Orthorhombic undoped κ-Ga₂O₃ epitaxial thin films for sensitive, fast, and stable direct X-ray detectors

Abstract

Photoelectronic properties of orthorhombic undoped κ-Ga₂O₃ epitaxial thin films, grown on sapphire substrates by metal–organic vapour phase epitaxy, were evaluated under X-ray irradiation (CuK_α line, 8.05 keV) for the first time. Photoresponse linearity at low dose-rates (varying in the 10–200 μGy s⁻¹ range), and excellent detection sensitivity (up to 342.3 μC Gy⁻¹ cm⁻³), were demonstrated even at very low applied electric fields (down to 0.001 V μm⁻¹). Photocurrent rise time was evaluated to be <0.5 s, and signal stability was assessed for exposure times up to 2 h, highlighting no degradation of the performance. These encouraging results, mostly due to the extremely low dark current measured (in the pA range), suggest that orthorhombic undoped κ-Ga₂O₃ is a promising material for the fabrication of sensitive and stable large-area X-ray detectors with minimum power consumption.

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Introduction

There is increasing interest in the development of high-performance direct X-ray detectors for applications in medical diagnostics, homeland security, environmental monitoring and industrial non-invasive inspection. Presently, the most popular solution makes use of indirect detectors with a scintillator material to convert X-ray energy into visible light, subsequently detected by well-established silicon-based devices. Direct X-ray detectors, on the other hand, use direct photoelectric conversion to generate electron–hole pairs into a suitable semiconductor, opening the possibility to achieve high resolution, high conversion efficiency, spectroscopic analysis and real-time imaging. Materials traditionally studied for direct X-ray conversion, such as Ge,¹ diamond,^2,3 and CZT (cadmium–zinc–telluride),⁴ are stable, sensitive, and radiation-hard, but not cost-effective; moreover, the required quality and thickness of the active films make them unsuitable for large area (and possibly flexible) devices. Conversely, the materials in the spotlight of research on X-ray direct detectors, such as organic semiconductors,⁵ perovskites,⁶ or semiconducting polymers blended with high-Z absorbers,⁷ ensure low-cost flexible devices with both large active areas and low bias, but their reliability under X-ray is still unsatisfactory, mostly due to significant radiation-induced modifications⁸ and chemical instability.⁹ Currently, the best active material for X-ray direct detectors (e.g., flat-panel detectors for X-ray imaging) is amorphous selenium (a-Se), thanks to its high sensitivity and the possibility of being deposited on large areas;¹⁰ however, a major drawback of a-Se is the very high energy (45 eV) necessary to create an electron–hole pair, implying a lower photogenerated charge per absorbed photon, and consequently the application of very high electric fields (in the 10–40 V μm⁻¹ range) for efficient signal collection.¹¹

In this context, gallium oxide (Ga₂O₃), one of the most promising wide-bandgap semiconductors for high power electronics and solar-blind UV photodetectors, can be considered as an attractive option, because it offers a convenient trade-off between different materials investigated as X-ray photoconductors: in particular, the combination of a wide bandgap (in the 4.7–5.2 eV range, depending on the considered polymorph), ensuring a very low dark current, with a high density (6.4 g cm⁻³), providing efficient X-ray absorption, makes Ga₂O₃ a sensitive and stable active material for X-ray detectors even at low applied electric fields.¹² Moreover, it is radiation-hard¹³ and it can be deposited over large areas.¹⁴

Ga₂O₃ is known since about seven decades, but only recently it has attracted renewed attention thanks to significant technological advancements: (1) high-quality single crystals (up to 4” diameter) can now be achieved using both edge-defined film-fed (EFG)¹⁵ and Czochralski¹⁶ growth techniques; (2) advanced epitaxial techniques have been developed, leading to good quality homo- and hetero-epitaxial films with reproducible properties; and (3) electrical properties can be tailored by appropriate doping techniques, providing conductivity ranging from high n-type to semi-insulating. A review of the most recent advances in the field of Ga₂O₃ can be found in ref. 17.

