Progress Semiconductor Technology

Recent progress in ZnO-based heterojunction ultravioletlight-emitting devices

Yichun Liu • Haiyang Xu • Chunyang Liu •

Weizhen Liu

Received: 31 March 2013 / Accepted: 23 May 2013 / Published online: 4 March 2014

� Science China Press and Springer-Verlag Berlin Heidelberg 2014

Abstract Wide bandgap (3.37 eV) and high exciton-

binding energy of ZnO (60 meV) make it a promising

candidate for ultraviolet light-emitting diodes (LEDs) and

low-threshold lasing diodes (LDs). However, the difficulty

in producing stable and reproducible high-quality p-type

ZnO has hindered the development of ZnO p–n homo-

junction LEDs. An alternative strategy for achieving ZnO

electroluminescence is to fabricate heterojunction devices

by employing other available p-type materials (such as

p-GaN) or building new device structures. In this article,

we will briefly review the recent progress in ZnO LEDs/

LDs based on p–n heterostructures and metal–insulator-

semiconductor heterostructures. Some methods to improve

device efficiency are also introduced in detail, including

the introduction of Ag localized surface plasmons and

single-crystalline nanowires into ZnO LEDs/LDs.

Keywords ZnO � Heterojunction � Ultraviolet light-

emitting devices � Progress

1 Introduction

ZnO, with a wide bandgap of 3.37 eV and a large exciton-

binding energy of 60 meV, is a promising material for

ultraviolet (UV) light-emitting diodes (LEDs) and low-

threshold lasing diodes (LDs). However, the difficulty in

producing stable and reproducible high-quality p-type ZnO

has hindered the development of ZnO homojunction LEDs,

though great progress has been made in this field [1–6].

Various group-I and -V dopants have been employed to

fabricate p-type ZnO. Among them, N is regarded as one of

the most promising p-type dopants because it has a similar

ionic radius to O, and the N3- ion substitution for O sub-

lattice can introduce a shallow acceptor level in ZnO

bandgap. Our group has developed a method of thermally

oxidizing Zn3N2 thin film to succeed in achieving p-type

nitrogen-doped ZnO (ZnO:N), and the stability of p-type

conductivity was investigated under the conditions of light

and thermal irradiation. The sample exhibited a stable

p-type characteristic in the darkness over a 1-year period

after deposition. However, when the p-type sample was

irradiated by 2.72 eV photons, it underwent a classic-

mixed conductivity transition from p-type to n-type, and it

took 24 h for the persisted n-type photoconductivity to fade

away in the dark and recover to original p-type. A local

potential fluctuation model induced by interface defects at

grain boundaries was used to explain the transient electric

behavior [7]. In addition, local chemical states and thermal

stabilities of N dopants in ZnO:N film are also investigated

by temperature-dependent X-ray photoelectron spectros-

copy (XPS). Different types of N local states were detec-

ted, including N2 molecules occupying O sites [(N2)O],

-NO species, and substitutional N atoms in O- and N-rich

local environments (a- and b-NO). Compared with the b-

NO, the a-NO shows a better thermal stability up to 723 K.

However, the transformation from a-NO acceptor to

undesirable (N2)O donor occurs at high temperature, which

degrades the p-type conductivity of ZnO:N film [8]. At the

current stage, the stability and controllability of p-type

ZnO are difficult to satisfy the practical applications. An

SPECIAL TOPIC: Wide Bandgap Semiconductor Materials and

Devices

Y. Liu (&) � H. Xu � C. Liu � W. Liu

Centre for Advanced Optoelectronic Functional Materials

Research and Key Laboratory for UV Light-Emitting Materials

and Technology of Ministry of Education, Northeast Normal

University, Changchun 130024, China

e-mail: ycliu@nenu.edu.cn

123

Chin. Sci. Bull. (2014) 59(12):1219–1227 csb.scichina.com

DOI 10.1007/s11434-014-0206-9 www.springer.com/scp

alternative strategy is to develop ZnO-based heterojunction

LEDs by employing other available p-type materials or

constructing new device structures. In this article, we will

present a brief review on the recent progress in ZnO LEDs/

LDs based on p–n heterostructures and metal–insulator-

semiconductor (MIS) heterostructures.

