P-type ZnO Films Research Articles

Giant photoresponse in p-type sodium-doped ZnO films

The detection of ultraviolet radiation is currently a topic that deserves the attention of society. This research shows that the addition of sodium (Na) to ZnO thin films enhances the generation and collection of photogenerated carriers at room temperature, making ZnO:Na a promising candidate for photosensors. This effect is particularly pronounced at 10 % Na-concentration, with a photoresponse around 104, suggesting a decrease in the role of deep traps. Additionally, as a defect engineering approach, annealing in different controlled atmospheres shows that the N2 atmosphere provides a markedly faster photoresponse. By introducing Na atoms into the ZnO crystal lattice, charge compensation levels are formed, inducing the p-type conductivity in some ranges of temperature, particularly at 300 K. Annealing in controlled atmospheres also influences both the transition from n-type to p-type conductivity and light-stimulated phenomena.

Electrical and Structural Properties of Semi-Polar-ZnO/a-Al2O3 and Polar-ZnO/c-Al2O3 Films: A Comparative Study.

In this work, the properties of ZnO films of 100 nm thickness, grown using atomic layer deposition (ALD) on a-(100) and c-(001) oriented Al2O3 substrate are reported. The films were grown in the same growth conditions and parameters at six different growth temperatures (Tg) ranging from 100 °C to 300 °C. All as-grown and annealed films were found to be polycrystalline, highly (001) oriented for the c-Al2O3 and highly (101) oriented for the a-Al2O3 substrate. The manifestation of semi-polar-(101) and polar (001)-oriented ZnO films on the same substrate provided the opportunity for a comparative study in terms of the influence of polarization on the electrical and structural properties of ZnO films. It was found that the concentration of hydrogen, carbon, and nitrogen impurities in polar (001)-oriented films was considerably higher than in semi-polar (101)-oriented ZnO films. The study showed that when transparent conductive oxide applications were considered, the ZnO layers could be deposited at a temperature of about 160 °C, because, at this growth temperature, the high electrical conductivity was accompanied by surface smoothness in the nanometer scale. On the contrary, semi-polar (101)-oriented films might offer a perspective for obtaining p-type ZnO films, because the concentration of carbon and hydrogen impurities is considerably lower than in polar films.

Temperature-induced potential barriers in piezoelectric semiconductor films through pyroelectric and thermoelastic couplings and their effects on currents

We study temperature effects in piezoelectric semiconductor thin films through pyroelectric and thermoelastic as well as piezoelectric couplings. Two-dimensional equations based on the macroscopic theory of piezoelectric semiconductors for extensional deformations of the films are used. A p-type ZnO film with an in-plane c-axis is analyzed. It is shown through a linear and theoretical analysis that a local temperature change produces a pair of a potential barrier and a well in the film. A nonlinear numerical analysis is then performed when a voltage is also applied, showing that the potential barrier blocks the in-plane motion of the holes and thus affects the current density distribution. The effects of various physical and geometric parameters are examined. The results obtained provide the theoretical foundation for thermally manipulating currents in piezoelectric semiconductor films for piezotronic applications.

A pathway for ZnO p-type transformation and its performance in solar cells

Aiming at the breakthrough of ZnO in solar cells, we investigate the theoretical rationality and experimental operability of its structural components. In this paper, First Principle Calculation is implemented to achieve the optimum doping ratio for ZnO p-type transformation. Then, Al-N co-doped ZnO films are prepared by magnetron sputtering. The quality of p-type ZnO films are controlled by changing the ratio of N2O to Ar in the sputtering atmosphere. Results shown that the p-type ZnO films with excellent photoelectrical properties can be successfully obtained in the condition of N2O:Ar = 1: 4, where the resistivity is as low asρ=3.678×10-5Ω·cm, the carrier concentration isN=1.3066×1021cm-3, the average transmittance of the visible region is about 90%. In addition, we first explore the feasibility on the fabrication of p-type Al-N co-doped ZnO/n-type Al-doped ZnO homojunction and its application in solar cell.

