Doped ZnO 1D nanostructures: synthesis, properties, and photodetector application.

reviews ZnO

Doped ZnO 1D Nanostructures: Synthesis, Properties, and Photodetector Application Cheng-Liang Hsu* and Shoou-Jinn Chang*

From the Contents 1. Introduction ........................................ 4563

In the past decades, the doping of ZnO one-dimensional

2. Synthesis of Doped ZnO 1D-NSs .......... 4563

nanostructures has attracted a great deal of attention due to the variety of possible morphologies, large surface-to-volume ratios, simple and low cost processing, and excellent physical properties for fabricating highperformance electronic, magnetic, and optoelectronic devices. This article mainly concentrates on recent advances regarding the doping of ZnO one-dimensional nanostructures, including a brief overview of the vapor phase transport method and hydrothermal method, as well as the fabrication process for photodetectors. The dopant elements include B, Al, Ga, In, N, P, As, Sb, Ag, Cu, Ti, Na, K, Li, La, C, F, Cl, H, Mg, Mn, S, and Sn. The various dopants which act as acceptors or donors to realize either p-type or n-type are discussed. Doping to alter optical properties is also considered. Lastly, the perspectives and future research outlook of doped ZnO nanostructures are summarized.

3. Fundamentals and Fabrication of ZnO 1D-NS Photodetectors .................................... 4567 4. Photodetection Properties of Doped ZnO 1DNSs ..................................................... 4571 5. Conclusion and Outlook....................... 4582

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Doped ZnO 1D Nanostructures: Synthesis, Properties, and Photodetector Application

1. Introduction One-dimensional (1D) nanoscale semiconductors have attracted a huge amount of attention within the past decade owing to their dimensionality-dependent chemical, physical, electrical, and magnetic properties. Many 1D nanoscale materials have been synthesized and investigated, such as carbon group,[1] III–V group,[2] II–VI group,[3] and oxide group materials.[4,5] Among the variety of 1D nanoscale oxides, zinc oxide (ZnO) has an n-type wurtzite structure with a large exciton binding energy (60 meV) and wide bandgap (3.37 eV) at room temperature. Oxygen vacancies or zinc interstitials play the role of n-type dopants and thereby affect the physical and chemical characteristics.[6–8] The various morphologies of ZnO 1D nanostructures (NSs) include nanowires (NWs),[9,10] nanotubes,[11] nanobelts,[12] and nanorods.[3,13] In recent years, ZnO 1D-NSs have had promising wide application in nanoscale electronic and photonic devices, such as photovoltaic cells,[9,14] light-emitting diodes (LEDs),[15] gas sensors,[16] biosensors,[17] piezoelectric nanogenerators,[18] field emission devices,[19] and photodetectors.[20–23] There are many articles reporting enhancements of physical properties from doping ZnO 1D-NSs, such as increased conductivity,[24] increased ferromagnetic properties,[25] increased transparency,[26] or a reduced work function.[27] It is well known that most appropriate dopant materials for the n-type doping of ZnO 1D-NSs are found in group III (i.e., boron (B),[28] aluminium (Al),[29] gallium (Ga)[30,31] and indium (In)[32]). Transparent conducting oxide (TCO) materials exhibit high conductivity and good optical transmission properties. Tin-doped indium oxide (ITO) has already been used extensively in our daily lives. Al-doped ZnO and Ga-doped ZnO are also potentially useful as cost-effective TCOs.[24,26] Some articles have reported n-type doping using hydrogen (H),[33] titanium (Ti),[34] and group VII elements such as fluorine (F)[35] and chlorine (Cl).[36] However, p-type ZnO 1D-NS materials are unstable and difficult to reproduce owing to some unknown mechanisms. Group V elements (nitrogen (N),[37] phosphorus (P),[38] arsenic (As),[39,40] and antimony (Sb)[41,42]) atoms are potentially useful doping materials for p-type ZnO. These elements could replace oxygen to achieve p-type ZnO. In comparison with group V, a few p-type materials have been reported using group IB (copper (Cu),[43] silver (Ag),[44] and gold (Au)[45]) and group I (lithium (Li),[27,46] sodium (Na),[47] potassium (K)[48]) atoms to replace Zn and to achieve a +1 valency. The group-IB doped in ZnO 1D-NSs could produce either a +1 or +2 valency, which is determined by the chemical structure. The group I elements were theoretically predicted to have a shallow acceptor level and were assumed to substitute Zn atoms. In recent years, rare earth elements have been widely studied as dopants for semiconductors to enhance their luminescence properties. Rare earth elements doped into ZnO 1D-NSs, such as lanthanum (La),[49] europium (Eu),[50] dysprosium (Dy),[51] and Erbium (Er),[52] have also been reported. Photoconductivity is the electrical conductivity increase due to absorption of incident light. The conductivity increases by light absorption when the light energy is larger small 2014, 10, No. 22, 4562–4585

than that of the material’s bandgap. The light absorption rapidly excites a lot of free electrons and holes for increased conductivity. Over the last decades, many photodetectors have been prepared with wide bandgap materials, including SiC,[53] diamond,[54] SiCN,[55] III–V compounds[56–58] and II–VI compounds.[59–62] These UV photodetectors have been reported with fabricated metal–semiconductor–metal (MSM) and p–n junction structures.[63–66] The MSM structure can form a Schottky barrier (rectified photodetector device) or ohmic contact (non-rectified photodetector device).[63,64] Currently, many photodetectors are designed with p–n junctions to increase their response speed, stable operation, and low noise. The p–n junctions can be divided into heterojunctions and homojunctions.[65,66] This review article will provide a helpful review on doping ZnO 1D-NSs. Firstly, it will introduce and discuss the synthesis methods and properties of doped ZnO 1D-NSs. Next, different unique, novel, and important doped ZnO 1D-NS photodetectors will be introduced, followed by the discussion of their structure, electrical, optical, and mechanical properties. Lastly, this article will summarize the reported results. A brief future outlook of doped ZnO 1D-NSs photodetectors will also be concluded.

2. Synthesis of Doped ZnO 1D-NSs Up to now, there are a few theoretical studies on bulk doped ZnO, its thin films, and clusters. A cluster often exhibits some special properties in comparison with the bulk or with thin films, owing to quantum confinement effects. Doped ZnO 1D-NSs fabricated from clusters could offer possible solutions for the incorporation and activation of the dopants.[67–69] The use of ZnO cluster structures makes it highly possible to investigate the interplay between intentionally inserted dopants and defects. The incorporation of dopants into a ZnO cluster has been calculated using the first principles method and density functional theory (DFT), which are useful to study doped ZnO clusters. Recently, some research groups have investigated the physical properties of various dopants on (ZnO)n (n = integer) clusters,[70–75] as shown in Figure 1. The effects of relaxation and electronic confinement can be studied with atomistic models. Doped ZnO 1D-NSs can be synthesized by several physical and chemical methods, such as vapor phase transport,[76,77] pulsed laser ablation,[78] molecular beam epitaxy (MBE),[79]

Prof. C.-L. Hsu Departments of Electrical Engineering National University of Tainan Tainan 700, Taiwan E-mail: [email protected] Prof. S.-J. Chang Institute of Microelectronics & Department of Electrical Engineering Advanced Optoelectronic Technology Center National Cheng Kung University Tainan 701, Taiwan E-mail: [email protected] DOI: 10.1002/smll.201401580


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electrochemical,[80] metal organic chemical vapor deposition (MOCVD),[81] and hydrothermal synthesis methods.[82] Among these methods, vapor phase transport and hydrothermal synthesis methods are the most commonly used to achieve the doping of ZnO 1D-NSs. Figure 2(a)–(i) and (j)– (o) display scanning electron microscopy (SEM) images of doped ZnO 1D-NSs synthesized by vapor phase transport and the hydrothermal method, respectively.[83–96] The Al-,[83] Cu-,[84] Ga-,[85] In-,[86] P-,[87] and Ti-doped[88] ZnO 1D-NSs are synthesized on glass substrates by our laboratory furnace, as shown in the top two rows of images in Figure 2. On the right-hand-side of Figure 2, Al-[92] and La-doped[94] ZnO 1D-NSs grown on polyimide substrates in our laboratory are shown. Our synthesis creates doped ZnO 1D-NSs with an almost vertical morphology: as the perpendicular state is easier to maintain, this synthesis is conducive to making easily processed devices. The crystalline lattice of the seed layer or substrate will affect the growth direction of the doped ZnO 1D-NSs.

