Improved electroluminescence from ZnO light-emitting diodes by p-type MgZnO electron blocking layer

We report on the effect of a p-type MgZnO electron blocking layer (EBL) on the electroluminescence from n-type ZnO/undoped ZnO/ptype ZnO light-emitting diodes (LEDs). The p-type Mg0.1Zn0.9O EBL was introduced between the undoped and p-type ZnO layers. The p-type Mg0.1Zn0.9O EBL increased the ultraviolet emission by 140% at 60 mA and decreased the broad deep-level emission from ZnO LEDs. The calculated band structures and carrier distribution in ZnO LEDs show that p-type Mg0.1Zn0.9O EBL effectively suppresses the electron overflow from undoped ZnO to p-type ZnO and increases the hole concentration in the undoped ZnO layer. ©2013 Optical Society of America OCIS codes: (160.6000) Semiconductor materials; (230.0230) Optical devices; (230.3670) Light-emitting diodes. References and links 1. K. Watanabe, T. Taniguchi, and H. Kanda, “Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal,” Nat. Mater. 3(6), 404–409 (2004). 2. Y. Narukawa, I. 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Introduction
Ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes are attractive for their potential use in solid-state white lighting, high-density information storage, secure communications, water and air sterilization, and chemical and biological detection systems [1][2][3][4][5][6].Recently, intensive research efforts have focused on finding materials to realize more efficient UV LEDs.Of the available wide-bandgap semiconductors, ZnO is a promising candidate for creating efficient UV-light emitters due to its large direct bandgap of 3.37 eV, low-power threshold for optical pumping, and large exciton binding energy of 60 meV [7][8][9].These attractive properties have drawn much attention to ZnO-based homojunction LEDs [10][11][12][13].However, the higher mobility of electrons than holes in ZnO LEDs causes electron overflow and increases recombination processes in the p-type ZnO region [11,14].To overcome this problem, undoped MgZnO was introduced as an energy barrier layer to confine the recombination processes to the active layer [14].The ZnO LEDs with an undoped MgZnO energy barrier layer showed enhanced UV emission and suppressed deep-level emission.However, the undoped MgZnO layer also decreases the hole transport from p-type ZnO to the ZnO active layer because its large bandgap provides an energy barrier to hole injection.One way to improve hole injection efficiency and the recombination rate in the active layer of LEDs is to use a p-type electron blocking layer (EBL) [15,16].The hole concentration in the undoped ZnO active layer can be increased because of the lower Fermi energy level in p-type MgZnO compared with undoped or n-type MgZnO.In this study, we have investigated the effect of a p-type MgZnO EBL on the performance of n-type ZnO/undoped ZnO/p-type ZnO LEDs.The p-type Mg 0.1 Zn 0.9 O EBL was introduced between the undoped ZnO and p-type ZnO layer to enhance UV emission by increasing the electron and hole concentrations in the undoped ZnO active layer.

