Enhanced ultraviolet emission of MgZnO / ZnO multiple quantum wells light-emitting diode by p-type MgZnO electron blocking layer

We report on the effect of a p-type MgZnO electron blocking layer (EBL) on the optical and electrical properties of MgZnO/ZnO multiple quantum wells (MQWs) light-emitting diodes (LEDs). The p-type Mg0.15Zn0.85O EBL was introduced between the MQWs and p-type Mg0.1Zn0.9O layers. The p-type Mg0.15Zn0.85O EBL increased the ultraviolet emission by 111.2% 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 Mg0.15Zn0.85O EBL effectively suppresses the electron overflow from MQWs to p-type Mg0.1Zn0.9O and increases the hole concentration in the MQWs. ©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. 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Introduction
Ultraviolet (UV) light-emitting diodes (LEDs) and laser diodes are attractive for use in solidstate 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.Among the available wide-bandgap semiconductors, ZnO is a promising candidate for creating efficient UV-light emitting devices 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 progress on ZnO LEDs has slowed dramatically because of the low emission efficiency and inadequate design of carrier flow and injection efficiency.Recently, we reported the effect of a p-type electron blocking layer (EBL) on the optical and electrical properties of p-i-n ZnO LEDs [14].The p-type EBL effectively increased carrier concentrations in the active layer and improved the emission intensity of ZnO LEDs [14][15][16].The electron energy barrier and high hole accumulation region generated between the p-type MgZnO EBL and active layer improved electron and hole concentrations in the active layer [14].To improve the efficiency of the ZnO LEDs, we have investigated the growth and optical properties of MgZnO/ZnO multiple quantum wells (MQWs).The MgZnO/ZnO MQWs were successfully grown by metalorganic chemical vapor deposition (MOCVD) and showed strong UV emissions at 370 nm [17].In this study, we have investigated the effect of a p-type MgZnO EBL on the performance of ZnO MQWs LEDs.The p-type Mg 0.15 Zn 0.85 O EBL introduced between the Mg 0.1 Zn 0.9 O/ZnO MQWs and p-type Mg 0.1 Zn 0.9 O ZnO layer enhanced UV emission by increasing the electron and hole concentrations in the MgZnO/ZnO MQWs.

Experiments
The ZnO LEDs were grown on the oxygen-polar face of n-type Ga-doped ZnO (ZnO:Ga) substrates by 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 mixed 1 cm away from the substrate to minimize gas phase parasitic reactions [18].Figure 1 shows the structure of the ZnO LED (300 × 300 μm 2 ) with a p-type Mg 0.15 Zn 0.85 O EBL.A 400 nm-thick n-type Mg 0.1 Zn 0.9 O layer was grown on the n-type ZnO substrate at 650 °C.The Mg 0.1 Zn 0.9 O/ZnO MQWs with five pairs of undoped ZnO wells (2 nm) and Mg 0.1 Zn 0.9 O barriers (5 nm) were grown on the n-type Mg 0.1 Zn 0.9 O layer at 670 °C [17].Then a 50 nm-thick p-type Mg 0.15 Zn 0.85 O EBL and a 600 nm-thick p-type Mg 0.1 Zn 0.9 O ZnO layer were grown at 600 °C.To estimate the Mg compositions of MgZnO in Table 1, the undoped Mg 0.1 Zn 0.9 O and Mg 0.15 Zn 0.85 ZnO films were grown on undoped ZnO templates and the Mg composition was determined by PL measurement at 10 K.The PL peak position of Mg 0.1 Zn 0.9 O and Mg 0.15 Zn 0.85 ZnO was 349.2 nm and 340.6 nm, respectively and the Mg compositions were estimated based on the PL data given in [19].As-grown Sb-doped layers exhibited semiinsulating 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, n-type Mg 0.1 Zn 0.9 O, p-type Mg 0.15 Zn 0.85 O, and p-type Mg 0.1 Zn 0.9 O layers.In order to measure the conductivity of n-type and p-type MgZnO layers, Hall effect measurement was conducted in a van der Pauw configuration with a film thickness of ~1 μm grown on the undoped ZnO template (electrically semi-insulator) which was grown on c-sapphire substrate.The hole concentration of p-type MgZnO layer decreased with increasing the Mg composition due to the increase of acceptor activation energy in p-type MgZnO with higher Mg composition [20].Ti (30 nm)/Au (100 nm) and Ni (30 nm)/Au (00 nm) were deposited on the n-type ZnO substrate and p-type Mg 0.1 Zn 0.9 O ZnO as n-type [21] 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.In addition, the integrated output power and integrated UV emission intensity of the LEDs with EBL drop more pronouncedly after 60mA than that without EBL.This behavior in optical output power of LEDs with EBL is closely related to the joule heating effect due to the highly resistive EBL.The joule heating in the LEDs is reported to reduce the quantum yield and the output power of LEDs [24].As shown in Figs.4(a

