Scholarly article on topic 'Al0.2Ga0.8As X-ray photodiodes for X-ray spectroscopy'

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Abstract of research paper on Physical sciences, author of scientific article — M.D.C. Whitaker, G. Lioliou, S. Butera, A.M. Barnett

Abstract Three custom-made Al0.2Ga0.8As p-i-n mesa X-ray photodiodes (200µm diameter, 3µm i layer) were electrically characterised and investigated for their response to illumination with soft X-rays from an 55Fe radioisotope X-ray source (Mn Kα = 5.9keV; Mn Kβ = 6.49keV). The AlGaAs photodiodes were shown to be suitable for photon counting X-ray spectroscopy at room temperature. When coupled to a custom-made low-noise charge-sensitive preamplifier, a mean energy resolution (as quantified by the full width at half maximum of the 5.9keV photopeak) of 1.24keV was measured at room temperature. Parameters such as the depletion width (1.92µm at 10V), charge trapping noise (61.7 e− rms ENC at 5V, negligible at 10V) and the electronic noise components (known dielectric noise (63.4 e− rms), series white noise (27.7 e− rms), parallel white noise (9.5 e− rms) and 1/f series noise (2.2 e− rms) at 10V reverse bias) affecting the achieved energy resolution were computed. The estimated charge trapping noise and mean energy resolution were compared to similar materials (e.g. Al0.8Ga0.2As) previously reported, and discussed. These results are the first demonstration of photon counting X-ray spectroscopy with Al0.2Ga0.8As reported to date.

Academic research paper on topic "Al0.2Ga0.8As X-ray photodiodes for X-ray spectroscopy"

Author's Accepted Manuscript

Al0.2Ga0.8As X-ray Photodiodes for X-ray Spectroscopy

M.D.C. Whitaker, G. Lioliou, S. Butera, A.M. Barnett

www.elsevier.com'locate/nim

PII: SO 168-9002(16)31042-7

DOI: http://dx.doi.org/10.1016/j.nima.2016.10.017

Reference: NIMA59376

To appear in: Nuclear Inst. and Methods in Physics Research, A

Received date: 27 June 2016 Revised date: 6 October 2016 Accepted date: 6 October 2016

Cite this article as: M.D.C. Whitaker, G. Lioliou, S. Butera and A.M. Barnett Al0.2Ga0.8As X-ray Photodiodes for X-ray Spectroscopy, Nuclear Inst. an Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2016.10.017

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Al02Ga08As X-ray Photodiodes for X-ray Spectroscopy

M.D.C. Whitaker*, G. Lioliou, S. Butera, A.M. Barnett

Semiconductor Materials and Devices Laboratory, Dept. Engineering and Design, Sch. of Engineering and Informatics, University of Sussex, Falmer, Brighton, BN1 9QT, UK Corresponding author, E-mail address: M.Whitaker@sussex.ac.uk Abstract

Three custom-made Alo.2Gao.8As p-i-n mesa X-ray photodiodes (200 |m diameter, 3 |m i layer) were electrically characterised and investigated for their response to illumination with soft X-rays from an 55Fe radioisotope X-ray source (Mn Ka = 5.9 keV; Mn Kp = 6.49 keV). The AlGaAs photodiodes were shown to be suitable for photon counting X-ray spectroscopy at room temperature. When coupled to a custom-made low-noise charge-sensitive preamplifier, a mean energy resolution (as quantified by the full width at half maximum of the 5.9 keV photopeak) of 1.24 keV was measured at room temperature. Parameters such as the depletion width (1.92 |m at 10 V), charge trapping noise (61.7 e" rms ENC at 5 V, negligible at 10 V) and the electronic noise components (known dielectric noise (63.4 e- rms), series white noise (27.7 e- rms), parallel white noise (9.5 e- rms) and 1f series noise (2.2 e- rms) at 10 V reverse bias) affecting the achieved energy resolution were computed. The estimated charge trapping noise and mean energy resolution were compared to similar materials (e.g. Al08Ga02As) previously reported, and discussed. These results are the first demonstration of photon counting X-ray spectroscopy with Al02Ga08As reported to date. Keywords: X-ray detector; photodiode; spectroscopy

