Optical and Electrophysical Properties of Thin Zinc Oxide Films Doped with Manganese Oxide and Obtained by Laser Deposition

Nanostructured thin films on a silicon substrate were obtained on a ceramic of zinc oxide doped with manganese oxide by high-frequency periodic pulsed laser action with f ~ 10–15 kHz, wavelength λ = 1.064 μm at power density q = 150 MW/cm2 in a vacuum chamber with p = 2.7 Pa. The surface morphology and the elemental composition of the obtained films were studied using atomic force microscopy, scanning electron microscopy, and X-ray spectral microanalysis. Features of the transmission spectra in the visible, near, and middle IR regions were determined. The electrophysical properties of the ZnO + 2% MnO2/Si heterostructure were analyzed.

The surface morphology of the samples was investigated with a Solver P47-Pro scanning probe microscope (NT-MDT, Russia) in the AFM regime. Contactless silicon cantilevers of the whisker type with stiffness coeffi cient 2.5-10 N/m, resonance frequency 115-190 kHz, and needle tip radius of curvature 1-3 nm were used. The AFM investigations were carried out in amplitude-frequency modulation mode by the constant force method [13].
The structure of the samples was investigated on a scanning electron microscope (SEM) with normal incidence of the beam on the surface of the sample. The signals of the refl ected and secondary electrons were recorded simultaneously at an accelerating potential of 20 kV. X-ray microanalysis was used to identify the elements and determine the elemental composition. The investigations were carried out on an Aztec Energy Advanced X-Max 80 energy-dispersive nitrogenfree spectrometer (Oxford Instruments, Great Britain), which provides an extended range of detectable elements (from beryllium to plutonium) and highly accurate determination of the concentration of the light elements in according to ISO 15632:2002 as well as high energy resolution. (The MnK α resolution is not worse than 125 eV.) In order to study the distribution of the elements over the surface of the sample, a given line was scanned with an electron beam.
The transmission of optical radiation in the near infrared region by the thin fi lms was measured on a Carry 500 Scan spectrophotometer. The transmission spectra in the mid-IR region were recorded on a NEXUS IR Fourier spectrometer (Thermo Nicolet) in the region of 400-4000 cm -1 . The sputtered ceramic targets were obtained by pressing at 500 MPa, and sintering was carried out in air in a laboratory chamber electric furnace at T = 1350 o C for 2 h. The relative density of the samples was 95% of the theoretical value. The CVC measurements were made on a Keithley series 2450 source meter with a multispectral source of laser radiation with wavelengths of 405, 450, 520, 660, 780, 808, 905, and 980 nm in the region of 405-980 nm based on semiconductor lasers of the LDI type with calibrated radiation power of 2 mW. The FVC measurements were made on a laboratory bench based on an E7-20 emittance meter at room temperature without illumination at a signal frequency of 100 kHz and 1 MHz.
Results and Discussion. A typical SEM of the microstructure of the initial target is shown in Fig. 12. By AFM it was established that a nanocrystalline structure is formed on the silicon substrate (Fig. 2). The main roughness parameters of the surface of the fi lm were determined by scanning a region measuring 20 × 20 μm at fi ve different points on the sample: mean height of surface relief of the fi lms 72 nm, average arithmetical mean of roughness 12.1 nm. Individual large particles with height of 100-350 nm and lateral dimension of 200-500 nm are observed on the surface of the fi lm ( Fig. 2a, b, d). Their average density is not greater than 1 particle/10 μm 2 . The lateral dimension of the structural elements is 25-30 nm (Fig. 2c). Figure 3 shows the SEM structure of the fi lms at various magnifi cations. The results of the investigation of the structure by the SEM method correlate with the results obtained by AFM: the fi lm is characterized by a nanocrystalline structure, and individual large particles are observed on the surface. By X-ray spectral microanalysis it was found that the MnO 2 dopant is distributed uniformly in the ZnO fi lm: during scanning with an electron beam the presence of manganese and oxygen was observed along the line both in the particles and in the regions between the particles (Fig. 3c). It is thus possible to obtain nanocrystalline fi lms of ZnO + 2% MnO 2 with uniform composition by laser deposition.
The transmission of the laser-deposited ZnO + 2% MnO 2 /Si fi lm in the near IR region of 2.2-2.6 μm amounts to ~2% (Fig. 4a), while in the middle IR region of 488-661 cm -1 (20.5-15.1 μm) T ~ 25% with a decrease in transmission to T = 18.6% at 611 cm -1 (Fig. 4b), which is a characteristic absorption band corresponding to vibration of the Mn-O bond [14]. The refl ection spectrum of the ZnO + 2% MnO 2 /Si fi lm on the silicon substrate in the visible and near IR regions is shown in Fig. 4c. The refl ection in the UV (200-400 nm) and visible regions is less than in the near-IR region. The region of transparency of the ZnO + 2% MnO 2 fi lm and absorption of the incident radiation is characteristic of zinc oxide fi lms [15]. Figure 5a shows the FVC of the ZnO + MnO 2 /Si structure. Irrespective of the frequency of the signal the FVC has the form characteristic of high-frequency dependence of the capacitance on the voltage of the MOS structure on a silicon substrate with p-type conductivity. The capacitance of the oxide has a fl at form at negative voltages. As seen, the capacitance decreases with increase of frequency and at low frequencies does not go into saturation mode at negative voltages. During investigation of the electric characteristics ZnO/Si systems are usually regarded as heterostructures since zinc oxide is a direct-gap n-type semiconductor. However, ZnO fi lms have a band gap of 3.37 eV, and in structures with the narrower band gap of monocrystalline silicon it can behave as a dielectric at high signal frequencies [16]. Hysteresis is not observed in the measured FCV characteristics, which indicates the absence of fi xed charge on the dielectric, but the fl at form at negative voltages in the capacitance modulation region with a signal frequency of 1 MHz indicates the presence of embedded surface states (traps) in the oxide fi lm and at the ZnO + 2% MnO 2 /Si interface. The charge carriers captured on these traps do not manage to recharge with increase of the frequency, and the total capacitance of the system therefore decreases [17].
The current-voltage characteristic of the ZnO + 2% MnO 2 /Si structure (Fig. 5b) is typical of a heterostructure: in the region of positive voltages it is possible to distinguish two sections each of which is described by exponential dependence of the current on the voltage I ~ U m . On the fi rst section the voltage is < 1.6 V (m = 0.87), and on the second  m ≈ 1, i.e., the conductivity is close to ohmic. As in the case of zinc oxide, the conductivity of the ZnO + 2% MnO 2 is determined by current restricted by space charge [16].
Since the doped zinc oxide exhibits photosensitivity over a wide spectral range [18], to determine the spectral sensitivity of the ZnO + 2% MnO 2 /Si structure the current-voltage characteristics were measured at positive voltages with the use of a multispectral laser source under the action of laser radiation with λ = 405-980 nm (Fig. 6a). The spectral dependence of the structure was constructed with a voltage shift of +2 V. (The highest photosensitivity occurs in this region of voltages.) The highest photosensitivity of 30.41 mA/W is observed with a voltage shift of +2 V at λ = 905 nm (Fig. 6b). Discovery of maximum photosensitivity in the IR region makes it possible to suppose that this effect is determined by the electron capture levels -by traps at the ZnO + MnO 2 -silicon interface.