Five Ga₂O₃ polymorphs are known, i.e., α, β, γ, δ, and κ, but research has been mostly concentrated on the monoclinic (β) one, as it is thermodynamically stable, it can be directly grown from the melt, and can withstand the high-temperature treatments necessary for the activation of the implanted dopants. However, in the past few years, Ga₂O₃ metastable polymorphs have attracted attention owing to their peculiar properties. For instance, the corundum (α) phase exhibits hexagonal symmetry and the widest bandgap (5.2 eV), making it attractive for power devices.¹⁸ The orthorhombic (κ) phase can be easily deposited by MOVPE (metal–organic vapour phase epitaxy) on commercial (0001)-oriented sapphire¹⁴ substrates as nominally undoped high-resistivity epilayers, which can conveniently be exploited in solar blind UV-C photodetectors.¹⁹ It is worth noting here that often the literature indifferently reports κ- and ε-Ga₂O₃; this ambiguity originates from the six-fold symmetry of early diffraction patterns and the consequent classification of such a Ga₂O₃ phase as “hexagonal”, hence the ε-Ga₂O₃ ascription. A comprehensive TEM study,²⁰ as well as other works recently published,^21–24 showed definitely that the so-called κ phase is indeed orthorhombic and not hexagonal. In analogy to the usual nomenclature of other metal-oxides, it is therefore correct to denominate the orthorhombic phase as κ, as it will be done in this paper. It was also demonstrated that κ-Ga₂O₃ exhibits a spontaneous polarization and a ferroelectric character,²⁵ that may turn out to be advantageous for the formation of a 2D electron gas in Ga₂O₃-based heterostructures.^26,27

In the field of Ga₂O₃-based X-ray detectors, only amorphous and nanocrystalline films or β-Ga₂O₃ bulk crystals (Fe-doped or undoped) have been tested so far,^28–32 but no investigations have been carried out on other Ga₂O₃ polymorphs. This, along with the very high resistivity measured on undoped κ-Ga₂O₃ grown by MOVPE, and the easy and cost-effective film deposition on a wide range of large-area commercial substrates (e.g., sapphire, GaN, and SiC), motivated the present investigation on the suitability of κ-Ga₂O₃ for X-ray detection. It is worth stressing here that although metastable, the κ phase is stable up to about 700 °C (then it fully converts to β at 900 °C^33–35), which is sufficient for the standard fabrication of ohmic and rectifying contacts, as well as for employing κ-Ga₂O₃-based devices under standard operating conditions.

In this work, we report for the first time on the photoresponse of κ-Ga₂O₃ epitaxial thin films to X-rays. Along with a very low dark current, the results point out a high detection sensitivity, linearity with dose rate, fast response, and stability with time even at very low applied electric fields. It will be shown that some of these properties are superior to those previously reported for X-ray detectors based on amorphous,²⁸ nanocrystalline,²⁹ and bulk Fe-doped β-Ga₂O₃ photoconductors,^30,31 or bulk undoped β-Ga₂O₃ Schottky diodes.³² These results are very encouraging, and pave the way to the application of orthorhombic undoped κ-Ga₂O₃ epitaxial thin films to direct X-ray detection.

Experimental

Epitaxial growth of κ-Ga₂O₃ thin films by MOVPE

κ-Ga₂O₃ epilayers were deposited on (0001) sapphire (Al₂O₃) substrates at 650 °C and a pressure of 100 mbar by means of MOVPE, according to the procedure described in ref. 36. Trimethylgallium (TMG) and H₂O were used as precursors, using a H₂O/TMG ratio of about 200. Helium (400 sccm) was used as a carrier gas to deliver the precursors to the growth chamber. The film thickness, as measured by optical interferometry, was about 440 nm, corresponding to a deposition time of 15 min. Surface roughness was evaluated to be ±3 nm. X-ray diffraction (2θ scan) exhibited sharp peaks at 19.17°, 38.85°, and 59.87°, typical of the 002, 004, and 006 reflections of c-oriented κ-Ga₂O₃ (see Fig. S1 in the ESI†). Crystallographic data and atomic parameters of epitaxial κ-Ga₂O₃ were previously reported by some of the present authors. A comprehensive investigation by X-ray and electron diffraction, and subsequent refinement, provided the following lattice parameters: a = 5.0463(15) Å, b = 8.7020(9) Å, and c = 9.2833(16) Å (the uncertainty beyond significant digits are shown in brackets, see Table 1 of ref. 20).

Table 1 Comparison of the X-ray photoresponse properties obtained in this work to the values reported in the last 3 years for direct X-ray detectors based on other Ga₂O₃ polymorphs. Operating electric field values were obtained from the ratio between the bias voltage and the interelectrode gap. Stability values have to be intended as the longest reported time intervals for which the value of a given X-ray photocurrent signal (corresponding to a specific dose rate and a specific bias voltage) showed no significant variations. Rise time, responsivity, sensitivity (normalized to the active volume of the detectors), and stability values are reported in the best case

Active material	Thickness (μm)	Bias voltage (V)	Operating electric field (V μm^−1)	Rise time (s)	Responsivity (μC Gy^−1)	Normalized sensitivity (μC mGy^−1 cm^−3)	Reported stability (min)	Year	Ref.
a-Ga2O3	0.25	5–100	1–20	15.5	—	271	20	2019	24
nc-β-Ga2O3	0.2	10–200	0.05–1	0.035	0.68	138.8	15	2021	25
sc-β-Ga2O3 (Fe)	1000	−1000 to −200	−1 to 0.2	<0.2	6.75	0.75	1	2021	27
sc-β-Ga2O3 (undoped)	1000	−25 to 0	0.025–0	13.8	1.8	—	5	2019	28
κ-Ga2O3	0.44	1–500	0.001–0.5	<0.5	2.66	342.3	120	2022	This work