Cheng-Liang Hsu received his PhD in electrical engineering from the National Cheng Kung University, Taiwan, in 2005. From 2000 to 2003, he was an Engineer at the Taiwan Semiconductor Manufacturing Company (TSMC). From 2003 to 2005, he was with the Material Research Laboratories, Industrial Technology Research Institute (ITRI). Currently, he is a Full Professor in the Department of Electrical Engineering, National University of Tainan, Taiwan. He has published more than 80 contributions in SCI papers. His research interests include nanoscale 1D semiconductors. Shoou-Jinn Chang received his B.S. degree from the National Cheng Kung University (NCKU), Tainan, Taiwan in 1983, a M.S. degree from the State University of New York, Stony Brook, in 1985, and a PhD from the University of California, Los Angeles, in 1989, all in electrical engineering. He is a Full Professor in the Department of Electri-

2.1. The Vapor Phase Transport Method

cal Engineering, NCKU. He served as the Drector of the Institute of Microelectron-

The vapor phase transport method has been used to grow doped ZnO 1D-NSs via the vapor–liquid–solid (VLS) mechanism or the self-catalytic VLS mechanism. The source materials, substrate, and temperature of the vapor phase transport are compared and summarized in Table 1.[97–110] Figure 3 illustrates the growth via VLS and self-catalytic VLS mechanisms. The VLS method is very widely used for the growth of doped ZnO 1D-NSs.[45,90,100] The VLS growth usually creates a coating of Au catalytic ultra-thin films or nanoparticles

ics, NCKU, from August 2008 to July 2011. Currently, he serves as the Deputy Director of the Advanced Optoelectronic Technology Center, NCKU. Professor Chang is the recipient of the outstanding research award from the National Science Council, Taiwan, in 2004. He is a fellow of SPIE, a fellow of OSA, and a fellow of IEEE.

(NPs) on a substrate. The Au ultra-thin film then forms NPs at above ∼300 °C. In the VLS growth of doped ZnO 1D-NSs, the Zn source and doping material were evaporated first, and

Figure 1. Geometry of the unrelaxed ZnO crystal structures with various dopants. (a) C element, Reproduced with permission.[70] Copyright 2011, IOP. (b) H element, Reproduced with permission.[71] Copyright 2014, IOP. (c) P element, Reproduced with permission.[72] Copyright 2011, Springer. (d) Li element, Reproduced with permission.[73] Copyright 2012, IOP. (e) Cu element, Reproduced with permission.[74] Copyright 2011, IOP. (f) N element, Reproduced with permission.[75] Copyright 2011, IOP.

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Figure 2. The SEM images of various doped ZnO 1D-NSs that were synthesized by (a–i) vapor phase transport, and (j–o) the hydrothermal method. Panel (g) reproduced with permission.[89] Copyright 2011, Nature Publishing Group. Panel (h) reproduced with permission.[90] Copyright 2008, ACS. Panel (i) reproduced with permission.[91] Copyright 2011, Springer. Panels (k) and (m) reproduced with permission.[93] Copyright 2010, ACS. Panel (n) reproduced with permission.[95] Copyright 2011, Wiley-VCH. Panel (o) reproduced with permission.[96] Copyright 2014, RSC.

then the gas-phase reactants are absorbed onto the Au NP catalyst, which creates eutectic alloy droplets. The reactant concentrations will keep increasing in the droplet, and then exceed the saturation point. The supersaturated liquid drop leads to the precipitation of the Zn and dopant, which are then mixed with oxygen to grow doped ZnO 1D-NSs. The Au alloy NPs catalytically direct the growth direction and normally stay at the top of the doped ZnO 1D-NSs. The growth speed of doped ZnO 1D-NSs is crystallographic-direction dependent, i.e., it is dependent on with c-axis [001] direction, which is much faster than growth in other directions. The diameter of the Au alloy NPs determines the 1D-NSs diameter size.

Many articles report that doping ZnO 1D-NSs can also be synthesized without any catalyst, likely via vapor–solid (VS) growth.[101–106] Some articles have defined this phenomenon as the ‘self-catalytic VLS’. Before self-catalytic VLS growth, radio frequency (RF) sputtering is used to deposit a ZnO crystalline seed layer on a substrate such as sapphire, silicon, or glass. The ZnO seed layer has a highly oriented columnar crystalline structure, as revealed via high-resolution transmission electron microscopy (HR-TEM). However, some research groups have inferred that Zn suboxides (ZnOx, where x < 1) act in a similar way to that of the Au catalyst in the nucleation of doped ZnO 1D-NSs, due to the fact that melting temperatures ∼419.5 °C for both Zn suboxides and Zn are lower than the synthesis

Figure 3. (a) VLS and (b) self-catalytic VLS growth processing steps. small 2014, 10, No. 22, 4562–4585



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Table 1. Listed various dopant source materials, temperature, substrate, catalytic and PL for vapor-phase transport method. Dopant n-type/p-type

Source Material

Temperature [°C]

Substrate

Catalyst

PL peaks Blue-shift (BS), Red-shift (RS)

Reference

Ga

n-type

ZnO powder, Ga2O3 powder mixed graphite powders

1100

n-Si

Ga

n-type

Zn powder, Ga powder

600

p-GaN/sapphire

384 nm RS, 500 nm

[31]

Ga

n-type

Zn powder, Ga powder

700

Si

380 nm BS, 500 nm

[97]

Al

n-type

ZnO powder, Al powder mixed graphite powders

900

ZnO/c-sapphire

[98]

Al

n-type

ZnO powder, Al2O3 powder mixed graphite powders

1100

Sapphire(0001)

[99]

In

n-type

In and Zn/ZnO powder

800–1000

Au

379.2 nm

[32]

In

n-type

Zn, In and ZnO powder

700

Zn

Au

381 nm BS, 540 nm

[100]

N

p-type

ZnO, N2O (diffusion)

950

Sapphire

Au

[30]

[90]

N

p-type

Zn powder, NH3

500

P

p-type

Zn powder, P2O5 powder

945

A-plane sapphire

P

p-type

ZnO powder, Zn3P2 powder

1050

ZnO/Si(001)

As

p-type

ZnO powder mixed graphite powders, GaAs

900

GaAs

382 nm BS, 510 nm

[40]

As

p-type

Zn powder, As powder

750

ZnO/n-Si(100)

At 10 K, 370 nm RS, 496 nm

[104]

Sb

p-type

Zn powder, Sb2O3 powder mixed graphite powders

930

Si

Sb

p-type

Zn powder, Sb2O3 powder

650

ZnO/glass

Sb

p-type

Zn powder, Sb powder

650

ZnO/c-sapphire

Sb

p-type

Zn powder, Sb2O3 powder

550

Si

Ag

p-type

Zn powder, silver (I) oxide powder

730

sapphire

Ag

p-type

ZnO powders and Ag2O with hot-walled pulsed laser

950

Au

p-type

Zn powder, Au

600

unknown

Cu

p-type

Zn powder, CuO powder mixed graphite powders

960

a-plane sapphire

Cu

p-type

Zn powder, Cu foil (hotwire)

600

ZnO/glass

Cu

p-type

ZnO powder mixed graphite powders, copper foil

950

ZnO/ Si(100)

Ti

n-type

Zn powder, Ti implanted on ZnO NWs with 30 keV

600

Si

Ti

n-type

Zn powder, Ti hotwire

600

ZnO/glass

Na

p-type

ZnO powder mixed graphite powders, Na2O

650

MWCNTs

Na

p-type

Zn powder, NaCl powder

unknown

unknown

temperature. Whether VLS or self-catalytic VLS growth is used, the substrates are inorganic materials, such as glass (corning), silicon, sapphire, or nitride. Because of the temperature for VLS or self-catalytic VLS growth is above 500 °C with oxygen, the soft point temperature of the substrates must be at least 500 °C. Most vapor phase transport experiments are completed within a furnace tube.

