herein, the degradation of EL performance of the unencapsulated diode was mainly ascribed to the adsorbed H2O, OH–, and O2 species in the surface of ZnO:As films due to the unideal storage

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environment where a mass of adsorbates existed.29 On the one hand, such contaminants incorporated in ZnO will cause the formation of undesirable surface states, which can serve as nonradiative recombination centers and quench the exciton-related NBE emission substantially.30,31 Therefore, the optical quality of ZnO:As films after 20 days duration will suffer from a degradation, as shown in Figure 5c. On the other hand, surface adspecies could also induce many donor-typedefects, which inevitably compensated the As-related acceptor in ZnO and deteriorated its p-type conductivity substantially. Figure 5d showed the I–V curves of the studied diode before and after 20 days storage. An obviously degraded rectification behavior can be observed compared with the fresh conditions, which was probably the main cause of emission decay of the studied diode. CONCLUSION In conclusion, we demonstrated the fabrication of a late-model p-ZnO:As/n-Si heterojunction diode with an elaborate hollow-shaped SiO2 patterned layer. Temperature-dependent I–V measurements are performed to determine the dominant carrier transport mechanisms. Several parameters such as I0, Φb, and n were deduced based on the thermionic emission model. The current transport mechanism in the heterojunction is limited by three types of processes depending on the applied bias voltage and temperature. Under forward bias, the diode displays an obvious UV-visible emission with an ultralow emission onset. The influence of operating temperature on the emission properties of the diode was also studied to test its sensitivity and compatibility for practical applications under harsh environments. A low emission onset and an acceptable operation stability positively prove the potential of our approach. In addition, we investigated the long term stability of the studied diode without encapsulation in air-exposure environment, and confirmed that the oxygen-related surface adsorption was responsible for the undesirable emission decay. The techniques involved in the studied LED are simple and compatible with current semiconductor technologies, and may provide a possible route for integration of optoelectronic devices based on Si materials and various hybrid material systems.

ASSOCIATED CONTENT Supporting Information SEM image and RT PL spectrum of p-ZnO:As films on Si substrate, and temperature dependence of parameter α in region II (0.5 V

Improved Electrical Transport and Electroluminescence Properties of p

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