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Structure, operating principle, and fabrication of the photoconductive X-ray detector

A 10 × 10 mm² sample was selected for the fabrication of the photoconductive X-ray detector. The device has a classical coplanar metal–semiconductor–metal (MSM) structure, as shown in Fig. 1(a), with two Au contact pads fabricated on the surface of the semiconducting κ-Ga₂O₃ epitaxial thin film, acting as the active medium. When X-ray photons are absorbed by the κ-Ga₂O₃ film in the interelectrodic volume, electron–hole pairs are photogenerated. A bias voltage is applied to one electrode, whereas the other one is grounded; in this way, photogenerated carriers are immediately separated by a transverse electric field, and collected by the metal contacts, thus generating current.

Fig. 1 (a) Operating scheme of the fabricated κ-Ga₂O₃ photoconductive X-ray detector; (b) picture of the device mounted on a PCB board and positioned right in front of the X-ray collimator output slit.

Prior to the deposition of the metal contacts, aimed at removing particulates and organic contaminants, the sample was cleaned in an ultrasonic bath in hot acetone for 5 min, then dipped in isopropanol for further 5 min, rinsed in deionized water, and finally dried in a pure N₂ flow. Then, metal contacts were fabricated in a single step by RF magnetron sputtering deposition of a 300 nm thick Au film. The contact geometry (two rectangular 4.2 × 1.6 mm² pads separated by a 1.0 mm-wide gap) was directly defined during deposition by the use of a stainless steel shadow mask positioned on the sample surface, with no need for photolithographic steps. The background pressure was 10⁻⁶ mbar, the target-to-substrate distance was 70 mm, and Ar was used as a sputter gas at a pressure of 2.5 × 10⁻² mbar. Finally, the detector was mounted on a standard printed circuit board (PCB) equipped with a SMA connector for biasing and current signal collection (Fig. 1(b)).

Characterization techniques and instrumentation

X-ray photocurrent measurements, as well as current–voltage characterization under dark conditions, were performed in continuous mode by using a low-noise electrometer (Keithley 487), simultaneously used as a voltage source for biasing the detector and as an ammeter. X-rays were produced by a standard Coolidge tube equipped with a Cu target (K_α line at 8.05 keV, K_β line at 8.91 keV). The detector was centered in front of the X-ray tube collimator output slit (Fig. 1(b)), resulting in a 10 mm diameter circular spot-size (checked with a fluorescent screen) irradiating the whole active area of the detector. Au contacts being only 300 nm-thick, they are not able to perturb the response of the underlying κ-Ga₂O₃ film to X-rays, therefore the active area of the detector was considered to be a 4.2 × 4.2 mm² square (i.e., the area where the applied electric field operates, below and between the contact pads). The X-ray tube accelerating voltage was fixed and set to be 40 kV. Conversely, the tube current was varied in the range of 2.5–40 mA, corresponding to a delivered dose rate in the range DR = 10–200 μGy s⁻¹, as recorded in a reference ionization chamber (Farmer mod. NE 2536/3C) connected to a commercial dosimeter (Farmer mod. NE 2670).

Spectral UV-Vis-NIR photoconductivity analysis in the 200–1000 nm range, used to investigate the role of defect states in photoconduction mechanisms, was performed by measuring the photocurrent generated by a monochromatic radiation focused on the 1 mm-wide gap between the two Au contact pads. Light came out from the output slit of a Newport Cornerstone 260 monochromator coupled to a Hg–Xe lamp. Aimed at reaching a higher sensitivity under sub-bandgap radiation, modulated measurements were performed. For this purpose, light was chopped at a frequency of 14 Hz by a SR540 chopper, and photogenerated current signals were converted using a low-noise transimpedance amplifier (Princeton Applied Research 181) into a voltage measured using a lock-in amplifier (EG&G 5209). Bias voltage was applied by means of a Keithley 487 electrometer, used as a voltage source only.

Results and discussion

Dark current measurements

Before irradiation under X-rays, the photoconductive detector was characterized under dark conditions to quantify the background noise level (i.e., the dark current) at different applied bias voltages, as well as to investigate on the presence of possible non-linear charge transport mechanisms (e.g., space charge limited current,³⁷ or Frenkel–Poole emission³⁸), which are well-known to negatively affect the performance of detectors based on the direct conversion of X-rays into electric charge.

