1. Introduction

Recently, ZnO-based compounds have gained significant importance to realize highly efficient ultraviolet (UV)/visible light emitting diodes (LEDs) and detectors. ZnO has some superior properties, such as its wide direct band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [1, 2]. By alloying with CdO, the band gap of ZnO can be shrunk from 3.3 to 1.8 eV making it useful to fabricate visible opto-electronic devices [3–5]. A number of research groups have attempted to fabricate ZnO based heterojunction LEDs with CdZnO as active layers by deploying GaN [6, 7] and SiC [8] materials as p-type layer. Research work has been channelized to realize the growth of high quality p-type ZnO films by doping with N [9, 10], As [11], P [12], and Li [13]. Our research group has recently demonstrated reliable p-type ZnO films by Sb doping [14]. In recent past, a number of successful demonstrations of achieving UV luminescence from ZnO-based light emitters have been reported [15–17]. However, demonstration of ZnO based LEDs for visible emissions are quite limited. In this paper, we demonstrated p-type Sb-doped ZnO (SZO)/i-CdZnO/n-type Ga-doped ZnO (GZO) double heterojunction blue LED grown on n-Si (100) substrate. Visible blue emissions were observed and scrupulously studied.

Dual ion beam sputtering deposition (DIBSD) system had been deployed to grow the p-i-n heterostructure [1]. To the best of authors’ knowledge, there is no report of growth and fabrication of ZnO-based blue LED structure by DIBSD yet. Therefore, the present study of realizing blue electroluminescence from ZnO-based LED is of utmost importance.

2. Experimental procedures

p-SZO/i-CdZnO/n-GZO heterojunction LED structure was grown on n-Si (100) substrate (1-5 ohm-cm) using DIBSD system [1]. The schematic diagram of this structure is shown in Fig. 1(a). Before inserting Si substrates inside the DIBSD growth chamber, these were rinsed in 10% HF solution to remove SiOx layer from substrate. Subsequently, the substrates were rinsed thoroughly with trichloroethylene, acetone, iso-propanol, and de-ionized (DI) water and purged by 5N-purity (99.999%) nitrogen gas in order to remove dust particles and various organic contaminants. Afterwards, 500 nm of GZO was grown on Si substrate by DIBSD system using a Ga-doped ZnO target with 3 atomic% of Ga-doping [2] at 400 °C, following an in situ annealing at 800 °C in vacuum. An active CdZnO layer with a thickness of 100 nm was grown using a Cd-doped ZnO target with calculated 11.8 atomic% of Cd in film [18] on top of GZO film at 400 °C and annealed in situ at 800 °C in vacuum. Finally, a layer of p-type SZO with a thickness of 700 nm was grown on top of CdZnO at 500 °C using a ZnO target with 5 atomic% doping of Sb [14]. The samples were then subjected to in situ annealing process for 20 minutes at 800 °C in oxygen atmosphere at a constant chamber pressure of ~3 × 10⁻⁵ mbar, in order to activate p-type dopant in SZO film. It should be mentioned here that prior to GZO film deposition, assist ion source was turned on for 10 minutes to perform substrate pre-cleaning by Ar + ion bombardment. During growth of all associated thin films, assist ion beam, consisting of plasma of Ar + ion, helped in the reduction of columnar growth of films and thereby enhancing growth uniformity and film adhesion. The discharge voltage and current of the assist ion source was kept constant at 70 volts and 600 mA, respectively at the time of film deposition. The growth of all films was done in Ar (99.999% purity) gas ambient with 500 V beam voltage, 29 mA beam current, and 40 W radio-frequency (RF) power of deposition source. The background pressure inside the deposition chamber was maintained at around 1 × 10-8 mbar, while the working pressure during film growth was kept constant at 2.34 × 10-4 mbar.

 

Fig. 1 (a) Heterojunction device structure under forward bias, (b) XRD pattern of CdZnO, SZO and GZO thin films, each grown on Si(100) substrates.

