Semiconductor characteristics of Nd doped PbO-Bi2O3-Ga2O3 films

NEW MATERIALS AND DEVICES

Semiconductor characteristics of Nd doped PbO-Bi₂O₃-Ga₂O₃ films

Luciana R. P. Kassab^I; Ronaldo D. Mansano^II; Luís da S. Zambom^I; Vanessa D. Del Cacho^II

^ILaboratório de Vidros e Datação, FATEC-SP/CEETEPS, Praça Cel. Fernando Prestes, 30 - CEP: 01124-060 - São Paulo - SP - Brazil

^IILaboratório de Sistemas Integráveis, Departamento de Engenharia de Sistemas Eletrônicos - EPUSP Av. Prof. Luciano Gualberto, 158, 05508-900, São Paulo, SP, Brazil

ABSTRACT

In this work optical and semiconductor characteristics of gallium oxide based thin films are studied. This material has important electro-optic applications and offers possibilities for semiconductor applications in heavy metal oxide semiconductor technology. In this study thin films of PbO-Bi₂O₃-Ga₂O₃ were deposited by reactive sputtering. Two targets with different concentrations of Nd₂O₃ (0.2 and 1.0 wt%) were prepared. Thin films were deposited by pure Ar plasma, at 5 mTorr pressure and RF power of only 20 W. Films were deposited on three inch diameter substrates of (100) silicon wafers. Optical UV-Vis-Nir transmittance and FTIR analyses showed that the films are transparent in the visible and middle infrared region of the electromagnetic spectrum. The refractive index of the films was of about 2.3, as measured by ellipsometry; Rutherford Backscattering Spectroscopy (RBS) analyses were employed for the compositional analysis of the films. For the electrical characterization it was used metallic electrodes, and the final structure obtained is a capacitor structure where the gallium oxide film plays the role of a dielectric. In the current versus voltage (IxV) analyses, the samples with 1.0 wt% of Nd₂O₃ show semiconductor characteristics similar to the ones obtained for the degenerated transistors. The electric characteristics of these films allow applications with electro-optic devices (phototransistors and photo detectors).

Keywords: PbO-Bi₂O₃-Ga₂O₃ films; Optical and semiconductor characteristics; Nd³+ doped

I. INTRODUCTION

The study of rare-earth doped glass for lasers and optical amplifiers has become a subject of renewed interest now that high power cw diode lasers are available for use as efficient pump sources. With the development of powerful diode lasers emitting around 800nm studies of laser materials doped with Nd³⁺ were renewed. Most solid state Nd laser systems pumped by high power diode lasers are now standard tools for many applications due to their high efficiencies. This has motivated investigations of Nd³⁺ doped glass thin film waveguides that offer a possible means of incorporating sources and amplifiers into integrated optical circuits. In this work we report studies of sputtered Nd:glass films based on heavy metal oxides (HMO) [1]. The films are produced using PbO-Bi₂O₃-Ga₂O₃ glass doped with Nd₂O₃ for the target. It has been shown recently that heavy metal oxide (HMO) glasses are very attractive hosts for rare-earth ions with applications in photonic and optoelectronic [2,3,4]. PbO and Bi₂O₃ oxides determine the optical properties and GaO favors the thermo stability and the electrical properties. The motivations for the use of this composition for the thin films are due to the following properties: large thermal expansion coefficient (123x10^-7/K), high refractive index (-2.3) with applications in photonics, good physical and chemical stability and large transmission window (550 nm to 8000 nm). We have to remember that wide infrared transmission window normally indicates that the vibrational phonon energy is small and this provides opportunities towards the realization of more efficient lasers. The other motivation for the production and study with these films is based on the previous results obtained with PbO-Bi₂O₃-Ga₂O₃ glasses doped with Nd₂O₃ [5].

II. EXPERIMENTAL

Target manufacturing: The target was manufactured from four 99wt% pure oxides: 12wt%Ga₂O₃-46wt%PbO-42wt%Bi₂O₃-(0.2wt% and 1wt%)Nd₂0₃. The reagents were melted in a platinum crucible at 1000 ^oC during 1 hour. Then the melt was poured into a graphite mould and annealed at 300 ^oC for 3 hours.

Substrates: 75 mm diameter silicon wafer were used as substrates. The silicon wafers have a resistivity between 1 and 10 Ohm.cm, p type, (100) orientation.

Deposition: The sputtering processes were performed at room temperature, 5mTorr pressure, RF (13.56 MHz) power of (only) 20 W, with pure argon (99.995%). Before deposition the base pressure was as low as 9 x 10^-7 Torr to minimize the number of impurities.

Analysis: the film thickness, were measured with a Height Step Meter Dektak 3030. The refractive index was determined with an ellipsometer with three wavelengths: 405 nm, 632.8 nm and 830 nm.The chemical composition was measured by RBS (Rutherford Backscattering Spectrometry) using a de He⁺⁺ beam with 2.2 MeV of energy. Optical transmission was measured with a Cary 5200 UV-Vis-NIR spectrometer, in the 220 nm to 2550 nm wavelength range. Transmission in the infrared region was performed in a JASCO FT-IR system. Photoluminescence was determined using an Argon laser at 514 nm. For photonics measurements, contacts were deposited at the front side of the wafer, by aluminium evaporation in order to obtain 300 nm thick layer. At the backside of the wafer, a 500-nm thick aluminium film was evaporated. The dark and light current was measured by HP 4140A picoamperemeter.

III. RESULTS AND DISCUSSION

The properties of the films were compared with those of the glass target [5] in terms of both physical and chemical composition. The deposition rate is of (1.1 ± 0.1) nm/min as can be observed by the results shown Table 1. The variations of the refractive index are within the experimental error. As the film absorbs light at 405 nm, it was not possible to measure its refractive index at that wavelength.

