Room Temperature Electroluminescence from n-ZnO ... - Springer Link

Electron. Mater. Lett., Vol. 10, No. 3 (2014), pp. 661-664 DOI: 10.1007/s13391-013-2206-3

Room Temperature Electroluminescence from n-ZnO:Ga/i-ZnO/p-GaN:Mg Heterojunction Device Grown by PLD Lichun Zhang,1,* Qingshan Li,1,2 Feifei Wang,1 Chong Qu,1 and Fengzhou Zhao1 1

School of Physics and Optoelectronic Engineering, Ludong University, Yantai, 264025, China 2 College of Physics and Engineering, Qufu Normal University, Qufu, 273165, China (received date: 31 October 2012 / accepted date: 13 September 2013 / published date: 10 May 2014)

The n-ZnO:Ga/p-GaN:Mg and n-ZnO:Ga/i-ZnO/p-GaN:Mg heterojunction light emitting diodes (LEDs) were fabricated by the pulsed laser deposition (PLD) technique. The blue electroluminescence (EL) of the n-ZnO:Ga/ p-GaN:Mg heterojunction LEDs is emitted mainly from the p-GaN layer instead of the n-ZnO:Ga layer, for the reason that the electron injection from n-ZnO:Ga prevailed over the hole injection from p-GaN:Mg due to the higher carrier concentration and carrier mobility in n-ZnO:Ga. On the other hand, the n-ZnO:Ga/i-ZnO/pGaN:Mg heterojunction LEDs exhibited dominant ultraviolet-blue emission. The reason for this difference is attributed to the inserted undoped i-ZnO layer between n-ZnO:Ga and p-GaN:Mg, in which the holes from pGaN:Mg and the electrons from n-ZnO:Ga are recombined. Keywords: light emitting diodes, pulsed laser deposition, electroluminescence, n-ZnO:Ga, p-GaN:Mg

1. INTRODUCTION Recently, ZnO has become considered as a promising material in the use of short-wavelength optoelectronic devices, since it has a direct bandgap of 3.37 eV and a large exciton binding energy (60 meV).[1-3] Owing to its relatively lower production cost and superior optical properties, ZnO seems to be the alternative to GaN in optoelectronic devices. Despite the great potential of ZnO in optoelectronic applications, there are few ZnO based devices, since it is difficult to obtain reproducible and high quality p-type ZnO,[4,5] hence, n-type ZnO films are grown on other p-type materials is an alternative for ZnO based devices.[6-8] Among the various ptype materials that have been used in ZnO-based heterojunction light emitting diodes (LEDs), GaN is regarded as the most promising candidate, due to having the same lattice structure (wurtzite) and relatively small lattice mismatch (1.8%) as ZnO. In previous work, different device architectures of ZnO/GaN have been fabricated using a variety of growth techniques, and various emission spectra from the ultraviolet to the visible spectra have been reported.[9-14] With the improved current confinement, the ZnO/GaN heterojunction LEDs could give rise to higher recombination and device efficiency. However, due to the higher carrier concentration and mobility in n-ZnO, the electron injection from n-ZnO would prevail over the hole injection from pGaN, and the electroluminescence (EL) of the n-ZnO/p-GaN *Corresponding author: [email protected] ©KIM and Springer

heterojunction LEDs is mainly emitted from p-GaN instead of n-ZnO. The blue-violet EL with peak wavelength of 430 nm from the n-ZnO/p-GaN heterojunction LEDs has been reported.[15] In this paper, the n-ZnO:Ga/p-GaN:Mg and the n-ZnO:Ga/ i-ZnO/p-GaN:Mg heterojunction LEDs were fabricated with the pulsed laser deposition (PLD) technique. It demonstrated that the holes from the p-GaN:Mg and the electrons from the n-ZnO:Ga will inject into the i-ZnO layer and recombine there. With an i-ZnO interlayer, the EL in the ultraviolet-blue region was observed under forward bias.

2. EXPERIMENTAL PROCEDURE The Mg-doped p-GaN epitaxial films, with a thickness of 1 µm, was deposited on sapphire (0001) wafers using metalorganic chemical vapor deposition (MOCVD) (from the Inlead Technology Corporation, Taiwan), which served as the substrate and hole injection layer. For the ZnO:Ga/ZnO/ GaN:Mg LED, a 20 nm thick undoped ZnO layer(with pure ZnO ceramic (99.999%) target) and a 200 nm thick ZnO:Ga layer (with Ga2O3-doped (5 wt. %) ZnO target) were grown on the p-GaN substrate with the PLD technique in the oxygen atmosphere (10 Pa). A KrF excimer laser (COMPexPro 201, Coherent Inc.) was employed, operating at a wavelength of 248 nm. For comparison, a ZnO:Ga/GaN heterojunction LED without an interlayer was fabricated under the same growth condition as the former. Pt (50 nm)/Ti (30 nm) on ZnO:Ga films and Pt (50 nm)/Ni (30 nm) on GaN substrates, that were deposited with PLD respectively, served as the