1. Introduction

Since the first reported detection of X-rays with AlGaAs [1], the material has received increased attention as a promising alternative for X-ray [1, 2, 3] and beta particle [4, 5] detection at room temperature and above. Results have been reported for a number of different Al fractions, but for X-ray spectroscopy, most work has concentrated on Al08Ga02As for high temperature devices due to its wide bandgap (2.09 eV [6]). Compared to narrow bandgap materials, wide bandgap detectors can possess superior resolutions at high temperature (>> 20 °C) due to lower thermally induced leakage currents [7] and thus lower parallel white noise [8]. This presents the possibility of reducing cooling requirements for X-ray detectors, with subsequent reduction of mass, volume, power requirements and hence, cost of the instrument; these improvements are important for applications including X-ray fluorescence spectroscopy for future space missions subjected to high temperature and intense radiation conditions, as well as terrestrial applications in industrial instrumentation.

Despite Al0.8Ga0.2As having been subject to extensive study, depending on the operating environment, lower Al fractions of AlxGa1-xAs could be more beneficial; varying the Al fraction adjusts the material's bandgap, where a reduction in Al fraction reduces the bandgap. Thus, for more modestly elevated temperatures, Al02Ga08As may provide a better solution than Al08Ga02As. This presumption is also based on the bandgap of Al02Ga08As (1.67 eV [9]) being close to the optimal for a room temperature X-ray detector (1.5 eV [10, 11]). In addition, the electron hole pair creation energy of Al02Ga08As is likely to be lower than those for higher Al fraction AlxGa1-xAs (e.g. 5.1 eV for Al08Ga02As at 294 K [12, 13]), leading to an improved Fano limited resolution for Al02Ga08As [13]. A reduction in Al fraction also leads to a larger X-ray linear attenuation coefficient (e.g. 787.8 cm-1 for Al02Ga08As c.f. 638.8 cm-1 for Al08Ga02As, at 5.9 keV). That is to say, more photons are absorbed and scattered within the detecting material due to the increased material density, whereby GaAs possesses a greater density (5.32 g cm-3) [14] in comparison to Al (2.69 g cm-3) [14].

Furthermore, previous work on Al08Ga02As for photon counting X-ray spectroscopy detectors has focused on thin i layers, e.g. 1 |m [4, 15] and 1.7 |m [2]. This is due to the lattice mismatch between Al08Ga02As and GaAs (the substrate material typically used for AlGaAs growth) which leads to relaxation when the wafer is cooled from growth temperatures. Whilst virtual substrate technology, in which graded Al fraction AlGaAs is grown on a GaAs substrate to provide a virtual Al08Ga02As substrate, may enable thick, high quality Al08Ga02As epilayers to be produced, comparatively thicker Al02Ga08As layers can be grown on a commercial GaAs substrate directly.

Here, results characterising three prototype Al02Ga08As X-ray p-i-n mesa photodiodes (200 |m diameter, 3 |m i layer) are presented. The devices were randomly selected from those grown and fabricated to the Authors' specifications at the EPSRC National Centre for III-V Technologies, Sheffield, UK.

2. Diode design

Alo.2Gao.8As p-i-n layers topped by a thin GaAs buffer layer were grown by metalorganic chemical vapour phase epitaxy (MOVPE) on a commercial 2 inch GaAs n+ substrate. The layer details are summarised in Table 1. Circular mesa structures (200 |m diameter) were formed using 1:1:1 H3PO4:H2O2:H2O solution followed by 10 s in 1:8:80 H2SO4:H2O2:H2O solution. An Ohmic contact consisting of 20 nm InGe and 200 nm Au was evaporated onto the rear substrate, and an Ohmic top contact of 20 nm Ti and 200 nm Au was evaporated onto the p+ side of the mesa devices; the top contact covered 45% of each diode's face. The devices were unpassivated.