2 p–n heterojunction light-emitting devices

Among all the available p-type materials (e. g., p-GaN,

p-Si, p-CuAlO2, p-NiO, p-type organics, etc.), GaN is

considered to be the most suitable one because of its

similar energy band structure and small lattice mismatch

(1.8 %) with ZnO [9–15]. In 2003, Alivov et al. [16] have

already fabricated the p-GaN/n-ZnO heterojunction by

molecular-beam epitaxy and chemical vapor deposition,

and obtained a 430-nm blue–violet electroluminescence

(EL). However, the wide emission band mainly originates

from radiative recombination related to Mg acceptor in

p-GaN. Analysis of the band alignment reveals that elec-

trons in n-ZnO and holes in p-GaN overcame almost equal

barriers to realize the carrier injection. Thereby, the origin

of EL would be mainly determined by the difference in

carrier mobilities between n-ZnO and p-GaN. Usually, the

electron mobility is higher than the hole mobility. There-

fore, the electron injection from n-ZnO to p-GaN domi-

nates the whole carrier transport process at the interface,

and the radiative recombination mainly occurs in the GaN

side of p-GaN/n-ZnO heterojunction.

To activate the excitonic emission from ZnO, two

improvement plans were suggested. One is to form p–n

junction with n-ZnO by employing other p-type semicon-

ductors with wider bandgap. For example, by fabricating

p-Al0.12Ga0.88N/n-ZnO heterojunction, the energy barrier

for electrons is increased to *0.45 eV, while the one for

holes is almost invariant (Fig. 1b). Thus, holes can pass

through the interface to radiatively recombine with elec-

trons blocked in n-ZnO region. A ZnO near-band-edge

(NBE) emission at 389 nm was obtained (Fig. 1a) [17].

Another plan is to insert a thin semi-insulating ZnO (i-

ZnO) layer between p-GaN and n-ZnO to form a p-GaN/i-

ZnO/n-ZnO ‘‘sandwich-like’’ structure (Fig. 2a) [18]. The

introduction of i-ZnO layer compensates for the difference

between electron and hole mobilities. Holes from p-GaN

and electrons from n-ZnO can be injected into the i-ZnO

region, where the radiative recombination occurs. As

shown in Fig. 2b, unlike in the case of the EL spectrum of

p-GaN/n-ZnO heterojunction, a UV emission at 3.21 eV,

associated with the NBE recombination of ZnO, was

observed in the EL spectrum of p–i–n heterojunction. In

addition, by adjusting the thickness and optical properties

of i-ZnO layer, the intensity ratio of deep-level (DL) vis-

ible emission to blue–violet emission can be tuned, and a

white light LED is achieved with a 20-nm-thick i-ZnO

layer [19]. In order to further enhance the electron con-

finement, MgZnO alloy, as the electron blocking layer, was

introduced into the n-ZnO region, forming p-GaN/n-ZnO/

n-MgZnO/n-ZnO heterojunction. With this device struc-

ture, the injected electrons can be effectively confined in

the n-ZnO side near p–n heterointerface. Thereby, com-

pared with the p–n junction, ZnO excitonic emission can be

easily obtained at lower injection current [20].

Although much progress has been made in ZnO-based

heterojunction LEDs, the energy barrier at the heterojunction

interface will inevitably reduce the carrier injection effi-

ciency, especially for heterostructures with large band off-

sets. Several methods have been proposed to enhance the

external quantum efficiency of LEDs, including designing

special photonic crystals, modifying the interface between

Fig. 1 a EL spectra of the p-Al0.12Ga0.88N/n-ZnO heterostructure LED at 300 and 500 K (injection current is 20 mA); b Anderson model energy

band diagram of p-Al0.12Ga0.88N/n-ZnO heterojunction. Reprinted from Ref. [17], Copyright � 2003 American Institute of Physics

1220 Chin. Sci. Bull. (2014) 59(12):1219–1227

123

Fig. 2 a I–V characteristics of the GaN/ZnO p–n and p–i–n heterojunctions; the inset shows the schematic diagram of p-GaN/i-ZnO/n-ZnO

heterojunction LED; b EL spectra of the GaN/ZnO p–n and p–i–n heterojunctions LEDs (injection current is 4 mA). Reprinted from Ref. [18],