2.2. The Hydrothermal Method The synthesis of doped ZnO 1D-NSs has been reported via various solution-based methods, which can be classified as hydrothermal, solvothermal, and electrodeposition methods according to the solvents used during the growth. The hydrothermal method uses water as the solvent. The source materials, substrate, and temperature for the hydrothermal method are compared and summarized in Table 2.[94,111–122] The typical reagents for electrodeposition are similar to those used for hydrothermal growth, but its growth ratio is higher by several orders of magnitude than that of the hydrothermal


380 nm RS, 630 nm

[101]

370 nm, 734∼742 nm

[102] [103]

[42] 390 nm RS, 531 nm

[105]

386 nm RS,

[106]

[89]

372.6 nm

[91]

Au

[107]

Au

[45] [43a]

Au

395 nm RS, 522 nm

[84]

380 nm, 510 nm

[108]

374 nm BS, 493 nm

[34]

372 nm RS, 511 nm

[88] [109]

370 nm, 496 nm

[110]

method.[123–126] Hydrothermal is the most common synthesis method for doped ZnO 1D-NSs out of the solution-based methods. The hydrothermal synthesis has a low cost, low growth temperature, and can be used for mass production. The doped ZnO 1D-NS morphology and dopant concentration are dependent on the solubility of Zn hydrate and the dopant material hydrate. Crystal growth is performed in hot water under high pressure in a steel autoclave. Much literature has reported that a ZnO seed layer must be deposited on the substrate before hydrothermal growth. The grains of the ZnO seed layer have a near-columnar crystalline structure, and the doped ZnO 1D-NSs grow along the columns. The thickness of the ZnO seed layer is about 10∼70 nm. Methenamine, commonly called hexamethylenetetramine (HMTA), is a nonionic tetradentate cyclic tertiary amine usually used in the hydrothermal method due to its high solubility in water. Hydroxyl ions are released by thermal degradation. These hydroxyl ions then react with Zn2+ ions and dopants to form doped ZnO. In hydrothermal synthesis, the substrate is immersed in an aqueous solution and placed in a sealed beaker. The growth temperature and time are ∼95 °C


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Table 2. Listed various dopant source materials, temperature, substrate and PL for hydrothernmal method. Dopant

n-type/p-type

Source Material

Temperature [°C]

Substrate

PL peaks Blue-shift (BS), Red-shift (RS) Reference

B

n-type

Zn(NO3)2·6H2O, ((CH3)CHO)3B

95

ZnO/p-Si

Ga

n-type

Zn(NO3)2·6H2O, Ga(NO3)3·xH2O

95

ZnO/p-Si

380 nm BS, 570 nm

[96]

Ga

n-type

Zn(NO3)2·6H2O, Ga(NO3)3·xH2O

90

ZnO/p-Si

375 RS, 580 nm, 760 nm

[111]

Ga

n-type

Zn(CH3COO)2·2H2O, Ga(NO3)3·xH2O

110

unknown

384 nm RS, 564 nm

[112]

Al

n-type

Zn(CH3COO)2·2H2O, Al(NO3)3

95

ZnO/glass

[28]

[113]

Al

n-type

Zn(NO3)2, Al(NO3)3

95

ZnO:Al/polyimide

Al

n-type

Zn(CH3COO)2·2H2O, AlCl3

95

AlZn/Ti/Si(100)

In

n-type

Zn(NO3)2·6H2O, InCl3

80

ZnO/Si

377 nm RS, 530 nm, 755 nm 368.7 nm BS,

381 nm RS, 556 nm, 755 nm

[92] [114] [115]

H

n-type

Zn(NO3)2·6H2O, NH3, H2 (425°C anneal)

97

ZnO/Si

F

n-type

Zn(NO3)2, ZnF(OH)

70

ZnO/FTO/glass

[33]

Cl

n-type

Zn(NO3)2, NH4Cl

90

Si

BS

[36a]

BS

[36b]

[35]

Cl

n-type

Zn(NO3)2·4H2O, NH4Cl

90

Si

N

p-type

Zn(CH3COO)2·2H2O, NH3 (530°C anneal)

95

ITO

Sb

p-type

Zn(NO3)2, (SbAc3)

90

Si

[41]

Sb

p-type

Zn(NO3)2, Sb(CH3COO)3

95

ZnO/flexible substrates

[117]

Sb

p-type

Zn(SO4)·7H2O, SbCl3

95

p-Si(100)

[93]

Ag

p-type

Zn(SO4)·7H2O, Ag(NO3)

95

p-Si(100)

[93]

Ag

p-type

Zn(NO3)2, Ag(NO3)

90

polyester

360 BS, 590 nm

Cu

p-type

Zn(NO3)2, CuCl2

90

p-GaN:Mg

394 nm RS,

Cu

p-type

Zn(Ac)2·2H2O, Cu(Ac)2·H2O,

140

[116]

[118] [95] [119]

K

p-type

ZnCl2, KOH

90

unknown

Li

p-type

Zn(CH3COO)2, Zn(NO3)2·6H2O Li(CH3COO), LiNO3

unknown

ITO/glass

410 nm RS, 500∼800 nm

[27]

375.3 nm RS

[121]

Li

p-type

Zn(NO3)2·6H2O, LiC2H3O2

450 K

Li

p-type

Zn(NO3)2·6H2O, Li(NO3)

92

A-plane sapphire

Na

p-type

Zn(NO3)2·6H2O, NaCl

92

ceramic AAO template

[120]

[46]

[47]

K

p-type

Zn(CH3COO)2, CH3CO2K

unknown

La

p-type

Zn(CH3COO)2, Zn(NO3)2·6H2O La(NO3)3·6H2O

150

La

p-type

Zn(NO3)2, La(NO3)3·6H2O

95

ZnO/polyimide

380 nm RS, 578 nm, 758 nm

[94]

Y

p-type

Zn(C2H3O2)2·2H2O, Y(C2H3O2)3·4H2O

90

ZnO/Si

375.8 nm RS

[122]

Eu

n-type

Zn(NO3)2·6H2O, Eu(NO3)3

80

sapphire

380 nm BS, 615 nm

[50]

and over 4 h, respectively. The growth temperature, at ∼95 °C, is lower than for other doped ZnO 1D-NS synthesis methods. The Zn source is zinc nitrate hexahydrate [Zn(NO3)2] or zinc acetate dehydrate [Zn(CH3COO)2•2H2O]. The most common dopant sources is nitrate hydrate. NaOH is added into the solution to adjust the pH to the desired value.

3. Fundamentals and Fabrication of ZnO 1D-NS Photodetectors In the last decade, doped ZnO 1D-NS photodetectors have been in focus, using various dopant elements, doping concentrations, and contact qualities. A common problem with doped nanostructures is that the introduction of increasing dopant concentrations during the synthesis process often leads to increased morphological changes.[85] Here we will small 2014, 10, No. 22, 4562–4585

383 nm, 510 nm

[48]

405 nm, 431 nm, 469 nm

[49]

give a brief introduction of the latest developments in doped ZnO 1D-NSs, and then a discussion of the contact technologies used for 1D-NS photodetector manufacturing.

3.1. Fundamentals of ZnO 1D-NS Photodetectors Photodetectors are widely used in daily life and have many various important potential applications, such as active-pixel sensors (APSs), charge-coupled devices (CCDs), space communication, flame detection, and military applications.[127–130] Figure 4(a) depicts the adsorption of molecular O2− species at the surface of n-type and p-type doped ZnO 1D-NSs in the dark.[131] Based on past reports, almost all of the adsorbed oxygen in the air is the O2− species at room temperature. The doped ZnO 1D-NSs could thus be used as gas-sensing devices because of their enhanced performance, which is due



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O2 ( gas) O2( ads.) O2( ads.) + e − O2−

Figure 4. Schematic depicts (a) the O2− adsorbed on doped ZnO 1D-NS surfaces, and (b) the band diagrams of doped ZnO 1D-NS NWs.

to their ultra large surface areas. Oxygen ions are adsorbed on the surface of doped ZnO 1D-NSs to catch free electrons. The surface of a ZnO 1D-NS has a large quantity of O2− species. The adsorption reaction is as follows:

Accordingly, the conductivity of n-type ZnO 1D-NSs is low, due to the formation of a depletion region on the surface.[132] The conductivity of p-type ZnO 1D-NS increases due to the increased hole concentration on the 1D-NS surface as the electrons disappear. Under UV light illumination with a photon energy higher than ZnO bandgap, light absorption occurs in doped ZnO 1D-NSs surface to produce a large amount of electron–hole pairs [hν→e− + h+]. The O2− species on the 1D-NS surface are then transformed into oxygen gas through redox reactions with the photogenerated holes.[23] Thus, the depletion region will disappear almost completely from the n-type ZnO 1D-NS surface upon UV exposure. The removed O2− species could release a significant amount of free electrons. This will enhance the electron concentration for the n-type ZnO and reduce the hole concentration (e− and h+ recombination) for the p-type ZnO. Figure 4(b) schematically depicts the band diagrams of n-type and p-type doped ZnO 1D-NSs. Compared with pure ZnO, the Fermi level (EF) of n-type doped ZnO 1D-NSs is shifted toward the conduction band (EC). The EF of p-type doped ZnO 1D-NSs is moved toward the valence band (EV). The upward band bending shown at the surface of the doped 1D-NSs surface is due to the adsorbed molecular O2− species.

