Dependence of electrical and optical properties of ZnO films on substrate temperature

ZnO films were prepared by filtered cathodic vacuum arc technique with Zn target at different substrate temperatures. The crystallinity is enhanced with increasing substrate temperature and preferably oriented at (1 0 3) direction when the substrate temperature is higher than 230°C. The PL emission corresponding to the exciton transition at 3.37 eV can be observed at room temperature, which indicates that high-quality films have been obtained by this technique. The Hall mobility, which increases with substrate temperature, is dominated by grain boundary scattering.     

Structural and optical properties of Zn<Subscript>1−x</Subscript>Mg<Subscript>x</Subscript>O thin films synthesized with metal organic chemical vapor deposition

We present the structural and optical properties of Zn_1?xMg_xO thin films studied using x-ray diffraction (XRD), extended x-ray absorption fine structure (EXAFS), and photoluminescence (PL) measurements. The Zn_1?xMg_xO films on sapphire [0001] substrates were fabricated with metal organic chemical vapor deposition (MOCVD). The XRD measurements showed that the Zn_1?xMg_xO films (x≤0.05) had a wurtzite structure without any MgO phase and were epitaxially grown along the c-axis of the Al₂O₃ substrate. The lattice constant of the Zn_0.95Mg_0.05O film shrank by 0.023 Å, compared with that of ZnO crystals. From the EXAFS measurements on the Zn_1?xMg_xO films at Zn K-edge, we found a substantial amount of distortion in the bond length of Zn-Zn pairs with a small amount of Mg substitution on the Zn site. The PL measurements showed a gradual increment of the main exciton transitions from 3.36 eV (x=0.0) to 3.57 eV (x=0.05) at 10 K. We also observed a strong deep-level emission near 2.3 eV from the specimen with x=0.05.     

Electrical and optical properties of lodine doped CdZnTe layers grown by metalorganic vapor phase epitaxy

Electrical and photoluminescence properties of iodine doped CdZnTe (CZT) layers grown by metalorganic vapor phase epitaxy have been studied. Doped layers showed an n-type conductivity from the Zn composition x=0 (CdTe) to 0.07. Above x=0.07, resistivities of doped layers increased steeply up to 10⁶ Ω-cm. Resistivities of doped CZT layers were higher than those of undoped layers above x=0.6. Photoluminescence intensity of doped layers increased compared to undoped layers. Doped CdTe and ZnTe layers showed neutral donor bound exciton emission lines at the exciton related region. Also, these layers showed an increase in emission intensity at the donor acceptor pair recombination bands. Sharp emission lines were observed in doped CZT layers at around 1.49 eV. These emission lines were considered to be originated from GaAs substrates which were optically excited by the intense emission from doped CZT layers.     

Photoluminescence measurements on undoped CdZnTe grown by the high-pressure bridgman method

The low temperature photoluminescence of Cd_0.91Zn_0.09Te grown by the high-pressure Bridgman (HPB) method exhibits a neutral donor bound exciton emission (D⁰X) at 1.65603 eV with its excited state (D⁰X^*) at 1.65798 eV and neutral acceptor bound exciton emissions (A⁰X) at 1.64566 eV and 1.65201 eV. Assuming a direct generation and subsequent relaxation of excitons at the D⁰X^* state, we demonstrate that the temporal evolution of the above emission bands is well reproduced by a set of rate equations. The resultant radiative-lifetime of 1.4 ns for the D⁰X and 1.5 and 2.0 ns for the A⁰Xs are compared with various CdZnTe's (CZTs) grown by the other methods to demonstrate the particular nature of the HPB CZT.     

Low temperature photoluminescence of lightly Si-doped and undoped MBE GaAs

A semiquantitative low temperature (~4 K) photoluminescence technique has been used to estimate the residual carbon concentration in MBE GaAs to be ~2 × 10¹⁴ cm^?3 . This value agrees well with the total compensating acceptor concentration calculated from electrical measurements and thus confirms that carbon is the dominant residual acceptor in MBE GaAs. In high purity MBE GaAs films grown at substrate temperatures between 545°C and 625°C a band of at least nine luminescence peaks is observed in the 1.471 eV to 1.491 eV spectral region. These peaks can be correlated with the most prominent "defect-induced" bound exciton peaks in the 1.504 eV to 1.511 eV spectral region and their transition energies follow the empirical relation $$hv_{C_{As}^o ,X} - hv_{d,X} = 0.38[hv_{e,C_{As}^o } - hv_{e,d} ].$$ This suggests that the luminescence in these two spectral regions have a common origin at a set of “defect-complexes” which involve carbon impurities.     

