Nitrogen effect on spin-coated ZnO-based p–n homojunctions: structural,optical and electrical characteristics

In this work, ZnO:Al–N/ZnO:Al and ZnO:Ag–N/ZnO:Al homojunctions were deposited by means of spin coating method using precursors obtained by sol gel chemistry. The optical, structural and electrical properties of spin coated undoped and M-doped ZnO thin films (M?=?Al, Ag–N and Al–N) using ammonium hydroxide as a nitrogen source are reported. The films showed the wurtzite type structure with a c-axis (002) preferential orientation. The films showed a surface morphology consisting of wrinkles, which were constituted of nanocrystals in the range of ～?20 nm. The thin films were highly transparent in the visible region of the electromagnetic spectrum. The optical band gap of the films was close to 3.30 eV. Hall Effect measurements indicated that undoped and Al doped ZnO thin films showed an n-type conductivity, whereas ZnO:Al–N and ZnO:Ag–N thin films exhibited p-type conductivity, probably related to the formation of dual acceptor complexes related to nitrogen. Two types of p–n homojunctions (ZnO:Al–N/ZnO:Al and ZnO:Ag–N/ZnO:Al) were fabricated by means of sol–gel spin-coating method. In both cases, a rectifying behavior was observed, as revealed by current–voltage measurements.     

注入剂量对离子注入法制备Al-N共掺杂P型ZnO的影响

采用射频磁控共溅射的方法制备出ZnO∶Al薄膜,以NO和O2为源气体(O2/O2+NO=75%)、等离子体浸没离子注入(PIII)方法在不同的工艺条件下得到了不同N+注入剂量的ZnO∶Al∶N薄膜,并在N2氛围下对样品进行了850℃退火处理。通过XRD图谱、霍尔效应(Hall)测试结果、光致发光光谱(PL)、紫外-可见光透射光谱等对样品的结构和性能进行了分析,着重研究了N+注入剂量对ZnO∶Al∶N薄膜性质的影响。结果表明,注入剂量控制在1015cm-2量级时,N可以通过占据O空位和替换O原子形成NO并与Al和Zn成键,对于ZnO薄膜实现p型反转是很关键的。实现p型反转的ZnO∶Al∶N薄膜载流子浓度可达2.16×1016cm-3,电阻率为8.90Ω.cm,霍耳迁移率为32.4cm2/V.s。     

Optical and electronic properties of highly stable and textured hydrogenated ZnO:Al thin films

We have experimentally investigated the effects of hydrogen-annealing on the structural, electrical, and optical properties of Al-doped ZnO (ZnO:Al) thin films prepared by RF magnetron sputtering at room temperature. From the X-ray diffraction observations, the orientation of ZnO:Al films was found to be a c-axis in the hexagonal structure. We found that intentionally incorporated hydrogen plays an important role in n-type conduction as a donor, improving free carrier concentration and electrical stability. We simultaneously obtained improved optical transmission and enhanced absorption edge of the ZnO:Al film due to hydrogen-annealing. Our experimental data suggest the hydrogen-annealing process as an important role in the enhancement of electrical and optical properties, which is promising as a back reflector material for thin-film solar cells.     

Effect of annealing on the formation of p-type conductivity in nano-Zn0.92Mn0.08O:N films

P-type conductive Mn-N co-doped ZnO films were prepared by annealing N(+)-implanted Zn0.92Mn0.08O films in a N2 ambient. Effect of the annealing on the structural, surface morphological, electrical and local chemical states of the films were investigated by X-ray diffraction (XRD), high-resolution field-emission scanning electron microscopy (FE-SEM), Hall-effect and X-ray photoelectron spectroscopy (XPS) measurements, respectively. The results indicate that all the samples were single phase and well oriented along the c-axis. The as-implanted samples were n-type semiconductors, while after thermal annealing at 650 degrees C ranging from 10 to 30 minutes, they were converted to p-type conductivity with the hole concentration of 10(16)-10(17) cm(-3). But with further increasing the annealing time or the temperature, it was observed that the p-type conductivity decreased and ultimately reverted to n-type conductivity again. The change of conductive type may be ascribed to the local chemical states evolution of nitrogen in the process of thermal annealing.     

