Effect of annealing temperature on structural, electrical and optical properties of B-N codoped ZnO thin films

The B-N codoped p-type ZnO thin films have been prepared by radio frequency magnetron sputtering using a mixture of nitrogen and oxygen as sputtering gas. The effect of annealing temperature on the structural, electrical and optical properties of B-N codoped films was investigated by using X-ray diffraction, Hall-effect, photoluminescence and optical transmission measurements. Results indicated that the electrical properties of the films were extremely sensitive to the annealing temperature and the conduction type could be changed dramatically from n-type to p-type, and finally changed to weak p-type in a range from 600 °C to 800 °C. The B-N codoped p-type ZnO film with good structural, electrical and optical properties can be obtained at an intermediate annealing temperature region (e.g., 650 °C). The codoped p-type ZnO had the lowest resistivity of 2.3 Ω cm, Hall mobility of 11 cm²/Vs and carrier concentration of 1.2 × 10¹⁷ cm^− 3.     

Raman analysis of nitrogen doped ZnO

The mechanism of nitrogen doping is essential for making p-type ZnO. This paper demonstrates that Raman characterization is a potentially powerful tool to study the mechanism of nitrogen doping. We have observed new Raman features near 280, 510, 570, 642, 773, 1360 and 1565 cm^− 1 shift in nitrogen doped ZnO (ZnO:N) thin films compared with undoped ZnO films. Peaks at 280, 510, 570, 642, and 773 cm^− 1 are attributed to the nitrogen related defect complex. The Raman peaks at 1360 cm^− 1 and 1565 cm^− 1 shift are assigned to D—(disordered) and G—(Graphitic) bands associated with the carbon-related defect complex, respectively. The intensity and the intensity ratio of peaks at 1360 cm^− 1 and 1565 cm^− 1 have been found to be sensitive parameters that reflect the conductivity type of ZnO:N. Explanations are presented which correlate the Raman features to the electric conductivity of the films. From this analysis, we found that at temperature lower than or at 400 °C, nitrogen incorporation will form the nitrogen or possible nitrogen carbon related defect complex. As the growth temperature increases to 500 °C, the features associated with nitrogen are difficult to distinguish and the features associated to carbon begin to emerge. This observation possibly indicates the decrease of the nitrogen content and the increase of the carbon content in the ZnO:N film. The increase of carbon content may affect the donor behavior of the film. This observation suggests that growth conditions should be controlled to avoid carbon into the film.     

AlN codoping and fabrication of ZnO homojunction by RF sputtering

The ZnO homojunction fabricated from undoped and 1 mol% AlN doped (codoped) ZnO targets by RF magnetron sputtering has been reported. The grown films on Si (100) substrate have been characterized by X-ray diffraction (XRD), Energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Photoluminescence (PL) and Hall measurements. The increase of d-space value (compared with unstressed bulk) found from XRD for AlN codoped ZnO film supports the formation of p-ZnO due to the N incorporation. The presence of N in the film has been confirmed by EDS and XPS analysis. Further, the p-conductivity in AlN codoped ZnO has been evidenced by low temperature PL (donor-acceptor-pair emission) and room temperature PL (red shift in near-band-edge emission). Hall measurement shows that 1 mol% AlN codoped ZnO has the hole concentration of 3.772 × 10¹⁹ cm⁻³. The fabricated homojunction with 1% AlN doped ZnO (p-type) and undoped ZnO (n-type) exhibits a typical rectification behavior with high breakdown voltage, and rectification ratio, 13.4. The junction parameters such as ideality factor, barrier height and series resistance have also been calculated for the fabricated p-n junction. The energy band diagram has been proposed for the fabricated homojunction.     