The RBS technique confirmed that the atomic composition of the deposited film corresponds to the one of the glass target employed (Figure 1). This may be attributed to the use of pure argon plasmas. The absorption spectrum of the Nd³⁺ is related to the electronic transitions from the ground state ⁴I_9/2, to various excited states. Figure 2 presents the absorption spectrum, at room temperature, for the glass doped with 1.0 wt% of Nd₂O₃ (2 mm thickness). In this spectrum we can observe the absorption bands with peaks at 874, 803, 747, 682,630 and 583 nm related, respectively to the ⁴F_3/2, ⁴F_5/2 F_9/2, ²H_11/2, ⁴G_5/2 and ⁴G_7/2 transitions of Nd³⁺ . The location, intensity and breadth of the absorption bands are determined by interaction of Nd^{3 +} ion with local crystalline fields. Each absorption band usually consists of a multiplicity of states. Unlike the local crystal field experienced by Nd³⁺ ion in crystalline hosts, the crystal field at sites in glasses varies from site to site. This distribution of crystal fields results in the inhomogeneous broadening of the absorption spectra of rare earth ions in amorphous media. Owing to this inhomogeneous broadening some bands are assumed to be superimposed because they are not resolved. The absorption spectra confirm the incorporation of Nd³⁺ and also indicate that the absorption edge, in the visible region, starts at 550 nm.

Figure 3 presents the transmission in the infrared region of the glass target; we note a large transmission window up to 8 µm. The band around 3 µm is related to the presence of OH^- radicals [5].

In our previous studies [5] we observed, for the emission measurements of the glasses (2 mm thickness), that, at larger neodymium concentrations, quenching by ion-ion or ion-impurities interaction become important. The maximum emission was measured for the sample with 1.0 wt% of Nd₂O_3; a quenching for 2.0 wt% of Nd₂O₃ was observed (Figure 4). These results lead us to use 1.0 wt% Nd₂O₃ as the maximum doping level for the production of the glass target.

Figure 5 presents the emission in the infrared region for the PbO-Bi₂O₃-Ga₂O₃ thin film with 1.0 wt% of Nd₂O₃, with peak at about 1100 nm, related to the ⁴F_3/2® ⁴I_11/2 transition of Nd³⁺ . This is the most important transition of Nd³⁺ because it is normally the most intense one and with applications in the second telecommunication window. The emission from the films is broadened and shifted to longer wavelength, when compared to the emission of the glass target. This indicates that there is a difference between the neodymium environment in the glass film and in the glass target, as predicted in the literature for films sputtered from phosphate glasses doped with Nd₂O₃ [6]. We remark that the emission of the film with 0.2 wt% was not measure because of the low intensity of the signal.

Figure 6 shows the photoconductivity of the PbO-Bi₂O₃- Ga₂O₃ thin films with 0.2 and 1.0 wt% of Nd₂O₃.

It is observed that the film with higher content of Nd₂O₃ has the current increased when illuminated by white light [7]. This effect is a characteristic of semiconductor materials but the same does not occur for the low doping thin films with 0.2 wt% Nd₂O₃. In this case the main characteristic is its low resistivity. Both effects, semiconductor and low resistivity, can be explained by doping concentration in each side of the junction Si - GaO. If the junction has one highly doped side (Si) and one lightly doped side (0.2 wt% Nd₂O₃), the lightly doped side determines the properties of the junction and low energy is sufficient to transport the carrier from one side to the other. That is why no difference is observed when the film is illuminated or not. When the junction is formed by two highly doped sides Si and 1.0 wt% Nd₂O₃, higher energy is necessary to transport the carrier from one side to the other. In this situation, light is used in order to give energy to the carrier to pass the one side to the other. Then its movement is observed as high current.

IV. CONCLUSIONS

In this work we present optical and semiconductor characteristics of gallium oxide based thin films. The sputtering deposition technique was an efficient method to obtain amorphous thin films from glass targets. The atomic composition of the deposited film correspond to the one of the glass target used; this may be attributed to the use of pure argon plasmas during the deposition. The deposition rate of 1 nm/min remained constant using 20 W RF power. The film with 1.0wt% of Nd₂O₃ presented emission around 1100 nm originated from Nd³⁺ ions, with applications in the second telecommunication window. The refractive index of the films was of about 2.3, as measured by ellipsometry; Rutherford Backscattering Spectroscopy (RBS) analyses were performed and confirmed that the atomic composition of the deposited film is very similar to the glass target. It is observed that the film with 1.0 wt% of Nd₂O₃ has characteristic of semiconductor materials when illuminated by white light. This effect was not observed for 0.2 wt% of Nd₂O₃. This is the first time that semiconductor characteristics are observed for PbO-Bi₂O₃-Ga₂O₃ thin films with 1.0 wt% of Nd₂O₃. The electric characteristics of these films indicate that they are potential candidates for applications with electro-optic devices (phototransistors and photo detectors).

Acknowledgements

The authors would like to thank to Physic Institute of the University of São Paulo and Msc. Marcia Ribeiro for the performance of the emission measurements and optical analysis, Msc. Ana Paula Mousinho for the helpful for the poster preparation, Msc Ronaldo Ruas for the RBS analysis, and Mr. Nelson Ordonez, Mr. Alexandre M. Camponucci for technical input and support. We also thank FAPESP, CAPES and CNPq for financial support.

Received on 4 April, 2005

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Publication Dates

• Publication in this collection
  06 July 2006
• Date of issue
  June 2006

History

• Received
  04 Apr 2005