P-type ZnO Films Research Articles

Fabrication and Study of P-Type ZnO Films by RF Magnetron Sputtering of ZnO: In<sub>2</sub>O<sub>3</sub> Powder Together with the N-Implantation

The N-In codoped p-type ZnO films with preferential orientation along (002) plane have been fabricated on quartz glass substrates using radio frequency magnetron sputtering technique of ZnO:In2O3 powder target combining with N-implantation. The samples annealed at 700°C deserved the optimal properties, the best of which exhibits electrical characteristics with the hole concentration of 4.04×1018 cm-3, the lowest resistivity of 1.15 Ωcm and Hall mobility of about 1.35 cm2V-1s-1. The effects of post-annealing on the microstructure and electronic properties of the codoped ZnO films is analyzed via SEM, XRD, XPS and Hall measurements system, and the trend of carrier concentration with annealing time is discussed theoretically.

Stable p-type conductivity and enhanced photoconductivity from nitrogen-doped annealed ZnO thin film

We report on the growth of p-type ZnO thin films with improved stability on various substrates and study the photoconductive property of the p-type ZnO films. The nitrogen doped ZnO (N:ZnO) thin films were grown on Si, quartz and alumina substrates by radio frequency magnetron sputtering followed by thermal annealing. Structural studies show that the N:ZnO films possess high crystallinity with c-axis orientation. The as-grown films possess higher lattice constants compared to the undoped films. Besides the high crystallinity, the Raman spectra show clear evidence of nitrogen incorporation in the doped ZnO lattice. A strong UV photoluminescence emission at ~380nm is observed from all the N:ZnO thin films. Prior to post-deposition annealing, p-type conductivity was found to be unstable at room temperature. Post-growth annealing of N:ZnO film on Si substrate shows a relatively stable p-type ZnO with room temperature resistivity of 0.2Ωcm, Hall mobility of 58cm2/Vs and hole concentration of 1.95×1017cm−3. A homo-junction p–n diode fabricated on the annealed p-type ZnO layer showed rectification behavior in the current–voltage characteristics demonstrating the p-type conduction of the doped layer. Doped ZnO films (annealed) show more than two orders of magnitude enhancement in the photoconductivity as compared to that of the undoped film. The transient photoconductivity measurement with UV light illumination on the doped ZnO film shows a slow photoresponse with bi-exponential growth and bi-exponential decay behaviors. Mechanism of improved photoconductivity and slow photoresponse is discussed based on high mobility of carriers and photodesorption of oxygen molecules in the N:ZnO film, respectively.

Growth of Na doped p-type non-polar a-plane ZnO films by pulsed laser deposition

Non-polar ZnO:Na films were deposited on r-plane sapphire substrates with different Na doping concentration by pulsed laser deposition. Based on the X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis, the ZnO:Na films with Na concentration from 1% to 2% exhibit unique non-polar <11–20> orientation and those films deposited with Na concentration above 2% present <0001> and <11–20> mixed orientations. Anisotropic stripes-like morphology can be observed for the Na content below 2%. A stable p-type non-polar ZnO:Na film was obtained by doping 2% Na, and X-ray photoelectron spectroscopy (XPS) exhibited the presentation of NaZn acceptor, which was the origin of p-type behavior.

Preparation of p-type ZnO Films by Alternate Deposition of ZnO and Mg3N2 Films

Films were deposited on a glass substrates heated at 100°C by alternate deposition of ZnO and Mg3N2 films in an atmosphere of N2 + 0.2% O2. p-type ZnO films with the resistivity of 9.9×103 - 4.5×103Ωcm were grown by Mg3N2 sputtering for 0.2s at 50-70W, whereas an n-type ZnO film was grown for 0.2s at 90W. Appearance of the n-type ZnO film is caused by a decrease of O2-partial pressure in an atmosphere. The contraction of the c-axis due to the incorporation of Mg atoms is found in these p-type and n-type ZnO films. When the Mg3N2-depostion period was extended to 5s, p-type ZnO films were grown in the alternate sputtering at 50W, whereas only insulator films were grown in the alternate sputtering at 70W. The optical band gap of the films increases with both a RF power and a period of Mg3N2-deposition.

Fabrication of p-Type ZnO Films Grown on Arsenic-Implanted Silicon via Thermal Diffusion at Various Substrate Temperatures

P-type ZnO films fabricated using the diffusion method with arsenic (As)-implanted silicon substrates as the source of dopants are investigated. The diffusion method is not restricted by the substrate and various dopants can be used in the implantation process. We demonstrate that stable p-type conduction appears at a proper substrate temperature (TS). The best crystallinity and electrical properties of the p-type ZnO films were obtained at TS = 400°C. The maximum carrier concentration of the As dopant was 6.7 × 1018 cm−3 and the resistivity reached 1.1×10−1 Ω-cm. The reproducible As-doped ZnO films can be obtained by this diffusion method.

