Durga Basak

Effect of substrate-induced strain on the structural, electrical, and optical properties of polycrystalline ZnO thin films

The effect of substrate-induced strain in polycrystalline ZnO thin films on different substrate, e.g., GaN epilayer, sapphire (0001), quartz glass, Si(111)∕SiO2, and glass deposited by sol-gel process, has been investigated by x-ray diffraction, scanning electron microscope, electrical resistivity, and photoluminescence measurements. A strong dependence of orientation, crystallite size, and electrical resistivity upon the substrate-induced strain along the c axis has been found. The results of structural and morphological studies indicate that relatively larger tensile strain exists in ZnO deposited on sapphire and glass, while a smaller compressive strain appears in film deposited on GaN and the strain is relaxed in larger crystallite size. The electrical resistivity of the films increases exponentially with increasing strain. The excitonic peak positions are found to shift slightly towards lower energy side with increasing strain. The analysis shows that GaN being a closely lattice-matched substrate pro...

Ultraviolet and visible photoresponse properties of n‐ZnO∕p‐Si heterojunction

A n‐ZnO∕p‐Si thin film heterojunction has been fabricated by a low cost sol-gel technique. The wavelength dependent photoresponse properties of the heterojunction is investigated in detail by studying the effect of light illumination on current-voltage (I‐V) characteristics, photocurrent, and photocapacitance spectra at room temperature. It shows good diode characteristics with IF∕IR=3.4×103 at 4V and reverse leakage current density of 7.6×10−5Acm−2 at −5V. From the photocurrent spectra, it is observed that the visible photons are absorbed in the depleted p‐Si under reverse bias conditions, while ultraviolet (UV) photons are absorbed in the depleted n‐ZnO under positive bias conditions. This indicates that such a sol-gel n‐ZnO∕p‐Si thin film heterojunction can be used to sense both UV and visible photons though the photoresponse for UV is much slower than that of visible. The photocapacitance measurements suggest the presence of a shallow defect level in the sol-gel derived ZnO film which acts as an elect...

Electrical and ultraviolet photoresponse properties of quasialigned ZnO nanowires/p-Si heterojunction

The authors report on the electrical and ultraviolet (UV) photoresponse properties of quasialigned ZnO nanowires (NWs)∕p-Si heterojunction grown by a low-temperature solvothermal technique. The current-voltage characteristic of a single ZnO NW∕p-Si junction measured by a scanning tunneling microscope shows a rectifying behavior with a rectification ratio IF∕IR of 33 at 5V. The current transport is dominated by the recombination-tunneling mechanism for 0.4V<V<1.5V while by the space-charge-limited current conduction beyond 1.5V. The heterojunction is sensitive to the UV light with the faster rise and decay time constants of 360 and 280ms, respectively.

Role of defects in the anomalous photoconductivity in ZnO nanowires

The anomalous photocurrent decay in aqueous solution grown ZnO nanowires (NWs) under steady ultraviolet light illumination have been investigated. The photocurrent growth-decay measurements using the above-band and subband gap light excitation energies in the as-grown and annealed NWs show that while a VZn-related defect complex is formed by the surface adsorbed H2O molecules, a faster carrier trapping by the surface adsorbed O2 molecules and a slower carrier recombination at the defect, Zni cause the photocurrent decay under steady illumination supported by the results of the photocurrent spectra and photoluminescence measurements. The predicted mechanism has been explained through a model.

Photoluminescence and Photoconductivity of ZnS-Coated ZnO Nanowires

ZnO nanowires (NWs) with a ZnS coating are synthesized in order to modify the surface without changing the diameter of the NWs. They have the wurtzite ZnO at the core and a cubic ZnS at the outer layer. The NWs show a sharp ultraviolet and a broad visible emission of the photoluminescence spectra. Surface modification has led to a change in the position of the maxima of the visible emission in ZnO-ZnS NWs. The photocarrier relaxation under steady UV illumination occurs in ZnO NW arrays but is absent in ZnO-ZnS NW arrays. The dark current value for both type of NWs are similar, whereas the photocurrent value is much higher in the surface-modified NWs. Higher photocurrent value indicates a transport of the photogenerated carriers from the ZnS layer to ZnO during UV illumination. The carrier transport mechanism is proposed through a model.