Figure 5. (a)–(c) SEM images of pure ZnO 1D-NSs and (d)–(i) various doped ZnO 1D-NSs. (a) Reproduced with permission.[133] Copyright 2007, Elsevier. (b) Reproduced with permission.[134] Copyright 2007, IEEE. (c) Reproduced with permission.[135] Copyright 2011, ACS. (d) Reproduced with permission.[136] Copyright 2011, ECS. (e) Reproduced with permission.[137] Copyright 2009, ACS. (f) Reproduced with permission.[138] Copyright 2009, ACS. (g) Reproduced with permission.[139] Copyright 2011, ACS. (h) Reproduced with permission.[140] Copyright 2012, Wiley-VCH. (i) Reproduced with permission.[141] Copyright 2010, AIP.



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3.2. The Fabrication Process of Doping ZnO 1D-NS Photodetectors Doped ZnO 1D-NS photodetectors have been fabricated via various synthesis and assembly technologies. The morphology of 1D-NS photodetectors is measurement by field-emission scanning electron microscopy (FE-SEM), such as revealed in Figure 5(a)–(i).[133–141] Figure 5(a)–(c) display single or multiple ZnO 1D-NS photodetectors fabricated by our laboratory. Figure 5(d)–(i) reveal various doped ZnO images from reports on 1D-NS photodetectors. The currently published literature can be broadly classified into multiple ZnO 1D-NS photodetectors and individual ZnO 1D-NS photodetectors.

3.2.1. Multiple ZnO 1D-NS Photodetectors Figure 6(a) depicts the fabrication processing steps of multiple ZnO 1D-NS lateral photodetectors.[133,140] The substrate must be an insulating material or a semiconductor substrate coated with an insulating layer. The ZnO seed layer or Au catalyst layer are deposited on substrate, with a photolithography/etching step used to define the pattern of this seed layer or Au catalyst layer. After first growing NWs, these ZnO 1D-NSs grow laterally staggered Figure 6. The multiple ZnO 1D-NS photodetector growth and processing steps. and interconnected to form the photodetector. Figure 6(b) depicts a different multiple ZnO 1D-NS these 1D-NSs in the alcohol solution, and then drops are photodetector fabrication method.[84,140] After ZnO 1D-NSs extracted onto the patterned electrodes/insulating subare grown on substrate, a a passivation layer is deposited or strate. Longer 1D-NSs have the possibility to cross two or spin-coated onto the 1D-NS sample to cover the surface. The more electrodes. The interface of the 1D-NS and electrodes passivation layer is an insulating material (such as polymethyl is stablized by a Pt contact, which is deposited by focused methacrylate (PMMA), spin of glass (SOG)) which fills the ion beam (FIB) equipment. Shorter 1D-NSs need more gaps between the NWs.[84,142] metallic patterns to connect them to electrodes by FIB,[143] Standard photolithography is then used to define the such as shown in Figure 7(a). Another method is to disperse electrode deposition area. Au is subsequently deposited to 1D-NSs randomly on an insulating substrate, and then use serve as a contact electrodes. In general, photodetectors fab- photoresist (PR) spin coating on the sample. Photolithogricated using the method shown in Figure 6(b) have larger raphy is used to define the PR pattern. The electrode metal leak currents compared with those using the method shown is deposited on the sample, and then the lift-off method is in Figure 6(a), due to the PMMA or SOG being difficult to applied to remove the PR for the patterned electrode metal. completely fill the high-density NW array. Nanosized porosity The details of this process are shown in Figure 7(b). Some will cause the leakage current problem and decrease photo- work has employed electron-beam lithography or photolidetector performance. thography to define the metal line pattern to connect the 1D-NSs and the neighboring electrodes. These methods are applicable to a single or small number of doped ZnO 1D-NS 3.2.2. Single ZnO 1D-NS Photodetectors photodetectors. Moreover, some research teams have modified the experimental patterns shown in Figure 6, changing Figure 7(a) and (b) reveal the fabrication processes of single the electrode pattern to line head to line head, or the tip ZnO 1D-NS photodetectors.[134–139] First, the electrodes to tip of the triangle. Doped ZnO 1D-NSs were synthesized are patterned onto an insulating substrate. Doped ZnO through the self-catalytic VLS method to grow across the 1D-NS are grown by various synthesis methods, then col- trench region from one electrode to another. The direct latlected in an alcohol solution. Sonication is used to disperse eral growth of ZnO 1D-NSs forms a single or a few 1D-NS small 2014, 10, No. 22, 4562–4585



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spectra of Ti-,[88] Cu-,[84] La-,[94] and Aldoped[92] ZnO 1D-NSs, respectively, fabricated by our laboratory. The structures in Figure 8(a),(b) and Figure 8(c),(d) are fabricated via vapor phase transport synthesis and hydrothermal synthesis, respectively. Based on past reports, the PL of pure ZnO 1D-NSs reveals different properties depending on whether vapor phase transport or hydrothermal synthesis was used. The PL properties of structures from vapor phase transport synthesis display a strong UV emission and an inferior green emission. The green emission is not easily observed. Many studies identified, through such a weak green emission situation, that no defect exists in these pure ZnO 1D-NSs. For hydrothermal synthesis, typical PL results show a strong UV emission and a weak yellow–orange emission band. Because the yellow–orange/UV emission peak ratio from the hydrothermal technique is higher than the green/UV emission peak ratio from vapor phase transport, we can identify that hydrothermally synthesized pure ZnO 1D-NSs have more defects than that fabricated via vapor phase transport. In general, the UV emission intensity is reduced by increasing the dopant concentration. The UV emission peak of doped ZnO 1D-NSs is blue shifted or red shifted, as compared to that observed from the pure ZnO 1D-NSs. The UV emission blue shift or red shift increases with an increasing doping concentration. The blue shift is attributed to the fact that dopants could generate a large amount of electrons Figure 7. Single ZnO 1D-NS photodetector growth and processing steps. to occupy the energy levels at the bottom photodetectors. Dependent on the substrate and seed layer of the conduction band.[88] When the doped ZnO 1D-NSs are materials, the structure of the photodetectors can be homo- optically excited, high energy excitons could jump to higher geneous or heterogeneous. For both single and multiple energy levels in the conduction band, as compared to those ZnO 1D-NS photodetector fabrication methods, often the occupied in the pure ZnO 1D-NSs. The Burstein–Moss effect success rate is not high, so a considerable number of devices is well known to cause an increased bandgap in the absorpneed to be produced on a substrate in order to select a suc- tion and luminescence spectra.[88,145] Many research groups have reported that the red shift originates from higher cessful test piece. dopant concentrations or a large defect density, which could both cause ZnO NW bandgap narrowing. The dopants, occupying either Zn or O sites in ZnO, act as acceptors or donors 3.3. Photoluminescence of Doped ZnO 1D-NS Photodetectors and could form new level states in the bandgap of ZnO. Deep The photoluminescence (PL) spectrum is typically a tool level emission (green band or yellow–orange band) peaks to detect defect and dopants in the emission band. The PL originate from oxygen vacancies and the incorporation of peak position, full width at half maximum (FWHM), and dopant impurities.[84,92,94] According to previously reported room-temperature PL UV-to-visible emission ratio could also be used to determine the quality of ZnO 1D-NSs. With a bandgap energy spectra, samples with large defect densities could still exhibit of ∼3.37 eV, ZnO emits UV light peaked at around 368 nm a strong UV peak with negligible defect emission at room in the PL spectrum. In general, the relevant UV emission temperature. Thus, low-temperature PL should be used to is attributed to exciton recombination through exciton– investigate the defect density and crystal quality of doped exciton collision processes.[144] Figure 8(a)–(d) shown PL ZnO 1D-NSs. Figure 9(a)–(d) display low-temperature PL