Fig. 2 reports the measured dark current I_d values as a function of the bias voltage V_BIAS, ranging from 0.1 to 500 V (corresponding approximately, by considering a 1 mm-wide interelectrode gap, to an applied electric field F varying in the range of 10⁻⁴–0.5 V μm⁻¹). As can be seen from Fig. 2(a) (linear plot), Au contacts show excellent linearity in the full range of applied bias voltage, with the absence of undesirable non-linear effects triggered at high V_BIAS values. Moreover, as can be inferred from Fig. 2(b) (log–log plot), linearity with the bias voltage extends for more than three decades, pointing out the ohmic behaviour of Au metal contacts on undoped κ-Ga₂O₃, differently from what was recently reported on n-type ε-Ga₂O₃, on which a Schottky junction was formed.³⁹ We have to indeed consider the very high resistivity of undoped κ-Ga₂O₃: this implies that Fermi energy is located deep into the bandgap, and that the material is fully depleted in practice, leading to negligible band-bending at the metal–semiconductor interface, i.e., to the formation of an ohmic contact. It is also worth mentioning here that both ohmic and Schottky contacts demonstrated to be effective for κ-Ga₂O₃-based MSM detectors:⁴⁰ in particular, ohmic contacts should be preferred to enhance the device responsivity, whereas Schottky contacts are better when the response speed is a key concern. In our case, Au ohmic contacts ensure extremely low dark current values, ranging from a few fA for V_BIAS < 1 V (Fig. 2(b)) to a maximum value of 6.7 pA at V_BIAS = 500 V (Fig. 2(a)), which confirms the very high resistivity ρ_d of the deposited epitaxial κ-Ga₂O₃ thin film. From the evaluation of the slope of the linear best-fit shown in Fig. 2(b), it is possible to obtain ρ_d = 3.15 × 10¹⁰ Ω cm, which exceeds by more than one order of magnitude the threshold requirement for semiconductor-based X-ray detectors (10⁸–10⁹ Ω cm).⁴¹

Fig. 2 Linear (a) and log–log (b) plots showing the current–voltage curves of the fabricated X-ray detector measured under dark conditions. Dashed line represents in both cases the best linear fit to the experimental data.

X-ray photoresponse evaluation

Transient photocurrent measurements under irradiation were performed at different bias voltages (in the range V_BIAS = 1–500 V) and dose rates (approximately in the range DR = 10–200 μGy s⁻¹) to evaluate within a single measurement session the main figures of merit of an X-ray detector: linearity with dose rate, sensitivity, response speed, and stability with time.

Fig. 3 shows the time-dependent net photocurrents measured under X-ray on/off cycles (duty cycle 50%, time period 240 s) at low (1–20 V, Fig. 3(a)) and high (20–500 V, Fig. 3(b)) bias voltages, by progressively decreasing the dose rate at each “X-ray on” period. The net photocurrent I_ph is given by I_ph = I_tot − I_d, where I_tot is the total current measured under irradiation. As expected, we can observe that: (1) at a given applied bias voltage, I_ph increases with increasing dose rate; (2) at a given dose rate, I_ph increases with increasing bias voltage. It is particularly worth noting the presence of a detectable signal even at V_BIAS = 1 V, which corresponds to an applied electric field of only 10⁻³ V μm⁻¹. Interestingly, the shape of the pulses is considerably different in the two operating regimes, as can be better appreciated in Fig. 4(a), showing a zoomed image of all the X-ray photocurrent pulses recorded in the bias voltage range of 1–500 V at the exemplary dose rate DR = 164 μGy s⁻¹. It can be seen that at low bias voltages (1–20 V) the photocurrent fairly retraces the X-ray pulse excitation: the signal rapidly rises when the X-ray tube is switched on, then immediately settles down to its stationary value, and finally falls quickly to zero when the tube is switched off. Conversely, at high bias voltages (V_BIAS > 20 V), after an initial overshoot followed by a rather fast decay, the signal increases very slowly towards its stationary value, which, however, is not even reached at the end of the “X-ray on” period (120 s); finally, an exponentially decaying residual signal is present after the tube is switched off.

Fig. 3 Time-dependent photocurrent of the fabricated photoconductive detector measured under X-ray on–off cycles at low (a) and high (b) bias voltages. The time period of a single on–off cycle was set to 240 s, with a 50% duty cycle. Dose rate progressively decreased from 189 to 10.3 μGy s⁻¹ at each “X-ray on” period by decreasing the X-ray tube current from 40 to 2.5 mA (at a fixed tube acceleration voltage of 40 kV).

Fig. 4 (a) Photocurrent signals recorded during a single “X-ray on” period (120 s) at different bias voltages; (b) stationary values of X-ray photocurrent as a function of the delivered dose rate at different bias voltages. Continuous lines represent the best power fit to the experimental data according to the Fowler's model (I_ph ∝ DR^Δ). The plot shown in the inset reports the estimated values of Fowler's linearity coefficient (Δ) for the different bias voltages.