Highly Transparent p-Type ZnO Thin Films Prepared by Non-Toxic Sol-Gel Process

In-N co-doped p-type ZnO thin films were demonstrated using a non-toxic sol-gel spin coating process. The resistivity of the p-type ZnO films is 4.43 Ω cm with the carrier concentration of 1.36 × 1018 cm−3 at room temperature. The X-ray photoelectron spectra of N 1s core level measured with an Al Kα photon line identify the existence of nitrogen in the p-type ZnO. The secondary ion mass spectrometry depth profile also confirms the nitrogen contents in the In-N co-doped films. In addition, the In-N co-doped film with a thickness of 200 nm has a high transparency about 90% in the visible region.

Effect of (O, As) dual implantation on p-type doping of ZnO films

Optical and electrical characteristics of ZnO films co-implanted with O and As ions have been investigated by photoluminescence (PL), Hall-effect, and current-voltage (I-V) measurements. 100-nm-thick ZnO films grown on n-type Si (100) wafers by RF sputtering have been implanted with various fluences of 30 keV O and 100 keV As ions at room temperature, and subsequently annealed at 800 °C for 20 min in a N2 ambient. The dually-implanted ZnO films show stable p-type characteristics for particular implant combinations, consistent with the observation of dominant PL peaks at 3.328 and 3.357 eV that are associated with the acceptor levels. For these dually-implanted p-type ZnO films/n-type Si diodes, the I-V curves show rectifying p-n junction behavior. Other singly (As)- or dually-implanted samples show n-type or indeterminable doping characteristics. These results suggest that O implantation plays a key role in forming p-type ZnO films by reducing the oxygen vacancy concentration and facilitating the formation of As-related acceptors in ZnO.

Ultraviolet-Visible Electroluminescence of a p-ZnO:As/n-Si Device Formed by the GaAs Interlayer Doping Method

Arsenic doped p-type ZnO films were grown on n-type silicon substrates using the GaAs interlayer doping method. Under our growth conditions the main doping element is arsenic, which was confirmed by x-ray photoelectroscopy. X-ray diffraction measurements revealed that the p-ZnO:As film was still in the (002) preferred orientation. The Hall test showed that the hole concentration of the p-ZnO:As film was 2.6 × 1017 cm−3. The acceptor level was located at 135 meV above the valance band maximum, according to the low-temperature photoluminescence results. We then fabricated a p-ZnO:As/n-Si heterojunction light-emitting device. Its current-voltage curve showed the typical rectifying behavior of a p—n diode. At forward current injections, the electroluminescence peaks, which cover the ultraviolet-to-visible region, could be clearly detected.

Microstructure and optical properties of ZnO/porous silicon nanocomposite films

In this study, porous silicon (PS) templates were formed by electrochemical anodization on p-type (100) silicon wafer and ZnO films were deposited on PS substrates using radio frequency (RF) reactive magnetron sputtering technique. The effects of oxygen partial pressures of growth ZnO films and annealing ambience on the microstructure and photoluminescence (PL) of the ZnO/PS nanocomposite films were systematically investigated by X-ray diffraction and fluorescence spectrophotometry. The results indicated that all ZnO/PS nanocomposite films were polycrystalline in nature with a hexagonal wurtzite structure and the (002) oriented ZnO films had the best crystal quality under O 2:Ar ratio of 10:10 sccm and annealing in vacuum. PL measurements at room temperature revealed that ZnO/PS nanocomposite systems formed a broad PL band including the blue and green emissions from ZnO and red-orange emission from the PS. The mechanism and interpretation of broadband PL of the nanocomposites were discussed in detail using an oxygen-bonding model in PS and a native defects model in ZnO.

P-type non-polar m-plane ZnO films grown by plasma-assisted molecular beam epitaxy

Non-polar ZnO thin films were grown epitaxially on m-plane sapphire substrates using plasma-assisted molecular beam epitaxy. The film is (1 0 1¯ 0) oriented ( m-plane) as identified by the X-ray diffraction pattern. The surface of ZnO film shows highly anisotropic morphology with stripes elongated along the c-axis. The nonintentionally doped non-polar ZnO film was grown under oxygen-rich condition, which exhibits weak p-type conductivity with a hole concentration of 1.3×10 16 cm −3, a Hall mobility of 0.314 cm 2 V −1 s −1, and a resistivity of 1536 Ω cm. Room-temperature photoluminescence of the p-type non-polar ZnO shows a strong UV emission at ∼3.307 eV and negligible deep level emission.