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Figure 8. Room-temperature PL spectra of (a) Ti-doped, (b) Cu-doped, (c) La-doped, and (d) Al-doped samples. (a) Reproduced with permission.[88] Copyright 2014, Elsevier. (b) Reproduced with permission.[84] Copyright 2014, ACS. (c) Reproduced with permission.[94] Copyright 2013, ACS. (d) Reproduced with permission.[92] Copyright 2014, RSC.

spectra of As-,[121] Ag-,[91] P-,[102] and Na-doped[110] ZnO 1D-NSs, respectively. In these spectra, the free exciton peak position can be used to evaluate the quality of the doped ZnO 1D-NSs PL at below 14 K. The low-temperature PL spectra of doped ZnO 1D-NSs occasionally displays a biexciton peak. Many research groups use low-temperature PL spectra of doped ZnO 1D-NSs to determine the ZnO defect density and/or to provide evidence for the presence of dopants. However, this requires high-resolution equipment to measure and accurately determine the location of bound exciton peaks. Some peak origins have not been determined due to the difficulty in assigning these peaks.

4. Photodetection Properties of Doped ZnO 1D-NSs The properties of doped ZnO 1D-NSs have been widely investigated both experimentally and theoretically. In this section, we will review recent articles about doped ZnO 1D-NS photodetectors, and provid an overview of the optical properties and measured results which are briefly summarized small 2014, 10, No. 22, 4562–4585

in Table 3.[146–160] The performance of doped ZnO 1D-NS photodetectors is greatly superior to ZnO or doped ZnO thin film photodetectors, but it still cannot compare to conventional top-down fabricated GaN-based photodetectors due to a very slow photoresponse time. So far, many research groups have been trying to increase the photocurrent gain (Iphoto/ Idark ratio) and decrease the photoresponse time of doped ZnO 1D-NS photodetectors, and different methods have been studied to improve their performance. The measured and analyzed external quantum efficiencies (EQE) of ZnO 1D-NSs are a major focus of solar cell research. The EQE of ZnO 1D-NS photodetectors is much less than that of solar cells. The EQE performances of doped ZnO 1D-NS photodetectors are difficult to optimize, due to the fact that their morphology and electronics paths are not unique, and dopant distribution is not uniformed compared with thin-film photodetectors. Currently, the EQE of doped ZnO 1D-NS photodetectors can be calculated using

η = R×

hc qλ



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Table 3. The photoresponse measured conditions and results of various doping ZnO 1D-NS photodetectors(MSM: metal–semiconductor–metal). Dopant Multiple/Single Ohmic/Schottky contact 1D-NS Heterojunction/ Homojunction

Light source

Bias [V]

Dark current

Photocurrent Iphoto/Idark ratio

Rise time

Decay time

External Ref. quantum efficiency

Al

Multiple

MSM-Ohmic

365 nm, 60 µW/cm2

1

0.1 nA

62 µA

6.2×105

0.1 s

20 s

[140]

Al

Multiple

MSM-Ohmic

365 nm

3

110 nA

5.3 µA

48.1

0.9 s

1.1 s

[146]

Al

Single

unknown

254 nm

5

1.94 µA

68 µA

35.0

4s

[147]

Al

Multiple

Heterojunction

254 nm

5

73 nA

10.7 µA

146.6

5s

[148]

As

Single

Homojunction-Schottky

30 W Xe lamp

0

24.4 pA

1.1 µA

∼45

C

Multiple

MSM-Ohmic

334 nm, 475 nm, 550 nm

1

0.31 nA 0.41 nA, 0.45 1.3 1.5 2.3 120 ms nA, 0.70 nA

C

Multiple

MSM-Ohmic

475 nm, 550 nm

1

0.77 nA 1.80 nA, 1.18 nA

Cu

Multiple

unknown

UV lamp

unknown

82 pA

23 µA

5.6×104

2.7 s

118 s

[151]

Cu

Single

MSM-Schottky

365 nm, 10 mW/cm2

10

10 pA

100 nA

1.0×104

9s

17 s

[152]

Cu

Multiple


365 nm, 0.25 mW/cm2

–10

0.465 µA

6.32 µA

13.6

Ga

Single

MSM-Ohmic

254 nm

1

105.5 µA

133.8 µA

1.3

H

Single

MSM-Schottky

300 nm, 1 mW/ cm2364 nm, 3 mW/ cm2640 nm, 0.3 mW/ cm2

–10

10 nA

In

Multiple

MSM

300 W Xe lamp

1

0.7 µA

50 µA

71.4

[154]

In

Single

MSM-Ohmic

360 nm

5

19 nA

458.5 nA

24.1

[136]

Li

Multiple

MSM-Schottky

30 W Xe lamp

5

0.484 mA

0.65 mA

1.3

[155]

La

Multiple

MSM-Ohmic

365 nm, 0.25 mW/cm2

10

0.304 µA

0.785 µA

2.6

[94]

Mg

Multiple

MSM-Schottky

365 nm

1

1.54 nA

8 µA

5.2

24.5 s

18.4 s

[156]

Mn

Single

MSM-Ohmic

532 nm, 400 mW/cm2

8

7 pA

0.34 nA

48.6

200 ms 250 ms

∼1200% [157]

S

Single

Homojunction-Ohmic

325 nm

3

0.04 nA

1.3 µA

3.25×104

2s 100 ms

[104] 80 ms

2.3 1.5

[149] [150]

[84] 0.53 s

14 s

400 nA, 40nA, 40.0, 4.0 .5 15nA

[153] [139]

74 s

[138]

Sb

Single

MSM-Schottky

325 nm, 5 mW

unknown

1 nA

23 nA

23.0

Sb

Multiple


UV lamp

–20

4.1 mA

7.2 mA

1.8.0

[158]

Sb

Single

MSM-Ohmic

254 nm

0.5

10.3 µA

11.6 µA

1.1

[159]

Sn

Multiple

MSM-Ohmic

UV lamp

5

0.3 µA

18 µA

60

[160]

where η denotes the quantum efficiency, R denotes the measured responsivity, q denotes the electron charge, λ denotes the incident light wavelength, h denotes the Planck constant, and c denotes the speed of light. It should be noted that these EQE results (shown in Table 3) were acquired without any optimization process. A much larger responsivity and EQE should be achieved for doped ZnO 1D-NSs by optimizing the fabrication parameters, such as the growth conditions and the design of the electrodes.

4.1. High Photocurrent Gain in Doped ZnO 1D-NS Photodetectors A higher photocurrent gain has been achieved by different methods, such as decreasing the dark current and increasing the photocurrent and absorption cross section of the material. S. Sarkar et al. reported the growth of a ZnO seed layer on a glass substrate via a DC magnetron sputtering technique, and a 2% Cu-doped ZnO nanorod array on a substrate via a hydrothermal method.[151] The UV photodetectors used


[141]

the multiple 1D-NSs method. Figure 10(a) and (b) reveal the photocurrent spectra and UV on/off functional switching of the Cu-doped ZnO and pure ZnO 1D-NSs.[151] The photocurrent spectra allow sufficient time to show the de-trapping yield, and this and an X-ray photoelectron spectroscopy (XPS) spectrum of a 2% Cu-doped sample are shown in Figure 10(c) and (d), respectively. It can be seen that the UV photocurrent gain of these Cu-doped ZnO nanorods could reach 2.8 × 105 (Iphoto = 23 µA/Idark = 85 pA), which is around 2 orders of magnitude larger than that observed from the pure ZnO nanorods. The multiple Cu-doped ZnO 1D-NSs had an ultra-low current of 85 pA, which is very helpful to increase the photocurrent gain. The photocurrent growth and decay time constants are 2.7 s and 118 s, respectively. The copper dopant had a 1.65 eV trap status, attributed to defects of the type [{CuZn+(3d10)}−—Zni+ (4s1)]0. The defect trapping density increased with increasing Cu dopant concentration. In the dark, a significant amount of carriers were trapped. These carriers could be activated under UV illumination. Thus, a higher Cu dopant concentration could result in a higher defect trapping density and higher photocurrent gain.