The low temperature growth of stable p-type ZnO films in HiPIMS

In this study, the influence of substrate temperature on properties of Al-N co-doped p-type ZnO films is explored. Benefitting from the high ionization rate in high-power impulsed magnetron sputtering, the concentration of ionized nitrogen N+ and ionized zinc Zn+ were increased, which promoted the formation of ZnO films and lowered the necessary substrate temperature. After optimization, a co-doped p-type ZnO thin film with a resistivity lower than 0.35 Ω cm and a hole concentration higher than 5.34 × 1018 cm−3 is grown at 280 °C. X-ray diffraction results confirm that Al-N co-doping does not destruct the ZnO wurtzite structure. X-ray photoelectron spectroscopy demonstrates that the presence of Al promotes the formation of acceptor (No) defects in ZnO films, and ensures the role of Al in stabilizing p-type ZnO.

Self-compensation reduction as first step of p-type ZnO synthesis

Abstract In this work, we used a new strategy, based on the reduction of self-compensation, to synthesize p-type ZnO thin films. This new protocol is based on the control of intrinsic defects especially the vacancies and the interstitials of both zinc and oxygen networks. It consists of using zinc nitrate as a precursor and varying its concentration on the one hand, and the addition of acetic acid on the other. Hall Effect measurements have shown a decrease in electron concentration as the nitrate concentration increases. A concrete transition in the conductivity of ZnO from n-type to p-type has been observed for concentration around 0,4M. Thermal Analyses (DTA-TGA), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), UV–visible spectrophotometry were used to study the structural, morphological and optical properties of the synthesized layers. All these measures confirm the appearance of a change in behavior when nitrate concentration is approaching 0,4M. This transition in conductivity-type in ZnO layers has been linked to a significant increase in the oxygen content as confirmed by an EDX analysis during films growth. The growth of p-type ZnO films based on zinc nitrate can potentially lead to the development of new transparent devices applicable in optoelectronics and photovoltaic fields.

Non-axial NO-VZn shallow acceptor complexes in nitrogen implanted p-type ZnO thin films

Over the past two decades, there have been numerous reports about the p-type behavior of N-doped ZnO. To date, however, its origin still remains mysterious, especially, N ion-implanted ZnO system. Herein, ZnO films were implanted with 70 keV N ions to a fluence of 1 × 1017 cm−2 at room temperature, followed by annealing in the range of 750–950 °C. The obtained p-type ZnO films have a widely hole concentration of 2.87 × 1015 ~ 2.64 × 1016 cm−3, a mobility of 1.37 ~ 7.27 cm2V-1s−1 and a resistivity of 148.3 ~ 299.4 Ω.cm. The thermal evolution of point defects and the possible shallow acceptors in N-implanted ZnO films were further investigated by means of Raman scattering, Photoluminescence (PL) and Electron paramagnetic resonance (EPR). The results show that abundant intrinsic-related defects including zinc interstitials (Zni), oxygen vacancies (VO) and zinc vacancies (VZn) were introduced during ion implantation. It is demonstrated that appropriate post-annealing can not only reduce the compensation of donor defects, but also facilitate the formation of N-related shallow acceptor complexes, both of which contribute to the p-type conduction transition of N-implanted ZnO films. The non-axial NO-VZn complexes are proposed to be a kind of potential and stable acceptors in N ion-implanted ZnO films.

Study on material properties of Sn- and Cu-doped ZnO thin films as n- and p-type thermoelectric materials based on wet solution synthesis

Sn-doped n-type and Cu-doped p-type ZnO films were fabricated by depositing solutions derrived by sol–gel route on glass substrates through dip-coating cycles. The X-ray diffractometry demonstrates the typical crystalline structure of hexagonal wurzite of ZnO. The surface morphology was made through scanning electronmicroscopy showing the density and shape of films’ grains of which size was determined between 40 and 55 µm. The results of Hall measurement indicate clearly the material nature of fabricated films at room temperature. The best electrical property of Sn-doped n-type ZnO films is corresponding to Sn content of 2 at.% with carrier concentration of 3.0 × 1018 cm−3. The formation of p-type ZnO thin films was elucidated by the manner of solution synthesis that glucose was added into the solube mixer of zinc acetate and copper(II) nitrate trihydrate and the way of thermal treatment for films in vacuum with pretty good hole concentration of about 1015 cm−3. The electrical transport properties characterizing thermoelectric (TE) behavior of all films were investigated between room temperature to 400 °C (or from 300 to 673 K). The obtained results of electrical conductivity, Seebeck coefficient and power factor revealed that Sn-doped n-type ZnO films have the quality and properties similar to Al-doped ones and Cu-doped ZnO films can be promising p-type materials for TE applications.