[Table. 1]

3. Experiment results

3.1. Capacitance as a function of applied reverse bias

Capacitance as a function of applied reverse bias was measured for each Al02Ga08As p-i-n photodiode, D1-D3, at a temperature of 22 °C, using an HP 4275A LCR Meter (signal magnitude 60 mV rms; frequency 1 MHz) and an HP 16065A EXT Voltage Bias Fixture. A Keithley 6487 picoammeter/voltage source was used to bias the detectors. The measured capacitances were consistent across all devices. Fig. 1 presents the capacitance as a function of applied reverse bias for one representative diode (D1); comparable results were found for the other devices. As the devices were measured after packaging, the capacitance of the package was removed by measuring the capacitance of four empty connections on the same package (0.65 pF ± 0.04 pF RMS) and deducting this from the total capacitance obtained for each diode. The capacitance of the bond wire of each detector was not individually separated from the packaging capacitance, but the subsequent analysis suggests that the bond wire capacitances were insignificant compared with the other system capacitances. The capacitances of the devices at 10V and 30V were 1.81 pF ± 0.02 pF and 1.14 pF ± 0.02 pF, respectively.

[Fig. 1]

From the measured depletion layer capacitance CDL(VR), the depletion width of the diodes as a function of applied reverse bias W(VR) was calculated using,

CDL(VR) = . (Eq. 1)

where s0 is the permittivity of free space, s is relative permittivity of the material (12.332 for Al0 2Ga0 8As [16]), and A is the area of the device [17]. From the measured depletion layer capacitance of the Al0 2Ga0 8As photodiode D1 shown in Fig. 1, depletion widths of 1.92 |m ± 0.05 |m and 3.06 |m ± 0.12 |m were calculated at reverse biases of 10 V and 30 V, respectively. Fig. 2 shows the calculated depletion width as a function of applied reverse bias for D1 Al02Ga08As. Beyond 30 V, the measured depletion layer capacitance and consequently the depletion width, remained constant, suggesting that the diodes were fully depleted at a reverse bias of 30 V. The carrier concentration in the i layer was determined from capacitance measurements to be 4*1015 cm-3. Further refinement and optimisation of the growth process may improve (reduce) the unintentional doping concentration in the i layer, which may lead to performance improvements.

[Fig. 2]

The implied detection efficiency of the Al02Ga08As diodes when reverse biased at 10 V (mean depletion width of 1.90 |m ± 0.05 |m) and 30 V (mean depletion width of 3.02 |m ± 0.12 |m) as functions of energy are shown in Fig. 3. The detection efficiencies of two previously reported Al08Ga02As devices [2, 4] are also plotted for reference. The detection efficiency (0.134 excluding bondpad attenuation c.f. 0.123 including bondpad attenuation at 5.9 keV) has been calculated under the conservative assumption that the only active region of the detector is the i layer. The greater X-ray linear attenuation coefficients of Al02Ga08As (e.g. 787.8 cm-1 at 5.9 keV) compared with Al08Ga02As (e.g. 638.8 cm-1 at 5.9 keV), together with the thicker i layer for the presently reported detectors, resulted in greater efficiency of the detectors compared with previous photon counting spectroscopic AlGaAs X-ray detectors. The attenuation due to device bondpads has not been included

[Fig. 3]

3.2. Leakage Current

The leakage current as a function of applied reverse bias of each photodiode was measured using a custom dark box and a Keithley 6487 Picoammeter/Voltage Source. National Instruments Labview software was used to automate the characterisation routine. The bias was applied in increments of 0.1 V at a rate of 1 increment per 2 seconds up to a maximum reverse bias of 30 V. The measurements were made in a dry N2 environment (<5%

relative humidity) to eliminate any humidity related effects [4]. All three diodes had comparable leakage currents across the measurement range, Fig. 4 presents the leakage current as a function of applied reverse bias for one representative device (D1); the leakage current profile does not represent ideal diode behaviour [17], suggesting that further investigation of the relative contributions of recombination-generation current and diffusion current would be valuable in devices of this type [17, 18]. The contributions of these mechanisms could be investigated through temperature dependence of the forward and reverse bias dark currents [18]. At 30 V, the reverse bias at which the detectors were fully depleted, the mean leakage current was 15.4 pA ± 0.4 pA (rms deviance), corresponding to a leakage current density of 49.0 nA cm-2 ± 1.3 nA cm-2. Device D2 recorded the lowest leakage current: 15.1 pA ± 0.4 pA, corresponding to 48.0 nA cm-2 ± 1.4 nA cm-2.