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Besides the p-i-n structure, separate growth of GZO film on p-Si (100) substrate, CdZnO film on p-Si (100) and SZO film on n-Si (100) were also carried out following the same growth conditions and post-growth treatments as deployed during the growth of p-i-n structure. Crystalline quality of grown GZO, CdZnO and SZO thin films was investigated using Bruker D8 Advance X-ray diffractometer (XRD) (Cu Kα, λ = 0.1541 nm). Room temperature electrical properties of these films were measured using four probe Hall measurement setup in Van der Pauw geometry with a magnetic field of 0.50 Tesla. DongWoo Optron photoluminescence (PL) set-up, affixed with a 20 mW continuous wave (CW) He-Cd laser (excitation wavelength = 325 nm, TEM00 mode), 320 mm and 150 mm focal length monochromators, chopper, lock-in amplifier, and a photomultiplier tube (PMT) detector, was deployed to conduct optical studies of individual thin films. Current-voltage (I-V) characteristics of the heterojunction LED was measured using Keithley 2612A source meter. Electroluminescence (EL) characterizations were performed by using measurement setup including lock-in amplifier, photomultiplier tube (PMT) detector, monochromators, and chopper.

3. Structural properties

Figure 1(b) demonstrated the XRD spectra of CdZnO, SZO and GZO films each grown on Si (100). The crystal structure of all films was identified to be (002) preferentially oriented. However, (100) and (004) plane orientations were observed for CdZnO along with much prominent (002) crystal orientation. The diffraction angles of (002) peak were 34.50°, 34.18°, and 34.08° for CdZnO, SZO, and GZO films, respectively. The (002) peak positions for these films were nearly close to angular position of ZnO powder peak (34.42°) [1] indicating the formation of almost stress-free CdZnO, SZO, and GZO films on Si. The full-width at half-maximum (FWHM) of (002) diffraction peaks were found to be 0.28°, 0.36° and 0.24° for CdZnO, SZO and GZO films, respectively. The (002) peak positions and FWHM of deposited CdZnO, SZO and GZO thin films revealed a good crystalline quality films suitable for growth of p-i-n LED structure. Moreover, no peaks corresponding to the presence of Cd or its oxide (CdO) in CdZnO film, Sb or its oxide (Sb₂O₃) in SZO film, and Ga or its oxide (Ga₂O₃) in GZO film were observed in XRD measurements [18–20]. This suggested that proper elementary doping took place in corresponding thin films.

4. Photoluminescence

The room-temperature PL spectra of n-type GZO, intrinsic CdZnO and p-type SZO films were shown in Fig. 2(a-c). The PL spectrum of GZO exhibited an intense near-band-edge (NBE) emission peak at ~383 nm. The emission at ~383 nm was probably the combination of two type emission mechanisms: (1) free-excitonic emission, and (2) emission due to the transition between radiative defects levels related to the interface traps existing at the grain boundaries and the valence band [2, 21]. These mechanisms were responsible for the occurrence of relatively larger emission linewidth. This emission peak was also slightly red-shifted from the undoped ZnO NBE emission peak (~375 nm) [1] due to emissive transition from an impurity donor level to valence band [21]. Figure 2(b) showed that NBE emission peak from CdZnO film was centered at ~440 nm. This blue PL emission peak was critical in order to realize blue electroluminescence from the p-i-n device. PL spectra of CdZnO film also produced broad emissions in visible spectra indicating the presence of defect‐related levels in the film. The broad emissions in the long wavelength region corresponded to transitions from oxygen vacancies (O_v) and/or zinc interstitials (Zn_i) to valance band [1, 22]. High-temperature in situ annealing process in vacuum condition might be responsible for such defect formation in CdZnO film [22]. PL spectra from SZO film also demonstrated a strong NBE emission peak at ~384 nm at room temperature. Additionally, SZO film represented a broad deep-level emission peak at ~556 nm that could be attributed to oxygen interstitial-related defects, which was consistent with the fact that p-type SZO was annealed in oxygen ambient [23]. Besides these two prominent peaks at 384 and 556 nm, one shoulder peak at ~405 nm was present in PL spectrum of SZO, which could be attributed to Zn vacancy related defects [14]. Our research group had already demonstrated previously that a single Sb atom in ZnO could substitute a Zn atom from its original lattice point and simultaneously associated the same with two Zn vacancies to form a Sb_Zn-2V_Zn complex, which was, in fact, a shallow acceptor in ZnO and responsible for p-type conduction in SZO films [14]. The presence of such Zn vacancy in SZO film which was responsible for the formation of Sb_Zn-2V_Zn complex was justified by the occurrence of shoulder peak at ~405 nm in PL spectrum.