P-type ZnO thin film deposited by spray pyrolysis technique: The effect of solution concentration

The aim of this research is to study the role of concentration variations on precursor solution of nitrogen doped ZnO (ZnO:N) thin films which has been prepared by spray pyrolysis technique. SEM micrographs show that ZnO:N films in 0.1 ML concentration have a mono-disperse surface with nano-spheres of 50 nm in diameter. In higher molarities the nano-spheres agglomerate leading to particle formation. For 0.4 ML concentrations this change is observed, where plume like particles are seen over the surface of ZnO:N thin film. This change corresponds also to changes observed in the XRD spectra, where crystal orientation of ZnO:N thin films changes from (002) to (100). All of the ZnO:N thin films have kept their sharp ultra violet absorption edge, but the transparency in visible spectra region decreases as the molarities in precursor solution increase. Photoluminescence spectra at room temperature revealed emissions at 2.33 eV, 2.54 eV and 3.16 eV that can be attributed to the presence of nitrogen in ZnO structure. We also observe that all samples analyzed show a p-type Hall effect behavior, and that as the molarities in the precursor solution increase, the electrical resistivity of the films decreases, due to an enhancement of free carriers, while the mobility decreases. These data prove the capability of spray pyrolysis as a viable technique in preparing p-type TCO materials and so, fully transparent CMOS-like devices.     

Deposition of Na–N dual acceptor doped p-type ZnO thin films and fabrication of p-ZnO:(Na,N)/n-ZnO:Eu homojunction

Sodium and nitrogen dual acceptor doped p-type ZnO (ZnO:(Na, N)) films have been prepared by spray pyrolysis technique at a substrate temperature of 623 K. The ZnO:(Na, N) films are grown at a fixed N doping concentration of 2 at.% and varying the nominal Na doping concentration from 0 to 8 at.%. The XRD results show that all the ZnO:(Na, N) films exhibited (0 0 2) preferential orientation. The EDX and elemental mapping analysis shows the presence and distribution of Zn, O, Na and N in the deposited films. The Hall measurement results demonstrate that the Na–N dual acceptor doped ZnO films show excellent p-type conduction. The p-type ZnO:(Na, N) films with comparatively low resistivity of 5.60 × 10⁻² Ω cm and relatively high carrier concentration of 3.15 × 10¹⁸ cm⁻³ are obtained at 6 at.%. ZnO based homojunction is fabricated by depositing n-type layer (Eu doped ZnO) grown over the p-type layer ZnO:(Na, N). The current–voltage (I–V) characteristics measured from the two-layer structure show typical rectifying characteristics of p-n junction with a low turn on voltage of about 1.69 V. The ZnO:(Na, N) films exhibit a high transmittance (about >90%) and the average reflectance is 8.9% in the visible region. PL measurement shows near-band-edge (NBE) emission and deep-level (DL) emission in the ZnO:(Na, N) thin films.     

p-Type ZnO nanowire arrays

Well-aligned ZnO nanowire (NW) arrays with durable and reproducible p-type conductivity were synthesized on alpha-sapphire substrates by using N2O as a dopant source via vapor-liquid-solid growth. The nitrogen-doped ZnO NWs are single-crystalline and grown predominantly along the [110] direction, in contrast to the [001] direction of undoped ZnO NWs. Electrical transport measurements reveal that the nondoped ZnO NWs exhibit n-type conductivity, whereas the nitrogen-doped ZnO NWs show compensated highly resistive n-type and finally p-type conductivity upon increasing N2O ratio in the reaction atmosphere. The electrical properties of p-type ZnO NWs are stable and reproducible with a hole concentration of (1-2) x 10(18) cm(-3) and a field-effect mobility of 10-17 cm2 V(-2) s(-1). Surface adsorptions have a significant effect on the transport properties of NWs. Temperature-dependent PL spectra of N-doped ZnO NWs show acceptor-bound-exciton emission, which corroborates the p-type conductivity. The realization of p-type ZnO NWs with durable and controlled transport properties is important for fabrication of nanoscale electronic and optoelectronic devices.     