Recently reported Al08Ga02As X-ray detectors (400 |m diameter; 1.7 |m i layer) had a leakage current density of 4.72 nA cm-2 ± 1.67 nA cm-2 at an average electric field strength of 29.4 kV/cm [2]. The presently reported Al02Ga08As detectors had a larger leakage current density of 9.1 nA cm-2 ± 2.1 nA cm-2 (a leakage current of 2.8 pA ± 0.7 pA) at the same average electric field strength (equivalent to a reverse bias of 8.8 V for the present detectors). It is also interesting to compare to GaAs mesa photodiodes: recently two 200 |m diameter, 7 |m i layer mesa photodiodes were reported which had leakage current densities of 17.4 nA cm-2 and 1.08 nA cm-2 respectively, at an average electric field strength of 22 kV/cm [18]. At this field strength (equivalent to an applied reverse bias of 6.6 V for the present devices), the Al02Ga08As detectors had a mean leakage current density of 7.1 nA cm-2 ± 2.7 nA cm-2; device D1 exhibited the lowest leakage current density of the three measured Al02Ga08As detectors (5.5 nA cm-2 ± 1.3 nA cm-2).

[Fig. 4]

3.3. X-ray Measurements

To investigate the performance of the photodiodes as detectors of soft X-rays, each diode was connected in turn to a custom-made low-noise charge-sensitive single channel preamplifier of feedback resistorless design similar to Ref. [19]. The preamplifier used a silicon JFET (2N4416A, capacitance = 2 pF) as the input transistor. The preamplifier was connected to an Ortec 571A shaping amplifier (shaping time = 1 |s, the optimum for the system used) and an Ortec 927 ASPEC multi-channel analyser (MCA). An 55Fe radioisotope X-ray source (225 MBq) emitting characteristic Mn Ka (5.9 keV) and Mn Kp (6.49 keV) X-rays was placed above the AlGaAs diodes. The diodes and preamplifier were operated at room temperature (22 °C) in a dry N2 environment (<5% relative humidity). Spectra were accumulated with the photodiodes reverse biased at 0 V, 5 V, 10 V, 15 V, 20 V, and 30 V. The live time limit for each spectrum was 1,000 s. The spectra were energy calibrated using the positions of the zero energy noise peak and the fitted Mn Ka 5.9 keV peak, with the assumption of a linear variation of detected charge with energy. A representative spectrum accumulated with device D1 reverse biased at 10 V is presented in Fig. 5. To minimise counts from the noise peak, a low energy discriminator threshold (3.1 keV) was set. The dashed lines are the Mn Ka and Mn Kp peaks fitted to the observed peak in the accepted ratio [20], accounting for the relative efficiency of the detector at the respective energies. The FWHM at 5.9 keV measured with D1 under these conditions was 1.3 keV ± 0.1 keV. FWHM at 5.9 keV of 1.2 keV ± 0.1 keV were measured for both D2 and D3. The impact ionization coefficients of Al02Ga08As as a function of average electric field were calculated and indicated that the diodes were operating within the non-avalanche regime. In addition, no shift in channel number of the Mn Ka 5.9 keV peak as a function of reverse bias was observed.

From Fig. 5, low energy tailing can be seen in the accumulated spectrum. This tailing is attributed to the partial collection of charge created by X-ray photons absorbed in the low-field regions of the photodiode [2]. The valley-to-peak (V/P) ratio can be used to quantify the amount of low energy tailing [2]. For the Al02Ga08As X-ray p-i-n mesa photodiodes reported, the mean V/P ratio at a reverse bias of 10 V was 0.08. This is comparable to that previously reported for Al08Ga02As devices [2]. For GaAs (7 |m i layer) at room temperature, an improved V/P ratio (0.05) has been previously shown [18]. As thicker i layer devices are produced, assuming that non-uniformities in the charge collection efficiency, especially at the device edges, are small, it is likely that the V/P ratio will improve due to a greater fraction of the X-ray photons illuminating the devices being absorbed in the active region compared with the low-field layers.

[Fig. 5]

The FWHM at 5.9 keV observed with each diode reverse biased at 0 V, 5 V, 10 V, 15 V, 20 V, and 30 V is presented in Fig. 6. The best mean FWHM at 5.9 keV (= 1.24 keV) was observed when the diodes were operated at 10 V and 20 V. An improving trend in FWHM from 0 V to 10 V was attributed to a reduction in capacitance and associated series white noise, in combination with a decrease in charge trapping noise, as discussed in section 4. Noise Analysis. Between 20 V and 30 V, an increase in FWHM indicated that the

leakage current and associated parallel white noise outweighed any positive aspects brought from operation at higher reverse bias.