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electrodes. The crystalline structures of the films were characterized by x-ray diffraction (XRD) (Rigaku D/MAX2500V diffractometer with Cu Kα radiation) and the cross-section was investigated using a Hitachi S-4800 Field-emission scanning electronic microscope (FE-SEM). The I-V characteristics were measured using a Keithley 2611A source meter. The photoluminescence (PL) measurements were carried out at room temperature and were excited by a He-Cd laser (CVI Melles Griot) operating at 325 nm. The emission spectra of PL and EL were collected with an ARS SP2557 monochromator.

leading to high quality oriented growth of the n-ZnO:Ga. The XRD patterns of the n-ZnO:Ga, i-ZnO and p-GaN films deposited on the sapphire substrate exhibited a highly coriented wurtzite structure, as shown in Fig. 3. Because of Ga doping, the full width at half maximum (FWHM) of the

3. RESULTS AND DISCUSSION The electrical properties of ZnO and GaN films that were grown directly on sapphire substrates were examined using Hall measurements, as shown in Table 1. The undoped ZnO film exhibited intrinsic semiconducting properties, while the ZnO:Ga film exhibited n-type conductivity. Figure 1 shows the schematic structures of the n-ZnO:Ga/p-GaN:Mg and nZnO:Ga/ZnO/p-GaN:Mg heterojunction LEDs. The cross section SEM images of the n-ZnO:Ga/p-GaN and n-ZnO:Ga/i-ZnO/p-GaN heterojunction LEDs are shown in Fig. 2(a) and (b), respectively. A three-layer structure can be clearly observed in the n-ZnO:Ga/i-ZnO/pGaN heterojunction LED. And the n-ZnO:Ga film grown on the i-ZnO layer has an improved perpendicular orientation. The difference can be attributed to the insertion of i-ZnO films. The undoped ZnO thin film acts as a nucleation sites for n-ZnO:Ga growth, and provides a much smaller lattice mismatch between the p-GaN layer and the n-ZnO:Ga,

Fig. 2. The cross section SEM images of the n-ZnO:Ga/p-GaN (a) and n-ZnO:Ga/i-ZnO/p-GaN (b) LEDs.

Table 1. The electrical properties of GaN:Mg and ZnO films. Conduction type

Concentration (cm−3)

Mobility (cm2/Vs)

GaN:Mg

p

1.0 × 1018

8.5

undoped ZnO

i

2.1 × 1017

3.3

n

20

Films

ZnO:Ga

8.6 × 10

13.8

Fig. 3. XRD patterns of n-ZnO:Ga/i-ZnO/p-GaN heterojunction, nZnO:Ga, i-ZnO and p-GaN film.

Fig. 1. Schematic diagrams of the n-ZnO:Ga/p-GaN (a) and n-ZnO:Ga/i-ZnO/p-GaN (b) heterojunction LEDs.

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Fig. 4. Room-temperature PL spectra of the n-ZnO:Ga, i-ZnO, and p-GaN.

n-ZnO:Ga (002) peak was wider than that of i-ZnO. For the n-ZnO:Ga/i-ZnO/p-GaN heterojunctions, the two diffraction peaks at ~33.81° and ~33.94° correspond to ZnO (002) and GaN (002), respectively. Furthermore, the FWHM of the XRD peak for the ZnO:Ga films in the p-i-n heterojunction was smaller than for the ZnO:Ga films on the sapphire substrate. The larger lattice mismatch (~18%) between the ZnO:Ga films and the sapphire (0001) substrate led to poor crystalline quality of ZnO:Ga films and large FWHW of the XRD peaks. However, the ZnO:Ga films in the p-i-n junction grew on the surface of the i-ZnO layer. The undoped ZnO thin film acts as a template for n-ZnO:Ga growth, and the very small lattice mismatch between n-ZnO:Ga and i-ZnO produces good epitaxial ZnO:Ga films on i-ZnO and a relatively small FWHW of the XRD peaks. Figure 4 presents the PL spectra of the n-ZnO:Ga, i-ZnO films and the p-GaN substrate measured at room temperature. The PL of p-GaN films exhibits the broad blue emission peak centered at 440 nm, which is generally attributed to the transition from the conduction band or shallow donors into deep Mg acceptor levels.[15-18] The PL spectra of the nZnO:Ga and i-ZnO films consists of an intense near-bandedge (NBE) ultraviolet (UV) emission and a much lower deep-level emission band. Moreover, the deep-level emission of n-ZnO:Ga was much stronger than for i-ZnO, which indicates a relatively higher concentration of native defect such as oxygen vacancies or zinc interstitials.[19,20] Figure 5 shows the I-V characteristics of n-ZnO:Ga/p-GaN and the n-ZnO:Ga/i-ZnO/p-GaN heterojunction LEDs at room temperature, where both devices exhibit rectifying, diode-like behavior. Obviously, the undoped i-ZnO layer in the n-ZnO:Ga/i-ZnO/p-GaN structure leads to a larger turnon voltage and a smaller reverse bias leakage current than the n-ZnO:Ga/p-GaN LED. As shown in the inset of Fig. 5, good ohmic contacts are formed for Pt/Ni/ n-ZnO:Ga and Pt/ Ti/p-GaN, and consequently the rectifying behavior of the