p-type doping of ZnO by means of high-density inductively coupled plasmas

A custom-designed inductively coupled plasma assisted radio-frequency magnetron sputtering deposition system has been used to fabricate N-doped p-type ZnO (ZnO:N) thin films on glass substrates from a sintered ZnO target in a reactive Ar + N₂ gas mixture. X-ray diffraction and scanning electron microscopy analyses show that the ZnO:N films feature a hexagonal crystal structure with a preferential (002) crystallographic orientation and grow as vertical columnar structures. Hall effect and X-ray photoelectron spectroscopy analyses show that N-doped ZnO thin films are p-type with a hole concentration of 3.32 × 10¹⁸ cm^− 3 and mobility of 1.31 cm² V^− 1 s^− 1. The current-voltage measurement of the two-layer structured ZnO p-n homojunction clearly reveals the rectifying ability of the p-n junction. The achievement of p-type ZnO:N thin films is attributed to the high dissociation ability of the high-density inductively coupled plasma source and effective plasma-surface interactions during the growth process.     

Enhancement-mode thin film transistor with nitrogen-doped ZnO channel layer deposited by laser molecular beam epitaxy

The thin film transistors (TFTs) based on nitrogen doped zinc oxide (ZnO) were investigated by laser molecular beam epitaxy. The increase of ZnO films' resistivity by nitrogen doping was found and applied in enhancement mode ZnO-TFTs. The ZnO-TFTs with a conventional bottom-gate structure were fabricated on thermally oxidized p-type silicon substrate. Electrical measurement has revealed that the devices operate as an n-channel enhancement mode and exhibit an on/off ratio of 10⁴. The threshold voltage is 5.15 V. The channel mobility on the order of 2.66 cm² V^− 1 s^− 1 has been determined.     

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 ~ 380 nm 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 0.2 Ω cm, Hall mobility of 58 cm²/V s and hole concentration of 1.95 × 10¹⁷ cm^− 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.     

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 A^oX spectral peak and the shift of bound exciton peak from D^oX to A^oX 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 × 10²⁰ cm^− 3 and long-term stability up to 180 days 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.     

Ultrathin Al-doped transparent conducting zinc oxide films fabricated by pulsed laser deposition

Al-doped transparent conducting zinc oxide (AZO) films, approximately 20-110 nm-thick, were deposited on glass substrates at substrate temperatures between 200 and 300 °C by pulsed laser deposition (PLD) using an ArF excimer laser (λ = 193 nm). When fabricated at a substrate temperature of 260 °C, a 40-nm-thick AZO film showed a low resistivity of 2.61 × 10^− 4 Ω·cm, carrier concentration of 8.64 × 10²⁰ cm^− 3, and Hall mobility of 27.7 cm²/V·s. Furthermore, for an ultrathin 20-nm-thick film, a resistivity of 3.91 × 10^− 4 Ω·cm, carrier concentration of 7.14 × 10²⁰ cm^− 3, and Hall mobility of 22.4 cm²/V·s were obtained. X-ray diffraction (XRD) spectra, obtained by the θ-2θ method, of the AZO films grown at a substrate temperature of 260 °C showed that the diffraction peak of the ZnO (0002) plane increased as the film thickness increased from 20 to 110 nm. The full-width-at-half-maximum (FWHM) values were 0.5500°, 0.3845°, and 0.2979° for film thicknesses of 20, 40, and 110 nm, respectively. For these films, the values of the average transmittance in visible light wavelengths (400-700 nm) were 95.1%, 94.2%, and 96.6%, respectively. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) observations showed that even the 20-nm-thick films did not show island structures. In addition, exfoliated areas or vacant and void spaces were not observed for any of the films.     

Structural properties of low-temperature grown ZnO thin films determined by X-ray diffraction and X-ray absorption spectroscopy

The epitaxial growth of ZnO thin films on Al₂O₃ (0001) substrates have been achieved at a low-substrate temperature of 150 °C using a dc reactive sputtering technique. The structures and crystallographic orientations of ZnO films varying thicknesses on sapphire (0001) were investigated using X-ray diffraction (XRD). We used angle-dependent X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to examine the variation of local structure. The XRD data showed that the crystallinity of the film is improved as the film thickness increases and the strain is fully released as the film thickness reached about 800 Å. The Zn K-edge XANES spectra of the ZnO films have a strong angle-dependent spectral feature resulting from the preferred c-axis orientation. The wurtzite structure of the ZnO films was explicitly shown by the XRD and EXAFS analysis. The carrier concentration, Hall mobility and resistivity of the 800 Å-thick ZnO film were 1.84 × 10¹⁹ cm^− 3, 24.62 cm²V^− 1s^− 1, and 1.38 × 10^− 2 Ω cm, respectively.     