[Fig. 6]

4. Noise Analysis

The fundamental (Fano-limited) energy resolution (AE) of a non-avalanche X-ray photodiode is given by,

A E = (Eq. 2)

where F is the Fano factor, E is the energy of the photon, and w is the average energy consumed in the generation of an electron-hole pair [2, 21]. The electron hole pair creation energy (w) can also be expressed in terms of the number of electron hole pairs (n) and the photon energy (E) by,

o = E/n. (Eq. 3)

Assuming a Fano factor of 0.12, and an electron hole pair creation energy of 4.4 eV (assuming a linear variation of rn with Al fraction between GaAs [22] and Al0 8Ga0 2As [4]), the expected Fano limited energy resolution (FWHM) at 5.9 keV would be 131 eV for Al02Ga08As at room temperature. However, it is often the case that the energy resolution of a detector is further degraded by electronic noise [2] and noise from the incomplete collection of charge (including charge trapping) [23], such that Eq. 2 becomes,

AE = 2. 3 5 5o J(^) + a 2 + r2, (Eq. 4)

where r is the incomplete charge collection noise in equivalent noise charge (in units of e- rms) and a is the electronic noise equivalent noise charge (in units of e- rms). Given that the experimentally observed energy resolutions (FWHM) of the diodes are much greater than the Fano-limit, it is important to consider the relative contributions of the additional noise sources.

Electronic noise contributions include parallel white noise, series white noise, induced gate drain current noise, 1/f series noise, and dielectric noise [2]. Fig. 7 presents the calculated values of these noise contributions, as per Refs. [24, 25, 26], for each diode when reverse biased at 10 V.

[Fig. 7]

In addition to the parallel white noise of the detector, the parallel white noise contribution from the JFET was also included within the total parallel white noise contribution, assuming that the leakage current of the JFET at room temperature was 1pA [27]. As the contribution from series white noise depends on the total capacitance load at the gate of the input transistor of the preamplifier, only a minimum estimate could be calculated. This is due to the prototype nature of the preamplifier, where, in addition to estimable capacitances, stray capacitances with unknown values are present. Similarly, dielectric noise contributions arising from the detector, JFET and feedback capacitor were readily estimated [2, 8], but additional noise from other lossy dielectrics in proximity to the preamplifier would also have added to the noise. Subtracting the expected Fano noise (the statistically limited resolution) and the electronic noise contributions (parallel white noise, known series white noise (including induced gate drain current noise), known dielectric noise, and 1/f noise) from the measured FWHM in quadrature, the remainder can be attributed to incomplete charge collection noise, and the unknown dielectric and stray series white noises [2].

From Fig. 7, the dominant source of noise across all diodes was this remaining noise. Assuming the remaining noise from unknown lossy dielectrics and stray series white noise was independent of reverse bias [24, 26], the reduction of this remaining noise as the reverse bias was increased from 0 V to 10 V can be attributed to a reduction in charge trapping noise (the prime constituent of incomplete charge collection noise broadening the energy resolution).

Given this assumption, a quantitative estimate of the reduction of charge trapping noise as a function of increased applied bias can be made by subtracting the known noise contributions that vary with applied reverse bias from the equivalent noise charge of the measured FWHM at each reverse bias in quadrature, and examining the change in the remainder as a function of applied reverse bias [24]. Therefore, it can be said that there was a mean additional charge trapping noise of 146 e- rms equivalent noise charge (ENC) at 5.9 keV when the