Figure 2 (
Figure 2(a) shows I-V curves of ZnO MQWs LEDs with and without the p-type Mg 0.15 Zn 0.85 O EBL.The I-V curves show good rectification for both MQWs LEDs.The forward voltage at 20 mA increased from 4.8 V to 6.2 V upon addition of the p-type Mg 0.15 Zn 0.85 O EBL.The large decrease of current is due to the increase in the turn-on voltage and series resistance resulting from the higher barrier height and high resistivity of p-type Mg 0.15 Zn 0.85 O EBL, compared with the p-type Mg 0.1 Zn 0.9 O layer [22].Figure 2(b) shows the EL spectra of LEDs with and without the p-type Mg 0.15 Zn 0.85 O EBL at an injection current of 50 mA.As shown in Fig. 2(b), the p-type Mg 0.15 Zn 0.85 O EBL enhances the UV emission intensity and decreases the deep-level emission.The improved UV emission is attributed to the increase in the radiative recombination rate in the MQWs.Moreover, the decrease of electron overflow to p-type region is responsible for the decrease of deep-level emission from the p-type ZnO layer.The deep level emissions are not observed on the photoluminescence (PL) spectra of ZnO/MgZnO MQWs and MgZnO, as shown in Fig. 2(c).The deep level emissions are observed only from Sb-doped p-type MgZnO layer.Therefore, it is believed that the deep level EL emissions in ZnO MQWs LEDs with p-type MgZnO EBL, as shown in Fig. 2(b) originate from the Sbrelated defects in p-type MgZnO EBL layer.The Sb-related defects and many point defects such as oxygen vacancies and zinc interstitials can be formed by doping the large-mismatched Sb into ZnO matrix [12, 14, 23].

Figure 2 (
Figure 2(a) shows I-V curves of ZnO MQWs LEDs with and without the p-type Mg 0.15 Zn 0.85 O EBL.The I-V curves show good rectification for both MQWs LEDs.The forward voltage at 20 mA increased from 4.8 V to 6.2 V upon addition of the p-type Mg 0.15 Zn 0.85 O EBL.The large decrease of current is due to the increase in the turn-on voltage and series resistance resulting from the higher barrier height and high resistivity of p-type Mg 0.15 Zn 0.85 O EBL, compared with the p-type Mg 0.1 Zn 0.9 O layer [22].Figure 2(b) shows the EL spectra of LEDs with and without the p-type Mg 0.15 Zn 0.85 O EBL at an injection current of 50 mA.As shown in Fig. 2(b), the p-type Mg 0.15 Zn 0.85 O EBL enhances the UV emission intensity and decreases the deep-level emission.The improved UV emission is attributed to the increase in the radiative recombination rate in the MQWs.Moreover, the decrease of electron overflow to p-type region is responsible for the decrease of deep-level emission from the p-type ZnO layer.The deep level emissions are not observed on the photoluminescence (PL) spectra of ZnO/MgZnO MQWs and MgZnO, as shown in Fig. 2(c).The deep level emissions are observed only from Sb-doped p-type MgZnO layer.Therefore, it is believed that the deep level EL emissions in ZnO MQWs LEDs with p-type MgZnO EBL, as shown in Fig. 2(b) originate from the Sbrelated defects in p-type MgZnO EBL layer.The Sb-related defects and many point defects such as oxygen vacancies and zinc interstitials can be formed by doping the large-mismatched Sb into ZnO matrix [12, 14, 23].