Ethylene glycol assisted hydrothermal synthesis of flower like ZnO architectures

We describe a simple route to flower like ZnO architectures, based on the decomposition of zinc acetate precursor in water-ethylene glycol solution at 140-160 °C for 1d through hydrothermal method. The PXRD pattern reveals that the ZnO crystals are of hexagonal wurtzite structure. Ethylene glycol plays a key role on the morphology control of ZnO crystals. The SEM images of ZnO products prepared at 140 °C and 160 °C mainly exhibit flower like architecture composed of many rods. Whereas, the product prepared at 180 °C shows bunches accompanying a few number of free rods. TEM results reveal that the rods resemble swords with decrease in size from one end to another. From Raman spectrum, the peaks at 437 cm^− 1, 382 cm^− 1 and 411 cm^− 1 correspond to E₂ (high), A₁ (TO) and E₁ (TO) of ZnO crystals respectively. The photoluminescence spectrum exhibits strong UV emission at ~ 397 nm, which comes from recombination of exciton. The possible mechanism for the formation of flower like ZnO architecture is proposed.     

Carrier transport in polycrystalline transparent conductive oxides: A comparative study of zinc oxide and indium oxide

Highly doped indium-tin oxide films exhibit resistivities ρ as low as  1.2 × 10^− 4 Ω cm, while for ZnO films resistivities in the range of 2 to 4 × 10^− 4 Ω cm are reported. This difference is unexpected, if ionized impurity scattering would be dominant for carrier concentrations above 10²⁰ cm^− 3. By comparing the dependences of the effective Hall mobility on the carrier concentration of ZnO and ITO it is found that grain barriers limit the carrier mobility in ZnO for carrier concentrations as high as 2 × 10²⁰ cm^− 3, independently, if the films were grown on amorphous or single crystalline substrates. Depending on the deposition method, grain barrier trap densities between 10¹² and 3 × 10¹³ cm^− 2 were estimated for ZnO layers. Also, crystallographic defects seem to reduce the mobility for highly doped ZnO films. On the other hand, for ITO films such an influence of the grain barriers was not observed down to carrier concentrations of about 10¹⁸ cm^− 3. Thus the grain barrier trap densities of ZnO and ITO are significantly different, which seems to be connected with the defect chemistry of the two oxides and especially with the piezoelectricity of zinc oxide.     

Enhanced conductivity of pulsed laser deposited n-InGaZn6O9 films and its rectifying characteristics with p-SiC

Wide band gap InGaZn₆O₉ films of thickness ~ 350 nm were deposited on sapphire (0001) at room temperature by using the pulsed laser deposition technique. The transparent films showed the optical transmission of > 80% with the room temperature Hall mobility of ~ 10 cm²/V s and conductivity of 4 × 10² S/cm at a carrier density > 10²⁰ cm^− 3. The electrical properties as a function of deposition temperatures revealed that the conductivity and mobility almost retained up to the deposition temperature of 200 °C. The films annealed in different atmospheres suggested oxygen vacancy plays an important role in determining the electrical conductivity of the compound. Room temperature grown heterostructure of n-InGaZn₆O₉/p-SiC showed a good rectifying behavior with a leakage current density of less than 10^− 9 A/cm², current rectifying ratio of 10⁵ with a forward turn on voltage ~ 3 V, and a breakdown voltage greater than 32 V.     