Effect of nonstoichiometry and doping on the photoconductivity spectra of GeSe layered crystals

The polarization photoconductivity spectra of Bi-doped nonstoichiometric GeSe layered crystals grown by static sublimation were investigated. Two strongly polarized maxima at the photon energies hν_max = 1.35 eV (E ∥ a) and 1.44 eV (E∥ b) due to the V ₁ ^V → V ₁ ^c and Δ ₂ ^v → Δ ₁ ^c optical transitions, respectively, were found in the spectra of nominally undoped GeSe crystals near the intrinsic absorption edge at 293 K. In the low-temperature region, an exciton photoconductivity band peaked at hν_max=1.32 eV, which is due to exciton dissociation at the cation vacancies, was revealed. With an increase in excess Se in crystals, a sharp increase in the intensity of the exciton peak in the photoconductivity spectra was observed. It is shown that doping of GeSe crystals with donor Bi impurity is an effective tool of the control of their electrical and photoelectric properties. Although introduction of Bi into germanium monoselenide does not lead to the conductivity conversion from the p to n type, a sharp increase in the resistivity is observed, the crystals become photosensitive, and a strong impurity band peaked at 1.11 eV arises in the photoconductivity spectra.     

Electron bombardment induced defect states in p-InP

A deep level transient spectroscopy (DLTS) study has been made of 1 MeV electron bombardment induced defect states in the lower half band gap of LEC grown Zn doped p-InP. One state was observed in the unirradiated material with a hole emission activation energy H of 0.15 eV. Irradiation resulted in two new states with H of 0.34 and 0.58 eV, and introduction rates dN_t/d? of 1 and 0.04 cm^?1, respectively. Annealing experiments revealed the appearance of an additional state with H(0.52 eV), and recovery of the 0.34 eV defect state above 150°C. The 0.52 eV and 0.58 eV states were found at the highest concentration near the metal-semiconductor interface. The implications of the large introduction rate of the 0.34 eV state are discussed.     

Characterization of sputtered ZnO films under different sputter-etching time of substrate

Polycrystalline ZnO films are prepared using radio frequency magnetron sputtering on glass substrates which are sputteretched for different time. Both the size of ZnO grains and the root-mean-square (RMS) roughness decrease, as the sputteretching time of the substrate increases. More Zn atoms are bound to O atoms in the films, and the defect concentration is decreased with increasing sputter-etching time of substrate. Meanwhile, the crystallinity and c-axis orientation are improved at longer sputter-etching time of the substrate. The Raman peaks at 99 cm⁻¹, 438 cm⁻¹ and 589 cm⁻¹ are identified as E₂(low), E₂(high) and E₁(LO) modes, respectively, and the position of E₁(LO) peak blue shifts at longer sputter-etching time. The transmittances of the films, which are deposited on the substrate and etched for 10 min and 20 min, are higher in the visible region than that of the films deposited under longer sputter-etching time of 30 min. The bandgap increases from 3.23 eV to 3.27 eV with the increase of the sputter-etching time of substrate.     

Structural and Optical Properties of ZnO Nanotips Grown on GaN Using Metalorganic Chemical Vapor Deposition

ZnO nanotips are grown on epitaxial GaN/c-sapphire templates by metalorganic chemical vapor deposition. X-ray diffraction (XRD) studies indicate that the epitaxial relationship between ZnO nanotips and the GaN layer is (0002)ZnO||(0002)GaN and (101̄0)ZnO||(101̄0)GaN. Temperature-dependent photoluminescence (PL) spectra have been measured. Sharp free exciton and donor-bound exciton peaks are observed at 4.4 K with photon energies of 3.380 eV, 3.369 eV, and 3.364 eV, confirming high optical quality of ZnO nanotips. Free exciton emission dominates at temperatures above 50 K. The thermal dissociation of these bound excitons forms free excitons and neutral donors. The thermal activation energies of the bound excitons at 3.369 eV and 3.364 eV are 11 meV and 16 meV, respectively. Temperature-dependent free A exciton peak emission is fitted to the Varshni’s equation to study the variation of energy bandgap versus temperature.     

Low-Temperature Photoluminescence Study of CdTe:In Crystals Annealed in Molten Bismuth

We used a low-temperature photoluminescence (PL) technique to investigate CdTe:In crystals after annealing in molten bismuth (Bi). The two annealed samples showed different resistivities after the treatment. For both samples, we observed very strong emissions in the excitonic spectral region and revealed fine structures of exciton emissions in the PL spectrum. In the sample with high resistivity, we found one ionized donor-bound exciton peak, (D⁺,X), that we ascribed to incorporated Bi atoms occupying Cd sites in the CdTe. The temperature dependence of the (D⁺,X) peak emission had an associated activation energy of 3.59 meV for the exciton bound to this ionized donor. Meanwhile, a donor–acceptor pair peak at 1.5315 eV, which was absent from the PL of the low-resistivity sample, suggested the likelihood of some Bi atoms occupying Te sites in the high-resistivity sample. Our findings highlight the need for detailed investigation of annealing conditions to ensure precise control of the electrical properties of the material during annealing in molten Bi.     