Figure 3 (
Figure 3(a) shows the total output power of ZnO LEDs with and without the p-type Mg 0.15 Zn 0.85 O EBL as a function of injection current.The total optical output power of ZnO MQWs LEDs was 2.14 μW at 60 mA and this increased by 21.5% to 2.60 μW with the p-type Mg 0.15 Zn 0.85 O EBL.The improved output power was attributed to the improved carrier recombination processes in the MQWs.Furthermore, the ZnO MQWs LEDs with the p-type Mg 0.15 Zn 0.85 O EBL shows a peak output power at 60 mA which is 10 mA higher than that of the ZnO MQWs LEDs without the p-type Mg 0.15 Zn 0.85 O EBL.This indicates that the p-type Mg 0.15 Zn 0.85 O EBL effectively confines the carriers in the ZnO quantum well layer of the MgZnO/ZnO MQWs LEDs at high injection currents, compared with ZnO MQWs LEDs without an EBL. Figure 3(b) shows the normalized integrated UV emission intensity of ZnO MQWs LEDs with and without a p-type Mg 0.15 Zn 0.85 O EBL as a function of injection current.The integrated UV emission intensity of ZnO MQWs LEDs with p-type Mg 0.15 Zn 0.85 O EBL is increased by 111.2% at 60 mA compared with that of the ZnO MQWs LEDs without p-type Mg 0.15 Zn 0.85 O EBL because of the improved carrier recombination process in the MQWs.Moreover, the ZnO MQWs LED with the p-type Mg 0.15 Zn 0.85 O EBL shows a peak UV emission intensity at 60 mA, while the ZnO LED without the p-type Mg 0.15 Zn 0.85 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 0.15 Zn 0.85 O EBL effectively confines the electrons and holes in the MQWs at high injection currents.In addition, the integrated output power and integrated UV emission intensity of the LEDs with EBL drop more pronouncedly after 60mA than that without EBL.This behavior in optical output power of LEDs with EBL is closely related to the joule heating effect due to the highly resistive EBL.The joule heating in the LEDs is reported to reduce the quantum yield and the output power of LEDs[24].As shown in Figs.4(a) and 4(b), the electron and hole concentrations in the MQWs are increased, while the electron concentrations in the p-type MgZnO EBL are reduced by the blocking effect of the p-type MgZnO EBL.As a result, the radiative recombination of the carriers occurs dominantly in MQWs, not in the p-type MgZnO EBL.This leads to the sharp improvement of UV emission from the MQWs (111.2%) and the slight reduction of broad deep-level emissions from the p-type MgZnO as shown in Fig. 2(b).Even though the UV emission is sharply increased by 111.2%, the overall optical output power is increased by 21.5% due to the narrow UV peak compared to the broad spectrum of ZnO LEDs as shown in Fig. 2(b).
Figure 3(a) shows the total output power of ZnO LEDs with and without the p-type Mg 0.15 Zn 0.85 O EBL as a function of injection current.The total optical output power of ZnO MQWs LEDs was 2.14 μW at 60 mA and this increased by 21.5% to 2.60 μW with the p-type Mg 0.15 Zn 0.85 O EBL.The improved output power was attributed to the improved carrier recombination processes in the MQWs.Furthermore, the ZnO MQWs LEDs with the p-type Mg 0.15 Zn 0.85 O EBL shows a peak output power at 60 mA which is 10 mA higher than that of the ZnO MQWs LEDs without the p-type Mg 0.15 Zn 0.85 O EBL.This indicates that the p-type Mg 0.15 Zn 0.85 O EBL effectively confines the carriers in the ZnO quantum well layer of the MgZnO/ZnO MQWs LEDs at high injection currents, compared with ZnO MQWs LEDs without an EBL. Figure 3(b) shows the normalized integrated UV emission intensity of ZnO MQWs LEDs with and without a p-type Mg 0.15 Zn 0.85 O EBL as a function of injection current.The integrated UV emission intensity of ZnO MQWs LEDs with p-type Mg 0.15 Zn 0.85 O EBL is increased by 111.2% at 60 mA compared with that of the ZnO MQWs LEDs without p-type Mg 0.15 Zn 0.85 O EBL because of the improved carrier recombination process in the MQWs.Moreover, the ZnO MQWs LED with the p-type Mg 0.15 Zn 0.85 O EBL shows a peak UV emission intensity at 60 mA, while the ZnO LED without the p-type Mg 0.15 Zn 0.85 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 0.15 Zn 0.85 O EBL effectively confines the electrons and holes in the MQWs at high injection currents.In addition, the integrated output power and integrated UV emission intensity of the LEDs with EBL drop more pronouncedly after 60mA than that without EBL.This behavior in optical output power of LEDs with EBL is closely related to the joule heating effect due to the highly resistive EBL.The joule heating in the LEDs is reported to reduce the quantum yield and the output power of LEDs[24].As shown in Figs.4(a) and 4(b), the electron and hole concentrations in the MQWs are increased, while the electron concentrations in the p-type MgZnO EBL are reduced by the blocking effect of the p-type MgZnO EBL.As a result, the radiative recombination of the carriers occurs dominantly in MQWs, not in the p-type MgZnO EBL.This leads to the sharp improvement of UV emission from the MQWs (111.2%) and the slight reduction of broad deep-level emissions from the p-type MgZnO as shown in Fig. 2(b).Even though the UV emission is sharply increased by 111.2%, the overall optical output power is increased by 21.5% due to the narrow UV peak compared to the broad spectrum of ZnO LEDs as shown in Fig. 2(b).

Fig. 3 .
Fig. 3. (a) Total output power of ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85OEBL as a function of injection current.(b) Normalized integrated UV emission intensity of ZnO MQWs LEDs with and without the p-type Mg0.15Zn0.85OEBL as a function of injection current.

Fig. 4 .
Fig. 4. Calculated (a) electron and (b) hole concentrations of ZnO MQWs LEDs without and with the p-type Mg0.15Zn0.85OEBL.(c) Conduction and (d) valence energy band diagrams with and without p-type Mg0.15Zn0.85OEBL.To further understand the carrier recombination process in the ZnO MQWs LEDs, we calculated the carrier distribution and energy band structures of ZnO MQWs LEDs using the