By considering the stationary X-ray net photocurrent values (extrapolated in the case V_BIAS > 20 V) obtained from Fig. 3 at the different applied bias voltages, it is possible to analyze the behavior of the induced signal with the delivered dose rate (Fig. 4(b)). As can be seen, the device exhibits a fairly linear photoresponse with dose rate. The dependence perfectly follows the Fowler's model⁴² for electrical conductivity of solid-state detectors, according to which I_ph ∝ DR^Δ, where Δ is the linearity coefficient. Values of Δ around unity are always desirable, even mandatory in the case of applications where linearity with the delivered dose rate is essential, such as dosimetry for radiotherapy. In the case of the device investigated in this work, Δ increases with increasing bias voltage in the range Δ = 0.76–0.99 (see inset of Fig. 4(b)). It is known from the literature on ionizing radiation detectors based on wide-bandgap semiconductors⁴³ that the value of Δ is strongly influenced by the presence of defects, the role of which in the dynamics of X-ray photogenerated charge transport is in turn significantly dependent on the intensity of the applied electric field. Indeed, when defects cause trapping of the photogenerated carriers, charge collection efficiency unavoidably decreases, unless the applied electric field is intense enough to induce detrapping, thus compensating for the temporary signal loss. This is most probably what happens in the case of κ-Ga₂O₃. At low fields, charge trapping results in a slightly sub-linear photoresponse with dose rate (Δ = 0.76–0.88 in the 1–10 V range), but linearity progressively improves at higher fields (Δ = 0.94–0.99 in the 20–500 V range), approaching the ideal value Δ = 1.

The fairly linear relationship between I_ph and DR allowed us to estimate the surface specific sensitivity of the device, defined as S = I_ph/(A·DR), where A = 4.2 × 4.2 mm² is the active area of the detector. The estimation was performed by fitting the photocurrent data reported in Fig. 4(b) with linear curves (not shown), and then evaluating the different slopes as a function of the bias voltage. Fig. 5 reports the values obtained for S, ranging from 0.50 ± 0.04 nC mGy⁻¹ cm⁻² at V_BIAS = 1 V to 15.06 ± 0.27 nC mGy⁻¹ cm⁻² at V_BIAS = 500 V. Note that the relative uncertainty decreases with increasing bias voltage, owing to the better linearity of the photoresponse. If we consider the film thickness (440 nm), the normalized sensitivity to the active volume of the detector ranges from 11.42 ± 1.02 μC mGy⁻¹ cm⁻³ at V_BIAS = 1 V to 342.29 ± 6.24 μC mGy⁻¹ cm⁻³ at V_BIAS = 500 V; it is worth stressing here that the latter is an unprecedented value for Ga₂O₃-based X-ray detectors.^28–32 Responsivity, defined as R = I_ph/DR, ranges from 0.09 ± 0.01 μC Gy⁻¹ at V_BIAS = 1 V to 2.66 ± 0.05 μC Gy⁻¹ at V_BIAS = 500 V, and is comparable to values reported in the literature for bulk single-crystal β-Ga₂O₃.^31,32

Fig. 5 Surface specific sensitivity of the fabricated X-ray κ-Ga₂O₃ detector as a function of the bias voltage. Error bars are related to the linear fitting of the experimental data shown in Fig. 4(b).

Unfortunately, the price to pay for a better linearity with dose rate and specific sensitivity at higher electric fields (in addition to the application of a high bias voltage, which is not desirable in any case) is a significantly longer time for the detector to reach a steady-state condition under irradiation, as it clearly appears from the 200 V and 500 V curves of Fig. 3(b) and 4(a). As can be seen, at high V_BIAS, a photocurrent overshoot is triggered immediately after the irradiation starts, the amplitude of which increases with both increasing dose rate and bias voltage, i.e., both when a large amount of charge carriers is photogenerated (at higher DR) and is effectively collected by the electrodes (at higher V_BIAS).

Photocurrent overshoots at the beginning of X-ray irradiation have already been observed in β-Ga₂O₃-based detectors,²⁹ but no possible explanation was provided. However, transient photocurrent peaks appearing in a wide range of photoconductive devices, from diamond-based X-ray detectors⁴⁴ to organic solar cells⁴⁵ and light-emitting diodes,⁴⁶ have been explained in the literature by trap-mediated space-charge effects. This is most likely what happens also in the case of κ-Ga₂O₃ at high bias voltages. At the beginning of the “X-ray on” period, the very fast rise of the photocurrent is due to the large amount of collected photogenerated carriers; however, we suppose that part of them gets trapped beneath the electrodes, causing space charge formation. As a consequence, an internal polarization electric field builds-up opposing the external bias, so that the charge collection efficiency is reduced, making the photocurrent decrease after peaking. A steady-state photocurrent should be reached at the end of the overshoot, soon after an equilibrium is established when polarization settles, but in our case the overshoot is followed by a very slow increase of the photocurrent, eventually exceeding (at the end of the “X-ray on” period) the overshoot peak value. This behavior may be explained by supposing that charge detrapping is triggered in the high-bias regime. To support this hypothesis, it is worth noting that the increase of photocurrent after the overshoot is more pronounced at higher bias voltages and higher dose rates, i.e., when the electric field is strong enough to promote charge detrapping and at the same time the radiation intensity is high enough to induce a significant trap-refilling.