Exciton characteristics of intercalated TlGaSe2 single crystal

It is shown that lithium-ion intercalation of TlGaSe₂ single crystal leads to a shift of the exciton peak associated with the direct edge toward longer wavelengths (ΔE=15 meV at 5 K). As a result, the temperature shift of the exciton peak in TlGaSe₂(∂E ^ex/∂T) decreases more than twofold in absolute value to −1.1×10⁻⁴ eV/K at 20≲T≲105 K and −0.25×10⁻⁴ eV/K at 5≲T≲20 K. Lithium-ion (Li⁺) intercalation of TlGaSe₂ has almost no effect on the energy position of the exciton associated with indirect transitions. Fiz. Tekh. Poluprovodn. 32, 145–147 (February 1998)     

Structural and electrical properties of brush plated ZnTe films

Zinc telluride thin films were deposited by the brush plating technique at a potential of −0.90 V (SCE) on conducting glass and titanium substrates at different temperatures in the range 30–90 °C. The films were polycrystalline in nature with peaks corresponding to the cubic phase. Direct band gap of 2.30 eV was observed. XPS studiers indicated the formation of ZnTe. Depth profiling studies indicated a uniform distribution of Zn and Te throughout the entire thickness. EDAX measurements were made on the films and it was found that there was a slight excess of Te. The carrier concentration was found to vary from 10¹⁴–10¹⁵ cm⁻³ with increase of substrate temperature. The mobility was found to vary from 5 to 60 cm² V⁻¹ s⁻¹.     

Characterization of deep defects in boron-doped CVD diamond films using transient photocapacitance method

We have developed a highly-sensitive transient photocapacitance measurement (TPM) system for deep defects in wide bandgap materials, and applied it to characterize the boron-doped diamond films grown on a high-pressure/high-temperature-synthesized Ib diamond substrate using high-power-density microwave-plasma chemical vapor deposition method. The developed TPM system has both a low detection limit of less than 0.5 fF for changes in the photocapacitance and a low measurement temperature drift of less than 0.03 K in 12 h. By using the TPM system, we have successfully found an acceptor-type defect around 1.2 eV above the valence-band maximum for the B-doped diamond film with a considerably high crystalline quality that had some strong exciton emission peaks in the cathodoluminescence spectra taken at ≈80 K. The photoionization cross section and the defect density estimated for the observed defect were 3.1×10^–15 cm² and 2.8×10¹⁶ cm⁻³, respectively.     

A detailed comparative study on the main electrical parameters of Au/n-Si and Au/PVA:Zn/n-Si Schottky barrier diodes

We have fabricated Au/n-Si and Au/PVA:Zn/n-Si Schottky barrier diodes (SBDs) to investigate the effect of organic interfacial layer on the main electrical characteristics. Zn doped poly(vinyl alcohol) (PVA:Zn) was successfully deposited on n-Si substrate by using the electrospinning system and surface morphology of PVA:Zn was presented by SEM images. The current–voltage (I–V) characteristics of these SBDs have been investigated at room temperature. The experimental results show that interfacial layer enhances the device performance in terms of ideality factor (n), zero-bias barrier height (Φ_B0), series resistance (R_s), and shunt resistance (R_sh) with values of 1.38, 0.75 eV, 97.64 Ω, and 203 MΩ whereas those of Au/n-Si SBD are found as 1.65, 0.62 eV, 164.15 Ω and 0.597 MΩ, respectively. Also, this interfacial layer at metal/semiconductor (M/S) interface leads to a decrease in the magnitude of leakage current and density of interface states (N_ss). The values of N_ss range from 1.36×10¹² at E_c—0.569 eV to 1.35×10¹³ eV^?1 cm^?2 at E_c—0.387 eV for Au/PVA:Zn/n-Si SBD and 3.34×10¹² at E_c—0.560 eV to 1.35×10¹³ eV^?1 cm^?2 at E_c—0.424 eV for Au/n-Si SBD. The analysis of experimental results reveals that the existence of PVA:Zn interfacial layer improves the performance of such devices.     

Diffusion and interdiffusion in Zn-disordered AlAs-GaAs superlattices

Several superlattices (SL's) with different layer thicknesses, grown by molecular beam epitaxy (MBE), were disordered via low temperature (550?600°C) Zn diffusions to investigate layer thickness effects on both the Zn diffusion process and the Al-Ga interdiffusion process. The Zn diffusion coefficients were measured using secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) and found to be ?10^?12 cm²/sec, increasing somewhat with decreasing layer thickness. The activation energy for the Zn diffusion process ranged from 3.1 eV for an 1100Å/period SL to 2.1 eV for a 320Å/period SL. The Al-Ga interdiffusion coefficient and the activation energy associated with the interdiffusion process were calculated from AES depth profiles. The coefficient is on the order of 10^?16 cm²/sec and the activation energy is approximately 1 eV, independent of the SL layer thickness.     