Photoluminescence properties of thin nitrogen- and phosphorus-doped ZnO films fabricated using pulsed laser deposition

The production of n- and p-type high-quality film structures is a foreground task in tackling the problem of growing the light-emitting p-n junctions based on zinc oxide. The ZnO:N and ZnO:P thin-film samples are produced from ceramic targets using the pulsed laser deposition. Zn3N2, MgO, and Zn3P2 are introduced in the ZnO ceramic targets for the fabrication of the p-type ZnO films. Gases O2 and N2O are used as buffer gases. The thermal annealing of the ZnO films is employed. The resistance and photoluminescence (PL) spectra of the ZnO films are measured prior to and after annealing. The dependence of the ZnO PL peak amplitude and position prior to and after annealing on the level of doping with nitrogen and phosphorus is established. The PL characteristics of the films are studied at cw optical excitation using a He-Cd laser with a radiation wavelength of 325 nm. The PL spectra in the interval 300–700 nm are recorded by an HR4000 Ocean Optics spectrometer in the temperature range 10–400 K. The effect of the conditions for the film deposition on the PL spectra is analyzed. The effect of the N- and P-doping level of the ZnO films on the PL intensity of the films and the position of the PL bands in the UV region is investigated. The short-wavelength (250–400 nm) transmission spectra of the ZnO:P films are measured. The effect of the P-doping level on the band gap of the ZnO films is studied.

Stable p-type ZnO films grown by atomic layer deposition on GaAs substrates and treated by post-deposition rapid thermal annealing

Long-term stable p-type ZnO films were grown by atomic layer deposition on semi-insulating GaAs substrates and followed by rapid thermal annealing (RTA) in oxygen ambient. Significant decrease in the electron concentration and increase in the hole concentration, together with the intensity enhancement of acceptor-related AoX spectral peak and the shift of bound exciton peak from DoX to AoX in the low-temperature photoluminescence spectra, were observed as the RTA temperature increased. Conversion of conductivity from intrinsic n-type to extrinsic p-type ZnO occurred at the RTA temperature of 600°C. The p-type ZnO film with a hole concentration as high as 3.44×1020cm−3 and long-term stability up to 180days was obtained as the RTA treatment was carried out at 700°C. The results were attributed to the diffusion of arsenic atoms from GaAs into ZnO as well as the activation of As-related acceptors by the post-RTA treatment.

Structure, Optical, Temperature Dependent Electrical Properties of P-Type Conduction in N–Al Codoped Zn&lt;sub&gt;1-X&lt;/sub&gt;Mg&lt;sub&gt;X&lt;/sub&gt;O Films by Ultrasonic Spray Pyrolysis

Four series of thin films have been deposited as the precursory sources of Zn(CH3COO)2, Mg(CH3COO)2, NH4CH3COO and AlCl3 aqueous solutions using ultrasonic spray pyrolysis (USP) method. The crystalline structure, morphology images, electrical, optical properties of the films are characterized by x-ray diffraction (XRD), field emission-scan electron image (FE-SEM), Hall-effect measurement and photoluminescence (PL). From the XRD patterns and SEM images, we can see that all the films present good crystallinity and surface uniformity. Hall-effect measurement results indicate that ZnO is n-type, while N-Al codoped ZnO and N-Al codoped Zn1-xMgxO exhibit p-type conduction. Temperature dependent of electrical measurement is carried out from 300K to 500K, then the conductive mechanism and carriers scattering are analysed. Furthermore, the photoluminescence peak of Zn1-xMgxO is tuned into shorten wavelength than pure ZnO (λ＝379－352＝27nm), and also the same phenomenon of the p-type Zn1-xMgxO film exhibits blue-shifted behavior from 378nm to 356nm compared with p-type ZnO film (λ＝378－356＝21nm). In other word, the p-type Zn1-xMgxO film shifts to a shorter wavelength of 356 nm while maintaining excellent electrical performances.

Ultraviolet Electroluminescence From ZnO-Based n-i-p Light-Emitting Diodes

The p-type ZnO films were obtained using codeposition of ZnO and sources by the vapor cooling condensation system. With an adequate postannealing temperature, the p-type conductive behaviors of the resulting ZnO films could be achieved due to the activation of Li-N dual acceptor. According to the emission energy of free electron to acceptor hole level, the acceptor binding energy of 137 meV was obtained. Furthermore, the ZnO-based n-i-p ultraviolet light-emitting diodes were deposited on sapphire substrates using the vapor cooling condensation system. A rectifying diode-like behavior and ultraviolet emission were observed from the ZnO-based n-i-p light-emitting diodes.

Effect of NH 4OH concentration on p-type doped ZnO film by solution based process

Highly oriented and large scale p-type ZnO:Al:N films have been successfully synthesized by a simple, solution-based approach. In this approach, p-type doping of zinc oxide was achieved using combination of aqueous ammonia, zinc acetate and aluminium chloride. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), photoluminescence and Hall measurements indicate that the structure of the ZnO:Al:N films were highly influenced by aqueous ammonia concentration. Excessive aqueous ammonia results in randomly oriented film with excessive precipitation.