A quantitative analysis of the timescales of the peak decay and the slow increase of photocurrent after the overshoot was performed, aimed at shedding light on the involved mechanisms. Two values of bias voltages were considered, namely, 200 V and 500 V. Curve fitting to the experimental data was performed by using the sum of two exponential functions, in the following form:

where (I₀ + I₁) is the photocurrent value corresponding to the overshoot peaking time, (I₀ + I₂) is the extrapolated steady-state value of the photocurrent, τ₁ is the time constant of the peak decay, and τ₂ is the time constant of the slow increase of the photocurrent after the overshoot. Results are reported in Fig. 6 for the exemplary case of DR = 164 μGy s⁻¹.

Fig. 6 Curve fitting to experimental data of X-ray photocurrent at V_BIAS = 500 V and (inset) V_BIAS = 200 V. Dashed lines represent the best fit curves obtained by using eqn (1). For the sake of simplicity, the overshoot peaking time was assumed to be t = 0.

The obtained values, averaged over all the dose rates in the range of 22.3–189 μGy s⁻¹ (where the overshoot is present), are τ₁ = 2.13 ± 0.07 s and τ₂ = 32.48 ± 0.57 s for 200 V, and τ₁ = 3.57 ± 0.09 s and τ₂ = 31.82 ± 0.41 s for 500 V. Such values are consistent with the time constants of charge trapping/detrapping mechanisms reported for both β-Ga₂O₃- and κ-Ga₂O₃-based MSM photodetectors for UV radiation.^47,48 It is worth recalling here that trapping/detrapping time constants are governed by the energy depth of the traps,⁴⁸ and that longer time constants are related to deeper traps. As can be inferred from our results, the timescale (few seconds) of the peak decay (τ₁) is consistent with charge trapping by shallow defects, responsible for the space charge formation after peaking, whereas the very high values (few tens of seconds) of τ₂ justify a predominant detrapping from deep levels in the second stage of the transient regime. Once trapping and detrapping rates are equal, and a constant electric field is established between the collecting electrodes as well, a steady-state photocurrent is eventually reached.

To further support our hypotheses on the role of sub-bandgap defects on charge transport, spectral photoconductivity measurements were performed in the 200–1000 nm range. Fig. 7 reports the spectral responsivity SR of the κ-Ga₂O₃ epitaxial thin film measured at the same bias voltages (1–500 V) used for the X-ray characterization session; SR was calculated as a function of the impinging wavelength λ as SR(λ) = |I_ph(λ)|/P(λ), where |I_ph| is the modulus of the measured modulated photocurrent, and P is the power of the monochromatic radiation focused on the active area of the device. As expected, responsivity increases with increasing bias voltage, denoting a progressively better charge collection efficiency. In addition, all spectra show the presence of shallow traps, appearing as band-tails for λ_gap < λ < 320 nm, where λ_gap ≈ 270 nm is the wavelength corresponding to the energy bandgap (≈4.6 eV). More interestingly, note that photoconduction mediated by deep traps (λ > 320 nm) is boosted in the high-bias regime, increasing considerably at V_BIAS = 50, 200, and 500 V; this experimentally confirms our hypothesis on detrapping from deep levels as the predominant photoconduction mechanism acting in the second stage of the X-ray photocurrent transient regime.

Fig. 7 Spectral responsivity of the κ-Ga₂O₃ epitaxial thin film in the 200–1000 nm range at different bias voltages. Dashed line indicates the wavelength λ_gap ≈ 270 nm corresponding to the energy bandgap (≈4.6 eV). Measurement points within the grey band have to be considered below the background noise level.