Energetic distribution of interface states extracted from photo-conductance of organic thin film transistors

In this paper, a new method is introduced to obtain the energetic distribution of the interface states (density of states; DOS) extracted from the photo-conductance of organic thin film transistors (OTFTs) which exhibit varied transfer characteristics under illumination with different photon energies. The method was applied to pentacene OTFTs, and the results were compared with existing data. The major findings were not only the existence of the well-known peaks of DOS at 1.82 eV (free exciton of pentacene), and at 1.49 eV (extrinsic exciton due to dihydropentacene) but also new peaks were found at 1.25 eV, 1.29 eV, 1.31 eV, and 1.35 eV in the mid-gap. The new peaks were strongly enhanced under exposure to oxygen, and thus seem to be related to the defects associated with the presence of oxygen.     

Effects of heat treatment on the 0.8 eV photoluminescence emission in GaAs grown by molecular beam epitaxy at low temperatures

We report 0.8 eV photoluminescence (PL) emission of GaAs grown at low temperatures between 325 and 400°C by molecular beam epitaxy. Effects of heat treatments of the 0.8 eV emission are compared with those of the 1.467 eV sharp bound exciton lines. This allows us to attribute the 0.8 eV emisson to the As-V_Ga center. We discuss the assigning of the As_i-V_Ga center to the well-known EL6. The PL intensity variation of 0.68 eV EL2 and 0.8 eV As_i-V_Ga seen in substrate materials is explained in terms of dislocation−mediated As_i-V_Ga transformation to EL2 whereas the PL intensity variation of 0.8 eV As_i-V_Ga for molecular beam epitaxy layers can be attributed to the growth condition.     

High performance aluminum arsenic intraband resonant microwave devices

We report on GaAs/AlAs triple-barrier quantum well intraband (TBQWI) heterostructures grown by molecular beam epitaxy (MBE) on n⁺ GaAs substrate. Heterostructure quality was evaluated by X-ray diffraction and photoluminescence spectrum measurements. The position of the broad peak near 65.84° corresponds well to the diffraction from the (4 0 0) face of AlAs layers assuming intensity of total AlAs spacers and barriers. The 10K photoluminescence (PL) data has a strong peak at 8140 Å. The PL spectrum is dominated by a sharp peak centered at the emission energy of 1.52 eV attributed to the energy of e1-hh bond exciton of GaAs layer. TBQWI heterostructures were grown and processed into resonant tunneling diode (RTD). Room temperature electrical measurement of the TBQWI RTD yielded maximum peak to valley current ratio (PVCR) of 120 with peak current density (J_p) of 2.1 kA/cm². The high PVCR of this GaAs/AlAs TBQWI RTD is, to the better of our knowledge, one of the higher PVCRs obtained in any intraband tunnel device.     

Photoluminescent properties of ZnO thin films grown on SiO2/Si(100) by metal-organic chemical vapor deposition

We report on the photoluminescent (PL) properties of ZnO thin films grown on SiO₂/Si(100) substrates using low pressure metal-organic chemical vapor deposition. The growth temperature of the films was as low as 400°C. From the PL spectra of the films at 10–300 K, strong PL peaks due to free and bound excitons were observed. The origin of the near bandedge emission peaks was investigated measuring temperature-dependent PL spectra. In addition, the Zn O films demonstrated a stimulated emission peak at room temperature. Upon illumination with an excitation density of 1 MW/cm², a strong, sharp peak was observed at 3.181 eV.     

Synthesis and characterization of nanostructure CdO:Zn thin films deposited by spray pyrolysis technique: Molarity and heat treatment effects

Preparation of transparent conducting cadmium oxide doped with various concentration of Zinc (3%, 6%, 10%) in the spray solution, on glass substrate by spray pyrolysis is reported. we have tried to improve some physical properties of CdO films by Zn doping, hence the electrical, optical and structural properties of Zn doped CdO films were investigated using X-ray diffraction, Scanning electron microscopy, Hall Effect and UV–Visible spectrophotometry. Optical band gap, refractive index and extinction coefficient are also determined for different concentration. results show 6% doping is appropriate between other doping concentration, thereupon we selected this value and increased substrate temperature to acquire optimal condition. Observations like as mobility increment up to 46.9 (cm²/V s), transmittance up to 82% and increase of band gap up to 2.62 (eV) state the sample with 400 °C substrate temperature is good candidate for transparent and conducting oxide application.