Tungsten-doped ZnO transparent conducting films deposited by direct current magnetron sputtering

Highly conducting and transparent thin films of tungsten-doped ZnO (ZnO:W) were prepared on glass substrates by direct current (DC) magnetron sputtering at low temperature. The effect of film thickness on the structural, electrical and optical properties of ZnO:W films was investigated. All the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity first decreases with film thickness, and then increases with further increase in film thickness. The lowest resistivity achieved was 6.97 × 10⁻⁴ Ω cm for a thickness of 332 nm with a Hall mobility of 6.7 cm² V⁻¹ s⁻¹ and a carrier concentration of 1.35 × 10²¹ cm⁻³. However, the average transmittance of the films does not change much with an increase in film thickness, and all the deposited films show a high transmittance of approximately 90% in the visible range.     

A study on the interface and bulk charge density of AlN films with sputtering pressure

An attempt to correlate deposition-induced effects and the morphological properties with the electrical properties of the aluminum nitride (AlN) films have been made. The AlN film was sputter deposited on silicon while increasing the pressure in steps from 2×10⁻³ to 8×10⁻³ mbar. An X-ray diffractogram revealed that the intensity of (0 0 2) orientation increased till 6×10⁻³ mbar pressure, but it changed to (1 0 0) orientation of the AlN film at 8×10⁻³ mbar. The FTIR spectra of the absorption band of the films were observed around 682 cm⁻¹ and became prominent at 6×10⁻³ mbar. A decrease in the grain size was seen by SEM images at 8×10⁻³ mbar. The AFM measurements revealed that the surface roughness varied from 1.56 to 3.24 nm with pressure. It was found that the insulator charge density (Q_in) increased from 1.4×10¹¹ cm⁻² to 1.3×10¹² cm⁻² with increase in pressure. On the other hand, the interface state density (D_it) was found minimum (7.3×10¹¹ eV⁻¹ cm⁻²) at 6×10⁻³ mbar. It is found that presence of the Q_in and D_it are primarily governed by the sputtering pressure of the AlN film.     

ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor

Recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interest. In this study, we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 and 250 °C. At typical growth conditions, pure ZnO thin films were obtained without any detectable carbon contamination. For comparison of key film properties including microstructure and chemical and electrical properties, ZnO films were also prepared by rf sputtering at room temperature. The microstructure analyses by X-ray diffraction have shown that both of the ALD and sputtered ZnO thin films have (002) preferred orientation. At low growth temperature T_s ≤ 125 °C, ALD ZnO films have high resistivity (> 10 Ω cm) with small mobility (< 3 cm²/V s), while the ones prepared at higher temperature have lower resistivity (< 0.02 Ω cm) with higher mobility (> 15 cm²/V s). Meanwhile, sputtered ZnO films have much higher resistivity than ALD ZnO at most of the growth conditions studied. Based upon the experimental results, the electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed focusing on the comparisons between ALD and sputtering.     

Deposition of Al-doped ZnO thin-films with radio frequency magnetron sputtering for a source/drain electrode for pentacene thin-film transistor

Al-doped ZnO thin-films were deposited with the radio frequency magnetron sputtering technique at various temperatures and sputtering powers for a source/drain electrode in the pentacene thin-film transistor. With the increase in the deposition temperature and the decrease in the radio frequency sputtering power, the crystallinity was increased and the surface roughness was decreased, which lead to the decrease in the electrical resistivity of the film. Al-doped ZnO film deposited at 200 °C and sputtering power of 50 W showed a low resistivity (9.73 × 10⁴ μΩcm), high crystallinity, low roughness and uniform surface morphology. The pentacene thin-film transistor fabricated with Al-doped ZnO film as a source/drain electrode showed a device performance, (mobility: 7.89 × 10^− 3 cm²/Vs and on/off ratio: ~ 5 × 10⁴) which is comparable with an indium tin oxide electrode grown at room temperature.     