Copyright � 2005 Springer-Verlag

Fig. 3 a Schematic diagram of the LSP-enhanced LED structure; b EL spectra of the LEDs with (blue line) and without (red line) Ag/MgO

interlayer obtained at the injection current of 1 mA, the inset shows Gaussian deconvolution analysis of near-UV EL spectrum; c 20 K TR-PL

spectra of ZnO films with (blue circle) and without (red square) Ag/MgO layer monitored at 380 nm, the solid lines are the fit to a biexponential

decay model; d arrhenius plots of the normalized integrated UV-PL intensity of ZnO films with (blue circle) and without (red square) Ag/MgO

layer. Reprinted from Ref. [28], Copyright � 2012 American Institute of Physics

Chin. Sci. Bull. (2014) 59(12):1219–1227 1221

123

emitter and air, and decreasing piezoelectric field at hetero-

interface [21–23]. In recent years, localized surface plas-

mons (LSPs), the collective oscillation of electrons at the

interface between metal nanoparticles (NPs) and dielectrics,

have been verified to be an effective method for improving

the efficiency of light-emitting materials and devices [24–

27]. When the energy of electron–hole pairs/excitons in a

semiconductor is close to the electron vibrational energy of

metal LSPs, the energy will be transferred to the LSPs and

finally be scattered into free space as radiation. Conse-

quently, an additional recombination path is created,

resulting in an increased spontaneous recombination rate.

Our group has manufactured LSP-enhanced UV LEDs by

introducing Ag NPs and MgO spacer layer into the p-GaN/i-

ZnO/n-ZnO heterostructures (Fig. 3a). By optimizing the

MgO thickness, which can suppress the undesired charge

transfer and nonradiative Forster resonant energy transfer

between Ag and ZnO, sevenfold EL enhancement was

achieved (Fig. 3b) [28]. Time-resolved and temperature-

dependent photoluminescence (TR-PL and TD-PL) mea-

surements reveal that both the spontaneous emission rate and

the internal quantum efficiency are increased as a result of

coupling between ZnO excitons and Ag LSPs. Theoretical

calculations, based on experimental data, also indicate that

most of the energy of LSP can be converted into the photon

energy (Fig. 3c, d).

Another effective method for improving the efficiency

of LEDs is to introduce one-dimensional single-crystalline

ZnO nanostructure as active layer into LEDs, since the

carrier injection rate has been observed to significantly

increase in nanosized junctions [29]. Park and Yi [30] and

Zhang et al. [31] have, respectively, reported obtaining

high-brightness yellow–green and UV-blue EL from ZnO

nanorod arrays (NRAs)/p-GaN film heterojunction LEDs.

However, surface defects and surface adsorption can seri-

ously degrade the optical quality of ZnO nanowires with

large surface-to-volume ratios, thus, affecting the device

performance and stability. Our group also manufactured

ZnO NRAs/p-GaN film LEDs with a 387-nm UV emission

via low-temperature hydrothermal synthesis (Fig. 4).

However, after the nanorod device was exposed to

ambient air over a 1-year period, the integrated intensity of

the entire EL spectrum and the intensity ratio of NBE to DL

emission (RNBE/DL) significantly decreased. That is, the

radiative recombination rate (grr) and UV emission effi-

ciency of the ZnO NRAs LED decrease, while nonradiative

recombination rate (gnr) increases with the increasing air-

exposure time. Assuming the grr for fresh device is 100 %,

it rapidly drops to *5 % in less than 6 months, and the

RNBE/DL decreases from 155 to 2.6 within 1 month (Fig. 5a–c).

A vacuum desorption was conducted by storing the long-

term air-exposed ZnO NRAs LED in a vacuum chamber

Fig. 4 a and b Tilted-view SEM images of uncoated and MgZnO-coated ZnO NRAs, the scale bars are 2 lm; c z-contrast scanning TEM image

of a single MgZnO-coated ZnO nanorod; d line-scan composition profiles of Mg and Zn elements along the radial direction of the core/shell

nanorod; e I–V curves of the uncoated (dashed line) and coated (solid line) LEDs, the near-linear I–V characteristic in the inset verifies an ohmic

contact between Ni/Au electrode and p-GaN; and f schematic diagram of a MgZnO-coated ZnO NRA/p-GaN heterojunction LED. Reprinted

from Ref. [32], Copyright � 2012 American Institute of Physics

1222 Chin. Sci. Bull. (2014) 59(12):1219–1227

123

(10-2–10-4 Pa) for 1 week. After desorption, both EL

intensity and RNBE/DL were observed to recover to a mod-

erate level (Fig. 5d), indicating a negative effect of surface

adsorption on the NRA LED performance. Through the

analysis of O 1s core-level XPS spectra of the fresh, exposed,

and desorbed ZnO NRAs (Fig. 5e), it is concluded that sur-

face-adsorbed O2 and OH- species, as acceptor and donor

surface states, respectively, quench UV EL and favor

undesirable surface-mediated nonradiative and DL recom-

bination (Fig. 5f) [32]. Surface coating is an effective

method to prevent surface adsorption and surface defects. By

coating MgZnO alloy onto ZnO NRAs, ZnO/MgZnO coaxial

NRAs were prepared. Well-defined core/shell heterostruc-

ture with high-quality interface and coherent epitaxial rela-

tionship were confirmed by Z-contrast scanning transmission

electron microscopy and line-scan compositional analyses

(Fig. 4b–d) [33]. The MgZnO coating can suppress the

oxygen-related surface adsorption, and thus block the sur-

face-trapping channel. As a result, the coated NRA LED

shows relatively stable performance (Fig. 5a–c) [32].