Experiments
The ZnO LEDs were grown on the oxygen-polar face of n-type Ga-doped ZnO (ZnO:Ga) substrates by metalorganic chemical vapor deposition (MOCVD).Diethylzinc (DEZn), trimethylantimony (TMSb), bis(cyclopentadienyl)magnesium (Cp 2 Mg), and O 2 gas (99.999% purity) were used as sources of Zn, Sb, Mg, and O, respectively.The metalorganic sources and O 2 gas were introduced into the reactor separately, and the source gases were mixed 1 cm ahead of the substrate to minimize gas phase parasitic reactions [17].Figure 1 shows the structure of the ZnO LED with a p-type Mg 0.1 Zn 0.9 O EBL.A 100 nmthick undoped ZnO layer was grown on the n-type ZnO substrate at 650 °C.Then a 50 nmthick p-type Mg 0.1 Zn 0.9 O EBL and a 600 nm-thick p-type ZnO layer were grown at 600 °C.As-grown Sb-doped layers exhibited semi-insulating electrical properties and these layers were converted to p-type ZnO by a rapid thermal annealing process at 500 °C under N 2 ambient conditions for 1 min.Table 1 shows the electrical properties of n-type ZnO:Ga substrate, undoped ZnO, p-type Mg 0.1 Zn 0.9 O, and p-type ZnO layers.Ti (30 nm)/Au (100 nm) and Ni (30 nm)/Au (100 nm) were deposited on the n-type ZnO substrate and p-type ZnO as n-type [18] and p-type metal electrodes [10], as shown in Fig. 1.The current-voltage (I-V) characteristics were measured at room temperature using an HP 4155 parameter analyzer.Electroluminescence (EL) spectra and integrated optical output power were measured using a UV-visible spectrometer (USB4000-UV-VIS Fiber Optic Spectrometer, Ocean Optics Inc.).The total output power of ZnO LEDs was measured by using an integrating sphere system.Figure 4(a) shows the total output power of ZnO LEDs with and without the p-type Mg 0.1 Zn 0.9 O EBL as a function of injection current.The total optical output power of ZnO LED without the p-type Mg 0.1 Zn 0.9 O EBL was 1.61 μW at 50 mA and it was increased to 1.94 μW with the addition of the p-type Mg 0.1 Zn 0.9 O EBL.The 20.2% increase in the total optical output power is attributed to the improved carrier recombination process in the undoped ZnO layer.Furthermore, the ZnO LED with the p-type Mg 0.1 Zn 0.9 O EBL shows a peak output power at 58 mA which is 8 mA higher than that of the ZnO LED without the p-type Mg 0.1 Zn 0.9 O EBL.This indicates that the p-type Mg 0.1 Zn 0.9 O EBL effectively confines the carriers in the undoped ZnO layer of the ZnO LED at high injection currents, compared with ZnO LED without EBL.Figure 4(b) shows the integrated UV emission intensity of ZnO LEDs with and without a p-type Mg 0.1 Zn 0.9 O EBL as a function of injection current.The UV emission intensity of ZnO LED with p-type Mg 0.1 Zn 0.9 O EBL is increased by 140% at 60 mA compared with that of the ZnO LED without p-type Mg 0.1 Zn 0.9 O EBL because of the improved carrier recombination process in the undoped ZnO layer.Moreover, the ZnO LED with the p-type Mg 0.1 Zn 0.9 O EBL shows a peak UV emission at 60 mA, while the ZnO LED without the p-type Mg 0.1 Zn 0.9 O EBL shows a peak UV emission at 50 mA.The large current shift of 10 mA for the peak intensity of UV emission also indicates that the p-type Mg 0.1 Zn 0.9 O EBL effectively confines the electron and holes in the undoped ZnO layer of the ZnO LED at high injection currents.The improved characteristics were reproducible for all devices with the p-type Mg 0.1 Zn 0.9 O EBL.To further understand the carrier recombination process in ZnO LEDs, we calculated the carrier distribution and energy band structures of ZnO LEDs using the LED simulator, SiLENSe 5.2. 1 [19, 20].In the simulation of energy band diagrams and carrier distributions of ZnO LEDs, we used the ZnO LED structure shown in Fig. 1 and electrical properties of films listed in Table 1.Figures 5(a

Summary
In summary, we have investigated the effect of a p-type MgZnO EBL on the properties of ZnO LEDs.The intensity of UV emission was enhanced and the deep-level emission was suppressed by the p-type Mg 0.1 Zn 0.9 O EBL.The intensity of UV emission of ZnO LEDs was increased by 140% at 60 mA by using p-type Mg 0.1 Zn 0.9 O EBL.The energy band structures and distribution of carrier concentrations show that p-type Mg 0.1 Zn 0.9 O EBL efficiently blocks electron overflow and increases the hole concentration in the active layer, increasing the UV output power of ZnO LEDs.

Fig. 1 .
Fig. 1.Structure of the ZnO LED with a p-type Mg 0.1 Zn 0.9 O EBL.

Fig. 2 .
Fig. 2. (a) I-V characteristics of ZnO LEDs with and without p-type Mg 0.1 Zn 0.9 O EBL.(b) EL spectra of the ZnO LEDs with and without the p-type Mg 0.1 Zn 0.9 O EBL, operating at a forward current of 50 mA.