In addition to sensitivity and linearity with the dose rate, another valuable figure of merit for X-ray detectors is the response speed. By taking a closer look at the shape of a photocurrent pulse recorded during a single “X-ray on” period (Fig. 8), it is possible to evaluate the response speed of the κ-Ga₂O₃ film in terms of rise time t_r (defined as the time required for the pulse to rise from 10% to 90% of its peak value) and decay time t_d (defined as the time required for the pulse to decay from 90% to 10% of its peak value). At low bias voltages (1–10 V), t_r and t_d are approximately equal, and vary in the range of 1–2 s; in particular, both t_r and t_d decrease slightly with increasing applied electric field (see Fig. 8(a) and (b)). Both the measured values (in the range of a few seconds) and the dependence on the bias voltage are in agreement with the typical electron/hole trapping effect, already reported in the literature for β-Ga₂O₃.⁴⁹ A higher bias voltage leads indeed to a faster charge collection and reduces the charge trapping probability, decreasing both the rise and the decay time; in this sense, narrowing the electrode spacing (by using interdigitated contacts, for example) may be surely helpful for speeding up the photoresponse in the low-bias regime. When V_BIAS = 20 V, t_r and t_d are both lower than 0.5 s; however, it is worth stressing here that this value should be considered as an upper limit for our device, because it represents the minimum delay time between two consecutive measurement points obtainable with the electrometer used for the experiments. Indeed, as recently reported on X-ray detectors based on ε-Ga₂O₃ epitaxial thin films grown by MOCVD,^50,51 and tested with a faster front-end electronics, the actual rise and decay times may be significantly lower (<50 ms).

Fig. 8 Rising (a)–(c) and decaying (b)–(d) dynamics of photocurrent pulses recorded during a single “X-ray on” period (120 s) at low (a), (b) and high (c), (d) bias voltages. Dose rate was set to 45.3 μGy s⁻¹, but similar results have been obtained at all other investigated dose rate values.

As can be inferred from Fig. 8, the biasing condition V_BIAS = 20 V ensures the best performance in terms of response speed and recovery time; the signal reaches its stationary value soon after X-ray irradiation starts, then immediately falls to zero when X-ray is shut down. At higher bias voltages (V_BIAS > 20 V), pulse rising and decaying dynamics gets progressively slower (see Fig. 8(c) and (d)). In this operating regime, the applied bias is indeed high enough to trigger both space charge formation and detrapping, thus requiring longer times to obtain a stable photocurrent when X-rays are on, as well as to recover the initial conditions at the end of the irradiation period. We can therefore deduce that V_BIAS = 20 V represents an optimal operating condition for the fabricated device, ensuring at the same time a fast response (t_r, t_d < 0.5 s) and a very good specific sensitivity (S = 5 nC mGy⁻¹ cm⁻²) to X-rays; most probably, the corresponding applied electric field is in this case high enough to ensure a fast and efficient charge collection, and at the same time low enough to avoid both photocurrent “crowding” beneath the electrodes (thus hampering space charge formation) and detrapping from deep levels.

Operation of the device at the optimal bias voltage (20 V) was then used to verify the signal reproducibility at a given dose rate, as well as to preliminarily assess the detector's response stability with time. Fig. 9 reports the X-ray photocurrent signal recorded at V_BIAS = 20 V in a 2 h measurement session at the highest investigated dose rate (189 μGy s⁻¹). In the first part of the session, the device was tested with 10 X-ray on–off cycles (duty cycle 50%, time period 360 s), and the recorded signal pulses demonstrated an excellent reproducibility. Then, the device was kept for 60 min under the maximum X-ray flux, producing a perfectly stable signal, only varying within ±1.5% of its average stationary value (165.5 pA, see Fig. 9).

Fig. 9 X-ray photocurrent recorded with the fabricated κ-Ga₂O₃-based detector in a measurement session consisting of two parts: (1) 10 on–off cycles (6 min each); (2) 1 h of continuous irradiation. Bias voltage and dose rate were set to 20 V and 189 μGy s⁻¹, respectively.

X-ray irradiation was finally profitably used to evaluate the average mobility-lifetime product μ_e,hτ_e,h of the photogenerated carriers, which is the most significant figure of merit for the charge transport properties of a semiconductor, being directly related to the concentration of defects acting as recombination centers for both electrons and holes. (i.e., it provides an estimate of the crystalline quality of the deposited film). A simple way to extract the μ_e,hτ_e,h product is by fitting the X-ray photocurrent I_phvs. V_BIAS curve according to Hecht's equation:⁵²

where d is half (0.5 mm) the spacing between the two Au metal contacts on the top surface of the κ-Ga₂O₃ film, which represents the average distance travelled by a photogenerated charge carrier before being collected.