detectors were operated at 0 V in comparison to 5 V reverse bias. Similarly, a mean additional charge trapping noise of 67 e" rms ENC at 5.9 keV was calculated at 5 V in comparison to 10 V. Beyond this reverse bias, any remaining charge trapping noise became insignificant compared with the other noise components. The calculated charge trapping noise was then subtracted from the unknown dielectrics, incomplete charge collection and additional series white noise in quadrature. The various noise components are presented in Fig. 8 for one representative diode (D1). The Al08Ga02As (400 ^m diameter, 1.7 ^m i layer) photodiodes reported in Ref. [2], had 26 e" rms ENC charge trapping noise at 5.9 keV at 5 V reverse bias; significantly less than the presently reported detectors (67 e" rms ENC at 5 V reverse bias). This is not surprising given the maturity of Al08Ga02As as a material for X-ray spectroscopy compared with that of Al02Ga08As. Additionally, as can be seen in Fig. 8, an apparent increase in the noise attributed to unknown lossy dielectrics and stray series white noise occurred between 20 V and 30 V reverse bias. One possible explanation for this is that rather than an increase in these particular noise components, there may have been an increase in parallel white noise from the preamplifier's input JFET as a result of the larger leakage current of the detector at 30 V compared with 20 V reverse bias. Such dependence of the JFET's performance was negligible at lower detector leakage currents but could have had a small effect at higher leakage currents due to the bias condition of the JFET being controlled in part by the leakage current of the detector in feedback resistorless preamplifiers [19].

[Fig. 8]

5. Conclusions and further work

For the first time, the material Al02Ga08As has been characterised for soft X-ray spectroscopy via a set of three p-i-n (200 ^m diameter, 3 ^m i layer) photodiodes. The material has been shown to be suitable for photon counting X-ray spectroscopy at room temperature with the performance of the system primarily limited by the performance of the preamplifier electronics rather than the material's inherent properties. The diodes were fully depleted at an applied reverse bias of 30 V, with a calculated mean depletion width of 3.02 ^m ± 0.12 ^m across the three diodes and a mean leakage current of 15.4 pA ± 0.4 pA (rms deviance), corresponding to a leakage current density of 49.0 nA cm-2 ± 1.3 nA cm-2. At 10 V (leakage current = 3.2 pA ± 0.5 pA) the mean energy resolution was calculated to be 1.24 keV FWHM at 5.9 keV. The measured energy resolutions (FWHM at 5.9 keV) of the Al02Ga0 8As diodes are comparable to recent reports using Al0.8Gao.2As, where a mean energy resolution of 1.27 keV ± 0.04 keV FWHM at 5.9 keV was found for 400 ^m devices [2]; however, the currently reported energy resolutions achieved with the Al02Ga08As X-ray photodiodes are modest compared with those recently achieved using state-of-the-art Silicon Drift Detectors (SDDs) coupled to state-of-the-art ultra-low-noise CMOS readout electronics, even at room temperature (FWHM = 141 eV at 5.9 keV [28]).The best reported energy resolution for non-avalanche AlGaAs X-ray detectors (200 ^m diameter; 1 ^m i layer) is currently 1.07 keV FWHM at 5.9 keV [15].

The results indicate that Al02Ga08As is a potentially promising material for room temperature photon counting X-ray spectroscopy. Al02Ga08As detectors with thicker i layers than previous Al08Ga02As photon counting X-ray detectors, and comparable energy resolutions have been demonstrated. Whilst the results are promising, in order to be competitive with GaAs detectors, the detection efficiency (thickness) will have to be increased and the energy resolution improved: GaAs detectors of thicker i layer (e.g. 7 ^m [18] and 40 ^m [29]) and better energy resolution (745 eV [18] and 266 eV [29]) have been demonstrated. SiC has also been shown to possess excellent energy resolution at room temperature (196 eV FWHM at 5.9 keV [30]), and despite its significantly lower linear attenuation coefficients (e.g. 348.2 cm-1 at 5.9 keV [31]) compared to those of AlGaAs (e.g. 787.8 cm-1 at 5.9 keV [31] for Al0.2Ga0.8As) and GaAs (e.g. 836.7 cm-1 at 5.9 keV [31]), SiC is still a highly competitive technology for soft X-ray spectroscopy with the availability of thicker structures offsetting the lower linear attenuation coefficients. Other wide band gap materials such as AlInP and AlInAs could also prove suitable for photon counting X-ray detection. Al052In048P (2 ^m i layer) was recently characterised at room temperature, where an energy resolution of 930 eV FWHM at 5.9 keV was reported [32], whilst maintaining <3 nA/cm2 leakage current densities [32]. Al052In048P avalanche photodiodes have also been characterised, where an energy resolution of 682 eV FWHM at 5.9 keV has been reported [33]. The increasing research in X-ray detection employing ternary semiconducting structures will undoubtedly yield further materials suitable for replacing Si photon counting X-ray detectors, better equipped to handle intense radiation and high temperature conditions.