Figure 2 (
Figure 2(a) shows I-V curves of ZnO LEDs with and without the p-type Mg 0.1 Zn 0.9 O EBL.The I-V characteristics show good rectification for both LEDs, but the forward voltage at 20 mA increased from 3.6 V to 4.5 V by the p-type Mg 0.1 Zn 0.9 O EBL, mainly due to the high resistivity and large bandgap of p-type Mg 0.1 Zn 0.9 O EBL. Figure2(b)shows the EL spectra of LEDs with and without the p-type Mg 0.1 Zn 0.9 O EBL at an injection current of 50 mA.The EL spectra show a sharp peak of UV emission at 382 nm and broad deep-level emissions around 720 nm.For ZnO LEDs, the emission at 382 nm is attributed to the near-band-edge emission of ZnO[10,11].The broad deep-level emission around 720 nm is attributed to Sb-related defects in the p-type ZnO layer[12].Figures3(a) and 3(b) show the EL spectrum of the ZnO LED without a p-type Mg 0.1 Zn 0.9 O EBL at an injection current of 50 mA and the photoluminescence (PL) spectra of the undoped ZnO and p-type ZnO layer, respectively.A comparison of the EL spectrum in Fig. 3(a) with the PL spectrum of a p-type ZnO layer in Fig. 3(b) suggests that the recombination of electrons and holes occurs mainly in the p-type ZnO layer.The carrier diffusion lengths of electrons in the p-type ZnO and holes in the n-type ZnO were estimated to be 199.5 and 18.5 nm, respectively.The large difference in electron and hole diffusion lengths also indicates that carrier recombination occurs mostly in the ptype ZnO layer which has poor crystal quality and has many defects.The problem of carrier recombination in p-type ZnO layer could be effectively solved by introducing a p-type Mg 0.1 Zn 0.9 O EBL between the undoped ZnO and the p-type ZnO layer.The p-type Mg 0.1 Zn 0.9 O EBL enhanced the UV emission intensity and decreased the deep-level emission by increasing the recombination rate of carriers in the undoped ZnO layer, as shown in Fig. 2(b).

Fig. 3 .
Fig. 3. (a) EL spectrum of ZnO LED at 50 mA and (b) PL spectra of undoped ZnO and p-type ZnO layers.

Fig. 4 .
Fig. 4. (a) Total output power of ZnO LEDs with and without the p-type Mg 0.1 Zn 0.9 O EBL as a function of injection current.(b) Integrated UV emission intensity of ZnO LEDs with and without the p-type Mg 0.1 Zn 0.9 O EBL as a function of injection current.

Fig. 5 .
Fig. 5. Calculated carrier concentrations of ZnO LEDs (a) without and (b) with the p-type Mg 0.1 Zn 0.9 O EBL. (c) Calculated energy band diagram of ZnO LED with the p-type Mg 0.1 Zn 0.9 O EBL.
) and 5(b) show the distribution of electron and hole concentrations in the n-type, undoped, and p-type regions of ZnO LEDs without and with the p-type Mg 0.1 Zn 0.9 O EBL at an input current of 20 mA, respectively.As shown in Fig. 5(b), the electron concentration is increased from 7.91 × 10 16 cm −3 to 5.98 × 10 17 cm −3 in the undoped ZnO region because of the blocking of electron overflow by the p-type Mg 0.1 Zn 0.9 O EBL.Therefore, the electron concentration in the p-type ZnO region is subsequently decreased from 7.82 × 10 15 cm −3 to 8.05 × 10 14 cm −3 .It is noteworthy that the p-type Mg 0.1 Zn 0.9 O EBL also increases the hole concentration from 7.84 × 10 16 cm −3 to 5.98 × 10 17 cm −3 in the undoped ZnO region, as shown in Fig. 5(b).

Figure 5 (
c) shows the energy band diagram of a ZnO LED with the p-type Mg 0.1 Zn 0.9 O EBL at 20 mA.The p-type Mg 0.1 Zn 0.9 O EBL shows a potential notch and spike in the valence band (circles in Fig. 5(c)) at the interfaces of the undoped ZnO/EBL/p-type ZnO due to the polarization-electric field in the layers [21, 22].The holes can be accumulated in the notch, leading to a high hole concentration in the undoped ZnO layer of the ZnO LED with the p-type Mg 0.1 Zn 0.9 O EBL, as shown in Fig. 5(b).Therefore, the improved EL property and optical output power of ZnO LEDs with a p-type #187297 -$15.00USD Received 18 Mar 2013; revised 30 Apr 2013; accepted 30 Apr 2013; published 6 May 2013 (C) 2013 OSA Mg 0.1 Zn 0.9 O EBL are attributed to the increased electron and hole concentrations in the undoped ZnO active layer.