3 MIS heterojunction light-emitting devices

As aforementioned, increased attention has been paid to p–n

heterojunctions to achieve ZnO-dominant UV EL. However,

the contribution from p-type layer is inevitable in their EL

spectra, and the advantage of large exciton-binding energy

of ZnO is not fully utilized. Thus, other types of

heterostructures are suggested to obtain the pure ZnO UV

emission. Recently, ZnO MIS-type heterojunction has

attracted increased attention. In fact, the MIS diode is not a

new product. For other wide bandgap semiconductors (e. g.,

Fig. 5 a EL spectra of uncoated (left) and coated (right) devices obtained at different air-exposure periods, the injection current fixed at 5 mA;

b and c the variations of grr, gnr, and RNBE/DL of both LEDs with air-exposure time, for clarity, with the inset in (c) exhibiting a magnified image

of the last four data points; d EL spectra of the long-term air-exposed, uncoated device before (solid line) and after (dashed line) vacuum

desorption, and the injection current is 3 mA; e O 1s core-level XPS spectra of the fresh, exposed, and desorbed ZnO NRAs; f schematic diagram

showing the surface-mediated carrier tunneling, trapping, and recombination processes in the uncoated ZnO NRA. Reprinted from Ref. [32],

Copyright � 2012 American Institute of Physics

ZnO

Insulator (MgO, SiO2, AlN…)

Metal

h

Hole injection

+ _

Fig. 6 The band alignment of MIS heterojunction with an insulator

(e. g., MgO and SiO2) as the I-layer under forward bias

Chin. Sci. Bull. (2014) 59(12):1219–1227 1223

123

GaN [34], ZnS [35] ), in the early stage of the development of

their LED devices, MIS diodes play an important role since

the p-type doping is very difficult during that time. Since the

1970s, ZnO-based MIS diodes have been studied. Early in

1973, Thomas et al. [36] reported the MIS LED grown on

ZnO:Li single crystal, and a blue emission at *420 nm was

observed. In the next year, near-UV EL was obtained from

the similar MIS heterojunction on single crystal substrate

under pulsed current injection [37]. However, due to the lack

of high-quality ZnO film, the ZnO MIS-type LEDs were

limited to the bulk materials, and the related research was

proceeding at a slow pace. In 1997, Tang et al. [38]

successfully achieved the optically pumped UV lasing of

ZnO film at room temperature, and since then the develop-

ment of ZnO LEDs has been going on the fast lane. The MIS

heterojunction, as a very important candidate, has motivated

intense research interest.

According to the difference in selecting I-layer materials,

ZnO MIS diodes can be divided into two categories. The first

type of MIS heterojunction is to use the high-resistance

intrinsic ZnO as the I-layer, forming the ‘‘metal/i-ZnO/n-

ZnO’’ heterojunction LED [39–42]. However, their EL

spectra are often dominated by the DL visible emission, and

it is hard to achieve pure ZnO UV EL in these MIS devices. In

Fig. 7 The EL spectra (a) and electrically pumped lasing spectra (b) of Au/SiO2/ZnO MIS heterojunctions. Reprinted from Ref. [43, 44],

Copyright � 2006 & 2007 American Institute of Physics

Fig. 8 The EL spectra of Au/MgO/ZnO film MIS diode a EL spectra under different injection currents, the inset shows the superlinear

dependence of integrated emission intensity on injection current density; b EL spectra detected from different angles, the inset drafts the EL

measurement configuration. Reprinted from the supporting information of Ref. [46], Copyright � 2011 American Institute of Physics

1224 Chin. Sci. Bull. (2014) 59(12):1219–1227

123

the other kind of ZnO MIS LEDs, some insulating materials

with very large bandgaps (e. g., SiO2, MgO, AlN, etc.) serve

as the I-layer. ZnO NBE emission can be realized from these

MIS heterojunctions, and so this kind of MIS heterostructure

has become more popular and attracted increased attention.

Its transport behavior and EL mechanism can be understood

in terms of the band alignment (Fig. 6). Due to the large

conduction band offset between ZnO and the insulating

layer, electrons would be blocked and accumulated at the

heterojunction interface under forward bias. While consid-

ering the dielectric nature of I-layer materials, most of the

bias would be applied on the insulating layer, and the local

electric field strength could be as high as *107 V/m therein.