Effects of Cu contents on defects formation in molecular dynamics simulations of ZnO:Cu films deposition

The development of p-type ZnO film is related to the acceptor doping and defects, which are difficult to assess. Copper doped ZnO (ZnO:Cu) film with varying Cu content on a ZnO(0 0 1) substrate were simulated using molecular dynamics simulations. The COMB3 potential was used to describe the interaction between the depositing and substrate atoms. The growth of the ZnO:Cu films on the ZnO(0 0 1) substrate was a 3D island mode. Numerous oxygen vacancies (VO), zinc vacancies (VZn) and dislocations existed in the deposition layers. Cu-Zn alloy clusters combined easily with Cu and Zn atoms, increasing the mass of the deposit. This resulted in the content of acceptor defect VZn in ZnO layers increasing with the increase of Cu atoms. This could explain the p-type characteristics of ZnO:Cu film.

Comprehensive study on Sb-doped zinc oxide films deposited on c-plane Al2O3 substrates

Undoped and Sb-doped p-type ZnO films were grown on (001) sapphire using metal organic chemical vapor deposition (MOCVD) technology. The crystal, optical, and electrical properties of the undoped and Sb-doped ZnO film were investigated by X-ray diffraction (XRD), photoluminescence, optical transmittance spectroscopy and Hall measurement system. The XRD patterns indicated both films exhibited preferred orientation along c axis and had a compressive stress. Strong near-band gap emissions of both films were observed in the room temperature photoluminescence spectra. Low temperature photoluminescence spectrum of the ZnO film was dominated by a donor-bound exciton with peak at 3.365 eV. To the Sb-doped ZnO film, donor-bound exciton was buried into the free exciton with peak at 3.371 eV, and emission intensities associated acceptor enhanced. The acceptor binding energy was calculated to be 0.19 eV. The optical bandgap of the Sb-doped ZnO film was estimated to be 3.302 eV. The Sb-doped ZnO film exhibited p-type conductivity with the resistivity of 0.6729 Ω cm, mobility of 6.150 cm2 V−1 s−1 and carrier concentration of 1.508 × 1018 cm−3.

Performance analysis of Schottky diodes based on Bi doped p-ZnO thin films

In the present paper, Bismuth (Bi) doped ZnO thin film have been deposited using sol gel spin coating technique to produce p-type ZnO films on n-Si substrate. The deposited Bi doped ZnO thin films were characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), hot point probe and hall measurement. The XRD and AFM characterization results shows uniform growth of the film over the entire substrate having polycrystalline nature of the film and Hall measurement and hot point probe method shows p-type nature of the deposited Bi doped ZnO film. Further, Palladium (Pd) and Nickel (Ni) metal contacts have been deposited over separate Bi doped ZnO thin films using vacuum coating method to fabricate Schottky diodes. The current–voltage characteristics have been analyzed by using conventional thermionic emission model, Cheung's method and Norde's technique to calculate the barrier height, ideality factor and series resistance of the fabricated Schottky diodes. From the calculations, it is observed that Cheung's method gives more realistic results for estimating diode parameters.

Oxygen vacancy-passivated ZnO thin film formed by atomic layer deposition using H2O2

Intrinsic defect oxygen vacancies, which can easily form in ZnO films and result in a compensation effect on p-type dopants, have long prevented the preparation of high-quality p-type ZnO; consequently, the application of ZnO in optoelectronic devices has been adversely affected. Therefore, in this investigation, the passivation of oxygen vacancies in undoped ZnO using H2O2 as an oxygen source is studied using atomic layer deposition (ALD). The ALD growth window ranged from 60 to 150 °C, and the use of H2O2 as an oxygen source, instead of H2O, changed the preferred growth orientation from coexisting a- and c-axes to only the c-axis, which indicated that H2O2 can provide an oxygen-rich environment for the growth of ZnO. Photoluminescence results indicated that oxygen vacancies in the ZnO film reduced significantly when H2O2 was used as the oxygen precursor instead of H2O for film preparation. Further, oxygen vacancies can be suppressed more efficiently using H2O2 when ZnO films were deposited at lower temperatures than at high temperatures. A decrease in the optical bandgap and an increase in the work function were observed when films were prepared using H2O2 due to a lowering of the Fermi level. Therefore, the use of H2O2 as an oxygen source is effective in providing an oxygen-rich environment and passivating oxygen vacancies in ZnO, which might be beneficial for the preparation of p-type ZnO films.