Comparative photoluminescence study on p-type and n-type ZnO films codoped by nitrogen and aluminium

We report on the temperature-dependent photoluminescence (PL) properties of n-type and p-type ZnO films codoped with N and Al. For the n-type film, the dominant emission at low temperature is exciton bound to neutral donors, while for the p-type film it is exciton bound to neutral acceptor at 3.33 eV. Four defect or impurity levels, including N acceptor, residual acceptor, and two doping-induced unknown deep acceptors, were identified. The energy level of the N acceptor was determined to be 0.23 eV. Excitation energy dependence of the PL was also investigated. It was found that at high excitation energy, the formation of exciton was suppressed by the formation of D⁺A⁻eh complexes.     

Nitrogen doped ZnO thin films prepared by photo-assisted metal-organic chemical vapor deposition

Nitrogen-doped ZnO (ZnO:N) films were prepared by photo-assisted metal-organic chemical vapor deposition technique using NH3 as N doping source. The effects of in-situ light irradiation on the properties of ZnO:N films were studied by Hall measurements, X-ray diffraction, Raman scattering, and X-ray photoelectron spectroscopy. The results show that stable p-type ZnO:N films with a hole concentration of 3.61 x 10(17) cm(-3) was successfully achieved. Moreover, introducing proper in-situ light irradiation during the growth process can not only effectively improve the crystalline quality of ZnO films, but also enhance the activity of (N)o (N occupies O site) acceptors by removing the undesirable hydrogen atoms from ZnO:N films. Both effects are benefit for the p-type conductivity formation. Our results indicate that photo-assisted MOCVD maybe an effective technology to realize device-quality p-type ZnO:N films.     

Effect of annealing temperature on ZnO:Al/p-Si heterojunctions

Al-doped, zinc oxide (ZnO:Al) films with a 1.2 at.% Al concentration were deposited on p-type silicon wafers using a sol-gel dip coating technique to produce a ZnO:Al/p-Si heterojunction. Following deposition and subsequent drying processes, the films were annealed in vacuum at five different temperatures between 550 and 900 °C for 1 h. The resistivity of the films decreased with increasing annealing temperature, and an annealing temperature of 700 °C provided controlled current flow through the ZnO:Al/p-Si heterojunction up to 20 V. The ZnO:Al film deposited on a p-type silicon wafer with 1.2 at.% Al concentration was concluded to have the potential for use in electronic devices as a diode after annealing at 700 °C.     

Growth of p-type ZnO thin films by using an atomic layer epitaxy technique and NH3 as a doping source

Nitrogen-doped, p-type ZnO thin films have been grown successfully on sapphire (0001) substrates by atomic layer epitaxy (ALE) using Zn(C₂H₅)₂ [Diethylzinc, DEZn], H₂O and NH₃ as a zinc precursor, an oxidant and a doping source gas, respectively. The lowest electrical resistivity of the p-type ZnO films grown by ALE was 210 Ω cm with a hole concentration of 3.41 × 10¹⁶ cm^− 3. Low temperature-photoluminescence analysis results support that the nitrogen ZnO after annealing is a p-type semiconductor. Also a model for change from n-type ZnO to p-type ZnO by annealing is proposed.     

Self-compensation in ZnO thin films: An insight from X-ray photoelectron spectroscopy, Raman spectroscopy and time-of-flight secondary ion mass spectroscopy analyses

As-grown ZnO typically exhibits n-type conductivity and the difficulty of synthesizing p-type ZnO for the realization of ZnO-based optoelectronic devices is mainly due to the compensation effect of a large background n-type carrier concentration. The cause of this self-compensation effect has not been conclusively identified although oxygen vacancies, zinc interstitials and hydrogen have been suggested. In this work, typical n-type ZnO thin films were prepared by sputtering and investigated using X-ray photoelectron spectroscopy, Raman spectroscopy and time-of-flight secondary ion mass spectroscopy to gain an insight on the possible cause of the self-compensation effect. The analyses found that the native defect that most likely behaved as the donor was zinc interstitial but some contribution of n-type conductivity could also come from the electronegative carbonates or hydrogen carbonates incorporated in the ZnO thin films.     