Thus, electrons and holes can be generated through the so-

called impact-ionization process in the insulating layer. The

generated holes would be driven into ZnO under forward bias

and radiatively recombine with the accumulated electrons.

As a result, NBE emission of ZnO can be achieved.

In 2006, Chen et al. [43] reported Au/SiO2/ZnO MIS

LEDs grown on Si substrates. The devices were fabricated

by the reactive sputtering and electron beam evaporation.

Under continuous current injection, fairly pure UV EL was

realized from these MIS diodes (Fig. 7a). Later, they suc-

ceeded in achieving electrically pumped ZnO lasing in the

same MIS heterostructure (Fig. 7b) [44]. Similarly, using

MgO as the insulating layer, Zhu et al. [45] fabricated ZnO

film-based MIS LDs with lower lasing threshold.

The excellent performance of ZnO MIS heterojunction

also attracts our attention. Our group has also grown ZnO

and MgO films on an ITO glass to construct a MIS LD

[46]. As shown in EL spectra of Fig. 8a, with the

increasing injection current, distinct sharp peaks appear

and superimpose on the spontaneous emission band, sug-

gesting a lasing behavior. The superlinear dependence of

integrated emission intensity on injection current density

(inset of Fig. 8a) provides another experimental evidence

Fig. 9 (Color online) a and c Tilted-view SEM images of the as-grown ZnO nanowires and nanowires covered by MgO, the inset in (a) clearly

shows the hexagonal cross section of ZnO nanowire; b the schematic device structure of ZnO/MgO core/shell nanowire MIS heterojunction;

d and e TEM images of ZnO/MgO core/shell nanowires and two EDX spectra obtained from upper and lower regions of a single nanowire.

Reprinted from Ref. [46], Copyright � 2011 American Institute of Physics

Chin. Sci. Bull. (2014) 59(12):1219–1227 1225

123

for lasing action, and determines the lasing threshold cur-

rent density of ZnO film MIS LD as 4.8 A/cm2. Similar

lasing spikes can be observed from different detection

angles in the EL measurements, demonstrating the random

laser mode (Fig. 8b).

However, as mentioned above, the energy band offset in

heterojunctions reduces the carrier injection rate as well as

the quantum efficiency. It is known that the nano-hetero-

junction is a feasible method to improve the device perfor-

mance. Therefore, we also fabricated a MIS heterostructure

based on ZnO/MgO core/shell nanowires [46], which were

grown by hydrothermal method combined with electron

beam evaporation technique. Figure 9 shows the morphol-

ogy of ZnO/MgO core/shell nanowires. The scanning elec-

tron microscopy (SEM) and transmission electron

microscopy (TEM) observations reveal that the MgO layer is

not only covered on the top of ZnO nanowires to form a

quasi-continuous film, but also coated on their side wall to

form a core/shell structure.

The MgO coating can passivate the surface defects of

ZnO nanowires, thereby suppressing surface nonradiative

recombination and surface-mediated DL traps. Thereby,

the luminescence properties of ZnO are improved. The EL

spectra of nanowire heterojunction are shown in Fig. 10a.

Similar electrically pumped lasing behavior is observed in

the nanowire MIS LD. The lasing threshold is determined

as 2.3 A/cm2, which is much smaller than that of the film

MIS LD. Moreover, the nanowire LD shows higher emis-

sion intensity at relatively low working current density

compared with the planar device (Fig. 10b).

4 Conclusion

Great research progress has been made in ZnO-based

materials and LEDs in the last decade. Herein, we give a

brief review on the development and current status of two

kinds of important heterojunction LEDs: p–n junction and

MIS junction. Some of the methods discussed above, such

as inserting i-ZnO between p-GaN and n-ZnO to activate

ZnO UV emission, and introducing ZnO nanostructures

and metal LSPs to improve the LED efficiency, are also

helpful for the device design and performance improve-

ment of ZnO homojunction LEDs. Currently, p-type dop-

ing of ZnO still remains a great challenge. Under this

situation, ZnO-based heterojunction devices may be an

effective strategy for realizing short-wavelength, low-

threshold LEDs/LDs in the future.

Acknowledgments This work was supported by the National Basic

Research Program of China (2012CB933703), the National High

Technology Research and Development Program of China

(2006AA03Z311), the National Natural Science Foundation of China

(51172041, 91233204 and 51372035), and the Program for New

Century Excellent Talents in University (NCET-11-0615).

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