Low temperature method to passivate oxygen vacancies in un-doped ZnO films using atomic layer deposition

Owing to oxygen vacancies, the as-prepared ZnO normally shows n-type semiconducting characteristic. This has restricted the preparation of high-quality p-type ZnO and the application of ZnO optoelectronic devices. Therefore, we studied a method of using H2O2 as an oxygen source to passivate oxygen vacancies (Vo) in ZnO films via atomic layer deposition (ALD). The temperature range for the self-limited growth of crystalline ZnO thin films by ALD using diethylzinc and H2O2 was found to be in the range of 80 to 150 °C. Our results show that the use of H2O2 as an oxygen source can provide an O-rich condition (instead of H2O) for the growth of ZnO film, with a total preferential (002) orientation of the growth plane and decreased grain size. Further, the O-rich growth environment can suppress the formation of Vo and zinc interstitials and decrease the carrier concentration in ZnO (from 2.525 × 1019 cm−3 to 1.695 × 1012 cm−3). This can lead to an increase in the film resistivity from 1.717 × 10−2 Ω·cm for a ZnO film prepared using H2O to 1.348 × 104 Ω·cm for a ZnO film prepared using H2O2. Thus, H2O2 could be used to passivate Vo in ZnO at a low temperature, and it could be beneficial for the preparation of p-type ZnO films.

P-type single-crystalline ZnO films obtained by (Na,N) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown by plasma-assisted molecular beam epitaxy technique on c-plane sapphire substrates. The films have been implanted with fixed fluence of 130 keV Na and 90 keV N ions at 460 °C. It is observed that dually-implanted single crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 1.24 × 1016–1.34 × 1017 cm−3, hole mobilities between 0.65 and 8.37 cm2 V−1 s−1, and resistivities in the range of 53.3–80.7 Ω cm by Hall-effect measurements. There are no other secondary phase appearing, with (0 0 2) (c-plane) orientation after ion implantation as identified by the X-ray diffraction pattern. It is obtained that Na and N ions were successfully implanted and activated as acceptors measured by XPS and SIMS results. Also compared to other similar studies, lower amount of Na and N ions make p-type characteristics excellent as others deposited by traditional techniques. It is concluded that Na and N ion implantation and dynamic annealing are essential in forming p-type single-crystalline ZnO films.

Effect of nanowires in microporous structures on the thermoelectric properties of oxidized Sb-doped ZnO film

Sb-doped ZnO thermoelectric films with microporous structures are fabricated by oxidizing evaporated Zn-Sb thin films in a leaf-like surface. High magnetic field (HMF) and Sb are employed to tune the formation of nanowires and nanorods in the microporous films and conduction type. Nanowire is formed in the film with Sb content of 3.0% and nanorod is formed with 4.6% Sb with the absence of HMF. P-type ZnO films with a wuterzite are formed. The resistivity of the films decreases by two orders of magnitude by increasing Sb content. The resistivity of films decreases 45% and 80% by forming nanowires and nanorods, respectively. The power factor of the nanorod structures increases by two orders of magnitude by comparison with others and reaches to 52.6μW/m K2. This indicates that the nanorod structures with a higher Sb content are easy to obtain stable p-type semiconductor with a higher power factor.

P-type single-crystalline ZnO films obtained by (Li, N) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy technique. The films have been implanted with fixed fluence of 30 keV Li and 80 keV N ions at 460 °C. It is shown that dually-implanted single crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 2.5 × 1015–2.3 × 1016 cm−3, hole mobilities between 0.8 and 1.04 cm2 V−1 s−1, and resistivities in the range of 328.9–2081 Ω cm by Hall-effect measurements. The ZnO film exhibit (0002) (c-plane) orientation with no secondary phase appearing after implantation as identified by the X-ray diffraction pattern. It is confirmed that Li and N ions were effectively implanted and successfully behave as acceptor measured by XPS and SIMS results. Also compared to other similar studies, lower amount of Li and N ions make p-type characteristics excellent. It is concluded that Li and N ion implantation and dynamic annealing play a critical role in forming p-type single-crystalline ZnO films.