Nitrogen-doped ZnO thin films grown on glass substrates by atomic layer deposition using NH3 as a doping source

Nitrogen-doped ZnO (ZnO:N) films were successfully grown on glass substrates by atomic layer deposition (ALD). NH3 was used as a doping source, and the substrate temperature was relatively low (90 approximately 210 degrees C). The main focus of the study was to report on the effect of the temperature on the electrical properties (e.g., carrier concentration, mobility, etc.) of the grown ZnO:N films. At all temperatures, the carrier was found to be n-type, and its electron concentration did not show much variation within the values between 3 x 10(16) and 6 x 10(16) cm-3; the mobility increased with the temperature (1 cm2/Vs at 110 degrees C, 5 cm2/Vs at 190 degrees C); and the resistivity decreased with the temperature (203 omegacm at 110 degrees C, 21 omegacm at 190 degrees C). The electrical properties are discussed in relation with the nitrogen concentration, crystallinity, crystal orientation, grain size, and surface morphology. The nitrogen concentration in the ZnO:N films was constant at all temperatures (approximately 2.5 atomic percent); the crystallinity and crystal orientation improved with the temperature; and the mean grain size increased with the temperature (13.2 nm at 110 degrees C, 35.3 nm at 190 degrees C). The results for the ZnO:N films were also compared with the results for the undoped ZnO films.     

Effect of solution concentration on the structural, optical and conductive properties of ZnO thin films prepared by sol–gel method

ZnO thin films with different solution concentrations (0.1–0.9 mol/L) were prepared by a simple sol–gel dip-coating technique. X-ray diffraction, ultraviolet–visible spectroscopy, Hall effect measurements and photoluminescence (PL) spectroscopy were employed to investigate the effect of solution concentration on the structural,optical and electrical conductive properties of the ZnO thin films. The results showed that the ZnO thin films preferentially oriented along the (002) direction at higher solution concentration. The careful study of the optical and electrical conductive properties showed that the resistivity decreased monotonously, while the transmittance increased first and then decreased when solution concentrations changed from 0.1 to 0.9 mol/L. Photoluminescence spectra indicated that the defect-related blue emission was increased with the enhancement of solution concentration. The mechanism of the blue emission, and the reasons why high solution concentration was favorable for forming high c-axis oriented ZnO thin films and obtaining low resistivity were also discussed in detail.     

Structural and optical properties of Cu doped ZnO thin films by co-sputtering

This paper reports on the structural and optical properties of ZnCuO thin films that were prepared by co-sputtering for the application of p-type-channel transparent thin-film transistors (TFTs). Pure ceramic ZnO and metal Cu targets were prepared for the co-sputtering of the ZnCuO thin films. The effects of the Cu concentration on the structural, optical, and electrical properties of the ZnCuO films were investigated after their heat treatment. It was observed from the XRD measurements that the ZnCuO films with a Cu concentration of 7% had ZnO(002), Cu2O(111), and Cu2O(200) planes. The 7% Cu-doped ZnO films also showed a band-gap energy of approximately 2.05 eV, an average transmittance of approximately 62%, and a p-type carrier density of approximately 1.33 x 10(19) cm-3 at room temperature. The bottom-gated TFTs that were fabricated with the ZnCuO thin film as a p-type channel exhibited an on-off ratio of approximately 6. These results indicate the possibility of applying ZnCuO thin films with variable band-gap energies to ZnO-based optoelectronic devices.     

Investigation of thin solid ZnO films prepared by sputtering

Zinc oxide (ZnO) thin films have attracted great attention in recent years due to their unique piezoelectric and piezooptic properties, making them suitable for various microelectronics and optoelectronics applications, such as surface acoustic wave devices, optical fibers, solar cells etc. ZnO is a semiconductor with a band gap of 3.3 eV and a large exciton binding energy of 60 meV. Undoped ZnO exhibits intrinsic n-type conductivity and it enables achieving high electron concentration. However, it may be doped to obtain low resistivity p-type thin films. Among group V of the periodic table, nitrogen is used as a popular p-type dopant due to its small atomic size. However, it is difficult to achieve p-type conduction in ZnO films due to the low solubility of nitrogen and its high intensity in self compensating process upon doping.Sputtering techniques enable us to form dense and homogeneous films due to the relatively high energy of the sputtered atoms. Thus we can grow high quality ZnO films with c-axis orientation, low growth temperature, high deposition rate, large area deposition, and availability in various growths ambient. In this work, the zinc oxide films were prepared using various DC sputtering methods in an atmosphere of pure argon and an atmosphere of mixed argon with nitrogen. Optical and electrical properties of the films were investigated.     