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Figure 9. Temperature-dependent PL spectra for the (a) As-doped, (b) Ag-doped, (c) P-doped and (d) Na-doped ZnO 1D-NSs. (e) Reproduced with permission.[121] Copyright 2011, Wiley-VCH. (f) Reproduced with permission.[91] Copyright 2011, Springer. (g) Reproduced with permission.[102] Copyright 2007, ACS. (h) Reproduced with permission.[110] Copyright 2010, ACS.

Figure 10. (a) Photocurrent spectra and (b) photocurrent decay of the pure ZnO and ZnO:Cu samples. (c) The photocurrent spectra of the pure ZnO and ZnO:Cu samples allowing sufficient time for detrapping yield (d) XPS spectrum of 2% Cu doped ZnO sample. Reproduced with permission.[151] Copyright 2013, AIP. small 2014, 10, No. 22, 4562–4585



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Figure 11. (a) An SEM image of ZnO:Cu NWs; the inset shows an optical microscopy image of a ZnO NW suspended across the gap between two terminals. (b) I–V characteristics of ZnO:Cu NWs. The measurements have been performed in the dark and under 365 nm and white-light irradiation. Reproduced with permission.[152] Copyright 2008, Wiley-VCH.

Figure 11(a) and (b) display a side-view SEM image of Cudoped ZnO NWs and its I–V characteristics measured in the dark, under UV (365 nm), and under white-light irradiation.[152] The Cu-doped ZnO NWs were grown by a vapor phase transport method. The UV photodetectors were fabricated using the single 1D-NS method. In the dark, the current is relatively small (10 pA at bias 10 V). The UV photocurrent gain is around 1.0 × 104 (Iphoto = 100 nA/Idark = 10 pA). The wavelength and power density of the UV lamp were 365 nm and 10 mW/cm2, respectively. The growth/decay time of the photocurrent was observed and the time constants are 9 s and 17 s, respectively. Figure 12(a) and (b) reveal a top view SEM image of single S-doped ZnO NWs, and the I–V curves are measured with UV on/off switching.[138] The S-doped ZnO NWs were also grown with vapor phase transport technology. UV photodetectors were fabricated using single 1D-NSs. The UV photocurrent gain is about 3.25 × 104 (Iphoto = 1.3 µA/Idark = 40 pA). The photocurrent growth and decay time constants are 2 s and 74 s, respectively. Figure 13(a) and (b) display an SEM image of an Aldoped ZnO NW photodetector,and its I-V curves, measured under UV lamp on/off switching (365 nm, ultra-low power density 60 µW/cm2).[140] The Al-doped ZnO NWs were grown via vapor phase transport technology at 650 °C. The UV photocurrent gain is record high ∼6.2 × 105 (Iphoto = 62 µA/Idark = 0.1 nA). The dynamic responses of the Al-doped ZnO NW

photodetector were measured under low power density UV exposure (60 µW/cm2) with 0.1∼1 V applied bias, as shown in Figure 13(c). The photo current growth and decay time constants are 0.1 s and 20 s, respectively. Figure 13(d) shows that spectral response curve of Al doped ZnO NWs photodetector measured at 1V and 2 V applied bias. The cutoff position of Al doped ZnO NWs photodetector was at around 370 nm, which is corresponding to ZnO bandgap. Because the photocurrent gain is calculated from the photocurrent vs. dark current ratio, these high photocurrent gain photodetectors have a very low dark current of 10 pA∼100 pA for increasing photocurrent gain. The dark current is related to the diameter, quantity, and conductivity of the 1D-NSs. These photocurrent gain reports are difficult to compare directly as the wavelengths and power densities of the light sources are not consistent. The photocurrent growth and decay time constants are around 0.1∼2 s and 17∼118 s, respectively, which is very slow and difficult to apply in electronic devices. The slow UV response is mainly ascribed to ZnO surface photodesorption and dark absorption of water molecules and molecular O2− species. High photocurrent gain has been shown for single ZnO 1D-NS photodetectors and a few multiple ZnO 1D-NS photodetectors, such as in Figure 6(a). The photocurrent gain is determined from the photocurrent and dark current ratio.

Figure 12. (a) SEM image of a back-gated ZnO:P NW (below) and its schematic structure (above). (b) The photocurrent response of the p–n homojunction diode with time-dependent UV (325 nm) exposure at a –3 V bias. Reproduced with permission.[138] Copyright 2009, ACS.



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Photocurrent gain decreases as the dark current increases. The multiple ZnO 1D-NS photodetector structure shown in Figure 6(b) has trouble overcoming the leakage current problem, which will increase the dark current by up to 1∼3 orders of magnitude and cause an overall performance degradation. 4.2. Humidity and Water Effects on Doped ZnO 1D-NS Photodetectors The conductivity of doed ZnO 1D-NSs is influenced by O2− species with environmental parameters, such as temperature, oxygen ions, reducing gas, and relative humidity (RH). It is difficult to avoid the humidity in the atmosphere. Under conditions of high RH, the density of water molecules is very high, which affects the photoresponse of 1D-NSs. It has been reported that the UV photoresponse of n-type ZnO is reduced by raising the RH. ZnO 1D-NSs are naturally n-type materials, which suggests an n-type doping reaction under high RH. Figure 14(a) and (b) show the SEM image of a multiple ZnO NW photodetector and its photoresponse under different RHs with on/off UV (365 nm, 1.4 mW/cm2).[161] The ZnO NWs were grown with a Au catalyst via the vapor phase transport method. In a high RH environment, the decay times of the ZnO NWs decreased drastically (from 106 s to 1.2 s), which indicates that water molecules were adsorbed onto the NW surface, changing the density and lifetime of the carriers. Figure 14(b) reveals how the dark current level increases with increasing RH: under high RH, the photocurrent was lower, as compared to that measured under low RH. Figure 15

displays a multiple ZnO nanofiber photodetector structure, the UV photoresponse, and RH relationship.[162] The diameter ofthese ZnO nanofibers was ∼200 nm. They were synthesized by electrospinning and calcination at 600 °C. Under high RH, the photocurrent gain decreased (Iphoto↓ / Idark↑) and photoresponse response speed increased, which is consistent with the results shown in Figure 14. Under high RH, the p-type ZnO exhibits a better photoresponse performance than n-type ZnO. This should be attributed to the fact that holes are the majority carriers in p-type semiconductors, instead of electrons. Figure 16(a) depicts a p-type La-doped ZnO 1D-NS photodetector structure.[94] Figure 16(b) shows the I-V curves of the p-ZnO:La sample measured in different RHs. The RH and temperature of the laboratory environment were 40% and 25 °C, respectively. In the dark, a higher RH reduced the current of p-ZnO:La 1D-NSs, which was reduced slowly with RH > 70%. When the applied bias was at 10 V, the minimum current ∼9.72 × 10−8 A occurred with the highest (98%) RH and attenuation ∼ 68% compared with the current measured at 40% RH. The highest RH indicated that a large amount of water vapor changed the conductivity of 1D-NSs. Figure 16(c) shows various RH I-V properties of p-ZnO:La 1D-NSs under UV exposure (365 nm) The photocurrent of p-ZnO:La 1D-NSs was increased (7.85 × 10−7∼4.68 × 10−6A) with increasing RH (40∼98%). Figure 16(d) reveals the UV photocurrent (IUV) to dark current (IDark) ratio with different RHs. The photocurrent gain (IUV/IDark ratio) increased with increasing RH. The maximum photocurrent gain was around 47.73 with 98% RH. The minimum photocurrent gain of ∼2.56 was found at

Figure 13. (a) SEM images of an Al-doped ZnO NW photodetector. (b) The I-V curve of the device was measured in the dark and under UV exposure. (c) Photocurrent characteristics at various biases under on/off UV exposure. (d) Photoresponsivity spectrum of the photodetector under UV exposure at 1 V and 2 V bias. Reproduced with permission.[140] Copyright 2012, Wiley-VCH. small 2014, 10, No. 22, 4562–4585



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Figure 14. (a) Optical microscope and SEM images of the ZnO nanowire UV detector. (b) The photoresponses of the ZnO NW UV detector with time at 5 V bias under different RHs. UV light (365 nm, power density 1.4 mW/cm2) was on/off switched. Reproduced with permission.[161] Copyright 2009, AIP.