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Fig. 5. I-V characteristics of the n-ZnO:Ga/i-ZnO/p-GaN and nZnO:Ga/p-GaN heterojunction LEDs. The inset shows the I-V curves of Ni/Pt and Ti/Pt Ohmic contacts to p-GaN and n-ZnO:Ga, respectively.

Fig. 6. Room-temperature EL spectra of the n-ZnO:Ga/p-GaN (a) and n-ZnO:Ga/i-ZnO/p-GaN (b) heterojunction LEDs at various injection currents.

LEDs originates from the n-ZnO:Ga/p-GaN and n-ZnO:Ga/ i-ZnO/p-GaN heterojunctions.


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Figure 6 shows the EL spectra of the n-ZnO:Ga/p-GaN and n-ZnO:Ga/i-ZnO/p-GaN heterojunction LEDs at various injection currents. All of the EL measurements were carried out under forward bias at room temperature. The n-ZnO:Ga/ p-GaN heterojunction LED exhibits a blue-violet emission centered at 425 nm, as shown in Fig. 6(a). Compared with the PL in Fig. 4, this EL emission results from deep-level carrier recombination in the p-GaN layer. Similar results and the origin of the EL have been previously reported.[15,16] For the n-ZnO:Ga/p-GaN heterojunction, due to the lower carrier concentration and the mobility of p-GaN than n-ZnO:Ga, along with a similar conduction band and valence band offsets, the radiative recombination mainly occurs in the pGaN instead of the n-ZnO:Ga. The blue shift of GaN emission may be caused by the recombination of the increasing kinetic energies of electrons and holes as the injection current increases. For the n-ZnO:Ga/i-ZnO/p-GaN heterojunction LED, a wide emission band from 350 nm to 500 nm was observed, as shown in Fig. 6(b). In order to understand the origin of the EL emission better, the Gaussian fitting of the EL spectra at injection current of 15 mA is presented. As shown in the inset of Fig. 6(b), the fitted EL spectra exhibit two independent emission band centered at 386 nm and 424 nm. The broad emission at 424 nm is attributable to the light emission from the Mg acceptor levels in p-GaN. And the strong UV emission at ~386 nm is similar to the PL result of the intrinsic ZnO layer. Though the EL intensity of the n-ZnO:Ga/i-ZnO/pGaN heterojunction LED is a little weaker than for the nZnO:Ga/p-GaN heterojunction LED, the intensity of the ultraviolet emission has been improved with the increase of the injection current. Among the three layers in the nZnO:Ga/i-ZnO/p-GaN heterojunction LED, i-ZnO has the lowest carrier concentration and mobility, and the carriers including holes from p-GaN and electrons from n-ZnO:Ga, can be injected into the i-ZnO layer, where radiative recombination occurs. The weaker blue-violet emission centered at 424 nm should be caused by the injection of the residual electrons from the i-ZnO layer into the p-GaN layer and recombination with the holes in the p-GaN region.[16]

4. CONCLUSIONS In summary, the fabrication and the properties of the nZnO:Ga/p-GaN and n-ZnO:Ga/i-ZnO/p-GaN heterojunction LEDs with the PLD technique was presented. The I-V curves of the two heterojunction LEDs exhibited diode-like rectifying characteristics. The inserted i-ZnO layer between n-ZnO:Ga and p-GaN resulted in a larger turn-on voltage. Due to the iZnO layer having the lowest electron concentration, the holes of p-GaN and the electrons from n-ZnO:Ga can be injected into the i-ZnO layer and produces ultraviolet emission, with a decrease in electron injection from n-ZnO:Ga into p-GaN.

ACKNOWLEDGEMENTS The authors wish to acknowledge the financial support of the National Natural Science Foundation of China (No. 11144010), Research Award Fund for Outstanding Middleaged Young Scientist of Shandong Province (Grant No. BS2011ZZ004).

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