Carrier relaxation through two-electron process during photoconduction in highly UV sensitive quasi-one-dimensional ZnO nanowires

We have investigated the carrier relaxation process during photoconduction in quasi-one-dimensional (Q1D) ZnO nanowires (NWs) of diameters 29–36nm on different substrates using photocurrent transient measurements. Ultraviolet (UV) sensitive NWs show around three to four orders of change in the photo-to-dark current ratio. Under steady UV illumination, the photocarrier relaxation occurs through two-electron process—carrier loss due to the trapping by the surface states and recombination at the deep defect states. The results demonstrate that the carrier relaxation during photoconduction in Q1D NWs of diameter comparable to the Debye length is also dominated by the surface states.We have investigated the carrier relaxation process during photoconduction in quasi-one-dimensional (Q1D) ZnO nanowires (NWs) of diameters 29–36nm on different substrates using photocurrent transient measurements. Ultraviolet (UV) sensitive NWs show around three to four orders of change in the photo-to-dark current ratio. Under steady UV illumination, the photocarrier relaxation occurs through two-electron process—carrier loss due to the trapping by the surface states and recombination at the deep defect states. The results demonstrate that the carrier relaxation during photoconduction in Q1D NWs of diameter comparable to the Debye length is also dominated by the surface states.

p-ZnO∕n-Si heterojunction: Sol-gel fabrication, photoresponse properties, and transport mechanism

p-ZnO∕n-Si heterojunction is achieved by depositing Al–N codoped p-type ZnO film on n-Si by low-cost sol-gel technique. The junction shows good diode characteristics with rectification ratio (IF∕IR)∼10 at 4V in the dark. The photoresponse of the heterojunction is investigated by studying the current-voltage characteristics under the ultraviolet (370nm) and visible light (450nm) illuminations. By fitting the experimental data, we have proposed the current transport mechanism to be dominated by the recombination tunneling at lower and by the space-charge limited current at higher forward voltages, which are further supported by the photocapacitance and photocurrent spectra.

Aluminium doped ZnO films: electrical, optical and photoresponse studies

A series of Al doped ZnO (ZnO : Al) films with different Al concentrations have been deposited on glass substrates using the sol–gel spin coating technique and the effect of Al concentrations on the structural, electrical, optical and photoresponse properties have been investigated. The XRD results show the presence of peaks due to the reflections of the planes from a wurtzite type of ZnO structure. The surface morphology shows that the grains become non-uniform and smaller in size as the Al doping level increases. For 1–2% Al doping, the film attains highest carrier concentration of about ~2.7 × 1019 cm−3 and lowest resistivity of ~2 × 10−2 Ω cm. The excitonic nature in the absorption spectrum disappears for doping above 1%. The band gap increases with the increase in the Al concentration. The photoconductivity studies show that although the photoresponse properties are degraded due to incorporation of Al atoms, the 1% Al doped film shows the best photoresponse properties within the doping limit up to 5%.

Investigation of a p-CuO/n-ZnO thin film heterojunction for H2 gas-sensor applications

A p-CuO/n-ZnO thin film heterojunction is fabricated on a glass substrate by the sol–gel technique. The crystallinity of the junction materials and microstructure of the top p-layer are examined by an x-ray diffractometer (XRD) and scanning electron microscope (SEM). The current–voltage (I–V) characteristics of the p–n heterojunction and its temperature dependence have been investigated in air and H2 ambient. Although the junction possesses linear I–V characteristics from room temperature (RT) to 150 °C in air, at higher temperatures (200 °C to 300 °C), it shows nonlinear rectifying behaviour. The forward current is greatly increased with increasing temperature while the reverse current is increased slightly resulting in a IF/IR ratio as high as 485. The ideality factor (n) is 4.88 at a temperature of 300 °C. The forward current is highly increased by the introduction of H2 gas at 300 °C. However, a simultaneous increase in the reverse current makes the IF/IR ratio 8.4. It is observed that H2 sensitivity of the heterojunction is increased with the increase in temperature as well as the thickness of CuO film. A sensitivity value as high as 266.5 is observed at 300 °C when biased at 3 V in the presence of approximately 3000 ppm of H2.

ZnO/polyaniline based inorganic/organic hybrid structure: Electrical and photoconductivity properties

Single layer ZnO/polyaniline (PANI) inorganic/organic hybrid structure fabricated on glass substrate by depositing polyaniline film on the sol-gel ZnO thin film shows a rectifying behavior indicating the formation of a diode. The current transport mechanism is modeled through an energy band diagram. The heterojunction is sensitive to UV illumination. The photo-to-dark current ratio of the junction is about 46 for −5V bias. The photoresponse parameters of the diode are found to be better than those of only ZnO film of similar thickness. The results indicate that the ZnO/PANI hybrid structure might be promising for the UV photodetection applications.