40% RH and is lower than in previous reports. In general, the doped ZnO 1D-NS photodetectors were measured under atmospheric conditions (∼40%). Figures 17 schematically depicts p-type and n-type ZnO 1D-NS under UV exposure and high RH.[94] The water

molecules can displace the O2− molecules and liberate free electrons [2H2O(ads.) + O2−(ads.) → 2H2O2 + e−].[162] The surfaces of the ZnO 1D-NS were coated with a lot of water molecules and a few O2− molecules. By removing the O2− molecules, the released electrons will recombine with the

Figure 15. (a) The schematic depicts the structure of an electrospun ZnO nanofiber photodetector. (b) Current of device measured at 40 V under different RHs. (c) The photoresponse was measured in the dark or under UV exposure with different RHs. (d) The rise and decay time of photocurrents under different RH conditions. Reproduced with permission.[162] Copyright 2013, RSC.



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Figure 16. (a) Schematic diagram of p-type ZnO:La NW UV photodetector. I–V curve of the ZnO:La sample measured (b) in the dark and (c) under UV with different RHs. (d) UV photocurrent (IUV) to dark current (Idark) ratio with various RHs. Reproduced with permission.[94] Copyright 2013, ACS

Figure 17. Schematic depicts NWs surfaces and its band diagrams under UV exposure at high RH. Reproduced with permission.[94] Copyright 2013, ACS. small 2014, 10, No. 22, 4562–4585



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holes in the p-type ZnO 1D-NS, resulting in a smaller conductivity. In contrast, the conductivity of n-type ZnO 1D-NS will become larger after the removal of O2− molecules due to the increased number of free electrons. The ZnO 1D-NSs absorb UV photons to produce a large amount of electron– hole pairs. The UV light-generated holes oxidize adsorbed O2− molecules, which become oxygen [O2−(ads.) + h+ → O2(gas)]. The O2− molecules disappear quickly under UV exposure. Figure 18(a) depicts water dropped onto p-ZnO:La 1D-NSs and the I–V characteristics measured from the p-ZnO:La 1D-NS photodetector under UV exposure.[94] In de-ionized (DI) water, the contrast ratio (Iphoto/Idark ratio) is ∼212.1, which is much larger than that measured with 40% RH ∼47.73. This should be attributed to the fact that water molecules displace the O2− species and thus reduce the dark hole concentration. Thus, water could minimize the dark current of p-type ZnO 1D-NSs. In contrast, water should enhance the dark current for n-type ZnO 1D-NSs. Indeed, it has been reported that the dark current could be enhanced by ∼10 times with increased RH. This could also result in reduced contrast ratios for n-type ZnO 1D-NS photodetectors.[161,162] The water and O2− molecules on the ZnO 1D-NS surface could react with UV light to generate e−, h+, oxidation–reduction reactions, and free radicals. The water molecule and h+ react to produce an •OH on the surface of ZnO.[163] The e− and h+ generate •OH species, which enhance the conductivity of water molecules and constitute a photocatalytic layer. The conductivity of the p-ZnO 1D-NSs was enhanced by this photocatalytic efficiency at high RH. The photocatalytic activities of doped ZnO 1D-NSs have also been suggested

to be a property of TiO2 NSs. Some past studies used these photocatalytic activities to assemble photoelectrochemical cells for photoelectrolysis. The photocatalytic reactions are as follows:[163] ZnO + hv → ZnO ( h + + e − )

h + + H 2 O → H + + •OH

h + + OH − → •OH

2 h + + 2H 2 O → 2H + + H 2 O 2

H 2 O 2 + e − → •OH + OH − The critical dopant material verified to enhance photocatalytic properties is La.[164,165] Some rare earth dopants provided higher photocatalytic activities to produce more e− and h+ exchanges in the water. Figure 18(b) and (c) show how water molecules act as photocatalytic layer under UV exposure. The photocatalytic layer under low RH was very thin and distributed discontinuously, which causes the lower photocurrent. A thicker photocatalytic layer exists when

Figure 18. (a) I-V curve of photodetector measured in DI water under UV exposure. Schematics depict the p-ZnO:La NW at (b) low RH and (c) water under UV exposure. Reproduced with permission.[94] Copyright 2013, ACS



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water covers the entire sample, providing more electron transfer paths, which further enhances the conductivity. The photocurrent of the fully wet sample was enhanced around 2 orders of magnitude, as compared to the low RH sample. Some literature has also discussed using a passivating coating of PMMA or Al2O3 or SOG as a barrier to water vapor and oxygen.[166–168] These photodetectors had a higher photoresponse speed and more stable operation, but higher dark current and lower photocurrent gain due to the oxygen being isolated (it cannot get through the barrier layer), and therefore there is no depletion region on the ZnO 1D-NS surface. A few groups have used atomic layer deposition (ALD) to improve the density of the insulation layer for better passivation.

4.3. Homojunctions of Doped ZnO 1D-NS Photodetectors In the past decades, many articles have discussed the assembly of heterojunction structures, including n-type doped ZnO 1D-NSs synthesized on p-type substrates (such as Si,[169] GaN,[170] SiC,[171] NiO,[172] and p-n-type core–shell 1D-NSs structures.[173–175] Among the p-type substrates, GaN has attracted much attention due to the fact that GaN and ZnO exhibit the same wurtzite structure with similar lattice constants. The n-type ZnO prepared on a p-type GaN substrate has become the most commonly used combination to achieve n-type ZnO 1D NS-based heterojunction structures. Some articles have reported heterojunctions fabricated with a p–n core–shell 1D-NS structure, fabricated using a p-type material covering the n-type ZnO 1D-NS. These p-type shell layers have included various organic (such as P3HT,[176] MEH-PPV,[177] and PFO[178]) and inorganic materials (such as CuO,[179] Cu2O,[175] PbS,[180] GaP[181] and NiO[182]). As pure ZnO 1D-NS is not within the scope of this article, it will not be discussed in further detail. Some homojunction reports have described the epitaxial growth of p-type and n-type ZnO thin film structures.[121,158,183] A few articles have also used p-type P,[38,184] As,[185] and Cu[84] dopants to achieve homojunction NWs.

Figure 19(a) reveals the I–V characteristics of the ZnO p–n homojunction. The vapor phase transport method was applied to synthesize p-ZnO:Sb NWs on an n-ZnO film.[158] The clear rectifying behavior can be observed under on/offswitched UV illumination. The possible formation of the graded p–n junction due to the impurity diffusion during NWs growth and the formation of the metal/NW schottky contact are the reasons for the small rectification ratio of the p–n homojunction. The UV light source is an Oriel Xe arc lamp. Figure 19(b) shows photocurrent spectra of the homojunction device operated under various reverse biases. It can be seen that the largest photocurrent occurred at a light wavelength ∼380 nm, which corresponds to the ZnO bandgap 3.26 eV. These photocurrents were enhanced with increasing reverse bias (–2, –4, and –6 V). There is a small peak at 570 nm (2.17 eV) in the spectra due to the presence of deep levels in ZnO. Figure 20(a) shows an SEM image of p–n homojunction NWs, which were synthesized on a ZnO/Si substrate by the vapor phase transport method.[104] The structure of the p–n homojunction NWs consists of a segment of n-type material and a segment of p-type material connected together. Figure 20(b) plots the low-temperature PL properties of As-doped p-type ZnO NWs and pure ZnO NWs. These PL spectra were measured at 10 K using a He–Cd laser (325 nm) as the excitation source. The pure ZnO NW PL measurements display a peak at ∼3.362 eV, which is from a neutral donor-bound exciton (D0, X). Another very weak peak is at 3.195 eV (donor–acceptor pair transition). The existence of a PL curve demonstrates that the pure ZnO NWs have a high crystalline quality. The PL measurements of p-ZnO:As NWs reveal a strong peak at ∼3.350 eV, which is an exciton bound to a neutral acceptor (A0, X). Another weaker peak indicates free-electron-to-bound-hole transitions (e-, A0) ∼3.320 eV.[186,187] The peaks corresponding to LO-phonon replicas (A0, X) and donor–acceptor pair (DAP) transitions are located at ∼3.280 eV and ∼3.195 eV, respectively. The strong (A0, X) PL peak ofthe ZnO:As NWs verifies expressly that the PL curve exists due to the As dopants in the ZnO. Figure 20(c) shows an optical