In future work, characterisation of Al02Ga08As detectors of different diameters and thicknesses will be reported, as will characterisation of their response to illumination with X-ray photons of different energy. The temperature dependence of the performance of the devices will also be investigated and measurements of the electron-hole pair creation energy will be reported.

Acknowledgments

This work was supported by Science and Technology Facilities Council, UK, Grants ST/M002772/1 and ST/M004635/1, and Royal Society, UK, Grant RS130515 (University of Sussex, A.M.B., PI). M.D.C.W. and G.L. each independently acknowledge funding received in the form of PhD scholarships from University of Sussex. The authors are grateful to B. Harrison, R. J. Airey, and S. Kumar at the EPSRC National Centre for III-V Technologies for material growth and fabrication.

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Fig. 1. Measured capacitance as a function of applied reverse bias for one representative Alo.2Gao.8As X-ray p-i-n mesa photodiode, D1, at room temperature. Comparable results were obtained for the other devices.

Fig. 2. Calculated depletion width as a function of applied reverse bias for D1 Al02Ga08As (200 |um diameter, 3 |um i layer). Comparable results were obtained for the other devices.

Fig. 3. Calculated detection efficiency as a function of energy for the Al02Ga08As X-ray p-i-n mesa photodiodes when operated at 30V (solid line) and 10V (long dashed line) reverse bias, respectively. For comparison, the

detection efficiencies of Alo.8Gao.2As photodiodes used in Refs. [2, 4] are also shown (dotted and short dashed lines respectively). The discontinuities are the Al K, Ga L, and As L X-ray absorption edges.

Fig. 4. Measured leakage current for one representative Al02Ga08As X-ray p-i-n mesa photodiode, D1 in a dry N2 atmosphere (<5% relative humidity) as a function of reverse bias. Leakage current density and average electric field strength are also shown for the convenience of the reader.

Fig. 5. Spectrum accumulated with Al02Ga08As device D1 at an applied reverse bias of 10 V when illuminated with an Fe radioisotope X-ray source. The dashed lines are the fitted Mn Ka and Mn Kp peaks.

Fig. 6. Measured FWHM at 5.9 keV as a function of applied reverse bias for D1-D3 Al02Ga08As: D1 (circles), D2 (+ symbol) and D3 (triangles).

Fig. 7. Calculated remaining noise contributions of each Al02Ga08As diode at an applied reverse bias of 10 V and a shaping time of 1 ^s: combined unknown dielectric noise, incomplete charge collection noise and any additional stray series white noise (crosses); known dielectric noise (stars); series white noise (triangles); Fano noise (diamonds); parallel white noise (circles); 1/f series noise (squares).

Fig. 8. Calculated remaining noise contributions (unknown dielectric noise, incomplete charge collection noise and additional series white noise) (circles), remaining noise contributions with charge trapping noise subtracted (stars), known dielectric noise (squares), series white noise (triangles), Fano noise (plus sign), parallel white noise (diamonds) and 1/f series noise (crosses) as a function of applied reverse bias, at a shaping time of 1 ^s, for one representative Al02Ga08As X-ray p-i-n mesa photodiode, D1, at room temperature. The dashed lines are guides for the eyes only.

Table 1. Layer details of the Al02Ga0.8As wafer.

Layer Material Thickness (lm) Dopant Dopant Type Doping Density (cm-3)

1 GaAs 0.01 Be P 1 x 1019

2 Al0.2Ga0.8As 0.5 Be P 2 x 1018

3 Al0.2Ga0.8As 3 Undoped 2 x 1018

4 Al0.2Ga0.8As 1 Si N

Substrate: GaAs

0 5 10 15 20 25 30

Reverse bias (V|

0 5 10 15 20 25 30

Reverse bias (V)

Energy (keV)

Average electric field strength (kV cm-1)

0 15 30 45 60 75 90

15 10 5 0

60 50 40 30 20 10 0

10 15 Reverse bias (V)

0 im im

0) c c ro .c u

2000 1500 1000 500 0

Energy (keV)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 Reverse bias (V)

0) ~ (5 E

120 100 80 60 40 20 0

1 - X i X i X

. X X X .

HI 6 0 e in

D1 D2 Diode number D3

5 10 15

Reverse bias (V)