ZnO薄膜p型掺杂的研究进展

ZnO是一种新型的II-VI族半导体材料,具有许多优异的性能.但由于ZnO存在诸多的本征施主缺陷(如空位氧V_o和间隙锌Zn_i),对受主产生高度自补偿作用,天然为n型半导体,难以实现p型转变.ZnO薄膜p型掺杂的实现是ZnO基光电器件的关键技术,也一直是ZnO研究中的主要课题,目前已取得重大进展,文章对此进行了详细阐述.     

P-type conductivity in doped and codoped ZnO thin films synthesized by RF magnetron sputtering

N-doped and Al–N codoped ZnO thin films with different volume ratios of N₂ reactive gas were deposited on plane glass substrates using the radio frequency magnetron sputtering method. The phase transition temperature and absorption edge of the ZnO powder were studied by differential scanning calorimetry at different heating rates and with Fourier transform infrared spectroscopy, respectively. The target used for the sputtering was synthesized using a palletize machine. It was sintered at 450 °C for 5 h. The X-ray diffraction results confirm that the thin films have wurtzite hexagonal structures with a very small distortion. The results indicate that the ZnO thin films have obviously enhanced transmittance of up to 80% on an average in the visible region. The Al–N codoped ZnO thin films exhibited the best p-type conductivity with a resistivity of 0.825 Ω-cm, a hole concentration of 6.55 × 10¹⁹ cm^?3, and a Hall mobility of 1.25 cm²/Vs. The p-type conductivity was observed after doping and codoping of the ZnO thin film.     

ZnO layers grown by Atomic Layer Deposition: A new material for transparent conductive oxide

We demonstrate possibility of a control (by selection of zinc precursors and variation of a growth temperature) of electrical properties of ZnO films grown by Atomic Layer Deposition (ALD). ZnO films grown by ALD are used in test photovoltaic devices (solar cells) as transparent conductive oxides for upper, transparent layer in inorganic and organic solar cells, and as n-type partners of p-type CdTe.     

Effect of Al concentrations on the electrodeposition and properties of transparent Al-doped ZnO thin films

Al-doped zinc oxide (AZO) thin films are prepared on polycrystalline fluorine-doped tin oxide-coated conducting glass substrates from nitrates baths by the electrodeposition process at 70 °C. The electrochemical, morphological, structural and optical properties of the AZO thin films were investigated in terms of different Al concentration in the starting solution. It was found that the carrier density of AZO thin films varied between ?3.11 and ?5.56 × 10²⁰ cm^?3 when the Al concentration was between 0 and 5 at.%. Atomic force microscopy images reveal that the concentration of Al has a very significant influence on the surface morphology and roughness of thin AZO. X-ray diffraction spectra demonstrate preferential (002) crystallographic orientation having c-axis perpendicular to the surface of the substrate and average crystallites size of the films was about 33–54 nm. With increasing Al doping, AZO films have a strong improved crystalline quality. As compared to pure ZnO, Al-doped ZnO exhibited lower crystallinity and there is a shift in the (002) diffraction peak to higher angles. Due to the doping of Al of any concentration, the films were found to be showing >80 % transparency. As Al concentration increased the optical band gap was also found to be increase from 3.22 to 3.47 eV. The room-temperature photoluminescence spectra indicated that the introduction of Al can improve the intensity of ultraviolet (UV) emission, thus suggesting its greater prospects in UV optoelectronic devices. A detailed comparison and apprehension of electrochemical, optical and structural properties of ZnO and ZnO:Al thin films is done for the determination of optimum concentration of Al doping.