Fig. 10(a) reports the experimental results obtained by irradiating the fabricated detector at different dose rates. Best fits returned μ_e,hτ_e,h = (1.91 ± 0.07) × 10⁻⁴ cm² V⁻¹ at DR = 189 μGy s⁻¹, μ_e,hτ_e,h = (2.37 ± 0.08) × 10⁻⁴ cm² V⁻¹ at DR = 91.9 μGy s⁻¹, and μ_e,hτ_e,h = (3.13 ± 0.12) × 10⁻⁴ cm² V⁻¹ at DR = 45.3 μGy s⁻¹. The first result to be noted is that all the estimated μ_e,hτ_e,h values are about one order of magnitude higher than those recently reported for single-crystal Fe-doped β-Ga₂O₃-based detectors tested under the same experimental conditions (type of X-ray source, accelerating voltage, and tube current).²⁶ This is most probably due to a higher lifetime of the photogenerated carriers before recombination; indeed, high Fe-doping of β-Ga₂O₃, necessary to increase the material resistivity and lower the dark current, greatly reduces both the carrier drift length and the carrier collection efficiency, due to the introduction of deep-level impurities.⁵³ Moreover, it is worth noting from Fig. 10(a) that μ_e,hτ_e,h decreases slightly with an increasing dose rate, presumably due to a higher density of photogenerated charge, enhancing carrier–carrier scattering effects which can limit carrier mobility.

Fig. 10 (a) Experimental results for the evaluation of the average mobility-lifetime product at different dose rates. Bias voltage was varied in the 1–500 V range. In all the cases, dashed lines show the best fit to data obtained by using Hecht's eqn (2); (b) the same plot as Fig. 10(a) zoomed in the 1–10 V range, aimed at showing the ohmic behaviour of the metal contact under X-ray irradiation. Dashed lines represent the best linear fit to experimental data.

It is also interesting to observe that the ohmicity of the metal contact remains even under X-ray irradiation, as can be inferred from Fig. 10(b), showing a zoomed image (in the 1–10 V range) of the plot reported in Fig. 10(a). In this regime of applied bias voltages, in which the photocurrent saturation is not started yet, the exponential term of Hecht's equation can be indeed neglected to a first approximation, and the relationship between I_ph and V_BIAS is linearized. In this way, the contact ohmicity can be correctly verified, as in our case. Moreover, it can be inferred from Fig. 10(b) that the slope of the linear fitting curve increases with increasing dose rate, as expected from a photoconductive device, the conductivity of which is directly proportional to the intensity of the absorbed radiation.

Conclusions

Epitaxial orthorhombic κ-Ga₂O₃ was tested for the first time under X-rays to assess its suitability as an active material for the development of photoconductive direct X-ray detectors. The results showed that κ-Ga₂O₃ offers an optimal combination of properties useful for the development of sensitive, fast, and stable X-ray detectors with minimum power consumption. This can be inferred from Table 1, comparing the main X-ray photoresponse properties of κ-Ga₂O₃ obtained in this work to those of other Ga₂O₃-based active materials recently reported in the literature.

It is worth highlighting that κ-Ga₂O₃ shows the highest normalized sensitivity to the active volume, and is the only polymorph which demonstrates excellent stability under 120 min of X-ray irradiation, without showing any degradation of the photoresponse. It shares with amorphous (a-) Ga₂O₃ and nanocrystalline (nc-) β-Ga₂O₃ the advantage, if compared to β-Ga₂O₃ single crystals, of being deposited as a thin film, allowing for the fabrication of large-area devices. It is slightly slower than nc-β-Ga₂O₃, but significantly faster than a-Ga₂O₃, and more responsive than both in a wider operating voltage range. Fe-doped single-crystal (sc-) β-Ga₂O₃ shows better properties than κ-Ga₂O₃ in terms of rise time and responsivity, but at the expense of very high bias voltage, which makes it unsuitable, for instance, for the implementation of low-power portable devices; conversely, undoped sc-β-Ga₂O₃ can operate at low bias voltages, but it shows both a slower response and a lower responsivity than κ-Ga₂O₃.

In conclusion, in comparison with the other members of the gallium oxide family, κ-Ga₂O₃ demonstrated to be the most versatile candidate for the fabrication of direct X-ray detectors, the performance of which can be tailored to the specific user's requirements, depending on the operating voltage. High bias regimes (V_BIAS > 20 V) are to be preferred to maximize sensitivity and linearity with X-ray dose rate regardless of the response speed, but low bias operation (down to 1 V) is also possible, with the advantage of a very low power consumption. In this work, it is shown that an intermediate bias voltage (V_BIAS = 20 V) is the optimal solution, ensuring the best trade-off between sensitivity and linearity with the dose rate, as well as a fast and rapidly stable photoresponse. Anyway, it is finally worth mentioning that, by reducing the gap between the electrodes at values lower than 1 mm (e.g., by the use of interdigitated electrodes fabricated by photolithographic techniques), it is possible to further reduce the value of the applied bias voltage while maintaining the same charge collection performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank Antonello Ranieri (IC-CNR, Istituto di Cristallografia, Consiglio Nazionale delle Ricerche) for the valuable technical support, Antonella Parisini, Maura Pavesi, Alessio Bosio, and Francesco Mezzadri (Università di Parma) for useful discussion and preliminary evaluation of the κ-Ga₂O₃ epitaxial films used in this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc05297k

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