Figure 19. (a) I-V curve of the ZnO NW/ZnO film device in the dark and under UV illumination. (b) Photocurrent spectra under different reverse biases. Reproduced with permission.[158] Copyright 2011, AIP small 2014, 10, No. 22, 4562–4585



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Figure 20. (a) SEM image of p–n ZnO homojunction NWs synthesized on a ZnO/Si substrate. (b) The PL was measured at 10 K. (c) Optical microscope image of a ZnO homojunction device. (d) I-V curve of a single ZnO homojunction NW in the dark and under 30 W Xe-lamp illumination. Reproduced with permission.[104] Copyright 2012, IOP

microscope image of a single NW connected to 4 electrodes. The I-V characteristics of the ZnO p–n homojunction NW were measured both in the dark and under light exposure, as shown in Figure 20(d). The I-V curve measured from the device reveals a standard rectification property owing to its ZnO p–n homojunction structure. This also suggests that the NWs form good ohmic contact with the electrodes. The turn-on voltage, ∼0.25 V, measured from the ZnO p–n homojunction single NW device is lower than that reported previously (0.8∼1 V) from a p-ZnO:P/n-ZnO homojunction NW.[38,186] The photovoltaic properties of the ZnO homojunction NWs were also measured and evaluated under a 30 W Xe-lamp (light intensity ∼0.03 kW/cm2). Under incident light exposure, it was found that the photocurrent was higher than the dark current by ∼45 times. The inset in Figure 20(d) shows its photovoltaic properties. It can be seen that the open-circuit voltage (Voc), short-circuit current (Isc), and fill factor (FF) of the fabricated device are ∼0.21 V, ∼1.1 µA and ∼35%, respectively. The output power of the p-ZnO:P/n-ZnO homojunction device is estimated to


be around ∼82 nW, which is similar to that reported from a ZnO NW/GaN film heterojunction.[188] Homojunction NWs of p-ZnO:Cu/n-ZnO were also synthesized by the vapor phase transport method with 1700 °C hotwires (as the furnace tube is heated to 600 °C, the Ta hotwire temperature is increased to around 1700 °C with a high-output power supply (~17 A, 220 V) in the same quartz tube).[84] The FE-SEM images reveal that high-density NWs were grown on ZnO/glass substrate, as shown in Figure 21(a). The inset shows an enlarged image. Figure 21(b) plots the I–V characteristics of the device, which were measured under UV illumination (365 nm, power density 0.25 mW/cm2) and in the dark. The rectifying I–V curves shown in the figure indicate the formation of a p–n junction diode. It was found that the photocurrent was 6.06 times larger than the dark current under –10 V reverse bias. Figure 21(c) shows the dynamic response of the photocurrent measured from the fabricated homojunction NW photodetector. It can be seen that the response is stable, with good reproducibility. The rise time and decay time are also shorter than those reported previously.[138]


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The built-in voltage Vbi is expressed as

Vbi =

kT ⎛ N A N D ⎞ ln ⎜ q ⎝ ni2 ⎟⎠

where k denotes the Boltzmann constant, T denotes the absolute temperature, q denotes the electron charge, NA and ND are the number of ionized donors and acceptors, respectively, and ni denotes the intrinsic carrier concentration. The intrinsic carrier density of ZnO is approximately ∼106 cm−3 at room temperature. The NA and ND were ∼1016 cm−3 and 1015 cm−3, respectively, as determined from Hall measurements. Knowing these values, it was found that Vbi was about 1.093 eV. The equation for the depletion width, W, is: ⎡ 2ε r ε 0 ⎛ 1 ⎤ 1 ⎞ + W ≈⎢ ⎜ ⎟ (Vbi − Vapplied )⎥ ⎣ q ⎝ N A ND ⎠ ⎦

1/ 2

where εr denotes ZnO dielectric permittivity ∼8.86, and Vapplied denotes the applied bias. Thus, the depletion width

W is around 1.086 µm, 2.563 µm, and 3.458 µm when applied bias is 0, –5, and –10 V, respectively. Figure 21(d) schematically depicts a band diagram of the homojunction NWs with a reverse bias of -5 V. Under UV illumination, photogenerated holes will oxidize the O2− molecules adsorbed onto the NW surface [O2−(ads.) + h+ → O2(gas)]. The depletion layer at the n-type ZnO surface will disappear due to the removal of the O2− molecules. Furthermore, the UV-excited electrons will jump from EV or ET to EC. In general, the equation giving the semiconductor conductivity, σ, is

σ = nqμ e + pqμ p where n represents the electron density, p is the hole density; q is the electron charge; µe represents the electron mobility, and µp represents the hole mobility. Some photoexcited electrons in the p-ZnO:Cu region pass through the p–n homojunction to the n-ZnO region when excited by UV light. However, improving and increasing the photocurrent gain of the above-mentioned doped ZnO 1D-NSs photodetectors has been attempted via various methods. These methods were

Figure 21. (a) FESEM images of NWs. The inset is the magnified image. (b) I-V curve of the NWs measured in the dark and under UV exposure. (c) Transient response of the NW with UV excitation switched on/off. (d) Band diagram of the homojunction NWs. Reproduced with permission.[84] Copyright 2014, ACS small 2014, 10, No. 22, 4562–4585



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to use various dopants in ZnO NSs for decreasing the dark current or increasing the photocurrent. The dark current of MSM-Ohmic and MSM-Schottky photodetectors structure is determined and controlled by semiconductor conductivity, which can be fine tuned by including various dopants. In general, the p-type doping decreases the dark current and increases the resistance of ZnO 1D-NSs, due to the fact that ZnO is originally an n-type material and does not easily change to p-type. Doping acceptors into ZnO 1D-NSs will increase the impedance and cause a lower dark current. Irrespective of doping, UV light incident on pure ZnO or doped ZnO 1D-NSs will produce a large amount electron–hole pairs: for the same UV illumination intensity, the photocurrent of doped ZnO 1D-NSs should be similar to that of pure ZnO 1D-NSs because their electron–hole pair amounts are very close. The photocurrent gain of p-type doped ZnO 1D-NSs is therefore increased by decreasing the dark current. However, n-type doped ZnO 1D-NSs usually have a lot free electrons contributed by donors, which causes the dark current to increase and decreases the photocurrent gain. A few studies have described the synthesis of an ultra-narrow-diameter n-type doped ZnO 1D-NSs to an increase in the surface-tovolume ratio, increasing the depletion region and reducing the dark current.[132] Some articles have reported that water molecules have been used as dopants to achieve higher photocatalytic efficiency and increase the photocurrent.[94] The p–n homojunction, p–n heterojunction and MSMSchottky of doped ZnO 1D-NS photodetectors uses a reverse bias to increase the p–n interface barrier or Schottky barrier, which induces a lower dark current (leakage current) and a high photocurrent gain. When UV light is absorbed by the surface of doped ZnO 1D-NSs, the excited NSs produce a large amount of electron–hole pairs to reduce the barrier and resistance for increasing the current. No matter what kind of pattern is used, doped ZnO 1D-NS photodetectors must minimize the dark current to achieve a maximum photocurrent gain.

5. Conclusion and Outlook This comprehensive review has assessed some important progress in doped ZnO 1D-NSs and the properties of photodetectors made from them. The synthesis of doped ZnO 1D-NSs containing various dopants have recently been via the vapor phase transport and hydrothermal method, the conitions of which are summarized for several important photodetectors in Tables 1 and 2. The fabrication process for single and multiple doped ZnO 1D-NS photodetectors have been illustrated and described in detail. Table 3 listed the measured photoresponse conditions for doped ZnO 1D-NSs containing various dopants. These photoresponse results demonstrate that doped ZnO 1D-NSs are really outstanding candidates for photodetectors owing to their high-performance properties. The higher photocurrent gain in some of these photodetectors is attributed to a very low dark current (

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