Optoelectronic properties of phosphorus doped p-type ZnO films grown by dual ion beam sputtering

We report highly conductive and stable p-type phosphorus doped ZnO (PZO) thin films fabricated by dual ion beam sputtering and subsequent thermal annealing. Hall measurements established that the annealed PZO films were p-type, which were also confirmed by typical diode-like rectifying current-voltage (I-V) characteristics of the p-PZO/n-Si heterojunction. The maximum hole concentration was evaluated to be 8.62 × 1019 cm−3 with a resistivity of 0.066 Ω cm and a mobility of 1.08 cm2/V s at room temperature. The stability of the p-type conduction was verified by Hall measurement performed again after one year of thin film fabrication resulting in a hole concentration of 3.77 × 1019 cm−3. Spectroscopic ellipsometry was employed to determine the complex dielectric function (ε=ε1+iε2) of p-type PZO films in the 1.2–6.4 eV energy range by a parameterized semiconductor oscillator model. Room temperature excitonic features were identified and the critical point energy was determined by second order derivative of imaginary part of dielectric function. The line shape analysis of ε resulted in a red shift of the energy positions of the critical point with an increase in hole concentration.

Characteristics of doping controllable ZnO films grown by photo-assisted metal organic chemical vapor deposition

Arsenic doped p-type ZnO films are prepared by the photo-assisted metal organic chemical vapor deposition method. Using the photo-assisted technique, the acceptor activation process is simplified. The arsenic doping level, which decides the carrier distribution, could be controlled by changing the thickness of the pre-deposited GaAs layer. The crystal and optical quality of the ZnO films is good. The acceptor is AsZn–2VZn. Its ionization energy could be slightly reduced by increasing the arsenic doping level. This finding is very helpful to improve the hole concentration. Our experiments provide a new method to grow high performance p-type ZnO based photoelectric devices.

Influence of molar ratio of Sb/Zn on the crystal, electrical and optical properties of Sb-doped ZnO films

Sb-doped p-type ZnO films were grown on sapphire substrate by metal organic chemical vapor deposition method. The influence of the molar ratio of Sb/Zn on the crystal, electrical and optical properties of the Sb-doped ZnO films was investigated by X-ray diffraction (XRD), Hall measurement system, optical absorption and photoluminescence spectroscopy. XRD patterns revealed all Sb-doped ZnO films were polycrystalline structures with strong c-axis orientation preferred. Electrical measurement results indicated Sb-doped ZnO films exhibited p-type electrical properties, and carrier concentrations were in the range of 10−17–10−18 cm−3. Low temperature photoluminescence measurement confirmed the existence of Sb in Sb-doped ZnO film. The photoluminescence spectrum of the Sb-doped ZnO film revealed the transition between the free electrons and acceptors peak at 3.243 eV, the acceptor-bound exciton peak at 3.319 eV and the donor-acceptor pair exciton at 3.200 eV. The thermal binding energy of the Sb accepter was estimated to be 0.19 eV.

Formation of p-type ZnO thin film through co-implantation

We present a study on the formation of p-type ZnO thin film through ion implantation. Group V dopants (N, P) with different ionic radii are implanted into chemical vapor deposition grown ZnO thin film on GaN/sapphire substrates prior to thermal activation. It is found that mono-doped ZnO by N+ implantation results in n-type conductivity under thermal activation. Dual-doped ZnO film with a N:P ion implantation dose ratio of 4:1 is found to be p-type under certain thermal activation conditions. Higher p-type activation levels (1019 cm−3) under a wider thermal activation range are found for the N/P dual-doped ZnO film co-implanted by additional oxygen ions. From high resolution x-ray diffraction and x-ray photoelectron spectroscopy it is concluded that the observed p-type conductivities are a result of the promoted formation of PZn–4NO complex defects via the concurrent substitution of nitrogen at oxygen sites and phosphorus at zinc sites. The enhanced solubility and stability of acceptor defects in oxygen co-implanted dual-doped ZnO film are related to the reduction of oxygen vacancy defects at the surface. Our study demonstrates the prospect of the formation of stable p-type ZnO film through co-implantation.