Transparent conducting Al and Y codoped ZnO thin film deposited by DC sputtering

Transparent conducting Al and Y codoped zinc oxide (AZOY) thin films with high transparency and low resistivity were deposited by DC magnetron sputtering. The effects of substrate temperature on the structural, electrical and optical properties of AZOY thin films deposited on glass substrates have been investigated. X-ray diffraction spectra indicate that no diffraction peak of Al₂O₃ or Y₂O₃ except that of ZnO (0 0 2) is observed. The AZOY thin film prepared at substrate temperature of 250 °C has the optimal crystal quality inferring from FWHM of ZnO (0 0 2) diffraction peak, but the AZOY thin film deposited at 300 °C has the lowest resistivity of 3.6 × 10⁻⁴ Ω-cm, the highest mobility of 30.7 cm² V⁻¹ s⁻¹ and the highest carrier concentration of 5.6 × 10²⁰ cm⁻³. The films obtained have disorderly polyhedral surface morphology indicating possible application in thin film solar cell with good quality and high haze factor without the need of post-deposition etching.     

Effects of sputtering power and pressure on properties of ZnO:Ga thin films prepared by oblique-angle deposition

The effects of power and pressure on radiofrequency (RF) diode sputtering in oblique-angle (80°) deposition arrangement are presented. Oblique-angle sputtering of ZnO:Ga (GZO) thin films resulted in a tilted columnar crystalline structure and inclination of the c-axis by an angle of approximately 9° with respect to the substrate. This improved their structural, electrical and optical properties in comparison with films deposited perpendicularly to the substrate. GZO films sputtered by an RF power of 600 W at room temperature of the substrate in Ar pressure 1.3 Pa showed strong crystalline (002) texture, lowest electrical resistivity 3.4 × 10^− 3 Ωcm, highest electron mobility 10 cm² V^− 1 s^− 1, high electron concentration 1.8 × 10²⁰ cm^− 3 and good optical transparency up to 88%. The small inclination angle of the film structure is caused by the high kinetic energy of sputtered species and additional energetic particle bombardment causes random surface diffusion, which is suppressing the shadow effect produced by oblique-angle sputtering.     

Post deposition annealing temperature effect on silicon quantum dots embedded in silicon nitride dielectric multilayer prepared by hot-wire chemical vapor deposition

The preparations of the 20-period of a Si quantum dot (QD)/SiN_x multilayer in a hot-wire chemical vapor deposition (HWCVD) chamber is presented in this paper. The changes in the properties of Si-QDs after the post deposition annealing treatment are studied in detail. Alternate a-Si:H and SiN_x layers are grown in a single SiN_x deposition chamber by cracking SiH₄, and SiH₄ + NH₃, respectively at 250 °C. The as-deposited samples are annealed in the temperature range of 800 °C to 950 °C to grow Si-QDs. All the samples are characterized by confocal micro Raman, transmission electron microscope (TEM), and photoluminescence (PL) to study the changes in the film structures after the annealing treatment. The micro Raman analysis of the samples shows the frequency line shifting from 482 cm^− 1 to 500 cm^− 1 indicating the Si transition from an amorphous to a crystalline phase. The TEM micrograph inspection indicates the formation of Si-QDs of size 3 to 5 nm and a density of 5 × 10¹²/cm². The high resolution TEM micrographs show an agglomeration of Si-QDs with an increase in the annealing temperature. The PL spectra show a peak shifting from 459 nm to 532 nm with increasing the annealing temperature of the film.     

Effect of thickness of ZnO active layer on ZnO-TFT's characteristics

We have investigated the electrical characteristics of ZnO thin film transistors with respect to the thickness of ZnO active layers. The ZnO layers with the thickness of 30 nm to 150 nm were deposited on bottom gate patterned Si substrate by RF sputtering at room temperature. The low-temperature oxide served as gate dielectric. As ZnO channel layer got thicker, the leakage current at V_DS = 30 V and V_G = 0 V greatly increased from 10^− 10 A to 10^− 6 A, while the threshold voltage decreased from 15 V to 10 V. On the other hand, the field effect mobility got around 0.15 cm²/V s except for the 30-nm-thick channel. Overall, the 55-nm-thick ZnO channel layer showed the best performance.