Growth and Characterization of alpha-PbO for Room Temperature Radiation Detection
Growth and Characterization of α-PbO for Room Temperature Radiation Detection
Erin Leigh Ford
Doctor of Philosophy in Engineering-Materials Science and Engineering University of California, Berkeley
Professor Eugene E. Haller, Chair
A global trading structure and high throughput of shipping containers into ports around the world increases the chance of nuclear terrorism via cargo containers. Harmless radioactive sources confuse and impede detection of the materials that pose a real threat, making spectroscopy difficult and requiring detectors with high resolution. The current methods that are used to check containers in ports have security flaws, and only 5% of all shipping containers are checked. The development of semiconductor γ-ray detectors is one of the protocols being advanced to alleviate this risk because they can function at room temperature and they are cost effective, easily produced, and have high resolution. This dissertation has addressed the current lack of "perfect" room temperature detector materials by investigating α-PbO, a novel material in this field. This includes the development of a growth process for α-PbO thin films, as well as its structural and performance characterization as a detector material.
Because we intend α-PbO to be a photoconductive detector, it should have certain properties. A photoconductive detector consists of a highly resistive material with a voltage bias across it. It absorbs incident γ-rays, creating electron-hole pairs that provide a signal. To function well, it must have a high atomic number and a high density in order to absorb high-energy photons via the photoelectric effect. It should also have a large resistivity and a wide band gap to avoid large leakage currents at room temperature. Finally, it must have good charge carrier transport properties and detector resolution in order to be able to determine the characteristic energy peaks of the radiation-emitting source. We chose α-PbO because it has a very high Z and a very high density and a band gap in the correct range. It also has a rich history of use as a photoconductor that reaches back to the 1950s.
Numerous methods have been used to grow thin films of α-PbO. However, rarely are those films single phase or highly oriented. Pulsed laser deposition provides a method to grow epitaxial thin films of α-PbO. The structure of the grown films was characterized using X-ray diffraction 2θ-ω scans, rocking curves, and reciprocal space mapping. Feedback from a parameterized study of the structural characterization enabled optimization of the growth process to improve the quality of the thin films. The parameters that were studied included: epitaxial strain, substrate temperature, oxygen background pressure, and fluence of the laser. The result of this process led to favorable growth parameters that included: KTO substrates, a substrate temperature of 495°C, an oxygen pressure of 10 mTorr, and a fluence of ~ 6 J/cm2. The resulting films showed peak broadening in the XRD scans that was only marginally greater than the peak broadening seen in scans of the single crystal substrate.
The methods used for the optical measurement of α-PbO films included absorption spectroscopy and ellipsometry. Determination of the spectral absorption coefficient was achieved by transmission spectroscopy and reflection spectroscopy via a PerkinElmer Lambda 950 UV- Vis spectrophotometer. Analysis of the square and square root of the absorption coefficient yielded values for the direct band gap and the indirect band gap of α-PbO. These values were Eg,dir = 3.2 eV and Eg,ind = 1.9 eV, respectively. To date, spectral ellipsometry measurements had never been performed on α-PbO. These initial measurements yielded the first recording of the spectral complex index of refraction. This is useful for analyzing how light interacts with α-PbO material. The data was used to help measure the thickness of the thin films.
Study of the electronic and transport properties of α-PbO is important in order to understand how the material will behave as a radiation detector. In general, photoconductive detectors have a very large applied voltage bias in order to ensure efficient charge collection. Therefore, high resistivity is needed to keep the resultant leakage current at a manageable level. Resistivity was determined current-voltage characteristics. The resistivity of the α-PbO thin films via this two-point probe measurement ranged from 1010 Ω cm to 1013 Ω cm. However, two-point probe analysis involves errors due to contact and probe resistance. The four-point probe measurements are underway.
Spectral photoconductivity was measured to ensure that α-PbO's response to light was large enough for it to be a useful detector material and to confirm the band gap measurements. The photocurrent onset occurred at the energy corresponding to the indirect band gap and it peaked at the energy corresponding to the direct band gap. The maximum photocurrent measured at a bias voltage of 60 volts was 4.7 nA. This value is four orders of magnitude greater than the photocurrent displayed at energies below the direct band gap.
In the field of detector materials, the μτ-product is commonly used as a figure of merit because it enables a measurement of the trapping length of the charge carriers within the detector. Many's equation, which is a derivation of the photocurrent with respect to the applied voltage across a wide band gap semiconductor, is one of the methods used to determine the μτ-product. The photocurrent voltage measurements were obtained from the 0.5 V to 80 V range. This data was difficult to fit with Many's equation over that whole range. Higher voltages displayed deviation from ideal behavior due to the contact effects, but at the lower voltages the data were unaffected. Fits to the lower voltage range, from 0.5 V to 10 V, yielded μτ = 6.8 x 10-4 cm2/V.
Room temperature photoconductors will ultimately be used to detect γ-rays; however, thin films do not have enough stopping power to absorb the total energy of a γ-ray. Therefore, we study the α-PbO detector response to radiation in the form of alpha particles because they are large, charged, and relatively easy to stop. SRIM calculation estimated that alpha particles have a range of up to 16 μm in α-PbO. The initial long-duration film growth yielded films that were ~ 8 μm thick. Therefore, a full energy peak from alpha particles was not seen in α-PbO. We did see a shoulder protruding out of the noise peak due to the charge carriers that were created before the alpha particles escaped the detector volume.