A reproducible route to p-ZnO films and their application in light-emitting devices

Although great efforts have been made, reproducible p-type doping is still one of the largest hurdles that hinders the optoelectronic applications of ZnO. In this Letter, a reproducible route to p-type ZnO films employing lithium-nitrogen as a dual-acceptor dopant has been demonstrated, and p-i-n structured light-emitting devices (LEDs) have been constructed. Obvious purple emissions have been observed from the LEDs, confirming the applicability of the p-type ZnO films in optoelectronic devices. The results reported in this Letter provide a reproducible route to p-type ZnO films, and thus may lay a solid ground for future optoelectronic applications of ZnO.

The p-type ZnO film realized by a hydrothermal treatment method

Reproducible and high quality N-doped ZnO (ZnO:N) films were achieved by a hydrothermal treatment method. The ZnO:N films exhibited p-type characteristics by means of the Hall-effect and the photoluminescence measurements. At room temperature, the electrical properties of ZnO:N film showed a hole concentration of 1×1016 cm−3 and hole mobility of 8.6 cm2 V−1 s−1. At 83 K two acceptor related emission peaks could be observed located at 3.353 and 3.237 eV, which were assigned to the acceptor-bound exciton and the donor-acceptor pair emissions. This result gave a direct evidence for the generation of the acceptor energy level after the hydrothermal treatment process. Also, a ZnO homojunction diode was fabricated by this method, which displayed a good rectification characteristic at room temperature. This study revealed that the hydrothermal treatment method was effective and practicable in producing p-type ZnO.

Electroluminescence of the p-ZnO:As/n-ZnO LEDs grown on ITO glass coated with GaAs interlayer

In this paper, we proposed a new p-type ZnO doping method with metal organic chemical vapor deposition (MOCVD) technology by inserting a GaAs interlayer between substrate and ZnO epitaxial layer. The doping concentration of p-type ZnO film is able to be controlled by adjusting the thickness of the GaAs interlayer. With this method, we fabricated n-ZnO/p-ZnO:As homojunction light-emitting diode (LED) on ITO-glass substrate pre-coated with 20nm GaAs interlayer. The device exhibits a typical rectifying behavior by current–voltage (I–V) measurement. When the device is forward biased, UV–vis electroluminescence (EL) emissions can be observed clearly.

Raman scattering studies of p-type Sb-doped ZnO thin films

Antimony doped p-type ZnO films were grown on Al2O3 (0001) substrate by pulsed laser deposition. The structural properties of Zn1−xSbxO (3% and 5%) thin films were investigated by Raman scattering studies. The softening of local lattice due to the formation of (SbZn−2VZn) acceptor complexes was detected as the shift in E2high mode toward lower frequency side in ZnSbO thin films. Additional optical modes observed at 277, 333, 483, and 534 cm−1 are due to the breaking of translational symmetry in w-ZnO by Sb doping. The Zn–Sb related local vibrational mode was detected around 237 cm−1 in 5% Sb doped ZnO thin film. Room temperature Hall measurements exhibited low resistivity of 0.017 Ω cm, high hole concentration of 6.25×1018 cm−3, and mobility of 57.44 cm2/V s in the 5% Sb-doped ZnO thin film.

Annealing effects on the p-type ZnO films fabricated on GaAs substrate by atmospheric pressure metal organic chemical vapor deposition

The effects of post-annealing conducted at 500–650 °C on structural, electrical and optical properties of ZnO film fabricated on GaAs (1 0 0) substrate by atmospheric pressure metal-organic chemical vapor deposition are investigated. X-ray diffraction analyses show that the Zn 3As 2 and ZnGa 2O 4 phases are produced for the specimens post-annealed at 500 °C and above. Hall measurements indicate that stable p-type ZnO films with hole concentration ranging from 4.7 × 10 18 to 8.7 × 10 19 cm −3 can be obtained by modulating the annealing temperature from 500 to 600 °C. In particular, room-temperature photoluminescence (PL) measurements indicate that the superior-quality p-type film could be achieved by a post-annealing treatment at 600 °C. Moreover, low temperature PL spectra at 10 K are dominated by the acceptor-related luminescence mechanisms for the films post-annealed at 550 °C and above. The ionization energy of acceptor was calculated to be 133–146 meV, which is in good agreement with that theoretically predicted for the As Zn–2V Zn complex in ZnO. The interdiffused arsenic atoms in the film post-annealed at 600 °C are suggested to form the As Zn–2V Zn complex quite effectively, resulting in the most enhanced p-type conductivity and improved material quality.