 

Fig. 2 Room-temperature PL spectra of separately grown (a) GZO, (b) CdZnO, and (c) SZO thin films grown on Si substrate.

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5. Electrical properties

I-V curve of p-i-n device was shown in linear and semi-logarithmic scales in Fig. 3(a-b). The device showed typical diode-like rectifying I-V characteristics. The turn-on voltage of this device was ~5 V, as calculated by using linear extrapolation method from Fig. 3(a). This large turn-on voltage of this device involved the effects from the voltage drop on the contacts, across SZO, CdZnO and GZO layers. Indium electrodes, annealed for 20 minutes in ultra-high vacuum at 400 °C, were used to form ohmic contacts on p-type SZO layer and n-Si substrate as shown I-V curves in the inset of Fig. 3 (a). The rectification ratio of the forward to reverse bias current was approximately 60000:1 at a bias voltage of ± 6 V, as calculated from semi-logarithmic I-V curve as shown in Fig. 2(b). The high ratio of forward to reverse bias current indicated that this device had low defect density. The electrical properties of SZO, CdZnO and GZO films were determined by Hall effect measurement. SZO film was observed to have a hole concentration of 1.36 × 10¹⁷ cm⁻³ and mobility of 6.75 cm²V⁻¹s⁻¹ whereas GZO film had an electron concentration of 2.88 × 10¹⁹ cm⁻³ and mobility of 14.16 cm²V⁻¹s⁻¹ at room temperature. CdZnO film had an electron concentration of 2.14 × 10¹⁶ cm⁻³ with a carrier mobility of 8.38 cm²V⁻¹s⁻¹ at room temperature. The existence of native donor defects in CdZnO was responsible for the measured electron concentration [1, 22]. From the Hall measurement results, it was evident that there would be large built-in potential at SZO/CdZnO interface [7, 15].

 

Fig. 3 I-V characteristic of p-i-n diode in (a) linear and (b) semi-logarithmic scale. The inset curve in linear I-V curve showed the ohmic contact formation with Indium contacts on SZO film and Si substrate. Energy band diagram of the heterojunction diode at (c) zero bias (d) forward bias.

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The energy band diagram of SZO/CdZnO/GZO/Si at zero bias was presented in Fig. 3(c). The band gap energy of SZO ($E_{g, SZO}$), CdZnO ($E_{g, CdZnO}$) and GZO ($E_{g, GZO}$) layers were 3.23, 2.82, and 3.24 eV, respectively at room temperature as calculated from their respective NBE peaks in PL spectra. The electron affinity of ZnO was 4.3 eV and ideally remained unaltered with doping and therefore the value was considered to be a material constant [8]. Thus, SZO (${\chi }_{SZO }$) and GZO (${\chi }_{GZO }$) were similarly assumed to have an electron affinity of 4.3 eV. The electron affinity of CdZnO layer was assumed to be linearly dependent on the Cd content and lies between 4.5 eV for CdO to 4.3 eV for ZnO, generating 4.324 eV for Cd_11.8Zn_88.2O film (${\chi }_{CdZnO}$) [8, 20, 24]. The electron affinity of Si (${\chi }_{Si }$) was 4.05 eV [25]. The band offset ΔE_C1 for electrons at SZO/CdZnO junction was:

The band offset ΔE_C3 for electrons at GZO/Si was:

6. Electroluminescence

Room-temperature (RT) EL spectra of the device under different injection currents are shown in Fig. 4(a). The RT EL spectra showed weak and broad EL emission centered around 446 nm under 50 mA injection current. At increased injection current of 70 mA, the EL emission was dominated by the NBE emission from CdZnO active layer centered around 449.95 nm, which corresponded to RT PL emission peak at ~440 nm from single CdZnO layer as shown in Fig. 2(b). The slight difference in peak positions of EL and PL spectra may be because of the fact that PL measurement was performed on single CdZnO film on Si substrate while EL emission was observed from the entire p-i-n device. There were also weak emissions at ~387.54 nm when injection current goes beyond 70 mA; it might be originated from some carrier recombination mechanism occurring in n-type GZO or p-type SZO layers. The relative intensity of NBE emission increased with the increase in the magnitude of injection current from 50 to 100 mA. The camera image of blue emission at injection current of 100 mA was shown as the inset of Fig. 4(a). The NBE emission peak red-shifted from 446 to 451.34 nm with the corresponding rise in injection current from 50 to 100 mA. The red-shifting of EL emission peak were due to heat induced band-gap shrinkage, which resulted from the enhanced thermal density at increased injection current during the operation of LED [26]. Comparing RT PL spectra of SZO, CdZnO and GZO layers and RT EL spectra of device, it could be inferred that the observed EL emission mainly originated from the radiative recombination in CdZnO active layer.

 

Fig. 4 (a) Room-temperature EL spectra at different injection current from 50 to 100 mA, (b) temperature dependent EL spectra of heterojunction diode from 80 to 300 K under injection current of 70 mA, (c) de-convoluted EL spectra of 80 K, and (c) Variation of NBE peak positions, as hollow red circles, against different temperatures and Varshni fitting is expressed as a solid blue line.

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Temperature dependent EL measurements from 80 to 300 K were carried out on the device at an injection current of 70 mA, to further confirm the NBE emission from CdZnO active layer as shown in Fig. 4(b). For EL measured at 80 K, the spectrum was dominated by a peak at 430.56 nm (2.88 eV). The EL emission peak wavelength was increased from 430.56 nm (2.88 eV) at 80 K to 449.95 nm (2.75 eV) at 300 K. The temperature dependence of the exciton energy in direct band gap material followed Varshni equation, E(T) = E(0)-αT²/(T + β) [27] where α and β were fitting parameters, which were calculated to be α = 0.00075 eV/K and β = 123 K. The values of parameters were close to reported results for ZnO-based material systems [3, 27]. The EL emission peak values agreed well with the fitting curve of Varshni equation as illustrated in Fig. 4(c). The EL spectrum at 80 K had been deconvoluted to further understand the origin of sub-peaks in the spectrum as shown in Fig. 4(d). Deconvoluted spectra generated three distinct, nearly Gaussian curves centered at 381.54 nm (green curve), 430.56 nm (red curve), and 471.15 nm (red curve). The low wavelength emission spectrum at ~381.54 nm was originated from carrier recombination mechanism occurring in n-type GZO or p-type SZO layers. The medium wavelength emission spectrum centered at ~430.56 nm was associated with the NBE emission from CdZnO active layer. The long wavelength emission spectrum centered at 471.15 nm corresponded to transitions from oxygen vacancies (O_v) to valance band levels [28]. These observations suggested that the EL emissions was caused by the NBE emission of CdZnO active layer and the red shifts of EL peak position with the increase in temperature, was primarily due to temperature induced band gap shrinkage.

7. Conclusion

In summary, ZnO based heterojunction LED with CdZnO active layer was fabricated on n-Si(100) substrate using DIBSD system. CdZnO layer was used between p-type SZO layer and n-type GZO layer to achieve smaller band gap necessitated for blue emission. Heterostructure LED showed good blue emissions at different injection currents and ambient temperatures. Temperature dependent EL measurement and RT PL spectra of different layers used in structure, confirmed that the blue emissions were primarily originated from the radiative recombinations taking place in CdZnO active layer. This study confirmed the demonstration of blue emission LED with SZO as p-type layer.